Preparation and Characterization of a Porphyrin Self-Assembled

A porphyrin-incorporated self-assembled monolayer (SAM) on gold, in which the macroplane of the porphyrin molecule assumes a nearly flat orientation w...
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Langmuir 2000, 16, 537-540

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Preparation and Characterization of a Porphyrin Self-Assembled Monolayer with a Controlled Orientation on Gold Zhijun Zhang,† Shifeng Hou, Zihua Zhu, and Zhongfan Liu* Center for Nanoscale Science and Technology, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China Received May 11, 1999. In Final Form: August 9, 1999 A porphyrin-incorporated self-assembled monolayer (SAM) on gold, in which the macroplane of the porphyrin molecule assumes a nearly flat orientation with respect to the surface of the Au substrate, was prepared by formation of a 4-pyridinethiol SAM on gold followed by axial ligation of metalloporphyrin (CoTPP) to it. The porphyrin SAM was characterized by surface-enhanced Raman scattering (SERS) and electrochemical techniques.

Introduction There have been continuous efforts toward the preparation of suitably tailored solid films of porphyrins and related compounds due to their great potentials in materials science for optical,1 photoelectrochemical,2 and chemical sensor3 applications, in bioseparation,4 in medicine for clinical applications,5 and as ideal models for electron transfer and electrocatalysis reactions.6-9 Among the many techniques used to fabricate thin films of porphyrins on solid substrates, that is, LangmuirBlodgett,10,11 dip-coating,12 spin-coating,13 electropolymerization,14 and thermoevaporation,15 the self-assembled monolayer (SAM) approach has the advantage over other methods in that it is capable of fabricating ultrathin solid films with well-controlled structures and superior thermal and mechanical stability.10 Moreover, incorporating porphyrin molecules in SAMs on solid substrates enables us * To whom correspondence should be addressed. Tel. and Fax: +86-10-62757157. E-mail: [email protected]. † Present address: Research Center for Materials Science, Nagoya Univeristy, Chikusa, Nagoya 464, Japan. E-mail: zjzhang@ chem2.chem.nagoya-u.ac.jp. (1) Nalwa, H. S. Adv. Mater. 1993, 5, 341. (2) Chau, L.-K.; Arbour, C.; Collins, G. E.; Nebesny, K. W.; Lee, P. A.; England, C. D.; Armstrong, N. R.; Barkinson, B. A. J. Phys. Chem. 1993, 97, 2690. (3) Malinski, T.; Taha, Z. Nature 1992, 358, 676. (4) Xiao, J.; Meyerhoff, M. E. Anal. Chem. 1996, 68, 2818. (5) Sinaasappel, M.; Ince, C. J. Appl. Physiol. 1996, 81, 2297. (6) Uosaki, K.; Kondo, T.; Zhang, X.-Q, Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (7) Zak, J.; Yuan, H. P.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (8) (a) Hutchison, J. E.; Postlehwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277. (b) Postlehwaite, T. A.; Hutchison, J. E.; Hathcock, K. W.; Murray, R. W. Langmuir 1995, 11, 4109. (c) Hutchison, J. E.; Postlehwaite, T. A.; Chen, C.-H.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W. Langmuir 1997, 13, 2143. (9) Durand, R. R.; Bencosme, C. S.; Collman, J. P.; Anson, F. C. J. Am. Chem. Soc. 1983, 105, 2710. (10) Ulman, A. An Introduction to Ultrathin Organic Films, from Langmuir-Blodgett Films to Self-Assembly; Academic Press: San Diego, CA, 1991. (11) Langmuir-Blodgett Films; Roberts, G. G. Ed.; Plenum Press: New York and London, 1990. (12) Araki, K.; Wagner, M. J.; Wrighton, M. S. Langmuir 1996, 12, 5393. (13) Kampas, F. J.; Yamashita, K.; Fajer, J. Nature 1980, 284, 40. (14) Curran, D.; Grimshaw, J.; Perera, S. D. Chem. Soc. Rev. 1991, 20, 391. (15) Manivannan, A.; Nagahara, L. A.; Hashimoto, K.; Fujishima, A.; Yanagi, H.; Kouzeki, T.; Ashida, M. Langmuir 1993, 9, 771.

