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Toward Real-Time Monitoring and Control of Single Nanoparticle Properties with a Microbubble Resonator Spectrometer

Cite this: ACS Nano 2019, 13, 11, 12743–12757
Publication Date (Web):October 15, 2019
https://doi.org/10.1021/acsnano.9b04702

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Abstract

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Optical microresonators have widespread application at the frontiers of nanophotonic technology, driven by their ability to confine light to the nanoscale and enhance light–matter interactions. Microresonators form the heart of a recently developed method for single-particle photothermal absorption spectroscopy, whereby the microresonators act as microscale thermometers to detect the heat dissipated by optically pumped, nonluminescent nanoscopic targets. However, translation of this technology to chemically dynamic systems requires a platform that is mechanically stable, solution compatible, and visibly transparent. We report microbubble absorption spectrometers as a versatile platform that meets these requirements. Microbubbles integrate a two-port microfluidic device within a whispering gallery mode microresonator, allowing for the facile exchange of chemical reagents within the resonator’s interior while maintaining a solution-free environment on its exterior. We first leverage these qualities to investigate the photoactivated etching of single gold nanorods by ferric chloride, providing a method for rapid acquisition of spatial and morphological information about nanoparticles as they undergo chemical reactions. We then demonstrate the ability to control nanorod orientation within a microbubble through optically exerted torque, a promising route toward the construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator spectrometer technology by permitting room-temperature, aqueous experimental conditions, which may be used for time-resolved single-particle experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.

KEYWORDS:
Optical microresonators, devices that confine light to microscopic volumes, have found widespread application within chemistry, biology, physics, and engineering. (1−5) A broad class of optical microresonators, whispering gallery mode (WGM) resonators, have exhibited superb sensitivity including the detection of single nanoparticles, (6,7) single molecules, (8−11) and even single metal ions. (12) However, the ability to perform spectroscopy on adsorbed objects would not only allow for label-free chemical identification but also allow the interrogation of single object properties, free from the static and dynamic blurring of typical ensemble measurements. To this end, we recently employed microtoroid resonators as single-particle absorption spectrometers, whereby the heat dissipated by optically pumped nano-objects such as gold nanorods (AuNRs), (13−16) carbon nanotubes, (17) or conductive polymers (18) is detected via small shifts in the WGM resonance condition. However, to harness the sensitivity of this method for chemically dynamic systems, a platform easily compatible with solution-phase measurements is necessary. Here, we report such a platform, the microbubble resonator, and use it to study the photoactivated chemical etching and reorientation of single AuNRs.
AuNRs (19) have important chemical and biological (20) applications such as bioimaging, (21) treatment of cancer (22,23) and infection, (24) label-free biosensing (25) down to single molecules, (26) surface-enhanced Raman spectroscopy, (27,28) fluorescence enhancement, (29) drug delivery, (30) and light harvesting to drive catalytic reactions. (31) These applications heavily rely on tuning the morphology-dependent optical features of AuNRs, necessitating precise tailoring of their dimensions. This result can be achieved during AuNR fabrication, where seed-mediated synthesis (32) can often tame the polydispersity that typically plagues samples. However, in many cases, polydisperse AuNR samples are still common, and post-synthetic modifications offer an attractive route to achieve a desired morphology. Furthermore, significant particle-to-particle variations of key optical properties of AuNRs both on surfaces (33) and in solution (34) highlight the heterogeneity within a population of nanoparticles, underscoring the need for single-particle inspection, including during nanoparticle synthesis and modification. A variety of optical methods exist for probing nonluminescent single nanoparticles and molecules via photothermal, (35−38) scattering, (39−41) and other techniques. (42−45) Observation of the chemical etching of single AuNRs has recently been accomplished with one-photon luminescence, (46,47) dark-field scattering, (48−54) and liquid transmission electron microscopy (TEM). (55) However, a highly sensitive absorption technique for monitoring such chemical dynamics is needed to compliment these methods and would be extremely valuable for accessing targets that are not luminescent or are too small for scattering experiments. WGM resonators are perfectly poised to fill this gap in methodology.
Various WGM microresonator geometries have been employed for sensing in solution, including microspheres, (9−11) microrings, (5,56) microtoroids, (6,57) microbubbles, (58,59) microdroplets, (60) microtubes, (61) and microbottles. (62,63) In particular, the variations and capabilities of hollow microresonators for sensing have been reviewed in detail elsewhere. (64) To adapt a microresonator for in-solution, visible-wavelength photothermal spectroscopy, three requirements must be met: (i) high sensitivity for interrogating nanoscopic analytes, (ii) resonator transparency at visible wavelengths to mitigate photothermal background, and (iii) robust performance in solution. Employing silica-on-silicon (SiO2-Si) microtoroids for photothermal spectroscopy, one can resolve attometer shifts of the WGM resonant wavelength from thermal fluxes of target nano-objects. (13) High backgrounds in SiO2-Si toroids can be mitigated with all-glass microtoroids, which can be used for visible spectroscopy. (15,16) However, immersing a WGM microresonator in water mandates the use of larger microresonators to avoid bending losses, (65) with consequent lower photothermal sensitivity. Furthermore, although tapers (66) and prisms (9) can be optically coupled to WGM microresonators in water, immersion of such couplers in solution may reduce mechanical stability and also result in fouling, particularly as more caustic reagents are employed for chemical studies. Therefore, an alternative platform is preferable for in-solution experiments. The microbubble WGM resonator, Figure 1B, which possesses a hollow, solution-accessible interior, while maintaining an air-glass exterior interface, meets the requirements for in-solution, visible spectroscopy of nanoscopic analytes.

Figure 1

Figure 1. Microbubble absorption spectroscopy. (A) Cartoon of instrumentation. PDH = Pound–Drever–Hall. LC = Liquid crystal. APD = Avalanche photodiode. (B) Optical micrographs of two microbubble resonators with different geometries. Scale bars 20 μm. (C) Photothermal maps of a microbubble resonator similar in geometry to the left microbubble in (B), both out-of-focus (left) and in-focus (right). Scale bars 20 μm.

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Microbubbles are fabricated from glass capillaries, resulting in low background signals at visible wavelengths, tunable fabrication, and two-port connectivity through which it is easy to flow reagents. Compared to a solid resonator immersed in solution, a microbubble maintains an air–glass interface on its exterior, enabling a higher refractive index contrast and allowing for smaller diameter resonators before bending losses occur. Additionally, the tapered optical fiber used for coupling light into the resonator can approach in air, reducing noise from the instability of coupling in-solution and eliminating solution contamination of the taper. Furthermore, the unique, thin-walled structure of the microbubble allows for high-order optical modes that exist almost entirely within the liquid-core of the resonator, a situation termed the “quasi-droplet regime”. Operating in the quasi-droplet regime, microbubbles have proven to be exceptional sensors, most recently for detecting polystyrene nanoparticles in aqueous solution with a sensitivity ∼280 times larger than similar experiments using microsphere resonators. (58) Together, these factors make microbubble resonators ideal for time-resolved spectroscopy of single-particle chemical reaction dynamics when exposed to solution. In this paper, we introduce microbubble absorption spectrometers for probing and controlling the chemical and rotational dynamics associated with the photoactivated chemical etching of AuNRs. This platform holds promise for elucidating mechanistic insights into nanoparticle reactions with a method orthogonal to existing techniques that rely on scattering or luminescence and is an attractive candidate for future single-particle and single-molecule studies.

Results/Discussion

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Experimental Design

WGM resonators operate via total internal reflection, wherein light propagates around a closed geometric loop, resulting in resonance conditions where only specific wavelengths propagate constructively. WGM resonances are interrogated by the “probe beam”, provided by a continuous wave (CW), narrow-line width, tunable laser coupled through a tapered optical fiber (67) (Figure 1A). Transmitted light through the tapered fiber is collected, and the probe beam is actively locked to a resonance by a Pound–Drever–Hall locking system. (6,13,68−70) The hollow core of the microbubble is filled with the desired reagents by attaching the microbubble capillary to a syringe pump. Two microbubbles are pictured in Figure 1B, highlighting the tunability of geometric parameters, and consequent versatility on optofluidic properties.
A second beam, the “pump beam”, is focused onto the microbubble surface to excite analytes. CW diode lasers at 532, 635, and 785 nm are coaligned using dichroic mirrors, permitting interrogation at different wavelengths. The linearly polarized pump beam is amplitude modulated at 433 Hz using an optical chopper, encoding the photothermal signal at this frequency and allowing for use of lock-in amplification to drastically lower the experimental noise floor. (13) Two galvanometer mirrors steer the pump beam through a relay lens system to a 40× objective with piezo-controlled focus, providing spatial control of the pump beam on the microbubble resonator. This spatial control is leveraged to photothermally map the interior surface of microbubble resonators, at low resolution for an entire resonator and high resolution for single diffraction-limited objects. Figure 1C shows two photothermal maps of a microbubble resonator at different objective foci, with the out-of-focus map indicating the curvature of the microbubble from the varied PSFs across the map. The polarization angle of the linearly polarized pump beam is rapidly scanned using a voltage-controlled liquid crystal, which is sandwiched between a polarizer and a quarter-waveplate (see Methods). This combination of wavelength, spatial, and polarization control permits thorough characterization of individual analytes bound to the resonator, realized at exquisite sensitivity due to the double-modulation scheme.

Operation of Single-Particle Microresonator Spectrometers

Microresonator sensing schemes generally rely on the reactive mechanism, (10) whereby binding of an analyte imparts a small refractive index change, shifting the resonance wavelength. Instead, microresonator-based photothermal spectroscopy relies on a resonance shift resulting from the heat plume generated by optically pumping a non-emissive object bound to the resonator surface. (17) The temperature rise accompanying this heat plume alters the resonator’s refractive index according to its thermo-optic coefficient (dn/dT), changing the WGM optical path length and shifting the resonance condition. The ability to detect this resonance shift is related to the figure of merit Q/V, the ratio of the resonator’s quality factor (Q) and mode volume (V). A resonator with minimized absorption, bending, and scattering losses allows photons to repeatedly circulate the resonator, resulting in a high Q, narrow line width resonance. This narrow line width increases the visibility of minute resonance shifts. A smaller resonator with consequent tighter confinement of light produces a smaller V, increasing the overlap between the thermal plume of the analyte and the optical mode. This increased overlap contributes a larger effective refractive index change and thus a larger resonance shift. (17) To properly examine the microbubble photothermal response, we employed finite-element simulations (COMSOL) of both the optical modes and the thermal properties of the microresonator. Simulated optical modes for a particular microbubble geometry are shown in Figure 2A. Varying mode numbers, defined in the traditional spherical geometric indices (polar, azimuthal, and radial, Supporting Information), clearly show the complicated mode structure inherent in the microbubble resonators. This complex mode structure gives rise to several important experimental considerations.

Figure 2

Figure 2. Optical resonances in microbubble resonators. (A) Simulated electric field distributions at 780 nm for first-, second-, and third-order radial modes, for both first- and second-order polar modes. All modes shown are transverse electric (TE). White curves are added to clearly indicate the position of the microbubble walls. (B) A 180 pm span of the mode spectrum of a microbubble resonator. (C) Left: The signal at the beginning of analyte pumping. Right: Signal once the resonator has reached a thermal equilibrium with its surroundings (theoretical). (D) Resonance shift from pumping a single AuNR with the 635 nm beam at decreasing powers (blue points). The red point indicates the signal for pump beam off. The inset is a zoom-out, showing signal linearity over orders of magnitude in pumping power. Further details in main text. Error bars are standard deviation of the mean.

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First, the efficient excitation of high-order modes leads to incredibly congested mode spectra. An illustrative 180 pm window of a water-filled microbubble’s resonance landscape is shown in Figure 2B. This high mode density stems from the highly prolate resonator geometry lifting the polar mode degeneracy relative to an ideal spherical resonator, (71) leading to varying effective resonator sizes for modes, as well as differing free-spectral ranges. (72) The differing free-spectral ranges cause spectral overlap of modes of different azimuthal mode order, (73) an effect that is compounded by the disparate dielectric environments experienced by different-order radial modes, which have different fractions of the electric field contained in glass, water, and air. Second, the burrowing of higher-order radial modes into the water-filled interior not only changes the effective refractive index of the mode but also yields tremendous variations in dn/dT. This varied dn/dT, which can even switch signs, produces very different thermal responsivities for modes. The combined congested mode spectrum and differential shifting from dn/dT variation amplifies experimental challenges, as photothermal heating or ambient temperature drifting can cause modes to shift through each other. Therefore, modes that are both thermally responsive and spectrally isolable are desirable.
Choosing a high thermal responsivity mode requires delving into the expected thermal response with finite element simulations. As described above, the radial mode order drastically alters the effective dn/dT, as glass has a small positive dn/dT of 9 × 10–6 K–1 and water a large negative dn/dT of −91 × 10–6 K–1. Thus, while glass-contained modes in a water-filled resonator offer Q values over 106 and show small positive resonance shifts upon heating, higher-order, water-contained modes offer Q values of mid-105 and show large negative resonance shifts captured both experimentally (Supporting Information) and in our simulations in Figure 2C. The “shark fin” shape results from the pump beam amplitude modulation. Interestingly, this modulation rides atop a rising baseline magnitude as heat builds over many modulation cycles (left panel) before thermal equilibrium is reached (right panel). This baseline stems from the lack of an effective, proximal heat sink in microbubbles with equilibration reached only after sufficient heat dissipation to the air, yielding a baseline shift about 10 times larger than the modulating shift, both experimentally and theoretically (Supporting Information).
Finding high-order water-contained radial modes to leverage their larger thermal response requires careful consideration of the coupling geometry. Specifically, the tapered fiber diameter (72) greatly impacts mode selectivity through phase matching conditions and evanescent field overlap. By translating along the length of the tapered fiber, this diameter was tuned until these water-contained modes were suitably excited. Then a thermally sensitive and spectrally isolated mode was selected by wavelength scanning. Importantly, the precise identity of this mode was not discerned, precluding direct relation of a resonance shift with an absolute absorption cross-section as in our previous experiments. (13,17) Identification is possible, (74−76) particularly when implementing procedures to simplify the mode structure, (73,77) but difficult in practice and was not pursued here.
Importantly, this lack of mode identification precludes the selection of the maximally thermally responsive mode. While the optimal mode is required for ideal sensor response, use of a less responsive resonance was sufficient for examination of AuNRs. The limit-of-detection of our system was investigated by optically pumping a single AuNR inside a resonator. The photothermal signal, averaged for 30 s with a time constant of 1 s (Figure 2D), was monitored at decreasing powers until it was indistinguishable from the signal obtained with the pump beam blocked. As the inset in Figure 2D shows, the signal remains linear over multiple orders of magnitude, flattening out at low powers as the noise floor is reached. The detection limit for this platform is in the low tens of attometers of wavelength shift (a comparison with microtoroids is made in the Supporting Information). For context, the typical photothermal response of a single AuNR at our pump fluxes is in the range of 10–100 fm, easily resolvable by many orders of magnitude. Additionally, this detection limit surpasses the expected femtometer photothermal shift for measuring a single chromophore. (17) As a first step toward monitoring reaction dynamics of molecules, we show below that microbubbles are well-suited for probing the chemical and spatial dynamics of single AuNRs.

Probing Photophysical Features of Single AuNRs

AuNRs exhibit optical features known as localized surface plasmon resonances (LSPRs), which result from light exciting collective oscillations of conduction band electrons. Two orthogonal LSPRs exist in AuNRs: the longitudinal plasmon band (LPB) and the transverse plasmon band (TPB), oriented parallel and perpendicular, respectively, to the long axis of the rod (Figure 3A). The LPB is at the longer wavelength in the bulk extinction spectra of the AuNRs used in this report (80 × 40 nm), Figure 3B. These spectral features are probed with the microbubble platform detailed in Figure 1A at the single AuNR level, at specific pump beam wavelengths (solid vertical lines Figure 3B). The LPB central wavelengths will likely be red-shifted compared to the bulk due to interaction with the glass surface. (39,78,79)

Figure 3

Figure 3. Probing photophysical features of single AuNRs. (A) Cartoon illustrating the photophysical features of a AuNR. LPB = Longitudinal plasmon band. TPB = Transverse plasmon band. (B) Bulk absorption spectrum of AuNRs, with the various laser beams in our experiment indicated by vertical lines. LPB and TPB indicated. (C) Example photothermal maps of a nanorod as pump polarization are varied in increments of 20°, as shown by the red arrow in the cartoon above the photothermal maps. Scale bar 1 μm. (D) Polarization fits for three different pump beams acquired using photothermal mapping. (E) Polarization traces for three different pump beams, acquired by recording photothermal signal as the linear pump polarization is quickly rotated 180° (∼10 s).

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After depositing AuNRs inside of a microbubble resonator (Methods), the resonator is photothermally mapped to find objects. To confirm successful deposition of single AuNRs, photothermal maps are acquired with the pump beam, linearly polarized from 0 to 180° (Figure 3C) at three different pump wavelengths. These maps are fit to extract an intensity at each polarization (Methods), shown as data points in Figure 3D. These polarization-dependent intensities are fit (dotted lines) to give a depth-of-modulation, M (Methods). The 635 and 785 nm traces, which probe the LPB, have a value of M close to unity for a single AuNR. A criterion of M ≥ 0.98 was used for classifying an object as a single AuNR. Small well-ordered aggregates could also exhibit high M values, but these are unlikely due to the presence of CTAB during deposition. Alternatively, a much faster method of probing AuNR orientation is to rapidly rotate the pump beam’s linear polarization while centered on an object, and fit the results to extract M. Although this method (Figure 3E) lacks background subtraction, it allows for hundreds of data points to be collected in a few seconds, resulting in quick determination of AuNR orientation and relative absorption cross-section at the pumping wavelength during reactions.
For both of the above polarization methods, the traces for the 635 and 785 nm pump beams align in peak angle (Figure 3D,E) because both of these wavelengths excite the LPB. In contrast, the 532 nm trace has a peak angle orthogonal to the other two traces because this wavelength excites the TPB at a pump polarization orthogonal to the LPB excitation. In addition, the 532 nm trace does not go to zero, because at that wavelength the pump beam is not only pumping the polarization-dependent TPB but also the interband transitions of gold, which are independent of pump beam polarization. Notably, the use of multiple wavelengths means that AuNRs can not only be localized but also studied spectroscopically. Herein, these capabilities are employed to study the etching of AuNRs in real-time.

Selecting an Etchant

The photophysical properties of AuNRs are well understood as a function of geometry, (80) and post-synthetic modifications are extremely useful for exerting control over these properties. Since 2002, when Jana and co-workers observed anisotropic etching of gold spheroids in both cyanide and persulfate solutions, (81) at least 20 other reagents have been reported to etch or accelerate the etching of AuNRs, often with spatial selectivity (see Supporting Information). Such reports include assays for facile detection of analytes at ultralow concentrations in both environmental and biological samples, indicating the utility of morphological control of AuNRs both in-the-field and at points-of-care.
Most of the aforementioned reports used spectrophotometers to study ensembles of AuNRs. Although some studies used TEM intermittently to verify nanorod morphology, this approach is limited in time resolution and generally requires stopping reactions for analysis. Optical monitoring of reactions of single AuNRs can also be accomplished in situ. Dark-field spectral imaging has been used to study anisotropic etching of individual AuNRs by hydrogen peroxide, (49) potassium iodide/iodine, (50) and gold(III). (53) In a different experimental design, luminescence was employed to study the cyanide etching of AuNRs. (47) Additionally, dissolution of AuNRs via substrate voltage tuning has been monitored using dark-field hyperspectral imaging. (48,54) Perhaps the most commonly reported reagent for etching single AuNRs in recent years is iron(III) chloride, starting with bulk studies in 2009. (82) Since then, FeCl3 etching of single AuNRs has been reported using dark-field monitoring, sometimes utilizing Le Chatlier’s principle to drive the reaction. (51,52) Ferric etching of single AuNRs using an electron beam, monitored by liquid-TEM, has also been reported. (55) Excepting the electron beam study, these reports evoked purely chemical mechanisms to explain their reported chemistries. However, the light-induced etching of AuNRs using FeCl3 has also been reported, both in bulk studies (83) and single AuNR experiments using one-photon luminescence. (46) Due to significant interest in ferric etching of AuNRs and the intriguing mechanistic parameter space, FeCl3 was employed for etching in this report.

Single AuNR Reactions

After single AuNRs deposited in the microbubble resonator were identified, they were chemically etched using FeCl3. The etching solution, ranging between 250 μM and 2 mM FeCl3 dissolved in dilute hydrochloric acid (pH ∼ 1.3) to prevent hydrolysis of the oxidant, was flowed into the microbubble. Due to differences in sensitivity resulting from microbubble geometries, mode selection, and even nanorod location within the same microbubble, the relative photothermal signals between nanorods cannot be directly compared. However, the relative signal of one AuNR reacting over time, using the same resonance, directly maps onto a change in absorption cross-section of the nanorod at the pump wavelength and thus its etching progress. Conveniently, nanorod etching was found to be photoactivated by the pump beam illumination (discussed further below), allowing controlled reaction initiation. The AuNRs were monitored by repeatedly rotating the linearly polarized pump, interrogating the relative absorption and orientation of the AuNR as it is etched. Importantly, before each polarization trace is taken, a beam-centering algorithm is used to mitigate any false signal decrease from spatial drift of the bubble. The centering also serves as a “dosing” period to enable AuNR etching between polarization traces. Each polarization data point required only 50 ms of data acquisition time, suggesting that fast chemical dynamics can be followed with our approach. A further discussion of time resolution and imaging of small gold nanoparticles (AuNPs) is presented below.
Figure 4A(i-iii) features three exemplary traces of a single AuNR reaction (additional examples in the Supporting Information). These three reactions were taken in different microbubbles on different days, confirming reproducibility of the experiment. A logarithmic version of reaction (i) (Figure 4B) readily shows the late stage continued reaction progression along with AuNR rotation. This behavior is better illustrated in the extracted maximum signal and angle traces from reaction (ii) seen in Figure 4C,D, respectively. “Exposure time” refers to the total time that the AuNR has been exposed to the laser beam, which does not include the “switching time” at the dotted lines, where the laser was turned off so that the power could be increased. The reactions slowed as they progressed, as seen by the plateauing effect in the maximum signal. This plateauing is a direct result of the photoactivation mechanism: As a nanorod shrinks, its absorption cross-section decreases, resulting in less light absorption and thus slower etching. This photoactivation is further confirmed by incrementally increasing the pump power (dotted vertical lines), quickening the reactivity before it plateaus once again. Although the three reactions in Figure 4A were taken at ferric chloride concentrations spanning almost an order of magnitude, the time scales of reaction vary by much less (discussion in Supporting Information). It is also evident that AuNRs sometimes rotate as they etch, especially late in reactions, as seen in Figure 4D and discussed later (see Single AuNR Rotations).

Figure 4

Figure 4. Etching single AuNRs. (A) Reaction series of polarization traces for three difference reactions, progressing in time from red traces to blue traces. Maximum signal is normalized by pump flux. Each trace was taken over the course of 10 s (0.05 s per point, 200 different angles), with a 1 s delay between traces (except when switching power) and 3 s for beam-centering between each trace. (B) The data for reaction (ii), but with signal shown logarithmically. (C) Maximum signal of polarization traces over the course of reaction (ii), showing the decrease in relative absorption cross section at 635 nm. Dashed lines indicate points in time at which pump power was increased. (D) Maximum angle of polarization traces over the course of reaction (ii), showing nanorod orientation. Dashed lines indicate points in time at which pump power was increased. Reaction conditions: dilute aqueous HCl (pH ∼ 1.3), room temperature, varied FeCl3 concentrations (i) 1 mM, (ii) 250 μM, (iii) 2 mM. Pump fluxes for reaction (i) were 2.7, 6.7, 11.4, 21.0, and 34.5 kW/cm2. Pump fluxes for reaction (ii) were 4.1, 11.9, and 35.4 kW/cm2. Pump fluxes for reaction (iii) were 6.4, 15.9, 31.6, 57.3, and 121 kW/cm2.

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A control experiment was performed with a nanorod-containing resonator filled with dilute hydrochloric acid (pH ∼ 1.3) without FeCl3, confirming that the acid alone is not enough to etch the nanorods under illumination. Additionally, when nanorods are left in etching solution for multiple days without laser illumination, they do not observably react, supporting a photoactivated etching mechanism. AuNRs exposed to etching solution for hours within a microbubble, with the probe beam on and locked to water-dominated modes but no pump beam on, also did not undergo significant etching, indicating that the probe beam is not sufficient to drive etching. Therefore, we hypothesize a photoactivated mechanism resulting from hot electrons generated from LPB decay (see Mechanistic Discussion). We also note that, occasionally, nanorods were “impervious” to photoactivated etching, as discussed further in the Supporting Information.
To exemplify the spectroscopic versatility of this platform, nanorod etching was also induced with the 532 nm pump beam. Because this wavelength pumps the TPB (as well as direct interband transitions), nanorod orientation can still be tracked. In Figure 5A, a single AuNR reaction time series is shown for 532 nm-driven (TPB-driven) conditions, with a logarithmic version of the data shown in Figure 5B for clear visualization of late-stage etching data. The polarization traces are conspicuously different than in Figure 4 because of the presence of polarization-independent interband transitions. In Figure 5C, maximum signals are extracted for this reaction and one other TPB-driven reaction in the same resonator. For direct comparison, a different nanorod in the same microbubble was reacted using the 635 nm pump beam (LPB-driven), with the extracted maximum signals shown in Figure 5D (note, this is the same reaction shown in Figure 4A(iii)). Overall, three such reactions were performed for each color in the same resonator to confirm reproducibility (Supporting Information). Although quantitative comparison of reaction rates between experiments is difficult due to the decreasing rate as absorption cross-section diminishes, it is clear from the extracted maximum signals that while the etching rate of the 532 nm-induced reactions is faster than the rate of the 635 nm-induced reactions, it is not multiple orders of magnitude faster, in contrast to previously reported bulk measurements, discussed further below. (83) The shapes of maximum signal traces for the LPB-driven and TPB-driven reactions are noticeably different, with the LPB-driven reactions yielding a concave up shape, and the TPB-driven reactions yielding a concave-down (Figure 5c(i)) or even sigmoidal shape (Figure 5c(ii)).

Figure 5

Figure 5. Etching reactions driven at two different pump wavelengths. (A) The reaction of a single AuNR being driven with the 532 nm pump beam, progressing in time from red traces to blue traces. Maximum signal is normalized by pump flux. Each trace was taken over the course of 10 s (0.05 s per point, 200 different angles), with a 1 s delay between traces (except when switching power), and 3 s for beam-centering between each trace. (B) The same data as in (A), but with signal shown logarithmically. (C) (i) Maximum signal of polarization traces for the reaction shown in (A). Polarization trace for indicated data point in inset. (ii) A similar trace for a different nanorod reacted in the same bubble using the 532 nm beam. Dashed lines indicate time points at which pump power was increased. (D) Maximum signal of polarization traces for the reaction of a nanorod in the same bubble, but using the 635 nm pump beam to drive the reaction. Dashed lines indicate time points at which pump power was increased. Reaction conditions: dilute aqueous HCl (pH ∼ 1.3), room temperature, 1 mM FeCl3. Pump fluxes for reaction (Ci) were 16.1, 39.2, and 63.3 kW/cm2. Pump fluxes for reaction (Cii) were 6.4, 31.6, and 57.3 kW/cm2. Pump fluxes for reaction (D) were 6.4, 15.9, 31.6, 57.3, 121 kW/cm2.

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We also use data from a TPB-driven reaction to discuss imaging of small AuNPs and compare to darkfield microscopy. Typical dark-field techniques can image AuNPs as small as 30 nm in diameter, (84) with more optimized approaches pushing down to 10 nm. (85) The threshold at which the absorption cross-section overtakes the scattering cross-section in magnitude is at a diameter of around 80 nm for gold nanospheres. (39) At diameters of 20 nm, this ratio of σabs/σscat is ∼100. (39) The initial sizes of the AuNRs in this report (approximate volume 4 × 105 nm2, as calculated for an 80 × 40 nm cylinder) are already well below the volume of an 80 nm nanosphere (2 × 106 nm2). Furthermore, after etching the AuNR, the volume is much lower, and we can estimate that volume. For this calculation, we use a 532 nm-driven reaction because the TPB is not expected to shift significantly in its center wavelength regardless of whether the etching is isotropic or anisotropic, meaning that the measured signal should scale proportionally with nanorod volume. (39) The inset of Figure 5c(i) shows a polarization trace from late in the reaction, with its corresponding data point indicated. Although later data points show even smaller signals, this data point was selected because the trace still clearly shows the polarization-dependence associated with the TPB. The signal at 180° in the inset is lower than the signal at 0° because the nanorod is reacting rapidly at the high pump flux (as indicated by the slope of Figure 5c(i)). The selected data point has a signal 34 times smaller than the initial signal, equivalent to a volume of 1.2 × 104 nm2, or a sphere with a diameter of 14 nm. Thus, even with time resolution of 50 ms per point, microbubble spectrometers are well-suited to study AuNPs of a size that is challenging to reach with dark-field measurements. Significant further increases to our sensitivity can be achieved with use of media with higher thermo-optic coefficients (37,86−88) or use of more optimized optical modes (see discussion above).

Mechanistic Discussion

Our proposed mechanism relies on the generation of hot electrons, as in previous reports of photoactivated AuNR etching. (82,89) Interest in such hot carrier chemical processes has exploded in recent years in a variety of applications, (90) especially photocatalysis. (91−93) The mechanisms of hot carrier generation and transfer have been extensively studied, (94−97) including efforts toward untangling the contributions of hot carrier effects and photothermal effects in nanoparticle synthesis (98) and plasmonic photocatalysis (99) and mapping hot carrier driven catalytic reactions nanoscopically. (100,101) Although there are still unresolved questions regarding plasmon-driven chemistry, hot carrier transfer can generally be predicted by using an energy overlap model. (102)
In the reported reactions, we expect a nominal temperature rise at the nanorod’s surface of <1 K as calculated from the known absorption cross-section and excitation intensity (Supporting Information), meaning that the observed etching mechanism should not be significantly influenced by photothermal heating. (46) With no noticeable dark reaction rate and a photothermal mechanism ruled out, a hot carrier mechanism, whereby the decay of LSPRs or excitation of interband transitions results in hot electrons that can transfer to ferric ions on the nanorod surface, must be invoked to explain the observed reactions. Hot electrons lower the Gibbs free energy of the etching reaction (52) and reduce the thermal activation barrier for electron transfer, (99) thus modifying both the thermodynamics and kinetics of the reaction. Photoexcitation of plasmonic NPs has been experimentally shown to lower the activation enthalpy for transferring electrons from gold nanoparticles to Fe3+ (103) and to lower the energy barrier for reaction at the surface of plasmonic NPs by affecting ligand-NP interactions. (104)
When the LSPRs of an AuNR decay, hot electrons are generated from the conduction band of the nanorod. These hot electrons are on average at lower energy than those generated by interband transitions, though a few carriers will be hotter in the LSPR decay case. (83) Although it was previously reported that interband pumping can drive etching orders of magnitude faster than LPB pumping, (83) this was not observed in our reported reactions. CTAB concentration and halide concentration play significant roles in AuNR etching, (82,89) and we attribute this observed discrepancy to different CTAB-mediated mechanisms. Our nanorod deposition procedure likely removes a significant part but not all of the CTAB (see Supporting Information). (105,106) Thus, the AuNRs in our study have significantly lower CTAB coverage than the report where pumping the interband transition resulted in order of magnitude increase in etch rate, where etching occurred in a medium with CTAB concentration greater than the critical micelle concentration (CMC). (83) Ultimately, the reaction relies on ferric ions binding to the nanorod before hot electron transfer can take place. Thus, a plausible mechanism for the reported reactions entails a two-step process, whereby slow ligand exchange of CTAB with Fe3+, or intercalation of the Fe3+ through the residual CTAB, is followed by fast photoactivated etching. A relative increase in the rate of the photoactivated step, as seen previously, (83) is then ultimately masked in the observed rate due to the slowness of the first ligand exchange step, as shown Figure 6. In the limit where ligand exchange is slow, increases in the rate of the photoinduced step would give a somewhat muted effect on the overall reaction rate, as observed.

Figure 6

Figure 6. Proposed mechanistic explanation for etching rates. A slow initial step requiring CTAB dissociation before ferric ions can bind determines the overall rate for the reaction, muting the effect of the higher rate for TPB excitation, even though hot electrons are more efficiently generated. Eventually, etching stops when the absorbed light falls below a threshold necessary for hot-electron-driven etching.

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To further understand the shape of the reaction profiles, we modeled the spectral changes of the TPB and LPB for a nanorod being etched for etching schemes ranging from tip-only etching to side-only etching (Supporting Information). Modeling was able to reproduce the concave-up shape observed in the LPB-driven reactions (Figure 4C). However, no combination of variables was able to capture the concave down or sigmoidal trend seen in some TPB-driven reactions (Figure 5C) or fully reproduce the threshold behavior observed. Thus, with a completely linear reaction mechanism ruled out, we can speculate on possible origins of nonlinearity in the etching mechanism. One possible origin stems from the evolution of nanorod morphology over time. For example, it was observed previously that for certain laser powers, nanorod LPBs would red-shift, then blue-shift. (46) Another possible origin derives from the changing surface concentration of CTAB, with a relatively dense coverage providing competitive inhibition for ferric ion binding at early times but AuNR etching resulting in easier access at later times. Future studies utilizing multiple pump beams could be valuable in studying these complex kinetics, as the evolution of the LPB and TPB spectra would yield important insight into the reaction mechanism. Thus, the reported microbubble platform may be used for spectroscopic, mechanistic studies into the wavelength dependence of hot-carrier-driven chemical dynamics in single plasmonic nanoparticles.

Single AuNR Rotations

Beyond using our microbubble spectrometer to monitor and control nanoparticle size, we can also use it to monitor and control nanoparticle orientation, adding significant utility to the microbubble spectrometer platform. Indicated by the shift in peak polarization during etching reactions (Figures 4 and 5), AuNRs can rotate while etching. Alternatively, active rotation can be induced with the pump beam, allowing for control over nanorod orientation. This control results from the optical torque exerted by linearly polarized light on an anisotropic, absorbing plasmonic structure (Figure 7a), a phenomenon that has been demonstrated experimentally (107) and theoretically (108,109) and is discussed further in the Supporting Information.

Figure 7

Figure 7. Orientation control of single AuNRs. (A) A cartoon illustrating the optically induced torque that a AuNR experiences under illumination with linearly polarized light, both from side-view (top) and top-view (bottom). (B) A series of pumping experiments showing optical control of nanorod orientation. (C) Trace showing the photothermal signal as two different AuNRs are pumped at increasing laser powers until the AuNRs dislodge slightly from the resonator wall and rotate, eventually settling down off-axis of the polarization.

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Theory predicts that the optical torque acting on the AuNR from the 635 nm pump beam will align the AuNR perpendicular to the polarization of the incident beam. Indeed, this perpendicular alignment is exhibited upon sufficient excitation power. In this way, nanorod orientation can be controlled to within approximately 10° (Figure 7b) as AuNR orientation is stepped through a ∼ 180° rotation. This control was accomplished by monitoring the photothermal signal, dislodging the nanorod with a large optical torque above some threshold incident power, and dithering the polarization until the photothermal signal was minimized at the desired polarization angle. This thresholding behavior was demonstrated further by a stepwise ramping of the pump laser intensity, resulting in an upward staircase of photothermal signal, until rotation was finally induced. As can be seen in the examples in Figure 7C, two different nanorods required significantly different pump powers to dislodge them from the microbubble surface. Following dislodgment, the signal quickly stabilized to around 70% of the maximum signal for the left nanorod trace, whereas it behaved semistochastically for the right trace, before settling at <40% of the maximum signal. These differences in orientational dynamics highlight the differences in the local environments around the two nanorods, including both Coulombic effects and refractive index differences. These staircase experiments were performed repeatedly for both nanorods in Figure 7C to confirm reproducibility (Supporting Information).
Light-induced rotation during reactions was observed more frequently as AuNR etching progressed. Likely, as Coulombic attractions between the AuNR and the resonator’s surface were weakened, hydrodynamic or optical torques were allowed to rotate the AuNR. Although rotation events could be forced in water-filled resonators, higher pump thresholds were generally required, and AuNRs were immune to rotation at powers that would result in rotation in ferric chloride solution. Therefore, it appears that the presence of etchant reduces the Coulombic attraction between nanorods and the resonator wall, possibly through charge screening, permitting facile rotation. Though optical rotation of nanorods has been seen in previous experiments, the coupling between evolving surface chemistry and propensity for rotation has not been explored to our knowledge.
Radiation pressure and optical gradient forces could also influence AuNRs, affecting the rotation power threshold. However, varying the pump beam focus position, which would change forces along the optical axis, did not significantly impact rotation thresholds. Therefore, it appears that the torque described above is the dominant driver of nanorod rotation. Although scattering forces in three-dimensional trapping can orient anisotropic plasmonic nanoparticles parallel to the optical axis of the excitation beam, (110) one would expect this to result in a highly stochastic signal over time in the polarization traces of Figures 3 and 4, as well as a revival of signal upon shutting off and turning back on the pump beam. Such behavior was not observed. Therefore, AuNRs likely remain parallel to the plane of the resonator’s surface during rotation, rotating only in two dimensions.

