Copper-manganese bimetallic oxide with excellent laccase-like activity for colorimetric detection of formaldehyde via the specific aldimine condensation reaction.

Laccase, known as the "green catalyst", holds significant promise for applications in the textile industry and pollutant detection. However, the use of natural laccase is constrained by challenges associated with complex preparation and inherent instability. The emergence of nanozymes provides a pathway for developing laccase-like mimics. This study focuses on the synthesis of copper-manganese oxide (Cu-doped Mn3O4) nanoparticles, which demonstrate significant laccase-like activity through the oxidation coupling reaction between 2,4-dichlorophenol (2,4-DCP) and 4-aminoantipyrine (4-AAP) with observable colorimetric change and an obvious absorption peak at 505 nm. Different copper-manganese oxides were synthesized by varying the proportion of metal salts during the synthesis process. A series of studies have demonstrated that copper doping enhances laccase catalytic activity by increasing the oxygen vacancy. Moreover, the introduction of formaldehyde (FA) results in a decrease of the peak at 505 nm, which is attributed to a condensation reaction between the aldehyde and amino groups under neutral conditions. Based on the laccase-like properties of Cu-doped Mn3O4, FA detection is achievable within the range of 100 nM to 100 μM, with a low detection limit of 39.4 nM. This research not only presents a novel laccase mimic with a simple synthesis route but also establishes a colorimetric method for FA detection, representing significant progress in food safety applications.