Molecularly imprinted polymers as a tool for separation in CEC

Molecularly imprinted polymers (MIPs) are synthesized in the presence of a template which results in the formation of specific recognition cavities complementary to the template in shape and chemical functionality. One of the most successful application areas of MIPs is chromatographic sorbents, which are tailor-made synthetic polymers for a given analyte. However, low efficiency of MIP columns is often observed because of slow kinetics of the template. CEC-based MIPs are thought to improve efficiency of MIP-based separation due to the enhanced flow dynamics of CEC. Another attractive feature is the miniaturized format of CEC, so that fewer templates or monomers for the molecular imprinting are consumed, a characteristic desired for 'green chemistry'. The small dimensions of a capillary demand the development of novel polymer formats that can be applied to a miniaturized system. This review discusses the various formats, i.e., the micro- or nanoparticle, the coating and the monolith, for application in CEC as well as the use in MIP syntheses and characteristics.     

Highly selective separations by capillary electrochromatography: molecular imprint polymer sorbents

Molecular imprint polymers (MIPs) are synthesized in the presence of a template, or 'imprint' molecule which results in the formation of specific recognition cavities complementary to the template in shape and chemical functionality. The resultant MIP then acts as a selective binding medium for the template molecule. The utility of MIPs lies in the selectivity of the rebinding process, which is based on molecular recognition. In many cases, the selectivity achieved with MIPs toward a particular molecule is comparable to that observed with antibodies. This has led to the application of MIPs to several areas of analytical chemistry including immunoassays, sensors and separations media. One of the most successful application areas of MIPs has been as chromatographic sorbents, where they have been utilized predominately in chiral separations. The use of MIP sorbents in CEC is attractive in that it combines the selectivity of a molecular recognition process with the enhanced flow dynamics of CEC, which can result in higher efficiency and shorter analysis times. This paper will review the use of molecular imprinted stationary phases in CEC. Following a brief introduction to molecular imprinting, various methodologies for preparation of MIP-CEC capillaries in addition to applications of the technique will be discussed.     

Molecularly imprinted polymers in capillary electrochromatography: recent developments and future trends

The developments in molecularly imprinted polymer (MIP)-based capillary electrochromatography (CEC) achieved during the past years are reviewed in this article. The MIP is prepared using a templated polymerization reaction and results in a material with a high selectivity towards a predetermined target. The selectivity of the MIP is comparable to that of the biological antibodies, however, the MIP is much more stable and is thus able to withstand extremely harsh conditions in terms of pH, temperature, and organic solvents. The high selectivity and stability of the MIP made it an interesting candidate for application as stationary phase sorbent in chromatography. However, due to slow kinetics the efficiency of the early MIP columns, which were predominantly applied in high-performance liquid chromatography (HPLC), were limited. The use of CEC was thought to improve the efficiency of the MIP-based separation system. The small dimensions of the capillary format employed in CEC have put demands on the polymer systems which have resulted in the development of many different polymer formats. Thus, this need for novel MIP formats for applications in CEC has contributed a lot to the general development of MIP formats as well as to the knowledge in MIP synthesis and characteristics.     

Porogen formulations for obtaining molecularly imprinted polymers with optimized binding properties

This paper examines the formulation of new porogenic mixtures used to prepare molecularly imprinted polymers (MIPs) in both thin film and bulk monolith formats. Films were cast by using spin coating to spread a pre-polymerization mixture onto a substrate, and rapid curing of the films was achieved with UV photolysis. The use of a low volatility solvent in combination with a linear polymer porogen resulted in a porous morphology and a 60-fold enhancement in the binding capacity, relative to a non-porous film prepared with a highly volatile solvent and in the absence of the polymer porogen. The opposite effect was seen in MIPs that were prepared in the traditional bulk monolith format, for which the binding efficiency of the MIP decreased monotonically with the concentration of the linear polymer porogen. Furthermore, bulk MIPs that were prepared in the presence of linear polymer porogens exhibited significantly decreased specific surface areas (from 620 to 8 m²/g for samples prepared with pure solvent and 50% polymer porogen, respectively). Despite the change in binding capacity and morphology, the selectivity of the bulk MIPs remained unaffected by the presence of the polymer porogens (approximately 50% chiral selectivity for all bulk MIPs considered). This difference in behavior of the two systems was attributed to the large difference in the kinetics of polymerization.     

