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 (Hg²⁺) and palladium (Pd²⁺) ions and an enantioselective detection of L/D-cysteine based on fluorescence turn off in the presence of Hg²⁺ and Pd²⁺ 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 Hg²⁺ and Pd²⁺ ions under UV illumination were acquired as input features for developing an intelligent machine learning model capable of detecting Hg²⁺ and Pd²⁺ using classification algorithms such as support vector machine (SVM), K-nearest neighbors (KNN), multilayer perceptron (MLP), and linear discriminant analysis (LDA).