Fluorescent chemosensor: recognition of metal ions in aqueous medium by fluorescence quenching

Abstract

A thiourea based benzothiazole–naphthalene (BTNP) receptor is synthesized and their photophysical properties and binding affinity with various metal ions is investigated in aqueous medium. UV–Vis absorption as well as emission spectroscopy in HEPES buffer exhibited the sensitivity and selectivity of BTNP relatively, more for Hg2+ ion in which the fluorescence intensity was significantly quenched (65–70%) and intensity of a fluorescent blue colored solution diminished (switch-off). The receptor BTNP has shown 2 ppm level detection limit for the metal ion. The proposed mechanism for the metal ion recognition by BTNP is realized through the studies based on FT-IR, 1H and 13C NMR spectroscopy. The results support the probability of Hg2+ ion induced irreversible desulphurization chemical reaction leading to the formation of urea derivative and also about complexation reaction with Cu2+ ion. Jobs plot analysis suggests about the complexation in 1:1 and 2:1 stoichiometry between BTNP and Hg2+ and Cu2+ metal ions respectively. The association constants and quenching efficiency were estimated to understand the unique naked-eye sensitivity of BTNP for the recognition of both the metal ions.