MnO2 polymorph selection for non-enzymatic glucose detection: An integrated experimental and density functional theory investigation

Herein, different polymorphs (alpha, beta, gamma and delta) of MnO2 have been synthesized and its electrochemical sensing behaviour was scrutinized with glucose as a probe molecule. Comparative morphology and structural features of all the polymorphs of MnO2 were investigated through Field emission scanning electron microscopy (FESEM), Xray diffraction (XRD), micro-Raman and X-ray photoelectron spectroscopy (XPS). Evaluation of electrocatalytic activity towards glucose oxidation was performed by cyclic voltammetry and chronoamperometry techniques. Among all, alpha- MnO2 has augmented sensitivity over the tested range (5 mu M-855 mu M) and the oxidation reaction is governed by a diffusion controlled process. To support our experimental findings, bonding and charge transfer mechanism of glucose molecules on different phases of MnO2 surfaces have been analysed by employing the state of the art Density Functional Theory (DFT) simulations. Higher binding energy of the glucose molecule and the maximum charge transfer from O 2p orbital of glucose to Mn 3d orbital of alpha- MnO2 justifies the higher glucose sensing activity of the alpha phase as observed in the experiment. Furthermore, a wide linear range (5 mu M to 855 mu M), good specificity and stability of the designed sensor widens its application in the future sensing platform.