to investigate the films without considering some troublesome issues, such as porphyrin aggregation16-19 and formation with oxygen of µ-oxo-complexes,20 which frequently occurred in the LB films and/or in solutions. There are several strategies that have been used to prepare porphyrin SAMs on gold through the synthesis of porphyrin-linked alkylthiols7,8,21,22 and isothiocyanosubstituted porphyrin23 and on oxide surfaces by a surface reaction of porphyrin with chemically modified silica4 or quartz.24-26 Construction of SAMs of porphyrins with a known orientation attracts considerable interest from researchers,7,8,24-26 especially those investigating the electrochemical and catalytic aspects of porphyrinincorporated SAMs.7,8 By formation of SAMs of thiolderivated porphyrins on a gold electrode, Porter et al.7 observed that the SAMs of porphyrin oriented perpendicular to the electrode surface show more efficient electrocatalysis of dioxygen reduction than those of porphyrin oriented parallel to the electrode surface. Murray and his colleagues8 have been investigating electrocatalytical dioxygen reduction of SAMs of porphyrin lying flat on the electrode surface. The porphyrin derivatives that form the oriented SAMs exploited by Porter (16) Gust, D.; Moore, T. A.; Moore, A. L.; Luttrull, D. K.; DeGraziano, J. M.; Boldt, N. J.; Auweraer, M. V.; De Schryver, F. C. Langmuir 1991, 7, 1483. (17) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (18) (a) Zhang, Z.-J.; Verma, A. L.; Yoneyama, M.; Nakashima, K.; Iriyama, K.; Ozaki, Y. Langmuir 1997, 13, 4422. (b) Zhang, Z.-J.; Verma, A. L.; Nakashima, K.; Yoneyama, M.; Iriyama, K.; Ozaki, Y. Langmuir 1997, 13, 5726. (19) (a) Katz, J. J.; Sheer, H. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 393. (b) Barber, D. C.; Freitag-Beeston, R. A.; Whitten, D. G. J. Phys. Chem. 1991, 95, 4074. (20) (a) Silver, J.; Lukas, B. Inorg. Chim. Acta 1983, 78, 219. (b) James, B. R. InThe Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 5, p 231. (21) (a) Akiyama, T.; Imahori, H.; Sakata, Y. Chem. Lett. 1994, 1447. (b) Imahori, H.; Norieda, H.; Ozawa, S.; Ushida, K.; Yamada, H.; Azuma, T.; Tamaki, K.; Sakata, Y. Langmuir 1998, 14, 5335. (22) Guo, L.-H.; Mclendon, G.; Razafitrimo, H.; Gao, Y. L. J. Mater. Chem. 1996, 6(3), 369. (23) Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L. Langmuir 1996, 12, 5742. (24) Li, D.; Moore, L. W.; Swanson, B. I. Langmuir 1994, 10, 1177. (25) Li, D.; Swanson, B. I.; Robinson, J. M.; Hoffbauer, M. A. J. Am. Chem. Soc. 1993, 115, 6975. (26) Pilloud, D. L.; Moser, C. C.; Reddy, K. S.; Dutton, P. L. Langmuir 1998, 14, 4809.

