Spatial Dispersion in Electrochemically Generated Surface

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Spatial Dispersion in Electrochemically Generated Surface Composition Gradients Visualized with Covalently Bound Fluorescent Nanospheres Susan T. Plummer and Paul W. Bohn* Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801 Received November 30, 2001. In Final Form: February 25, 2002 Surface electrochemical potential gradients were prepared by poising the two ends of a thin Au strip at different potentials relative to a common solution Ag/AgCl reference couple. The small resistivity of the Au film resulted in a linear gradient in surface electrochemical potential, enabling spatially resolved electrochemistry. With no applied potential, a self-assembled monolayer (SAM) of an organothiol was formed on the electrode surface. Upon potential application the SAM was stripped in regions with local potentials cathodic of the reductive desorption potential, while regions sufficiently anodic of that potential maintained full monolayer coverage. A transition region was produced between the bare areas and those retaining full monolayer coverage. Carboxylic acid modified, fluorescently doped, polystyrene nanospheres can be reacted with amine-terminated thiols through the use of a water-soluble carbodiimide and N-hydroxysuccinimide. Fluorescence microscopy was used to image the nanosphere distribution on the surface, from which information on the position and width of the gradient transition was obtained. The gradient center and width were found to depend on the offset potential and magnitude of the applied potential window. The gradient center potential shifts cathodic as the center of the potential window shifts anodic, and the gradient width increases with a decrease in the width of the applied potential window. Since in-plane composition control results in the control of physical properties, the gradient surfaces produced and studied here can be used in applications requiring anisotropy in in-plane chemical or physical properties.

Introduction A great deal of interest exists in producing surface architectures with controllable nonuniform composition in order to influence the behavior of molecular and supermolecular objects in the vicinity of the surface.1-4 As a special case, surfaces with lateral variations in coverage are interesting due to their possible application to wettability and chemotaxis. Anisotropic in-plane forces large enough to move supermolecular objects against the combined forces of gravity and friction, for example, the movement of liquid droplets using surface chemical gradients, have been demonstrated previously.5,6 In our laboratory, the electrosorption behavior of selfassembled monolayers (SAMs) of ω-functionalized alkanethiols on coinage metal surfaces displaying spatially varying electrochemical potentials has been exploited to produce gradients in surface composition. Adsorbed thiols on Au, RS-Au, are electrochemically stripped from metal surfaces by reductive desorption, a 1 e- process resulting in the release of RS-.7-14 By coupling the thiol stripping reaction to a nonuniform in-plane electrochemical poten* To whom correspondence should be addressed: e-mail [email protected]. (1) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. N.; Wang, D. I.; Whitesides, G. M.; Ingber, D. E. Science 1994, 264, 696-698. (2) Welin-Klintstrom, S.; Lestelius, M.; Liedberg, B.; Tengvall, P. Colloid Surf. B: Biointerfaces 1999, 15, 81-87. (3) Hypolite, C. L.; Huang, C. C.; Distefano, M. D.; Hu, W. S. Mol. Biol. Cell 1995, 6, 272-272. (4) Herbert, C. B.; McLernon, T. L.; Hypolite, C. L.; Adams, D. N.; Pikus, L.; Huang, C. C.; Fields, G. B.; Letourneau, P. C.; Distefano, M. D.; Hu, W. S. Chem. Biol. 1997, 4, 731-737. (5) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 15391541. (6) 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.

tial distribution, V(x), thiols can be stripped from regions of the surface where the local potential is cathodic of the desorption potential, V(x) < E0des. The resulting gradient in surface potential produces a spatial variation in coverage, Γ(x). The bare regions can then be back-filled with another thiol, creating a two-component full monolayer SAM, Γ1(x) + Γ2(x) ) Γsat’n. Two-component gradients have been produced with thiols of varying chain length and varying terminal functional groups.15,16 Under most experimentally important conditions the spatial distribution of the first desorbed thiol controls the two-component distribution; i.e., the second thiol, R′SH, undergoes adsorption much more rapidly than surface displacement, so the net effect is for R′SH to fill in the areas denuded by desorption of the initial thiol, RSH. Because the general strategy for creating two-component gradient structures is largely independent of the identity of the ω-functional group of the thiol, molecular recognition motifs may be (7) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (8) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-2693. (9) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860-5862. (10) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 65706577. (11) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243249. (12) Hobara, D.; Miyake, K.; Imabayashi. S.-I.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590-3596. (13) Hatchett, D. W.; Uibel, R. H.; Stevenson, K. J.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1998, 120, 1062-1069. (14) Stevenson, K. J.; Mitchell, M.; White, H. S. J. Phys. Chem. B 1998, 102, 1235-1240. (15) Balss, K. M.; Coleman, B. D.; Lansford, C. H.; Haasch, R. T.; Bohn, P. W. J. Phys. Chem. B 2001, 105, 8970-8978. (16) Terrill, R. H.; Balss, K. M.; Zhang, Y.; Bohn, P. W. J. Am. Chem. Soc. 2000, 122, 988-989.

