Elucidation of Electrostatic Interaction between Cationic Dyes and Ag

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Elucidation of Electrostatic Interaction between Cationic Dyes and Ag Nanoparticles Generating Enormous SERS Enhancement in Aqueous Solution Masayuki Futamata,* Yingying Yu, and Toru Yajima Graduate School of Science and Engineering, Saitama University, Saitama, 338-8570 Japan

bS Supporting Information ABSTRACT: Addition of a trace quantity of cationic triphenylmethane (TPMþ) dyes to isolated Ag nanoparticles in an aqueous solution yielded Ag flocculates composed of a few closely adjacent suspended nanoparticles. This change was evidenced by coupled LSP peaks emerged at 600-800 nm. However, neutral para-rosaniline (p-RA) molecules as well as neutral rhodamine 123 did not cause the Ag flocculation in contrast to their cations, indicating a crucial role of electrostatic interaction between cationic dyes and negatively charged Ag surfaces for the flocculation. Accordingly, cationic dye molecules are located in the nanogap between closely adjacent Ag nanoparticles which evoked enormous SERS intensity. The formation of the Ag flocculates was insensitive to steric hindrance by different amino groups -NH2, -N(CH3)2, and -N(C2H5)2 in TPM dyes. This observation is consistent with dominant role of the electrostatic interaction. Nevertheless, distinct red shifts of fluorescence peaks were observed depending on the molecular structures such as coplanar and propeller phenyl rings, suggesting perturbed electronic states of TPM dyes upon adsorption through the amino groups.

1. INTRODUCTION Recently, particular interest has been aroused in the electromagnetic (EM) enhancement of surface-enhanced Raman scattering (SERS).1-4 It markedly enhanced electric field intensity by a factor of 103-104, corresponding to 106-108 as a SERS enhancement factor, through localized surface plasmon (LSP) resonance, which is available for various molecules independent of their structures or electronic states. From extensive exploration into SERS active nanostructures, we have a consensus, albeit without relevant evidence from single molecule experiments, that molecules located between adjacent metal nanoparticles show enormous SERS intensities through the coupled LSP field.5-12 In addition, noticeable Raman enhancement of 102-103 was obtained for specific molecules and vibrational modes in the chemical enhancement (CE), which results from electron transfer resonance between metal and molecules.2 Concerning the CE mechanism, Otto et al. have extensively investigated and elucidated the atomically roughened active sites that provide the first layer SERS effect through static and dynamic charge transfer (CT) under the ultrahigh vacuum (UHV) condition.2 Although the CE mechanism has been investigated also under the ambient conditions,13,14 using electrode potential dependent SERS intensities or density functional theory, a coherent picture of the underlying process has not been fully evolved. Accordingly, accumulation of relevant empirical information on EM and CE mechanisms is essential in establishing single molecule (SM) SERS spectroscopy that requires 1010 or larger enhancement for versatile applications such as biomedical and environmental analysis.5 Indeed, we are not assured that a trace quantity of adsorbed dyes is preferentially located at the nanogaps between Ag nanoparticles. The use of optical trapping was suggested by r 2011 undefined

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theoretical calculations to collect dye molecules at the nanogap by markedly enhanced electric field gradient under the LSP resonance.11 Optical trapping is, however, not always effective in collecting dye molecules there due to insufficient electric field under single molecule experiments as described in Section 3.2.2.11,15 Thus, although it is heuristically pertinent that dye molecules with enormous SERS intensities are located at the nanogap between adjacent Ag particles, further convincing evidence is essential for deeply understanding the crucial role of the nanogaps in collecting dye molecules. Concerning the location of adsorbates at the nanogap, we have recently found that closely adjacent AgNPs are formed in an aqueous solution by the addition of a trace quantity of cationic xanthene dyes at a concentration of 10-7-10-8 M.15 Note that the concentration of the cationic dye is much lower than that necessary for cations like Naþ (50 mM or more) to cause coagulation of AgNPs as described in Section 3.1.1. AgNPs incubated in a 10 mM NaCl solution are negatively charged and thereby will be flocculated, but not be coagulated, by the addition of cationic dye molecules like R6Gþ through attractive electrostatic forces. Indeed, we reported that injection of rhodamine B cation (RBþ) generates the coupled LSP peaks and enormous SERS enhancement only in acidic conditions.15 In a neutral pH solution, Ag flocculation does not occur owing to repulsion between negatively charged AgNPs and carboxylate anions of RBþ. In the present paper, we disclose convincing evidence for the crucial role of the electrostatic interaction in Ag flocculation Received: October 2, 2010 Revised: February 19, 2011 Published: March 11, 2011 dx.doi.org/10.1021/jp110146y | J. Phys. Chem. C 2011, 115, 5271–5279

