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A thiocarbonohydrazone locked salicylidene based macrocycle ligand L has been synthesized and its ion sensing properties were examined by UV–visible and fluorescence spectroscopy. The macrocycle serves as a highly selective colorimetric sensor for Hg2+ ions while it acts as an excellent fluorescent sensor for Zn2+ ions by exhibiting a green fluorescence at 498 nm even in the presence of interfering ions. A detailed analysis of binding characteristics such as complex stoichiometry, association constant and detection limits of L toward Hg2+ and Zn2+ ions were evaluated by UV–visible and fluorescence experiments which revealed a stronger binding affinity and higher detection limit of L toward the mercury ions. Further, the sequential displacement strategy for the chromofluorogenic detection of Zn2+, Hg2+ and S2− ions by ligand L, has been studied comprehensively. Finally, the ion-responsive fluorescence output signal of L were employed to design a molecular keypad lock which could be accessible by two users having two different set of chemical passwords (inputs) through distinguishable optical trajectories.

Two chromofluorescent coumarin-functionalized salicylidene based probes (3a and 3b) have been synthesized and evaluated for selective detection of fluoride and cyanide ions. Probes 3a and 3b selectively detect fluoride ions in acetonitrile medium via H-bond interaction and subsequent deprotonation to elicit a distinct visual colour change from colourless to deep red with a significant enhancement in their emission intensity to “turn on” a yellow fluorescence. In addition, probe 3a could optically discriminate the presence of cyanide ions over other anions by a colour change from colourless to deep yellow with an enhancement of green fluorescence in aqueous-acetonitrile medium (1:1 v/v). Job’s plot experiments revealed a 1:2 binding stoichiometry between the probes and fluoride or cyanide ions. A detailed analysis of the binding characteristics of probe 3a with fluoride and cyanide ions have been further carried out by 1H NMR titration studies which indicates a good correlation between colorimetric, UV–vis and 1H NMR observations. These experimental results were further corroborated from the theoretical models using quantum chemical calculations.

The blue color of butterfly pea (Clitoria ternatea) was extracted by Milli-Q water and evaluated for selective detection of bisulphate (HSO4−) ions. The stability of the Clitoria ternatea extract was established by UV–visible and fluorescence techniques. The blue water extract from Clitoria ternatea selectively recognizes HSO4− ions over various anions via a distinct visual color change from blue to purple with a significant hypsochromic shift of 68 nm in the UV–visible absorption spectra. Thus Clitoria ternatea extract provides a selective real time colorimetric monitoring of HSO4− ions which would pave the way for the development of low cost green analytical tool. This type of detection technique enhances the environmental and economic benefits and can emerge as an alternative form of synthetic chelating sensors.