Azo-hydrazone tautomerism in a simple coumarin azo dye and its contribution to the naked-eye detection of Cu2+ and other potential applications.

A new tailor-made azo dye of coumarin connected to phenolic derivative is presented herein. Azo-hydrazone tautomerism in aqueous solution of the dye was observed and studied using spectroscopic assays such as 1H NMR, absorption and emission assays, and theoretical studies. Tautomerism was attributed to the presence of a labile phenolic hydrogen in the ortho position to the azo functionality and the hydrazone was found to be the more dominant tautomer. Influence of metal ions on the azo-hydrazone chemical equilibrium and how the accompanying colour and spectroscopic changes can be exploited for various functions, especially the detection and quantification of Cu2+ in aqueous environments was explored. The presence of Cu2+ affects the azo-hydrazone equilibrium resulting in visual appearance and spectroscopic changes and the likely binding sites for Cu2+ were evaluated. Cu2+ pushes the azo-hydrazone equilibrium towards the more conjugated form and the presence of other metal ions does not have any perceivable impact on this mechanism. The dye showed potential applications as a sensor in colorimetric and spectroscopic detection and quantification of Cu2+ in domestic and environmental water samples, photo-imprinting and as a logic gate. The limits of detection (LOD) and quantification (LOQ) for Cu2+ were found to be 0.0779 mg/L and 0.236 mg/L, respectively, much lower than the World Health Organization (WHO) guideline limit for Cu2+ levels in drinking water.

A novel multidentate pyridyl ligand: A turn-on fluorescent chemosensor for Hg2+ and its potential application in real sample analysis.

A novel pyridyl-based ligand with multiple binding sites was developed as potential turn on fluorescent probe for mercuric ion. In comparison with other transition metal ions, the ligand displayed a significant optical selectivity and sensitivity for Hg2+ in aqueous solution with a remarkable fluorescence enhancement. The obtained spectroscopic response was related to the inhibition of the photo-chemical mechanism known as photo-induced electron transfer (PET) in the ligand and CN isomerization by Hg2+ binding. A good linearity between fluorescence responses and Hg2+ concentration was obtained in the range 3.3 × 10-9 M-1.6 × 10-8 M and a nanomolar level limit of detection (LOD) (1.4 × 10-9 M ~ 0.28 ppb) and limit of quantification (LOQ) (4.8 × 10-9 M ~ 0.93 ppb) were obtained. Both LOD and LOQ values are very low compared to the reported permissible Hg2+ level in drinking water (2 ppb) by US Environmental Protection Agency (EPA). The possible binding mode between ligand and Hg2+ were determined using Job's plot analysis and density functional theory (DFT) calculations and a complex with 1:1 stoichiometric ratio was suggested. The response of the pyridyl ligand upon Hg2+ addition was noted to be fast without any time delay and reversible. The performance of the ligand at nanomolar level of Hg2+ and real sample application of the proposed method was investigated and satisfactory results were obtained.

A New Highly Selective Colorimetric and Fluorometric Coumarin-based Chemosensor for Hg2.

A novel colorimetric and fluorometric method based on coumarin as signalling unit was developed for Hg2+ recognition and quantification. Initially, the alkyne functionality was incorporated into a coumarin system and the resulting molecule showed higher specificity and sensitivity for Hg2+ over other cations in both absorption and emission sensing assays. The Hg2+ recognition was detected as visible colour change from colourless to yellow and as fluorescence quenching. The colour change was assigned to the increased intramolecular charge transfer (ICT) in the signalling unit upon Hg2+ binding whereas a decline in the fluorescence intensity was ascribed to the heavy atom effect from Hg2+. In order to generate a material with high sensing performance level, alkyne-functionalized molecule was hosted into a polymeric material. The resulting functionalized polymer showed higher sensitivity and selectivity for Hg2+ over its corresponding coumarin molecule. The investigation of the possible binding modes for Hg2+ suggested both alkyne and triazole functionalities as potential binding sites for Hg2+. The limit of detection (LOD) and limit of quantification (LOQ) of the proposed method were evaluated and values less than a recommended maximum level of Hg2+contaminant in drinking water (2.00 µg/L) were obtained (LOD = 0.44 µg/L and LOQ = 1.33µg/L). The real-life application of the method was investigated using natural water samples containing Hg2+ levels equivalent to the maximum tolerable concentration of Hg2+ in drinking water. The outcomes suggested that the method could be used in the sensing and determination of Hg2+ level of contaminant in the environment.

Fluorescent Polymer Incorporating Triazolyl Coumarin Units for Cu2+ Detection via Planarization of Ict-Based Fluorophore.

A novel fluorescent polymer with pendant triazolyl coumarin units was synthesized through radical polymerization. The polymer showed reasonable sensitivity and selectivity towards Cu2+ in acetonitrile in comparison to other tested metal ions with a significant quenching effect on fluorescence and blue shifting in the range of 20 nm. The blue shift was assigned to the conformation changes of the diethylamino group from the coumarin moiety which led to planarization of the triazolyl coumarin units. The possible binding modes for Cu2+ towards the polymer were determined through the comparison of the emission responses of the polymer, starting vinyl monomer and reference compound, and the triazole ring was identified as one of the possible binding sites for Cu2+. The detection limits of the polymer and vinyl monomer towards Cu2+ were determined from fluorescence titration experiments and a higher sensitivity (35 times) was observed for the polymer compared with its starting monomer.