Influence of template basicity and hydrophobicity on the molecular recognition properties of molecularly imprinted polymers

Oxoanion Imprinting Combining Cationic and Urea Binding Groups: A Potent Glyphosate Adsorber

The use of polymerizable hosts in anion imprinting has led to powerful receptors with high oxyanion affinity and specificity in both aqueous and non-aqueous environments. As demonstrated in previous reports, a carefully tuned combination of orthogonally interacting binding groups, for example, positively charged and neutral hydrogen bonding monomers, allows receptors to be constructed for use in either organic or aqueous environments, in spite of the polymer being prepared in non-competitive solvent systems. We here report on a detailed experimental design of phenylphosphonic and benzoic acid-imprinted polymer libraries prepared using either urea- or thiourea-based host monomers in the presence or absence of cationic comonomers for charge-assisted anion recognition. A comparison of hydrophobic and hydrophilic crosslinking monomers allowed optimum conditions to be identified for oxyanion binding in non-aqueous, fully aqueous, or high-salt media. This showed that recognition improved with the water content for thiourea-based molecularly imprinted polymers (MIPs) based on hydrophobic EGDMA with an opposite behavior shown by the polymers prepared using the more hydrophilic crosslinker PETA. While the affinity of thiourea-based MIPs increased with the water content, the opposite was observed for the oxourea counterparts. Binding to the latter could however be enhanced by raising the pH or by the introduction of cationic amine- or Na⁺-complexing crown ether-based comonomers. Use of high-salt media as expected suppressed the amine-based charge assistance, whereas it enhanced the effect of the crown ether function. Use of the optimized receptors for removing the ubiquitous pesticide glyphosate from urine finally demonstrated their practical utility.

A Tapered Capillary-Based Contactless Atmospheric Pressure Ionization Mass Spectrometry for On-Line Preconcentration and Separation of Small Organics

Capillary electrophoresis (CE) is an effective technique for the separation of different analytes. Moreover, online preconcentration of trace analytes in the capillary for CE analysis has been demonstrated. CE and capillary electrochromatography (CEC) are suitable for the separation of analytes with similar polarities. Given that CE and CEC are only used to separate small-volume samples, sensitive mass spectrometry (MS) is a suitable detection tool for CE and CEC. Contactless atmospheric pressure ionization (C-API) is a continuous flow ion source that only uses a short capillary as the ionization emitter operated at atmospheric pressure for MS analysis. In this study, we demonstrated the feasibility of hyphenating CE/CEC with C-API-MS by using a short and tapered capillary as the interface. The short capillary (a few centimeters) can function as the separation/preconcentration tube and the ionization emitter. This hyphenated technique can be used to analyze small organics within a few minutes. The suitability of using the hyphenated technique for online preconcentration, separation, and quantitative analysis for small organics is demonstrated in this study.

Development of a selective sorbent for the solid-phase extraction of terbuthylazine in olive oil samples: a molecular imprinting strategy

Aiming to implement an analytical methodology that is highly selective for the extraction and quantification of terbuthylazine from olive oil, we successfully achieved: (i) the development of a molecularly imprinted polymer by bulk polymerization using terbuthylazine as template molecule, methacrylic acid as functional monomer, ethylene glycol dimethacrylate as cross-linker, and dichloromethane as porogen; (ii) characterization of the imprinting material using Fourier transform infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption at 77 K, and scanning electron microscopy; (iii) their molecular recognition for the template molecule using high-performance liquid chromatography, and (iv) optimization of a solid-phase extraction procedure using as sorbent the synthesized molecularly imprinted polymer for the selective extraction and clean-up of terbuthylazine from spiked organic olive oil and further quantification of the pesticide levels by high-performance liquid chromatography. The suitability of the implemented analytical methodology was demonstrated, as concentrations of terbuthylazine below the tolerated maximum residue limits in the spiked organic olive oil samples could be satisfactorily analyzed with good precision/accuracy with high recovery rates (96%). Overall, the implemented methodology has proven to be reliable and robust and is highly promising in the field of sample preparation, particularly for the isolation/preconcentration of terbuthylazine in complex food samples.

