Selective Functionalization of Gold Microstructures with Ferrocenyl Derivatives via Reaction with Thiols or Disulfides: Characterization by Electrochemistry and Auger Electron Spectroscopy

Auger electron spectroscopy and electrochemistry show that ferrocenyl thiols or disulfides can be used to modify selectively the Au microelectrodes (~2μm×90μm ×0.1 μm) on a Si₃N₄ substrate. The Auger technique shows the selectivity of chemisorption of thiol (-SH) or disulfide (-SS-) groups on Au relative to their physisorption on Si₃N₄ to be at least 100:1. Immersion of Au electrodes into solutions containing (1-mercapto-3,6-dithiaheptanyl)octamethylferrocene (I), 11-mercaptoundecanoylferrocene (II), or bis[10-(ferrocenylcarbonyl)decyl] disulfide (III) yields modified Au electrodes showing about one monolayer of a reversibly redox active ferrocene reagent. Compound I yields an E_1/2 about 550 mV more negative than that for II or III (which both have the same E_1/2 values). The electroactive layer from I on Au is far more labile than that from II or III. Loss of I takes place with a t_1/2 of ~2 h at 298 K in a hexane solution; little or no loss of electroactive material derived from II or III occurs on the same time scale. When Au electrodes modified with I are immersed in hexane solutions of II, replacement of I by II occurs cleanly and at a rate that is the same as loss of I in hexane only. Detailed procedures for Auger element mapping of organic monolayers at high lateral resolution (~0.1 μm) are presented, and such maps give a reliable image of the distribution of molecules on the surface.