Nitrogen-doped carbon quantum dots as a fluorescent probe to detect copper ions, glutathione, and intracellular pH.

A facile one-step hydrothermal method was developed to synthesize nitrogen-doped carbon quantum dots (N-CQDs) by utilizing hexamethylenetetramine as the carbon and nitrogen source. The quantum yield (QY) of 21.7% was under the excitation wavelength of 420 nm with maximum emission at 508 nm. This N-CQD fluorescent probe has been successfully applied to selectively determine the concentration of copper ion (Cu2+) with a linear range of 0.1-40 μM and a limit of detection of 0.09 μM. In addition, the fluorescence of N-CQDs could be effectively quenched by Cu2+ and specifically recovered by glutathione (GSH), which render the N-CQDs as a premium fluorescent probe for GSH detection. This fluorescence "turn-on" protocol was applied to determine GSH with a linear range of 0.1-30 μM as well as a detection limit of 0.05 μM. For pH detection, there is good linearity in the pH range of 2.87-7.24. Furthermore, N-CQD is a promising and convenient fluorescent pH, Cu2+, and glutathione sensor with brilliant biocompatibility and low cytotoxicity in environmental monitoring and bioimaging applications.