A Dual-Functional BSA-MnO₂ Quantum Dots-Based Ratiometric Sensor for the Detection of Acid Phosphatase

Ratiometric fluorescence sensors with quantum dots (QDs) as signal reporters have been widely used in biomarkers testing. However, there are few reports on the construction of a ratio fluorescence sensor based on the dual action of enzyme reaction and QDs. In this study, a "two-in-one" multifunctional catalytic QDs (BSA-MnO2 QDs) developed by hydrothermal method displayed excellent fluorescent properties and high oxidase-like activity. Its optimal quantum yield and Michaelis–Menten kinetics constants Km were 10.7% and 0.072 mM, respectively. In the presence of an enzyme substrate, the ratiometric signal can be generated indirectly by BSA-MnO2 QDs catalyzing, which was more conducive to the construction of a ratio fluorescence biosensor and made the experiment result more accurate. Based on these advantages, an acid phosphatase (ACP) ratiometric fluorescence biosensor was designed. In the absence of ACP, o-phenylenediamine (OPD) was catalyzed to produce 2,3-diaminophenothiazine (DAP) (the oxidized fluorescent product of OPD) that has an inner-filter effect with BSA-MnO2 QDs. Conversely, in the presence of ACP, l-ascorbic acid-2-phosphate was dephosphorylated to produce ascorbic acid, which inhibited the oxidase-like activity of BSA-MnO2 QDs and prevented the catalytic oxidation of OPD to DAP. The method can achieve the assay of ACP with a detection limit of 0.038 mU/mL. In addition, this effect can be achieved when other natural enzymes catalyze substrates to produce reducing substances. In this study, a universal ratio fluorescence probe (BSA-MnO2 QDs) was discovered and prepared for the first time, which has wide application prospects.