Nanoplasmonic Array Enhancement of Two-Photon Absorption in a

May 23, 2012 - ... to load: https://cdn.mathjax.org/mathjax/contrib/a11y/accessibility-menu.js .... We investigate the enhancement of the effective tw...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCC

Nanoplasmonic Array Enhancement of Two-Photon Absorption in a Dye Film Jarrett H. Vella†,‡ and Augustine M. Urbas*,† †

United States Air Force Research Laboratory, Materials and Manufacturing Directorate, 3005 Hobson Way, Wright-Patterson Air Force Base, Ohio 45433, United States ‡ General Dynamics Information Technology, Fairfax, Virginia 22030, United States S Supporting Information *

ABSTRACT: We investigate the enhancement of the effective two-photon absorption cross section of a film of an organic dye by a plasmonic triangular prism array through finite element calculations and experimental measurements. Hexagonal arrays of plasmonic triangular prism arrays were prepared using nanosphere lithography so that their localized surface plasmon resonance (LSPR) is at 800 nm. A dye, AF455, with significant two-photon fluorescence when excited at 800 nm was spin-coated onto the plasmonic array. Several film thicknesses of AF455 were prepared, ranging from 40 to 184 nm. The dependence of the effective two-photon absorption cross section σ(2) of AF455 on the thickness of the dye layer was measured using a newly applied technique. Because two-photon fluorescence is only sensitive to light absorbed by the chromophore, absorption from the nanostructure, thermal effects, and other parasitic optical mechanisms that could indicate anomalously high σ(2) enhancement values are eliminated from the measured enhancement. The results quantitatively agreed with the σ(2) enhancement values predicted by finite element method (FEM) calculations. The simulations show that the σ(2) enhancement observed was due to the plasmonic triangular prism arrays’ LSPR and that the dependence of σ(2) of AF455 on the gold nanostructure is influenced by an optical reflection pattern generated by the plasmonic array in addition to the near-field enhancement. The combination of theory and experiments validates the application of the technique to σ(2) enhancement measurements.

■

INTRODUCTION In 1981, Glass1 described the ability of nanoparticulate silver films to enhance the two-photon absorption cross section, σ(2), of Rhodamine B by 150-fold relative to the dye in solution. Glass developed a model describing the effects of metallic nanostructures on the excitation rate, R, of a molecule in close proximity to them (1), where N represents the number density of the absorbing species, σ(1) is the linear absorption cross section, σ(2) is the two-photon absorption cross section, and I is the intensity of the excitation light with wavelength λi. R = Nσ (1)f 2 (λi)I(λi) + Nσ (2)f 4 (λi)I 2(λi)

that the two-photon absorption spectrum of the dye must overlap the localized surface plasmon resonance (LSPR) spectrum of the patterned metal film. More recently, several other researchers have demonstrated the ability of metal nanostructures to increase σ(2) of organic chromophores.2−5 Kano reported a 90-fold enhancement of the two-photon fluorescence intensity of LD490 on a planar silver film,4 while Perry claims a 1000-fold enhancement in the twophoton absorption efficacy of a stillbene derivative on silver fractal clusters.5 The studies of Glass, Kano, Perry, and others rely on the metal’s ability to support localized surface plasmon resonance. A localized surface plasmon resonance is the optical frequency collective oscillation of the electrons in a metal nanostructure. Their frequency and features are defined by the size and shape of the metal nanostructures, whose dimensions are often smaller than the wavelength of the incident light. The LSPRs are excited directly by incident light in contrast to surface plasmons on planar films which require wave vector matching.6 This has a few consequences. The LSPR is confined

(1)

The first term in eq 1 describes linear absorption, and the second term describes the rate of two-photon absorption. The rate of two-photon absorption is increased when σ(2) is multiplied by a factor f. This factor is the local field factor and describes the strength of the electric field contributing to the excitation of the chromophore. It suggests that an increase in the electric field experienced by the chromophore will increase R by acting as a scalar for σ(2); in other words, an increase in the local electric field experienced by the chromophore will result in an increase in σ(2) proportional to the fourth power of the electric field. This will be referred to as σ(2) enhancement. In this case, a necessary condition for σ(2) enhancement to occur is © 2012 American Chemical Society

