Highly selective turn-on fluorescent chemosensor for the detection of Cr (III) ion in drinking water

Abstract

An effective and simple carbazole-based Schiff base (CrbPy2) chemosensor for selective recognition of Cr(III) was designed and synthesized. The molecular structure of CrbPy2 was confirmed by NMR and FT-IR spectroscopies, its crystal structure was investigated by XRD analysis and the results were in good agreement with the DFT calculations. The CrbPy2 probe showed high selectivity toward Cr3+ ions in THF/H2O (95/5%) solution. Adding Cr3+ ions to CrbPy2 dilute solution induced a naked eye color change from colorless to yellow accompanied with a fluorescence turn-on. The proposed sensing mechanism was related to chelation-enhanced fluorescence (CHEF) process. The stoichiometry ratio of CrbPy2-Cr3+ complex is 1:1 with an association constant exceeding 3 × 105 M-1. The study of the sensing properties in different pH showed that CrbPy2 can tolerate the detection of Cr3+ ions in a pH range from 6 to 8. The chelating sites of the chemosensor and the coordination geometry of the complex were determined using 1H NMR titrations and MEP calculations. The detection was completed within 2–3 min and the detection limit reaches 1.08 μM. The linear recognition range of Cr(III) is from 1.08 μM to 4 μM making CrbPy2 a suitable probe for real-time identification of unsafe Cr3+ concentration in drinking water.