Which molecularly imprinted polymer is better?

Molecularly imprinted polymers (MIP) have been successfully synthesized toward many different compounds in the last decades. The mechanistic details of selective binding at binding sites are not yet well understood. For this reason the characterization of MIP binding has been mostly phenomenological and this makes the transfer of results between different laboratories or between different types of applications difficult. In this paper we analyze the relationship between different types of characterization like isotherms, binding site models, chromatographic k and α values, etc. as they relate to different applications like HPLC, solid phase extraction (SPE), binding assays, batch extraction and sensors. It is shown that α values determined by elution chromatography depend on seemingly irrelevant factors as the length and diameter of the column, respectively. The determination of distribution ratios or partition coefficients is proposed as an easily understandable and useful quantity in the characterization of novel MIPs. Data used for the characterization of a MIP should be transferable between different applications but the qualification of MIPs as better or worse will depend on the application in case.     

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.     

Monodispersed, molecularly imprinted polymers for cinchonidine by precipitation polymerization

Three monodispersed, molecularly imprinted polymers (MIPs) for cinchonidine (CD) have been synthesized by precipitation polymerization. MIP1 was prepared using methacrylic acid (MAA) as a functional monomer and divinylbenzene (DVB) as a cross-linker and MIP2 was prepared with further addition of 2-hydroxyethyl methacrylate (HEMA) as a co-monomer. For the preparation of MIP3, core-shell type MIP, monodispersed DVB homopolymers, which are prepared by precipitation polymerization, were used as a core and CD-imprinted MAA-DVB copolymer phases were coated onto the core. Three MIPs synthesized gave monodispersed, spherical beads in micrometer sizes. The binding characteristics and molecular recognition properties of MIP1-3 were examined by Scatchard analysis and chromatographic studies. The association constant of CD with MIP1 was the highest among MIPs prepared, while that with MIP3 was the lowest. The template molecule, CD, was more retained than its stereoisomer, cinchonine, on the three MIPs, and the stereoseparation factor of 38 was obtained with MIP3.     

Molecularly Imprinted Electrochemical Sensors

In this review, the applications of molecularly imprinted polymer (MIP) materials in the area of electrochemical sensors have been explored. The designs of the MIPs containing different polymers, their preparation and their immobilization on the transducer surface have been discussed. Further, the employment of various transducers containing the MIPs based on different electrochemical techniques for determining analytes has been assessed. In addition, the general protocols for getting the electrochemical signal based on the binding ability of analyte with the MIPs have been given. The review ends with describing scope and limitations of the above electrochemical based MIP sensors.     

Molecular Recognition Ability of Molecularly Imprinted Polymer Nano- and Micro-Particles by Reversible Addition-Fragmentation Chain Transfer Polymerization

The role of molecularly imprinted polymers (MIPs) is changing from academic to applied researches. Challenging problems about MIP will be more highlighted in applicable uses and solving these problems is vital. The controlled/“living” radical polymerization (CLRP) techniques are applicable to solve the challenging problems in MIPs. The “living” nature of CLRP helps to improve the heterogeneity of binding sites in MIPs as a main challenge where precise control over sizes, compositions, and surface functionalities is achieved. Among different techniques of CLRP, reversible addition-fragmentation chain transfer (RAFT) technique presents distinguished benefits such as compatibility and tolerance to a wide range of functional monomers and mild reaction conditions rather than other CLRP techniques. In this review, in order to obtain more insights into the potential benefits of RAFT polymerization in fabrication of nano and micro MIP networks, recent research in advanced MIP materials for different templates with improved morphology, efficiency, and binding capacities with respect to traditional free radical polymerization (FRP) will be discussed. MIPs prepared via RAFT method have advantages of MIPs as high performance molecular recognition devices and CLRP as controllable polymerization mechanism, simultaneously.     

