Magnetic γ-cyclodextrin polymer with compatible cavity promote the magnetic solid-phase extraction of microcystins in water samples

doi:10.1016/j.aca.2018.12.028

Analytica Chimica Acta

Volume 1054, 25 April 2019, Pages 38-46

https://doi.org/10.1016/j.aca.2018.12.028 Get rights and content

Highlights

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A facile MSPE method was applied for preconcentration microcystins using Fe₃O₄@PDA@γ-CDP microspheres.
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Good aqueous dispersibility and compatible inner cavity endow Fe₃O₄@PDA@γ-CDP with high enrichment performance.
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Two different kinds of MCs could be detected in lake and sea water samples.

Abstract

Microcystins (MCs), which are produced by eruptive cyanobacteria, seriously threaten the health of humans for their poisonousness. Herein, a facile strategy was introduced to synthesize the magnetic γ-cyclodextrin polymer (Fe₃O₄@PDA@γ-CDP) composite and set it as a novel adsorbent with excellent properties for the magnetic solid-phase extraction (MSPE) of MCs. The prepared Fe₃O₄@PDA@γ-CDP microspheres exhibit good aqueous dispersibility and highly affinity for MCs, thus contribute to an excellent extraction performance for trace MCs in water samples. Combined with HPLC–MS/MS, a handy, sensitive and efficient method was developed for detection of MCs in water samples, which shows good linearity (R² ≥ 0.9992) in the range of 1.0–1000 pg mL⁻¹, low limits of detection (0.8–2.0 pg mL⁻¹, S/N = 3), satisfactory repeatability with the relative standard deviations (RSDs) lower than 6.2% (n = 5). At last, the proposed method was successfully applied for the extraction and detection of MCs in fresh water and sea water samples, which provides great potential of trace-level MCs determination in lake and sea water.

Graphical abstract

Introduction

With the increasingly serious eutrophication of water, water blooms and red tides often break out. During the whole process of algae growth, lots of algal toxins generate. Among them, microcystins (MCs) are biologically active monocyclic heptapeptide toxins which have attracted much attention for their acute lethal toxicity to human and animals [1]. Up to date, there are over 100 variants of MCs have been discovered in nature, such as the most commonly studied MC-YR, MC-RR and MC-LR [2,3]. The concentration of documented MC in whole water of Lake Taihu was 23.26 μg L⁻¹ [4] which is much exceed the upper limit of the safe value for human exposure (1.0 μg L⁻¹) recommend by WHO [5]. Hence, it's of great urgent to establish a reliable method for effective and rapid detection of MCs from water sources.

As one of the most powerful technology, HPLC–MS/MS has been used as routine analysis of MCs [[6], [7], [8]]. Before the HPLC–MS/MS analysis of MCs, an efficient sample pretreatment strategy is essential because of their trace concentrations and high levels of salt in seawater. Some useful pretreatment methods including solid-phase extraction (SPE) [[9], [10], [11]], solid-phase microextraction (SPME) [12], matrix solid-phase dispersion (MSPD) [13], dispersive liquid–liquid microextraction (DLLME) [14] and magnetic solid-phase extraction (MSPE) [7] have been studied. In the past several years, MSPE has drawn increasing interest for its rapid and convenient isolation by an external magnet after enrichment of adsorbed species [[15], [16], [17], [18]]. A prerequisite for MSPE procedure with outstanding performance is a suitable magnetic sorbent with strong affinity, well dispersibility and good stability.

