Chemical and Electrochemical Ag Deposition onto Preformed Au

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Langmuir 1996, 12, 810-817

Chemical and Electrochemical Ag Deposition onto Preformed Au Colloid Monolayers: Approaches to Uniformly-Sized Surface Features with Ag-Like Optical Properties Robin M. Bright, Daniel G. Walter, Michael D. Musick, Michael A. Jackson, Keith J. Allison, and Michael J. Natan* Department of Chemistry, The Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802 Received May 31, 1995. In Final Form: October 5, 1995X Two approaches to preparation of Ag-clad Au colloid monolayers are described. Each begins with a preformed monolayer of colloidal Au particles on glass, Au-coated quartz, or In-doped SnO2. Chemical reduction of Ag+ from a commercial Ag coating formulation (LI Silver) or electrochemical reduction of Ag+ leads to surfaces for which the amount of Ag deposited can be controlled. On transparent substrates, Ag deposition can be followed in real time by UV-visible spectroscopy. When the substrate is an electrode, electrochemical deposition can be monitored by coulometry; for the chemical process, anodic stripping voltammetry yields accurate values of the amount of Ag present, as confirmed by quartz crystal microgravimetry (QCM). Chemical reduction yields surfaces that are extremely enhancing for surface enhanced Raman scattering (SERS), although the deposition process required must be precisely tuned. In contrast, electrochemical deposition, while affording more accurate control of the reduction rate, produces only mildly enhancing surfaces. Atomic force microscopy (AFM) images of electrochemically-produced surfaces show formation of very large nonuniform Ag particles that are distinct from the Au colloid monolayer, while those prepared by LI Silver show both growth of Ag on Au and formation of smaller colloidal Ag particles attached to Ag-coated Au particles.

Introduction Choosing a substrate for surface enhanced Raman scattering (SERS)1 presents an interesting problem. Ag and Au are by far the most widely used metals, and each has some uniquely useful attributes. Each metal also has physical properties that compromise its utility. For example, elucidating details of metalloprotein active sites by SERS2 typically requires the additional enhancement afforded by resonant excitation (SERRS).1 Since the overwhelming majority of biologically relevant metal complexes absorb electromagnetic radiation below 600 nm, a region where Au is SERS-inactive, metalloprotein SERRS requires Ag substrates.2 Unfortunately, the ease of Ag f Ag+ oxidation (E°′ (Ag+/Ag) ) 0.5 V vs saturated calomel electrode (SCE)),3 the tendency of Ag to denature proteins,4 and the need for Cl- ion for significant SERS * Author to whom correspondence should be addressed. Telephone: (814) 863-7832. (814) 865-1924. Electronic Mail: [email protected]. Chemistry Department Telephone (814) 8656553. Chemistry Department Fax: (814) 865-3314. X Abstract published in Advance ACS Abstracts, December 15, 1995. (1) (a) Brandt, E. S.; Cotton, T. M. In Investigations of Surfaces and Interfaces-Part B, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; Vol. IXB, Chapter 8. (b) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783-826. (c) Birke, R. L.; Lu, T.; Lombardi, J. R. In Techniques for Characterization of Electrodes and Electrochemical Processes: Varma, R., Selman, J. R., Eds.; John Wiley & Sons: New York, 1991; Chapter 5. (d) Smith, W. E. Methods Enzymol. 1993, 226, 482-495. (e) Nabiev, I. R.; Sokolov, K. V.; Manfait, M. Biomolecular Spectroscopy, Part A; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons Ltd.: Chicester, 1993; Vol. 20, Chapter 7. (f) Garrell, R. L. Anal. Chem. 1989, 61, 401A-411A. (g) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys: Condens. Matter 1992, 4, 1143-1212. (2) (a) Broderick, J. B.; Natan, M. J.; O’Halloran, T. V.; Van Duyne, R. P. Biochemistry 1993, 32, 13771-13776. (b) Hildebrandt, P.; Stockburger, M. Biochemistry 1989, 28, 6710-6721. (c) Silin, V. I.; Gaigalas, A. K. J. Raman Spectrosc. 1994, 25, 903-906. (d) Cotton, T. M.; Kim, J.-H.; Chumanov, G. D. J. Raman Spectrosc. 1991, 22, 729742. (3) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980.

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activity1 all present obstacles to using Ag as a substrate for SERS of biomolecules. Aggregates of colloidal Au and Ag nanoparticles are widely used alternatives to solid substrates for SERS.5,6 Colloidal particles possess significant advantages over macroscopic surfaces with respect to preparation,7 characterization,8 and modification:9 it is our belief that the future of SERS lies with organized assemblies of uniformlysized colloidal particles. Here, too, enhancement factor advantages for Ag are offset by difficulties associated with production of monodisperse particles in the 3-50 nm regime, a problem solved for colloidal Au.10 Our approach (4) (a) Holt, R. E.; Cotton, T. M. J. Am. Chem. Soc. 1989, 111, 28152821. (b) Lee, N.-S.; Sheng, R.-s.; Morris, M. D.; Schopfer, L. M. J. Am. Chem. Soc. 1986, 108, 6179-6183. (5) (a) Fornasiero, D.; Grieser, F. J. Chem. Phys. 1987, 87, 32133217. (b) Nabiev, I.; Baranov, A.; Chourpa, I.; Beljebbar, A.; Sockalingum, G. D.; Manfait, M. J. Phys. Chem. 1995, 99, 1608-1613. (c) Park, S. S.; Kim, K.; Kim, M. S. Chem. Phys. Lett. 1994, 230, 171-176. (d) Liang, E. J.; Engert, C.; Kiefer, W. J. Raman Spectrosc. 1993, 24, 775-779. (e) Kamyshny, A. L.; Zakharov, V. N.; Fedorov, Y. V.; Galashin, A. E.; Aslanov, L. A. J. Colloid Interface Sci. 1993, 158, 171-182. (f) Cermakova, K.; Sestak, O.; Matejka, P.; Baumruk, V.; Vlckova, B. Collect. Czech. Chem. Commun. 1993, 58, 2682-2694. (6) (a) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790-798. (b) Ahern, A. M.; Garrell, R. L. Langmuir 1991, 7, 254-261. (c) Wetzel, H.; Gerischer, H. Chem. Phys. Lett. 1980, 76, 460-464. (d) Angel, S. M.; Katz, L. F.; Archibald, D. D.; Honigs, D. E. Appl. Spectrosc. 1989, 43, 367-372. (e) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435-455. (7) (a) Kotov, N. A.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 8827-8830. (b) Esumi, K.; Sato, N.; Torigoe, K.; Meguro, K. J. Colloid Interface Sci. 1992, 149, 295-298. (c) Markowitz, M. A.; Chow, G.-M.; Singh, A. Langmuir 1994, 10, 4095-4102. (d) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475-478. (e) Quinn, M.; Mills, G. J. Phys. Chem. 1994, 98, 9840-9844. (f) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1994, 10, 4726-4730. (g) Collins, I. R.; Taylor, S. E. J. Dispersion Sci. Technol. 1991, 12, 403-415. (h) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C. Chem. Mater. 1992, 4, 1234-1239. (i) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 7, 801-802. (j) Barnickel, P.; Wokaun, A.; Sager, W.; Eicke, H.-F. J. Colloid Interface Sci. 1992, 148, 80-90. (k) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301-2309. (l) Nagata, Y.; Watananabe, Y.; Fujita, S.-i.; Dohmaru, T.; Taniguchi, S. J. Chem. Soc., Chem. Commun. 1992, 1620-1622.

