Sensitive Fluorescent Assay for Determination of Cu2+ in Aqueous Solution Using Isorhamnetin-β-cyclodextrin Inclusion

Abstract

Abstract A fluorescent probe based on isorhamnetin (Iso)-cyclodextrin (CD) inclusion for detecting Cu2+ was synthesized. A major fluorescence emission peak at 542 nm was observed for isorhamnetin (Iso) in buffer solution (pH 7.40) when the excitation wavelength was 390 nm. It was surprisingly found that (2-hydroxypropyl)-β-cyclodextrin (β-CD) could enhance the fluorescence intensity of Iso, and the major fluorescence peak was shifted to 557 nm. UV-vis spectrum showed that the Iso-β-CD system was formed by hydrogen bond and hydrophobic interaction. And the fluorescence intensity of Iso-β-CD system was stronger and more stable compared with Iso. Further study indicated that Iso-β-CD system had good selectivity and sensitivity toward Cu2+. When Cu2+ was added, the fluorescent emission intensity of Iso-β-CD system was quenched, while other metal ions could not bring about obvious change, which meant that the Iso-β-CD system had good selectivity toward Cu2+. The fluorescence titration spectra indicated that the concentration of Cu2+ was inversely proportional to fluorescence intensity, and fluorescence emission intensity was linearly correlated with the concentration of Cu2+ in the range of 0.05–6.0 μM. The calibration equation was y = −11.66x + 781.13 (R2 = 0.996), with a limit of detection of 0.017 μM (S/N = 3). The possible reaction mechanism was studied by UV-vis spectroscopy and Job's plot method. The results indicated that Cu2+ could combine with Iso-β-CD system. The stoichiometric ratio for the complex of Iso and Cu2+ was 2:1. The formation of Iso-β-CD-Cu(II) complex led to the extension of the conjugated system and the intramolecular charge transfer (ICT) took place. Therefore, fluorescence of Iso-β-CD system was quenched. At last, the probe was successfully applied to determination of Cu2+ in water samples, vegetables and fruits with good recoveries, and the results coincided with those obtained by ICP-AES or AAS methods.