Phenylalanine-embedded carbazole-based fluorescent 'turn-off' chemosensor for the detection of metal ions

Fluorescent chemosensors are highly important for various applications including medical diagnostics, environmental monitoring, and industrial processing. Significant advancements have been made to produce sensors capable of detecting biologically and environmentally relevant ions. Specifically, carbazole-derived fluorophores are chemically stable agents with the ability to detect anions, cations, and small bioorganic molecules. However, most carbazole-based fluorescent probes for the detection of metal ions are Schiff bases and require stringent pH control to prevent hydrolysis. On the other hand, amide-based sensors that utilize stable amino acid scaffolds provide a robust sensing platform as well as a soft-chemical environment for detecting both soft and heavy metal ions. Herein, we explored an aromatic amino acid Phe-containing carbazole-based "turn-off" fluorescent chemosensor to improve the sensor specificity using π-conjugation and additional binding sites. The structure of the novel chemosensor was characterized by electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy. In addition, the sensing properties towards metal ions were studied using UV-vis and fluorescence spectroscopy. Among the various metal ions tested, the chemosensor showed high selectivity and sensitivity towards Co²⁺, Ni²⁺, and Cu²⁺ ions. The detection limits for Co²⁺, Ni²⁺, and Cu²⁺ ions were found to be 4.78 µM, 3.50 µM, and 5.17 µM respectively. Furthermore, the interaction of Phe-amino-carbazole with the various tested metal ions resulted in a flakes-like supramolecular structure, similar to the native Phe-amino-carbazole, whereas the interaction of the designed chemosensor with the Pb²⁺ metal ion resulted in a uniform 3D-circular disc-like supramolecular structure, as confirmed by electron microscopy experiment. This highlights the potential of the Phe-containing carbazole-derived chemosensor for the detection of multiple cations with a decrease in the fluorescence response with a lower detection limit.