Influence of Gold Topography on Carboxylic Acid Terminated Self

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Influence of Gold Topography on Carboxylic Acid Terminated Self-Assembled Monolayers Michael C. Leopold,† Joseph A. Black, and Edmond F. Bowden* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received November 16, 2001 Film permeability and double layer capacitance (Cdl) results are reported for mercaptotetradecanoic acid self-assembled monolayers (C13COOH SAMs) on gold substrates spanning a range of topography. Whereas film permeability was observed to decrease with increasing substrate smoothness, indicative of lower defect density in the SAMs, the capacitance, unexpectedly, was observed to increase as the topography became smoother. To explain these results, a simple structural model is proposed in which the extent of hydrogen bonding (H-bonding) among carboxylic acid endgroups is related to the underlying gold substrate topography. The model predicts that more extensive H-bonding will occur as substrates become smoother which, in turn, impacts the dielectric properties of the film. The model also provides an explanation for the discordant pKa results that have heretofore been reported for COOH SAMs. Altered surface acidity of up to 3 pKa units appears to be attributable to substrate topography.

As part of our ongoing investigation of the electron transfer (ET) reactions of cytochrome c adsorbed on goldsupported carboxylic acid terminated self-assembled monolayers (COOH SAMs),1 we have examined the possible role played by substrate topography. Some unexpected capacitive behavior was observed that leads us to propose a physical model in which gold substrate topography significantly impacts the extent of interfacial hydrogen bonding (H-bonding) and the resulting film structure. We furthermore propose that topographical variability is the primary cause of the lack of agreement among pKa values that have been reported for homogeneous COOH SAMs (i.e., pKa = 5.2 T 11.5).2 Prior work has shown that the surface pKa (more accurately, pK1/2) values of COOH SAMs typically fall alkaline of the solution values (pKa ) 4-5) due to several factors including a lower effective dielectric constant for the flanking alkane region, Coulombic repulsion among carboxylate groups, doublelayer potential effects, and H-bonding.3,4 In this paper, we present an account of how gold substrate topography appears to dramatically influence interfacial H-bonding, film structure, and acid/base properties of COOH SAMs. * To whom correspondence should be addressed. Email: [email protected]. Phone: (919) 515-7069. Fax: (919) 515-5079. † Present address: Department of Chemistry, University of North Carolina, CB# 3290, Kenan Laboratories, Chapel Hill, NC 27599. (1) (a) Bowden, E. F. Interface 1997, 6, 40-44. (b) Leopold, M. C.; Bowden, E. F. Langmuir, in press. (2) (a) Smalley, J. F.; Chalfant, K.; Feldberg, S. W.; Nahir, T. M.; Bowden, E. F. J. Phys. Chem. B 1999, 103, 1676-1685. (b) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114-5119. (c) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224-5227. (d) Sugihara, K.; Shimazu, K.; Uosaki, K. Langmuir 2000, 16, 7101-7105. (e) Shimazu, K.; Teranishi, T.; Sugihara, K.; Uosaki, K. Chem. Lett. 1998, 669-670. (f) Kakiuchi, T.; Iida, M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397-5401. (g) Godinez, L. A.; Castro, R.; Kaifer, A. E. Langmuir 1996, 12, 5087-5092. (h) Zhou, J.; Luo L.; Yang, X.; Wang, E.; Dong, S. Electroanalysis 1999, 11, 1108-1111. (i) Kane, V.; Mulvaney, P. Langmuir 1998, 14, 3303-3311. (3) (a) Aoki, K.; Kakiuchi, T. J. Electroanal. Chem. 1999, 478, 101107. (b) White, H. S.; Peterson, J. D.; Cui, Q.; Stevenson, K. J. J. Phys. Chem. B 1998, 102, 2930-2934. (c) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675-3683. (d) Fawcett, W. R.; Andreu, R. J. Phys. Chem. 1994, 97, 12753. (e) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1-3. (4) (a) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378. (b) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741-749.