The Journal of Physical Chemistry C

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(EV), respectively, in Supporting Information, all of which were purchased from Wako Pure Chemical Ltd. (Osaka). These dyes were used without further purification. Extinction spectra of AgNPs in an aqueous solution with and without NaCl (10 mM), NaOH (10 mM), and each dye (10-7-10-8 M) were measured using a JASCO V-530 UV-vis spectrometer. SERS and fluorescence spectra were observed for the dyes adsorbed on AgNPs with NaCl (10 mM) in a thin layer solution cuvette composed of a coverslip, an ∼1 mm thick rubber spacer, and a Si plate. Renishaw micro Raman system with a CCD, an Arþ (NEC, GLG 550) and He-Ne laser (632.8 nm, NEOARK, NEO-30MS), and a modified Olympus microscope (normal, dark-field type BXFM, equipped with a 50 objective) were used for these spectral measurements. A decrease in absorbance ΔA/A, here ΔA and A denote decreased and original absorbance of the solution, respectively, at 538 nm for cationic p-RAþ and at 242 nm for neutral p-RA molecules after centrifugation of all the AgNPs in solutions was used to estimate adsorbed quantity of p-RA. For this purpose, a centrifuge (ASONE-Pasolina) was used at 900010 000 rpm for 15 min to coagulate entire AgNPs in sample solutions with cationic or neutral R123 molecules at concentrations between 1 10-7 and 6 10-6 M.

3. RESULTS AND DISCUSSION 3.1. Changes in Extinction Spectra of AgNPs by the Addition of TPM Dyes. 3.1.1. Cationic TPMþ Dyes. As-prepared

Figure 1. Molecular structure of (a) MGþ, (b) p-RAþ and p-RA neutral molecule, and (c) CVþ.

using neutral TPM and xanthene dye molecules such as p-rosaniline (p-RA) and rhodamine123 (R123). Extinction and SERS spectra for the neutral molecules are compared with those for the cationic forms to get insight into the formation mechanism of Ag flocculates. Indeed, TPMþ dyes as well as cationic xanthene dyes gave coupled LSP peaks emerged at 600-800 nm and prominent red shifts of fluorescence peaks. The latter observation suggests a perturbed electronic state of the dyes relating to the CE mechanism. Because the dyes adsorb on Ag surfaces through amino groups, molecular structures of the cationic dyes likely affect their fluorescence spectra. Thus, we investigated the effect of size and the number of amino groups -NH2, -N(CH3)2, and -N(C2H5)2 in TPMþ dyes on the Ag flocculation and the fluorescence spectra.

2. EXPERIMENTAL SECTION Experimental details are the same as those previously reported.15 Briefly, AgNPs were prepared by the Lee Meisel method.16 A trisodium citrate (1%, 10 mL) aqueous solution was added into a boiling aqueous AgNO3 (90 mg, 200 mL) solution from which oxygen was purged by nitrogen gas prior to the reaction and was refluxed for 60 min to generate AgNPs. This AgNP solution was diluted with water by a factor of 10 and used in the sample preparation. Figure 1a-c depicts the TPM dyes we used, p-rosaniline, malachite green (MG), and crystal violet (CV); see also Figures S1a and S1b for brilliant green (BG) and ethyl violet

