Investigating fluorescence quenching of cysteine-functionalized carbon quantum dots by heavy metal ions: Experimental and QSPR studies

Carbon quantum dots (CQDs) have potential application as fluorescent probes for the heavy metal detection. But there are few reports on the investigation of the quenching mechanism of CQDs by heavy metal ions. In this study, CQDs were prepared by directly pyrolyzing turmeric powder and functionalized using the thiol functionality of cysteine. Then the fluorescence quenching of cysteine-functionalized CQDs (Cys-CQDs) by 25 heavy metal ions were experimentally measured. Then the effect of the physical and chemical properties of heavy metal ions on the fluorescence quenching of CQDs was investigated by using the quantitative structure–property relationship (QSPR) method. Applying different machine learning method including genetic algorithm, enhanced replacement method, support vector machine, and multi linear regression, some reliable and predictive models were developed. Based on the variables included in the final QSPR model, covalent interactions of metal ions with functional groups on CQDs surface and the atomic environment were significant effect on the quenching of CQDs by heavy metal ions. Briefly, the tendency of metal ion to accept electrons from a donor ligand and the hard and soft acids and bases affinities can be considered as a primary factor governing the mechanism of quenching of CQDs by heavy metal ions.