Improved Electrical Wiring of Glucose Oxidase Enzyme with an <i>in-Situ</i> Immobilized Mn(1,10-Phenanthroline)<sub>2</sub>Cl<sub>2</sub>‑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 [Mn<sub>2</sub><sup>III</sup>(phen)<sub>4</sub>(O)­(Cl)<sub>2</sub>]<sup>2+</sup> complex/Nafion-immobilized carboxylic acid-functionalized multiwalled carbon nanotube modified glassy carbon electrode (GCE/f-MWCNT@Mn<sub>2</sub>(Phen)<sub>4</sub>O­(Cl)<sub>2</sub>-Nf, phen = 1,10-phenanthroline), prepared by an <i>in-situ</i> electrochemical method using the precursor, Mn­(phen)<sub>2</sub>Cl<sub>2</sub>, 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, [Mn<sub>2</sub><sup>III</sup>(phen)<sub>4</sub>(O)­(Cl)<sub>2</sub>]<sup>2+</sup>, 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 <i>E</i>°′ = 0.55 V vs Ag/AgCl in neutral solution with a surface excess (Γ<sub>Mn</sub>) value of 5.6 × 10<sup>–9</sup> mol cm<sup>–2</sup>. 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<sup>–1</sup>. Amperometric <i>i</i>–<i>t</i> analysis of glucose showed a calibration plot in a linear range of 50–550 μM and with current sensitivity of 316.7 μA mM<sup>–1</sup> cm<sup>–2</sup>. 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<sup>–2</sup>.