AgNPs show a single prominent peak at ∼400 nm for the particle size 35-40 nm in diameter according to their isolated state.15,17,18 These nanoparticles become unstable with increasing ionic strength in the solution as anticipated by DLVO (Derjaguin-Landau-Verwey-Overbeek) theory.19 This change is observed by the addition of NaCl to isolated AgNPs in water, which is prerequisite for removals of obstructing surface residuals such as citrate, a-carbon or Ag2O layer. At the concentration of NaCl over 5 mM, these residuals on Ag surfaces are substituted with Cl- anions by forming AgCl or partially AgCl2(Cl--treated AgNP), thus leaving AgNPs isolated due to repulsive forces between counter Naþ cations in the diffusion layer. However at the concentration of NaCl over 50 mM, AgNPs themselves are coagulated, mostly precipitated to form mutually merged dendritelike structures as evidenced by featureless LSP peaks and by scanning electron microscopy (SEM) measurements.15 In contrast to the featureless LSP peaks for coagulated AgNPs at high NaCl concentration,15 injection of a trace quantity of cationic TPM dyes of MGþ or CVþ (10-8-10-7 M) into a solution of AgNPs and NaCl (10 mM) generated new extinction peaks at 500-800 nm in addition to the invariant 400 nm peak as shown in Figure 2a-c (see also Supporting Information Figures S2a and S2b for BGþ and EVþ, respectively). The new extinction peaks originate from the coupled LSP of AgNPs, indicating the formation of flocculates of AgNPs.15,17,18 These Ag flocculates, suspended as small aggregates, are assembled and stabilized by the electrostatic forces between cationic dyes and negatively charged AgNPs; we provide convincing evidence for the crucial role of the electrostatic interaction in the formation of Ag flocculates favored by cationic dyes in the next section (Section 3.1.2). Here we definitely distinguish flocculates from coagulates. Since flocculates were exclusively formed by the addition of a trace quantity of cationic dyes at 10-7-10-8 M to isolated Cl-treated AgNPs, the formation of them is not due to neutralization of their surface charges nor due to a decrease in thickness of a 5272

dx.doi.org/10.1021/jp110146y |J. Phys. Chem. C 2011, 115, 5271–5279


Figure 2. LSP extinction spectra of a mixed solution (a) of AgNPs, NaCl (10 mM) and MGþ (1 10-8 to 2 10-7 M), and (b) of AgNPs, NaCl (10 mM) and CVþ (1 10-8 to 3 10-6 M). (c) Temporal change in the extinction spectra in panel a. Absorption spectra of MGþ and CVþ in aqueous solution are depicted at the bottom of a and b, respectively.

double layer on AgNPs, both of which are predicted for coagulation at electrolyte concentrations much higher than 10-7-10-8 M in DLVO theory. Indeed, AgNP coagulates, whose size is much larger than flocculates, are exclusively formed at concentrations higher than 20-30 mM of NaCl (Figure 1 in ref 9). Concerning the flocculates of AgNPs, an outstanding example was recently reported by Dadosh et al.,12 which is a Cl--treated AgNP dimer combined with a R123þ cation formed under the precisely controlled conditions of constituents, temperature, and centrifugation. Although the dye molecules we used show absorption maxima in the visible region (at 540-620 nm), they are unambiguously distinguished from the LSP maxima as shown in Figures 2a-c for MGþ and CVþ. A weak shoulderlike LSP peak grew to distinct peaks with increasing concentration of the cationic dyes from 10-8 to 10-6 M, as exemplified in Figure 2a. Obviously no precipitation was recognized even at high concentrations of the dyes up to ∼10-4 M, which is limited by their solubility, although a trace quantity of gray-to-black suspended materials appeared. Chemical stability of closely adjacent AgNPs is assured by the concentration of cationic dyes on AgNPs (∼400 molecules/particle), which is much lower than that of Cl- ions on AgNPs (3 104 ions/particle15) and of Naþ ions in the diffusion layer. This condition was satisfied by the markedly different concentrations between Cl- (10 mM) and the dyes (10-7-10-8 M). Finally, the solutions containing AgNPs, NaCl, and various cationic dyes commonly gave pronounced coupled LSP peaks at 500 nm and 700-800 nm in addition to the intrinsic LSP peak at 400 nm as shown in Figure 2a and Supporting Information Figure S2a,b. We found that the positions of coupled LSP peaks for TPMþ dyes are the same as those for xanthene dyes,15 although they are very sensitive to the nanogap size of AgNPs and the number of

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Figure 3. Theoretical LSP resonance and local electric field distribution. (a) Scattering cross section of closely adjacent AgNPs in an aqueous solution; (b) local electric field at the gap size 1 nm and (c) at 2 nm, both at 470 nm excitation. The symbols in panel a denote the number of adjacent AgNPs and the gap size (G), for example, “2AgG1 nm” means two adjacent AgNPs at the gap size 1 nm.