10.1021/la990570g CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/1999

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Figure 1. Chemical structure of CoTPP.

and Murray have two or more thiol appendages on the same side of the porphyrin plane and are difficult to synthesize and separate from the other atropisomers.7,8,27,28 Here, we report a simple and widely applicable strategy for preparing porphyrin monolayer films on gold via ligation of metalloporphyrin to a 4-pyridinethiolate SAM on a gold surface, by modification of the method described previously by Li and his collaborators.24 Surface-enhanced Raman scattering and electrochemical techniques were employed to characterize the structure of the film, and we have found that the porphyrin plane in the SAM assumes a flat orientation to the surface of the gold substrate. We intend to employ such an oriented porphyrin film on a metal surface as a model system to study molecular interactions such as π-π interactions, using the CFM technique developed by Lieber and co-workers.29,30 The work on molecular interaction is now in progress and will be reported in a separate paper. Experimental Section Chemicals and Solvents. Cobalt(II) 5,10,15,20-tetrakisphenyl-porphyrin (CoTPP) (as shown in Figure 1) and 4-pyridinethiol were purchased from Aldrich and used as received. Ethanol, chloroform, and other organic solvents used were spectroscopic grade. Ultrapure water (>16 MΩ‚cm) was used throughout the experiment. Formation of a SAM of 4-Pyridinethiol on Gold. Gold films 200 nm thick sputtered on silicon substrates were cleaned in piranha solution (concentrated sulfuric acid:30% H2O2 3:1 in volume) at 90 °C for 5 min then rinsed thoroughly with ultrapure water and dried under pure N2. Thus-treated substrates were immersed into an ethanol solution of 4-pyridinethiol (1 × 10-3 M) for 5 min. The prepared pyridinethiol SAMs on gold underwent 3 min sonication in ethanol before being used for reacting with the CoTPP solution. Preparation of Porphyrin Thin Films by Axial Coordination. Ligation of CoTPP with the pyridine SAMs on gold was achieved by immersing the 4-pyridinethiolate SAMs on gold into a chloroform solution of CoTPP (1 × 10-4 M) for ca. 72 h. Then, the porphyrin-covalently bonded SAMs were rinsed thoroughly with chloroform. The samples were then ultrasonically treated in chloroform for 3 min. The freshly prepared samples were immediately used for SERS and electrochemical measurements. SERS Measurement. The SERS-active samples were prepared according to the previous literature.31,32 Briefly, the porphyrin-based SAMs on gold were immersed for 6 h at room temperature into a colloidal solution of gold having a size of ca. (27) Collman, J. P.; Groh, S. E. J. Am. Chem. Soc. 1982, 104, 1391. (28) Young, R.; Chang, C. K. J. Am. Chem. Soc. 1985, 107, 898. (29) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (b) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (30) Akari, S.; Horn, D.; Keller, H.; Schrepp, W. Adv. Mater. 1995, 7, 549.

Figure 2. SERS spectrum of a 4-pyridinethiolate SAM on a gold substrate with a Au nanoparticles assemble overlayer. 40 nm, characteristic of a peak at 531 nm in the UV-vis spectrum, which was prepared according to the sodium citrate procedure proposed by Frens.31 Then they were withdrawn, rinsed with ultrapure water, and dried with N2 gas. The samples treated in this manner were used immediately for Raman measurement. Raman spectra were taken on a Renishaw System 1000 Raman imaging microscope (Renishaw plc., UK) equipped with a 25 mW (632.8 nm) He-Ne laser (model 127-25 RP, Spectra-Physics) and a Peltier-cooled CCD detector (576 pixels × 384 pixels). A 50× objective mounted on an Olympus BH-2 microscope was used to focus the laser beam onto a spot of approximately 1 µm in diameter (the laser power was 0.06 mW at the sample) and to collect the backscattered light from the sample. Electrochemistry. All electrochemical measurements were performed with a CH Instruments Electrochemical Workstation at 25 ( 1 °C, using Ag/AgCl as the reference electrode, a platinum wire as the counter electrode, and the CoTPP-bonded 4-pyridinethiolate SAM on the gold plate as the working electrode. The geometric area of the electrode of 0.135 cm2 was determined by the method described by Inzelt33 and used to estimate the surface coverage of the porphyrin SAMs on Au. The supporting electrolyte was 0.1 mol/L HClO4 solution.

