Voltage Regulation of Fluorescence Emission of ... - ACS Publications

We report experiments indicating that (1) the dye emission wavelength is red-shifted (10−30 nm) by applying an external low voltage (1−10 V) and t...
0 downloads 0 Views 133KB Size
Download PDF
NANO LETTERS

Voltage Regulation of Fluorescence Emission of Single Dyes Bound to Gold Nanoparticles

2007 Vol. 7, No. 4 1070-1075

F. Cannone,† M. Collini,† L. D’Alfonso,† G. Baldini,† G. Chirico,*,† G. Tallarida,‡ and P. Pallavicini§ Laboratory for AdVanced BioSpectroscopy (LABS), INFM-CNR, Dipartimento di Fisica, UniVersita` degli Studi di Milano-Bicocca, Milano, I-20126, Italy, MDM-INFM-CNR, Agrate, Milano, Italy, and Chemistry Department, UniVersita` degli Studi di PaVia, PaVia, I-27100, Italy Received December 4, 2006; Revised Manuscript Received December 29, 2006

ABSTRACT An organic dye, SAMSA, bound to gold nanoparticles, displays random photoactivated fluorescence blinking whose rate depends on the size of the nanoparticles. We report experiments indicating that (1) the dye emission wavelength is red-shifted (10−30 nm) by applying an external low voltage (1−10 V) and that (2) the fluorescence emission of single dyes can be resonantly driven by tuning the alternating external bias frequency from 1 to 3 Hz, depending on the nanoparticle size. These properties appear highly valuable and promising for devising light emitting nanostructures.

In recent years, a large fraction of supramolecular chemistry research has been devoted to studying molecular assemblies that could be used as devices. Noble metal nanoclusters display many interesting optical and electronic properties that are size-dependent.1-7 These materials have potential applications in developing biological nanosensors and optoelectronic nanodevices.8-13 The nanoparticles (NP) of noble metals, in fact, exhibit increased photochemical activity because of their high surface/volume ratio and unusual electronic properties. Particular interest plays the possibility of tailoring the metal nanocluster surface with threedimensional molecular arrangement consisting of electro- and photoactive moieties. In fact, NP complexed to dyes and organic compounds (fullerenes for example) present interesting properties as light-harvesting systems.14 We have employed here a fluoresceine derivative, SAMSA, that can be found in several prototropic states and can be linked to gold surfaces by mean of an activable thiol group. The gold NP investigated (diameter g5 nm, see Figure 1A) support surface plasmon resonances12 that can couple with the dye, whose fluorescence dynamics is largely affected, as indicated by the changes in the blinking times and excited-state lifetimes. The excited-state lifetime of SAMSA at the emission wavelength λem ) 520 nm decreases from τ ) 4.1 ( 0.1 ns, * Corresponding author. E-mail: [email protected]. † Laboratory for Advanced BioSpectroscopy (LABS), Dipartimento di Fisica, Universita` degli Studi di Milano-Bicocca. ‡ MDM-INFM-CNR. § Chemistry Department, Universita ` degli Studi di Pavia. 10.1021/nl0628293 CCC: $37.00 Published on Web 03/09/2007

© 2007 American Chemical Society

when free in solution or bound to glass substrates,15 to 1.62 ( 0.07 and 0.57 ( 0.06 ns, when bound to NP with diameters 5 and 20 nm, respectively.5 The fluorescence emission of SAMSA bound to gold NP blinks with rates = 0.1-1 Hz. It has been suggested5 that blinking is due to strong coupling between the vibroelectronic configuration of the dye and the plasmonic levels of the metal nanoparticles (see Figure 1B). In fact, while single SAMSA molecules bound to gold NP show blinking and reduced values of the excited-state lifetime, single SAMSA molecules bound to chemically etched glasses hydrated with high purity solvents do not exhibit fluorescence intermittency.5 The characteristic rate for the electron/energy transfer occurring between the higher excited state (S2 in Figure 1B) and the gold NP plasmons is drastically lower (less than few hundreds of kHz) than the nonradiative relaxation rate (0.1GHz), which is related to an interaction between the dye first excited state (S0 in Figure 1B) and the bound gold NP states.5 This suggests that the process leading to fluorescence blinking is fundamentally different (charge transfer) from that determining the excited-state lifetime (energy transfer). The purpose of the experiments reported here is twofold. First, we attempt to elucidate the origin of the blinking dynamics in terms of the dye-gold nanoparticle interaction and, second, we investigate possible ways to regulate the fluorescence blinking dynamics by modulating the dye-gold interaction. The main result of the report is that the fluorescence emission of SAMSA can be modulated, in amplitude and spectral position, by an applied external bias.

