Synthetic peptide receptor mimics: highly stereoselective recognition in non-covalent molecularly imprinted polymers

Label-Free Bioanalyte Detection from Nanometer to Micrometer Dimensions-Molecular Imprinting and QCMs †

Modern diagnostic tools and immunoassay protocols urges direct analyte recognition based on its intrinsic behavior without using any labeling indicator. This not only improves the detection reliability, but also reduces sample preparation time and complexity involved during labeling step. Label-free biosensor devices are capable of monitoring analyte physiochemical properties such as binding sensitivity and selectivity, affinity constants and other dynamics of molecular recognition. The interface of a typical biosensor could range from natural antibodies to synthetic receptors for example molecular imprinted polymers (MIPs). The foremost advantages of using MIPs are their high binding selectivity comparable to natural antibodies, straightforward synthesis in short time, high thermal/chemical stability and compatibility with different transducers. Quartz crystal microbalance (QCM) resonators are leading acoustic devices that are extensively used for mass-sensitive measurements. Highlight features of QCM devices include low cost fabrication, room temperature operation, and most importantly ability to monitor extremely low mass shifts, thus potentially a universal transducer. The combination of MIPs with quartz QCM has turned out as a prominent sensing system for label-free recognition of diverse bioanalytes. In this article, we shall encompass the potential applications of MIP-QCM sensors exclusively label-free recognition of bacteria and virus species as representative micro and nanosized bioanalytes.

Imprinting of Microorganisms for Biosensor Applications

There is a growing need for selective recognition of microorganisms in complex samples due to the rapidly emerging importance of detecting them in various matrices. Most of the conventional methods used to identify microorganisms are time-consuming, laborious and expensive. In recent years, many efforts have been put forth to develop alternative methods for the detection of microorganisms. These methods include use of various components such as silica nanoparticles, microfluidics, liquid crystals, carbon nanotubes which could be integrated with sensor technology in order to detect microorganisms. In many of these publications antibodies were used as recognition elements by means of specific interactions between the target cell and the binding site of the antibody for the purpose of cell recognition and detection. Even though natural antibodies have high selectivity and sensitivity, they have limited stability and tend to denature in conditions outside the physiological range. Among different approaches, biomimetic materials having superior properties have been used in creating artificial systems. Molecular imprinting is a well suited technique serving the purpose to develop highly selective sensing devices. Molecularly imprinted polymers defined as artificial recognition elements are of growing interest for applications in several sectors of life science involving the investigations on detecting molecules of specific interest. These polymers have attractive properties such as high bio-recognition capability, mechanical and chemical stability, easy preparation and low cost which make them superior over natural recognition reagents. This review summarizes the recent advances in the detection and quantification of microorganisms by emphasizing the molecular imprinting technology and its applications in the development of sensor strategies.

Characterization of the Binding Properties of Molecularly Imprinted Polymers

The defining characteristic of the binding sites of any particular molecularly imprinted material is heterogeneity: that is, they are not all identical. Nonetheless, it is useful to study their fundamental binding properties, and to obtain average properties. In particular, it has been instructive to compare the binding properties of imprinted and non-imprinted materials. This chapter begins by considering the origins of this site heterogeneity. Next, the properties of interest of imprinted binding sites are described in brief: affinity, selectivity, and kinetics. The binding/adsorption isotherm, the graph of concentration of analyte bound to a MIP versus concentration of free analyte at equilibrium, over a range of total concentrations, is described in some detail. Following this, the techniques for studying the imprinted sites are described (batch-binding assays, radioligand binding assays, zonal chromatography, frontal chromatography, calorimetry, and others). Thereafter, the parameters that influence affinity, selectivity and kinetics are discussed (solvent, modifiers of organic solvents, pH of aqueous solvents, temperature). Finally, mathematical attempts to fit the adsorption isotherms for imprinted materials, so as to obtain information about the range of binding affinities characterizing the imprinted sites, are summarized.