Results and Discussion Preparation of SAMs with CoTPP as an Overlayer. Formation of the porphyrin-linked 4-pyridinethiolate SAMs on the gold surface can be divided into two steps. As the first step, we prepared 4-pyridinethiolate SAMs and identified them by SERS measurement, following our previous work.32 A SERS spectrum of a Au nanoparticle layer coupled with a pyridinethiolate SAM on a gold surface is given as Figure 2. Peaks at 1004 cm-1, assignable to pyridine ring breathing; 1096 cm-1, the X-sensitive band; 1206 cm-1, due to the in-plane C-H deformation of the pyridine ring; and ca. 1616 cm-1, attributable to the CdC stretching vibration, are clearly seen in the SERS spectrum. The SERS spectrum, in the present case, is much different from the Raman spectrum of the bulk 4-pyridinethiol but is consistent with previous reports on SERS spectra of 4-pyridinethiolate SAMs on Au electrodes,32,34,35 indicating formation of a 4-pyridinethiolate SAM on gold. (31) Frens, G. Nat. Phys. Sci. 1973, 20. (32) Zhu, T.; Zhang, X.; Wang, J.; Fu, X. Y.; Liu, Z. F. Thin Solid Films 1998, 327-329, 595. (33) Inzelt, G. in Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994. (34) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (35) (a) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J. Electroanal. Chem. 1985, 186, 299. (b) Taniguchi, I.; Yoshimoto, S.; Nishiyama, K. Chem. Lett. 1997, 353.

A Porphyrin Self-Assembled Monolayer on Gold

Figure 3. (a) SERS spectrum of the CoTPP-based SAM on Au covered with a Au nanoparticles assemble overlayer and (b) Raman spectrum of bulk CoTPP. Table 1. Raman Frequencies (cm-1) and Assignments for CoTPP CoTPP solid

SERS of the CoTPP SAM

1008 1021 1079 1253 1365 1381 1470 1504 1543 1576

1003 1025 1087 1245 1356 1372 1497 1563

band assignment ν(CR-Cm) phenyl δ(Cβ-H) ν(CR-N) δ(Cβ-H) + ν(CR-N) ν(CR-Cβ) ν(CR-Cβ) + δ(Cβ-H) ν(Cβ-Cβ) ν(CR-Cβ) ν(Cβ-Cβ) + δ(Cβ-H)

Figure 3 depicts (a) a SERS spectrum of a CoTPPpyridinethiolate SAM on gold covered with a layer of Au nanoparticles and (b) a Raman spectrum of the bulk CoTPP. The Raman frequencies and their assignments are given in Table 1.36 The resemblance of the Raman spectrum of solid CoTPP to the SERS spectrum of the porphyrin overlayer film suggests that the porphyrin monolayer was successfully formed onto the SAM of 4-pyridinethiol on gold. It is well-known that the SERS technique is a useful tool in understanding molecular identities and the orientation of adsorbed species on a metal surface.35,36 Variations in some bands in the SERS spectrum compared to the corresponding solid spectrum indicate that the structure of the porphyrin SAM on gold is different from that of the porphyrin in the solid state. Deduction of the information regarding porphyrin orientation by the SERS measurement in the present case becomes rather difficult due to possible coupling between the Au nanoparticles overlayer and the Au film underlayer and to that between electromagnetic and chemical enhancement. The porphyrin molecules are chemically bonded to the metal surface through a pyridinethiolate bridge. Other techniques are required to get a clear understanding of the porphyrin orientation (vide infra). The present work also reveals that the existence of the Au nanoparticles overlayer is mainly, if not completely, responsible for the observation of the SERS spectra of the porphyrin monolayer, since we did not observe a discernible SERS effect for the same porphyrin SAM on gold without the Au nanoparticles overlayer. (36) (a) Houston, C. M.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1992, 96, 6585. (b) Cotton, T. M.; Schultz, S. G.; Van Duyne, R. P. J. Am. Chem. Soc. 1982, 104, 6528. (c) Stein, P.; Ulman, A.; Spiro, T. G. J. Phys. Chem. 1984, 88, 369.

