The direct electron transfer of iron-containing superoxide dismutase (Fe-SOD) and its catalysis for the oxygen reduction reaction (ORR) in room temperature ionic liquids (RTILs) on a gold electrode

In this work, for the first time, the direct electron transfer of iron-containing superoxide dismutase (Fe-SOD) was observed by cyclic voltammetry on a gold (Au) electrode in three RTILs, i.e., 1 -ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), 1-n-propyl-3-methylimidazolium tetra fluoroborate (PMIBF4) and 1 -n-butyl-3 -methylimidazoliurn tetrafluoroborate (BMIBF4). And the results demonstrate that when the scan rate was as low as 1 mV/s, a pair of well-defined quasi-reversible peaks of Fe-SOD was presented, while as the potential scan rate was above 10 mV/s, the reduction peak of Fe-SOD disappeared though its oxidation peak could be clearly observed even as the potential scan rate was up to 400 mV/s, strongly indicating that these CVs we observed were attributable to Fe-SOD rather than the impurities in RTILs. Its catalysis for oxygen reduction reaction (ORR) was directly verified by the shifting of formal potential, E-0', of ORR, to the positive direction though the value of standard rate constant, kappa(0), corresponding to ORR, was not much enhanced. In PMIBF4, for the multi-walled carbon nanotubes (MWCNTs)-modified gold electrode, both the reduction peak current and oxidation peak current for oxygen redox reaction were all dramatically enhanced compared to the case of a bare gold electrode, and the value Of K 0 was also increased from 3.1 x 10(-3) cm s(-1) for the bare gold electrode, to 17.5 x 10(-3) cm s(-1). Hence, in the presence of Fe-SOD in RTILs, MWCNTs, showing catalysis for the electron transfer process of ORR, coupled with Fe-SOD, leading to the shifting of formal potential corresponding to ORR to the positive direction, presented us a satisfactory catalysis for ORR in RTILs. Some reasons available for this catalysis behavior stemming from Fe-SOD, and MWCNTs as well, for ORR are discussed based on the previously developed proposition.