An insight into molecularly imprinted polymers for capillary electrochromatography

Molecularly imprinted polymers (MIPs) are actively being developed as a practical tool for affinity chromatographic supports. From the viewpoint of separation science, capillary electrochromatography (CEC) might be one of the more promising chromatographic techniques to be used in combination with the MIPs. However, up to the present, very little MIP work has involved CEC. This review gives a full overview of MIP including current trends in MIP, methods for the characterization of MIP, and methods for the preparation of MIP with particular emphasis on application of the resulting materials in CEC. To prepare MIPs with selectivity predetermined for a particular substance or group of structural analogues is an important factor for the development of a new format of CEC. From the fundamental research with the batch method, a better knowledge of imprint formation and imprint recognition will be helpful for expanding the application area of the combination of MIPs with CEC.     

Synthesis and Evaluation of Molecularly Imprinted Polymers as Sorbents for Selective Extraction of Coumarins

Coumarin, 7-hydroxycoumarin and dicoumarol molecularly imprinted polymers (MIP) were synthesized by bulk polymerization. Methacrylic acid and 4-vinylpyridine were tested as functional monomers and methanol, ethanol, acetonitrile, toluene and chloroform were tested as porogens. The binding capabilities of the imprinted polymers were assessed by equilibrium binding analysis. Highest binding capacity was obtained for MIP prepared for the template 7-hydroxycoumarin synthesized in methacrylic acid as functional monomer, chloroform as porogen and methanol/water as analyte solvent. Scanning electron microscopy analysis documented its appropriate morphology. ATR-FTIR spectra confirmed successful polymerization of MIP. Coumarin structural analogues were employed to evaluate the polymer selectivity and it was found that polymer prepared for 7-hydroxycoumarin was selective for its template molecule. Kinetic studies showed relatively fast adsorption of analytes to MIPs (1 h). Rebinding properties of MIPs were evaluated by adsorption isotherms. The calculated data fitted well with experimental data showing that Freundlich isotherm is suitable for modelling the adsorption of tested coumarins on prepared MIPs. Applicability of polymer prepared for 7-hydroxycoumarin was tested for the selective extraction of coumarins from the sample of chicory.     

Water-compatible molecularly imprinted polymers for efficient direct injection on-line solid-phase extraction of ropivacaine and bupivacaine from human plasma

Two novel molecularly imprinted polymers (MIPs) selected from a combinatorial library of bupivacaine imprinted polymers were used for selective on-line solid-phase extraction of bupivacaine and ropivacaine from human plasma. The MIPs were prepared using methacrylic acid as the functional monomer, ethylene glycol dimethacrylate as the cross-linking monomer and in addition hydroxyethylmethacrylate to render the polymer surface hydrophilic. The novel MIPs showed high selectivity for the analytes and required fewer and lower concentrations of additives to suppress non-specific adsorption compared with a conventional MIP. This enabled the development of an on-line system for direct extraction of buffered plasma. Selective extraction was achieved without the use of time-consuming solvent switch steps, and transfer of the analytes from the MIP column to the analytical column was carried out under aqueous conditions fully compatible with reversed-phase LC gradient separation of analyte and internal standard. The MIPs showed excellent aqueous compatibility and yielded extractions with acceptable recovery and high selectivity.     

Dual‐templates molecularly imprinted monolithic columns for the evaluation of serotonin and histamine in CEC

To extend the application of molecularly imprinted polymers, the dual‐templates molecularly imprinted monolithic columns were developed in a capillary format. Two templates serotonin and histamine were simultaneously imprinted using two different functional monomers such as methacrylic acid (MAA) and methylenesuccinic acid (MSA) in a mixture of ethylene glycol dimethacrylate (EDMA) as a cross‐linker and AIBN as polymerization initiator dissolved in DMF as porogen. The resulting molecular imprinted polymers (MIPs) were characterized based on their performance in the CEC separation of two imprinted templates. The optimization parameters such as pH, ACN composition, and concentration of the eluent were varied to achieve best resolution and efficiency for CEC separation of templates with each MIP column. It was found that the MIP monolith column fabricated using MSA offered better resolution and separation efficiency compared to column fabricated with MAA. This work utilized the dual‐templates imprinting approach successfully and broadens the scope of multi‐templates imprinting capabilities in capillary format in CEC application.     