10.1021/la011742o CCC: $22.00 © 2002 American Chemical Society Published on Web 04/12/2002

Spatial Dispersion in Surface Composition Gradients

Langmuir, Vol. 18, No. 10, 2002 4143 Scheme 1

inserted at the thiol terminus. For example, the ligands for the integrin class of cell surface receptors might be inserted and thus used to effect directed cellular motion in two dimensions. Gradients in surface potential are formed by the injection of an in-plane current through a thin Au film (5 nm e d e 80 nm).16 The nonnegligible film resistivity and current densities of 0.5 A/cm2 result in an in-plane potential variation. The potential profile as a function of position depends on the local resistivity of the film according to

V(x) ) V0 +

∫

iF(l) dl A

(1)

where V0 is the offset potential, i is the current, A is the cross-sectional area, and F(l) is the position-dependent resistivity given by

F(x) ) F0 + δ(x) + κΓ(x)

(2)

where F0 is the bulk resistivity, δ(x) is a stochastic term associated with variations in the film morphology, κ is an adsorbate-dependent constant, and Γ(x) is the local adsorbate coverage. The resistivity of a thin Au film typically increases by -OH > -COOH thiol termination, with ΓCH3 ∼ 108 cm-2 and ΓCOOH ∼ ΓAu. The order likely reflects electrostatic repulsion of partially ionized unreacted carboxylate moieties on the nanosphere surface by negative charges resident on the SAM surface. It is expected that due to the hydrophobic nature of the -CH3 SAM physisorption onto that surface would be greatest. The reaction parameters for nanosphere assembly were optimized with respect to nanosphere concentration, activating agents, and assembly time. As expected, the nanosphere coverage increased with increasing solution concentration at low activating agent concentrations, from 106 at 5.3 × 109 mL-1 to 109 at 5.3 × 1010 mL-1 for a 1:1:1 mole ratio of COOH:EDC:NHS. As the concentration of activating agents was increased, the coverage initially increased but then decreased from 109 at 1:1:1 COOH: EDC:NHS to 108 at 1:50:50 with a solution nanosphere concentration of 2.56 × 1010 mL-1. The observed decrease in nanosphere coverage at 1:50:50 COOH:EDC:NHS mole ratio is correlated with agglomeration in solution, which likely results from a decrease in nanosphere surface charge density due to conversion of a larger fraction of the COOH moieties to the succinimidyl ester. At the 1:50:50 conditions

(27) Jiang, L.; Glidle, A.; Griffith, A.; McNeil, C. J.; Cooper, J. M. Bioelectrochem. Bioenerg. 1997, 42, 15-23. (28) Luo, J.; Isied, S. S. Langmuir 1998, 14, 3602-3606. (29) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Anal. Chem. 1999, 71, 119-125.

(30) Kretschmann, E.; Raether, H. Z. Naturforsch. A 1968, 23, 21352136.