Recognizing Amino Acid Chirality with Surface-Imprinted Polymers Prepared in W/O Emulsions

A molecularly imprinted polymer was prepared by a surface molecular imprinting technique in water-in-oil (W/O) emulsion. In this technique, the solid polymer, which is molecularly imprinted at the internal cavity surface, is prepared by polymerizing W/O emulsions consisting of a water-soluble imprinted molecule, a functional host molecule, an emulsion stabilizer, and a crosslinking agent. Dioleoyl phosphate was used as an emulsion stabilizer, and this compound also acted as a monomer and a host functional group in the imprinted cavity. Divinylbenzene was used as a crosslinker. Tryptophan methyl ester and phenylalanine methyl ester were used as the target template materials. These imprinted polymers exhibited enantiomeric selectivity in absorption experiments, and the maximum separation factor was 1.58. The enantiomeric selectivity with tryptophan methyl ester was higher than that with phenylalanine methyl ester.

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.

Artificial receptors

Herein I will provide a brief overview of artificial receptors with emphasis on molecularly imprinted polymers (MIPs) and their applications. Alternative techniques to produce artificial receptors such as in silico designed and modelled polymers as well as different receptors designed using libraries of more or less natural composition will also be mentioned. Examples of these include aptamers and bio-nanocomposites. The physical presentation of the receptors is important and may depend on the application. Block polymerization of MIPs and grinding to particles of suitable size used to be the preferred technique, but today beaded materials can be produced in sizes down to nanobeads and also nanofibers can be used to increase available surface area and thereby capacity. For sensor applications it may be attractive to include the artificial receptors in surface coatings or in membrane structures. Different composite designs can be used to provide additional desirable properties. MIPs and other artificial receptors are gaining rapidly increasing attention in very shifting application areas and an attempt to provide a systematic account for current applications has been made with examples from separation, solid-phase extraction, analysis, carbohydrate specific experiments, and MIPs-directed synthesis.

Chiral recognition applications of molecularly imprinted polymers: a critical review

Molecular imprinting technology offers the unique opportunity to tailor chiral stationary phases with predefined chiral recognition properties by employing the enantiomers of interest as binding-site-forming templates. Added advantages, such as ease of preparation, chemical robustness, low-cost production, and the possibility of shaping molecularly imprinted polymers (MIPs) in various self-supporting formats, render them attractive materials for a broad range of chiral recognition applications. In this review a critical overview on recent developments in the field of MIP-based chiral recognition applications is given, focusing on separation techniques and molecular sensing. Inherent limitations associated with the use of enantioselective MIP materials in high-performance separation techniques are outlined, including binding site heterogeneity and slow mass transfer characteristics. The prospects of MIP materials as versatile recognition elements for the design of enantioselective sensor systems are highlighted.

Non-covalent molecular imprinting with emphasis on its application in separation and drug development

The molecular imprinting technique can be defined as the formation of specific nano-sized cavities by means of template-directed synthesis. The resulting molecularly imprinted polymers (MIPs), which often have an affinity and a selectivity approaching those of antibody-antigen systems, have thus been coined "artificial antibodies." MIPs are characterized by their high specificity, ease of preparation, and their thermal and chemical stability. They have been widely studied in connection with many potential applications, including their use for separation and isolation purposes, as antibody mimics (biomimetic assays and sensors), as enzyme mimics, in organic synthesis, and in drug delivery. The non-covalent imprinting approach, developed mainly in Lund, has proven to be more versatile than the alternative covalent approach because of its preparation being less complicated and of the broad selection of functional monomers and possible target molecules that are available. The paper presents a review of studies of this versatile technique in the areas of separation and drug development, with emphasis being placed on work carried out in our laboratory.

Molecular imprinting science and technology: a survey of the literature for the years up to and including 2003

Over 1450 references to original papers, reviews and monographs have herein been collected to document the development of molecular imprinting science and technology from the serendipitous discovery of Polyakov in 1931 to recent attempts to implement and understand the principles underlying the technique and its use in a range of application areas. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by papers dealing with fundamental aspects of molecular imprinting and the development of novel polymer formats. Thereafter, literature describing attempts to apply these polymeric materials to a range of application areas is presented.

