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Monodisperse Sub-100 nm Au Nanoshells for Low-Fluence Deep-Tissue Photoacoustic Imaging
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Monodisperse Sub-100 nm Au Nanoshells for Low-Fluence Deep-Tissue Photoacoustic Imaging
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Cite this: Nano Lett. 2023, 23, 16, 7334–7340
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https://doi.org/10.1021/acs.nanolett.3c01696
Published August 4, 2023

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Nanoparticles with high absorption cross sections will advance therapeutic and bioimaging nanomedicine technologies. While Au nanoshells have shown great promise in nanomedicine, state-of-the-art synthesis methods result in scattering-dominant particles, mitigating their efficacy in absorption-based techniques that leverage the photothermal effect, such as photoacoustic (PA) imaging. We introduce a highly reproducible synthesis route to monodisperse sub-100 nm Au nanoshells with an absorption-dominant optical response. Au nanoshells with 48 nm SiO2 cores and 7 nm Au shells show a 14-fold increase in their volumetric absorption coefficient compared to commercial Au nanoshells with dimensions commonly used in nanomedicine. PA imaging with Au nanoshell contrast agents showed a 50% improvement in imaging depth for sub-100 nm Au nanoshells compared with the smallest commercially available nanoshells in a turbid phantom. Furthermore, the high PA signal at low fluences, enabled by sub-100 nm nanoshells, will aid the deployment of low-cost, low-fluence light-emitting diodes for PA imaging.

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Emerging therapeutic and bioimaging technologies leverage nanoparticles with optical resonances in the near-infrared (NIR) wavelength regime, i.e., the biological window, to efficiently target deep tissue. (1) This coupling of nanotechnology and biomedicine is commonly referred to as nanomedicine. (2) Prominent examples include photothermal therapy (PTT), (3) optical coherence tomography (OCT), (4) photoacoustic (PA) imaging, (5) and diffuse optical tomography. (6) Although these techniques rely on absorption and scattering by endogenous tissue components, exogenous contrast agents can significantly augment signal generation. (2) Plasmonic nanoparticles, with their strong light–matter interactions and biocompatibility, e.g., Au, make excellent exogenous agents for these applications. (2) Consequently, Au nanoparticles of different shapes, e.g., nanorods, (7) nanocages, (8) bipyramids, (9) and nanoshells, (10) are promising candidates for nanomedicine. Among Au nanoparticles of different shapes, spherical particles have the lowest surface-to-volume ratio, which can result in lower toxicity. (11) Consequently, several foundational studies in nanomedicine use Au spheres, albeit limited to the visible light regime. Au nanoshells are also spherical, support NIR resonances in the biological window, and have been used in pioneering nanomedicine work. (12−14)

Au nanoshells are inorganic structures with a SiO2 core covered by a thin Au shell. Nanoshells were developed in the late 90s by the Halas group and rely on coupling of the local surface plasmons resonances between the inner and outer surfaces to offer tunable absorption and scattering in the NIR. (10,15) There has been significant progress in using nanoshells for nanomedicine applications. Hirsch et al. used nanoshells to destroy cancer cells via PTT in the early 2000s, (16) and Food and Drug Administration (FDA)-approved clinical trials followed. (17) Despite their promise for nanomedicine, Au nanoshells can be significantly improved for absorption-based applications. (18,19) To date, reported studies use large nanoshells that scatter more than they absorb. (12,13,16,18,20−22) Absorption-dominance in nanoshells requires particles with sub-100 nm diameter given synthetically achievable shell thicknesses. (18,23,24) The lack of examples in the literature of sub-100 nm nanoshells stems from major synthetic challenges resulting from poor particle stability as the core size decreases. (20) Additionally, there are physical limitations on how thin the Au shell can be relative to the SiO2 core. (20) As a result, scattering-dominant nanoshells are pervasive in absorption-based nanomedicine. Figure 1a highlights the advantages of moving from large scattering-dominant nanoshells to smaller absorption-dominant nanoshells for absorption-based nanomedicine techniques such as PA imaging.

Figure 1

Figure 1. (a) Schematic showing that decreasing nanoshell size leads to absorption dominant optical behavior and improved performance in absorption-based nanomedicine techniques. (b) Simulated absorption efficiency of 5 nm Au shells on 20 (green), 50 (purple), 80 (black), and 110 nm (red) SiO2 cores indicates 60 nm overall diameter nanoshells have the highest values in the biological window. (c) Simulated volumetric absorption coefficients of nanoshells of different core diameter with shell thicknesses of 5(blue), 10 (green), and 15 nm (red). The volumetric absorption decreases with both core diameters and shell thickness increase, with nanoshells of sub-100 nm diameters having significantly larger volumetric absorption.

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The benefits of synthesizing sub-100 nm nanoshells include improved light absorption and improved transport in tissue. (11) The improved light absorption can be seen in Figure 1b and Figure 1c, which compare nanoshells of different sizes using Mie theory simulations; Figure 1b shows that nanoshells with an overall diameter of 60 nm, made from a 50 nm core and a 5 nm shell, have higher absorption efficiency than the larger nanoshells commonly seen in the literature. Figure 1c shows that sub-100 nm nanoshells can have significantly higher volumetric absorption than larger diameter nanoshells. Higher volumetric absorption stems from increased absorption efficiency and the number of particles that can fit a given volume as the dimensions decrease. Higher volumetric absorption contributes to more signal generation in absorption-based nanomedicine. Beyond benefiting the absorption properties, reducing the nanoshell size results in improved cellular uptake, (11) evidenced by studies on spherical gold nanoparticles which show that smaller diameters near 50 nm are better internalized by cells. (25) Particles that are too small have higher energy requirements, while larger particles diffuse slowly. (11) Additionally, particles near 50 nm have more optimal clearance pathways and have fewer long-term toxicological implications due to higher clearance rates. (11,26)

Here, we overcome the optical tunability limits of nanoshells and optimize them for absorption-based applications by developing a synthesis method to decrease their overall diameter to less than 100 nm. The smallest nanoshells we synthesized have a 48 nm SiO2 core diameter and a 7 nm Au shell thickness, with a total diameter of 62 nm. These nanoshells are absorption dominant and achieve a 14-fold increase in the volumetric absorption coefficient compared to commercial Au nanoshells with dimensions commonly used in nanomedicine. We show the direct implication of their optimized absorption profile by comparing their performance as PA imaging contrast agents with conventional (i.e., >100 nm diameter) nanoshells, and we show that sub-100 nm Au nanoshells have improved performance with a 50% increase in PA imaging depth in a turbid phantom when compared to commercial nanoshells.

The first step in the synthesis is the preparation of SiO2 core particles and Au seed particles. The SiO2 core particles react with 3-aminopropyltriethoxysilane (APTES) to obtain sites where the Au seeds adsorb in the next step. Before the Au adsorption step, centrifugation cleaning cycles ensure that the nucleation sites for Au shell growth are on the SiO2 surface. These cleaning cycles eliminate excess APTES, thus preventing the seeds from attaching to APTES in the solution. After seeding, additional cleaning cycles to remove unbound Au seeds are performed. The final step in the synthesis is the Au shell growth on seeded SiO2. We used formaldehyde as the reducing agent in a K2CO3-aged HAuCl4 medium in the presence of NH4OH. The inception of NH4OH is the major novelty of our process, and it is discussed further below. Attempts to synthesize sub-100 nm nanoshells without the addition of NH4OH yielded poor results. Previous attempts at modifying the synthesis process did not allow the consistent synthesis of absorption-dominant sub-100 nm nanoshells. (27−29)

To consistently synthesize sub-100 nm nanoshells, it is necessary to overcome key challenges during the steps listed above. Of these challenges, improving the shell growth step is paramount. Shell growth for sub-100 nm nanoshells is more difficult than conventional large nanoshells due to the decrease in particle stability as the core size decreases. (20) Sub-100 nm nanoshells also require thinner shells to achieve NIR resonance. (24,30,31) The core-to-shell ratio dictates resonance wavelength and is highly sensitive to changes in shell thicknesses at small core diameters. (24,30,31) Therefore, Au shell thickness precision is critical at smaller size regimes. Departure from traditional nanoshell synthesis recipes by adding NH4OH during shell growth allowed us to synthesize sub-100 nm nanoshells consistently.

The introduction of NH4OH during Au shell growth was inspired by its role as a stabilizer during the Stober synthesis of SiO2. (32) The addition of NH4OH promotes the formation of hydrogen bonding to the surface of the seeded silica particles through residual THPC ligands on the gold seeds. Consequently, there is an improvement in the hydrophilicity of the particles, leading to improved stability. Figure 2 confirms that this addition does indeed prevent agglomeration. Extinction spectra in Figure 2a, without NH4OH, and Figure 2d, with NH4OH, show that adding NH4OH reduced the full width at half-maximum (fwhm) of the nanoshell extinction spectra. Electron micrographs of drop-cast samples from the two suspensions further support the conclusion that adding NH4OH reduces the agglomeration of the nanoshells. Figure 2b,c shows that in the absence of NH4OH, significant agglomeration of the nanoshells occurs. The Au shell growth is uncontrollable in the absence of NH4OH, with significant Au growth in solution; by contrast, adding NH4OH results in improved shell growth and negligible agglomeration, as highlighted in Figure 2e and Figure 2f. The micrographs also indicate that NH4OH inhibits the formation of Au in the solution. Adding NH4OH raises the pH to >10.1, where Au(OH)4– is the major species present. (33) Out of the possible gold complexes that can be present, Au(OH)4– has the lowest redox potential and slowest reaction rate making Au growth in solution less likely. (33) The Supporting Information provides a complete discussion of the strategies we implemented to address issues at the different steps in the synthesis.

Figure 2

Figure 2. Effect of NH4OH addition during Au shell growth. Extinction spectra of Au nanoshell colloids grown (a) without NH4OH addition and (d) with 6.88 mM NH4OH; electron micrographs of drop cast Au nanoshells (b, c) without NH4OH and (e, f) with 6.88 mM NH4OH. The addition of NH4OH resulted in improved shell growth with narrower peak bandwidth and less aggregation.

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Following the above experimental guidelines, we synthesized scattering-dominant and absorption-dominant nanoshells. The electron micrographs in Figure 3a and Figure 3b highlight their size differences. The scattering-dominant nanoshells in Figure 3a have an 80 nm core diameter and an 11 nm shell. In comparison, the absorption-dominant nanoshells in Figure 3b have a 48 nm core and a 7 nm Au shell. We calculated the shell thickness from the difference in the diameters before and after shell growth using electron micrographs and a disk centrifuge photosedimentometer (see Figure S1). The extinction spectra from the two nanoshell dimensions are nearly overlapping; see Figure 3c, which enables us to better compare their absorption and scattering fractions.

Figure 3

Figure 3. Electron micrographs and optical properties of scattering- and absorption-dominant nanoshells. Electron micrographs of drop-cast nanoshells with outer diameters of (a) 102 nm and (b) 62 nm. (c) Nearly overlapping extinction spectra from suspensions of nanoshells in (a) and (b). (d, e) Absorption and scattering for overall diameters: (d) 102 nm scattering dominant nanoshells, (e) 62 nm absorption dominant nanoshells; (f) absorption comparison for 102 nm vs 62 nm nanoshells, where 62 nm nanoshells have higher and more redshifted absorption.

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To experimentally show how the nanoshell dimensions affect their optical behavior, Figure 3d,e shows the spectra of the nanoshells shown in the micrographs. The larger nanoshells are confirmed to be scattering, while the smaller nanoshells are absorption dominant. Typical spectrophotometric measurements on colloidal samples measure extinction only via light transmission to the detector through the sample. Extinction is the summation of absorbed and scattered light. We separate the nanoshell absorption and scattering effects using an integrating sphere with a center-mounted cuvette, enabling the detector to collect light from all directions. Our method involves a two-step measurement (see Supporting Information Figure S4). In the first step, we allow transmitted and scattered light to reach the detector. In the second step, a light trap opposite the entrance of the integrating sphere prevents transmitted light from reaching the detector. The second measurement provides the scattered fraction. The difference between the two measurements determines the transmitted fraction. We then calculate the absorbed fraction using Kirchoff‘s rule, which states that the absorbed, scattered, and transmitted fractions sum to 1.

As predicted by Mie’s theory in Figure 1b, smaller nanoshells absorb light more efficiently than larger nanoshells. Table 1 shows the measured maximum volumetric absorption in the NIR of four nanoshells with varying dimensions: the two synthesized nanoshells shown above, and two commercial nanoshells. The commercial nanoshell with a 118 nm core and 15 nm shell represents the most commonly used dimensions in absorption-based nanomedicine reports, (12,13,15,16) while 81 nm core diameter and 20 nm shell were the smallest commercially available nanoshells. According to Bohren and Huffman, volumetric absorption, defined as normalized absorption cross section per particle volume, is the most practical way to measure efficiency toward applications. (30,34) From Table 1, the 62 nm absorption dominant nanoshells have the highest volumetric absorption, which is 14-fold larger than the volumetric absorption of the nanoshells used in the literature.

Table 1. Measured Maximum Volumetric Absorption of Two Synthesized Nanoshells Compared to Two Larger Commercial Nanoshells
core diameter (nm)shell thickness (nm)total diameter (nm)λabsa (nm)Vabsb (μm–1)
48 ± 5762 ± 5686304.08
80 ± 711102 ± 767667.49
81 ± 820125 ± 965023.54
118 ± 415147 ± 782321.80
a

λabs represents the maximum NIR absorption wavelength.

b

Vabs represents the maximum volumetric absorption.

