In-Situ Investigation of Binary-Component Self-Assembled Monolayers

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In-Situ Investigation of Binary-Component Self-Assembled Monolayers: A SERS-Based Spectroelectrochemical Study of the Effects of Monolayer Composition on Interfacial Structure Vanessa Oklejas and Joel M. Harris* Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850 Received November 18, 2002. In Final Form: April 24, 2003 SERS-based spectroelectrochemistry was used to investigate interfacial structural properties of mixed self-assembled monolayers (SAMs) as a function of their composition. Mixed monolayers were formed from ethanolic solutions containing 12-mercaptododecanenitrile and n-heptanethiol. The ratio of the twocomponent concentrations in solutions from which the SAMs were assembled was varied to provide a series of SAMs with varying surface concentrations of the terminal nitrile group. The intensities of Raman scattering from vibrational modes associated with the nitrile and methyl termini were used to determine the relative composition of the mixed monolayer versus component concentrations in the assembly solutions. Stark tuning of the nitrile group served as a localized probe of the interfacial environment. Stark tuning rates of the nitrile probe molecule exhibited a dependence on monolayer composition comprising two distinct regions: Stark tuning rates at high nitrile surface coverages were large and did not exhibit significant composition dependence, while Stark tuning at low nitrile coverage was weaker and varied with SAM composition. Raman scattering obtained in the C-H stretching spectral region is modeled using ab initio electronic structure calculations and used to determine conformational order within the SAM as a function of applied potential.

Introduction Compositional flexibility is one of the major benefits afforded by alkanethiol-based self-assembled monolayer (SAM) chemistry: terminal functionality and chain length of the alkanethiol can be altered to yield SAMs with unique or desirable properties. SAMs composed of two (or more) molecular species, or mixed monolayers, exhibit properties characteristic of both molecular constituents. For instance, varying relative concentrations of the hydrophobic/hydrophilic mixed SAMs introduces proportional changes in wetting properties.1 Accordingly, mixed monolayers are used to achieve surfaces with tunable functionality for investigations into interfacial phenomena (e.g., protein adsorption, molecular recognition, interfacial binding events, structure of electrochemical interfaces, electron transfer, etc.).2 Previous researchers have demonstrated that the practical functionality of SAM-modified surfaces is dependent on monolayer composition and structure. The conformational order, spatial distribution, and surface coverage of each molecular species determine the chemical, biological, and electronic properties of mixed SAM * To whom correspondence should be addressed. E-mail: harrisj@ chem.utah.edu. (1) (a) Terrill, R. H.; Balss, K. M.; Zhang, Y.; Bohn, P. W. J. Am. Chem. Soc. 2000, 122, 988. (b) Chaudhury, M.; Whitesides, G. Science 1992, 255, 1230. (c) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821. (2) (a) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 3464. (b) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (c) Chaki, N. K.; Vijayamohanan. Biosens. Bioelectron. 2002, 17, 1. (d) Pope, J. M.; Buttry, D. A. J. Electroanal. Chem. 2001. (e) Nelson, K. E.; Gamble, L.; Jung, L. S.; Boeckl, M. S.; Naeemi, E.; Golledge, S. L.; Sasaki, T.; Castner, D. G.; Campbell, C. T.; Stayton, P. S. Langmuir 2001, 17, 2807. (f) Svedhem, S.; Oeberg, L.; Borrelli, S.; Valiokas, R.; Andersson, M.; Oscarson, S.; Svensson, S. C. T.; Liedberg, B.; Konradsson, P. Langmuir 2002, 18, 2848. (g) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (h) Chidsey, C. E. D.; Bertozzi, C. R.; Putwinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301.

