Electrochemical Properties of Colloidal Au-Based ... - ACS Publications

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Langmuir 1999, 15, 844-850

Electrochemical Properties of Colloidal Au-Based Surfaces: Multilayer Assemblies and Seeded Colloid Films Michael D. Musick,† David J. Pen˜a,† Steven L. Botsko,† Todd M. McEvoy,‡ John N. Richardson,‡ and Michael J. Natan*,† Department of Chemistry, The Pennsylvania State University, 152 Davey Lab, University Park, Pennsylvania 16802-6300, and Department of Chemistry, Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania 17257-2299 Received July 20, 1998. In Final Form: December 2, 1998 The preparation, characterization, and electrochemical properties of two types of conductive Au films are described. Both films are made entirely by wet chemical procedures. In the first, successive treatment of a Au colloid monolayer/glass substrate with (i) 2-mercaptoethylamine and (ii) colloidal Au in solution leads to systematic buildup of a Au colloid multilayer. After seven to eight layers of Au nanoparticles have been deposited, the multilayers become conductive. Cyclic voltammograms of several different redox couples show that the peak-to-peak separation decreases as the number of layers increases. In the second type of film, a solution of hydroxylamine and Au3+ are used to selectively enlarge the size of a preimmobilized colloidal Au monolayer. Once the particles coalesce, the resulting film can be used to generate voltammograms with narrow peak separations. The ability to form conductive Au films using entirely wet-chemical steps may be valuable for fabrication of electrodes with complex shapes.

Introduction Increasingly, materials chemistry is having an impact on analytical chemistry, especially when interfacial chemistry is involved. For example, bundles of optical fibers whose cores can be selectively etched1 are finding use in the analysis of fluorescently encoded beads.2 Dendrimers3 and hyperbranched polymers4 are being confined to electrode surfaces for a variety of sensor applications.5,6 Colloidal metal nanoparticles have received much attention in this regard. Mirkin and co-workers have used DNA-tethered colloidal Au particles to develop an optical method for detection of DNA hybridization.7-9 Emory and Nie have studied surface-enhanced Raman scattering (SERS) from single, surface-confined Au and Ag nanoparticles.10 Monolayers of colloidal Au nanoparticles11 on SnO2 have been shown to facilitate electron transfer to * To whom all correspondence should be addressed via email at [email protected]. † The Pennsylvania State University. ‡ Shippensburg University. (1) Pantano, P.; Walt, D. R. Chem. Mater. 1996, 8, 2832. (2) Michael, K. L.; Taylor, L. C.; Schultz, S. L.; Walt, D. R. Anal. Chem. 1998, 70, 1242. (3) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (4) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (5) Tokuhisa, H.; Zhao, M. Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120, 4492. (6) Liu, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Agnew Chem., Int. Ed. Engl. 1997, 36, 2114. (7) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (8) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (9) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (10) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (11) (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. (b) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148.

