In situ Observation of Direct Electron Transfer Reaction of Cytochrome c Immobilized on ITO Electrode Modified with 11-{2-[2-(2-Methoxyethoxy)- ethoxy]ethoxy}undecylphosphonic Acid Self-assembled Monolayer Film by Electrochemical Slab Optical Waveguide Spectroscopy

To immobilize cytochrome c (cyt. c ) on an ITO electrode while keeping its direct electron transfer (DET) functionality, the ITO electrode surface was modified with 11-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}undecylphosphonic acid (CH 3 O (CH 2 CH 2 O) 3 C 11 H 22 PO(OH) 2 , M-EG 3 -UPA) self-assembled monolayer (SAM) film. After a 100-times washing process to exchange a phosphate buffer saline solution surrounding cyt. c and ITO electrode to a fresh one, an in situ observation of visible absorption spectral change with slab optical waveguide (SOWG) spectroscopy showed that 87.7% of the cyt. c adsorbed on the M-EG3-UPA modified ITO electrode remained on the ITO electrode. The SOWG absorption spectra corresponding to oxidized and reduced cyt. c were observed with setting the ITO electrode potential at 0.3 and–0.3 V vs . Ag/AgCl, respectively, while probing the DET reaction between cyt. c and ITO electrode occurred. The amount of cyt. c was evaluated to be about 19.4% of a monolayer coverage based on the coulomb amount in oxidation and reduction peaks on cyclic voltammetry (CV) data. The CV peak current maintained to be 83.4% compared with the initial value for a M-EG 3 -UPA modified ITO electrode after 60 min continuous scan with 0.1 V/s between 0.3 and–0.3 V vs . Ag/AgCl.