Elsevier

Talanta

Volume 85, Issue 1, 15 July 2011, Pages 282-289
Talanta

Molecularly imprinted polymers by reversible addition–fragmentation chain transfer precipitation polymerization for preconcentration of atrazine in food matrices

https://doi.org/10.1016/j.talanta.2011.03.060Get rights and content

Abstract

Controlled/living free radical polymerization (CLRP) has been accepted as an effective technique in preparation of polymers because of its inherent advantages over traditional free radical polymerization. In this work, reversible addition–fragmentation chain transfer (RAFT) polymerization, the ideal candidate for CLRP, was applied to prepare atrazine molecularly imprinted polymers (MIPs) by precipitation polymerization. The resultant RAFT-MIPs demonstrated uniform spherical shape with rough surface containing significant amounts of micropores, leading to an improvement in imprinting efficiency compared with that of the MIPs prepared by traditional precipitation polymerization (TR-MIPs). The maximum binding capacities of the RAFT-MIPs and TR-MIPs were 2.89 mg g−1 and 1.53 mg g−1, respectively. The recoveries ranging from 81.5% to 100.9% were achieved by one-step extraction by using RAFT-MIPs for preconcentration and selective separation of atrazine in spiked lettuce and corn samples. These results provided the possibility for the separation and enrichment of atrazine from complicated matrices by RAFT-MIPs.

Introduction

The family of triazines comprises of the most widely employed herbicides in the world. Atrazine, with better efficiency for control of weeds, is probably the most widely used herbicides of this class. The prolonged utilization of atrazine results in its accumulation in environment and represents a threat to the environment and human health. As an endocrine disruptor, atrazine has high carcinogenicity and mutagenicity, especially after biomagnification [1]. It has been reported that atrazine can cause biological effects of model animals even at much lower the regulated safe dose levels [2]. Analysis of atrazine residues is complicated because of their trace level presence, world-wide distribution and great matrix effects. Therefore, high-efficiency pretreatment procedures and high selective and sensitive analysis methods are urgently required for monitoring the presence and determining the levels of atrazine.

The most frequently used methods for the determination of atrazine are high-performance liquid chromatography (HPLC) [3], [4] and gas chromatography (GC) [5], [6], which always involves in traditional sample pretreatment procedures, such as solid phase extraction (SPE) and solid phase microextraction (SPME). The main problem associated with traditional sorbents of SPE/SPME is the low selectivity and/or low adsorption capacity. A new type of high efficiency adsorbents, molecularly imprinted polymers (MIPs), owing to high sample load capacity, high selectivity, low cost and easy preparation, have been widely applied for preconcentration and high efficient separation of trace analytes in diverse matrices, such as natural, agricultural and food products and environmental samples [7], [8], [9], [10].

MIPs are prepared by copolymerization of functional monomers and cross-linkers in the presence of target analytes which act as template molecule. After removal of template, recognition sites complementary in size, shape, and functionality to the template are formed in the three-dimensional polymer network. The mechanism of MIPs formation mainly includes free-radical polymerization and sol–gel process; the former is the more popular, typically including bulk polymerization, suspension polymerization [11], emulsion polymerization [12], and precipitation polymerization [13]. Up to now, bulk polymerization and suspension polymerization have been used for the preparation of MIPs for the determination of triazines [14], [15], [16]. However, the resulting MIPs are associated with some problems, involving complicated after-treatment workup, heterogeneous binding sites and slow mass transfer, etc., which restrict its imprinting capacity and extraction efficiency. Recently, controlled/living free radical polymerization (CLRP), has been widely utilized to synthesize MIPs, including reversible addition–fragmentation chain transfer (RAFT) polymerization, metal-catalyzed atom transfer radical polymerization (ATRP), nitroxide-mediate polymerization (NMP) and iniferter, due to their intrinsic advantages over conventional free radical polymerization, such as producing well-defined polymers with predictable molecular, low polydispersity, controlled composition and functionality [17]. RAFT polymerization, the ideal candidate for CLRP has also been applied for MIPs preparation because of its versatility and simplicity. For example, Titirici and Sellergren [18] have prepared l-phenylalanine anilide imprinted core-shell microspheres by RAFT polymerization on silica beads. The usage of RAFT prevented visible gel formation. Subsequently, the same approach was exploited by Lu and coworkers [19]. They immobilized chain transfer agent on silica nanoparticles and synthesized 2,4-dichlorophenoxyacetic acid imprinted microspheres by surface RAFT polymerization. The forming of uniform MIP shells with adjustable thicknesses can be attributed to the intrinsic characteristics of RAFT polymerization. Pan and coworkers [20] have proved that MIPs prepared by RAFT precipitation polymerization (RAFT-MIPs) possess improved binding capacity and larger binding constant than MIPs prepared by traditional precipitation polymerization (TR-MIPs).

In this work, the development of RAFT precipitation polymerization for the preparation of atrazine MIPs with higher imprinting efficiency and binding capacity was described. To the best of our knowledge, this was the first demonstration for the preconcentration of atrazine in food matrix using RAFT-MIPs. Some variable parameters influencing the final characteristics of the obtained materials in terms of capacity, affinity and selectivity for target analyte, such as the nature of functional monomers, cross-linkers and solvent, the amount and the nature of chain transfer agent (CTA) were investigated in detail. The performances of RAFT-MIPs for extraction of atrazine in spiked lettuce and corn samples were also evaluated.

Section snippets

Reagents

Bromobenzene, magnesium, benzyl bromide, tetrahydrofuran (THF), carbon disulfide, 2-chloropropane and petroleum ether were purchased from Shanghai Chemical Reagents Company (Shanghai, China). Methacrylic acid (MAA), 4-vinlpyridine (4-VP), ethyleneglycol dimethacrylate (EGDMA), trimethylolpropane trimethacrylate (TRIM) and divinylbenzene (DVB) were purchased from Sigma–Aldrich and distilled in vacuum prior to use in order to remove stabilizers. 2,2′-azo-bis-isobutyronitrile (AIBN) were purchased

RAFT precipitation polymerization for preparation of MIPs

The imprinting process of RAFT polymerization for atrazine is illustrated in Scheme 1b. Homogeneous morphology and excellent recognition properties are the most significant indicators of well functioning MIPs, which are mainly the result of optimization of typical factors such as type and proportion of porogens, monomers and cross-linkers. Especially, for the proposed RAFT precipitation polymerization, the unique factor, CTA, is also considered.

Conclusions

An alternative general method was presented to prepare atrazine MIPs by RAFT precipitation polymerization. The resultant RAFT-MIPs exhibited excellent characteristics, such as highly spherical and uniform morphology, higher binding capacity and selectivity for atrazine. The analytical method based on RAFT-MIPs was successfully applied for atrazine analysis in spiked lettuce and corn samples. Given the advantages of “living polymerization” of the RAFT polymerization method, we expect it can be

Acknowledgements

Financial support from the National Natural Science Foundation of China (20975089), the Chinese Academy of Sciences (KZCX2-EW-206), Department of Science and Technology of Shandong Province of China (2008GG20005005), the 100 Talents Program of the Chinese Academy of Sciences, and the Department of Science and Technology of Yantai City of China (2010158) is gratefully acknowledged.

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