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ArticleMarch 18, 2009

Rational Selection of Gold Nanorod Geometry for Label-Free Plasmonic Biosensors
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Cite this: ACS Nano 2009, 3, 4, 795–806

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Published March 18, 2009

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Abstract

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We present the development of an analytical model that can be used for the rational design of a biosensor based on shifts in the local surface plasmon resonance (LSPR) of individual gold nanoparticles. The model relates the peak wavelength of light scattered by an individual plasmonic nanoparticle to the number of bound analyte molecules and provides an analytical formulation that predicts relevant figures-of-merit of the sensor such as the molecular detection limit (MDL) and dynamic range as a function of nanoparticle geometry and detection system parameters. The model calculates LSPR shifts for individual molecules bound by a nanorod, so that the MDL is defined as the smallest number of bound molecules that is measurable by the system, and the dynamic range is defined as the maximum number of molecules that can be detected by a single nanorod. This model is useful because it will allow a priori design of an LSPR sensor with figures-of-merit that can be optimized for the target analyte. This model was used to design an LSPR sensor based on biotin-functionalized gold nanorods that offers the lowest MDL for this class of sensors. The model predicts a MDL of 18 streptavidin molecules for this sensor, which is in good agreement with experiments and estimates. Further, we discuss how the model can be utilized to guide the development of future generations of LSPR biosensors.

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The unique optical properties of plasmonic nanoparticles have led to the development of a novel class of label-free biomolecular sensors. These biosensors exploit the phenomenon of localized surface plasmon resonance (LSPR) which endows unique light scattering and absorption properties to noble metal nanoparticles that are a function of the local environment. This behavior transduces binding events at the nanoparticle surface into a macroscopically measurable optical signal (1-6) and forms the basis of LSPR biosensors. Herein, we report the derivation and experimental verification of an analytical model that predicts the figures-of-merit of nanoparticle LSPR biosensors operating at the single particle limit and provides guidance for the rational design of LSPR biosensors.

LSPR arises from the resonant oscillation of conduction electrons on the surface of metal nanoparticles. The energy associated with this resonance is a function of the nanoparticle composition, size, shape, and the surrounding dielectric environment. (7-12) Noble metal nanoparticles with features in the 10−100 nm size scale exhibit plasmonic resonances at optical frequencies. Consequently, such nanoparticles exhibit a characteristic LSPR spectrum, with one or more peaks in the visible light range that correspond to specific electron oscillation resonances. The LSPR peak location and intensity are sensitive to the local refractive index (RI) surrounding the nanoparticle and are the basis of their utility as biosensors. Binding of analyte molecules to nanoparticles that are decorated with a receptor specific for that analyte alters the local RI in the vicinity of the plasmonic nanostructure, resulting in a shift of the LSPR spectrum. This shift can be measured either as a change in the peak intensity of the scattered light or as a shift in the LSPR peak wavelength (λ*).

This approach to biomolecular detection was initially proposed for plasmonic particles in suspension, (13) was then demonstrated for an ensemble of gold nanoparticles that were immobilized on a transparent substrate in a chip-based format by our group, (14, 15) and was subsequently validated by other groups (16-18) and extended to the limit of single nanoparticles. (5, 19-25) By miniaturizing the sensor down to a single nanoparticle, the detection system is reduced to the size-scale that is commensurate with the size of biomolecular analytes. To further understand the mechanics of single-nanoparticle detection, investigators have also been studying the specific details of plasmonic nanostructures that are responsible for their use in sensors such as the distance dependence of sensitivity (26-28) and the resonant electric field enhancement distribution. (29, 30) However, to date, these developments have not been integrated into a coherent quantitative framework that allows the effect of these parameters on relevant figures-of-merit of biosensors, such as their limit of detection and dynamic range, to be predicted.

Motivated by the goal of rationally designing LSPR sensors, in this paper, we integrate recent progress in understanding the structural details of metal nanoparticles that control their plasmonic behavior with various measurement system parameters that impact measurement uncertainty into a quantitative model that is capable of predicting the response of a single, receptor-functionalized nanoparticle to discrete analyte binding events. The result is an analytical model that quantifies the LSPR shift of a gold nanorod caused by the local refractive index increase from the presence of a target biomolecule. The principle of this work is similar to that of Stemberg et al. who describe an analytical model to quantify the surface concentration of bound protein on plasmon resonant planar gold (31) and to that of Vollmer et al. who estimate the surface density of bound proteins based on wavelength shifts of a resonant microcavity. (32)

The utility of this model is two-fold. First, it provides an analytical model that allows a priori design of an LSPR sensor with figures-of-merit, such as the molecular detection limit (MDL) and dynamic range, that can be analytically calculated completely as a function of sensor components for a given receptor−analyte pair. Because the model calculates LSPR shifts for individual bound molecules, the MDL is defined as the smallest number of bound molecules that is measurable by the system, and the dynamic range is the maximum number of detectable molecules. To illustrate its experimental utility, the model was used to identify the optimal gold nanorod geometry (length and diameter) that was predicted by the model to yield the lowest MDL for the detection of streptavidin by gold nanorod LSPR sensors. An LSPR sensor was fabricated from biotin-functionalized gold nanorods, and the experimental results were compared with the prediction of the model to investigate its accuracy. Second, this model is useful because it allows the contribution of different system parameters to overall sensitivity to be individually parsed. With the ultimate goal of label-free, single molecule detection by LSPR sensors, we conclude with a discussion of optimization of the system within realistic physical and current technological constraints that might allow the fabrication of an LSPR sensor and dark-field microspectrometer that are capable of detecting the binding of individual analyte molecules.

Results and Discussion

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As a first approximation, the MDL of nanoparticle sensors is determined by their composition, size, and shape of the NPs. Recognizing that the development of a generalized model that could account for the dependence of the MDL of an LSPR sensor on all of these parameters was likely to prove computationally intensive and possibly intractable, we focused instead on developing a model that was applicable to gold nanorods as plasmonic transducers of binding events for the following reasons. First, although silver particles are more sensitive than gold particles of the same shape and size, (33) the greater reactivity of silver as compared to gold makes it less suitable for use in biologically relevant media as silver can be easily oxidized, altering the plasmonic behavior of the particle. Second, gold nanorods can be conveniently synthesized with a range of dimensions via established chemical synthesis methods, which allows the model to be experimentally tested—and the sensor optimized—at the structural level. (34-36) Third, gold nanorods can be conveniently synthesized to exhibit plasmon resonances with peak wavelengths ranging from 700 to 900 nm and beyond simply by tuning their aspect ratio and size. (34, 37-40) This spectral region is particularly useful for biosensing because the background absorption and scattering of endogeneous chromophores from biological mixtures (e.g., serum and blood) and of water are minimal in this wavelength range. (41, 42) We note that LSPR of several other geometries of gold nanoparticles such as nanoshells, nanodiscs, (43) and nanorings (44) can also be tuned via synthesis methods to exhibit plasmon bands in the wavelength range of gold nanorods. Nanorods, however, have a higher bulk RI sensitivity (45) and also a narrower line width than these geometries, (43, 44, 46) which allows more accurate determination of peak shifts as described later in the model details. Additionally, the line widths of gold nanorod scattering spectra have been shown to be a function of the length and aspect ratio of the gold nanorods (47) as well as end-cap shape, (48) parameters that can be controlled in their chemical synthesis.

Model Development

We sought to develop an analytical model that estimates the MDL of an LSPR sensor for a specific analyte−receptor pair based on the geometric dimensions of the gold nanorod using a specified spectral detection system. Thus, the model is an equation that predicts the minimum number of detectable analyte molecules based on input parameters that consist of the nanorod dimensions and optical system parameters. Building the model involved several steps; first, the spectral detection system and data analysis algorithms were analyzed to determine the measurement uncertainty in detecting LSPR peak wavelengths. Then, the average number of analyte molecules that must bind to induce an LSPR shift equal to the measurement uncertainty for a given nanorod geometry was determined. This is the smallest number of bound molecules that can be reliably be detected by the proposed system and is defined as its MDL. It is important to note that we define the MDL by the amount of material bound to the surface of the nanoparticle, and not the concentration of the analyte in the surrounding medium. This choice was made so that the focus of the model is the interaction of the bound analyte with the plasmonic nanoparticles and the subsequent LSPR signal generated, and not the mass transport kinetics of the sensor system. Therefore, the optimal nanoparticle can be determined for a proposed analyte strictly by using the model to predict which geometry will offer the lowest MDL.

The first step in the derivation of the model was estimation of the minimum LSPR shift that can be reliably measured for a particular detection system. We assumed that the total uncertainty is the sum of uncertainties induced by two factors as represented in eq 1: uncertainty introduced by the detection system, U_system, and uncertainty introduced by the peak fitting to the gathered nanoparticle scattering spectrum, U_fit.

(1)

A thorough discussion of the U_system of the microspectroscopy system utilized in this work has been reported elsewhere. (49) That study, which incorporated considerations of image focus, physical sample stability, and overall repeatability of the measurements, concluded that U_system was ∼0.3 nm for the optical detection system described in that work. The same experimental setup was used in this study; hence we assume that U_system is 0.3 nm.

Next, uncertainty due to the data analysis method must be considered. The measurement noise was modeled as a Gaussian distribution. Although the physical model of noise in an optical measurement is a Poisson distribution due to shot noise, it can be accurately and conveniently modeled by a Gaussian distribution for a large number of photons because it is merely a counting problem. This yields the following relationship for U_fit

(2) where FWHM is the line width of the spectrum being fit and SNR is the signal-to-noise ratio of the data being fit. (49) The SNR depends on photoelectron signal magnitude, M, the integration time, t, and the spectrometer camera’s dark current specification, N_d, and read noise, N_r, according to the following:

(3)

For a shot noise-limited measurement (SNR = √M), the shot noise dominates over the dark and read noise. The system used for this study has a dark current of less than 0.0025 electrons·pixel⁻¹·s⁻¹ and read noise of less than 4 electrons RMS such that a shot noise-limited measurement (defined here as a measurement for which shot noise exceeds dark and read noise sources by an order of magnitude) will have an SNR > 13 for a typical integration time of 20 s. The role of the nanoparticle geometry in affecting measurement uncertainty becomes apparent as the signal level; that is, the amount of light scattered is proportional to the nanoparticle’s scattering cross section, C_sca. Thus in the shot noise-limited regime, the following relationship holds for SNR:

(4) where A is a constant that accounts for the input photon flux, the integration time, and the quantum efficiency of the measurement system. The parameter A can be calculated for a microspectroscopy system by making a shot noise-limited measurement of the SNR of a nanoparticle with known C_sca. (50) We measured the scattering spectrum of an 80 nm diameter gold sphere under typical operating parameters and observed a SNR of 100. Using the C_sca predicted by Mie theory of the sphere, a value of 0.385 is determined for A for the microspectroscopy system described in this work. With the value for A known, the SNR for an arbitrary nanoparticle of known C_sca may be calculated by rewriting eq 7 as follows:

(5)

Now, with eqs 2 and 5, it is possible to calculate total peak measurement uncertainty for an arbitrary particle based on the FWHM and C_sca of the particle, even if the measurement is not shot noise-limited. C_sca can be determined analytically as a function of nanorod geometry by applying the model developed by Kuwata et al. (51) Further details of the specific application of this model are reported below. Therefore, U can now be calculated entirely as a function of nanorod length and diameter. It is worth noting that the constant A is dependent upon the measurement system, but not upon the measured nanoparticle. Thus, it must only be empirically determined once for a specific experimental setup.

The next step in the development of the model was to analytically translate this uncertainty limit from units of wavelength shift to number of bound molecules as a function of nanorod geometry. In order to formulate this relationship in a manner that is analytically simple, several approximations and assumptions were made. First, we assumed that the nanorods are cylindrical in shape with a length (l) and diameter (d). We recognize that the assumption that the nanorods are perfect cylinders is an approximation because transmission electron microscopy (TEM) images of chemically synthesized nanorods indicate that the nanorods are only approximately cylindrical, as they have “end-caps” with visible curvature on their ends. This approximation will introduce some error as it has been shown that end-cap geometry has an impact on the optical scattering spectra of nanorods. (48, 52) We also assumed that the LSPR peak shift, Δλ_LSPR, resulting from a bound analyte is proportional to the total LSPR shift that is expected if the entire surrounding medium increased to the RI of the analyte. The proportionality constant is the ratio of the analyte volume to the total sensing volume of the nanorod. This assumption yields the following relationship:

(6) where V_S is the total sensing volume of the nanorod, V_D is the volume of the detected analyte, S(r) is the spatially dependent RI sensitivity of the nanoparticle, and r is the distance of the bound molecule from the surface of the nanorod. Because the RI sensitivity decreases away from the nanoparticle surface, the sensing volume is defined as the fixed volume surrounding the nanorod that contains 95% of its sensitivity. Recognizing that the optical mass of the bound molecules is responsible for inducing the observed LSPR shifts, the optical mass increase induced by bound analyte is defined as the product V_D·ΔRI, where V_D is the volume of analyte bound to the nanorod and ΔRI is the difference between the RI of the analyte and that of the surrounding medium. In the case where the approximate size of the analyte is known, the detection volume V_D can be replaced with the product N·V_A where V_A is the volume of the analyte molecule and N is the number of bound molecules to the nanorod. Substituting this parameter in eq 6 and rearranging for Δλ_LSPR yields an expression relating the measured peak shift of the LSPR spectrum to the optical detection mass:

(7)

Solving eq 7 for N yields an expression that can be used to translate an observed LSPR peak shift into the number of bound molecules.

(8)

In order to determine the MDL of the system, we replace Δλ_LSPR in eq 8 with U, the uncertainty of the optical system in detecting wavelength shifts. Thus, the right-hand side of eq 8 becomes an expression of the bound optical mass that would induce the minimum detectable LSPR wavelength shift. In this case, solving for N yields an expression for L_M, the MDL of the system in terms of the minimum detectable number of bound biomolecules.

(9) where L_M is a function of the length l and diameter d of the nanorod. From eq 9, we observe that the function L_M is determined by several parameters on the right-hand side. Careful examination of these parameters is required to fully understand the dependence of each on the nanorod geometry and the effect each has on the overall MDL of the system. We note that L_M is dependent upon the detection system employed for measuring the LSPR shifts of single nanoparticles because of the dependence on the peak measurement uncertainty U. L_M is also a function of the spatially dependent RI sensitivity S(r) of the nanoparticle, where r is the location of the bound molecule with respect to the surface of the nanorod.

In order to determine the function L_M analytically, we next sought to define the parameters U and S(r) based solely on the length and diameter of the nanorod, as follows: first, we utilized a model developed by Kuwata and co-workers to approximate the wavelength-dependent scattering cross section (C_sca) of gold nanorods as a function of nanorod geometry. This model provides the complete LSPR scattering spectrum of the nanorod with a simple analytical formula based on nanorod length and diameter. The model is based on fits to finite-difference time-domain with the depolarization factor calculated by electrostatic approximation. (51) We employed this formula to determine the location of the LSPR peak (λ*) and scattering cross section C_sca at the peak wavelength for nanorods of arbitrary length and diameter. A second analytical model, based on Gans’s extension of Mie theory that simulates LSPR spectra as a function of the nanorod aspect ratio, (53-55) was also used to simulate spectra as a means of validation. These two models provided consistent results within the geometric range of nanorods studied (length 50−100 nm and diameter 15−50 nm). These predictions were then further verified experimentally by comparison to scattering spectra of individual, chemically synthesized nanorods as shown in Figure 1. The good agreement between experimentally collected LSPR spectra and simulated spectra further indicates the utility of the analytical model.

Figure 1. Comparison of simulated *versus* experimental spectra. Scattering spectra collected from single nanorods from synthesized batches characterized by TEM to be (63.3 ± 8.2) × (24.9 ± 4.9) nm (n = 319, yellow dotted line) and (74 ± 9) × (33 ± 6) nm (n > 100, green dotted line). Simulated spectra for nanorods with dimensions of 64.5 × 24.7 nm (blue line) and 75 × 32 nm (red line) *via* the Kuwata method are also shown.

It has been shown that LSPR shifts induced by local RI have a strong distance dependence as a result of the exponential decrease in field enhancement further from the nanoparticle surface. (26) This distance dependence of sensitivity has been experimentally measured for nanospheres (56) as well as anisotropic triangular nanoparticles. (57) Additionally, it has been shown that the LSPR associated electric field is enhanced near the ends of nanorods. (29, 30) These observations suggest that the location at which a target analyte binds to the nanorod (i.e., along the sides or at the ends) will affect the magnitude of the induced LSPR shift. The detector−analyte system described in this work provides reasonable certainty in estimating the average binding distance, which is determined by the length of the receptor conjugated to the nanorod (biotin in the experimental case tested herein) and its binding site. Hence, we model sensitivity as being a function solely of the distance r from the surface of the nanorod S(r). The effect of this assumption is that the model effectively outputs the LSPR response of the average bound analyte at a fixed distance r from the surface of the nanorod. This is a simplification of the physical phenomenon because of the known complexity of the electric field enhancement distribution. Nevertheless, this model is the closest representation to the actual detection experiments because the actual binding locations cannot be controlled nor can the fraction of analytes that bind along the nanorod ends versus sides be accurately estimated.

With these assumptions clarified, we sought to generate an analytical function that describes S(r) purely as a function of nanorod geometry. This was accomplished by first experimentally measuring S(r) for several nanorod geometries spanning the dimensions of interest. This was done by using a procedure previously reported to probe the sensing volume of gold nanospheres by adsorbing sequential layers of oppositely charged polyelectrolytes to the nanoparticle surface. (14) These experiments provided data on the dependence of S(r) on nanorod geometry. These data were then curve-fit to describe S(r) for nanorods of arbitrary length and diameter. From this fit, the sensing volume of the nanorod V_S and the decay length of electric field enhancement l_d were also determined as a function of nanorod geometry. V_S is defined as the fixed distance from the nanorod surface containing 95% of S(r), and the decay length is the distance from the nanorod surface at which the electric field enhancement is reduced by a factor of e. Figure 2 shows the excellent agreement obtained between the model and experimental data. Details of this fitting procedure are further described in the Supporting Information. With the results of this fit, it is possible to write every parameter on the right-hand side of eq 8 as an analytical function of the length and diameter of the nanorod such that the MDL, L_M, of an arbitrary nanorod can be analytically estimated from the length and diameter of the nanorod.

(10) where V_S is the sensing volume, V_A is the analyte volume, ΔRI is the RI difference between the analyte and the surrounding medium, U_system is the peak measurement uncertainty resulting from uncertainty in the physical detection of the LSPR peak, U_fit is peak determination uncertainty due to data fitting, S₀ is the bulk RI sensitivity, r is the distance from the nanorod surface that the analyte binds, and l_d is the decay length of the resonant electric field.

Figure 2. Right axis: experimental wavelength shift of a nanorod (63.3 ± 8.2 nm × 24.9 ± 4.9 nm) as a function of deposited polyelectrolyte thickness (red dots) and shifts predicted by eq 7 (green line). Left axis: corresponding calculated distance dependence of the nanorod LSPR sensitivity (blue line).

Equation 10 is especially useful because it allows an analysis of the overall performance of a single nanoparticle based on a variety of parameters, only one of which is the nanoparticle bulk RI sensitivity S₀. We emphasize this point because much work in the field has been devoted to synthesizing nanoparticles with different geometries and compositions in order to optimize the RI sensitivity of the nanoparticle. (45, 58, 59) Clearly, the nanoparticle bulk sensitivity is an important contributor to the overall MDL; however, it is not necessarily the most important. In fact, Miller et al. have shown that the bulk RI sensitivities of nanoparticles can be predicted from the wavelength of the LSPR peak, independent of nanoparticle shape. (60, 61) The parameter U is also clearly important in determining MDL but has largely been ignored on discussions of MDL. (49) Obviously the sensitivity of the detection system depends not only on the signal generated by the nanoparticles but also how precisely it can be measured. This parameter is primarily dependent on optical detection system parameters and data analysis techniques. However, U also depends on nanoparticle geometry because spectra for nanoparticles with higher C_sca can be measured more accurately due to their increased scattering signal as seen in eq 10. Other parameters, such as the effective sensing volume of the nanoparticles (i.e., the electric field enhancement localization) can vary a great deal across geometries and have a large impact on the sensing characteristics of a nanoparticle. Nanoparticles with more confined electric field enhancements maintain smaller sensing volumes, resulting in larger shifts upon analyte binding within the sensing volume. However, nanoparticles with smaller detection volumes have a smaller dynamic range because they saturate upon the binding of fewer analytes. The proposed model offers a means to collectively analyze the effects of these parameters on overall sensor performance to deduce the optimal nanoparticle geometry for an LSPR sensor that is designed for a specific application.

Figure 3A shows the theoretical MDL for streptavidin binding to biotin decorated gold nanorods as predicted by eq 10 for single nanorods of arbitrary dimensions when measured in the described micro-spectroscopy system for which U has been previously characterized. We chose this analyte−receptor pair because their interaction is one of the best characterized and most commonly used model receptor−analyte systems. The simulation was restricted to nanoparticles that have LSPR peak wavelengths in the range of 300−900 nm because that is the range observable in the microspectroscopy system. We also applied a minimum SNR threshold of 30 to exclude particles with small C_sca that cannot produce enough scattered light to be visualized in the dark-field microscopy setup. For this reason, no data are shown for nanorods with LSPR peaks outside of this range or with SNR below the cutoff of 30. We assumed the streptavidin molecules have a volume (62) of 114 nm³ and RI of 1.57. (63) From Figure 3A, we can see that nanorods with lengths between 55 and 65 nm and diameters between 25 and 33 nm offer the lowest MDL of ∼18 streptavidin molecules. It is important to note that the nanorod MDLs predicted by eq 10 are a monotonic function of several detection system and analyte parameters. Although the absolute value of the MDL will vary across detection systems and receptor−analyte pairs, the relative performance of a nanorod with a specified geometry will not vary, so that once the optimal rod geometry has been identified for a target analyte it will always offer the lowest MDL across detection systems and receptor−ligand configurations.

