Single-atom Co-N-C nanozymes with excellent oxidase-like activity for colorimetric detection of l-cysteine.

l-Cysteine (l-Cys) is a significant biomarker for various diseases, including liver disease, skin disorders, and Parkinson's disease, making its detection crucial for diagnostic purposes. In this study, we present a novel colorimetric sensing platform for l-Cys detection. Using ZIF-8 as a precursor, a cobalt-based single-atom catalyst (Co-N-C) was synthesized through pyrolysis, adsorption, and activation processes. The resulting Co-N-C catalyst exhibited remarkable oxidase-like (OXD-like) activity, effectively facilitating the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to form oxTMB, which resulted in a distinct blue color. However, due to its strong reducibility, l-Cys can scavenge reactive oxygen species (ROS) generated by the Co-N-C catalyst, thus inhibiting the oxidation of TMB. By monitoring the color change in the solution, we developed a highly sensitive colorimetric assay for l-Cys detection. The assay demonstrates excellent selectivity and has a detection limit of 0.1 µM, with a linear detection range from 0.1 to 10 µM. This work offers an efficient approach for detecting l-Cys and establishes a new framework for designing high-performance nanozymes.