Au@NH2-MIL-125(Ti) heterostructure as light-responsive oxidase-like mimic for colorimetric sensing of cysteine

Abstract Photo-responsive oxidase-like nanozymes are attractive and highly desirable because they can directly activate O2 instead of using unstable and destructive H2O2 as oxidant and their activities are photo-switchable by turning light on or off. However, their practical applications are confronted with challenges such low photocatalytic efficiencies. Regulation of charge-carriers migration and separation efficiency is an efficient strategy to enhance photocatalytic performance. Herein, Au@NH2-MIL-125(Ti) nanocomposite was constructed by loading Au nanoparticles on the NH2-MIL-125(Ti) semiconductor for enhanced photocatalytic activity and explored as a photo-responsive oxidase-like mimic. Its oxidase-mimicking activity is about 3 times higher than that of the pristine NH2-MIL-125(Ti) towards colorimetric substrate 3,3′,5,5′-tetramethylbenzidine (TMB) under light irradiation, probably due to the formation of Schottky barrier at the junction between the NH2-MIL-125(Ti) and the Au nanoparticles. The enhanced charge transfer and separation in Au@NH2-MIL-125(Ti) are verified by photoluminescence spectroscopy and photoelectrochemical experiments. Meanwhile, kinetic analysis demonstrates that the photo-responsive oxidase-like mimic exhibits lower Km value (0.026 mM) and higher Vmax (13.56 nM s−1) than the natural horseradish peroxidase (Km = 0.43 mM, Vmax = 10 nM s−1). Radical trapping experiments assured the ·OH and h+ were the major reactive species in the photocatalytic oxidations. Based on its photo-responsive oxidase-like activity and inhibition effect of cysteine, a green colorimetric platform with high sensitivity and selectivity was developed for cysteine detection, with a good linear relationship (1–10 μM) and a very low detection limit of 0.15 μM. This work may shed light on the development of MOF-based photo-responsive oxidase-like mimics for environmentally benign colorimetric biosensing.