Oxygen Reduction Reaction of Hemoglobin on Indium-Tin-Oxide Electrodes and Its Application to Biofuel Cell Cathode

Biofuel cells (BFCs) are nature-inspired electrochemical systems. Natural bio-materials (e.g. glucose as the fuel and enzymes as the electrocatalysts) are used under mild conditions at around body temperature at near-neutral pH. We have developed BFCs with hemoglobin (Hb), [1, 2] making it possible for use as body implantable power supplies applied to medical devices such as pacemakers for cardiac resynchronization therapy and deep brain stimulation. In this study, indium tin oxide (ITO) thin films were synthesized on glass plates with different metal molar ratio by dip-coating methods and used as bio-electrodes for Hb. Electrochemical evaluation was conducted under a three-electrode system consisting of the synthesized ITO working, a platinum wire counter, and an Ag|AgCl|KCl(satd.) reference electrode. Cyclic voltammograms (CVs) in a Hb containing solution with pH 7.4 revealed a pair of well-defined redox peaks at around -0.15 V vs. Ag|AgCl|KCl(satd.) on the ITO electrode with In:Sn=19:1. The quantity of electrochemically active Hb molecules was estimated from the oxidation current of the CV curve. The estimated value of 9.1 pmol in this study corresponds well with the theoretical monolayer deposition of 8.3 pmol, assuming one Hb molecule occupies 20 nm² [3]. On the other hand, it was found that remarkably large redox current was observed for the ITO electrode with In:Sn=1:1, 222.3 pmol of Hb was estimated to be electrochemically active. It indicates that the electrochemically active species were not only Hb molecules in monolayer deposition on the ITO electrode but also included adsorbed multilayer Hb molecules, suggesting the charge transfer through several Hb molecules in multilayer deposition. Hb molecules exhibited oxygen reduction reaction (ORR) activity on all ITO electrodes with different metal molar ratio. The ITO with In:Sn=1:1 showed the highest ORR activity. These results prove that the Hb-ITO system can be used as the BFC cathode. These findings are great advantages for practical use of the Hb-ITO system, in particular for body implantable power supplies, as the BFC ITO cathode may be inserted into blood vessels.

References

[1] Y. Ayato, K. Sakurai, S. Fukunaga, T. Suganuma, K. Yamagiwa, H. Shiroishi, J. Kuwano, Biosens Bioelectron, 55 (2014) 14.

[2] Y. Ayato, N. Matsuda, Energies, 7(1) (2014) 1.

[3] M. Revenga-Parra, E. Lorenzo, F. Pariente, Sens. Actuat. B, 107 (2005) 678.