Triple-enzyme mimetic activity of Fe3O4@C@MnO2 composites derived from metal-organic frameworks and their application to colorimetric biosensing of dopamine.

Novel Fe3O4@C@MnO2 composites were successfully synthesized for the first time via an interfacial reaction between magnetic porous carbon and KMnO4, in which the magnetic porous carbon was derived from the pyrolysis of Fe-MIL-88A under N2 atmosphere. Interestingly, the obtained Fe3O4@C@MnO2 composites were found to have triple-enzyme mimetic activity including peroxidase-like, catalase-like, and oxidase-like activity. As a peroxidase mimic, Fe3O4@C@MnO2 composites could catalyze the oxidation of TMB into a blue oxidized product by H2O2. As a catalase mimic, Fe3O4@C@MnO2 could catalyze the decomposition of H2O2 to generate O2 and H2O. As an oxidase mimic, Fe3O4@C@MnO2 could catalyze the direct oxidation of TMB to produce a blue oxidized product without H2O2. Reactive oxygen species measurements revealed that the oxidase-like activity originated from 1O2 and O2-∙and little∙OH generated by the dissolved oxygen, which was catalyzed by the Fe3O4@C@MnO2 in the TMB oxidation reaction. The oxidase-like activity of Fe3O4@C@MnO2 was investigated in detail. Under the optimized conditions, a rapid, sensitive, visual colorimetric method for dopamine detection was developed based on the inhibitory effect of dopamine on the oxidase-like activity. The proposed method allows for dopamine detection with a limit of detection of 0.034 μM and a linear range of 0.125-10 μM. This new colorimetric method was successfully used for the determination of dopamine in human blood samples.