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 An-sputtered quartz c rystals (9 MHz) by oxidizing iron(II) tris(5-amino-1,10-phenanthroline) (de noted Fe(phen-NH2)(3)(2+)) 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 polymerizati on. The resulting polymer (denoted poly[Fe(phen-NH2)(3)](2+)) as prepared o n the electrodes showed two redox waves in aqueous solutions. One was ascri bed 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 po lymer, poly[Fe(phen-NH2)(3)](2+/+). For the latter waver, the peak potentia l 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-12 appr oximate to 120 mV). a one-electon-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 (MV2+o), in addition, revealed that this pH dependence could be employed to regu late the electron transfer taking place through the polyFe(phen-NH2)(3)(2+) 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 M V2+ and the polymeric Fe(phen-NH2)(3)(2+) film was also found. The associat ed 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 diffus e away from the electrode. In consequence, a vivid ion-off pattern in terms of current Versus time was found during the variation in pH. (C) 2001 Else vier Science B.V. All rights reserved.