A novel method for the preparation of solvent-free, microwave-assisted and nitrogen-doped carbon dots as fluorescent probes for chromium(vi) detection and bioimaging.

Chromium(vi) [Cr(vi)] has been shown to be toxic to organisms due to its mutagenicity and carcinogenicity. Therefore, the exploitation of probes with low toxicity and high sensitivity for Cr(vi) detection is needed. In this study, a one-step, solvent-free, and microwave-assisted method was developed for the preparation of nitrogen-doped carbon dots (N-CDs). The reaction could be finished in just three minutes, and the yield of the dots could reach 58.5%; the as-prepared N-CDs exhibited excellent water solubility, emitted bright cyan fluorescence with a high quantum yield of 38.88%, and possessed excitation- and concentration-dependent characteristics. The N-CDs could be effectively applied to Cr(vi) detection with a linear range of 1-100 μM, and the detection limit could be as low as 0.12 μM. The quenching mechanism was responsible for the inner filter effect, and the quenched fluorescence could be recovered with a linear range of 5-100 μM by the addition of ascorbic acid. We showed that the fluorescent probes could even be employed for the detection of Cr(vi) in river water and for bio-imaging because of their nearly zero cytotoxicity; this showed the potential application of these probes in ion detection and cellular bioimaging. Herein, we have provided an effective strategy to rapidly obtain high-quality N-CDs using a solid-phase microwave method, and the as-prepared N-CDs exhibit various potential applications in environmental and biological fields.