Ratiometric Fluorescence Chiral Sensors for pH Monitoring and Its Applications in Indicator Paper and Bioimaging

Chirality is a prevalent phenomenon in nature and living organisms and plays a significant role in biological systems. The chirality of nanomaterials represents one of the most exciting and rapidly expanding fields of science. Comparing the pH response of enantiomers of a particular chiral species remains a challenge in the field of chemistry. In this work, chiral d/l-Cdots were prepared by post-modifying an achiral Cdots core with arginine enantiomers as chiral ligands for comparing the pH sensitivity of enantiomers. In this case, modified silicon quantum dots (SiQDs) were employed to harness a sensitive and accurate ratiometric method. The fluorescence response of chiral sensors is based on an "on-off" strategy, with different pH ranges for d- and l-probes. Surprisingly, the d- and l-probes show different sensitivities to pH, with a linear range of pH 4.0-8.0 for d-probes and 3.5-7.5 for l-probes. The response mechanism is based on aggregation caused quenching (ACQ) due to protonated-deprotonated d-Cdots/Ca-SiQDs and l-Cdots/Ca-SiQDs probes in acidic and basic environments. Quantum mechanical (QM) calculations were in excellent agreement with the experimental results, confirming the stronger binding affinity of d-Cdots to protonated and aggregation in acidic conditions compared to l-Cdots. Crucially, these chiral probes have been successfully utilized as indicator paper and accurately detect intracellular pH (pH_i) in SK-BR-3 breast cancer cells.