Tuning surface states to achieve the modulated fluorescence of carbon dots for probing the activity of alkaline phosphatase and immunoassay of α-fetoprotein

摘要

Graphical abstractA novel concept to reversibly modulate the fluorescence of carbon dots through redox chemistry is achieved. This reversibly fluorescent nanoswitch is demonstrated to have versatile biosensing applications for probing the enzymatic reaction of ALP and immunoassay of AFP (taken as a model analyte) with high sensitivity. This research may broaden the applications of CDs in versatile bioassays.Display OmittedHighlights•A novel way to modulate the fluorescence of CDs for reversibly fluorescent nanoswitch.•This nanoswitch is achieved through facile redox chemistry.•It can be used for versatile biosensing applications with high sensitivity.•It may broaden the applications of CDs in versatile bioassays.AbstractA convenient redox reaction was utilized to tune the surface states of carbon dots (CDs), leading to a reversibly fluorescent nanoswitch. The oxidizing treatment of CDs by KMnO4brought about partial oxidation of hydroxyl groups to carboxyl groups, resulting in effective fluorescence quenching; reversely, the subsequent addition of ascorbic acid (AA) restored the quenched fluorescence by the reduction of the previously oxidized carboxyl groups to hydroxyl groups. Based on the redox modulated fluorescence of CDs, quantitative evaluation of alkaline phosphatase (ALP) activity in a wide range from 0.002 to 250U/L with a low detection limit of 6.0×10−4U/L was realized. In addition, this testing protocol was also expanded to an immunoassay (taking α-fetoprotein (AFP) as an example) by the cascade amplification strategy combing the hybridization chain reaction (HCR) and the ALP–based biocatalysis. A wide linear range of 1.0 fg/mL to 1.0ng/mL with a quite low detection limit of 0.2 fg/mL were obtained for the AFP assay. This study opens up a new way to modulate the fluorescence of native CDs for reversible nano-switch, which may broaden the applications of CDs in versatile bioassays.