Received: March 15, 2012 Revised: May 21, 2012 Published: May 23, 2012 17169

dx.doi.org/10.1021/jp302517b | J. Phys. Chem. C 2012, 116, 17169−17173

The Journal of Physical Chemistry C

Article

150 fs, 1 mJ diode-pumped, Ti:Al2O3 regenerative amplifier (Spectra-Physics Hurricane) with a 1000 Hz repetition rate. Single-photon excitation of the AF455 films at 400 nm was accomplished by frequency doubling the 800 nm output from the laser. The undoubled, 800 nm light was removed from the 400 nm light using a combination of dichroic mirrors and shortpass filters. The sample’s fluorescence was collected, collomated, and filtered with a 720 nm short-pass Semrock filter (FF01-720/SP25) and focused onto a linear fiber bundle, which was coupled into a Jobin-Yvon SPEX 270 M imaging monochromator. It was equipped with a 150 g/mm, 500 nm blaze grating and a Princeton Instruments PIXIS 400B back-thinned CCD camera. For all emission measurements, the grating was centered at 460 nm, the fluorescence maximum of AF455.

to a nanoparticle and does not propagate as with a planar metal film, leading to a strong increase in the local electric field around the nanostructure in the surrounding dielectric medium.7 This phenomenon has been referred to as the “lightening rod effect”8 and is one of the mechanisms employed to use localized surface plasmon resonances to enhance molecular properties.9−11 A consequence of the “lightening rod effect” is that the absorption cross section1 and fluorescence decay rate12 of a chromophore can be increased without synthetic alterations. A second consequence of nanoparticle-based plasmonics is that the wavelength at which the LSPR has a maximum can be tailored to the size and shape of the nanoparticle.7,13−15 This feature of nanoparticle-based LSPRs makes it facile to develop a plasmonic nanostructure whose resonance overlaps a molecule’s fluorescence spectrum, in the case of plasmonic fluorescence enhancement,16 or twophoton absorption spectrum, in the case of two-photon absorption cross-section enhancement.1 In this work, a hexagonal array of triangular gold nanoprisms, with an LSPR at 800 nm, was prepared using nanosphere lithography (NSL), a technique capable of creating large area plasmonic nanostructures with tailorable LSPR maxima.17−19 A dye which forms a neat glassy film, AF455, was spin-coated onto the plasmonic triangular prism array. This dye is well characterized, and it is known to exhibit two-photon fluorescence arising from a strong σ(2) (10−20 cm4/GW) at 800 nm.20 The reader is directed to refs 20 and 21 for more information on AF455. This enables overlap between the dye’s two-photon absorption region and the LSPR extinction spectrum of the plasmonic triangular prism array. This interaction was quantified by comparing σ(2) of AF455 on a plasmonic triangular prism array to σ(2) of AF455 on glass using the method of Hermann and Ducuing.22

■

RESULTS AND DISCUSSION Two-Photon Absorption Cross-Section Enhancement Simulation Parameters. The expected two-photon absorption cross-section enhancement of AF455 on plasmonic triangular prism arrays was predicted using COMSOL Multiphysics, a finite element software package. The complex index of refraction for gold was obtained from Johnson and Christy.24 The index of refraction for glass was obtained from the Sellmeier equation,25 and since glass is transparent throughout the region studied, its absorption coefficient was assumed to be constant at 10−7. The complex index of refraction for AF455 was calculated from the experimentally obtained absorption and reflection spectra as described by Seoudi.26 A plot of the index of refraction and absorption coefficient for AF455 versus wavelength can be found in Figure S-1 of the Supporting Information, and its wavelength-dependent absorptivity can be found in Figure S-2. An illustration of the simulation geometry can be found in Figure S-3. The experimentally obtained plasmonic triangular prism arrays had a bisector of 100 nm, a tip−tip separation of 250 nm, and a height of 22 nm in a hexagonal pattern, to obtain an LSPR at 800 nm. Due to variations in the actual index of refraction between the gold as deposited for this study and Johnson’s and Christy’s data, the real part of the index of refraction measured by Johnson and Christy was altered to 0.5 from the literature value of 0.2 at 800 nm, although simulations suggested any value from 0.3 to 0.5 matches the measured resonance. This was necessary so that the LSPR maximum of the modeled triangles matched the LSPR maximum of the experimentally obtained triangles. The plasmonic triangular prism arrays were modeled on a glass substrate covered by a layer of AF455, whose thickness corresponded to the measured thickness of the AF455 layer in the experimentally obtained samples. The computational unit cell consisted of four whole triangular prisms and was sized so that simple periodic boundary conditions in x and y would return the appropriate hexagonal lattice. The unit cell and dimensions are contained in the Supporting Information. The 800 nm light was set to propagate in the z direction as a plane wave (see Figure S-3 for placement of the axes and unit cell definition). The electric field concentrated by a plasmonic triangular prism array was quantified by performing a volume integration of the electric field in the entire AF455 layer for each dye thickness; the electric field present in the AF455 layer without a plasmonic triangular prism array was similarly quantified. Two-Photon Absorption Cross-Section Enhancement Simulation Results. The two-photon fluorescence of the