Conclusions

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We have demonstrated microbubble resonators as a robust platform for studying chemical dynamics in solution via single-particle absorption spectroscopy. We used a microbubble spectrometer to observe changes in the optical properties of AuNRs as they were controllably etched by ferric chloride via a photoinduced mechanism. The sizes of the etched AuNRs push the limit of what can be imaged via dark-field scattering. The hypothesis that ligand-exchange-limited etching is the reason for muted wavelength dependence of etching will require more experiments in the future to confirm and understand. Additionally, we monitored and controlled the orientation of the AuNRs using optical torque.
With this demonstration, we lay the groundwork for studying more complex reaction dynamics of single particles and molecules. Thus, this technique provides a complementary measurement to the luminescence and dark-field methods previously used to observe similar reactions as reported here. In particular, the demonstrated exquisite sensitivity offers prospects of examining non-emissive objects inaccessible with fluorescence and too small to observe with scattering, which scales as 1/volume2, a more severe penalty than in absorption measurements, which scale more favorably as 1/volume. Furthermore, rotational control could be used to estimate Coulombic forces attaching deposited objects to the resonator, helping to understand the interface between nanoparticles and the surface. This knowledge, combined with structured light-field manipulation of nanoparticles, (111) might be used to arrange arrays of plasmonic nanoparticles as desired. The optical control of plasmonic nanoparticles within a microbubble resonator may allow for in-solution, photonic-plasmonic assembly, and live control of emergent optical properties in such coupled systems. Additionally, by providing a direct thermal readout, our method could be used to untangle the respective contributions of photothermal heating and hot carrier generation for nanoparticle reactions, aiding the design of improved nanocatalysts. Overall, there is a compelling case for the use of microbubbles in materials studies, sensing, and chemical kinetics, and even hybridizing them with plasmonic or acoustic sensing schemes for further applications. Microbubble absorption spectrometers thus hold great potential for pushing the frontiers of absorption spectroscopy at the nanoscale.

Methods

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Microbubble Fabrication

Microbubble resonators were fabricated according to the method reported by Yang and co-workers. (112) First, a glass capillary (Polymicro Technologies, TSP250350) is tapered using a heat-and-pull method, until it is approximately 25 μm in diameter. Next, counter-propagating CO2 laser beams are focused onto the capillary, while positive pressure is applied from the inside of the capillary using an inert gas. The heat from the laser beams softens the capillary, allowing for the local expansion of the capillary to 50–100 μm in diameter, depending on the experimental parameters. Eventually, radiative cooling from the expanded glass outcompetes the expansion process, and the bubble’s size stops increasing. To operate in the quasi-droplet regime, a wall thickness close to the wavelength of the laser beam used for WGM excitation is desirable. Microbubble wall thickness is determined by an equation reported by Henze and co-workers (113) and validated separately by others. (114)

Tapered Optical Fiber Fabrication

Single-mode optical fiber was purchased from Corning (HI 780C). Tapered fibers are made by removing the polymer sheath, cleaning the fiber, and tapering using a heat-and-pull method with a hydrogen torch and motorized actuators (Thorlabs Z825B) until the fiber returns to single-mode, as determined using a 785 nm diode laser (Thorlabs LPS-785-FC) and optical power meter.

Instrumentation for Photothermal Spectroscopy

A tunable, ultranarrow line width, fiber-coupled CW laser (Newport TLB-6712) with a wavelength range of 765–781 nm was used for coupling into resonators. Pound–Drever–Hall (PDH) locking electronics were constructed as previously reported, (13) except for the use of a different voltage-controlled oscillator (Mini Circuits ZX95-310A+) and different lithium niobate phase-modulator (EOSPACE PM-0S5-01-PFA-PFA-765/782). PDH feedback was applied to the tunable laser using high-speed servo controller (Newport LB1005). The optical output from the experiment was collected using an APD (Thorlabs APD430A), and the photothermal signal was extracted using a lock-in amplifier (Ametek 7265). The resulting signal was collected using a data acquisition (DAQ) card (National Instruments BNC2120) for later processing. Custom LabVIEW code was used for instrumentation control. For photothermal mapping, a lock-in time constant of 20 or 50 ms was used. For polarization traces, a lock-in time constant of 50 ms was used.
Diode lasers were used for pump beams, with the wavelengths 532 nm (FTEC2 532–20), 635 nm (FTEC2 635-50), and 785 nm (Thorlabs LPS-785-FC). The pump beam was amplitude modulated using an optical chopper system (Thorlabs MC200B) and steered using galvanometer mirrors (Thorlabs GVS212) run by outputs from the DAQ mentioned above, modified using custom electronics. The pump beam was focused using a piezo-controlled (Thorlabs DRV517) objective (Nikon Plan 40×, 0.65 NA). Pump beam polarization was controlled using a three-optic system, comprised of a linear polarizer (LPVISE100-A), followed by a liquid crystal variable retarder (Thorlabs LCC1423-A, LCC25) with its fast axis set 45° relative to the polarization axis, followed by a zero-order achromatic quarter-wave plate (Thorlabs AQWP05M-600) with its fast axis set 45° relative to the liquid crystal’s fast axis. In this design, tuning of the liquid crystal voltage results in a rotation of linearly polarized light at the output of the three-optic system.

Polarization Plots

Shown in Figure 3C,D, AuNRs are first identified by photothermally mapping them at different pump polarizations and processing these maps with a 2D-Gaussian fit, which results in background subtraction. Then, the maximum signals for each plot are together fit to provide a depth-of-modulation (M), using eq 1, which also gives the maximum signal (σmax) plotted in Figure 4C, and the polarization angle of the maximum signal (θmax) plotted in Figure 4D.
(1)
For the data in Figure 3E, the liquid crystal is used to rapidly collect many data points that are then fit to eq 1 to obtain M. This method does not include background subtraction.

Bulk/UV–vis Studies

The bulk absorption spectrum in Figure 3B was taken using a UV–visible spectrophotometer (Varian Cary 50). Additionally, studies were conducted to confirm the effects of CTAB concentration and FeCl3 concentration on bulk nanorod etching. The effects of added NaCl were studied to further examine the impacts of chloride concentration. Results and further discussion are in the Supporting Information.

Nanorod Deposition in Microbubbles

All chemicals were purchased through Sigma-Aldrich unless otherwise noted. To deposit nanorods in a microbubble, a 500× serial dilution is made of AuNRs (Nanopartz A12-40-650-CTAB-DIH-1-25, size: 80 × 40 nm, ζ potential: 35 mV, stock pH: 7, stock CTAB concentration: 5 mM) in a solution of 200× diluted HCl and 25 μM CTAB in water. The low CTAB concentration prevents nanorod aggregation during deposition, but keeps the CTAB concentration well below the CMC of ∼1 mM. Dilute hydrochloric acid, which results in a pH of around 1.3, encourages binding of the nanorods on the resonator interior by enhancing Coulombic interactions. (9) For deposition, water is first flowed through the resonator using a syringe pump attached to the first port of the resonator’s capillary. Then, dilute HCl is flowed through the resonator to prime the glass surface for deposition. Next, deposition solution is backfilled through the second capillary port, which is cut to a much shorter length to reduce deposition of AuNRs to the capillary’s interior walls. Following this, dilute HCl is flowed through the resonator, followed by water, through the first port of the capillary to push out the deposition solution while maintaining a pH gradient. Water is flowed through the resonator for at least several minutes to ensure the removal of nonbound objects and remove excess CTAB from the nanorod surfaces.

Reactions in Microbubbles

All chemicals were purchased from Sigma-Aldrich. Reaction mixtures are made by dissolving and serial diluting ferric chloride hexahydrate in 1/200 dilute hydrochloric acid, resulting in a solution pH of around 1.3. While reaction solution is flowed into the bubble, resonances shift as the refractive index being probed by the WGM changes. Complete filling of the microbubble with reaction mixture is indicated when the resonances have stopped shifting. Following this stabilization, the syringe pump pressure is released, resulting in a microbubble primed for etching experiments.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b04702.

  • Geometric parameters of microbubbles, COMSOL simulations, comparison with microtoroids, static offset vs. modulated signal, mode shifting for different dn/dT values (experimental), modeling of LPB and TPB during etching, thermal expansion, diagram for mode indices, additional single AuNR etching data, additional rotation data, bulk reaction results, brief discussion on extracting reaction kinetics, background on AuNR etchants, impervious nanorods, discussion of role of CTAB in etching mechanism, estimation of CTAB remaining on AuNRs after deposition, estimation of nanorod temperature increase, theory of nanorod rotation, Matlab code for modeling plasmon changes during etching (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Levi T. Hogan - Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States;  Orcidhttp://orcid.org/0000-0001-5349-8656
    • Erik H. Horak - Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
    • Jonathan M. Ward - Light-Matter Interactions for Quantum Technologies Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa 904-0495, Japan
    • Kassandra A. Knapper - Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States;  Orcidhttp://orcid.org/0000-0001-5171-5792
    • Síle Nic Chormaic - Light-Matter Interactions for Quantum Technologies Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa 904-0495, Japan;  Orcidhttp://orcid.org/0000-0003-4276-2014
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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R.H.G. acknowledges support from NIH (GM127957) and NSF (DBI-1556241). S.N.C. and J.M.W. acknowledge support from the Okinawa Institute of Science and Technology Graduate University. We thank J. Millstone and C. Murphy for enlightening conversations about nanoparticle chemistry.