Figure 1. Scheme A: SAMSA binding to gold NP through the activated thiol group. An indication of the orientation of the major absorption dipole µ488 is also given. Scheme B: energy levels of SAMSA and possible absorption transitions (vertical arrows). The vertical dashed line indicates the de-excitation from the first excited state S1. knr and kBfD indicate the nonradiative transition rate and the dye-gold NP coupling rate at the second excited state S2. Image C: fluorescence image under single photon excitation of SAMSA bound to gold NP (λexc ) 488 nm, λem ) 535/30 nm). Field of view 5 µm × 5 µm, pixel dwell time 1ms. Average excitation intensity 2.5 kW/cm2. Image D: back scattering image of the same field of view as in Image C (λexc ) 800 nm, high pass filter at 670 nm). Same dwell time as in C, excitation intensity 190 kW/cm2.

This optical response has a resonance behavior with characteristic frequencies of the order of few Hz, depending on the size of the nanoparticle. SAMSA, when diluted in salt buffer at pH > 7, absorbs light primarily5 at 488 nm (one photon excitation) or 800 nm (two-photon excitation) with emission at 505 nm. The dye emission shifts to 520 nm (see Supporting Information) when it is bound to gold NP via its activable thiol group (Figure 1A). The fluorescence emission of single gold-NP SAMSA complexes was studied on highly diluted spin coated samples; 5-10 well separated objects on a 5 µm × 5 µm field of view were typically visible under λexc ) 488 nm in confocal detection mode (Figure 1C). The position of the gold NP was checked before and after a long acquisition sequence by performing backscattering (Figure 1D). Very similar results are obtained under two-photon excitation at λexc ) 800 nm or second harmonic generation imaging (λexc ) 960 nm, emission filter 480/20 nm, see Supporting Information). Gold NP fluorescence can be primed by singlephoton and two-photon excitation at 540 and 980 nm, respectively, with a corresponding dim emission16 at = 680 nm. When the sample is excited at 488 nm (one-photon excitation) or at 800 nm (two-photon excitation), some of the gold NP do not give rise to a fluorescence signal at =535 nm (Figure 1), thereby indicating that some colloids are not carrying SAMSA molecules. On the other hand, some of Nano Lett., Vol. 7, No. 4, 2007

the fluorescence spots (SAMSA molecules) are not associated with light scattering (gold NP). We estimate that the ratio of complexed to free SAMSA molecules in our samples is =5 ( 1. The results regarding SAMSA fluorescence dynamics reported hereafter refer to one-photon excitation at λexc ) 488 nm. SAMSA molecules are probably bound to small aggregates of gold nanoparticles. The histogram of the image levels acquired by exciting fluorescence emission of gold NP at 543 nm (emission at 680 nm) indicates (see Supporting Information) the presence of evenly spaced peaks. From this analysis, an average aggregation number =4 can be inferred. Only 15% of the spots visible in a backscattering image (Figure 1D) can actually be ascribed to single gold nanoparticles. SAMSA bleaching time, TB, is proportional to the average excitation intensity Iexc, with a typical value TB = 70 s at Iexc = 2.5 kW/cm2 (λexc ) 488 nm). The single sudden downstep in the fluorescence emission is usually taken as a fingerprint of single molecule behavior.17 However, the assignment of a fluorescence spot to single molecule emission is made after the examination of the evenly spaced peaks in the histogram of the image fluorescence levels.18 Only those spots associated with the lowest fluorescence peak in the histogram and whose size was approximately equal to the optical point spread function were ascribed to single molecule emission,17 in agreement with a more detailed analysis based on antibunching experiments.5 Besides this single molecule selection procedure, the fluorescence was acquired through a narrow bandpass filter centered at 535 nm (535 ( 15 nm, Chroma Tech., Brattelboro, VT) in order to focus on the bound SAMSA dynamics. In fact (see Supporting Information), the fluorescence emission has a maximum at λem ) 505 ((8) and 520 ((20) nm for the free and gold-complexed dye, respectively. It is known that electric fields19 affect semiconductor nanoparticles luminescence. For colloidal semiconductor QDs and quantum rods, the application of an external field leads to large spectral shifts due to surface-charge induced internal electric fields20,21 and to reversible emission switching.22 However, not much seems to be known on the electric field effect on noble metal nanoparticles7,14 and to its possible application. Fluorescence microscopy was then performed on single SAMSA bound to gold NP (5 and 20 nm in diameter) deposited on a glass surface between gold electrodes with electrode-electrode spacing of =700 µm (Figure 2 and Supporting Information). The sample was hydrated by a drop of pH ) 8 phosphate buffer and a low ac voltage, V ) ∆V cos(2πν0t), with ν0 in the range 0.1-10 Hz, was applied. Emission spectra averaged over fields of view that contained single SAMSA molecules both free and bound to gold NP, show that, by increasing the amplitude of the bias from 0 to 2.5 V, only SAMSA molecules bound to gold NP display a sensible (=50 nm), linear red-shifted emission (Figure 2B) and a =tenfold decrease of the fluorescence emission, whereas no appreciable change was found in the emission spectrum of free SAMSA and of uncomplexed gold NP. The red-shifted emission of SAMSA bound to gold NP, which corresponds to a maximum energy decrease of =170 1071

