Truncated Power Law Analysis of Blinking SERS of Thiacyanine

Apr 15, 2013 - From blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine adsorbed on single Ag nanoaggregates, the electromagnetic...
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Truncated Power Law Analysis of Blinking SERS of Thiacyanine Molecules Adsorbed on Single Silver Nanoaggregates by Excitation at Various Wavelengths Yasutaka Kitahama,*,† Ai Enogaki,† Yuhei Tanaka,† Tamitake Itoh,‡ and Yukihiro Ozaki† †

Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan Nano-bioanalysis Research Group, Health Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Takamatsu, Kagawa 761-0395, Japan

‡

S Supporting Information *

ABSTRACT: From blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine adsorbed on single Ag nanoaggregates, the electromagnetic field and the molecular behavior in a nonemissive state were investigated by a truncated power law analysis. The power law that reproduces probability distribution of dark SERS events versus duration time was not truncated often by excitation at long wavelengths; otherwise it was truncated at the long tail. The truncation suggests a high energy barrier from nonemissive to emissive state and a short passage time of molecular random walk to overcome the energy barrier. The energy barrier in blinking SERS likely originates from a nanometer-ordered periodic optical trapping potential well, namely, electromagnetic field around a junction of the Ag nanoaggregate due to coupling of multipolar surface plasmon resonance, which is hardly induced by excitation at long wavelengths. This is consistent with the experimental excitation wavelength dependence of the truncation. At a low concentration of anionic thiacyanine, the power law was truncated at the short tail. The reason may be the short passage time of the molecule on the Ag surface adsorbing a small number of obstacles to reach the junction.

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INTRODUCTION On a noble metal nanoparticle, an electromagnetic (EM) field is enhanced by resonance of plasmon with excitation light due to dipolar oscillation of the conduction band electrons (localized surface plasmon resonance, LSPR). The EM field is enormously enhanced at a junction of aggregate of the nanoparticles and then causes surface-enhanced Raman scattering (SERS). In SERS, detailed information about molecular structure and dynamics can be provided by the sharp peaks of the vibrational modes of molecules adsorbed on the noble metal nanoaggregate.1−4 At the single molecule level, a small number of molecules enter and exit the enhanced EM field at the junction, which can be as small as a molecule, and then blinking SERS takes place.1−17 However, there is a possibility that the blinking SERS is induced by fluctuation of the enhanced EM field at the junction (hot spot).8 Blinking SERS deteriorates the reproducibility and signal-to-noise ratio of obtained spectra, making it difficult to identify the analyte and investigate its molecular structure at the single molecule level. Thus, the application of SERS to single molecule detection is disturbed by the blinking. The blinking statistics of SERS have been analyzed by using a power law,11−14 which originates from distribution of the first passage time required for a random walker to return to its starting point.18 It suggests that the blinking SERS is due to © 2013 American Chemical Society

random walk of the molecule adsorbed on the Ag nanoaggregate. In an opposite way of blinking fluorescence from a single semiconductor quantum dot (QD),19−21 the probability distributions of dark SERS events against their durations are reproduced by a truncated power law, namely a power law with an exponential function.12,13 The truncation in the power law indicates that the luminous body such as a QD quickly changes between a nonemissive and an emissive state via a high energy barrier.22 In the case of blinking SERS, a molecule on an Ag surface (nonemissive state) may go to a junction (emissive state) quickly via nanometer-ordered periodic optical trapping potential well (energy barrier) due to the surface-plasmonenhanced EM field around the junction.12,13 Quite recently, the periodic EM field has been observed and may be due to coupling of multipolar surface plasmon resonance.23−25 The energy barrier due to the periodicity may be high enough to induce the truncation, because it is considered that the EM field around the junction can optically trap a single molecule.26 Thus, the truncation is observed in dark SERS events, and the truncated power law analysis can investigate the molecular random walk in the nonemissive state, which can be barely Received: December 19, 2012 Revised: April 12, 2013 Published: April 15, 2013 9397