systems.2e,3 Fundamental studies of mixed monolayer structure and composition, however, have been outpaced by investigations of their application. A wealth of mixed SAM systems with complex functional groups have been developed for their unique properties and response characteristics.4 In contrast, investigations into the structure of mixed monolayers are limited to comparatively few systems.2b,5 Previous investigations of mixed monolayer structure and composition have utilized an assortment of analytical techniques: cyclic voltammetry,5c,6 X-ray photoelectron spectroscopy (XPS),7 contact angle measurements,8 infrared-based spectroscopies,2b,5d,g,8b,9 and scanning probe (3) (a) Lahav, M.; Katz, E.; Willner, I. Langmuir 2001, 17, 7387. (b) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426. (c) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. Langmuir 2000, 16, 9287. (d)Wimalasena, R. L.; Wilson, G. S. J. Chromatogr. 1991, 572, 85. (4) (a) Steel, A. B.; Herne, T. M.; Tarlov, M. J. Anal. Chem. 1998, 70, 4670. (b) Rickert, J.; Goepel, W.; Beck, W.; Jung, G.; Heiduschka, P. Biosens. Bioelectron. 1996, 11, 757. (c) Yousaf, M. N.; Chan, E. W. L.; Mrksich, M. Angew. Chem. 2002, 39 (11), 1943. (5) For examples, see: (a) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (b) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (c) Hobara, D.; Ota, M.; Imabayashi, S.-I.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113. (d) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (e) Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.-I.; Niki, K. Langmuir 2001, 17, 1599. (f) Kwon, Y.; Mrksich, M. J. Am. Chem. Soc. 2002, 124, 806. (g) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (h) Dunbar, T. D.; Cygan, M. T.; Bumm, L. A.; McCarty, G. S.; Burgin, T. P.; Reinerth, W. A.; Jones, L.; Jackiw, J. J.; Tour, J. M.; Weiss, P. S.; Allara, D. L. J. Phys. Chem. B 2000, 104, 4880. (i) Auletta, T.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 2002, 18, 1288. (6) (a) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (b) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (7) (a) Heeg, J.; Schubert, U.; Kuechenmeister, F. Fresnius J. Anal. Chem. 1999, 365, 272. (b) Wang, Z.; Shi, Y.-L.; Li, H.-L. J. Can. Chem. 2001, 79, 328. (c) Cotton, C.; Glidle, A.; Beamson, G.; Cooper, J. M. Langmuir 1998, 14, 5139.

10.1021/la020916e CCC: $25.00 © 2003 American Chemical Society Published on Web 06/12/2003

Binary-Component Self-Assembled Monolayers

microscopies (e.g., STM and AFM)3c,10 are among the most commonly used. Scanning probe microscopic investigations of mixed SAMs yield compelling results, in which SAM order, orientation, and spatial distribution are directly observed.5b-c,e,10 Cyclic voltammetry (CV) has also been used to provide indirect characterization of SAM order in mixed monolayer systems.6 XPS studies are commonly utilized to determine the surface composition of mixed SAMs. Contact angle measurements have also been used to infer structural changes in mixed monolayer systems as a function of monolayer composition.8b Infrared spectroscopy can be used to detect conformational changes of mixed SAMs as a function of monolayer composition.9 However, with the exception of cyclic voltammetry and IR spectroscopy, all of the above investigations must be performed ex situ. The utility of ex-situ measurements depends on the structure of mixed SAMs being independent of the surrounding environment. Given that most applications of SAM-modified surfaces are carried out in solution, insitu structural information is important for the development and optimization of mixed SAMs for practical applications.2g,h,5i,11 For many SAM systems, CV does offer the advantage of in-situ characterization, but it usually cannot provide direct information on the SAM structure. CV-based studies of composition and spatial heterogeneity of mixed monolayers have been performed but require the incorporation of bulky, redox-active groups in the film.2g,h,12 In-situ FT-IR studies of SAMs have not been heavily pursued due to the interference from water, as well as the ambiguity of interpreting difference spectra. Accordingly, many studies of mixed monolayers employ multiple techniques to obtain a combination of structural information and in-situ characterization of mixed SAMs.5a,h,7c,12 Surface-enhanced Raman scattering (SERS) spectroscopy has proved a potent tool for characterization of SAMmodified surfaces. Sandroff et al. were the first to utilize SERS for the structural study of hexadecanethiol SAMs assembled on Ag island films.13 Perhaps the most intriguing aspect of SERS measurements is the accessibility of a large number of vibrational transitions, where modes associated with specific molecular conformations allow direct characterization of conformational structure and molecular composition.14 Previous research has shown that the frequency of vibrational modes can also yield structural information, where the change in frequency can be correlated with changes in the local environment of the probe molecule.5h,15 Similarly, a change in vibrational frequency due to an applied electric field, the vibrational (8) (a) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (b) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (c) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (9) (a) Stole, S. M.; Porter, M. D. Langmuir 1994, 6, 1199. (b) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638. (c) Everett, W. R.; Fritsch-Faules, I. Anal. Chem. Acta 1995, 307, 253. (d) Everett, W. R.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292. (10) (a) Pflaum, J.; Bracco, G.; Schreiber, F.; Colorado, R.; Shmakova, O. E.; Lee, T. D.; Scoles, G.; Kahn, A. Surf. Sci. 2002, 498, 89. (11) Ye, S.; Haba, T.; Shimazu, K.; Uosaki, K. Phys. Chem. Chem. Phys. 1999, 1, 3653. (12) (a) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (b) Zhang, H.-L.; Chen, M.; Li, H.-L. J. Phys. Chem. B 2000, 104, 28. (13) Sandroff, C. J.; Garoff, S.; Leung, K. P. Chem. Phys. Lett. 1983, 96, 547. (14) (a) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (b) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (15) (a) Orendorff, C. J.; Ducey, M. W., Jr.; Pemberton, J. E. J. Phys. Chem. A, in press. (b) Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1945; pp 534-537.