adjacent particles. Figure 3a shows that scattering cross section calculated by a FDTD (finite difference time domain)20 method provided a coupled LSP peak at 490 nm for the gap size d = 2 nm and 570 nm for d = 1 nm in addition to intrinsic LSP peaks around 400-420 nm for two closely adjacent AgNPs with a diameter of 30 nm in water. Only a 1 nm difference in the gap size provides 100 nm of LSP peak shift as observed.10,15 These peaks red shifted with increasing the number of aligned particles or decreasing gap sizes, for example, three and four aligned particles give the coupled LSP peaks at 600 nm (510 nm) and at 630 nm (540 nm) for d = 1 nm (d = 2 nm). The intensity of electric field was enormously enhanced at the nanogap under coupled LSP resonance for the aligned particles compared with LSP resonance for isolated particles, that is, 400 nm for the present AgNPs. The coupled LSP peaks were experimentally observed at 500550 nm and ∼750 nm independent of dye molecules. These results suggest that the gap size and the number of nanoparticles in the Ag flocculates for cationic TPMþ dyes are mostly equivalent to those for cationic xanthene dyes. For instance, experimental peaks at 520 and 720 nm for coupled LSP resonance in Figure 2a correspond to 3-4 particles with the gap size d = 2 nm and 7-8 particles with d = 1 nm, respectively. Furthermore, Figure 3b,c shows that the gap size 1-2 nm was sufficiently small to provide huge enhancement in the electric field by a factor of 105 at 470 nm excitation close to the experimental excitation at 488 nm.26 Note that the actual gap size should be determined by TEM observations for the flocculates immobilized on substrates, while retaining their adjacent state in solution. Yet, preliminary evaluation for the gap size (12 nm) and the number of Ag nanoparticles (3-8 particles) in 5273



Figure 4. (a) Absorption spectra of cationic p-RAþ (black trace, 1 10-5 M) and neutral p-RA molecules (red trace, 1 10-5 M and 10 mM NaOH) in aqueous solutions. (b) LSP extinction spectra of the solution containing AgNPs, NaCl (10 mM) and p-RAþ (1 10-6 M) with/ without NaOH (10 mM, see the text in detail). Absorption peaks at 470 and 538 nm in panel b are from p-RAþ identical to those at 494 and 538 nm in panel a.

each flocculate allows us to attribute safely enormous SERS intensity to the coupled LSP. 3.1.2. Neutral TPM and Xanthene Dyes. We examined the effect of positive charge on the Ag flocculation using p-RAþ with amino groups of -NH2 and dNH2þ, whose positive charge is readily eliminated in a solution with pH above pKa. The pKa of dNH2þ in p-RAþ has not experimentally been determined but is likely to be smaller than 9.4, because it must be lower than that for dN(CH3)2þ in CVþ (pKa = 9.421) and dN(C2H5)2þ in EVþ cations. Indeed, p-RAþ was neutralized in a 10 mM NaOH solution (pH = 12). Figure 4a shows that the neutralization was verified through a change in color of the dye solution from rose pink to colorless transparent. In such high pH solutions, a hydroxide anion attaches to the central carbon atom of p-RAþ cations, where a cation is located in one of resonance structures. Accordingly, the central carbon of p-RA in the neutral form takes no more planar sp2 but tetrahedral sp3 structure (Figure 1b). The neutralization markedly reduces the length of conjugated double bonds from three phenyl rings linked by the central carbon to the single isolated phenyl ring. Correspondingly, neutral p-RA molecule showed absorption peaks only in the UV region (Figure 4a). Note that Ag flocculates were not formed by the injection of neutral p-RA molecules (1 10-6 M at the highest) to a mixed solution of AgNPs, NaCl (10 mM), and NaOH (10 mM). This observation is in striking contrast to the result of coupled LSP peaks at 500 and 700-800 nm for a mixed solution of AgNPs, NaCl (1-10 mM), and p-RAþ (1 10-6 M, Figure 4b). Here we estimated the number of cationic and neutral dyes adsorbed on AgNPs using a centrifuge. For cationic state, the decrease in absorbance ΔA/A at 538 nm, where ΔA and A denote decreased and the initial absorbance of the solution, was 0.035/0.0070 at a concentration of 1 10-7 M. This ratio corresponds to ∼400 molecules of p-RAþ cations adsorbed on each AgNP. Assuming homogeneous distribution of adsorbed p-RAþ cations on AgNPs, only a few molecules are located at the nanogap region.15 In contrast, only a negligible difference in ΔA/A (50 mM) in coagulates, (d) closely adjacent AgNPs bridged by the addition of a trace quantity of cationic dyes, and (e) expanded image of panel d showing presumable orientation of chemisorbed dyes on AgNPs. Adsorbed R123 molecules are homogeneously located on the entire AuNP surfaces. The molecular plane of the dyes at the nanogap should be tilted by ∼45° to the coordinate that pierces neighboring two Ag NPs.