A critical examination of the use of the Freundlich isotherm in characterizing molecularly imprinted polymers (MIPs)

The Freundlich isotherm (FI) has previously been shown to be useful in modeling the binding properties of non-covalent molecularly imprinted polymers (MIPs). The advantage of the FI is that it is a heterogeneous binding model that can accommodate and measure the heterogeneity inherent in MIPs. However, it is often difficult to verify whether the FI is an appropriate binding model for a particular MIP because only a narrow portion of the binding isotherm is experimentally measurable. This study takes a critical examination of whether the FI is an appropriate binding model for MIPs and explores its limitations in characterizing a (+)-cinchonine (CN) imprinted polymer and a non-imprinted polymer (NIP). A wider portion of the binding isotherm can be examined by systematically measuring a series of isotherms at different polymer concentrations. This strategy verified that FI can yield an accurate measure of the heterogeneity in the cinchonine MIP and the NIP. However, in cases of extremely high polymer loading, saturation behavior was observed, and the FI yielded inaccurate measures of the binding properties even though the experimental isotherm appears to be well modeled by the FI. Further, these studies indicate that the FI accurately predicts the heterogeneity index for more homogeneous compared to heterogeneous polymers over a wider concentration range but is subject to considerable error as saturation conditions are approached. These studies demonstrate the importance of correctly applying the FI to the lowest concentration portion of the binding isotherm that is experimentally measurable.     

Chromatographic characterization of molecularly imprinted polymers

Recent efforts in the investigation of chromatographic characterization of molecularly imprinted polymers (MIPs) have focused mainly on the nature of heterogeneous binding sites. More data on the thermodynamics than on the kinetic features of MIP columns have been published. The present article addresses the sources of peak broadening and tailing, which are the main drawbacks often associated with imprinted polymers in chromatography for practical applications. With use of the theory of nonlinear chromatography, the peak properties of a MIP column, including the retention and peak broadening and tailing, can be well interpreted. Efforts to improve chromatographic efficiency using MIPs prepared by approaches different from the conventional method, including covalent imprinting and the format of uniformly sized spherical microbeads, are reviewed and discussed. This review leads to the conclusion that nonlinear chromatography theory is useful for characterizing chromatographic features of MIP columns, since a MIP is essentially an affinity-based chromatographic stationary phase. We expect more theoretical and experimental studies on the kinetic aspects of MIP columns, especially the factors influencing the apparent rate constant, as well as the analysis of the influences of mobile-phase composition on the chromatographic performance. In addition to revealing the affinity interaction by molecular recognition, slow nonspecific interactions which may be inherited from the imperfect imprinting and may be involved in the rebinding of the template to MIPs also need to be characterized. Figure The peak broadening and tailing associated often with molecularly imprinted polymers (MIPs) in column chromatography for practical applications can be well characterized by the theory of nonlinear chromatography.     

Synthetic creatinine receptor: imprinting of a Lewis acidic zinc(II)cyclen binding site to shape its molecular recognition selectivity

Molecularly imprinted polymers (MIPs) from polymerizable Lewis acidic zinc(II)cyclen complexes and ethylene glycol dimethyl acrylate have been prepared. For the imprinting process the template molecule creatinine is reversibly coordinated to the zinc atom. The high strength of this interaction allows analyte binding to the MIP from aqueous solution with high affinity. Its pH dependence is used for controlled guest release with nearly quantitative analyte recovery rate. The binding capacity and selectivity profile of the MIP remains constant through several pH controlled binding and release cycles. MIPs missing a suitable metal binding site showed no significant affinity for thymine or creatinine. Flavin adsorbs nonspecifically to all polymers. The imprinting process reverses the binding selectivity of zinc(II)cyclen for creatinine and thymine from 1:34 in homogeneous solution to 3.5:1 in the MIP. Scatchard plot analysis of creatinine binding isotherms reveals uniform binding of the imprint, with fits indicating a one-site model; however, similar analysis for thymine indicate high and low affinity sites. This corresponds to unrestricted coordination sites freely accessible for thymine, e.g., at the polymer surface, and misshaped imprinted sites, which still can accommodate thymine. More than 50% of all binding sites exclusively bind creatinine and are not accessible to thymine. The binding properties of a copolymer of polymerizable zinc(II)cyclen and ethylene glycol dimethyl acrylate missing the creatinine template, which match the binding selectivity of the complex in solution, confirm that the origin of altered selectivities is the imprinting process. With binding ability at physiological pH, the MIPs are applicable for tasks in medicinal diagnostics or biotechnology. Imprinted zinc(II)cyclen complexes provide, like a metalloenzyme binding motif, high binding affinity by reversible coordination while the surrounding macromolecule determines binding selectivity.     

