CoO/Co-graphene quantum dots as an oxidative mimetic nanozyme for the colorimetric detection of l-cysteine

Abstract

The preparation of cobalt-based nanozymes with high oxidase-like activity still needs more efforts. In this paper, we report the synthesis of a CoO/Co–tryptophan-functional graphene quantum dot hybrid (CoO/Co–Try-GQD). Firstly, cobalt ions coordinate with the indole nitrogen on Try-GQD to form a complex, followed by thermal reduction and oxidation. The resulting hybrid presents a three-dimensional network structure, and CoO/Co nanoparticles are uniformly dispersed on the graphene sheet with an average size of 10 ± 0.24 nm. This unique structure improved the oxidase-like activity of the hybrid, enabling it to catalyze the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to rapidly produce deep blue ox-TMB with a strong absorbance at 652 nm (A₆₅₂). A colorimetric method was developed for the highly sensitive determination of L-cysteine (L-cys) based on the inhibition of the hybrid's oxidase-like activity and low A₆₅₂ caused by the binding of L-cys with Co atoms on CoO/Co via the Co–S bond. The A₆₅₂ linearly decreased with increasing L-cys concentration in the range of 0.05–2 μM, and the detection limit was 0.032 μM. Further, the established method has been successfully applied to the determination of L-cys in milk.