■

EXPERIMENTAL SECTION Materials. The plasmonic triangular prism arrays and AF455 were prepared according to published literature procedures.21,23 Thin films of neat AF455 were spin-coated from pure toluene on the appropriate substrate at 3000 rpm for 60 s. The AF455 film thickness was varied by changing the concentration of AF455 in the casting solution, and the films were allowed to dry overnight at room temperature and pressure before use. Optical Measurements. Steady-state ultraviolet−visible (UV−vis) absorption spectra were recorded on a Carey 500 UV−vis−NIR spectrophotometer. Reflection measurements were made on the Carey 500 using an integrating sphere. The reflection spectrum of clean glass was subtracted from the reflection spectrum of AF455 on glass. The index of AF455 is close to that of the substrate, making differences in first surface reflection minimal at longer wavelengths. Absorption measurements were acquired using the same instrument without the integrating sphere. Using a Dektak 6 M stylus profilometer, the absorption of AF455 at 426 nm was correlated to the film thickness for a series of variable-thickness AF455 films as an absorption−thickness calibration plot. The thicknesses of the reported AF455 films were determined from this plot. The σ(2) of AF455 on plasmonic triangular prism arrays relative to σ(2) of AF455 on planar glass was calculated using the method of Hermann and Ducuing.22 Only the ratio of the two respective cross sections will be reported. Two-photon excitation of the AF455 films at 800 nm was achieved using a 17170

dx.doi.org/10.1021/jp302517b | J. Phys. Chem. C 2012, 116, 17169−17173

The Journal of Physical Chemistry C

Article

sample will be proportional to the square or the intensity or the local average electric field from the incident pump beam to the fourth power. To characterize the enhancement of the effective two-photon absorption cross section from the simulated system, we will take the ratio of the local average electric fields, each to the fourth power (2). The average electric field was calculated by performing a volume integration over the AF455 layer while excluding the volume of the metal prism regions. This value was then raised to the fourth power in eq 2. enhancement =

E(AF455 + Au)4 E(AF455 + glass)4

of AF455 on glass, at a thickness of 86.1 nm. Finally, when the film thickness is 184.3 nm, the σ(2) of AF455 on plasmonic triangular prism arrays is expected to recover to 0.87 of the value on glass. A similar trend was observed by Sandoghdar in his study of LSPR-mediated fluorescence enhancement.27 As mentioned previously, the LSPR electric field is strongest at the tips of the triangles and weak in the spaces between the triangles. It is important to note that the σ(2) of AF455 at the tip of a triangle can be expected to increase by a factor of 645 in a 40.3 nm thick AF455 film. The focus of this work is on the effective increase in the entire volume of the film. The sensitive, large area collection experimental methods used do not give us access to local peak enhancement values. Because the tips of the triangles cover a relatively small fraction of the overall surface area of the substrate, the strong enhancement at the tips average over the volume of the film, producing an average expected enhancement of 1.73 for the whole 40.3 nm thick film. This effect is manifested in the expected σ(2) enhancement values for the thicker films as well. To probe the origin of the σ(2) decrease in the thicker films, a 400 nm thick AF455 layer was simulated on plasmonic triangular prism arrays and on glass. A plane integration of the electric field in AF455 was taken at various distances from the substrates as a way of qualitatively describing the evolution of the electric field in the AF455 layer in the films. The results are given in Figure 3. Figures S-4a and S-4b are both a cross

(2)

With an LSPR at 800 nm, the plasmonic triangular prism arrays have the electric field profile shown in Figure 1. The

Figure 1. Localized surface plasmon electric field of plasmonic triangular prism arrays, resonant at 800 nm, as predicted using the finite element method. The electric field is plotted on a 0−10 scale, with 10 being the most intense electric field.