References

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    Label-free bio-sensing is a crit. functionality underlying a variety of health- and security-related applications. Micro-/nano-photonic devices are well suited for this purpose and have emerged as promising platforms in recent years. Here we propose and demonstrate an approach that utilizes the optical spring effect in a high-Q coherent optomech. oscillator to dramatically enhance the sensing resoln. by orders of magnitude compared with conventional approaches, allowing us to detect single bovine serum albumin proteins with a mol. wt. of 66 kDa at a signal-to-noise ratio of 16.8. The unique optical spring sensing approach opens up a distinctive avenue that not only enables biomol. sensing and recognition at individual level, but is also of great promise for broad phys. sensing applications that rely on sensitive detection of optical cavity resonance shift to probe external phys. parameters.
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  12. 12
    Baaske, M. D.; Vollmer, F. Optical Observation of Single Atomic Ions Interacting with Plasmonic Nanorods in Aqueous Solution. Nat. Photonics 2016, 10, 733– 739,  DOI: 10.1038/nphoton.2016.177
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    12
    Optical observation of single atomic ions interacting with plasmonic nanorods in aqueous solution
    Baaske, Martin D.; Vollmer, Frank
    Nature Photonics (2016), 10 (11), 733-739CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    Plasmonic nanoparticles provide the basis for a multitude of applications in chem., health care and optics because of their unique properties. Nanoparticle-based techniques have evolved into powerful tools for studying mol. interactions with single-mol. resoln. Here, we show that this sensing capability can be used to detect single at. ions in aq. medium. We monitored interactions of single zinc and mercury ions with plasmonic gold nanorods (NRs) resonantly coupled to our whispering gallery mode sensor. Our system's ability to discern permanent binding and transient interaction allows us to study the different interaction kinetics of both ion species. The detection of transient interactions enables us to confirm statistically that the sensor signals originate from single ions. Furthermore, we reveal how the ion-NR interactions evolve with respect to the medium's ionic strength as mercury ions amalgamate with gold and zinc ions eventually turn into probes of highly localized surface potentials.
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  13. 13
    Heylman, K. D.; Thakkar, N.; Horak, E. H.; Quillin, S. C.; Cherqui, C.; Knapper, K. A.; Masiello, D. J.; Goldsmith, R. H. Optical Microresonators as Single-Particle Absorption Spectrometers. Nat. Photonics 2016, 10, 788– 795,  DOI: 10.1038/nphoton.2016.217
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    13
    Optical microresonators as single-particle absorption spectrometers
    Heylman, Kevin D.; Thakkar, Niket; Horak, Erik H.; Quillin, Steven C.; Cherqui, Charles; Knapper, Kassandra A.; Masiello, David J.; Goldsmith, Randall H.
    Nature Photonics (2016), 10 (12), 788-795CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    Optical measurements of nanoscale objects offer major insights into fundamental biol., material and photonic properties. In absorption spectroscopy, sensitivity limits applications at the nanoscale. Here, we present a new single-particle double-modulation photothermal absorption spectroscopy method that employs on-chip optical whispering-gallery-mode (WGM) microresonators as ultrasensitive thermometers. Optical excitation of a nanoscale object on the microresonator produces increased local temps. that are proportional to the absorption cross-section of the object. We resolve photothermal shifts in the resonance frequency of the microresonator that are smaller than 100 Hz, orders of magnitude smaller than previous WGM sensing schemes. The application of our new technique to single gold nanorods reveals a dense array of sharp Fano resonances arising from the coupling between the localized surface plasmon of the gold nanorod and the WGMs of the resonator, allowing for the exploration of plasmonic-photonic hybridization. In terms of the wider applicability, our approach adds label-free spectroscopic identification to microresonator-based detection schemes.
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  14. 14
    Thakkar, N.; Rea, M. T.; Smith, K. C.; Heylman, K. D.; Quillin, S. C.; Knapper, K. A.; Horak, E. H.; Masiello, D. J.; Goldsmith, R. H. Sculpting Fano Resonances to Control Photonic-Plasmonic Hybridization. Nano Lett. 2017, 17, 6927– 6934,  DOI: 10.1021/acs.nanolett.7b03332
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    14
    Sculpting Fano Resonances To Control Photonic-Plasmonic Hybridization
    Thakkar, Niket; Rea, Morgan T.; Smith, Kevin C.; Heylman, Kevin D.; Quillin, Steven C.; Knapper, Kassandra A.; Horak, Erik H.; Masiello, David J.; Goldsmith, Randall H.
    Nano Letters (2017), 17 (11), 6927-6934CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their resp. components have either high-quality factors or low mode vols. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode vols. to maximize the system's photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune crit. system parameters. In this Letter, we present a general and practical method to det. the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on exptl. measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticle's localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters crit. to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode vol. and ultimately maximize the Purcell factor to be 104.
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  15. 15
    Knapper, K. A.; Heylman, K. D.; Horak, E. H.; Goldsmith, R. H. Chip-Scale Fabrication of High-Q All-Glass Toroidal Microresonators for Single-Particle Label-Free Imaging. Adv. Mater. 2016, 28, 2945– 2950,  DOI: 10.1002/adma.201504976
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    15
    Chip-Scale Fabrication of High-Q All-Glass Toroidal Microresonators for Single-Particle Label-Free Imaging
    Knapper, Kassandra A.; Heylman, Kevin D.; Horak, Erik H.; Goldsmith, Randall H.
    Advanced Materials (Weinheim, Germany) (2016), 28 (15), 2945-2950CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We have fabricated all-glass microtoroid resonators, preserving the morphol. leading to the high QJV of the SiO2-On-Si microtoroid, while addressing their fabrication, biocompatibility, and optical challenges by eliminating the silicon pillar and substrate. Most importantly, all-glass toroids are reflowed using a high-temp. furnace anneal step, which provides a wafer-scale and highly uniform reflow mechanism, while preserving surface tension-induced smoothness.
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  16. 16
    Knapper, K. A.; Pan, F.; Rea, M. T.; Horak, E. H.; Rogers, J. D.; Goldsmith, R. H. Single-Particle Photothermal Imaging via Inverted Excitation through High-Q All-Glass Toroidal Microresonators. Opt. Express 2018, 26, 25020– 25030,  DOI: 10.1364/OE.26.025020
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    16
    Single-particle photothermal imaging via inverted excitation through high-Q all-glass toroidal microresonators
    Knapper, Kassandra A.; Pan, Feng; Rea, Morgan T.; Horak, Erik H.; Rogers, Jeremy D.; Goldsmith, Randall H.
    Optics Express (2018), 26 (19), 25020-25030CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
    In this work, we fabricate all-glass toroidal microresonators on a coverslip thickness (∼170μm) substrate, enabling excitation delivery through the sample, simplifying optical integration. Further, we demonstrate the application of this new geometry for single-particle photothermal imaging. Finally, we discover and develop simulations to explain a non-trivial astigmatism in the point spread function (PSF) arising from the curvature of the resonator.
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    Heylman, K. D.; Knapper, K. A.; Goldsmith, R. H. Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators. J. Phys. Chem. Lett. 2014, 5, 1917– 1923,  DOI: 10.1021/jz500781g
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    17
    Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators
    Heylman, Kevin D.; Knapper, Kassandra A.; Goldsmith, Randall H.
    Journal of Physical Chemistry Letters (2014), 5 (11), 1917-1923CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the crit. detecting element. The approach is photothermal in nature as the microresonators were used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, the authors' approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chem. information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled C nanotubes are spatially mapped, and the per-atom absorption cross section is detd. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to mols. is laid out, leading to a potent new method of performing measurements on individual mols.
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  18. 18
    Horak, E. H.; Rea, M. T.; Heylman, K. D.; Gelbwaser-Klimovsky, D.; Saikin, S. K.; Thompson, B. J.; Kohler, D. D.; Knapper, K. A.; Wei, W.; Pan, F.; Gopalan, P.; Wright, J. C.; Aspuru-Guzik, A.; Goldsmith, R. H. Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy. Nano Lett. 2018, 18, 1600– 1607,  DOI: 10.1021/acs.nanolett.7b04211
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    18
    Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy
    Horak, Erik H.; Rea, Morgan T.; Heylman, Kevin D.; Gelbwaser-Klimovsky, David; Saikin, Semion K.; Thompson, Blaise J.; Kohler, Daniel D.; Knapper, Kassandra A.; Wei, Wei; Pan, Feng; Gopalan, Padma; Wright, John C.; Aspuru-Guzik, Alan; Goldsmith, Randall H.
    Nano Letters (2018), 18 (3), 1600-1607CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    PEDOT:PSS, a transparent elec. conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased cond., a detailed understanding of the coupled electronic and morphol. landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examn. of electronic structure and morphol. ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theor. calcns. and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. We find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.
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    Chen, H. J.; Shao, L.; Li, Q.; Wang, J. F. Gold Nanorods and Their Plasmonic Properties. Chem. Soc. Rev. 2013, 42, 2679– 2724,  DOI: 10.1039/C2CS35367A
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    19
    Gold nanorods and their plasmonic properties
    Chen, Huanjun; Shao, Lei; Li, Qian; Wang, Jianfang
    Chemical Society Reviews (2013), 42 (7), 2679-2724CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the prepn. of their hybrid structures. The plasmonic properties and the assocd. applications of Au nanorods will also be discussed in detail.
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    Dreaden, E. C.; Alkilany, A. M.; Huang, X. H.; Murphy, C. J.; El-Sayed, M. A. The Golden Age: Gold Nanoparticles for Biomedicine. Chem. Soc. Rev. 2012, 41, 2740– 2779,  DOI: 10.1039/C1CS15237H
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    20
    The golden age: gold nanoparticles for biomedicine
    Dreaden, Erik C.; Alkilany, Alaaldin M.; Huang, Xiaohua; Murphy, Catherine J.; El-Sayed, Mostafa A.
    Chemical Society Reviews (2012), 41 (7), 2740-2779CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Gold nanoparticles have been used in biomedical applications since their first colloidal syntheses more than three centuries ago. However, over the past two decades, their beautiful colors and unique electronic properties have also attracted tremendous attention due to their historical applications in art and ancient medicine and current applications in enhanced optoelectronics and photovoltaics. In spite of their modest alchem. beginnings, gold nanoparticles exhibit phys. properties that are truly different from both small mols. and bulk materials, as well as from other nanoscale particles. Their unique combination of properties is just beginning to be fully realized in range of medical diagnostic and therapeutic applications. This crit. review will provide insights into the design, synthesis, functionalization, and applications of these artificial mols. in biomedicine and discuss their tailored interactions with biol. systems to achieve improved patient health. Further, we provide a survey of the rapidly expanding body of literature on this topic and argue that gold nanotechnol.-enabled biomedicine is not simply an act of gilding the (nanomedicinal) lily', but that a new Golden Age' of biomedical nanotechnol. is truly upon us. Moving forward, the most challenging nanoscience ahead of us will be to find new chem. and phys. methods of functionalizing gold nanoparticles with compds. that can promote efficient binding, clearance, and biocompatibility and to assess their safety to other biol. systems and their long-term term effects on human health and reprodn. (472 refs.).
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    Huang, X. H.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods. J. Am. Chem. Soc. 2006, 128, 2115– 2120,  DOI: 10.1021/ja057254a
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    21
    Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods
    Huang, Xiaohua; El-Sayed, Ivan H.; Qian, Wei; El-Sayed, Mostafa A.
    Journal of the American Chemical Society (2006), 128 (6), 2115-2120CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Due to strong elec. fields at the surface, the absorption and scattering of electromagnetic radiation by noble metal nanoparticles are strongly enhanced. These unique properties provide the potential of designing novel optically active reagents for simultaneous mol. imaging and photothermal cancer therapy. It is desirable to use agents that are active in the near-IR (NIR) region of the radiation spectrum to minimize the light extinction by intrinsic chromophores in native tissue. Gold nanorods with suitable aspect ratios (length divided by width) can absorb and scatter strongly in the NIR region (650-900 nm). In the present work, we provide an in vitro demonstration of gold nanorods as novel contrast agents for both mol. imaging and photothermal cancer therapy. Nanorods are synthesized and conjugated to anti-epidermal growth factor receptor (anti-EGFR) monoclonal antibodies and incubated in cell cultures with a nonmalignant epithelial cell line (HaCat) and two malignant oral epithelial cell lines (HOC 313 clone 8 and HSC 3). The anti-EGFR antibody-conjugated nanorods bind specifically to the surface of the malignant-type cells with a much higher affinity due to the overexpressed EGFR on the cytoplasmic membrane of the malignant cells. As a result of the strongly scattered red light from gold nanorods in dark field, obsd. using a lab. microscope, the malignant cells are clearly visualized and diagnosed from the nonmalignant cells. It is found that, after exposure to continuous red laser at 800 nm, malignant cells require about half the laser energy to be photothermally destroyed than the nonmalignant cells. Thus, both efficient cancer cell diagnostics and selective photothermal therapy are realized at the same time.
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    Yin, D. Y.; Li, X. L.; Ma, Y. Y.; Liu, Z. Targeted Cancer Imaging and Photothermal Therapy via Monosaccharide-Imprinted Gold Nanorods. Chem. Commun. 2017, 53, 6716– 6719,  DOI: 10.1039/C7CC02247F
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    Targeted cancer imaging and photothermal therapy via monosaccharide-imprinted gold nanorods
    Yin, Danyang; Li, Xinglin; Ma, Yanyan; Liu, Zhen
    Chemical Communications (Cambridge, United Kingdom) (2017), 53 (50), 6716-6719CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Plasmonic nanomaterials have been widely used for photothermal therapy (PTT) of cancer, but their recognition specificity remains challenging. We prepd. monosaccharide-imprinted gold nanorods (AuNRs) for targeted cancer PTT, using sialic acid (SA) as a representative monosaccharide. The SA-imprinted AuNRs exhibited good specificity, enabling the killing of cancer cells without damaging healthy cells.
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  23. 23
    Ali, M. R. K.; Wu, Y.; Ghosh, D.; Do, B. H.; Chen, K.; Dawson, M. R.; Fang, N.; Sulchek, T. A.; El-Sayed, M. A. Nuclear Membrane-Targeted Gold Nanoparticles Inhibit Cancer Cell Migration and Invasion. ACS Nano 2017, 11, 3716– 3726,  DOI: 10.1021/acsnano.6b08345
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    Nuclear Membrane-Targeted Gold Nanoparticles Inhibit Cancer Cell Migration and Invasion
    Ali, Moustafa R. K.; Wu, Yue; Ghosh, Deepraj; Do, Brian H.; Chen, Kuangcai; Dawson, Michelle R.; Fang, Ning; Sulchek, Todd A.; El-Sayed, Mostafa A.
    ACS Nano (2017), 11 (4), 3716-3726CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Most cancer patients die from metastasis. Recent studies have shown that gold nanoparticles (AuNPs) can slow down the migration/invasion speed of cancer cells and suppress metastasis. Since nuclear stiffness of the cell largely decreases cell migration, our hypothesis is that targeting AuNPs to the cell nucleus region could enhance nuclear stiffness, and therefore inhibit cell migration and invasion. Our results showed that upon nuclear targeting of AuNPs, the ovarian cancer cell motilities decrease significantly, compared with nontargeted AuNPs. Furthermore, using at. force microscopy, we obsd. an enhanced cell nuclear stiffness. In order to understand the mechanism of cancer cell migration/invasion inhibition, the exact locations of the targeted AuNPs were clearly imaged using a high-resoln. three-dimensional imaging microscope, which showed that the AuNPs were trapped at the nuclear membrane. In addn., we obsd. a greatly increased expression level of lamin A/C protein, which is located in the inner nuclear membrane and functions as a structural component of the nuclear lamina to enhance nuclear stiffness. We propose that the AuNPs that are trapped at the nuclear membrane both (1) add to the mech. stiffness of the nucleus and (2) stimulate the overexpression of lamin A/C located around the nuclear membrane, thus increasing nuclear stiffness and slowing cancer cell migration and invasion.
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    Meeker, D. G.; Chen, J. Y.; Smeltzer, M. S. Could Targeted, Antibiotic-Loaded Gold Nanoconstructs Be a New Magic Bullet to Fight Infection?. Nanomedicine 2016, 11, 2379– 2382,  DOI: 10.2217/nnm-2016-0260
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    Could targeted, antibiotic-loaded gold nanoconstructs be a new magic bullet to fight infection?
    Meeker, Daniel G.; Chen, Jingyi; Smeltzer, Mark S.
    Nanomedicine (London, United Kingdom) (2016), 11 (18), 2379-2382CODEN: NLUKAC; ISSN:1743-5889. (Future Medicine Ltd.)
    There is no expanded citation for this reference.
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    Cao, J.; Sun, T.; Grattan, K. T. V. Gold Nanorod-Based Localized Surface Plasmon Resonance Biosensors: A Review. Sens. Actuators, B 2014, 195, 332– 351,  DOI: 10.1016/j.snb.2014.01.056
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    Gold nanorod-based localized surface plasmon resonance biosensors: A review
    Cao, Jie; Sun, Tong; Grattan, Kenneth T. V.
    Sensors and Actuators, B: Chemical (2014), 195 (), 332-351CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    A review. Noble metal nanoparticle-based localized surface plasmon resonance (LSPR) is an advanced and powerful label-free biosensing technique which is well-known for its high sensitivity to the surrounding refractive index change in the local environment caused by the biomol. interactions around the sensing area. The characteristics of the LSPR effect in such sensors are highly dependent on the size, shape and nature of the material properties of the metallic nanoparticles considered. Among the various types of metallic nanoparticles used in studies employing the LSPR technique, the use of gold nanorods (GNRs) has attracted particular attention for the development of sensitive LSPR biosensors, this arising from the unique and intriguing optical properties of the material. This paper provides a detailed review of the key underpinning science for such systems and of recent progress in the development of a no. of LSPR-based biosensors which use GNR as the active element, including an overview of the sensing principle, the synthesis of GNRs, the fabrication of a no. of biosensors, techniques for surface modification of GNRs and finally their performance in several biosensing applications. The review ends with a consideration of key advances in GNR-based LSPR sensing and prospects for future research and advances for the development of the GNR-based LSPR biosensors.
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    Taylor, A. B.; Zijlstra, P. Single-Molecule Plasmon Sensing: Current Status and Future Prospects. ACS Sens. 2017, 2, 1103– 1122,  DOI: 10.1021/acssensors.7b00382
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    Single-Molecule Plasmon Sensing: Current Status and Future Prospects
    Taylor, Adam B.; Zijlstra, Peter
    ACS Sensors (2017), 2 (8), 1103-1122CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
    A review. Single-mol. detection has long relied on fluorescent labeling with high quantum-yield fluorophores. Plasmon-enhanced detection circumvents the need for labeling by allowing direct optical detection of weakly emitting and completely nonfluorescent species. This review focuses on recent advances in single mol. detection using plasmonic metal nanostructures as a sensing platform, particularly using a single particle-single mol. approach. In the past decade two mechanisms for plasmon-enhanced single-mol. detection have been demonstrated: (1) by plasmonically enhancing the emission of weakly fluorescent biomols., or (2) by monitoring shifts of the plasmon resonance induced by single-mol. interactions. We begin with a motivation regarding the importance of single mol. detection, and advantages plasmonic detection offers. We describe both detection mechanisms and discuss challenges and potential solns. We finalize by highlighting the exciting possibilities in anal. chem. and medical diagnostics.
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    Lin, K. Q.; Yi, J.; Hu, S.; Liu, B. J.; Liu, J. Y.; Wang, X.; Ren, B. Size Effect on SERS of Gold Nanorods Demonstrated via Single Nanoparticle Spectroscopy. J. Phys. Chem. C 2016, 120, 20806– 20813,  DOI: 10.1021/acs.jpcc.6b02098
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    Size Effect on SERS of Gold Nanorods Demonstrated via Single Nanoparticle Spectroscopy
    Lin, Kai-Qiang; Yi, Jun; Hu, Shu; Liu, Bi-Ju; Liu, Jun-Yang; Wang, Xiang; Ren, Bin
    Journal of Physical Chemistry C (2016), 120 (37), 20806-20813CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Surface-enhanced Raman spectroscopy (SERS) has attracted tremendous interest as a label-free highly sensitive anal. method. For optimization of SERS activity, it is highly important to systematically investigate the size effect of nanoparticles on the SERS enhancement, which appears to be challenging in expt., as the localized surface plasmon resonance (LSPR) of nanoparticles also changes with the change of the particle size. This challenge can be overcome by utilizing the unique property of gold nanorods, whose LSPR wavelength can be controlled to be the same by properly choosing the size and aspect ratio of the nanorods. We obtained the correlated SEM images, scattering spectra, and SERS spectra on a home-built single nanoparticle spectroscopy system and systematically investigate the size effect on SERS of individual gold nanorods using the adsorbed malachite green isothiocyanate (MGITC) mol. as the probe mol. The dark field scattering intensity was found to increase with the increase of the size of nanoparticles, whereas the SERS intensity increases with the decrease of the size as a result of the stronger lightning rod effect and weaker radiation damping. We further explored the size-dependent effect for the coupled nanorod dimer system. The SERS activity was also found to increase with a decrease of the particle size when the excitation is close to the LSPR wavelength. Understanding of the size effect on the local field enhancement may help to design and fabricate SERS substrate and TERS tips with high SERS activity.
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  28. 28
    Gao, Z.; Burrows, N. D.; Valley, N. A.; Schatz, G. C.; Murphy, C. J.; Haynes, C. L. In Solution SERS Sensing Using Mesoporous Silica-Coated Gold Nanorods. Analyst 2016, 141, 5088– 5095,  DOI: 10.1039/C6AN01159D
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    28
    In solution SERS sensing using mesoporous silica-coated gold nanorods
    Gao, Zhe; Burrows, Nathan D.; Valley, Nicholas A.; Schatz, George C.; Murphy, Catherine J.; Haynes, Christy L.
    Analyst (Cambridge, United Kingdom) (2016), 141 (17), 5088-5095CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
    Mesoporous silica-coated gold nanorods (AuNR@MS) act as a colloidally stable Raman sensing platform with a built-in analyte size cutoff. Herein, these core-shell plasmonic nanostructures were presented with a range of thiolated Raman-active mols. to probe the limits of this platform for SERS sensing. The exptl. results show generally, that the transport of mols. through the mesopores is highly dependent on the size of the mol. and specifically, that AuNR@MS with pores of ∼4 nm diam. are able to sense analytes with mol. dimensions smaller than 1.5 nm. This sensing platform will likely find broad use, performing well even in complex media based on the high colloidal stability imbued by the mesoporous silica shell.
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  29. 29
    Khatua, S.; Paulo, P. M. R.; Yuan, H. F.; Gupta, A.; Zijlstra, P.; Orrit, M. Resonant Plasmonic Enhancement of Single-Molecule Fluorescence by Individual Gold Nanorods. ACS Nano 2014, 8, 4440– 4449,  DOI: 10.1021/nn406434y
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    29
    Resonant Plasmonic Enhancement of Single-Molecule Fluorescence by Individual Gold Nanorods
    Khatua, Saumyakanti; Paulo, Pedro M. R.; Yuan, Haifeng; Gupta, Ankur; Zijlstra, Peter; Orrit, Michel
    ACS Nano (2014), 8 (5), 4440-4449CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Enhancing the fluorescence of a weak emitter is important to further extend the reach of single-mol. fluorescence imaging to many unexplored systems. Here the authors study fluorescence enhancement by isolated Au nanorods and explore the role of the surface plasmon resonance (SPR) on the obsd. enhancements. Au nanorods can be cheaply synthesized in large vols., yet similar fluorescence enhancements as literature reports on lithog. fabricated nanoparticle assemblies were found. The fluorescence of a weak emitter, crystal violet, can be enhanced >1000-fold by a single nanorod with its SPR at 629 nm excited at 633 nm. This strong enhancement results from both an excitation rate enhancement of ∼130 and an effective emission enhancement of ∼9. The fluorescence enhancement, however, decreases sharply when the SPR wavelength moves away from the excitation laser wavelength or when the SPR has only a partial overlap with the emission spectrum of the fluorophore. The reported measurements of fluorescence enhancement by 11 nanorods with varying SPR wavelengths are consistent with numerical simulations.
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  30. 30
    Nima, Z. A.; Alwbari, A. M.; Dantuluri, V.; Hamzah, R. N.; Sra, N.; Motwani, P.; Arnaoutakis, K.; Levy, R. A.; Bohliqa, A. F.; Nedosekin, D.; Zharov, V. P.; Makhoul, I.; Biris, A. S. Targeting Nano Drug Delivery to Cancer Cells Using Tunable, Multi-Layer, Silver-Decorated Gold Nanorods. J. Appl. Toxicol. 2017, 37, 1370– 1378,  DOI: 10.1002/jat.3495
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    Targeting nano drug delivery to cancer cells using tunable, multi-layer, silver-decorated gold nanorods
    Nima, Zeid A.; Alwbari, Ahmed M.; Dantuluri, Vijayalakshmi; Hamzah, Rabab N.; Sra, Natasha; Motwani, Pooja; Arnaoutakis, Konstantinos; Levy, Rebecca A.; Bohliqa, Amani F.; Nedosekin, Dmitry; Zharov, Vladimir P.; Makhoul, Issam; Biris, Alexandru S.
    Journal of Applied Toxicology (2017), 37 (12), 1370-1378CODEN: JJATDK; ISSN:0260-437X. (John Wiley & Sons Ltd.)
    Multifunctional nanoparticles have high potential as targeting delivery vehicles for cancer chemotherapy. In this study, silver-decorated gold nanorods (AuNR\Ag) have been successfully used to deliver specific, targeted chemotherapy against breast cancer (MCF7) and prostate carcinoma (PC3) cell lines. Doxorubicin, a commonly used chemotherapy, and anti-Epithelial cell adhesion mol. (anti-EpCAM) antibodies were covalently bonded to thiolated polyethylene glycol-coated AuNR\Ag, and the resultant system was used to deliver the drugs to cancer cells in vitro. Furthermore, these nanoparticles have a unique spectral signature by surface enhanced Raman spectroscopy (SERS), which enables reliable detection and monitoring of the distribution of these chemotherapy constructs inside cells. The development of interest in a plasmonic nano drugs system with unique spectroscopic signatures could result in a clin. approach to the precise targeting and visualization of cells and solid tumors while delivering mols. for the enhanced treatment of cancerous tumors.
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    Wang, F.; Li, C. H.; Chen, H. J.; Jiang, R. B.; Sun, L. D.; Li, Q.; Wang, J. F.; Yu, J. C.; Yan, C. H. Plasmonic Harvesting of Light Energy for Suzuki Coupling Reactions. J. Am. Chem. Soc. 2013, 135, 5588– 5601,  DOI: 10.1021/ja310501y
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    Plasmonic Harvesting of Light Energy for Suzuki Coupling Reactions
    Wang, Feng; Li, Chuanhao; Chen, Huanjun; Jiang, Ruibin; Sun, Ling-Dong; Li, Quan; Wang, Jianfang; Yu, Jimmy C.; Yan, Chun-Hua
    Journal of the American Chemical Society (2013), 135 (15), 5588-5601CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The efficient use of solar energy has received wide interest due to increasing energy and environmental concerns. A potential means in chem. is sunlight-driven catalytic reactions. The authors report on the direct harvesting of visible-to-near-IR light for chem. reactions by use of plasmonic Au-Pd nanostructures. The intimate integration of plasmonic Au nanorods with catalytic Pd nanoparticles through seeded growth enabled efficient light harvesting for catalytic reactions on the nanostructures. Upon plasmon excitation, catalytic reactions were induced and accelerated through both plasmonic photocatalysis and photothermal conversion. Under the illumination of an 809 nm laser at 1.68 W, the yield of the Suzuki coupling reaction was ∼2 times that obtained when the reaction was thermally heated to the same temp. Moreover, the yield was also ∼2 times that obtained from Au-TiOx-Pd nanostructures under the same laser illumination, where a 25-nm-thick TiOx shell was introduced to prevent the photocatalysis process. This is a more direct comparison between the effect of joint plasmonic photocatalysis and photothermal conversion with that of sole photothermal conversion. The contribution of plasmonic photocatalysis became larger when the laser illumination was at the plasmon resonance wavelength. It increased when the power of the incident laser at the plasmon resonance was raised. Differently sized Au-Pd nanostructures were further designed and mixed together to make the mixt. light-responsive over the visible to near-IR region. In the presence of the mixt., the reactions were completed within 2 h under sunlight, while almost no reactions occurred in the dark.
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    Gole, A.; Murphy, C. J. Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and Nature of the Seed. Chem. Mater. 2004, 16, 3633– 3640,  DOI: 10.1021/cm0492336
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    Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and Nature of the Seed
    Gole, Anand; Murphy, Catherine J.
    Chemistry of Materials (2004), 16 (19), 3633-3640CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    We report studies on the synthesis of gold nanorods by a three-step seeding protocol method using a variety of different gold seeds. The synthetic method is adapted from one we published earlier (Jana et al. J. Phys. Chem. B 2001, 105, 4065). The seeds chosen for these studies have av. diams. in the range from 4 to 18 nm, with pos. charged as well as neg. charged surface groups. In all the cases, along with a large concn. of long rods, a small no. of different shapes such as triangles, hexagons, and small rods are obsd. The proportion of small rods increases with an increase in the seed size used for nanorod synthesis. For long nanorods synthesized by different seeds a comparison of various parameters such as length, width, and aspect ratio has been made. A dependence of the nanorod aspect ratio on the size of the seed is obsd. Increasing the seed size results in lowering of the gold nanorod aspect ratios for a const. concn. of reagents. The charge on the seed also plays a role in detg. the nanorod aspect ratio. For pos. charged seeds variation in the aspect ratio is not as pronounced as that for neg. charged seeds. The gold nanorods synthesized were characterized by transmission electron microscopy (TEM), UV-vis spectroscopy, and Fourier transform IR spectroscopy. The role of seed size in the size and shape evolution of the nanocrystal, at different growth stages, has been studied by TEM.
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    Baida, H.; Christofilos, D.; Maioli, P.; Crut, A.; Del Fatti, N.; Vallee, F. Surface Plasmon Resonance Spectroscopy of Single Surfactant-Stabilized Gold Nanoparticles. Eur. Phys. J. D 2011, 63, 293– 299,  DOI: 10.1140/epjd/e2010-10594-y
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    Surface plasmon resonance spectroscopy of single surfactant-stabilized gold nanoparticles
    Baida, H.; Christofilos, D.; Maioli, P.; Crut, A.; Del Fatti, N.; Vallee, F.
    European Physical Journal D: Atomic, Molecular, Optical and Plasma Physics (2011), 63 (2), 293-299CODEN: EPJDF6; ISSN:1434-6060. (Springer)
    The optical extinction spectra of single gold nanoparticles stabilized by surfactant mols. are investigated using the spatial modulation spectroscopy technique. The exptl. results are compared to computed spectra, focusing on the width of the surface plasmon resonance. It is shown to strongly vary from particle to particle, independently of their size and dielec. environment. This demonstrates the key role of the interface conditions on the width of the surface plasmon resonance for surfactant-stabilized metal nanoparticles.
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    Li, Z. M.; Mao, W. Z.; Devadas, M. S.; Hartland, G. V. Absorption Spectroscopy of Single Optically Trapped Gold Nanorods. Nano Lett. 2015, 15, 7731– 7735,  DOI: 10.1021/acs.nanolett.5b03833
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    Absorption Spectroscopy of Single Optically Trapped Gold Nanorods
    Li, Zhongming; Mao, Weizhi; Devadas, Mary Sajini; Hartland, Gregory V.
    Nano Letters (2015), 15 (11), 7731-7735CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Extinction spectra of single gold nanorods optically trapped in water were measured by spatial modulation spectroscopy. Comparison of the extinction cross sections and resonance frequencies to finite element calcns. allows us to det. the dimensions of the nanorod and est. the contribution of radiation damping to the LSPR line width. Subtracting the radiation damping and bulk contributions from the measured line widths yields the electron-surface scattering contribution. The results show that the surfactant coating for the nanorods causes large electron-surface scattering effects with significant particle-to-particle variations. These effects are more pronounced than those seen for substrate-supported particles in previous single particle studies. Indeed, the measured line widths are only slightly narrower than that of the ensemble spectrum. These results show the importance of removing surfactant for sensing applications of these materials.
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    Yorulmaz, M.; Nizzero, S.; Hoggard, A.; Wang, L. Y.; Cai, Y. Y.; Su, M. N.; Chang, W. S.; Link, S. Single-Particle Absorption Spectroscopy by Photothermal Contrast. Nano Lett. 2015, 15, 3041– 3047,  DOI: 10.1021/nl504992h
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    Single-Particle Absorption Spectroscopy by Photothermal Contrast
    Yorulmaz, Mustafa; Nizzero, Sara; Hoggard, Anneli; Wang, Lin-Yung; Cai, Yi-Yu; Su, Man-Nung; Chang, Wei-Shun; Link, Stephan
    Nano Letters (2015), 15 (5), 3041-3047CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Removing effects of sample heterogeneity through single-mol. and single-particle techniques has advanced many fields. While background free luminescence and scattering spectroscopy is widely used, recording the absorption spectrum only is rather difficult. Here the authors present an approach capable of recording pure absorption spectra of individual nanostructures. The authors demonstrate the implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers. Comparison of the absorption spectra to the scattering spectra of the same individual gold nanoparticles reveals the blueshift of the absorption spectra, as predicted by Mie theory but previously not detectable in extinction measurements that measure the sum of absorption and scattering. By covering a wavelength range of 300 nm, the authors are also able to record absorption spectra of single gold nanorods with different aspect ratios. The spectral shift between absorption and scattering for the longitudinal plasmon resonance decreases as a function of nanorod aspect ratio, which is in agreement with simulations.
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    Berciaud, S.; Cognet, L.; Tamarat, P.; Lounis, B. Observation of Intrinsic Size Effects in the Optical Response of Individual Gold Nanoparticles. Nano Lett. 2005, 5, 515– 518,  DOI: 10.1021/nl050062t
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    Observation of Intrinsic Size Effects in the Optical Response of Individual Gold Nanoparticles
    Berciaud, Stephane; Cognet, Laurent; Tamarat, Philippe; Lounis, Brahim
    Nano Letters (2005), 5 (3), 515-518CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The photothermal heterodyne imaging method is used to study for the first time the absorption spectra of individual gold nanoparticles with diams. down to 5 nm. Intrinsic size effects that result in a broadening of the surface plasmon resonance are unambiguously obsd. Dispersions in the peak energies and homogeneous widths of the single-particle resonances are revealed. The exptl. results are analyzed within the frame of Mie theory.
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    Gaiduk, A.; Yorulmaz, M.; Ruijgrok, P. V.; Orrit, M. Room-Temperature Detection of a Single Molecule’s Absorption by Photothermal Contrast. Science 2010, 330, 353– 356,  DOI: 10.1126/science.1195475
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    Room-Temperature Detection of a Single Molecule's Absorption by Photothermal Contrast
    Gaiduk, A.; Yorulmaz, M.; Ruijgrok, P. V.; Orrit, M.
    Science (Washington, DC, United States) (2010), 330 (6002), 353-356CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    So far, single-mol. imaging has predominantly relied on fluorescence detection. The authors imaged single nonfluorescent azo dye mols. in room-temp. glycerol by the refractive effect of the heat that they release in their environment upon intense illumination. This photothermal technique provides contrast for the absorbing objects only, irresp. of scattering by defects or roughness, with a signal-to-noise ratio of ~10 for a single mol. in an integration time of 300 ms. In the absence of O, virtually no bleaching event was obsd., even after >10 min of illumination. In a soln. satd. with O, the av. bleaching time was of the order of 1 min. No blinking was obsd. in the absorption signal. From bleaching steps, the authors obtained an av. absorption cross section of 4 Å2 for a single chromophore.
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    Chien, M. H.; Brameshuber, M.; Rossboth, B. K.; Schutz, G. J.; Schmid, S. Single-Molecule Optical Absorption Imaging by Nanomechanical Photothermal Sensing. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 11150– 11155,  DOI: 10.1073/pnas.1804174115
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    Single-molecule optical absorption imaging by nanomechanical photothermal sensing
    Chien, Miao-Hsuan; Brameshuber, Mario; Rossboth, Benedikt K.; SchA1/4tz, Gerhard J.; Schmid, Silvan
    Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (44), 11150-11155CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Absorption microscopy is a promising alternative to fluorescence microscopy for single-mol. imaging. So far, mol. absorption has been probed optically via the attenuation of a probing laser or via photothermal effects. The sensitivity of optical probing is not only restricted by background scattering but it is fundamentally limited by laser shot noise, which minimizes the achievable single-mol. signal-to-noise ratio. Here, we present nanomech. photothermal microscopy, which overcomes the scattering and shot-noise limit by detecting the photothermal heating of the sample directly with a temp.-sensitive substrate. We use nanomech. silicon nitride drums, whose resonant frequency detunes with local heating. Individual Au nanoparticles with diams. from 10 to 200 nm and single mols. (Atto 633) are scanned with a heating laser with a peak irradiance of 354 +- 45μm2 using 50A~, long-working-distance objective. With a stress-optimized drum we reach a sensitivity of 16 fW/Hz1/2 at room temp., resulting in a single-mol. signal-to-noise ratio of >70. The high sensitivity combined with the inherent wavelength independence of the nanomech. sensor presents a competitive alternative to established tools for the anal. and localization of nonfluorescent single mols. and nanoparticles.
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    Crut, A.; Maioli, P.; Del Fatti, N.; Vallee, F. Optical Absorption and Scattering Spectroscopies of Single Nano-Objects. Chem. Soc. Rev. 2014, 43, 3921– 3956,  DOI: 10.1039/c3cs60367a
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    Optical absorption and scattering spectroscopies of single nano-objects
    Crut, Aurelien; Maioli, Paolo; Del Fatti, Natalia; Vallee, Fabrice
    Chemical Society Reviews (2014), 43 (11), 3921-3956CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Developments of optical detection and spectroscopy methods for single nano-objects are key advances for applications and fundamental understanding of the novel properties exhibited by nanosize systems. These methods are reviewed, focusing on far-field optical approaches based on light absorption and elastic scattering. The principles of the main linear and nonlinear methods are described and exptl. results are illustrated in the case of metal nanoparticles, stressing the key role played by the object environment, such as the presence of a substrate, bound surface mols. or other nano-objects. Special attention is devoted to quant. methods and correlation of the measured optical spectra of a nano-object with its morphol., characterized either optically or by electron microscopy, as this permits precise comparison with theor. models. Application of these methods to optical detection and spectroscopy for single semiconductor nanowires and carbon nanotubes is also presented. Extension to ultrafast nonlinear extinction or scattering spectroscopies of single nano-objects is finally discussed in the context of investigation of their nonlinear optical response and their electronic, acoustic and thermal properties.
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    Zrimsek, A. B.; Wong, N. L.; Van Duyne, R. P. Single Molecule Surface-Enhanced Raman Spectroscopy: A Critical Analysis of the Bianalyte versus Isotopologue Proof. J. Phys. Chem. C 2016, 120, 5133– 5142,  DOI: 10.1021/acs.jpcc.6b00606
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    Single Molecule Surface-Enhanced Raman Spectroscopy: A Critical Analysis of the Bianalyte versus Isotopologue Proof
    Zrimsek, Alyssa B.; Wong, Nolan L.; Van Duyne, Richard P.
    Journal of Physical Chemistry C (2016), 120 (9), 5133-5142CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Verification of single-mol. (SM) detection for surface-enhanced Raman spectroscopy (SERS) requires the use of two analytes via either the bianalyte or isotopologue approach. For both approaches, the preferential observation of the individual analytes over a combination of both analytes is used to conclude that SM detection has been achieved. Isotopologues are preferred because they have identical surface binding affinities and Raman cross sections, whereas bianalyte pairs typically do not. We conducted multianalyte SERS studies to investigate the limitations of the bianalyte approach. The bianalyte partners, Rhodamine 6G (R6G-d0) and crystal violet (CV-d0), were directly compared, while SM detection was verified (or disproved) using their corresponding isotopologues (R6G-d4, CV-d12). We found that the significant difference in counts between R6G and CV can provide misleading evidence for SMSERS. We then rationalized these results using a joint Poisson-binomial model with unequal detection probabilities and adjusted the relative concns. of R6G and CV to achieve a comparable distribution of SMSERS counts. Using this information, we outlined the necessary considerations, such as accounting for the differences in mol. properties, for reliable SMSERS proofs. Moreover, we showed that multianalyte expts. at the SM level are achievable, opening the opportunity for new types of SM studies.
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    Young, G.; Kukura, P. Interferometric Scattering Microscopy. Annu. Rev. Phys. Chem. 2019, 70, 301– 322,  DOI: 10.1146/annurev-physchem-050317-021247
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    Interferometric Scattering Microscopy
    Young, Gavin; Kukura, Philipp
    Annual Review of Physical Chemistry (2019), 70 (), 301-322CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews)
    Interferometric scattering microscopy (iSCAT) is an extremely sensitive imaging method based on the efficient detection of light scattered by nanoscopic objects. The ability to, at least in principle, maintain high imaging contrast independent of the exposure time or the scattering cross section of the object allows for unique applications in single-particle tracking, label-free imaging of nanoscopic (dis)assembly, and quant. single-mol. characterization. We illustrate these capabilities in areas as diverse as mechanistic studies of motor protein function, viral capsid assembly, and single-mol. mass measurement in soln. We anticipate that iSCAT will become a widely used approach to unravel previously hidden details of biomol. dynamics and interactions.
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    Celebrano, M.; Kukura, P.; Renn, A.; Sandoghdar, V. Single-Molecule Imaging by Optical Absorption. Nat. Photonics 2011, 5, 95– 98,  DOI: 10.1038/nphoton.2010.290
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    Single-molecule imaging by optical absorption
    Celebrano, Michele; Kukura, Philipp; Renn, Alois; Sandoghdar, Vahid
    Nature Photonics (2011), 5 (2), 95-98CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    To date, optical studies of single mols. at room temp. have relied on the use of materials with high fluorescence quantum yield combined with efficient spectral rejection of background light. To extend single-mol. studies to a much larger pallet of substances that absorb but do not fluoresce, scientists have explored the photothermal effect, interferometry, direct attenuation and stimulated emission. Indeed, very recently, three groups have succeeded in achieving single-mol. sensitivity in absorption. Here, we apply modulation-free transmission measurements known from absorption spectrometers to image single mols. under ambient conditions both in the emissive and strongly quenched states. We arrive at quant. values for the absorption cross-section of single mols. at different wavelengths and thereby set the ground for single-mol. absorption spectroscopy. Our work has important implications for research ranging from absorption and IR spectroscopy to sensing of unlabeled proteins at the single-mol. level.
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    Chong, S. S.; Min, W.; Xie, X. S. Ground-State Depletion Microscopy: Detection Sensitivity of Single-Molecule Optical Absorption at Room Temperature. J. Phys. Chem. Lett. 2010, 1, 3316– 3322,  DOI: 10.1021/jz1014289
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    Ground-State Depletion Microscopy: Detection Sensitivity of Single-Molecule Optical Absorption at Room Temperature
    Chong, Shasha; Min, Wei; Xie, X. Sunney
    Journal of Physical Chemistry Letters (2010), 1 (23), 3316-3322CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Optical studies of single mols. in ambient environments, which led to broad applications, are primarily based on fluorescence detection. Direct detection of optical absorption with single-mol. sensitivity at room temp. is difficult because absorption is not a background-free measurement and is often complicated by sample scattering. Here the authors report ground-state depletion microscopy for ultrasensitive detection of absorption contrast. The authors image 20 nm Au nanoparticles as an initial demonstration of this microscopy. The authors then demonstrate the detection of an absorption signal from a single chromophore mol. at room temp. This is accomplished by using 2 tightly focused collinear continuous-wave laser beams at different wavelengths, both within a mol. absorption band, 1 of which is intensity modulated at a high frequency (>MHz). The transmission of the other beam is modulated at the same frequency due to ground state depletion. The signal of single chromophore mols. scanned across the common laser foci can be detected with shot-noise limited sensitivity. This measurement represents the ultimate detection sensitivity of nonlinear optical spectroscopy at room temp.
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    Maley, A. M.; Lu, G. J.; Shapiro, M. G.; Corn, R. M. Characterizing Single Polymeric and Protein Nanoparticles with Surface Plasmon Resonance Imaging Measurements. ACS Nano 2017, 11, 7447– 7456,  DOI: 10.1021/acsnano.7b03859
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    44
    Characterizing Single Polymeric and Protein Nanoparticles with Surface Plasmon Resonance Imaging Measurements
    Maley, Adam M.; Lu, George J.; Shapiro, Mikhail G.; Corn, Robert M.
    ACS Nano (2017), 11 (7), 7447-7456CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Near-IR surface plasmon resonance imaging (SPRI) microscopy was used to detect and characterize the adsorption of single polymeric and protein nanoparticles (PPNPs) onto chem. modified gold thin films in real time. The single-nanoparticle SPRI responses, Δ%RNP, from several hundred adsorbed nanoparticles are collected in a single SPRI adsorption measurement. Anal. of Δ%RNP frequency distribution histograms was used to provide information on the size, material content, and interparticle interactions of the PPNPs. Examples include the measurement of log-normal Δ%RNP distributions for mixts. of polystyrene nanoparticles, the quantitation of bioaffinity uptake into and aggregation of porous NIPAm-based (N-isopropylacrylamide) hydrogel nanoparticles specifically engineered to bind peptides and proteins, and the characterization of the neg. single-nanoparticle SPRI response and log-normal Δ%RNP distributions obtained for three different types of genetically encoded gas-filled protein nanostructures derived from bacteria.
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    Jiang, D.; Jiang, Y. Y.; Li, Z. M.; Liu, T.; Wo, X.; Fang, Y. M.; Tao, N. J.; Wang, W.; Chen, H. Y. Optical Imaging of Phase Transition and Li-Ion Diffusion Kinetics of Single LiCoO2 Nanoparticles During Electrochemical Cycling. J. Am. Chem. Soc. 2017, 139, 186– 192,  DOI: 10.1021/jacs.6b08923
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    Optical Imaging of Phase Transition and Li-Ion Diffusion Kinetics of Single LiCoO2 Nanoparticles During Electrochemical Cycling
    Jiang, Dan; Jiang, Yingyan; Li, Zhimin; Liu, Tao; Wo, Xiang; Fang, Yimin; Tao, Nongjian; Wang, Wei; Chen, Hong-Yuan
    Journal of the American Chemical Society (2017), 139 (1), 186-192CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Understanding the phase transition and Li-ion diffusion kinetics of Li-ion storage nanomaterials holds promising keys to further improve the cycle life and charge rate of the Li-ion battery. Traditional electrochem. studies were often based on a bulk electrode consisting of billions of electroactive nanoparticles, which washed out the intrinsic heterogeneity among individuals. Here, we employ optical microscopy, termed surface plasmon resonance microscopy (SPRM), to image electrochem. current of single LiCoO2 nanoparticles down to 50 fA during electrochem. cycling, from which the phase transition and Li-ion diffusion kinetics can be quant. resolved in a single nanoparticle, in operando and high throughput manner. SPRM maps the refractive index (RI) of single LiCoO2 nanoparticles, which significantly decreases with the gradual extn. of Li-ions, enabling the optical read-out of single nanoparticle electrochem. Further SEM characterization of the same batch of nanoparticles led to a bottom-up strategy for studying the structure-activity relationship. As RI is an intrinsic property of any material, the present approach is anticipated to be applicable for versatile kinds of anode and cathode materials, and to facilitate the rational design and optimization toward durable and fast-charging electrode materials.
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    Thambi, V.; Kar, A.; Ghosh, P.; Khatua, S. Light-Controlled In Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl3. J. Phys. Chem. C 2018, 122, 24885– 24890,  DOI: 10.1021/acs.jpcc.8b06679
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    Light-Controlled in Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl3
    Thambi, Varsha; Kar, Ashish; Ghosh, Piue; Khatua, Saumyakanti
    Journal of Physical Chemistry C (2018), 122 (43), 24885-24890CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    We report on light-controlled in situ bidirectional tuning of longitudinal surface plasmon resonance (LSPR) of single gold nanorods via oxidative etching with ferric chloride. By removing the surfactant layer from the surface of a gold nanorod, we demonstrate that the etching happens only in the presence of an excitation laser, and the etching rate and directionality can be controlled by the intensity of excitation light. At a low excitation power, a blue shift of a nanorod's LSPR of up to 50 nm was obsd., which indicates preferential etching from its tips. Whereas at a high power, we see a red shift of the nanorod's LSPR of up to 140 nm indicating etching from sides. These results present a new approach for in situ finer adjustments of a selected nanorod's plasmon resonance.
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    Carattino, A.; Khatua, S.; Orrit, M. In Situ Tuning of Gold Nanorod Plasmon through Oxidative Cyanide Etching. Phys. Chem. Chem. Phys. 2016, 18, 15619– 15624,  DOI: 10.1039/C6CP01679K
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    In situ tuning of gold nanorod plasmon through oxidative cyanide etching
    Carattino, Aquiles; Khatua, Saumyakanti; Orrit, Michel
    Physical Chemistry Chemical Physics (2016), 18 (23), 15619-15624CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Single gold nanorods exhibit great opportunities for bio-sensing, enhanced spectroscopies and photothermal therapy. A key property of these particles is the surface plasmon resonance, that is strongly dependent on their shape. Methods for tuning this resonance after the synthesis of the particles are of great interest for many applications. In this work we show that, through very well known chem. between gold atoms and cyanide ions, it is possible to tune the surface plasmon of single 25 × 50 nm rods by more than 100 nm towards longer wavelengths. This is achieved by slowly etching gold atoms from the surface of the particles, preserving their specific optical properties.
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    Al-Zubeidi, A.; Hoener, B. S.; Collins, S. S. E.; Wang, W.; Kirchner, S. R.; Hosseini Jebeli, S. A.; Joplin, A.; Chang, W.-S.; Link, S.; Landes, C. F. Hot Holes Assist Plasmonic Nanoelectrode Dissolution. Nano Lett. 2019, 19, 1301– 1306,  DOI: 10.1021/acs.nanolett.8b04894
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    Hot Holes Assist Plasmonic Nanoelectrode Dissolution
    Al-Zubeidi, Alexander; Hoener, Benjamin S.; Collins, Sean S. E.; Wang, Wenxiao; Kirchner, Silke R.; Hosseini Jebeli, Seyyed Ali; Joplin, Anneli; Chang, Wei-Shun; Link, Stephan; Landes, Christy F.
    Nano Letters (2019), 19 (2), 1301-1306CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Strong light-absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. We demonstrate the role of photogenerated hot holes in the oxidative dissoln. of individual gold nanorods with millisecond time resoln. while tuning charge-carrier d. and photon energy using snapshot hyperspectral imaging. We show that light-induced hot charge carriers enhance the rate of gold oxidn. and subsequent electrodissoln. Importantly, we distinguish how hot holes generated from interband transitions vs. hot holes around the Fermi level contribute to photooxidative dissoln. The results provide new insights into hot-hole-driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of nonequil. processes on innately heterogeneous nanoparticle supports.
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    Cheng, J.; Liu, Y.; Cheng, X. D.; He, Y.; Yeung, E. S. Real Time Observation of Chemical Reactions of Individual Metal Nanoparticles with High-Throughput Single Molecule Spectral Microscopy. Anal. Chem. 2010, 82, 8744– 8749,  DOI: 10.1021/ac101933y
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    Real Time Observation of Chemical Reactions of Individual Metal Nanoparticles with High-Throughput Single Molecule Spectral Microscopy
    Cheng, Jing; Liu, Yang; Cheng, Xiaodong; He, Yan; Yeung, Edward S.
    Analytical Chemistry (Washington, DC, United States) (2010), 82 (20), 8744-8749CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    Real time observation of chem. reactions of individual noble metal nanoparticles (MNPs) is fundamentally important to their controlled synthesis, chem. sensing, and catalysis applications. Here, with a simple and high-throughput single-mol. dark-field spectral imaging technique, the authors demonstrate that the reaction-induced plasmonic resonance variations of multiple MNPs could be monitored in parallel. Oxidn. kinetics of individual gold nanorods (AuNRs), either immobilized on a glass substrate or moving freely in homogeneous soln., was recorded successfully. Heterogeneous reaction pathways and intermediate states unobservable in ensemble UV-visible measurements were revealed. Interestingly, the oxidn. rate of individual immobilized AuNRs was much slower than that of the bulk AuNR soln., which implies the existence of a novel self-catalysis mechanism. This high-throughput dark-field spectral imaging technique could be applied to chem. reaction kinetics and heterogeneous catalysis studies of other MNPs at single particle level.
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    Sun, S. S.; Gao, M. X.; Lei, G.; Zou, H. Y.; Ma, J.; Huang, C. Z. Visually Monitoring the Etching Process of Gold Nanoparticles by Ki/I2 at Single-Nanoparticle Level Using Scattered-Light Dark-Field Microscopic Imaging. Nano Res. 2016, 9, 1125– 1134,  DOI: 10.1007/s12274-016-1007-z
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    Visually monitoring the etching process of gold nanoparticles by KI/I2 at single-nanoparticle level using scattered-light dark-field microscopic imaging
    Sun, Shanshan; Gao, Mingxuan; Lei, Gang; Zou, Hongyan; Ma, Jun; Huang, Chengzhi
    Nano Research (2016), 9 (4), 1125-1134CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
    Real-time monitoring of reaction processes is helpful for understanding the reaction mechanisms. In this study, we investigated the etching mechanism of gold nanoparticles (AuNPs) by iodine on a single-nanoparticle level because AuNPs have become important nanoprobes with applications in sensing and bioimaging fields owing to their specific localized surface plasmon resonance (LSPR) properties. By using a scattered-light dark-field microscopic imaging (iDFM) technique, the in situ KI/I2-treated etching processes of various shapes of AuNPs, including nanospheres (AuNSs), nanorods (AuNRs), and nanotrigonal prisms (AuNTs), were monitored in real time. It was found that the scattered light of the different shapes of AuNPs exhibited noticeable color changes upon exposure to the etching soln. The scattering spectra during the etching process showed obvious blue-shifts with decreasing scattered intensity owing to the oxidn. of Au atoms into [AuI2]-. Both finite-difference time-domain (FDTD) simulations and monitoring of morphol. variations proved that the etching was a thermodn.-dependent process through a chamfering mechanism coupled with layer-by-layer peeling, resulting in isotropic spheres with decreased particle sizes. [Figure not available: see fulltext.].
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    Wang, J.; Zhang, H. Z.; Liu, J. J.; Yuan, D.; Li, R. S.; Huang, C. Z. Time-Resolved Visual Detection of Heparin by Accelerated Etching of Gold Nanorods. Analyst 2018, 143, 824– 828,  DOI: 10.1039/C7AN01923H
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    Time-resolved visual detection of heparin by accelerated etching of gold nanorods
    Wang, Jian; Zhang, Hong Zhi; Liu, Jia Jun; Yuan, Dan; Li, Rong Sheng; Huang, Cheng Zhi
    Analyst (Cambridge, United Kingdom) (2018), 143 (4), 824-828CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
    Plasmonic gold nanorods are promising and sensitive light scattering probes, which can reach the single particle level. Herein, we present the light scattering properties of gold nanorods for time-resolved visual detection of heparin based on the rapid etching of gold nanorods under dark-field microscopy.
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    Zhang, H. Z.; Li, R. S.; Gao, P. F.; Wang, N.; Lei, G.; Huang, C. Z.; Wang, J. Real-Time Dark-Field Light Scattering Imaging to Monitor the Coupling Reaction with Gold Nanorods as an Optical Probe. Nanoscale 2017, 9, 3568– 3575,  DOI: 10.1039/C6NR09453H
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    Real-time dark-field light scattering imaging to monitor the coupling reaction with gold nanorods as an optical probe
    Zhang, Hong Zhi; Li, Rong Sheng; Gao, Peng Fei; Wang, Ni; Lei, Gang; Huang, Cheng Zhi; Wang, Jian
    Nanoscale (2017), 9 (10), 3568-3575CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Gold nanorods (GNRs) have opened up promising applications based on their reshaping, due to the fact that a tiny change in shape or size could directly lead to optical changes. Herein, we report chem.reshaping of GNRs induced by the coupling reaction between Au, ferric chloride and thiourea. In the coupling reaction, Fe3+ oxidizes the GNRs to yield Au(I), which complexes with the thiourea ligand, lowering the Gibbs free energy of the gold species and promoting the reaction equil.to enable the chem.reshaping of the GNRs. This coupling reaction process was monitored using a light-scattering dark-field microscopy (DFM) imaging technique and SEM (SEM). The light scattering underwent a color change from bright red to yellow and finally to green, and the GNRs underwent a morphol.change from rod-shaped to fusiform and finally to spherical, which is somewhat different from the results of other chem.etching processes of GNRs. It is believed that the coupling reaction induced chem.reshaping of GNRs not only provides an alternative way to monitor the coupling reaction, but also offers a facile way to obtain a desirable GNR morphol., which is important for the prepn.of fusiform nanostructures.
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    Xie, T.; Jing, C.; Ma, W.; Ding, Z. F.; Gross, A. J.; Long, Y. T. Real-Time Monitoring for the Morphological Variations of Single Gold Nanorods. Nanoscale 2015, 7, 511– 517,  DOI: 10.1039/C4NR05080K
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    Real-time monitoring for the morphological variations of single gold nanorods
    Xie, Tao; Jing, Chao; Ma, Wei; Ding, Zhifeng; Gross, Andrew James; Long, Yi-Tao
    Nanoscale (2015), 7 (2), 511-517CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The morphol. characteristics of metal nanoparticles, particularly the shape, play an essential role in the optical, phys. and chem. properties. In this work, we reported a transverse etching process to investigate the morphol. variations of single gold nanorods (GNRs). Dark-field microscopy and Rayleigh scattering spectroscopy were used as complementary technologies to track the transverse etching process. Dark-field imaging with high spatial and temporal resoln. could easily monitor the transverse etching process of GNRs in situ and in real time. Interactions between the scattering spectrum and the morphol. variations were judiciously calcd. within the dipole approxn. by the Drude function. The calcd. peak shift of GNRs (Δλmax = 17 nm) was obtained via the ratio of the long axis and short axis (aspect ratio) of GNRs from transmission electron microscopy. The av. scattering peak shift (Δλmax = 22 nm) from Rayleigh scattering spectroscopy was in good agreement with the calcd. peak shift. Monitoring the morphol. variations of single GNRs enables us to track the transverse etching of GNRs at arbitrary time. This promises to be a useful method for the study of different nanomaterials and their spectral properties.
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    Flatebo, C.; Collins, S. S. E.; Hoener, B. S.; Cai, Y.-y.; Link, S.; Landes, C. F. Electrodissolution Inhibition of Gold Nanorods with Oxoanions. J. Phys. Chem. C 2019, 123, 13983– 13992,  DOI: 10.1021/acs.jpcc.9b01575
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    Electrodissolution Inhibition of Gold Nanorods with Oxoanions
    Flatebo, Charlotte; Collins, Sean S. E.; Hoener, Benjamin S.; Cai, Yi-yu; Link, Stephan; Landes, Christy F.
    Journal of Physical Chemistry C (2019), 123 (22), 13983-13992CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Metal nanoparticles experience varied chem. environments that can cause corrosion and dissoln. in electronics, electrocatalysis, and sensing applications. Understanding oxidative dissoln. is crit. for plasmonic nanoparticles because their optical properties strongly depend on size and shape. The addn. of low relative concns. of oxoanions to aq. halide electrolyte solns. improves the morphol. stability of plasmonic Au nanorods at anodic electrochem. potentials that otherwise induce complete oxidative electrodissoln. Single particle hyperspectral dark-field imaging and correlated SEM show that oxoanions alter the electrodissoln. onset potential, electrodissoln. pathway, and nanoparticle reaction heterogeneity, as compared to chloride-only electrolyte solns. The authors identify five mechanistic contributors to the corrosion inhibition capabilities of oxoanions in the presence of chloride ions, with the aim of expanding the range of electrochem. sensing and catalysis applications for plasmonic metal nanoparticles. Of the contributors studied, the pH, adsorption potential, and ionicity of the oxoanion are the most influential factors, supporting the superior corrosion inhibition obsd. with bicarbonate and phosphate.
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    Ye, X. C.; Jones, M. R.; Frechette, L. B.; Chen, Q.; Powers, A. S.; Ercius, P.; Dunn, G.; Rotskoff, G. M.; Nguyen, S. C.; Adiga, V. P.; Zettl, A.; Rabani, E.; Geissler, P. L.; Alivisatos, A. P. Single-Particle Mapping of Nonequilibrium Nanocrystal Transformations. Science 2016, 354, 874– 877,  DOI: 10.1126/science.aah4434
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    Single-particle mapping of nonequilibrium nanocrystal transformations
    Ye, Xingchen; Jones, Matthew R.; Frechette, Layne B.; Chen, Qian; Powers, Alexander S.; Ercius, Peter; Dunn, Gabriel; Rotskoff, Grant M.; Nguyen, Son C.; Adiga, Vivekananda P.; Zettl, Alex; Rabani, Eran; Geissler, Phillip L.; Alivisatos, A. Paul
    Science (Washington, DC, United States) (2016), 354 (6314), 874-877CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The control of the shape and size of metal nanoparticles can be sensitive to the growth conditions of the particles. Ye et al. studied the reverse process: They tracked the dissoln. of gold nanoparticles in a redox environment inside a liq. cell within an electron microscope, controlling the particle dissoln. with the electron beam. Tracking short-lived particle shapes revealed structures of greater or lesser stability. The findings suggest kinetic routes to particle sizes and shapes that would otherwise be difficult to generate.
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    Sun, Y. Z.; Fan, X. D. Optical Ring Resonators for Biochemical and Chemical Sensing. Anal. Bioanal. Chem. 2011, 399, 205– 211,  DOI: 10.1007/s00216-010-4237-z
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    Optical ring resonators for biochemical and chemical sensing
    Sun, Yuze; Fan, Xudong
    Analytical and Bioanalytical Chemistry (2011), 399 (1), 205-211CODEN: ABCNBP; ISSN:1618-2642. (Springer)
    A review. In the past few years optical ring resonators have emerged as a new sensing technol. for highly sensitive detection of analytes in liq. or gas. This article introduces the ring resonator sensing principle, describes various ring resonator sensor designs, reviews the current state of the field, and presents an outlook of possible applications and related research and development directions.
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    Lu, T.; Lee, H.; Chen, T.; Herchak, S.; Kim, J. H.; Fraser, S. E.; Flagan, R. C.; Vahala, K. High Sensitivity Nanoparticle Detection Using Optical Microcavities. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 5976– 5979,  DOI: 10.1073/pnas.1017962108
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    High sensitivity nanoparticle detection using optical microcavities
    Lu, Tao; Lee, Hansuek; Chen, Tong; Herchak, Steven; Kim, Ji-Hun; Fraser, Scott E.; Flagan, Richard C.; Vahala, Kerry
    Proceedings of the National Academy of Sciences of the United States of America (2011), 108 (15), 5976-5979CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    We demonstrate a highly sensitive nanoparticle and virus detection method by using a thermal-stabilized ref. interferometer in conjunction with an ultrahigh-Q microcavity. Sensitivity is sufficient to resolve shifts caused by binding of individual nanobeads in soln. down to a record radius of 12.5 nm, a size approaching that of single protein mols. A histogram of wavelength shift vs. nanoparticle radius shows that particle size can be inferred from shift maxima. Addnl., the signal-to-noise ratio for detection of Influenza A virus is enhanced to 38:1 from the previously reported 3:1. The method does not use feedback stabilization of the probe laser. It is also obsd. that the conjunction of particle-induced backscatter and optical-path-induced shifts can be used to enhance detection signal-to-noise.
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    Ward, J. M.; Yang, Y.; Lei, F. C.; Yu, X. C.; Xiao, Y. F.; Nic Chormaic, S. Nanoparticle Sensing Beyond Evanescent Field Interaction with a Quasi-Droplet Microcavity. Optica 2018, 5, 674– 677,  DOI: 10.1364/OPTICA.5.000674
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    Nanoparticle sensing beyond evanescent field interaction with a quasi-droplet microcavity
    Ward, Jonathan M.; Yang, Yong; Lei, Fuchuan; Yu, Xiao-Chong; Xiao, Yun-Feng; Chormaic, Sile Nic
    Optica (2018), 5 (6), 674-677CODEN: OPTIC8; ISSN:2334-2536. (Optical Society of America)
    Sensing with whispering gallery resonators (WGRs) is largely limited by the weak perturbation of the whispering gallery mode (WGM) via the evanescent field. A new sensing regime using quasi-droplet WGMs allows WGRs to move beyond the limitation of the evanescent field and push the detection sensitivity to new heights. We present exptl. results on the detection of 100 nm and 500 nm polystyrene particles in aq. soln. using thin-walled, hollow WGRs supporting quasi-droplet modes. The detection sensitivity in terms of mode shift and broadening is measured, with mode shifts of 400 MHz obsd. for 100 nm particles. In terms of the no. of linewidths, this is 276 times larger than similar expts. with microsphere WGRs, thus showing a significant increase in detection sensitivity beyond the capability of std. evanescent field sensing with WGRs.
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    Barucci, A.; Berneschi, S.; Giannetti, A.; Baldini, F.; Cosci, A.; Pelli, S.; Farnesi, D.; Righini, G. C.; Soria, S.; Conti, G. N. Optical Microbubble Resonators with High Refractive Index Inner Coating for Bio-Sensing Applications: An Analytical Approach. Sensors 2016, 16, 1992,  DOI: 10.3390/s16121992
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    Optical microbubble resonators with high refractive index inner coating for bio-sensing applications: an analytical approach
    Barucci, Andrea; Berneschi, Simone; Giannetti, Ambra; Baldini, Francesco; Cosci, Alessandro; Pelli, Stefano; Farnesi, Daniele; Righini, Giancarlo C.; Soria, Silvia; Conti, Gualtiero Nunzi
    Sensors (2016), 16 (12), 1992/1-1992/19CODEN: SENSC9; ISSN:1424-8220. (MDPI AG)
    The design of Whispering Gallery Mode Resonators (WGMRs) used as an optical transducer for biosensing represents the first and crucial step towards the optimization of the final device performance in terms of sensitivity and Limit of Detection (LoD). Here, we propose an anal. method for the design of an optical microbubble resonator (OMBR)-based biosensor. In order to enhance the OMBR sensing performance, we consider a polymeric layer of high refractive index as an inner coating for the OMBR. The effect of this layer and other optical/geometrical parameters on the mode field distribution, sensitivity and LoD of the OMBR is assessed and discussed, both for transverse elec. (TE) and transverse magnetic (TM) polarization. The obtained results do provide phys. insights for the development of OMBR-based biosensor.
