A (μ-oxo) dicopper complex anchoring graphitized mesoporous carbon surface prepared by an in situ electrochemical method for bioinspired electrocatalytic reduction of nitrite to ammonia and sensing

Abstract

The design and development of functional biomimetic systems that resemble natural enzymes is highly challenging. Cytochrome c nitrite reductase (ccNiR), a specific enzyme containing multiple heme sites, functions as a catalyst for converting nitrite to ammonium through a complex six-electron transfer reaction under physiological conditions. In the literature, only a few homogeneous biomimetic metal complexes catalyzing nitrite reduction were reported, all under non-physiological conditions (organic and acid media). Herein, we developed a bioinspired copper(II)–BPA(μ-oxo) complex (BPA = bis(pyrid-2-ylmethyl)amine), denoted as {Cu₂(μ-O)₂}, immobilized on a graphitized mesoporous carbon modified glassy carbon electrode, GCE/GMC@{Cu₂(μ-O)₂}, which was prepared by an in situ electrochemical reaction of a precursor complex, {Cu(BPA)Cl}. The as-prepared GCE/GMC@{Cu₂(μ-O)₂} worked as an efficient biomimetic heterogeneous electrocatalyst for the reduction of nitrite into ammonia, in an aqueous medium and at neutral pH. This bioinspired system showed a stable and well-defined redox system with E^o′ = −220 mV vs. Ag/AgCl, and a surface-excess value of 83.9 nmol cm⁻². The peak potential is much lower (>500 mV) than that of the unmodified electrode. In the presence of nitrite, a well-defined irreversible reduction peak at −0.250 V vs. Ag/AgCl is developed, which was assigned to the electrocatalytic reduction of nitrite. The molecular reaction and product formation were confirmed by SEM, TEM, Raman, IR, electrochemical quartz crystal microbalance (EQCM), and ESI-MS. A strong π–π interaction between the graphitic sp² carbons and the aromatic sp² in the BPA-pyridyl moiety of the metal complex favors good stability. To prove its functional application, the electroanalytical performance and bulk electrolysis conversion of nitrite into ammonia (faradaic efficiency of 92 ± 1%) were demonstrated.