Figure 1. Fluorescence microscopy images of the surface after the nanosphere reaction with (a) AET-coated Au, (b) bare Au, and (c) a 25 µm wide strip of glass exposed between AET-coated Au films. In (c) regions I and III are composed of AET-Au, while region II contains the stripe of uncoated glass.

electrostatic forces are no longer strong enough to facilitate suspension of the nanospheres. The reaction conditions are optimal when the activating agent concentration is in a 1:1 molar ratio with the COOH groups on the nanospheres at a concentration of 2.65 × 1010 mL-1, and the coverage was observed to saturate after an assembly time of 3 h (assembly kinetics not shown). The nanosphere assembly was also followed using surface plasmon reflectometry (SPR),31 as demonstrated in Figure 2. Because surface plasmons are sensitive to

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the high-frequency dielectric response function in the nearsurface region, the assembly of thiols and nanospheres can be monitored in real time.32,33 The theoretical shift of the SPR resonance for a surface adsorbate layer of known thickness and dielectric constant can be calculated from the Fresnel equations and compared to the observed shift. For a full monolayer of AET, with a thickness of 0.6 nm and a dielectric constant of 2.1, the theoretical shift is 0.04°, which was compared to experimental shifts using the measured CCD camera calibration of 0.0033°/pixel. Figure 2a shows the shift in resonance position with the assembly of AET. The resonance shift resulting from nanosphere assembly was large, and in order to observe the shift more readily, a nanosphere reaction solution 5 times more dilute than the optimal concentration (5.3 × 109 mL-1) was used in the SPR studies. This resulted in a slower assembly than that observed under optimal conditions. As seen in Figure 2b, the assembly of nanospheres onto the thiol monolayer for 1 h produced an angle shift of 0.38° and a nanosphere surface coverage of 108 cm-2, as determined by fluorescence microscopy (Figure 2d). This angle shift value is consistent with the observed coverage when the effective index of refraction for the nanosphere layer is calculated using effective medium theory. A monolayer of nanospheres was assumed, with sphere ) 2.40 and H2O ) 1.777. The percentage coverage of nanospheres in the monolayer was calculated from the sphere volume and surface coverage, assuming a sphere radius of 0.1 µm. At a coverage of 108 cm-2, layer ) 1.790 with a thickness of 0.2 µm. It is apparent that the reaction did not reach completion in 1 h, since the SPR minimum did not plateau. The predicted angle shift for a saturation coverage of 109 cm-2 is 2.56° (∼770 pixels), a shift larger than can be observed with the CCD camera used. To demonstrate the selectivity of the nanosphere reaction further, the SPR minimum was monitored as a bare Au surface was exposed to activated nanospheres (viz. Figure 2c,e). The total pixel shift was zero, indicating negligible physisorption, consistent with the results of fluorescence microscopy control experiments. A coverage of 105 cm-2 does not significantly deviate the dielectric constant of the nanosphere layer from 1.777, such that the assembly cannot be detected with this SPR system. Nanosphere Visualization of AET Gradients. The tagging reaction was employed to provide an optical marker for the spatial distribution of thiols remaining after application of a linear electrochemical potential profile to an AET-covered Au film. Gradient formation depends strongly on the detailed electrochemistry of AET on Au. Using cyclic voltammetry, AET reductive desorption occurs at -710 mV, as seen in Figure 3a. Integration of the reduction peak results in a surface coverage of 5 × 10-10 mol cm-2, which is consistent with the literature value (6 × 10-10 mol cm-2).34 Surface plasmon resonance (SPR) imaging with in situ electrochemistry (data not shown) indicates that desorption occurs at potentials ∼100 mV more positive under the static electrolysis conditions pertinent to gradient formation, consistent with the removal of AET from the Au surface at locations corresponding to a local potential V(x) ∼ -600 mV. This anodic shift in the desorption potential has been observed previously with SPR imaging of octanethiol gradients.16 Modeling of surface composition assuming Nernstian (31) Raether, H. Surface Plasmon, Springer Tracts in Modem Physics; Springer-Verlag: Berlin, 1988; Vol. 111. (32) Gordon, J. G.; Swalen, J. D. Opt. Commun. 1977, 22, 374-376. (33) Frutos, A. G.; Corn, R. M. Anal. Chem. 1998, 70, 449A-455A. (34) Wirde, M.; Gelius, U.; Nyholm, L. Langmuir 1999, 15, 63706378.