Figure 3. (A) Calculated molecular limits of detection for gold nanorods of arbitrary dimension based on eq 10. (B) Composite figures of merit (FOM) for nanorods of arbitrary dimensions. The composite FOM is calculated as the maximum number of bound molecules divided by the minimum number of detectable molecules. The surrounding gray regions in both are indicative of nanorod geometries that were not considered in this model because they either had resonances outside the visible spectrum or had scattering cross sections that are insufficient to collect spectra with SNR greater than 30.

In addition to the absolute MDL, the dynamic range (DR) is an important figure-of-merit (FOM) of a biomolecular sensor. For the purposes of this model, the DR is defined as the theoretical maximum number of analyte molecules that are detectable by a single nanorod. This definition was chosen because it is consistent with the model output, which is quantified as the number of bound molecules. The DR was determined by calculating the total surface area available for binding and dividing it by the footprint of a bound analyte molecule. This value was then scaled by a factor of 0.9, which assumes a hexagonal packing density of hard spheres yielding the highest possible coverage that could be achieved in practice. By this definition, a larger nanorod will obviously exhibit a higher DR because of its larger surface area. However, larger nanorods also tend to have higher MDLs because of the increased sensing volume. To balance these considerations, we calculate a composite FOM for nanorods that includes both the DR and MDL of these nanorod plasmonic sensors operating in the single nanorod mode. The composite FOM is calculated simply as the ratio of the DR versus its MDL. Figure 3B shows the composite FOM for nanorods of arbitrary dimensions. Figure 3B shows that rods with a length of 85 nm and a diameter of 40 nm offer the highest composite FOM with the potential to bind ∼35 times as many molecules as is required to generate its lowest detectable signal. So although nanorods of those dimensions may exhibit a MDL of ∼22, their higher dynamic range indicates they may be more useful as a streptavidin sensor over a wider range of analyte concentrations.

Experimental Application of Model

To experimentally test the results provided by the model, gold nanorods were synthesized with dimensions that were as close to the geometry predicted by eq 9 (and visually represented by Figure 3A) to have the lowest streptavidin MDL. To experimentally test the predictions of the model, streptavidin binding experiments were performed using biotin-functionalized gold nanorods that were shown by TEM to have a length of 63.3 ± 8.2 nm and a diameter of 24.9 ± 4.9 nm (n = 319) (Figure 4). Equation 10 calculates that these rods have a mean MDL of 20 molecules and mean FOM of 22.

Figure 4. Left: TEM of rods used in this study. Nanorod length is measured to be 63.3 ± 8.2 nm and diameter 24.9 ± 4.9 nm (n = 319). Scale bar corresponds to 50 nm. Right: LSPR peak shift of individual, biotin-activated gold nanorods *versus* concentration of streptavidin. Open circles are the mean LSPR shift of approximately 15 nanorods per measurement at each concentration, and the error bars indicate 95% confidence interval. The solid line is a sigmoidal fit to the data. The dotted line indicates where the fit line crosses 0.3 nm shift, the measurement detection uncertainty as calculated above. This occurs at a concentration of 160 pM.

A dose−response curve was determined by incubating identical samples of biotin-functionalized gold nanorods that were chemisorbed on glass slides in streptavidin solutions that spanned a range of protein concentration. Figure 4 (right panel) shows the steady-state LSPR shift of single nanorods as a function of streptavidin concentration. A sigmoidal fit was applied to these data, and the concentration at which the fit crosses the LSPR peak measurement uncertainty of 0.3 nm was defined as the experimental detection limit. For the experiments shown in Figure 4, the detection limit is 160 pM.

To investigate the relationship between the experimental detection limit measured by analyte concentration and the MDL predicted by eq 10, we specifically looked at the detection of 100 pM streptavidin. Figure 5 shows fits to scattering spectra collected from a single nanorod incubated in 100 pM streptavidin. The blue line is the nanorod LSPR scattered spectrum in water, the red line is the spectrum after the binding of biotin, and the black line is the spectrum after incubation in 100 pM streptavidin. The ∼3 nm shift between the blue and green lines observed upon biotin functionalization is consistent with the adsorption of a thin dielectric layer. The length of the biotin−amine is estimated to be 2.39 nm (Chem3d, Cambridgesoft) and the SAM to which the biotin is conjugated has a thickness of 2.78 nm and RI of 1.464. (64) Using eq 6 to simulate the LSPR response of the addition of dielectric layers yields an expected LSPR peak shift of 3.59 nm. This value is in good agreement with the experimental mean LSPR shift of 3.23 ± 1.2 nm observed upon the incubation of biotin for all nanorods used in the streptavidin detection experiments.

Figure 5. Fits to scattering spectra of a single nanorod in water (blue), after biotin conjugation (green), and subsequently in 100 pM streptavidin (red). The 0.52 nm shift corresponds to approximately 27 streptavidin molecules by eq 8.

A 0.52 nm shift was observed upon incubation of the biotin-functionalized gold nanorods in 100 pM streptavidin, which translates to ∼27 streptavidin molecules bound by a single nanorod, by use of eq 8. Lower streptavidin concentrations were tested in order to establish the detection limit in terms of concentration but did not yield results that were statistically significant from negative control experiments, (19) so the lowest streptavidin concentration that was experimentally detected is 100 pM. This experimentally detected minimum of approximately 27 molecules is reasonably close to the theoretical MDL of 20 streptavidin molecules per nanorod predicted by the model for the nanorods used. Thus, by utilizing the proposed model (eq 10), specifically, to select the optimal nanorod geometry for streptavidin detection, we are able to observe binding in the picomolar range. To our knowledge, this is the lowest reported concentration of biomolecule detected with a single particle LSPR sensor.

As a further check of the relevance of the MDLs predicted by our model, we observed that the mean LSPR shift at saturation is 5.4 nm (Figure 4). Using eq 8, we calculate that this shift arises from the binding of approximately 280 streptavidin molecules to the nanorod surface. For the nanorods used in these experiments, approximately 8000 nm² of biotin-activated surface is available for the binding of streptavidin molecules. Each streptavidin molecule is a tetramer of four identical biotin binding subunits, and the entire tetramer is roughly ellipsoidal with axes of 5.4 nm × 5.8 nm × 4.8 nm. (65, 66) Thus, the footprint that a streptavidin molecule would occupy on the binding surface is approximately 25 nm². In this orientation, a maximum of only 320 molecules would be expected to fit on the nanorod surface. Approximating the streptavidin molecules as hard spheres, the model of random sequential adsorption (67) predicts a packing ratio of 0.54, which would translate to only 170 bound molecules, whereas the highly ordered method of hexagonal packing predicts a packing ratio of 0.9, allowing for up to 290 streptavidin molecules. Therefore, the 280 molecules bound at saturation as determined by eq 8 is reasonable and suggests a more ordered arrangement of bound streptavidin molecules.

Additionally, we compare the mass of streptavidin presumed bound at saturation by eq 8 with that of streptavidin adlayers that were experimentally measured. Densities of 240 and 231 ng/cm² have been experimentally determined for streptavidin layers formed over biotin layers on flat substrates. (68, 69) These binding densities yield an estimated ∼220 bound streptavidin molecules to the available nanorod binding area of 8000 nm². Again, these estimates are slightly lower than the 280 molecules estimated from eq 8 to be responsible for the 5.29 nm shift observed at saturation but are close enough to justify the validity of our calculations.

Prospects for Single Molecule Detection

We systematically explored the parameters in eq 10 within physically realistic limits to determine which parameters can be modified in order to develop an LSPR sensor with a single molecule MDL.

The product V_A·ΔRI is a parameter of the analyte and of the surrounding medium. For proteins with a RI commonly near 1.57, detection in water (RI = 1.33) yields a typical ΔRI of 0.24. By drying the samples and taking measurements in air (RI = 1.00), ΔRI can be increased to 0.57. Thus, a factor of 2.4 improvement in MDL is possible through this method. We note that this approach would preclude real-time detection and require further processing steps of the sample, which detracts from the strength of label-free detection. However, this drying methodology has been successfully utilized by Van Duyne and co-workers. (16)

The total sensing volume V_S of the nanoparticle is determined by the nanoparticle shape and decay length l_d. Smaller V_S and l_d are indicative of nanostructures with small, intense electric field enhancements. In general, the sensing volume will vary proportionally with the cube of the decay length simply because it is a three-dimensional volume subtended by the decay length. In the case of rods, smaller rods have shorter decay lengths. So ideally, one would want to use the smallest nanorods possible. However, smaller rods also exhibit smaller scattering cross sections C_sca, which means they scatter less light under dark-field illumination. For rods in particular, C_sca varies approximately as the square of the rod length and proportionately to rod diameter. (51) So if the length and diameter of a nanorod were both reduced by a factor of 2, C_sca would be reduced by about a factor of 8. On the other hand, to maintain an adequate SNR of the collected spectra, the intensity of the illumination source must be increased. An upper limit to this intensity exists because at some point the nanorods will melt due to photothermal effects. (70, 71) The use of a broadband illumination source, such as a supercontinuum fiber laser, would allow the complete scattering spectrum to be collected and would provide a light output that is 6−10-fold more intense than the tungsten filament used in the experimental setup used here. Hence, eq 10 predicts that a physically realistic increase in illumination intensity by a factor of 10 could enable the visualization of smaller nanorods, effectively reducing the size of observable nanorods by about 1.5, thereby reducing the sensing volume and decay length, which would ultimately reduce the MDL by a factor of 7.

The decay length of the electric field enhancement l_d is determined by the geometry of the nanoparticles. Equation 10 assumes a uniform decay length for nanorods. The actual electric field enhancement has a more complicated geometry and, in particular, has been shown to exhibit larger enhancements near the ends of the rod. (30, 55, 72) Thus, analyte molecules that bind near the end of the nanorod will cause a greater LSPR shift than those binding on the lateral portion. For this reason, eq 10 is applicable to a MDL that stems from the LSPR response of individual biomolecules that are spatially averaged around the nanorod surface. It is reasonable to make this approximation for the experiments described here because the streptavidin molecules are adsorbed a fixed distance r from the nanorod surface, but no further spatial localization is possible. It has been shown through finite element modeling that the ends of nanorods exhibit much higher field enhancement than the spatial average. (73) Thus, if analyte binding can be limited to only within these regions, it is reasonable to assume a concurrent sensitivity increase greater than a factor of 10 due to the reduction of sensing volume. Some recent reports offer compelling end-to-end assembly of nanorods, (74) which could soon provide a route to site-specific functionalization of nanorods.

The binding distance r is the physical distance that the analyte binds from the surface of the nanoparticle. It is determined by the length of the receptor, which is approximated to be 2.4 nm for the experiments presented herein. EDC-NHS coupling is used to conjugate amine-terminated biotin to the carboxyl-terminated monolayer applied to the gold surface. We employ this binding moiety because it offers easy translation to other amine-terminated receptors, such as antibodies or aptamers. (36) It would be possible to employ a physically shorter receptor that would cause the target analyte to bind closer to the surface of the nanoparticle where the field enhancement, and thus RI sensitivity, is greater. For example, using thiol-terminated biotin has been successfully employed as a receptor for streptavidin detection with single nanoparticles. (75) This would reduce r by as much as a factor of 2, which in return reduces the overall MDL by a factor of exp(l_d⁻¹). For the case of nanorods with decay length l_d generally on the order of 10−20 nm, this results in a factor of 1.1 improvement (i.e., 10% decrease in MDL). In most practical situations, however, the size of the receptor−analyte pair is likely to be significantly larger than the model biotin−streptavidin pair used here (e.g., antibody−protein), so that some of the gains realized from the optimization of the parameters discussed here are likely to be offset by the size of the receptor−analyte pair. Nevertheless, these results suggest that minimizing the size of the receptor while retaining high binding affinity and specificity for its analyte is an important factor in extracting the minimum MDL of which LSPR sensors are capable. In this regard, the use of smaller receptors such as aptamers or smaller, engineered antibody fragments such as single chain antibodies are preferable to intact antibodies, which to date are the most commonly used class of receptors in biosensors.

The bulk RI sensitivity of a nanostructure S₀ is dependent on its size, shape, and material composition. However, Miller and Lazarides have observed that S₀ is correlated with the LSPR peak wavelength, regardless of nanoparticle shape. (60, 61) Thus, for a nanostructure of known material composition, its bulk RI sensitivity can be predicted simply by characterizing its LSPR peak. Because longer wavelength resonances exhibit higher bulk RI sensitivities, the highest sensitivities will be from particles with LSPR peaks in the red end of the spectrum. Assuming the detection is to remain in the visible light spectrum, LSPR peaks at wavelengths up to 800 nm could be measured. Thus, bulk RI sensitivities as high as 600 nm/RIU could be expected. This is more than a two-fold improvement over the 260 nm/RIU sensitivity of nanorods used in this study. However, it is worth noting that, for the case of nanorods, longer resonances correspond to larger rods with larger sensing volumes and longer decay lengths. Equation 10 was used to balance the contributions for these effects to optimize sensor performance using the nanorod geometry. Thus, for this factor of 2 increase to be realistic, an alternative nanoparticle geometry would be sought that could indeed have a resonance far into the red without such large geometries. Additional sensitivity could be achieved by structuring the sensor to operate in near-IR wavelengths where the bulk RI sensitivities are higher, although the optics and instrumentation could be potentially more challenging. Also, silver nanoparticles exhibit sensitivities typically 1.5-fold higher than gold particles at similar resonant wavelengths. However, as mentioned above, the high reactivity of silver makes it less suitable as a sensor for use in biologically relevant media.

The peak measurement uncertainties U_fit and U_system represent the most direct way at improving sensor detection limits. Obviously, the smaller the wavelength shifts that can be reliably detected, the greater the accuracy and lower the overall MDL will be. For the system used in these experiments, U_fit was found to be approximately 0.02 nm while U_system was found to be 0.3 nm. Thus, the contribution to MDL is dominated by U_system, whereas U_fit acts more as an absolute limit for noise levels as discussed above. We performed an in-depth analysis of the contributing factors to U_system and found that a large portion of the uncertainty is the result of physical system stability. (49) In particular, it was found that stability in microscope focus and sample stage drift account for the largest contributions in uncertainty. It is proposed that, by using active feedback hardware to control microscope focus and XY-sample location, U_system can be reduced to the order of 0.005 nm. (49) Thus, the total uncertainty would then be dominated by U_fit resulting in a 15-fold decrease in MDL.

These possible enhancement factors are displayed in Table 1. From here, we see that the estimated possible enhancement factors could potentially provide an 800-fold improvement over the 18 molecule detection limit described herein. Thus, we show that the ultimate limit of label-free detection of single molecule binding events is theoretically possible within the framework of eq 10.

Table 1. Theoretical Improvement Factors That Can Reduce the Molecular Detection Limit of a Label-Free, Single Particle LSPR Sensor Based on the Modulation of Parameters from Equation 10

technique summary	variable from eq 10 involved	potential enhancement factor
drying	ΔRI	2.4
brighter illumination source	C_sca, V_S, and l_d	7
shorter binding moiety	r	1.1
LSPR peaks in IR	S₀	2
silver nanoparticles	S₀	1.5
high spectral resolution detection system	U_system	15
	total	∼800

In conclusion, we have presented a simple mathematical model that analytically relates physical detection parameters of a single nanoparticle LSPR sensor to the minimum number of detectable analyte molecules. The utility of this model is two-fold. It can be used to select the optimum nanoparticle geometry for a desired detection system completely analytically, forgoing otherwise necessary comprehensive, experimental characterization. The minimum number of detectable molecules can be estimated as well as the number of molecules detected at saturation, which provides insight into the dynamic range of the system. Additionally, the model provides a framework through which the effects of potential system improvements can be assessed. Analysis of the theoretical limits of the dependent variables of eq 10 indicates that an 800-fold reduction of the MDL for the detection system described herein is possible. Clearly, not all of these potential avenues for MDL reduction can be realized concurrently. However, because our current MDL is 18 streptavidin molecules, we propose it is theoretically possible to design a system capable of detecting single molecule binding events by careful optimization of system parameters, as described herein.

Experimental Section

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Materials

Hydrogen tetrachloroaurate trihydrate (HAuCl₄), sodium borohydride, ascorbic acid, phosphate buffered saline (PBS) tablets, poly(allylamine hydrochloride) (PAH), poly(4-styrenesulfonate, sodium salt) (PSS), silver nitrate, and mercaptohexadecanoic acid (MHA) were purchased from Sigma. Glass coverslips, ethanol, methanol, sodium chloride (NaCl), and hydrochloric acid (HCl) were purchased from VWR. Cetyltrimethylammonium bromide (CTAB) was purchased from Fluka. (+)-Biotinyl-3,6,9-trioxaundecanediamine (biotin−amine), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), N-hydroxysuccinimide (NHS), and streptavidin were purchased from Pierce. Mercaptopropyltriethoxysilane (MPTES) was purchased from Gelest. (1-Mercaptoundec-11-yl)tri(ethylene glycol) (EG₃SH) was purchased from Prochimia. Distilled water purified by a reverse-osmosis filtration system (18 MΩ·cm, PureFlow Inc.) was used for all experiments.

Nanorod Synthesis

Gold nanorods were chemically synthesized by a seed-mediated growth procedure (34, 76) similar to that described previously. (19, 36) Spherical gold seed particles were prepared as follows: to a mixture of 7.5 mL of 0.1 M CTAB in water and 0.250 mL of 0.01 M HAuCl₄ was added under vigorous stirring 0.6 mL of chilled 0.01 M NaBH₄. The brown suspension was then gently heated and stirred for a few minutes. Gold nanorods were synthesized in a water bath at 29 °C as follows. To 95 mL of 0.1 M CTAB in water were added 0.6 mL of silver nitrate and 0.64 mL of 0.1 M ascorbic acid. The mixture was swirled after the addition of each reagent to ensure mixing. Sixty microliters of gold seed particles was added, and the mixture was inverted and allowed to sit overnight, resulting in a purple-colored suspension of gold nanorods. Excess CTAB was removed from the gold rod suspension by centrifugation twice at 4500 rpm for 30 min. The gold nanorods were resuspended in water to a total volume of 10 mL and stored at room temperature until further use.

Polyelectrolyte Deposition

Number 1 thickness rectangular 9 × 26 mm coverglasses were cleaned via the same method reported previously by Nath et al. for producing nanosphere biosensor chips. (15) First, they were cleaned by sonication for 20 min in a 1:1 HCl/methanol mixture. Then the coverglasses were rinsed with water and ethanol and then dried overnight at 65 °C. They were then incubated in a solution of 10% (v/v) MPTES in ethanol for 15 min followed by rinsing in ethanol and drying at 120 °C for 3 h. The coverglasses were incubated in the nanorod suspension overnight to ensure optimal coverage. The resulting nanorod-covered substrates were immersed in a solution of PAH (0.003 monomol/L in 1 M NaCl) for 30 min, rinsed with water, rinsed with 1 M NaCl, then immersed in a solution of PSS (0.003 monomol/L in 1 M NaCl). This procedure was repeated 13 times to deposit a total of 26 polymer layers. Each polyelectrolyte layer deposited is approximately 2 nm thick and has a RI of 1.52. (14, 77, 78) Between each polymer incubation step, an extinction spectrum was collected with a Varian Cary 300Bio UV−visible spectrophotometer. In this manner, plots were generated of λ*versus adsorbed layer thickness for two samples from each of the four different geometries for a total of eight plots. These eight plots were then used as the basis for the determination of S(r).

Single Particle Detection

Scattering spectra were collected from individual nanorods using the Zeiss Axiovert 200 (Plan Neofluor 100X objective) dark-field illumination microspectroscopy system described previously. (19, 50) Scattering spectra of individual nanorods were collected by imaging the microscope field of view to a line-imaging spectrometer (Acton Research SpectraPro 2150i). Streptavidin detection experiments with single gold nanorods were also performed as previously reported. (19) Briefly, the nanorods were immobilized onto 25 mm diameter, round No.1 glass coverslips, which were cleaned and incubated in MPTES as described above. The nanorod suspension was diluted in water to 1:15 (v/v), and then a 20 μL aliquot of the suspension was pipetted onto the center of the coverglass and immediately rinsed off with water. The nanorods were then modified by immersion of the chip in an ethanol solution of 0.5 mM EG₃SH and 0.5 mM MHA. The EG₃SH molecules of the mixed monolayer serve two purposes. They prevent nonspecific adsorption as well as modulate the surface density of the MHA molecules and hence of biotin that is subsequently chemically conjugated to the −COOH groups. Biotin was conjugated to the nanorods by incubating the SAM-functionalized nanorods in an aqueous solution of 0.4 M EDC and 0.1 M NHS for 7 min to convert the COOH groups to NHS esters. The chips were rinsed with water and were then immediately incubated in biotin−amine for 2 h and rinsed with water again. Streptavidin binding was performed by incubating the biotin-functionalized nanorods with a continuous flow of a streptavidin solution in PBS for 2 h. LSPR shifts were measured as the difference between the λ* of fits to the measured scattering spectra. To ensure the observed LSPR shifts resulted from specific interaction between biotin and streptavidin, controls for the binding experiments were performed by incubating biotin-functionalized gold nanorods in streptavidin solution that had been presaturated with excess (1 mM) free biotin. (19)

Supporting Information

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An in-depth derivation of the distance dependence of nanoparticle sensitivity to local refractive index is available. This material is available free of charge via the Internet at http://pubs.acs.org.

nn8006465_si_001.pdf (67.01 kb)

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Author Information

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Corresponding Author
- Ashutosh Chilkoti - Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708; Email: [email protected]
Authors
- Greg J. Nusz - Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708
- Adam C. Curry - Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708
- Stella M. Marinakos - Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708
- Adam Wax - Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708

Acknowledgment

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This work was supported by a grant from the Centers for Disease Control (NCID; R01 CI-00097) to A.C. G.J.N. acknowledges the support of a graduate fellowship from the NIH through a Biotechnology training grant (GM8555) awarded to the Center of Biomolecular and Tissue Engineering at Duke University. The authors also wish to thank the anonymous reviewers for their constructive suggestions that, we believe, helped improve this paper.

