A reversible fluorescence nanoswitch based on dynamic covalent B–O bonds using functional carbon quantum dots and its application for α-glucosidase activity monitoring

Dynamic covalent B–O bonds are introduced to the design of a reversible fluorescence nanoswitch in response to the external stimulus of pH. This nanoswitch is based on a phenylboronic acid functionalized carbon quantum dot (PBA–CQD) nanoprobe, and constructed with reference to the two facts that the PBA–CQD probe can bind to p-nitrophenol to form a non-fluorescent conjugate via B–O bonds, and that the making and breaking of the B–O bonds between them can be controlled by pH changes. Excellent reversibility of this nanoswitch is illustrated by switching of the pH from 4 to 8. The reaction between the PBA–CQD nanoprobe and the p-nitrophenol resulting in switching off the fluorescence is further utilized to design a general detection strategy for enzyme activity when substrates that can generate p-nitrophenol through enzymatic reactions are chosen. The feasibility of the detection strategy is qualitatively assessed using α-glucosidase and β-galactosidase, and its practicability to quantitatively monitor enzyme activity is also demonstrated by taking α-glucosidase as an example. The detection limit of this method can be as low as 0.33 U L−1, which is much lower than those reported previously and is sufficiently low to be capable of α-glucosidase level detection in practical human samples. This study demonstrates excellent usability of dynamic covalent B–O bonds in the design of reversible switches and in the general detection of enzyme activity, and provides a sensitive, real-time assay for α-glucosidase based on carbon quantum dots.