Fluorescent carbon quantum dots synthesized using phenylalanine and citric acid for selective detection of Fe³⁺ ions

A facile, economical and one-step hydrothermal method was used to synthesize fluorescent carbon dots by utilizing citric acid as carbon source and phenylalanine to provide nitrogen. The as-prepared fluorescence carbon dots had strong blue light emission around 440 nm. As confirmed by UVvis absorption, X-ray photoelectron spectroscopic, Fourier transform infrared spectroscopy and transmission electron microscope characterization, the carbon dots were small and very stable in water for using as a fluorescent probe. It was also found that the fluorescence of the carbon dots could be quenched in the presence of Fe³⁺ ions, and the quenching rate was linear with the concentration of Fe³⁺ ions. We here proposed a static quenching mechanism about the fluorescence of the Phe-CDs could be selectively quenched by Fe³⁺ ions, which was because these Fe³⁺ ions could easily combine with the hydroxyl or carboxyl groups on the surface of Phe-CDs and induced aggregation. In addition, the pH had little effect on the fluorescence intensity of the Phe-CDs and maintained excellent fluorescence intensity even under extreme pH value conditions and could be used for the detection of Fe³⁺ ions. We have demonstrated that the method using the carbon dots for Fe³⁺ ions detection was rapid, reliable, and selective with a detection limit as low as 0.720 μM and a dynamic range from 5.0 to 500.0 μM. Moreover, the results of determination Fe³⁺ ions in tap water samples indicated that the presented method has potential for practical application in environmental metal analysis.