A new fluorescent nitrogen-doped carbon dot system modified by the fluorophore-labeled ssDNA for the analysis of 6-mercaptopurine and Hg (II).

A simple, environmentally friendly hydrothermal method was used to prepare strongly luminescent, nitrogen-doped carbon dots (NCDs) with the use of Chinese yams as a source of carbon and nitrogen. Such NCDs have an average size of 2.7±1.4 nm; they emit blue light at 420 nm and have a quantum yield of up to 9.3%. Thus, carboxyfluorescein (FAM)-DNA macro-molecules were assembled on the surfaces of the NCDs, and stabilised by strong π-π stacking; the so formed hybrid nano-sensors were found to have an ultra-sensitive response to 6-mercaptopurine (6-MP). A strong emission and enhancement of yellow radiation was observed from FAM. Furthermore, due to the specific interactions between DNA and Hg(2+), which resulted in the formation of the T-Hg(2+)-T (T: thymine base) complex - a large, conjugated system, which formed between NCDs, DNA and 6-MP, was broken up. Thus, the fluorescence from FAM was quenched. The detection limits for 6-MP and Hg(2+) were 0.67 and 1.26 nM, respectively. The proposed method was applied for the determination of 6-MP in human serum and Hg(2+) in water samples with satisfactory results.