Active Spatiotemporal Control of Electrochemical Reactions by Coupling to In−Plane Potential Gradients

Abstract

Active spatiotemporal control of electrochemical reactions through dynamic electrochemical potential gradients was explored by investigating three different types of reactions on Au: alkanethiol SAM electrosorption, Cu deposition and stripping, and O2 evolution from H2O2 oxidation. Counterpropagating gradients composed of two different thiols differing either in terminal functionality or in chain length were prepared, and their kinetic and environmental stability was inferred from spatially resolved contact angle measurements for samples stored under varying environmental conditions for periods up to one month. Chain length was found to correlate strongly with stabilitya requirement for stability being that at least one of the chains be at least C8 or longer. Spatially directed Cu deposition on Au was demonstrated by forming Cu stripes on Au, establishing that a sequence of different potential gradients could be used to define an area of deposition in the center of a working electrode. Dynamic spatiotemporal control of Cu deposition on Au was achieved by translating a potential window, which encompassed the Cu redox waves, across the Au surface. The position of the Cu/Au transition was constant at a potential intermediate between the two waves, and the width of the transition region in the SPR images was narrower than either of the two electron transfer waves. Spatially directed oxidation of H2O2 was demonstrated by monitoring the formation of oxygen bubbles near the electrode. Consistent with predictions of the Butler−Volmer equation, the rate of bubble formation was found to depend on spatial position (overpotential) in these experiments.

Nanotechnology for Cell–Substrate Interactions

“…A gradient of alkanethiols, and therefore surface adhesiveness, can be achieved using a variety of methods. 12,84,95,104 As one example, cross-diffusion of different alkanethiols was used to generate a one-dimensional gradient of surface-immobilized fibronectin on a SAM surface. 117 Endothelial cells seeded on the gradient migrated towards regions of higher ECM density, and displayed an increase in directed, but not random, migration speed.…”

Au–Pd Alloy Gradients Prepared by Laterally Controlled Template Synthesis

“…Several methods for generating such gradients have been described, [18] including linear motion for the production of gradients over a significant distance (millimeters to centimeters), [19] contact printing, [20] replacement lithography using a scanning tunneling microscope, [4] or modification of an electrode under a lateral voltage gradient. [5][6][7]10,[21][22][23] Application of the latter scheme to template electrodeposition in insulating nanoporous alumina membranes (NAMs) resulted in graded materials comprising an ensemble of metal nanowires of continuously variable lengths. [24] Lateral gradients of tens of micrometers in nanowire length were obtained by template electrodeposition or electrodissolution of Cu in NAMs under an applied lateral voltage gradient.…”

Spatiotemporally Controlled Formation of Two‐Component Counterpropagating Lateral Graft Density Gradients of Mixed Polymer Brushes on Planar Au Surfaces

“…Electrochemical-potential gradients are able to generate chemical-composition gradients of organothiol SAMs on thin Au electrodes, [5,16,17,[35][36][37][38][39][40] with tunable gradient properties, such as the position and slope of the transition region on samples with a broad range of physical sizes. The electrochemical-gradient approach is extended here to generate mixed polymer-brush gradients of poly(N-isopropylacrylamide) (PNIPAAm) and poly(2-hydroxyethyl methacrylate) (PHEMA), as shown in Scheme 1.…”