Highly Blue-fluorescent Carbon Quantum Dots Obtained from Medlar Seed for Hg²⁺ Determination in Real Water Samples

The carbon quantum dots (CQDs) have been prepared from medlar seeds with pyrolysis method in an oven at 300 °C. UV-vis absorption spectroscopy, fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR) spectroscopy, x-ray diffraction (XRD) technique, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used in the characterization of CQDs. CQDs, give a strong blue fluorescence under UV lamp (at 365 nm), have a quantum yield of 12.2%. The influence of metal ions such as K⁺, Mg²⁺, Ca²⁺, Be²⁺, Cr³⁺, Mn²⁺, Ni²⁺, Ag⁺, Hg²⁺, and Al³⁺ on the fluorescence properties of the CQDs was investigated by means of emission spectrophotometry. CQDs altering fluorescence characteristics depending on the excitation wavelength show selectivity for Hg²⁺ ions with outstanding fluorescence quenching among the tested metal ions. Based on these results, a new fluorimetric method has been developed for the determination of Hg²⁺ in real water samples. The linear range of method is 1.0 to 5.0 mgL^- 1. Limit of detection and limit of quantification are 0.26 and 0.79 mgL^- 1, respectively. The proposed method has been successfully used in determination of Hg²⁺ ions in tap, sea, and stream water samples with application of addition-recovery experiments.