3.2. SERS and Emission Spectra of Cationic Dyes Located in the Nanogap between Closely Adjacent AgNPs in Aqueous Solutions. 3.2.1. SERS Spectra. Because cationic dyes is

located in the nanogap between Ag particles while forming the flocculates as described in Section 3.1.2, pronounced SERS and modified fluorescence spectra are expected by the coupled LSP resonance and additionally by electronic interaction with Ag surfaces. FDTD simulations anticipated prominent enhancement for the electric field intensity (|EL|2/|E0|2, where |EL|, |E0| denote electric field amplitude at the nanogap and incident light), respectively. The expected enhancement factor was ∼105 at the nanogap (d) between closely adjacent AgNPs (r = 16 nm and d = 1-2 nm), when excited at 470 nm with the polarization parallel to the connecting axis. Thus, the SERS enhancement of the dyes at the nanogap was estimated to be 1010 by exploiting the LSP in a scattering channel as well as in an incident channel. Indeed, SERS spectra were dramatically enhanced together with the appearance of the coupled LSP peaks under 10 mM NaCl and 10-7-10-8 M MGþ and CVþ cations (see Figures 2, 6, and 7). The SERS peaks were observed at 1625, 1596, 1492, 1400, 1370, 1298, 1225, 1182, 922, 805, and 537 cm-1 for MGþ (Figure 6a) and 1650, 1605, 1554, 1398, 1380, 1310, 1187, 991, 959, 924, 812, 733, 619, 573, 528, 462, and 434 cm-1 for CVþ (Figure 7a). These bands were partly assigned to νCringN/δs(CH3) (1548), δ(CCC)ring/νas(CCcenterC)/δ(CH) (1383, 1307), νas(CCcenterC) (1192), ν (CN) (733), and δ(CNC) (528, 434) based on the DFT calculations for CVþ in literature.23 These SERS spectra

correspond well with the Raman spectra of these dyes in a compressed pellet at 1624, 1600, 1494, 1402, 1375, 1298, 1227, 1184, 1000, 932, 807, 739, 530, and 426 cm-1 (MGþ, Supporting Information Figure S5); 1627, 1597, 1551, 1380, 1307, 1187, 920, 812, 735, 565, 532, and 432 cm-1 (CVþ, Supporting Information Figure S5). Noticeable shifts were not observed even for C-N str (Δν = þ2 cm-1) and CNC bend modes (Δν e (4 cm-1) suggesting that adsorption of these dyes do not markedly perturb their molecular structures. Pronounced SERS spectra were observed also for BGþ (Supporting Information Figure S3a) and EVþ (Supporting Information Figure S4a). Consequently, various dyes with distinct amino groups of -NH2 (p-RAþ, Figure 8), -N(CH3)2 (CVþ, MGþ), and -N(C2H5)2 (EVþ, BGþ) exhibited enormous SERS enhancement, consistent with their coupled LSP peaks commonly emerged at 500 nm and 700-800 nm. By contrast, neutral p-RA molecule formed in a 10 mM NaOH solution did not show the coupled LPS peak. Accordingly, the neutral dye molecules did not bind negatively charged AgNPs together and thus isolated AgNPs were retained with a subtle quantity of weakly physisorbed R6G. Isolated AgNPs showed no detectable SERS signals for adsorbed dyes. Weak Raman peaks in the lower trace of Figure 8 are attributed to a-carbon that remains on Ag surfaces. Similar faint SERS spectra were observed for neutral rhodamine 123 molecules in an alkaline solution (10 mM NaOH, pH = 12) as opposed to those in neutral pH solutions (Supporting Information Figure S7). The crucial role of the 5275



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Figure 6. (a) SERS and (b) emission spectra of a mixed solution of Ag NPs, NaCl (10 mM) and MGþ cations (8 10-8-1 10-6 M). (c) Emission spectra of MGþ in aqueous solutions at various concentrations 10-4-10-6 M.

