An Interesting Class of Analytical Reagents: Flavonoids
Introduction
Flavonoids, benzo-γ -pyron derivatives with several hydroxyl groups attached to ring structures C6-C3-C6, are one of the most important groups of compounds occurring in plants. This family includes the categories of flavonols, flavanones, flavones, anthocyanidins, and isoflavones (as shown in Fig.1), and is known for its biological effects, including antibacterial, anti-inflammatory, anti-allergic, and antithrombotic actions. But interestingly, they are also a good class of analytical reagents for detecting metal ions. This is because all types of flavonoids possess three domains able to react with metal ions: the 3 -4 -dihydroxy system located on the B ring and the 3-hydroxy or 5-hydroxy groups and 4-carbonyl group in the C ring, which makes flavonoids can form complexes with some p-, d-, and f-electron metals. The formed complexes could be employed in the determination of these metal ions in different kinds of samples by various techniques[1]. Until now, various applications of flavonoid compounds as analytical reagents have been reported.
Fig. 1. Chemical structures of the common classes of flavonoids. Representative compounds are shown in brackets.
Analytical Applications of Flavonoid Compounds
The flavonoid compounds can act as chromogenic agents in spectrophotometric and fluorometric detection of metals, and can act as complexing modifiers in solid phase extraction for preconcentration and separation of metal ions and in adsorptive voltammetry for the determination of metals [1].
Fig. 2. Schemes of possible sites for complexation of metal ions.
Chromogenic Agents: Flavonoids exhibit two major absorption bands in the ultra-violet/visible region (cinnamoyl system, band I; benzoyl system, band II) (as shown in Fig. 2). When the flavonoid compounds form complexes with the metal, these absorption spectra are shifted to the long-wavelength region, which make the basis of the determination of metals by UV-VIS spectrophotometry and spectrofluorimetry. Morin and quercetin can serve mainly as the chromogenic agents in analytical procedures for determination of Al(III), Cr(III), Fe(III), Zr(IV), Mo(VI), etc. Selected examples of application of morin and quercetin as chromogenic agents for determination of metal ions are presented in Table 1.
Table 1. Selected examples of application of morin and quercetin as chromogenic agents for determination of metal ions.
| Flavonoids | Metal ions | Conditions | λmax (nm) | ε×103 (L/mol·cm) | Dynamic range (µg/mL) | Detection
limit (µg/L) | Sample |
|---|
| Morin | Al(III) | 50% (v/v) EtOH | 421 | 5.3 | 0.01–5 | 6.0 | Alloys, environmental waters, biological samples |
| Al(III) | pH 4.5 | Ex: 418; Em: 490 | | 0.0001-1.6 | 0.16 | Waters, tea infusions, bovine serum, human hair |
| Cr(III) | pH 3.4-4.2; EFA | 435 | 113 | 0.03-0.46 | 16 | Steel, wastewater, nonferrous alloys |
| Fe(III) | CPB | 412 | | 0.9–1.5 | 40 | Alloys |
| Au(III) | pH 2–3 | 291 | 20.2 | 0.2–12 | 200 | Cosmetic cream samples |
| Quercetin | Cr(III) | pH 4–4.6; CBP | 441 | 102 | 0.14–2.1 | 920 | Steel, wastewater, nonferrous alloys |
| Zr(IV) | Oxalate + PVP | 320 | 73.1 | up to 1.46 | 60 | Industrial wastewaters |
| Ge(IV) | Dodecyl ether | Ex: 432; Em: 552 | | 7.4–150 | 7.4 | Drugs, wholemeal oats |
| Fe(III) | 0.3 M HCl | 291 | 7.5 | 0.1–15 | 70 | Pharmaceutical products |
Complexing Modifiers in Solid Phase Extraction: Solid-phase extraction (SPE) using various sorbents has been developed for preconcentration and/or selective separation of the analyte before its determination, which is frequently necessary in analytical procedures in order to reduce the effect of interference present in the matrix. Among the many sorbents, complexing resins are especially interesting due to their higher selectivity. Flavonoid molecule can act as complexing reagents (or complexing modifiers) to produce complexing resins to separate and preconcentrate metals before analysis. For example, a sorbent of controlled-pore silica glass functionalized with quercetin was reported by Abou-El-Sherbini and Hassanien[2]. This sorbent was used for the separation and preconcentration of Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) at pH 7.5–8.5. In addition, silica gel sorbents with immobilized quercetin and morin were synthesized for sorption-spectrophotometric determination of Sn(IV) and Zr(IV), respectively.
Fig. 3. Immobilization of quercetin on controlled-pore silica glass.
Adsorption Voltammetry for Determination of Metals: Adsorptive stripping voltammetry (AdSV) is an extremely sensitive electrochemical technique for determination of trace metals in various matrices with high selectivity. In this process, morin is the most frequently used as a complexing reagent to form chelates with metals for trace amount determination of metal ions by AdSV. The formed metal chelates are adsorbed and accumulated on the working electrode by a non-electrolysis process. Then, depending on the oxidation-reduction properties of the accumulated analyte, the qualitative and quantitative determination is carried out by scanning the potential in the appropriate negative or positive direction. Except for the possibility to determine a single metal ion with high sensitivity, morin was successfully used for simultaneous determination of bismuth and copper as well as copper, zinc, and lead in some natural and synthetic samples with satisfactory results.
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References
- Pyrzynska K. and Pękal A. Flavonoids as analytical reagents[J]. Critical Reviews in Analytical Chemistry, 2011, 41(4): 335-345.
- Abou-El-Sherbini K. S. and Hassanien M. M. Synthesis of controlled-pore silica glass functionalized with quercetin and its application for the separation and preconcentration of Mn (II), Co (II), Ni (II), Cu (II), and Zn (II)[J]. Separation science and technology, 2005, 39(5): 1177-1201.
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