In-situ growth of cobalt oxyhydroxide on graphitic-phase C3N4 nanosheets for fluorescence turn-on detection and imaging of ascorbic acid in living cells.

Cobalt oxyhydroxide (CoOOH) was grown on the surface of graphitic-phase C3N4 nanosheets to obtain an activatable fluorescent nanoprobe for ascorbic acid (AA). The probe was applied to the detection of AA in biological fluids and to image AA in HeLa cells. The negatively charged nanosheets first adsorb Co2+, and then the CoOOH nanoflakes are generated in-situ on the surface of g-C3N4. This results in the quenching of the blue fluorescence (with excitation/emission maxima of 345/435 nm) via fluorescence resonance energy transfer from g-C3N4 to CoOOH. The AA-induced redox reaction reduces the trivalent cobalt ion in CoOOH to Co2+ which then becomes released from the nanosheets. This leads to the recovery of fluorescence. The method can quantify AA in the 1.0 to 800 µM concentration range at near neutral pH values. When applied to cell extracts, the limit of detection is 0.14 µM. The nanoprobe was successfully applied to the determination of AA in serum and urine, and to image AA in living HeLa cells. Additional attractive features include the ease of preparation, low cytotoxicity, rapid fluorometric turn-on response, and good biocompatibility. Graphical abstract Schematic presentation of an activatable fluorescent nanoprobe. It consists of CoOOH nanoflakes that were modified withg-C3N4 nanosheets. It enables monitoring of AA in the biological samples as well as imaging of AA in living cells.