Preparation and evaluation of molecularly imprinted polymer liquid chromatography column for the separation of Cathine enantiomers

In this study molecular imprinting technology was employed to prepare a specific affinity sorbent for the resolution of Cathine, a chiral drug product. The molecularly imprinted polymer (MIP) was prepared by non-covalent molecular imprinting with either (+) or (-)-Cathine (threo-2-amino-1-hydroxy-1-phenyl propane; norpseudoephedrine) as the template. Methacrylic acid and ethylene glycol di-methacrylate were copolymerized in the presence of the template molecule. The bulk polymerization was carried out in chloroform with 2,2'-azobisisobutyronitrile as the initiator, at 5 °C and under UV radiation. The resulting MIP was ground into powders, which were slurry packed into analytical columns. After removal of template molecules, the MIP-packed columns were found to be effective for the resolution of (±)-Cathine racemates. The separation factor for the enantiomers ranged between 1.5 and 2.4 when the column was packed with MIP prepared with (+)-Cathine as the template. A separation factor ranging from 1.6 to 2.9 could be achieved from the column packed with MIP, prepared with (-)-Cathine as the template. Although the separation factor was higher with that previously obtained from reversed-phase column chromatography following derivatization with a chiral agent, elution peaks were broader due to the heterogeneity of binding sites on MIP particles and the possible non-specific interaction.

Studies on the Process of Formation, Nature and Stability of Binding Sites in Molecularly Imprinted Polymers

In Molecular Imprinting the nature of the templated binding sites and the mechanism of their formation are still poorly understood. For this reason our groups are carrying out fundamental studies concerning known imprinting protocols, with the primary aim of shedding light on the role of the template in the different steps of the polymerisation, from the formation of primary chains to the build-up of the porous structure. In this paper we report our initial results concerning copolymers of methacrylic acid (MAA) and ethyleneglycol dimethacrylate (EDMA) and their formation in presence or absence of the templates 9-ethyladenine, ametryn or terbutylazine. Monitoring the monomer disappearance by 1H-NMR showed that the presence of templates such as 9-ethyladenine significantly retarded the polymerisation but otherwise had minor influence on the relative reactivity of the monomers. The latter appeared in most cases to be stoichiometrically incorporated into the polymer. The signals arising from the template experienced little or no shift in the early stage of the polymerisation, although pronounced broadening was observed. By delaying the addition of the template, it was observed that binding sites with high selectivity could be induced more than one hour after the gel point of the system had been passed. Finally, the results of post-polymerisation curing on the dry and swollen state porosities and the recognition properties of terbutylazine imprinted polymers are reported. This treatment when performed at temperatures between 100-120°C, slightly enhanced the selectivity of the polymers, whereas at higher temperatures the polymers lost their molecular recognition properties. Swollen state porosity derived from inverse size exclusion chromatography (ISEC) revealed an interesting sharpening of the pore size distribution for the imprinted compared to the non-imprinted polymers.

Dielectric constants are not enough: principal component analysis of the influence of solvent properties on molecularly imprinted polymer-ligand rebinding

The influence of the physical properties of incubation medium on the rebinding of template to bupivacaine molecularly imprinted and non-imprinted methacrylic acid-ethylene dimethacrylate co-polymers has been studied. Principal component analysis (PCA) was employed to identify the factors with the greatest influence on binding. While the dielectric constant (D) made a significant contribution to describing the observed binding, the influence of polarity as reflected in the Snyder polarity index (SPI) was also demonstrated to make a significant contribution. The use of solvents containing hydroxyl functionality in particular was observed to exert unique effects on recognition. The variation in solvent influence on binding at constant D motivates more complex analyses when studying MIP-ligand recognition.