cytochrome c in solution.12 The heightened interparticle electric field13,14 in deliberately prepared metal nanoparticle-analyte-metal nanoparticle sandwiches has been used to generate strong SERS signals from the sandwiched analyte.15 Electrochemical sensors for peroxide and other small molecules have been developed using surfaceconfined protein/Au colloid complexes.16 Binding of colloidal Au nanoparticles to a smooth, thin Au film has been shown to dramatically perturb reflectivity,17 allowing amplification of surface plasmon resonance signals.18 Of course, colloidal Au has enjoyed a long history as an electron dense marker in histochemistry and cytochemistry.19 We describe herein the preparation, characterization, and electrochemical properties of two types of electrodes derived from self-assembled monolayers of colloidal Au nanoparticles on glass.20-22 Schemes 1 and 2 depict the synthetic approaches employed. Scheme 1 illustrates fabrication of colloidal Au multilayers by repeated stepwise derivatization of a colloidal Au monolayer with (i) a (12) Brown, K. R.; Fox, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1154-1157. (13) Sestak, O.; Matejka, P.; Vlckova, B. Collect. Czech. Chem. Commun. 1996, 61, 59. (14) Keating, C. D.; Kovaleski, K. K.; Natan, M. J. J. Phys. Chem. B 1998, 102, 9414-9425. (15) Brandt, E. S.; Cotton, T. M. Surface-enhanced Raman Scattering. In Investigations of Surfaces and InterfacessPart B; Rossiter, B. W., Ed.; John Wiley & Sons: New York, 1993; Vol. IXB, p 633. (16) (a) Zhao, J.; Henkens, R. W.; Stonehuerner, J.; O’Daly, J. P.; Crumbliss, A. L. J. Electroanal. Chem. 1992, 327, 109. (b) Henkens, R. W.; Kitchel, B. S.; O’Daly, J. P.; Perine, S. C.; Crumbliss, A. L. Recl. Trav. Chim. Pays-Bas. 1987, 106, 298. (17) Lyon, L. A.; Musick, M. D.; Smith, P. C.; Reiss, B. D.; Pen˜a, D. J.; Natan, M. J. Sens. Actuators, B, in press. (18) (a) Lofas, S. Pure Appl. Chem. 1995, 67, 829. (b) Hutchinson, A. M. Mol. Biotechnol. 1995, 3, 47. (19) Hayat, M. A., Ed. Colloidal Gold: Principles, Methods, and Applications; Academic Press: New York, 1989; Vol. 2. (20) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (21) Bright, R. M.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695-5701. (22) Grabar, K. C.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S. Natan, M. J. Anal. Chem. 1997, 69, 471.

10.1021/la980911a CCC: $18.00 © 1999 American Chemical Society Published on Web 01/14/1999

Colloidal Au-Based Films Scheme 1. Colloidal Au Multilayer

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There are numerous approaches to fabrication of conducting metal wires, surfaces, and/or electrodes,26-35 and there exists a growing body of important work on electrical and electrochemical data on colloidal metal nanoparticles and nanoparticle arrays.36-42 The colloidal Au-based electrodes described in Schemes 1 and 2 combine elements of both fields but exhibit the following distinguishing characteristics. (i) Both types of electrodes are fabricated entirely in solution, removing essentially all constraints on size or shape. (ii) Both types of electrodes exhibit extremely high bulk conductivities and can be used for conventional macroscopic voltammetric measurements. (iii) Both types of electrodes are discontinuous, exhibiting at least some degree of porosity. (iv) Both routes offer good control over electrode film thickness and morphology. (v) Because they are prepared in solution, large batches of identical electrodes can be prepared simultaneously. (vi) Since they can be prepared on glass substrates, both electrode types are amenable to characterization by optical spectroscopy and by atomic force microscopy (AFM). Colloidal Au-derived electrodes constitute an interesting intersection between electrochemistry and nanoscale science: although their electrochemical behavior is best described in macroelectrode terms, and their conductivity is essentially equivalent to bulk Au, they are clearly composed of a collection of nanoparticles. As such, they are a good example of macroscale properties controlled via nanoscale construction. Experimental Section

Scheme 2. Procedure for Electroless Deposition of Au onto Immobilized Colloidal Au

bifunctional organic cross-linker and (ii) colloidal Au nanoparticles.23 In this fashion, conductive Au films can be generated one step at a time. In Scheme 2, selective electroless deposition of Au onto Au nanoparticles in a two-dimensional (2-D) array leads to particle coalescence and bulk conductivity.24,25 (23) Musick, M. D.; Keating, C. D.; Keefe, M. H.; Natan, M. J. Chem. Mater. 1997, 9, 1499. (24) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726. (25) Brown, K. R.; Lyon, L. A.; Fox, A. P.; Reiss B. D.; Natan, M. J. Submitted for publication.

