Abstracts Biological Sciences

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

by Darren Robert Dunphy

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.