Development and evaluation of spherical molecularly imprinted polymer beads

The majority of studies on molecularly imprinted polymers has until now been carried out on irregularly shaped particles prepared by grinding of polymer monoliths. The preparation procedures are time- and labor-consuming and produce particles of wide size distributions. To answer the need for fast and straightforward routes to spherical molecularly imprinted polymer beads, we have developed a method comprising the formation of droplets of pre-polymerization solution directly in mineral oil by vigorous mixing followed by transformation of the droplets into solid spherical beads by photoinduced free-radical polymerization. No detergents or stabilizers were required for the droplet formation. Factors influencing the bead synthesis have been investigated and are detailed here. The beads were evaluated in parallel with corresponding irregularly shaped particles prepared from polymer monoliths. Conditions for the synthesis of propranolol-imprinted poly(methacrylic acid-co-trimethylolpropane trimethacrylate) beads in the size range of 1-100 microm in almost quantitative yield are described. The beads were applied as the recognition element in a 96-well plate format radioligand assay of propranolol in human serum.

Virtual imprinting as a tool to design efficient MIPs for photosynthesis-inhibiting herbicides

Molecular modelling and computational screening were used to identify functional monomers capable of interacting with several different photosynthesis-inhibiting herbicides. The process involved the design of a virtual library of molecular models of functional monomers containing polymerizable residues and residues able to interact with the template through electrostatic, hydrophobic, Van der Waals forces and dipole-dipole interactions. Each of the entries in the virtual library was probed for its possible interactions with molecular models of the template molecules. It was anticipated that the monomers giving the highest binding score would represent good candidates for the preparation of affinity polymers. Strong interactions were computationally determined between acidic functional monomers like methacrylic acid (MAA) or itaconic acid (IA) with triazines, and between vinylimidazole with bentazone and bromoxynil. Nevertheless, weaker interactions were seen with phenylureas. The corresponding blank polymers were prepared using the selected monomers and tested in the solid phase extraction (SPE) of herbicides from chloroform solutions. A good correlation was found between the binding score of the monomers and the affinities of the corresponding polymers. The use of computationally designed blanks can potentially eliminate the need for molecular imprinting, (adding a template to the monomer mixture to create specific binding sites). Data also showed that some monomers have a natural selectivity for some herbicides, which can be further enhanced by imprinting. Thus, in regard to retention on the blank polymer, we can estimate if the resulting imprinted polymer will be effective or not.

Systematic cross-selectivity study of the factors influencing template receptor interactions in molecularly imprinted nitrogen heterocycles

A systematic cross-selectivity study involving a series of structurally related N-methylated and non-methylated substituted pyridines was performed with the aim of evaluating the parameters responsible for template receptor binding in molecularly imprinted polymers. Variation in binding of substrate structure permitted evaluation of the steric restraints of the imprinted cavity. The electrostatic effects, primarily hydrogen-bonding, were investigated through rebinding in chloroform and acetonitrile. All species were non-covalently imprinted in thermoinduced methacrylic acid-ethylene glycol dimethacrylate co-polymers. Evaluation of template properties indicate that a correlation exists between non-specific binding and template basicity for a series of structural isomers. A correlation between non-specific binding and hydrophobicity was also identified for templates increasing in alkyl character. However no overall correlation was observed, as it was speculated that these factors may be competing. All species imprinted, with the exception of 2-dimethylaminopyridine, produced a selective response for the template species. Varying degrees of cross-selectivity were observed for each imprinted polymer. Polymers imprinted with templates of higher basicity demonstrated a greater degree of cross-selectivity relative to those of lower basicity. While overall binding was reduced in acetonitrile relative to chloroform, specificity was increased. This highlights the intrinsic difference in binding sites within imprinted and non-imprinted sites of the polymer. Finally, while the ability of the template species to form a co-operative interaction may be advantageous in producing a selective imprint it is not a prerequisite. For species based on this co-operative interaction the steric environment in the immediate proximity to the binding functionalities are critical to recognition. Steric hindrance of non-functionally active groups can dramatically impair the formation of interactions.