Figure 7. (a) SERS and (b) emission spectra of a mixed solution of Ag NPs, NaCl (10 mM) and CVþ cations (1 10-8-1 10-6 M). (c) Emission spectra of CVþ in aqueous solutions at various concentrations 10-4-10-6 M.

positive charge in the adsorbed dyes was again validated in SERS measurements for the formation of closely adjacent AgNPs, whose surface was negatively charge by precedent halide addition.

3.2.2. Emission Spectra. Fluorescence spectra of various TPMþ dyes adsorbed in the Ag flocculates were observed together with enormous SERS enhancement. Furthermore, fluorescence 5276



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spectra of these dyes showed distinct peak shifts (Figures 6b and 7b and Supporting Information Figures S3b and S4b) depending on their molecular structures. Table 1 summarizes that MGþ and BGþ cations with two amino groups exhibited prominent red shift by 50-60 nm, that is, 670 (free in solution) to 720 nm (on Ag) for MGþ and 670 (free in solution) to 730 nm (on Ag) for BGþ cations. These large red shifts are similar to those observed for cationic xanthene dyes in the Ag flocculates.15 Compared with the observation for MGþ and BGþ cations, much smaller red shift was observed for p-RAþ (þ30 nm), CVþ (-30 nm), and EVþ cations (1 nm) of adjacent two AgNPs (r = 40 nm).25 The predicted shift of 10 nm is only modest compared with our observation of ∼50 nm for MGþ and 70 nm for R6Gþ, which is raised by predominant enhancement at a longer wavelength part in the board fluorescence band. However, much larger shift can be obtained under other LSP resonance conditions such as at gap sizes smaller than 0.5 nm or at particle sizes larger than r = 20 nm. Indeed, the effect of LSP resonance peak on the apparent fluorescence shift was experimentally ensured for different dye molecules using gold nanostructures fabricated by electron beam lithography, and gold nanoparticles linked by bulky biomolecules.28 For instance, methylene blue, crystal violet, and cyanine fluorescent (Cy3) dyes dramatically change their fluorescence peak profile, which is reproduced by convolution of a LSP resonance of the metal nanostructures and fluorescence spectra of the adsorbed dyes.28 In addition to a contribution of the coupled LSP resonance to the observed fluorescence shift, we argue for the last possibility of the perturbation of Ag surfaces to an electronic state of dyes upon adsorption.29-31 The electronic perturbation is affected by detailed adsorption nature, for example, position and population of metal bands and molecular orbitals responsible for the adsorption. Such interaction often results in electron transfer through the interface with modified molecular structure and orientation.32 The distinct spectral shifts that depended on the number of amino groups in Table 1 are likely due to different orientations of phenyl rings in TPMþ dyes. For instance, three aniline rings in p-RAþ, CVþ and EVþ dyes are twisted to form a propeller-like structure with an inclined angle of ∼30° to eliminate specific repulsion of their ortho-hydrogen in a solid phase and in solution.33 In contrast, two aniline rings in MGþ and BGþ dyes are coplanar similar to those in xanthene dyes with less steric hindrance to the adsorption. The coplanar structure likely provides larger electronic interaction between the amino groups

Figure 8. SERS spectra of a mixed solution of AgNPs, NaCl (10 mM) and p-RAþ (1 10-6 M) with NaOH (lower trace, 10 mM; neutral p-RA molecules, p-RA0) and without NaOH (upper trace, cationic p-RAþ forms).

Table 1. Formation of Ag Flocculates, SERS Activation versus Molecular Structuresa molecules TPM

xanthene

amino groups

coupled LSP peaks

SERS activity

fluorescence shift (nm)

MGþ

2 dNþ(CH3)2

O

O

50

BGþ

2 dNþ(C2H5)2

O

O

55-60

p-RAþ

dNþH2, 2 -NH2

O

O

30

p-RA neutralb

dNH, -NH2 (neutral)

CVþ

3 dNþ(CH3)2

O

O

-30

EVþ R6Gþ

3 dNþ(C2H5)2 2 dNþ(H)C2H5

O O

O O

70

R123þ

dNþH2, -NH2

O

O

90

R123 neutralb

dNH, -NH2 (neutral)

RBþ zwitter ionc

2 dNþ(H)C2H5, -COO-

RBþd

2 dNþ(H)C2H5, -COOH (cationic)

O

O

50

a

See details in the text. The symbols of open circle and cross bar indicate coupled LSP peak and enormous SERS are observed, not observed, respectively. b Alkaline condition to deprotonate amino groups in dye. c In neutral solution, where carboxylic groups are deprotonated. d Acidic condition to hold protons at carboxylic groups in dye. 5277