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Figure 2. Surface plasmon resonance minimum position as a function of time for various surface treatments. (a) Assembly of AET. First arrow indicates the introduction of 25 µM AET in H2O, and second arrow indicates rinsing with pure H2O. (b) Assembly of AET followed by the nanosphere reaction. First two arrows as in panel (a). Third arrow indicates introduction of 5.30 × 109 mL-1 nanospheres, derivatized with a 1:1:1 mole ratio of COOH:EDC:NHS. Fourth arrow indicates rinsing with pure H2O. (c) Nanosphere reaction on bare Au. First arrow indicates the introduction of 5.30 × 109 mL-1 nanospheres, derivatized in a 1:1:1 mole ratio of COOH:EDC:NHS, and the second arrow indicates rinsing with pure H2O. Fluorescence images (d) and (e) correspond to the results of surface treatments (b) and (c), respectively.

behavior also indicates that desorption should occur positive of the formal potential for reductive desorption, E0des. Once a gradient of AET had been formed, nanospheres were used to tag the thiol molecules and were observed with fluorescence microscopy (viz. Figure 3b). The nanospheres are not present at locations corresponding to potentials negative of the reduction potential and are present at locations corresponding to more positive potentials. The anodic shift of the desorption region from the location V(x) ∼ E0des puts the transition region in Figure 3b at a location expected for the oxidative adsorption potential in the static electrolysis situation, i.e., ∼100 mV positive of the anodic edge of the oxidative adsorption wave in the cyclic voltammogram (cf. Figure 3a). Figure 3c shows the normalized fluorescence intensity profile as a function of position across the film. The profile was fit to a sigmoid function of the form

l(x) ) Ib +

Imax 1 + e(x0-x)/r

(3)

where Ib is the base intensity, Imax is the normalized maximum intensity, x0 is the inflection point of the slope region, and r is a spatial rate constant related to the slope. The width of the gradient, W, was determined from the full width at half-maximum of the derivative of the fit function, I′(x). The x0 and W values were then converted

to potential values using the magnitude of the applied potential window, ∆V, and the total length of the film. For example, the gradient in Figure 5b was measured to have a center potential of -405 mV with a width of 92 mV. Gradient Formation Characteristics. The working hypothesis is that the gradient formation characteristics are determined by the local value of the electrochemical potential, V(x), as given by eqs 1 and 2. Experimentally, the voltage offset, V0, and the magnitude of the injected current, i, jointly determine the electrochemical potential profile. These parameters were adjusted to vary the width of the potential window, ∆V, and its center potential, V(x0), to learn how the gradient formation characteristics changed as the local potential, V(x), was moved relative to the physical frame of the active region. Because the extent of reductive desorption of AET is governed, to first order, by V(x), the fraction of thiols remaining at a given position after electrolysis should also be determined by V(x), independent of where the desorption is located in physical-space in the active region. Furthermore, as the width of the applied potential window, ∆V, decreases, the potential drop per unit length decreases, meaning that the transition region would occupy a larger physical area of the film, although its width in potential space would remain constant. To test these hypotheses, experiments were performed in which the width of the potential window, ∆V, the center of the window, V(x0), or both were adjusted while characterizing the spatial properties of


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Figure 3. (a) Cyclic voltammogram for AET/Au vs Ag/AgCl acquired at 100 mV s-1 in 0.5 M aqueous KOH. (b) Fluorescence image of a nanosphere-tagged gradient produced between -1000 mV (left) e V(x) e -200 mV (right). (c) Intensity profile of the image in (b). The spatial position x-axes in panels (b) and (c) correspond to the potential axis in panel (a). Bright regions in the fluorescence microscope image correspond to regions of high fluorescent intensity and therefore nanosphere coverage.

the nanosphere-tagged gradient by fluorescence microscopy. In the first experiment both ∆V and V(x0) were adjusted, as shown in Figure 4, by keeping the right end of the sample at -200 mV and moving the left boundary to successively more anodic potentials. The values of the gradient center and width in potential space, obtained by averaging the center and width values for multiple samples produced under the same conditions, are listed in Table 1. It is apparent from the fluorescence images in Figure 4 that the transition region shifts to the left in physical space as ∆V is decreased. The fits to eq 3 also indicate that the gradient center shifts to somewhat more negative potentials as ∆V decreases. The shift of the gradient center in potential space is significant, with an observed shift ∼150 mV from the conditions of Figure 4a to the conditions of Figure 4c. The linear correlation coefficient (r) for this trend is >0.99. The change in the gradient width is more difficult to interpret due to the large error associated with replicate measurements but increases with a decrease in ∆V with r ) 0.70. Because the quasi-linear potential gradient model implied in eqs 1 and 2 predicts that the gradient center would shift in physical space, but not in potential space, under the potential window variations given in Table 1, these shifts were investigated further. Fluorescence images were next obtained from gradients formed with varying ∆V but maintaining a constant center