Reagents. The following reagents were purchased from commercial vendors and used without further purification. HAuCl4‚3H2O, NH2OH‚HCl, 2-mercaptoethylamine (MEA), Na3citrate, Na2SO4, and KCl were purchased from Fisher Co., Aldrich, or Acros. Redox probes included potassium ferricyanide (Sigma), hexaamineruthenium(III) chloride (Strem), hydroxymethylferrocene (Strem), and hydroquinone (Eastman). United Chemical Technologies, Inc. supplied 3-mercaptopropyltrimethoxysilane (MPTMS) and 3-aminopropyltrimethoxysilane (APTMS). Methanol, HCl, HNO3, and H2SO4 were sourced from EM Science. J. (26) Ugo, P.; Moretto, L. M.; Bellomi, S.; Menon, V. P.; Martin, C. R. Anal. Chem. 1996, 9, 4160. (27) Kumar, A. Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 9, 1498. (28) Dressick, W. J.; Dulcey, C. S.; Georger, J. H., Jr.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210. (29) Vossmeyer, T.; DeIonno, E.; Heath, J. R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1080. (30) Wang, J.; Naser, N.; Angnes, L. Wu, H. Chen, L. Anal. Chem. 1992, 64, 1285. (31) Yogev, D.; Efrima, S. J. Colloid Interface Sci. 1991, 147, 88. (32) Ducey, M. W., Jr.; Meyerhoff, M. E. Electroanalysis 1998, 10, 157. (33) Moberg, P.; McCarley, R. L. J. Electrochem. Soc. 1997, 144, 151. (34) Xia, Y.; Venkateswaran, N.; Qin, D.; Tien, J.; Whitesides, G. M. Langmuir 1998, 14, 363-371. (35) Bradley, J.-C.; Chen, H.-M.; Crawford, J.; Eckert, J.; Ernazarova, K.; Kurzeja, T.; Lin, M.; McGee, M.; Nadler, W.; Stephens, S. G. Nature 1997, 389, 268. (36) Bethell, D.; Brust, M.;, Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (37) (a) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (b) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (38) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (39) Fox, A. P.; Hooper, A. E.; Allara, D. L.; Natan, M. J. In preparation. (40) (a) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (b) Heath, J. R. Science 1995, 270, 1315. (41) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (42) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313.

846 Langmuir, Vol. 15, No. 3, 1999 T. Baker supplied H2O2. All solutions were made using filtered deionized water (>18 MΩ) purified by either a Barnstead Nanopure or a Barnstead MP-3A Megapure system. Colloidal Au nanoparticles, prepared and analyzed as described previously,20 were imaged using transmission electron microscopy. A mean particle diameter and standard deviation (major axis ) 12.4 ( 1.6 nm and minor axis ) 11.5 ( 1.3 nm) were determined by tunneling electron microscopy (TEM) image analysis using a free program, NIH Image (v. 1.5). Instrumentation. Optical spectra were acquired using either a HP8452 UV-vis or Perkin-Elmer Lambda 9 UV-vis/NIR spectrophotometer. Film resistance was determined with a Fluke 77 multimeter or a Keithley 6587 high-resistance electrometer. A Digital Instruments Nanoscope IIIa operated in tapping mode at 1.5 Hz and 512 lines per inch using TESP cantilevers was used to collect AFM images. All electrochemical measurements were performed using a PAR model 273A potentiostat/galvanostat. A saturates calomel electrode (SCE) or Ag/AgCl (Bioanalytical Systems) reference electrode with a Pt foil or coil counter electrode was used for all experiments. Au disk electrodes (1.6 mm diameter) for control experiments were purchased from Bioanalytical Systems. Heterogeneous electron-transfer rate constants were measured by comparing cyclic voltammograms to simulations generated using Butler-Volmer kinetic theory via the program, Compare, provided by Dr. Stephen Feldberg. Preparation of Colloidal Multilayer Electrodes and Microband Electrodes. Microscope slides (Fisher Premium) were cut to 2.5 cm × 0.8 cm dimensions and sonicated for 20 min in each of the following solvents: soapy dH2O; dH2O; acetone; and methanol. Care was taken to rinse with the next step’s solvent prior to each sonication. Slides were immersed in a 10% (v:v) solution of MPTMS in methanol and rinsed thoroughly with methanol before immersion into colloidal Au. Slides were removed from colloid when an absorbance of 0.145-0.155 at λ ) 526 nm was reached. Multilayers were then constructed by immersion of a colloid monolayer into a 10 mM solution of MEA for 15 min followed by copious rinsing with dH2O and a subsequent 50 min exposure to colloidal Au solution. The process was repeated until the desired number of layers was reached. Slides were dried under Ar, and a Cu contact was affixed to the end of each slide using silver paint (Planned Products). The Cu wire was fed down a Pasteur pipet to provide protection and stability. The pipet tip and any exposed Cu wire or silver paint was covered using white Dexter Epoxi-Patch. Finally, a damp swab was used to wipe the back of the substrate clean. Microband electrodes were fabricated from multilayer electrodes by immersion of the conductive surface into thermally curable epoxy. After hardening, films were scored with a scribe, broken, and checked for voltammetric response. Preparation of Hydroxlamine-Reduced Au Films. Colloidal Au monolayers were prepared as above using APTMS to derivatize the substrate. One side of the substrate was wiped clean of colloid, and the optical spectra were taken of the surface immersed in dH2O. All monolayers were transferred to 148.5 mL of a 0.40 mM solution of NH2OH in a 4 in. Pyrex crystallizing dish and placed on a Lab-Line orbital shaker operated at 120 rpm. A 1% solution of HAuCl4 (1.5 mL) was added to initiate the reaction. After removal, surfaces were immediately washed with dH2O, dried in a stream of Ar gas, and stored. For each surface, an optical spectrum was recorded, after which two electrical contacts for resistance measurements were deposited using a Circuit Works conductive pen (Planned Products). The surfaces were imaged by tapping-mode AFM; a 5 µm × 5 µm scan and three 1 µm × 1 µm images were collected from each surface. Robot Preparation of Au Colloid Multilayers. A HP Analytical Systems radial arm type robot (model 89810A) was used to complete the repetitive dipping necessary to produce colloidal Au multilayers of significant thickness. Although originally designed to carry out ELISA assays, it was programmed using the provided MDS 1.0 software to perform this new task. The glass substrates were functionalized in a 1% methanolic APTMS solution in vials, in the same way that the substrates were made above. All polymeric materials that come in contact with the colloidal Au were first washed in 50% aqua regia and rinsed thoroughly in 18 MΩ H2O. They were then transferred to a home-built PTFE rack capable of holding 2.54 cm × 10.16