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Ag Deposition onto Preformed Au Colloid Monolayers Chart 1

to resolving this enigma is to prepare composite particles that combine desirable features of both colloidal Au and Ag. One such useful composite would possess the monodispersity of Au with the optical properties of Ag. The layer of Ag could be extremely thin (I in Chart 1) or reasonably thick (II in Chart 1). The optical properties of composite metal particles like I and II have been modeled theoretically.11 For thin coats, optical properties are dominated by the Au core, while for thicker coats, properties of the Ag coat dominate the observed spectra. To explore the properties of I, we have recently prepared solutions of colloidal Au particles coated with a monolayer or less of Ag.12 These composite particles are prepared by adsorption of small quantities of Ag+ onto colloidal Au, followed by chemical reduction. The aggregation and SERS behavior of the resulting Ag-clad Au particles differ dramatically from typical colloidal Au preparations. For example, deposition of submonolayers of Ag on colloidal Au leads to a doubling of SERS intensity with 647 nm excitation.12 Particles like II can be prepared by controlled, slow deposition of Ag onto immobilized colloidal Au particles; in histochemistry and cytochemistry, this technique is called “autometallography” and is widely used to enhance the visibility of intracellular, ultrasmall Au particles by transmission electron microscopy (TEM).10,13 There are numerous recent studies on deposition of Ag on Au, of Au on Ag, and of either metal onto macroscopic host metal substrates14-21 The resulting surfaces have been investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy,14 second harmonic gen(8) (a) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035-2040. (b) Fischer, C.-H.; Weller, H.; Fojtik, A.; Lume-Pereira, C.; Janata, E.; Henglein, A. Ber. Busen-Ges. Phys. Chem. 1986, 90, 46-49. (c) Duff, D. G.; Baiker, A.; Gameson, I.; Edwards, P. P. Langmuir 1993, 9, 2310-2317. (d) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545-555. (e) Boxall, C.; Kelsall, G. H. J. Chem. Soc., Faraday Trans. 1991, 87, 3537-3545. (9) (a) Schafer, D. A.; Gelles, J.; Sheetz, M. P.; Landick, R. Nature (London) 1991, 352, 444-448. (b) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780-781. (c) Amiens, C.; de Caro, D.; Chaudret, B.; Bradley, J. S. J. Am. Chem. Soc. 1993, 115, 11638-11639. (d) Duteil, A.; Schmid, G.; Meyer-Zaika, W. J. Chem. Soc., Chem. Commun. 1995, 31-32. (10) Hayat, M. A., Ed. Colloidal Gold: Priciples, Methods and Applications; Academic Press, Inc.: New York, 1989; Vols. 1, 2. (11) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley Interscience: New York, 1983. (12) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem., in press. (13) (a) Dansher, G. Histochemistry 1984, 81, 331-335. (b) Dansher, G.; Norgaard, J. O. R. J. Histochem. Cytochem. 1985, 33, 706-710. (c) Dansher, G.; Norgaard, J. O. R.; Baatrup, E. Histochemistry 1987, 86, 465-469. (14) (a) Samant, M. G.; Borges, G.; Melroy, O. R. J. Electrochem. Soc. 1993, 140, 421-425. (b) White, J. H.; Albarelli, M. J.; Abrun˜a, H. D.; Blum, L.; Melroy, O. R.; Samant, M. G.; Borges, G. L.; Gordon, J. G., II. J. Phys. Chem. 1988, 92, 4432-4436. (15) Koos, D. A.; Richmond, G. L. J. Phys. Chem. 1992, 96, 37703775.