LSPR electric field is concentrated at the tips and edges of the triangles, with a low-strength electric field in the interstitial space. The expected σ(2) of AF455 on plasmonic triangular prism arrays relative to σ(2) of AF455 on glass is calculated as described above, with the results plotted in Figure 2. For AF455 on plasmonic triangular prism arrays, the model predicts a rapidly decreasing σ(2) enhancement at film thickness under 58.3 nm, at which point there is no expected σ(2) enhancement. The expected σ(2) of AF455 on plasmonic triangular prism arrays is expected to decrease by a factor of 0.85, relative to σ(2)

Figure 3. Electric field vs distance from the substrate for AF455 on glass (- - -) and on plasmonic triangular prism arrays ().

section of the same plane in the two simulations. Figure S-4a depicts the electric field present in AF455 on planar glass, while Figure S-4b represents the electric field generated by a representative plasmonic triangular prism array. The plasmonic triangular prism array generates an electric field pattern indicating a strong reflection off of the surface, in contrast to the electric field present without the array. The reflected light produces a null which results in the minimum effective enhancements seen in Figures 2 and 3. The null reduces the effective average electric field inside the AF455 film for certain thicknesses, producing an apparent decrease in its two-photon absorption cross section. Two-Photon Absorption Cross-Section Enhancement Measurements. The ultraviolet−visible absorption (UV−vis) spectra for AF455 on plasmonic triangular prism arrays, AF455 on glass, as well as the clean plasmonic triangular prism arrays, can be found in Figure 4. The clean plasmonic triangular prism arrays have an LSPR resonance near 575 nm, which broadens

Figure 2. Calculated (●) and measured (■) two-photon absorption cross-section enhancement of AF455 on plasmonic triangular prism arrays, relative to AF455 on glass. 17171

dx.doi.org/10.1021/jp302517b | J. Phys. Chem. C 2012, 116, 17169−17173

The Journal of Physical Chemistry C

Article

and two-photon excitation, so the effects of dye quenching by the metallic nanoparticle are eliminated. These features of eq 3 results in reported σ(2) enhancement values that are inherently relatively free from the effects of nanoparticle-based scattering, metal nonlinear absorption, and fluorescence quenching, producing highly reliable enhancement values. Although the Hermann and Ducuing method works with only one set of fluorescence intensities, a more thorough approach involves taking many one- and two-photon fluorescence measurements over a range of excitation powers, as seen in Figures S-5−S-8. The corresponding fluorescence spectra are given in Figures S-9−S-12. Analysis of the slopes in Figures S-4−S-7 reveals that the fluorescence intensity varies linearly with the excitation flux density for one-photon excitation of AF455 on plasmonic triangular prism arrays and glass. Similarly, the fluorescence intensity of both films varies with the square of the excitation flux density for two-photon excitation. All of the films obey the proper power law regarding the mechanism of excitation as expected. The σ(2) enhancement of AF455 was calculated by taking the ratio of σ(2) of AF455 on plasmonic triangular prism arrays to σ(2) of AF455 on planar glass, using the σ(2) values calculated using the data in Figures S-5−S-8. The σ(2) enhancement values are shown in Figure 2. Five different AF455 thicknesses were experimentally obtained: 40.3, 66.1, 86.1, 131, and 184.3 nm, giving measured σ(2) enhancement factors of 1.56 ± 0.19, 0.93 ± 0.14, 0.83 ± 0.22, 0.84 ± 0.15, and 0.93 ± 0.13, respectively. From the simulations discussed above, the expected σ(2) enhancement factors for each film were 1.73, 0.95, 0.85, 0.89, and 0.87, respectively. The measured and expected enhancements agreed with each other within the standard deviation of the measured values but are unarguably low. Literature values of σ(2) enhancement factors for various systems range anywhere from 90 to 1000, as stated in the Introduction.4,5 The σ(2) enhancement factors measured in this report are no larger than 1.56 ± 0.19, producing an obvious discrepancy. Using COMSOL to calculate the surface areas of the plasmonic triangular prism arrays and of the bare substrate, the triangles occupy only 2.9% of the total surface area of the substrate. It has been shown that a σ(2) enhancement factor of 645 can be expected at the tips of the triangle. By using far-field collection optics, the measured enhancement factor represents mostly the enhancement between the triangles, which is considerably lower. Most experiments in the literature feature systems with a significantly higher surface coverage of metallic nanoparticles and is perhaps the major cause of the difference. A second reason for variation between the literature values and the measured enhancement values lies in the method used. As previously discussed, the Hermann and Ducuing method minimizes the effects of the nanoparticle scattering and nonlinear absorption, leading to a potentially lower, but more reliable, measurement of the σ(2) enhancement factor.

