Voltammetric determination of folic acid at physiological pH values by using a glassy carbon electrode modified with a multilayer composite consisting of polyoxometalate (H8P2Mo16V2O62) and reduced graphene oxide and prepared via layer-by-layer self-assembly and in-situ photoreduction

Vanadium-substituted polyoxometalate clusters of type H8P2Mo16V2O62 and reduced graphene oxide (rGO) were incorporated into a multilayer composite film via combining layer-by-layer self-assembly (using polyethyleneimine) and in-situ UV photoreduction. The growth of the multilayer film was monitored by UV-vis spectroscopy. The UV-induced conversion of graphene oxide to rGO in the multilayer film is due to the photocatalytic activity of P2Mo16V2 and was proven by X-ray photoelectron spectroscopy. The material was placed on a glassy carbon electrode (GCE) where it displayed good electrocatalytic activity towards oxidation of folic acid (FA) as demonstrated by cyclic voltammetry and differential pulse voltammetry. The modified GCE (best operated at a working voltage of 0.75 V vs Ag/AgCl and a scan rate of 50 mV s−1) compared to films containing either P2Mo16V2 or rGO only. The effects of the number of bilayers and of pH values on the electro-oxidation were investigated. The modified GCE was applied to selective determination of FA even in the presence of dopamine (DA) and uric acid (UA) at physiological pH. In fact, the modified GCE may also be applied to the simultaneous determination of DA, UA and FA. The amperometric response to FA increases linearly in the 10 nM to 0.8 mM FA concentration range, with a 0.28 nM detection limit. The sensor was successfully applied to the determination of FA in spiked human serum samples.