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eration,15 electrochemistry,16 scanning probe microscopy methods,17 diffraction,18 optical spectroscopy,19 XPS,20 and SERS.21 Composite22 and alloyed23 colloidal Au/Ag particles have also been previously described, but their SERS behavior has never been explored. SERS studies of particles of types I and II are greatly facilitated by the ready availability of monolayers of colloidal Au particles.24-27 Prepared by self-assembly of colloidal Au in solution onto polymer-coated substrates, these macroscopic surfaces are SERS-active, easy to characterize on both the centimeter and nanometer scales, electrochemically addressable, and highly reproducible.24 In this manuscript we describe deposition of Ag onto preformed Au colloid monolayers. The two systems considered are selective chemical reduction of Ag+ from solution onto surface-bound Au particles and electrochemical Ag+ reduction onto Au colloid-derivatized Indoped SnO2 electrodes. Both methods lead to arrays of Ag-clad Au colloid particles that can be characterized by AFM, quartz crystal microgravimetry (QCM), electrochemistry, UV-vis spectroscopy, and SERS. While LI Silver deposition is more selective and leads to greater SERS enhancements than the electrochemical process, it is also more difficult to control. It should be noted that since Ag is not biocompatible, our long range plans rest not with II but rather with III, which incorporates a thin Au overlayer on Ag-clad Au. Such particles are currently unknown. Moreover, it should be recognized that II represents an ideal case with regard to uniformity of coating: under certain conditions, particles like IV may also form. (16) (a) Rodriguez, J. F.; Taylor, D. L.; Abrun˜a, H. D. Electrochim. Acta 1993, 38, 235-244. (b) Taylor, D. L.; Abrun˜a, H. D. J. Electrochem. Soc. 1993, 140, 3402-3408. (c) Harford, S. T.; Taylor, D. L.; Abrun˜a, H. D. J. Electrochem. Soc. 1994, 141, 3394-3403. (d) Vaskevich, A.; Rosenblum, M.; Gileadi, E. J. Electroanal. Chem. 1995, 383, 167-174. (17) (a) Chen, C.-h.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451-458. (b) Corcoran, S. G.; Chakarova, G. S.; Sieradzki, K. Phys. Rev. Lett. 1993, 71, 1585-1588. (c) Hachiya, T.; Itay, K. Ultramicroscopy 1992, 42-44, 445-452. (d) Mason, M. G.; Hansen, J. C. J. Vac. Sci. Technol. A 1994, 12, 2023-2028. (e) Corcoran, S. G.; Chakarova, G. S.; Sieradzki, K. J. Electroanal. Chem. 1994, 377, 8590. (f) Sugita, S.; Abe, T.; Itaya, K. J. Phys. Chem. 1993, 97, 8780-8785. (18) (a) Lipphardt, U.; Engelhard, H.; Westhof, J.; Goldmann, A.; Witzel, S. Surf. Sci. 1993, 294, 84-92. (b) Hirschorn, E. S.; Miller, T.; Sieger, M.; Chiang, T.-C. Surf. Sci. Lett. 1993, 295, L1045-L1049. (19) Watanabe, M.; Uchida, H.; Miura, M.; Ikedo, N. J. Electroanal. Chem. 1995, 384, 191-195. (20) Tarlov, M. J. Langmuir 1992, 8, 80-89. (21) (a) Tuo, L. C.; Chen, T. T.; Chou, Y. C.; Loung, N. T. Surf. Sci. 1989, 222, L861-L870. (b) Schneider, S.; Grau, H.; Halbig, P.; Nickel, U. Analyst 1993, 118, 689-694. (c) Loo, B. H.; Furtak, T. E. Chem. Phys. Lett. 1980, 71, 68-71. (d) Furtak, T. E.; Trott, G.; Loo, B. H. Surf. Sci. 1980, 101, 374-380. (e) Kester, J. J.; Furtak, T. E. Solid State Commun. 1982, 41, 457-460. (f) Murray, C. A. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 371-382. (g) Takahashi, M.; Ito, M. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 913-922. (h) Marinyuk, V. V.; Lazorenko-Manevich, R. M.; Kolotyrkin, Y. M. Solid State Commun. 1982, 43, 721-725. (i) Pemberton, J. E. J. Electroanal. Chem. Interfacial Electrochem. 1984, 167, 317-323. (22) (a) Henglein, F.; Henglein, A.; Mulvaney, P. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 180-189. (b) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061-7064. (c) Sato, T.; Kuroda, S.; Takami, A.; Yonezawa, Y.; Hada, H. Appl. Organomet. Chem. 1991, 5, 261-268. (d) Morriss, R. H.; Collins, L. F. J. Chem. Phys. 1964, 41, 3357-3363. (23) (a) Teo, B. K.; Keating, K.; Kao, Y.-H. J. Am. Chem. Soc. 1987, 109, 3494-3495. (b) Papavassiliou, G. C. J. Phys. F: Metal Phys. 1976, 6, L103-L105. (c) Ripken, K. Z. Phys. 1972, 250, 228-234. (24) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-1632. (b) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735-743. (25) Grabar, K. C.; Davis, J. E.; Natan, M. J. Polym. Prepr. 1995, 36, 69-70. (26) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Dolan, C. M.; Freeman, R. G.; Fox, A. P.; Musick, M. D.; Natan, M. J. Langmuir, in press. (27) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313-1317.

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Experimental Section Materials. The following materials were obtained form Aldrich: H2O2, HAuCl4‚3H2O, Na2SO4, trisodium citrate dihydrate, pyridine, and N,N-dimethyl-4-nitrosoaniline (p-NDMA). The following organosilanes were obtained from Hu¨ls America, Inc., and used without further purification: (2-(diphenylphosphino)ethyl)triethoxysilane (DPPETES), (3-aminopropyl)trimethoxysilane (APTMS), (3-mercaptopropyl)methyldimethoxysilane (MPMDMS) and (2-(trimethoxysilyl)ethyl)-2-pyridine (PETMS). APTMS from Aldrich was also used. KCl, HCl, H2SO4, and HNO3 were obtained from J. T. Baker. [Ru(NH3)6]Cl3 and Ag2SO4 were obtained from Johnson Matthey. LI Silver was obtained from Nanoprobes, Inc. CH3OH and acetone (spectrophotometric grade) were obtained from EM Sciences. Absolute C2H5OH was obtained from Aaper Alcohol and Chemical Company. All chemicals were used as received. All H2O was 18 MΩ, distilled through a Barnstead Nanopure purification system. Single component Ag epoxy was obtained from Epoxy Technologies, Inc., and insulating white epoxy (Epoxy-Patch) from Dexter Corporation. Glass microscope slides were obtained from Fisher Scientific, quartz slides from Technical Glass Products, Sb-doped SnO2 from Delta Technologies, and In-doped SnO2 from PPG Industries, Inc. Colloidal Au particles with 12-, 18-, and 40-nm diameters were prepared and characterized by UV-vis spectroscopy and transmission electron microscopy as described previously.24,26 Instrumentation. SERS spectra were obtained with a Spectra-Physics Model 127 He:Ne ion laser and other equipment previously described.12,24 Optical spectra were obtained using a Hewlett-Packard 8452A diode array spectrophotometer with 2-nm spectral resolution and 1-s integration time. Electrochemical measurements made use of Cypress Systems CS87, a Princeton Applied Research (PAR) 173/175 combination, a PAR 273/270 combination, and BAS CV27 potentiostats, as well as an NGI Servogor 790 X-Y recorder and a Linseis L6512B strip chart recorder. Atomic force microscopy (AFM) was performed on either a Digital Instruments Nanoscope III Multimode AFM in tapping mode or a Burleigh Instruments ARIS-3300 with an ARIS-3370 Mode II option. In both cases, air-dried 1 cm × 1 cm samples were utilized. Scan areas varied from 25 000 nm2 to 9 µm2. QCM measurements were carried out using a home-built oscillating circuit, a Heathkit (Model IP-18) regulated power supply, and a Fluke Timer/Counter (Model PM-6680B) equipped with a high stability oven oscillator time base. Data was collected via a National Instruments IEEE-488 interface to a PCcompatible computer. All experiments were conducted inside a Faraday cage to reduce electrical interference. 9.0 MHz polished AT-cut quartz crystals (surface features ≈ 0.02 µm in depth)28 with Au electrodes were obtained from International Crystal Manufacturing (Oklahoma City, OK). Only one side of the crystal was exposed to solution. Preparation of Au Colloid Monolayers on Glass/Quartz. Cut (25 mm × 9 mm) glass and quartz microscope slides were cleaned in a bath of 4 parts H2SO4 to 1 part 30% H2O2 at 70 °C. After being rinsed in H2O, slides were cleaned in a bath of 3:1 HCl:HNO3 (aqua regia). Slides were rinsed in H2O and stored in CH3OH until needed. Clean surfaces were placed in 2% (by volume) organosilane/CH3OH mixture (DPPETES or PETMS). Solutions were shaken vigorously and then allowed to react for 24 h. Substrates were then removed from solution and rinsed thoroughly in CH3OH and subsequently rinsed and stored in H2O. Silanized surfaces were immersed in 12-nm colloidal Au for 24 h. Surfaces were then removed, rinsed several times in H2O, and stored in H2O. For QCM crystals, the aqua regia step was omitted, and C2H5OH was used instead of CH3OH. APTMS derivatization was from a 20% solution for 1 h, again with an C2H5OH rinse. After being dried with Ar, the frequency was determined in air. The crystal was then exposed to 12-nm-diameter colloidal Au particles for 2 h and dried with Ar, and the new frequency was determined. Electrode Fabrication. Sb-doped or In-doped SnO2 (henceforth SnO2 for both) was cut to an approximate area of 3.0 cm2. A Cu wire was adhered to SnO2 with Ag epoxy. The copper wire was encased in glass tubing and sealed with Epoxy-Patch. The (28) Yang, M.; Thompson, M. Langmuir 1993, 9, 1990-1994.

