A new disulfide Schiff base as a versatile “OFF–ON–OFF” fluorescent–colorimetric chemosensor for sequential detection of CN− and Fe3+ ions: combined experimental and theoretical studies

Abstract

A new naphthaldehyde based disulfide fluorogenic and colorimetric chemosensor, N,N′-disulfandiylbis((2,1-phenylene))bis(1-(naphthalene-1-yl)methaneimine) (L), has been synthesized, and characterized by CHN analyses, X-ray crystallography and different spectroscopic methods. The detection of cyanide is accompanied by the cleavage of the S–S bond and nucleophilic attack of the cyanide ion on the imine group of the disulfide chemosensor with a 1 : 2 binding stoichiometry, as confirmed by ¹H NMR studies. The specific reaction results in a significant fluorescence enhancement and a color change from pale yellow to orange. The LOD for cyanide detection is 2.5 × 10⁻⁷ M. The L chemosensor is used successfully for the detection of cyanide in electroplating wastewater and the fabrication of colorimetric test strips. The colorimetric and fluorogenic sensing mechanism of L toward CN⁻ is also investigated by DFT calculations and ESI-MS experiments. The emission of the [L-CN⁻] system is quenched by Fe³⁺ ions and can be used as a sensitive and selective “ON–OFF” fluorescent sensor for the detection of Fe³⁺. The fluorogenic sensing can be applied for the fabrication of an INHIBIT type molecular logic gate.