Enhanced selectivity of carbon quantum dots for metal ion detection through surface modification by heteroatom doping: A study on optical properties and theoretical approach

Water contamination by toxic metal ions has become a significant issue, requiring the development of effective ion detection methods. Traditional analytical techniques often involve toxic elements or complex devices. Carbon quantum dots (CQDs) have emerged as a promising alternative for optic ion detection due to their unique properties and compatibility with living organisms. This study focuses on synthesizing and functionalizing CQDs with various heteroatoms (N, S) to enhance their optical properties and ion selectivity. CQDs were synthesized using citric acid as the carbon source and modified with l-cysteine, ethylenediamine, and diethylenetriamine. The structural and optical properties of the CQDs were determined using several techniques, including FT-IR, TEM, UV–Vis, and Fluorescence Spectroscopy. The results indicate that doping with heteroatoms significantly alters the absorption and emission properties of CQDs. Particularly, nitrogen-doped CQDs (NCQDs) exhibited the highest absorption and emission intensities, making them ideal for sensor applications. The study also demonstrated that functionalization with sulfur could modulate emission frequencies, enhancing the detection capabilities for specific ions. Fluorescence quenching studies revealed that NCQDs and S-CQDs have a high selectivity for Hg²⁺ ions, attributed both electrostatic and covalent interactions formed between the CQDs and Hg²⁺. Computational studies supported these findings, showing that the interaction with Hg²⁺ significantly affects the energy gap of the CQDs, enhancing their sensitivity. This research contributes to the field of environmental monitoring by providing a practical solution for the detection of free metal ions in water through the development of advanced CQD-based sensors.