Synthesis of Silver Nanoparticles with Controllable Surface Charge and Their Application to Surface-Enhanced Raman Scattering Ramon A. Alvarez-Puebla and Ricardo F. Aroca* Materials and Surface Science Group, Faculty of Sciences, University of Windsor, Windsor, ON, Canada, N9B 3P4 The objective of the present work was to explore new methods of synthesis of silver nanocolloids using amino acids as reducing agents. The goal of the study was to fabricate nanostructures with controllable surface charge (zeta potential) that may allow optimizing the adsorption of target molecules for ultrasensitive analysis using surface- enhanced Raman scattering (SERS). The average SERS properties of these colloids are tested with two organic analytes and compared with those obtained by using the most commonly used citrate Ag sols. Surface-enhanced Raman scattering (SERS) is a powerful analytical technique for ultrasensitive chemical or biochemical analysis. 1 Since the first reported SERS on silver and gold colloidal solutions in 1979, 2 metal colloids have become one of the most commonly used nanostructures for SERS. The localized surface plasmon resonances (LSPR) 3 in colloidal metal particles have served as a testing ground for the most thorough of theoretical modeling and, with the achievement of the single molecule detection (SMD), 4-6 they have become central to single molecule Raman spectroscopy. For analytical applications, it is important to distinguish two types of SERS signals when using colloidal nanoparticles. 7 First, the “average SERS” enhanced spectra, 1 i.e., the SERS spectrum of a given analyte obtained from an ensemble of colloidal particles and aggregates and characterized by a stable intensity pattern, with well defined and reproducible frequencies and bandwidths. Second, SERS intensities obtained from silver or gold nanostructures sustaining a “hot spot” (large enhancement factors), which permits the detection of a few molecules with fluctuating spectral charateristics. 8,9 Average SERS is nowadays a quantitative analytical tool, 10 and today the work is turning to the specifics of tuning the experimental conditions for a given analyte. For example, enhancement factors (EF) reported for organic acids and alcohols are not competitive with those achieved with thiols and amines. 11 Of course, for a proper comparison, it is the first order of business to examine the corresponding scattering cross sections of the analytes. However, the adsorption properties of the analyte remain as a central issue, and the adsorption is partly controlled by the surface charge of the colloidal metal particles. Recently, it was demonstrated that with variation of the pH, one can gain control of the surface charge of the colloidal suspensions. 12,13 Thereby, it is possible to maximize the adsorption. Silver and gold colloids used in SERS have surface charge (i.e., zeta potential, ) varying between -60 mV at pH ) 10 and 5 mV at pH ) 2. 12,13 In the present work, new methods of synthesis of silver nanocolloids are carried out using amino acids as reducing agents. The colloidal nanostructures show optimal optical properties for SERS and a controllable  ranging from - 60 to 30 mV. Amino acids are also retained at the surface, stabilizing the colloids as predicted by the double diffuse layer theory. 14 SERS properties of these colloids are tested with two different analytes at different conditions (average and hot-spot SERS), and results are compared with those obtained by using the most commonly used citrate Ag sols. EXPERIMENTAL METHODS All the chemicals were purchased from Aldrich and used without further purification. Reducing agent solutions of 1% (w/v) were prepared with glycine (Gly), D-cysteine, (Cys), D-lysine (Lys), and citrate. These solutions were adjusted at pH 9 to ensure that they were in their carboxylate forms. Silver colloids were pre- pared by adding 6 mL of the reducing agent (2 mL in the case of citrate) to 100 mL of boiling solution of Ag + 10 -3 M under vigorous stirring. The solution was kept boiling during 30 min. After that, aliquots of the colloidal dispersion were adjusted at the desired pH (1-11) by adding HNO 3 or NaOH 1 M (Metrohm Titrino 702SM autoburet). Colloids were then * Author to whom correspondence should be addressed. E-mail: raroca1@ cogeco.ca. Fax: 1-519-973-7098. Phone: 1-519-253-3000 (ext. 3528). (1) Aroca, R. Surface-Enhanced Vibrational Spectroscopy; John Wiley & Sons: Chichester, U.K., 2006. (2) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. 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G.; Garrido, J. J.; Aroca, R. F. J. Phys. Chem. B 2005, 109, 3787–3792. (13) Aroca, R. F.; Alvarez-Puebla, R. A.; Pieczonka, N.; Sanchez-Cortez, S.; Garcia- Ramos, J. V. Adv. Colloid Interface Sci. 2005, 116, 45–61. (14) Shaw, D. J. Introduction to Colloid and Surface Chemistry; Butterworths: London, 1980. Anal. Chem. 2009, 81, 2280–2285 10.1021/ac8024416 CCC: $40.75  2009 American Chemical Society 2280 Analytical Chemistry, Vol. 81, No. 6, March 15, 2009 Published on Web 02/17/2009