A high efficient adsorbent for plant growth regulators based on ionic liquid and β-cyclodextrin functionalized magnetic graphene oxide

Highlights

• The ionic liquid functionalized magnetic β-cyclodextrin/graphene oxide nanoparticle was firstly synthesized.

• Fe₃O₄@SiO₂/GO/β-CD/IL was used as MSPE adsorbent for plant growth regulators in vegetable samples.

• Fe₃O₄@SiO₂/GO/β-CD/IL showed a high extraction efficiency toward 7 PGRs among 23 PGRs.

• The parameters influenced the extraction efficiency were optimized in detail.

Abstract

Four types of ionic liquids which composed of cation with imidazole ring and anion with benzene, naphthalene, anthraquinone and naphthoquinone ring were prepared and named VOIm⁺BenSO₃^-, VOIm⁺NapSO₃^-, VOIm⁺AQSO₃^- and VOIm⁺NQSO₃^-, respectively. Then, ionic liquids and β-cyclodextrin functionalized magnetic graphene oxide materials were firstly synthesized for the extraction of 23 plant growth regulators in vegetable samples. The synthesized materials anticipated the unique properties of ionic liquids, β-cyclodextrin, graphene oxide and Fe₃O₄@SiO₂, including high surface area, high supramolecular recognition capability, superparamagnetism and large delocalized π-electron system, which were successfully characterized. Among four kinds of ionic liquids functionalized adsorbents, the VOIm⁺AQSO₃^- modified material exhibited high adsorption capability to 7 plant growth regulators compared with Fe₃O₄@SiO₂/GO/β-CD, Fe₃O₄@SiO₂/GO and other three types of ionic liquids based adsorbents. Parameters that could affect the recoveries of 7 plant growth regulators were investigated, such as mass ratio of Fe₃O₄@SiO₂/GO/β-CD to ionic liquids, salt concentration, pH, amount of adsorbent, extraction time and desorption solvent. Under the optimal condition, the magnetic solid phase extraction method using VOIm⁺AQSO₃^- and β-cyclodextrin functionalized magnetic graphene oxide material (Fe₃O₄@SiO₂/GO/β-CD/IL) as adsorbent was developed and coupled with ultra-high performance liquid chromatography-triple quadrupole linear ion trap mass spectrometry for the determination of 7 plant growth regulators in vegetable samples. Validation results showed that the limit of detection and limit of quantitation were in the range of 0.01–0.18 μg/kg and 0.03–0.58 μg/kg, respectively. The satisfactory linearities were acquired in the concentration of 2–50 μg/kg with the correlation coefficients higher than 0.9982. Taken together, the synthesized Fe₃O₄@SiO₂/GO/β-CD/IL was a highly effective magnetic solid phase extraction adsorbent for the enrichment of trace level of plant growth regulators in vegetable samples.

Introduction

Solid phase extraction (SPE) is one of the most widely used pre-concentration technique for the chemical and biological analysis, especially when dealing with samples with complex matrix. However, the traditional SPE techniques required column-packing, sample loading, washing and eluting steps, which were tedious, time consumption, labor intensive and relatively expensive. Magnetic solid phase extraction (MSPE), a novel derivate of SPE, based on the use of magnetic adsorbents, has attracted a tremendous amount of attentions due to the merits of high adsorption efficiency, easy operation, fast separation, time saving and low cost [1], [2]. In MSPE, the magnetic adsorbents composed of magnetic inorganic materials and non-magnetic materials played a crucial role. The adsorbents dispersed in sample solution for extraction and enrichment of the target analytes could be rapidly separated by an external magnetic field owing to the presence of the magnetic core of adsorbents, which greatly simplified the SPE procedure. The non-magnetic materials could improve the dispersibility and the adsorption efficiency of adsorbents. Thus, in recent researches, great attentions have been focused on synthesis of new nano-magnetic adsorbents and exploring their potential application in MSPE [3], [4], [5]. So far, plenty of nanomaterials involving nanotubes [6], nanofibers [7], metal-organic frameworks [8],quantum dots [9], graphene and graphene oxide have been investigated [10]. Among them, graphene oxide (GO) is an oxidized form of graphene (G), which contains the various functional groups such as carboxyl (-COOH), hydroxyl (-OH) and epoxy groups (-C-O-C-) on its chemical structure. As an adsorbent, the high specific surface area of up to 2630 m²/g, large delocalized π-electron system and oxygen-containing groups imported GO strong affinity with aromatic compounds via Van der Waals forces, π-π interactions, hydrophobic interactions, hydrogen bonds and other noncovalent interactions [11], [12]. However, it was challenge for separation of GO from sample solution by conventional separation methods such as centrifugation and filtration, which would restrict GO apply in dispersive solid phase extraction. To overcome this shortage, some literatures about modification of GO by magnetic nanoparticles have been reported [13], [14], [15]. The magnetic adsorbents based on GO exhibited high adsorption capability of GO and the fast separation property of magnetic nanoparticles.