分子印迹聚合物颗粒在毛细管电色谱中的应用

依据分子印迹技术(MIT)制备的分子印迹聚合物(MIP)颗粒对模板分子及其结构类似物具有特异性识别和选择性吸附作用,同时具有较大的比表面积和快速的传质动力学特性,因而被广泛用作液相色谱固定相和固相萃取材料。将MIP颗粒作为固定相应用于毛细管电色谱(CEC),结合了CEC的快速、高效和MIP的高亲和性、高选择性的特点,成为分析科学领域最具有发展前景的分离技术之一。MIP颗粒在CEC领域有几种不同的应用形式: 作为填充材料填充到毛细管柱中;作为嵌入材料嵌入到毛细管柱内部不同基质的骨架中;作为准固定相添加到CEC运行缓冲溶液中。本文综述了近几年MIP颗粒在CEC领域应用的发展,对该领域今后的发展前景进行了展望。     

Size-exclusion chromatography in 1,1,1,3,3,3-hexafluoro-2-propanol

Two molecularly imprinted polymers (MIPs) have been synthesised for the selective extraction of 4-nitrophenol (4-NP) from water samples. One polymer was synthesised via a non-covalent approach and the other via a semi-covalent approach. The selectivity of the polymers for 4-NP was evaluated when these polymers were applied in on-line solid-phase extraction (MISPE) coupled to reversed-phase HPLC. The MISPE conditions for both MIPs were optimised and a clean-up step was included to eliminate non-specific interactions. Differences between the two MIPs were observed with the non-covalent MIP being the more selective of the two, whereas the recoveries were slightly higher for the semi-covalent MIP. The performance of the imprinted polymers in the MISPE of real water samples was also evaluated.     

Molecular imprinting of natural flavonoid antioxidants: application in solid-phase extraction for the sample pretreatment of natural products prior to HPLC analysis

As shown in the past years, SPE based on molecularly imprinted polymers (MIPs) may provide significant enhancement of selectivity in sample preparation and analyte preconcentration. The objective of this work was the fabrication of MIPs for the specific adsorption of rutin and quercetin. The two flavonoids were used as the template molecules for the preparation of MIP phases in a self-assembly (noncovalent) approach. The produced MIPs were validated with regard to the imprinting efficiency as media for LC and SPE. The retention behavior of several flavonoid compounds was studied using as stationary phases imprinted, control nonimprinted polymers, and commercial silica-based materials. MIPs were applied as materials for the selective SPE and preconcentration of the flavonoids from white and red wine, orange juice, and tea. The collected fractions were analyzed by high-pressure LC. MIP-SPE facilitated specific analyte isolation and effective sample clean-up. The results show that molecularly imprinted SPE can be a useful tool for the simple, selective, and cost-effective pretreatment of samples containing natural antioxidants.     

Selective extraction of organophosphorus nerve agent degradation products by molecularly imprinted solid-phase extraction

The analysis of alkyl alkylphosphonic acids, the degradation products of V and G nerve agents as VX, Sarin or Soman, is an important task for the verification of compliance to the Chemical Weapons Convention. The detection of these contaminants at low concentration levels is often difficult in complex matrices due to the amount of interfering substances. Molecularly imprinted solid-phase extraction technique should allow a selective extraction of these compounds from complex samples, and thus make their detection easier. Two molecularly imprinted polymers (MIPs) prepared with methacrylic acid (MAA) as monomer and pinacolyl methylphosphonic acid (PMPA) as template molecule were synthesised and tested. The first polymer, MIP A, was prepared with ethylene glycol dimethacrylate (EGDMA) in dichloromethane. The second polymer, MIP B, was synthesised using trimethylolpropane trimethacrylate (TRIM) in acetonitrile. To evaluate the selectivity provided by these MIPs, the retention of the ethyl methylphosphonic acid (EMPA) target molecule was studied in parallel on a non-imprinted polymer (NIP). While MIP A does not show any difference compared to NIP A, a good selectivity was obtained for MIP B. After the optimisation of the extraction process, 60% of EMPA can be removed from the NIP B without affecting the retention on the MIP B. A recovery of extraction of 93% was then obtained on the MIP B. Its capacity was then measured and corresponds to 97 microg of EMPA per gram of MIP. Finally, the selectivity of MIP B was clearly demonstrated by applying it to the clean-up of a soil extract spiked with EMPA.     

