A critical review of molecularly imprinted polymers for the analysis of organic pollutants in environmental water samples
Introduction
The ever-increasing release of chemicals from human activities like agriculture and industrial processes has impacted the environment and human health, for example, through exposure to cancer causing or promoting agents [1,2]. Environmental matrices affected include water, soil, and atmosphere, as well as flora and fauna; all of these are the concern of analytical chemists. Numerous analytical methods, usually including liquid chromatography (LC) and gas chromatography (GC), have been reported for the quantification of pollutants in different samples. Although direct detection methods can improve throughput and reduce errors associated with sample handling, low analyte concentrations, and sample complexity limit their accuracy and suitability. Therefore, simple, rapid, cheap and reliable techniques for clean-up, isolation, and preconcentration of desired compounds from environmental samples are necessary before instrumental analysis [3,4].
The simplest routine preconcentration method used widely for environmental analysis is liquid–liquid extraction (LLE) [5]. LLE is time-consuming, labor-intensive, and expensive (labor and solvent cost); it also requires large sample volumes and toxic organic solvents. To overcome these disadvantages, extraction techniques such as solid phase extraction (SPE) [6], [7], [8], solid phase microextraction (SPME) [9,10], stir bar sorptive extraction (SBSE) [11,12], liquid phase microextraction (LPME) [13,14], and cloud point extraction (CPE) [15,16] have been developed for isolation and preconcentration of contaminants from environmental samples. Though these methods advance extraction protocols by reducing reagent and sample volumes and time, which have led to adoption in routine analyses, they suffer from lack of selectivity against interfering compounds and their performance is sensitive to matrix composition [17]. Selective sample preparation methods improve sensitivity and reproducibility by decreasing matrix effects.
One approach to achieving this is by using molecularly imprinted polymers (MIPs), which is the focus of this paper. MIPs, which are frequently described as plastic antibodies and analogous to naturally-occurring antibodies, feature selective recognition properties for target molecules. These synthetic molecular recognition systems are chemically robust and relatively easy to tailor to new analytical targets. MIPs are synthesized (Fig. 1) through the copolymerization of a functional monomer and a crosslinker, such as methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA), respectively, in the presence of the template molecule (the target analyte or an analogue with similar chemistry and shape). Polymerization is usually induced with thermal or UV activation of an initiator. After polymerization, the template molecules are removed leaving a polymer containing cavities complementary in shape, size and functional groups to the target molecule. The excellent recognition properties of MIPs compared to non-imprinted polymers (NIPs) are derived from the interactions between the template molecule and a monomer functional group that is present in the pre-polymerization complex. These interactions are easily re-established when the MIPs are exposed to the analyte in the sample matrix. It is this feature that gives MIPs a significant advantage over traditional non-selective sorbents [18]. MIPs which have been utilized in wide range applications including chromatography [19], sensors [20], [21], [22], [23], drug delivery [24,25] and catalysis [26], [27], [28], can be implemented as selective extraction phases for sample pretreatment and preconcentration [29], [30], [31]. In theory, the selectivity of MIPs should increase the sensitivity and repeatability of water analysis by diminishing the co-extraction of matrix interferences. This selectivity reduces overlapping chromatographic peaks and matrix effects at detection, particularly ion suppression in mass spectrometry (MS) [32].
Due to the broad applicability of MIPs, comprehensive reviews on principles of MIPs synthesis, formats, and their applications have been published [33], [34], [35]. Chen et al. [36] provided a focused review of the components of MIPs, and novel technologies to prepare MIPs for selective analyte recognition in sample preparation, chromatography, and sensors. SPE, which is a routine and global preconcentration method generally performed using non-selective extraction phases in packed cartridges, was reviewed specifically by Caro et al. [37]. Application of MIP-SPE allows for selective adsorption of target analytes during sample loading and removal of matrix components. Following the successful application for SPE packing, MIPs have been developed for use as the extraction phase in other formats. These techniques are promising for miniaturization, ease of use, direct sampling, and automation. For example, Hu et al. [38] published a review of synthesis of MIPs on magnetic beads, in membranes, and in the forms of SPME and SBSE. Sarafraz-Yazdi and Razavi [39] presented a review of the preparation of MIPs applied in SPME including coated and monolithic fibers, in-tube SPME (coatings and packings), membranes, and sol–gel MIPs. Nanoparticles, which offer a large surface area with the possibility for functionalization, can be used as the supports to fabricate MIP sorbents. Thus MIPs-coated nanoparticles have also been discussed in several reviews [40], [41], [42] Ansari's review [42] was particularly useful, with configurations and preparation methods for magnetic molecularly imprinted polymers (MMIPs) discussed in detail. The magnetic properties of MMIPs provide an advantage for dispersive solid phase extraction (DSPE), allowing for fast and efficient collection of sorbent particles [43]. Other nanoparticles such as silica, quantum dots, carbon dots, and gold or silver nanoparticles have been used in core-shell MIPs [44]. Although it has been demonstrated that MIPs introduce selective recognition of analytes in a variety of formats, it is also essential to assess their performance in the context of real sample matrices.
