CREATING AND IMAGING MICROSTRUCTURES ON SURFACES

We have investigated self-assembled monolayers (SAMs) of alkyl thiols on gold for their use in modification of an electrode surface. By derivatization of a pendent functional group of the SAM, a variety of functionalities or chemicals, such as enzymes, could be attached to the surface. Instead of derivatizing all pendent groups, our goal is to restrict derivatization to pendent groups located within microscopically-sized domains on the surface. We have used two approaches to modify the SAM surface, 1) removal of SAMs and 2) derivatization of pendent amines of SAMs within micrometer-sized regions. In the first, an electron transfer mediator, tris(2,2’-bipyridyl)ruthenium(II) (Ru(bpy)3 2+ ), was used to create micrometer-sized “holes” or “defects” in an SAM on a polycrystalline gold electrode by a technique we refer to as “localized-reductive desorption.” In this technique, Ru(bpy)3 2+ was reduced at a carbon microelectrode positioned within 2 =µm of the SAM surface. The reduced Ru(bpy)3 2+ diffused to the SAM, transferred its electrons to the adsorbed thiols causing the thiols to desorb within a microscopic domain. Electrogenerated chemiluminescence imaging (ECL) with luminol in alkaline peroxide was used with a microscopebased imaging system to observe the deliberately-produced “defect” sites or “holes” in the alkyl thiol coating. The effectiveness of the reductive desorption decreased significantly when oxygen was present. Reduction of oxygen at -1.0 V vs. Ag/AgCI, prior to mediator generation at -2.0 V, yielded more effective thiol desorption. The precathodization step reduced dissolved oxygen preventing the oxygen from scavenging electrons from the reduced Ru(bpy)3