Closely Adjacent Ag Nanoparticles Formed by Cationic Dyes in

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Closely Adjacent Ag Nanoparticles Formed by Cationic Dyes in Solution Generating Enormous SERS Enhancement† Masayuki Futamata,* Yei-Yei Yu, Tomomi Yanatori, and Takeshi Kokubun Department of Chemistry, Faculty of Science, Saitama UniVersity, Saitama, 338-8570 Japan ReceiVed: December 1, 2009; ReVised Manuscript ReceiVed: February 11, 2010

Explicit evidence is reported to prove that huge surface enhanced Raman scattering (SERS) signal from cationic dye results from its adsorption at the nanogap of neighboring Ag particles. The addition of extremely small amount of cationic dye such as 10-8 M R6G+ provokes the formation of closely adjacent Ag nanoparticles in aqueous solutions exploiting electrostatic forces, of which surface residue was substituted by Cl- anions (10 mM) in advance to have negative charges. This is ensured by the appearance of a coupled localized surface plasmon (LSP) peak at 600-700 nm in addition to that for isolated particles at about 400 nm. The adjacent state of the Ag nanoparticles, which does not render coagulated precipitates but flocculates, suspended particles, is quite stable for more than 24 h as confirmed by invariable extinction spectra. Simultaneously, the linked Ag nanoparticles confer enormous SERS signal and prominent red-shifted fluorescence of the dye molecules. Consequently, markedly enhanced electric field under the coupled plasmon resonance leads to huge SERS signal for the dye sitting at the nanogap as well as their specific interaction with the Ag surfaces. In addition, the isolated state is recovered by injection of the SERS quenching anions such as S2O32- according to their exclusive adsorption to the Ag surfaces. Primary role of electrostatic interaction was also convinced by the fact that the identical flocculation and SERS activation for rhodamine B+ cation were observed only at low pH, i.e., pH 3, by inhibiting dissociation of its carboxylic group. Introduction In the early history of SERS studies, rather modest enhancement by 104-106 was reported from population averaged measurements for various sizes or shaped metal nanostructures.1–3 Recent developments in electron beam and ion beam technologies as well as sophisticated chemical synthesis enable us to fabricate optimum nanostructures with their arrays generating high SERS activity.4,5 Consequently, enormous enhancement of the incident and scattered electric field is explored toward single molecule detection,6,7 nanometer spatial resolution using near-field Raman spectroscopy,8,9 and quantitative analysis of bio- and medical systems.10–12 In these investigations, primary interest is focused on the electromagnetic (EM) enhancement using localized surface plasmon (LSP) resonances,1,3,13 because it produces largely enhanced electric field available for various molecules, independent of their structures and electronic states. From extensive studies of SERS active particles in various morphologies and aggregation state, we have a common understanding of the EM mechanism that molecules located in the nanogap between adjacent metal nanoparticles give enormous SERS intensity through the coupled LSP field.4,6,7,13–19 In addition, noticeable enhancement is expected for specific species in the chemical enhancement (CE), because it is based on specific electronic resonance between metal and molecules.2 In terms of the CE mechanism involving halide ions, we have reported that16,17 (1) residual species such as a-carbons and oxides adsorbed on the Ag nanoparticles are entirely replaced by Cl-, Br- or SCN- ions to render the surfaces of Ag particles negatively charged, and (2) cationic dyes chemisorbed on the Ag particles, most probably located at the nanogap by electro†

Part of the “Martin Moskovits Festschrift”. * To whom correspondence should be addressed. E-mail: futamata@ chem.saitama-u.ac.jp.