Figure 2. Panel A: Fluorescence emission spectra obtained by combining the emission spectra on single SAMSA molecules (Iexc ) 62 kW/cm2, λexc ) 488 nm) and the gold NP emission spectra (λexc ) 543 nm, Iexc ) 25 kW/cm2) of aggregates of =10 SAMSA molecules bound to 5 nm gold NP under alternating external bias of amplitude ∆V and frequency 1 Hz. Rayleigh scattering at 488 and 543 nm is substracted from the spectra for clarity reasons. The spectra are obtained on aggregates of =10 SAMSA bound to gold NP. Spectral resolution is obtained by repeated scanning of the sample while varying the observation wavelength. Bandwidth ) 8 nm. The peak-to-peak applied voltage is ∆V ) 0, 1, 1.5, 2, and 2.5 V from top to bottom, respectively. Panel B: Maximum emission wavelength of SAMSA bound to 5 nm gold NP (λexc ) 488 nm, filled squares, aggregate number 10 SAMSA-gold NP) and of gold NP (λexc ) 543 nm, open circles) vs the amplitude ∆V of the external alternating bias (frequency 1 Hz). Solid lines are best linear fit to the data. The image show the detail of the gold chip used to apply the electric bias. The squares are the gold electrodes and the dashed vertical line indicates 15.5 mm (see Supporting Information). Panel C: Fluorescence emission (Iexc ) 2.5 kW/cm2, λexc ) 488 nm) in different wavelength windows (see right labels), of a molecular aggregate of =20 SAMSA dyes on a 5 nm gold colloid under an external alternating bias, V ) ∆V cos(2πν0t) with ν0 ) 1 Hz. The amplitude ∆V was varied (∆V ) 0, 1.5, and 2.5 V) every 5 s. The filled squares, open triangles, and squares represent the emission in the 515-525, 535-545, and 555-565 nm spectral windows. The solid line in the bottom panel represents the amplitude of the applied bias. The fluorescence signal of the spot, estimated on the images, decreases with ∆V in agreement with Panel A.

meV at ∆V ) 2.5 V, suggests a picture in which an excitonlike quasiparticle22 is formed on the SAMSA bound to the gold NP. The sensitivity of SAMSA emission to a low electric field can be exploited to devise tunable nanosources. As an example, a small aggregate of SAMSA molecules bound to gold NP can be driven to emit in the windows 515-525, 535-545, and 555-565 nm, by applying voltages with 1072

Figure 3. Panel A: Fluorescence emission (λexc ) 488 nm, λem ) 535/30 nm) of single SAMSA molecules bound to 5 nm gold colloids vs time at increasing values of the average excitation intensity on the focal plane (sampling time is 1ms). The excitation intensities are 2.5, 5, 10, and 12.5 kW/cm2 from top to bottom. The data in the upper panel are multiplied by a factor of 2 for clarity. Panel B: Fluorescence emission (λexc ) 488 nm, λem ) 535/30 nm) of single SAMSA molecules bound to 20 nm gold colloids vs the average excitation intensity on the focal plane (sampling time is 1ms). The excitation intensity is 15, 20, 25, and 30 kW/cm2, from top to bottom. Panel C: Fourier transform spectrum (smoothed by adjacent averaging over 15 points) computed on a set of 100 single dye bound to 5 nm gold NP. Solid line is a Lorentzian fit to the first harmonic peak at =1 Hz. Inset: details of the fluorescence dynamics (λexc ) 488 nm, λem ) 515/30 nm) of single SAMSA molecules bound to 5 nm gold colloids (excitation intensity 20 kW/cm2). Sampling time is 1ms. Three repeated events spaced by =1 s are evident at the times =10.3, 15, and 18 s.

