Chiral Fluorescent Carbon Dots as Multi-Phased Sensors for Hg ²⁺ , Pd ²⁺ , and Cysteine Enantiomers

This work presents a novel chiral fluorescence sensor for the selective and sensitive detection of mercury (Hg 2+ ) and palladium (Pd 2+ ) ions and an enantioselective detection of L/D-cysteine based on fluorescence turn off in the presence of Hg 2+ and Pd 2+ and turn on by adding L(D)-cysteine using highly fluorescent blue emissive chiral L&D-carbon dots abbreviated as L&D-CDs. Importantly, solid-state sensors were prepared using hydrogel and paper-based techniques and observed a significant turn “on–off–on” behavior for hydrogel and turn “on–off” behavior for paper sensors. Molecular logic gates were constructed based on the observed on–off–on fluorescence switching. To expand the potential for smartphone-based sensing, an efficient RGB detection method was also used for both liquid and solid-state sensors. We subsequently explored an alternative approach where digital fluorescence images of L- and D-CDs exposed to varying concentrations of Hg 2+ and Pd 2+ ions under UV illumination were acquired as input features for developing an intelligent machine learning model capable of detecting Hg 2+ and Pd 2+ using classification algorithms such as support vector machine (SVM), K-nearest neighbors (KNN), multilayer perceptron (MLP), and linear discriminant analysis (LDA).