To better understand the role of scattering and absorption in nanomedicine-enabled bioimaging modalities, we tested absorption-dominant and scattering-dominant nanoshells as exogenous contrast agents in PA imaging. Motivated by prior work using plasmonic nanoparticles as contrast agents for PA imaging, we chose PA as the model application. (35) PA imaging employs the absorption of nanosecond pulsed light to generate ultrasound waves via the thermoelastic effect. (36) PA signal generation is governed by the equation below:

PA=ΓηthμaF
(1)
where Γ is the Grüneisen parameter, ηth is the thermal conversion efficiency, μa is the absorption coefficient, and F is the local fluence.

A comparison between absorption-dominant and scattering dominant nanoshells was conducted at the same extinction optical density of 1 to better elucidate the role of light absorption. Note that the optical density was confirmed using a UV/vis/NIR spectrometer and prior to imaging with an in-house plate reader.

The photostability of the absorption-dominant particles was first assessed, and the PA signal was linear up to fluence values of 125 mJ/cm2, with a lack of hysteresis in the signal as the fluence returned to 40 mJ/cm2 (Figure S7). We measured the PA imaging depth for the different Au nanoshells by placing the nanoshell dispersions in a tube placed diagonally under a turbid phantom, e.g., 1 wt % aqueous dispersion of 1 μm diameter polystyrene spheres. The sketch in Figure 4a details the experimental setup for the PA depth measurements. The diagonal orientation of the tube allows the optical path to be varied by adjusting the imaging plane (IP) along the length of the tube. Parts b–e of Figure 4 are the PA images collected from the tube carrying the 62 nm (parts b and d) and 102 nm (parts c and e) nanoshells at depths of 3 cm (parts d and e) and 6 cm (parts b and c), respectively. Figure 4f plots the PA signal generated by the Au nanoshells at different depths within the turbid phantom. The mean PA signal generated by the absorption-dominant nanoshells outperforms the scattering dominant nanoshells at all imaging depths. PA signals generated at a depth of 4.5 cm from the absorption-dominant particles are similar to those generated by the scattering particles at a shallower depth of 3 cm. This 50% increase in imaging depth motivates the use of the absorbing particles for deep tissue imaging and their ability to generate higher PA signals at lower fluence values.

Figure 4

Figure 4. Exploring maximum imaging depth with Au nanoshells in a turbid phantom of suspended polystyrene spheres (PS). A tube carrying the Au nanoshells was placed diagonally under a turbid phantom. This allows the optical path to be varied by adjusting the imaging plane (IP) along the length of the tube. (b)–(e) describe the PA images collected from the tube carrying the 62 nm (b, d) and 102 nm (c, e) nanoshells at depths of 3 cm (d, e) and 6 cm (b, c), respectively. (f) Plot of PA image signal generated by the Au nanoshells at different depths within the turbid phantom. PA signals generated by the 62 nm at a depth of 4.5 cm are similar to those generated at a depth of 3 cm by the 102 nm Au nanoshells.

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Improved PA imaging performance at low fluences has several merits. Laser fluence decreases significantly as tissue depth increases. (37,38) For example, Raijan et al. report a 4-order magnitude change in fluence at 3 cm tissue depth. (39) In a scenario where fluence values drop significantly, absorption dominant nanoshells can enhance contrast. Additionally, PA systems are moving toward light-emitting diodes (LED) and pulse laser diodes (PLD) as inexpensive alternatives to commonly used solid-state light sources. (40−42) These systems can translate better to clinical applications since they are less bulky and affordable. (43) However, PLD and LED light sources operate at lower fluence values, which can result in poor PA image quality. (44) Absorption-dominant nanoshells can prove beneficial in such applications.

Figure 5a–c compares the photoacoustic performance of our sub-100 nm nanoshells to the smallest commercially available nanoshells (NANOCOMPOSIX) at low fluences of 2 mJ/cm2, again restricting the comparison to nanoshells with similar extinction peaks. Figure 5d–f also shows the respective sizes, absorption, and scattering spectra for the various nanoshells. Here we highlight the trend of photoacoustic signals at low fluences, showing a positive correlation between photoacoustic signals and absorption. Figure 5a–c shows a gradual increase in the photoacoustic signal as the corresponding absorption shown in Figure 5d–f increases. The commercial nanoshells of 81 nm core and 20 nm shells have the lowest photoacoustic signals and absorb the least light; see Figure 5a,d. The poor image quality in Figure 5a results from a low contrast-to-noise ratio attributed to a negligible PA signal from 2 mJ/cm2, the minimum fluence that PA images could be obtained using commercial nanoshells. On the other hand, absorption-dominant nanoshells show a significantly higher PA signal (Figure 5c) and consequently 50% improvement in PA imaging depth over the smallest commercial Au nanoshell demonstrated in Figure 4f.

Figure 5

Figure 5. Comparison of low fluence photoacoustic performance and optical properties for sub-100 nm synthesized nanoshells to the smallest available commercial nanoshells. (a–c) PA images of the highest signals at 2 mJ/cm2 for (a) commercial nanoshells, (b) scattering-nanoshells, and (c) sub-100 nm absorption-nanoshells. (d–f) Absorption and scattering spectra of nanoshells, with dimensions shown in the inset: (d) commercial, 81 nm core and 20 nm shell; (e) scattering-dominant nanoshells, 80 nm core and 11 nm shell; (f) absorption-dominant nanoshells, 48 nm core and 7 nm shell.

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In summary, we have shown how to overcome the current limitations of Au nanoshells, addressing issues at all stages of the synthesis process and leading to the successful realization of absorption-dominant sub-100 nm nanoshells. The synthesized sub-100 nm nanoshells resulted in a 14-fold increase in volumetric absorption coefficient compared to commercial Au nanoshells with dimensions commonly used in absorption-based nanomedicine. Furthermore, we demonstrated the benefits of absorption dominant Au nanoshells on PA imaging by testing sub-100 nm nanoshells and conventional scattering-dominant nanoshells. Our results showed that absorption-dominant sub-100 nm nanoshells outperform conventional scattering nanoshells at low fluences. Consequently, sub-100 nm nanoshells can yield a 50% increase in PA imaging depth in turbid phantoms, as compared to the smallest commercially available nanoshell, and facilitate the use of low-cost PA light sources. Similar studies could be extended to other absorption-based nanomedicine applications that rely on the photothermal effect.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.3c01696.

  • Materials, nanoshell synthesis procedure and challenges, size analysis results, and optical and photoacoustic characterization methods including setup schemes (PDF)

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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
    • Luis D. B. Manuel - Gordon and Mary Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States;  Orcidhttps://orcid.org/0000-0002-9314-6396
    • Vinoin Devpaul Vincely - Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana 70118, United States
    • Carolyn L. Bayer - Department of Biomedical Engineering, Tulane University, New Orleans, Louisiana 70118, United States;  Orcidhttps://orcid.org/0000-0003-0947-6892
  • Author Contributions

    K.M.M. and L.D.B.M conceived the idea. L.D.B.M synthesized the Au nanoshells, characterized their size and optical properties, and performed the Mie theory calculations. C.L.B. and V.D.V. designed the photoacoustic imaging experiments with input from L.D.B.M and K.M.M. V.D.V. performed the photoacoustic imaging experiments and analyzed the results. L.D.B.M. wrote the manuscript with contributions from all coauthors. K.M.M and C.L.B. supervised and edited the manuscript. All authors have given approval to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors acknowledge Dr. Daniel E. Willis for helpful discussions on the integrating sphere, Dr. James A. Dorman for allowing us access to his PerkinElmer Lambda 900 UV/vis/NIR spectrometer, Jonathan Ellis at CPS Instruments for assistance with our disk centrifuge, and the LSU Chemical Engineering machine shop staff Nick Lombardo, Joe Bell, and Alan Nguyen.