amplitudes ranging from 0 to 2.5 V (Figure 2C). It must be noted that the above-mentioned properties apply to small aggregates as well as to single SAMSA molecules bound to gold NP. The emission of single SAMSA molecules bound to gold NP under moderate-intermediate laser excitation intensities (2.5-12.5 kW/cm2 at 488 nm) displays random fluorescence blinking events5 (Figure 3). On the other hand, no blinking events have been detected on free SAMSA molecules on silanized glasses hydrated with high purity solvents.5 The blinking dynamics can be characterized by on (ton) and off times (toff), defined as the duration of the continuous fluorescence emission time stretch and of the consequent dark state. Both ton and toff are distributed according to singleexponential decays5 in absence of external perturbations, as Nano Lett., Vol. 7, No. 4, 2007

Figure 4. Blinking dynamics of single SAMSA molecules bound to gold NP under the action of an external alternating bias, V ) ∆V cos(2πν0t) with ν0 ) 1 Hz. Panel A: Blinking frequency, γon, vs the excitation intensity (single photon excitation at 488 nm) under external bias ∆V ) 0, 1, 1.5, and 2 V, from bottom to top. The solid lines are power law fits to the data, γon ) kIδexc, with δ = 2 (see panel B). Panel B, right axis: Best fit power law exponent δ of the data reported in panel A vs the applied voltage amplitude. Panel B, left axis: Best fit parameter a ) γon/Iδexc of the data reported in panel A vs the applied voltage amplitude. The line is the best fit of the parameter a according to the law a ) R + β∆V. The best fit parameters are R ) 0.0017 ( 0.0008 Hz (kW/cm2)-δ and β ) 0.005 ( 0.001 Hz V-1 (kW/cm2)-δ. Panel C: Fluorescence vs time for four different SAMSA molecules bound to 5 nm gold nanoparticles, under single-photon excitation at 488 nm and observed through a 515/30 nm emission filter (excitation intensity 27 kW/cm2). An external alternating bias V ) ∆V cos(2πν0 t) is applied with ν0 ) 1 Hz and amplitude ∆V ) 0, 1, 1.5, 2 V from top to bottom. Panel D: Fourier transform (FT) of the fluorescence dynamics of SAMSA bound to gold NP under the action of an external alternate bias V ) ∆V cos(2πν0t) is applied with ν0 ) 1 Hz and amplitude ∆V ) 0, 1, 1.5, 2 V from top to bottom. The FT spectrum is averaged over 30 fluorescence trajectories acquired on different single molecules. The solid line is a Gaussian function, exp[-ln(2)(ν - νR)2/σ2], which fits at best the first harmonic peak found at νR = 1 Hz.

expected. The average toff times are 〈toff〉 = 90 ( 10 ms and 〈toff〉 = 360 ( 40 ms for SAMSA bound to the 5 and 20 nm gold NP, respectively, nearly independent of the excitation intensity. On the other hand, the bright state duration, ton, of single SAMSA molecules is 〈ton〉 = 7.0 ( 0.5 s and 〈ton〉 = 2.5 ( 0.5 s (at Iexc = 7.5 kW/cm2) when bound to the 5 and 20 nm gold NPs, respectively, (Figure 3A,B) and depends on the excitation intensity. The average blinking frequency, γon ) 1/〈ton〉 (note that 〈toff〉 , 〈ton〉), has a power law dependence on the excitation intensity, γon = Iexc1.9 (Figure 4). This nonlinear behavior suggests that blinking may involve wave function superposition between high-energy excited states of SAMSA and surface plasmons of the gold NP.5 It is noteworthy also that, as the excitation intensity increases above =20-25 kW/cm2, some rare events are found in the fluorescence dynamics (Figure 3C, inset) consisting of few closely and evenly spaced blinking events that occur with a repetition time of =1 s and =0.4 s for the dyes bound to the 5 and 20 nm gold NP, respectively. The Nano Lett., Vol. 7, No. 4, 2007

