Poly[Fe(phen-NH₂)₃]²⁺ modified electrodes: Proton-gated charge transfer reactions and applications in current rectification

Poly[Fe(phen-NH₂)₃]²⁺ electrodes were prepared on Au-sputtered quartz crystals (9 MHz) by oxidizing iron(II) tris(5-amino-1,10-phenanthroline) (denoted Fe(phen-NH₂)₃²⁺) complex in acetonitrile. According to the study with the electrochemical quartz crystal microbalance (EQCM) technique, the decisive step in this electrode preparation involved an anodic polymerization. The resulting polymer (denoted poly[Fe(phen-NH₂)₃]²⁺) as prepared on the electrodes showed two redox waves in aqueous solutions. One was ascribed to the electron removal from the metal center (Fe^II), poly[Fe(phen-NH₂)₃]^3+/2+, and the other one to an electron addition to the resulting polymer, poly[Fe(phen-NH₂)₃]^2+/+. For the latter waves, the peak potential shifted to more negative values in a linear manner with an increased pH. From the slope (ca. 72 mV pH^-1) and the half-height-peak width (W_1/2≈120 mV), a one-electron-one-proton transfer reaction was ascribed. Because of this property, the poly[Fe(phen-NH₂)₃]²⁺ electrode behaved as a pH sensor. The sensitivity covered a pH range from 2 to 10. Energetic probing with Fe(CN)₆^3-/4-, Ru(NH₃)₆^3+/2+ and methylviologen (MV^2+/+/0), in addition, revealed that this pH dependence could be employed to regulate the electron transfer taking place through the polyFe(phen-NH₂)₃²⁺ film. As a result, current rectifications for Fe(CN)₆^3-/4- and Ru(NH₃)₆^3+/2+ were achieved. In this study a short-range interaction between MV²⁺ and the polymeric Fe(phen-NH₂)₃²⁺ film was also found. The associated Gibbs energy change was estimated to be -33 kJ. Long-term experiments, in addition, suggested that although a decrease in pH might deactivate the electrochemical activity of the adsorbate, the adsorbed MV²⁺ did not diffuse away from the electrode. In consequence, a vivid 'on-off' pattern in terms of current versus time was found during the variation in pH.