Self-Assembly of Peptide Nanostructures onto an Electrode Surface for Nonenzymatic Oxygen Sensing

Abstract

We report here the fabrication of a biomimetic sensor for direct oxygen reduction; the sensor consists of multicopper oxidases derived from cyclic-tetrameric copper(II) species containing the ligand (4-imidazolyl)ethylene-2-amino-1-ethylpyridine (apyhist) that are self-assembled with l-diphenylalanine micro/nanostructures (FF-MNTs). The [Cu4(apyhist)4]4+/FF-MNT complex was immobilized onto the surface of a glassy carbon (GC) electrode by poly ion complex formation with a Nafion film. This hybrid membrane allows regular proton transport to a Cu-based molecular oxygen reduction reaction catalyst, and the imidazole group in the imine ligand (apyhist) acts as a local buffer in the vicinity of the O2 reducing center, thus aiding the catalyst in retaining its selectivity for 4e–/4H+ oxygen reduction reaction. This nanocomposite provided improved sensing characteristics in the electrode interface with respect to the electroactive surface area, the diffusion coefficient, and the electron transfer kinetics. In addition, the hybrid film [Cu4(apyhist)4]4+/FF-MNT-coated GC electrode was successfully used as an enzymeless electrochemical sensor for the detection of dissolved oxygen in aqueous media at two concentration intervals, viz., 0.2–3.0 mg L–1 and greater than 3.0 mg L–1, with sensitivities of 25.0 and 80.2 μA L mg–1 cm–2, respectively, and a detection limit of 0.1 mg L–1. Evaluated in terms of relative standard deviation, the repeatability of the proposed sensor was less than 9.0% for ten measurements of a solution of 6.5 mg L–1 oxygen. Experimental efforts were conducted to use this proposed platform for O2 determination with real samples. Results from theoretical investigations using density functional theory support the hypothesis that the [Cu4(apyhist)4]4+ complex can act as the sole source of protons and electrons in the O2 reduction reaction.