Concentration dependent atrazine–atrazine complex formation promotes selectivity in atrazine imprinted polymers

An atrazine (ATR) molecularly imprinted polymer (MIP) was prepared using a non-covalent strategy. The affinity and selectivity of the polymer was initially evaluated under non-equilibrium conditions and the polymer was shown to possess good template selectivity. The selectivity of the polymer was further investigated under equilibrium conditions and over a range of concentrations using Scatchard plots and Hill plots and by assessing distribution coefficients and normalised selectivity values. It was observed that both selectivity and affinity were dependent on the concentration of the ligand and that unusually selectivity and affinity were better at higher atrazine concentrations. It was concluded that this phenomenon resulted from the formation of atrazine-atrazine complexes during the pre-polymerisation stage and during rebinding and that the polymer demonstrated improved atrazine affinity when the conditions favoured complex formation.

Molecularly imprinted polymers for the determination of a pharmaceutical development compound in plasma using 96-well MISPE technology

The use of molecularly imprinted polymers (MIPs) as sorbents for the solid phase extraction (SPE) of a pharmaceutical compound in development, prior to quantitative analysis was investigated. Three MIPs were synthesised using a structural analogue as the template molecule. Each polymer was prepared with different monomers and porogens. The MIPs were then tested for their performance both in organic and aqueous environments, the final aim being to load plasma directly onto the polymers. At an early development stage, there is a limited amount of compound available. Due to this limitation, reducing the amount of template required for imprinting was investigated. A MIP capable of extracting the analyte directly from plasma was produced. The specificity of the polymer allowed the method to be validated at a lower sensitivity than a more conventional SPE assay. For the first time, MIPs were packed into 96-well blocks enabling high throughput analysis. The analytical method was fully validated for imprecision and inaccuracy down to 4 ng/ml in plasma.

Water-Compatible Molecularly Imprinted Polymers Obtained via High-Throughput Synthesis and Experimental Design

A technique allowing high-throughput synthesis and evaluation of molecularly imprinted polymer sorbents at a reduced scale (mini-MIPs) was developed and used for the optimization of MIPs for use in pure aqueous environments. The technique incorporated a 4-port liquid-handling robot for the rapid dispensing of monomers, templates, solvents and initiator into the reaction vessels of a 96-well plate. A library of 80 polymers, each ca. 50 mg, could thus be prepared in 24 h. The MIP rebinding capacity and selectivity could be rapidly assessed in the batch mode by quantifying nonbound fractions in parallel using a UV monochromator plate reader. This allowed a complete evaluation of the binding characteristics of an 80 polymer library in approximately 1 week. With the objective of optimizing a polymer imprinted with the local anaesthetic Bupivacaine for use in pure aqueous systems, a polymer library was prepared by varying the original poly(MAA-co-EDMA) MIP composition. The variable factors were the added amount of the hydrophilic comonomer, 2-hydroxyethyl methacrylate (HEMA), the cross-linking ratio, and the porogen. This optimization resulted in polymers showing high imprinting factors (IF = K(MIP)/K(NIP)) in water as a result, mainly, of reduced binding to the nonimprinted polymer. Normal scale batches of these materials showed strong retention of the template and low nonspecific binding when assessed as chromatographic stationary phases using pure phosphate buffer, pH 7.4, as mobile phase, by equilibrium batch rebinding experiments and as sorbents for extractions of the analyte from blood plasma samples.