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Figure 4. Cyclic voltammograms of (a) bare Au, (b) a SAM of 4-pyridinethiol on Au, and (c) a CoTPP-bonded 4-pyridinethiolate SAM on Au.

Electrochemistry of the Porphyrin-Overlayered SAMs. Surface-confined porphyrins and related compounds have been studied in regard to electrochemical charge transfer and electrocatalysis.6-8,21 Electrochemical measurements can also provide insight into the structure of the electroactive films adsorbed onto the electrode surface. Figure 4 presents cyclic voltmmograms of (a) bare Au, (b) 4-pyridinethiolate SAM on Au, and (c) CoTPPappended 4-pyridinethiolate SAM on Au in 0.1 mmol/L HClO4 solution. It is clearly seen that the formation of a pyridine SAM on Au remarkably reduced the capacitive charging current of the gold electrode, a phenomenon often observed for alkanethiol-modified metal electrodes.38 There was no reduction or oxidation peak at the pyridinethiolate Au electrode within the potential range examined. In contrast, a pair of reduction and oxidation peaks appeared for the pyridinethiolate SAM after immersion in a chloroform solution of CoTPP for several days, suggesting formation of a porphyrin film on Au. The reduction and oxidation peak potentials are 0.065 and 0.502 V, respectively. The peak potentials were found to remain unaltered with an increasing scan rate of the potential, and the peak currents are proportional to the scan rates over the range of 10 to 100 mV/s, indicative of a surface electrochemical reaction. The cyclic voltammograms of the CoTPP SAM electrode remained stable after 200 scans, suggesting electrochemical stability of the porphyrin-coated film. The surface coverage of redox-active centers on the electrode (Γ) was calculated to be (8.17 ( 1.6) × 10-11 mol/cm2 by integrating charges (Q) passing on each reduction peak at 10 mV/s, according to the following equation:38

Γ ) Q/nFA

(1)

where A is the electrode surface area, and F and n are Faraday’s constant and the number of electrons involved in the electrode reaction, respectively. The measured value of CoTPP coverage is fully consistent with that for the parallel-oriented porphyrin film on metal surfaces deduced from electrochemical8,39 and (37) (a) Brolo, A. G.; Irish, D. E.; Lipkowski, J. J. Phys. Chem. 1997, 101, 3906. (b) Brolo, A. G.; Smith, B. D.; Irish, D. E. J. Mol. Struct. 1997, 101, 3906. (c) Murty, K. V. G. K.; Venkataramanan, M.; Prodeep, T. Langmuir 1998, 14, 5446. (38) (a) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996. (b) Hamann, C. H.; Hamnett, A.; Vielstich, W. Electrochemistry; Wiley-VCH: Weinheim, New York, Chichester, Brisbane, Singapore, and Toronto, 1998. (39) Van Galen, D. A.; Majda, M. Anal. Chem. 1988, 60, 1549.