Musick et al. cm × 0.1 cm microscope slides. The rack was then placed in a polycarbonate tank containing an approximately 14 nm diameter polydisperse colloidal Au solution. After immersion in the tank for 1 h, it was removed by the robot and rinsed in a flowing 7 MΩ H2O bath for 5 min. The robot proceeded to place the samples into a Delrin tank containing 10 mM MEA for 8 min. The samples were removed, rinsed, and placed back in the colloidal Au. The robot repeated this until 25 layers of colloidal Au were laid down.

Results and Discussion Sample Fabrication and Characterization. A useful aspect of Au colloid monolayers20,43 is that the vast majority of the surface area of the immobilized Au nanoparticles is not involved in bonding interactions with the substrate. Thus, in contrast with colloidal Au particles immobilized in gels or other 3-D matrixes,44,45 which are partly or wholly inaccessible to solution reagents, particles in the monolayer can be chemically addressed. Thus, Au colloid monolayers on SnO2 were shown to be electrochemically addressable43 and to promote direct electrochemistry of cytochrome c in solution.12 Likewise, SERS spectra of analytes adsorbed from solution have been reported.10,21 This chemical accessibility has been exploited to form multilayers of colloidal particles, using the protocol described in Scheme 1.23 Adsorption of 2-mercaptoethylamine (MEA)sa molecule with a functional group at each end that binds strongly to Ausonto a Au colloid monolayer allows a second layer of colloidal Au particles to bind upon reintroduction of the sample into a solution of colloidal Au. Note that in the absence of this (or other46) crosslinker, only a submonolayer of Au nanoparticles will bind to a silanized glass slide. Moreover, incomplete rinsing of the sample after adsorption of MEA leads to wholescale aggregation of the colloidal Au solution. However, when done carefully, this process can be repeated as many times as desired.47 Figure 1 shows the optical spectrum for five different colloidal Au films, harboring 1 (i.e., a monolayer), 4, 8, 14, and 21 layers of colloidal Au. Note that in percent transmission mode, the characteristic surface plasmon band of colloidal Au particle in a monolayer (at ν ≈ 520 nm) appears upside down. As the number of layers increases, there is decreased transmission in the visible and in the near-IR: the 21-layer sample is e25% transmissive from 400 to 2500 nm. The high degree of reflectivity over a broad wavelength range is suggestive of metallic behavior, and indeed colloidal Au multilayers with g15 layers look to the eye like evaporated Au films (Supporting Information). AFM images of Au colloid multilayers (Figure 2) reveal that the film is discontinuous: there are spots with multiple layers of colloidal Au nanoparticles, and other spots with no particles whatsoever. This is true even for samples immersed 20 times in colloidal Au solution. The optical spectra and AFM of an unannealed and annealed 25-layer multilayers are shown in Supporting Information.47 The optical spectra resemble those of Figure 1, with broad, poorly defined (43) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353-2361. (44) Bharathi, S.; Lev, O. Chem. Commun. 1997, 2303-2304. (45) Matsuoka, J.; Mizutani, R.; Kaneko, S.; Nasu, H.; Kamiya, K.; Kadono, K.; Sakaguci, T.; Miya, M. J. Ceram. Soc. Jpn. 1993, 101, 53-58. (46) Pen˜a, D. J.; Musick, M. D.; Botsko, S. L.; Keating, C. D.; Keefe, M. H.; Natan, M. J. In preparation. (47) The repetitive dip approach has been automated using a simple laboratory robot. The Supporting Information shows a photograph of the robot used. Also shown are AFM images and optical spectra before and after annealing for 10 min at 320 °C under N2 of a robotically prepared 25-layer Au colloid multilayer film.