Effect of solvents on the selectivity of terbutylazine imprinted polymer sorbents used in solid-phase extraction

A solid-phase extraction sample preparation method using a molecularly imprinted polymer (MIP) selective for the triazine type pesticide terbutylazine has been developed. The method involves preconcentration from large volumes of water samples on a C18 disk coupled to selective clean-up on the MIP. The method has been optimised by studying the recovery and retention of terbutylazine and some other structurally related triazine derivates as a function of the selective washing solvent used. The effect of the water content of the selective washing solvent was also investigated on the recovery of the MIP. River water samples were analysed with the coupled technique, and efficient clean-up of the samples was observed.

Towards ochratoxin A selective molecularly imprinted polymers for solid-phase extraction

Molecularly imprinted polymers (MIPs) displaying selective binding properties for the mycotoxin ochratoxin A (OTA) in polar/protic media were prepared. Crucial to the success of these efforts was the implementation of rationally designed OTA mimics as templates and a set of novel basic and neutral functional monomers, allowing the maximization of the template-functional monomer association via ion-pairing, hydrophobic and steric interactions. MIPs prepared with a 20:1:1:3 molar ratio of cross-linking agent, template mimic, basic functional monomer and hydrophobic auxiliary monomer produced polymers with superior recognition properties compared to materials generated with other stoichiometries. Chromatographic evaluation using the OTA mimics, OTA and a set of structurally closely related compounds as analytes revealed pronounced substrate selectivity of these MIPs in polar/protic media, the templates and OTA being bound with significantly higher affinities. Complementary substrate selectivities/affinities were observed in aprotic and apolar solvents. The possibility of solvent-dependent tuning of substrate selectivity/affinity and the high binding capacity recommend the developed MIPs as promising solid-phase extraction adsorbents for clean-up and pre-concentration of OTA from various biologically relevant matrices.

Evaluation of a multidimensional solid-phase extraction platform for highly selective on-line cleanup and high-throughput LC-MS analysis of triazines in river water samples using molecularly imprinted polymers

A novel highly selective sample cleanup procedure based on the use of molecularly imprinted polymers (MIPs) as solid-phase extraction materials has been evaluated with respect to its applicability and routine use in environmental analysis. The method comprises the combination of a restricted access material (RAM) and a MIP allowing a selective sample preparation to be achieved in the online mode. This combination is called the size-selective sample separation and solvent switch (six-SPE). The RAM column combines size exclusion and adsorption chromatography, reducing the concentration of matrix molecules by a cutoff of 15 kDa. The MIP column selectively retains the triazine analytes whereas the residual matrix is not retained and separated completely. Thus, the automated RAM-MIP is capable of excluding all matrix and nontarget compounds. The cleaned and enriched extract is subsequently eluted to an HPLC column and analyzed by LC-MS. A complete on-line analysis cycle including multidimensional solid-phase extraction, separation, and detection takes less than 15 min. Terbuthylazine, atrazine, propazine, simazine, ametryn, prometryn, irgarol, and also the metabolites deethylatrazine and deisopropylatrazine can be determined without any matrix interferences, e.g., by humic acids. The whole setup is fully automated and may be continuously operated. Nonspecific interactions with the polymer are below 1% in all cases. The accuracy of the LC-MIP-LC-MS system was controlled using a certified reference material (Aquacheck). The applicability of the method to the cleanup of real samples was demonstrated by injection of contaminated river water samples. The stability of different polymers was tested by consecutive injections, and it was shown that the performance of the materials did not vary even after more than 300 enrichment and desorption cycles.

New synthetic ways for the preparation of high-performance liquid chromatography supports

The latest developments and in particular important synthetic aspects for the preparation of modern HPLC supports are reviewed. In this context, the chemistry of inorganic supports based on silica, zirconia, titania or aluminum oxide as well as of organic supports based on poly(styrene-divinylbenzene), acrylates, methacrylates and other, more specialized polymers is covered. Special consideration is given to modern approaches such as sol-gel technology, molecular imprinting, perfusion chromatography, the preparation of monolithic separation media as well as to organic HPLC supports prepared by new polymer technologies such as ring-opening metathesis polymerization. Synthetic particularities relevant for the corresponding applications are outlined.