A study on the electro-reductive cycle of amino-functionalized graphene quantum dots immobilized on graphene oxide for amperometric determination of oxalic acid.

Amino-functionalized graphene quantum dots (NH2-GQD) are described for the amperometric determination of oxalic acid. The NH2-GQD were synthesized via a hydrothermal method using hexamethylenetetramine as the source for nitrogen. The average particle size of the GQD is ∼30 nm, which is also supported by TEM. Electrochemical analysis of the NH2-GQD-GO composite on a glassy carbon electrode at pH 7.4 showed a faint reduction peak at -0.6 V vs. SCE, which was enhanced in the presence of oxalic acid. This variation in cathodic current density is an interesting deviation from the usually studied anodic current density for the electrochemical sensors. This is also supported by cyclic voltammetry and time-based amperometric measurements. The electrode has a linear response in the 0.5-2.0 mM and 2.0-55 mM oxalate concentration ranges and a 50 μM detection limit (at S/N = 3). The electrode was successfully applied to the determination of oxalate in spiked urine samples. Graphical abstract Schematic representation of the fabrication of amino-functionalized graphene quantum dots and graphene oxide composite coated on glassy carbon electrode for utilizing the electro-reduction peak in cyclic voltammetry at around -0.6 V for the quantitative determination of oxalic acid.