occurrence of these rare events and the corresponding repetition frequency can be better assessed by performing a Fourier analysis on =100 single molecule traces. As an example, the Fourier amplitude spectrum shown in Figure 3C (lower panel) confirms the presence of a frequency =1 Hz in the emission of SAMSA bound to the 5 nm gold NP. The corresponding repetition frequency for SAMSA bound to the 20 nm gold NP is about 2.6 Hz (data not shown). The average blinking rate of single SAMSA molecules, γon, increases also with the amplitude of the alternating bias, ∆V (Figure 4), according to the law γon ) (R + β∆V)Iδexc, where R ) 0.0017 ( 0.0008 Hz (kW/cm2)-δ and β ) 0.005 ( 0.001 Hz V-1 (kW/cm2)-δ. The slope parameter δ decreases slightly with ∆V with average value δ ) 1.9 ( 0.1. The double dependence of γon on the excitation intensity and the amplitude of the applied bias suggests that blinking may be due to two independent events: the photoinduced excitation to a high-energy vibronic state of SAMSA and the enhanced interaction between the excited SAMSA and the gold NP induced by the action of the bias. On the basis of these observations, we may envision the following mechanism (Figure 1B). (1) Photons are absorbed by SAMSA with a probability branching: either a single photon excitation from S0 to S1, leading eventually to radiative decay, or a two photon excitation (δ = 2) from S0 to S2. (2) In both cases, the excited SAMSA may form an exciplex with the gold NP. (3) The excitation of the S2 state may then relax, likely by a charge transfer (CT), to the gold NP. SAMSA, by transferring energy to the gold NP, may take a different electronic structure, SAMSA/, which is dark: the dye may then undergo further excitation, but no appreciable radiative decay occurs. (4) The excited gold NP may lose its energy through thermal paths and interaction with the ground state of SAMSA/ (as confirmed by the lack of dependence of toff on the excitation intensity and the external bias), which then goes back in its original electronic structure. The dye is then again in its bright electronic configuration. The applied voltage could then be seen as a modulator of the height of the barrier between the S2-excited SAMSA and the dark SAMSA/ (Figure 1B). Besides the increase in the blinking average frequency, also the statistics of the fluorescence blinking dynamics is dramatically affected by the external bias. The stochastic fluorescence blinking found for ∆V ) 0 V becomes, when ∆V = 2V, a clear regular and periodic oscillation (Figure 4C) that lasts approximately for the whole emitting life (bleaching time) of the dye. As the voltage bias approaches ∆V ) 2 V, the values of ton gather around a value characteristic of the NP size: ton = 1 s and ton = 0.4 s for the 5 and 20 nm gold NP, respectively (〈toff〉 does not change appreciably with ∆V). It must be noted that these values are close to the time spacing of the rare blinking events observed at high excitation intensity under no external bias (Figure 3C). It is also worth noting that the fluorescence rate of the bright state is not affected by the applied bias. The decrease in the average fluorescence emission (see Figure 2) is then mostly due to the total permanence time in the dark state, which increases both with ∆V and Iexc. To describe the 1073

Figure 5. Panel A: Distribution of the SAMSA-gold NP (5 nm diameter) ton times under applied alternating bias (∆V ) 2 V) vs the modulation frequency ν0 (indicated in the panels). Panel B: Relative half-height width (σ/νR) of the Gaussian fit to the first harmonic peak of the FT spectrum of the fluorescence dynamics of single SAMSA bound to gold NP of 5 (open circles) and 20 nm (filled squares). The solid lines are Lorentzian best fit functions, B - 2AπΓ/(4(ν - νR0)2 + Γ2), to the data: B ) 0.57 ( 0.03, νR0 ) 2.6 ( 0.02 Hz, Γ ) 0.52 ( 0.07 Hz, A ) 0.31 ( 0.06 for the 20 nm diameter NP and B ) 0.55 ( 0.04, νR0 ) 0.9 ( 0.01 Hz, Γ ) 0.40 ( 0.08 Hz, A ) 0.26 ( 0.06 for the 5 nm diameter gold NP.

transition from a stochastic to a regular blinking, it is convenient to resort to a Fourier transform (FT) of the original fluorescence data versus time. The FT spectrum, averaged over =100 single molecules trajectories (Figure 4D), is characterized by a peak centered at the frequency νR with relative half-height width (width/νR ) σ/νR) = 0.05, superimposed on a smooth decrease =1/ν (Figure 4D). The shape of the FFT spectrum indicates the presence of a resonance at =1 and =2.6 Hz for the 5 and 20 nm (data not shown) gold NP-dye complexes, respectively. The fluorescence blinking is also largely affected by the field modulation frequency ν0, with distributions of ton that change from an exponential, at ν0 = 0.4 Hz, to a Gaussian function peaked at about ton ) 1 s (gold NP 5 nm in diameter, Figure 5A) at ν0 = 1.0 Hz. On the contrary, 〈toff〉 does not change appreciably with the modulation frequency. The dependence of the blinking statistics on the modulation frequency is also visible from the FT analysis. The relative width σ changes versus the modulation frequency of the applied bias, ν0, according to a Lorentzian function centered at νR = 0.90 ( 0.01 Hz, for the gold nanoparticles with average diameter =5 nm and at νR ) 2.6 Hz ( 0.02 for gold nanoparticles with 20 nm diameter (Figure 5B). The Lorentzian shape of the response again suggests that a resonance process is induced by the applied voltage. The increase observed in the resonant frequency νR with the gold NP diameter suggests an effect of the NP size on the fluorescence dynamics. This could be due to the change of 1074