Film permeability and double layer capacitance (Cdl) were measured for mercaptotetradecanoic acid (HS(CH2)13COOH) SAMs on gold substrates of varied topography. In order of decreasing roughness, the following substrates were used: electrochemically pretreated evaporated gold films, annealed bulk gold foil, Au(111) single crystal, and epitaxial gold on mica. These four substrates were characterized by electrochemical roughness factor (Rf) determination, scanning probe microscopy (SPM), and lead underpotential deposition (UPD) stripping voltammetry. Rf, the ratio of real to geometric surface area, was determined by established electrochemical techniques5 based on measurement of the charge required to strip a gold oxide layer. Surface topography was also evaluated using SPM techniques and found to be in good agreement with the electrochemical results. Pb UPD results confirmed that the predominant orientation at these substrate surfaces was Au(111). More details on these topics can be found in Supporting Information. Film permeability was evaluated by cyclic voltammetry (CV) of hydroxymethyl ferrocene (HMFc), a sensitive solution probe of permeability and defect density in SAMs.6 Enhanced blocking of the HMFc ET reaction is manifested by attenuation of faradaic current and a positive shift of the anodic peak potential (Ep,a). This characteristic behavior was observed at all four topographic types of gold modified by C13COOH. CVs of HMFc at the modified gold substrates exhibited an almost completely suppressed cathodic peak and an attenuated and positively shifted anodic peak. The Ep,a values in Table 1 reveal that the presence of a COOH SAM causes the anodic overpotential to increase and the permeability to therefore decrease as the gold becomes smoother. This finding is not surprising in light of the conventional view that smoother substrates typically result in more uniform and densely packed SAMs.7 (5) (a) Oesch, U.; Janata, J. Electrochim. Acta 1983, 28, 1237-1246. (b) Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1990, 35, 117-125. (6) Creager, S.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854861. (7) (a) Finklea, H. O. Electroanal. Chem. 1996, 19, 114-118. (b) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884. (c) Guo, L.; Facci, J. S.; McLendon, G.; Mosher, R. Langmuir 1994, 10, 4588-4593.

10.1021/la011683e CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002

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Table 1. HMFc Voltammetry and Cdl Results for COOH SAMs on Goldd substrate evap. Au (ref) C14OOH/evap. Au C14OOH/bulk Au C14OOH/Au(111) C14OOH/Au/mica

HMFc Cdl Cdl Rfa Ep,ab (mV)c CVc (µF/cm2) EISc (µF/cm2) 2.1 2.1 1.5 1.1 1.0

230 348 388 384 419

2.8 5.4 7.6 15.0

1.9 3.9 5.5 13.5

Rf values were normalized to Au/mica (Rf ≡ 1.0). bThe relative standard deviation of Ep,a was ∼2%; 0.10 M HClO4. c4.4 mM potassium phosphate buffer (µ ) 10 mM; pH ) 7). dSee Supporting Information for additional experimental details. a

Figure 1. Cdl measurements made by CV for HS(CH2)13COOH SAMs on electrochemically cleaned evaporated gold (9), annealed bulk gold (b), Au(111) single crystal (2), and gold epitaxially grown on mica ([).

Table 1 also lists Cdl values obtained by both CV and electrochemical impedance spectroscopy (EIS). As gold surfaces became smoother, Cdl was observed to increase (also see Figure 1). This finding was unexpected. The permeability and capacitance results seem contradictory if smoother gold is assumed to result in more densely packed films with fewer defects. Such an assumption would lead to prediction of the observed permeability trend but not the unusual capacitance trend. In contrast to these COOH SAM results, we observed no unusual capacitive behavior when SAMs comprised of unsubstituted alkanethiols (HS(CH2)nCH3) were examined for a range of substrates (see Supporting Information).8 Clearly, the COOH group is able to exert a profound influence on film structure and properties. Although the chain-length dependence of these phenomena has yet to be ascertained in any detail, identical capacitance and permeability trends were detected at the one other chain length tested, namely, HS(CH2)10COOH. The trends were, however, less pronounced. C10COOH SAMs were, in fact, tested first, but effort soon shifted to the longer C13COOH SAMs, which were found to exhibit a greater sensitivity to substrate variability. To rationalize these trends, we propose the simple model for COOH-terminated SAMs shown in Figure 2. For smoother substrates, we expect the acid termini to be spatially restricted to a tighter molecular plane, a situation presumed to engender more extensive hydrogen bonding (8) (a) Leopold, M. C. PhD Dissertation, North Carolina State University, 2000. This document is available electronically at http:// www.lib.ncsu.edu/etd/author/l.html. (b) For example, using bulk gold foil, Rf ) 1.2 after hydrogen flame annealing and 1.7 after electrochemical pretreatment.

