Highly Redox-Active Hematin-Functionalized Carbon Mesoporous Nanomaterial for Electrocatalytic Reduction Applications in Neutral Media

Hematin is a hydroxyl group linked heme site (hydroxyl heme) of the natural enzymes/proteins like hemoglobin, cytochrome c, catalase, and horseradish peroxidase, and it has an important role in the physiological function. Because of problems like poor electron-transfer functionality (on solid electrodes), poor solubility, and molecular aggregation in aqueous solution, limited electrochemical studies have been reported in the literature. A new electrode modification method for hematin using graphitized mesoporous carbon nanomaterial and chitosan for enhanced redox-active and efficient electrocatalytic reductions of hydrogen peroxide and dissolved oxygen in neutral pH was demonstrated in this work. The hematin-modified electrode showed a highly stable redox peak at E°′ = −0.390 V versus Ag/AgCl with a heterogeneous rate constant value of 1.34 s–1. Calculated hematin-active loading concentration (Γhemat = 126 × 10–10 mol cm–2) is ∼20 times higher than the reported values. Physicochemical and electrochemical characterizations revealed trapping of the hematin via axial bond coordination and intermolecular hydrogen bonding with amino functional groups of chitosan and π–π interactions with the graphitic site of mesoporous carbon within the matrix. The new hematin electrode showed ∼400 mV reduction in the overpotential with current sensitivity/detection range of 570 nA μM–1/100–900 μM and 6.7 μA ppm–1/1–10 ppm, respectively, for H2O2 and dissolved oxygen reduction reactions in pH 7 phosphate buffer solution. Michaelis–Menten kinetics were applied for the H2O2 reduction reaction and estimated the rate constant values as KM = 0.78 mM and ks = 1.15 s–1. No marked interference was noticed with common biochemicals such as nitrite, nitrate, glucose, uric acid, ascorbic acid, xanthine, hypoxanthine, cysteine, and dopamine on amperometric i–t detection of H2O2 indicating the activity similar to the heme-based proteins/enzymes.