Co nanoparticles confined in N-doped hollow carbon nanospheres as multifunctional oxidase mimic for colorimetric detection of L-Cysteine and Hg²⁺

In contrast to peroxidase-mimicking nanozymes, oxidase mimics directly employ molecular oxygen (O₂) as oxidant to generate reactive oxygen species (ROS). Generally, transition metal-based nanozymes exhibit poor oxidase-like activity. We herein constructed a colorimetric sensor based on Co-based oxidase mimic, where the Co nanoparticles were encapsulated in hollow N-doped carbon nanospheres (Co-N/C). Co nanoparticles confined catalyst effectively anchored the active part, which greatly influenced the catalytic activity of the oxidase-mimic nanozymes. Due to the antioxidative property of L-Cysteine (L-Cys) and the specific complexation between L-Cys and Hg²⁺, the rationally designed Co-N/C could be utilized to detect L-Cys and Hg²⁺. The proposed sensing platform exhibited wide linear ranges of 1–30 μM for L-Cys and 0.6–30 μM for Hg²⁺, respectively. The excellent selectivity of the constructed sensor showed the reliability and practicality for the assay of L-Cys and Hg²⁺ in fetal bovine serum. The spatial confinement can prevent the aggregation of Co nanoparticles, boosting the stability and catalytic performance in colorimetric sensing.