The Journal of Physical Chemistry C and Ag surfaces due to increased overlap of their wave function30 compared to that in the propeller structure. Our presumption of the electronic interaction has also been evidenced by other experimental observations. First, femtosecond dynamics was measured for the excited electron on metal and adsorbates system. Shibamoto et al. reported that adsorption of CVþ on SERS active Au films opens a new relaxation channel within 200 fs for a photoexcited electron via CT from Au to CVþ.37 Second, the excitation profile of SERS intensity depends on electrode potentials or the potential dependence of SERS intensity is inherently related to an excitation wavelength, both of which are a well established method for identifying the CT effect.2,34 Lombardi et al. recently estimated CT contribution to be 0.50.6 in total SERS enhancement of CVþ at 514.5 nm using the experimental data on the Ag electrode by Pettinger35 and the unified theoretical approach.36 Detailed potential dependence of SERS for MGþ and CVþ adsorbed on closely adjacent AgNPs should be investigated to elucidate adsorbed state and electronic interaction between them. In short, the observed fluorescence shift depending on molecular structures suggests the contribution of their distinct electronic interaction with Ag surfaces.

4. CONCLUSION We elucidated the crucial role of the electrostatic interaction between cationic TPMþ and xanthene dyes and AgNPs to form closely adjacent nanoparticles. The dyes combining AgNPs were accordingly located in the nanogap, which yielded enormous SERS enhancement and significant fluorescence shift. The red shift was attributed to the coupled LSP resonance and also presumably to the electronic interaction between adsorbed dyes and AgNPs. Bulky nature of amino groups in the dyes did not affect the Ag flocculation due to the dominant role of the electrostatic interaction. On the other hand, the number of amino groups gave distinct fluorescence red shift, suggesting the propeller structure with three aniline rings has smaller electronic interaction with the Ag surfaces. ’ ASSOCIATED CONTENT

bS

Supporting Information. Section S.1 gives additional experimental results that BGþ and EVþ with slightly different molecular structures at their amino groups caused the flocculation of AgNPs, which support the arguments on the predominant role of the electrostatic interaction between negatively charged AgNPs and cationic dyes. Indeed as described in Section S.2, the addition of neutral R-123 molecules at a concentration of 1 10-6 M to the mixed solution of AgNP and NaCl (10 mM) did not provide the coupled LSP peak, which are identical to those for neutral p-RA molecules in the main text. These data indicate the neutral xanthene dye as well as TPM dye do not adsorb on Ag surfaces or form the AgNP flocculates due to faint interaction. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported by grant-in-aid for Scientific Research (B) 21310071 by Japan Society for the Promotion of

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Science (JSPS), and for Priority-Area-Research “Strong photonsmolecules coupling fields (470)” from the Ministry of Educations, Science, Sports and Culture, Japan (No. 21020008). We are also indebted to Iketani Science and Technology Foundation (Grant 0211069, 2009) and Salt Science Foundation (Grant 1014, 2010). The authors thank Dr. Mitsuru Ishikawa (National Institute for Advanced Industrial Science and Technology, Takamatsu, Japan) for useful discussion.

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assuming that an area around a nanogap is 6 nm in diameter on the Ag surfaces. Thus, the Coulomb potential is estimated to be V = -4.5 10-20 J for ndye = 3 and nAgNP = 10, which is large enough to immobilize the dyes and to stabilize closely adjacent AgNPs. Since the surface coverage of Cl- on AgNPs is at monolayer level, average surface charge must be sufficiently large to overcome thermal diffusion and to trap cationic molecules at the nanogap. Obviously, eq 1 is only intuitive, but valid for a rough estimation of electrostatic interaction between cationic dyes and AgNPs. Detailed theoretical calculation is awaited for more robust evaluation that involves microscopic distributions of Cl-, Naþ and water molecules. V ¼ -

q

0

q

iðdyeÞ jðAgNPÞ ∑i, j 4πε ε r 0 H2 O ij

¼ - ndye

1 nAgNP ð1:602 10-19 Þ2 2 4πð8:8 10-12 Þð78Þð1 10-19 Þ

¼ - 1:49 10-21 ndye nAgNP

ðJÞ

ð1Þ

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Elucidation of Electrostatic Interaction between Cationic Dyes and Ag

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