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Nath, Nidhi; Chilkoti, Ashutosh
Analytical Chemistry (2002), 74 (3), 504-509CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
This paper presents a new label-free optical method to study biomol. interactions in real time at the surface of an optically transparent substrate. The method relies on the change in the absorbance spectrum of a self-assembled monolayer of colloidal gold on glass, as a function of biomol. binding to the surface of the immobilized colloids. Using this approach, we demonstrate proof of principle of a label-free optical biosensor to quantify biomol. interactions in real time on a surface in a com. available UV-visible spectrophotometer and of a colorimetric end-point assay using an optical scanner. The spectrophotometric sensor shows concn.-dependent binding and a detection limit of 16 nM for streptavidin. The sensor is easy to fabricate, is reproducible in its performance, has minimal technol. requirements, namely, the availability of an UV-visible spectrophotometer or an optical scanner, and will enable high-throughput screening of biomol. interactions in real time in an array-based format.
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Haes, A. J.; Stuart, D. A.; Nie, S. M.; Van Duyne, R. P. Using Solution-Phase Nanoparticles, Surface-Confined Nanoparticle Arrays and Single Nanoparticles as Biological Sensing Platforms J. Fluoresc. 2004, 14, 355– 367
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Using Solution-Phase Nanoparticles, Surface-Confined Nanoparticle Arrays and Single Nanoparticles as Biological Sensing Platforms
Haes, Amanda J.; Stuart, Douglas A.; Nie, Shuming; Van Duyne, Richard P.
Journal of Fluorescence (2004), 14 (4), 355-367CODEN: JOFLEN; ISSN:1053-0509. (Kluwer Academic/Plenum Publishers)
A review. The intense colors of noble metal nanoparticles have inspired artists and fascinated scientists for hundreds of years. In this review, we describe three sensing platforms based on the tunability of the localized surface plasmon resonance (LSPR) of gold and silver nanoparticles. Specifically, the color assocd. with soln.-phase nanoparticles, surface-confined nanoparticle arrays, and single nanoparticles will be shown to be tunable and useful as platforms for biol. sensing.
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Frederix, F.; Friedt, J. M.; Choi, K. H.; Laureyn, W.; Campitelli, A.; Mondelaers, D.; Maes, G.; Borghs, G. Biosensing Based on Light Absorption of Nanoscaled Gold and Silver Particles Anal. Chem. 2003, 75, 6894– 6900
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Dahlin, A.; Zach, M.; Rindzevicius, T.; Kall, M.; Sutherland, D. S.; Hook, F. Localized Surface Plasmon Resonance Sensing of Lipid-Membrane-Mediated Biorecognition Events J. Am. Chem. Soc. 2005, 127, 5043– 5048
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Nusz, G. J.; Marinakos, S. M.; Curry, A. C.; Dahlin, A.; Höök, F.; Wax, A.; Chilkoti, A. Label-Free Plasmonic Detection of Biomolecular Binding by a Single Gold Nanorod Anal. Chem. 2008, 80, 984– 989
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Label-Free Plasmonic Detection of Biomolecular Binding by a Single Gold Nanorod
Nusz, Greg J.; Marinakos, Stella M.; Curry, Adam C.; Dahlin, Andreas; Hoeoek, Fredrik; Wax, Adam; Chilkoti, Ashutosh
Analytical Chemistry (Washington, DC, United States) (2008), 80 (4), 984-989CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
The authors report the use of individual gold nanorods as plasmonic transducers to detect the binding of streptavidin to individual biotin-conjugated nanorods in real time on a surface. Label-free detection at the single-nanorod level was performed by tracking the wavelength shift of the nanorod-localized surface plasmon resonant scattering spectrum using a dark-field microspectroscopy system. The lowest streptavidin concn. that was exptl. measured was 1 nM, which is a factor of 1000-fold lower than the previously reported detection limit for streptavidin binding by biotinylated single plasmonic nanostructures. The authors believe that the current optical setup is able to reliably measure wavelength shifts as small as 0.3 nm. Binding of streptavidin at 1 nM concn. induces a mean resonant wavelength shift of 0.59 nm suggesting that the authors are currently operating at close to the limit of detection of the system.
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Raschke, G.; Kowarik, S.; Franzl, T.; Sonnichsen, C.; Klar, T. A.; Feldmann, J.; Nichtl, A.; Kurzinger, K. Biomolecular Recognition Based on Single Gold Nanoparticle Light Scattering Nano Lett. 2003, 3, 935– 938
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McFarland, A. D.; Van Duyne, R. P. Single Silver Nanoparticles as Real-Time Optical Sensors with Zeptomole Sensitivity Nano Lett. 2003, 3, 1057– 1062
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Single Silver Nanoparticles as Real-Time Optical Sensors with Zeptomole Sensitivity
McFarland, Adam D.; Van Duyne, Richard P.
Nano Letters (2003), 3 (8), 1057-1062CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
This work utilizes dark-field optical microscopy to demonstrate the localized surface plasmon resonance λmax response of individual Ag nanoparticles to the formation of a monolayer of small-mol. adsorbates. The adsorption of fewer than 60 000 1-hexadecanethiol mols. on single Ag nanoparticles results in a localized surface plasmon resonance shift of 40.7 nm. Addnl., the kinetics of the single nanoparticle response was shown to be comparable to that of other real-time sensor technologies.
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Rindzevicius, T.; Alaverdyan, Y.; Dahlin, A.; Hook, F.; Sutherland, D. S.; Kall, M. Plasmonic Sensing Characteristics of Single Nanometric Holes Nano Lett. 2005, 5, 2335– 2339
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Sherry, L. J.; Jin, R. C.; Mirkin, C. A.; Schatz, G. C.; Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms Nano Lett. 2006, 6, 2060– 2065
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Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms
Sherry, Leif J.; Jin, Rongchao; Mirkin, Chad A.; Schatz, George C.; Van Duyne, Richard P.
Nano Letters (2006), 6 (9), 2060-2065CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
The plasmonic properties of single silver triangular nanoprisms were investigated by using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) were obsd. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approxn. (DDA). The dipole resonance is very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chem. sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is as high as 205 nm RIU-1 and the plasmon line width as narrow as ∼0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU-1/line width (eV), as high as 3.3. In addn., the LSPR shift response to alkanethiol chain length was linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.
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van Dijk, M. A.; Lippitz, M.; Orrit, M. Far-Field Optical Microscopy of Single Metal Nanoparticles Acc. Chem. Res. 2005, 38, 594– 601
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Far-Field Optical Microscopy of Single Metal Nanoparticles
van Dijk, Meindert A.; Lippitz, Markus; Orrit, Michel
Accounts of Chemical Research (2005), 38 (7), 594-601CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. Individual noble-metal particles, with sizes ranging from a few tenths to some hundreds of nanometers, can now be detected by far-field optics. Single-particle microscopy gives access to inhomogeneity, distributions, and fluctuations, which were previously hidden in ensemble expts. Scattering methods rely on dark-field illumination, spectral signatures of the metal particles, or both. More advanced techniques provide high sensitivity and improved selectivity with respect to other scatterers by isolating metal-specific signals, for example the refractive index change due to heating of the environment by a pump beam or the time-resolved optical response of the particle to a short pump pulse. The authors review and compare linear and nonlinear methods in far-field optical microscopy that have reached the single-particle regime by scattered light, thermal effects, photoluminescence, or nonlinear frequency generation.
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Sonnichsen, C.; Geier, S.; Hecker, N. E.; von Plessen, G.; Feldmann, J.; Ditlbacher, H.; Lamprecht, B.; Krenn, J. R.; Aussenegg, F. R.; Chan, V. Z. H.; Spatz, J. P.; Moller, M. Spectroscopy of Single Metallic Nanoparticles Using Total Internal Reflection Microscopy Appl. Phys. Lett. 2000, 77, 2949– 2951
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Haes, A. J.; Van Duyne, R. P.; Zou, S. L.; Schatz, G. C. Nanoscale Optical Biosensor: Short Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles J. Phys. Chem. B 2004, 108, 6961– 6968
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Nanoscale Optical Biosensor: Short Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles
Haes, Amanda J.; Zou, Shengli; Schatz, George C.; Van Duyne, Richard P.
Journal of Physical Chemistry B (2004), 108 (22), 6961-6968CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
Silver and gold nanotriangles were fabricated by nanosphere lithog. (NSL) and their localized surface plasmon resonance (LSPR) spectra were measured by UV-vis extinction spectroscopy. It is demonstrated that the short range (viz., 0-2 nm) distance dependence of the electromagnetic fields that surround these nanoparticles when resonantly excited can be systematically tuned by changing their size, structure, and compn. This is accomplished by measuring the shift in the peak wavelength, λmax, of their LSPR spectra caused by the adsorption of hexadecanethiol as a function of nanoparticle size (in-plane width, out-of-plane height, and aspect ratio), shape (truncated tetrahedron vs. hemisphere), and compn. (silver vs. gold). We find that the hexadecanethiol-induced LSPR shift for Ag triangles decreases when in-plane width is increased at fixed out-of-plane height or when height is increased at fixed width. These trends are the opposite to what was seen in an earlier study of the long range distance dependence in which 30 nm thick layers were examd. (Haes et al. J. Phys. Chem. B 2004, 108, 109), but both the long and short range results are confirmed by a theor. anal. based on finite element electrodynamics. The theory results also indicate that the short range results are primarily sensitive to hot spots (regions of high induced elec. field) near the tips of the triangles, so this provides an example where enhanced local fields play an important role in extinction spectra. Our measurements further show that the hexadecanethiol-induced LSPR peak shift is larger for nanotriangles than for hemispheres with equal vols. and is larger for Ag nanotriangles than for Au nanotriangles with the same in-plane widths and out-of-plane heights. The dependence of the alkanethiol-induced LSPR peak shift on chain length for Ag nanotriangles is approx. size-independent. We anticipate that the improved understanding of the short range dependence of the adsorbate-induced LSPR peak shift on nanoparticle structure and compn. reported here will translate to significant improvements in the sensitivity of refractive-index-based nanoparticle nanosensors.
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Haes, A. J.; Van Duyne, R. P.; Zou, S. L.; Schatz, G. C. A Nanoscale Optical Biosensor: The Long Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles J. Phys. Chem. B 2004, 108, 109– 116
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A Nanoscale Optical Biosensor: The Long Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles
Haes, Amanda J.; Zou, Shengli; Schatz, George C.; Van Duyne, Richard P.
Journal of Physical Chemistry B (2004), 108 (1), 109-116CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The elucidation of the long range distance dependence of the localized surface plasmon resonance (LSPR) of surface-confined noble metal nanoparticles is the aim of this work. It was suspected that the linear distance dependence found in Me(CH2)xSH self-assembled monolayer (SAM) formation was the thin shell limit of a longer range, nonlinear dependence. To verify this, multilayer SAM shells based on the interaction of HOOC(CH2)10SH and Cu2+ were assembled onto surface-confined noble metal nanoparticles and were monitored using UV-visible spectroscopy. Measurement of the LSPR extinction peak shift vs. no. of layers and adsorbate thickness is nonlinear and has a sensing range that is dependent on the compn., shape, in-plane width, and out-of-plane height of the nanoparticles. Theor. calcns. based on an accurate electrodynamics description of the metal nanoparticle plus surrounding layered material indicate plasmon resonance wavelength shifts that are in excellent agreement with the measurements. The calcns. show that the sensing range is detd. by falloff of the av. induced elec. field around the nanoparticle. This detailed set of expts. coupled with an excellent theory vs. expt. comparison prove that the sensing capabilities of noble metal nanoparticles can be size tuned to match the dimensions of biol. and chem. analytes by adjusting the aforementioned properties. The optimization of the LSPR nanosensor for a specific analyte will significantly improve an already sensitive nanoparticle-based sensor.
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Evanoff, D. D.; White, R. L.; Chumanov, G. Measuring the Distance Dependence of the Local Electromagnetic Field from Silver Nanoparticles J. Phys. Chem. B 2004, 108, 1522– 1524
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Measuring the Distance Dependence of the Local Electromagnetic Field from Silver Nanoparticles
Evanoff, David D., Jr.; White, Ryan L.; Chumanov, George
Journal of Physical Chemistry B (2004), 108 (5), 1522-1524CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The distance dependence of the local EM field assocd. with 84 ± 5 nm silver particles was characterized by monitoring spectral shifts in the plasmon resonance due to changes in local dielec. environment, which was controlled by coating the particles with silica layers of various thicknesses. The local EM field that extends beyond the phys. boundaries of the particles is defined as that providing feedback between the local environment and the electron oscillations in a particle.
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Hao, E.; Schatz, G. C. Electromagnetic Fields around Silver Nanoparticles and Dimers J. Chem. Phys. 2004, 120, 357– 366
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Electromagnetic fields around silver nanoparticles and dimers
Hao, Encai; Schatz, George C.
Journal of Chemical Physics (2004), 120 (1), 357-366CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
We use the discrete dipole approxn. to investigate the electromagnetic fields induced by optical excitation of localized surface plasmon resonances of silver nanoparticles, including monomers and dimers, with emphasis on what size, shape, and arrangement leads to the largest local elec. field (E-field) enhancement near the particle surfaces. The results are used to det. what conditions are most favorable for producing enhancements large enough to observe single mol. surface enhanced Raman spectroscopy. Most of the calcns. refer to triangular prisms, which exhibit distinct dipole and quadrupole resonances that can easily be controlled by varying particle size. In addn., for the dimer calcns. we study the influence of dimer sepn. and orientation, esp. for dimers that are sepd. by a few nanometers. We find that the largest |E|2 values for dimers are about a factor of 10 larger than those for all the monomers examd. For all particles and particle orientations, the plasmon resonances which lead to the largest E-fields are those with the longest wavelength dipolar excitation. The spacing of the particles in the dimer plays a crucial role, and we find that the spacing needed to achieve a given |E|2 is proportional to nanoparticle size for particles below 100 nm in size. Particle shape and curvature are of lesser importance, with a head to tail configuration of two triangles giving enhanced fields comparable to head to head, or rounded head to tail. The largest |E|2 values we have calcd. for spacings of 2 nm or more is ∼105.
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Imura, K.; Okamoto, H.; Nagahra, T. Plasmon Mode Imaging of Single Gold Nanorods J. Am. Chem. Soc. 2004, 126, 12730– 12731
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Plasmon Mode Imaging of Single Gold Nanorods
Imura, Kohei; Nagahara, Tetsuhiko; Okamoto, Hiromi
Journal of the American Chemical Society (2004), 126 (40), 12730-12731CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
We have investigated two-photon-induced photoluminescence images and spectra of single gold nanorods by using an apertured scanning near-field optical microscope. The obsd. PL spectrum of single gold nanorod can be explained by the radiative recombination of the electron-hole pair near the X and L symmetry points. PL images reveal characteristic features reflecting an eigenfunction of a specific plasmon mode as well as elec. field distributions around the nanorod.
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Stenberg, E.; Persson, B.; Roos, H.; Urbaniczky, C. Quantitative Determination of Surface Concentration of Protein with Surface Plasmon Resonance Using Radiolabeled Proteins J. Colloid Interface Sci. 1991, 143, 513– 526
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Quantitative determination of surface concentration of protein with surface plasmon resonance using radiolabeled proteins
Stenberg, Esa; Persson, Bjoern; Roos, Haakan; Urbaniczky, Csaba
Journal of Colloid and Interface Science (1991), 143 (2), 513-26CODEN: JCISA5; ISSN:0021-9797.
A methodol. to correlate the abs. surface concn. of protein to the surface plasmon resonance (SPR) response is described. The thickness and the optical consts. for each layer on the sensor chip used were detd. with different optical techniques. In a flow injection system, the steady-state SPR response was correlated to the abs. amt. of radiolabeled protein adsorbed by using a surface scintillation counter. The proteins used, 14C-labeled human transferrin and chymotrypsinogen A, as well as in vivo 35S-labeled monoclonal antibodies, were adsorbed via electrostatic interaction to a carboxymethylated dextran hydrogen on the sensor chip. For these proteins, surface concns. from 2 to 50 ng/mm2 correspond linearly to the SPR response, with specific response in the range 0.10 ± 0.01° (ng/mm2)-1, independent of protein size. The min. detectable surface concn. of protein is estd. to be 50 pg/mm2 with this SPR instrument. Optical models used developed to describe how the SPR response depends on the distribution of the adsorbed protein within the hydrogel vol. at the surface. With a thin-film optical program, the theor. SPR response for the different models were calcd. Comparison with exptl. data shows that the protein is distributed within an ∼100-mm-thick dextran hydrogel layer.
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Vollmer, F.; Braun, D.; Libchaber, A.; Khoshsima, M.; Teraoka, I.; Arnold, S. Protein Detection by Optical Shift of a Resonant Microcavity Appl. Phys. Lett. 2002, 80, 4057– 4059
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Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. Colloidal Metal Films as a Substrate for Surface-Enhanced Spectroscopy J. Phys. Chem. 1995, 99, 9466– 9471
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Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method Chem. Mater. 2003, 15, 1957– 1962
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Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method
Nikoobakht, Babak; El-Sayed, Mostafa A.
Chemistry of Materials (2003), 15 (10), 1957-1962CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
A method is used for prepg. gold nanorods with aspect ratios of 1.5-10 for which the surface plasmon absorption maxima are between 600 and 1300 nm. This method has been adapted from a previously published seed-mediated growth method of N. Jana et al. (2001). The disadvantages and limitations of the earlier method (formation of noncylindrical nanorods, φ-shaped particles, and formation of a large fraction of spherical particles) have been overcome by using a hexadecyltrimethylammonium bromide (CTAB)-capped seed instead of a citrate-capped one. In a single-component surfactant system, the silver content of the growth soln. was used to grow Au nanorods to a desired length. This results in reproducible formation of nanorods with aspect ratios ranging from 1.5 to 4.5. To grow longer nanorods with aspect ratios of 4.6-10, a binary surfactant mixt. composed of benzyldimethylhexadecylammonium chloride and CTAB was used. Nanorods are grown in this mixt. either by aging or by addn. of a growth soln. suitable for shorter nanorods. Effects of the silver ions and co-surfactant along with the growth mechanism of the nanorods are discussed.
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Chen, C.-D.; Cheng, S.-F.; Chau, L.-K.; Wang, C. R. C. Sensing Capability of the Localized Surface Plasmon Resonance of Gold Nanorods Biosens. Bioelectron. 2007, 22, 926– 932
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Sensing capability of the localized surface plasmon resonance of gold nanorods
Chen, Cheng-Dah; Cheng, Shu-Fang; Chau, Lai-Kwan; Wang, C. R. Chris
Biosensors & Bioelectronics (2007), 22 (6), 926-932CODEN: BBIOE4; ISSN:0956-5663. (Elsevier B.V.)
We demonstrate the feasibility of using the longitudinal component of gold nanorod's surface plasmon resonance in biomol. sensing. The sensitive dependence of the absorption max. on the dielec. const. of the particle interfacial region makes gold nanorods a promise for constructing a biomol. sensing scheme. The sensor contg. gold nanorods, with a mean aspect ratio of 5.2, exhibits a sensitivity of ∼366 nm/RIU (refractive index unit), which increases accordingly with the increase of the particle mean aspect ratios. Such a biosensor was further modified to demonstrate its effectiveness in quant. detection for selective binding events, such as biotin/streptavidin pairs, through a process in which biotin mols. were chem. attached to the gold nanorods' surface prior to detection measurements. Results showed that the spectral λmax shifts linearly to the concns. of the streptavidin. The results from both expt. and model calcns. strongly indicate the efficiency of the longitudinal surface plasmon absorption band in biosensing.
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Marinakos, S. M.; Chen, S.; Chilkoti, A. Plasmonic Detection of a Model Analyte in Serum by a Gold Nanorod Sensor Anal. Chem. 2007, 79, 5278– 5283
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Plasmonic Detection of a Model Analyte in Serum by a Gold Nanorod Sensor
Marinakos, Stella M.; Chen, Sihai; Chilkoti, Ashutosh
Analytical Chemistry (Washington, DC, United States) (2007), 79 (14), 5278-5283CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
The authors describe the fabrication of a label-free, chip-based biosensor based on the localized surface plasmon resonance (LSPR) of gold nanorods. Gold nanorods were chemisorbed onto a mercaptosilane-modified glass substrate, followed by conjugation of biotin to the nanorods. Streptavidin binding to biotin was monitored by the wavelength shift of the LSPR peak in the UV-vis extinction spectrum of the immobilized gold nanorods due to the change in local refractive index at the gold nanorod surface induced by streptavidin binding. The limit of detection of the sensor is 0.005 μg/mL (94 pM) in PBS and 1 μg/mL (19 nM) in serum, and the dynamic range spans 94 pM to 0.19 μM. The advantages of the nanorod-based sensor over an LSPR sensor that the authors had previously fabricated from gold nanospheres (Nath, N., and Chilkoti, A., 2002 and 2004) are the significantly lower detection limit and the internal self-ref. that the signal of the nanorod sensor provides based on the measurement of peak wavelength shift.
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Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods J. Phys. Chem. B 2001, 105, 4065– 4067
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Wet chemical synthesis of high aspect ratio cylindrical gold nanorods