Figure 4. Fluorescence images of nanosphere-tagged AET gradients with potential windows of (a) -1000 mV (left) e V(x) e -200 mV (right), (b) -900 mV (left) e V(x) e -200 mV (right), and (c) -800 mV (left) e V(x) e -200 mV (right). (d) Normalized intensity as a function of position for each image, with dotted lines corresponding to data and solid lines to fits of the data.

Figure 5. Normalized intensity as a function of distance for seven gradient samples with a potential range of -900 mV (left) e V(x) e -200 mV (right).

potential. The average gradient center and width values in potential space are listed in Table 2. In physical space the gradients shift slightly to the left and increase in breadth with decreasing ∆V. The fits to eq 3 show that the

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Table 1. Gradient Center and Width Values for the Gradients in Figure 4 width window potential range potential center (mV) window (mV) (mV) -200 to -1000 -200 to -900 -200 to -800

800 700 600

-600 -550 -500

gradient center (mV)a

gradient width (mV)a

-407 ( 67 126 ( 44 -482 ( 58 112 ( 56 -555 ( 48 162 ( 161

a Errors represent (1 standard deviation of replicate measurements.

Table 2. Gradient Center and Width Values for the Constant Window Center Experiment potential range (mV) -100 to -900 -150 to -850 -200 to -800 -250 to -750 -300 to -700

width window potential center window (mV) (mV) 800 700 600 500 400

-500 -500 -500 -500 -500



-446 -438 ( 35 -555 ( 48 -623 -552 ( 12

89 126 ( 6.7 162 ( 161 128 165 ( 51


Table 3. Gradient Center and Width Values for the Constant Potential Window Experiment potential range (mV) -400 to -1000 -300 to -900 -200 to -800 -100 to -700

width window potential center window (mV) (mV) 600 600 600 600

-700 -600 -500 -400



-499 -428 ( 29 -555 ( 48 -561 ( 37

122 162 ( 82 162 ( 161 360 ( 83


gradient transition regions shift to more cathodic potentials with decreasing ∆V with r ) 0.79. In the complementary experiment, the center potential was varied, while ∆V was held constant (viz. Table 3). As expected, the transition regions shift to the left in physical space and broaden as the potential window shifts positive. Fit values indicate that the gradient center shifts negative and its width increases as the potential window shifts positive with r ) 0.65 and 0.86, respectively. On the basis of these experiments, it is apparent that the gradient center shifts to more cathodic values as either ∆V decreases or the window shifts positive. Nonlinearities in V(x) are one possible cause of these shifts. In fact, the third term in eq 2, when coupled with the Nernst equation for local concentration, predicts a nonlinear potential profile. However, modeling studies show that the magnitude of the adsorbate-induced nonlinearity in V(x)sthe maximum adsorbate-induced change in resistivity is ∼5%sis too small to be significant in determining the coverage profile. When the surface coverage, Γ(x), takes on a sigmoid function and κ ) 0.05F0, the resulting potential profile is linear to a very high degree of approximation. Another contribution to the observed shifts of the transition region could arise from the kinetics of thiol desorption. It is known that the reductive desorption of thiols from a Au surface follows a nucleation and growth mechanism35 involving three steps:36 (1) incorporation of cations into the monolayer, (2) electron transfer, and (3) (35) Yang, D. F.; Morin, M. J. Electroanal. Chem. 1997, 429, 1-5. (36) Thirsk, H. R.; Harrison, J. A. A Guide to the Study of Electrode Kinetics; Academic Press: New York, 1972.