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RESULTS AND DISCUSSION Figure 1 shows a SERS spectrum of TC adsorbed on an isolated single Ag nanoaggregate that consists of a few Ag

detected even by a super-resolution imaging of SERS-active molecule.17 In our previous study, the excitation laser intensity dependence of blinking SERS of a dye molecule adsorbed on a single Ag nanoaggregate has been analyzed by using a truncated power law.13 Despite changing the laser intensity, the truncation times in the power law for the dark SERS events were almost constant,13 although the power law for the bright fluorescence events from a single QD was truncated at the shorter tail under higher laser intensity.19,20 It suggests that blinking SERS is affected not only by photodependent random walk of the molecule but also by the surface-plasmon-enhanced EM field on the single Ag nanoaggregate. In the present study, SERS from a single Ag nanoaggregate with adsorbed anionic thiacyanine molecules, which also have positively charged nitrogen atoms, at different concentrations was measured by excitation at various wavelengths. In this system, the temporally fluctuated spectra were observed by Coulomb repulsion between the anionic dyes and citrate anion that covered the colloidal Ag nanoaggregates,15 and then the blinking SERS was analyzed by using a truncated power law. The concentration and the excitation wavelength that is longer than the wavelength of an absorption band of the adsorbed molecules may not influence the enhanced EM field on the single Ag nanoaggregate and a random walk of the molecules, respectively. As a result, the concentration and the excitation wavelength dependence of the truncation were found. It is noted that the latter can be explained by the periodic EM field due to coupling of multipolar surface plasmon resonance, which is hardly induced by excitation at long wavelengths.25

Figure 1. Conventional Raman (top) and SERS (bottom) spectra of 3,3′-disulfopropylthiacyanine powder and adsorbed on single Ag nanoaggregate, respectively. Inset: chemical structure of 3,3′disulfopropylthiacyanine triethylamine.

nanoparticles. Prominent peaks were observed at almost the same wavenumbers as those in a conventional Raman spectrum of TC powder excited at 458 nm. The behavior of the SERS peaks, namely their disappearance and the slight shift from the corresponding Raman peak, are similar to those in previous reports.1−16 Thus, these observations indicate that the blinking are not attributed to the SERS of amorphous carbon formed by photo- and thermodegradation27 but to that of TC. It has been reported that totally symmetric peaks shows different blinking behavior from nontotally symmetric peaks in the case of various analytes, and this can be interpreted in terms of vibronic coupling.16 On the other hand, there is no cross correlation among blinking SERS peaks of Fe-protoporphyrin IX at 1480, 1570, and 1620 cm−1,11 which are assignable to ν3, ν2 (totally symmetric modes), and ν10 (nontotally symmetric mode), respectively.28−30 By using a power law without an exponential function, their exponents for the dark SERS events are derived to be similar values (−1.46, −1.56, and −1.51, respectively).11 Thus, it is interesting that intensities of each blinking peaks are analyzed by a truncated power law. In the present study, however, the integrated total emission intensity was used for the analysis, because we focused on the translational random walk of the molecules on the metal between the emissive and nonemissive states, which may be influenced by the enhanced EM field. The blinking SERS intensity can be represented by a time profile of the integrated total emission intensity from a single Ag nanoaggregate with adsorbed TC as shown in Figure 2a, because the excitation wavelengths (λ ≥ 458 nm) are longer than the wavelength of an absorption band of a TC molecule,15 the molecules on a metallic surface hardly emit fluorescence due to the energy transfer to the metal,31 and the SERS signal coincides with the background emission.32 From the baseline of the time profile, an averaged intensity, Ibase, and a standard deviation, σ, were evaluated. We defined bright and dark SERS events as the events showing larger and

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EXPERIMENTS We used 3,3′-disulfopropylthiacyanine (TC) triethylamine as purchased from Hayashibara Biochemical Laboratories (Japan). A stock aqueous solution of TC dyes (10 or 25 μM), a NaCl aqueous solution (100 mM), and a citrate-reduced Ag colloidal solution were mixed at a volume ratio of 1:1:2 at room temperature. The sample solutions were spin-coated on a glass plate, which was then rinsed with water and acetone. An aliquot of a 1 M NaCl solution was dropped on the glass plate to immobilize the sample Ag nanoaggregates on the surface. This glass plate was covered with another glass plate to prevent the solution from evaporating. The details of the experimental setup are described elsewhere.14 In brief, the Ag nanoaggregates with adsorbed TC on an inverted microscope (Olympus, IX70) were excited using a 458, 514, or 568 nm line of an Ar or Kr ion laser whose intensity of 10 mW corresponding to power density of 100 W cm−2. The SERS emissions of a single Ag nanoaggregate with adsorbed TC were collected with an objective lens (Olympus, LCPlanFl 60×, NA 0.7) and led to a polychromator (Acton, Pro-275) coupled to a thermoelectrically cooled CCD (Andor, DV434-FI) through a notch filter and a pinhole. Videos of the blinking SERS were taken for 5− 20 min by the inverted microscope coupled with a cooled digital CCD camera (Hamamatsu, ORCA-AG), which had a time resolution of 61−183 ms. We calculated a near-field image of the EM field using a finite-difference time-domain (FDTD) calculation. The software used was PLANC-FDTD (Information and Mathematical Science Laboratory Inc., Version 6.2). For the calculation of the EM field, the mesh size was set at 0.5 nm inside and outside an Ag nanoaggregate. 9398