Approaches to molecular imprinting based selectivity in capillary electrochromatography

The work done during the past decade in order to adapt molecularly imprinted polymers (MIPs) to the capillary format and subsequently use these highly selective matrices for capillary electrochromatography (CEC) are reviewed in this article. MIPs are prepared utilizing a templated polymer synthesis where the template addresses the selectivity of the resulting polymer. These polymers possess binding characteristics that are comparable to the biological antibodies. Due to the polyclonality of the binding sites in the MIP, the separation result in severe peak broadening and tailing when performed in the isocratic mode. This was seen early in the development of MIPs as selective stationary phases in liquid chromatography (LC). As a mean of decreasing these problems, much effort was put into adapting the MIP to fit in CEC systems, that offers an efficiency that is superior to that in LC. Aiming to increase the efficiency of the MIP-CEC systems, different MIP formats have been developed that can be divided into three conceptually different categories, i.e., the monolithic, the microparticle and the coating. The strive for MIP formats that can be used in small bore capillaries has led to the development of MIP formats applicable to miniaturized systems approaching the chip format. Although prepared in order to perform MIP-CEC mediated separations, these formats can be used in a broad range of applications were the characteristics of the MIP, e.g. stability, selectivity and cost efficiency, could offer an interesting solution to cover the needs.     

Photopolymerization and photostructuring of molecularly imprinted polymers for sensor applications—A review

Biosensors are already well established in modern analytical chemistry, and have become important tools for clinical diagnostics, environmental analysis, production monitoring, drug detection or screening. They are based on the specific molecular recognition of a target molecule by a biological receptor such as an antibody or an enzyme. Synthetic biomimetic receptors like molecularly imprinted polymers (MIPs) have been shown to be a potential alternative to biomolecules as recognition element for biosensing. Produced by a templating process at the molecular level, MIPs are capable of recognizing and binding target molecules with similar specificity and selectivity to their natural analogues. One of the main challenges in MIP sensor development is the miniaturization of MIP structures and their interfacing with the transducer or with a microchip. Photostructuring appears thereby as one of the most suitable methods for patterning MIPs at the micro and nano scale, directly on the transducer surface. In the present review, a general overview on MIPs in biosensing applications is given, and the photopolymerization and photopatterning of MIPs are particularly described.     

An Update on the Use of Molecularly Imprinted Polymers in Beta-Blocker Drug Analysis as a Selective Separation Method in Biological and Environmental Analysis

Beta-blockers are antihypertensive drugs and can be abused by athletes in some sport competitions; it is therefore necessary to monitor beta-blocker levels in biological samples. In addition, beta-blocker levels in environmental samples need to be monitored to determine whether there are contaminants from the activities of the pharmaceutical industry. Several extraction methods have been developed to separate beta-blocker drugs in a sample, one of which is molecularly imprinted polymer solid-phase extraction (MIP-SPE). MIPs have some advantages, including good selectivity, high affinity, ease of synthesis, and low cost. This review provides an overview of the polymerization methods for synthesizing MIPs of beta-blocker groups. The methods that are still widely used to synthesize MIPs for beta-blockers are the bulk polymerization method and the precipitation polymerization method. MIPs for beta-blockers still need further development, especially since many types of beta-blockers have not been used as templates in the MIP synthesis process and modification of the MIP sorbent is required, to obtain high throughput analysis.     

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.     