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  6. 6
    Pogue, B. W.; Patterson, M. S.; Jiang, H.; Paulsen, K. D. Initial Assessment of a Simple System for Frequency Domain Diffuse Optical Tomography. Phys. Med. Biol. 1995, 40 (10), 1709,  DOI: 10.1088/0031-9155/40/10/011
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    Initial assessment of a simple system for frequency domain diffuse optical tomography
    Pogue B W; Patterson M S; Jiang H; Paulsen K D
    Physics in medicine and biology (1995), 40 (10), 1709-29 ISSN:0031-9155.
    Diffuse optical tomography is an imaging technique whereby spatial maps of absorption and scattering coefficients are derived from the characteristics of multiply scattered light transmitted through the object. The system described here used four intensity-modulated light sources and measurements of the intensity and phase (relative to each source) at 16 or 20 detectors on the surface of a 10 cm diameter cylinder. An iterative Newton-Raphson algorithm was used to estimate the absorption and scattering coefficients at each pixel in a 17 x 17 array minimizing the difference between measured and calculated values of the intensity and phase at the measurement sites. Forward calculations of the intensity and phase were based on a multigrid finite-difference solution of the frequency domain diffusion equation. Numerical simulations were used to examine the resolution, contrast, and accuracy of the reconstructions as well as the effects of measurement noise, systematic uncertainties in source-detector location, and accuracy of the initial estimates for the optical properties. Experimental tests also confirmed that the system could identify and locate both scattering and absorbing inhomogeneities in a tissue-simulating phantom.
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    Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. Gold Nanorods: Electrochemical Synthesis and Optical Properties. J. Phys. Chem. B 1997, 101 (34), 6661– 6664,  DOI: 10.1021/jp971656q
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    Gold nanorods: electrochemical synthesis and optical properties
    Yu, Yu-Ying; Chang, Ser-Sing; Lee, Chien-Liang; Wang, C. R. Chris
    Journal of Physical Chemistry B (1997), 101 (34), 6661-6664CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Aq. solns. contg. a high yield of suspended gold nanorods were successfully synthesized via an electrochem. method. The control of prepg. gold nanorods with different aspect ratios can be attained. Their absorption spectral features show a dominant surface plasma band corresponding to the longitudinal resonance, SPl, and its λmax shifts markedly to the red as the aspect ratio is increased. Meanwhile, the dependence of λmax for longitudinal resonance on the mean aspect ratio shows a deviation from the classical electrostatic model prediction at mean aspect ratios around 4 ± 1, where it limits the validity of the classical electrostatic approxn.
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    Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science (80-.) 2002, 298 (5601), 2176– 2179,  DOI: 10.1126/science.1077229
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    8
    Shape-Controlled Synthesis of Gold and Silver Nanoparticles
    Sun, Yugang; Xia, Younan
    Science (Washington, DC, United States) (2002), 298 (5601), 2176-2179CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in detg. the geometric shape and size of the product. The silver cubes could serve as sacrificial templates to generate single-cryst. nanoboxes of gold: hollow polyhedra bounded by six {100} and eight {111} facets. Controlling the size, shape, and structure of metal nanoparticles is technol. important because of the strong correlation between these parameters and optical, elec., and catalytic properties.
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    Liu, M.; Guyot-Sionnest, P. Mechanism of Silver (I)-Assisted Growth of Gold Nanorods and Bipyramids. J. Phys. Chem. B 2005, 109 (47), 22192– 22200,  DOI: 10.1021/jp054808n
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    Mechanism of Silver(I)-Assisted Growth of Gold Nanorods and Bipyramids
    Liu, Mingzhao; Guyot-Sionnest, Philippe
    Journal of Physical Chemistry B (2005), 109 (47), 22192-22200CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The seed-mediated growth of gold nanostructures is strongly dependent on the gold seed nanocrystal structure. The gold seed solns. can be prepd. such that the seeds are either single cryst. or multiply twinned. With added silver(I) in the cetyltrimethylammonium bromide (CTAB) aq. growth solns., the two types of seeds yield either nanorods or elongated bipyramidal nanoparticles, in good yields. The gold nanorods are single cryst., with a structure similar to those synthesized electrochem. (Yu, Y. Y. et al. J. Phys. Chem. B 1997, 101, 6661). In contrast, the gold bipyramids are penta-twinned. These bipyramids are strikingly monodisperse in shape. This leads to the sharpest ensemble longitudinal plasmon resonance reported so far for metal colloid solns., with an inhomogeneous width as narrow as 0.13 eV for a resonance at ∼1.5 eV. Ag(I) plays an essential role in the growth mechanism. Ag(I) slows down the growth of the gold nanostructures. Ag(I) also leads to high-energy side facets that are {110} for the single cryst. gold nanorods and unusually highly stepped {11n} (n ∼ 7) for the bipyramid. To rationalize these observations, it is the underpotential deposition of Ag(I) that leads to the dominance of the facets with the more open surface structures. This forms the basis for the one-dimensional growth mechanism of single crystal nanorods, while it affects the shape of the nanostructures growing along a single twinning axis.
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  10. 10
    Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Nanoengineering of Optical Resonances. Chem. Phys. Lett. 1998, 288 (2), 243– 247,  DOI: 10.1016/S0009-2614(98)00277-2
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    10
    Nanoengineering of optical resonances
    Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J.
    Chemical Physics Letters (1998), 288 (2,3,4), 243-247CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    Metal nanoshells, consisting of a dielec. core with a metallic shell of nanometer thickness, are a new, composite nanoparticle whose optical resonance can be ''designed in'' in a controlled manner. By varying the relative dimensions of the core and shell, the optical resonance of these nanoparticles can be varied over hundreds of nanometers in wavelength, across the visible and into the IR region of the spectrum. The authors report a general approach to the making of metal nanoshell composite nanoparticles based on mol. self-assembly and colloid redn. chem.
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  11. 11
    Li, B.; Lane, L. A. Probing the Biological Obstacles of Nanomedicine with Gold Nanoparticles. Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 2019, 11 (3), e1542– e1542a,  DOI: 10.1002/wnan.1542
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    There is no corresponding record for this reference.
  12. 12
    Wang, Y.; Xie, X.; Wang, X.; Ku, G.; Gill, K. L.; O’Neal, D. P.; Stoica, G.; Wang, L. V. Photoacoustic Tomography of a Nanoshell Contrast Agent in the in Vivo Rat Brain. Nano Lett. 2004, 4 (9), 1689– 1692,  DOI: 10.1021/nl049126a
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    12
    Photoacoustic Tomography of a Nanoshell Contrast Agent in the in Vivo Rat Brain
    Wang, Yiwen; Xie, Xueyi; Wang, Xueding; Ku, Geng; Gill, Kelly L.; O'Neal, D. Patrick; Stoica, George; Wang, Lihong V.
    Nano Letters (2004), 4 (9), 1689-1692CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    This study demonstrates the feasibility of using nanoshells in vivo as a new contrast-enhancing agent for photoacoustic tomog. Deep penetrating near-IR light was employed to image the in vivo distribution of poly(ethylene glycol)-coated nanoshells circulating in the vasculature of a rat brain. The images, captured after three sequential administrations of nanoshells, present a gradual enhancement of the optical absorption in the brain vessels by up to 63%. Subsequent clearance of the nanoshells from the blood was imaged for ∼6 h after the administrations.
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  13. 13
    Gobin, A. M.; Lee, M. H.; Halas, N. J.; James, W. D.; Drezek, R. A.; West, J. L. Near-Infrared Resonant Nanoshells for Combined Optical Imaging and Photothermal Cancer Therapy. Nano Lett. 2007, 7 (7), 1929– 1934,  DOI: 10.1021/nl070610y
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    13
    Near-Infrared Resonant Nanoshells for Combined Optical Imaging and Photothermal Cancer Therapy
    Gobin, Andre M.; Lee, Min Ho; Halas, Naomi J.; James, William D.; Drezek, Rebekah A.; West, Jennifer L.
    Nano Letters (2007), 7 (7), 1929-1934CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Metal nanoshells are core/shell nanoparticles that can be designed to either strongly absorb or scatter within the near-IR (NIR) wavelength region (∼650-950 nm). Nanoshells were designed that possess both absorption and scattering properties in the NIR to provide optical contrast for improved diagnostic imaging and, at higher light intensity, rapid heating for photothermal therapy. Using these in a mouse model, the authors have demonstrated dramatic contrast enhancement for optical coherence tomog. (OCT) and effective photothermal ablation of tumors.
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  14. 14
    Wu, C.; Liang, X.; Jiang, H. Metal Nanoshells as a Contrast Agent in Near-Infrared Diffuse Optical Tomography. Opt. Commun. 2005, 253 (1), 214– 221,  DOI: 10.1016/j.optcom.2005.04.057
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    14
    Metal nanoshells as a contrast agent in near-infrared diffuse optical tomography
    Wu, Changfeng; Liang, Xiaoping; Jiang, Huabei
    Optics Communications (2005), 253 (1-3), 214-221CODEN: OPCOB8; ISSN:0030-4018. (Elsevier B.V.)
    The plasma resonance peak of metal nanoshells can be tuned to the near-IR region (700-900 nm), making them have great potential in biol. and biomedical applications. Gold nanoshells has been synthesized and characterized as a contrast agent for diffuse optical tomog. A unique advantage of the nanoshells is their tremendous absorptivity. Spectral measurements indicate that the absorption cross-section of each nanoshell is 40,000 times larger than that of an Indocyanine Green (ICG) mol., suggesting that the nanoshells are a much more efficient absorption agent than ICG mols. Tissue-like phantom expts. using the nanoshells were performed using our diffuse optical imaging system and absorption images were successfully obtained through a finite element based reconstruction algorithm.
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  15. 15
    Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. A Hybridization Model for the Plasmon Response of Complex Nanostructures. Science 2003, 302 (5644), 419– 422,  DOI: 10.1126/science.1089171
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    15
    A hybridization model for the plasmon response of complex nanostructures
    Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P.
    Science (Washington, DC, United States) (2003), 302 (5644), 419-422CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The authors present a simple and intuitive picture, an electromagnetic analog of MO theory, that describes the plasmon response of complex nanostructures of arbitrary shape. The authors' model can be understood as the interaction or hybridization of elementary plasmons supported by nanostructures of elementary geometries. As an example, the approach is applied to the important case of a four-layer concentric nanoshell, where the hybridization of the plasmons of the inner and outer nanoshells dets. the resonant frequencies of the multilayer nanostructure.
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  16. 16
    Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Nanoshell-Mediated near-Infrared Thermal Therapy of Tumors under Magnetic Resonance Guidance. Proc. Natl. Acad. Sci. U. S. A. 2003, 100 (23), 13549– 13554,  DOI: 10.1073/pnas.2232479100
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    16
    Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance
    Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L.
    Proceedings of the National Academy of Sciences of the United States of America (2003), 100 (23), 13549-13554CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Metal nanoshells are a class of nanoparticles with tunable optical resonances. In this article, an application of this technol. to thermal ablative therapy for cancer is described. By tuning the nanoshells to strongly absorb light in the near IR, where optical transmission through tissue is optimal, a distribution of nanoshells at depth in tissue can be used to deliver a therapeutic dose of heat by using moderately low exposures of extracorporeally applied near-IR (NIR) light. Human breast carcinoma cells incubated with nanoshells in vitro were found to have undergone photothermally induced morbidity on exposure to NIR light (820 nm, 35 W/cm2), as detd. by using a fluorescent viability stain. Cells without nanoshells displayed no loss in viability after the same periods and conditions of NIR illumination. Likewise, in vivo studies under magnetic resonance guidance revealed that exposure to low doses of NIR light (820 nm, 4 W/cm2) in solid tumors treated with metal nanoshells reached av. max. temps. capable of inducing irreversible tissue damage (ΔT = 37.4 ± 6.6°C) within 4-6 min. Controls treated without nanoshells demonstrated significantly lower av. temps. on exposure to NIR light (ΔT < 10°C). These findings demonstrated good correlation with histol. findings. Tissues heated above the thermal damage threshold displayed coagulation, cell shrinkage, and loss of nuclear staining, which are indicators of irreversible thermal damage. Control tissues appeared undamaged.
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  17. 17
    Rastinehad, A. R.; Anastos, H.; Wajswol, E.; Winoker, J. S.; Sfakianos, J. P.; Doppalapudi, S. K.; Carrick, M. R.; Knauer, C. J.; Taouli, B.; Lewis, S. C.; Tewari, A. K.; Schwartz, J. A.; Canfield, S. E.; George, A. K.; West, J. L.; Halas, N. J. Gold Nanoshell-Localized Photothermal Ablation of Prostate Tumors in a Clinical Pilot Device Study. Proc. Natl. Acad. Sci. U. S. A. 2019, 116 (37), 18590– 18596,  DOI: 10.1073/pnas.1906929116
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    17
    Gold nanoshell-localized photothermal ablation of prostate tumors in a clinical pilot device study
    Rastinehad, Ardeshir R.; Anastos, Harry; Wajswol, Ethan; Winoker, Jared S.; Sfakianos, John P.; Doppalapudi, Sai K.; Carrick, Michael R.; Knauer, Cynthia J.; Taouli, Bachir; Lewis, Sara C.; Tewari, Ashutosh K.; Schwartz, Jon A.; Canfield, Steven E.; George, Arvin K.; West, Jennifer L.; Halas, Naomi J.
    Proceedings of the National Academy of Sciences of the United States of America (2019), 116 (37), 18590-18596CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Biocompatible gold nanoparticles designed to absorb light at wavelengths of high tissue transparency have been of particular interest for biomedical applications. The ability of such nanoparticles to convert absorbed near-IR light to heat and induce highly localized hyperthermia has been shown to be highly effective for photothermal cancer therapy, resulting in cell death and tumor remission in a multitude of preclin. animal models. Here we report the initial results of a clin. trial in which laser-excited gold-silica nanoshells (GSNs) were used in combination with magnetic resonance ultrasound fusion imaging to focally ablate low-intermediate-grade tumors within the prostate. The overall goal is to provide highly localized regional control of prostate cancer that also results in greatly reduced patient morbidity and improved functional outcomes. This pilot device study reports feasibility and safety data from 16 cases of patients diagnosed with low- or intermediate-risk localized prostate cancer. After GSN infusion and high-precision laser ablation, patients underwent multiparametric MRI of the prostate at 48 to 72 h, followed by postprocedure mpMRI/ultrasound targeted fusion biopsies at 3 and 12 mo, as well as a std. 12-core systematic biopsy at 12 mo. GSN-mediated focal laser ablation was successfully achieved in 94% (15/16) of patients, with no significant difference in International Prostate Symptom Score or Sexual Health Inventory for Men obsd. after treatment. This treatment protocol appears to be feasible and safe in men with low- or intermediate-risk localized prostate cancer without serious complications or deleterious changes in genitourinary function.
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  18. 18
    Jauffred, L.; Samadi, A.; Klingberg, H.; Bendix, P. M.; Oddershede, L. B. Plasmonic Heating of Nanostructures. Chem. Rev. 2019, 119 (13), 8087– 8130,  DOI: 10.1021/acs.chemrev.8b00738
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    18
    Plasmonic Heating of Nanostructures
    Jauffred, Liselotte; Samadi, Akbar; Klingberg, Henrik; Bendix, Poul Martin; Oddershede, Lene B.