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Stark effect, can be an effective structural probe for diverse systems: electrochemical interfaces, cell membranes, protein structure, and catalytic sites in zeolites.2d,16 The vibrational Stark effect is a perturbation of the energy of vibrational transitions (ν) due to the presence of an electric field (E). The vibrational Stark effect requires large fields to be observed and is ideally suited for metal/ solution interfaces: ions in solution organize at charged metal surfaces in order to balance the presence of excess surface metallic charge. This localization of the solutionphase ions at the metal surface (i.e., the diffuse doublelayer region) creates a rapid change in potential between the metal surface and solution, in which the surface potential is dissipated within several angstroms. The small size of these ions (and concomitant small length of the diffuse double-layer region) is the origin of the enormous interfacial E fields (∼108 V m-1) observed at metal/solution interfaces.17 The magnitude of the resulting E field is highly dependent on the concentration of solution-phase ions, as well as metallic surface and interfacial solvent structure.17a,18 Consequently, the magnitude of the Stark response (i.e., the Stark tuning rate, dν/dE) has been successfully utilized as a probe of interfacial environments.17b,19 Previous research in this laboratory has established that Stark tuning rates associated with a probe molecule immobilized at Ag electrode surfaces were sufficiently sensitive to report structural changes within the diffuse double-layer region.20 In this previous work, Stark tuning rates associated with the pendent nitrile group, immobilized within a mixed monolayer composed of mercaptododecanenitrile and heptanethiol, provided an immobilized interfacial reporter group. The Stark tuning rates of the nitrile stretching frequency reported changes in the interfacial E fields within the diffuse double layer as a function of ionic strength of aqueous solution, thereby establishing that the nitrile probe was capable of detecting changes within the diffuse double-layer region. Stark tuning rates associated with the nitrile group may also reflect changes in the structure of the mixed SAMs to which they are attached: the composition of mixed monolayers may modify the structure and position of the Stark probe relative to the diffuse double layer, which, in turn, responds to these spatial changes in interfacial aqueous structure. Theoretical predictions of interfacial E fields at SAM-modified electrodes suggest that applied potential decays linearly from the metal surface through the dielectric SAM layer until it reaches the SAM/aqueous solution interface.17b This distance dependence then assumes an exponential decay form at the SAM/solution interface, where mobile ions rapidly quench the interfacial potential over short distances. Accordingly, the position of the Stark probe (i.e., nitrile stretching mode) relative to the SAM/aqueous interface (16) (a) Korzeniewski, C.; Pons, S.; Schmidt, P. P.; Severson, M. W. J. Chem. Phys. 1986, 85, 4153. (b) Luo, H.; Weaver, M. J. Langmuir 1999, 15, 8743. (c) Kao, W. Y.; Davis, C. E.; Kim, Y. I.; Beach, J. M. Biophys. J. 2001, 81, 1163. (d) Loew, L. M.; Simpson, L. M. Biophys. J. 1981, 34, 353. (e) Park, E. S.; Thomas, M. R.; Boxer, S. G. J. Am. Chem. Soc. 2000, 122, 12297. (f) Park, E. S.; Andrews, S. S.; Hu, R. B.; Boxer, S. G. J. Phys. Chem. B 1999, 103, 9813. (g) Bublitz, Ge. U.; Boxer, S. G. Stark spectroscopy: applications in chemistry, biology, and materials science. Annu. Rev. Phys. Chem. 1997, 48, 213C. (h) Xiang, P. L.; Grassian, V. H.; Larsen, S. C. J. Phys. Chem. B 1999, 103, 5058. (17) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley and Sons: New York, 1980. (b) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (18) Glosli, J. N.; Philpott, M. R. Electrochim. Acta 1996, 41, 2145. (19) (a) Weaver, M. J.; Wasileski, S. A. Langmuir 2001, 17, 3039. (b) Russell, A. E.; Pons, S. A.; Anderson, M. R. Chem. Phys. 1990, 141, 41. (20) Oklejas, V.; Sjostrom, C.; Harris, J. M. J. Am. Chem. Soc. 2002, 124, 2408.

