Covalent Coupling-Regulated rGO/VN Nanocomposite Enabling Nitrogen Defects to Remarkably Boost the Peroxidase-Like Catalytic Efficiency for the Ultrasensitive Colorimetric Assay of Uric Acid.

It remains challenging to rationally design superior nanozymes and understand the underlying mechanism. Herein, a facile covalent coupling-modulated nitrogen defect is reported for significantly boosting peroxidase (POD)-like activity. Vanadium nitride (VN) nanoparticles are grown on graphene oxide (GO) via C-N bonding to form VN/rGO nanocomposites by varying with the VO/GO ratio. The initial increasing GO amount enables formation of the C-N bond, dramatically boosting POD-like activity. Nevertheless, with a higher GO amount, the nitrogen defects decrease due to forming mainly VO. The defect-rich VN/rGO nanocomposite with 20 wt % GO (VG-2) exhibits the best catalytic efficiency (/ = 0.0187 s), which is 778-fold higher than that of natural horseradish peroxidase. Theoretical calculations and structure characterization reveal that the rich-N defects originate from VN covalent binding onto rGO with an rich-electron structure, impeding VN agglomeration, which greatly reduces the energy barrier of the rate-determining step of the catalytic reaction. Finally, coupling urate oxidase with VG-2 as an enzyme cascade, an ultrasensitive and selective colorimetric detection was developed for uric acid (UA), one of the indicators of kidney function or gout attacks, with a linear detection ranging 1-100 μM and 0.1-2.5 mM with a limit of detection of 0.24 μM UA (S/N = 3). The proposed method was applicable to detecting UA in human serum samples satisfactorily. This work could inspire more effective insights into designing other robust nanozymes through covalent coupling for a variety of biochemical analysis and biocatalysis applications.