    Chemical Reviews (Washington, DC, United States) (2019), 119 (13), 8087-8130CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The absorption of light by plasmonic nanostructures and their assocd. temp. increase are exquisitely sensitive to the shape and compn. of the structure and to the wavelength of light. Therefore, much effort is put into synthesizing novel nanostructures for optimized interaction with the incident light. The successful synthesis and characterization of high quality and biocompatible plasmonic colloidal nanoparticles has fostered numerous and expanding applications, esp. in biomedical contexts, where such particles are highly promising for general drug delivery and for tomorrow's cancer treatment. We review the thermoplasmonic properties of the most commonly used plasmonic nanoparticles, including solid or composite metallic nanoparticles of various dimensions and geometries. Common methods for synthesizing plasmonic particles are presented with the overall goal of providing the reader with a guide for designing or choosing nanostructures with optimal thermoplasmonic properties for a given application. Finally, the biocompatibility and biol. tolerance of structures are critically discussed along with novel applications of plasmonic nanoparticles in the life sciences.
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  19. 19
    Jørgensen, J. T.; Norregaard, K.; Tian, P.; Bendix, P. M.; Kjaer, A.; Oddershede, L. B. Single Particle and PET-Based Platform for Identifying Optimal Plasmonic Nano-Heaters for Photothermal Cancer Therapy. Sci. Rep. 2016, 6 (1), 30076,  DOI: 10.1038/srep30076
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    19
    Single Particle and PET-based Platform for Identifying Optimal Plasmonic Nano-Heaters for Photothermal Cancer Therapy
    Jorgensen Jesper Tranekjaer; Norregaard Kamilla; Kjaer Andreas; Norregaard Kamilla; Tian Pengfei; Bendix Poul Martin; Oddershede Lene B; Tian Pengfei
    Scientific reports (2016), 6 (), 30076 ISSN:.
    Plasmonic nanoparticle-based photothermal cancer therapy is a promising new tool to inflict localized and irreversible damage to tumor tissue by hyperthermia, without harming surrounding healthy tissue. We developed a single particle and positron emission tomography (PET)-based platform to quantitatively correlate the heat generation of plasmonic nanoparticles with their potential as cancer killing agents. In vitro, the heat generation and absorption cross-section of single irradiated nanoparticles were quantified using a temperature sensitive lipid-based assay and compared to their theoretically predicted photo-absorption. In vivo, the heat generation of irradiated nanoparticles was evaluated in human tumor xenografts in mice using 2-deoxy-2-[F-18]fluoro-D-glucose ((18)F-FDG) PET imaging. To validate the use of this platform, we quantified the photothermal efficiency of near infrared resonant silica-gold nanoshells (AuNSs) and benchmarked this against the heating of colloidal spherical, solid gold nanoparticles (AuNPs). As expected, both in vitro and in vivo the heat generation of the resonant AuNSs performed superior compared to the non-resonant AuNPs. Furthermore, the results showed that PET imaging could be reliably used to monitor early treatment response of photothermal treatment. This multidisciplinary approach provides a much needed platform to benchmark the emerging plethora of novel plasmonic nanoparticles for their potential for photothermal cancer therapy.
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  20. 20
    Rasch, M. R.; Sokolov, K. V.; Korgel, B. A. Limitations on the Optical Tunability of Small Diameter Gold Nanoshells. Langmuir 2009, 25 (19), 11777– 11785,  DOI: 10.1021/la901249j
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    20
    Limitations on the Optical Tunability of Small Diameter Gold Nanoshells
    Rasch, Michael R.; Sokolov, Konstantin V.; Korgel, Brian A.
    Langmuir (2009), 25 (19), 11777-11785CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Au nanoshells were grown on SiO2 nanoparticles with differing av. diams., ranging from 30 to 120 nm. Au nanoshells were also formed on SiO2 spheres encapsulating 5 nm diam. magnetic Fe oxide nanocrystals. The optical absorbance spectra of these Au nanoshells are reported. The plasmon resonance wavelengths of the smaller diam. nanoshells were significantly less tunable than those of the larger diam. nanoshells. This is due to a reduced range of accessible core-shell ratio, the geometric factor that dets. the plasmon peak position, as the SiO2 core diam. shrinks. The smaller diam. nanoshells also are highly prone to aggregation, which broadens the plasmon absorption peak. Model calcns. of dispersion stability as a function of SiO2 core diam. reveal that smaller diam. Au shells exhibit more aggregation because of the size-dependence of the electrostatic double-layer potential.
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  21. 21
    García-Soto, M. J.; González-Ortega, O. Synthesis of Silica-Core Gold Nanoshells and Some Modifications/Variations. Gold Bull. 2016, 49 (3), 111– 131,  DOI: 10.1007/s13404-016-0188-2
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    21
    Synthesis of silica-core gold nanoshells and some modifications/variations
    Garcia-Soto, Mariano J.; Gonzalez-Ortega, Omar
    Gold Bulletin (Berlin, Germany) (2016), 49 (3-4), 111-131CODEN: GOBUFW; ISSN:2190-7579. (Springer)
    Gold nanoshells are particles usually composed of a spherical silica core coated with a thin gold layer. Their chem. and optical properties make them suitable and attractive for medical applications, namely cancer treatment and diagnosis, as they have been studied for biosensing, imaging, and photothermal ablation. For their synthesis, most of the reported methods are based on the first reported by Oldenburg et al. In this method, silica nanoparticles are first produced and then modified to incorporate amino groups aimed to adsorb small gold clusters, which in turn act as the nucleation sites for the redn. of addnl. gold until complete gold shells are formed. In this review, we examine some common conditions to synthesize gold nanoshells based on this process, along with important aspects that need to be followed to ensure the prodn. of gold nanoshells having homogeneous size and shape that render suspensions with consistent properties in the near-IR region of the electromagnetic spectrum. Since the customary method is laborious and time-consuming, three addnl. processes intended to simplify or reduce some steps are described as well. Finally, fundamental aspects on the chem. of the synthesis and their variations involved in all the revised processes are also presented. Figures from our own findings are included to support these descriptions. Please notice that this review focuses on the synthesis; other reviews focus in optical properties and applications.
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  22. 22
    Ayala-Orozco, C.; Urban, C.; Knight, M. W.; Urban, A. S.; Neumann, O.; Bishnoi, S. W.; Mukherjee, S.; Goodman, A. M.; Charron, H.; Mitchell, T. Au Nanomatryoshkas as Efficient Near-Infrared Photothermal Transducers for Cancer Treatment: Benchmarking against Nanoshells. ACS Nano 2014, 8 (6), 6372– 6381,  DOI: 10.1021/nn501871d
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    22
    Au Nanomatryoshkas as Efficient Near-Infrared Photothermal Transducers for Cancer Treatment: Benchmarking against Nanoshells
    Ayala-Orozco, Ciceron; Urban, Cordula; Knight, Mark W.; Urban, Alexander Skyrme; Neumann, Oara; Bishnoi, Sandra W.; Mukherjee, Shaunak; Goodman, Amanda M.; Charron, Heather; Mitchell, Tamika; Shea, Martin; Roy, Ronita; Nanda, Sarmistha; Schiff, Rachel; Halas, Naomi J.; Joshi, Amit
    ACS Nano (2014), 8 (6), 6372-6381CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Au nanoparticles with plasmon resonances in the near-IR (NIR) region of the spectrum efficiently convert light into heat, a property useful for the photothermal ablation of cancerous tumors subsequent to nanoparticle uptake at the tumor site. A crit. aspect of this process is nanoparticle size, which influences both tumor uptake and photothermal efficiency. Here, we report a direct comparative study of ∼90 nm diam. Au nanomatryoshkas (Au/SiO2/Au) and ∼150 nm diam. Au nanoshells for photothermal therapeutic efficacy in highly aggressive triple neg. breast cancer (TNBC) tumors in mice. Au nanomatryoshkas are strong light absorbers with 77% absorption efficiency, while the nanoshells are weaker absorbers with only 15% absorption efficiency. After an i.v. injection of Au nanomatryoshkas followed by a single NIR laser dose of 2 W/cm2 for 5 min, 83% of the TNBC tumor-bearing mice appeared healthy and tumor free >60 days later, while only 33% of mice treated with nanoshells survived the same period. The smaller size and larger absorption cross section of Au nanomatryoshkas combine to make this nanoparticle more effective than Au nanoshells for photothermal cancer therapy.
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  23. 23
    Hogan, N. J.; Urban, A. S.; Ayala-Orozco, C.; Pimpinelli, A.; Nordlander, P.; Halas, N. J. Nanoparticles Heat through Light Localization. Nano Lett. 2014, 14 (8), 4640– 4645,  DOI: 10.1021/nl5016975
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    23
    Nanoparticles Heat through light localization
    Hogan, Nathaniel J.; Urban, Alexander S.; Ayala-Orozco, Ciceron; Pimpinelli, Alberto; Nordlander, Peter; Halas, Naomi J.
    Nano Letters (2014), 14 (8), 4640-4645CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Aq. solns. contg. light-absorbing nanoparticles have recently been shown to produce steam at high efficiencies upon solar illumination, even when the temp. of the bulk fluid vol. remains far below its b.p. Here we show that this phenomenon is due to a collective effect mediated by multiple light scattering from the dispersed nanoparticles. Randomly positioned nanoparticles that both scatter and absorb light are able to conc. light energy into mesoscale vols. near the illuminated surface of the liq. The resulting light absorption creates intense localized heating and efficient vaporization of the surrounding liq. Light trapping-induced localized heating provides the mechanism for low-temp. light-induced steam generation and is consistent with classical heat transfer.
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  24. 24
    Tuersun, P.; Han, X. Optical Absorption Analysis and Optimization of Gold Nanoshells. Appl. Opt. 2013, 52 (6), 1325– 1329,  DOI: 10.1364/AO.52.001325
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    24
    Optical absorption analysis and optimization of gold nanoshells
    Tuersun, Paerhatijiang; Han, Xiang'e
    Applied Optics (2013), 52 (6), 1325-1329CODEN: APOPAI; ISSN:1559-128X. (Optical Society of America)
    Gold nanoshells, consisting of a nanoscale dielec. core coated with an ultrathin gold shell, have wide biomedical applications due to their strong optical absorption properties. Gold nanoshells with high absorption efficiencies can help to improve these applications. We investigate the effects of the core material, surrounding medium, core radius, and shell thickness on the absorption spectra of gold nanoshells by using the light-scattering theory of a coated sphere. Our results show that the position and intensity of the absorption peak can be tuned over a wide range by manipulating the above-mentioned parameters. We also obtain the optimal absorption efficiencies and structures of hollow gold nanoshells and gold-coated SiO2 nanoshells embedded in water at wavelengths of 800, 820, and 1064 nm. The results show that hollow gold nanoshells possess the max. absorption efficiency (5.42) at a wavelength of 800 nm; the corresponding shell thickness and core radius are 4.8 and 38.9 nm, resp. They can be used as the ideal photothermal conversation particles for biomedical applications.
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  25. 25
    Chithrani, B. D.; Ghazani, A. A.; Chan, W. C. W. Determining the Size and Shape Dependence of Gold Nanoparticle Uptake into Mammalian Cells. Nano Lett. 2006, 6 (4), 662– 668,  DOI: 10.1021/nl052396o
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    25
    Determining the Size and Shape Dependence of Gold Nanoparticle Uptake into Mammalian Cells
    Chithrani, B. Devika; Ghazani, Arezou A.; Chan, Warren C. W.
    Nano Letters (2006), 6 (4), 662-668CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We investigated the intracellular uptake of different sized and shaped colloidal gold nanoparticles. We showed that kinetics and satn. concns. are highly dependent upon the phys. dimensions of the nanoparticles (e.g., uptake half-life of 14, 50, and 74 nm nanoparticles is 2.10, 1.90, and 2.24 h, resp.). The findings from this study will have implications in the chem. design of nanostructures for biomedical applications (e.g., tuning intracellular delivery rates and amts. by nanoscale dimensions and engineering complex, multifunctional nanostructures for imaging and therapeutics).
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  26. 26
    Sykes, E. A.; Chen, J.; Zheng, G.; Chan, W. C. W. Investigating the Impact of Nanoparticle Size on Active and Passive Tumor Targeting Efficiency. ACS Nano 2014, 8 (6), 5696– 5706,  DOI: 10.1021/nn500299p
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    26
    Investigating the Impact of Nanoparticle Size on Active and Passive Tumor Targeting Efficiency
    Sykes, Edward A.; Chen, Juan; Zheng, Gang; Chan, Warren C. W.
    ACS Nano (2014), 8 (6), 5696-5706CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Understanding the principles governing the design of nanoparticles for tumor targeting is essential for the effective diagnosis and treatment of solid tumors. There is currently a poor understanding of how to rationally engineer nanoparticles for tumor targeting. Here, we engineered different-sized spherical gold nanoparticles to discern the effect of particle diam. on passive (poly(ethylene glycol)-coated) and active (transferrin-coated) targeting of MDA-MB-435 orthotopic tumor xenografts. Tumor accumulation of actively targeted nanoparticles was found to be 5 times faster and approx. 2-fold higher relative to their passive counterparts within the 60 nm diam. range. For 15, 30, and 100 nm, we obsd. no significant differences. We hypothesize that such enhancements are the result of an increased capacity to penetrate into tumors and preferentially assoc. with cancer cells. We also use computational modeling to explore the mechanistic parameters that can impact tumor accumulation efficacy. We demonstrate that tumor accumulation can be mediated by high nanoparticle avidity and are weakly dependent on their plasma clearance rate. Such findings suggest that empirical models can be used to rapidly screen novel nanomaterials for relative differences in tumor targeting without the need for animal work. Although our findings are specific to MDA-MB-435 tumor xenografts, our exptl. and computational findings help to enrich knowledge of design considerations that will aid in the optimal engineering of spherical gold nanoparticles for cancer applications in the future.
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  27. 27
    Phonthammachai, N.; Kah, J. C. Y.; Jun, G.; Sheppard, C. J. R.; Olivo, M. C.; Mhaisalkar, S. G.; White, T. J. Synthesis of Contiguous Silica– Gold Core– Shell Structures: Critical Parameters and Processes. Langmuir 2008, 24 (9), 5109– 5112,  DOI: 10.1021/la703580r
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    27
    Synthesis of Contiguous Silica-Gold Core-Shell Structures: Critical Parameters and Processes
    Phonthammachai, Nopphawan; Kah, James C. Y.; Jun, Guo; Sheppard, Colin J. R.; Olivo, Malini C.; Mhaisalkar, Subodh G.; White, Timothy J.
    Langmuir (2008), 24 (9), 5109-5112CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    A direct process for prepg. contiguous Au shells (15-25 nm thick) over amorphous SiO2 spheres (200 nm) is described. In this method, Au seeds are synthesized from HAuCl4 in a dil. NaOH soln. using deposition-pptn. with subsequent metalization by Na borohydride (NaBH4). The ease of dispersing Au nanocrystals on spheres of bare SiO2 and spheres after grafting with NH3 was studied as a function of pH (4-8), reaction temp. (65-96°), and time (5-30 min). Addnl. parameters requiring optimization included the quantity of NaBH4 and the HAuCl4 in K2CO3 soln. to SiO2 vol. ratio. The evolution of Au nanocrystal growth was monitored by TEM, and the bathochromic shift of UV-visible absorption was correlated with shell perfection and thickness.
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  28. 28
    Lim, Y. T.; Park, O. O.; Jung, H.-T. Gold Nanolayer-Encapsulated Silica Particles Synthesized by Surface Seeding and Shell Growing Method: Near Infrared Responsive Materials. J. Colloid Interface Sci. 2003, 263 (2), 449– 453,  DOI: 10.1016/S0021-9797(03)00322-9
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    28
    Gold nanolayer-encapsulated silica particles synthesized by surface seeding and shell growing method: near infrared responsive materials
    Lim, Yong Taik; Park, O. Ok; Jung, Hee-Tae
    Journal of Colloid and Interface Science (2003), 263 (2), 449-453CODEN: JCISA5; ISSN:0021-9797. (Elsevier Science)
    Au nanolayer-encapsulated SiO2 particles whose optical resonance is located in 750-900 nm spectral region were synthesized by combining Sn-surface seeding and a shell growing process. The synthesized composite particles can be potentially used in wide biol. fields, due to biocompatibility and a known bioconjugation technique of Au layer. Sn atoms, which can act not only as a catalytic surface for redn. of Au but also as a linker between SiO2 surface and Au nanoparticles, were chem. deposited on hydroxylated SiO2 particles. Then, the authors introduced another reductant with Au chloride to produce a multilayer of Au shell. In the process, Au shells grew by the redn. of addnl. Au ions on the Sn-functionalized SiO2 surface and resulted in the subsequent coalescence and growth of the deposited Au nanoparticles. Finally, a complete Au nanoshell was formed on the SiO2 surface by the 1-step method, without a repeated coating process. The deposition of a Au nanolayer on the SiO2 particles was easily controlled by the concn. ratio of Sn-functionalized SiO2 particles and Au chloride solns. TEM images and optical extinction spectra clearly showed that Au nanolayers were successfully deposited on the SiO2 surface by the novel method. As the Au colloids attached on the SiO2 surface grew, their optical plasmon peak became red shifted until complete a Au shell was formed. After the Au shell was completed, the optical plasmon resonance became blue-shifted and the extinction spectra were functions of a relative ratio of the core to shell thickness.