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Figure 1. UV-vis/near-IR transmission spectra of colloidal Au multilayers on MPTMS-coated glass to which an initial layer of 12 nm diameter colloidal Au had been bound followed by successive treatments in 10 mM 2-mercaptoethylamine and exposure to 12 nm diameter colloidal Au (number of treatments indicated on graph).

Figure 3. UV-vis/near-IR transmission spectra of 14 nm diameter colloidal Au monolayers immobilized on APTMScoated glass immersed in 0.4 mM NH2OH/0.01% Au3+ mixture for (a) 7.5, (b) 10, (c) 12.5, (d) 15, (e) 17.5, and (f) 20 min.

Figure 2. Tapping-mode AFM images (1 µm × 1 µm) of representative multilayer electrodes formed as in Figure 1. The number of treatments in 2-mercaptoethylamine/colloidal Au is noted next to each image.

absorbance over the entire visible region. Interestingly, while the AFM data of the unannealed film are similar to that shown in Figure 2, the AFM image of the annealed sample indicates that the colloidal Au particles have fused together to form a continuous, interconnected structure. This complex, porous morphology of the multilayers is potentially interesting for electrochemical measurements: the electrochemical properties of microporous Au electrodes have recently been described.32 Another, more direct route to formation of continuous Au structures involves the straightforward protocol shown in Scheme 2. Immersion of a Au colloid monolayer on glass into a solution of hydroxylamine (NH2OH‚HCl) and Au3+ (added as HAuCl4) leads to selective deposition of Au onto

the colloidal metal particles. As a result, the particles grow in size and eventually coalesce. During this time (≈20 min, a value that depends on the initial particle spacing25), the film transmission decreases dramatically (Figure 3). From ν ≈ 700 to 2500 nm, only a small percentage of the light is transmitted; again, these samples are indistinguishable to the eye from evaporated Au films. Interestingly, the films exhibit a quite sharp maximum in transmission at ν ≈ 500 nm. Comparison to Figure 1 indicates that the same maximum is present in the colloidal Au multilayer films, though not as pronounced. The growth of the NH2OH-treated films can be monitored conveniently by AFM studies of samples removed at various treatment times, as shown in Figure 4. Starting with a submonolayer of 14 nm diameter colloidal Au particles, the particles grow and fuse together over time, with the overall film height remaining e50 nm. The chemistry delineated in both Scheme 1 and Scheme 2 produces thin, ostensibly conductive Au films possessing complex architectures. One advantage to these protocols is that each step, including formation of the initial Au colloid monolayer, is carried out in solution. Thus, it should be possible to grow conductive films in geometries that might otherwise be difficult. For example, it should be easy to produce a consistent coating of Au along the length of a cylindrical optical fiber; at the moment, such films are generated by continuous fiber rotation in the working area of an evaporator. Alternatively, these methods can be used to prepare batches of macroscopic Au electrodes. Electrochemical characterization of the resulting species follows. Electrochemical Properties. Electrochemical responses of colloidal Au multilayer electrodes to various redox probes were evaluated by acquiring cyclic voltammograms for K3[Fe(CN)6], hydroxymethylferrocene, and hydroquinone in aqueous electrolyte. Illustrated in Figure 5 are positive feedback IR-corrected voltammograms obtained using each of these redox probes at 100 mV/s in aqueous 0.1 M Na2SO4. These are well-defined diffusion-

