Electrochemical sensing of mercury ions in electrolyte solutions by nitrogen-doped graphene quantum dot electrodes at ultralow concentrations

Electrochemical detection of mercury ions in aqueous solution was investigated at indium tin oxide (ITO) conducting glass electrode modified by nitrogen-doped graphene quantum dots (N-doped GQDs). The N-doped GQDs with an average particle size of 4.5 nm were synthesized through an infrared-assisted pyrolysis of citric acid and urea at 250 °C. The GQD sample contains high oxidation and amidation level, i.e., O/C and N/C atomic ratios: 37.6% and 30.7%, respectively. The electrochemical sensing toward Hg²⁺ ions was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Based on the CV and EIS analyses, both the reductive and oxidative peak currents as well as the equivalent series resistance demonstrate a decreasing trend with increased Hg²⁺ concentration. The detection limit of N-doped GQD/ITO electrodes toward Hg²⁺ ions reached 10 ppb with the accumulation time of 32 s. The GQD/ITO electrodes also exhibit superior selectivity toward the target contaminant (i.e. Hg²⁺ ion). Accordingly, the functionalized GQDs pave the way for engineering the electrochemical sensors capable of detecting toxic Hg²⁺ ions with superb sensitivity and selectivity.