Direct Electron Transfer of Hemoglobin on Indium-Tin Oxide Electrodes Synthesized By Dip-Coating Methods Under Different Sintering Temperature

Abstract

The direct electron transfer (DET) between electrodes and redox proteins is important when considering practical applications such as body implantable devices for cardiac resynchronization therapy (CRT) and deep brain stimulation (DBS), since such electrical devices should require biocompatibility, simplicity of electrodes, potential difference in electrochemical cells, etc. We have investigated the DET properties of hemoglobin (Hb) molecules on indium-tin-oxide (ITO) electrodes [1-3]. In our previous study, the ITO electrodes were synthesized on glass plates by using dip-coating solution with different Sn/(Sn+In) molar ratio, and Hb molecules showed the DET activity on the synthesized ITO electrodes [3]. It was found that the DET activities of Hb molecules were different at ITO electrodes synthesized from different Sn ratio. The largest DET current was observed on the ITO electrode synthesized from a Sn ratio of 0.50. The results also suggested from previous studies with different Sn ratio that the active sites and electrical properties of ITO might be related with the DET behavior with Hb molecules. To understand the relationship between crystallinity of ITO and DET of Hb molecules on electrode surfaces, in this study, ITO electrodes were prepared on quartz plates under different sintering temperature from 350°C to 950°C for 15 min by using dip-coating solution with the Sn ratio of 0.50. The coating procedure was repeated ten times. X-ray diffraction (XRD) peaks attributed to Sn-doped In2O3 were observed, with the peak intensity increasing with increasing sintering temperature. Cyclic voltammetry (CV) was conducted with a three-electrode system in 39 mM phosphate buffered saline (PBS; pH 7.4) with or without 100 μM Hb solution. ITO sintered at 350°C and 950°C behaved as poor electrodes due to their high electrical resistance. Hb molecules showed good DET activities on the ITO electrodes sintered at 500°C, 650°C, and 800°C. The results indicated that there were optimum crystallinity of the ITO electrodes for showing DET activity to Hb molecules.