Electrochemical synthesis of Au-MnO₂ on electrophoretically prepared graphene nanocomposite for high performance supercapacitor and biosensor applications

Herein, we report a facile electrochemical synthesis of an Au-MnO₂ nanocomposite highly dispersed on an electrophoretically prepared graphene surface for the first time. Fascinatingly, we obtained a nanowires-like morphology for the MnO₂ by using a simple in situ electrochemical deposition method. The as-synthesized Au-MnO₂-graphene nanocomposite is characterized by various analytical and spectroscopic techniques viz. SEM, EDX, TEM, XRD, Raman spectroscopy and XPS. The as-prepared nanocomposite is employed in an electrochemical supercapacitor and for the sensitive detection of epinephrine. The supercapacitor performance is evaluated in 0.5 M NaOH by both cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) methods. The MnO₂:Au ratio during deposition plays a vital role to influence the capacitance properties. The highest specific capacitance of 575 F g^-1 for 1:0.01 (MnO₂:Au) at a current density of 2.5 A g^-1 has been obtained. The effect of current density, MnO₂:Au ratio, scan rate, mass loading and electrolyte concentration were also optimized and good cycle stability was demonstrated. The comparison of specific capacitance over MnO₂-graphene and Au-MnO₂-graphene nanocomposites suggests that the incorporation of Au nanoparticles on MnO₂-graphene surfaces has a highly substantial effect for enhancement of capacitive behaviour. Furthermore, the epinephrine sensor performance of an Au-MnO₂-graphene nanocomposite modified glassy carbon electrode is evaluated by CV and differential pulse voltammetry (DPV) techniques. Interestingly, the DPV sensor exhibited a very low detection limit of 24 nM and an excellent current sensitivity value of 35.6 μA μM^-1 cm^-2, surpassing several related modified electrodes and demonstrating several practical industrial applications.