Far‐Red BODIPY‐Based Fluorescent Probe for the Selective Detection of Cysteine and Hydrogen Sulfide: Applications in Blood Serum Analysis and Live‐Cell Imaging

Abstract Cysteine (Cys) is a crucial biomolecule involved in protein synthesis, antioxidant defense, and cellular signaling, while H 2 S serves as a gasotransmitter. The imbalance of these thiols is linked to a range of pathological conditions, highlighting the need for precise and reliable detection methods. Herein, we have developed a bright far‐red fluorescent probe (BYN‐DNS) for the selective and sensitive detection of Cys and H 2 S. The probe is synthesized using methoxy‐1‐naphthaldehyde modified BODIPY molecule (BYN‐OH) functionalized with a 2,4‐dinitrobenzene sulfonyl (DNS) moiety. The probe produces very low fluorescence due to ICT between the DNS group and the BYN‐OH derivative. In the presence of Cys and H 2 S, the DNS group cleaved by nucleophilic substitution reaction, releasing the far‐red fluorophore BYN‐OH and resulting in an 81‐ and 122 fold increase in fluorescence at 610 nm, respectively. The probe exhibits significant selectivity for Cys and H 2 S compared to structurally similar thiol, including homocysteine (Hcy) and glutathione (GSH). The detection limits were calculated to be 85 nM for Cys and 68 nM for H 2 S. The quantum yield of the probe in the presence of H 2 S was calculated to be 0.63. The practical applicability and biocompatibility of BYN‐DNS were confirmed via human blood serum analysis and live‐cell fluorescence imaging.