Colorimetric assay of NO₂⁻ and Hg²⁺ through on–off strategy of the Cu, Mo-CDs nanozyme

Until now, there have been many reports on the regulation of inorganic metal ions on the catalytic activity of nanozymes, but few reports on the regulation of anions (such as NO₂⁻) on enzyme activity. In this work, a copper-molybdenum doped carbon dots (Cu, Mo-CDs) with peroxidase-like (POD) activity was synthesized by simple one-step microwave digestion method. The Cu, Mo-CDs + NO₂⁻ system can promote 3,3′,5,5′-tetramethylbenzidine (TMB) oxidizing to blue oxidative product (oxTMB) due to the formation of ∙NO₂ free radical. While Hg²⁺ was introduced into the system, the POD-like activity of Cu, Mo-CDs + NO₂⁻ system was inhibited due to the strong interaction and the electron transfer between Cu, Mo-CDs and Hg²⁺. The formation of ∙NO₂ and the electron transfer capability of Cu, Mo-CDs + Hg²⁺ were confirmed through ESR spectra and FMN experiments. The bioenzyme-free colorimetric assay based on Cu, Mo-CDs nanozyme is used for detection of NO₂⁻ and Hg²⁺, showing lower detection limit (LOD) of 1.63 µg/L for NO₂⁻, 0.13 µg/L for Hg²⁺, respectively and good recovery from 90.79 to 108.90%. The work paves a new way to design a nanozyme-based colorimetric protocol for traces NO₂⁻ and Hg²⁺ residues in food analysis.