Figure 2. Hypothetical model for COOH SAMs highlighting the proposed role of microscopic substrate topography. (A) On smooth substrates, spatial constraints engender extensive hydrogen bonding interactions among COOH groups (and water molecules2c) giving rise to a high degree of structural order at the surface of the SAM. The alkane portion of the SAM is believed to be in a state of relative disorder, a scenario depicted by the broken lines. (B) On rough substrates, less extensive interfacial hydrogen bonding results in a greater structural role for chain-chain interactions, giving rise to a higher degree of order in the alkane portion of the SAM. On the right, chains are oriented at the nominal 30° tilt angle characteristic of crystalline regions of COOH SAMs. See: Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D. Langmuir 1992, 8, 27072714.

among COOH groups. As H-bonding interactions strengthen, we expect the film structure to become progressively less dependent upon chain-chain interactions, which are responsible for crystallinity in longer chain SAMs.7 In this view, when the cohesive energy associated with interfacial H-bonding exceeds that due to chainchain interactions, the alkyl chains become less densely packed and consequently more susceptible to solvent and/ or ion penetration. Thus, the model predicts that higher capacitance arises on smoother substrates due to degraded film dielectric properties. These thoughts are also in line with the gentler trends noted above for a thinner SAM, C10COOH, which would exhibit less intrinsic crystallinity than the C13COOH SAM. To explain the decreased film permeability that occurs with increasing substrate smoothness, we furthermore propose that the extensively Hbonded interfacial network acts as a kinetic barrier to HMFc transfer. For substrates with substantial atomic scale roughness, on the other hand, the probability of attaining a highly uniform COOH “plane” and an extensively H-bonded interface seem less likely. Two additional observations lend support to the proposed model. First, mixed CnCOOH/CmOH SAMs with m ) n - 2 or n - 3 always resulted in Cdl values smaller than those of the homogeneous CnCOOH SAM, a result consistent with a denser chain packing for the mixed SAMs (see Supporting Information, section 6, for data). Presumably, dilution of the COOH groups results in less extensive H-bonding, and chain-chain interactions are able to play a more dominant role in determining film structure of the mixed SAMs. Second, when any of the four substrate types used in this work were intentionally roughened or smoothed prior to SAM modification, Cdl values were observed to decrease and increase, respectively.8 The impact of endgroups on SAM structure and properties has been widely recognized9 including the important role that H-bonding can play.10 Abnormally high viscoelasticity arising from extensive interfacial H-bonding was proposed as a model to explain unexpected quartz crystal microbalance (QCM) behavior observed for COOH SAMs.2c Lateral H-bonding among COOH endgroups has furthermore been identified spectroscopically in gas-phase (9) For example: (a) Chidsey, C. E. D.; Loiacono, D. Langmuir 1990, 6, 682-691. (b) Delamarche, E.; Michel, B.; Biebuyck, H. A. Adv. Mater. 1996, 8, 719-729. (c) Delamarche, E.; Michel, B. Thin Solid Films 1996, 273, 54-60.

