A new strategy for synthesis of an in-tube molecularly imprinted polymer-solid phase microextraction device: Selective off-line extraction of 4-nitrophenol as an example of priority pollutants from environmental water samples
Graphical abstract
Introduction
Since the introduction of artificial antibody hypothesis in 1942 and the first use of a molecularly imprinted polymer (MIP) in 1972, these materials have been extensively studied [1], [2], [3]. MIP operates as an artificial specific receptor, owing to it's imprinting according to the size, shape and functional groups of a template molecule. In addition to the high selectivity of MIPs, their simplicity of production and furthermore, less strict operation conditions compared to immunosorbents, make their applications remarkably widespread.
Solid-phase microextraction (SPME) is a simple, fast, portable sampling and sample preparation method that uses smaller volumes of solvent and is solvent-free in some cases. Since its introduction in early 1990s [4], SPME has been developed in different types, including coated fibers and in-tube devices. Due to the fairly unspecific absorption or adsorption mechanisms in current SPME techniques, the enrichment of a broad variety of substances is the preferable application field of SPME. In general, commercial SPME polymer coatings possess a low selectivity toward a special target compound.
A combination of the selectivity of MIP materials with the simplicity of SPME can enable a tailor-made extraction performance of target molecules of special concern. Various methods were developed for preparing MIP-coated SPME fibers, which were used for analysis of brombuterol [5], triazines [6], [7], Sudan dyes [8], [9], ascorbic acid [10], tetracyclines [11], bisphenol A (BPA) [12], chloroacetanilide herbicides [13], anabolic steroids [14] and estrogens [15]. MIP-monolithic SPME fibers were evaluated via selective determination of triazines [16], [17], [18], methamphetamine [19], diacetylmorphine [20], ephedrine [21] and parabens [22]. In all these applications, the limited mechanic stability of the MIP fibers may cause short lifetimes and problems for multiple uses.
One approach for overcoming this problem would be in-tube SPME technique. Besides, handling in-tube techniques may be easier for automated on-line methods. First report for in-tube MIP-SPME was released by Pawliszyn and coworkers [23] for selective determination of propranolol. The MIP selected particles were used to pack a polyether ether ketone (PEEK) tube. Since then, different methods like monolith in-tube MIP-SPME [24], [25], [26], [27], MIP modified-polypropylene hollow fiber [28], multiple fibers packed in-PEEK tube [29] were developed.
Due to its high toxicity impact on environment and human health, 4-nitrophenol is regulated as one of the priority pollutants by the US Environmental Protection Agency (EPA) [30]. 4-Nitrophenol has been used widely as an intermediate or precursor for the production of insecticides, pharmaceuticals and dyes. Thus the determination of 4-nitrophenol in complex samples has attracted much attention. Commercially available SPME fibers [31], [32], [33], [34], [35], [36] and new generated SPME fibers [37], [38], [39] were used for its determination in water samples. Molecularly imprinted polymers were synthesized for 4-nitrophenol and evaluated for its determination in honey [40] and water [41], [42], [43], [44], [45] samples.
The objective of this study was to develop simple, inexpensive and reusable open-tubular MIP-capillaries for application in in-tube MIP-SPME. The novelty in contrast to the above mentioned packed or surface modified MIP-SPME is a new approach in synthesizing the MIPs as monolithic tube directly inside a glass capillary. This in-situ synthesis allows a wide variability of the MIP composition. The synthesized MIP-capillaries were evaluated for in-tube MIP-SPME extraction with HPLC-UV analysis of 4-nitrophenol from real water samples.
Section snippets
Materials
The chemicals used for polymer synthesis and extraction experiments were 4-nitrophenol, acetic acid, hydrochloric acid (HCl), methanol and acetonitrile (ACN) from MERCK (Darmstadt, Germany). Ethylene glycol dimethacrylate (EGDMA), methacrylic acid (MAA), 2,2′-azobisisobutyronitrile (AIBN) and EPA 604 phenols mixture containing 4-chloro-3-methylphenol (4-C-3MP), 2-chlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP), 2,4-dimethylphenol (2,4-DMP), 2,4-dinitrophenol (2,4-DNP),
The volume and the surface of the polymer
For the extraction, the type and the amount of the adsorbent are the most important parameters that must be considered carefully to get a high recovery and a complete and fast desorption [31], [32]. Coating–water distribution constants (Kcw) of polydimethylsiloxane (PDMS) and polyacrylate (PA) for phenol compounds are listed in Table 1, where nitrophenol compounds are among the chemicals that need polar coatings for efficient extraction. Furthermore, the volume and the surface area of the
Conclusions
A novel preparation protocol was developed for the construction of MIP-capillaries and evaluated via in-tube MIP-SPME extraction of polar compound from water. Besides its simple preparation and because of the glass cover, this new polymer tube has high in robustness and mechanical stability. Depending to the experimental conditions, an array of the tubes can be connected to increase the extraction recovery and number of concerned targets. Inherent potential of the tubes for automation use is
Acknowledgement
The authors thank Michael Rueckert (synlab Umweltinstitut GmbH, Leipzig-Markkleeberg, Germany) for providing the real sample of a monitoring program.
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