Improved Electrical Wiring of Glucose Oxidase Enzyme with an in-Situ Immobilized Mn(1,10-Phenanthroline)₂Cl₂‑Complex/Multiwalled Carbon Nanotube-Modified Electrode Displaying Superior Performance to Os-Complex for High-Current Sensitivity Bioelectrocatalytic and Biofuel Cell Applications

The search for a new and efficient transducer that can electrically connect enzyme active sites, like flavin adenine dinucleotide in glucose oxidase (GOx), with the electrode surface is a cutting-edge research area. Currently, Os­(bpy)-complex pendent polyvinyl­pyridine/polyvinyl imidazole/pyridinium hydrogel based chemically modified electrodes have been widely used for this purpose (bpy = 2,2’-bipyridine). Herein, we report, a [Mn₂^III(phen)₄(O)­(Cl)₂]²⁺ complex/Nafion-immobilized carboxylic acid-functionalized multiwalled carbon nanotube modified glassy carbon electrode (GCE/f-MWCNT@Mn₂(Phen)₄O­(Cl)₂-Nf, phen = 1,10-phenanthroline), prepared by an in-situ electrochemical method using the precursor, Mn­(phen)₂Cl₂, as an efficient and low cost alternate to the Os-complex transducer, for the glucose oxidase enzyme (GOx) based bio-electro-catalytic system. The existence of the key active site, [Mn₂^III(phen)₄(O)­(Cl)₂]²⁺, on the modified electrode was confirmed by physicochemical characterizations using transmission electron microscope, Raman, infrared, and UV–vis spectroscopes and electrospray ionization mass spectrometry techniques. The Mn-complex modified electrode showed a redox peak at E°′ = 0.55 V vs Ag/AgCl in neutral solution with a surface excess (Γ_Mn) value of 5.6 × 10^–9 mol cm^–2. The GOx enzyme bioanode prepared by adsorbing GOx on the Mn-complex modified electrode has shown an efficient bioelectrocatalytic oxidation of glucose with a Tafel slope value of 111 mV dec^–1. Amperometric i–t analysis of glucose showed a calibration plot in a linear range of 50–550 μM and with current sensitivity of 316.7 μA mM^–1 cm^–2. The current sensitivity value obtained here is about 2–80 000 times higher than that of the Os­(bpy)-complex based transducers used for GOx based bio-electro-catalytic applications. Utilizing this new bioanode system along with a Pt-based oxygen reduction electrode, a new biofuel cell was constructed and achieved a power density value 7.5 μW cm^–2.