Discriminative Detection of Dopamine and Tyrosinase Based on Polydopamine Dots Triggered by Fenton-like Activity of Mn₃O₄ Nanoparticles

In this work, the inherent Fenton-like activity of Mn₃O₄ nanoparticles (Mn₃O₄ NPs) was explored to catalyze the polymerization of dopamine to blue-emitting polydopamine dots (PDA-dots), which can be used for selective detection of dopamine (DA) and tyrosinase (TYR). It was demonstrated that Mn₃O₄ NPs sparklingly oxidized DA to its quinone derivatives at the same time decomposing hydrogen peroxide to hydroxyl radicals that controlled the polymerization to PDA-dots. Interestingly, Mn₃O₄ NPs retained high activity to produce PDA-dots after repeated use of five cycles. It was found that with the polymerization of DA, the FL intensity of the produced PDA-dots changes in a dose-dependent on dopamine concentration. Therefore, under the optimum conditions, a turn-on fluorometric detection was thereby established to detect dopamine, showing a linear range of dopamine concentration from 0.050 μM to 300 μM. The Mn₃O₄ NPs-assisted preparation of fluorescent PDA-dots was further exploited to detect TYR by using tyramine as its model substrate. This simple strategy showed a limit of detection of 0.0048 U/mL toward TYR and displayed a linear response in the concentration range from 0.021 to 13.43 U/mL. Importantly, this study not only provides insight into how to quickly synthesize polydopamine dots that would be explored in biomedical applications but also establish a method to obtain highly discriminative and sensitive detection of dopamine and tyrosinase.