diffusion of thiols away from the surface.37,38 The effects of ion incorporation and thiol diffusion are evidenced by asymmetry in the chronoamperometric current-time transients during desorption and by the effect of alkanethiol chain length on the desorption kinetics.37 Asymmetry in the transients has been explained by taking into consideration chain-chain interactions between neighboring thiols39 and the convolution of thiol reduction at island edges and the growth of holes inside a thiol island.38 The formation and growth of holes are therefore dependent upon both the applied potential and the chemical environment. Thus, the probability of desorption depends on both the local electrochemical potential and the number of nearest neighbors.40,41 According to this picture, adsorbed thiols are more likely to desorb from the edges of an island than from the center at a given local potential. More negative applied potential windows would be associated with a larger fraction of smaller islandssenhancing the number of edge molecules relative to those on the interior of an islandscompared to samples with more positive potential windows. The increased probability of desorption from smaller islands would result in an apparent shift of the transition region to more anodic potentials as well as a sharpening of the gradient width, which is consistent with the observed behavior. To place the magnitude of the observed changes in context, the error associated with the gradient center and width measurements was established by comparing seven gradient samples with a potential window of -200 to -900 mV. The intensity profiles (Figure 5) show an average gradient center value of -482 ( 58 mV, i.e., a relative standard error of 8.3% of ∆V, consistent with the gradient center position errors noted in Tables 1-3. This error is likely caused by potential drop variations in the active area, especially from sample-to-sample differences in positioning of the press connections on the thick Au pads. While 95% of the potential drop nominally occurs across the active area, varying the position of the contact wires by 1 mm reduces the percentage to 92% and causes a shift of 12 mV in the apparent position of the gradient center. The average width of the replicate gradients was 112 ( 56 mV. These disparities in gradient widths can also be attributed to differences in the potential profiles. Small defects such as gradients that possess some slope or curvature in the direction perpendicular to the gradient direction may also lead to differences in the slope, since the intensity is averaged across the entire width of the gradient. Despite the indeterminate fluctuations in gradient widths in potential space, fluorescence images demonstrate that the width of the gradients in physical space does change significantly when ∆V is varied. Thus, the spatial width of a gradient can easily be manipulated by decreasing the magnitude of the applied potential window, an advantageous property when designing gradients for use in studying directed motion. By controlling the physical steepness of the gradient, the forces needed to move supermolecular objects can be studied. (37) Yang, D. F.; Morin, M. J. Electroanal. Chem. 1998, 441, 173181. (38) Mulder, W. H.; Calvente, J. J.; Andreu, R. Langmuir 2001, 17, 3273-3280. (39) Vinokurov, I. A.; Morin, M.; Kankare, J. J. Phys. Chem. B 2000, 104, 5790-5796. (40) Aoki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 452, 187192. (41) Vela, M. E.; Martin, H.; Vericat, C.; Andreasen, G.; Creus, A. H.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 11878-11882.


The advantage of electrochemically generated gradients over other forms of in-plane composition control is the ability to control composition in both space and time. By shifting the potential window, the position and width of the gradient can be adjusted in situ after its initial formation. The possibility of using fluorescent nanospheremodified thiols to study this spatiotemporal control is attractive due to the ease of fluorescent nanosphere detection. Unfortunately, it is difficult to desorb covalently tagged nanospheres after initial adsorption, probably due to the fact that multivalent binding interactions shift the desorption potential beyond the accessible range, thus preventing the application of fluorescent nanospheres in studying moveable electrochemical gradients. Conclusion The reaction of carboxylic acid-modified, polystyrene nanospheres to a SAM of AET has been demonstrated and used to tag thiols remaining on the surface of a thin film working electrode after applying a linear potential gradient spanning the reductive desorption potential. Fluorescence microscopy of the tagged gradients yielded spatial intensity profiles which were well described by a

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sigmoid function, from which the center potential and width could be recovered and used to study gradient behavior as a function of the processing parameters. Contrary to our expectations, the center potential and width were found to be functions of the size and placement of the potential window, with smaller and more positive potential windows resulting in steeper gradients. These discrepancies from the simple quasi-linear model developed in eqs 1 and 2 can be explained by kinetic effects which affect the rates at which thiols desorb from islands of varying size. The ability to produce gradients with differing physical widths and positions has been demonstrated, and the understanding of gradient formation gained from these nanosphere tagging experiments can now be used to design gradient systems for the study of directed motion on surfaces. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE 99-10236 and by the Department of Energy through Grant FG02 91ER45439. LA011742O

Spatial Dispersion in Electrochemically Generated Surface

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