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 rho-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 rho-nitrophenol through enzymatic reactions are chosen. The feasibility of the detection strategy is qualitatively assessed using alpha-glucosidase and beta-galactosidase, and its practicability to quantitatively monitor enzyme activity is also demonstrated by taking alpha-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 alpha-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 alpha-glucosidase based on carbon quantum dots.