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where n(t) is the number of bright or dark SERS events against their duration times, respectively. Figure 2b shows the log−log plot for the normalized probability distributions of dark SERS events versus their duration. The log−log plot yields the line truncated at the tail. This is reproduced by a truncated power law as ⎛ −t ⎞ Poff (t ) = At αoff exp⎜ ⎟ ⎝ τ ⎠

(2)

where Poff(t) is probability distribution of the dark SERS events, A is a coefficient for normalization, αoff is a power law exponent for the dark SERS events, and τ is a truncation time in the power law. On the other hand, the probability distributions of bright SERS events versus their duration are given by Pon(t ) = At αon

We checked an influence on the truncated power law analysis by changing the threshold. By using the higher threshold, the number of the duration times (points in the log−log plot) for the bright and dark SERS events are decreased and similar (see Figure S1a and S1b in Supporting Information), respectively, because the bright SERS events for long duration times are barely counted. However, most of the exponents (αon and αoff) and the truncation times on different conditions are increased equally by the higher threshold (see Supporting Information Figure S1cS1e) and thus the dependence of blinking SERS on concentration, excitation wavelength, and so on will be similar despite changing the threshold. The reason may be as follows. The probabilities of the dark SERS events for long duration times are increased by the higher threshold; namely, the points on the lower right in the log−log plot (like Figure 2b) go upward. Then the exponent and the truncation time are

Figure 2. (a) A time-profile of the SERS intensity from the single Ag nanoaggregate. (b) Probability distributions of the dark SERS events against the duration times.

smaller intensities than a threshold of Ibase + 3σ, respectively. Probability distribution for a time duration t1 of the bright or dark SERS events is given by P(t1) =

∑ t = t1

n(t ) t1

(3)

(1)

Figure 3. Histograms of exponents in the power law for the bright and dark SERS events (αon and αoff, respectively) of TC adsorbed on single Ag nanoaggregates from the solution at (a) 10 and (b) 25 μM by excitation at various wavelengths. 9399

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increased. On the other hand, the bright SERS event for longest duration time is divided into a few events for long duration times rather than many events for short duration times by the higher threshold. In other words, the rightmost point in the log−log plot vanishes, and the neighbor points go upward. Then the power law exponent is increased. Figure 3 shows histograms of the exponents in the power law that reproduces probability distributions of the bright and dark SERS events versus their duration for TC adsorbed on 32 163 single Ag nanoaggregates with various sizes and shapes. At the higher concentrations of TC, the power law exponents were increased or decreased (see also Figure S2 in Supporting Information; the rearranged histograms of Figure 3). Thus, the concentration dependence of the exponents was not confirmed. On the other hand, the maxima of the histograms of αon and αoff were increased and decreased by excitation at longer wavelength except for αoff at 514 nm, respectively. The averaged exponents for bright and dark SERS events show similar behavior as summarized in Tables 1 and 2, respectively. This Table 1. Averaged Exponents in the Power Law for the Bright SERS Events (αon) of TC Adsorbed on Single Ag Nanoaggregatesa 10 μM 25 μM a

458 nm

514 nm

568 nm

−2.34 (0.05) −2.48 (0.03)

−2.30 (0.09) −2.24 (0.05)

−2.18 (0.06) −2.16 (0.06)

Figure 4. Histograms of truncation times in the power law for the dark SERS events of TC adsorbed on single Ag nanoaggregates from the solution at different concentration by excitation at (a) 458, (b) 514, and (c) 568 nm. Rearranged histograms of those from the solution at (d) 10 and (e) 25 μM by excitation at various wavelengths.

higher concentrations from 10 to 25 μM as summarized in Tables 3 and 4, respectively.

The values in parentheses are their standard errors.