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  29. 29
    Abdollahi, S. N.; Naderi, M.; Amoabediny, G. Synthesis and Physicochemical Characterization of Tunable Silica–Gold Nanoshells via Seed Growth Method. Colloids Surfaces A Physicochem. Eng. Asp. 2012, 414, 345– 351,  DOI: 10.1016/j.colsurfa.2012.08.043
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    Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine. J. Phys. Chem. B 2006, 110 (14), 7238– 7248,  DOI: 10.1021/jp057170o
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    30
    Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine
    Jain, Prashant K.; Lee, Kyeong Seok; El-Sayed, Ivan H.; El-Sayed, Mostafa A.
    Journal of Physical Chemistry B (2006), 110 (14), 7238-7248CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The selection of nanoparticles for achieving efficient contrast for biol. and cell imaging applications, as well as for photothermal therapeutic applications, is based on the optical properties of the nanoparticles. The authors use Mie theory and discrete dipole approxn. method to calc. absorption and scattering efficiencies and optical resonance wavelengths for three commonly used classes of nanoparticles: gold nanospheres, silica-gold nanoshells, and gold nanorods. The calcd. spectra clearly reflect the well-known dependence of nanoparticle optical properties, viz. the resonance wavelength, the extinction cross-section, and the ratio of scattering to absorption, on the nanoparticle dimensions. A systematic quant. study of the various trends is presented. By increasing the size of gold nanospheres from 20 to 80 nm, the magnitude of extinction as well as the relative contribution of scattering to the extinction rapidly increases. Gold nanospheres in the size range commonly employed (∼40 nm) show an absorption cross-section 5 orders higher than conventional absorbing dyes, while the magnitude of light scattering by 80-nm gold nanospheres is 5 orders higher than the light emission from strongly fluorescing dyes. The variation in the plasmon wavelength max. of nanospheres, i.e., from ∼520 to 550 nm, is however too limited to be useful for in vivo applications. Gold nanoshells are found to have optical cross-sections comparable to and even higher than the nanospheres. Addnl., their optical resonances lie favorably in the near-IR region. The resonance wavelength can be rapidly increased by either increasing the total nanoshell size or increasing the ratio of the core-to-shell radius. The total extinction of nanoshells shows a linear dependence on their total size, however, it is independent of the core/shell radius ratio. The relative scattering contribution to the extinction can be rapidly increased by increasing the nanoshell size or decreasing the ratio of the core/shell radius. Gold nanorods show optical cross-sections comparable to nanospheres and nanoshells, however, at much smaller effective size. Their optical resonance can be linearly tuned across the near-IR region by changing either the effective size or the aspect ratio of the nanorods. The total extinction as well as the relative scattering contribution increases rapidly with the effective size, however, they are independent of the aspect ratio. To compare the effectiveness of nanoparticles of different sizes for real biomedical applications, size-normalized optical cross-sections or per μ coeffs. are calcd. Gold nanorods show per μ absorption and scattering coeffs. that are an order of magnitude higher than those for nanoshells and nanospheres. While nanorods with a higher aspect ratio along with a smaller effective radius are the best photoabsorbing nanoparticles, the highest scattering contrast for imaging applications is obtained from nanorods of high aspect ratio with a larger effective radius.
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    Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J. Infrared Extinction Properties of Gold Nanoshells. Appl. Phys. Lett. 1999, 75 (19), 2897– 2899,  DOI: 10.1063/1.125183
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    Infrared extinction properties of gold nanoshells
    Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J.
    Applied Physics Letters (1999), 75 (19), 2897-2899CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Au nanoshells, nanoparticles consisting of a SiO2 core coated with a thin Au shell, exhibit a strong optical resonance that depends sensitively on their core radius and shell thickness. Au nanoshells were fabricated with a peak optical extinction that can be varied across the near-IR region of the spectrum (800 nm-2.2 μm). Multipolar plasmon resonances are clearly resolvable in the extinction spectra and agree well with electromagnetic theory. Addnl. resonances due to particle aggregation are also obsd. The frequency agile IR properties of these nanoparticles make them particularly attractive for a range of technol. important applications.
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    Stöber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26 (1), 62– 69,  DOI: 10.1016/0021-9797(68)90272-5
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    Usher, A.; McPhail, D. C.; Brugger, J. A Spectrophotometric Study of Aqueous Au (III) Halide–Hydroxide Complexes at 25–80 C. Geochim. Cosmochim. Acta 2009, 73 (11), 3359– 3380,  DOI: 10.1016/j.gca.2009.01.036
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    A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25-80°C
    Usher, Al; McPhail, D. C.; Brugger, Joel
    Geochimica et Cosmochimica Acta (2009), 73 (11), 3359-3380CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
    The mobility and transport of gold in low-temp. waters and brines is affected by the aq. speciation of gold, which is sensitive in particular to pH, oxidn. and halide concns. UV-Vis spectrophotometry was used to identify and det. the thermodn. properties of Au(III) aq. complexes with chloride, bromide and hydroxide. Au(III) forms stable square planar complexes with hydroxide and halide ligands. Based on systematic changes in the absorption spectra of solns. in three binary systems NaCl-NaBr, NaCl-NaOH and NaBr-NaOH at 25 °C, log dissocn. consts. were derived for the following mixed and end-member halide and hydroxide complexes: [AuCl3Br]-, [AuCl2Br2]-, [AuBr3Cl]- and [AuBr4]-; [AuCl3(OH)]-, [AuCl2(OH)2]-, [AuCl(OH)3]- and [Au(OH)4]-; and [AuBr3(OH)]-, [AuBr2(OH)2]- and [AuBr(OH)3]-. These are the first reported results for the mixed chloride-bromide complexes. Increasing temp. to 80° resulted in an increase in the stability of the mixed chloride-bromide complexes, relative to the end-member chloride and bromide complexes. For the [AuCl(4-n)(OH)n]- series of complexes (n = 0-4), there is an excellent agreement between obtained spectrophotometric results and previous electrochem. results of Chateau et al. (1964). In other expts., the iodide ion (I-) was found to be unstable in the presence of Au(III), oxidizing rapidly to I2(g) and causing Au to ppt. Predicted Au(III) speciation indicates that Au(III) chloride-bromide complexes can be important in transporting gold in brines with high bromide-chloride ratios (e.g., >0.05), under oxidizing (atm.), acidic (pH < 5) conditions. Native gold soly. under atm. oxygen conditions is predicted to increase with decreasing pH in acidic conditions, increasing pH in alk. conditions, increasing chloride, esp. at acid pH, and increasing bromide for bromide/chloride ratios greater than 0.05. Study results increase the understanding of gold aq. geochem., with the potential to lead to new methods for mineral exploration, hydrometallurgy and medicine.
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    Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley & Sons: Hoboken, NJ, 2008.
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    Das, D.; Sharma, A.; Rajendran, P.; Pramanik, M. Another Decade of Photoacoustic Imaging. Phys. Med. Biol. 2021, 66 (5), 05TR01,  DOI: 10.1088/1361-6560/abd669
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    Manohar, S.; Razansky, D. Photoacoustics: A Historical Review. Adv. Opt. photonics 2016, 8 (4), 586– 617,  DOI: 10.1364/AOP.8.000586
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    Patterson, M. S.; Wilson, B. C.; Wyman, D. R. The Propagation of Optical Radiation in Tissue. II: Optical Properties of Tissues and Resulting Fluence Distributions. Lasers Med. Sci. 1991, 6 (4), 379– 390,  DOI: 10.1007/BF02042460
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    Dimofte, A.; Zhu, T. C.; Solonenko, M.; Hahn, S. M. Comparison of Depth Dependence of Light Fluence Rate in Intralipid for Wavelengths between 532-730 Nm. In Proceedings of the 22nd Annual International Conference of the IEEE Engineering in Medicine and Biology Society (Cat. No. 00CH37143); IEEE, 2000; Vol. 2, pp 884– 887.
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    Rajian, J. R.; Li, R.; Wang, P.; Cheng, J.-X. Vibrational Photoacoustic Tomography: Chemical Imaging beyond the Ballistic Regime. J. Phys. Chem. Lett. 2013, 4 (19), 3211– 3215,  DOI: 10.1021/jz401638e
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    Vibrational Photoacoustic Tomography: Chemical Imaging beyond the Ballistic Regime
    Rajian, Justin Rajesh; Li, Rui; Wang, Pu; Cheng, Ji-Xin
    Journal of Physical Chemistry Letters (2013), 4 (19), 3211-3215CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Proof-of-concept of vibrational photoacoustic tomog. is demonstrated with a home-built Raman laser generating greater than 100 mJ of energy per pulse at a 1197 nm wavelength. The authors employed this system for excitation of the second overtone transition of C-H bonds. The vibrational photoacoustic signal from a C-H-rich polyethylene tube phantom placed under 3 cm thick chicken breast tissue was obtained with a signal-to-noise ratio of 2.5. Further, the authors recorded a photoacoustic image of a polyethylene ring placed under 5 mm chicken tissue with excellent contrast. This development opens new opportunities of performing label-free vibrational imaging in the deep tissue regime.
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    Zeng, L.; Liu, G.; Yang, D.; Ji, X. Portable Optical-Resolution Photoacoustic Microscopy with a Pulsed Laser Diode Excitation. Appl. Phys. Lett. 2013, 102 (5), 053704,  DOI: 10.1063/1.4791566
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    Portable optical-resolution photoacoustic microscopy with a pulsed laser diode excitation
    Zeng, Lvming; Liu, Guodong; Yang, Diwu; Ji, Xuanrong
    Applied Physics Letters (2013), 102 (5), 053704/1-053704/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Optical-resoln. photoacoustic microscopy (OR-PAM) was significantly improved in terms of spatial resoln., detection sensitivity, imaging speed, and penetration depth. However, the popularity of OR-PAM system is still limited by the size and cost of solid-state laser excitation. Here, the authors developed a portable laser-diode-based OR-PAM (LD-OR-PAM) system using a pulsed semiconductor laser source, which was operated at 905 ± 15 nm with a pulse energy ≥4.9 μJ. The measured lateral resoln. was improved to ∼1.5 μm from hundreds of microns. The compact and inexpensive natures of LD-OR-PAM would promote the potential clin. applications such as in dermatol. (c) 2013 American Institute of Physics.
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    Roth, K. B.; Neeves, K. B.; Squier, J.; Marr, D. W. M. Imaging of a Linear Diode Bar for an Optical Cell Stretcher. Biomed. Opt. Express 2015, 6 (3), 807– 814,  DOI: 10.1364/BOE.6.000807
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    Imaging of a linear diode bar for an optical cell stretcher
    Roth K B; Marr D W M; Neeves K B; Squier J
    Biomedical optics express (2015), 6 (3), 807-14 ISSN:2156-7085.
    We present a simplified approach for imaging a linear diode bar laser for application as an optical stretcher within a microfluidic geometry. We have recently shown that these linear sources can be used to measure cell mechanical properties; however, the source geometry creates imaging challenges. To minimize intensity losses and simplify implementation within microfluidic systems without the use of expensive objectives, we combine aspheric and cylindrical lenses to create a 1:1 image of the source at the stretcher focal plane and demonstrate effectiveness by measuring the deformation of human red blood cells and neutrophils.
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    Hariri, A.; Lemaster, J.; Wang, J.; Jeevarathinam, A. S.; Chao, D. L.; Jokerst, J. V. The Characterization of an Economic and Portable LED-Based Photoacoustic Imaging System to Facilitate Molecular Imaging. Photoacoustics 2018, 9, 10– 20,  DOI: 10.1016/j.pacs.2017.11.001
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    The characterization of an economic and portable LED-based photoacoustic imaging system to facilitate molecular imaging
    Hariri Ali; Lemaster Jeanne; Wang Junxin; Jeevarathinam AnanthaKrishnan S; Jokerst Jesse V; Chao Daniel L; Jokerst Jesse V; Jokerst Jesse V
    Photoacoustics (2018), 9 (), 10-20 ISSN:2213-5979.
    Photoacoustic imaging (PAI) is a non-invasive, high-resolution hybrid imaging modality that combines optical excitation and ultrasound detection. PAI can image endogenous chromophores (melanin, hemoglobin, etc.) and exogenous contrast agents in different medical applications. However, most current equipment uses sophisticated and complicated OPO lasers with tuning and stability features inconsistent with broad clinical deployment. As the number of applications of PAI in medicine increases, there is an urgent need to make the imaging equipment more compact, portable, and affordable. Here, portable light emitting diode - based photoacoustic imaging (PLED-PAI) was introduced and characterized in terms of system specifications, light source characterizations, photoacoustic spatial/temporal resolution, and penetration. The system uses two LED arrays attached to the sides of a conventional ultrasound transducer. The LED pulse repetition rate is tunable between 1 K Hz, 2 K Hz, 3 K Hz, and 4 K Hz. The axial resolution was 0.268 mm, and the lateral resolution was between 0.55 and 0.59 mm. The system could detect optical absorber (pencil lead) at a depth of 3.2 cm and the detection limits of indocyanine green (ICG) and methylene blue (MB) were 9 μM and 0.78 mM. In vivo imaging of labeled human mesenchymal stem cells was achieved to confirm compatibility with small animal imaging. The characterization we report here may have value to other groups evaluating commercially available photoacoustic imaging equipment.
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    Erfanzadeh, M.; Zhu, Q. Photoacoustic Imaging with Low-Cost Sources; A Review. Photoacoustics 2019, 14, 1– 11,  DOI: 10.1016/j.pacs.2019.01.004
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    Photoacoustic imaging with low-cost sources; A review
    Erfanzadeh Mohsen; Zhu Quing; Zhu Quing
    Photoacoustics (2019), 14 (), 1-11 ISSN:2213-5979.
    Benefitting from advantages of optical and ultrasound imaging, photoacoustic imaging (PAI) has demonstrated potentials in a wide range of medical applications. In order to facilitate clinical applications of PAI and encourage its application in low-resource settings, research on low-cost photoacoustic imaging with inexpensive optical sources has gained attention. Here, we review the advances made in photoacoustic imaging with low-cost sources.
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  44. 44
    Hariri, A.; Alipour, K.; Mantri, Y.; Schulze, J. P.; Jokerst, J. V. Deep Learning Improves Contrast in Low-Fluence Photoacoustic Imaging. Biomed. Opt. Express 2020, 11 (6), 3360– 3373,  DOI: 10.1364/BOE.395683
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    44
    Deep learning improves contrast in low-fluence photoacoustic imaging
    Hariri, Ali; Alipour, Kamran; Mantri, Yash; Schulze, Jurgen P.; Jokerst, Jesse V.
    Biomedical Optics Express (2020), 11 (6), 3360-3373CODEN: BOEICL; ISSN:2156-7085. (Optical Society of America)
    Low fluence illumination sources can facilitate clin. transition of photoacoustic imaging because they are rugged, portable, affordable, and safe. However, these sources also decrease image quality due to their low fluence. Here, we propose a denoising method using a multi-level wavelet-convolutional neural network to map low fluence illumination source images to its corresponding high fluence excitation map. Quant. and qual. results show a significant potential to remove the background noise and preserve the structures of target. Substantial improvements up to 2.20, 2.25, and 4.3-fold for PSNR, SSIM, and CNR metrics were obsd., resp. We also obsd. enhanced contrast (up to 1.76-fold) in an in vivo application using our proposed methods. We suggest that this tool can improve the value of such sources in photoacoustic imaging.
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Published August 4, 2023