Rapid preparation of molecularly imprinted polymer by frontal polymerization

Frontal polymerization was successfully applied, for the first time, to obtain molecularly imprinted polymers (MIPs). The method provides a solvent-free polymerization mode, and the reaction can be completed in 30 min. By this approach, MIPs were synthesized using a mixture of levofloxacin (template), methacrylic acid, and divinylbenzene. The effect of template concentration and the amount of comonomer on the imprinting effect of the resulting MIPs was investigated. The textural and morphological parameters of the MIP particles were also characterized by mercury intrusion porosimetry, nitrogen adsorption isotherms, and scanning electron microscopy, providing evidence concerning median pore diameter, pore volumes, and pore size distributions. The levofloxacin-imprinted polymer formed in frontal polymerization mode showed high selectivity, with an imprinting factor of 5.78. The results suggest that frontal polymerization provides an alternative means to prepare MIPs that are difficult to synthesize and may open up new perspectives in the field of MIPs.

Molecularly Imprinted Polymers for Solid Phase Extraction

The combination of molecularly imprinted polymers (MIPs) and solid phase extraction (SPE) is reviewed. MIPs, which have high selectivity and affinity for a predetermined molecule (template), have been used as sorbents for SPE to selectively isolate analytes from biological, pharmaceutical, and environmental samples. Solid phase extraction with molecularly imprinted polymers (MIP–SPE) is a promising technique which allows specific analytes to be selectively extracted from complex matrices. This survey summarizes the characteristics, development and application of MIP–SPE in recent years. Existed problems and the future direction of MIP–SPE are also discussed.     

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.     

Preparation and computational investigation of molecular imprinted polymers for Clidinium Bromide

Molecular imprinted polymers (MIPs) are polymers that possess recognition sites specific for a predetermined target molecule (Template). Inspired by the idea of biological natural receptors, they behave like synthetic molecular recognition elements. They have been developed into a promising tool in several crucial applications, including analytical methods, drug delivery, and catalysis. The non-covalent imprinting is more commonly used approach in the preparation of MIPs because of its simplicity. In this approach, intermolecular interactions between the template molecule (T) and the functional monomer (FM) are the forces that govern the performance of the resulting MIP. Hence, studying these interactions is very important to elucidate and understand the imprinting mechanism. This paper focuses on preparation of two MIPs for a Clidinium Bromide (CB), using two different types of FMs. These MIPs are characterized by using IR and SEM techniques. Adsorption isotherm properties to CB are assayed for them. Then the structures of the pre-polymerization complexes of prepared MIPs were investigated using Density Functional Theory (DFT) calculations at B3LYP/6-31G level in a vacuum and other media. Finally, Bader's Quantum Theory of Atoms in Molecules (QTAIM) was used to prove the existence and nature of intermolecular interactions between CB and FM. The theoretical results were in complete agreement with experiments and indicated that the use of AM as FM is preferred over MA in the MIP preparation for CB.     

Characterization of molecularly imprinted polymers for the extraction of tobacco alkaloids and their metabolites in human urine

Molecularly imprinted polymers (MIPs) are synthetic polymers designed to selectively extract target analytes from complex matrices (including biological matrices). The literature shows that MIPs have a degree of cross-selectivity from analytes within the same class of compounds. A commercially available MIP for tobacco-specific nitrosamines (TSNAs) is designed to be class selective for four TSNA compounds. This study sought to characterize the extent of cross-selectivity of the TSNA MIPs with other tobacco alkaloids. Cross-selectivity and recovery of the SupelMIP™ TSNA SPE cartridges was assessed with N-nitrosonornicotine (NNN), nicotine, cotinine and morphine. Their recoveries were compared with the recoveries of a nonimprinted polymer SPE cartridge, and two traditional SPE cartridges: a Waters mixed-mode cation exchange cartridge and a Waters hydrophilic–lipophilic balance cartridge. NNN and cotinine had the highest recoveries with the MIP cartridge, over 80%, and cotinine samples in urine had >80% recoveries. Nicotine had highly variable recoveries, possibly owing to differing chemical properties from the TSNAs. All three analytes had significantly different recoveries with the MIP cartridges compared with the traditional SPE cartridges. Morphine displayed nonspecific interactions with the MIP cartridges. Utilization of the TSNAs’ cross-selectivity allows for simultaneous extraction and identification of multiple tobacco biomarkers using one extraction technique.     