Jana, Nikhil R.; Gearheart, Latha; Murphy, Catherine J.
Journal of Physical Chemistry B (2001), 105 (19), 4065-4067CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
Gold nanorods with aspect ratios of 4.6 ± 1.2, 13 ± 2, and 18 ± 2.5 (all with 16 ± 3 nm short axis) were prepd. by using a seeding growth approach in the presence of an aq. micelle template. Citrate-capped 3.5 nm diam. gold particles, prepd. by the redn. of HAuCl4 with borohydride, are used as the seed. The aspect ratio of the nanorods is controlled by varying the ratio of seed to metal salt. The long rods are isolated from spherical particles by centrifuging.
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Busbee, B. D.; Obare, S. O.; Murphy, C. J. An Improved Synthesis of High-Aspect-Ratio Gold Nanorods Adv. Mater. 2003, 15, 414– 416
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An improved synthesis of high-aspect-ratio gold nanorods
Busbee, Brantley D.; Obare, Sherine O.; Murphy, Catherine J.
Advanced Materials (Weinheim, Germany) (2003), 15 (5), 414-416CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
An improved synthetic methodol. that produces monodisperse gold nanorods of high aspect ratio in ∼90% yield is described. The method involves a three-step seed-mediated growth process. In the first step, 4 nm gold nanoparticles are produced by the redn. of HAuCl4 with sodium borohydride in the presence of sodium citrate. These gold nanoparticles are used as seed for the next growth steps. The seed is added to a growth soln. contg. the surfactant cetyltrimethylammonium bromide, HAuCl4, ascorbic acid, and sodium hydroxide. This new method requires only one round of purifn. to produce gold nanorods of aspect ratio ∼18 at pH 3.5. Gold nanorods with an aspect ratio of ∼20 are obtained in the presence of heptane.
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Sau, T. K.; Murphy, C. J. Room Temperature, High-Yield Synthesis of Multiple Shapes of Gold Nanoparticles in Aqueous Solution J. Am. Chem. Soc. 2004, 126, 8648– 8649
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Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution
Sau, Tapan K.; Murphy, Catherine J.
Journal of the American Chemical Society (2004), 126 (28), 8648-8649CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A seed-mediated growth method was used to control the morphol. and dimensions of Au nanocrystals by manipulation of the exptl. parameters in aq. soln. at room temp. This chem. route produces various structural architectures with rod-, rectangle-, hexagon-, cube-, triangle-, and starlike profiles and branched (such as bi-, tri-, tetra-, and multipod) Au nanocrystals of various dimensions in high yield in the presence of a single surfactant, cetyltrimethylammonium bromide.
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Link, S.; Mohamed, M. B.; El-Sayed, M. A. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant J. Phys. Chem. B 1999, 103, 3073– 3077
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40
Simulation of the optical absorption spectra of gold nanorods as a function of their aspect ratio and the effect of the medium dielectric constant
Link, S.; Mohamed, M. B.; El-Sayed, M. A.
Journal of Physical Chemistry B (1999), 103 (16), 3073-3077CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
Gold nanorods with different aspect ratios are prepd. in micelles by the electrochem. method and their absorption spectra are modeled by theory. Exptl., a linear relationship is found between the absorption max. of the longitudinal plasmon resonance and the mean aspect ratio as detd. from TEM. It is shown here that such a linear dependence is also predicted theor. However, calcns. also show that the absorption max. of the longitudinal plasmon resonance depends on the medium dielec. const. in a linear fashion for a fixed aspect ratio. Attempts to fit the calcns. to the exptl. values indicate that the medium dielec. const. has to vary with the aspect ratio in a nonlinear way. Chem., this suggests that the structure of the micelle capping the gold nanorods is size dependent. Furthermore, comparison with the results obtained for rods of different aspect ratios made by systematic thermal decompn. of the long rods further suggests that the medium dielec. const. is also temp. dependent. This is attributed to thermal annealing of the structure of the micelles around the nanorods.
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Weissleder, R. A Clearer Vision for In Vivo Imaging Nat. Biotechnol. 2001, 19, 316– 317
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A clearer vision for in vivo imaging
Weissleder, Ralph
Nature Biotechnology (2001), 19 (4), 316-317CODEN: NABIF9; ISSN:1087-0156. (Nature America Inc.)
There is no expanded citation for this reference.
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Jackson, J. B.; Westcott, S. L.; Hirsch, L. R.; West, J. L.; Halas, N. J. Controlling the Surface Enhanced Raman Effect via the Nanoshell Geometry Appl. Phys. Lett. 2003, 82, 257– 259
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43
Hanarp, P.; Kall, M.; Sutherland, D. S. Optical Properties of Short Range Ordered Arrays of Nanometer Gold Disks Prepared by Colloidal Lithography J. Phys. Chem. B 2003, 107, 5768– 5772
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Optical Properties of Short Range Ordered Arrays of Nanometer Gold Disks Prepared by Colloidal Lithography
Hanarp, Per; Kaell, Mikael; Sutherland, Duncan S.
Journal of Physical Chemistry B (2003), 107 (24), 5768-5772CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The optical properties of Au nanodisk arrays prepd. by colloidal lithog. are studied exptl. The arrays exhibit short range translational order and weak interparticle interactions. Tunable localized surface plasmon resonances are achieved by varying the diam. of the disks at const. disk height. The macroscopic optical properties are well-described by modeling the Au disks as oblate spheroids in the electrostatic limit. The optical sensing capabilities of the disks are studied by varying the surrounding refractive index. It is found, in agreement with theory, that more oblate disk shapes have higher sensitivity. Probably nanodisks prepd. by colloidal lithog. are of interest as substrates for optimizing optical biosensing methods at the nanometer scale.
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Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Kall, M.; Bryant, G. W.; de Abajo, F. J. G. Optical Properties of Gold Nanorings Phys. Rev. Lett. 2003, 90
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45
Raschke, G.; Brogl, S.; Susha, A. S.; Rogach, A. L.; Klar, T. A.; Feldmann, J.; Fieres, B.; Petkov, N.; Bein, T.; Nichtl, A.; Kurzinger, K. Gold Nanoshells Improve Single Nanoparticle Molecular Sensors Nano Lett. 2004, 4, 1853– 1857
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Hafner, J. H.; Nehl, C. L.; Grady, N. K.; Goodrich, G. P.; Tam, F.; Halas, N. J. Scattering Spectra of Single Gold Nanoshells Nano Lett. 2004, 4, 2355– 2359
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47
Novo, C.; Gomez, D.; Perez-Juste, J.; Zhang, Z. Y.; Petrova, H.; Reismann, M.; Mulvaney, P.; Hartland, G. V. Contributions from Radiation Damping and Surface Scattering to the Linewidth of the Longitudinal Plasmon Band of Gold Nanorods: A Single Particle Study Phys. Chem. Chem. Phys. 2006, 8, 3540– 3546
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Contributions from radiation damping and surface scattering to the linewidth of the longitudinal plasmon band of gold nanorods: a single particle study
Novo, Carolina; Gomez, Daniel; Perez-Juste, Jorge; Zhang, Zhenyuan; Petrova, Hristina; Reismann, Maximilian; Mulvaney, Paul; Hartland, Gregory V.
Physical Chemistry Chemical Physics (2006), 8 (30), 3540-3546CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
The scattering spectra of single Au nanorods with aspect ratios of 2-4 were examd. by dark field microscopy. The longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Anal. of the data using γ = γbulk + AνF/Leff, where Leff is the effective path length of the electrons and νF is the Fermi velocity, allows one to det. a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were examd. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in agreement with recent work on spherical Au nanoparticles by C. Sonnichsen et al. (2002) and by S. Berciaud et al. (2005).
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Prescott, S. W.; Mulvaney, P. Gold Nanorod Extinction Spectra J. Appl. Phys. 2006, 99
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49
Curry, A.; Nusz, G.; Chilkoti, A.; Wax, A. Analysis of Total Uncertainty in Spectral Peak Measurements for Plasmonic Nanoparticle-Based Biosensors Appl. Opt. 2007, 46, 1931– 1939
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Analysis of total uncertainty in spectral peak measurements for plasmonic nanoparticle-based biosensors
Curry Adam; Nusz Gregory; Chilkoti Ashutosh; Wax Adam
Applied optics (2007), 46 (10), 1931-9 ISSN:0003-6935.
One goal of recent research on plasmonic nanoparticle-based sensors is maximizing nanoparticle sensitivity or shift of resonance peak wavelength per refractive index change. Equally important is a measurement system's peak location uncertainty or shift resolution. We provide systematic analyses and discuss optimization of factors that determine peak location uncertainty, reporting values as low as 0.3 nm for the presented scheme. This type of analysis is important, in part, because it provides a means of evaluating detection thresholds for biosensor applications such as analyte binding. We estimate thresholds of 310 streptavidin molecules for the presented scheme and 20 molecules with system improvements.
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Curry, A.; Nusz, G.; Chilkoti, A.; Wax, A. Substrate Effect on Refractive Index Dependence of Plasmon Resonance for Individual Silver Nanoparticles Observed Using Darkfield Micro-Spectroscopy Opt. Express 2005, 13, 2668– 2677
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Substrate effect on refractive index dependence of plasmon resonance for individual silver nanoparticles observed using darkfield micro-spectroscopy
Curry, A.; Nusz, G.; Chilkoti, A.; Wax, A.
Optics Express (2005), 13 (7), 2668-2677CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
We use optical darkfield micro-spectroscopy to characterize the plasmon resonance of individual silver nanoparticles in the presence of a substrate. The optical system permits multiple individual nanoparticles to be identified visually for simultaneous spectroscopic study. For silver particles bound to a silanated glass substrate, we observe changes in the plasmon resonance due to induced variations in the local refractive index. The shifts in the plasmon resonance are investigated using a simple anal. theory in which the contributions from the substrate and environment are weighted with distance from the nanoparticle. The theory is compared with exptl. results to det. a weighting factor which facilitates modeling of environmental refractive index changes using std. Mie code. Use of the optical system for characterizing nanoparticles attached to substrates for biosensing applications is discussed.
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Kuwata, H.; Tamaru, H.; Esumi, K.; Miyano, K. Resonant Light Scattering from Metal Nanoparticles: Practical Analysis Beyond Rayleigh Approximation Appl. Phys. Lett. 2003, 83, 4625– 4627
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Resonant light scattering from metal nanoparticles: Practical analysis beyond Rayleigh approximation
Kuwata, Hitoshi; Tamaru, Hiroharu; Esumi, Kunio; Miyano, Kenjiro
Applied Physics Letters (2003), 83 (22), 4625-4627CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
We propose a simple anal. formula that can quant. predict resonant light scattering from metal nanoparticles of arbitrary shape, whose sizes are too large for Rayleigh approxn. to be applicable. The formula has been derived as an empirical extension of Mie's rigorous calcn. for light scattering from spheres. It can very well reproduce the exptl. characteristics of light scattering from Au nanorods.
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Xu, X. D.; Cortie, M. B. Shape Change and Color Gamut in Gold Nanorods, Dumbbells, and Dog Bones Adv. Funct. Mater. 2006, 16, 2170– 2176
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Shape change and color gamut in gold nanorods, dumbbells, and dog bones
Xu, Xiaoda; Cortie, Michael B.
Advanced Functional Materials (2006), 16 (16), 2170-2176CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
Deviations from a prolate ellipsoidal shape have a significant effect on the optical properties of Au nanorods. Transitions from rods to 'dumbbell'- or 'phi'-shaped particles lead to a shift in the longitudinal plasmon peak in the blue and red directions, resp. Development of 'dog-bone' shapes leads to a red-shift and to the development of a 3rd peak. A broad and flexible color gamut can be obtained.
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Yan, B. H.; Yang, Y.; Wang, Y. C. Comment On “Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant” J. Phys. Chem. B 2003, 107, 9159– 9159
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54
Link, S.; El-Sayed, M. A.; Mohamed, M. B. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant (Vol 103b, Pg 3073, 1999) J. Phys. Chem. B 2005, 109, 10531– 10532
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There is no corresponding record for this reference.
55
Link, S.; Mohamed, M. B.; El-Sayed, M. A. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant J. Phys. Chem. B 1999, 103, 3073– 3077
Google Scholar
55
Simulation of the optical absorption spectra of gold nanorods as a function of their aspect ratio and the effect of the medium dielectric constant
Link, S.; Mohamed, M. B.; El-Sayed, M. A.
Journal of Physical Chemistry B (1999), 103 (16), 3073-3077CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
Gold nanorods with different aspect ratios are prepd. in micelles by the electrochem. method and their absorption spectra are modeled by theory. Exptl., a linear relationship is found between the absorption max. of the longitudinal plasmon resonance and the mean aspect ratio as detd. from TEM. It is shown here that such a linear dependence is also predicted theor. However, calcns. also show that the absorption max. of the longitudinal plasmon resonance depends on the medium dielec. const. in a linear fashion for a fixed aspect ratio. Attempts to fit the calcns. to the exptl. values indicate that the medium dielec. const. has to vary with the aspect ratio in a nonlinear way. Chem., this suggests that the structure of the micelle capping the gold nanorods is size dependent. Furthermore, comparison with the results obtained for rods of different aspect ratios made by systematic thermal decompn. of the long rods further suggests that the medium dielec. const. is also temp. dependent. This is attributed to thermal annealing of the structure of the micelles around the nanorods.
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Nath, N.; Chilkoti, A. Label-Free Biosensing by Surface Plasmon Resonance of Nanoparticles on Glass: Optimization of Nanoparticle Size Anal. Chem. 2004, 76, 5370– 5378
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56
Label-Free Biosensing by Surface Plasmon Resonance of Nanoparticles on Glass: Optimization of Nanoparticle Size
Nath, Nidhi; Chilkoti, Ashutosh
Analytical Chemistry (2004), 76 (18), 5370-5378CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
The unique optical properties of noble metal nanoparticles have been used to design a label-free biosensor in a chip format. In this paper, we demonstrate that the size of gold nanoparticles significantly affects the sensitivity of the biosensor. Gold nanoparticles with diams. in the range of 12-48 nm were synthesized in soln. and sensor chips were fabricated by chemisorption of these nanoparticles on amine-functionalized glass. Sensors fabricated from 39-nm-diam. gold nanoparticles exhibited max. sensitivity to the change of the bulk refractive index and the largest "anal. vol.", defined as the region around the nanoparticle within which a change in refractive index causes a change in the optical properties of the immobilized nanoparticles. The detection limit for streptavidin-biotin binding of a sensor fabricated from 39-nm-diam. nanoparticles was 20-fold better than a previously reported sensor fabricated from 13-nm-diam. gold nanoparticles. We also discuss several other factors that could improve the performance of the next generation of these immobilized metal nanoparticle sensors.
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Whitney, A. V.; Elam, J. W.; Zou, S. L.; Zinovev, A. V.; Stair, P. C.; Schatz, G. C.; Van Duyne, R. P. Localized Surface Plasmon Resonance Nanosensor: A High-Resolution Distance-Dependence Study Using Atomic Layer Deposition J. Phys. Chem. B 2005, 109, 20522– 20528
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Localized Surface Plasmon Resonance Nanosensor: A High-Resolution Distance-Dependence Study Using Atomic Layer Deposition
Whitney, Alyson V.; Elam, Jeffrey W.; Zou, Shengli; Zinovev, Alex V.; Stair, Peter C.; Schatz, George C.; Van Duyne, Richard P.
Journal of Physical Chemistry B (2005), 109 (43), 20522-20528CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
At. layer deposition (ALD) is used to deposit 1-600 monolayers of Al2O3 on Ag nanotriangles fabricated by nanosphere lithog. (NSL). Each monolayer of Al2O3 has a thickness of 1.1 Å. It is demonstrated that the localized surface plasmon resonance (LSPR) nanosensor can detect Al2O3 film growth with at. spatial resoln. normal to the nanoparticle surface. This is approx. 10 times greater spatial resoln. than that in our previous long-range distance-dependence study using multilayer self-assembled monolayer shells. The use of ALD enables the study of both the long- and short-range distance dependence of the LSPR nanosensor in a single unified expt. Ag nanoparticles with fixed in-plane widths and decreasing heights yield larger sensing distances. XPS, variable angle spectroscopic ellipsometry, and quartz crystal microbalance measurements are used to study the growth mechanism. It is proposed that the growth of Al2O3 is initiated by the decompn. of trimethylaluminum on Ag. Semiquant. theor. calcns. were compared with the exptl. results and yield excellent agreement.
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Hicks, E. M.; Zhang, X. Y.; Zou, S. L.; Lyandres, O.; Spears, K. G.; Schatz, G. C.; Van Duyne, R. P. Plasmonic Properties of Film over Nanowell Surfaces Fabricated by Nanosphere Lithography J. Phys. Chem. B 2005, 109, 22351– 22358
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Plasmonic Properties of Film over Nanowell Surfaces Fabricated by Nanosphere Lithography
Hicks, Erin M.; Zhang, Xiaoyu; Zou, Shengli; Lyandres, Olga; Spears, Kenneth G.; Schatz, George C.; Van Duyne, Richard P.
Journal of Physical Chemistry B (2005), 109 (47), 22351-22358CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
In this work, a detailed and systematic study of the plasmonic properties of a novel film over nanowell surface is investigated. These nanostructures are fabricated using nanosphere lithog. and reactive ion etching and structurally characterized by AFM and SEM. The resulting structures show remarkably narrow plasmon bands in reflectance spectra (as little as 0.10 eV) and greater sensitivity to external dielec. environment than has been seen in other nanoparticle systems, resulting in an improvement in the figure of merit (FOM = refractive index sensitivity (eV·RIU-1)/full width at half-max. (eV)) for refractive index sensing. Theor. modeling for the plasmon spectra of these nanostructures is done using discrete dipole approxn. code under periodic boundary conditions. The modeling results match the measurements accurately in aspects of the variation of the plasmon line shape with altering internanowell distance and dielec. environment.
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Haes, A. J.; Van Duyne, R. P. A Nanoscale Optical Biosensor: Sensitivity and Selectivity of an Approach Based on the Localized Surface Plasmon Resonance Spectroscopy of Triangular Silver Nanoparticles J. Am. Chem. Soc. 2002, 124, 10596– 10604
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A nanoscale optical biosensor: Sensitivity and selectivity of an approach based on the localized surface plasmon resonance spectroscopy of triangular silver nanoparticles
Haes, Amanda J.; Van Duyne, Richard P.
Journal of the American Chemical Society (2002), 124 (35), 10596-10604CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Triangular silver nanoparticles (∼100 nm wide and 50 nm high) have remarkable optical properties. In particular, the peak extinction wavelength, λmax of their localized surface plasmon resonance (LSPR) spectrum is unexpectedly sensitive to nanoparticle size, shape, and local (∼10-30 nm) external dielec. environment. This sensitivity of the LSPR λmax to the nanoenvironment has allowed us to develop a new class of nanoscale affinity biosensors. The essential characteristics and operational principles of these LSPR nanobiosensors will be illustrated using the well-studied biotin-streptavidin system. Exposure of biotin-functionalized Ag nanotriangles to 100 nM streptavidin (SA) caused a 27.0 nm red-shift in the LSPR λmax. The LSPR λmax shift, ΔR/ΔRmax, vs. [SA] response curve was measured over the concn. range 10-15 M < [SA] < 10-6 M. Comparison of the data with the theor. normalized response expected for 1:1 binding of a ligand to a multivalent receptor with different sites but invariant affinities yielded approx. values for the satn. response, ΔRmax = 26.5 nm, and the surface-confined thermodn. binding const. Ka,surf = 1011 M-1. At present, the limit of detection (LOD) for the LSPR nanobiosensor is found to be in the low-picomolar to high-femtomolar region. A strategy to amplify the response of the LSPR nanobiosensor using biotinylated Au colloids and thereby further improve the LOD is demonstrated. Several control expts. were performed to define the LSPR nanobiosensor's response to nonspecific binding as well as to demonstrate its response to the specific binding of another protein. These include the following: (1) electrostatic binding of SA to a nonbiotinylated surface, (2) nonspecific interactions of prebiotinylated SA to a biotinylated surface, (3) nonspecific interactions of bovine serum albumin to a biotinylated surface, and (4) specific binding of anti-biotin to a biotinylated surface. The LSPR nanobiosensor provides a pathway to ultrasensitive biodetection expts. with extremely simple, small, light, robust, low-cost instrumentation that will greatly facilitate field-portable environmental or point-of-service medical diagnostic applications.
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Miller, M. M.; Lazarides, A. A. Sensitivity of Metal Nanoparticle Plasmon Resonance Band Position to the Dielectric Environment as Observed in Scattering J. Opt. A: Pure Appl. Opt. 2006, 8, S239– S249
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Miller, M. M.; Lazarides, A. A. Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment J. Phys. Chem. B 2005, 109, 21556– 21565
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Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment
Miller, Molly M.; Lazarides, Anne A.
Journal of Physical Chemistry B (2005), 109 (46), 21556-21565CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