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spectroscopic8 measurements of porphyrins similar to ours. This leads us to a conclusion that the porphyrin molecules in the SAM were oriented nearly flatly to the electrode surface, that is, to the gold surface. A perpendicular orientation of the porphyrin molecule to the electrode surface seems unlikely since it will give a much higher surface coverage of 3.3 × 10-10 mol/cm2.39 It should be pointed out that a true electrode area, instead of the geometric surface area, is required in estimating the surface coverage from which the molecular orientation can be deduced. A true electrode area is, however, difficult to determine due to microscopic roughness of the Au electrode surface, so the surface coverage obtained has some uncertainty. Also considering the differences in the structure of the porphyrins and the methods employed by us from others mentioned above, it has to be noted that further experiments, including IR and XPS, combined with our electrochemistry and SERS data, are needed to obtain more accurate information regarding porphyrin orientation in the SAMs on gold. Orientation of Pyridine Molecules with Respect to the Au Surface. There is no doubt that the orientation of porphyrin molecules is largely dependent on that of the pyridine molecule to the Au surface, namely, on the position of the nitrogen atom of the pyridine that coordinates to the central metal Co of the porphyrin core. SAMs of 4-pyridinethiols on metal surfaces attract considerable interest from bioelectrochemists because of their ability to facilitate direct electron transfer of metalloproteins such as cytochrome c.34,40-42 The orientation of the pyridine ring with respect to the electrode surface reported by different research groups, however, is not unambiguous. Osawa et al.43 reported, from their STM results, that in the SAMs of 4-pyridinethiolate on Au(111), the pyridine plane lies nearly parallel to the Au surface. Another Japanese group,44 however, based also on their STM observation of 4-pyridinethiolate SAMs on Au(111), proposed that the pyridine ring is oriented almost vertical to the (111) surface, and the molecular axis through the N and S atoms is tilted appreciably to the surface normal. The latter conclusion is in good agreement (40) Christensen, P. A.; Hamnett, A. J. Electroanal. Chem. 1991, 318, 407. (41) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736. (42) Taniguchi, I.; Yoshimoto, S.; Nishiyama, K. Chem. Lett. 1997, 353. (43) Wan, L. J.; Hara, Y.; Noda, H.; Osawa, M. J. Phys. Chem. B 1998, 102, 5943. (44) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Langmuir 1998, 14, 3565.

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with other previous literature results for the SAMs of 4-pyridinethiolate32,34,40-42,45 and related compounds37c,46 on Au or Ag surfaces. Although it is difficult to verify the pyridine orientation directly from our result, we are still able to draw some useful information regarding the pyridine orientation. Considering the geometry of tetraphenylporphyrins and the M-N bond length of 1.9-2.2 Å for the central metal (M) and the N atom of the axial ligand,47 the ligation of the Co atom of the porphyrin core with the N atom of pyridine indicates that the pyridine plane is not oriented parallel to the surface. The nitrogen atom of pyridine should be in a position that allows access of the metalloporphyrin to it. Taniguchi et al.35b investigated the effect of the pyridine structure on the interaction between cytochrome c and 4- and 2-pyridinethiol SAM on a flat Au electrode and stated that the nitrogen atom of the pyridine faced to the solution interacted with cytochrome c at the electrode in the case of 4-pyridinethiol SAM on Au, while no interaction occurred in the case of the 2-pyridinethiol SAM on gold. The nearly flat orientation of the porphyrin in the present work suggests that both the pyridine plane and the axis through the N and S atoms is tilted, at least to some extent, to the Au surface. We are as yet uncertain as to what influences the chemical bonding of the porphyrin to the pyridine have on the structures of the pyridine SAM and how the features of SERS of the SAM compared with its solid spectrum is related to such structural changes. In conclusion, we have prepared porphyrin-overlayered SAMs on gold, in which the porphyrin molecules assume a roughly flat orientation with respect to the gold surface. Given the fact that it is easy to modify the structure of the porphyrin and the axial ligands used, the method assumes general significance for fabricating porphyrin monolayer films with a desired arrangement and properties as models for various purposes of study. In addition, we discussed the orientation of the pyridine on the basis of our experiments, which may help in clarifying the existing debate on the issue. Work in progress is addressing issues such as orientations of the pyridine and porphyrin monolayer films on the metal surface, taking account of uncertainties that are not so clear in the present paper, by means of IR and XPS techniques. LA990570G (45) Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1990, 292, 245. (46) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (47) Salzmann, R.; Ziegler, C. J.; Godbout, N.; McMahon, M. T.; Suslick, K. S.; Oldfield, E. J. Am. Chem. Soc. 1998, 120, 11323.