Cyclodextrins (CD) are oligosaccharides with ring-shaped, three-dimensional structure and a hydrophobic cavity in the center [19]. Such characteristic features endow cyclodextrin molecules with the capability of accommodating a lipophilic “guest” molecule if its size and shape are compatible [20]. The outer surface of CD is hydrophilic which ensures the compatibility of CD with aqueous solution. It has been found that there is a strong host-guest interaction between hydrophobic inner cavity of CD and the Adda moiety of MCs (see Fig. S1) [21]. Therefore, functional composites based on CD could be a good candidate sorbent for enrichment of MCs from real samples. In our previous work, magnetic porous β-CD-containing polymer microspheres were applied as the adsorbent for MSPE of the MCs and achieved pretty good extraction efficiency [22]. However, the long pretreatment time and high limits of detection (LODs) of the method are still blocking their application for real sample. The main reasons should be ascribe to the deficiency of the low aqueous solubility (<1.8 g/100 g, 25 °C) and small inner cavity (ID. = 6.5 Å) of β-CD which prohibit the accessibility of target molecule into the cavity. The γ-CD has bigger inner cavity (ID. = 8.5 Å) and much better hydrophily (23.2 g/100 g water, 25 °C) than β-CD [23], and such properties in γ-CD are believed to not only facilitate the dispersibility of adsorbent and accelerate MCs molecules diffusion but also render highly affinity to promote the enrichment performance. Additionally the magnetic composites can achieve rapid separation, it's reasonable to suppose that the analytical method based on porous γ-CD-containing polymer will shorter the pretreatment time and has lower LODs than that based on β-CD-containing polymer.

Herein, the magnetic particles were well coated with polydopamine (PDA) firstly, then the magnetic γ-CDP (denoted as Fe₃O₄@PDA@γ-CDP) microspheres were synthesized via a facile synthetic method and set as the sorbent for MSPE for MCs. The adsorption behavior of magnetic γ-CDP microspheres for three MCs was explored and some main MSPE conditions were optimized. Based on the compatible cavity and numerous hydrogen bonds interaction, the highly dispersed Fe₃O₄@PDA@γ-CDP particles exhibit excellent enrichment of MCs in water samples. Subsequently, MSPE with the newly prepared magnetic γ-CDP coupled with HPLC–MS/MS was successfully used for the extraction and determination of the three MCs in lake and sea water. Such a simple, efficient and sensitive method developed in this study will pave the way to new techniques for the determination and monitoring of MCs in water sources.

Section snippets

Chemicals and reagents

All reagents were of analytical grade or better. Trisodium citrate dihydrate (Na₃Cit·2H₂O), sodium acetate (NaOAc), Ferric chloride hexahydrate (FeCl₃·6H₂O) and ethylene glycol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The γ-CD (>98%) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Ultrapure water (18.2 MΩ cm) was prepared from a Milli-Q water purification system (Millipore, Millipore, USA).

MCs (≥95%) were obtained from Enzo Life Sciences,

Synthesis and characterization of Fe₃O₄@PDA@γ-CDP microspheres

The Fe₃O₄@PDA@γ-CDP microspheres were prepared via simple two-steps, and the schematic representation of the fabrication is shown in Fig. 1a. The ∼200 nm Fe₃O₄ was used as the core of the core-shell structure, embedded into a ∼35 nm γ-CDP shell (Fig. 1b and d and Fig. S2). Between them, the coated thin PDA layer of ∼10 nm was not only act as protective layer for Fe₃O₄, but also used as linker for further copolymerization with γ-CD (Fig. 1c). A HAADF–STEM image further confirmed that core-shell

Conclusion

In this work, the core-shell Fe₃O₄@PDA@γ-CDP microspheres were successfully synthesized via a simple synthetic method and used as the MSPE adsorbents for enrichment of the MCs in real water samples. The synthesized Fe₃O₄@PDA@γ-CDP microspheres owning the following advantages: good hydrophily, large specific surface area, good chemical stability and simultaneously containing suitable mesopores. The multiple interactions of host-guest interaction, strong π-π affinity, and hydrogen-bond

Declaration of interest statement

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

Acknowledgements

This work completed with the support of the National Nature Sciences Foundation of China (21575028, 21605022), the Program for Changjiang Scholar and Innovative Research Team in University (No. IRT15R11), China.

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Magnetic γ-cyclodextrin polymer with compatible cavity promote the magnetic solid-phase extraction of microcystins in water samples

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemicals and reagents

Synthesis and characterization of Fe3O4@PDA@γ-CDP microspheres

Conclusion

Declaration of interest statement

Acknowledgements

Toxicology

Water Res.

J. Clean. Prod.

Ecol. Indicat.

J. Hazard Mater.

J. Chromatogr. A

J. Chromatogr. A

J. Chromatogr. A

Toxicon

Talanta

J. Chromatogr. A

Anal. Chim. Acta

Anal. Chim. Acta

Synthesis and characterization of Fe₃O₄@PDA@γ-CDP microspheres