Ingenious Multifunctional MnO2 Quantum Dot Nanozymes with Superior Catechol Oxidase-like Activity for Highly Selective Sensing of Redox-Active Dopamine Based on an Interfacial Passivation Strategy

The single function and weak catalytic activity of nonprecious metal nanozymes distinctly hinder their practical applications, so it is intriguing to develop multifunctional nanozymes via a facile and green route. Here, for the first time, "bottom–up" strategies were used to realize the efficient synthesis of MnO2 quantum dots (QDs) with simple operation and mild reaction conditions based on proteins, avoiding the operation complexity of the traditional "up–down" approach. More interestingly, MnO2 QDs exhibit extraordinarily outstanding catechol oxidase-like activity. QD nanozymes can catalyze the oxidation of dopamine (DA) into DA quinone in just 2 min, and then DA quinone polymerizes to form PDA in more than 10 min at 37 °C over a broad pH range (above pH 7.4). Based on the catechol oxidase-like activity and the interaction between QDs with a DA quinone-induced passivation effect, a "turn-on" fluorescence sensor based on two sequential recognitions for DA with self-signal output is developed without using the commonly used chromogenic reactions. The dual recognition provides the sensor with extremely good selectivity, which removes the interference from common reducible substances and metal ions. The detection limit can reach 0.038 μM, and the sensor has good specificity and a short detection time (just 2 min) under mild conditions. This offers a flexible guide for rational design and fabrication of FL turn-on biosensors for redox-active dopamine and enriches our understanding of QD nanozymes in bioanalytical fields.