static forces, provide enormous SERS enhancement and prominent red-shift of the fluorescence of the R6G+ cation by >70 nm. The observation of the red-shift provides convincing evidence for electronic interaction between the dye and Ag surfaces. Consequently, these halide ions contribute adsorption of dye molecules for the CE and EM enhancement. In addition, (3) a drastic blue shift of the coupled LSP peak was observed from 600-700 to 500-600 nm, which is provoked by the halide addition by slightly dissolving Ag surfaces. (Apparently the opposite red shift of the LSP peak is reported here, which is caused by the formation of the closely adjacent Ag nanoparticles that yields enormous SERS signal from the adsorbed dye in the nanogap as explained in the following sections.) Thus, the SERS activation by halide and cationic dye molecules are explained by adsorption of the dye to the nanogap of neighboring Ag particles to utilize the coupled LSP most efficiently. However, because only several dye molecules per particle are incubated in the above experiments, they are not assured to preferentially sit on the nanogap. Thus, only less than 1% of the immobilized particles exhibit the huge SERS enhancement.13–16 With respect to this point, preferred adsorption or optical trapping of the molecules at the nanogap was suggested by extremely large electric field gradient under LSP resonance. The R6G+ cations adsorbed on Ag nanoparticles are trapped by the potential barrier of ∼0.5 kBT at sufficiently high laser power density such as 1 mW/µm2.18 Much weaker power density is actually used in single molecule experiments, e.g., 1 µW/µm2 in our experiments, resulting in 3 × 10-3 kBT which allows adsorbed molecules to freely diffuse on the Ag surfaces. The optical trapping is not always effective in collecting dye molecules at the nanogap in the single molecule experiments. Thus, although it is heuristically pertinent that the dye molecules with enormous SERS enhancement are located at the nanogap

10.1021/jp9113877  2010 American Chemical Society Published on Web 02/25/2010

Closely Adjacent Ag Nanoparticles of the adjacent Ag particles, we have an issue of no convincing evidence to prove it. To remove this issue, several groups fabricated nanogap structures consisting of metal substrates-metal particles, or two metal particles, by using a chemical bond with aminothiol or dithiol molecules.19–22 For example, Au particle (with a radius of 6 nm) dimers, trimers, and tetramers were assembled by adjusting the ratio of dithiol molecules to nanoparticles.20 The dimer particles were then trapped at the nanogap electrode, and their electrical conductance nature such as Coulomb blockade was observed to confirm that the single dithiol molecule efficiently binds the two Au particles. This is a robust method for delivering a single molecule at the nanogap of the metal nanoparticles and the nanogap electrode; however, adsorbed species are currently restricted to dithiol or related species. Essentially the same approach was done to detect much larger biomolecules, whose ends were again modified by thiol or thiolate.19 Unfortunately, rather long chain length such as 5-6 nm of the biomolecules may suppress the field enhancement of the gap LSP mode, thereby requiring Raman labels by using dye molecules to detect SERS signal and identify each biomolecule.21 Another approach is to replace charged adsorbates with neutral species to control repulsive forces between metal colloid and thereby to form aggregates.22 In this paper, we report on an alternative method for binding of Ag nanoparticles by electrostatic forces. Namely, Ag nanoparticles incubated in NaCl (10 mM) solution are negatively charged and thus can be flocculated using cationic dye molecules like R6G+ through the electrostatic attractive force. This approach will provide convincing evidence that the cationic dye molecules are indeed located at the nanogap of the adjacent Ag nanoparticles immobilized on the substrate to evoke enormous SERS activity and specific electronic interactions with Ag surfaces. For this approach, we implemented the following 3-fold experiments. First, the LSP extinction spectra were measured before/after addition of the cationic dye to the Ag dispersed solution, in which 10 mM NaCl was added in advance to substitute the residual surface species.16,17 If it induces the formation of closely adjacent Ag particles, the coupled LSP peak should be observed at much longer wavelength than that for the isolated particles at around 400 nm as anticipated by the theoretical considerations.25,26 The results are compared with those for coagulated (precipitated) particles induced by concentrated NaCl solution. Second, SERS and emission were measured for the same samples in solution, which could be largely enhanced if dye molecules are sitting at the nanogap of the closely adjacent Ag particles under the coupled LSP resonances. Last, several xanthene dye molecules were investigated to obtain a deep insight into the mechanism of the formation of the closely adjacent Ag nanoparticles. Experimental Section Silver nanoparticles were prepared based on the Lee Meisel method.24 Briefly, trisodium citrate (100 mg) aqueous solution was added into boiling AgNO3 (90 mg) aqueous solution (200 mL), in which oxygen gas was purged by nitrogen gas prior to the reaction, and refluxed for 60 min. Xanthene dye molecules of rhodamine 6G (R6G), rhodamine 123 (R123), and rhodamine B (RB) were purchased from Wako Pure Chemical Ltd. (Osaka) and used without further purification. Extinction spectra of Ag nanoparticles (diluted by 10 times) in aqueous solution with and without NaCl and/or dye molecules were measured in a square quartz cell (10 mm × 10 mm) using a JASCO V-530 UV-vis spectrometer. SERS and fluorescence spectra were