Bright et al. electrodes were sonicated in neat acetone, soap solution, and H2O for approximately 15 min each and then were soaked in approximately 3 M NaOH for over 2 h. Clean electrodes were placed in aqueous APTMS (1% by weight) for 15 min. Silanized electrodes were then rinsed with H2O and air-dried for 48 h. The electrodes were placed in Au colloid solution for 2 h. Colloidcoated electrodes were then rinsed and stored in H2O. Derivatization with MPMDMS was accomplished by immersion of SnO2 samples in neat MPMDMS for approximately 5 h. Substrates were then rinsed with CH3OH and dried in air. To achieve high colloid coverages, these substrates were then placed into freshly prepared Au colloidal solutions for at least 8 h. The Au colloidcoated surfaces were then removed, rinsed with triply-distilled H2O, and air-dried. SnO2 and Au-derivatized SnO2 electrodes were characterized by measuring a cyclic voltammogram of 5 mM Ru(NH3)6Cl3 in degassed 0.1 M Na2SO4 under N2. A saturated calomel electrode (SCE) was used as the reference, and Pt gauze was used as the counter electrode for these and all other electrochemical measurements. Ag Deposition. Chemical reduction of Ag+ onto Au colloid monolayers on glass or Au-coated quartz was accomplished by immersing the substrates into a solution of equal volumes of LI Silver enhancer and initiator solutions. The reaction time was varied between 5 and 30 min and always measured with a stopwatch. Surfaces were then rinsed and stored in H2O. Electrochemical deposition made use of a degassed 1 mM Ag2SO4/0.1 M Na2SO4 solution. Initially, a cyclic voltammogram from +1 to 0 V vs SCE was recorded at 50 mV/s under N2. A constant potential of +0.27 V was then applied to the electrode until a predetermined number of coulombs had passed. The electrodes were then removed and stored in H2O. All Ag-coated Au colloid monolayers were characterized by UV-vis spectroscopy in H2O. SERS spectra of Ag-coated Au monolayers on SnO2 were obtained from 0.5 M pyridine/0.1 M KCl solutions. SERS spectra on surfaces made by LI Silver reduction were obtained using 0.5 mM solutions of p-NDMA in CH3OH. Unless otherwise noted, all SERS experiments utilized 20 mW of 632.8-nm excitation, 7.5-cm-1 bandpass, 1-cm-1 steps, and 1-s integration times. Theoretical Calculations. Simulated optical spectra were calculated using an adaptation of the programs COATME and BHCOAT.11 Literature values for Ag and Au optical constants22d were fit to polynomial equations for use by the programs.

Results and Discussion Chemical Deposition of Ag using LI Silver. LI Silver is a commercially available, two-component mixture designed specifically to grow Ag on colloidal Au. While the composition of LI Silver is proprietary, the open literature suggests that it contains a reducing agent (such as a hydroquinone) and a polymeric material to control the rate of Ag+ reduction.29 The formulation is designed to operate at room temperature, and there are numerous examples of its successful application in histochemistry and cytochemistry.13 We have shown elsewhere that Ag+ ion adsorbs onto colloidal Au particles at room temperature, but with citrate as a reductant, heating to 65-70 °C is required to initiate reduction.30 A weak SERS spectrum results when a glass surface derivatized with DPPETES and 18-nm-diameter colloidal Au particles is placed in 0.5 mM p-NDMA in CH3OH and exposed to 632.8-nm excitation (Figure 1). A close correlation between SERS intensities and the magnitude of the multiparticle surface plasmon resonance has previously been demonstrated.1,24 The absence of such a feature in the optical spectrum for this sample (not shown) is consistent with the minimal surface enhancement observed. Immersion of an optically similar substrate into a solution of LI Silver for 10 min, followed by (29) (a) Danscher, G. Histochemistry 1981, 71, 1-16. (b) Scopsi, L.; Larrson, L.-I. Histochemistry 1985, 82, 321-329. (30) Hommer, M. B.; Natan, M. J. Unpublished results. (31) Calculated using R ) F(L/A) where R is resistance in Ω, F is resistivity in Ω‚cm, L is the length of a uniform conductor in cm, and A is its cross-sectional area in cm2.