β-cyclodextrin (β-CD) is a torus-shaped cyclic oligosaccharide composed of seven D-glucose units, which possesses hydrophobic cavity and hydrophilic wall. The hydrophobic inner cavity has remarkable capacity to selectively form host-gust inclusion complex with various molecules of a certain size and polarity. In addition, β-CD is an environmentally friendly and water-soluble molecule, having the virtue of improving the solubility and biocompatibility of functional materials. It has been widely used as adsorbent for selective separation and extraction analytes with particular chemical structures by modified with other materials. For example, Wang et al. synthesized a β-cyclodextrin/ethylenediamine/magnetic/graphene oxide for Cr(VI) removal; Zhao et al. reported an EDTA-cross-linked β-cyclodextrin (EDTA-β-CD) bifunctional adsorbent for simultaneous adsorption of metals and cationic dyes; Wang et al. prepared a β-cyclodextrin-graphene oxide nanocomposite for p-phenylenediamine adsorption and removal [16], [17], [18].

Ionic liquids (ILs), a class of organic salts which are liquid below 100 °C or even at room temperature, have been considered to be “green” solvents owing to their unique properties including negligible vapor pressure, good chemical and thermal stability, variable viscosity, desirable solubility for organic and inorganic compounds, as well as environmental friendliness. Additionally, the chemical and physical properties of ILs could be easily tuned and modulated by varying the cation or anion [19]. The combination of ILs with other materials would enhance the performance of corresponding materials and broaden their application field. Wang et al. reported that the ILs based on imidazolium could change the surface charge of the material because of the high atomic percentage of nitrogen. Zhang et al. imparted that there were van der Waals forces and π-π interactions between aromatic IL and graphene layer [20], [21]. Thus, the modification of IL with graphene or graphene oxide have attracted significant attention in the field of separation and electrode. For instance, Atta et al. prepared a cyclodextrin/ionic liquid crystal/graphene composite electrode for the determination of some neurotransmitters in urine; Zhou et al. synthesized an ionic liquid modified graphene oxide composite for the adsorption of phthalates from aqueous solution; Li et al. reported a magnetic ionic liquid/chitosan/graphene oxide composite and application for water treatment [22], [23], [24]. The ILs modified on surface of materials obviously improved the adsorption efficiency of adsorbents.

Plant growth regulators (PGRs) are widely used in agriculture to improve the production at a low concentration. Nevertheless, the PGRs residues exhibited potential toxicity to humans and animals, involving carcinogenicity, acute toxicity and neurotoxicity. Trace determination of PGRs in complex matrices for minimizing the health risk from dietary PGRs exposure required a rapid, simple, sensitive and selective sample pre-treatment technique prior to instrumental analysis [25], [26]. Herein, in this study, a novel material composed of ionic liquid (IL) and β-cyclodextrin modified magnetic graphene oxide (Fe₃O₄@SiO₂/GO/β-CD/IL) was firstly fabricated and applied as an adsorbent for the extraction of seven plant growth regulators (PGRs). The characteristic results of Fourier transform infrared spectrums (FT-IR), field emission scanning electron micrographs (FESEM), transmission electron micrographs (TEM), X-ray photoelectron spectra (XPS), vibrating sample magnetometry (VSM) and BET (Brunauer-Emmet-Teller) analysis proved that Fe₃O₄@SiO₂/GO/β-CD/IL was successfully prepared with high surface area, magnetic property and plenty of functional groups in its structure, which would present the merits of Fe₃O₄@SiO₂, GO, β-CD and IL for the adsorption of target analytes. Under the optimum condition, in comparison with Fe₃O₄@SiO₂/GO, Fe₃O₄@SiO₂/GO/β-CD and Fe₃O₄@SiO₂/GO/IL, the final synthesized Fe₃O₄@SiO₂/GO/β-CD/IL exhibited the best extraction efficiency toward 7 PGRs, indicating the ability of IL for improvement the adsorption capability of adsorbent. The adsorption mechanism between material and PGRs could be ascribed to the π-π interactions, hydrophobic interactions, electrostatic interactions, host-guest inclusion complexes formation, hydrogen bonds and other non-covalent bonds. Based on the optimized experimental parameters, a reliable MSPE procedure coupled with ultra-high performance liquid chromatography-triple quadrupole linear ion trap mass spectrometry (UHPLC-MS/MS) was developed for analysis of 7 PGRs in vegetables. To the best of our knowledge, this was the first time for synthesis of VOIm⁺AQSO₃^- functionalized β-cyclodextrin/magnetic graphene oxide material and used as MSPE adsorbent.