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experiments.10d In a lateral force microscopy study, the interface of COOH SAMs was characterized as being “stiffer” than SAMs with other endgroups,10a a result consistent with a more cohesive surface due to H-bonding. Recently, the ability of patterned COOH SAM regions to withstand displacement by unsubstituted alkanethiols was reported, with H-bonding among COOH groups proposed as the major contribution to the enthalpy of adsorption.10c The model we propose here for COOH SAMs may also resolve the unexplained inconsistency among reported pKa measurements. Because COOH groups engaged in Hbonding are more difficult to deprotonate, we predict that COOH SAMs on smoother substrates will exhibit higher pKa values. In the literature we have identified eight reports2,11 in which aqueous pKa values have been measured for homogeneous COOH SAMs using a variety of techniques. While recognizing that pKa decreases with increasing ionic strength (µ)2a,c,3c,12b and increases with SAM chain length (nCH2),2e-f,3c,12b we conclude that the wide range of reported values can be rationalized on the basis of gold topographical variability.13 With one apparent exception (see following paragraph), studies utilizing “rough”13 substrates report pKa values in the range of 5.46.3 (average ) 5.8 ( 0.4)2a,d,e,h,i whereas studies using “smooth” substrates yield a range of 7.7-10.3 (average ) 8.7 ( 1.4).2b,f,g It thus appears that variation in substrate topography can lead to alterations in surface acidity of up to 3 pKa units. One weakness of the preceding analysis is the crudeness of the classification scheme, which categorizes surface topography as either “smooth” or “rough”.13 We believe that the one apparent exception to the analysis presented in the preceding paragraph is actually a reflection of this weakness. Ward and co-workers2c obtained basic pKa values (∼8-9) in QCM experiments for unannealed (10) (a) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825-831. (b) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Phys. Chem. 1996, 100, 2089-2092. (c) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024-1032. (d) Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1993, 9, 1775-1780. (e) Delamarche, E.; Sprik, M.; Michel, B.; Rothlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116-4130. (f) Clegg, R. S.; Hutchinson, J. E. Langmuir 1996, 12, 5239-5243. (g) Tengrall, P.; Liedberg, B.; Lestelius, N. Langmuir 1997, 13, 5900-5908. (11) Reports based on atomic force microscopy adhesion techniques12a-c and gas-phase spectroscopic techniques12d-f were excluded due to the fundamentally different interfacial environments. (12) (a) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. 1997, 101, 9563-9569. (b) Smith, D. A.; Wallwork, M. L. Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; M. Wong J. Phys. Chem. B 2000, 104, 88628870. (c) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006-2015. (d) Wells, M., Dermody, D. L.; Yang, H. C.; Kim, T.; Crooks, R. M.; Ricco, A. J. Langmuir 1996, 12, 19891996. (e) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. Langmuir 1996, 11, 1190-1195. (f) Bagg, J.; Haber, M. D.; Gregor, H. P. J. Colloid Interface Sci. 1966, 22, 138-143. (13) Since most reports do not characterize Rf, we classified surfaces qualitatively as “rough” or “smooth” based on available descriptions of substrates and pretreatments. Unannealed evaporated gold films were classified as “rough”.

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evaporated gold films on quartz. Lacking any information to the contrary, we classified13 unannealed evaporated gold films as “rough” substrates predicted to give rise to more acidic pKa values. We hypothesize that the QCM substrates2c were incorrectly classified and were, in reality, much smoother than typical “rough” substrates.2a,d,e,h,i The fact that more recent QCM results from a different laboratory2d,e (pKa ∼ 6) fall in line with our model lends support to this view. Notably, these two disparate pKa findings correspond to qualitatively different QCM behaviors. Shimazu and co-workers2d,e observed ca. 2 Hz decrease in frequency upon full ionization of the COOH layer, a result consistent with a simple mass increase due to proton replacement by sodium counterions. Ward and co-workers, on the other hand, obtained the striking result of a 103 Hz increase in frequency upon full ionization. To explain this result, they proposed a large increase in interfacial viscoelasticity due to the existence of an extensively H-bonded water layer in the protonated state of the film, but not the deprotonated state. Our work is supportive of their model and we furthermore believe that the discrepancy in QCM results between the Ward and Shimazu groups is a reflection of topographical differences between their gold substrates. Finally, we would like to bring to the reader’s attention a separate observation concerning cytochrome c adsorption that appears to be largely a manifestation of the proposed dependence of pKa on substrate topography. In a separate contribution,1b we report that cytochrome c adsorption at neutral pH becomes greatly inhibited when COOH films are prepared on smooth gold substrates. This observation is consistent with the proposed model, which postulates a major alkaline shift of the SAM pKa at smooth gold substrates. For sufficiently alkaline pKa values, extensive ionization of the COOH surface would be precluded at pH 7, and Coulombic binding of cytochrome c would thus be compromised. Currently work is underway to obtain FTIR spectroscopic data that should clarify the validity of the proposed model. Continued investigation of the acid/base properties of COOH SAMs is also being pursued using electrochemical and surface wetting techniques. Acknowledgment. We acknowledge the U.S. National Science Foundation for generously supporting this work (CHE-9816268) and also for supporting J. A. Black under the Research Experiences for Undergraduates Program (CHE-9610196). We thank Professors Michael D. Ward and James D. Martin for helpful discussions. Supporting Information Available: Details of gold substrate characterization and pretreatments, Rf determination, capacitance results for unsubstituted SAMs and mixed SAMs, and other experimental aspects. This material is available free of charge via the Internet at http://pubs.acs.org. LA011683E