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In this study, the electrochemical detection of Se(IV) on a glassy carbon electrode (GCE) modified with nitrogen-doped graphene (NG) is reported. NG was synthesized from graphene oxide (GO) by thermal annealing of GO in ammonia. Structural and morphological studies of the synthesized NG were conducted using field emission scanning electron microscopy (FESEM), Raman spectroscopy, high resolution-transmission electron microscopy (HR-TEM), Fourier Transform -infrared spectroscopy (FT-IR) and a CHNS analyzer. Electrochemical characterization of the unmodified GCE and the NG modified GCE (GCE-NG) was conducted using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The newly developed GCE-NG sensor demonstrated improved electrochemical properties when compared to the bare GCE. Square wave anodic stripping voltammetry (SWASV) was employed to optimize the proposed sensors' detection parameters: 0.1 M HClO4 supporting electrolyte, -0.8 V deposition potential and 50 s deposition. The calibration graph of Se(IV) concentrations and current response demonstrated linearity during the calibration of the sensor in the concentration range 1 -120 ppb, with a limit of detection (LOD) of 0.092 ppb. The proposed electrochemical sensor was applied in the analysis of real water samples and inductively coupled plasmaoptical emission spectroscopy (ICP-OES) was used to validate the results.

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An aptasensor was prepared by immobilising a biotinylated aptamer selected for Cd(II) on an activated carbon nanofiber (CNF) and streptavidin modified a screen-printed electrode. The acid activated CNF was characterised by FTIR, CHNS elemental analysis and electron microscopy. Cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy (EIS) were used to characterise each step in the aptasensor preparation electrochemically. A detection limit of 0.11 ppb and a linear range of 2-100 ppb were obtained for Cd(II) with tolerance for other interfering species. The aptasensor was applied in real water samples and validated with ICPOES.