It can be noted from Fig. 4A that the reduction peak currents are directly linear to the scan rates in the range from 30 to 280 mV/s for 100 lM FA in 0.04 M B-R solution (pH 7.0), suggesting that the electrochemical reduction of FA undergoes an adsorption controlled process. Inset of Fig. 4A displays the linear sweep voltammetry grams (LSVs) of OMC/GCE at different scan rates. It can be seen that the reduction peak currents (Ip) increase and the peak potentials (Ep) shift negatively along with the increase of the scan
rates, respectively. For an irreversible process, the Ep can be expressed by the following equation [36]: Ep = E0’ + m [0.78 + ln D1/2 k1 s –0.5ln m] + (m/2) ln v (m = RT/[(1–a)nF]). The calibration equation is Ep = 0.636–0.0127 ln v (R = 0.998), as can be seen from Fig. 4B. Then the number of electrons transferred (n) can be obtained according to the slope (b) of the plot: b = RT/2(1–a)nF, where a of the reduction process is assumed as 0.5, and n is estimated to be about 2.