Preparation and characterization of redox active molecular assemblies on microelectrode arrays

Microelectrode arrays, consisting of six or eight individually addressable Au or Pt microelectrodes about 2 μm wide, 50 μm long, and 0.1 μm thick separated by about 2 μm on a Si₃N₄ substrate, can be modified by immersion into a solution containing molecules having thiol, dithiocarbamate, or disulfide functional groups. The functional groups yield selective modification of the Au or Pt, not the Si₃N₄, with about one monolayer of molecular reagents. Electrochemical and Auger electron spectroscopy (AES) data are summarized to illustrate that the dithiocarbamate functional group can be used to link redox active molecules to Au or Pt surfaces. Results are presented to illustrate that secondary ion mass spectrometry (SIMS) can be used to characterize organic monolayers on the microelectrodes. Preliminary findings are presented showing the esters of lipoic acid, a five-membered cyclic disulfide, will selectively modify Au surfaces versus Si₃N₄, and the cyclic disulfide will kinetically compete with a linear disulfide for sites on a Au surfaces. In a competition with the linear disulfide, the cyclic disulfide is at least ten times more reactive towards Au. Overall, the studies define classes of experiments needed to develop rational approaches to the modification of surfaces using spontaneous self-assembly methods by taking advantage of selective surface coordination chemistry of molecules having appropriate functional groups.