Hydrothermal synthesis of green fluorescent nitrogen doped carbon dots for the detection of nitrite and multicolor cellular imaging

Highlights: • Nitrogen doped carbon dots (NCDs) were prepared by hydrothermal treatment of citric acid and p-phenylenediamine. • The NCDs showed selective and sensitive nitrite determination property. • The NCDs were successfully used to detect the content of nitrite in food samples. • The NCDs possessed high efficiency for multicolor cellular imaging. -- Abstract: A fluorescent probe for the determination of nitrite (NO₂⁻) was fabricated by using green fluorescent nitrogen doped carbon dots (NCDs). The NCDs were synthesized via a one-pot hydrothermal carbonization of citric acid in the presence of p-phenylenediamine as the nitrogen source. The N content of the NCDs was high to 17.09% and consisted of a variety of functional groups on the NCDs surface, including sp²-hybridized CN, porphyrin C–N–C and amino N in N-(C) ₃ or H–N-(C) ₂ et al. N atoms were also doped within the framework of the NCDs. The almost monodisperse NCDs (average particle diameter = 3.67 nm) exhibited green photoluminescence (PL) with excitation/emission maxima of 360/505 nm. The PL of the NCDs was dependent on both excitation wavelength and solution pH. The NCDs showed a strong PL quenching response to NO₂⁻ under acidic conditions (pH = 2.5). The PL intensity of the NCDs was inversely proportional to the concentration of NO₂⁻ between 0.02 and 40 μM (R² = 0.992), with a detection limit of 21.2 nM. The practical use of the nanoprobe for NO₂⁻ determination in food samples was also demonstrated, successfully. NCD-nitroso compounds formed because of reaction between the abundant amide groups on the surface of NCDs with the NO₂⁻, which caused an inner filter effect and static PL quenching. Importantly, the NCDs had low cellular toxicity and were successfully used as a multicolor cellular imaging agent for Hepg2 cells.