Many authors have reported using MIPs for analysis of environmental, food, and biological samples. Ansari and Karimi [45] detail the synthesis of MIPs in SPE, SPME, and ultrasonic-assisted SPE, sensors, and magnetic separations for a suite of applications for the analysis of drugs in biological and environmental samples. Speltini et al. [46] published an updated review of MIPs applications (2014–2017) in which different formats of MIPs, including offline and online SPE, SPME, SBSE, and DSPE were used for the analysis of contaminants in food and environmental samples. Murray and Örmeci [47] and Huang et al. [48] provided applications of MIPs in water and wastewater treatment. MIPs can be implemented for selective extraction of organic contaminants from environmental samples [49]. The examples of such applications include polycyclic aromatic hydrocarbons (PAHs) [50], endocrine disrupting chemicals (EDCs) [51], pharmaceuticals [52] and pesticides [53]. There is a wealth of research on such applications, and further details will be presented in this review. Though there are many good reviews of MIP technology in which the authors discuss the applications and novel developments of the materials, MIP technology in water analysis has not been critically assessed.
This review aims to provide a comprehensive evaluation of the selectivity and efficiency of MIPs used in sample preparation step for analysis of organic pollutants in water samples. To achieve this aim, the following topics will be reviewed: synthetic strategies, characterization methodologies, and MIP formula and sample preparation parameter optimization for water analysis, especially environmental waters. The versatility of MIPs has led to the development of several techniques for selective extraction of organic contaminants from aqueous samples, such as MIP-SPE, MIP-DSPE, MIP-SPME, MIP-SBSE, and membrane protected MIPs, which will be discussed and evaluated. The factors that limit the applicability of MIPs for selective extraction and isolation of pollutants from aqueous matrices will be identified. These shortcomings are incompatibility of MIPs with aqueous matrices, poor imprinting effects for water-soluble compounds, heterogeneous and non-specific binding sites, and adsorption of interferences. Novel strategies can be adopted in the synthesis of MIPs to overcome these limitations. Finally, some novel and highlighted applications of MIPs that allow for reducing sample manipulation, and automation of analysis such as direct analysis will be explained.
Section snippets
Covalent imprinting
MIPs based on the formation of covalent bonds between the template and monomer in pre-polymerization solutions demonstrate the most homogenous structures with well-defined binding sites and cavities because of the fixed stoichiometric ratio of functional monomer to template molecules arising from the specificity of the bond formation [39]. The main drawback of covalent-based MIPs is the need for an appropriate monomer-template complex that could form easily reversible covalent bonds with
Adsorption studies
To evaluate the performance of MIPs, adsorption properties such as equilibrium adsorption capacity of the synthesized MIP particles should be considered. To obtain the equilibrium adsorption capacity, MIPs are exposed to the analyte in aqueous matrices [63] or in organic solvents such as acetonitrile (ACN) [64] methanol (MeOH) [65], dichloromethane (DCM) [66] or water mixed with an organic solvent [67] for an experimentally determine interval, which is long enough to ensure equilibration. The
Optimization of MIP formulae
There are several factors in preparation of MIPs which can be optimized to improve the selectivity, including the type of template, monomer(s), and crosslinker(s) as well as their relative ratios. The type of porogenic solvent and its volume is also important. For this purpose, MIPs with different formulae can be prepared and used for binding experiments to determine the optimum composition of the MIP formula leading to the highest selectivity [81].
