Multi-functional carbon dots-based nanoprobe for ratiometric enzyme reaction monitoring and biothiol analysis

In this work, red-emissive carbon dots (CDs) were prepared through microwave-assisted heating and then conjugated with fluorescein isothiocyanate (FITC) to construct a ratiometric multi-functional nanoprobe (CDs-F) for both pH sensing and biothiols analysis. After coupling with FITC, the as-obtained CDs-F shows both green and red emissions. Based on the pH sensitivity of FITC, the CDs-F can be used for ratiometric pH sensing. On the other hand, the red emission of CDs can be quenched significantly by Ag⁺. Studies of quenching mechanism demonstrate that Ag⁺ could coordinate with the functional groups of CDs resulting in the formation of the CDs-Ag⁺ complex and also charge transfer between the CDs and Ag⁺. Simultaneous static and dynamic quenching processes impart the maximum quenching. Based on the strong affinity between thiol groups and Ag⁺, the CDs-F with dual emissions can be used for ratiometric biothiols analysis with the detection limits of tens of nanomole per liter. Furthermore, the strategy has been successfully applied to monitor proton-related enzymatic catalytic reactions and to determine biothiols in fetal bovine serum samples with good recoveries.