Preparation and application of solvent-modulated self-doped N-S multicolour fluorescence carbon quantum dots

In this paper, two types of carbon quantum dot (CQDs) were prepared using biocompatible l-methionine as the carbon source and urea as the nitrogen source and a one-step hydrothermal treatment. By changing the reaction solvents (deionized (DI) water and dimethylformamide (DMF)), the maximum emission of the resulting CQDs shifted from blue to red light. Specifically, the emission wavelength of the CQDs moved from 433 nm to 625 nm following embedding of a new functional group (-CONH-) on the surface of the CQDs. Photoluminescence quantum yields of the CQDs with blue and red emission reached 64% and 61%, respectively. The R-CQDs were used to detect metal ions and a linear relationship was demonstrated between ln(F/F₀ ) and Fe³⁺ concentration in the range 0-0.5 mmol/L with a detection limit of 0.067 μM. Therefore these R-CQDs have great potential as fluorescent probes for Fe³⁺ detection. We expect that the excellent water-soluble, biocompatible and optical properties of the CQDs developed in this work mean that they will be widely used to detect biological cells.