Figure 4. Ultraviolet−visible absorption spectra of AF455 on glass (), of AF455 on plasmonic triangular prism arrays (− •• −), and of clean plasmonic triangular prism arrays (− • −).

and red-shifts upon spin-coating of the AF455. The AF455 UV−vis spectrum features a structured absorption with a maximum at 426 nm. The AF455 films whose spectra are shown in Figure 4 are representative examples of the films studied. The absorbance at 426 nm of AF455 on glass is 0.94, while the absorbance at 426 nm of AF455 on plasmonic triangular prism arrays is 0.98, corresponding to film thicknesses of 149.5 and 155.8 nm, respectively. As described below, two-photon fluorescence spectra will be used to measure σ(2) of AF455 on glass and on plasmonic triangular prism array substrates, so it is necessary for both films to have similar thicknesses; since the thickness of both films differ by 6.3 nm and because they have absorbances approaching 1.0, the effects of variable film thickness on the σ(2) enhancement will be negligible. In addition to the popular Z-scan technique to measure σ(2),2 an alternative method is that of Hermann and Ducuing.22 In the latter method, the two-photon fluorescence of a dye with unknown σ(2) is compared to the one-photon fluorescence of the same dye with a known σ(1) using (3), where ϕ2ω is the flux density of the linear excitation beam, ϕω is the flux density of the two-photon excitation beam, and I(2PL) and I(1PL) are the two-photon and one-photon fluorescence intensities, respectively. σ (2) = σ (1)

ϕ2ωI(2PL) ϕω 2I(1PL)

(3)

The application of Hermann’s and Ducuing’s technique to measure σ(2) enhancement results in an enhancement value that is free from a number of effects that would yield an otherwise artificially high enhancement value. The plasmonic triangular prism arrays will scatter excitation light, increasing the effective optical path length of the film; regardless of the technique used, this would result in an artificially high measure of absorption by the AF455. By taking the ratio of the two-photon fluorescence intensity to the one-photon fluorescence intensity and correcting for the differences in the power densities, this scattering effect can be minimized. The gold itself is also capable of nonlinear absorption, and by measuring dye fluorescence, the nonlinear absorption of gold is neglected.28−30 Furthermore, it is known that metallic nanoparticles can quench the fluorescence of dyes.31−33 From Kasha’s rule,34 the photophysics of the excited state of a dye are identical for linear

■

CONCLUSION We have calculated and measured the effective two-photon absorption cross-section enhancement of a dye film of AF455 in contact with a plasmonic triangular prism array through experimental measurements of enhanced two-photon fluorescence. The measured effective two-photon absorption cross section of AF455 on plasmonic triangular prism arrays, 1.56 ± 0.19, which quantatively agreed with the predicted factor of 1.73 for a 40.3 nm thick AF455 film. This measurement shows that an effective enhancement of the properties of the whole 17172

dx.doi.org/10.1021/jp302517b | J. Phys. Chem. C 2012, 116, 17169−17173

The Journal of Physical Chemistry C

Article

film, while modest, is possible though the use of plasmonic structures. This configuration and property is an alternative to more typically reported hot spot or singularity based enhancements where only values for highly enhanced microscopy regions are reported. The results also validate the method of using two-photon fluorescence of a dye to measure plasmonic nanostructure-mediated two-photon absorption cross-section enhancement. This isolates the enhanced absorption of the dye from the nonlinear absorption of the nanostructure, scattering effects, and other parasitic optical mechanisms that would otherwise suggest an artificially high increase in the dye’s twophoton absorption cross section. The applications of these techniques can allow for tailored modification of TPA cross sections in plasmonically enhanced nonlinear materials.

