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J. Am. Chem. Soc. 2000, 122, 988-989
Dynamic Monolayer Gradients: Active Spatiotemporal Control of Alkanethiol Coatings on Thin Gold Films Roger H. Terrill,† Karin M. Balss, Yumo Zhang, and Paul W. Bohn* Department of Chemistry, Beckman Institute and Materials Research Laboratory UniVersity of Illinois at Urbana-Champaign 600 South Mathews AVenue, Urbana, Illinois 61801 ReceiVed September 23, 1999 The chemical modification of coinage metal surfaces with ω-functionalized alkanethiols has proven to be a popular system for studies of wetting,1-3 adhesion,4-7 chemical affinity,8-12 and electron transfer.13-15 Varying the composition of binary hydrophilic-hydrophobic self-assembled monolayers (SAMs) can vary the wetting properties of a surface in a continuous manner,12,16 a fact that has been exploited to distribute surface-active molecules inhomogeneously under mass-transport control.17,18 One goal of these experiments is to create surfaces which permit supermolecular objects to be manipulated under external control, as recently demonstrated at air-liquid interfaces.19 Here we report chemical potential distributions of alkanethiols, which can be manipulated in both space and time under active electrochemical control. In-plane current passed in a thin (5 nm e d e 80 nm) Au film of resistivity, F(l),20 and cross-section, A, produces an in-plane potential, V(x),
V(x) ) V0 +
∫0x
iF(l) dl A
(1)
whose magnitude and position, relative to a solution reference couple, can be tuned by adjusting the magnitude of the current, i, and the potentiostat voltage offset, V0, respectively. Electro* To whom correspondence should be addressed. † Current address: Department of Chemistry, San Jose State University, One Washington Square, San Jose, CA 95192-0101. (1) Tao, Y.-T.; Lin, W.-L.; Hietpas, G. D.; Allara, D. L. J. Phys. Chem. B 1997, 101, 9732-9740. (2) Laibinis, P.; Whitesides, G.; Allara, D.; Tao, Y. T.; Parikh, A.; Nuzzo, R. J. Am. Chem. Soc. 1991, 113, 7152-7167. (3) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 19901995. (4) Tidwell, C.; Ertel, S.; Ratner, B.; Tarasevich, B.; Atre, S.; Allara, D. Langmuir 1997, 13, 3404-3413. (5) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009-12010. (6) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 1071410721. (7) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877-5878. (8) Arduengo, A.; Moran, J.; Rodriguez-Parada, J.; Ward, M. J. Am. Chem. Soc. 1990, 112, 6153-6154. (9) Sayre, C.; Collard, D. Langmuir 1995, 11, 302-306. (10) Evans, S.; Ulman, A.; Goppert-Berarducci, K.; Gerenser, L. J. Am. Chem. Soc. 1991, 113, 5867-5868. (11) Chailapakul, O.; Crooks, R. Langmuir 1995, 11, 1329-1340. (12) Chaudhury, M.; Whitesides, G. Science 1992, 255, 1230-1232. (13) Chidsey, C.; Bertozzi, C.; Putvinski, T.; Mujsce, A. J. Am. Chem. Soc. 1990, 112, 4301-4306. (14) Finklea, H.; Snider, D.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660-3667. (15) Sun, L.; Crooks, R. Langmuir 1993, 9, 1951-1954. (16) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821-3827. (17) Thome, J.; Himmelhaus, M.; Zharnikov, M.; Grunze, M. Langmuir 1998, 14, 7435. (18) Chaudhury, M.; Whitesides, G. Science 1992, 256, 1539-1541. (19) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science 1999, 283, 57-60. (20) The resistivity itself is a function of position21,22 according to the following, F(x) ) F0 + δ(x) + κΓ(x), where δ(x) represents a positiondependent stochastic fluctuation in the local resistivty, Γ(x) is the surface number density, and κ is a constant that depends on the chemical identity of the adsorbate. However, the stochastic and adsorption terms comprise typically an effect e 5%, so they are neglected in this treatment.
chemical phenomena typically produce currents 103 smaller than the in-plane current required to achieve, say ∆V ∼ 500 mV. However, the voltage noise produced by passing the in-plane current is some 104 smaller than the in-plane voltage,21,22 so electrochemical phenomena can be followed and spatial gradients in electrochemical potential transformed into surface chemical potential gradients. The electrochemical desorption of alkanethiols in methanolic KOH at potentials V < -0.8 V vs Ag/AgCl23-30 was used to establish static gradients in two-dimensional (surface) chemical potential. A 500 Å Au film was contacted by deoxygenated 0.5 M KOH/5 mM octanethiol (OT) in CH3OH in a flow-cell configuration, and cyclic voltammetry was performed to verify the potential of the cathodic stripping and anodic adsorption waves, cf. Figure 1.31 OT gradients were formed during 5-min electrolyses performed with a 15 mV mm-1 in-plane potential gradient, as implied by the top and bottom axes in Figure 1. The composition gradient was then captured by rapidly (