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Figure 4. Tapping-mode AFM images (1 µm × 1 µm) illustrating the growth of a colloidal Au monolayer in a 0.4 mM NH2OH/0.01% Au3+ mixture at (A) 0, (B) 7.5, (C) 12.5, (D) 15, and (E) 20 min.

limited voltammograms, as evidenced by classical t-1/2 current decays and linear ipeak vs square root of potential sweep rate plots (Supporting Information). Peak potential differences (∆Epeak) ranging from 85 mV for K3[Fe(CN)6] to 133 mV for hydroquinone indicate nearly reversible to quasi-reversible electron-transfer kinetics. To investigate the dependence of the electrochemical response on the number of colloidal layers, cyclic voltammograms of 5 mM [Ru(NH3)6]3+ in aqueous 0.1 M Na2SO4 were obtained at colloidal Au working electrodes ranging from 3 to 15 layers, as shown in Figure 6. Below seven layers (not shown), the voltammetry is resistive and poorly defined. However, with a coverage of seven colloid layers the voltammetry becomes sigmoidal, such as that observed at a microelectrode due to radial diffusion of the redox species to the electrode surface. Given the relatively large currents observed, this electrode is likely composed of many small conductive “islands” with nonoverlapping diffusion layers over the time scale of the experiment and thus behaves as a microelectrode array.48 In contrast, current transients of electrodes with greater than seven layers exhibit linear diffusion characteristic of a planar macroelectrode. Here, the electrode may be modeled as either a continuous metal surface or a microelectrode array whose component diffusion layers overlap over the time frame of the experiment due to the higher density of gold “islands” on the electrode surface. (48) Preliminary examination of the electrode surface using electric force microscopy (EFM), a technique capable of discerning conductive and nonconductive areas with nanometer resolution, indicates the presence of conductive islands surrounded by nonconductive material for similarly prepared multilayer samples (five to eight layers). Presumably conductive paths beneath the surface provide electrical contact between islands.

Figure 5. Cyclic voltammograms of multilayer electrodes in 0.1 M Na2SO4 at 100 mV/s for (a) 10 colloidal Au layers in 5 mM [Fe(CN)6]3-, (b) 13 colloidal Au layers in 1 mM hydroxymethylferrocene, and (c) 13 colloidal Au layers in 1 mM benzoquinone. For all electrodes, substrates were MPTMS-coated glass with a monolayer of 12 nm diameter colloidal Au.

The 12-layer voltammogram in Figure 6 yields a peak potential difference (∆Epeak) at 100 mV/s of 92 mV using positive feedback IR compensation, compared to 238 mV without IR compensation. This observations leads to two important conclusions: (1) voltammetry at these electrodes can approach the nearly reversible behavior observed for [Ru(NH3)6]3+ at a bulk Au electrode (∆Epeak ) 78 mV), and (2) colloid multilayer electrodes are resistive enough that IR compensation is required to achieve this approximate reversibility. The fact that IR compensation is successful in lowering ∆Epeak indicates that intrinsic barriers to electron transfer are not the major contributor to peak separations. Two possible sources of the resistivity are poor electrical contact to the Au surface and/or poor conductivity through the cross-linker that binds the colloidal particles together. Previous measurements of dc resistance in these types of samples indicated a resistivity about 80-100 times greater than single-crystal Au of the same thickness.23 Estimated values of k0, the heterogeneous electron-transfer rate constant, for Au colloid

