JOURNAL OF NANJING FORESTRY UNIVERSITY ›› 2023, Vol. 47 ›› Issue (4): 209-218.doi: 10.12302/j.issn.1000-2006.202111045

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Fluorescent properties and application for recognizing copper ions based on kaempferol and kaempferol-cyclodextrin inclusion

YANG Shilong1(), JIANG Guobin2, XU Li3,*(), SUN Lu3, JIA Yunxuan3, WU Yu3   

  1. 1. Advanced Analysis and Testing Center, Nanjing Forestry University, Nanjing 210037, China
    2. College of Forestry, Nanjing Forestry University, Nanjing 210037, China
    3. College of Science, Nanjing Forestry University, Nanjing 210037, China
  • Received:2021-11-28 Revised:2022-02-21 Online:2023-07-30 Published:2023-07-20

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

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