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  60. 60
    Giorgini, A.; Avino, S.; Malara, P.; De Natale, P.; Gagliardi, G. Liquid Droplet Microresonators. Sensors 2019, 19, 473,  DOI: 10.3390/s19030473
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    Liquid droplet microresonators
    Giorgini, Antonio; Avino, Saverio; Malara, Pietro; De Natale, Paolo; Gagliardi, Gianluca
    Sensors (2019), 19 (3), 473/1-473/20CODEN: SENSC9; ISSN:1424-8220. (MDPI AG)
    A review. We provide here an overview of passive optical micro-cavities made of droplets in the liq. phase. We focus on resonators that are naturally created and suspended under gravity thanks to interfacial forces, illustrating simple ways to excite whispering-gallery modes in various slow-evapn. liqs. using free-space optics. Similar to solid resonators, frequency locking of near-IR and visible lasers to resonant modes is performed exploiting either phase-sensitive detection of the leakage cavity field or multiple interference between whispering-gallery modes in the scattered light. As opposed to conventional micro-cavity sensors, each droplet acts simultaneously as the sensor and the sample, whereby the internal light can detect dissolved compds. and particles. Optical quality factors up to 107-108 are obsd. in liq.-polymer droplets through photon lifetime measurements. First attempts in using single water droplets are also reported. These achievements point out their huge potential for direct spectroscopy and bio-chem. sensing in liq. environments. Finally, the first expts. of cavity optomechanics with surface acoustic waves in nanolitre droplets are presented. The possibility to perform studies of viscous-elastic properties points to a new paradigm: a droplet device as an opto-fluid-mechanics lab. on table-top scale under controlled environmental conditions.
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  61. 61
    Madani, A.; Harazim, S. M.; Quinones, V. A. B.; Kleinert, M.; Finn, A.; Naz, E. S. G.; Ma, L. B.; Schmidt, O. G. Optical Microtube Cavities Monolithically Integrated on Photonic Chips for Optofluidic Sensing. Opt. Lett. 2017, 42, 486– 489,  DOI: 10.1364/OL.42.000486
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    61
    Optical microtube cavities monolithically integrated on photonic chips for optofluidic sensing
    Madani, Abbas; Harazim, Stefan M.; Quinones, Vladimir A. Bolanos; Kleinert, Moritz; Finn, Andreas; Naz, Ehsan Saei Ghareh; Ma, Libo; Schmidt, Oliver G.
    Optics Letters (2017), 42 (3), 486-489CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
    Microtubular optical resonators are monolithically integrated on photonic chips to demonstrate optofluidic functionality. Due to the compact subwavelength-thin tube wall and a well-defined nanogap between polymer photonic waveguides and the microtube, excellent optical coupling with extinction ratios up ro 32 dB are obsd. in the telecommunication relevant wavelength range. For the first time, optofluidic applications of fully on-chip integrated microtubular systems are investigated both by filling the core of the microtube and by the microtube being covered by a liq. droplet. Total shifts over the full free spectral range are obsd. in response to the presence of the liq. medium in the vicinity of the microtube resonators. This work provides a vertical coupling scheme for optofluidic applications in monolithically integrated so-called "lab-in-a-tube" systems.
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    Han, K. W.; Kim, J.; Bahl, G. High-Throughput Sensing of Freely Flowing Particles with Optomechanofluidics. Optica 2016, 3, 585– 591,  DOI: 10.1364/OPTICA.3.000585
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    High-throughput sensing of freely flowing particles with optomechanofluidics
    Han, Kewen; Kim, Junhwan; Bahl, Gaurav
    Optica (2016), 3 (6), 585-591CODEN: OPTIC8; ISSN:2334-2536. (Optical Society of America)
    High-Q photonic microcavity sensors have enabled the label-free measurement of nanoparticles, such as single viruses and large mols., close to the fundamental limits of detection. However, key scientific challenges persist: (1) photons do not directly couple to mech. parameters such as mass d., compressibility, or viscoelasticity, and (2) current techniques cannot measure all particles in a fluid sample due to the reliance on random diffusion to bring analytes to the sensing region. Here, we present a new, label-free microfluidic optomech. sensor that addresses both challenges, enabling, for the first time, the rapid photonic sensing of the mech. properties of freely flowing particles in a fluid. Sensing is enabled by optomech. coupling of photons to long-range phonons that cast a near-perfect net deep inside the device. Our opto-mechano-fluidic approach enables the measurement of particle mass d., mech. compressibility, and viscoelasticity at rates potentially exceeding 10,000 particles/s. Uniquely, we show that the sensitivity of this high-Q microcavity sensor is highest when the analytes are located furthest from the optical mode, at the center of the device, where the flow is fastest. Our results enable till-date inaccessible mech. anal. of flowing particles at speeds comparable to com. flow cytometry.
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    Stoian, R.-I.; Bui, K. V.; Rosenberger, A. Silica Hollow Bottle Resonators for Use as Whispering Gallery Mode Based Chemical Sensors. J. Opt. 2015, 17, 125011,  DOI: 10.1088/2040-8978/17/12/125011
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    Silica hollow bottle resonators for use as whispering gallery mode based chemical sensors
    Stoian, Razvan-Ionut; Bui, Khoa V.; Rosenberger, A. T.
    Journal of Optics (Bristol, United Kingdom) (2015), 17 (12), 125011/1-125011/7CODEN: JOOPCA; ISSN:2040-8978. (IOP Publishing Ltd.)
    A simple three-step method for making silica hollow bottle resonators (HBRs) was developed. This procedure is advantageous because it uses com. available materials, is cost effective, and is easy to implement. Addnl., the use of these HBRs as whispering gallery mode based chem. sensors is demonstrated by preliminary absorption sensing results in the near IR (1580-1660 nm) using a trace gas (CH4) in air at atm. pressure and a dye (SDA2072) in methanol soln.
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    Ward, J. M.; Dhasmana, N.; Nic Chormaic, S. Hollow Core, Whispering Gallery Resonator Sensors. Eur. Phys. J.: Spec. Top. 2014, 223, 1917– 1935,  DOI: 10.1140/epjst/e2014-02236-5
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    Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J. Demonstration of Ultra-High-Q Small Mode Volume Toroid Microcavities on a Chip. Appl. Phys. Lett. 2004, 85, 6113– 6115,  DOI: 10.1063/1.1833556
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    Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip
    Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J.
    Applied Physics Letters (2004), 85 (25), 6113-6115CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Optical microcavities confine light spatially and temporally and find application in a wide range of fundamental and applied studies. In many areas, the microcavity figure of merit is not only detd. by photon lifetime (or the equiv. quality-factor, Q), but also by simultaneous achievement of small mode vol. (V). Here we demonstrate ultra-high Q-factor small mode vol. toroid microcavities on-a-chip, which exhibit a Q/V factor of more than 106 (λ/n)-3. These values are the highest reported to date for any chip-based microcavity. A corresponding Purcell factor in excess of 200 000 and a cavity finesse of °2.8 × 106 is achieved, demonstrating that toroid microcavities are promising candidates for studies of the Purcell effect, cavity QED or biochem. sensing.
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    Armani, A. M.; Vahala, K. J. Heavy Water Detection Using Ultra-High-Q Microcavities. Opt. Lett. 2006, 31, 1896– 1898,  DOI: 10.1364/OL.31.001896
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    Heavy water detection using ultra-high-Q microcavities
    Armani, Andrea M.; Vahala, Kerry J.
    Optics Letters (2006), 31 (12), 1896-1898CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
    Ultra-high-Q optical microcavities (Q > 107) provide one method for distinguishing chem. similar species. Resonators immersed in H2O have lower quality factors than those immersed in D2O due to the difference in optical absorption. This difference can be used to create a D2O detector. This effect is most noticeable at 1300 nm, where the Q(H2O) is 106 and the Q(D2O) is 107. By monitoring Q, concns. of 0.0001% [1 part in 106 per vol.] of D2O in H2O have been detected. This sensitivity represents an order of magnitude improvement over previous techniques. Reversible detection was also demonstrated by cyclic introduction and flushing of D2O.
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    Armani, D. K.; Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J. Ultra-High-Q Toroid Microcavity on a Chip. Nature 2003, 421, 925– 928,  DOI: 10.1038/nature01371
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    Ultra-high-Q toroid microcavity on a chip
    Armani, D. K.; Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J.
    Nature (London, United Kingdom) (2003), 421 (6926), 925-928CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The circulation of light within dielec. vols. enables storage of optical power near specific resonant frequencies and is important in a wide range of fields including cavity quantum electrodynamics, photonics, biosensing and nonlinear optics. Optical trajectories occur near the interface of the vol. with its surroundings, making their performance strongly dependent upon interface quality. With a nearly at.-scale surface finish, surface-tension-induced microcavities such as liq. droplets or spheres are superior to all other dielec. microresonant structures when comparing photon lifetime or, equivalently, cavity Q factor. Despite these advantageous properties, the phys. characteristics of such systems are not easily controlled during fabrication. It is known that wafer-based processing of resonators can achieve parallel processing and control, as well as integration with other functions. However, such resonators-on-a-chip suffer from Q factors that are many orders of magnitude lower than for surface-tension-induced microcavities, making them unsuitable for ultra-high-Q expts. Here the authors demonstrate a process for producing SiO2 toroid-shaped microresonators-on-a-chip with Q factors >100 million using a combination of lithog., dry etching and a selective reflow process. Such a high Q value was previously attainable only by droplets or microspheres and represents an improvement of nearly four orders of magnitude over previous chip-based resonators.
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    Black, E. D. An Introduction to Pound-Drever-Hall Laser Frequency Stabilization. Am. J. Phys. 2001, 69, 79– 87,  DOI: 10.1119/1.1286663
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    Barnes, J. A.; Gagliardi, G.; Loock, H. P. Absolute Absorption Cross-Section Measurement of a Submonolayer Film on a Silica Microresonator. Optica 2014, 1, 75– 83,  DOI: 10.1364/OPTICA.1.000075
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    Absolute absorption cross-section measurement of a submonolayer film on a silica microresonator
    Barnes, Jack A.; Gagliardi, Gianluca; Loock, Hans-Peter
    Optica (2014), 1 (2), 75-83CODEN: OPTIC8; ISSN:2334-2536. (Optical Society of America)
    Conventional absorption spectroscopy is not nearly sensitive enough for quant. overtone measurements on submonolayer coatings. While cavity-enhanced absorption detection methods using microresonators have the potential to provide quant. absorption cross sections of even weakly absorbing submonolayer films, this potential has not yet been fully realized. To det. the absorption cross section of a submonolayer film of ethylene diamine (EDA) on a silica microsphere resonator, we use phase-shift cavity ringdown spectroscopy simultaneously on near-IR radiation that is Rayleigh backscattered from the microsphere and transmitted through the coupling fiber taper. We then independently det. both the coupling coeff. and the optical loss within the resonator. Together with a coincident measurement of the wavelength frequency shift, an abs. overtone absorption cross section of adsorbed EDA, at submonolayer coverage, was obtained and was compared to the bulk value. The smallest quantifiable absorption cross section is σmin 2.7 × 10-12 cm2. This absorption cross section is comparable to the extinction coeffs. of, e.g., single gold nanoparticles or aerosol particles. We therefore propose that the present method is also a viable route to abs. extinction measurements of single particles.
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    Carmon, T.; Kippenberg, T. J.; Yang, L.; Rokhsari, H.; Spillane, S.; Vahala, K. J. Feedback Control of Ultra-High-Q Microcavities: Application to Micro-Raman Lasers and Microparametric Oscillators. Opt. Express 2005, 13, 3558– 3566,  DOI: 10.1364/OPEX.13.003558
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    70
    Feedback control of ultra-high-Q microcavities: application to micro-Raman lasers and microparametric oscillators
    Carmon Tal; Kippenberg Tobias; Yang Lan; Rokhsari Hosein; Spillane Sean; Vahala Kerry
    Optics express (2005), 13 (9), 3558-66 ISSN:.
    We demonstrate locking of an on-chip, high-Q toroidal-cavity to a pump laser using two, distinct methods: coupled power stabilization and wavelength locking of pump laser to the microcavity. In addition to improvements in operation of previously demonstrated micro-Raman and micro-OPO lasers, these techniques have enabled observation of a continuous, cascaded nonlinear process in which photons generated by optical parametric oscillations (OPO) function as a pump for Raman lasing. Dynamical behavior of the feedback control systems is also shown including the interplay between the control loop and the thermal nonlinearity. The demonstrated stabilization loop is essential for studying generation of nonclassical states using a microcavity optical parametric oscillator.
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    Murugan, G. S.; Petrovich, M. N.; Jung, Y.; Wilkinson, J. S.; Zervas, M. N. Hollow-Bottle Optical Microresonators. Opt. Express 2011, 19, 20773– 20784,  DOI: 10.1364/OE.19.020773
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    Nasir, M. N. M.; Murugan, G. S.; Zervas, M. N. Spectral Cleaning and Output Modal Transformations in Whispering-Gallery-Mode Microresonators. J. Opt. Soc. Am. B 2016, 33, 1963– 1970,  DOI: 10.1364/JOSAB.33.001963
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    Spectral cleaning and output modal transformations in whispering-gallery-mode microresonators
    Nasir, Mohd Narizee Mohd; Murugan, G. Senthil; Zervas, Michalis N.
    Journal of the Optical Society of America B: Optical Physics (2016), 33 (9), 1963-1970CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)
    A systematic study on the effects of microtaper fiber diams. on the spectral characteristics of a whispering-gallery-mode (WGM) microbottle resonator (MBR) is presented. Progressively cleaner and simpler spectra of the MBR were obsd. when the utilized microtaper fiber waist diam. (Dt) was increased from 2 to 10 μm. The max. transmission depth at resonance varies with different microtaper fiber utilized from ∼20 dB (Dt=2 μm) to ∼4 dB (Dt = 10 μm). The loaded Q-factors were obsd. to be unaffected by the increase of Dt with values of >106 being measured in all cases. Mode transformation of MBR was also exptl. investigated and compared to a microdisc finite-difference time-domain simulation by studying near-field images of the output beam on the waist of the microtaper fibers. For the first time, exptl. observation of mode transformation from LP01 to LP11 across scanned WGM resonances is being reported.
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    Ding, M.; Murugan, G. S.; Brambilla, G.; Zervas, M. N. Whispering Gallery Mode Selection in Optical Bottle Microresonators. Appl. Phys. Lett. 2012, 100, 081108,  DOI: 10.1063/1.3688601
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    Whispering gallery mode selection in optical bottle microresonators
    Ding, Ming; Murugan, Ganapathy Senthil; Brambilla, Gilberto; Zervas, Michalis N.
    Applied Physics Letters (2012), 100 (8), 081108/1-081108/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The authors demonstrated a method to excite selected whispering gallery modes in optical bottle microresonators (BMR) by inscribing microgroove scars on their surface by focused ion beam milling. Substantial spectral clean-up is obtained in appropriately scarred BMRs, providing the potential for high performance sensors and other optical devices. (c) 2012 American Institute of Physics.
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    Schunk, G.; Furst, J. U.; Fortsch, M.; Strekalov, D. V.; Vogl, U.; Sedlmeir, F.; Schwefel, H. G. L.; Leuchs, G.; Marquardt, C. Identifying Modes of Large Whispering-Gallery Mode Resonators from the Spectrum and Emission Pattern. Opt. Express 2014, 22, 30795– 30806,  DOI: 10.1364/OE.22.030795
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    Identifying modes of large whispering-gallery mode resonators from the spectrum and emission pattern
    Schunk Gerhard; Furst Josef U; Fortsch Michael; Strekalov Dmitry V; Vogl Ulrich; Sedlmeir Florian; Schwefel Harald G L; Leuchs Gerd; Marquardt Christoph
    Optics express (2014), 22 (25), 30795-806 ISSN:.
    Identifying the mode numbers in whispering-gallery mode resonators (WGMRs) is important for tailoring them to experimental needs. Here we report on a novel experimental mode analysis technique based on the combination of frequency analysis and far-field imaging for high mode numbers of large WGMRs. The radial mode numbers q and the angular mode numbers p = ℒ-m are identified and labeled via far-field imaging. The polar mode numbers ℒ are determined unambiguously by fitting the frequency differences between individual whispering gallery modes (WGMs). This allows for the accurate determination of the geometry and the refractive index at different temperatures of the WGMR. For future applications in classical and quantum optics, this mode analysis enables one to control the narrow-band phase-matching conditions in nonlinear processes such as second-harmonic generation or parametric down-conversion.
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    Ward, J. M.; Yang, Y.; Nic Chormaic, S. Highly Sensitive Temperature Measurements with Liquid-Core Microbubble Resonators. IEEE Photonics Technol. Lett. 2013, 25, 2350– 2353,  DOI: 10.1109/LPT.2013.2283732
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    Hall, J. M. M.; Francois, A.; Afshar, V. S.; Riesen, N.; Henderson, M. R.; Reynolds, T.; Monro, T. M. Determining the Geometric Parameters of Microbubble Resonators from Their Spectra. J. Opt. Soc. Am. B 2017, 34, 44– 51,  DOI: 10.1364/JOSAB.34.000044
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    Determining the geometric parameters of microbubble resonators from their spectra
    Hall, Jonathan M. M.; Francois, Alexandre; Shahraam, Afshar V.; Riesen, Nicolas; Henderson, Matthew R.; Reynolds, Tess; Monro, Tanya M.
    Journal of the Optical Society of America B: Optical Physics (2017), 34 (1), 44-51CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)
    A method for detg. the diams. and shell thickness of microbubble resonators is presented; it entails simulating whispering gallery mode (WGM) spectra using a newly developed finite-difference time-domain (FDTD)-based toolkit. Spectra for a range of shell thicknesses are simulated using FDTD, assuming a linear dependence of the free spectral range on the diam., and the free spectral ranges and positions of the prominent modes are matched to those of the measured spectrum. This method improves upon existing techniques for extg. the diam. and thickness, such as SEM imaging, which typically require the microbubble to be dissected or otherwise rendered unusable for subsequent use. The model allows a variety of methods of mode excitation to be simulated. Dye coatings are simulated by placing a layer of dipole sources on the surface of the resonator, yielding mode couplings comparable to those measured in expts. The model is tested for a small-diam. silica glass microbubble, with the free spectral range being simulated for a range of diams. and shell thicknesses. The numerically simulated spectra are then compared to the exptl. measured spectrum. The ability to det. the geometric parameters of such resonators directly from their WGM spectra represents a step forward in the characterization of microbubble resonators. Furthermore, the model opens the way to previously unstudied spectral behavior of microbubbles with small diams. and thin shell thicknesses.
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    Murugan, G. S.; Wilkinson, J. S.; Zervas, M. N. Selective Excitation of Whispering Gallery Modes in a Novel Bottle Microresonator. Opt. Express 2009, 17, 11916– 11925,  DOI: 10.1364/OE.17.011916
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    Selective excitation of whispering gallery modes in a novel bottle microresonator
    Murugan, Ganapathy Senthil; Wilkinson, James S.; Zervas, Michalis N.
    Optics Express (2009), 17 (14), 11916-11925CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
    Selective excitation of spheroidal whispering gallery modes and bottle modes in a robust bottle microresonator fabricated straightforwardly from a short section of optical fiber is demonstrated. Characteristic resonance spectra of long-cavity bottle modes were obtained by using a tapered fiber to excite evanescently bottle microresonator at different points along its axis. Compared to bare-fiber cylindrical resonators, the bottle microresonator results in a 35× increase of the obsd. Q factor.
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    Davletshin, Y. R.; Lombardi, A.; Cardinal, M. F.; Juve, V.; Crut, A.; Maioli, P.; Liz-Marzan, L. M.; Vallee, F.; Del Fatti, N.; Kumaradas, J. C. A Quantitative Study of the Environmental Effects on the Optical Response of Gold Nanorods. ACS Nano 2012, 6, 8183– 8193,  DOI: 10.1021/nn302869v
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    A Quantitative Study of the Environmental Effects on the Optical Response of Gold Nanorods
    Davletshin, Yevgeniy R.; Lombardi, Anna; Cardinal, M. Fernanda; Juve, Vincent; Crut, Aurelien; Maioli, Paolo; Liz-Marzan, Luis M.; Vallee, Fabrice; Fatti, Natalia Del; Kumaradas, J. Carl
    ACS Nano (2012), 6 (9), 8183-8193CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The effects of the dielec. environment on the optical extinction spectra of Au nanorods were quant. studied using individual bare and SiO2-coated nanorods. The dispersion and amplitude of their extinction cross-section, dominated by absorption for the studied sizes, were measured using spatial modulation spectroscopy (SMS). The exptl. results were compared to calcns. from a numerical model that included environmental features present in the measurements and the morphol. and size of the corresponding nanorods measured by TEM. The combination of these exptl. and theor. tools permits a detailed interpretation of the optical properties of the individual nanorods. The measured optical extinction spectra and the extinction cross-section amplitudes were well reproduced by the numerical model for SiO2-coated Au nanorods, for which the SiO2 shell provides a controlled environment. But addnl. environmental factors had to be assumed in the model for bare nanorods, stressing the importance of controlling and characterizing the exptl. conditions when measuring the optical response of bare surface-deposited single metal nanoparticles.
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  79. 79
    Ni, W. H.; Chen, H. J.; Kou, X. S.; Yeung, M. H.; Wang, J. F. Optical Fiber-Excited Surface Plasmon Resonance Spectroscopy of Single and Ensemble Gold Nanorods. J. Phys. Chem. C 2008, 112, 8105– 8109,  DOI: 10.1021/jp801579m
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    79
    Optical Fiber-Excited Surface Plasmon Resonance Spectroscopy of Single and Ensemble Gold Nanorods
    Ni, Weihai; Chen, Huanjun; Kou, Xiaoshan; Yeung, Man Hau; Wang, Jianfang
    Journal of Physical Chemistry C (2008), 112 (22), 8105-8109CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Au nanorods are deposited on the core surface of an etched optical fiber, and their surface plasmon resonance is excited by the evanescent wave from the optical fiber. The excitation of the surface plasmon produces strong light scattering from individual nanorods, allowing for single-particle scattering-based imaging and spectroscopy. The authors systematically characterize the dependence of the scattering spectra of individual nanorods on the refractive index of the surrounding medium. As the refractive index is increased, the scattering spectra red-shift steadily. The refractive index sensitivity and sensing figure of merit, which is the index sensitivity divided by the scattering peak line width, reach 200 nm/RIU (RIU = refractive index unit) and 3.8, resp. The authors further study the ensemble response of the plasmon resonance of Au nanorods to varying dielec. environments by measuring the intensity and spectrum of the light transmitted through the fiber. The ensemble index sensitivity and figure of merit are 138 nm/RIU and 1.2, resp. These results suggest the potential for combining the high index sensitivities of Au nanorods together with the attractive sensing features of fiber-based sensors to develop real-time, low-cost, ultrasensitive, and multiplexed sensors.
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  80. 80
    Park, K.; Biswas, S.; Kanel, S.; Nepal, D.; Vaia, R. A. Engineering the Optical Properties of Gold Nanorods: Independent Tuning of Surface Plasmon Energy, Extinction Coefficient, and Scattering Cross Section. J. Phys. Chem. C 2014, 118, 5918– 5926,  DOI: 10.1021/jp5013279
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    80
    Engineering the Optical Properties of Gold Nanorods: Independent Tuning of Surface Plasmon Energy, Extinction Coefficient, and Scattering Cross Section
    Park, Kyoungweon; Biswas, Sushmita; Kanel, Sushil; Nepal, Dhriti; Vaia, Richard A.
    Journal of Physical Chemistry C (2014), 118 (11), 5918-5926CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The future integration of plasmonic nanoparticles, such as Au nanorods (Au NRs), into applications requires the ability to tune the components of their optical properties to optimize performance for the underlying technol. Verifying techniques that model the resonance energy and assocd. extinction, scattering, and absorption cross sections necessitate exptl. data from series of Au NRs where structural features are independently tuned. Here, the extinction cross section and scattering efficiency are presented for Au NR series with high compositional and structural purity where effective vol., aspect ratio, length, and diam. are independently varied by factors of 25, 3, 2, and 4, resp. The extinction cross sections quant. agree with prior calcns., confirming that the vol. of the rod is the dominant factor. Comparisons of the scattering efficiency however are less precise, with both quant. and qual. differences between the role of rod vol. and aspect ratio. Such extensive exptl. data sets provide a crit. platform to improve quant. structure-property correlations, and thus enable design optimization of plasmonic nanoparticles for emerging applications.
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  81. 81
    Jana, N. R.; Gearheart, L.; Obare, S. O.; Murphy, C. J. Anisotropic Chemical Reactivity of Gold Spheroids and Nanorods. Langmuir 2002, 18, 922– 927,  DOI: 10.1021/la0114530
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    81
    Anisotropic chemical reactivity of gold spheroids and nanorods
    Jana, Nikhil R.; Gearheart, Latha; Obare, Sherine O.; Murphy, Catherine J.
    Langmuir (2002), 18 (3), 922-927CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    We obsd. anisotropic chem. reactivity of gold spheroids during cyanide dissoln. and reaction with persulfate. Cyanide dissoln. of 2-5 aspect ratio spheroids (with 12-30 nm short axis) starts at the ends of the long axis and leads to intermediate lower aspect ratio spheroids and spheres. Persulfate converts the spheroids into spheres. In contrast, cyanide dissoln. of 18 aspect ratio nanorods (with 16 nm short axis) starts simultaneously at many sites and does not change the nanorod length throughout dissoln. Spheroids are thermally less stable than rods and slowly convert to spheres with time, and thus anisotropic chem. reactivity is presumably due to their lower stability.
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    Zou, R. X.; Guo, X.; Yang, J.; Li, D. D.; Peng, F.; Zhang, L.; Wang, H. J.; Yu, H. Selective Etching of Gold Nanorods by Ferric Chloride at Room Temperature. CrystEngComm 2009, 11, 2797– 2803,  DOI: 10.1039/b911902g
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    82
    Selective etching of gold nanorods by ferric chloride at room temperature
    Zou, Renxian; Guo, Xia; Yang, Jian; Li, Dandan; Peng, Feng; Zhang, Lei; Wang, Hongjuan; Yu, Hao
    CrystEngComm (2009), 11 (12), 2797-2803CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
    The chem. stability of gold nanorods in different environments is important for a variety of practical applications, because the tiny change in shape and size could directly affect the optical properties of the gold nanorods. Herein we report a chem. etching reaction of the gold nanorods induced by ferric chloride at room temp. In the chem. etching, halide ions act as a ligand to reduce the electron potential of the gold species, which enables ferric ions to oxidize the gold nanorods and results in the etching of the gold nanorods. The redox etching leads to a significant decrease of the gold nanorods in length but little change in diam., which could be attributed to less surface passivation or higher chem. reactivity of the tips of the gold nanorods. It is believed that the selective shortening not only provides an alternative way to obtain the gold nanorods with desirable aspect ratios and specific optical properties, but also offers a safe way to remove gold nanostructures, which is important for the prepn. of hollow nanostructures with gold as a template.
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  83. 83
    Zhao, J.; Nguyen, S. C.; Ye, R.; Ye, B. H.; Weller, H.; Somorjai, G. A.; Alivisatos, A. P.; Toste, F. D. A Comparison of Photocatalytic Activities of Gold Nanoparticles Following Plasmonic and Interband Excitation and a Strategy for Harnessing Interband Hot Carriers for Solution Phase Photocatalysis. ACS Cent. Sci. 2017, 3, 482– 488,  DOI: 10.1021/acscentsci.7b00122
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    83
    A Comparison of Photocatalytic Activities of Gold Nanoparticles Following Plasmonic and Interband Excitation and a Strategy for Harnessing Interband Hot Carriers for Solution Phase Photocatalysis
    Zhao, Jie; Nguyen, Son C.; Ye, Rong; Ye, Baihua; Weller, Horst; Somorjai, Gabor A.; Alivisatos, A. Paul; Toste, F. Dean
    ACS Central Science (2017), 3 (5), 482-488CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Light driven excitation of gold nanoparticles (GNPs) has emerged as a potential strategy to generate hot carriers for photocatalysis through excitation of localized surface plasmon resonance (LSPR). In contrast, carrier generation through excitation of interband transitions remains a less explored and underestimated pathway for photocatalytic activity. Photoinduced oxidative etching of GNPs with FeCl3 was investigated as a model reaction in order to elucidate the effects of both types of transitions. The quant. results show that interband transitions more efficiently generate hot carriers and that those carriers exhibit higher reactivity as compared to those generated solely by LSPR. Further, leveraging the strong π-acidic character of the resulting photogenerated Au+ hole, an interband transition induced cyclization reaction of alkynylphenols was developed. Notably, alkyne coordination to the Au+ hole intercepts the classic oxidn. event and leads to the formation of the catalytically active gold clusters on subnanometer scale.
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    Zijlstra, P.; Orrit, M. Single Metal Nanoparticles: Optical Detection, Spectroscopy and Applications. Rep. Prog. Phys. 2011, 74, 106401,  DOI: 10.1088/0034-4885/74/10/106401
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    Single metal nanoparticles: optical detection, spectroscopy and applications
    Zijlstra, P.; Orrit, M.
    Reports on Progress in Physics (2011), 74 (10), 106401/1-106401/55CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
    A review. Since the first report on the far-field optical detection of single metal nanoparticles in the late 1990s, the field has rapidly developed and new methods and concepts have been introduced. Eliminating averaging over the broad size, shape and crystallinity distributions produced by even the best of current synthesis methods, these techniques have proven extremely useful for gaining a deeper insight into many of the properties of metal nanoparticles. These techniques have already led to the first applications specifically directed at using single particles. In this review the authors describe far-field optical techniques (both linear and nonlinear) that have sufficient sensitivity to detect single metal particles. They further discuss emerging applications, and emphasize the importance of single-particle detection techniques in their development.
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    Weigel, A.; Sebesta, A.; Kukura, P. Dark Field Microspectroscopy with Single Molecule Fluorescence Sensitivity. ACS Photonics 2014, 1, 848– 856,  DOI: 10.1021/ph500138u
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    85
    Dark field microspectroscopy with single molecule fluorescence sensitivity
    Weigel, Alexander; Sebesta, Aleksandar; Kukura, Philipp
    ACS Photonics (2014), 1 (9), 848-856CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Dark field microscopy directly detects scattering from a sample by rejecting excitation light. The technique has been extensively used for spectral characterization of nanoscopic particles, but its sensitivity has been limited by residual stray light. Here, the authors present a simple geometry based on wide field illumination under normal incidence capable of background suppression by more than seven orders of magnitude. The setup is optimized for spectrally resolved wide-field detection with white light illumination. They record images and spectra of single 10 nm gold particles binding to a functionalized surface, demonstrating a more than two order of magnitude improvement in sensitivity over the current state of the art. Their level of stray light rejection allows one to record single mol. fluorescence images with broadband excitation without any filters in the detection path. The approach is ideally suited for investigations of truly nanoscopic objects with applications in single mol. and nanoparticle spectroscopy, plasmonic sensing, and ultrafast spectroscopy.
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    Chang, W. S.; Link, S. Enhancing the Sensitivity of Single-Particle Photothermal Imaging with Thermotropic Liquid Crystals. J. Phys. Chem. Lett. 2012, 3, 1393– 1399,  DOI: 10.1021/jz300342p
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    86
    Enhancing the Sensitivity of Single-Particle Photothermal Imaging with Thermotropic Liquid Crystals
    Chang, Wei-Shun; Link, Stephan
    Journal of Physical Chemistry Letters (2012), 3 (10), 1393-1399CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Individual mols. and nanoparticles can be imaged based on their absorption using photothermal microscopy. This technique relies on the heating-induced changes in the refractive index of the surrounding medium. Here, the authors demonstrate an order of magnitude larger enhancement of the signal-to-noise ratio in photothermal imaging of 20 nm gold nanoparticles when using a thermotropic liq. crystal (5CB). The authors show quant. that this increase is due to the large change in the thermooptical properties of 5CB mainly along the nematic director. Enhancing the sensitivity is important for the further development of absorption-based single-mol. spectroscopy techniques.
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    Parra-Vasquez, A. N. G.; Oudjedi, L.; Cognet, L.; Lounis, B. Nanoscale Thermotropic Phase Transitions Enhancing Photothermal Microscopy Signals. J. Phys. Chem. Lett. 2012, 3, 1400– 1403,  DOI: 10.1021/jz300369d
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    87
    Nanoscale Thermotropic Phase Transitions Enhancing Photothermal Microscopy Signals
    Parra-Vasquez, A. Nicholas G.; Oudjedi, Laura; Cognet, Laurent; Lounis, Brahim
    Journal of Physical Chemistry Letters (2012), 3 (10), 1400-1403CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The photothermal heterodyne imaging technique enables studies of individual weakly absorbing nano-objects in various environments. It uses a photoinduced change in the refractive index of the environment. Taking advantage of the dramatic index of refraction change occurring around a thermotropic liq.-cryst. phase transition, a 40-fold signal-to-noise ratio enhancement for Au nanoparticles imaged in 4-cyano-4'-pentylbiphenyl (5CB) liq. crystals over those in a H2O environment is demonstrated. The photothermal signal was studied as a function of probe laser polarization, heating power, and sample temp. quantifying the optimal enhancement. This study established photothermal microscopy as a valuable technique for inducing and/or detecting local phase transitions at the nanometer scales.
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  88. 88
    Ding, T. N. X.; Hou, L.; van der Meer, H.; Alivisatos, A. P.; Orrit, M. Hundreds-Fold Sensitivity Enhancement of Photothermal Microscopy in near-Critical Xenon. J. Phys. Chem. Lett. 2016, 7, 2524– 2529,  DOI: 10.1021/acs.jpclett.6b00964
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    88
    Hundreds-fold Sensitivity Enhancement of Photothermal Microscopy in Near-Critical Xenon
    Ding, Tina X.; Hou, Lei; Meer, Harmen van der; Alivisatos, A. Paul; Orrit, Michel
    Journal of Physical Chemistry Letters (2016), 7 (13), 2524-2529CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Photothermal absorption microscopy of single Au nanoparticles was conducted at temps. and pressures near the crit. point of Xe (Tc = 16.583°, Pc = 5.842 MPa). The divergence of the thermal expansion coeff. at the crit. point makes the refractive index highly sensitive to changes in temp., which directly translates to a large enhancement of the photothermal signal. Measurements taken near the crit. point of Xe give a signal enhancement factor of up to 440 ± 130 over those taken in glycerol. The highest sensitivity recorded here corresponds to power dissipation of 64 pW, achieving a signal-to-noise ratio of 9.4 for 5 nm Au nanoparticles with an integration time of 50 ms, making this the most sensitive of any absorption microscopy technique reported to date. Enhancing the sensitivity of absorption microscopy lowers the operating heating power, allowing the technique to be more compatible with absorbers with absorption coeff. and photochem. stability lower than that of Au.
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    Rodriguez-Fernandez, J.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Spatially-Directed Oxidation of Gold Nanoparticles by Au(III)-CTAB Complexes. J. Phys. Chem. B 2005, 109, 14257– 14261,  DOI: 10.1021/jp052516g
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    89
    Spatially-Directed Oxidation of Gold Nanoparticles by Au(III)-CTAB Complexes
    Rodriguez-Fernandez, Jessica; Perez-Juste, Jorge; Mulvaney, Paul; Liz-Marzan, Luis M.
    Journal of Physical Chemistry B (2005), 109 (30), 14257-14261CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Gold nanoparticles are readily oxidized by Au(III) in the presence of cetyl-trimethylammonium bromide (CTAB). Oxidn. occurs preferentially at surface sites with higher curvature. Conversely, oxidn. with cyanide ions in the absence of CTAB leads to uniform oxidn. over the whole surface. Examples of the spatially directed oxidn. are provided using large, irregular spheres, nanocubes, and nanorods. We conclude that the mechanism of oxidn. depends on whether the oxidant is attached to CTAB micelles. The CTAB micelles approach the nanoparticles preferentially at the tips, leading to spatially directed oxidn.
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    Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-Induced Hot Carrier Science and Technology. Nat. Nanotechnol. 2015, 10, 25– 34,  DOI: 10.1038/nnano.2014.311
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    Plasmon-induced hot carrier science and technology
    Brongersma, Mark L.; Halas, Naomi J.; Nordlander, Peter
    Nature Nanotechnology (2015), 10 (1), 25-34CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    A review. The discovery of the photoelec. effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technol. In the early 1900s it played a crit. role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of phys. and chem. processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to elec. dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.
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    Zhang, Y. C.; He, S.; Guo, W. X.; Hu, Y.; Huang, J. W.; Mulcahy, J. R.; Wei, W. D. Surface-Plasmon-Driven Hot Electron Photochemistry. Chem. Rev. 2018, 118, 2927– 2954,  DOI: 10.1021/acs.chemrev.7b00430
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    91
    Surface-plasmon-driven hot electron photochemistry
    Zhang, Yuchao; He, Shuai; Guo, Wenxiao; Hu, Yue; Huang, Jiawei; Mulcahy, Justin R.; Wei, Wei David
    Chemical Reviews (Washington, DC, United States) (2018), 118 (6), 2927-2954CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Visible-light-driven photochem. has continued to attract heightened interest due to its capacity to efficiently harvest solar energy and its potential to solve the global energy crisis. Plasmonic nanostructures boast broadly tunable optical properties coupled with catalytically active surfaces that offer a unique opportunity for solar photochem. Resonant optical excitation of surface plasmons produces energetic hot electrons that can be collected to facilitate chem. reactions. This review sums up recent theor. and exptl. approaches for understanding the underlying photophys. processes in hot electron generation and discusses various electron-transfer models on both plasmonic metal nanostructures and plasmonic metal/semiconductor heterostructures. Following that are highlights of recent examples of plasmon-driven hot electron photochem. reactions within the context of both cases. The review concludes with a discussion about the remaining challenges in the field and future opportunities for addressing the low reaction efficiencies in hot-electron-induced photochem.
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    Kale, M. J.; Avanesian, T.; Christopher, P. Direct Photocatalysis by Plasmonic Nanostructures. ACS Catal. 2014, 4, 116– 128,  DOI: 10.1021/cs400993w
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    92
    Direct Photocatalysis by Plasmonic Nanostructures
    Kale, Matthew J.; Avanesian, Talin; Christopher, Phillip
    ACS Catalysis (2014), 4 (1), 116-128CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A review. Recent reports have shown that plasmonic nanostructures can be used to drive direct photocatalysis with visible photons, where nanostructures act as the light absorber and the catalytic active site. These reports have showcased direct plasmon driven photocatalysis as a route to conc. and channel the energy of low intensity visible light into adsorbed mols., enhancing the rates of chem. transformations, and offering pathways to control reaction selectivity. In this perspective, the authors will discuss the fundamental photophysics of localized surface plasmon resonance (LSPR) excitation in the context of driving chem. transformations. The various demonstrated chem. conversions executed using direct plasmonic photocatalysis will be reviewed. Exptl. observations, such as the dependence of photocatalytic rate on illumination intensity and photon energy, will be related to microscopic mechanisms of photocatalysis. In addn., theor. treatments of various mechanisms within the process of direct plasmonic photocatalysis will be discussed and related to exptl. studies. Throughout the Perspective, the possibility of activating targeted adsorbate bonds to allow rational manipulation of reaction selectivity in direct plasmonic photocatalysis will be discussed.
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    Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Photochemical Transformations on Plasmonic Metal Nanoparticles. Nat. Mater. 2015, 14, 567– 576,  DOI: 10.1038/nmat4281
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    Photochemical transformations on plasmonic metal nanoparticles
    Linic, Suljo; Aslam, Umar; Boerigter, Calvin; Morabito, Matthew
    Nature Materials (2015), 14 (6), 567-576CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    A review. The strong interaction of electromagnetic fields with plasmonic nanomaterials offers opportunities in various technologies that take advantage of photophys. processes amplified by this light-matter interaction. Recently, it has been shown that in addn. to photophys. processes, optically excited plasmonic nanoparticles can also activate chem. transformations directly on their surfaces. This potentially offers a no. of opportunities in the field of selective chem. synthesis. In this Review we summarize recent progress in the field of photochem. catalysis on plasmonic metallic nanostructures. We discuss the underlying phys. mechanisms responsible for the obsd. chem. activity, and the issues that must be better understood to see progress in the field of plasmon-mediated photocatalysis.
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  94. 94
    Besteiro, L. V.; Kong, X. T.; Wang, Z. M.; Hartland, G.; Govorov, A. O. Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms. ACS Photonics 2017, 4, 2759– 2781,  DOI: 10.1021/acsphotonics.7b00751
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    94
    Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms
    Besteiro, Lucas V.; Kong, Xiang-Tian; Wang, Zhiming; Hartland, Gregory; Govorov, Alexander O.
    ACS Photonics (2017), 4 (11), 2759-2781CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications in which excited electrons play an important role. However, the central questions in the problem of plasmonic hot electrons are the no. of optically-excited energetic electrons in a nanocrystal and how to ext. such electrons. Here we develop a theory describing the generation rates and the energy-distributions of hot electrons in nanocrystals with various geometries. In our theory, hot electrons are generated owing to surfaces and hot spots. As expected, the formalism predicts that large optically-excited nanocrystals show the excitation of mostly low-energy Drude electrons, whereas plasmons in small nanocrystals involve mostly high-energy (hot) electrons. We obtain anal. expressions for the distribution functions of excited carriers for simple shapes. For complex shapes and for small quantum nanocrystals, our results are computational. By looking at the energy distributions of electrons in an optically-excited nanocrystal, we see how the quantum many-body state in small particles evolves towards the classical state described by the Drude model when increasing nanocrystal size. We show that the rate of surface decay of plasmons in nanocrystals is directly related to the rate of generation of hot electrons. Based on a detailed many-body theory involving kinetic coeffs., we formulate a simple scheme describing how the plasmon in a nanocrystal dephases over time. In most nanocrystals, the main decay mechanism of a plasmon is the Drude friction-like process and the secondary path comes from generation of hot electrons due to surfaces and electromagnetic hot spots. The hot-electron path strongly depends on the material system and on its shape. Correspondingly, the efficiency of hot-electron prodn. in a nanocrystal strongly varies with size, shape and material. The results in the paper can be used to guide the design of plasmonic nanomaterials for photochem. and photodetectors.
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  95. 95
    Hartland, G. V.; Besteiro, L. V.; Johns, P.; Govorov, A. O. What’s So Hot About Electrons in Metal Nanoparticles?. ACS Energy Lett. 2017, 2, 1641– 1653,  DOI: 10.1021/acsenergylett.7b00333
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    95
    What's so Hot about Electrons in Metal Nanoparticles?
    Hartland, Gregory V.; Besteiro, Lucas V.; Johns, Paul; Govorov, Alexander O.
    ACS Energy Letters (2017), 2 (7), 1641-1653CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    A review. Metal nanoparticles are excellent light absorbers. The absorption processes create highly excited electron-hole pairs, and recently there has been interest in harnessing these hot charge carriers for photocatalysis and solar energy conversion applications. The goal of this Perspective is to describe the dynamics and energy distribution of the charge carriers produced by photon absorption and the implications for the photocatalysis mechanism. The authors will also discuss how spectroscopy can be used to provide insight into the coupling between plasmons and mol. resonances. In particular, the anal. shows that the choice of material and shape of the nanocrystal can play a crucial role in hot electron generation and coupling between plasmons and mol. transitions. The detection and even calcn. of many-body hot-electron processes in the plasmonic systems with continuous spectra of electrons and short lifetimes are challenging, but at the same time they are very interesting from the points of view of both potential applications and fundamental science. The authors propose that developing an understanding of these processes will provide a pathway for improving the efficiency of plasmon-induced photocatalysis.
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  96. 96
    Wu, K.; Chen, J.; McBride, J. R.; Lian, T. Efficient Hot-Electron Transfer by a Plasmon-Induced Interfacial Charge-Transfer Transition. Science 2015, 349, 632– 635,  DOI: 10.1126/science.aac5443
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    96
    Efficient hot-electron transfer by a plasmon-induced interfacial charge-transfer transition
    Wu, K.; Chen, J.; McBride, J. R.; Lian, T.
    Science (Washington, DC, United States) (2015), 349 (6248), 632-635CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Plasmon-induced hot-electron transfer from metal nanostructures is a potential new paradigm for solar energy conversion; however, the reported efficiencies of devices based on this concept are often low because of the loss of hot electrons via ultrafast electron-electron scattering. The authors propose a pathway, called the plasmon-induced interfacial charge-transfer transition (PICTT), that enables the decay of a plasmon by directly exciting an electron from the metal to a strongly coupled acceptor. The authors demonstrated this concept in Cd selenide nanorods with Au tips, in which the Au plasmon was strongly damped by Cd selenide through interfacial electron transfer. The quantum efficiency of the PICTT process was high (>24%), independent of excitation photon energy over a ∼1-eV range, and dependent on the excitation polarization.
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  97. 97
    Minutella, E.; Schulz, F.; Lange, H. Excitation-Dependence of Plasmon-Induced Hot Electrons in Gold Nanoparticles. J. Phys. Chem. Lett. 2017, 8, 4925– 4929,  DOI: 10.1021/acs.jpclett.7b02043
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    97
    Excitation-Dependence of Plasmon-Induced Hot Electrons in Gold Nanoparticles
    Minutella, Emanuele; Schulz, Florian; Lange, Holger
    Journal of Physical Chemistry Letters (2017), 8 (19), 4925-4929CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The decay of a plasmon leads to a hot electron distribution in metallic nanoparticles. Depending on the processes involved in the excitation, different distributions are obtained, which thermalize differently. The authors exptl. study excitation-wavelength and size-dependences on the generation and thermalization of the hot-electrons. The absence of size-dependences is confirmed, and clearly 2 regimes are obsd. with significantly different relaxation dynamics depending on the photon energy. The hot electron generation is more efficient when exciting with light that enables interband transitions.
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  98. 98
    Kamarudheen, R.; Castellanos, G. W.; Kamp, L. P. J.; Clercx, H. J. H.; Baldi, A. Quantifying Photothermal and Hot Charge Carrier Effects in Plasmon-Driven Nanoparticle Syntheses. ACS Nano 2018, 12, 8447– 8455,  DOI: 10.1021/acsnano.8b03929
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    98
    Quantifying Photothermal and Hot Charge Carrier Effects in Plasmon-Driven Nanoparticle Syntheses
    Kamarudheen, Rifat; Castellanos, Gabriel W.; Kamp, Leon P. J.; Clercx, Herman J. H.; Baldi, Andrea
    ACS Nano (2018), 12 (8), 8447-8455CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The excitation of localized surface plasmon resonances in Au and Ag colloids can be used to drive the synthesis of complex nanostructures, such as anisotropic prisms, bipyramids, and core@shell nanoparticles. Yet, after two decades of research, it is challenging to paint a complete picture of the mechanisms driving such light-induced chem. transformations. In particular, whereas the injection of hot charge carriers from the metal nanoparticles is usually proposed as the dominant mechanism, the contribution of plasmon-induced heating can often not be neglected. Here, we tackle this uncertainty and quantify the contribution of different activation mechanisms using a temp.-sensitive synthesis of Au@Ag core@shell nanoparticles. We compare the rate of Ag shell growth in the dark at different temps. with the one under plasmon excitation with varying laser intensities. Our controlled illumination geometry, coupled to numerical modeling of light propagation and heat diffusion in the reaction vol., allows us to quantify both localized and collective heating effects and det. their contribution to the total growth rate of the nanoparticles. We find that nonthermal effects can be dominant, and their relative contribution depends on the fraction of nanoparticle suspension under irradn. Understanding the mechanism of plasmon-activated chem. at the surface of metal nanoparticles is of paramount importance for a wide range of applications, from the rational design of novel light-assisted nanoparticle syntheses to the development of plasmonic nanostructures for catalytic and therapeutic purposes.
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  99. 99
    Zhou, L. A.; Swearer, D. F.; Zhang, C.; Robatjazi, H.; Zhao, H. Q.; Henderson, L.; Dong, L. L.; Christopher, P.; Carter, E. A.; Nordlander, P.; Halas, N. J. Quantifying Hot Carrier and Thermal Contributions in Plasmonic Photocatalysis. Science 2018, 362, 69– 72,  DOI: 10.1126/science.aat6967
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    99
    Quantifying hot carrier and thermal contributions in plasmonic photocatalysis
    Zhou, Linan; Swearer, Dayne F.; Zhang, Chao; Robatjazi, Hossein; Zhao, Hangqi; Henderson, Luke; Dong, Liangliang; Christopher, Phillip; Carter, Emily A.; Nordlander, Peter; Halas, Naomi J.
    Science (Washington, DC, United States) (2018), 362 (6410), 69-72CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Photocatalysis based on optically active, "plasmonic" metal nanoparticles has emerged as a promising approach to facilitate light-driven chem. conversions under far milder conditions than thermal catalysis. However, an understanding of the relation between thermal and electronic excitations has been lacking. We report the substantial light-induced redn. of the thermal activation barrier for ammonia decompn. on a plasmonic photocatalyst. We introduce the concept of a light-dependent activation barrier to account for the effect of light illumination on electronic and thermal excitations in a single unified picture. This framework provides insight into the specific role of hot carriers in plasmon-mediated photochem., which is critically important for designing energy-efficient plasmonic photocatalysts.
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  100. 100
    Wu, C. Y.; Wolf, W. J.; Levartovsky, Y.; Bechtel, H. A.; Martin, M. C.; Toste, F. D.; Gross, E. High-Spatial-Resolution Mapping of Catalytic Reactions on Single Particles. Nature 2017, 541, 511– 515,  DOI: 10.1038/nature20795
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    100
    High-spatial-resolution mapping of catalytic reactions on single particles
    Wu, Chung-Yeh; Wolf, William J.; Levartovsky, Yehonatan; Bechtel, Hans A.; Martin, Michael C.; Toste, F. Dean; Gross, Elad
    Nature (London, United Kingdom) (2017), 541 (7638), 511-515CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The crit. role in surface reactions and heterogeneous catalysis of metal atoms with low coordination nos., such as found at at. steps and surface defects, is firmly established. But despite the growing availability of tools that enable detailed in situ characterization, so far it has not been possible to document this role directly. Surface properties can be mapped with high spatial resoln., and catalytic conversion can be tracked with a clear chem. signature; however, the combination of the two, which would enable high-spatial-resoln. detection of reactions on catalytic surfaces, has rarely been achieved. Single-mol. fluorescence spectroscopy has been used to image and characterize single turnover sites at catalytic surfaces, but is restricted to reactions that generate highly fluorescing product mols. Herein the chem. conversion of N-heterocyclic carbene mols. attached to catalytic particles is mapped using synchrotron-radiation-based IR nanospectroscopy with a spatial resoln. of 25 nm, which enabled particle regions that differ in reactivity to be distinguished. These observations demonstrate that, compared to the flat regions on top of the particles, the peripheries of the particles-which contain metal atoms with low coordination nos.-are more active in catalyzing oxidn. and redn. of chem. active groups in surface-anchored N-heterocyclic carbene mols.
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  101. 101
    Cortes, E.; Xie, W.; Cambiasso, J.; Jermyn, A. S.; Sundararaman, R.; Narang, P.; Schlucker, S.; Maier, S. A. Plasmonic Hot Electron Transport Drives Nano-Localized Chemistry. Nat. Commun. 2017, 8, 14880,  DOI: 10.1038/ncomms14880
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    101
    Plasmonic hot electron transport drives nano-localized chemistry
    Cortes, Emiliano; Xie, Wei; Cambiasso, Javier; Jermyn, Adam S.; Sundararaman, Ravishankar; Narang, Prineha; Schlucker, Sebastian; Maier, Stefan A.
    Nature Communications (2017), 8 (), 14880CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Nanoscale localization of electromagnetic fields near metallic nanostructures underpins the fundamentals and applications of plasmonics. The unavoidable energy loss from plasmon decay, initially seen as a detriment, has now expanded the scope of plasmonic applications to exploit the generated hot carriers. However, quant. understanding of the spatial localization of these hot carriers, akin to electromagnetic near-field maps, has been elusive. Here we spatially map hot-electron-driven redn. chem. with 15 nm resoln. as a function of time and electromagnetic field polarization for different plasmonic nanostructures. We combine expts. employing a six-electron photo-recycling process that modify the terminal group of a self-assembled monolayer on plasmonic silver nanoantennas, with theor. predictions from first-principles calcns. of non-equil. hot-carrier transport in these systems. The resulting localization of reactive regions, detd. by hot-carrier transport from high-field regions, paves the way for improving efficiency in hot-carrier extn. science and nanoscale regio-selective surface chem.
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  102. 102
    Schlather, A. E.; Manjavacas, A.; Lauchner, A.; Marangoni, V. S.; DeSantis, C. J.; Nordlander, P.; Halas, N. J. Hot Hole Photoelectrochemistry on Au@SiO2@Au Nanoparticles. J. Phys. Chem. Lett. 2017, 8, 2060– 2067,  DOI: 10.1021/acs.jpclett.7b00563
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    102
    Hot Hole Photoelectrochemistry on Au@SiO2@Au Nanoparticles
    Schlather, Andrea E.; Manjavacas, Alejandro; Lauchner, Adam; Marangoni, Valeria S.; De Santis, Christopher J.; Nordlander, Peter; Halas, Naomi J.
    Journal of Physical Chemistry Letters (2017), 8 (9), 2060-2067CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    There is currently a worldwide need to develop efficient photocatalytic materials that can reduce the high-energy cost of common industrial chem. processes. One possible soln. focuses on metallic nanoparticles (NPs) that can act as efficient absorbers of light due to their surface plasmon resonance. Recent work indicates that small NPs, when photoexcited, may allow for efficient electron or hole transfer necessary for photocatalysis. Here we investigate the mechanisms behind hot hole carrier dynamics by studying the photodriven oxidn. of citrate ions on Au@SiO2@Au core-shell NPs. We find that charge transfer to adsorbed mols. is most efficient at higher photon energies but still present with lower plasmon energy. On the basis of these exptl. results, we develop a simple theor. model for the probability of hot carrier-adsorbate interactions across the NP surface. These results provide a foundation for understanding charge transfer in plasmonic photocatalytic materials, which could allow for further design and optimization of photocatalytic processes.
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  103. 103
    Kim, Y.; Torres, D. D.; Jain, P. K. Activation Energies of Plasmonic Catalysts. Nano Lett. 2016, 16, 3399– 3407,  DOI: 10.1021/acs.nanolett.6b01373
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    103
    Activation Energies of Plasmonic Catalysts
    Kim, Youngsoo; Dumett Torres, Daniel; Jain, Prashant K.
    Nano Letters (2016), 16 (5), 3399-3407CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The activation energy of a catalytic reaction serves not only as a metric of the efficacy of a catalyst but also as a potential indicator of mechanistic differences between the catalytic and noncatalytic reaction. However, activation energies are quite underutilized in the field of photocatalysis. We characterize in detail the effect of visible light excitation on the activation enthalpy of an electron transfer reaction photocatalyzed by plasmonic Au nanoparticles. We find that in the presence of visible light photoexcitation, the activation enthalpy of the Au nanoparticle-catalyzed electron transfer reaction is significantly reduced. The redn. in the activation enthalpy depends on the excitation wavelength, the incident laser power, and the strength of a hole scavenger. On the basis of these results, we argue that the activation enthalpy redn. is directly related to the photoelectrochem. potential built-up on the Au nanoparticle under steady-state light excitation, analogous to electrochem. activation. Under optimum light excitation conditions, a potential as high as 240 mV is measured. The findings constitute more precise insights into the mechanistic role and energetic contribution of plasmonic excitation to chem. reactions catalyzed by transition metal nanoparticles.
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  104. 104
    Smith, J. G.; Jain, P. K. The Ligand Shell as an Energy Barrier in Surface Reactions on Transition Metal Nanoparticles. J. Am. Chem. Soc. 2016, 138, 6765– 6773,  DOI: 10.1021/jacs.6b00179
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    104
    The Ligand Shell as an Energy Barrier in Surface Reactions on Transition Metal Nanoparticles
    Smith, Jeremy G.; Jain, Prashant K.
    Journal of the American Chemical Society (2016), 138 (21), 6765-6773CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Transition metal nanoparticles, including those employed in catalytic, electrocatalytic, and photocatalytic conversions, have surfaces that are typically coated with a layer of short or long-chain ligands. There is little systematic understanding of how much this ligand layer affects the reactivity of the underlying surface. We show for Ag nanoparticles that a surface-adsorbed thiol layer greatly impedes the kinetics of an ionic chem. reaction taking place on the Ag surface. The model reaction studied is the galvanic exchange of Ag with Au3+ ions, the kinetics of which is measured on individual thiol-coated nanoparticles using in situ optical scattering spectroscopy. We observe a systematic lowering of the reactivity of the nanoparticle as the chain length of the thiol is increased, from which we deduce that the ligand layer serves as an energy barrier to the transport of incoming/outgoing reactive ions. This barrier effect can be decreased by light irradn., resulting from weakened binding of the thiol layer to the metal surface. We find that the influence of the surface ligand layer on reactivity is much stronger than factors such as nanoparticle size, shape, or crystallinity. These findings provide improved understanding of the role of ligand or adsorbates in colloidal catalysis and photocatalysis and have important implications for the transport of reactants and ions to surfaces and for engineering the reactivity of nanoparticles using surface passivation.
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  105. 105
    Burrows, N. D.; Lin, W.; Hinman, J. G.; Dennison, J. M.; Vartanian, A. M.; Abadeer, N. S.; Grzincic, E. M.; Jacob, L. M.; Li, J.; Murphy, C. J. Surface Chemistry of Gold Nanorods. Langmuir 2016, 32, 9905– 9921,  DOI: 10.1021/acs.langmuir.6b02706
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    105
    Surface Chemistry of Gold Nanorods
    Burrows, Nathan D.; Lin, Wayne; Hinman, Joshua G.; Dennison, Jordan M.; Vartanian, Ariane M.; Abadeer, Nardine S.; Grzincic, Elissa M.; Jacob, Lisa M.; Li, Ji; Murphy, Catherine J.
    Langmuir (2016), 32 (39), 9905-9921CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    A review. The current understanding of the surface chem. of as-synthesized gold nanorods, methods of tailoring the surface chem. of gold nanorods with various inorg. and org. coatings/ligands, and the techniques employed to characterize ligands on the surface of gold nanorods, as well as the assocd. measurement challenges, are reviewed. Specifically, we address the challenges of detg. how thick the ligand shell is, how many ligands per nanorod are present on the surface, and where the ligands are located in regiospecific and mixed-ligand systems. The review concludes with an outlook on the development of the surface chem. of gold nanorods leading to the development of a synthetic nanoparticle surface chem. toolbox analogous to that of synthetic org. chem. and natural product synthesis.
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  106. 106
    He, J.; Unser, S.; Bruzas, I.; Cary, R.; Shi, Z. W.; Mehra, R.; Aron, K.; Sagle, L. The Facile Removal of CTAB from the Surface of Gold Nanorods. Colloids Surf., B 2018, 163, 140– 145,  DOI: 10.1016/j.colsurfb.2017.12.019
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    106
    The facile removal of CTAB from the surface of gold nanorods
    He, Jie; Unser, Sarah; Bruzas, Ian; Cary, ReJeana; Shi, Zhiwei; Mehra, Rajesh; Aron, Kenneth; Sagle, Laura
    Colloids and Surfaces, B: Biointerfaces (2018), 163 (), 140-145CODEN: CSBBEQ; ISSN:0927-7765. (Elsevier B.V.)
    A common capping agent for gold nanorods, Cetyl trimethylammonium bromide (CTAB), is particularly problematic for biol. studies because of its cytotoxicity. Several procedures have been developed to remove the CTAB from the surface of the gold nanorods, but most are lengthy, involving many steps, and use expensive reagents. Here, we present a simple, one-pot method for the complete removal of CTAB from the surface of gold nanorods, so that particles can be more effectively utilized in biol. in vivo studies. The procedure involves first adding sodium borohydride to remove the CTAB, quickly followed by a replacement ligand, such as mercaptoundecanoic acid (MUA). Both the CTAB removal and MUA replacement were monitored by FTIR, surface enhanced Raman spectroscopy (SERS) and XPS and compared to com. available citrate-capped gold nanorods. The procedure presented herein is shown to be as effective at removing CTAB and replacing it with MUA as com. available gold nanorod samples.
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  107. 107
    Ruijgrok, P. V.; Verhart, N. R.; Zijlstra, P.; Tchebotareva, A. L.; Orrit, M. Brownian Fluctuations and Heating of an Optically Aligned Gold Nanorod. Phys. Rev. Lett. 2011, 107, 037401,  DOI: 10.1103/PhysRevLett.107.037401
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    107
    Brownian Fluctuations and Heating of an Optically Aligned Gold Nanorod
    Ruijgrok, P. V.; Verhart, N. R.; Zijlstra, P.; Tchebotareva, A. L.; Orrit, M.
    Physical Review Letters (2011), 107 (3), 037401/1-037401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    The authors present the 1st quant. measurements of the torque exerted on a single Au nanorod in a polarized 3-dimensional optical trap. The authors detd. the torque both by observing the time-averaged orientation distribution and by measuring the dynamics of the rotational Brownian fluctuations. The measurements are in good agreement with calcns., where the temp. profile around the hot nanorod gives rise to a reduced, effective viscosity. The max. torque on a 60. nm × 25 nm nanorod was 100 pN nm, large enough to address single-mol. processes in soft and biol. matter.
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  108. 108
    Trojek, J.; Chvatal, L.; Zemanek, P. Optical Alignment and Confinement of an Ellipsoidal Nanorod in Optical Tweezers: A Theoretical Study. J. Opt. Soc. Am. A 2012, 29, 1224– 1236,  DOI: 10.1364/JOSAA.29.001224
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    108
    Optical alignment and confinement of an ellipsoidal nanorod in optical tweezers: a theoretical study
    Trojek Jan; Chvatal Lukas; Zemanek Pavel
    Journal of the Optical Society of America. A, Optics, image science, and vision (2012), 29 (7), 1224-36 ISSN:.
    Within the Rayleigh approximation, we investigate the behavior of an individual ellipsoidal metal nanorod that is optically confined in three dimensions using a single focused laser beam. We focus on the description of the optical torque and optical force acting upon the nanorod placed into a linearly polarized Gaussian beam (scalar description of the electric field) or a strongly focused beam (vector field description). The study comprises the influence of the trapping laser wavelength, the angular aperture of focusing optics, the orientation of the ellipsoidal nanorod, and the aspect ratio of its principal axes. The results reveal a significantly different behavior of the nanorod if the trapping wavelength is longer or shorter than the wavelength corresponding to the longitudinal plasmon resonance mode. Published experimental observations are compared with our theoretical predictions with satisfactory results.
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  109. 109
    Xu, X. H.; Cheng, C.; Zhang, Y.; Lei, H. X.; Li, B. J. Scattering and Extinction Torques: How Plasmon Resonances Affect the Orientation Behavior of a Nanorod in Linearly Polarized Light. J. Phys. Chem. Lett. 2016, 7, 314– 319,  DOI: 10.1021/acs.jpclett.5b02375
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    109
    Scattering and Extinction Torques: How Plasmon Resonances Affect the Orientation Behavior of a Nanorod in Linearly Polarized Light
    Xu, Xiaohao; Cheng, Chang; Zhang, Yao; Lei, Hongxiang; Li, Baojun
    Journal of Physical Chemistry Letters (2016), 7 (2), 314-319CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Linearly polarized light can exert an orienting torque on plasmonic nanorods. The torque direction has generally been considered to change when the light wavelength passes through a plasmon longitudinal resonance. Here, we use the Maxwell stress tensor to evaluate this torque in general terms. According to distinct light-matter interaction processes, the total torque is decompd. into scattering and extinction torques. The scattering torque tends to orient plasmonic nanorods parallel to the light polarization, independent of the choice of light wavelength. The direction of the extinction torque is not only closely tied to the excitation of plasmon resonance but also depends on the specific plasmon mode around which the light wavelength is tuned. Our findings show that the conventional wisdom that simply assocs. the total torque with the plasmon longitudinal resonances needs to be replaced with an understanding based on the different torque components and the details of spectral distribution.
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  110. 110
    Tong, L. M.; Miljkovic, V. D.; Kall, M. Alignment, Rotation, and Spinning of Single Plasmonic Nanoparticles and Nanowires Using Polarization Dependent Optical Forces. Nano Lett. 2010, 10, 268– 273,  DOI: 10.1021/nl9034434
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    110
    Alignment, Rotation, and Spinning of Single Plasmonic Nanoparticles and Nanowires using Polarization Dependent Optical Forces
    Tong, Lianming; Miljkovic, Vladimir D.; Kaell, Mikael
    Nano Letters (2010), 10 (1), 268-273CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors demonstrate optical alignment and rotation of individual plasmonic nanostructures with lengths from tens of nanometers to several micrometers using a single beam of linearly polarized near-IR laser light. Ag nanorods and dimers of Au nanoparticles align parallel to the laser polarization because of the high long-axis dipole polarizability. Ag nanowires, in contrast, spontaneously turn perpendicular to the incident polarization and predominantly attach at the wire ends, in agreement with electrodynamics simulations. Wires, rods, and dimers all rotate if the incident polarization is turned. In the case of nanowires, the authors demonstrate spinning at an angular frequency of ∼1 Hz due to transfer of spin angular momentum from circularly polarized light.
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  • Abstract

