Synergistic effect-mediated fluorescence switching of nitrogen-doped carbon dots for visual detection of alkaline phosphatase

The natural enzyme alkaline phosphatase (ALP), which is associated with a large number of diseases, plays an essential role in clinicopathological examinations. Therefore, it is of great significance to quantify the ALP activity levels. In this work, a switchable fluorescence assay of detecting alkaline phosphatase (ALP) activity was proposed using Fe3+ to adjust the fluorescence of nitrogen-doped carbon dots (CDs). Wherein, CDs chelated with Fe3+, leading to a remarkable and dynamic fluorescence quenching. However, ascorbic acid (AA) and phosphate group (Pi), resulting from the enzymatic reaction between ascorbic acid-2-phosphate (AAP) and ALP, enabled the great fluorescence recovery, which was possibly related with the synergistic effect including the chemical redox of AA towards Fe3+ and the formation of a stable FePO4 complex. Based on this synergistic effect-mediated switching mechanism, the indirect detection of ALP activity was developed in the range of 15–445 U/L with the limit of detection at 2.576 U/L (3σ/k) and the limit of quantitation at 8.587 U/L (10σ/k), which could be observed by the naked-eyes. Then, the proposed method was further applied to the detection of ALP activity in human serum and urine samples with satisfactory results, which was in good agreement with the result obtained by ALP commercial kit. This proposed synergistic effect from AA and Pi provides a new insight for ALP activity detection.