Table 2. Averaged Exponents in the Power Law for the Dark SERS Events (αoff) of TC Adsorbed on Single Ag Nanoaggregatesa 10 μM 25 μM a

458 nm

514 nm

568 nm

−1.44 (0.03) −1.46 (0.02)

−1.22 (0.04) −1.42 (0.02)

−1.59 (0.03) −1.48 (0.03)

Table 3. Averaged Truncation Times in the Power Law for the Dark SERS Events (τ) of TC Adsorbed on Single Ag Nanoaggregatesa 10 μM 25 μM

The values in parentheses are their standard errors. a

458 nm

514 nm

568 nm

74 (10) s 85 (7) s

162 (26) s 256 (20) s

186 (19) s 221 (24) s

The values in parentheses are their standard errors.

Table 4. Medians of Truncation Times in the Power Law for the Dark SERS Events (τ) of TC Adsorbed on Single Ag Nanoaggregates

behavior is similar to the LSPR wavelength dependence of the power law exponent in which the αon and αoff were increased and decreased by deeper surface-plasmon-enhanced optical trapping potential well as the LSPR peaks approaches the excitation wavelength, respectively.14 In the present case, the deeper optical trapping potential well may be induced by the excitation light at the longer wavelength (especially at 568 nm) through LSPR, because it has been confirmed that the LSPR peak of the single SERS-active nanodimer mostly appears at 530−750 nm by a scanning electron microscope image, a Rayleigh scattering spectrum, and a FDTD calculation of the same Ag nanodimer.33 Since even the LSPR peak of the nanodimer appears at 530−750 nm, those of the larger nanoaggregates will appear at longer wavelengths than 530− 750 nm. Figure 4 displays histograms of the truncation times in the power law for the dark SERS events. The truncation times were lengthened at a higher concentration of TC. Figure 4a show a larger percentage of the events of τ = 50−100 s by the excitation at 458 nm for 25 μM than that for 10 μM. Figure 4b,c reveals that the maxima of the histograms by the excitation at 514 and 568 nm for 25 μM appear at a longer truncation time than those for 10 μM, respectively. Indeed, the averages and medians of the truncation times were increased at the

10 μM 25 μM

458 nm

514 nm

568 nm

40 s 51 s

119 s 190 s

117 s 142 s

In terms of excitation wavelength dependence, the truncation times for the dark SERS events were lengthened by the excitation at the long wavelengths, although those for the bright fluorescence events from a single QD were similar except for near-ultraviolet excitation.21 Figure 4d,e shows that the maxima of the histograms by excitation at 514 and 568 nm appear around at 100−150 s, although the maxima at 458 nm appear around at 0−50 s. The averages and medians of the truncation times were increased by excitation at the long wavelengths of 514 and 568 nm compared with 458 nm as summarized in Tables 3 and 4, respectively. Moreover, the probability distribution for the dark SERS event excited at the long excitation wavelengths became difficult to be reproduced by the truncated power law; namely, the truncation times were often deduced to be very long with large errors. By the excitations at 568 and 514 nm, the percentages of the events whose 9400

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probability distribution cannot be reproduced by using the power law with an exponential function are larger than those at 458 nm as summarized in Table 5. Thus, the power law was truncated at the long tail or not truncated by the excitation at the long wavelengths. Table 5. Percentages of the Dark SERS Events Whose Probability Distribution Cannot Be Reproduced by a Power Law with an Exponential Function 10 μM 25 μM

458 nm

514 nm

568 nm

7% (= 6/81) 8% (= 15/178)

29% (= 13/45) 29% (= 44/154)

34% (= 53/155) 17% (= 17/99)

The truncation at a tail of the power law for the blinking is produced by random walk on parabolic potential surfaces against reaction coordinate for emissive and nonemissive states. According to the differential equation for classical diffusion on a harmonic potential of a diffusion-controlled electron-transfer model for a single QD,22 a truncation is induced by the high energy barrier and a fast random walk to overcome the energy barrier, and the truncation time is given by τ=

Figure 5. (a) Calculated spatial distribution of enhancement factor of the electromagnetic field around a gap (2 nm) of Ag nanoparticles of 20 nm in diameter excited by horizontal polarizations at 458 nm. (b,c) Calculated intensities of the optical trapping potential wells along the vertical broken line in (a).