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  • Abstract

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    Figure 1

    Figure 1. (a) Schematic showing that decreasing nanoshell size leads to absorption dominant optical behavior and improved performance in absorption-based nanomedicine techniques. (b) Simulated absorption efficiency of 5 nm Au shells on 20 (green), 50 (purple), 80 (black), and 110 nm (red) SiO2 cores indicates 60 nm overall diameter nanoshells have the highest values in the biological window. (c) Simulated volumetric absorption coefficients of nanoshells of different core diameter with shell thicknesses of 5(blue), 10 (green), and 15 nm (red). The volumetric absorption decreases with both core diameters and shell thickness increase, with nanoshells of sub-100 nm diameters having significantly larger volumetric absorption.

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    Figure 2

    Figure 2. Effect of NH4OH addition during Au shell growth. Extinction spectra of Au nanoshell colloids grown (a) without NH4OH addition and (d) with 6.88 mM NH4OH; electron micrographs of drop cast Au nanoshells (b, c) without NH4OH and (e, f) with 6.88 mM NH4OH. The addition of NH4OH resulted in improved shell growth with narrower peak bandwidth and less aggregation.

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    Figure 3

    Figure 3. Electron micrographs and optical properties of scattering- and absorption-dominant nanoshells. Electron micrographs of drop-cast nanoshells with outer diameters of (a) 102 nm and (b) 62 nm. (c) Nearly overlapping extinction spectra from suspensions of nanoshells in (a) and (b). (d, e) Absorption and scattering for overall diameters: (d) 102 nm scattering dominant nanoshells, (e) 62 nm absorption dominant nanoshells; (f) absorption comparison for 102 nm vs 62 nm nanoshells, where 62 nm nanoshells have higher and more redshifted absorption.

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    Figure 4

    Figure 4. Exploring maximum imaging depth with Au nanoshells in a turbid phantom of suspended polystyrene spheres (PS). A tube carrying the Au nanoshells was placed diagonally under a turbid phantom. This allows the optical path to be varied by adjusting the imaging plane (IP) along the length of the tube. (b)–(e) describe the PA images collected from the tube carrying the 62 nm (b, d) and 102 nm (c, e) nanoshells at depths of 3 cm (d, e) and 6 cm (b, c), respectively. (f) Plot of PA image signal generated by the Au nanoshells at different depths within the turbid phantom. PA signals generated by the 62 nm at a depth of 4.5 cm are similar to those generated at a depth of 3 cm by the 102 nm Au nanoshells.

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    Figure 5

    Figure 5. Comparison of low fluence photoacoustic performance and optical properties for sub-100 nm synthesized nanoshells to the smallest available commercial nanoshells. (a–c) PA images of the highest signals at 2 mJ/cm2 for (a) commercial nanoshells, (b) scattering-nanoshells, and (c) sub-100 nm absorption-nanoshells. (d–f) Absorption and scattering spectra of nanoshells, with dimensions shown in the inset: (d) commercial, 81 nm core and 20 nm shell; (e) scattering-dominant nanoshells, 80 nm core and 11 nm shell; (f) absorption-dominant nanoshells, 48 nm core and 7 nm shell.

