Generation of Molecular-Scale Compositional Gradients in Self-Assembled Monolayers

Gradients of surface chemical composition were prepared by performing a selective photo-oxidation of self-assembled monolayers (SAMs) of alkanethiols adsorbed on a gold surface, using an ultraviolet laser (244 nm) coupled to an etched optical fiber. In the center of the exposed region, uniform oxidation occurs facilitating uniform replacement of the oxidation products with a second, contrasting thiol, while at the perimeter of the exposed region a gradient of exposure occurs, leading to a gradient of photochemical reaction and, following immersion of the sample in a solution of a contrasting thiol, a gradient of composition. Variation in the exposure is possible by varying the time or the power, and both the height and the width of the resulting gradient may be controlled straightforwardly yielding a simple but highly effective means of generating molecular-scale chemical gradients.