the excitation cross-section for the S0 f S2 transition of SAMSA when bound to gold NP of different diameters or to the increase in the transition rate to the dark form SAMSA/ while the dye is at the S2 excited state. In conclusion, the optical behavior of single molecules of SAMSA bound to gold NP is strongly affected by tiny (510 V/cm) electric fields. The external field provides direct control for reversible on-off optical switching of single dye-NP complexes that can be resonantly driven at frequencies in the 1-3 Hz range, depending on the size of the gold NP. The observed control of the fluorescence emission by an external bias suggests the involvement of higher excitation states of the dye interacting with the gold NP, in agreement with the steep dependence of the blinking rate on the excitation intensity. At the same time, the simultaneous redshift of the dye fluorescence emission under the applied voltage indicates exciplex formation. The remarkable field-induced behavior of the dye-gold NP complex could be a valuable tool for devising optical biosensors of electrical activity. Additionally, such complexes could be considered for applications in wavelength-tunable optical switching devices. Acknowledgment. The research reported in this work has been partially funded by the project of Fondazione Cariplo 2006-2007, code 2005-1079. G.T. acknowledges M. Alia (MDM) for his support in device fabrication. Supporting Information Available: Materials and methods, spectral decomposition of the images, nonlinear imaging of SAMSA-gold nanoparticles, nanoparticles aggregation number. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Kamat, P. V. J. Phys. Chem. B. 2002, 106, 7729. (2) Wang, Y.; Herron, N. Science. 1996, 273, 632. (3) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.;Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature. 1996, 383, 802. (4) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (5) Cannone, F.; Chirico, G.; Bizzarri, A. R.; Cannistraro, S. J. Phys. Chem. B 2006, 110, 16491. (6) Akiyama, T.; Nakada, M.; Terasaki, N.; Yamada, S. Chem. Commun. 2006, 395, http://dx.doi.org/10.1039/b511487j. (7) Ouyang, J.; Chu, C-W.; Sieves, D.; Yang, Y. Appl. Phys. Lett. 2005, 86, 123507. (8) Gomez-Romero, P. AdV. Mater. 2001, 13, 163. (9) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (10) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (11) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (12) Lakowicz, J. R. Plasmonics 2006, 1, 5. (13) Oyuang, J.; Chu, C.; Szmada, C.; Yang, Y. Nat. Mater. 2004, 3, 918. (14) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 2005, 127, 1216. (15) Sjoerback, R.; Nygren, J.; Kubista, M. Spectrochim. Acta, Part A 1995, 51, L7. (16) Geddes, C. D.; Parfenov, A.; Gryczynski, I.; Lakowicz, J. R. Chem. Phys. Lett. 2003, 380, 269. (17) Chirico, G.; Cannone, F.; Beretta, S.; Baldini, G.; Diaspro, A. Microsc. Res. Tech. 2001, 55, 359.

Nano Lett., Vol. 7, No. 4, 2007

(18) Cannone, F.; Caccia, M.; Bologna, S.; Diaspro, A.; Chirico, G. Microsc. Res. Tech. 2004, 65, 186. (19) Miller, D. A. B.; Chemla, D. S.; Damen, T. C.; Gossard, A. C.; Weigmann, W.; Wood, T. H.; Burrus, C. A. Phys. ReV. Lett. 1984, 53, 2173. (20) Weman, H.; Martinet, E.; Dupertuis, M. A.; Rudra, A.; Leifer, K.;Kapon, E. Appl. Phys. Lett. 1999, 74, 2334.

Nano Lett., Vol. 7, No. 4, 2007

(21) Empedocles, S. A.; Bawendi, M. G. Science 1997, 278, 2114. (22) Rothenberg, E.; Kazes, M., Shaviv, E.; Uri Banin, U. Nano Lett. 2005, 5, 1581.

NL0628293

1075