Enhanced selectivity of hydrogel-based molecularly imprinted polymers (HydroMIPs) following buffer conditioning

We have investigated the effect of buffer solution composition and pH during the preparation, washing and re-loading phases within a family of acrylamide-based molecularly imprinted polymers (MIPs) for bovine haemoglobin (BHb), equine myoglobin (EMb) and bovine catalyse (BCat). We investigated water, phosphate buffer saline (PBS), tris(hydroxymethyl)aminomethane (Tris) buffer and succinate buffer. Throughout the study MIP selectivity was highest for acrylamide, followed by N-hydroxymethylacrylamide, and then N-iso-propylacrylamide MIPs. The selectivity of the MIPs when compared with the NIPs decreased depending on the buffer conditions and pH in the order of Tris > PBS > succinate. The Tris buffer provided optimum imprinting conditions at 50 mM and pH 7.4, and MIP selectivities for the imprinting of BHb in polyacrylamide increased from an initial 8:1 to a 128:1 ratio. It was noted that the buffer conditions for the re-loading stage was important for determining MIP selectivity and the buffer conditions for the preparation stage was found to be less critical. We demonstrated that once MIPs are conditioned using Tris or PBS buffers (pH7.4) protein reloading in water should be avoided as negative effects on the MIP's imprinting capability results in low selectivities of 0.8:1. Furthermore, acidifying the pH of the buffer solution below pH 5.9 also has a negative impact on MIP selectivity especially for proteins with high isoelectric points. These buffer conditioning effects have also been successfully demonstrated in terms of MIP efficiency in real biological samples, namely plasma and serum.     

On-line solid-phase extraction with molecularly imprinted polymers to selectively extract substituted 4-chlorophenols and 4-nitrophenol from water

Three polymers have been synthesised using 4-chlorophenol (4-CP) as the template, following different protocols (non-covalent and semi-covalent) and using different functional co-monomers, 4-vinylpyridine (4-VP) and methacrylic acid (MAA). The polymers were evaluated to check their selectivity as molecularly imprinted polymers (MIPs) in solid-phase extraction (SPE) coupled on-line to liquid chromatography. The solid-phase extraction procedure using MIPs (MISPE), including the clean-up step to remove any interferences, was optimised. The 4-VP non-covalent polymer was the only one which showed a clear imprint effect. This MIP also showed cross-reactivity for the 4-chloro-substituted phenols and for 4-nitrophenol (4-NP) from a mixture containing the 11 priority EPA (Environmental Protection Agency) phenolic compounds and 4-chlorophenol. The MIP was applied to selectively extract the 4-chloro-substituted compounds and 4-NP from river water samples.     

The complete replacement of antibodies by protein-imprinted poly(ethylene-co-vinyl alcohol) in sandwich fluoroimmunoassays

The replacement of antibodies by molecularly imprinted polymers (MIPs) has been investigated for many decades. However, indirect protocols (including natural primary and secondary antibodies) are still utilized to evaluate the ability of MIP thin films to recognize target molecules. MIPs can be prepared as either a thin film or as particles, and cavities that are complementary to the template can be generated on their surfaces. We have prepared thin film MIPs and particle MIPs prepared by solvent evaporation and phase inversion, respectively, from solutions of poly(ethylene-co-vinyl alcohol) (pEVAL) in the presence of the target analytes amylase, lysozyme, and lipase. These were first adsorbed on MIP thin films and by MIP particles that contain fluorescent quantum dots. Sandwich fluoroimmunoassays were then conducted to quantify them in MIP-coated 96-well microplates. The method was applied to determine amylase in saliva, and results were compared with a commercial analytical system.