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    Figure 1

    Figure 1. Microbubble absorption spectroscopy. (A) Cartoon of instrumentation. PDH = Pound–Drever–Hall. LC = Liquid crystal. APD = Avalanche photodiode. (B) Optical micrographs of two microbubble resonators with different geometries. Scale bars 20 μm. (C) Photothermal maps of a microbubble resonator similar in geometry to the left microbubble in (B), both out-of-focus (left) and in-focus (right). Scale bars 20 μm.

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    Figure 2

    Figure 2. Optical resonances in microbubble resonators. (A) Simulated electric field distributions at 780 nm for first-, second-, and third-order radial modes, for both first- and second-order polar modes. All modes shown are transverse electric (TE). White curves are added to clearly indicate the position of the microbubble walls. (B) A 180 pm span of the mode spectrum of a microbubble resonator. (C) Left: The signal at the beginning of analyte pumping. Right: Signal once the resonator has reached a thermal equilibrium with its surroundings (theoretical). (D) Resonance shift from pumping a single AuNR with the 635 nm beam at decreasing powers (blue points). The red point indicates the signal for pump beam off. The inset is a zoom-out, showing signal linearity over orders of magnitude in pumping power. Further details in main text. Error bars are standard deviation of the mean.

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    Figure 3

    Figure 3. Probing photophysical features of single AuNRs. (A) Cartoon illustrating the photophysical features of a AuNR. LPB = Longitudinal plasmon band. TPB = Transverse plasmon band. (B) Bulk absorption spectrum of AuNRs, with the various laser beams in our experiment indicated by vertical lines. LPB and TPB indicated. (C) Example photothermal maps of a nanorod as pump polarization are varied in increments of 20°, as shown by the red arrow in the cartoon above the photothermal maps. Scale bar 1 μm. (D) Polarization fits for three different pump beams acquired using photothermal mapping. (E) Polarization traces for three different pump beams, acquired by recording photothermal signal as the linear pump polarization is quickly rotated 180° (∼10 s).