Γ Ea 2kBT

(4)

where Γ is the random walk time to overcome the energy barrier between emissive and nonemissive states, Ea. In the case of a QD, the energy barrier between emissive (electron−hole pair) and nonemissive states (photoionized state) is derived from Marcus theory.22 In blinking SERS, it is not likely that the concentration of dye influences the energy barrier, which may be due to a periodic surface-plasmon-enhanced EM field around the junction. Thus, the short truncation time at the low concentration suggests short time to reach the junction for the molecules on the Ag surface adsorbing a small number of obstacles. In a different way of the SERS with resonance Raman effect of thiacarbocyanine excited at 514 and 568 nm,13 a TC molecule barely absorbs the present excitation light (λ ≥ 458 nm).15 Thus, the excitation wavelength dependence of the truncation for TC is not attributed to photoinduced random walk of the molecule. The excitation wavelength dependence suggests that the nanometer-ordered periodic EM field is hardly formed by the excitation at the long wavelengths. Indeed, the periodic EM field around a gap of an Au nanoblock pair seems to be observed by a scattering-type scanning near-field optical microscope.23,24 It is thought that the periodic EM field may be due to coupling of multipolar surface plasmon resonance, which is induced by the excitation at short wavelengths.25 Figure 5 shows a near-field image and intensities of the EM field around a gap of Ag nanoparticles by a FDTD calculation. Also in Figure 5b,c, it is exhibited that the periodic optical trapping potential well due to the EM field is hardly formed by the excitation at the long wavelengths. Because SERS is narrowly emitted from a single molecule by the enormous enhancement at the hot spot and is quadratically weakened by a decrease in an optical trapping potential,33,34 the molecule not at the hot spot (the deepest bottom of the periodic optical trapping potential well), but in metastable state of the potential well, may not emit SERS light but be optically trapped. In short, the metastable state may be a nonemissive state. When the molecule quickly goes from the metastable

state to the emissive state, it must overcome the energy barrier between them. Thus, the truncation in the power law for the dark SERS events is induced by the periodic optical trapping potential well as the energy barrier; namely, τ is shortened by large Ea (see eq 4). On the other hand, the molecule cannot quickly escape to the nonemissive state due to the stronger optical trapping potential in the emissive state than that in the metastable state. In other words, Γ lengthens in the bright SERS, and then τ becomes very long (see eq 4). Thus, the truncation for the bright SERS events hardly occurs. From the assumed parameters of a polarizability of a molecule (α = 10−37 J−1 C2 m2),35 an adequate SERS enhancement factor to detect a single molecule (E4/E04 = 10812) and this laser power density (E02 = 100 W cm−2), an optical trapping potential is derived to be 0.005−0.00005 kBT at 300 K by αE2/2, where α is a polarizability of a molecule, E is an enhanced electric field of light, and E0 is an original electric field of light. The periodicity of the optical trapping potential, which is much smaller than the thermal energy, can work as the energy barrier to a small number of the molecules not in solution but electrically adsorbed on the colloidal metal surface, because it has been reported that even the potential of ∼0.1 kBT at 77 K works to immobilize entirely an adsorbate onto the junction by suppression of the blinking.10 Furthermore, its effective polarizability on the metal is increased by effect of mirror image dipole.36 Incidentally, quantitative relation between the height of the energy barrier and the excitation wavelength has not been found yet. Moreover, the truncation times are intricately influenced by the periodic surface-plasmonenhanced optical trapping potential in terms of the energy barrier and the molecular random walk.13 Thus, it is difficult to explain quantitatively the excitation wavelength dependence of the truncation (Tables 3−5) so far. It seems that the periodic EM fields were much stronger than the monotonous EM fields as shown in Figure 5b,c. Thus, there is a possibility that periodicity attendant on an original EM field 9401

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excitation wavelength dependence of the truncation times in the power law for the blinking SERS of TC.

is enormously enhanced by dipolar LSPR. In other words, the truncation, which originates from the periodic EM field, could happen at shorter tail of the power law as the LSPR wavelength approaches the excitation wavelength. We already have analyzed blinking SERS of cationic thiacyanine (3,3′-diethylthiacyanine iodine salt), which also barely absorbs the excitation light at longer wavelength than 458 nm,37 at various LSPR wavelengths of the single Ag nanoaggregates with the adsorbed dye.14 In the our previous report,14 however, the LSPR wavelength dependence of the truncation times has not been shown. Here, we display the truncation times in the power law plotted versus the LSPR wavelengths in Figure 6. The

