Enhanced Specificity and Sensitivity for the Determination of Nickel(II) by Square-wave Adsorptive Cathodic Stripping Voltammetry at Disposable Graphene-modified Pencil Graphite Electrodes

Abstract

Chemical sensors relying on graphene-based materials have been widely used for electrochemical determination of metal ions and have demonstrated excellent signal amplification. This study reports an electrochemically reduced graphene oxide (ERGO)/mercury film (HgF) nanocomposite-modified pencil graphite electrode (PGE) prepared through successive electrochemical reduction of graphene oxide (GO) sheets and an in situ plated HgF. The ERGO-PG-HgFE, in combination with dimethylglyoxime (DMG) and square-wave adsorptive cathodic stripping voltammetry (SW-AdCSV), was evaluated for the determination of Ni²⁺ in tap and natural river water samples. A single-step electrode pre-concentration approach was employed for the in situ Hg-film electroplating, metal-chelate complex formation, and non-electrolytic adsorption at –0.7 V. The current response due to nickel-dimethylglyoxime [Ni(II)-DMG₂] complex reduction was studied as a function of experimental paratmeters including the accumulation potential, accumulation time, rotation speed, frequency and amplitude, and carefully optimized for the determination of Ni²⁺ at low concentration levels (μg L⁻¹) in pH 9.4 of 0.1 M NH₃–NH₄Cl buffer. The reduction peak currents were linear with the Ni²⁺ concentration between 2 and 16 μg L⁻¹. The limits of detection and quantitation were 0.120 ± 0.002 µg L⁻¹ and 0.401 ± 0.007 µg L⁻¹ respectively, for the determination of Ni²⁺ at an accumulation time of 120 s. The ERGO-PG-HgFE further demonstrated a highly selective stripping response toward Ni²⁺ determination compared to Co²⁺. The electrode was found to be sufficiently sensitive to determine metal ions in water samples at 0.1 µg L⁻¹, well below the World Health Organization standards.

Keywords:

• Dimethylglyoxime (DMG)
• electrochemically reduced graphene oxide (ERGO)
• graphene oxide (GO)
• mercury film (HgF)
• pencil graphite electrode (PGE)
• square-wave adsorptive cathodic stripping voltammetry (SW-AdCSV)

Acknowledgments

The following work was performed in collaboration with SensorLab, the University of the Western Cape and the National Research Foundation (NRF) of South Africa.