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    Figure 4

    Figure 4. Etching single AuNRs. (A) Reaction series of polarization traces for three difference reactions, progressing in time from red traces to blue traces. Maximum signal is normalized by pump flux. Each trace was taken over the course of 10 s (0.05 s per point, 200 different angles), with a 1 s delay between traces (except when switching power) and 3 s for beam-centering between each trace. (B) The data for reaction (ii), but with signal shown logarithmically. (C) Maximum signal of polarization traces over the course of reaction (ii), showing the decrease in relative absorption cross section at 635 nm. Dashed lines indicate points in time at which pump power was increased. (D) Maximum angle of polarization traces over the course of reaction (ii), showing nanorod orientation. Dashed lines indicate points in time at which pump power was increased. Reaction conditions: dilute aqueous HCl (pH ∼ 1.3), room temperature, varied FeCl3 concentrations (i) 1 mM, (ii) 250 μM, (iii) 2 mM. Pump fluxes for reaction (i) were 2.7, 6.7, 11.4, 21.0, and 34.5 kW/cm2. Pump fluxes for reaction (ii) were 4.1, 11.9, and 35.4 kW/cm2. Pump fluxes for reaction (iii) were 6.4, 15.9, 31.6, 57.3, and 121 kW/cm2.

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    Figure 5

    Figure 5. Etching reactions driven at two different pump wavelengths. (A) The reaction of a single AuNR being driven with the 532 nm pump beam, progressing in time from red traces to blue traces. Maximum signal is normalized by pump flux. Each trace was taken over the course of 10 s (0.05 s per point, 200 different angles), with a 1 s delay between traces (except when switching power), and 3 s for beam-centering between each trace. (B) The same data as in (A), but with signal shown logarithmically. (C) (i) Maximum signal of polarization traces for the reaction shown in (A). Polarization trace for indicated data point in inset. (ii) A similar trace for a different nanorod reacted in the same bubble using the 532 nm beam. Dashed lines indicate time points at which pump power was increased. (D) Maximum signal of polarization traces for the reaction of a nanorod in the same bubble, but using the 635 nm pump beam to drive the reaction. Dashed lines indicate time points at which pump power was increased. Reaction conditions: dilute aqueous HCl (pH ∼ 1.3), room temperature, 1 mM FeCl3. Pump fluxes for reaction (Ci) were 16.1, 39.2, and 63.3 kW/cm2. Pump fluxes for reaction (Cii) were 6.4, 31.6, and 57.3 kW/cm2. Pump fluxes for reaction (D) were 6.4, 15.9, 31.6, 57.3, 121 kW/cm2.

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    Figure 6

    Figure 6. Proposed mechanistic explanation for etching rates. A slow initial step requiring CTAB dissociation before ferric ions can bind determines the overall rate for the reaction, muting the effect of the higher rate for TPB excitation, even though hot electrons are more efficiently generated. Eventually, etching stops when the absorbed light falls below a threshold necessary for hot-electron-driven etching.

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    Figure 7

    Figure 7. Orientation control of single AuNRs. (A) A cartoon illustrating the optically induced torque that a AuNR experiences under illumination with linearly polarized light, both from side-view (top) and top-view (bottom). (B) A series of pumping experiments showing optical control of nanorod orientation. (C) Trace showing the photothermal signal as two different AuNRs are pumped at increasing laser powers until the AuNRs dislodge slightly from the resonator wall and rotate, eventually settling down off-axis of the polarization.