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Figure 1. LSP extinction spectra of Ag nanoparticles (NP): (a) in Ag NP + NaCl solutions, (b) in Ag NP + R6G+ solutions, (c) in Ag NP + NaCl (10 mM) + R6G+ (4 × 10-8-8 × 10-8 M), and (d) release of closely adjacent state by SERS quenching I- (7.5 mM) or S2O32anions (4 mM). The absorption spectra of the R6G+ dye in 2 × 10-5 M aqueous solution was depicted with a dotted black line at the lower part of panel b.

obtained for dye molecules adsorbed on Ag nanoparticles with NaCl (10 mM) in a thin layer solution cell composed of a coverslip, a ∼1 mm thick rubber spacer, and a Si plate. Renishaw micro Raman system equipped with a CCD detector, an Ar+ laser (NEC, GLG 550), and a modified Olympus microscope (normal type, equipped with a × 50 objective) were used for these spectral measurements. Scanning electron microscope, Hitachi S4100, was employed to characterize flocculated and coagulated Ag nanoparticles on Si substrates. Results and Discussion 1. Changes in Extinction Spectra of Ag Nanoparticles in Aqueous Solutions by the Addition of NaCl and Dye Molecules. The formation of closely adjacent Ag particles by NaCl and R6G+ dye was investigated using the extinction spectra of Ag particles. First of all, solely NaCl or R6G+ were added to Ag particles in solution to elucidate the effect of their adsorption or ionic strength. Ag nanoparticles (radius r ) 15-20 nm) in water conferred only one LSP peak at around 400 nm (Figure 1a), corresponding to isolated state of the particles.25,26 Our Ag particle surfaces are covered by residual citrate, its decomposed amorphous carbon, or naturally formed silver oxide layers as reported before.16,17 Subsequent addition of NaCl (>5 mM) into a solution of Ag particles thoroughly removed these residual species. This observation was evidenced by XPS or SERS measurements.16,17 The LSP peak intensity at 400 nm decreased with increasing NaCl concentration and finally coagulated at g50 mM as shown in Figure 1a. After that, modest and featureless extinction bands were observed in the entire visible wavelength (blue line). Obviously, the yellowish color of the Ag dispersed solution for isolated state varied to be almost colorless and transparent with gray precipitates on the cell bottom. The coagulation of negatively charged Ag nanoparticles results from increased ionic strength by NaCl addition. The

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Figure 2. SEM images of Ag nanoparticles on Si substrate: (a) after coagulation by NaCl addition (50 mM) and (b) after flocculation by NaCl (10 mM) + R6G+ (5 × 10-8 M).