Electrodynamic simulations of gold nanoparticle spectra were used to investigate the sensitivity of localized surface plasmon band position to the refractive index, n, of the medium for nanoparticles of various shapes and nanoshells of various structures. Among single-component nanoparticles less than 130 nm in size, sensitivities of dipole resonance positions to bulk refractive index are found to depend only upon the wavelength of the resonance and the dielec. properties of the metal and the medium. Among particle plasmons that peak in the frequency range where the real part of the metal dielec. function varies linearly with wavelength and the imaginary part is small and slowly varying, the sensitivity of the peak wavelength, λ*, to refractive index, n, is found to be a linearly increasing function of λ*, regardless of the structural features of the particle that det. λ*. Quasistatic theory is used to derive an anal. expression for the refractive index sensitivity of small particle plasmon peaks. Through this anal., the dependence of sensitivity on band position is found to be detd. by the wavelength dependence of the real part, ε', of the particle dielec. function, and the sensitivity results are found to extend to all particles with resonance conditions of the form, ε'* = -2χn2, where χ is a function of geometric parameters and other consts. The sensitivity results obsd. using accurate computational methods for dipolar plasmon bands of gold nanodisks, nanorods, and hollow nanoshells extend, therefore, to particles of other shapes (such as hexagonal and chopped tetrahedral), composed of other metals, and to higher-order modes. The bulk refractive index sensitivity yielded by the theory serves as an upper bound to sensitivities of nanoparticles on dielec. substrates and sensitivities of nanoparticles to local refractive index changes, such as those assocd. with biomol. sensing.
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Neish, C. S.; Martin, I. L.; Henderson, R. M.; Edwardson, J. M. Direct Visualization of Ligand-Protein Interactions Using Atomic Force Microscopy Br. J. Pharmacol. 2002, 135, 1943– 1950
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Direct visualization of ligand-protein interactions using atomic force microscopy
Neish, Calum S.; Martin, Ian L.; Henderson, Robert M.; Edwardson, J. Michael
British Journal of Pharmacology (2002), 135 (8), 1943-1950CODEN: BJPCBM; ISSN:0007-1188. (Nature Publishing Group)
1 Streptavidin is a 60-kDa tetramer which binds four mols. of biotin with extremely high affinity (KA ∼ 1014 M-1). We have used at. force microscopy (AFM) to visualize this ligand-protein interaction directly. 2 Biotin was tagged with a short (152-basepair; 50-nm) DNA rod and incubated with streptavidin. The resulting complexes were then imaged by AFM. The mol. vol. of streptavidin calcd. from the dimensions of the protein particles (105±3 nm3) was in close agreement with the value calcd. from its mol. mass (114 nm3). Biotinylation increased the apparent size of streptavidin (to 133±2 nm3), concomitant with an increase in the thermal stability of the tetramer. 3 Images of streptavidin with one to four mols. of DNA-biotin bound were obtained. When two ligands were bound, the angle between the DNA rods was either acute or obtuse, as expected from the relative orientations of the biotin binding sites. The ratio of acute: obtuse angles (1: 3) was lower than the expected value (1: 2), indicating a degree of steric hindrance in the binding of the DNA-biotin. The slight under-representation of higher occupancy states supported this idea. 4 Streptavidin with a single mol. of DNA-biotin bound was used to tag biotinylated β-galactosidase, a model multimeric enzyme. 5 The ability to image directly the binding of a ligand to its protein target by AFM provides useful information about the nature of the interaction, and about the effect of complex formation on the structure of the protein. Furthermore, the use of DNA-biotin/streptavidin tags could potentially shed light on the architecture of multi-subunit proteins.
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Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S. Quantitative Interpretation of the Response of Surface Plasmon Resonance Sensors to Adsorbed Films Langmuir 1998, 14, 5636– 5648
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Quantitative Interpretation of the Response of Surface Plasmon Resonance Sensors to Adsorbed Films
Jung, Linda S.; Campbell, Charles T.; Chinowsky, Timothy M.; Mar, Mimi N.; Yee, Sinclair S.
Langmuir (1998), 14 (19), 5636-5648CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
A simple but quant. math. formalism for interpretation of surface plasmon resonance (SPR) signals from adsorbed films of a wide variety of structures is presented. It can be used to est. adsorbed film thicknesses, surface coverages, or surface concns. from the SPR response over the entire range of film thicknesses without relying on calibration curves of response vs. known thicknesses or surface concns. This formalism is compared to more complex optical simulations. It is further tested by (1) calibrating the response of two SPR spectrometers to changes in bulk index of refraction, (2) using these calibrations with this formalism to predict responses to several well-characterized adlayer structures (alkanethiolates and serum albumin on gold, propylamine on COOH-functionalized gold), and then (3) comparing these predictions to measured SPR responses. Methods for estg. the refractive index of the adlayer material are also discussed. Detection limits in both bulk and adsorption-based analyses are discussed. The planar system used here has a detection limit of ∼0.003 nm in av. film thickness for adsorbates whose refractive index differs from that of the solvent by only 0.1. The temp. sensitivities of these two SPR spectrometers are characterized and discussed in terms of detection limits.
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Structural origins of high-affinity biotin binding to streptavidin
Weber, Patricia C.; Ohlendorf, D. H.; Wendoloski, J. J.; Salemme, F. R.
Science (Washington, DC, United States) (1989), 243 (4887), 85-8CODEN: SCIEAS; ISSN:0036-8075.
The high affinity of the noncovalent interaction between biotin and streptavidin forms the basis for many diagnostic assays that require the formation of an irreversible and specific linkage between biol. macromols. Comparison of the refined crystal structures of apo and a streptavidin:biotin complex shows that the high affinity results from several factors. These factors include the formation of multiple H bonds and van der Waals interactions between biotin and the protein, together with the ordering of surface polypeptide loops that bury the biotin in the protein interior. Structural alterations at the biotin-binding site produce quaternary changes in the streptavidin tetramer. These changes apparently propagate through cooperative deformations in the twisted β sheets that link tetramer subunits.
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The Shape Transition of Gold Nanorods
Chang, Ser-Sing; Shih, Chao-Wen; Chen, Cheng-Dah; Lai, Wei-Cheng; Wang, C. R. Chris
Langmuir (1999), 15 (3), 701-709CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
The authors report a revised synthetic procedure based on an electrochem. method for prepg. an aq. soln. contg. suspended Au nanorods. The mean aspect ratios of the Au nanorods can be exptl. adjusted between 1 and 7. The evolution of the longitudinal surface plasmon bands shows an eminently sensitive dependence on the aspect ratios of the nanorods. Their dependence is accordingly described by classical-electrostatic-model predictions. The shape transition of the nanorod particles was studied by varying some key influencing factors such as the wavelength, the laser fluence, and matrix effects. The nanorods were exposed to laser lines at 532 and 1064 nm, frequencies which correspond closely to the short- and long-axis plasmon resonances, resp. A photon-induced shape transition process was evidenced, and the corresponding rod-to-sphere conversion contributed by a photoannealing process was obsd. in both cases. 010000. Meanwhile, the authors obsd. a new type of φ-shaped Au nanostructure in the case of 1064-nm irradn., which possibly represents the early stage of the shape transition and indicates that the starting location of the at.-scale restructuring is at the centroid of the Au nanorod. The results of laser fluence-dependence measurements state that an efficient shape transition occurs via a multiphoton process. The authors also demonstrate the fabrication of the Au nanorod@silica nanostructures for preliminary studies of the matrix effects. As a result of the higher rigidity of the thin-silica-coating layer, the assocd. shape transition requires higher energy and proceeds less efficiently as compared with the cases for the micelle-stabilized Au nanorods.
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Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nano-dots and Nano-rods
Link, Stephan; El-Sayed, Mostafa A.
Journal of Physical Chemistry B (1999), 103 (40), 8410-8426CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
Review with 137 refs. The field of nanoparticle research has drawn much attention in the past decade as a result of the search for new materials. Size confinement results in new electronic and optical properties, possibly suitable for many electronic and optoelectronic applications. A characteristic feature of noble metal nanoparticles is the strong color of their colloidal solns., which is caused by the surface plasmon absorption. This article describes the studies of the properties of the surface plasmon absorption in metal nanoparticles that range in size between 10 and 100 nm. The effects of size, shape, and compn. on the plasmon absorption max. and its bandwidth are discussed. also, the optical response of the surface plasmon absorption due to excitation with femtosecond laser pulses allowed the authors to follow the electron dynamics (electron-electron and electron-phonon scattering) in these metal nanoparticles. The electron-phonon relaxation processes in nanoparticles, which are smaller than the electron mean free path, are independent of their size or shape. Intense laser heating of the electrons in these particles is also found to cause a shape transformation (photoisomerization of the rods into spheres or fragmentation), which depends on the laser pulse energy and pulse width.
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Muskens, O. L.; Bachelier, G.; Del Fatti, N.; Vallee, F.; Brioude, A.; Jiang, X. C.; Pileni, M. P. Quantitative Absorption Spectroscopy of a Single Gold Nanorod J. Phys. Chem. C 2008, 112, 8917– 8921
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Quantitative Absorption Spectroscopy of a Single Gold Nanorod
Muskens, Otto L.; Bachelier, Guillaume; Del Fatti, Natalia; Vallee, Fabrice; Brioude, Arnaud; Jiang, Xuchuan; Pileni, Marie-Paule
Journal of Physical Chemistry C (2008), 112 (24), 8917-8921CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The spectrally- and polarization-resolved absorption cross sections of a single Au nanorod were studied using the spatial modulation spectroscopy technique. The ensemble of its optical features, i.e., longitudinal and transverse surface plasmon resonances and interband absorption, was quant. characterized. The results are compared with numerical simulations using the discrete dipole approxn. and the finite element method, yielding information on the studied nanorod size and shape.
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Park, H. S.; Agarwal, A.; Kotov, N. A.; Lavrentovich, O. D. Controllable Side-by-Side and End-to-End Assembly of Au Nanorods by Lyotropic Chromonic Materials Langmuir 2008, 24, 13833– 13837
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Controllable side-by-side and end-to-end assembly of Au nanorods by lyotropic chromonic materials
Park, Heung-Shik; Agarwal, Ashish; Kotov, Nicholas A.; Lavrentovich, Oleg D.
Langmuir (2008), 24 (24), 13833-13837CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
A simple and universal technique for assembling gold nanorods by using self-assembled stacks of lyotropic chromonic materials, such as disodium chromoglycate, without covalent bonding between nanorods and linking agent is presented. The anisotropic electrostatic interaction between the chromonic stacks and nanorods allows achievement of either side-by-side or end-to-end assembly, depending on the surface charge of the nanorods. The assembled structures are stable within an extended temp. range. The degree of nanorod aggregation can be controlled by a no. of factors influencing the self-assembly of chromonic materials, such as the concn. and pH of the soln.
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Dahlin, A. B.; Tegenfeldt, J. O.; Hook, F. Improving the Instrumental Resolution of Sensors Based on Localized Surface Plasmon Resonance Anal. Chem. 2006, 78, 4416– 4423
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Improving the Instrumental Resolution of Sensors Based on Localized Surface Plasmon Resonance
Dahlin, Andreas B.; Tegenfeldt, Jonas O.; Hoeoek, Fredrik
Analytical Chemistry (2006), 78 (13), 4416-4423CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
The colorimetric variations induced upon changes in interfacial refractive index of nanoscale noble metal structures exhibiting localized surface plasmon resonance (LSPR) provides a convenient means of label-free, affinity-based detection of biomol. recognition reactions. However, despite being similar in nature to conventional SPR, LSPR has so far suffered from significantly lower data quality in terms of its signal-to-noise ratio (S/N) in typical biomol. recognition anal. In this work, generic data anal. algorithms and a simple exptl. setup that provide a S/N upon protein binding that is comparable to that of state-of-the art SPR systems are presented. Specifically, it is demonstrated how temporal variations (rate ∼0.5 Hz) in parameters proportional to the resonance peak position can be recorded simultaneously, yielding a peak position precision of <5 × 10-4 nm and an extinction noise level of <5 × 10-6 absorbance units (Abs). This, in turn, is shown to provide a S/N of ∼2000 (equiv. to a detection limit of <0.1 ng/cm2) for typical protein binding reactions. Furthermore, the importance of utilizing changes in both peak position and magnitude is highlighted by comparing different LSPR active noble metal architectures that respond differently to bulk and interfacial refractive index changes.
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Jana, N. R.; Gearheart, L.; Murphy, C. J. Seed-Mediated Growth Approach for Shape-Controlled Synthesis of Spheroidal and Rod-Like Gold Nanoparticles Using a Surfactant Template Adv. Mater. 2001, 13, 1389– 1393
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Seed-mediated growth approach for shape-controlled synthesis of spheroidal and rod-like gold nanoparticles using a surfactant template
Jana, Nikhil R.; Gearheart, Latha; Murphy, Catherine J.
Advanced Materials (Weinheim, Germany) (2001), 13 (18), 1389-1393CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
A two-step self-control synthesis of gold nanoparticles is developed. The synthesis consists of (1)prepn. of small size (3-4 nm) spherical gold nanoparticles, and (2)growth of the prepd. spherical particle in rod-like micellar environment. Roughly spherical borohydride-reduced gold nanoparticle seeds with a mean diam. of 3-4 nm were prepd. and added to the growth soln. contg. gold salt, cetyltrimethylammonium bromide (CTAB), a rod-shaped micellar template, ascorbic acid, and a small amt. of silver ions. The particles were studied by TEM after sepg. the surfactant by centrifugation. Spheroidal or rod-like gold particles with aspect ratios that were dependent on the gold seed-to-salt ratio were obtained. Initial addn. of preformed seeds increases the overall reaction rate, and hence the growth rate. The use of AgNO3 in the growth soln. was helpful in control of the spheroids/rods aspect ratio.
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Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mohwald, H. Electrostatic Self-Assembly of Silica Nanoparticle-Polyelectrolyte Multilayers on Polystyrene Latex Particles J. Am. Chem. Soc. 1998, 120, 8523– 8524
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Electrostatic Self-Assembly of Silica Nanoparticle-Polyelectrolyte Multilayers on Polystyrene Latex Particles
Caruso, Frank; Lichtenfeld, Heinz; Giersig, Michael; Moehwald, Helmuth
Journal of the American Chemical Society (1998), 120 (33), 8523-8524CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The construction of composite multilayers of 25 nm diam. silica particles and poly(diallyldimethylammonium chloride) was carried out on submicrometer-sized polystyrene latex particles via the sequential electrostatic adsorption of SiO2 and PDADMAC from dil. soln.
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Buscher, K.; Graf, K.; Ahrens, H.; Helm, C. A. Influence of Adsorption Conditions on the Structure of Polyelectrolyte Multilayers Langmuir 2002, 18, 3585– 3591
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Abstract
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Figure 1
Figure 1. Comparison of simulated versus experimental spectra. Scattering spectra collected from single nanorods from synthesized batches characterized by TEM to be (63.3 ± 8.2) × (24.9 ± 4.9) nm (n = 319, yellow dotted line) and (74 ± 9) × (33 ± 6) nm (n > 100, green dotted line). Simulated spectra for nanorods with dimensions of 64.5 × 24.7 nm (blue line) and 75 × 32 nm (red line) via the Kuwata method are also shown.
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Figure 2
Figure 2. Right axis: experimental wavelength shift of a nanorod (63.3 ± 8.2 nm × 24.9 ± 4.9 nm) as a function of deposited polyelectrolyte thickness (red dots) and shifts predicted by eq 7 (green line). Left axis: corresponding calculated distance dependence of the nanorod LSPR sensitivity (blue line).
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Figure 3
Figure 3. (A) Calculated molecular limits of detection for gold nanorods of arbitrary dimension based on eq 10. (B) Composite figures of merit (FOM) for nanorods of arbitrary dimensions. The composite FOM is calculated as the maximum number of bound molecules divided by the minimum number of detectable molecules. The surrounding gray regions in both are indicative of nanorod geometries that were not considered in this model because they either had resonances outside the visible spectrum or had scattering cross sections that are insufficient to collect spectra with SNR greater than 30.
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Figure 4
Figure 4. Left: TEM of rods used in this study. Nanorod length is measured to be 63.3 ± 8.2 nm and diameter 24.9 ± 4.9 nm (n = 319). Scale bar corresponds to 50 nm. Right: LSPR peak shift of individual, biotin-activated gold nanorods versus concentration of streptavidin. Open circles are the mean LSPR shift of approximately 15 nanorods per measurement at each concentration, and the error bars indicate 95% confidence interval. The solid line is a sigmoidal fit to the data. The dotted line indicates where the fit line crosses 0.3 nm shift, the measurement detection uncertainty as calculated above. This occurs at a concentration of 160 pM.
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Figure 5
Figure 5. Fits to scattering spectra of a single nanorod in water (blue), after biotin conjugation (green), and subsequently in 100 pM streptavidin (red). The 0.52 nm shift corresponds to approximately 27 streptavidin molecules by eq 8.
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References
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  A review. This review covers the emerging field of nanobiotechnol., in which nanoparticles are applied to the anal. of biomols. Nanoparticles can be used in a variety of bioanal. formats, and this review discusses four classes of use. First, nanoparticles as quantitation tags, such as the optical detection of quantum dots and the electrochem. detection of metallic nanoparticles. Second, encoded nanoparticles as substrates for multiplexed bioassays, such as striped metallic nanoparticles. Third, nanoparticles that leverage signal transduction, for example in colloidal gold-based aggregation assays. Fourth, functional nanoparticles that exploit specific phys. or chem. properties of nanoparticles to carry out novel functions, such as the catalysis of a biol. reaction. In addn., the review discusses the next generation of nanoparticles that will be utilized in the life sciences, such as nanodots and carbon nanotubes.
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4. 4
  Iqbal, S. S.; Mayo, M. W.; Bruno, J. G.; Bronk, B. V.; Batt, C. A.; Chambers, J. P. A Review of Molecular Recognition Technologies for Detection of Biological Threat Agents Biosens. Bioelectron. 2000, 15, 549– 578
  There is no corresponding record for this reference.
5. 5
  Mirkin, C. A.; Ivanisevic, A.; Taton, T. A.; Letsinger, R. L.; Viswanadham, G. PCR-Less Detection of Genomic DNA with Nanoparticle Probes. Abstracts of Papers; American Chemical Society: Washington, DC, 2001; Vol. 222, p U578.
  There is no corresponding record for this reference.
6. 6
  Schultz, D. A.; Mock, J. J.; Schultz, S.; Smith, D. R. Single-Target Molecule Detection with Nonbleaching Multicolor Optical Immunolabels Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 996– 1001
  6
  Single-target molecule detection with nonbleaching multicolor optical immunolabels
  Schultz, Sheldon; Smith, David R.; Mock, Jack J.; Schultz, David A.
  Proceedings of the National Academy of Sciences of the United States of America (2000), 97 (3), 996-1001CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  We introduce and demonstrate the use of colloidal silver plasmon-resonant particles (PRPs) as optical reporters in typical biol. assays. PRPs are ultrabright, nanosized optical scatterers, which scatter light elastically and can be prepd. with a scattering peak at any color in the visible spectrum. PRPs are readily obsd. individually with a microscope configured for dark-field microscopy, with white-light illumination of typical power. Here we illustrate the use of PRPs, surface coated with std. ligands, as target-specific labels in an in situ hybridization and an immunocytol. assay. We propose that PRPs can replace or complement established labels, such as those based on radioactivity, fluorescence, chemiluminescence, or enzymic/colorimetric detection that are used routinely in biochem., cell biol., and medical diagnostic applications. Moreover, because PRP labels are nonbleaching and bright enough to be rapidly identified and counted, an ultrasensitive assay format based on single-target mol. detection is now practical. We also present the results of a model sandwich immunoassay for goat anti-biotin antibody, in which the no. of PRP labels counted in an image constitutes the measured signal.
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7. 7
  Mie, G. Beiträge Zur Optik Trüber Medien, Speziell Kolloidaler Metallösungen Ann. Phys. 1908, 25, 377– 445
  There is no corresponding record for this reference.
8. 8
  Yguerabide, J.; Yguerabide, E. E. Light-Scattering Submicroscopic Particles as Highly Fluorescent Analogs and Their Use as Tracer Labels in Clinical and Biological Applications. I. Theory Anal. Biochem. 1998, 262, 137– 156
  There is no corresponding record for this reference.
9. 9
  Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment J. Phys. Chem. B 2003, 107, 668– 677
  9
  The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment
  Kelly, K. Lance; Coronado, Eduardo; Zhao, Lin Lin; Schatz, George C.
  Journal of Physical Chemistry B (2003), 107 (3), 668-677CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The optical properties of metal nanoparticles have long been of interest in phys. chem., starting with Faraday's studies of colloidal Au in the middle 1800s. More recently, new lithog. techniques as well as improvements to classical wet chem. methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielec. environments. In this feature article, the authors describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qual. features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of anal. and numerical methods for calcg. extinction and scattering cross sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangular Ag particles and related shapes.
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10. 10
  Khlebtsov, N. G.; Trachuk, L. A.; Mel’nikov, A. G. The Effect of the Size, Shape, and Structure of Metal Nanoparticles on the Dependence of Their Optical Properties on the Refractive Index of a Disperse Medium Opt. Spectrosc. 2005, 98, 77– 83
  10
  The Effect of the Size, Shape, and Structure of Metal Nanoparticles on the Dependence of Their Optical Properties on the Refractive Index of a Disperse Medium
  Khlebtsov, N. G.; Trachuk, L. A.; Mel'nikov, A. G.
  Optics and Spectroscopy (2005), 98 (1), 77-83CODEN: OPSUA3; ISSN:0030-400X. (MAIK Nauka/Interperiodica Publishing)
