Nitrogen and copper (II) co-doped carbon dots for applications in ascorbic acid determination by non-oxidation reduction strategy and cellular imaging

Nitrogen and copper co-doped carbon dots (Cu-NCDs) were prepared by solvothermal carbonization of folic acid and CuCl2. The N-containing groups including sp2-hybridized CN, porphyrin C–N–C and amino N in N-(C) 3 or H–N-(C) 2, and the Cu-containing group, N–Cu–N, have formed on the surface and framework of Cu-NCDs. The Cu-NCDs were monodisperse with the average particle diameter of 3.57 nm and exhibited dual fluorescence emission peaks at 410 and 470 nm with the excitation wavelength of 340 nm. The new emission center of 410 nm might originate from the Cu doping which changed the structural network and surface state of the carbon dots. The Cu-NCDs exhibited good fluorescence quenching response towards ascorbic acid (AA) from 0.02 to 40 μM (R2 = 0.992), and the limit of detection was 17.8 nM. The mechanism of the quenching process is non-oxidation reduction strategy based static quenching (SQE). Cu doping can improve the selectivity and sensitivity for Cu-NCDs towards AA benefiting from its chelation effect towards multi-hydroxyl in AA. Cu and N doping cause positively charged surface of Cu-NCDs, improving the interaction towards AA and then the stable Cu-NCD-based non-luminescent compounds formed, which resulted SQE. The Cu-NCDs possessed low cellular toxicity and showed good uptake by HepG2 cells.