counter cations, Na+ here, are distributed in the diffusion layer to be compatible with Poisson’s equations and thermal diffusion and retain the isolated Ag particles by electrostatic repulsion. Thickness of the diffusion layer decreases with increasing salt concentration as expressed by the Debye length 1/κ ) 0.304/ CNaCl (nm),27 where κ and CNaCl denote the Debye constant and molar concentration of NaCl, respectively. The thickness of the diffusion layer is roughly estimated to be 1.5 nm for 50 mM and 3.0 nm for 10 mM NaCl solution using this equation. As empirically indicated here, the thickness 1.5 nm is sufficiently short for van der Waals attractive forces to cause the coagulation, whereas 3.0 nm is still long to retain the isolated state. Hence, the critical concentration for the coagulation is between 10 mM and 50 mM for NaCl. (We will be back to this point.) Such coagulated (precipitated) Ag particles in concentrated NaCl solution were merged yielding large dendrite-like structures (Figure 2a) and thereby exhibiting a featureless extinction spectrum in the 400-900 nm region.28 Addition of R6G+ to the AgNP solutions without NaCl did not manifest the LSP spectral changes even at 10-4 M (Figure 1b). The invariant spectra provide evidence for the isolated state of Ag nanoaprticles. The retained isolated state is because R6G+ cations are not chemisorbed on the Ag surfaces but physisorbed outside the residual a-carbon or Ag2O. Marked bulk-like fluorescence peak of R6G+ from this sample was observed at 550 nm with modest Raman signal (data not shown) in contrast to the peak at ca. 620 nm for chemisorbed R6G+ supporting the physisorption17 (as explained in the next section). These observations suggest that addition of concentrated NaCl (g50 mM) or solely R6G+ to Ag nanoparticles are intrinsically different from that of diluted NaCl and R6G+ to them as discussed in the following section. Indeed, when a small amount of R6G+ (10-8-10-7 M) was injected into the solution containing Ag particles and NaCl (10 mM45), a new extinction peak emerged at 600-800 nm together with the invariant 400 nm peak from isolated particles (Figure 1c). It is well-known that the aggregated metal nanoparticles provide such longer wavelength peak than that for isolated particles.25,26 For instance, incompletely coagulated Ag particles in concentrated NaCl solution showed the broad peak at 600-800 nm (green line at 33 mM in Figure 1a). Although the dye molecules show absorption maxima at 526 (R6G, Figure 1b), 500 (R123, Figure 3a), and 553 nm (RB, Figure 3b), they are unambiguously distinguished from the LSP peaks; for example for R6G+ in Figure 1c. Accordingly the additional peak observed at 600-800 nm is originated from the coupling of the LSP of each Ag nanoparticle, namely from closely adjacent particles, (AgNP)---R6G+--(AgNP)-. These Ag flocculates, floating small aggregates, are assembled and stabilized by the electrostatic forces between the negatively charged Ag nanoparticles with Cl- substitution16,17 and the cationic dye. In

Figure 3. LSP extinction spectra of Ag nanoparticles (NP): (a) in Ag NP + NaCl (10 mM) + Rhodamine 123+ solutions (1 × 10-7-5 × 10-7 M), (b) in Ag NP + NaCl (10 mM) + Rhodamine B+ (RB+) solutions (3 × 10-7-1 × 10-6 M), and (c) in Ag NP + NaCl (10 mM) + H2SO4 (1 mM) + RB+ solutions (1 × 10-8-1 × 10-7 M). The absorption spectra of R123+ in 3 × 10-7 M solution and RB+ dye in 1 × 10-6 M aqueous solution were depicted with a dotted black line at the lower part of panels a (R123+) and b (RB+), respectively.

accordance with these observations, the neighboring Ag nanoparticles with a gap size of 1 nm renders the coupled LSP peak at 570 nm - 630 nm, depending on the number of aligned particles as anticipated by theoretical simulation (for example as Figure 11 in ref 17). The new coupled LSP peaks grew with the R6G+ concentration in 10-8-10-7 M range (Figure 1c). Even at higher R6G+ concentration, obviously no precipitation was recognized although faint amount of gray-to-black floating materials emerged. Finally, the solution gave the coupled LSP peak at 500 nm (as a shoulder, and more clearly observed at 535 nm for RB in Figure 3c) and significant peaks at 650-700 nm in addition to the moderate LSP peak at 400 nm. The identical LSP peak observed for different dye indicates that the Ag flocculates consist of a limited and similar number of particles. The LSP peaks at this region are preferable to gain large SERS activity with a conventional visible-wavelength laser. We noted in SEM images that the Ag particles from this solution showed chained bead structures while retaining their spherical shapes (see Figure 2b). Note that the SEM images do not provide exactly the number of particles in a single flocculate because drying process induces apparent coagulation. Still, distinct interparticle binding is fairly displayed. No sintering or precipitation occurred during the formation of adjacent particles by cationic dye in contrast to those coagulated (saltout) by the addition of NaCl (Figure 2a). The cationic dye molecules together with Cl- anions likely block the interparticle diffusion of Ag atoms as suggested by Otto29 because they are sandwiched between the naked Ag surfaces. Thus, the pretreatment by NaCl solution is prerequisite to eliminate surface residue and to have negative charges for the Ag surfaces, which is inherently sensitive to ionic strength and relative abundance of Cl- to Ag particles in solutions. For example, a much higher concentration of Ag particles, e.g., 10 times, facilitated coagula-