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Figure 6. Cyclic voltammograms of colloidal Au multilayers in 5 mM [Ru(NH3)6]3+ with 0.1 M Na2SO4 supporting electrolyte recorded at 100 mV/s as a function of number of colloidal Au layers (number of colloid layers indicated on graph). Table 1. Comparison of Electrode Area Using Geometric and Electrochemicala Values no. of layers

Ageom (cm2)

Acalc, [Ru(NH3)6]3+ (cm2)

Acalc, [Fe(CN)6]3(cm2)

13 15

1.1 0.94

1.59 1.57

1.66 1.34

a Electrochemical area calculated from potential step experiments (see text).

Scheme 3. Conversion of Colloidal Multilayers to Planar Electrodes and Microelectrodes

mulitlayer electrodes were obtained by comparing experimental values of ∆Epeak to simulation-generated values (using Butler-Volmer kinetic theory)49 as the dimensionless rate constant was varied. The resulting values were reasonably close to values obtained at bulk Au electrodes. For example, we measured a value of 7.9 × 10-3 cm/s for [Fe(CN)6]3- at a bulk Au electrode and 7.6 × 10-3 cm/s at a 10-layer colloidal Au film. A final measure of electrochemical characterization of the multilayer electrodes was afforded through area determination using the Cottrell equation.49 These data are summarized in Table 1 and indicate that the active electrochemical areas are typically 40-60% larger than the geometric areas. The magnitude of current is larger than the geometric area suggests. This is explained by (49) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980.

Figure 7. Cyclic voltammograms of a microband electrode fabricated by coating an Au colloid multilayer surface with an insulating epoxy, breaking, and polishing the end. Each voltammogram was recorded in 5 mM [Ru(NH3)6]3+ in 0.1 M Na2SO4 at the potential sweep rate indicated to the left of each trace. The inset shows a plot of peak current vs the log of sweep rate for a microband electrode.

the model used to determine the active area. The Cottrell model assumes a planar macroscopic electrode and does not consider microscopic roughness or the possibility of complex diffusion patterns that accompany the presence of passivated and highly conducting regimes on the electrode. Indeed, recent experiments have shown that colloid multilayers display fractal properties due to surface roughness or passivation. A detailed fractal analysis that calculates electrode dimensionality through application of a generalized Cottrell equation50 is forthcoming and should offer new insight in active area determinations.51 However, electron transfer through the short MEA spacer is facile, a result consistent with our voltammetry for monolayers of colloidal Au on short organothiol selfassembled monolayers.39 Moreover, the notion that the actual electrode surface area is larger than the geometric area is consistent with the AFM data in Figure 2, which shows a complex morphology. The solution-based approach to formation of conductive Au electrodes can also be extended to the realm of mircoband electrodes by following the additional steps shown in Scheme 3. Briefly, as-prepared Au colloid multilayers were dipped into nonviscous epoxy and thermally cured. Microbands were obtained by producing an exposed end-on Au colloid surface through sanding or cleaving of the encapsulated glass slide. Generally, scoring and cleaving gave a more ideal microelectrode behavior; sanding produced large charging currents and thin-layer faradaic responses consistent with leakage between the epoxy and the colloid surface. Examples of microelectrode voltammetry acquired at such a band electrode are shown in Figure 7. These voltammograms were obtained in 5 mM [Ru(NH3)6]3+ in 0.1 M aqueous Na2SO4, and exhibit classical limiting currents indicative of radial diffusion at (50) Pajkossy, T.; Nyikos, L. New J. Chem. 1990, 14, 233-237. (b) Pajkossy, T. J. Electroanal. Chem. 1991, 300, 1-11. (51) Wehmeyer, K. R.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1985, 57, 1913-1916. (52) (a) Brandow, S. L.; Chen, M.-S.; Wang, T.; Dulcey, C. S.; Calvert, J. M.; Bohland, J. F.; Calabrese, G. S.; Dressick, W. J. J. Electrochem. Soc. 1997, 144, 3425-3434. (b) Hokke, J. W.; Wel, H. V.; With, G. D. J. Electrochem. Soc. 1993, 140, 682-686. (c) Bradley, J.-C.; Crawford, J.; Ernazarnova, K.; McGee, M.; Stephend, S. G. Adv. Mater. 1997, 9, 1168-1171. (d) Fleury, V. J. Mater. Res. 1991, 6, 1169-1174.

