Oxygen Vacancy-Enriched NiO Nanozymes Achieved by Facile Annealing in Argon for Detection of L-Cys

Nickel oxide (NiO) nanozymes, as an excellent oxidase mimic, have been widely used in fluorescence biological detection, water pollutant analysis, food safety and cell imaging. However, a great challenge in fully realising these applications is regulating their crystalline micro-/nano-structure and composites to achieve high enzyme activity and high specific surface area. Herein, we applied a very simple thermal annealing treatment to restructure the calcined precursor of NiO. Importantly, it was found that the oxygen vacancy (OV) concentration of the targeted NiO nanozyme significantly increased when the annealing atmosphere was argon rather than air. Moreover, the as-prepared novel NiO sample (NiO-OV) nanosheets achieved about 2-fold enhancement in their specific surface area. It is believed that a higher OV concentration and larger specific surface area increase enzyme activity by accelerating the electron transfer rate and improving catalytic interfaces. The significant improvement in the enzyme activity of NiO-OV was verified using the fluorescence "turn-on" experiment of Amplex Red (AR). Finally, using the NiO-OV/AR system, we constructed a highly sensitive enzyme sensor on L-Cys with a detection limit of 37.8 nM. The sensor also displayed excellent specificity for ten typical amino acid interferents.