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Scheme 1. Theoretical Profile of the Potential Applied at a Metal Surface, through the Dielectric SAM, and into the Aqueous Phase: Behavior of Potential Decay across a Mixed SAM (A) with High Nitrile Coverage and (B) with Low Nitrile Coverage

will determine the magnitude of the Stark tuning rate: nitrile groups positioned at the SAM/solution interface should experience the greatest E field and exhibit the largest Stark tuning rates. Conversely, nitrile groups that are placed deeply inside the diffuse double layer, where both the potential and potential gradient approach zero, should experience far less E field with proportionally smaller tuning rates. The relative position of the probe with respect to the diffuse double layer should depend on the relative surface concentrations of both the longer mercaptdodecanenitrile and the shorter heptanethiol. Accordingly, Stark tuning rates of the pendent nitrile group serve as a probe of the interfacial environment as a function of SAM composition: small tuning rates are associated with a mixed SAM in which the pendent nitrile groups are well separated and are exposed to an aqueous environment (Scheme 1). Surface-enhanced Raman scattering (SERS) spectroscopy provides several key advantages for in-situ investigations of mixed monolayers. This technique interrogates numerous vibrational modes, which allows direct determination of relative monolayer composition, as well as insight into the conformational structure of the alkane chains within the mixed SAMs. The vibrational Stark effect, which is influenced by local electric fields, may provide a route to the characterization of the environment surrounding the pendent nitrile and associated SAM structure. Furthermore, SERS investigations can be easily adapted for in-situ studies in aqueous solution, due to large local surface enhancement and relatively weak Raman scattering by water. This paper presents a SERS-based method for in-situ characterization of mixed monolayers. Two structural parameters are the focus of this study: (1) the ratio of the intensity of the nitrile stretching mode to that of the vibrational mode associated with the methyl-terminated group and (2) the dependence of the Stark tuning rates on monolayer composition. The former is used to assess compositional changes within the SAM as a function of assembly solution composition, while the latter is examined in order to characterize the environment surrounding the pendent nitrile in order to gain insight into the structure of mixed SAMs as a function of composition. Raman modes obtained in the C-H stretching region of SERS spectra are assigned on the basis of ab initio electronic structure calculations of n-heptanethiol and 12-

Oklejas and Harris

mercaptododecanethiol. These assignments are used to determine the structural order within the SAM as a function of applied potential. Experimental Section Synthesis of Mercaptododecanenitrile. HS(CH2)11CN was synthesized according to a published procedure.21 Dibromoundecane (98%, Aldrich) was refluxed with an equimolar amount of thiolacetic acid (97%, Aldrich) in freshly distilled methanol under nitrogen for 6 h. The reaction was quenched with the addition of brine and hexanes. The organic phase was washed three times [15 mM sodium bicarbonate (once) and doubly distilled deionized water (twice)] and dried over magnesium sulfate. The desired monoacetylthiol product was purified using a silica column (60 Å, pore size) with 19:1 hexanes:diethyl ether as eluent. The purified product was allowed to react with equimolar amounts of sodium cyanide in freshly distilled methanol for 30 min at 60 °C. The reaction was cooled to room temperature and quenched with addition of diethyl ether and water. The organic reaction mixture was extracted with water, and the solvent was reduced under vaccuum. The acetylthiol group was deprotected upon reduction by sodium bicarbonate to yield the desired mercaptan. The reaction mixture was purified using a silica column (60 Å, pore size) with 1:1 hexanes:diethyl ether as eluent. The final product, mercaptododecanenitrile (