Ultrasensitive spectroelectrochemistry of monolayer and submonolayer thin films using an electroactive integrated optical waveguide

To increase the applicability of spectroelectrochemistry to ultrathin films at a transparent semiconductor electrode, a single-mode, step-index electroactive integrated optical waveguide (the EA-IOW) incorporating an indium tin oxide top layer as an electrode was developed. The EA-IOW is much more sensitive to absorbance by molecular adlayer species than previous electroactive waveguide designs; a sensitivity increase of ca. 4000 relative to a single-pass transmission experiment has been measured by monitoring the reduction of a surface-adsorbed dye molecule. An important characteristic of the present three-layer EA-IOW structure is that its design is close to being optimized in terms of maximizing sensitivity while maintaining acceptable optical losses, as determined by theoretical modeling. Before the EA-IOW can be applied to measure absorbance changes arising from electron transfer in ultrathin films, the background optical changes that occur as a function of potential must be understood. There is a linear decrease in outcoupled intensity as the EA-IOW is scanned negative which is a result of an increase in the number of free carriers inside the ITO, a highly reproducible effect. There is also a poorly reproducible non-linear component to the optical background, accompanied by a hysteresis between the forward and reverse potential scans, that disappears after conditioning the EA-IOW in electrolyte solution for a period of several days. It is hypothesized that his effect is due to hydroxylation of the ITO network. To test the EA-IOW experimentally, the reduction of surface-adsorption methylene blue was monitored, along with the formation of Prussian blue during the electrochemistry of ferricyanide. Two experimental applications of the EA-IOW will be reviewed; first, the EA-IOW was used to measure the spectroelectrochemistry of submonolayer films of phthalocyanine polymeric assemblies to compare the electrochemistry at submonolayer and multilayer coverages. Finally, the use of the EA-IOW in protein electrochemistry will be discussed. The dichroic ratio of cytochrome c adsorbed to indium tin oxide was measured as a function of potential, and found to be consistent with an orientation of the heme ligand that is almost parallel to the electrode surface. Also, a change in heme orientation was detected during reduction of the protein.