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      Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance
      Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L.
      Proceedings of the National Academy of Sciences of the United States of America (2003), 100 (23), 13549-13554CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Metal nanoshells are a class of nanoparticles with tunable optical resonances. In this article, an application of this technol. to thermal ablative therapy for cancer is described. By tuning the nanoshells to strongly absorb light in the near IR, where optical transmission through tissue is optimal, a distribution of nanoshells at depth in tissue can be used to deliver a therapeutic dose of heat by using moderately low exposures of extracorporeally applied near-IR (NIR) light. Human breast carcinoma cells incubated with nanoshells in vitro were found to have undergone photothermally induced morbidity on exposure to NIR light (820 nm, 35 W/cm2), as detd. by using a fluorescent viability stain. Cells without nanoshells displayed no loss in viability after the same periods and conditions of NIR illumination. Likewise, in vivo studies under magnetic resonance guidance revealed that exposure to low doses of NIR light (820 nm, 4 W/cm2) in solid tumors treated with metal nanoshells reached av. max. temps. capable of inducing irreversible tissue damage (ΔT = 37.4 ± 6.6°C) within 4-6 min. Controls treated without nanoshells demonstrated significantly lower av. temps. on exposure to NIR light (ΔT < 10°C). These findings demonstrated good correlation with histol. findings. Tissues heated above the thermal damage threshold displayed coagulation, cell shrinkage, and loss of nuclear staining, which are indicators of irreversible thermal damage. Control tissues appeared undamaged.
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    17. 17
      Rastinehad, A. R.; Anastos, H.; Wajswol, E.; Winoker, J. S.; Sfakianos, J. P.; Doppalapudi, S. K.; Carrick, M. R.; Knauer, C. J.; Taouli, B.; Lewis, S. C.; Tewari, A. K.; Schwartz, J. A.; Canfield, S. E.; George, A. K.; West, J. L.; Halas, N. J. Gold Nanoshell-Localized Photothermal Ablation of Prostate Tumors in a Clinical Pilot Device Study. Proc. Natl. Acad. Sci. U. S. A. 2019, 116 (37), 18590– 18596,  DOI: 10.1073/pnas.1906929116
      17
      Gold nanoshell-localized photothermal ablation of prostate tumors in a clinical pilot device study
      Rastinehad, Ardeshir R.; Anastos, Harry; Wajswol, Ethan; Winoker, Jared S.; Sfakianos, John P.; Doppalapudi, Sai K.; Carrick, Michael R.; Knauer, Cynthia J.; Taouli, Bachir; Lewis, Sara C.; Tewari, Ashutosh K.; Schwartz, Jon A.; Canfield, Steven E.; George, Arvin K.; West, Jennifer L.; Halas, Naomi J.
      Proceedings of the National Academy of Sciences of the United States of America (2019), 116 (37), 18590-18596CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Biocompatible gold nanoparticles designed to absorb light at wavelengths of high tissue transparency have been of particular interest for biomedical applications. The ability of such nanoparticles to convert absorbed near-IR light to heat and induce highly localized hyperthermia has been shown to be highly effective for photothermal cancer therapy, resulting in cell death and tumor remission in a multitude of preclin. animal models. Here we report the initial results of a clin. trial in which laser-excited gold-silica nanoshells (GSNs) were used in combination with magnetic resonance ultrasound fusion imaging to focally ablate low-intermediate-grade tumors within the prostate. The overall goal is to provide highly localized regional control of prostate cancer that also results in greatly reduced patient morbidity and improved functional outcomes. This pilot device study reports feasibility and safety data from 16 cases of patients diagnosed with low- or intermediate-risk localized prostate cancer. After GSN infusion and high-precision laser ablation, patients underwent multiparametric MRI of the prostate at 48 to 72 h, followed by postprocedure mpMRI/ultrasound targeted fusion biopsies at 3 and 12 mo, as well as a std. 12-core systematic biopsy at 12 mo. GSN-mediated focal laser ablation was successfully achieved in 94% (15/16) of patients, with no significant difference in International Prostate Symptom Score or Sexual Health Inventory for Men obsd. after treatment. This treatment protocol appears to be feasible and safe in men with low- or intermediate-risk localized prostate cancer without serious complications or deleterious changes in genitourinary function.
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    18. 18
      Jauffred, L.; Samadi, A.; Klingberg, H.; Bendix, P. M.; Oddershede, L. B. Plasmonic Heating of Nanostructures. Chem. Rev. 2019, 119 (13), 8087– 8130,  DOI: 10.1021/acs.chemrev.8b00738
      18
      Plasmonic Heating of Nanostructures
      Jauffred, Liselotte; Samadi, Akbar; Klingberg, Henrik; Bendix, Poul Martin; Oddershede, Lene B.
      Chemical Reviews (Washington, DC, United States) (2019), 119 (13), 8087-8130CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The absorption of light by plasmonic nanostructures and their assocd. temp. increase are exquisitely sensitive to the shape and compn. of the structure and to the wavelength of light. Therefore, much effort is put into synthesizing novel nanostructures for optimized interaction with the incident light. The successful synthesis and characterization of high quality and biocompatible plasmonic colloidal nanoparticles has fostered numerous and expanding applications, esp. in biomedical contexts, where such particles are highly promising for general drug delivery and for tomorrow's cancer treatment. We review the thermoplasmonic properties of the most commonly used plasmonic nanoparticles, including solid or composite metallic nanoparticles of various dimensions and geometries. Common methods for synthesizing plasmonic particles are presented with the overall goal of providing the reader with a guide for designing or choosing nanostructures with optimal thermoplasmonic properties for a given application. Finally, the biocompatibility and biol. tolerance of structures are critically discussed along with novel applications of plasmonic nanoparticles in the life sciences.
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    19. 19
      Jørgensen, J. T.; Norregaard, K.; Tian, P.; Bendix, P. M.; Kjaer, A.; Oddershede, L. B. Single Particle and PET-Based Platform for Identifying Optimal Plasmonic Nano-Heaters for Photothermal Cancer Therapy. Sci. Rep. 2016, 6 (1), 30076,  DOI: 10.1038/srep30076
      19
      Single Particle and PET-based Platform for Identifying Optimal Plasmonic Nano-Heaters for Photothermal Cancer Therapy
      Jorgensen Jesper Tranekjaer; Norregaard Kamilla; Kjaer Andreas; Norregaard Kamilla; Tian Pengfei; Bendix Poul Martin; Oddershede Lene B; Tian Pengfei
      Scientific reports (2016), 6 (), 30076 ISSN:.
      Plasmonic nanoparticle-based photothermal cancer therapy is a promising new tool to inflict localized and irreversible damage to tumor tissue by hyperthermia, without harming surrounding healthy tissue. We developed a single particle and positron emission tomography (PET)-based platform to quantitatively correlate the heat generation of plasmonic nanoparticles with their potential as cancer killing agents. In vitro, the heat generation and absorption cross-section of single irradiated nanoparticles were quantified using a temperature sensitive lipid-based assay and compared to their theoretically predicted photo-absorption. In vivo, the heat generation of irradiated nanoparticles was evaluated in human tumor xenografts in mice using 2-deoxy-2-[F-18]fluoro-D-glucose ((18)F-FDG) PET imaging. To validate the use of this platform, we quantified the photothermal efficiency of near infrared resonant silica-gold nanoshells (AuNSs) and benchmarked this against the heating of colloidal spherical, solid gold nanoparticles (AuNPs). As expected, both in vitro and in vivo the heat generation of the resonant AuNSs performed superior compared to the non-resonant AuNPs. Furthermore, the results showed that PET imaging could be reliably used to monitor early treatment response of photothermal treatment. This multidisciplinary approach provides a much needed platform to benchmark the emerging plethora of novel plasmonic nanoparticles for their potential for photothermal cancer therapy.
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    20. 20
      Rasch, M. R.; Sokolov, K. V.; Korgel, B. A. Limitations on the Optical Tunability of Small Diameter Gold Nanoshells. Langmuir 2009, 25 (19), 11777– 11785,  DOI: 10.1021/la901249j
      20
      Limitations on the Optical Tunability of Small Diameter Gold Nanoshells
      Rasch, Michael R.; Sokolov, Konstantin V.; Korgel, Brian A.
      Langmuir (2009), 25 (19), 11777-11785CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Au nanoshells were grown on SiO2 nanoparticles with differing av. diams., ranging from 30 to 120 nm. Au nanoshells were also formed on SiO2 spheres encapsulating 5 nm diam. magnetic Fe oxide nanocrystals. The optical absorbance spectra of these Au nanoshells are reported. The plasmon resonance wavelengths of the smaller diam. nanoshells were significantly less tunable than those of the larger diam. nanoshells. This is due to a reduced range of accessible core-shell ratio, the geometric factor that dets. the plasmon peak position, as the SiO2 core diam. shrinks. The smaller diam. nanoshells also are highly prone to aggregation, which broadens the plasmon absorption peak. Model calcns. of dispersion stability as a function of SiO2 core diam. reveal that smaller diam. Au shells exhibit more aggregation because of the size-dependence of the electrostatic double-layer potential.
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    21. 21
      García-Soto, M. J.; González-Ortega, O. Synthesis of Silica-Core Gold Nanoshells and Some Modifications/Variations. Gold Bull. 2016, 49 (3), 111– 131,  DOI: 10.1007/s13404-016-0188-2
      21
      Synthesis of silica-core gold nanoshells and some modifications/variations
      Garcia-Soto, Mariano J.; Gonzalez-Ortega, Omar
      Gold Bulletin (Berlin, Germany) (2016), 49 (3-4), 111-131CODEN: GOBUFW; ISSN:2190-7579. (Springer)
      Gold nanoshells are particles usually composed of a spherical silica core coated with a thin gold layer. Their chem. and optical properties make them suitable and attractive for medical applications, namely cancer treatment and diagnosis, as they have been studied for biosensing, imaging, and photothermal ablation. For their synthesis, most of the reported methods are based on the first reported by Oldenburg et al. In this method, silica nanoparticles are first produced and then modified to incorporate amino groups aimed to adsorb small gold clusters, which in turn act as the nucleation sites for the redn. of addnl. gold until complete gold shells are formed. In this review, we examine some common conditions to synthesize gold nanoshells based on this process, along with important aspects that need to be followed to ensure the prodn. of gold nanoshells having homogeneous size and shape that render suspensions with consistent properties in the near-IR region of the electromagnetic spectrum. Since the customary method is laborious and time-consuming, three addnl. processes intended to simplify or reduce some steps are described as well. Finally, fundamental aspects on the chem. of the synthesis and their variations involved in all the revised processes are also presented. Figures from our own findings are included to support these descriptions. Please notice that this review focuses on the synthesis; other reviews focus in optical properties and applications.
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    22. 22
      Ayala-Orozco, C.; Urban, C.; Knight, M. W.; Urban, A. S.; Neumann, O.; Bishnoi, S. W.; Mukherjee, S.; Goodman, A. M.; Charron, H.; Mitchell, T. Au Nanomatryoshkas as Efficient Near-Infrared Photothermal Transducers for Cancer Treatment: Benchmarking against Nanoshells. ACS Nano 2014, 8 (6), 6372– 6381,  DOI: 10.1021/nn501871d
      22
      Au Nanomatryoshkas as Efficient Near-Infrared Photothermal Transducers for Cancer Treatment: Benchmarking against Nanoshells
      Ayala-Orozco, Ciceron; Urban, Cordula; Knight, Mark W.; Urban, Alexander Skyrme; Neumann, Oara; Bishnoi, Sandra W.; Mukherjee, Shaunak; Goodman, Amanda M.; Charron, Heather; Mitchell, Tamika; Shea, Martin; Roy, Ronita; Nanda, Sarmistha; Schiff, Rachel; Halas, Naomi J.; Joshi, Amit
      ACS Nano (2014), 8 (6), 6372-6381CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Au nanoparticles with plasmon resonances in the near-IR (NIR) region of the spectrum efficiently convert light into heat, a property useful for the photothermal ablation of cancerous tumors subsequent to nanoparticle uptake at the tumor site. A crit. aspect of this process is nanoparticle size, which influences both tumor uptake and photothermal efficiency. Here, we report a direct comparative study of ∼90 nm diam. Au nanomatryoshkas (Au/SiO2/Au) and ∼150 nm diam. Au nanoshells for photothermal therapeutic efficacy in highly aggressive triple neg. breast cancer (TNBC) tumors in mice. Au nanomatryoshkas are strong light absorbers with 77% absorption efficiency, while the nanoshells are weaker absorbers with only 15% absorption efficiency. After an i.v. injection of Au nanomatryoshkas followed by a single NIR laser dose of 2 W/cm2 for 5 min, 83% of the TNBC tumor-bearing mice appeared healthy and tumor free >60 days later, while only 33% of mice treated with nanoshells survived the same period. The smaller size and larger absorption cross section of Au nanomatryoshkas combine to make this nanoparticle more effective than Au nanoshells for photothermal cancer therapy.
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    23. 23
      Hogan, N. J.; Urban, A. S.; Ayala-Orozco, C.; Pimpinelli, A.; Nordlander, P.; Halas, N. J. Nanoparticles Heat through Light Localization. Nano Lett. 2014, 14 (8), 4640– 4645,  DOI: 10.1021/nl5016975
      23
      Nanoparticles Heat through light localization
      Hogan, Nathaniel J.; Urban, Alexander S.; Ayala-Orozco, Ciceron; Pimpinelli, Alberto; Nordlander, Peter; Halas, Naomi J.
      Nano Letters (2014), 14 (8), 4640-4645CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Aq. solns. contg. light-absorbing nanoparticles have recently been shown to produce steam at high efficiencies upon solar illumination, even when the temp. of the bulk fluid vol. remains far below its b.p. Here we show that this phenomenon is due to a collective effect mediated by multiple light scattering from the dispersed nanoparticles. Randomly positioned nanoparticles that both scatter and absorb light are able to conc. light energy into mesoscale vols. near the illuminated surface of the liq. The resulting light absorption creates intense localized heating and efficient vaporization of the surrounding liq. Light trapping-induced localized heating provides the mechanism for low-temp. light-induced steam generation and is consistent with classical heat transfer.
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    24. 24
      Tuersun, P.; Han, X. Optical Absorption Analysis and Optimization of Gold Nanoshells. Appl. Opt. 2013, 52 (6), 1325– 1329,  DOI: 10.1364/AO.52.001325
      24
      Optical absorption analysis and optimization of gold nanoshells
      Tuersun, Paerhatijiang; Han, Xiang'e
      Applied Optics (2013), 52 (6), 1325-1329CODEN: APOPAI; ISSN:1559-128X. (Optical Society of America)
      Gold nanoshells, consisting of a nanoscale dielec. core coated with an ultrathin gold shell, have wide biomedical applications due to their strong optical absorption properties. Gold nanoshells with high absorption efficiencies can help to improve these applications. We investigate the effects of the core material, surrounding medium, core radius, and shell thickness on the absorption spectra of gold nanoshells by using the light-scattering theory of a coated sphere. Our results show that the position and intensity of the absorption peak can be tuned over a wide range by manipulating the above-mentioned parameters. We also obtain the optimal absorption efficiencies and structures of hollow gold nanoshells and gold-coated SiO2 nanoshells embedded in water at wavelengths of 800, 820, and 1064 nm. The results show that hollow gold nanoshells possess the max. absorption efficiency (5.42) at a wavelength of 800 nm; the corresponding shell thickness and core radius are 4.8 and 38.9 nm, resp. They can be used as the ideal photothermal conversation particles for biomedical applications.
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    25. 25
      Chithrani, B. D.; Ghazani, A. A.; Chan, W. C. W. Determining the Size and Shape Dependence of Gold Nanoparticle Uptake into Mammalian Cells. Nano Lett. 2006, 6 (4), 662– 668,  DOI: 10.1021/nl052396o
      25
      Determining the Size and Shape Dependence of Gold Nanoparticle Uptake into Mammalian Cells
      Chithrani, B. Devika; Ghazani, Arezou A.; Chan, Warren C. W.
      Nano Letters (2006), 6 (4), 662-668CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We investigated the intracellular uptake of different sized and shaped colloidal gold nanoparticles. We showed that kinetics and satn. concns. are highly dependent upon the phys. dimensions of the nanoparticles (e.g., uptake half-life of 14, 50, and 74 nm nanoparticles is 2.10, 1.90, and 2.24 h, resp.). The findings from this study will have implications in the chem. design of nanostructures for biomedical applications (e.g., tuning intracellular delivery rates and amts. by nanoscale dimensions and engineering complex, multifunctional nanostructures for imaging and therapeutics).
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    26. 26
      Sykes, E. A.; Chen, J.; Zheng, G.; Chan, W. C. W. Investigating the Impact of Nanoparticle Size on Active and Passive Tumor Targeting Efficiency. ACS Nano 2014, 8 (6), 5696– 5706,  DOI: 10.1021/nn500299p
      26
      Investigating the Impact of Nanoparticle Size on Active and Passive Tumor Targeting Efficiency
      Sykes, Edward A.; Chen, Juan; Zheng, Gang; Chan, Warren C. W.
      ACS Nano (2014), 8 (6), 5696-5706CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Understanding the principles governing the design of nanoparticles for tumor targeting is essential for the effective diagnosis and treatment of solid tumors. There is currently a poor understanding of how to rationally engineer nanoparticles for tumor targeting. Here, we engineered different-sized spherical gold nanoparticles to discern the effect of particle diam. on passive (poly(ethylene glycol)-coated) and active (transferrin-coated) targeting of MDA-MB-435 orthotopic tumor xenografts. Tumor accumulation of actively targeted nanoparticles was found to be 5 times faster and approx. 2-fold higher relative to their passive counterparts within the 60 nm diam. range. For 15, 30, and 100 nm, we obsd. no significant differences. We hypothesize that such enhancements are the result of an increased capacity to penetrate into tumors and preferentially assoc. with cancer cells. We also use computational modeling to explore the mechanistic parameters that can impact tumor accumulation efficacy. We demonstrate that tumor accumulation can be mediated by high nanoparticle avidity and are weakly dependent on their plasma clearance rate. Such findings suggest that empirical models can be used to rapidly screen novel nanomaterials for relative differences in tumor targeting without the need for animal work. Although our findings are specific to MDA-MB-435 tumor xenografts, our exptl. and computational findings help to enrich knowledge of design considerations that will aid in the optimal engineering of spherical gold nanoparticles for cancer applications in the future.
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    27. 27
      Phonthammachai, N.; Kah, J. C. Y.; Jun, G.; Sheppard, C. J. R.; Olivo, M. C.; Mhaisalkar, S. G.; White, T. J. Synthesis of Contiguous Silica– Gold Core– Shell Structures: Critical Parameters and Processes. Langmuir 2008, 24 (9), 5109– 5112,  DOI: 10.1021/la703580r
      27
      Synthesis of Contiguous Silica-Gold Core-Shell Structures: Critical Parameters and Processes
