Thermosensitive molecularly imprinted polymers on porous carriers: Preparation, characterization and properties as novel adsorbents for bisphenol A
Graphical abstract
Introduction
Phenolic environmental estrogens (PEEs) have received considerable concerns in the field of pollution monitoring and remediation, which may change the function of the endocrine systems and consequently elicit negative health effects [1], [2]. Bisphenol A (BPA), one of PEEs, has been widely used as raw material of plastics and resins, antioxidants and polymerization inhibitors [3], [4]. However, prolonged utilization and abuse of BPA leads to its accumulation in environment and thereafter causes environmental and health threats [3], [4], [5]. Some hazards and adverse influences have been identified in model animals exposed to low doses of BPA even much lower than the regulated safe dose levels [5]. Meanwhile, complicated matrix effects make the residue analysis of BPA face crucial challenges [6]. So, it is urgently required to develop simple, fast and high sensitive methods for identifying the presence and determining the levels of BPA.
The most frequently used methods for separation and determination of BPA mainly include high performance liquid chromatography (HPLC), HPLC–mass spectrometry (MS), and gas chromatography–MS (GC–MS), generally following suitable sample pretreatment processes such as the widely used liquid–liquid extraction (LLE) and solid-phase extraction (SPE) [7], [8], [9], [10], [11]. The main problem associated with traditional sorbents of SPE is the low selectivity and thereby low adsorption capacity and enrichment efficiency. Therefore, developing specific materials with high selectivity is necessary. Molecularly imprinted polymers (MIPs) are gaining popularity [12]. For example, Wang et al. synthesized MIPs via ring-opening metathesis polymerization [13] and Li et al. prepared hollow porous MIPs using tetrabromobisphenol A as dummy template [14], which were both used as SPE sorbents to enrich BPA combining with HPLC–UV determination.
Besides, another promising material, stimuli-responsive polymers (SRPs), also known as environmental responsive polymers or smart polymers, has also attracted widespread interest [15], [16]. SRPs are able to respond to specific external stimuli with considerable changes in their physicochemical properties, including molecular chain structure, solubility, surface structure, swelling or dissociation behavior, etc., and various stimuli signals are available such as temperature, pH, magnetism and light [17]. By combining MIPs with SRPs, the resultant seductive functional materials, stimuli responsive MIPs (SR-MIPs) have been developed, occupying the advantages of molecular recognition ability for template/target species and responsive ability to external stimuli [17], [18], [19]. That is, the release and adsorption of template/target molecules can be achieved through external stimuli regulation. Excitedly, a number of SR-MIPs have been designed and synthesized for recognition and detection of BPA [17], [20], [21], [22], [23]. For instance, Griffete et al. presented inverse opals of imprinted hydrogels for the detection of BPA and displayed large responses to external pH stimuli related to the thickness of hydrogel film [20]. Liu et al. described a novel core-shell BPA imprinted nanoparticle by polychloromethylstyrene coated superparamagnetic material via surface initiated atom transfer radical polymerization [21]. Lin et al. adopted bisphenol F as a dummy template to prepare MIPs for BPA with a magnetic supporter which could increase the extraction efficiency [22]. Lu et al. fabricated a photoelectrochemical (PEC) sensor to detect BPA based on vertically aligned TiO2 nanotubes with surface molecularly imprinted polypyrrole, and a highly sensitive PEC response for BPA was obtained [23]. It is found that magnetic SR-MIPs are mostly discussed; other type ones are rarely mentioned. Meanwhile, to the best of our knowledge, temperature responsive SR-MIPs have never been reported for BPA till now.
N-Isopropylacrylamide (NIPAM) is a well-known intelligent material that is responsive to the temperature, which has the property of a soluble (hydrophilic)–insoluble (hydrophobic) transition at a lower critical solution temperature (LCST) of about 33 °C [24]. It displays a coil (soluble) state when the solution temperature is below the LCST, as well as a collapsed (insoluble) state at above the LCST [24]. The thermally responsive behavior of NIPAM can be used to design smart MIPs; this may be the ideal way to controlled adsorption and release of template molecules according to temperature changes.
In this work, we firstly synthesized the smart material of thermosensitive MIPs (T-MIPs) by using NIPAM as temperature responsive functional monomer, for selective recognition and extraction of BPA from real water and food samples. A simple synthesis protocol was used by grafting two types of functional monomers on porous polymers carrier, and plenty of hollow pores were beneficial for mass transfer. The synergy effect of dual monomers of NIPAM and 4-vinylpyridine (VP) would contribute greatly to the performances of T-MIPs. Binding property, imprinting ability and temperature-regulated behavior were systematically investigated. Accordingly, the obtained smart T-MIPs were successfully applied to SPE for BPA in seawater and yogurt samples, indicating great potential for the analysis/removal of the PEEs in complicated matrices.
Section snippets
Materials and instruments
Poly-vinylpyrrolidone (PVP), poly-vinylalcohol (PVA), N-isopropylacrylamide (NIPAM, 98%) and N,N′-methylenebis(acrylamide) (MBAA, 99%) were purchased from Aladdin (Shanghai, China). Ethylene glycol dimethacrylate (EGDMA, 98%), divinylbenzene (DVB, 80%), styrene (99%), thioglycolic acid, 2,2-bis(4-hydroxyphenyl) (BPA), 17β-estradiol (β-E2, 99%), phenol (PE, 99.6%) and cholesterol (CE) were obtained from Aladdin (Shanghai, China). Estriol (E3, 99.5%), estrone (ET, 99%), 4-vinylpyridine (VP, 95%)
Preparation of T-MIPs for BPA
T-MIPs were synthesized using two types of functional monomers via their synergistic effect, on a porous carrier material. The preparation process and possible imprinting mechanism are schematically illustrated in Fig. 1 and the condition optimization recipes are listed in Table S1. HPS nanoparticles were first prepared by dispersion polymerization followed by the thermosensitive imprinted polymers grafted onto the cavity in the carrier. Meanwhile, four groups of monomers (NIPAM-VP, NIPAM-MAA,
Conclusions
In summary, the design and synthesis of thermosensitive MIPs materials for selective recognition and controlled adsorption and release of BPA are demonstrated in this study, via synergy of dual functional monomers. The morphology and structure characterization results clearly confirmed that T-MIPs were successfully prepared with high polymerization efficiency. And the T-MIPs presented high adsorption capacity, fast binding kinetics and good selectivity as well as good stability and reusability.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (21105117, 31160317, 21201098 and 21275158).
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