Dual-function sensor based on NH2-MIL-101(Fe)@Cu/CeO2 nanozyme for colorimetric and fluorescence detection of heavy metals

Heavy metal ions exhibit significant toxicity and pose threats to ecosystems and human health. Therefore, the development of sensitive detection methods is crucial. Nanozyme-based heavy metal sensors have garnered attention, yet existing nanozymes often suffer from low catalytic activity, limited functionality, and unclear mechanisms. To address these issues, this study successfully synthesized an organic-inorganic hybrid nanozyme (NH2-MIL-101(Fe)@Cu/CeO2) using doping and composite techniques, creating a dual-functional sensor. It's worth noting that in NH2-MIL-101(Fe)@Cu/CeO2, copper doping and heterojunction formation may increase oxygen vacancy concentration, promoting electron transfer. Compared to individual components like amino-terephthalic acid iron (III) metal-organic framework (NH2-MIL-101(Fe)) and copper-doped cerium dioxide (Cu/CeO2), this composite material exhibited enhanced peroxidase-like activity (oxidase-like, peroxidase-mimic, catalase-mimic, and superoxide dismutase-mimic). Particularly, NH2-MIL-101(Fe)@Cu/CeO2 displayed outstanding peroxidase-like activity, with Km values of 0.02 mM for TMB and 0.49 mM for H2O2, surpassing most nanozymes, along with high reaction rate (Vmax = 3.82 × 10−5 mM/s). Leveraging NH2-MIL-101(Fe)@Cu/CeO2's peroxidase-like behavior, TMB oxidation turned blue, inhibited by glutathione causing fading. Importantly, Hg2+ complexed with glutathione's thiol group forming GSH-Hg2+, restoring the blue color. Utilizing these properties, a colorimetric sensor for Hg2+ detection was developed. Furthermore, NH2-MIL-101(Fe)@Cu/CeO2 also exhibited remarkable fluorescence performance; Cu2+ binding with amino groups quenched fluorescence. Exploiting this phenomenon, a fluorescent sensor for Cu2+ detection was devised. Results demonstrated this dual-functional sensor's advantages: high sensitivity (LODHg2+ = 0.7 nM, LODCu2+ = 0.1 µM), strong selectivity, wide linear range (0.01 ∼ 4 µM and 8 ∼ 600 µM), and robust stability (90 % activity retained after three cycles).