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ASSOCIATED CONTENT

S Supporting Information *

Influence on the truncated power law analysis by changing the threshold for the definition of the bright and dark SERS events; rearranged histograms of exponents in the power law for the bright and dark SERS events of TC adsorbed on single Ag nanoaggregates from the solution at different concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-79-565-9077. Tel: +81-79-565-8349. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI (Grant-in-Aid for Young Scientists B) (No. 23750025) and a Support Project to Assist Private Universities in Developing Bases for Research (Research Center for Single Molecule Vibrational Spectroscopy) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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REFERENCES

(1) Qian, X. -M.; Nie, S. M. Single-molecule and single-nanoparticle SERS: from fundamental mechanism to biomedical applications. Chem. Soc. Rev. 2008, 37, 912−920. (2) Pieczonka, N. P. W.; Aroca, R. F. Single molecule analysis by surfaced-enhanced Raman scattering. Chem. Soc. Rev. 2008, 37, 946− 954. (3) Kneipp, J.; Kneipp, H.; Kneipp, K. SERSa single-molecule and nanoscale tool for bioanalysis. Chem. Soc. Rev 2008, 37, 1052−1060. (4) Etchegoin, P. G.; Le Ru, E. C. A perspective on single molecule SERS: current status and future challenges. Phys. Chem. Chem. Phys. 2008, 10, 6079−6089. (5) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (6) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667− 1670. (7) Weiss, A.; Haran, G. Time-Dependent Single-Molecule Raman Scattering as a Probe of Surface Dynamics. J. Phys. Chem. B 2001, 105, 12348−12354. (8) Emory, S. R.; Jensen, R. A.; Wenda, T.; Han, M.; Nie, S. Reexamining the origins of spectral blinking in single-molecule and single-nanoparticle SERS. Faraday Discuss. 2006, 132, 249−259. (9) Habuchi, S.; Cotlet, M.; Gronheid, R.; Dirix, G.; Michiels, J.; Vanderleyden, J.; De Schryver, F. V.; Hofkens, J. Single-Molecule Surface Enhanced Resonance Raman Spectroscopy of the Enhanced Green Fluorescent Protein. J. Am. Chem. Soc. 2003, 125, 8446−8447. (10) Maruyama, Y.; Ishikawa, M.; Futamata, M. Thermal Activation of Blinking in SERS signal. J. Phys. Chem. B 2004, 108, 673−678. (11) Bizzarri, A. R.; Cannistraro, S. Lévy statistics of Vibrational Mode Fluctuations of Single Molecules from Surface-Enhanced Raman Scattering. Phys. Rev. Lett. 2005, 94, 068303. (12) Kitahama, Y.; Tanaka, Y.; Itoh, T.; Ozaki, Y. Power-law statistics in blinking SERS of thiacyanine adsorbed on a single silver nanoaggregate. Phys. Chem. Chem. Phys. 2010, 12, 7457−7460.

Figure 6. Truncation times for the power law for the dark SERS events of cationic thiacyanine adsorbed on single Ag nanoaggregates by excitation at (a) 458 and (b) 514 nm plotted against their LSPR wavelengths.

truncation times were independent of the LSPR wavelengths. Thus, it does not matter that the various sized and shaped single Ag nanoaggregates, whose LSPR peaks appear at different wavelengths, were used for the present study. On the other hand, the truncation times in Figure 6a (τ = 5−25 s) were shorter than those in Figure 6b (τ = 20−50 s); namely, they were shortened by excitation at the short wavelength in a similar way of the anionic TC as shown in Figure 4d,e.

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CONCLUSION The truncation times in the power law for the dark SERS events are shortened at lower concentrations of TC. This suggests that the molecules on the Ag surface reach the junction more quickly as the number of obstacles on the surface decreases. By excitation at a long wavelength, the truncation times are lengthened, and moreover, the number of the dark SERS events that show no truncation is increased. The truncation indicates the energy barrier from the nonemissive to the emissive states on the Ag nanoaggregate, namely, the molecule quickly goes from the metastable state of the periodic optical trapping potential well to the bottom via the energy barrier. Thus, the energy barrier likely originates from nanometer-ordered periodic EM field around the junction. Quite recently, the periodicity seems to be due to coupling of multipolar surface plasmon resonance, which is hardly induced by excitation at a long wavelength.25 This is consistent with the 9402

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp312530j | J. Phys. Chem. C 2013, 117, 9397−9403