      Phonthammachai, Nopphawan; Kah, James C. Y.; Jun, Guo; Sheppard, Colin J. R.; Olivo, Malini C.; Mhaisalkar, Subodh G.; White, Timothy J.
      Langmuir (2008), 24 (9), 5109-5112CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      A direct process for prepg. contiguous Au shells (15-25 nm thick) over amorphous SiO2 spheres (200 nm) is described. In this method, Au seeds are synthesized from HAuCl4 in a dil. NaOH soln. using deposition-pptn. with subsequent metalization by Na borohydride (NaBH4). The ease of dispersing Au nanocrystals on spheres of bare SiO2 and spheres after grafting with NH3 was studied as a function of pH (4-8), reaction temp. (65-96°), and time (5-30 min). Addnl. parameters requiring optimization included the quantity of NaBH4 and the HAuCl4 in K2CO3 soln. to SiO2 vol. ratio. The evolution of Au nanocrystal growth was monitored by TEM, and the bathochromic shift of UV-visible absorption was correlated with shell perfection and thickness.
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    28. 28
      Lim, Y. T.; Park, O. O.; Jung, H.-T. Gold Nanolayer-Encapsulated Silica Particles Synthesized by Surface Seeding and Shell Growing Method: Near Infrared Responsive Materials. J. Colloid Interface Sci. 2003, 263 (2), 449– 453,  DOI: 10.1016/S0021-9797(03)00322-9
      28
      Gold nanolayer-encapsulated silica particles synthesized by surface seeding and shell growing method: near infrared responsive materials
      Lim, Yong Taik; Park, O. Ok; Jung, Hee-Tae
      Journal of Colloid and Interface Science (2003), 263 (2), 449-453CODEN: JCISA5; ISSN:0021-9797. (Elsevier Science)
      Au nanolayer-encapsulated SiO2 particles whose optical resonance is located in 750-900 nm spectral region were synthesized by combining Sn-surface seeding and a shell growing process. The synthesized composite particles can be potentially used in wide biol. fields, due to biocompatibility and a known bioconjugation technique of Au layer. Sn atoms, which can act not only as a catalytic surface for redn. of Au but also as a linker between SiO2 surface and Au nanoparticles, were chem. deposited on hydroxylated SiO2 particles. Then, the authors introduced another reductant with Au chloride to produce a multilayer of Au shell. In the process, Au shells grew by the redn. of addnl. Au ions on the Sn-functionalized SiO2 surface and resulted in the subsequent coalescence and growth of the deposited Au nanoparticles. Finally, a complete Au nanoshell was formed on the SiO2 surface by the 1-step method, without a repeated coating process. The deposition of a Au nanolayer on the SiO2 particles was easily controlled by the concn. ratio of Sn-functionalized SiO2 particles and Au chloride solns. TEM images and optical extinction spectra clearly showed that Au nanolayers were successfully deposited on the SiO2 surface by the novel method. As the Au colloids attached on the SiO2 surface grew, their optical plasmon peak became red shifted until complete a Au shell was formed. After the Au shell was completed, the optical plasmon resonance became blue-shifted and the extinction spectra were functions of a relative ratio of the core to shell thickness.
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    29. 29
      Abdollahi, S. N.; Naderi, M.; Amoabediny, G. Synthesis and Physicochemical Characterization of Tunable Silica–Gold Nanoshells via Seed Growth Method. Colloids Surfaces A Physicochem. Eng. Asp. 2012, 414, 345– 351,  DOI: 10.1016/j.colsurfa.2012.08.043
      There is no corresponding record for this reference.
    30. 30
      Jain, P. K.; Lee, K. S.; El-Sayed, I. H.; El-Sayed, M. A. Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine. J. Phys. Chem. B 2006, 110 (14), 7238– 7248,  DOI: 10.1021/jp057170o
      30
      Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine
      Jain, Prashant K.; Lee, Kyeong Seok; El-Sayed, Ivan H.; El-Sayed, Mostafa A.
      Journal of Physical Chemistry B (2006), 110 (14), 7238-7248CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The selection of nanoparticles for achieving efficient contrast for biol. and cell imaging applications, as well as for photothermal therapeutic applications, is based on the optical properties of the nanoparticles. The authors use Mie theory and discrete dipole approxn. method to calc. absorption and scattering efficiencies and optical resonance wavelengths for three commonly used classes of nanoparticles: gold nanospheres, silica-gold nanoshells, and gold nanorods. The calcd. spectra clearly reflect the well-known dependence of nanoparticle optical properties, viz. the resonance wavelength, the extinction cross-section, and the ratio of scattering to absorption, on the nanoparticle dimensions. A systematic quant. study of the various trends is presented. By increasing the size of gold nanospheres from 20 to 80 nm, the magnitude of extinction as well as the relative contribution of scattering to the extinction rapidly increases. Gold nanospheres in the size range commonly employed (∼40 nm) show an absorption cross-section 5 orders higher than conventional absorbing dyes, while the magnitude of light scattering by 80-nm gold nanospheres is 5 orders higher than the light emission from strongly fluorescing dyes. The variation in the plasmon wavelength max. of nanospheres, i.e., from ∼520 to 550 nm, is however too limited to be useful for in vivo applications. Gold nanoshells are found to have optical cross-sections comparable to and even higher than the nanospheres. Addnl., their optical resonances lie favorably in the near-IR region. The resonance wavelength can be rapidly increased by either increasing the total nanoshell size or increasing the ratio of the core-to-shell radius. The total extinction of nanoshells shows a linear dependence on their total size, however, it is independent of the core/shell radius ratio. The relative scattering contribution to the extinction can be rapidly increased by increasing the nanoshell size or decreasing the ratio of the core/shell radius. Gold nanorods show optical cross-sections comparable to nanospheres and nanoshells, however, at much smaller effective size. Their optical resonance can be linearly tuned across the near-IR region by changing either the effective size or the aspect ratio of the nanorods. The total extinction as well as the relative scattering contribution increases rapidly with the effective size, however, they are independent of the aspect ratio. To compare the effectiveness of nanoparticles of different sizes for real biomedical applications, size-normalized optical cross-sections or per μ coeffs. are calcd. Gold nanorods show per μ absorption and scattering coeffs. that are an order of magnitude higher than those for nanoshells and nanospheres. While nanorods with a higher aspect ratio along with a smaller effective radius are the best photoabsorbing nanoparticles, the highest scattering contrast for imaging applications is obtained from nanorods of high aspect ratio with a larger effective radius.
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    31. 31
      Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J. Infrared Extinction Properties of Gold Nanoshells. Appl. Phys. Lett. 1999, 75 (19), 2897– 2899,  DOI: 10.1063/1.125183
      31
      Infrared extinction properties of gold nanoshells
      Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J.
      Applied Physics Letters (1999), 75 (19), 2897-2899CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Au nanoshells, nanoparticles consisting of a SiO2 core coated with a thin Au shell, exhibit a strong optical resonance that depends sensitively on their core radius and shell thickness. Au nanoshells were fabricated with a peak optical extinction that can be varied across the near-IR region of the spectrum (800 nm-2.2 μm). Multipolar plasmon resonances are clearly resolvable in the extinction spectra and agree well with electromagnetic theory. Addnl. resonances due to particle aggregation are also obsd. The frequency agile IR properties of these nanoparticles make them particularly attractive for a range of technol. important applications.
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    32. 32
      Stöber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26 (1), 62– 69,  DOI: 10.1016/0021-9797(68)90272-5
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      Usher, A.; McPhail, D. C.; Brugger, J. A Spectrophotometric Study of Aqueous Au (III) Halide–Hydroxide Complexes at 25–80 C. Geochim. Cosmochim. Acta 2009, 73 (11), 3359– 3380,  DOI: 10.1016/j.gca.2009.01.036
      33
      A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25-80°C
      Usher, Al; McPhail, D. C.; Brugger, Joel
      Geochimica et Cosmochimica Acta (2009), 73 (11), 3359-3380CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
      The mobility and transport of gold in low-temp. waters and brines is affected by the aq. speciation of gold, which is sensitive in particular to pH, oxidn. and halide concns. UV-Vis spectrophotometry was used to identify and det. the thermodn. properties of Au(III) aq. complexes with chloride, bromide and hydroxide. Au(III) forms stable square planar complexes with hydroxide and halide ligands. Based on systematic changes in the absorption spectra of solns. in three binary systems NaCl-NaBr, NaCl-NaOH and NaBr-NaOH at 25 °C, log dissocn. consts. were derived for the following mixed and end-member halide and hydroxide complexes: [AuCl3Br]-, [AuCl2Br2]-, [AuBr3Cl]- and [AuBr4]-; [AuCl3(OH)]-, [AuCl2(OH)2]-, [AuCl(OH)3]- and [Au(OH)4]-; and [AuBr3(OH)]-, [AuBr2(OH)2]- and [AuBr(OH)3]-. These are the first reported results for the mixed chloride-bromide complexes. Increasing temp. to 80° resulted in an increase in the stability of the mixed chloride-bromide complexes, relative to the end-member chloride and bromide complexes. For the [AuCl(4-n)(OH)n]- series of complexes (n = 0-4), there is an excellent agreement between obtained spectrophotometric results and previous electrochem. results of Chateau et al. (1964). In other expts., the iodide ion (I-) was found to be unstable in the presence of Au(III), oxidizing rapidly to I2(g) and causing Au to ppt. Predicted Au(III) speciation indicates that Au(III) chloride-bromide complexes can be important in transporting gold in brines with high bromide-chloride ratios (e.g., >0.05), under oxidizing (atm.), acidic (pH < 5) conditions. Native gold soly. under atm. oxygen conditions is predicted to increase with decreasing pH in acidic conditions, increasing pH in alk. conditions, increasing chloride, esp. at acid pH, and increasing bromide for bromide/chloride ratios greater than 0.05. Study results increase the understanding of gold aq. geochem., with the potential to lead to new methods for mineral exploration, hydrometallurgy and medicine.
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    34. 34
      Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; John Wiley & Sons: Hoboken, NJ, 2008.
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      Das, D.; Sharma, A.; Rajendran, P.; Pramanik, M. Another Decade of Photoacoustic Imaging. Phys. Med. Biol. 2021, 66 (5), 05TR01,  DOI: 10.1088/1361-6560/abd669
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      Manohar, S.; Razansky, D. Photoacoustics: A Historical Review. Adv. Opt. photonics 2016, 8 (4), 586– 617,  DOI: 10.1364/AOP.8.000586
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      Patterson, M. S.; Wilson, B. C.; Wyman, D. R. The Propagation of Optical Radiation in Tissue. II: Optical Properties of Tissues and Resulting Fluence Distributions. Lasers Med. Sci. 1991, 6 (4), 379– 390,  DOI: 10.1007/BF02042460
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      Dimofte, A.; Zhu, T. C.; Solonenko, M.; Hahn, S. M. Comparison of Depth Dependence of Light Fluence Rate in Intralipid for Wavelengths between 532-730 Nm. In Proceedings of the 22nd Annual International Conference of the IEEE Engineering in Medicine and Biology Society (Cat. No. 00CH37143); IEEE, 2000; Vol. 2, pp 884– 887.
      There is no corresponding record for this reference.
    39. 39
      Rajian, J. R.; Li, R.; Wang, P.; Cheng, J.-X. Vibrational Photoacoustic Tomography: Chemical Imaging beyond the Ballistic Regime. J. Phys. Chem. Lett. 2013, 4 (19), 3211– 3215,  DOI: 10.1021/jz401638e
      39
      Vibrational Photoacoustic Tomography: Chemical Imaging beyond the Ballistic Regime
      Rajian, Justin Rajesh; Li, Rui; Wang, Pu; Cheng, Ji-Xin
      Journal of Physical Chemistry Letters (2013), 4 (19), 3211-3215CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Proof-of-concept of vibrational photoacoustic tomog. is demonstrated with a home-built Raman laser generating greater than 100 mJ of energy per pulse at a 1197 nm wavelength. The authors employed this system for excitation of the second overtone transition of C-H bonds. The vibrational photoacoustic signal from a C-H-rich polyethylene tube phantom placed under 3 cm thick chicken breast tissue was obtained with a signal-to-noise ratio of 2.5. Further, the authors recorded a photoacoustic image of a polyethylene ring placed under 5 mm chicken tissue with excellent contrast. This development opens new opportunities of performing label-free vibrational imaging in the deep tissue regime.
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    40. 40
      Zeng, L.; Liu, G.; Yang, D.; Ji, X. Portable Optical-Resolution Photoacoustic Microscopy with a Pulsed Laser Diode Excitation. Appl. Phys. Lett. 2013, 102 (5), 053704,  DOI: 10.1063/1.4791566
      40
      Portable optical-resolution photoacoustic microscopy with a pulsed laser diode excitation
      Zeng, Lvming; Liu, Guodong; Yang, Diwu; Ji, Xuanrong
      Applied Physics Letters (2013), 102 (5), 053704/1-053704/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Optical-resoln. photoacoustic microscopy (OR-PAM) was significantly improved in terms of spatial resoln., detection sensitivity, imaging speed, and penetration depth. However, the popularity of OR-PAM system is still limited by the size and cost of solid-state laser excitation. Here, the authors developed a portable laser-diode-based OR-PAM (LD-OR-PAM) system using a pulsed semiconductor laser source, which was operated at 905 ± 15 nm with a pulse energy ≥4.9 μJ. The measured lateral resoln. was improved to ∼1.5 μm from hundreds of microns. The compact and inexpensive natures of LD-OR-PAM would promote the potential clin. applications such as in dermatol. (c) 2013 American Institute of Physics.
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    41. 41
      Roth, K. B.; Neeves, K. B.; Squier, J.; Marr, D. W. M. Imaging of a Linear Diode Bar for an Optical Cell Stretcher. Biomed. Opt. Express 2015, 6 (3), 807– 814,  DOI: 10.1364/BOE.6.000807
      41
      Imaging of a linear diode bar for an optical cell stretcher
      Roth K B; Marr D W M; Neeves K B; Squier J
      Biomedical optics express (2015), 6 (3), 807-14 ISSN:2156-7085.
      We present a simplified approach for imaging a linear diode bar laser for application as an optical stretcher within a microfluidic geometry. We have recently shown that these linear sources can be used to measure cell mechanical properties; however, the source geometry creates imaging challenges. To minimize intensity losses and simplify implementation within microfluidic systems without the use of expensive objectives, we combine aspheric and cylindrical lenses to create a 1:1 image of the source at the stretcher focal plane and demonstrate effectiveness by measuring the deformation of human red blood cells and neutrophils.
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    42. 42
      Hariri, A.; Lemaster, J.; Wang, J.; Jeevarathinam, A. S.; Chao, D. L.; Jokerst, J. V. The Characterization of an Economic and Portable LED-Based Photoacoustic Imaging System to Facilitate Molecular Imaging. Photoacoustics 2018, 9, 10– 20,  DOI: 10.1016/j.pacs.2017.11.001
      42
      The characterization of an economic and portable LED-based photoacoustic imaging system to facilitate molecular imaging
      Hariri Ali; Lemaster Jeanne; Wang Junxin; Jeevarathinam AnanthaKrishnan S; Jokerst Jesse V; Chao Daniel L; Jokerst Jesse V; Jokerst Jesse V
      Photoacoustics (2018), 9 (), 10-20 ISSN:2213-5979.
      Photoacoustic imaging (PAI) is a non-invasive, high-resolution hybrid imaging modality that combines optical excitation and ultrasound detection. PAI can image endogenous chromophores (melanin, hemoglobin, etc.) and exogenous contrast agents in different medical applications. However, most current equipment uses sophisticated and complicated OPO lasers with tuning and stability features inconsistent with broad clinical deployment. As the number of applications of PAI in medicine increases, there is an urgent need to make the imaging equipment more compact, portable, and affordable. Here, portable light emitting diode - based photoacoustic imaging (PLED-PAI) was introduced and characterized in terms of system specifications, light source characterizations, photoacoustic spatial/temporal resolution, and penetration. The system uses two LED arrays attached to the sides of a conventional ultrasound transducer. The LED pulse repetition rate is tunable between 1 K Hz, 2 K Hz, 3 K Hz, and 4 K Hz. The axial resolution was 0.268 mm, and the lateral resolution was between 0.55 and 0.59 mm. The system could detect optical absorber (pencil lead) at a depth of 3.2 cm and the detection limits of indocyanine green (ICG) and methylene blue (MB) were 9 μM and 0.78 mM. In vivo imaging of labeled human mesenchymal stem cells was achieved to confirm compatibility with small animal imaging. The characterization we report here may have value to other groups evaluating commercially available photoacoustic imaging equipment.
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    43. 43
      Erfanzadeh, M.; Zhu, Q. Photoacoustic Imaging with Low-Cost Sources; A Review. Photoacoustics 2019, 14, 1– 11,  DOI: 10.1016/j.pacs.2019.01.004
      43
      Photoacoustic imaging with low-cost sources; A review
      Erfanzadeh Mohsen; Zhu Quing; Zhu Quing
      Photoacoustics (2019), 14 (), 1-11 ISSN:2213-5979.
      Benefitting from advantages of optical and ultrasound imaging, photoacoustic imaging (PAI) has demonstrated potentials in a wide range of medical applications. In order to facilitate clinical applications of PAI and encourage its application in low-resource settings, research on low-cost photoacoustic imaging with inexpensive optical sources has gained attention. Here, we review the advances made in photoacoustic imaging with low-cost sources.
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    44. 44
      Hariri, A.; Alipour, K.; Mantri, Y.; Schulze, J. P.; Jokerst, J. V. Deep Learning Improves Contrast in Low-Fluence Photoacoustic Imaging. Biomed. Opt. Express 2020, 11 (6), 3360– 3373,  DOI: 10.1364/BOE.395683
      44
      Deep learning improves contrast in low-fluence photoacoustic imaging
      Hariri, Ali; Alipour, Kamran; Mantri, Yash; Schulze, Jurgen P.; Jokerst, Jesse V.
      Biomedical Optics Express (2020), 11 (6), 3360-3373CODEN: BOEICL; ISSN:2156-7085. (Optical Society of America)
      Low fluence illumination sources can facilitate clin. transition of photoacoustic imaging because they are rugged, portable, affordable, and safe. However, these sources also decrease image quality due to their low fluence. Here, we propose a denoising method using a multi-level wavelet-convolutional neural network to map low fluence illumination source images to its corresponding high fluence excitation map. Quant. and qual. results show a significant potential to remove the background noise and preserve the structures of target. Substantial improvements up to 2.20, 2.25, and 4.3-fold for PSNR, SSIM, and CNR metrics were obsd., resp. We also obsd. enhanced contrast (up to 1.76-fold) in an in vivo application using our proposed methods. We suggest that this tool can improve the value of such sources in photoacoustic imaging.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.3c01696.

    • Materials, nanoshell synthesis procedure and challenges, size analysis results, and optical and photoacoustic characterization methods including setup schemes (PDF)


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