A comparative study of different electrodeposited NiCo2O4 microspheres anchored on a reduced graphene oxide platform: electrochemical sensor for anti-depressant drug venlafaxine

Abstract

Two different fabrication methods were performed and compared for preparation of binary metallic oxide microstructures supported on a reduced graphene oxide (rGO) modified graphite electrode. Nickel-Cobalt oxide microspheres (NiCo₂O₄ MSs) were prepared by two different deposition methods: wet chemical and in situ-electrical deposited methods. Different characterization methods were conducted, including cyclic voltammetry (CV), scanning emission microscopy (SEM), electrochemical impedance spectroscopy (EIS) and Raman spectroscopy. The deposition methods of NiCo₂O₄ MSs were found to affect the electrochemical behavior of the modified electrodes towards the oxidation of venlafaxine (VEN), an anti-depressant drug. The fabricated electrode showed linearity over the range 5-500 nmol L^-1 and an excellent sensitivity with a limit of detection (LOD) and limit of quantitation (LOQ) of 3.4 and 10.3 nmol L^-1, respectively. It was revealed that the wet-NiCo₂O₄@rGO modified electrode prepared by the wet chemical method showed an improved electrochemical behavior for determination of VEN in pharmaceuticals and human plasma with high recovery results in the range of 96.7-98.6% and 96.0-100.7%, respectively without any interference from the co-existing components.

Fig. 1. Square wave voltammograms of 0.1 μmol L⁻¹ VEN solution at (a) bare GCE, (b) bare PGE, (c) in situ-NiCo₂O₄@rGO and (d) wet-NiCo₂O₄@rGO in 0.04 mol L⁻¹ BR buffer (pH = 10.5) under optimum conditions. The upper inset: cyclic voltammograms of 0.1 μmol L⁻¹ VEN recorded at different electrodes.

Fig. 2. SEM images of (A) bare PGE, (B) in situ-NiCo₂O₄@rGO, (C) wet-NiCo₂O₄@rGO surfaces and (D) Raman spectra of GO, rGO, in situ and wet-NiCo₂O₄/rGO composite.

Fig. 3. Nyquist plots of electrochemical responses of 1.0 mmol L⁻¹ Fe(CN)₆^3−/4− in 0.5 mol L⁻¹ KCl recorded at (a) bare PGE, (b) in situ-NiCo₂O₄@rGO and (c) wet-NiCo₂O₄@rGO at scan rates 100 mV s⁻¹. The upper inset: enlarged view at low impedance range.

Fig. 4. Cyclic voltammograms of 0.1 μmol L⁻¹ VEN solution at different scan rates (0.05 to 1.0 V s⁻¹) recorded at wet-NiCo₂O₄@rGO electrode. Inset: relation between scan rate and oxidation peak current of VEN.

Fig. 5. Dependence of the logarithm of peak current (log I_P/μA) and the oxidation peak potential (E/V) of VEN on logarithm of scan rate (V s⁻¹).

Fig. 6. (A) Effect of different pH values on square wave voltammograms of 0.1 μmol L⁻¹ VEN solution in 0.04 mol L⁻¹ BR buffer. The upper inset: histogram of potential peak position against pH values. (B) Linear plot between peak potential (V) and pH values of supporting electrolyte.

Scheme 1. The probable mechanism of VEN electro-oxidation.

Fig. 7. Square wave voltammograms of different concentrations of VEN (0.5 to 50 × 10⁻⁸ mol L⁻¹) recorded at wet-NiCo₂O₄@rGO. Optimum parameters: 0.04 mol L⁻¹ BR buffer (pH = 10.5), E_acc = −0.5 V, frequency = 225 Hz, pulse height = 5 mV, step height = 15 mV, t_acc = 30 s. Inset: calibration curve between VEN concentration and peak current.