Abstract
Two azo dyes (E)-4-((6-hydroxyquinolin-5-yl)diazenyl)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one (6QD) and (E)-4-((8-hydroxyquinolin-5-yl)diazenyl)-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one (8QD) have been designed and synthesized. The structure of both dyes was confirmed by FTIR, NMR spectroscopy and mass spectrometry. The absorption and emission spectral analysis of 6QD and 8QD confirmed their selective turn-on behaviour towards Al3+ and Co2+ ions, respectively in MeOH:H2O (v/v = 1:1, HEPES buffer, pH = 7.2) at room temperature. The emission intensity of 6QD enhanced remarkably by 148-fold upon binding with Al3+ ion due to chelation-enhanced fluorescence (CHEF) effect and inhibition of photoinduced electron transfer (PET) phenomenon in the molecule. Emission behaviour of 8QD operates on CHEF and intramolecular charge transfer (ICT) mechanisms in presence of Co2+ ion which induced 26-fold enhancement in emission intensity of 8QD. Colorimetric studies showed prominent visual response for Al3+ and Co2+ ions allowing the real time analysis of these ions in their corresponding solutions. Other coexisting metal ions do not interfere with the detection of these metal ions. Job’s plot analyses revealed 1:1 binding stoichiometry for complexes 6QD-Al3+ and 8QD-Co2+. Association constants of complexes 6QD-Al3+ and 8QD-Co2+ were found to be 1.2 × 106 and 4 × 106 M−1, respectively which suggest good association of both probes with respective metal ions. The limit of detection of 6QD and 8QD was determined to be 1 × 10−8 M and 9.1 × 10−8 M respectively, which demonstrates the high sensitivity towards corresponding metal ions. Reversible studies confirmed that both probes 6QD and 8QD could be used as secondary sensor for EDTA2− anion. Furthermore, pH based absorption studies revealed that 6QD and 8QD can act as naked eye colorimetric probe for detection of hydroxyl ions at pH > 9.
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