Axial Coordination Site-Turned Surface Confinement, Electron Transfer, and Bio-Electrocatalytic Applications of a...

Understanding the relation between the chemical bonding and the electron-transfer (ET) reaction of surface-confined hemin (a five-coordinated Fe-porphyrin-with-chlorine complex) is a special interest in the biomimicking studies of heme proteins. Owing to the difficulty in ET function, scanty electrochemical reports of hemin in aqueous solution were reported. It has been noticed that in most of the reported procedures, the sixth axial coordination position of the hemin complex has been unknowingly turned by attaching with water molecules (potential cycling in alkaline conditions or heating), solvents such as ethanol and dimethyl sulfoxide, and nitrogen-donating compounds that have helped for the heme ET reaction. In this work, a systematic effort has been taken to find out the contribution of hemin and its axial bond coordination with π-π interaction, hydrogen bonding, and hydrophobic binding systems toward the ET reaction. Various graphitic carbons such as graphitized mesoporous carbon (GMC), mesoporous carbon-hydrophilic and hydrophobic units, graphite nanopowder, graphene oxide, single-walled carbon, multiwalled carbon nanotube (MWCNT), and carboxylic acid-functionalized MWCNT (as a source for π-π interaction, hydrogen bonding, and hydrophobic environment) along with the amino functional group of chitosan (Chit; as an axial site coordinating system) have been tested by modifying them as a hemin hybrid on a glassy carbon electrode (GCE). In addition, a gold nanoparticle (Au_nano) system was combined with the above matrix as a molecular wiring agent, and its role was examined. A highly stable and well-defined redox peak at an apparent formal potential (E^o') of -320 mV versus Ag/AgCl with the highest surface excess of 120 × 10^-10 mol cm^-2 was noticed with the GCE/Au_nano-GMC@hemin-Chit hybrid system, wherein all interactive features have been utilized. Omitting any of the individual interactions resulted in either decreased (with Au_nano) or nil current response. As applications, efficient bio-electrocatalytic reduction and sensing of dissolved oxygen and hydrogen peroxide have been demonstrated.