  The effect of the size, shape, and structure of gold and silver nanoparticles on the dependence of their extinction and integral scattering spectra on the dielec. environment has been investigated. Calcns. were performed using the Mie theory for spheres and nanoshells and the T-matrix method for chaotically oriented bispheres, spheroids, and s cylinders with hemispherical ends. The sensitivity of plasmon resonances to variations in the refractive index of the environment in the range 1.3-1.7 for particles of different equivolume size, as well as to variations in the thickness of the metal layer of nanoshells, was studied. For nanoparticles with an equivolume diam. of 15 nm, the maximal shifts of plasmon resonances due to variation in the refractive index of the environment are obsd. for bispheres and the shifts decrease in the series nanoshells, s cylinders or spheroids, and spheres. For particles 60 nm in diam., the largest shifts of plasmon resonances occur for nanoshells and the shifts decrease in the series bispheres, s cylinders or spheroids, and spheres. All other conditions being the same, silver nanoparticles are more sensitive to the resonance tuning due to a change in the dielec. environment.
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11. 11
  Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. Shape Effects in Plasmon Resonance of Individual Colloidal Silver Nanoparticles J. Chem. Phys. 2002, 116
  There is no corresponding record for this reference.
12. 12
  Mock, J. J.; Smith, D. R.; Schultz, S. Local Refractive Index Dependence of Plasmon Resonance Spectra from Individual Nanoparticles Nano Lett. 2003, 3, 485– 491
  12
  Local Refractive Index Dependence of Plasmon Resonance Spectra from Individual Nanoparticles
  Mock, Jack J.; Smith, David R.; Schultz, Sheldon
  Nano Letters (2003), 3 (4), 485-491CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  The authors present an exptl. optical darkfield microscope study of the dependence of the plasmon resonance spectrum of individual Ag nanoparticles on the local index of refraction. The authors systematically characterize the position of the resonance peaks assocd. with the same set of individual Ag nanoparticles embedded sequentially in index oils with increasing refractive index. This technique effectively allows the local refractive index to be stepped in increments of 0.04. As the local index is increased, the spectrum from each of the nanoparticles generally undergoes a very regular and reproducible red shift; however, the amt. of red shift per index increase varies depending on the shape of the nanoparticle and the mode of excitation. In particular, the spectral peak that occurs in triangular nanoparticles exhibits a noticeably larger red shift than that assocd. with the dipole mode corresponding to spherical nanoparticles. The authors' results are consistent with expts. performed on ensembles of similar nanoparticles and suggest that individual nanoparticles may be used in biosensing applications where currently ensembles are being studied.
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13. 13
  Englebienne, P. Use of Colloidal Gold Surface Plasmon Resonance Peak Shift to Infer Affinity Constants from the Interactions between Protein Antigens and Antibodies Specific for Single or Multiple Epitopes Analyst 1998, 123, 1599– 1603
  13
  Use of colloidal gold surface plasmon resonance peak shift to infer affinity constants from the interactions between protein antigens and antibodies specific for single or multiple epitopes
  Englebienne, Patrick
  Analyst (Cambridge, United Kingdom) (1998), 123 (7), 1599-1603CODEN: ANALAO; ISSN:0003-2654. (Royal Society of Chemistry)
  The surface plasmon resonance (SPR) wavelength of colloidal gold particles coated with a monoclonal antibody is red-shifted when the antibody interacts with its specific ligand. This shift results from the change in the refractive index of the particles as induced by ligand binding. This property is used to monitor in real-time the assocn. and dissocn. kinetics of the interaction in soln. The monitoring is performed in a clin. chem. automated analyzer during a few minutes of incubation at 37°. Data treatment allows calcn. of the affinity const. of the interaction. The SPR wavelength shift does not necessarily require agglutination or aggregation of the particles to occur since particles coated with one monoclonal antibody specific for a single epitope on the ligand can be used in the procedure. The affinity consts. measured by this procedure correlate with those calcd. from Scatchard plots or BIAcore data.
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14. 14
  Nath, N.; Chilkoti, A. Label Free Colorimetric Biosensing Using Nanoparticles J. Fluoresc. 2004, 14, 377– 389
  14
  Label Free Colorimetric Biosensing Using Nanoparticles
  Nath, Nidhi; Chilkoti, Ashutosh
  Journal of Fluorescence (2004), 14 (4), 377-389CODEN: JOFLEN; ISSN:1053-0509. (Kluwer Academic/Plenum Publishers)
  A review. In this review article, we discuss a class of biosensors that exploit the change in the colorimetric properties of noble metal nanoparticles in response to biomol. binding at their surface. Several sensor fabrication techniques as well as sensor configurations are discussed with an emphasis on their strengths and limitations. We conclude by presenting the future prospects and challenges for the successful transition of this technol. from the lab. to a com. product.
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15. 15
  Nath, N.; Chilkoti, A. A Colorimetric Gold Nanoparticle Sensor to Interrogate Biomolecular Interactions in Real Time on a Surface Anal. Chem. 2002, 74, 504– 509
  15
  A colorimetric gold nanoparticle sensor to interrogate biomolecular interactions in real time on a surface
  Nath, Nidhi; Chilkoti, Ashutosh
  Analytical Chemistry (2002), 74 (3), 504-509CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
  This paper presents a new label-free optical method to study biomol. interactions in real time at the surface of an optically transparent substrate. The method relies on the change in the absorbance spectrum of a self-assembled monolayer of colloidal gold on glass, as a function of biomol. binding to the surface of the immobilized colloids. Using this approach, we demonstrate proof of principle of a label-free optical biosensor to quantify biomol. interactions in real time on a surface in a com. available UV-visible spectrophotometer and of a colorimetric end-point assay using an optical scanner. The spectrophotometric sensor shows concn.-dependent binding and a detection limit of 16 nM for streptavidin. The sensor is easy to fabricate, is reproducible in its performance, has minimal technol. requirements, namely, the availability of an UV-visible spectrophotometer or an optical scanner, and will enable high-throughput screening of biomol. interactions in real time in an array-based format.
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16. 16
  Haes, A. J.; Stuart, D. A.; Nie, S. M.; Van Duyne, R. P. Using Solution-Phase Nanoparticles, Surface-Confined Nanoparticle Arrays and Single Nanoparticles as Biological Sensing Platforms J. Fluoresc. 2004, 14, 355– 367
  16
  Using Solution-Phase Nanoparticles, Surface-Confined Nanoparticle Arrays and Single Nanoparticles as Biological Sensing Platforms
  Haes, Amanda J.; Stuart, Douglas A.; Nie, Shuming; Van Duyne, Richard P.
  Journal of Fluorescence (2004), 14 (4), 355-367CODEN: JOFLEN; ISSN:1053-0509. (Kluwer Academic/Plenum Publishers)
  A review. The intense colors of noble metal nanoparticles have inspired artists and fascinated scientists for hundreds of years. In this review, we describe three sensing platforms based on the tunability of the localized surface plasmon resonance (LSPR) of gold and silver nanoparticles. Specifically, the color assocd. with soln.-phase nanoparticles, surface-confined nanoparticle arrays, and single nanoparticles will be shown to be tunable and useful as platforms for biol. sensing.
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17. 17
  Frederix, F.; Friedt, J. M.; Choi, K. H.; Laureyn, W.; Campitelli, A.; Mondelaers, D.; Maes, G.; Borghs, G. Biosensing Based on Light Absorption of Nanoscaled Gold and Silver Particles Anal. Chem. 2003, 75, 6894– 6900
  There is no corresponding record for this reference.
18. 18
  Dahlin, A.; Zach, M.; Rindzevicius, T.; Kall, M.; Sutherland, D. S.; Hook, F. Localized Surface Plasmon Resonance Sensing of Lipid-Membrane-Mediated Biorecognition Events J. Am. Chem. Soc. 2005, 127, 5043– 5048
  There is no corresponding record for this reference.
19. 19
  Nusz, G. J.; Marinakos, S. M.; Curry, A. C.; Dahlin, A.; Höök, F.; Wax, A.; Chilkoti, A. Label-Free Plasmonic Detection of Biomolecular Binding by a Single Gold Nanorod Anal. Chem. 2008, 80, 984– 989
  19
  Label-Free Plasmonic Detection of Biomolecular Binding by a Single Gold Nanorod
  Nusz, Greg J.; Marinakos, Stella M.; Curry, Adam C.; Dahlin, Andreas; Hoeoek, Fredrik; Wax, Adam; Chilkoti, Ashutosh
  Analytical Chemistry (Washington, DC, United States) (2008), 80 (4), 984-989CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
  The authors report the use of individual gold nanorods as plasmonic transducers to detect the binding of streptavidin to individual biotin-conjugated nanorods in real time on a surface. Label-free detection at the single-nanorod level was performed by tracking the wavelength shift of the nanorod-localized surface plasmon resonant scattering spectrum using a dark-field microspectroscopy system. The lowest streptavidin concn. that was exptl. measured was 1 nM, which is a factor of 1000-fold lower than the previously reported detection limit for streptavidin binding by biotinylated single plasmonic nanostructures. The authors believe that the current optical setup is able to reliably measure wavelength shifts as small as 0.3 nm. Binding of streptavidin at 1 nM concn. induces a mean resonant wavelength shift of 0.59 nm suggesting that the authors are currently operating at close to the limit of detection of the system.
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20. 20
  Raschke, G.; Kowarik, S.; Franzl, T.; Sonnichsen, C.; Klar, T. A.; Feldmann, J.; Nichtl, A.; Kurzinger, K. Biomolecular Recognition Based on Single Gold Nanoparticle Light Scattering Nano Lett. 2003, 3, 935– 938
  There is no corresponding record for this reference.
21. 21
  McFarland, A. D.; Van Duyne, R. P. Single Silver Nanoparticles as Real-Time Optical Sensors with Zeptomole Sensitivity Nano Lett. 2003, 3, 1057– 1062
  21
  Single Silver Nanoparticles as Real-Time Optical Sensors with Zeptomole Sensitivity
  McFarland, Adam D.; Van Duyne, Richard P.
  Nano Letters (2003), 3 (8), 1057-1062CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  This work utilizes dark-field optical microscopy to demonstrate the localized surface plasmon resonance λmax response of individual Ag nanoparticles to the formation of a monolayer of small-mol. adsorbates. The adsorption of fewer than 60 000 1-hexadecanethiol mols. on single Ag nanoparticles results in a localized surface plasmon resonance shift of 40.7 nm. Addnl., the kinetics of the single nanoparticle response was shown to be comparable to that of other real-time sensor technologies.
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22. 22
  Rindzevicius, T.; Alaverdyan, Y.; Dahlin, A.; Hook, F.; Sutherland, D. S.; Kall, M. Plasmonic Sensing Characteristics of Single Nanometric Holes Nano Lett. 2005, 5, 2335– 2339
  There is no corresponding record for this reference.
23. 23
  Sherry, L. J.; Jin, R. C.; Mirkin, C. A.; Schatz, G. C.; Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms Nano Lett. 2006, 6, 2060– 2065
  23
  Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms
  Sherry, Leif J.; Jin, Rongchao; Mirkin, Chad A.; Schatz, George C.; Van Duyne, Richard P.
  Nano Letters (2006), 6 (9), 2060-2065CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
  The plasmonic properties of single silver triangular nanoprisms were investigated by using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) were obsd. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approxn. (DDA). The dipole resonance is very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chem. sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is as high as 205 nm RIU-1 and the plasmon line width as narrow as ∼0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU-1/line width (eV), as high as 3.3. In addn., the LSPR shift response to alkanethiol chain length was linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.
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24. 24
  van Dijk, M. A.; Lippitz, M.; Orrit, M. Far-Field Optical Microscopy of Single Metal Nanoparticles Acc. Chem. Res. 2005, 38, 594– 601
  24
  Far-Field Optical Microscopy of Single Metal Nanoparticles
  van Dijk, Meindert A.; Lippitz, Markus; Orrit, Michel
  Accounts of Chemical Research (2005), 38 (7), 594-601CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Individual noble-metal particles, with sizes ranging from a few tenths to some hundreds of nanometers, can now be detected by far-field optics. Single-particle microscopy gives access to inhomogeneity, distributions, and fluctuations, which were previously hidden in ensemble expts. Scattering methods rely on dark-field illumination, spectral signatures of the metal particles, or both. More advanced techniques provide high sensitivity and improved selectivity with respect to other scatterers by isolating metal-specific signals, for example the refractive index change due to heating of the environment by a pump beam or the time-resolved optical response of the particle to a short pump pulse. The authors review and compare linear and nonlinear methods in far-field optical microscopy that have reached the single-particle regime by scattered light, thermal effects, photoluminescence, or nonlinear frequency generation.
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25. 25
  Sonnichsen, C.; Geier, S.; Hecker, N. E.; von Plessen, G.; Feldmann, J.; Ditlbacher, H.; Lamprecht, B.; Krenn, J. R.; Aussenegg, F. R.; Chan, V. Z. H.; Spatz, J. P.; Moller, M. Spectroscopy of Single Metallic Nanoparticles Using Total Internal Reflection Microscopy Appl. Phys. Lett. 2000, 77, 2949– 2951
  There is no corresponding record for this reference.
26. 26
  Haes, A. J.; Van Duyne, R. P.; Zou, S. L.; Schatz, G. C. Nanoscale Optical Biosensor: Short Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles J. Phys. Chem. B 2004, 108, 6961– 6968
  26
  Nanoscale Optical Biosensor: Short Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles
  Haes, Amanda J.; Zou, Shengli; Schatz, George C.; Van Duyne, Richard P.
  Journal of Physical Chemistry B (2004), 108 (22), 6961-6968CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  Silver and gold nanotriangles were fabricated by nanosphere lithog. (NSL) and their localized surface plasmon resonance (LSPR) spectra were measured by UV-vis extinction spectroscopy. It is demonstrated that the short range (viz., 0-2 nm) distance dependence of the electromagnetic fields that surround these nanoparticles when resonantly excited can be systematically tuned by changing their size, structure, and compn. This is accomplished by measuring the shift in the peak wavelength, λmax, of their LSPR spectra caused by the adsorption of hexadecanethiol as a function of nanoparticle size (in-plane width, out-of-plane height, and aspect ratio), shape (truncated tetrahedron vs. hemisphere), and compn. (silver vs. gold). We find that the hexadecanethiol-induced LSPR shift for Ag triangles decreases when in-plane width is increased at fixed out-of-plane height or when height is increased at fixed width. These trends are the opposite to what was seen in an earlier study of the long range distance dependence in which 30 nm thick layers were examd. (Haes et al. J. Phys. Chem. B 2004, 108, 109), but both the long and short range results are confirmed by a theor. anal. based on finite element electrodynamics. The theory results also indicate that the short range results are primarily sensitive to hot spots (regions of high induced elec. field) near the tips of the triangles, so this provides an example where enhanced local fields play an important role in extinction spectra. Our measurements further show that the hexadecanethiol-induced LSPR peak shift is larger for nanotriangles than for hemispheres with equal vols. and is larger for Ag nanotriangles than for Au nanotriangles with the same in-plane widths and out-of-plane heights. The dependence of the alkanethiol-induced LSPR peak shift on chain length for Ag nanotriangles is approx. size-independent. We anticipate that the improved understanding of the short range dependence of the adsorbate-induced LSPR peak shift on nanoparticle structure and compn. reported here will translate to significant improvements in the sensitivity of refractive-index-based nanoparticle nanosensors.
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27. 27
  Haes, A. J.; Van Duyne, R. P.; Zou, S. L.; Schatz, G. C. A Nanoscale Optical Biosensor: The Long Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles J. Phys. Chem. B 2004, 108, 109– 116
  27
  A Nanoscale Optical Biosensor: The Long Range Distance Dependence of the Localized Surface Plasmon Resonance of Noble Metal Nanoparticles
  Haes, Amanda J.; Zou, Shengli; Schatz, George C.; Van Duyne, Richard P.
  Journal of Physical Chemistry B (2004), 108 (1), 109-116CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The elucidation of the long range distance dependence of the localized surface plasmon resonance (LSPR) of surface-confined noble metal nanoparticles is the aim of this work. It was suspected that the linear distance dependence found in Me(CH2)xSH self-assembled monolayer (SAM) formation was the thin shell limit of a longer range, nonlinear dependence. To verify this, multilayer SAM shells based on the interaction of HOOC(CH2)10SH and Cu2+ were assembled onto surface-confined noble metal nanoparticles and were monitored using UV-visible spectroscopy. Measurement of the LSPR extinction peak shift vs. no. of layers and adsorbate thickness is nonlinear and has a sensing range that is dependent on the compn., shape, in-plane width, and out-of-plane height of the nanoparticles. Theor. calcns. based on an accurate electrodynamics description of the metal nanoparticle plus surrounding layered material indicate plasmon resonance wavelength shifts that are in excellent agreement with the measurements. The calcns. show that the sensing range is detd. by falloff of the av. induced elec. field around the nanoparticle. This detailed set of expts. coupled with an excellent theory vs. expt. comparison prove that the sensing capabilities of noble metal nanoparticles can be size tuned to match the dimensions of biol. and chem. analytes by adjusting the aforementioned properties. The optimization of the LSPR nanosensor for a specific analyte will significantly improve an already sensitive nanoparticle-based sensor.
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28. 28
  Evanoff, D. D.; White, R. L.; Chumanov, G. Measuring the Distance Dependence of the Local Electromagnetic Field from Silver Nanoparticles J. Phys. Chem. B 2004, 108, 1522– 1524
  28
  Measuring the Distance Dependence of the Local Electromagnetic Field from Silver Nanoparticles
  Evanoff, David D., Jr.; White, Ryan L.; Chumanov, George
  Journal of Physical Chemistry B (2004), 108 (5), 1522-1524CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The distance dependence of the local EM field assocd. with 84 ± 5 nm silver particles was characterized by monitoring spectral shifts in the plasmon resonance due to changes in local dielec. environment, which was controlled by coating the particles with silica layers of various thicknesses. The local EM field that extends beyond the phys. boundaries of the particles is defined as that providing feedback between the local environment and the electron oscillations in a particle.
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29. 29
  Hao, E.; Schatz, G. C. Electromagnetic Fields around Silver Nanoparticles and Dimers J. Chem. Phys. 2004, 120, 357– 366
  29
  Electromagnetic fields around silver nanoparticles and dimers
  Hao, Encai; Schatz, George C.
  Journal of Chemical Physics (2004), 120 (1), 357-366CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  We use the discrete dipole approxn. to investigate the electromagnetic fields induced by optical excitation of localized surface plasmon resonances of silver nanoparticles, including monomers and dimers, with emphasis on what size, shape, and arrangement leads to the largest local elec. field (E-field) enhancement near the particle surfaces. The results are used to det. what conditions are most favorable for producing enhancements large enough to observe single mol. surface enhanced Raman spectroscopy. Most of the calcns. refer to triangular prisms, which exhibit distinct dipole and quadrupole resonances that can easily be controlled by varying particle size. In addn., for the dimer calcns. we study the influence of dimer sepn. and orientation, esp. for dimers that are sepd. by a few nanometers. We find that the largest |E|2 values for dimers are about a factor of 10 larger than those for all the monomers examd. For all particles and particle orientations, the plasmon resonances which lead to the largest E-fields are those with the longest wavelength dipolar excitation. The spacing of the particles in the dimer plays a crucial role, and we find that the spacing needed to achieve a given |E|2 is proportional to nanoparticle size for particles below 100 nm in size. Particle shape and curvature are of lesser importance, with a head to tail configuration of two triangles giving enhanced fields comparable to head to head, or rounded head to tail. The largest |E|2 values we have calcd. for spacings of 2 nm or more is ∼105.
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30. 30
  Imura, K.; Okamoto, H.; Nagahra, T. Plasmon Mode Imaging of Single Gold Nanorods J. Am. Chem. Soc. 2004, 126, 12730– 12731
  30
  Plasmon Mode Imaging of Single Gold Nanorods
  Imura, Kohei; Nagahara, Tetsuhiko; Okamoto, Hiromi
  Journal of the American Chemical Society (2004), 126 (40), 12730-12731CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  We have investigated two-photon-induced photoluminescence images and spectra of single gold nanorods by using an apertured scanning near-field optical microscope. The obsd. PL spectrum of single gold nanorod can be explained by the radiative recombination of the electron-hole pair near the X and L symmetry points. PL images reveal characteristic features reflecting an eigenfunction of a specific plasmon mode as well as elec. field distributions around the nanorod.
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31. 31
  Stenberg, E.; Persson, B.; Roos, H.; Urbaniczky, C. Quantitative Determination of Surface Concentration of Protein with Surface Plasmon Resonance Using Radiolabeled Proteins J. Colloid Interface Sci. 1991, 143, 513– 526
  31
  Quantitative determination of surface concentration of protein with surface plasmon resonance using radiolabeled proteins
  Stenberg, Esa; Persson, Bjoern; Roos, Haakan; Urbaniczky, Csaba
  Journal of Colloid and Interface Science (1991), 143 (2), 513-26CODEN: JCISA5; ISSN:0021-9797.
  A methodol. to correlate the abs. surface concn. of protein to the surface plasmon resonance (SPR) response is described. The thickness and the optical consts. for each layer on the sensor chip used were detd. with different optical techniques. In a flow injection system, the steady-state SPR response was correlated to the abs. amt. of radiolabeled protein adsorbed by using a surface scintillation counter. The proteins used, 14C-labeled human transferrin and chymotrypsinogen A, as well as in vivo 35S-labeled monoclonal antibodies, were adsorbed via electrostatic interaction to a carboxymethylated dextran hydrogen on the sensor chip. For these proteins, surface concns. from 2 to 50 ng/mm2 correspond linearly to the SPR response, with specific response in the range 0.10 ± 0.01° (ng/mm2)-1, independent of protein size. The min. detectable surface concn. of protein is estd. to be 50 pg/mm2 with this SPR instrument. Optical models used developed to describe how the SPR response depends on the distribution of the adsorbed protein within the hydrogel vol. at the surface. With a thin-film optical program, the theor. SPR response for the different models were calcd. Comparison with exptl. data shows that the protein is distributed within an ∼100-mm-thick dextran hydrogel layer.