CEC separation of ofloxacin enantiomers using imprinted microparticles prepared in molecular crowding conditions

Molecular crowding is a new concept to obtain molecularly imprinted polymers (MIPs) with greater capacity and selectivity, which could shift the equilibrium of a print molecule reacting with functional monomers in the direction of complex formation side. In this work, molecular crowding agent was first applied to the preparation of MIPs microparticles by precipitation polymerization. A new system of molecular crowding surrounding was developed, composed of polystyrene and tetrahydrofuran, in the presence of the template (S)-ofloxacin. Partial filling capillary electrochromatography (CEC) was utilized to evaluate imprinting effect of the resulting microparticles by chiral separations of ofloxacin. Some important parameters in the preparation, i.e. template to monomer ratio, influence of cross-linking monomers and functional monomer composition on the CEC separation of MIP microparticles were investigated. Baseline separation of ofloxacin (R(s) =1.53) was obtained under optimized conditions and the highest theory plate of the later eluent (S)-ofloxacin was 5400. The textural and morphological parameters for imprinted particles, such as Brunauer-Emmett-Teller surface areas, pore volumes and pore size distributions have also been determined. Compared to the MIP microparticle prepared by conventional precipitation polymerization, the (S)-ofloxacin-imprinted particles formed under molecular crowding conditions showed higher selectivity (α=1.09) and separation efficiency (<25 min) in the CEC mode.     

Developing imprinted polymer nanoparticles for the selective separation of antidiabetic drugs

In this study, new molecularly imprinted polymer (MIP) nanoparticles are designed for selective recognition of different drugs used for the treatment of type 2 diabetes mellitus, i.e. sitagliptin (SG) and metformin (MF). The SG‐ and MF‐imprinted polymer nanoparticles are synthesized by free‐radical initiated polymerization of the functional monomers: methacrylic acid and methyl methacrylate; and the crosslinker: ethylene glycol dimethacrylate. The surface morphology of resultant MIP nanoparticles is studied by atomic force microscopy. Fourier transform infrared spectra of MIP nanoparticles suggest the presence of reversible, non‐covalent interactions between the template and the polymer. The effect of pH on the rebinding of antidiabetic drugs with SG‐ and MF‐imprinted polymers is investigated to determine the optimal experimental conditions. The molecular recognition characteristics of SG‐ and MF‐imprinted polymers for the respective drug targets are determined at low concentrations of SG (50–150 ppm) and MF (5–100 ppm). In both cases, the MIP nanoparticles exhibit higher binding response compared to non‐imprinted polymers. Furthermore, the MIPs demonstrate high selectivity with four fold higher responses toward imprinted drugs targets, respectively. Recycled MIP nanoparticles retain 90% of their drug‐binding efficiency, which makes them suitable for successive analyses with significantly preserved recognition features.     

An unexpected selectivity of a propranolol-derived molecular imprint for tamoxifen.

During the evaluation of molecular imprinted polymers (MIPs) prepared against the drug tamoxifen a propranolol-derived MIP was used as a positive control. Surprisingly the propranolol-derived MIP showed considerable selectivity towards tamoxifen, and was indeed much more selective than the MIP prepared using tamoxifen as the imprint molecule. The consequences of this unexpected, cross reactivity for the use of MIPs in analytical chemistry is discussed.     

Preparation and characterization of molecularly imprinted monolithic column based on 4-hydroxybenzoic acid for the molecular recognition in capillary electrochromatography

A novel prepared method of molecularly imprinted monolithic polymers (MIPs) using 4-hydroxybenzoic acid (4-HBA) as templates for capillary electrochromatography (CEC) was developed. A strategy of high concentration of monomers in the pre-polymerization mixture was used to fulfil the solubility of polar imprinted molecule and reduction of the interference during complex formation. The imprinted polymer capillary monolithic column was synthesized by an in situ therm-initiated copolymerization of methacrylic acid (MAA) and ethylene glycol dimethacrylate with a mixture of toluene-isooctane as a porogenic solvent in the presence of a polar model imprinting molecule, 4-HBA. On the resultant MIP monolithic column, the effect of parameter of CEC on electroosmotic flow (EOF) and the retention of 4-HBA was investigated. The column efficiency of the imprinted molecule, 4-HBA, was 13,000 plates/m. The resolution of isomers of HBA was 5.0 and good molecular recognition was achieved for 4-HBA.     

