Abstract:

【Objective】Kaempferol (Kae) is a flavonoid that is widely present in plants. By investigating the fluorescence properties and applications of Kae, this study seeks to reveal the recognition mechanisms of metal ions and provide a basis for the development of flavonoid resources. 【Method】 Kae was extracted from ginkgo leaves. The experimental conditions for obtaining the fluorescence emission spectra of Kae were optimized by adjusting the pH and preparing the inclusion complex with (2-hydroxypropyl)-β-cyclodextrin (CD). The responses of Kae and Kae-CD on different metal ions were explored by studying the change of fluorescence. 【Result】 Interestingly, the fluorescent emission of Kae was observed in the CH₃OH-PBS buffer solution (volume ratio 1∶99, pH 7.40), however, the fluorescence intensity and stability were poor. When the inclusion complex was formed with Kae and CD, the maximum emission peak shifted from 538 nm to 552 nm, and the Stokes shift increased. Moreover, the fluorescence intensity was greater than that of Kae and more stable. The results of selectivity experiments indicated that fluorescence of Kae was quenched after adding Cu²⁺, fluorescence intensity of Kae decreased in some degree after adding Fe²⁺. And other metal ions had no obvious influence on fluorescence intensity of Kae. Fluorescence titration experiments demonstrated fluorescence intensity of Kae was inversely proportional to Cu²⁺ concentrations. The calibration curve between fluorescence intensity (y) and Cu²⁺ concentration (x) was defined as y = -10.61x + 225.8 (R² = 0.998) with a linear range from 1.0× 10^-8 to 1.7 × 10^-6 mol/L. The detection limit was estimated to be 4.2 × 10^-9 mol/L. In the Kae-CD solution, fluorescence quenching occurred after adding Cu²⁺, and the other metal ions had no obvious effect on fluorescence intensity, which indicated that the Kae-CD solution had a higher selectivity for Cu²⁺. Fluorescence titration experiments demonstrated that the linear relationship between fluorescence intensity (y) of Kae-CD solution and Cu²⁺ concentration (x) was defined as y= -8.54 x + 708.55 (R² = 0.997) with a linear range from 5.0 × 10^-8 to 5.0 × 10^-6 mol/L and detection limit was 1.5 × 10^-8 mol/L. UV-Vis spectra, FT-IR spectra, and Job's plots were used to characterize the mechanisms of Cu²⁺ recognition. These experiments demonstrated that Kae and Kae-CD formed complexes with Cu²⁺ in a stoichiometric ratio of 2∶1. When the complex was formed, intramolecular charge transfer (ICT) occurred due to the extension of the conjugated system, thereby causing fluorescence quenching. 【Conclusion】As shown by the present study, this method can be successfully applied to determine the concentration of Cu²⁺ in rivers or lakes. Compared with the ICP-MS or ICP-OES methods, the results obtained using Kae and Kae-CD were more accurate and stable, thereby providing a novel, fast and convenient method to quantify Cu²⁺ presence.

Key words: kaempferol, (2-hydroxypropyl)-β-cyclodextrin, fluorescent probe