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

A 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 KMnO4 brought 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 250 U/L with a low detection limit of 6.0 x 10(-4) U/L was realized. In addition, this testing protocol was also expanded to an immunoassay (taking a-fetoprotein (AFP) as an example) by the cascade amplification strategy combing the hybridization chain reaction (HCR) and the ALPbased biocatalysis. A wide linear range of 1.0 fg/mL to 1.0 ng/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. (C) 2017 Elsevier B.V. All rights reserved.