A naked-eye colorimetric sensor for methanol and 'turn-on' fluorescence detection of Al3+

The current study demonstrates a simple and economical synthetic approach for the development of a multifunctional Schiff base compound (NRSB) through the condensation reaction between 2-hydroxy-naphthaldehyde and a five-membered heterocycle 3-aminorhodanine. The rhodanine unit was chosen due to the presence of an active electron-deficient carbonyl group. A facile chemical transformation from the closed to the open form was noticeable for rhodanine derivatives in the presence of a suitable nucleophile or an electron-rich system. Such a unique feature was invoked to develop a versatile molecular platform for the colorimetric sensing of methanol and the fluorometric detection of Al3+ ions. The nucleophilic attack by methanol on NRSB led to the formation of an open-ring compound, NRSB-O, with a distinct change in color from colorless to yellow. NRSB showed noticeable sensitivity towards methanol and the detection limit was found to be 0.43 wt%. The methanol-induced chemical transformation from NRSB to NRSB-O was probed through X-ray diffraction studies. The strong CH-pi interaction between the methylene protons of the rhodanine unit and the pi-electron cloud of the naphthalene unit (3.03 angstrom) and the pi-pi interaction between naphthalene and the rhodanine moiety (3.51 angstrom and 3.55 angstrom) lead to a two-dimensional (2D) supramolecular structure in the NRSB crystal. NRSB-O possesses strong CH-pi interactions between molecules with an intermolecular distance of 2.49 angstrom, leading to 2D-supramolecular self-assembly. The crystal structure revealed the scope of the chelation of NRSB-O with metal ions due to the presence of different N, O, and S donor centers. Interestingly, NRSB-O exhibited a turn-on fluorescence response specifically towards Al3+ through the chelation-enhanced fluorescence (CHEF) mechanism over other competitive metal ions. The turn 'on-off' fluorescence switching was demonstrated for multiple cycles through the alternative addition of Al3+ and EDTA. The low-cost, simple design strategy of NRSB and NRSB-O as delineated in the current study would contribute to further development of task-specific molecular sensors and switches.