Ag Deposition onto Preformed Au Colloid Monolayers

Figure 1. SERS spectra of 0.5 mM p-NDMA in CH3OH at (a) glass/DPPETES/18-nm-diameter colloidal Au and (b) the same surface after 10 min reaction time with LI Silver. Acquisition parameters: 22 mW, 632.8-nm excitation; 1-cm-1 step, 1-s integration; 7.5-cm-1 bandpass.

immersion into the 0.5 mM p-NDMA solution, leads to a significant increase in SERS intensity for adsorbed p-NDMA. Peak integration for these spectra, which were acquired under identical conditions, shows a 15-fold increase in Raman signal for the Ag-coated samples. Increases of this magnitude cannot be accounted for by increased surface area/roughness and consequently must arise from superior enhancement factors at this wavelength for Ag relative to Au. A 7-cm-1 shift in the phenylnitroso stretch from 1159 cm-1 for the colloidal Au surface to 1166 cm-1 for Ag-coated colloidal Au, as previously observed,12 provides further evidence that p-NDMA is adsorbed to a site with Ag-like character (presumably Ag°). Three control experiments show that the results of Figure 1 do not result from formation of discrete Ag particles on the polymer surface. (1) Immersion of glass slides coated with DPPETES, PETMS, or APTMS (but not colloidal Au) into an LI Silver solution did not lead to evolution of SERS activity on any time scale. (2) Leaving out either of the two solutions that, when mixed together, comprise LI Silver leads to no increase in SERS intensity and no peak shifts for p-NDMA. (3) These results could not be duplicated in the presence of Ag+ ion, citrate, and a derivatized colloidal Au at room temperature (although at higher temperatures, Ag deposition does occur).12,30 The results of these control experiments all indicate specific production of Ag on colloidal Au. However, these experiments cannot distinguish between particle types II and IV; such a distinction requires nanometer scale imaging (vide infra). The transformation from a Au colloid-based surface to one composed of Ag-clad Au can be monitored in real time on transparent surfaces by optical spectroscopy. Figure 2 profiles the optical changes that result over time from immersion of an Au/PETMS/quartz slide into a solution of LI Silver. Optical spectra were recorded every minute and exhibit a continuous change; only six are shown in Figure 2. Prior to immersion (curve a), only the Au surface plasmon mode is present.6 Ag reduction leads first to an increase in absorbance, followed by a shift of the surface plasmon excitation to that of colloidal Ag.5 Using a previously described model,11,24 the optical changes shown in Figure 2 are for curves c and d consistent with formation of a roughly 4-nm coat of Ag on Au, yielding 26-nmdiameter particles. At higher coverages, the major change

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Figure 2. Optical spectra of glass/PETMS/18-nm-diameter colloidal Au after immersion in LI Silver for (a) 0 min (initial spectrum), (b) 5 min, (c) 10 min, (d) 15 min, (e) 20 min, and (f) 25 min.

Figure 3. SERS spectra of 0.5 mM p-NDMA in CH3OH at glass/PETMS/18-nm-diameter colloidal Au after immersion in LI Silver for (a) 9 min, (b) 9.5 min, (c) 10 min, (d) 10.5 min, (e) 11 min, (f) 11.5 min, (g) 12 min, (h) 12.5 min, and (i) 13 min. Acquisition parameters were identical to those described in Figure 1.

observed is an increase in scattering over the entire spectral region. Ag deposition can also be followed by SERS; these data are shown in Figure 3, with p-NDMA as the adsorbate. SERS intensity increases over time, with a maximal intensity reached for this particular sample at 13 min. The time leading to maximum enhancement clearly depends on initial particle size and spacing, and we have made no efforts to optimize these parameters. Moreover, the optimum time for samples made simultaneously with that described in Figure 3 (as judged by the SERS intensity upon immersion in p-NDMA) varied by as much as 2 min. This variation may result from the critical dependence of the growth rate on the solution concentration of LI Silver. In any case, for each deposition there is a clearly-defined maximum, and immersion for longer times leads to significant decreases in intensity. It should also be noted that, unlike previously-described metal colloid monolayers,24-26 these surfaces do exhibit an intrinsic, timedependent SERS spectrum. While we have not pursued identification of the species responsible, they undoubtedly are associated with the LI Silver formulation. Impor-

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Figure 4. Linear sweep voltammograms of SnO2/APTMS/12nm-diameter colloidal Au after immersion in LI Silver for the times indicated. Inset: Linear sweep voltammogram of Aucoated 9-MHz quartz crystal/APTMS/12-nm-diameter colloidal Au after immersion in LI Silver for 13 min.

tantly, they are displaced by p-NDMA and other adsorbates. Prolonged immersion of Au colloid monolayers on glass into LI Silver leads to production of surfaces that exhibit electrical conductivity. Applying a voltage (V) to both ends of a 2 cm × 0.9 cm overcoated glass slide and measuring current flow (i) leads to a linear i-V response (data not shown). From the slope, a resistance of 1 MΩ is obtained. For comparison, the sheet resistance of a pure Ag of 50nm thickness in the same geometry is 7 mΩ.32 While these values seem quite disparate, it is worth noting that pure Ag is the most conductive metal and that our calculation is based on an even 50-nm coating of Ag, when in fact we have colloidal Au particles interspersed. The sheet resistance of our sample, based on these assumptions,32 is 2 Ω‚cm. Accordingly, these surfaces are adequate for faradaic processes, as evidenced by the expected Ag/Ag+ stripping wave obtained when an overcoated Ag surface is scanned from 0.0 to +0.8 V in aqueous 0.1 M Na2SO4. Interestingly, this overcoated, quasimetallic surface has no measurable SERS activity. On conductive substrates, the stripping wave can be used to monitor Ag coverages obtained using LI Silver. We have carried out such experiments on SnO2 and on 9-MHz Au-coated QCM crystals. In both cases, submonolayers of 12-nm-diameter colloidal Au were immobilized onto APTMS-coated substrates. For SnO2, a set of four surfaces with absorbances between 0.1 and 0.2 were immobilized in LI Silver for 3, 6, 9, and 13 min; integration of linear sweep voltammograms obtained (Figure 4) yielded 0.69, 1.51, 2.18, and 5.29 mC/cm2, respectively. Linear sweep voltammetry of an Au colloid monolayer on APTMS/ SnO2 that had not been exposed to LI Silver showed no anodic current. All deposited Ag was removed in the first scan, as second scans on each of the four samples gave no anodic signal. It is apparent from the shapes of the (32) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 71 ed.; CRC Press: Boca Raton, 1990; pp 12-24.