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  • References

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      10
      Label-Free Detection of Single Protein Using a Nanoplasmonic-Photonic Hybrid Microcavity
      Dantham, Venkata R.; Holler, Stephen; Barbre, Curtis; Keng, David; Kolchenko, Vasily; Arnold, Stephen
      Nano Letters (2013), 13 (7), 3347-3351CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Recently the authors reported the detection and sizing of the smallest RNA virus MS2 with a mass of 6 ag from the resonance frequency shift of a whispering gallery mode-nanoshell hybrid resonator (WGM-h) upon adsorption on the nanoshell and anticipated that single protein >0.4 ag should be detectable but with considerably smaller signals. Here, the authors report the detection of single thyroid cancer marker (Thyroglobulin, TG) and bovine serum albumin (BSA) proteins with masses of only 1 ag and 0.11 ag (66 kDa), resp. However, the wavelength shifts are enhanced beyond those anticipated in the authors' earlier work by 240% for TG and 1500% for BSA. This surprising sensitivity is traced to a short-range reactive field near the surface of the authors' Au nanoshell receptor due to intrinsic random bumps of protein size, leading to an unanticipated increase in sensitivity to single protein, which grows larger as the protein diminishes in size. As a consequence of the largest signal-to-noise ratio in the authors' BSA expts. (S/N ≈ 13), the authors conservatively estd. a new protein limit of detection for the authors' WGM-h of 5 kDa.
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    11. 11
      Yu, W. Y.; Jiang, W. C.; Lin, Q.; Lu, T. Cavity Optomechanical Spring Sensing of Single Molecules. Nat. Commun. 2016, 7, 12311,  DOI: 10.1038/ncomms12311
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      Cavity optomechanical spring sensing of single molecules
      Yu, Wenyan; Jiang, Wei C.; Lin, Qiang; Lu, Tao
      Nature Communications (2016), 7 (), 12311CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Label-free bio-sensing is a crit. functionality underlying a variety of health- and security-related applications. Micro-/nano-photonic devices are well suited for this purpose and have emerged as promising platforms in recent years. Here we propose and demonstrate an approach that utilizes the optical spring effect in a high-Q coherent optomech. oscillator to dramatically enhance the sensing resoln. by orders of magnitude compared with conventional approaches, allowing us to detect single bovine serum albumin proteins with a mol. wt. of 66 kDa at a signal-to-noise ratio of 16.8. The unique optical spring sensing approach opens up a distinctive avenue that not only enables biomol. sensing and recognition at individual level, but is also of great promise for broad phys. sensing applications that rely on sensitive detection of optical cavity resonance shift to probe external phys. parameters.
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    12. 12
      Baaske, M. D.; Vollmer, F. Optical Observation of Single Atomic Ions Interacting with Plasmonic Nanorods in Aqueous Solution. Nat. Photonics 2016, 10, 733– 739,  DOI: 10.1038/nphoton.2016.177
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      12
      Optical observation of single atomic ions interacting with plasmonic nanorods in aqueous solution
      Baaske, Martin D.; Vollmer, Frank
      Nature Photonics (2016), 10 (11), 733-739CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      Plasmonic nanoparticles provide the basis for a multitude of applications in chem., health care and optics because of their unique properties. Nanoparticle-based techniques have evolved into powerful tools for studying mol. interactions with single-mol. resoln. Here, we show that this sensing capability can be used to detect single at. ions in aq. medium. We monitored interactions of single zinc and mercury ions with plasmonic gold nanorods (NRs) resonantly coupled to our whispering gallery mode sensor. Our system's ability to discern permanent binding and transient interaction allows us to study the different interaction kinetics of both ion species. The detection of transient interactions enables us to confirm statistically that the sensor signals originate from single ions. Furthermore, we reveal how the ion-NR interactions evolve with respect to the medium's ionic strength as mercury ions amalgamate with gold and zinc ions eventually turn into probes of highly localized surface potentials.
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    13. 13
      Heylman, K. D.; Thakkar, N.; Horak, E. H.; Quillin, S. C.; Cherqui, C.; Knapper, K. A.; Masiello, D. J.; Goldsmith, R. H. Optical Microresonators as Single-Particle Absorption Spectrometers. Nat. Photonics 2016, 10, 788– 795,  DOI: 10.1038/nphoton.2016.217
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      13
      Optical microresonators as single-particle absorption spectrometers
      Heylman, Kevin D.; Thakkar, Niket; Horak, Erik H.; Quillin, Steven C.; Cherqui, Charles; Knapper, Kassandra A.; Masiello, David J.; Goldsmith, Randall H.
      Nature Photonics (2016), 10 (12), 788-795CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      Optical measurements of nanoscale objects offer major insights into fundamental biol., material and photonic properties. In absorption spectroscopy, sensitivity limits applications at the nanoscale. Here, we present a new single-particle double-modulation photothermal absorption spectroscopy method that employs on-chip optical whispering-gallery-mode (WGM) microresonators as ultrasensitive thermometers. Optical excitation of a nanoscale object on the microresonator produces increased local temps. that are proportional to the absorption cross-section of the object. We resolve photothermal shifts in the resonance frequency of the microresonator that are smaller than 100 Hz, orders of magnitude smaller than previous WGM sensing schemes. The application of our new technique to single gold nanorods reveals a dense array of sharp Fano resonances arising from the coupling between the localized surface plasmon of the gold nanorod and the WGMs of the resonator, allowing for the exploration of plasmonic-photonic hybridization. In terms of the wider applicability, our approach adds label-free spectroscopic identification to microresonator-based detection schemes.
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    14. 14
      Thakkar, N.; Rea, M. T.; Smith, K. C.; Heylman, K. D.; Quillin, S. C.; Knapper, K. A.; Horak, E. H.; Masiello, D. J.; Goldsmith, R. H. Sculpting Fano Resonances to Control Photonic-Plasmonic Hybridization. Nano Lett. 2017, 17, 6927– 6934,  DOI: 10.1021/acs.nanolett.7b03332
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      14
      Sculpting Fano Resonances To Control Photonic-Plasmonic Hybridization
      Thakkar, Niket; Rea, Morgan T.; Smith, Kevin C.; Heylman, Kevin D.; Quillin, Steven C.; Knapper, Kassandra A.; Horak, Erik H.; Masiello, David J.; Goldsmith, Randall H.
      Nano Letters (2017), 17 (11), 6927-6934CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their resp. components have either high-quality factors or low mode vols. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode vols. to maximize the system's photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune crit. system parameters. In this Letter, we present a general and practical method to det. the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on exptl. measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticle's localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters crit. to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode vol. and ultimately maximize the Purcell factor to be 104.
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    15. 15
      Knapper, K. A.; Heylman, K. D.; Horak, E. H.; Goldsmith, R. H. Chip-Scale Fabrication of High-Q All-Glass Toroidal Microresonators for Single-Particle Label-Free Imaging. Adv. Mater. 2016, 28, 2945– 2950,  DOI: 10.1002/adma.201504976
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      15
      Chip-Scale Fabrication of High-Q All-Glass Toroidal Microresonators for Single-Particle Label-Free Imaging
      Knapper, Kassandra A.; Heylman, Kevin D.; Horak, Erik H.; Goldsmith, Randall H.
      Advanced Materials (Weinheim, Germany) (2016), 28 (15), 2945-2950CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We have fabricated all-glass microtoroid resonators, preserving the morphol. leading to the high QJV of the SiO2-On-Si microtoroid, while addressing their fabrication, biocompatibility, and optical challenges by eliminating the silicon pillar and substrate. Most importantly, all-glass toroids are reflowed using a high-temp. furnace anneal step, which provides a wafer-scale and highly uniform reflow mechanism, while preserving surface tension-induced smoothness.
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    16. 16
      Knapper, K. A.; Pan, F.; Rea, M. T.; Horak, E. H.; Rogers, J. D.; Goldsmith, R. H. Single-Particle Photothermal Imaging via Inverted Excitation through High-Q All-Glass Toroidal Microresonators. Opt. Express 2018, 26, 25020– 25030,  DOI: 10.1364/OE.26.025020
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      16
      Single-particle photothermal imaging via inverted excitation through high-Q all-glass toroidal microresonators
      Knapper, Kassandra A.; Pan, Feng; Rea, Morgan T.; Horak, Erik H.; Rogers, Jeremy D.; Goldsmith, Randall H.
      Optics Express (2018), 26 (19), 25020-25030CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
      In this work, we fabricate all-glass toroidal microresonators on a coverslip thickness (∼170μm) substrate, enabling excitation delivery through the sample, simplifying optical integration. Further, we demonstrate the application of this new geometry for single-particle photothermal imaging. Finally, we discover and develop simulations to explain a non-trivial astigmatism in the point spread function (PSF) arising from the curvature of the resonator.
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    17. 17
      Heylman, K. D.; Knapper, K. A.; Goldsmith, R. H. Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators. J. Phys. Chem. Lett. 2014, 5, 1917– 1923,  DOI: 10.1021/jz500781g
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      17
      Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators
      Heylman, Kevin D.; Knapper, Kassandra A.; Goldsmith, Randall H.
      Journal of Physical Chemistry Letters (2014), 5 (11), 1917-1923CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the crit. detecting element. The approach is photothermal in nature as the microresonators were used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, the authors' approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chem. information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled C nanotubes are spatially mapped, and the per-atom absorption cross section is detd. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to mols. is laid out, leading to a potent new method of performing measurements on individual mols.
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    18. 18
      Horak, E. H.; Rea, M. T.; Heylman, K. D.; Gelbwaser-Klimovsky, D.; Saikin, S. K.; Thompson, B. J.; Kohler, D. D.; Knapper, K. A.; Wei, W.; Pan, F.; Gopalan, P.; Wright, J. C.; Aspuru-Guzik, A.; Goldsmith, R. H. Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy. Nano Lett. 2018, 18, 1600– 1607,  DOI: 10.1021/acs.nanolett.7b04211
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      18
      Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy
      Horak, Erik H.; Rea, Morgan T.; Heylman, Kevin D.; Gelbwaser-Klimovsky, David; Saikin, Semion K.; Thompson, Blaise J.; Kohler, Daniel D.; Knapper, Kassandra A.; Wei, Wei; Pan, Feng; Gopalan, Padma; Wright, John C.; Aspuru-Guzik, Alan; Goldsmith, Randall H.
      Nano Letters (2018), 18 (3), 1600-1607CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      PEDOT:PSS, a transparent elec. conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased cond., a detailed understanding of the coupled electronic and morphol. landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examn. of electronic structure and morphol. ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theor. calcns. and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. We find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.
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    19. 19
      Chen, H. J.; Shao, L.; Li, Q.; Wang, J. F. Gold Nanorods and Their Plasmonic Properties. Chem. Soc. Rev. 2013, 42, 2679– 2724,  DOI: 10.1039/C2CS35367A
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      19
      Gold nanorods and their plasmonic properties
      Chen, Huanjun; Shao, Lei; Li, Qian; Wang, Jianfang
      Chemical Society Reviews (2013), 42 (7), 2679-2724CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the prepn. of their hybrid structures. The plasmonic properties and the assocd. applications of Au nanorods will also be discussed in detail.
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    20. 20
      Dreaden, E. C.; Alkilany, A. M.; Huang, X. H.; Murphy, C. J.; El-Sayed, M. A. The Golden Age: Gold Nanoparticles for Biomedicine. Chem. Soc. Rev. 2012, 41, 2740– 2779,  DOI: 10.1039/C1CS15237H
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      20
      The golden age: gold nanoparticles for biomedicine
      Dreaden, Erik C.; Alkilany, Alaaldin M.; Huang, Xiaohua; Murphy, Catherine J.; El-Sayed, Mostafa A.
      Chemical Society Reviews (2012), 41 (7), 2740-2779CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Gold nanoparticles have been used in biomedical applications since their first colloidal syntheses more than three centuries ago. However, over the past two decades, their beautiful colors and unique electronic properties have also attracted tremendous attention due to their historical applications in art and ancient medicine and current applications in enhanced optoelectronics and photovoltaics. In spite of their modest alchem. beginnings, gold nanoparticles exhibit phys. properties that are truly different from both small mols. and bulk materials, as well as from other nanoscale particles. Their unique combination of properties is just beginning to be fully realized in range of medical diagnostic and therapeutic applications. This crit. review will provide insights into the design, synthesis, functionalization, and applications of these artificial mols. in biomedicine and discuss their tailored interactions with biol. systems to achieve improved patient health. Further, we provide a survey of the rapidly expanding body of literature on this topic and argue that gold nanotechnol.-enabled biomedicine is not simply an act of gilding the (nanomedicinal) lily', but that a new Golden Age' of biomedical nanotechnol. is truly upon us. Moving forward, the most challenging nanoscience ahead of us will be to find new chem. and phys. methods of functionalizing gold nanoparticles with compds. that can promote efficient binding, clearance, and biocompatibility and to assess their safety to other biol. systems and their long-term term effects on human health and reprodn. (472 refs.).
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    21. 21
      Huang, X. H.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods. J. Am. Chem. Soc. 2006, 128, 2115– 2120,  DOI: 10.1021/ja057254a
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      21
      Cancer Cell Imaging and Photothermal Therapy in the Near-Infrared Region by Using Gold Nanorods
      Huang, Xiaohua; El-Sayed, Ivan H.; Qian, Wei; El-Sayed, Mostafa A.
      Journal of the American Chemical Society (2006), 128 (6), 2115-2120CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Due to strong elec. fields at the surface, the absorption and scattering of electromagnetic radiation by noble metal nanoparticles are strongly enhanced. These unique properties provide the potential of designing novel optically active reagents for simultaneous mol. imaging and photothermal cancer therapy. It is desirable to use agents that are active in the near-IR (NIR) region of the radiation spectrum to minimize the light extinction by intrinsic chromophores in native tissue. Gold nanorods with suitable aspect ratios (length divided by width) can absorb and scatter strongly in the NIR region (650-900 nm). In the present work, we provide an in vitro demonstration of gold nanorods as novel contrast agents for both mol. imaging and photothermal cancer therapy. Nanorods are synthesized and conjugated to anti-epidermal growth factor receptor (anti-EGFR) monoclonal antibodies and incubated in cell cultures with a nonmalignant epithelial cell line (HaCat) and two malignant oral epithelial cell lines (HOC 313 clone 8 and HSC 3). The anti-EGFR antibody-conjugated nanorods bind specifically to the surface of the malignant-type cells with a much higher affinity due to the overexpressed EGFR on the cytoplasmic membrane of the malignant cells. As a result of the strongly scattered red light from gold nanorods in dark field, obsd. using a lab. microscope, the malignant cells are clearly visualized and diagnosed from the nonmalignant cells. It is found that, after exposure to continuous red laser at 800 nm, malignant cells require about half the laser energy to be photothermally destroyed than the nonmalignant cells. Thus, both efficient cancer cell diagnostics and selective photothermal therapy are realized at the same time.
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    22. 22
      Yin, D. Y.; Li, X. L.; Ma, Y. Y.; Liu, Z. Targeted Cancer Imaging and Photothermal Therapy via Monosaccharide-Imprinted Gold Nanorods. Chem. Commun. 2017, 53, 6716– 6719,  DOI: 10.1039/C7CC02247F
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      22
      Targeted cancer imaging and photothermal therapy via monosaccharide-imprinted gold nanorods
      Yin, Danyang; Li, Xinglin; Ma, Yanyan; Liu, Zhen
      Chemical Communications (Cambridge, United Kingdom) (2017), 53 (50), 6716-6719CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Plasmonic nanomaterials have been widely used for photothermal therapy (PTT) of cancer, but their recognition specificity remains challenging. We prepd. monosaccharide-imprinted gold nanorods (AuNRs) for targeted cancer PTT, using sialic acid (SA) as a representative monosaccharide. The SA-imprinted AuNRs exhibited good specificity, enabling the killing of cancer cells without damaging healthy cells.
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    23. 23
      Ali, M. R. K.; Wu, Y.; Ghosh, D.; Do, B. H.; Chen, K.; Dawson, M. R.; Fang, N.; Sulchek, T. A.; El-Sayed, M. A. Nuclear Membrane-Targeted Gold Nanoparticles Inhibit Cancer Cell Migration and Invasion. ACS Nano 2017, 11, 3716– 3726,  DOI: 10.1021/acsnano.6b08345
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      23
      Nuclear Membrane-Targeted Gold Nanoparticles Inhibit Cancer Cell Migration and Invasion
      Ali, Moustafa R. K.; Wu, Yue; Ghosh, Deepraj; Do, Brian H.; Chen, Kuangcai; Dawson, Michelle R.; Fang, Ning; Sulchek, Todd A.; El-Sayed, Mostafa A.
      ACS Nano (2017), 11 (4), 3716-3726CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Most cancer patients die from metastasis. Recent studies have shown that gold nanoparticles (AuNPs) can slow down the migration/invasion speed of cancer cells and suppress metastasis. Since nuclear stiffness of the cell largely decreases cell migration, our hypothesis is that targeting AuNPs to the cell nucleus region could enhance nuclear stiffness, and therefore inhibit cell migration and invasion. Our results showed that upon nuclear targeting of AuNPs, the ovarian cancer cell motilities decrease significantly, compared with nontargeted AuNPs. Furthermore, using at. force microscopy, we obsd. an enhanced cell nuclear stiffness. In order to understand the mechanism of cancer cell migration/invasion inhibition, the exact locations of the targeted AuNPs were clearly imaged using a high-resoln. three-dimensional imaging microscope, which showed that the AuNPs were trapped at the nuclear membrane. In addn., we obsd. a greatly increased expression level of lamin A/C protein, which is located in the inner nuclear membrane and functions as a structural component of the nuclear lamina to enhance nuclear stiffness. We propose that the AuNPs that are trapped at the nuclear membrane both (1) add to the mech. stiffness of the nucleus and (2) stimulate the overexpression of lamin A/C located around the nuclear membrane, thus increasing nuclear stiffness and slowing cancer cell migration and invasion.
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      Meeker, D. G.; Chen, J. Y.; Smeltzer, M. S. Could Targeted, Antibiotic-Loaded Gold Nanoconstructs Be a New Magic Bullet to Fight Infection?. Nanomedicine 2016, 11, 2379– 2382,  DOI: 10.2217/nnm-2016-0260
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      24
      Could targeted, antibiotic-loaded gold nanoconstructs be a new magic bullet to fight infection?
      Meeker, Daniel G.; Chen, Jingyi; Smeltzer, Mark S.
      Nanomedicine (London, United Kingdom) (2016), 11 (18), 2379-2382CODEN: NLUKAC; ISSN:1743-5889. (Future Medicine Ltd.)
      There is no expanded citation for this reference.
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      Cao, J.; Sun, T.; Grattan, K. T. V. Gold Nanorod-Based Localized Surface Plasmon Resonance Biosensors: A Review. Sens. Actuators, B 2014, 195, 332– 351,  DOI: 10.1016/j.snb.2014.01.056
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      Gold nanorod-based localized surface plasmon resonance biosensors: A review
      Cao, Jie; Sun, Tong; Grattan, Kenneth T. V.
      Sensors and Actuators, B: Chemical (2014), 195 (), 332-351CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      A review. Noble metal nanoparticle-based localized surface plasmon resonance (LSPR) is an advanced and powerful label-free biosensing technique which is well-known for its high sensitivity to the surrounding refractive index change in the local environment caused by the biomol. interactions around the sensing area. The characteristics of the LSPR effect in such sensors are highly dependent on the size, shape and nature of the material properties of the metallic nanoparticles considered. Among the various types of metallic nanoparticles used in studies employing the LSPR technique, the use of gold nanorods (GNRs) has attracted particular attention for the development of sensitive LSPR biosensors, this arising from the unique and intriguing optical properties of the material. This paper provides a detailed review of the key underpinning science for such systems and of recent progress in the development of a no. of LSPR-based biosensors which use GNR as the active element, including an overview of the sensing principle, the synthesis of GNRs, the fabrication of a no. of biosensors, techniques for surface modification of GNRs and finally their performance in several biosensing applications. The review ends with a consideration of key advances in GNR-based LSPR sensing and prospects for future research and advances for the development of the GNR-based LSPR biosensors.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXjslOkt7c%253D&md5=3b3c7f5cb9f1a1250fa301e885ef8ad5
    26. 26
      Taylor, A. B.; Zijlstra, P. Single-Molecule Plasmon Sensing: Current Status and Future Prospects. ACS Sens. 2017, 2, 1103– 1122,  DOI: 10.1021/acssensors.7b00382
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      26
      Single-Molecule Plasmon Sensing: Current Status and Future Prospects
      Taylor, Adam B.; Zijlstra, Peter
      ACS Sensors (2017), 2 (8), 1103-1122CODEN: ASCEFJ; ISSN:2379-3694. (American Chemical Society)
      A review. Single-mol. detection has long relied on fluorescent labeling with high quantum-yield fluorophores. Plasmon-enhanced detection circumvents the need for labeling by allowing direct optical detection of weakly emitting and completely nonfluorescent species. This review focuses on recent advances in single mol. detection using plasmonic metal nanostructures as a sensing platform, particularly using a single particle-single mol. approach. In the past decade two mechanisms for plasmon-enhanced single-mol. detection have been demonstrated: (1) by plasmonically enhancing the emission of weakly fluorescent biomols., or (2) by monitoring shifts of the plasmon resonance induced by single-mol. interactions. We begin with a motivation regarding the importance of single mol. detection, and advantages plasmonic detection offers. We describe both detection mechanisms and discuss challenges and potential solns. We finalize by highlighting the exciting possibilities in anal. chem. and medical diagnostics.
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    27. 27
      Lin, K. Q.; Yi, J.; Hu, S.; Liu, B. J.; Liu, J. Y.; Wang, X.; Ren, B. Size Effect on SERS of Gold Nanorods Demonstrated via Single Nanoparticle Spectroscopy. J. Phys. Chem. C 2016, 120, 20806– 20813,  DOI: 10.1021/acs.jpcc.6b02098
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      27
      Size Effect on SERS of Gold Nanorods Demonstrated via Single Nanoparticle Spectroscopy
      Lin, Kai-Qiang; Yi, Jun; Hu, Shu; Liu, Bi-Ju; Liu, Jun-Yang; Wang, Xiang; Ren, Bin
      Journal of Physical Chemistry C (2016), 120 (37), 20806-20813CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Surface-enhanced Raman spectroscopy (SERS) has attracted tremendous interest as a label-free highly sensitive anal. method. For optimization of SERS activity, it is highly important to systematically investigate the size effect of nanoparticles on the SERS enhancement, which appears to be challenging in expt., as the localized surface plasmon resonance (LSPR) of nanoparticles also changes with the change of the particle size. This challenge can be overcome by utilizing the unique property of gold nanorods, whose LSPR wavelength can be controlled to be the same by properly choosing the size and aspect ratio of the nanorods. We obtained the correlated SEM images, scattering spectra, and SERS spectra on a home-built single nanoparticle spectroscopy system and systematically investigate the size effect on SERS of individual gold nanorods using the adsorbed malachite green isothiocyanate (MGITC) mol. as the probe mol. The dark field scattering intensity was found to increase with the increase of the size of nanoparticles, whereas the SERS intensity increases with the decrease of the size as a result of the stronger lightning rod effect and weaker radiation damping. We further explored the size-dependent effect for the coupled nanorod dimer system. The SERS activity was also found to increase with a decrease of the particle size when the excitation is close to the LSPR wavelength. Understanding of the size effect on the local field enhancement may help to design and fabricate SERS substrate and TERS tips with high SERS activity.
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    28. 28
      Gao, Z.; Burrows, N. D.; Valley, N. A.; Schatz, G. C.; Murphy, C. J.; Haynes, C. L. In Solution SERS Sensing Using Mesoporous Silica-Coated Gold Nanorods. Analyst 2016, 141, 5088– 5095,  DOI: 10.1039/C6AN01159D
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      28
      In solution SERS sensing using mesoporous silica-coated gold nanorods
      Gao, Zhe; Burrows, Nathan D.; Valley, Nicholas A.; Schatz, George C.; Murphy, Catherine J.; Haynes, Christy L.
      Analyst (Cambridge, United Kingdom) (2016), 141 (17), 5088-5095CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
      Mesoporous silica-coated gold nanorods (AuNR@MS) act as a colloidally stable Raman sensing platform with a built-in analyte size cutoff. Herein, these core-shell plasmonic nanostructures were presented with a range of thiolated Raman-active mols. to probe the limits of this platform for SERS sensing. The exptl. results show generally, that the transport of mols. through the mesopores is highly dependent on the size of the mol. and specifically, that AuNR@MS with pores of ∼4 nm diam. are able to sense analytes with mol. dimensions smaller than 1.5 nm. This sensing platform will likely find broad use, performing well even in complex media based on the high colloidal stability imbued by the mesoporous silica shell.
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    29. 29
      Khatua, S.; Paulo, P. M. R.; Yuan, H. F.; Gupta, A.; Zijlstra, P.; Orrit, M. Resonant Plasmonic Enhancement of Single-Molecule Fluorescence by Individual Gold Nanorods. ACS Nano 2014, 8, 4440– 4449,  DOI: 10.1021/nn406434y
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      29
      Resonant Plasmonic Enhancement of Single-Molecule Fluorescence by Individual Gold Nanorods
      Khatua, Saumyakanti; Paulo, Pedro M. R.; Yuan, Haifeng; Gupta, Ankur; Zijlstra, Peter; Orrit, Michel
      ACS Nano (2014), 8 (5), 4440-4449CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Enhancing the fluorescence of a weak emitter is important to further extend the reach of single-mol. fluorescence imaging to many unexplored systems. Here the authors study fluorescence enhancement by isolated Au nanorods and explore the role of the surface plasmon resonance (SPR) on the obsd. enhancements. Au nanorods can be cheaply synthesized in large vols., yet similar fluorescence enhancements as literature reports on lithog. fabricated nanoparticle assemblies were found. The fluorescence of a weak emitter, crystal violet, can be enhanced >1000-fold by a single nanorod with its SPR at 629 nm excited at 633 nm. This strong enhancement results from both an excitation rate enhancement of ∼130 and an effective emission enhancement of ∼9. The fluorescence enhancement, however, decreases sharply when the SPR wavelength moves away from the excitation laser wavelength or when the SPR has only a partial overlap with the emission spectrum of the fluorophore. The reported measurements of fluorescence enhancement by 11 nanorods with varying SPR wavelengths are consistent with numerical simulations.
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    30. 30
      Nima, Z. A.; Alwbari, A. M.; Dantuluri, V.; Hamzah, R. N.; Sra, N.; Motwani, P.; Arnaoutakis, K.; Levy, R. A.; Bohliqa, A. F.; Nedosekin, D.; Zharov, V. P.; Makhoul, I.; Biris, A. S. Targeting Nano Drug Delivery to Cancer Cells Using Tunable, Multi-Layer, Silver-Decorated Gold Nanorods. J. Appl. Toxicol. 2017, 37, 1370– 1378,  DOI: 10.1002/jat.3495
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      30
      Targeting nano drug delivery to cancer cells using tunable, multi-layer, silver-decorated gold nanorods
      Nima, Zeid A.; Alwbari, Ahmed M.; Dantuluri, Vijayalakshmi; Hamzah, Rabab N.; Sra, Natasha; Motwani, Pooja; Arnaoutakis, Konstantinos; Levy, Rebecca A.; Bohliqa, Amani F.; Nedosekin, Dmitry; Zharov, Vladimir P.; Makhoul, Issam; Biris, Alexandru S.
      Journal of Applied Toxicology (2017), 37 (12), 1370-1378CODEN: JJATDK; ISSN:0260-437X. (John Wiley & Sons Ltd.)
      Multifunctional nanoparticles have high potential as targeting delivery vehicles for cancer chemotherapy. In this study, silver-decorated gold nanorods (AuNR\Ag) have been successfully used to deliver specific, targeted chemotherapy against breast cancer (MCF7) and prostate carcinoma (PC3) cell lines. Doxorubicin, a commonly used chemotherapy, and anti-Epithelial cell adhesion mol. (anti-EpCAM) antibodies were covalently bonded to thiolated polyethylene glycol-coated AuNR\Ag, and the resultant system was used to deliver the drugs to cancer cells in vitro. Furthermore, these nanoparticles have a unique spectral signature by surface enhanced Raman spectroscopy (SERS), which enables reliable detection and monitoring of the distribution of these chemotherapy constructs inside cells. The development of interest in a plasmonic nano drugs system with unique spectroscopic signatures could result in a clin. approach to the precise targeting and visualization of cells and solid tumors while delivering mols. for the enhanced treatment of cancerous tumors.
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    31. 31
      Wang, F.; Li, C. H.; Chen, H. J.; Jiang, R. B.; Sun, L. D.; Li, Q.; Wang, J. F.; Yu, J. C.; Yan, C. H. Plasmonic Harvesting of Light Energy for Suzuki Coupling Reactions. J. Am. Chem. Soc. 2013, 135, 5588– 5601,  DOI: 10.1021/ja310501y
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      31
      Plasmonic Harvesting of Light Energy for Suzuki Coupling Reactions
      Wang, Feng; Li, Chuanhao; Chen, Huanjun; Jiang, Ruibin; Sun, Ling-Dong; Li, Quan; Wang, Jianfang; Yu, Jimmy C.; Yan, Chun-Hua
      Journal of the American Chemical Society (2013), 135 (15), 5588-5601CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The efficient use of solar energy has received wide interest due to increasing energy and environmental concerns. A potential means in chem. is sunlight-driven catalytic reactions. The authors report on the direct harvesting of visible-to-near-IR light for chem. reactions by use of plasmonic Au-Pd nanostructures. The intimate integration of plasmonic Au nanorods with catalytic Pd nanoparticles through seeded growth enabled efficient light harvesting for catalytic reactions on the nanostructures. Upon plasmon excitation, catalytic reactions were induced and accelerated through both plasmonic photocatalysis and photothermal conversion. Under the illumination of an 809 nm laser at 1.68 W, the yield of the Suzuki coupling reaction was ∼2 times that obtained when the reaction was thermally heated to the same temp. Moreover, the yield was also ∼2 times that obtained from Au-TiOx-Pd nanostructures under the same laser illumination, where a 25-nm-thick TiOx shell was introduced to prevent the photocatalysis process. This is a more direct comparison between the effect of joint plasmonic photocatalysis and photothermal conversion with that of sole photothermal conversion. The contribution of plasmonic photocatalysis became larger when the laser illumination was at the plasmon resonance wavelength. It increased when the power of the incident laser at the plasmon resonance was raised. Differently sized Au-Pd nanostructures were further designed and mixed together to make the mixt. light-responsive over the visible to near-IR region. In the presence of the mixt., the reactions were completed within 2 h under sunlight, while almost no reactions occurred in the dark.
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    32. 32
      Gole, A.; Murphy, C. J. Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and Nature of the Seed. Chem. Mater. 2004, 16, 3633– 3640,  DOI: 10.1021/cm0492336
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      32
      Seed-Mediated Synthesis of Gold Nanorods: Role of the Size and Nature of the Seed
      Gole, Anand; Murphy, Catherine J.
      Chemistry of Materials (2004), 16 (19), 3633-3640CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      We report studies on the synthesis of gold nanorods by a three-step seeding protocol method using a variety of different gold seeds. The synthetic method is adapted from one we published earlier (Jana et al. J. Phys. Chem. B 2001, 105, 4065). The seeds chosen for these studies have av. diams. in the range from 4 to 18 nm, with pos. charged as well as neg. charged surface groups. In all the cases, along with a large concn. of long rods, a small no. of different shapes such as triangles, hexagons, and small rods are obsd. The proportion of small rods increases with an increase in the seed size used for nanorod synthesis. For long nanorods synthesized by different seeds a comparison of various parameters such as length, width, and aspect ratio has been made. A dependence of the nanorod aspect ratio on the size of the seed is obsd. Increasing the seed size results in lowering of the gold nanorod aspect ratios for a const. concn. of reagents. The charge on the seed also plays a role in detg. the nanorod aspect ratio. For pos. charged seeds variation in the aspect ratio is not as pronounced as that for neg. charged seeds. The gold nanorods synthesized were characterized by transmission electron microscopy (TEM), UV-vis spectroscopy, and Fourier transform IR spectroscopy. The role of seed size in the size and shape evolution of the nanocrystal, at different growth stages, has been studied by TEM.
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    33. 33
      Baida, H.; Christofilos, D.; Maioli, P.; Crut, A.; Del Fatti, N.; Vallee, F. Surface Plasmon Resonance Spectroscopy of Single Surfactant-Stabilized Gold Nanoparticles. Eur. Phys. J. D 2011, 63, 293– 299,  DOI: 10.1140/epjd/e2010-10594-y
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      Surface plasmon resonance spectroscopy of single surfactant-stabilized gold nanoparticles
      Baida, H.; Christofilos, D.; Maioli, P.; Crut, A.; Del Fatti, N.; Vallee, F.
      European Physical Journal D: Atomic, Molecular, Optical and Plasma Physics (2011), 63 (2), 293-299CODEN: EPJDF6; ISSN:1434-6060. (Springer)
      The optical extinction spectra of single gold nanoparticles stabilized by surfactant mols. are investigated using the spatial modulation spectroscopy technique. The exptl. results are compared to computed spectra, focusing on the width of the surface plasmon resonance. It is shown to strongly vary from particle to particle, independently of their size and dielec. environment. This demonstrates the key role of the interface conditions on the width of the surface plasmon resonance for surfactant-stabilized metal nanoparticles.
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    34. 34
      Li, Z. M.; Mao, W. Z.; Devadas, M. S.; Hartland, G. V. Absorption Spectroscopy of Single Optically Trapped Gold Nanorods. Nano Lett. 2015, 15, 7731– 7735,  DOI: 10.1021/acs.nanolett.5b03833
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      34
      Absorption Spectroscopy of Single Optically Trapped Gold Nanorods
      Li, Zhongming; Mao, Weizhi; Devadas, Mary Sajini; Hartland, Gregory V.
      Nano Letters (2015), 15 (11), 7731-7735CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Extinction spectra of single gold nanorods optically trapped in water were measured by spatial modulation spectroscopy. Comparison of the extinction cross sections and resonance frequencies to finite element calcns. allows us to det. the dimensions of the nanorod and est. the contribution of radiation damping to the LSPR line width. Subtracting the radiation damping and bulk contributions from the measured line widths yields the electron-surface scattering contribution. The results show that the surfactant coating for the nanorods causes large electron-surface scattering effects with significant particle-to-particle variations. These effects are more pronounced than those seen for substrate-supported particles in previous single particle studies. Indeed, the measured line widths are only slightly narrower than that of the ensemble spectrum. These results show the importance of removing surfactant for sensing applications of these materials.
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    35. 35
      Yorulmaz, M.; Nizzero, S.; Hoggard, A.; Wang, L. Y.; Cai, Y. Y.; Su, M. N.; Chang, W. S.; Link, S. Single-Particle Absorption Spectroscopy by Photothermal Contrast. Nano Lett. 2015, 15, 3041– 3047,  DOI: 10.1021/nl504992h
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      Single-Particle Absorption Spectroscopy by Photothermal Contrast
      Yorulmaz, Mustafa; Nizzero, Sara; Hoggard, Anneli; Wang, Lin-Yung; Cai, Yi-Yu; Su, Man-Nung; Chang, Wei-Shun; Link, Stephan
      Nano Letters (2015), 15 (5), 3041-3047CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Removing effects of sample heterogeneity through single-mol. and single-particle techniques has advanced many fields. While background free luminescence and scattering spectroscopy is widely used, recording the absorption spectrum only is rather difficult. Here the authors present an approach capable of recording pure absorption spectra of individual nanostructures. The authors demonstrate the implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers. Comparison of the absorption spectra to the scattering spectra of the same individual gold nanoparticles reveals the blueshift of the absorption spectra, as predicted by Mie theory but previously not detectable in extinction measurements that measure the sum of absorption and scattering. By covering a wavelength range of 300 nm, the authors are also able to record absorption spectra of single gold nanorods with different aspect ratios. The spectral shift between absorption and scattering for the longitudinal plasmon resonance decreases as a function of nanorod aspect ratio, which is in agreement with simulations.
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    36. 36
      Berciaud, S.; Cognet, L.; Tamarat, P.; Lounis, B. Observation of Intrinsic Size Effects in the Optical Response of Individual Gold Nanoparticles. Nano Lett. 2005, 5, 515– 518,  DOI: 10.1021/nl050062t
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      Observation of Intrinsic Size Effects in the Optical Response of Individual Gold Nanoparticles
      Berciaud, Stephane; Cognet, Laurent; Tamarat, Philippe; Lounis, Brahim
      Nano Letters (2005), 5 (3), 515-518CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The photothermal heterodyne imaging method is used to study for the first time the absorption spectra of individual gold nanoparticles with diams. down to 5 nm. Intrinsic size effects that result in a broadening of the surface plasmon resonance are unambiguously obsd. Dispersions in the peak energies and homogeneous widths of the single-particle resonances are revealed. The exptl. results are analyzed within the frame of Mie theory.
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      Gaiduk, A.; Yorulmaz, M.; Ruijgrok, P. V.; Orrit, M. Room-Temperature Detection of a Single Molecule’s Absorption by Photothermal Contrast. Science 2010, 330, 353– 356,  DOI: 10.1126/science.1195475
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      Room-Temperature Detection of a Single Molecule's Absorption by Photothermal Contrast
      Gaiduk, A.; Yorulmaz, M.; Ruijgrok, P. V.; Orrit, M.
      Science (Washington, DC, United States) (2010), 330 (6002), 353-356CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      So far, single-mol. imaging has predominantly relied on fluorescence detection. The authors imaged single nonfluorescent azo dye mols. in room-temp. glycerol by the refractive effect of the heat that they release in their environment upon intense illumination. This photothermal technique provides contrast for the absorbing objects only, irresp. of scattering by defects or roughness, with a signal-to-noise ratio of ~10 for a single mol. in an integration time of 300 ms. In the absence of O, virtually no bleaching event was obsd., even after >10 min of illumination. In a soln. satd. with O, the av. bleaching time was of the order of 1 min. No blinking was obsd. in the absorption signal. From bleaching steps, the authors obtained an av. absorption cross section of 4 Å2 for a single chromophore.
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    38. 38
      Chien, M. H.; Brameshuber, M.; Rossboth, B. K.; Schutz, G. J.; Schmid, S. Single-Molecule Optical Absorption Imaging by Nanomechanical Photothermal Sensing. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 11150– 11155,  DOI: 10.1073/pnas.1804174115
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      Single-molecule optical absorption imaging by nanomechanical photothermal sensing
      Chien, Miao-Hsuan; Brameshuber, Mario; Rossboth, Benedikt K.; SchA1/4tz, Gerhard J.; Schmid, Silvan
      Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (44), 11150-11155CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Absorption microscopy is a promising alternative to fluorescence microscopy for single-mol. imaging. So far, mol. absorption has been probed optically via the attenuation of a probing laser or via photothermal effects. The sensitivity of optical probing is not only restricted by background scattering but it is fundamentally limited by laser shot noise, which minimizes the achievable single-mol. signal-to-noise ratio. Here, we present nanomech. photothermal microscopy, which overcomes the scattering and shot-noise limit by detecting the photothermal heating of the sample directly with a temp.-sensitive substrate. We use nanomech. silicon nitride drums, whose resonant frequency detunes with local heating. Individual Au nanoparticles with diams. from 10 to 200 nm and single mols. (Atto 633) are scanned with a heating laser with a peak irradiance of 354 +- 45μm2 using 50A~, long-working-distance objective. With a stress-optimized drum we reach a sensitivity of 16 fW/Hz1/2 at room temp., resulting in a single-mol. signal-to-noise ratio of >70. The high sensitivity combined with the inherent wavelength independence of the nanomech. sensor presents a competitive alternative to established tools for the anal. and localization of nonfluorescent single mols. and nanoparticles.
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    39. 39
      Crut, A.; Maioli, P.; Del Fatti, N.; Vallee, F. Optical Absorption and Scattering Spectroscopies of Single Nano-Objects. Chem. Soc. Rev. 2014, 43, 3921– 3956,  DOI: 10.1039/c3cs60367a
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      39
      Optical absorption and scattering spectroscopies of single nano-objects
      Crut, Aurelien; Maioli, Paolo; Del Fatti, Natalia; Vallee, Fabrice
      Chemical Society Reviews (2014), 43 (11), 3921-3956CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Developments of optical detection and spectroscopy methods for single nano-objects are key advances for applications and fundamental understanding of the novel properties exhibited by nanosize systems. These methods are reviewed, focusing on far-field optical approaches based on light absorption and elastic scattering. The principles of the main linear and nonlinear methods are described and exptl. results are illustrated in the case of metal nanoparticles, stressing the key role played by the object environment, such as the presence of a substrate, bound surface mols. or other nano-objects. Special attention is devoted to quant. methods and correlation of the measured optical spectra of a nano-object with its morphol., characterized either optically or by electron microscopy, as this permits precise comparison with theor. models. Application of these methods to optical detection and spectroscopy for single semiconductor nanowires and carbon nanotubes is also presented. Extension to ultrafast nonlinear extinction or scattering spectroscopies of single nano-objects is finally discussed in the context of investigation of their nonlinear optical response and their electronic, acoustic and thermal properties.
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    40. 40
      Zrimsek, A. B.; Wong, N. L.; Van Duyne, R. P. Single Molecule Surface-Enhanced Raman Spectroscopy: A Critical Analysis of the Bianalyte versus Isotopologue Proof. J. Phys. Chem. C 2016, 120, 5133– 5142,  DOI: 10.1021/acs.jpcc.6b00606
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      40
      Single Molecule Surface-Enhanced Raman Spectroscopy: A Critical Analysis of the Bianalyte versus Isotopologue Proof
      Zrimsek, Alyssa B.; Wong, Nolan L.; Van Duyne, Richard P.
      Journal of Physical Chemistry C (2016), 120 (9), 5133-5142CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Verification of single-mol. (SM) detection for surface-enhanced Raman spectroscopy (SERS) requires the use of two analytes via either the bianalyte or isotopologue approach. For both approaches, the preferential observation of the individual analytes over a combination of both analytes is used to conclude that SM detection has been achieved. Isotopologues are preferred because they have identical surface binding affinities and Raman cross sections, whereas bianalyte pairs typically do not. We conducted multianalyte SERS studies to investigate the limitations of the bianalyte approach. The bianalyte partners, Rhodamine 6G (R6G-d0) and crystal violet (CV-d0), were directly compared, while SM detection was verified (or disproved) using their corresponding isotopologues (R6G-d4, CV-d12). We found that the significant difference in counts between R6G and CV can provide misleading evidence for SMSERS. We then rationalized these results using a joint Poisson-binomial model with unequal detection probabilities and adjusted the relative concns. of R6G and CV to achieve a comparable distribution of SMSERS counts. Using this information, we outlined the necessary considerations, such as accounting for the differences in mol. properties, for reliable SMSERS proofs. Moreover, we showed that multianalyte expts. at the SM level are achievable, opening the opportunity for new types of SM studies.
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      Young, G.; Kukura, P. Interferometric Scattering Microscopy. Annu. Rev. Phys. Chem. 2019, 70, 301– 322,  DOI: 10.1146/annurev-physchem-050317-021247
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      Interferometric Scattering Microscopy
      Young, Gavin; Kukura, Philipp
      Annual Review of Physical Chemistry (2019), 70 (), 301-322CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews)
      Interferometric scattering microscopy (iSCAT) is an extremely sensitive imaging method based on the efficient detection of light scattered by nanoscopic objects. The ability to, at least in principle, maintain high imaging contrast independent of the exposure time or the scattering cross section of the object allows for unique applications in single-particle tracking, label-free imaging of nanoscopic (dis)assembly, and quant. single-mol. characterization. We illustrate these capabilities in areas as diverse as mechanistic studies of motor protein function, viral capsid assembly, and single-mol. mass measurement in soln. We anticipate that iSCAT will become a widely used approach to unravel previously hidden details of biomol. dynamics and interactions.
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      Celebrano, M.; Kukura, P.; Renn, A.; Sandoghdar, V. Single-Molecule Imaging by Optical Absorption. Nat. Photonics 2011, 5, 95– 98,  DOI: 10.1038/nphoton.2010.290
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      Single-molecule imaging by optical absorption
      Celebrano, Michele; Kukura, Philipp; Renn, Alois; Sandoghdar, Vahid
      Nature Photonics (2011), 5 (2), 95-98CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      To date, optical studies of single mols. at room temp. have relied on the use of materials with high fluorescence quantum yield combined with efficient spectral rejection of background light. To extend single-mol. studies to a much larger pallet of substances that absorb but do not fluoresce, scientists have explored the photothermal effect, interferometry, direct attenuation and stimulated emission. Indeed, very recently, three groups have succeeded in achieving single-mol. sensitivity in absorption. Here, we apply modulation-free transmission measurements known from absorption spectrometers to image single mols. under ambient conditions both in the emissive and strongly quenched states. We arrive at quant. values for the absorption cross-section of single mols. at different wavelengths and thereby set the ground for single-mol. absorption spectroscopy. Our work has important implications for research ranging from absorption and IR spectroscopy to sensing of unlabeled proteins at the single-mol. level.
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      Chong, S. S.; Min, W.; Xie, X. S. Ground-State Depletion Microscopy: Detection Sensitivity of Single-Molecule Optical Absorption at Room Temperature. J. Phys. Chem. Lett. 2010, 1, 3316– 3322,  DOI: 10.1021/jz1014289
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      Ground-State Depletion Microscopy: Detection Sensitivity of Single-Molecule Optical Absorption at Room Temperature
      Chong, Shasha; Min, Wei; Xie, X. Sunney
      Journal of Physical Chemistry Letters (2010), 1 (23), 3316-3322CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Optical studies of single mols. in ambient environments, which led to broad applications, are primarily based on fluorescence detection. Direct detection of optical absorption with single-mol. sensitivity at room temp. is difficult because absorption is not a background-free measurement and is often complicated by sample scattering. Here the authors report ground-state depletion microscopy for ultrasensitive detection of absorption contrast. The authors image 20 nm Au nanoparticles as an initial demonstration of this microscopy. The authors then demonstrate the detection of an absorption signal from a single chromophore mol. at room temp. This is accomplished by using 2 tightly focused collinear continuous-wave laser beams at different wavelengths, both within a mol. absorption band, 1 of which is intensity modulated at a high frequency (>MHz). The transmission of the other beam is modulated at the same frequency due to ground state depletion. The signal of single chromophore mols. scanned across the common laser foci can be detected with shot-noise limited sensitivity. This measurement represents the ultimate detection sensitivity of nonlinear optical spectroscopy at room temp.
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      Maley, A. M.; Lu, G. J.; Shapiro, M. G.; Corn, R. M. Characterizing Single Polymeric and Protein Nanoparticles with Surface Plasmon Resonance Imaging Measurements. ACS Nano 2017, 11, 7447– 7456,  DOI: 10.1021/acsnano.7b03859
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      Characterizing Single Polymeric and Protein Nanoparticles with Surface Plasmon Resonance Imaging Measurements
      Maley, Adam M.; Lu, George J.; Shapiro, Mikhail G.; Corn, Robert M.
      ACS Nano (2017), 11 (7), 7447-7456CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Near-IR surface plasmon resonance imaging (SPRI) microscopy was used to detect and characterize the adsorption of single polymeric and protein nanoparticles (PPNPs) onto chem. modified gold thin films in real time. The single-nanoparticle SPRI responses, Δ%RNP, from several hundred adsorbed nanoparticles are collected in a single SPRI adsorption measurement. Anal. of Δ%RNP frequency distribution histograms was used to provide information on the size, material content, and interparticle interactions of the PPNPs. Examples include the measurement of log-normal Δ%RNP distributions for mixts. of polystyrene nanoparticles, the quantitation of bioaffinity uptake into and aggregation of porous NIPAm-based (N-isopropylacrylamide) hydrogel nanoparticles specifically engineered to bind peptides and proteins, and the characterization of the neg. single-nanoparticle SPRI response and log-normal Δ%RNP distributions obtained for three different types of genetically encoded gas-filled protein nanostructures derived from bacteria.
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      Jiang, D.; Jiang, Y. Y.; Li, Z. M.; Liu, T.; Wo, X.; Fang, Y. M.; Tao, N. J.; Wang, W.; Chen, H. Y. Optical Imaging of Phase Transition and Li-Ion Diffusion Kinetics of Single LiCoO2 Nanoparticles During Electrochemical Cycling. J. Am. Chem. Soc. 2017, 139, 186– 192,  DOI: 10.1021/jacs.6b08923
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      Optical Imaging of Phase Transition and Li-Ion Diffusion Kinetics of Single LiCoO2 Nanoparticles During Electrochemical Cycling
      Jiang, Dan; Jiang, Yingyan; Li, Zhimin; Liu, Tao; Wo, Xiang; Fang, Yimin; Tao, Nongjian; Wang, Wei; Chen, Hong-Yuan
      Journal of the American Chemical Society (2017), 139 (1), 186-192CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Understanding the phase transition and Li-ion diffusion kinetics of Li-ion storage nanomaterials holds promising keys to further improve the cycle life and charge rate of the Li-ion battery. Traditional electrochem. studies were often based on a bulk electrode consisting of billions of electroactive nanoparticles, which washed out the intrinsic heterogeneity among individuals. Here, we employ optical microscopy, termed surface plasmon resonance microscopy (SPRM), to image electrochem. current of single LiCoO2 nanoparticles down to 50 fA during electrochem. cycling, from which the phase transition and Li-ion diffusion kinetics can be quant. resolved in a single nanoparticle, in operando and high throughput manner. SPRM maps the refractive index (RI) of single LiCoO2 nanoparticles, which significantly decreases with the gradual extn. of Li-ions, enabling the optical read-out of single nanoparticle electrochem. Further SEM characterization of the same batch of nanoparticles led to a bottom-up strategy for studying the structure-activity relationship. As RI is an intrinsic property of any material, the present approach is anticipated to be applicable for versatile kinds of anode and cathode materials, and to facilitate the rational design and optimization toward durable and fast-charging electrode materials.
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      Thambi, V.; Kar, A.; Ghosh, P.; Khatua, S. Light-Controlled In Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl3. J. Phys. Chem. C 2018, 122, 24885– 24890,  DOI: 10.1021/acs.jpcc.8b06679
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      Light-Controlled in Situ Bidirectional Tuning and Monitoring of Gold Nanorod Plasmon via Oxidative Etching with FeCl3
      Thambi, Varsha; Kar, Ashish; Ghosh, Piue; Khatua, Saumyakanti
      Journal of Physical Chemistry C (2018), 122 (43), 24885-24890CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      We report on light-controlled in situ bidirectional tuning of longitudinal surface plasmon resonance (LSPR) of single gold nanorods via oxidative etching with ferric chloride. By removing the surfactant layer from the surface of a gold nanorod, we demonstrate that the etching happens only in the presence of an excitation laser, and the etching rate and directionality can be controlled by the intensity of excitation light. At a low excitation power, a blue shift of a nanorod's LSPR of up to 50 nm was obsd., which indicates preferential etching from its tips. Whereas at a high power, we see a red shift of the nanorod's LSPR of up to 140 nm indicating etching from sides. These results present a new approach for in situ finer adjustments of a selected nanorod's plasmon resonance.
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      Carattino, A.; Khatua, S.; Orrit, M. In Situ Tuning of Gold Nanorod Plasmon through Oxidative Cyanide Etching. Phys. Chem. Chem. Phys. 2016, 18, 15619– 15624,  DOI: 10.1039/C6CP01679K
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      In situ tuning of gold nanorod plasmon through oxidative cyanide etching
      Carattino, Aquiles; Khatua, Saumyakanti; Orrit, Michel
