Building an eco-friendly, biocompatible, and ratiometric NIR fluorescent sensor for the rapid detection of trace Pd2+ in real samples and living cells

The increasing industrial use of palladium has led to its environmental accumulation, raising concerns about its toxicity to aquatic life and human health. Therefore, fluorescent probes capable of detecting Pd2+ are highly beneficial. With this objective, a new near-infrared (NIR) fluorescent probe based on a cyanine dye, 2 ((E)-2-((E)-2-((dimethylcarbamothioyl)oxy)-3-(2-((Z)-1,3,3-trimethylindolin-2ylidene)ethylidene)cyclohex −1-en-1-yl)vinyl)-1,3,3-trimethyl-3H-indol-1-ium iodide (CNS), was synthesized for selective and rapid detection of Pd2+. The detection reaction followed the elimination of thiocarbamate moiety, leading to the highly fluorescent product. CNS demonstrated remarkable sensitivity (detection limit: 0.105 μM), high selectivity, short response time (1.0 min), long lifetime (0.88 ns), and easily detectable color changes upon Pd2+ exposure. A CNS-loaded TLC strip integrated with a smartphone detection system was able to detect Pd2+ in solutions, soil, and drug samples. In addition, CNS enabled concentration-dependent detection of Pd2+ in onion roots and epidermis. Because of low cytotoxicity, good membrane permeability, NIR fluorescence, and high contrast, CNS has been successfully applied to Pd2+ bioimaging in living cells, targeting mitochondria. Compared to existing probes, CNS offers superior sensitivity, selectivity, and adaptability for sensing applications.