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voltammograms that there are two pools of Ag, one with a voltammetric peak at +0.25 V vs SCE and another with peaks between +0.4 and +0.55 V vs SCE. The magnitude of the first peak is nearly constant, with only the second growing as a function of immersion time. Carrying out the same experiment on a QCM crystal allows the integrated currents to be cross-checked by mass changes. The inset to Figure 4 shows the linear sweep voltammogram obtained for a Au-coated QCM crystal derivatized with APTMS and 12-nm-diameter colloidal Au and immersed in LI Silver for 13 min. There are several interesting features to the data, the most obvious of which is the reversal of the magnitudes of the currents for the first and second stripping waves. Linear sweep voltammetry gave a Ag coverage of 12.9 mC/cm2 for a surface with a particle coverage of 5.4 × 1011/cm2 (also confirmed by QCM). Note that this integrated current is more than a factor of 2 greater than that observed on SnO2 and the Au particle coverage is roughly a factor of 4 greater than that typically found for glass/APTMS/colloidal Au.24 However, the QCM experiment is internally consistent: the frequency change for the 13 min LI Silver immersion was 2785.4 Hz, corresponding to 13.4 mC/cm2, and agreeing to within 5% with the value obtained by voltammetry. The good agreement between QCM and electrochemical data suggests that the difference in Au coverage between SnO2 and Au must be reconciled: our hypothesis is that, because there is no covalent bonding between the organosilane and Au, a very different morphology and polymer thickness are obtained on the quartz crystal sample than we usually obtain on SnO2, glass, or quartz. In addition, some of this difference can be accounted for by surface roughness.26 The nanometerscale structure of these samples is currently under investigation. For the QCM sample, it is possible to estimate the diameter of the Ag-clad Au particles. This calculation requires two assumptions: (i) that all charge consumed goes into production of Ag° bound to colloidal particles and (ii) that bound particles are spherically coated (i.e., that the final particles are spherical). This last assumption is almost certainly not the case but is reasonable for the purpose of approximation. Converting integrated currents to a Ag volume per particle and factoring in the volume of a 12-nm Au core particle yields a new particle diameter of 18.8 nm for the Au/QCM electrode. This corresponds to a 3.4-nm Ag coat. We have obtained preliminary AFM data on particle size distribution in LI Silver-treated samples. For these experiments, a monolayer of 40-nm-diameter colloidal Au particles on MPMDMS/glass was used. AFM images of this surface reveal, as expected, predominantly isolated particles that vary between 40 and 50 nm high on the z-scale. Despite the increased particle size, the sample was also coated with LI Silver for 13 min, since for a monolayer of close-packed spherical particles, the total surface area is not a function of the particle diameter. Thus, as a first approximation, we expected a Ag coverage equal to that obtained on 12-nm particles. A bimodal distribution in particle sizes was found: 45-55 nm features as well as a clearly discernible but small subset of 5-15 nm diameter particles. The latter feature corresponds to the formation of Ag colloids or Ag islands. Importantly, all of these smaller features are associated with larger particles. Nevertheless, their consistent presence on images acquired from various spots on the samples suggest a less than perfectly uniform Ag deposition, with most but not all of the Ag depositing evenly on the particles. Moreover, there may be a connection between the two Ag environments imaged by AFM and

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Figure 5. Optical spectra of SnO2/MPMDMS/12-nm-diameter colloidal Au in 0.1 M Na2SO4 at 0, -0.1, -0.2, -0.3, -0.4, -0.5, -0.6, -0.7, -0.8, -0.9, and -1.0 V vs SCE.

the two pools of Ag seen in linear stripping voltammetry. Even so, the ability to prepare particles that are roughly 50 nm in diameter and have Ag-like optical properties is unique in Ag colloid SERS, especially given the realization that the diameter of the colloidal Au core is tunable from 3 to 100 nm.10 Electrochemical Deposition of Ag. Colloidal Au particles bound via an organosilane film to a conducting substrate are electrochemically addressable.24 Thus, electrochemical generation of Ag on Au colloid monolayers is feasible. In principle, this approach should be superior to autometallography, because electrochemical potential can be used to control the rate, and coulometry can be used to monitor the exact quantity of Ag deposited. Moreover, the transparency of Ag+ ion in the visible region of the spectrum means that electrochemical deposition can be monitored on transparent substrates by UV-vis. For the UV-vis data to be interpretable, it was necessary to show that surface-confined Au colloidal particles themselves exhibit no potential-dependent optical changes. The results of one such experiment with a Au colloid multilayer are shown in Figure 5. At open circuit in H2O, the colloidal film exhibits a single broad peak at ≈690 nm. From previous work in the literature on aggregates of colloidal Au,6 as well as our own data on correlation of optical properties with nanostructure using AFM and field emission scanning electron microscopy,33 this type of optical spectrum is characteristic of small aggregates. Our interest in such a surface stems from the fact that there are two forces binding colloidal Au: covalent attachment to the surface, and interparticle electrostatic forces binding particles beyond the first monolayer. Using a standard UV-vis cell whose top was enlarged to accommodate roughly 20 mL of solution, optical spectra were measured in 0.1 M Na2SO4 at open circuit and every -100 mV from 0.0 to -1.0 V vs SCE. The data show no change in optical properties, i.e., no further aggregation or deaggregation, meaning that the strength of the electric field at the interface is insufficient to disrupt existing interactions. Completely potential-independent optical properties were obtained for submonolayers of colloidal Au as well. We have not carried out these measurements in organic solvents, but such an investigation may be worth pursuing in light of a recent report describing laser electric field-induced coagulation of nanometer-sized colloidal Au in organic solvents.34 Electrochemical deposition of Ag onto preformed 12nm-diameter Au colloid monolayers bound to APTMS/ SnO2 was carried out in a solution of 1 mM Ag2SO4/0.1 M Na2SO4. These conditions differ from standard conditions (Ag+ in strong acid) used for deposition onto single crystals but were necessitated by the instability of colloid mono(33) Grabar, K. C.; Smith, P. C.; Jackson, M. A.; Davis, J. A.; Guthrie, A. P.; Musick, M. D.; Walter, D. G.; Natan, M. J. J. Am. Chem. Soc., in press. (34) Kimura, K. J. Phys. Chem. 1994, 98, 11997-12002.