      Physical Chemistry Chemical Physics (2016), 18 (23), 15619-15624CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Single gold nanorods exhibit great opportunities for bio-sensing, enhanced spectroscopies and photothermal therapy. A key property of these particles is the surface plasmon resonance, that is strongly dependent on their shape. Methods for tuning this resonance after the synthesis of the particles are of great interest for many applications. In this work we show that, through very well known chem. between gold atoms and cyanide ions, it is possible to tune the surface plasmon of single 25 × 50 nm rods by more than 100 nm towards longer wavelengths. This is achieved by slowly etching gold atoms from the surface of the particles, preserving their specific optical properties.
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      Al-Zubeidi, A.; Hoener, B. S.; Collins, S. S. E.; Wang, W.; Kirchner, S. R.; Hosseini Jebeli, S. A.; Joplin, A.; Chang, W.-S.; Link, S.; Landes, C. F. Hot Holes Assist Plasmonic Nanoelectrode Dissolution. Nano Lett. 2019, 19, 1301– 1306,  DOI: 10.1021/acs.nanolett.8b04894
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      Hot Holes Assist Plasmonic Nanoelectrode Dissolution
      Al-Zubeidi, Alexander; Hoener, Benjamin S.; Collins, Sean S. E.; Wang, Wenxiao; Kirchner, Silke R.; Hosseini Jebeli, Seyyed Ali; Joplin, Anneli; Chang, Wei-Shun; Link, Stephan; Landes, Christy F.
      Nano Letters (2019), 19 (2), 1301-1306CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Strong light-absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. We demonstrate the role of photogenerated hot holes in the oxidative dissoln. of individual gold nanorods with millisecond time resoln. while tuning charge-carrier d. and photon energy using snapshot hyperspectral imaging. We show that light-induced hot charge carriers enhance the rate of gold oxidn. and subsequent electrodissoln. Importantly, we distinguish how hot holes generated from interband transitions vs. hot holes around the Fermi level contribute to photooxidative dissoln. The results provide new insights into hot-hole-driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of nonequil. processes on innately heterogeneous nanoparticle supports.
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      Cheng, J.; Liu, Y.; Cheng, X. D.; He, Y.; Yeung, E. S. Real Time Observation of Chemical Reactions of Individual Metal Nanoparticles with High-Throughput Single Molecule Spectral Microscopy. Anal. Chem. 2010, 82, 8744– 8749,  DOI: 10.1021/ac101933y
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      Real Time Observation of Chemical Reactions of Individual Metal Nanoparticles with High-Throughput Single Molecule Spectral Microscopy
      Cheng, Jing; Liu, Yang; Cheng, Xiaodong; He, Yan; Yeung, Edward S.
      Analytical Chemistry (Washington, DC, United States) (2010), 82 (20), 8744-8749CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Real time observation of chem. reactions of individual noble metal nanoparticles (MNPs) is fundamentally important to their controlled synthesis, chem. sensing, and catalysis applications. Here, with a simple and high-throughput single-mol. dark-field spectral imaging technique, the authors demonstrate that the reaction-induced plasmonic resonance variations of multiple MNPs could be monitored in parallel. Oxidn. kinetics of individual gold nanorods (AuNRs), either immobilized on a glass substrate or moving freely in homogeneous soln., was recorded successfully. Heterogeneous reaction pathways and intermediate states unobservable in ensemble UV-visible measurements were revealed. Interestingly, the oxidn. rate of individual immobilized AuNRs was much slower than that of the bulk AuNR soln., which implies the existence of a novel self-catalysis mechanism. This high-throughput dark-field spectral imaging technique could be applied to chem. reaction kinetics and heterogeneous catalysis studies of other MNPs at single particle level.
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      Sun, S. S.; Gao, M. X.; Lei, G.; Zou, H. Y.; Ma, J.; Huang, C. Z. Visually Monitoring the Etching Process of Gold Nanoparticles by Ki/I2 at Single-Nanoparticle Level Using Scattered-Light Dark-Field Microscopic Imaging. Nano Res. 2016, 9, 1125– 1134,  DOI: 10.1007/s12274-016-1007-z
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      Visually monitoring the etching process of gold nanoparticles by KI/I2 at single-nanoparticle level using scattered-light dark-field microscopic imaging
      Sun, Shanshan; Gao, Mingxuan; Lei, Gang; Zou, Hongyan; Ma, Jun; Huang, Chengzhi
      Nano Research (2016), 9 (4), 1125-1134CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)
      Real-time monitoring of reaction processes is helpful for understanding the reaction mechanisms. In this study, we investigated the etching mechanism of gold nanoparticles (AuNPs) by iodine on a single-nanoparticle level because AuNPs have become important nanoprobes with applications in sensing and bioimaging fields owing to their specific localized surface plasmon resonance (LSPR) properties. By using a scattered-light dark-field microscopic imaging (iDFM) technique, the in situ KI/I2-treated etching processes of various shapes of AuNPs, including nanospheres (AuNSs), nanorods (AuNRs), and nanotrigonal prisms (AuNTs), were monitored in real time. It was found that the scattered light of the different shapes of AuNPs exhibited noticeable color changes upon exposure to the etching soln. The scattering spectra during the etching process showed obvious blue-shifts with decreasing scattered intensity owing to the oxidn. of Au atoms into [AuI2]-. Both finite-difference time-domain (FDTD) simulations and monitoring of morphol. variations proved that the etching was a thermodn.-dependent process through a chamfering mechanism coupled with layer-by-layer peeling, resulting in isotropic spheres with decreased particle sizes. [Figure not available: see fulltext.].
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      Wang, J.; Zhang, H. Z.; Liu, J. J.; Yuan, D.; Li, R. S.; Huang, C. Z. Time-Resolved Visual Detection of Heparin by Accelerated Etching of Gold Nanorods. Analyst 2018, 143, 824– 828,  DOI: 10.1039/C7AN01923H
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      Time-resolved visual detection of heparin by accelerated etching of gold nanorods
      Wang, Jian; Zhang, Hong Zhi; Liu, Jia Jun; Yuan, Dan; Li, Rong Sheng; Huang, Cheng Zhi
      Analyst (Cambridge, United Kingdom) (2018), 143 (4), 824-828CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
      Plasmonic gold nanorods are promising and sensitive light scattering probes, which can reach the single particle level. Herein, we present the light scattering properties of gold nanorods for time-resolved visual detection of heparin based on the rapid etching of gold nanorods under dark-field microscopy.
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      Zhang, H. Z.; Li, R. S.; Gao, P. F.; Wang, N.; Lei, G.; Huang, C. Z.; Wang, J. Real-Time Dark-Field Light Scattering Imaging to Monitor the Coupling Reaction with Gold Nanorods as an Optical Probe. Nanoscale 2017, 9, 3568– 3575,  DOI: 10.1039/C6NR09453H
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      Real-time dark-field light scattering imaging to monitor the coupling reaction with gold nanorods as an optical probe
      Zhang, Hong Zhi; Li, Rong Sheng; Gao, Peng Fei; Wang, Ni; Lei, Gang; Huang, Cheng Zhi; Wang, Jian
      Nanoscale (2017), 9 (10), 3568-3575CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Gold nanorods (GNRs) have opened up promising applications based on their reshaping, due to the fact that a tiny change in shape or size could directly lead to optical changes. Herein, we report chem.reshaping of GNRs induced by the coupling reaction between Au, ferric chloride and thiourea. In the coupling reaction, Fe3+ oxidizes the GNRs to yield Au(I), which complexes with the thiourea ligand, lowering the Gibbs free energy of the gold species and promoting the reaction equil.to enable the chem.reshaping of the GNRs. This coupling reaction process was monitored using a light-scattering dark-field microscopy (DFM) imaging technique and SEM (SEM). The light scattering underwent a color change from bright red to yellow and finally to green, and the GNRs underwent a morphol.change from rod-shaped to fusiform and finally to spherical, which is somewhat different from the results of other chem.etching processes of GNRs. It is believed that the coupling reaction induced chem.reshaping of GNRs not only provides an alternative way to monitor the coupling reaction, but also offers a facile way to obtain a desirable GNR morphol., which is important for the prepn.of fusiform nanostructures.
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      Xie, T.; Jing, C.; Ma, W.; Ding, Z. F.; Gross, A. J.; Long, Y. T. Real-Time Monitoring for the Morphological Variations of Single Gold Nanorods. Nanoscale 2015, 7, 511– 517,  DOI: 10.1039/C4NR05080K
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      Real-time monitoring for the morphological variations of single gold nanorods
      Xie, Tao; Jing, Chao; Ma, Wei; Ding, Zhifeng; Gross, Andrew James; Long, Yi-Tao
      Nanoscale (2015), 7 (2), 511-517CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      The morphol. characteristics of metal nanoparticles, particularly the shape, play an essential role in the optical, phys. and chem. properties. In this work, we reported a transverse etching process to investigate the morphol. variations of single gold nanorods (GNRs). Dark-field microscopy and Rayleigh scattering spectroscopy were used as complementary technologies to track the transverse etching process. Dark-field imaging with high spatial and temporal resoln. could easily monitor the transverse etching process of GNRs in situ and in real time. Interactions between the scattering spectrum and the morphol. variations were judiciously calcd. within the dipole approxn. by the Drude function. The calcd. peak shift of GNRs (Δλmax = 17 nm) was obtained via the ratio of the long axis and short axis (aspect ratio) of GNRs from transmission electron microscopy. The av. scattering peak shift (Δλmax = 22 nm) from Rayleigh scattering spectroscopy was in good agreement with the calcd. peak shift. Monitoring the morphol. variations of single GNRs enables us to track the transverse etching of GNRs at arbitrary time. This promises to be a useful method for the study of different nanomaterials and their spectral properties.
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      Flatebo, C.; Collins, S. S. E.; Hoener, B. S.; Cai, Y.-y.; Link, S.; Landes, C. F. Electrodissolution Inhibition of Gold Nanorods with Oxoanions. J. Phys. Chem. C 2019, 123, 13983– 13992,  DOI: 10.1021/acs.jpcc.9b01575
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      Electrodissolution Inhibition of Gold Nanorods with Oxoanions
      Flatebo, Charlotte; Collins, Sean S. E.; Hoener, Benjamin S.; Cai, Yi-yu; Link, Stephan; Landes, Christy F.
      Journal of Physical Chemistry C (2019), 123 (22), 13983-13992CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Metal nanoparticles experience varied chem. environments that can cause corrosion and dissoln. in electronics, electrocatalysis, and sensing applications. Understanding oxidative dissoln. is crit. for plasmonic nanoparticles because their optical properties strongly depend on size and shape. The addn. of low relative concns. of oxoanions to aq. halide electrolyte solns. improves the morphol. stability of plasmonic Au nanorods at anodic electrochem. potentials that otherwise induce complete oxidative electrodissoln. Single particle hyperspectral dark-field imaging and correlated SEM show that oxoanions alter the electrodissoln. onset potential, electrodissoln. pathway, and nanoparticle reaction heterogeneity, as compared to chloride-only electrolyte solns. The authors identify five mechanistic contributors to the corrosion inhibition capabilities of oxoanions in the presence of chloride ions, with the aim of expanding the range of electrochem. sensing and catalysis applications for plasmonic metal nanoparticles. Of the contributors studied, the pH, adsorption potential, and ionicity of the oxoanion are the most influential factors, supporting the superior corrosion inhibition obsd. with bicarbonate and phosphate.
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    55. 55
      Ye, X. C.; Jones, M. R.; Frechette, L. B.; Chen, Q.; Powers, A. S.; Ercius, P.; Dunn, G.; Rotskoff, G. M.; Nguyen, S. C.; Adiga, V. P.; Zettl, A.; Rabani, E.; Geissler, P. L.; Alivisatos, A. P. Single-Particle Mapping of Nonequilibrium Nanocrystal Transformations. Science 2016, 354, 874– 877,  DOI: 10.1126/science.aah4434
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      Single-particle mapping of nonequilibrium nanocrystal transformations
      Ye, Xingchen; Jones, Matthew R.; Frechette, Layne B.; Chen, Qian; Powers, Alexander S.; Ercius, Peter; Dunn, Gabriel; Rotskoff, Grant M.; Nguyen, Son C.; Adiga, Vivekananda P.; Zettl, Alex; Rabani, Eran; Geissler, Phillip L.; Alivisatos, A. Paul
      Science (Washington, DC, United States) (2016), 354 (6314), 874-877CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The control of the shape and size of metal nanoparticles can be sensitive to the growth conditions of the particles. Ye et al. studied the reverse process: They tracked the dissoln. of gold nanoparticles in a redox environment inside a liq. cell within an electron microscope, controlling the particle dissoln. with the electron beam. Tracking short-lived particle shapes revealed structures of greater or lesser stability. The findings suggest kinetic routes to particle sizes and shapes that would otherwise be difficult to generate.
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      Sun, Y. Z.; Fan, X. D. Optical Ring Resonators for Biochemical and Chemical Sensing. Anal. Bioanal. Chem. 2011, 399, 205– 211,  DOI: 10.1007/s00216-010-4237-z
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      Optical ring resonators for biochemical and chemical sensing
      Sun, Yuze; Fan, Xudong
      Analytical and Bioanalytical Chemistry (2011), 399 (1), 205-211CODEN: ABCNBP; ISSN:1618-2642. (Springer)
      A review. In the past few years optical ring resonators have emerged as a new sensing technol. for highly sensitive detection of analytes in liq. or gas. This article introduces the ring resonator sensing principle, describes various ring resonator sensor designs, reviews the current state of the field, and presents an outlook of possible applications and related research and development directions.
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      Lu, T.; Lee, H.; Chen, T.; Herchak, S.; Kim, J. H.; Fraser, S. E.; Flagan, R. C.; Vahala, K. High Sensitivity Nanoparticle Detection Using Optical Microcavities. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 5976– 5979,  DOI: 10.1073/pnas.1017962108
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      High sensitivity nanoparticle detection using optical microcavities
      Lu, Tao; Lee, Hansuek; Chen, Tong; Herchak, Steven; Kim, Ji-Hun; Fraser, Scott E.; Flagan, Richard C.; Vahala, Kerry
      Proceedings of the National Academy of Sciences of the United States of America (2011), 108 (15), 5976-5979CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      We demonstrate a highly sensitive nanoparticle and virus detection method by using a thermal-stabilized ref. interferometer in conjunction with an ultrahigh-Q microcavity. Sensitivity is sufficient to resolve shifts caused by binding of individual nanobeads in soln. down to a record radius of 12.5 nm, a size approaching that of single protein mols. A histogram of wavelength shift vs. nanoparticle radius shows that particle size can be inferred from shift maxima. Addnl., the signal-to-noise ratio for detection of Influenza A virus is enhanced to 38:1 from the previously reported 3:1. The method does not use feedback stabilization of the probe laser. It is also obsd. that the conjunction of particle-induced backscatter and optical-path-induced shifts can be used to enhance detection signal-to-noise.
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    58. 58
      Ward, J. M.; Yang, Y.; Lei, F. C.; Yu, X. C.; Xiao, Y. F.; Nic Chormaic, S. Nanoparticle Sensing Beyond Evanescent Field Interaction with a Quasi-Droplet Microcavity. Optica 2018, 5, 674– 677,  DOI: 10.1364/OPTICA.5.000674
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      Nanoparticle sensing beyond evanescent field interaction with a quasi-droplet microcavity
      Ward, Jonathan M.; Yang, Yong; Lei, Fuchuan; Yu, Xiao-Chong; Xiao, Yun-Feng; Chormaic, Sile Nic
      Optica (2018), 5 (6), 674-677CODEN: OPTIC8; ISSN:2334-2536. (Optical Society of America)
      Sensing with whispering gallery resonators (WGRs) is largely limited by the weak perturbation of the whispering gallery mode (WGM) via the evanescent field. A new sensing regime using quasi-droplet WGMs allows WGRs to move beyond the limitation of the evanescent field and push the detection sensitivity to new heights. We present exptl. results on the detection of 100 nm and 500 nm polystyrene particles in aq. soln. using thin-walled, hollow WGRs supporting quasi-droplet modes. The detection sensitivity in terms of mode shift and broadening is measured, with mode shifts of 400 MHz obsd. for 100 nm particles. In terms of the no. of linewidths, this is 276 times larger than similar expts. with microsphere WGRs, thus showing a significant increase in detection sensitivity beyond the capability of std. evanescent field sensing with WGRs.
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    59. 59
      Barucci, A.; Berneschi, S.; Giannetti, A.; Baldini, F.; Cosci, A.; Pelli, S.; Farnesi, D.; Righini, G. C.; Soria, S.; Conti, G. N. Optical Microbubble Resonators with High Refractive Index Inner Coating for Bio-Sensing Applications: An Analytical Approach. Sensors 2016, 16, 1992,  DOI: 10.3390/s16121992
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      Optical microbubble resonators with high refractive index inner coating for bio-sensing applications: an analytical approach
      Barucci, Andrea; Berneschi, Simone; Giannetti, Ambra; Baldini, Francesco; Cosci, Alessandro; Pelli, Stefano; Farnesi, Daniele; Righini, Giancarlo C.; Soria, Silvia; Conti, Gualtiero Nunzi
      Sensors (2016), 16 (12), 1992/1-1992/19CODEN: SENSC9; ISSN:1424-8220. (MDPI AG)
      The design of Whispering Gallery Mode Resonators (WGMRs) used as an optical transducer for biosensing represents the first and crucial step towards the optimization of the final device performance in terms of sensitivity and Limit of Detection (LoD). Here, we propose an anal. method for the design of an optical microbubble resonator (OMBR)-based biosensor. In order to enhance the OMBR sensing performance, we consider a polymeric layer of high refractive index as an inner coating for the OMBR. The effect of this layer and other optical/geometrical parameters on the mode field distribution, sensitivity and LoD of the OMBR is assessed and discussed, both for transverse elec. (TE) and transverse magnetic (TM) polarization. The obtained results do provide phys. insights for the development of OMBR-based biosensor.
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      Giorgini, A.; Avino, S.; Malara, P.; De Natale, P.; Gagliardi, G. Liquid Droplet Microresonators. Sensors 2019, 19, 473,  DOI: 10.3390/s19030473
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      Liquid droplet microresonators
      Giorgini, Antonio; Avino, Saverio; Malara, Pietro; De Natale, Paolo; Gagliardi, Gianluca
      Sensors (2019), 19 (3), 473/1-473/20CODEN: SENSC9; ISSN:1424-8220. (MDPI AG)
      A review. We provide here an overview of passive optical micro-cavities made of droplets in the liq. phase. We focus on resonators that are naturally created and suspended under gravity thanks to interfacial forces, illustrating simple ways to excite whispering-gallery modes in various slow-evapn. liqs. using free-space optics. Similar to solid resonators, frequency locking of near-IR and visible lasers to resonant modes is performed exploiting either phase-sensitive detection of the leakage cavity field or multiple interference between whispering-gallery modes in the scattered light. As opposed to conventional micro-cavity sensors, each droplet acts simultaneously as the sensor and the sample, whereby the internal light can detect dissolved compds. and particles. Optical quality factors up to 107-108 are obsd. in liq.-polymer droplets through photon lifetime measurements. First attempts in using single water droplets are also reported. These achievements point out their huge potential for direct spectroscopy and bio-chem. sensing in liq. environments. Finally, the first expts. of cavity optomechanics with surface acoustic waves in nanolitre droplets are presented. The possibility to perform studies of viscous-elastic properties points to a new paradigm: a droplet device as an opto-fluid-mechanics lab. on table-top scale under controlled environmental conditions.
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    61. 61
      Madani, A.; Harazim, S. M.; Quinones, V. A. B.; Kleinert, M.; Finn, A.; Naz, E. S. G.; Ma, L. B.; Schmidt, O. G. Optical Microtube Cavities Monolithically Integrated on Photonic Chips for Optofluidic Sensing. Opt. Lett. 2017, 42, 486– 489,  DOI: 10.1364/OL.42.000486
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      Optical microtube cavities monolithically integrated on photonic chips for optofluidic sensing
      Madani, Abbas; Harazim, Stefan M.; Quinones, Vladimir A. Bolanos; Kleinert, Moritz; Finn, Andreas; Naz, Ehsan Saei Ghareh; Ma, Libo; Schmidt, Oliver G.
      Optics Letters (2017), 42 (3), 486-489CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
      Microtubular optical resonators are monolithically integrated on photonic chips to demonstrate optofluidic functionality. Due to the compact subwavelength-thin tube wall and a well-defined nanogap between polymer photonic waveguides and the microtube, excellent optical coupling with extinction ratios up ro 32 dB are obsd. in the telecommunication relevant wavelength range. For the first time, optofluidic applications of fully on-chip integrated microtubular systems are investigated both by filling the core of the microtube and by the microtube being covered by a liq. droplet. Total shifts over the full free spectral range are obsd. in response to the presence of the liq. medium in the vicinity of the microtube resonators. This work provides a vertical coupling scheme for optofluidic applications in monolithically integrated so-called "lab-in-a-tube" systems.
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    62. 62
      Han, K. W.; Kim, J.; Bahl, G. High-Throughput Sensing of Freely Flowing Particles with Optomechanofluidics. Optica 2016, 3, 585– 591,  DOI: 10.1364/OPTICA.3.000585
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      High-throughput sensing of freely flowing particles with optomechanofluidics
      Han, Kewen; Kim, Junhwan; Bahl, Gaurav
      Optica (2016), 3 (6), 585-591CODEN: OPTIC8; ISSN:2334-2536. (Optical Society of America)
      High-Q photonic microcavity sensors have enabled the label-free measurement of nanoparticles, such as single viruses and large mols., close to the fundamental limits of detection. However, key scientific challenges persist: (1) photons do not directly couple to mech. parameters such as mass d., compressibility, or viscoelasticity, and (2) current techniques cannot measure all particles in a fluid sample due to the reliance on random diffusion to bring analytes to the sensing region. Here, we present a new, label-free microfluidic optomech. sensor that addresses both challenges, enabling, for the first time, the rapid photonic sensing of the mech. properties of freely flowing particles in a fluid. Sensing is enabled by optomech. coupling of photons to long-range phonons that cast a near-perfect net deep inside the device. Our opto-mechano-fluidic approach enables the measurement of particle mass d., mech. compressibility, and viscoelasticity at rates potentially exceeding 10,000 particles/s. Uniquely, we show that the sensitivity of this high-Q microcavity sensor is highest when the analytes are located furthest from the optical mode, at the center of the device, where the flow is fastest. Our results enable till-date inaccessible mech. anal. of flowing particles at speeds comparable to com. flow cytometry.
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      Stoian, R.-I.; Bui, K. V.; Rosenberger, A. Silica Hollow Bottle Resonators for Use as Whispering Gallery Mode Based Chemical Sensors. J. Opt. 2015, 17, 125011,  DOI: 10.1088/2040-8978/17/12/125011
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      Silica hollow bottle resonators for use as whispering gallery mode based chemical sensors
      Stoian, Razvan-Ionut; Bui, Khoa V.; Rosenberger, A. T.
      Journal of Optics (Bristol, United Kingdom) (2015), 17 (12), 125011/1-125011/7CODEN: JOOPCA; ISSN:2040-8978. (IOP Publishing Ltd.)
      A simple three-step method for making silica hollow bottle resonators (HBRs) was developed. This procedure is advantageous because it uses com. available materials, is cost effective, and is easy to implement. Addnl., the use of these HBRs as whispering gallery mode based chem. sensors is demonstrated by preliminary absorption sensing results in the near IR (1580-1660 nm) using a trace gas (CH4) in air at atm. pressure and a dye (SDA2072) in methanol soln.
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      Ward, J. M.; Dhasmana, N.; Nic Chormaic, S. Hollow Core, Whispering Gallery Resonator Sensors. Eur. Phys. J.: Spec. Top. 2014, 223, 1917– 1935,  DOI: 10.1140/epjst/e2014-02236-5
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      Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J. Demonstration of Ultra-High-Q Small Mode Volume Toroid Microcavities on a Chip. Appl. Phys. Lett. 2004, 85, 6113– 6115,  DOI: 10.1063/1.1833556
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      Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip
      Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J.
      Applied Physics Letters (2004), 85 (25), 6113-6115CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Optical microcavities confine light spatially and temporally and find application in a wide range of fundamental and applied studies. In many areas, the microcavity figure of merit is not only detd. by photon lifetime (or the equiv. quality-factor, Q), but also by simultaneous achievement of small mode vol. (V). Here we demonstrate ultra-high Q-factor small mode vol. toroid microcavities on-a-chip, which exhibit a Q/V factor of more than 106 (λ/n)-3. These values are the highest reported to date for any chip-based microcavity. A corresponding Purcell factor in excess of 200 000 and a cavity finesse of °2.8 × 106 is achieved, demonstrating that toroid microcavities are promising candidates for studies of the Purcell effect, cavity QED or biochem. sensing.
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      Armani, A. M.; Vahala, K. J. Heavy Water Detection Using Ultra-High-Q Microcavities. Opt. Lett. 2006, 31, 1896– 1898,  DOI: 10.1364/OL.31.001896
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      Heavy water detection using ultra-high-Q microcavities
      Armani, Andrea M.; Vahala, Kerry J.
      Optics Letters (2006), 31 (12), 1896-1898CODEN: OPLEDP; ISSN:0146-9592. (Optical Society of America)
      Ultra-high-Q optical microcavities (Q > 107) provide one method for distinguishing chem. similar species. Resonators immersed in H2O have lower quality factors than those immersed in D2O due to the difference in optical absorption. This difference can be used to create a D2O detector. This effect is most noticeable at 1300 nm, where the Q(H2O) is 106 and the Q(D2O) is 107. By monitoring Q, concns. of 0.0001% [1 part in 106 per vol.] of D2O in H2O have been detected. This sensitivity represents an order of magnitude improvement over previous techniques. Reversible detection was also demonstrated by cyclic introduction and flushing of D2O.
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      Armani, D. K.; Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J. Ultra-High-Q Toroid Microcavity on a Chip. Nature 2003, 421, 925– 928,  DOI: 10.1038/nature01371
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      Ultra-high-Q toroid microcavity on a chip
      Armani, D. K.; Kippenberg, T. J.; Spillane, S. M.; Vahala, K. J.
      Nature (London, United Kingdom) (2003), 421 (6926), 925-928CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      The circulation of light within dielec. vols. enables storage of optical power near specific resonant frequencies and is important in a wide range of fields including cavity quantum electrodynamics, photonics, biosensing and nonlinear optics. Optical trajectories occur near the interface of the vol. with its surroundings, making their performance strongly dependent upon interface quality. With a nearly at.-scale surface finish, surface-tension-induced microcavities such as liq. droplets or spheres are superior to all other dielec. microresonant structures when comparing photon lifetime or, equivalently, cavity Q factor. Despite these advantageous properties, the phys. characteristics of such systems are not easily controlled during fabrication. It is known that wafer-based processing of resonators can achieve parallel processing and control, as well as integration with other functions. However, such resonators-on-a-chip suffer from Q factors that are many orders of magnitude lower than for surface-tension-induced microcavities, making them unsuitable for ultra-high-Q expts. Here the authors demonstrate a process for producing SiO2 toroid-shaped microresonators-on-a-chip with Q factors >100 million using a combination of lithog., dry etching and a selective reflow process. Such a high Q value was previously attainable only by droplets or microspheres and represents an improvement of nearly four orders of magnitude over previous chip-based resonators.
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      Black, E. D. An Introduction to Pound-Drever-Hall Laser Frequency Stabilization. Am. J. Phys. 2001, 69, 79– 87,  DOI: 10.1119/1.1286663
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      Barnes, J. A.; Gagliardi, G.; Loock, H. P. Absolute Absorption Cross-Section Measurement of a Submonolayer Film on a Silica Microresonator. Optica 2014, 1, 75– 83,  DOI: 10.1364/OPTICA.1.000075
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      Absolute absorption cross-section measurement of a submonolayer film on a silica microresonator
      Barnes, Jack A.; Gagliardi, Gianluca; Loock, Hans-Peter
      Optica (2014), 1 (2), 75-83CODEN: OPTIC8; ISSN:2334-2536. (Optical Society of America)
      Conventional absorption spectroscopy is not nearly sensitive enough for quant. overtone measurements on submonolayer coatings. While cavity-enhanced absorption detection methods using microresonators have the potential to provide quant. absorption cross sections of even weakly absorbing submonolayer films, this potential has not yet been fully realized. To det. the absorption cross section of a submonolayer film of ethylene diamine (EDA) on a silica microsphere resonator, we use phase-shift cavity ringdown spectroscopy simultaneously on near-IR radiation that is Rayleigh backscattered from the microsphere and transmitted through the coupling fiber taper. We then independently det. both the coupling coeff. and the optical loss within the resonator. Together with a coincident measurement of the wavelength frequency shift, an abs. overtone absorption cross section of adsorbed EDA, at submonolayer coverage, was obtained and was compared to the bulk value. The smallest quantifiable absorption cross section is σmin 2.7 × 10-12 cm2. This absorption cross section is comparable to the extinction coeffs. of, e.g., single gold nanoparticles or aerosol particles. We therefore propose that the present method is also a viable route to abs. extinction measurements of single particles.
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      Carmon, T.; Kippenberg, T. J.; Yang, L.; Rokhsari, H.; Spillane, S.; Vahala, K. J. Feedback Control of Ultra-High-Q Microcavities: Application to Micro-Raman Lasers and Microparametric Oscillators. Opt. Express 2005, 13, 3558– 3566,  DOI: 10.1364/OPEX.13.003558
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      Feedback control of ultra-high-Q microcavities: application to micro-Raman lasers and microparametric oscillators
      Carmon Tal; Kippenberg Tobias; Yang Lan; Rokhsari Hosein; Spillane Sean; Vahala Kerry
      Optics express (2005), 13 (9), 3558-66 ISSN:.
      We demonstrate locking of an on-chip, high-Q toroidal-cavity to a pump laser using two, distinct methods: coupled power stabilization and wavelength locking of pump laser to the microcavity. In addition to improvements in operation of previously demonstrated micro-Raman and micro-OPO lasers, these techniques have enabled observation of a continuous, cascaded nonlinear process in which photons generated by optical parametric oscillations (OPO) function as a pump for Raman lasing. Dynamical behavior of the feedback control systems is also shown including the interplay between the control loop and the thermal nonlinearity. The demonstrated stabilization loop is essential for studying generation of nonclassical states using a microcavity optical parametric oscillator.
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      Murugan, G. S.; Petrovich, M. N.; Jung, Y.; Wilkinson, J. S.; Zervas, M. N. Hollow-Bottle Optical Microresonators. Opt. Express 2011, 19, 20773– 20784,  DOI: 10.1364/OE.19.020773
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      Nasir, M. N. M.; Murugan, G. S.; Zervas, M. N. Spectral Cleaning and Output Modal Transformations in Whispering-Gallery-Mode Microresonators. J. Opt. Soc. Am. B 2016, 33, 1963– 1970,  DOI: 10.1364/JOSAB.33.001963
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      Spectral cleaning and output modal transformations in whispering-gallery-mode microresonators
      Nasir, Mohd Narizee Mohd; Murugan, G. Senthil; Zervas, Michalis N.
      Journal of the Optical Society of America B: Optical Physics (2016), 33 (9), 1963-1970CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)
      A systematic study on the effects of microtaper fiber diams. on the spectral characteristics of a whispering-gallery-mode (WGM) microbottle resonator (MBR) is presented. Progressively cleaner and simpler spectra of the MBR were obsd. when the utilized microtaper fiber waist diam. (Dt) was increased from 2 to 10 μm. The max. transmission depth at resonance varies with different microtaper fiber utilized from ∼20 dB (Dt=2 μm) to ∼4 dB (Dt = 10 μm). The loaded Q-factors were obsd. to be unaffected by the increase of Dt with values of >106 being measured in all cases. Mode transformation of MBR was also exptl. investigated and compared to a microdisc finite-difference time-domain simulation by studying near-field images of the output beam on the waist of the microtaper fibers. For the first time, exptl. observation of mode transformation from LP01 to LP11 across scanned WGM resonances is being reported.
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      Ding, M.; Murugan, G. S.; Brambilla, G.; Zervas, M. N. Whispering Gallery Mode Selection in Optical Bottle Microresonators. Appl. Phys. Lett. 2012, 100, 081108,  DOI: 10.1063/1.3688601
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      Whispering gallery mode selection in optical bottle microresonators
      Ding, Ming; Murugan, Ganapathy Senthil; Brambilla, Gilberto; Zervas, Michalis N.
      Applied Physics Letters (2012), 100 (8), 081108/1-081108/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The authors demonstrated a method to excite selected whispering gallery modes in optical bottle microresonators (BMR) by inscribing microgroove scars on their surface by focused ion beam milling. Substantial spectral clean-up is obtained in appropriately scarred BMRs, providing the potential for high performance sensors and other optical devices. (c) 2012 American Institute of Physics.
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    74. 74
      Schunk, G.; Furst, J. U.; Fortsch, M.; Strekalov, D. V.; Vogl, U.; Sedlmeir, F.; Schwefel, H. G. L.; Leuchs, G.; Marquardt, C. Identifying Modes of Large Whispering-Gallery Mode Resonators from the Spectrum and Emission Pattern. Opt. Express 2014, 22, 30795– 30806,  DOI: 10.1364/OE.22.030795
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      Identifying modes of large whispering-gallery mode resonators from the spectrum and emission pattern
      Schunk Gerhard; Furst Josef U; Fortsch Michael; Strekalov Dmitry V; Vogl Ulrich; Sedlmeir Florian; Schwefel Harald G L; Leuchs Gerd; Marquardt Christoph
      Optics express (2014), 22 (25), 30795-806 ISSN:.
      Identifying the mode numbers in whispering-gallery mode resonators (WGMRs) is important for tailoring them to experimental needs. Here we report on a novel experimental mode analysis technique based on the combination of frequency analysis and far-field imaging for high mode numbers of large WGMRs. The radial mode numbers q and the angular mode numbers p = ℒ-m are identified and labeled via far-field imaging. The polar mode numbers ℒ are determined unambiguously by fitting the frequency differences between individual whispering gallery modes (WGMs). This allows for the accurate determination of the geometry and the refractive index at different temperatures of the WGMR. For future applications in classical and quantum optics, this mode analysis enables one to control the narrow-band phase-matching conditions in nonlinear processes such as second-harmonic generation or parametric down-conversion.
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      Ward, J. M.; Yang, Y.; Nic Chormaic, S. Highly Sensitive Temperature Measurements with Liquid-Core Microbubble Resonators. IEEE Photonics Technol. Lett. 2013, 25, 2350– 2353,  DOI: 10.1109/LPT.2013.2283732
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      Hall, J. M. M.; Francois, A.; Afshar, V. S.; Riesen, N.; Henderson, M. R.; Reynolds, T.; Monro, T. M. Determining the Geometric Parameters of Microbubble Resonators from Their Spectra. J. Opt. Soc. Am. B 2017, 34, 44– 51,  DOI: 10.1364/JOSAB.34.000044
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      Determining the geometric parameters of microbubble resonators from their spectra
      Hall, Jonathan M. M.; Francois, Alexandre; Shahraam, Afshar V.; Riesen, Nicolas; Henderson, Matthew R.; Reynolds, Tess; Monro, Tanya M.
      Journal of the Optical Society of America B: Optical Physics (2017), 34 (1), 44-51CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)
      A method for detg. the diams. and shell thickness of microbubble resonators is presented; it entails simulating whispering gallery mode (WGM) spectra using a newly developed finite-difference time-domain (FDTD)-based toolkit. Spectra for a range of shell thicknesses are simulated using FDTD, assuming a linear dependence of the free spectral range on the diam., and the free spectral ranges and positions of the prominent modes are matched to those of the measured spectrum. This method improves upon existing techniques for extg. the diam. and thickness, such as SEM imaging, which typically require the microbubble to be dissected or otherwise rendered unusable for subsequent use. The model allows a variety of methods of mode excitation to be simulated. Dye coatings are simulated by placing a layer of dipole sources on the surface of the resonator, yielding mode couplings comparable to those measured in expts. The model is tested for a small-diam. silica glass microbubble, with the free spectral range being simulated for a range of diams. and shell thicknesses. The numerically simulated spectra are then compared to the exptl. measured spectrum. The ability to det. the geometric parameters of such resonators directly from their WGM spectra represents a step forward in the characterization of microbubble resonators. Furthermore, the model opens the way to previously unstudied spectral behavior of microbubbles with small diams. and thin shell thicknesses.
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    77. 77
      Murugan, G. S.; Wilkinson, J. S.; Zervas, M. N. Selective Excitation of Whispering Gallery Modes in a Novel Bottle Microresonator. Opt. Express 2009, 17, 11916– 11925,  DOI: 10.1364/OE.17.011916
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      Selective excitation of whispering gallery modes in a novel bottle microresonator
      Murugan, Ganapathy Senthil; Wilkinson, James S.; Zervas, Michalis N.
      Optics Express (2009), 17 (14), 11916-11925CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
      Selective excitation of spheroidal whispering gallery modes and bottle modes in a robust bottle microresonator fabricated straightforwardly from a short section of optical fiber is demonstrated. Characteristic resonance spectra of long-cavity bottle modes were obtained by using a tapered fiber to excite evanescently bottle microresonator at different points along its axis. Compared to bare-fiber cylindrical resonators, the bottle microresonator results in a 35× increase of the obsd. Q factor.
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    78. 78
      Davletshin, Y. R.; Lombardi, A.; Cardinal, M. F.; Juve, V.; Crut, A.; Maioli, P.; Liz-Marzan, L. M.; Vallee, F.; Del Fatti, N.; Kumaradas, J. C. A Quantitative Study of the Environmental Effects on the Optical Response of Gold Nanorods. ACS Nano 2012, 6, 8183– 8193,  DOI: 10.1021/nn302869v
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      78
      A Quantitative Study of the Environmental Effects on the Optical Response of Gold Nanorods
      Davletshin, Yevgeniy R.; Lombardi, Anna; Cardinal, M. Fernanda; Juve, Vincent; Crut, Aurelien; Maioli, Paolo; Liz-Marzan, Luis M.; Vallee, Fabrice; Fatti, Natalia Del; Kumaradas, J. Carl
      ACS Nano (2012), 6 (9), 8183-8193CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The effects of the dielec. environment on the optical extinction spectra of Au nanorods were quant. studied using individual bare and SiO2-coated nanorods. The dispersion and amplitude of their extinction cross-section, dominated by absorption for the studied sizes, were measured using spatial modulation spectroscopy (SMS). The exptl. results were compared to calcns. from a numerical model that included environmental features present in the measurements and the morphol. and size of the corresponding nanorods measured by TEM. The combination of these exptl. and theor. tools permits a detailed interpretation of the optical properties of the individual nanorods. The measured optical extinction spectra and the extinction cross-section amplitudes were well reproduced by the numerical model for SiO2-coated Au nanorods, for which the SiO2 shell provides a controlled environment. But addnl. environmental factors had to be assumed in the model for bare nanorods, stressing the importance of controlling and characterizing the exptl. conditions when measuring the optical response of bare surface-deposited single metal nanoparticles.
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    79. 79
      Ni, W. H.; Chen, H. J.; Kou, X. S.; Yeung, M. H.; Wang, J. F. Optical Fiber-Excited Surface Plasmon Resonance Spectroscopy of Single and Ensemble Gold Nanorods. J. Phys. Chem. C 2008, 112, 8105– 8109,  DOI: 10.1021/jp801579m
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      79
      Optical Fiber-Excited Surface Plasmon Resonance Spectroscopy of Single and Ensemble Gold Nanorods
      Ni, Weihai; Chen, Huanjun; Kou, Xiaoshan; Yeung, Man Hau; Wang, Jianfang
      Journal of Physical Chemistry C (2008), 112 (22), 8105-8109CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Au nanorods are deposited on the core surface of an etched optical fiber, and their surface plasmon resonance is excited by the evanescent wave from the optical fiber. The excitation of the surface plasmon produces strong light scattering from individual nanorods, allowing for single-particle scattering-based imaging and spectroscopy. The authors systematically characterize the dependence of the scattering spectra of individual nanorods on the refractive index of the surrounding medium. As the refractive index is increased, the scattering spectra red-shift steadily. The refractive index sensitivity and sensing figure of merit, which is the index sensitivity divided by the scattering peak line width, reach 200 nm/RIU (RIU = refractive index unit) and 3.8, resp. The authors further study the ensemble response of the plasmon resonance of Au nanorods to varying dielec. environments by measuring the intensity and spectrum of the light transmitted through the fiber. The ensemble index sensitivity and figure of merit are 138 nm/RIU and 1.2, resp. These results suggest the potential for combining the high index sensitivities of Au nanorods together with the attractive sensing features of fiber-based sensors to develop real-time, low-cost, ultrasensitive, and multiplexed sensors.
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    80. 80
      Park, K.; Biswas, S.; Kanel, S.; Nepal, D.; Vaia, R. A. Engineering the Optical Properties of Gold Nanorods: Independent Tuning of Surface Plasmon Energy, Extinction Coefficient, and Scattering Cross Section. J. Phys. Chem. C 2014, 118, 5918– 5926,  DOI: 10.1021/jp5013279
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      80
      Engineering the Optical Properties of Gold Nanorods: Independent Tuning of Surface Plasmon Energy, Extinction Coefficient, and Scattering Cross Section
      Park, Kyoungweon; Biswas, Sushmita; Kanel, Sushil; Nepal, Dhriti; Vaia, Richard A.
      Journal of Physical Chemistry C (2014), 118 (11), 5918-5926CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The future integration of plasmonic nanoparticles, such as Au nanorods (Au NRs), into applications requires the ability to tune the components of their optical properties to optimize performance for the underlying technol. Verifying techniques that model the resonance energy and assocd. extinction, scattering, and absorption cross sections necessitate exptl. data from series of Au NRs where structural features are independently tuned. Here, the extinction cross section and scattering efficiency are presented for Au NR series with high compositional and structural purity where effective vol., aspect ratio, length, and diam. are independently varied by factors of 25, 3, 2, and 4, resp. The extinction cross sections quant. agree with prior calcns., confirming that the vol. of the rod is the dominant factor. Comparisons of the scattering efficiency however are less precise, with both quant. and qual. differences between the role of rod vol. and aspect ratio. Such extensive exptl. data sets provide a crit. platform to improve quant. structure-property correlations, and thus enable design optimization of plasmonic nanoparticles for emerging applications.
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    81. 81
      Jana, N. R.; Gearheart, L.; Obare, S. O.; Murphy, C. J. Anisotropic Chemical Reactivity of Gold Spheroids and Nanorods. Langmuir 2002, 18, 922– 927,  DOI: 10.1021/la0114530
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      81
      Anisotropic chemical reactivity of gold spheroids and nanorods
      Jana, Nikhil R.; Gearheart, Latha; Obare, Sherine O.; Murphy, Catherine J.
      Langmuir (2002), 18 (3), 922-927CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      We obsd. anisotropic chem. reactivity of gold spheroids during cyanide dissoln. and reaction with persulfate. Cyanide dissoln. of 2-5 aspect ratio spheroids (with 12-30 nm short axis) starts at the ends of the long axis and leads to intermediate lower aspect ratio spheroids and spheres. Persulfate converts the spheroids into spheres. In contrast, cyanide dissoln. of 18 aspect ratio nanorods (with 16 nm short axis) starts simultaneously at many sites and does not change the nanorod length throughout dissoln. Spheroids are thermally less stable than rods and slowly convert to spheres with time, and thus anisotropic chem. reactivity is presumably due to their lower stability.
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    82. 82
      Zou, R. X.; Guo, X.; Yang, J.; Li, D. D.; Peng, F.; Zhang, L.; Wang, H. J.; Yu, H. Selective Etching of Gold Nanorods by Ferric Chloride at Room Temperature. CrystEngComm 2009, 11, 2797– 2803,  DOI: 10.1039/b911902g
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      82
      Selective etching of gold nanorods by ferric chloride at room temperature
      Zou, Renxian; Guo, Xia; Yang, Jian; Li, Dandan; Peng, Feng; Zhang, Lei; Wang, Hongjuan; Yu, Hao
      CrystEngComm (2009), 11 (12), 2797-2803CODEN: CRECF4; ISSN:1466-8033. (Royal Society of Chemistry)
      The chem. stability of gold nanorods in different environments is important for a variety of practical applications, because the tiny change in shape and size could directly affect the optical properties of the gold nanorods. Herein we report a chem. etching reaction of the gold nanorods induced by ferric chloride at room temp. In the chem. etching, halide ions act as a ligand to reduce the electron potential of the gold species, which enables ferric ions to oxidize the gold nanorods and results in the etching of the gold nanorods. The redox etching leads to a significant decrease of the gold nanorods in length but little change in diam., which could be attributed to less surface passivation or higher chem. reactivity of the tips of the gold nanorods. It is believed that the selective shortening not only provides an alternative way to obtain the gold nanorods with desirable aspect ratios and specific optical properties, but also offers a safe way to remove gold nanostructures, which is important for the prepn. of hollow nanostructures with gold as a template.
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    83. 83
      Zhao, J.; Nguyen, S. C.; Ye, R.; Ye, B. H.; Weller, H.; Somorjai, G. A.; Alivisatos, A. P.; Toste, F. D. A Comparison of Photocatalytic Activities of Gold Nanoparticles Following Plasmonic and Interband Excitation and a Strategy for Harnessing Interband Hot Carriers for Solution Phase Photocatalysis. ACS Cent. Sci. 2017, 3, 482– 488,  DOI: 10.1021/acscentsci.7b00122
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      83
      A Comparison of Photocatalytic Activities of Gold Nanoparticles Following Plasmonic and Interband Excitation and a Strategy for Harnessing Interband Hot Carriers for Solution Phase Photocatalysis
      Zhao, Jie; Nguyen, Son C.; Ye, Rong; Ye, Baihua; Weller, Horst; Somorjai, Gabor A.; Alivisatos, A. Paul; Toste, F. Dean
      ACS Central Science (2017), 3 (5), 482-488CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      Light driven excitation of gold nanoparticles (GNPs) has emerged as a potential strategy to generate hot carriers for photocatalysis through excitation of localized surface plasmon resonance (LSPR). In contrast, carrier generation through excitation of interband transitions remains a less explored and underestimated pathway for photocatalytic activity. Photoinduced oxidative etching of GNPs with FeCl3 was investigated as a model reaction in order to elucidate the effects of both types of transitions. The quant. results show that interband transitions more efficiently generate hot carriers and that those carriers exhibit higher reactivity as compared to those generated solely by LSPR. Further, leveraging the strong π-acidic character of the resulting photogenerated Au+ hole, an interband transition induced cyclization reaction of alkynylphenols was developed. Notably, alkyne coordination to the Au+ hole intercepts the classic oxidn. event and leads to the formation of the catalytically active gold clusters on subnanometer scale.
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    84. 84
      Zijlstra, P.; Orrit, M. Single Metal Nanoparticles: Optical Detection, Spectroscopy and Applications. Rep. Prog. Phys. 2011, 74, 106401,  DOI: 10.1088/0034-4885/74/10/106401
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      Single metal nanoparticles: optical detection, spectroscopy and applications
      Zijlstra, P.; Orrit, M.
      Reports on Progress in Physics (2011), 74 (10), 106401/1-106401/55CODEN: RPPHAG; ISSN:0034-4885. (Institute of Physics Publishing)
      A review. Since the first report on the far-field optical detection of single metal nanoparticles in the late 1990s, the field has rapidly developed and new methods and concepts have been introduced. Eliminating averaging over the broad size, shape and crystallinity distributions produced by even the best of current synthesis methods, these techniques have proven extremely useful for gaining a deeper insight into many of the properties of metal nanoparticles. These techniques have already led to the first applications specifically directed at using single particles. In this review the authors describe far-field optical techniques (both linear and nonlinear) that have sufficient sensitivity to detect single metal particles. They further discuss emerging applications, and emphasize the importance of single-particle detection techniques in their development.
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    85. 85
      Weigel, A.; Sebesta, A.; Kukura, P. Dark Field Microspectroscopy with Single Molecule Fluorescence Sensitivity. ACS Photonics 2014, 1, 848– 856,  DOI: 10.1021/ph500138u
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      85
      Dark field microspectroscopy with single molecule fluorescence sensitivity
      Weigel, Alexander; Sebesta, Aleksandar; Kukura, Philipp
      ACS Photonics (2014), 1 (9), 848-856CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Dark field microscopy directly detects scattering from a sample by rejecting excitation light. The technique has been extensively used for spectral characterization of nanoscopic particles, but its sensitivity has been limited by residual stray light. Here, the authors present a simple geometry based on wide field illumination under normal incidence capable of background suppression by more than seven orders of magnitude. The setup is optimized for spectrally resolved wide-field detection with white light illumination. They record images and spectra of single 10 nm gold particles binding to a functionalized surface, demonstrating a more than two order of magnitude improvement in sensitivity over the current state of the art. Their level of stray light rejection allows one to record single mol. fluorescence images with broadband excitation without any filters in the detection path. The approach is ideally suited for investigations of truly nanoscopic objects with applications in single mol. and nanoparticle spectroscopy, plasmonic sensing, and ultrafast spectroscopy.
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    86. 86
      Chang, W. S.; Link, S. Enhancing the Sensitivity of Single-Particle Photothermal Imaging with Thermotropic Liquid Crystals. J. Phys. Chem. Lett. 2012, 3, 1393– 1399,  DOI: 10.1021/jz300342p
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      86
      Enhancing the Sensitivity of Single-Particle Photothermal Imaging with Thermotropic Liquid Crystals
      Chang, Wei-Shun; Link, Stephan
      Journal of Physical Chemistry Letters (2012), 3 (10), 1393-1399CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Individual mols. and nanoparticles can be imaged based on their absorption using photothermal microscopy. This technique relies on the heating-induced changes in the refractive index of the surrounding medium. Here, the authors demonstrate an order of magnitude larger enhancement of the signal-to-noise ratio in photothermal imaging of 20 nm gold nanoparticles when using a thermotropic liq. crystal (5CB). The authors show quant. that this increase is due to the large change in the thermooptical properties of 5CB mainly along the nematic director. Enhancing the sensitivity is important for the further development of absorption-based single-mol. spectroscopy techniques.
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    87. 87
      Parra-Vasquez, A. N. G.; Oudjedi, L.; Cognet, L.; Lounis, B. Nanoscale Thermotropic Phase Transitions Enhancing Photothermal Microscopy Signals. J. Phys. Chem. Lett. 2012, 3, 1400– 1403,  DOI: 10.1021/jz300369d
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      87
      Nanoscale Thermotropic Phase Transitions Enhancing Photothermal Microscopy Signals
      Parra-Vasquez, A. Nicholas G.; Oudjedi, Laura; Cognet, Laurent; Lounis, Brahim
      Journal of Physical Chemistry Letters (2012), 3 (10), 1400-1403CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The photothermal heterodyne imaging technique enables studies of individual weakly absorbing nano-objects in various environments. It uses a photoinduced change in the refractive index of the environment. Taking advantage of the dramatic index of refraction change occurring around a thermotropic liq.-cryst. phase transition, a 40-fold signal-to-noise ratio enhancement for Au nanoparticles imaged in 4-cyano-4'-pentylbiphenyl (5CB) liq. crystals over those in a H2O environment is demonstrated. The photothermal signal was studied as a function of probe laser polarization, heating power, and sample temp. quantifying the optimal enhancement. This study established photothermal microscopy as a valuable technique for inducing and/or detecting local phase transitions at the nanometer scales.
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    88. 88
      Ding, T. N. X.; Hou, L.; van der Meer, H.; Alivisatos, A. P.; Orrit, M. Hundreds-Fold Sensitivity Enhancement of Photothermal Microscopy in near-Critical Xenon. J. Phys. Chem. Lett. 2016, 7, 2524– 2529,  DOI: 10.1021/acs.jpclett.6b00964
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      88
      Hundreds-fold Sensitivity Enhancement of Photothermal Microscopy in Near-Critical Xenon
      Ding, Tina X.; Hou, Lei; Meer, Harmen van der; Alivisatos, A. Paul; Orrit, Michel
      Journal of Physical Chemistry Letters (2016), 7 (13), 2524-2529CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Photothermal absorption microscopy of single Au nanoparticles was conducted at temps. and pressures near the crit. point of Xe (Tc = 16.583°, Pc = 5.842 MPa). The divergence of the thermal expansion coeff. at the crit. point makes the refractive index highly sensitive to changes in temp., which directly translates to a large enhancement of the photothermal signal. Measurements taken near the crit. point of Xe give a signal enhancement factor of up to 440 ± 130 over those taken in glycerol. The highest sensitivity recorded here corresponds to power dissipation of 64 pW, achieving a signal-to-noise ratio of 9.4 for 5 nm Au nanoparticles with an integration time of 50 ms, making this the most sensitive of any absorption microscopy technique reported to date. Enhancing the sensitivity of absorption microscopy lowers the operating heating power, allowing the technique to be more compatible with absorbers with absorption coeff. and photochem. stability lower than that of Au.
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    89. 89
      Rodriguez-Fernandez, J.; Perez-Juste, J.; Mulvaney, P.<