The selection of template is a crucial step
MIP-SPE
SPE is a routine sample preparation method for water analysis and is used to perform clean-up, preconcentration, class fraction and extraction of analytes. In this technique, materials such as C18, HLB, and ion-exchange stationary phases have been applied to extract compounds with a wide range of physicochemical properties (i.e., solubility, pKa, logP, and functional groups) [88]. To enhance selectivity in SPE and analytical reproducibility and sensitivity, MIPs can be deployed as sorbent [52].
MIP-DSPE
MIP-SPE cartridges have been widely used for selective extraction of analytes from environmental water samples. However, the packing of MIPs in SPE cartridges has potential drawbacks, such as clogged columns when using complex matrices and swelling of the MIPs in organic solvents, both of which can increase the back pressure and consumption of organic solvents [99]. DSPE overcomes these issues by allowing for dispersion of MIP particles into the sample. There has been lots of applications of
MIP-SPME
SPE, a validated alternative to LLE, is commonly used by the US-EPA, and valued for using less organic solvents than LLE. The application of this method has been restricted by the main drawback, analyte breakthrough when large volumes of samples are analyzed. The other drawback is the loss of analyte during the filtration process when such a process is required for real samples, especially hydrophobic ones [153]. Arthur and Pawliszyn [154] introduced SPME as a miniaturized extraction technique,
MIP-SBSE
SPME is a common microextraction technique for the solvent-less measurement of pollutants in environmental samples based on simplicity and portability of this device [9]; however, there are several disadvantages associated with SPME technique such as low extraction efficiency due to the low amount of sorbent coated on SPME fibers, fragility and lack of robustness and reproducibility of SPME fibers [170,171]. SBSE demonstrated in Fig. 4d is another solvent-less microextraction technique and
Membrane-related MIPs
LPME is an alternative to traditional extraction methods and is performed using a very small amount (µL) of extraction solvent instead of large amounts of solvents associated with most LLE methods. In hollow fiber-liquid phase microextraction (HF-LPME) the analytes are extracted based on distribution between the aqueous solution and the extraction solvent (donor and acceptor phase, respectively), which are separated by a polypropylene hollow fiber. HF-LPME provides several benefits such as
Optimization of MIP-based microextraction techniques
There are many factors in MIP-based microextraction techniques that need to be optimized for maximum sensitivity of the analysis. The key parameters influencing sample preparation are sample agitation, extraction time, extraction temperature, pH, ionic strength, and volume of sample solution as well as desorption conditions.
After the preparation of MIPs (i.e., MIP-fiber or MIP-stir bar), a solvent treatment removes template molecules and results in selective binding sites [168]. Besides, a
Extraction of water-soluble compounds
Selective rebinding of water-soluble compounds from environmental water samples is problematic. This issue raised from the weak interactions during template-monomer complexation due to the disturbance of the hydrogen bonding by polar solvents. To solve this problem, different strategies can be adopted. One solution is polymerization of hydrophilic monomers in a water/MeOH porogenic system to increase the interaction with the water-soluble templates. Polymerization of 1-(α-methyl
Prospects for direct and online measurements using MIPs in environmental analysis
MIPs have shown great potential for the analysis of organic contaminants during the last decade. These sorptive materials are not only selective but also deployable for online and direct analysis. Using an online extraction procedure allows for simultaneous preconcentration and determination of analytes [235]. Watabe et al. [236] used online MIP-SPE coupled with LC for the determination of 17β-E2 in river water. By loading the water sample onto a MIP column, which was the pretreatment column,
Conclusion
Arising from the rapidly growing interest in selective sorbents for sample preparation, development of MIPs used for different sample preparation techniques, especially MIP-SPE will remain one of the main themes in the sample preparation field. MIPs for online SPE and µ-SPE is an attractive application which can improve water analysis using automated extraction and quantitation. The high efficiency associated with the combination of nanoparticles with MIPs especially MMIPs, encourages
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by Atlantic Innovation Fund (AIF) from Atlantic Canada Opportunities Agency (ACOA); Newfoundland and Labrador Department of Tourism, Culture, Industry and Innovation; the Natural Sciences and Engineering Research Council of Canada (NSERC); the Department of Chemistry; School of Graduate Studies (SGS); and the Memorial University of Newfoundland (MUN).
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