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32. 32
  Vollmer, F.; Braun, D.; Libchaber, A.; Khoshsima, M.; Teraoka, I.; Arnold, S. Protein Detection by Optical Shift of a Resonant Microcavity Appl. Phys. Lett. 2002, 80, 4057– 4059
  There is no corresponding record for this reference.
33. 33
  Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. Colloidal Metal Films as a Substrate for Surface-Enhanced Spectroscopy J. Phys. Chem. 1995, 99, 9466– 9471
  There is no corresponding record for this reference.
34. 34
  Nikoobakht, B.; El-Sayed, M. A. Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method Chem. Mater. 2003, 15, 1957– 1962
  34
  Preparation and growth mechanism of gold nanorods (NRs) using seed-mediated growth method
  Nikoobakht, Babak; El-Sayed, Mostafa A.
  Chemistry of Materials (2003), 15 (10), 1957-1962CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  A method is used for prepg. gold nanorods with aspect ratios of 1.5-10 for which the surface plasmon absorption maxima are between 600 and 1300 nm. This method has been adapted from a previously published seed-mediated growth method of N. Jana et al. (2001). The disadvantages and limitations of the earlier method (formation of noncylindrical nanorods, φ-shaped particles, and formation of a large fraction of spherical particles) have been overcome by using a hexadecyltrimethylammonium bromide (CTAB)-capped seed instead of a citrate-capped one. In a single-component surfactant system, the silver content of the growth soln. was used to grow Au nanorods to a desired length. This results in reproducible formation of nanorods with aspect ratios ranging from 1.5 to 4.5. To grow longer nanorods with aspect ratios of 4.6-10, a binary surfactant mixt. composed of benzyldimethylhexadecylammonium chloride and CTAB was used. Nanorods are grown in this mixt. either by aging or by addn. of a growth soln. suitable for shorter nanorods. Effects of the silver ions and co-surfactant along with the growth mechanism of the nanorods are discussed.
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35. 35
  Chen, C.-D.; Cheng, S.-F.; Chau, L.-K.; Wang, C. R. C. Sensing Capability of the Localized Surface Plasmon Resonance of Gold Nanorods Biosens. Bioelectron. 2007, 22, 926– 932
  35
  Sensing capability of the localized surface plasmon resonance of gold nanorods
  Chen, Cheng-Dah; Cheng, Shu-Fang; Chau, Lai-Kwan; Wang, C. R. Chris
  Biosensors & Bioelectronics (2007), 22 (6), 926-932CODEN: BBIOE4; ISSN:0956-5663. (Elsevier B.V.)
  We demonstrate the feasibility of using the longitudinal component of gold nanorod's surface plasmon resonance in biomol. sensing. The sensitive dependence of the absorption max. on the dielec. const. of the particle interfacial region makes gold nanorods a promise for constructing a biomol. sensing scheme. The sensor contg. gold nanorods, with a mean aspect ratio of 5.2, exhibits a sensitivity of ∼366 nm/RIU (refractive index unit), which increases accordingly with the increase of the particle mean aspect ratios. Such a biosensor was further modified to demonstrate its effectiveness in quant. detection for selective binding events, such as biotin/streptavidin pairs, through a process in which biotin mols. were chem. attached to the gold nanorods' surface prior to detection measurements. Results showed that the spectral λmax shifts linearly to the concns. of the streptavidin. The results from both expt. and model calcns. strongly indicate the efficiency of the longitudinal surface plasmon absorption band in biosensing.
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36. 36
  Marinakos, S. M.; Chen, S.; Chilkoti, A. Plasmonic Detection of a Model Analyte in Serum by a Gold Nanorod Sensor Anal. Chem. 2007, 79, 5278– 5283
  36
  Plasmonic Detection of a Model Analyte in Serum by a Gold Nanorod Sensor
  Marinakos, Stella M.; Chen, Sihai; Chilkoti, Ashutosh
  Analytical Chemistry (Washington, DC, United States) (2007), 79 (14), 5278-5283CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
  The authors describe the fabrication of a label-free, chip-based biosensor based on the localized surface plasmon resonance (LSPR) of gold nanorods. Gold nanorods were chemisorbed onto a mercaptosilane-modified glass substrate, followed by conjugation of biotin to the nanorods. Streptavidin binding to biotin was monitored by the wavelength shift of the LSPR peak in the UV-vis extinction spectrum of the immobilized gold nanorods due to the change in local refractive index at the gold nanorod surface induced by streptavidin binding. The limit of detection of the sensor is 0.005 μg/mL (94 pM) in PBS and 1 μg/mL (19 nM) in serum, and the dynamic range spans 94 pM to 0.19 μM. The advantages of the nanorod-based sensor over an LSPR sensor that the authors had previously fabricated from gold nanospheres (Nath, N., and Chilkoti, A., 2002 and 2004) are the significantly lower detection limit and the internal self-ref. that the signal of the nanorod sensor provides based on the measurement of peak wavelength shift.
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37. 37
  Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods J. Phys. Chem. B 2001, 105, 4065– 4067
  37
  Wet chemical synthesis of high aspect ratio cylindrical gold nanorods
  Jana, Nikhil R.; Gearheart, Latha; Murphy, Catherine J.
  Journal of Physical Chemistry B (2001), 105 (19), 4065-4067CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  Gold nanorods with aspect ratios of 4.6 ± 1.2, 13 ± 2, and 18 ± 2.5 (all with 16 ± 3 nm short axis) were prepd. by using a seeding growth approach in the presence of an aq. micelle template. Citrate-capped 3.5 nm diam. gold particles, prepd. by the redn. of HAuCl4 with borohydride, are used as the seed. The aspect ratio of the nanorods is controlled by varying the ratio of seed to metal salt. The long rods are isolated from spherical particles by centrifuging.
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38. 38
  Busbee, B. D.; Obare, S. O.; Murphy, C. J. An Improved Synthesis of High-Aspect-Ratio Gold Nanorods Adv. Mater. 2003, 15, 414– 416
  38
  An improved synthesis of high-aspect-ratio gold nanorods
  Busbee, Brantley D.; Obare, Sherine O.; Murphy, Catherine J.
  Advanced Materials (Weinheim, Germany) (2003), 15 (5), 414-416CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  An improved synthetic methodol. that produces monodisperse gold nanorods of high aspect ratio in ∼90% yield is described. The method involves a three-step seed-mediated growth process. In the first step, 4 nm gold nanoparticles are produced by the redn. of HAuCl4 with sodium borohydride in the presence of sodium citrate. These gold nanoparticles are used as seed for the next growth steps. The seed is added to a growth soln. contg. the surfactant cetyltrimethylammonium bromide, HAuCl4, ascorbic acid, and sodium hydroxide. This new method requires only one round of purifn. to produce gold nanorods of aspect ratio ∼18 at pH 3.5. Gold nanorods with an aspect ratio of ∼20 are obtained in the presence of heptane.
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39. 39
  Sau, T. K.; Murphy, C. J. Room Temperature, High-Yield Synthesis of Multiple Shapes of Gold Nanoparticles in Aqueous Solution J. Am. Chem. Soc. 2004, 126, 8648– 8649
  39
  Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution
  Sau, Tapan K.; Murphy, Catherine J.
  Journal of the American Chemical Society (2004), 126 (28), 8648-8649CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A seed-mediated growth method was used to control the morphol. and dimensions of Au nanocrystals by manipulation of the exptl. parameters in aq. soln. at room temp. This chem. route produces various structural architectures with rod-, rectangle-, hexagon-, cube-, triangle-, and starlike profiles and branched (such as bi-, tri-, tetra-, and multipod) Au nanocrystals of various dimensions in high yield in the presence of a single surfactant, cetyltrimethylammonium bromide.
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40. 40
  Link, S.; Mohamed, M. B.; El-Sayed, M. A. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant J. Phys. Chem. B 1999, 103, 3073– 3077
  40
  Simulation of the optical absorption spectra of gold nanorods as a function of their aspect ratio and the effect of the medium dielectric constant
  Link, S.; Mohamed, M. B.; El-Sayed, M. A.
  Journal of Physical Chemistry B (1999), 103 (16), 3073-3077CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  Gold nanorods with different aspect ratios are prepd. in micelles by the electrochem. method and their absorption spectra are modeled by theory. Exptl., a linear relationship is found between the absorption max. of the longitudinal plasmon resonance and the mean aspect ratio as detd. from TEM. It is shown here that such a linear dependence is also predicted theor. However, calcns. also show that the absorption max. of the longitudinal plasmon resonance depends on the medium dielec. const. in a linear fashion for a fixed aspect ratio. Attempts to fit the calcns. to the exptl. values indicate that the medium dielec. const. has to vary with the aspect ratio in a nonlinear way. Chem., this suggests that the structure of the micelle capping the gold nanorods is size dependent. Furthermore, comparison with the results obtained for rods of different aspect ratios made by systematic thermal decompn. of the long rods further suggests that the medium dielec. const. is also temp. dependent. This is attributed to thermal annealing of the structure of the micelles around the nanorods.
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41. 41
  Weissleder, R. A Clearer Vision for In Vivo Imaging Nat. Biotechnol. 2001, 19, 316– 317
  41
  A clearer vision for in vivo imaging
  Weissleder, Ralph
  Nature Biotechnology (2001), 19 (4), 316-317CODEN: NABIF9; ISSN:1087-0156. (Nature America Inc.)
  There is no expanded citation for this reference.
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42. 42
  Jackson, J. B.; Westcott, S. L.; Hirsch, L. R.; West, J. L.; Halas, N. J. Controlling the Surface Enhanced Raman Effect via the Nanoshell Geometry Appl. Phys. Lett. 2003, 82, 257– 259
  There is no corresponding record for this reference.
43. 43
  Hanarp, P.; Kall, M.; Sutherland, D. S. Optical Properties of Short Range Ordered Arrays of Nanometer Gold Disks Prepared by Colloidal Lithography J. Phys. Chem. B 2003, 107, 5768– 5772
  43
  Optical Properties of Short Range Ordered Arrays of Nanometer Gold Disks Prepared by Colloidal Lithography
  Hanarp, Per; Kaell, Mikael; Sutherland, Duncan S.
  Journal of Physical Chemistry B (2003), 107 (24), 5768-5772CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The optical properties of Au nanodisk arrays prepd. by colloidal lithog. are studied exptl. The arrays exhibit short range translational order and weak interparticle interactions. Tunable localized surface plasmon resonances are achieved by varying the diam. of the disks at const. disk height. The macroscopic optical properties are well-described by modeling the Au disks as oblate spheroids in the electrostatic limit. The optical sensing capabilities of the disks are studied by varying the surrounding refractive index. It is found, in agreement with theory, that more oblate disk shapes have higher sensitivity. Probably nanodisks prepd. by colloidal lithog. are of interest as substrates for optimizing optical biosensing methods at the nanometer scale.
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44. 44
  Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Kall, M.; Bryant, G. W.; de Abajo, F. J. G. Optical Properties of Gold Nanorings Phys. Rev. Lett. 2003, 90
  There is no corresponding record for this reference.
45. 45
  Raschke, G.; Brogl, S.; Susha, A. S.; Rogach, A. L.; Klar, T. A.; Feldmann, J.; Fieres, B.; Petkov, N.; Bein, T.; Nichtl, A.; Kurzinger, K. Gold Nanoshells Improve Single Nanoparticle Molecular Sensors Nano Lett. 2004, 4, 1853– 1857
  There is no corresponding record for this reference.
46. 46
  Hafner, J. H.; Nehl, C. L.; Grady, N. K.; Goodrich, G. P.; Tam, F.; Halas, N. J. Scattering Spectra of Single Gold Nanoshells Nano Lett. 2004, 4, 2355– 2359
  There is no corresponding record for this reference.
47. 47
  Novo, C.; Gomez, D.; Perez-Juste, J.; Zhang, Z. Y.; Petrova, H.; Reismann, M.; Mulvaney, P.; Hartland, G. V. Contributions from Radiation Damping and Surface Scattering to the Linewidth of the Longitudinal Plasmon Band of Gold Nanorods: A Single Particle Study Phys. Chem. Chem. Phys. 2006, 8, 3540– 3546
  47
  Contributions from radiation damping and surface scattering to the linewidth of the longitudinal plasmon band of gold nanorods: a single particle study
  Novo, Carolina; Gomez, Daniel; Perez-Juste, Jorge; Zhang, Zhenyuan; Petrova, Hristina; Reismann, Maximilian; Mulvaney, Paul; Hartland, Gregory V.
  Physical Chemistry Chemical Physics (2006), 8 (30), 3540-3546CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  The scattering spectra of single Au nanorods with aspect ratios of 2-4 were examd. by dark field microscopy. The longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Anal. of the data using γ = γbulk + AνF/Leff, where Leff is the effective path length of the electrons and νF is the Fermi velocity, allows one to det. a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were examd. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in agreement with recent work on spherical Au nanoparticles by C. Sonnichsen et al. (2002) and by S. Berciaud et al. (2005).
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48. 48
  Prescott, S. W.; Mulvaney, P. Gold Nanorod Extinction Spectra J. Appl. Phys. 2006, 99
  There is no corresponding record for this reference.
49. 49
  Curry, A.; Nusz, G.; Chilkoti, A.; Wax, A. Analysis of Total Uncertainty in Spectral Peak Measurements for Plasmonic Nanoparticle-Based Biosensors Appl. Opt. 2007, 46, 1931– 1939
  49
  Analysis of total uncertainty in spectral peak measurements for plasmonic nanoparticle-based biosensors
  Curry Adam; Nusz Gregory; Chilkoti Ashutosh; Wax Adam
  Applied optics (2007), 46 (10), 1931-9 ISSN:0003-6935.
  One goal of recent research on plasmonic nanoparticle-based sensors is maximizing nanoparticle sensitivity or shift of resonance peak wavelength per refractive index change. Equally important is a measurement system's peak location uncertainty or shift resolution. We provide systematic analyses and discuss optimization of factors that determine peak location uncertainty, reporting values as low as 0.3 nm for the presented scheme. This type of analysis is important, in part, because it provides a means of evaluating detection thresholds for biosensor applications such as analyte binding. We estimate thresholds of 310 streptavidin molecules for the presented scheme and 20 molecules with system improvements.
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50. 50
  Curry, A.; Nusz, G.; Chilkoti, A.; Wax, A. Substrate Effect on Refractive Index Dependence of Plasmon Resonance for Individual Silver Nanoparticles Observed Using Darkfield Micro-Spectroscopy Opt. Express 2005, 13, 2668– 2677
  50
  Substrate effect on refractive index dependence of plasmon resonance for individual silver nanoparticles observed using darkfield micro-spectroscopy
  Curry, A.; Nusz, G.; Chilkoti, A.; Wax, A.
  Optics Express (2005), 13 (7), 2668-2677CODEN: OPEXFF; ISSN:1094-4087. (Optical Society of America)
  We use optical darkfield micro-spectroscopy to characterize the plasmon resonance of individual silver nanoparticles in the presence of a substrate. The optical system permits multiple individual nanoparticles to be identified visually for simultaneous spectroscopic study. For silver particles bound to a silanated glass substrate, we observe changes in the plasmon resonance due to induced variations in the local refractive index. The shifts in the plasmon resonance are investigated using a simple anal. theory in which the contributions from the substrate and environment are weighted with distance from the nanoparticle. The theory is compared with exptl. results to det. a weighting factor which facilitates modeling of environmental refractive index changes using std. Mie code. Use of the optical system for characterizing nanoparticles attached to substrates for biosensing applications is discussed.
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51. 51
  Kuwata, H.; Tamaru, H.; Esumi, K.; Miyano, K. Resonant Light Scattering from Metal Nanoparticles: Practical Analysis Beyond Rayleigh Approximation Appl. Phys. Lett. 2003, 83, 4625– 4627
  51
  Resonant light scattering from metal nanoparticles: Practical analysis beyond Rayleigh approximation
  Kuwata, Hitoshi; Tamaru, Hiroharu; Esumi, Kunio; Miyano, Kenjiro
  Applied Physics Letters (2003), 83 (22), 4625-4627CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
  We propose a simple anal. formula that can quant. predict resonant light scattering from metal nanoparticles of arbitrary shape, whose sizes are too large for Rayleigh approxn. to be applicable. The formula has been derived as an empirical extension of Mie's rigorous calcn. for light scattering from spheres. It can very well reproduce the exptl. characteristics of light scattering from Au nanorods.
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52. 52
  Xu, X. D.; Cortie, M. B. Shape Change and Color Gamut in Gold Nanorods, Dumbbells, and Dog Bones Adv. Funct. Mater. 2006, 16, 2170– 2176
  52
  Shape change and color gamut in gold nanorods, dumbbells, and dog bones
  Xu, Xiaoda; Cortie, Michael B.
  Advanced Functional Materials (2006), 16 (16), 2170-2176CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Deviations from a prolate ellipsoidal shape have a significant effect on the optical properties of Au nanorods. Transitions from rods to 'dumbbell'- or 'phi'-shaped particles lead to a shift in the longitudinal plasmon peak in the blue and red directions, resp. Development of 'dog-bone' shapes leads to a red-shift and to the development of a 3rd peak. A broad and flexible color gamut can be obtained.
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53. 53
  Yan, B. H.; Yang, Y.; Wang, Y. C. Comment On “Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant” J. Phys. Chem. B 2003, 107, 9159– 9159
  There is no corresponding record for this reference.
54. 54
  Link, S.; El-Sayed, M. A.; Mohamed, M. B. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant (Vol 103b, Pg 3073, 1999) J. Phys. Chem. B 2005, 109, 10531– 10532
  There is no corresponding record for this reference.
55. 55
  Link, S.; Mohamed, M. B.; El-Sayed, M. A. Simulation of the Optical Absorption Spectra of Gold Nanorods as a Function of Their Aspect Ratio and the Effect of the Medium Dielectric Constant J. Phys. Chem. B 1999, 103, 3073– 3077
  55
  Simulation of the optical absorption spectra of gold nanorods as a function of their aspect ratio and the effect of the medium dielectric constant
  Link, S.; Mohamed, M. B.; El-Sayed, M. A.
  Journal of Physical Chemistry B (1999), 103 (16), 3073-3077CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  Gold nanorods with different aspect ratios are prepd. in micelles by the electrochem. method and their absorption spectra are modeled by theory. Exptl., a linear relationship is found between the absorption max. of the longitudinal plasmon resonance and the mean aspect ratio as detd. from TEM. It is shown here that such a linear dependence is also predicted theor. However, calcns. also show that the absorption max. of the longitudinal plasmon resonance depends on the medium dielec. const. in a linear fashion for a fixed aspect ratio. Attempts to fit the calcns. to the exptl. values indicate that the medium dielec. const. has to vary with the aspect ratio in a nonlinear way. Chem., this suggests that the structure of the micelle capping the gold nanorods is size dependent. Furthermore, comparison with the results obtained for rods of different aspect ratios made by systematic thermal decompn. of the long rods further suggests that the medium dielec. const. is also temp. dependent. This is attributed to thermal annealing of the structure of the micelles around the nanorods.
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56. 56
  Nath, N.; Chilkoti, A. Label-Free Biosensing by Surface Plasmon Resonance of Nanoparticles on Glass: Optimization of Nanoparticle Size Anal. Chem. 2004, 76, 5370– 5378
  56
  Label-Free Biosensing by Surface Plasmon Resonance of Nanoparticles on Glass: Optimization of Nanoparticle Size
  Nath, Nidhi; Chilkoti, Ashutosh
  Analytical Chemistry (2004), 76 (18), 5370-5378CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
  The unique optical properties of noble metal nanoparticles have been used to design a label-free biosensor in a chip format. In this paper, we demonstrate that the size of gold nanoparticles significantly affects the sensitivity of the biosensor. Gold nanoparticles with diams. in the range of 12-48 nm were synthesized in soln. and sensor chips were fabricated by chemisorption of these nanoparticles on amine-functionalized glass. Sensors fabricated from 39-nm-diam. gold nanoparticles exhibited max. sensitivity to the change of the bulk refractive index and the largest "anal. vol.", defined as the region around the nanoparticle within which a change in refractive index causes a change in the optical properties of the immobilized nanoparticles. The detection limit for streptavidin-biotin binding of a sensor fabricated from 39-nm-diam. nanoparticles was 20-fold better than a previously reported sensor fabricated from 13-nm-diam. gold nanoparticles. We also discuss several other factors that could improve the performance of the next generation of these immobilized metal nanoparticle sensors.
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57. 57
  Whitney, A. V.; Elam, J. W.; Zou, S. L.; Zinovev, A. V.; Stair, P. C.; Schatz, G. C.; Van Duyne, R. P. Localized Surface Plasmon Resonance Nanosensor: A High-Resolution Distance-Dependence Study Using Atomic Layer Deposition J. Phys. Chem. B 2005, 109, 20522– 20528
  57
  Localized Surface Plasmon Resonance Nanosensor: A High-Resolution Distance-Dependence Study Using Atomic Layer Deposition
  Whitney, Alyson V.; Elam, Jeffrey W.; Zou, Shengli; Zinovev, Alex V.; Stair, Peter C.; Schatz, George C.; Van Duyne, Richard P.
  Journal of Physical Chemistry B (2005), 109 (43), 20522-20528CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  At. layer deposition (ALD) is used to deposit 1-600 monolayers of Al2O3 on Ag nanotriangles fabricated by nanosphere lithog. (NSL). Each monolayer of Al2O3 has a thickness of 1.1 Å. It is demonstrated that the localized surface plasmon resonance (LSPR) nanosensor can detect Al2O3 film growth with at. spatial resoln. normal to the nanoparticle surface. This is approx. 10 times greater spatial resoln. than that in our previous long-range distance-dependence study using multilayer self-assembled monolayer shells. The use of ALD enables the study of both the long- and short-range distance dependence of the LSPR nanosensor in a single unified expt. Ag nanoparticles with fixed in-plane widths and decreasing heights yield larger sensing distances. XPS, variable angle spectroscopic ellipsometry, and quartz crystal microbalance measurements are used to study the growth mechanism. It is proposed that the growth of Al2O3 is initiated by the decompn. of trimethylaluminum on Ag. Semiquant. theor. calcns. were compared with the exptl. results and yield excellent agreement.
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58. 58
  Hicks, E. M.; Zhang, X. Y.; Zou, S. L.; Lyandres, O.; Spears, K. G.; Schatz, G. C.; Van Duyne, R. P. Plasmonic Properties of Film over Nanowell Surfaces Fabricated by Nanosphere Lithography J. Phys. Chem. B 2005, 109, 22351– 22358
  58
  Plasmonic Properties of Film over Nanowell Surfaces Fabricated by Nanosphere Lithography
  Hicks, Erin M.; Zhang, Xiaoyu; Zou, Shengli; Lyandres, Olga; Spears, Kenneth G.; Schatz, George C.; Van Duyne, Richard P.