■

(18) Kosiorek, A.; Kandulski, W.; Chudzinski, P.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1359. (19) Jensen, T. R.; Duval, M. L.; Kelly, K. L.; Lazarides, A. A.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 9846. (20) Rogers, J. E.; Slagle, J. E.; McLean, D. G.; Sutherland, R. L.; Sankaran, B.; Kannan, R.; Tan, L.-S.; Fleitz, P. A. J. Phys. Chem. A 2004, 108, 5514. (21) Kannan, R.; He, G. S.; Lin, T.-C.; Prasad, P. N.; Vaia, R. A.; Tan, L.-S. Chem. Mater. 2003, 16, 185. (22) Hermann, J. P.; Ducuing, J. Phys. Rev. A 1972, 5, 2557. (23) Tabor, C.; Qian, W.; El-Sayed, M. A. J. Phys. Chem. C 2007, 111, 8934. (24) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370. (25) Malitson, I. H. J. Opt. Soc. Am. 1965, 55, 1205. (26) Seoudi, R.; El-Bahy, G. S.; El Sayed, Z. A. Opt. Mater. 2006, 29, 304. (27) Kühn, S.; Håkanson, U.; Rogobete, L.; Sandoghdar, V. Phys. Rev. Lett. 2006, 97, 017402. (28) West, R.; Wang, Y.; Goodson, T. J. Phys. Chem. B 2003, 107, 3419. (29) Qu, S.; Gao, Y.; Jiang, X.; Zeng, H.; Song, Y.; Qiu, J.; Zhu, C.; Hirao, K. Opt. Commun. 2003, 224, 321. (30) Chen, D.-J.; Ding, S.; Han, J.-B.; Zhou, H.-J.; Xiao, S.; Xiong, G.G.; Wang, Q.-Q. Chin. Phys. Lett. 2005, 22, 2286. (31) Anger, P.; Bharadwaj, P.; Novotny, L. Phys. Rev. Lett. 2006, 96, 113002. (32) Dulkeith, E.; Morteani, A. C.; Niedereichholz, T.; Klar, T. A.; Feldmann, J.; Levi, S. A.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Möller, M.; Gittins, D. I. Phys. Rev. Lett. 2002, 89, 203002. (33) Horimoto, N. N.; Imura, K.; Okamoto, H. Chem. Phys. Lett. 2008, 467, 105. (34) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Sausalito, CA, 1991.

ASSOCIATED CONTENT

S Supporting Information *

Fluorescence power scans, supplemental fluorescence spectra, and finite element method simulation screen captures. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Chris Tabor of the Air Force Research Laboratory is gratefully thanked for the plasmonic plasmonic triangular prism array. Loon-Seng Tan is acknowledged for the supplying the AF455.

■

REFERENCES

(1) Glass, A. M.; Wokaun, A.; Heritage, J. P.; Bergman, J. G.; Liao, P. F.; Olson, D. H. Phys. Rev. B 1981, 24, 4906. (2) Cohanoschi, I.; Hernández, F. E. J. Phys. Chem. B 2005, 109, 14506. (3) Jung, J.-M.; Yoo, H.-W.; Stellacci, F.; Jung, H.-T. Adv. Mater. 2010, 22, 2542. (4) Kano, H.; Kawata, S. Opt. Lett. 1996, 21, 1848. (5) Wenseleers, W.; Stellacci, F.; Meyer-Friedrichsen, T.; Mangel, T.; Bauer, C. A.; Pond, S. J. K.; Marder, S. R.; Perry, J. W. J. Phys. Chem. B 2002, 106, 6853. (6) Lakowicz, J. R. Anal. Biochem. 2005, 337, 171. (7) Mayer, K. M.; Hafner, J. H. Chem. Rev. 2011, 111, 3828. (8) Mohamed, M. B.; Volkov, V.; Link, S.; El-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517. (9) Lal, S.; Grady, N. K.; Kundu, J.; Levin, C. S.; Lassiter, J. B.; Halas, N. J. Chem. Soc. Rev. 2008, 37, 898. (10) Fromm, D. P.; Sundaramurthy, A.; Schuck, P. J.; Kino, G.; Moerner, W. E. Nano Lett. 2004, 4, 957. (11) Chen, X.; Li, S.; Xue, C.; Banholzer, M. J.; Schatz, G. C.; Mirkin, C. A. ACS Nano 2008, 3, 87. (12) Barnes, W. L. J. Mod. Opt. 1998, 45, 661. (13) Ha, T. H.; Koo, H.-J.; Chung, B. H. J. Phys. Chem. C 2006, 111, 1123. (14) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 10531. (15) Shuford, K. L.; Ratner, M. A.; Schatz, G. C. J. Chem. Phys. 2005, 123, 114713. (16) Bardhan, R.; Grady, N. K.; Cole, J. R.; Joshi, A.; Halas, N. J. ACS Nano 2009, 3, 744. (17) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599. 17173

dx.doi.org/10.1021/jp302517b | J. Phys. Chem. C 2012, 116, 17169−17173