Figure 6. Current vs time profile for electrochemical Ag deposition on a 1.8 cm2 SnO2/APTMS/12-nm-diameter colloidal Au electrode held at +0.27 V vs SCE in a solution of 1 mM Ag2SO4 /0.1 M Na2SO4. Inset: CV of deposition solution at 50 mV/s using a SnO2/APTMS/12-nm-diameter colloidal Au electrode prior to electrochemical deposition.

Figure 7. (A) Optical spectra of an SnO2/APTMS/12-nm colloidal Au electrode after deposition of (a) 5.0 × 10-3, (b) 1.0 × 10-2, (c) 1.5 × 10-2, (d) 2.5 × 10-2, and (e) 3.0 × 10-2 C/cm2 Ag. The dashed line is the initial optical spectrum for the substrate. (B) Calculated optical spectra for a 12-nm-diameter Au particle encased in (a) a 1.5, (b) a 3, (c) a 4.5, and (d) a 7.5-nm Ag coat. The dashed line is the calculated optical spectrum for an uncoated 12-nm-diameter Au particle.

layers at pH values below 2. The inset to Figure 6 shows the cyclic voltammogram at 50 mV/s of a Au colloid monolayer immersed in this solution. The cathodic and anodic waves correspond to Ag deposition and stripping, respectively. The position of the cathodic wave was used to choose the electrochemical potential for a controlled potential electrolytic deposition, +0.27 V vs SCE. Figure 6 shows the current vs time profile, revealing a very large initial current that rapidly declines over the course of 30 s to a steady state value of ≈10 µA. The integrated current for this deposition was 1.83 × 10-2 C. These data show that electrochemical deposition of Ag on Au colloid monolayers is well-behaved and easily quantified by coulometry. Changes in the optical spectrum of the substrate can be monitored as a function of charge passed (Figure 7, panel A). The optical spectrum of a 12-nm-diameter Au colloid submonolayer on APTMS/SnO2 prior to Ag deposition is denoted by the dotted line. We have previously shown that UV-vis can be used to estimate coverages of 12-nm-diameter Au on transparent substrates.24,33 From

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the absorbance maximum of 0.25 at 522 nm, a particle coverage of ≈0.2 monolayers is estimated. This value can be used to calculate the total Au surface area (≈5.4 × 1015 Å2/cm2 of SnO2) and amount of charge required to theoretically deposit a monolayer of Ag on Au (≈3.9 × 10-4 C/cm2 of SnO2). Deposition of what formally corresponds 13 and 26 monolayers of Ag (curves a and b) leads to increased extinction of the 522-nm peak, but without evolution of new features that could be ascribed to bulk Ag. Interestingly, continued deposition (1.5 × 10-2 C/cm2, curve c) leads to a decrease in absorbance, again with no hint of separate Ag-linked absorbances in the visible region of the spectrum. A further 60% increase in deposited Ag (to a total of 2.5 × 10-2 C/cm2, curve d). leads to emergence of a large peak at 406 nm and corresponds to the wellknown Ag surface plasmon resonance. The final deposition of Ag yields a surface with a coulometricallydetermined (total) Ag coverage of 3.0 × 10-2 C/cm2 (curve e). While this value corresponds to only a 20% increase over curve d in deposited Ag, it leads to complete elimination of the Ag surface plasmon resonance at 406 nm. These surprising optical changes do not correspond to theory and have been repeated exactly in separate experiments on several occasions, eliminating the possibility that they constitute anomalies. To help understand the nature of these effects, we used a computer program to model the optical spectrum of isolated, concentric, two-layer particles. Panel B of Figure 7 shows the calculated optical spectra for a 12-nm Au particle uniformly coated with metallic Ag to yield effective diameters of 12 nm (i.e., no Ag; dashed line), 15.1 nm (a), 18 nm (b), 21 nm (c), and 27 nm (d). The spectra are plotted in units of relative scattering, which are adequate to observe the evolution of an intense Ag plasmon absorbance. When the Ag film thickness reaches just a few nanometers, the Ag resonance effectively masks the Au surface plasmon. For particles with a 12-nm-diameter Au core surrounded by a 7.5-nm-thick shell of Ag (curve d), the optical spectrum looks exactly like that of pure Ag particles (although the total scattering/extinction is slightly diminished).12 These calculated spectra, when combined with the estimated Au particle coverage, provide insight into the fate of Ag atoms upon deposition. For example, to surround each of the 12-nm colloidal Au particles on the APTMS/SnO2 surface with a 7.5-nm diameter shell would require 1 × 10-2 C/cm2 of charge. However, the observed spectrum corresponding to this passage of charge does not exhibit an intense or even a distinct Ag band but rather an increased absorbance at the 522-nm peak, which correlates with much smaller coverages of Ag on Au.12 Only when 3 times more charge is passed does the optical spectrum yield the predicted peak at 406 nm. In other words, it is clear that the Ag atoms are not depositing exclusively on the colloidal Au particles. The finding that Ag deposition is not confined to Au and the observation that none of the surfaces prepared by electrochemical deposition of Ag on Au colloid monolayers are particularly SERS-activesboth aggregated Ag-clad Au particles in solution12 and Ag-clad Au colloid monolayers prepared by chemical deposition with LI Silver are significantly more enhancingsled us to explore the question of whether adsorbates bind to Ag atoms, as was shown for LI Silver (in Figure 1) and Ag-clad Au particles in solution. These issues are addressed by Figure 8, which shows the potential dependence of the pyridine SERS spectrum on a Ag-coated Au colloid monolayer on APTMS/ SnO2. The lower SERS intensities compared to LI Silvercoated surfaces is apparent, since the data in Figures 8 and 3 were acquired under identical conditions. Curves

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Figure 8. SERS spectra in 50 mM pyridine/0.1 M KCl on an Ag-clad (2.5 × 10-2 C/cm2) SnO2/APTMS/12-nm-diameter colloidal Au at (a) 0.0, (b) -0.1, (c) -0.2, (d) -0.3, (e) -0.4, (f) -0.5, (g) -0.6, (h) -0.7, (i) -0.8, and (j) -0.9 V vs SCE. Inset: Peak maximum vs electrochemical potential. Acquisition parameters: 22 mW, 632.8-nm excitation; 0.5-cm-1 step, 1-s integration; 7.5-cm-1 bandpass.