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prepared is monodispersed with less than 10% size deviation and can be covalently attached to an appropriately functionalized substrate. Second, the electrode surface is not covered with an organic adsorbate, as is the case with Au colloid multilayers (i.e., MEA). Third, the number of steps required to generate these electrodes (Scheme 2) is very small compared to the multidip mutilayer films. Finally, and most importantly, no IR compensation was required. Previous data have shown that films grown by this method are extraordinarily conductive (i.e., within a factor of 10 of pure Au, and far more conductive than SnO2, C, etc.),24 and this is borne out in the very sharp voltammetry. Conclusions

Figure 8. Cyclic voltammograms of Au electrodes made by exposing a colloidal Au monolayer to a 0.4 mM NH2OH/0.01% Au3+ mixture for (a) 7.5, (b) 10, (c) 12.5, (d) 17.5, (e) 20 min taken in 5 mM [Ru(NH3)6]3+ in 0.1 M Na2SO4 at a sweep rate of 50 mV/s. Inset shows the peak current vs square root of the sweep rate for the electrode developed in the NH2OH/Au3+ mixture for 7.5 min.

sweep rates ranging from 20 to 500 mV/s. The band electrodes display a scan rate dependence in which contributions from both radial and linear diffusion are significant as shown in the inset to Figure 7.51 Colloidal Au films prepared by NH2OH-mediated particle growth and coalescence also exhibit well-behaved macroelectrode voltammetry. Figure 8 shows voltammetry for [Ru(NH3)6]3+ at Au electrodes prepared by treatment of Au colloid monolayers on silanized glass with Au3+/ NH2OH for increasing lengths of time. Two phenomena are evident: a factor of 5 increase in peak currents, and a narrowing of the peak-to-peak separation (from 150 to under 90 mV). The inset to Figure 8 shows that the expected ν1/2 dependence on peak currents was observed. These colloidal Au films offer several advantages as bulk electrodes. Other deposition methods depend on the nucleation of Pd catalyst onto the surface before further metal reduction can occur52a or require the presence of two spatially separated electrodes.52b Substrates for Pd activation require extensive pretreatment, suffer from substrate-film adhesion failure, and lack film reproducibility due to particle polydispersity. Colloidal Au as

Two solution-based approaches to prepare conductive Au films on glass are reported. Both types of films (Au colloid multilayers and hydroxylamine-Au3+ grown Au particles) exhibit optical properties consistent with bulk conductivity. This is verified by voltammetric measurements on several redox probes, which demonstrate that viable Au electrodes can be prepared by either method. A band microelectrode was fabricated by insulating the top face of a Au colloid multilayer and exposed the electrode end. Conductive films produced by NH2OH-mediated growth of Au colloid monolayers yielded excellent voltammetry. Together, these results suggest that particulate nanomaterials, especially when they are conductivesas is the case herescomprise good building blocks for electrodes. When one considers the voluminous amount of published material on protein/Au colloid complexes,19 the opportunities for stepwise construction of electrochemical biosensors using the appropriate protein-coated nanometallic building blocks are substantial. Work along these lines is in progress. Acknowledgment. Support from NSF (CHE-9256692, CHE-9627338), NIH (GM55312-01), and the Beckman Foundation is gratefully acknowledged. Acknowledgment is also made to the Electron Microscopy Facility for the Life Sciences in the Biotechnology Institute at The Pennsylvania State University, and to SmithKline Beecham for the gift of the benchtop robot. Supporting Information Available: Photographs of Au colloid multilayers and of robot, optical spectra, and AFM images of robot-generated Au colloid multilayers before and after annealing, and scan rate dependence of peak current for Au colloid multilayers. This material is available free of charge via the Internet at http://pubs.acs.org. LA980911A