分子印迹聚合物在毛细管电色谱拆分手性药物中的研究进展

手性药物通过与生物体内生物大分子之间的手性匹配与分子识别来发挥药理作用。两个对映体与体内手性环境相互作用的不同导致每个对映体表现出不同的药理活性、代谢过程、代谢速率及毒性等药代动力学特征。因此发展手性药物的拆分方法,对于手性药物的开发和生产过程的质量监控具有重要意义。分子印迹聚合物（MIPs）是以目标分子作为模板而制备的高分子聚合物,它具有特定的空间分子结构和官能团,对目标分子具有高度的特异性识别能力。基于该特点,MIPs非常适合于手性药物的拆分和纯化。毛细管电色谱（CEC）可同时基于毛细管电泳和液相色谱的分离机理对目标物进行分离,因此具有高分离效率和高选择性的特点。将MIPs材料作为CEC的固定相,可将这两种技术的优势结合,从而实现对手性药物的高效拆分。MIPs材料在1994年首次应用于CEC手性拆分,此后该研究领域开始获得关注和发展。MIPs材料主要通过4种模式在CEC中实现手性拆分,分别是作为开管柱、填充柱和整体柱的固定相以及分离介质中的准固定相。该综述以这4种模式作为分类基准,根据MIPs制备所需的材料和分离对象对其在CEC手性拆分中的应用进行了总结,揭示了MIPs在CEC手性拆分中的潜力,同时评述了这4种模式各自的优势与不足,并对将来MIPs在CEC手性拆分中的发展进行了展望。     

In situ preparation of powder and the sorption behaviors of molecularly imprinted polymers through the complexation between polymer ion of methyl methacrylate/acrylic acid and Ca ion

Molecularly imprinted polymer (MIP) powders were prepared using a simple complexation strategy between the polymer carboxylate groups and template molecule followed by metal cation cross-linking of residual polymer carboxylates. Polymer powders were formed in situ by templating carboxylic acid containing polymers with 4-ethylaniline (4-EA), followed by addition of an aqueous CaCl₂ solution. The solution remained homogeneous. The powders were prepared by precipitation by slowly adding a non-solvent, H₂O, to the mixture. The resulting particles were very porous with uptake capacity that approached the theoretical value. We suggest two types of complexes are formed between the template, 4-EA, and polymer. The isolated entry type forms well defined cavities for the template with high specific selectivity, while the adjacent entry type forms wider binding sites without specific sorption for isomeric molecules. To evaluate conditions for forming materials with high affinity and selectivity, three MIPs were prepared containing 0.5, 1.0, and 1.5 equivalents of template to the base polymer. The MIP containing 0.5 eq showed higher specific selectivity to 4-EA, but the MIP containing 1.5 eq had noticeably lower selectivity. The lower selectivity is attributed to poorly formed binding sites with little selective sorption to any isomer when the higher ratio of template was used. However at the lower ratio of template the isolated entry is preferably formed to produce well defined binding cavities with higher selectivity to template.     

Recent advancement of carbon nanomaterials engrained molecular imprinted polymer for environmental matrix

Sample preparation techniques have always been considered as a complex issue in the analytical process. Most of the sample preparation techniques show a lack of selectivity. Molecularly imprinted polymer (MIP) is a synthetic approach for sample preparation technique that has the ability of selective extractions. Generally, MIPs are selective sorbent, MIPs are capable of binding a molecule or its geometrical analogues. The imprinted polymers own particular voids exclusively framed for the aimed target analytes. These MIPs have been synthesized through a complex route of polymerization using a dedicated crosslinker, a template and function bound specific monomers (mainly interacting with the template). Despite having various pros like selectivity, morphological predictability, chemical & thermal stability, points alike binding site heterogeneity, partial template removal, and limited application pose a challenge. In this regard, a relatively newer carbon-based MIP method is explored as the molecular imprinting technique in various environmental samples. This paper describes the current scenario in the field of molecular-based imprinting technology using different carbon engrained materials and highlights the latest applications in this field and suggest proposals for the prospect in the area of the MIP.     

Via protoporphyrin to the synthesis of levofloxacin‐imprinted polymer

A new molecularly imprinted polymer (MIP) for levofloxacin was prepared by the combined use of methacrylic acid and protoporphyrin as functional monomers. The adsorption properties of resultant imprinted polymers were evaluated by equilibrium rebinding experiments. The highest binding capacity of levofloxacin achieved from the optimized imprinted polymer in acetonitrile was 246.26 µmol/g with an imprinting factor of 2.05. A ?uorescence quenching effect was observed when a protoporphyrin‐based imprinted polymer was incubated in the solutions of levofloxacin. The results indicated that the protoporphyrin‐based MIPs were able to create higher binding cavities for template compared with MIPs using only methacrylic acid as a functional monomer. It should be expected that the cooperative use of the protoporphyrin with supplemental different functional monomers may be an alternative to obtain MIP with the improvement of the selectivity. Copyright © 2010 John Wiley & Sons, Ltd.