Figure 9. AFM images and section analyses of dry 1-µm2 substrates. The average sample heights were taken to be zero, and the Z-axis is 300 nm. (A) Bare SnO2. (B) SnO2/APTMS/ 12-nm-diameter colloidal Au. (C) 2.5 × 10-2 C/cm2 Ag deposited on SnO2/APTMS/12-nm-diameter colloidal Au. (D) 3.0 × 10-2 C/cm2 Ag deposited on SnO2/APTMS/12-nm-diameter colloidal Au. (E) 2.5 × 10-2 C/cm2 Ag deposited on SnO2/APTMS. (F) Sectional analysis of B. G. Sectional analysis of D.

a-j show an increase in SERS signal as the electrochemical potential is moved from 0.0 V vs SCE to -0.9 V vs SCE. A slight change in the position of the pyridine ring breathing mode at 1010 cm-1 is also observed as the electrochemical potential is changed (inset to Figure 8). Both of these effects have been previously observed in the literature35 and strongly suggest that Ag is being deposited directly onto Au. AFM images of Au colloid monolayers before and after Ag deposition (Figure 9) provide direct confirmation of Ag deposition and provide an explanation for the anomalous optical properties and relatively weak SERS enhancement. (35) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1-20.

Ag Deposition onto Preformed Au Colloid Monolayers

Part A is an AFM image of bare dry SnO2. Part B depicts a Au colloid monolayer on SnO2. C and D are Au colloid surfaces upon which 2.5 × 10-2 C/cm2 and 3 × 10-2 C/cm2 Ag had been deposited, respectively, and E shows an APTMS-coated SnO2 surface (lacking colloid) upon which 2.5 × 10-2 C/cm2 had been deposited. For A-E, the Z-axis is 300 nm. F and G are line-scan section analyses of the samples imaged in B and D, respectively. It is important to point out that each of the AFM images reflects convolution of the AFM tip with the true image: although the colloidal Au particles are known to be 12-nm-diameter spheres, the AFM image suggests that they are 30-35 nm wide. As a result, the image suggests that the particles are touching. However, comparison of field emission SEM and AFM data on identical samples shows that this is not the case. A number of other interesting features are revealed by these images. The roughness of In-doped SnO2 is predominantly less than 10 nm, a value not significantly greater than that of glass or quartz.36 In addition, there are a few isolated features that are on the order of 50-60 nm. Fortunately, these do not preclude us from obtaining images of Au colloid monolayers assembled with 12-nmdiameter particles (B, F). Image B shows that there is a single submonolayer layer of colloidal Au particles, in accord with our previous AFM and field emission SEM measurements on glass. The line scan F shows that AFM can accurately determine particle height and can apparently laterally resolve individual particles. The nonplanarity of the particles, resulting from the intrinsic substrate roughness, is also evident. Line scan F and AFM image B both show what appears to be a close-packed monolayer, with an average particle width of roughly 40 nm. Since it is known from independent TEM analysis that these particles are separated 12-nm-diameter spheres, it is clear that the image represents a convolution of the true image and the AFM tip profile. The AFM image corresponding to 2.5 × 10-2 C/cm2 (C) shows a markedly nonuniform coverage, with several large features hundreds of nanometers in size as well as numerous smaller 50-nm-sized objects. An image of the surface that results from depositing the same amount of Ag in the absence of colloidal Au is shown in E. The presence of islands verifies the finding that Ag derivatization is nonspecific under these conditions. Deposition of another 5 mC to an Ag-clad Au sample already harboring 2.5 × 10-2 C/cm2 of Ag leads to a major surface recon(36) (a) Dressick, W. J.; Dulcey, C. S.; Georger, J. H., Jr.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210-220. (b) Karrasch, S.; Dolder, M.; Schabert, F.; Ramsden, J.; Engel, A. Biophys. J. 1993, 65, 2437-2446.

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struction; the density of large features increases, with a concomitant decrease of the number density of smaller features. Indeed, the smaller features closely resemble in size the initial Au colloid sample (B), but are more triangular. A line scan of these features (G) indicates that they are oblong particles roughly 250 nm wide and at least 200 nm high, far too large to provide optimum SERS enhancement with red excitation.1 One explanation for the extreme nonuniformity of Ag deposition is that the high Ag+ concentration near the electrode, coupled with the ability to pass electric charge through the organosilane film, leads to random deposition on the surface. Subsequent island coagulation leads to the extremely large particles observed. A superior protocol might be to adsorb Ag+ ion from solution onto colloidal Au, withdraw the substrate, and then electrochemically reduce the surface-bound ions in a Ag+-free solution of supporting electrolyte. While a bit cumbersome, this process could be repeated as many times as needed. Conclusions Chemical or electrochemical reduction of Ag+ onto colloidal Au in preformed films leads to Ag-clad Au colloids, a process that can be followed by optical spectroscopy, QCM, electrochemistry, SERS, and AFM. Chemical reduction leads to a surface with exceptional SERS characteristics, but the deposition process must be closely monitored. By and large, Ag is deposited on colloidal Au particles, thereby achieving the goal of preparing particles with Ag-like optical properties and Au-like morphology. In contrast, electrochemically-deposited Ag on Au colloid monolayers does not lead to a significant enhancement in SERS intensity, primarily a result of nonselective deposition. Acknowledgment is made to the National Science Foundation (CHE 92-08614, CHE 92-56692, and CHE9307485) and the Beckman Foundation (Young Investigator Program) for partial support of this research. Acknowledgment is also made to The Pennsylvania State University (PSU) (through a Teas undergraduate fellowship to D.G.W.) for partial support of this research, to PPG Industries, Inc., for a gift of In-doped SnO2, to the Electron Microscopy Facility for the Life Sciences in the Biotechnology Institute at PSU, and to Craig T. Bohren (PSU) for the computer programs BHMIE and BHCOAT. We thank Burleigh Instruments, Inc., for collaboration in determining the nanometer-scale structure of Au colloid monolayers. LA950429H