Polyoxometalate Nanostructures Decorated with CuO Nanoparticles for Sensing Ascorbic Acid and Fe²⁺ Ions

Nanozymes are considered as ideal substitutes for nature enzymes; however, the enzyme activity and the relevant detection sensitivity in actual samples still need to be further improved. Here, a nanomaterial of CuO NP-POM composed of polyoxometalate (POM) decorated with copper oxide nanoparticles (CuO NPs), which possessed tetraenzyme mimetic activities including oxidase (OXD)-, peroxidase (POD)-, catalase (CAT)-, and glutathione peroxidase (GPx)-like activities was developed. The corresponding catalytic performances, mechanisms, and kinetics were systematically investigated. Based on the POD-like activity of a CuO NP-POM nanocomposite, a colorimetric sensing platform for ascorbic acid (AA) was built with the detection limit of 15 nM, depending on the fact that AA can efficiently react with the oxidation product of TMB, which was catalyzed by CuO NP-POM on the basis of the POD-like activity. A simple fluorometric sensor for Fe2+ was developed on the basis of the GPx-like activity of CuO NP-POM with the detection limit of 8.0 nM. CuO NP-POM can weaken the fluorescence of the isoindole derivative derived from GSH and o-phthalaldehyde (OPA) due to the excellent GPx-like activity, which can accelerate the catalysis of GSH into GSSG in the presence of H2O2. However, in the presence of Fe2+, the fluorescence was recovered owing to the fact that the Fenton reaction between Fe2+ and H2O2 could reduce GSSG into GSH. The proposed colorimetric and fluorometric sensors have been successfully applied in detecting AA and Fe2+ in several food and biological samples. This work not only developed a nanocomposite with enhanced multinanozyme activity but also offered rapidly detection methods for the analysis of specific targets via the special nanozyme activities.