Mercury ion-imprinted polymer nanoparticles (Hg-IP-NPs) were synthesized via precipitation polymerization by using itaconic acid as a functional monomer. A carbon paste electrode was impregnated with the synthesized Hg-IP-NPs and MWCNTs to obtain a highly sensitive and selective electrode for determination of Hg(II). Mercury ion is first accumulated on the electrode surface via an open circuit procedure. After reduction of Hg(II) ions to its metallic form at a negative pre-potential, square wave anodic stripping voltammetry was applied to generate the electrochemical signal. The high affinity of the Hg-IP-NPs for Hg(II) was substantiated by comparing of the signals of electrodes with imprinted and non-imprinted polymer. The beneficial effect of MWCNTs on the voltammetric signal is also demonstrated. Under the optimized conditions and at a typical working potential of +0.05 V (vs. Ag/AgCl), the electrode has a linear response in the 0.1-20 nmol L Hg(II) concentration range and a 29 pM detection limit. The electrochemical sensitivity is as high as 1441 A·M·cm which is among the best values known. The electrode was applied to the determination of Hg(II) in water samples. Graphical abstract Schematic representation of the sensor electrode modified with mercury-imprinted polymer nanoparticles, and the recognition and voltammetric determination steps.

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Nanosized graphitic carbon nitride (nano-g-C 3 N 4 ) was synthesized using the thermal polymerization of melamine and utilized as a novel electrocatalyst for electrooxidation of oxalic acid (OA). The nano-g-C 3 N 4 was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The electrocatalytic performance of the g-C 3 N 4 -modified carbon paste electrode (g-C 3 N 4 /CPE) was investigated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The modified electrode showed excellent electrocatalytic activity towards the oxidation of OA. The effects of electrode composition, pH and scan rate on the electrooxidation response of OA were studied. Under optimized conditions, the differential pulse voltammetric response of the electrode was linearly related to OA concentrations between 1 and 1000 mM, with a limit of detection (LOD) of 7.5 Â 10 À7 M. The electrode exhibited very high sensitivity of 1945 mA mM À1 cm À2 for OA assay. The developed method was successfully applied for the determination of OA in urine samples with satisfactory results. Experimental Apparatus and chemicalsThe surface morphology of the prepared g-C 3 N 4 was examined by means of FE-SEM (TESCAN MIRA, Czech). The crystalline

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