A hydrazine-based unsymmetrical bis-imine-Schiff base as a chemosensor for turn-off fluorescence and naked-eye detection of Cu2+ ion: Application in aqueous media using test strips

A new hydrazine-based unsymmetrical bis-imine-Schiff base 1-((((5-bromothiophen-2-yl)methylene)hydrazono)methyl)naphthalen-2-ol (L1) was synthesized and characterized with FT-IR, 1H NMR, 13C NMR, UV–visible spectroscopy, ESI-MS, and elemental analysis. The solution of L1 in dichloromethane/methanol (9.8:0.2,v/v) mixture shows a rapid change in color from yellow to yellowish brown in the presence of Cu2+ ions. In addition, L1 displays “turn-off” fluorescence preferentially for Cu2+ ions which was attributed to the Chelation Enhanced Quenching (CHEQ) effect caused due to the reversible binding of L1 to paramagnetic Cu2+ in a 2:1 stoichiometric ratio. No interference was caused in the sensing process by other metal ions such as Ag+, Na+, Ni2+, Co2+, Hg2+, Cr3+ Ba2+, Ca2+, Cd2+, Pb2+, Sn2+, Zn2+, Fe3+, Al3+, K+, and Sr2+. The limit of detection was determined to be 2.809 µM. Furthermore, the test strips coated with L1 show a change in color from pale yellow to brownish-yellow in the presence of Cu2+ions in the aqueous medium, and the limit of visual detection was found to be 15 µM, which is well under the maximum recommended limit (31.5 µM) of Cu2+ for drinking water as per WHO guidelines. These observations establish L1 as an efficient reversible chemosensor for sensing Cu2+ in both aqueous and non-aqueous media with high sensitivity and selectivity.