CDs-MnO2-TPPS Ternary System for Ratiometric Fluorescence Detection of Ascorbic Acid and Alkaline Phosphatase

Manganese dioxide (MnO2) nanosheet-based fluorescence sensors often use oxidase-like activity or wide absorption spectrum for detection of antioxidants. In those strategies, MnO2 nanosheets were reduced to Mn2+ by antioxidants. However, few strategies emphasize the role of Mn2+ obtained from MnO2 reduction in the design of the fluorescence sensor. Herein, we expanded the application of a MnO2 nanosheet-based fluorescence sensor by involving Mn2+ in the detection process using ascorbic acid (AA) as a model target. In this strategy, carbon dots (CDs), MnO2 nanosheets, and tetraphenylporphyrin tetrasulfonic acid (TPPS) comprise a ternary system for ratiometric fluorescence detection of AA. Initially, CDs were quenched by MnO2 nanosheets based on the inner filter effect, while TPPS maintained its fluorescence intensity. After the addition of AA, MnO2 nanosheets were reduced to Mn2+ so that the fluorescence intensity of CDs was recovered and TTPS was quenched by coordination with Mn2+. Overall, AA triggered an emission intensity increase at 440 nm for CDs and a decrease at 640 nm for TPPS. The ratio intensity of CDs to TPPS (F440/F640) showed a good linear relationship from 0.5 to 40 mu M, with a low detection limit of 0.13 mu M for AA detection. By means of the alkaline phosphatase (ALP)-triggered generation of AA, this strategy can be applied for the detection of ALP in the range of 0.1-100 mU/mL, with a detection limit of 0.04 mU/mL. Furthermore, this sensor was applied to detect AA and ALP in real, complex samples with ideal recovery. This novel platform extended the application of MnO2 nanosheet-based fluorescence sensors.