Bionic multi-tentacled ionic liquid-modified silica gel for adsorption and separation of polyphenols from green tea (Camellia sinensis) leaves
Graphical abstract
Introduction
Polyphenols in tea leaves (TP) have multiple preventive and therapeutic roles, which can reduce the risk of a variety of illnesses including cancer and coronary heart disease (Cimpoiu, Cristea, Hosu, Sandru, & Seserman, 2011). As a typical natural antioxidant with enormous variety, these phenolic constituents have been found in most plants and daily foods (Granato et al., 2016, Zielinski et al., 2015). Various methods have been developed to separate them for their use in pharmaceutical, food and chemical industry. For instance, pressure-assisted solvent extraction (300–500 MPa) was used to extract polyphenols from green tea through rapid washing together with slow diffusion (Xi, He, & Yan, 2015), and macroporous resin combined with polyamide column chromatography was employed to separate and purify epigallocatechin-3-gallate (EGCG) in tea leaves (Jin et al., 2015).
In the past decade, many functional materials have been invented through modification for inorganic particles of SiO2/TiO2 and employed for various separation tasks (Obaid et al., 2016, Reddy et al., 2014, Reddy et al., 2015, Reddy et al., 2011, Reddy et al., 2016, Showkat et al., 2007). Among them, ionic liquid-modified silica gel or immobilized/supported ionic liquid (SIL) on silica is getting much attraction, which has been increasingly applied as a new adsorbent material in separation of carbon dioxide, metal ions, benzene series pollutants, proteins/peptides, dyes/pigments, antibiotics and bioactive natural compounds. Compared with other inorganic supports (e.g. aluminum oxide, aluminum silicate, porous clays, activated carbon or carbon fibre), silica gel combines the advantages of low cost, little dead sorption resulting from complexation, easier preparation, modification and functionalization, etc. So it has become the most commonly used support in chromatographic fields (Rasmus, Anders, & Marco, 2014). A dicationic IL-modified silica stationary phase based on the most common imidazolium ionic liquids was evaluated in the separation of a series of organic and inorganic anions under reversed-phase/anion-exchange mixed-mode (Sun et al., 2014). In another study, a kind of imidazolium-embedded N,N-dimethylaminopropyl-functionalized silica has been used as hydrophilic interaction/reversed-phase mixed-mode stationary phase (Liu et al., 2015). Qiu et al. also developed a new copolymerization method to immobilize the cation of [C18VIm]+ and anion of p-styrene sulphonate simultaneously on mercaptyl-modified silica (Qiu et al., 2014). Meanwhile, many other kinds of ILs are being used to modify the silica gel, and some of them include tropine-type (for metal ions and racemic amino acids), quaternary ammonium-type (for palladium ions) and polytype ILs (for carbon dioxide) (Cheng et al., 2015, Qian et al., 2015, Sharma et al., 2016), etc. It should be mentioned that the innovation of new structural supporting materials is also current research hotspot. S. Mohammed and coworkers successfully immobilized 1-benzyl-3-(trimethoxysilylpropyl) imidazolium chloride on the surface of a cyano functionalized silica-titania oxide sol–gel for solid-phase extraction (SPE) of hazardous chlorophenols (Bakhshaei, Kamboh, Mohamad, Zaina, & Ma'amor, 2016).
Currently, the isolation of bioactive constituents from the complex extracts via a wider variety of ionic liquid-modified silica gels has been studied for different purposes. In the reported separation process, phenolic constituents were found to have strong and discrepant interactions with the cations/anions of ILs (Zhang et al., 2009, Du et al., 2011, Nie et al., 2015). Furthermore, more and more attention is being paid for IL@SiO2 with unique structures in the preparative separation for bioactive phenols. Meanwhile, there is still some difficulty in the rapid separation of TP from coexisting constituents (e.g. purine-type alkaloids, polysaccharide, tannins, vitamins and amino acids) in the tea extracts. Three kinds of commercial polymeric resins (Amberlite XAD-7, XAD-4 and Dianion HP-2MG) have been applied to adsorb (+)-catechin, (−)-epicatechin, (+)-gallocatechin together with other similar compounds (Gogoi, Saikia, Dutta, & Rao, 2010). It was found Dianion HP-2MG showed intermediate affinity for these adsorbates as a non-ionic aliphatic acrylic polymer (Wang, Gong, Chen, Han, & Li, 2012). Besides that, cross-linked agarose gel and cellulose acetate–titanium composite ultrafiltration membrane also showed good performance in previous reports (Li et al., 2005, Xu et al., 2006). However, to our knowledge, there is no report about the design and application of new SIL adsorbent for the separation of above mentioned compounds.
Polyvinyl alcohol (PVA) is a type of easy-processable and biocompatible shape memory polymer with temperature sensitivity. The cross-linked network and undisturbed crystal domain contribute to the permanent phase, while the amorphous phase acts as the temperature-sensitive phase. Inspired by the morphological and behaviour characteristics of a kind of marine organism named as Fungia repanda (see Fig. 1a), PVA was used to prepare novel multi-tentacled ionic liquid-modified silica in this work. According to the preliminary screening, N-methyl imidazolium cation combined with l-proline anion showed ideal interaction with TP. So the particle surface of spherical silica was firstly modified with the IL via PVA long chains by covalent bonds (see Fig. 1b). After comprehensive characterization by various analytical methods and instruments, the new multi-tentacled ionic liquid-modified silica was used for the selective adsorption and separation of major polyphenols from the tea extracts for the first time. Moreover, detailed investigation was focused on the selectivity, mechanism and influence of separation process on free radical scavenging capacity of total polyphenols. Finally, the composition of various separated fractions and reusability of this ionic liquid-modified silica were also explored. The work is expected to develop new solid-phase extraction and column chromatography media based on unique SIL and provide more meaningful reference about their potential application in the separation and purification of natural bioactive products.
Section snippets
Materials
l-proline, glutaraldehyde, sodium hydroxide, methanol, absolute ethanol, acetonitrile, ethyl acetate, diethyl ether, hydrochloric acid and PVA-1799 (degree of polymerization: 1700, degree of alcoholysis: 99, M.Wt.: 25,000–35,000) were purchased from Kelong chemical reagent factory (Chengdu, China). All solvents and reagents used were of analytical grade or higher. N-methylimidazole and 201 × 7 (717) strongly basic styrene-type anion-exchange resin (particle size: 0.4–0.7 mm, total exchange
Elemental analysis and morphology
The elemental content of the synthesized tentacle type sorbent was 33.68% of C, 6.82% of H and 6.41% of N, respectively; meanwhile N content of blank activated silica was determined as 0.0%. As the result, the content of ionic liquid (Im+·Pro−) attached to the tentacle silica surface was calculated to be 1.53 mmol g−1 from the percentage amounts of nitrogen (N%), and the mass percentage of ionic liquid was determined as 30.15%. Compared with previous immobilized IL without PVA tentacles, the SiO2
Conclusions
In the present work, the selective adsorption performance of SiO2·PVA·Im+·Pro− as sorbent for tea polyphenols was evaluated and related effects of main operating conditions were all investigated. The adsorption capacity could reach up to 236.84 mg g−1 after RSM optimization, which was several times higher than traditional adsorbents such as those common resins. The adsorption kinetics was well described by pseudo-second-order model and the adsorption data fitted well with the Langmuir model. The
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
Preparation of this paper was supported by the National Natural Science Foundation of China (No. 81373284, 81673316) and 2013 scientific research foundation of Sichuan University for Outstanding Young Scholars.
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