  Journal of Physical Chemistry B (2005), 109 (47), 22351-22358CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  In this work, a detailed and systematic study of the plasmonic properties of a novel film over nanowell surface is investigated. These nanostructures are fabricated using nanosphere lithog. and reactive ion etching and structurally characterized by AFM and SEM. The resulting structures show remarkably narrow plasmon bands in reflectance spectra (as little as 0.10 eV) and greater sensitivity to external dielec. environment than has been seen in other nanoparticle systems, resulting in an improvement in the figure of merit (FOM = refractive index sensitivity (eV·RIU-1)/full width at half-max. (eV)) for refractive index sensing. Theor. modeling for the plasmon spectra of these nanostructures is done using discrete dipole approxn. code under periodic boundary conditions. The modeling results match the measurements accurately in aspects of the variation of the plasmon line shape with altering internanowell distance and dielec. environment.
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59. 59
  Haes, A. J.; Van Duyne, R. P. A Nanoscale Optical Biosensor: Sensitivity and Selectivity of an Approach Based on the Localized Surface Plasmon Resonance Spectroscopy of Triangular Silver Nanoparticles J. Am. Chem. Soc. 2002, 124, 10596– 10604
  59
  A nanoscale optical biosensor: Sensitivity and selectivity of an approach based on the localized surface plasmon resonance spectroscopy of triangular silver nanoparticles
  Haes, Amanda J.; Van Duyne, Richard P.
  Journal of the American Chemical Society (2002), 124 (35), 10596-10604CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Triangular silver nanoparticles (∼100 nm wide and 50 nm high) have remarkable optical properties. In particular, the peak extinction wavelength, λmax of their localized surface plasmon resonance (LSPR) spectrum is unexpectedly sensitive to nanoparticle size, shape, and local (∼10-30 nm) external dielec. environment. This sensitivity of the LSPR λmax to the nanoenvironment has allowed us to develop a new class of nanoscale affinity biosensors. The essential characteristics and operational principles of these LSPR nanobiosensors will be illustrated using the well-studied biotin-streptavidin system. Exposure of biotin-functionalized Ag nanotriangles to 100 nM streptavidin (SA) caused a 27.0 nm red-shift in the LSPR λmax. The LSPR λmax shift, ΔR/ΔRmax, vs. [SA] response curve was measured over the concn. range 10-15 M < [SA] < 10-6 M. Comparison of the data with the theor. normalized response expected for 1:1 binding of a ligand to a multivalent receptor with different sites but invariant affinities yielded approx. values for the satn. response, ΔRmax = 26.5 nm, and the surface-confined thermodn. binding const. Ka,surf = 1011 M-1. At present, the limit of detection (LOD) for the LSPR nanobiosensor is found to be in the low-picomolar to high-femtomolar region. A strategy to amplify the response of the LSPR nanobiosensor using biotinylated Au colloids and thereby further improve the LOD is demonstrated. Several control expts. were performed to define the LSPR nanobiosensor's response to nonspecific binding as well as to demonstrate its response to the specific binding of another protein. These include the following: (1) electrostatic binding of SA to a nonbiotinylated surface, (2) nonspecific interactions of prebiotinylated SA to a biotinylated surface, (3) nonspecific interactions of bovine serum albumin to a biotinylated surface, and (4) specific binding of anti-biotin to a biotinylated surface. The LSPR nanobiosensor provides a pathway to ultrasensitive biodetection expts. with extremely simple, small, light, robust, low-cost instrumentation that will greatly facilitate field-portable environmental or point-of-service medical diagnostic applications.
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60. 60
  Miller, M. M.; Lazarides, A. A. Sensitivity of Metal Nanoparticle Plasmon Resonance Band Position to the Dielectric Environment as Observed in Scattering J. Opt. A: Pure Appl. Opt. 2006, 8, S239– S249
  There is no corresponding record for this reference.
61. 61
  Miller, M. M.; Lazarides, A. A. Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment J. Phys. Chem. B 2005, 109, 21556– 21565
  61
  Sensitivity of Metal Nanoparticle Surface Plasmon Resonance to the Dielectric Environment
  Miller, Molly M.; Lazarides, Anne A.
  Journal of Physical Chemistry B (2005), 109 (46), 21556-21565CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  Electrodynamic simulations of gold nanoparticle spectra were used to investigate the sensitivity of localized surface plasmon band position to the refractive index, n, of the medium for nanoparticles of various shapes and nanoshells of various structures. Among single-component nanoparticles less than 130 nm in size, sensitivities of dipole resonance positions to bulk refractive index are found to depend only upon the wavelength of the resonance and the dielec. properties of the metal and the medium. Among particle plasmons that peak in the frequency range where the real part of the metal dielec. function varies linearly with wavelength and the imaginary part is small and slowly varying, the sensitivity of the peak wavelength, λ*, to refractive index, n, is found to be a linearly increasing function of λ*, regardless of the structural features of the particle that det. λ*. Quasistatic theory is used to derive an anal. expression for the refractive index sensitivity of small particle plasmon peaks. Through this anal., the dependence of sensitivity on band position is found to be detd. by the wavelength dependence of the real part, ε', of the particle dielec. function, and the sensitivity results are found to extend to all particles with resonance conditions of the form, ε'* = -2χn2, where χ is a function of geometric parameters and other consts. The sensitivity results obsd. using accurate computational methods for dipolar plasmon bands of gold nanodisks, nanorods, and hollow nanoshells extend, therefore, to particles of other shapes (such as hexagonal and chopped tetrahedral), composed of other metals, and to higher-order modes. The bulk refractive index sensitivity yielded by the theory serves as an upper bound to sensitivities of nanoparticles on dielec. substrates and sensitivities of nanoparticles to local refractive index changes, such as those assocd. with biomol. sensing.
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62. 62
  Neish, C. S.; Martin, I. L.; Henderson, R. M.; Edwardson, J. M. Direct Visualization of Ligand-Protein Interactions Using Atomic Force Microscopy Br. J. Pharmacol. 2002, 135, 1943– 1950
  62
  Direct visualization of ligand-protein interactions using atomic force microscopy
  Neish, Calum S.; Martin, Ian L.; Henderson, Robert M.; Edwardson, J. Michael
  British Journal of Pharmacology (2002), 135 (8), 1943-1950CODEN: BJPCBM; ISSN:0007-1188. (Nature Publishing Group)
  1 Streptavidin is a 60-kDa tetramer which binds four mols. of biotin with extremely high affinity (KA ∼ 1014 M-1). We have used at. force microscopy (AFM) to visualize this ligand-protein interaction directly. 2 Biotin was tagged with a short (152-basepair; 50-nm) DNA rod and incubated with streptavidin. The resulting complexes were then imaged by AFM. The mol. vol. of streptavidin calcd. from the dimensions of the protein particles (105±3 nm3) was in close agreement with the value calcd. from its mol. mass (114 nm3). Biotinylation increased the apparent size of streptavidin (to 133±2 nm3), concomitant with an increase in the thermal stability of the tetramer. 3 Images of streptavidin with one to four mols. of DNA-biotin bound were obtained. When two ligands were bound, the angle between the DNA rods was either acute or obtuse, as expected from the relative orientations of the biotin binding sites. The ratio of acute: obtuse angles (1: 3) was lower than the expected value (1: 2), indicating a degree of steric hindrance in the binding of the DNA-biotin. The slight under-representation of higher occupancy states supported this idea. 4 Streptavidin with a single mol. of DNA-biotin bound was used to tag biotinylated β-galactosidase, a model multimeric enzyme. 5 The ability to image directly the binding of a ligand to its protein target by AFM provides useful information about the nature of the interaction, and about the effect of complex formation on the structure of the protein. Furthermore, the use of DNA-biotin/streptavidin tags could potentially shed light on the architecture of multi-subunit proteins.
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63. 63
  Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S. Quantitative Interpretation of the Response of Surface Plasmon Resonance Sensors to Adsorbed Films Langmuir 1998, 14, 5636– 5648
  63
  Quantitative Interpretation of the Response of Surface Plasmon Resonance Sensors to Adsorbed Films
  Jung, Linda S.; Campbell, Charles T.; Chinowsky, Timothy M.; Mar, Mimi N.; Yee, Sinclair S.
  Langmuir (1998), 14 (19), 5636-5648CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
  A simple but quant. math. formalism for interpretation of surface plasmon resonance (SPR) signals from adsorbed films of a wide variety of structures is presented. It can be used to est. adsorbed film thicknesses, surface coverages, or surface concns. from the SPR response over the entire range of film thicknesses without relying on calibration curves of response vs. known thicknesses or surface concns. This formalism is compared to more complex optical simulations. It is further tested by (1) calibrating the response of two SPR spectrometers to changes in bulk index of refraction, (2) using these calibrations with this formalism to predict responses to several well-characterized adlayer structures (alkanethiolates and serum albumin on gold, propylamine on COOH-functionalized gold), and then (3) comparing these predictions to measured SPR responses. Methods for estg. the refractive index of the adlayer material are also discussed. Detection limits in both bulk and adsorption-based analyses are discussed. The planar system used here has a detection limit of ∼0.003 nm in av. film thickness for adsorbates whose refractive index differs from that of the solvent by only 0.1. The temp. sensitivities of these two SPR spectrometers are characterized and discussed in terms of detection limits.
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64. 64
  Chien, F. C.; Chen, S. J. Direct Determination of the Refractive Index and Thickness of a Biolayer Based on Coupled Waveguide-Surface Plasmon Resonance Mode Opt. Lett. 2006, 31, 187– 189
  There is no corresponding record for this reference.
65. 65
  Weber, P. C.; Ohlendorf, D. H.; Wendoloski, J. J.; Salemme, F. R. Structural Origins of High-Affinity Biotin Binding to Streptavidin Science 1989, 243, 85– 88
  65
  Structural origins of high-affinity biotin binding to streptavidin
  Weber, Patricia C.; Ohlendorf, D. H.; Wendoloski, J. J.; Salemme, F. R.
  Science (Washington, DC, United States) (1989), 243 (4887), 85-8CODEN: SCIEAS; ISSN:0036-8075.
  The high affinity of the noncovalent interaction between biotin and streptavidin forms the basis for many diagnostic assays that require the formation of an irreversible and specific linkage between biol. macromols. Comparison of the refined crystal structures of apo and a streptavidin:biotin complex shows that the high affinity results from several factors. These factors include the formation of multiple H bonds and van der Waals interactions between biotin and the protein, together with the ordering of surface polypeptide loops that bury the biotin in the protein interior. Structural alterations at the biotin-binding site produce quaternary changes in the streptavidin tetramer. These changes apparently propagate through cooperative deformations in the twisted β sheets that link tetramer subunits.
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66. 66
  Hendrickson, W. A.; Pahler, A.; Smith, J. L.; Satow, Y.; Merritt, E. A.; Phizackerley, R. P. Crystal-Structure of Core Streptavidin Determined from Multiwavelength Anomalous Diffraction of Synchrotron Radiation Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 2190– 2194
  There is no corresponding record for this reference.
67. 67
  Hinrichsen, E. L.; Feder, J.; Jossang, T. Geometry of Random Sequential Adsorption J. Stat. Phys. 1986, 44, 793– 827
  There is no corresponding record for this reference.
68. 68
  Jung, L. S.; Nelson, K. E.; Stayton, P. S.; Campbell, C. T. Binding and Dissociation Kinetics of Wild-Type and Mutant Streptavidins on Mixed Biotin-Containing Alkylthiolate Monolayers Langmuir 2000, 16, 9421– 9432
  There is no corresponding record for this reference.
69. 69
  Kim, Y. P.; Hong, M. Y.; Kim, J.; Oh, E.; Shon, H. K.; Moon, D. W.; Kim, H. S.; Lee, T. G. Quantitative Analysis of Surface-Immobilized Protein by Tof-Sims: Effects of Protein Orientation and Trehalose Additive Anal. Chem. 2007, 79, 1377– 1385
  There is no corresponding record for this reference.
70. 70
  Link, S.; El-Sayed, M. A. Spectroscopic Determination of the Melting Energy of a Gold Nanorod J. Chem. Phys. 2001, 114, 2362– 2368
  There is no corresponding record for this reference.
71. 71
  Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai, W. C.; Wang, C. R. C. The Shape Transition of Gold Nanorods Langmuir 1999, 15, 701– 709
  71
  The Shape Transition of Gold Nanorods
  Chang, Ser-Sing; Shih, Chao-Wen; Chen, Cheng-Dah; Lai, Wei-Cheng; Wang, C. R. Chris
  Langmuir (1999), 15 (3), 701-709CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
  The authors report a revised synthetic procedure based on an electrochem. method for prepg. an aq. soln. contg. suspended Au nanorods. The mean aspect ratios of the Au nanorods can be exptl. adjusted between 1 and 7. The evolution of the longitudinal surface plasmon bands shows an eminently sensitive dependence on the aspect ratios of the nanorods. Their dependence is accordingly described by classical-electrostatic-model predictions. The shape transition of the nanorod particles was studied by varying some key influencing factors such as the wavelength, the laser fluence, and matrix effects. The nanorods were exposed to laser lines at 532 and 1064 nm, frequencies which correspond closely to the short- and long-axis plasmon resonances, resp. A photon-induced shape transition process was evidenced, and the corresponding rod-to-sphere conversion contributed by a photoannealing process was obsd. in both cases. 010000. Meanwhile, the authors obsd. a new type of φ-shaped Au nanostructure in the case of 1064-nm irradn., which possibly represents the early stage of the shape transition and indicates that the starting location of the at.-scale restructuring is at the centroid of the Au nanorod. The results of laser fluence-dependence measurements state that an efficient shape transition occurs via a multiphoton process. The authors also demonstrate the fabrication of the Au nanorod@silica nanostructures for preliminary studies of the matrix effects. As a result of the higher rigidity of the thin-silica-coating layer, the assocd. shape transition requires higher energy and proceeds less efficiently as compared with the cases for the micelle-stabilized Au nanorods.
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72. 72
  Link, S.; El-Sayed, M. A. Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods J. Phys. Chem. B 1999, 103, 8410– 8426
  72
  Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nano-dots and Nano-rods
  Link, Stephan; El-Sayed, Mostafa A.
  Journal of Physical Chemistry B (1999), 103 (40), 8410-8426CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  Review with 137 refs. The field of nanoparticle research has drawn much attention in the past decade as a result of the search for new materials. Size confinement results in new electronic and optical properties, possibly suitable for many electronic and optoelectronic applications. A characteristic feature of noble metal nanoparticles is the strong color of their colloidal solns., which is caused by the surface plasmon absorption. This article describes the studies of the properties of the surface plasmon absorption in metal nanoparticles that range in size between 10 and 100 nm. The effects of size, shape, and compn. on the plasmon absorption max. and its bandwidth are discussed. also, the optical response of the surface plasmon absorption due to excitation with femtosecond laser pulses allowed the authors to follow the electron dynamics (electron-electron and electron-phonon scattering) in these metal nanoparticles. The electron-phonon relaxation processes in nanoparticles, which are smaller than the electron mean free path, are independent of their size or shape. Intense laser heating of the electrons in these particles is also found to cause a shape transformation (photoisomerization of the rods into spheres or fragmentation), which depends on the laser pulse energy and pulse width.
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73. 73
  Muskens, O. L.; Bachelier, G.; Del Fatti, N.; Vallee, F.; Brioude, A.; Jiang, X. C.; Pileni, M. P. Quantitative Absorption Spectroscopy of a Single Gold Nanorod J. Phys. Chem. C 2008, 112, 8917– 8921
  73
  Quantitative Absorption Spectroscopy of a Single Gold Nanorod
  Muskens, Otto L.; Bachelier, Guillaume; Del Fatti, Natalia; Vallee, Fabrice; Brioude, Arnaud; Jiang, Xuchuan; Pileni, Marie-Paule
  Journal of Physical Chemistry C (2008), 112 (24), 8917-8921CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The spectrally- and polarization-resolved absorption cross sections of a single Au nanorod were studied using the spatial modulation spectroscopy technique. The ensemble of its optical features, i.e., longitudinal and transverse surface plasmon resonances and interband absorption, was quant. characterized. The results are compared with numerical simulations using the discrete dipole approxn. and the finite element method, yielding information on the studied nanorod size and shape.
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74. 74
  Park, H. S.; Agarwal, A.; Kotov, N. A.; Lavrentovich, O. D. Controllable Side-by-Side and End-to-End Assembly of Au Nanorods by Lyotropic Chromonic Materials Langmuir 2008, 24, 13833– 13837
  74
  Controllable side-by-side and end-to-end assembly of Au nanorods by lyotropic chromonic materials
  Park, Heung-Shik; Agarwal, Ashish; Kotov, Nicholas A.; Lavrentovich, Oleg D.
  Langmuir (2008), 24 (24), 13833-13837CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
  A simple and universal technique for assembling gold nanorods by using self-assembled stacks of lyotropic chromonic materials, such as disodium chromoglycate, without covalent bonding between nanorods and linking agent is presented. The anisotropic electrostatic interaction between the chromonic stacks and nanorods allows achievement of either side-by-side or end-to-end assembly, depending on the surface charge of the nanorods. The assembled structures are stable within an extended temp. range. The degree of nanorod aggregation can be controlled by a no. of factors influencing the self-assembly of chromonic materials, such as the concn. and pH of the soln.
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75. 75
  Dahlin, A. B.; Tegenfeldt, J. O.; Hook, F. Improving the Instrumental Resolution of Sensors Based on Localized Surface Plasmon Resonance Anal. Chem. 2006, 78, 4416– 4423
  75
  Improving the Instrumental Resolution of Sensors Based on Localized Surface Plasmon Resonance
  Dahlin, Andreas B.; Tegenfeldt, Jonas O.; Hoeoek, Fredrik
  Analytical Chemistry (2006), 78 (13), 4416-4423CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
  The colorimetric variations induced upon changes in interfacial refractive index of nanoscale noble metal structures exhibiting localized surface plasmon resonance (LSPR) provides a convenient means of label-free, affinity-based detection of biomol. recognition reactions. However, despite being similar in nature to conventional SPR, LSPR has so far suffered from significantly lower data quality in terms of its signal-to-noise ratio (S/N) in typical biomol. recognition anal. In this work, generic data anal. algorithms and a simple exptl. setup that provide a S/N upon protein binding that is comparable to that of state-of-the art SPR systems are presented. Specifically, it is demonstrated how temporal variations (rate ∼0.5 Hz) in parameters proportional to the resonance peak position can be recorded simultaneously, yielding a peak position precision of <5 × 10-4 nm and an extinction noise level of <5 × 10-6 absorbance units (Abs). This, in turn, is shown to provide a S/N of ∼2000 (equiv. to a detection limit of <0.1 ng/cm2) for typical protein binding reactions. Furthermore, the importance of utilizing changes in both peak position and magnitude is highlighted by comparing different LSPR active noble metal architectures that respond differently to bulk and interfacial refractive index changes.
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76. 76
  Jana, N. R.; Gearheart, L.; Murphy, C. J. Seed-Mediated Growth Approach for Shape-Controlled Synthesis of Spheroidal and Rod-Like Gold Nanoparticles Using a Surfactant Template Adv. Mater. 2001, 13, 1389– 1393
  76
  Seed-mediated growth approach for shape-controlled synthesis of spheroidal and rod-like gold nanoparticles using a surfactant template
  Jana, Nikhil R.; Gearheart, Latha; Murphy, Catherine J.
  Advanced Materials (Weinheim, Germany) (2001), 13 (18), 1389-1393CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
  A two-step self-control synthesis of gold nanoparticles is developed. The synthesis consists of (1)prepn. of small size (3-4 nm) spherical gold nanoparticles, and (2)growth of the prepd. spherical particle in rod-like micellar environment. Roughly spherical borohydride-reduced gold nanoparticle seeds with a mean diam. of 3-4 nm were prepd. and added to the growth soln. contg. gold salt, cetyltrimethylammonium bromide (CTAB), a rod-shaped micellar template, ascorbic acid, and a small amt. of silver ions. The particles were studied by TEM after sepg. the surfactant by centrifugation. Spheroidal or rod-like gold particles with aspect ratios that were dependent on the gold seed-to-salt ratio were obtained. Initial addn. of preformed seeds increases the overall reaction rate, and hence the growth rate. The use of AgNO3 in the growth soln. was helpful in control of the spheroids/rods aspect ratio.
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77. 77
  Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mohwald, H. Electrostatic Self-Assembly of Silica Nanoparticle-Polyelectrolyte Multilayers on Polystyrene Latex Particles J. Am. Chem. Soc. 1998, 120, 8523– 8524
  77
  Electrostatic Self-Assembly of Silica Nanoparticle-Polyelectrolyte Multilayers on Polystyrene Latex Particles
  Caruso, Frank; Lichtenfeld, Heinz; Giersig, Michael; Moehwald, Helmuth
  Journal of the American Chemical Society (1998), 120 (33), 8523-8524CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The construction of composite multilayers of 25 nm diam. silica particles and poly(diallyldimethylammonium chloride) was carried out on submicrometer-sized polystyrene latex particles via the sequential electrostatic adsorption of SiO2 and PDADMAC from dil. soln.
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78. 78
  Buscher, K.; Graf, K.; Ahrens, H.; Helm, C. A. Influence of Adsorption Conditions on the Structure of Polyelectrolyte Multilayers Langmuir 2002, 18, 3585– 3591
  There is no corresponding record for this reference.
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An in-depth derivation of the distance dependence of nanoparticle sensitivity to local refractive index is available. This material is available free of charge via the Internet at http://pubs.acs.org.
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Rational Selection of Gold Nanorod Geometry for Label-Free Plasmonic BiosensorsClick to copy article linkArticle link copied!

ACS Nano

Abstract

Results and Discussion

Model Development

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Experimental Application of Model

Figure 4

Figure 5

Prospects for Single Molecule Detection

Experimental Section

Materials

Nanorod Synthesis

Polyelectrolyte Deposition

Single Particle Detection

Supporting Information

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Acknowledgment

References

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Abstract

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Rational Selection of Gold Nanorod Geometry for Label-Free Plasmonic Biosensors
Click to copy article linkArticle link copied!