Closely Adjacent Ag Nanoparticles tion even at 70 nm) of the fluorescence band of the dye were observed. The primary role of electrostatic interaction was convincingly demonstrated by the identical flocculation and SERS activation for rhodamine B observed only at low pH, i.e. pH 3, to inhibit dissociation of its carboxylic group. It was also found that SERS quenching anions like S2O32- release the flocculates to isolated particles by their exclusive adsorption to Ag surfaces. These results are inherently the same as those observed for Ag nanoparticles immobilized on the Si substrate. Thus, the location of small amount of the cationic dye at the nanogap was empirically manifested to yield the enormous SERS intensity and significant electronic state. Acknowledgment. This research was financially supported by grant-in-aid for Scientific Research (B) 21310071 by Japan Society for the Promotion of Science (JSPS), and for PriorityArea-Research “Strong photons-molecules coupling fields (470)” from the Ministry of Educations, Science, Sports and Culture, Japan (No. 21020008), and by Iketani Science and Technology Foundation (Grant No. 0211069, 2009). One (M.F.) of the authors thanks Prof. Andreas Otto (Du¨sseldorf) and Mr. Y. Maruyama (Hamamatsu Photonics K. K.) for their collaboration, in particular useful discussions to inspire the presented subject. M.F. also thanks Prof. Mitsuru Ishikawa (AIST) for valuable discussions. References and Notes (1) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783–826. (2) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143–1212. (3) Kerker, K. Surface Enhanced Raman Scattering; SPIE, Bellingham, 1990; vol. MS10. (4) Kneipp, K.; Moskovits, M.; Kneipp, H. Surface-Enhanced Raman Scattering; Top. Appl. Phys.; Springer: New York, 2006; vol. 103. (5) Lal, S.; Grady, N. K.; Kundu, J.; Levin, C. S.; Lassiter, J. B.; Halas, N. J. Chem. Soc. ReV. 2008, 37, 898–911. (6) Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. J. Am. Chem. Soc. 1999, 121, 9208–9214. (7) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. ReV. 1999, 99, 2957–2975. (8) Kawata, S., Shalaev, V. M., Eds.; Tip Enhancement; Elsevier: Amsterdam, 2007. (9) Novotny, L.; Hecht, B. Principles of Nano-Optics; Cambridge Press: Cambridge, 2006. (10) Graham, D. Chem. Soc. ReV. 2008, 37, 883–884. (11) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442–453. (12) Lal, S.; Clare, S. E.; Halas, N. J. Acc. Chem. Res. 2008, 41, 1842– 1851. (13) Le Ru, E. C.; Etchegoin, P. G. Principles of Surface-Enhanced Raman Spectroscopy; Elsevier: Amsterdam, 2009. (14) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964–9972. (15) Xu, H.; Aizpurua, J.; Ka¨ll, M.; Apell, P. Phys. ReV. E 2000, 62, 4318–4324. (16) Futamata, M.; Maruyama, Y. Anal. Bioanal. 2007, 388, 89–102. (17) Futamata, M.; Maruyama, Y. Appl. Phys. B: Laser Opt. 2008, 93, 117–130. (18) Xu, H.; Ka¨ll, M. Phys. ReV. Lett. 2002, 89, 246802.

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