Stereoselective peptide analysis

The stereochemistry of a peptide determines its spatial features and can profoundly influence its chemical properties and biological activity. Thus, the analysis of the stereochemical properties of a peptide is an important aspect of its characterisation. For such investigations a "selector" that engages in stereoselective interactions with the peptide analytes is often used. A substantiated knowledge of the underlying molecular recognition mechanism will therefore be helpful in understanding existing and developing new stereoselective analysis systems. After a short introduction concerning the fundamentals of peptide stereoisomers and their biological implications, the stereoselective peptide analysis methods described in the literature are comprehensively reviewed. The characteristics and applications of the employed methods based on various techniques including chromatography (pressure- and electrokinetically driven), capillary electrophoresis, nuclear magnetic resonance spectroscopy and mass spectrometry are discussed. The various selectors that have been utilised to discriminate peptide enantiomers and/or diastereomers are described concurrently. The review concludes with an overview of combinations and comparisons of techniques that have been applied to the analysis of peptide stereoisomers and constitute a trend for further developments.

Molecular imprinting: a dynamic technique for diverse applications in analytical chemistry

Continuous advances in analyzing complex matrices, improving reliability and simplicity, and performing multiple simultaneous assays with extreme sensitivity are increasing. Several techniques have been developed for the quantitative assays of analytes at low concentrations (e.g., high-pressure liquid chromatography, gas chromatography, immunoassay and the polymerase chain reaction technique). To achieve highly specific and sensitive analysis, high affinity, stable, and specific recognition agents are needed. Although biological recognition agents are very specific and sensitive they are labile and/or have a low density of binding sites. During the past decade molecular imprinting has emerged as an attractive and highly accepted tool for the development of artificial recognition agents. Molecular imprinting is achieved by the interaction, either noncovalent or covalent, between complementary groups in a template molecule and functional monomer units through polymerization or polycondensation. These molecularly imprinted polymers have been widely employed for diverse applications (e.g., in chromatographic separation, drug screening, chemosensors, catalysis, immunoassays etc.) owing to their specificity towards the target molecules and high stability against physicochemical perturbations. In this review the advantages, applications, and recent developments in molecular imprinting technology are highlighted.

Analysis of NAD(P)+/NAD(P)H cofactors by imprinted polymer membranes associated with ion-sensitive field-effect transistor devices and Au-quartz crystals

Specific recognition sites for the NAD(P)+ and NAD(P)H cofactors are imprinted in a cross-linked acrylamide-acrylamidophenylboronic acid copolymer membrane. The imprinted membranes, associated with pH-sensitive field-effect transistors (ISFETs) or Au-quartz piezoelectric crystals, enable the potentiometric or microgravimetric analysis of the oxidized NAD(P)+ cofactors and the reduced NAD(P)H cofactors, respectively. The NAD+- and NADP+-imprinted membranes associated with the ISFET allow the analysis of NAD+ and NADP+ with sensitivities that correspond to 15.0 and 18.0 mVdecade(-1) and detection limits of 4 x 10(-7) and 2 x 10(-7) M, respectively. The NADH- and NADPH-imprinted membranes associated with the ISFET device enable the analysis of NADH and NADPH with sensitivities that correspond to 24.2 and 21.8 mV x decade(-1) and lower detection limits that are 1 x 10(-7) and 2 x 10(-7) M, respectively. The ISFET devices functionalized with the NADH and NADPH membranes are employed in the analysis of the biocatalyzed oxidation of lactic acid and ethanol in the presence of lactate dehydrogenase and alcohol dehydrogenase, respectively.

Imprinting of nucleotide and monosaccharide recognition sites in acrylamidephenylboronic acid-acrylamide copolymer membranes associated with electronic transducers

Molecular recognition sites for the nucleotides adenosine 5'-monophosphate (1), guanosine 5'-monophosphate (2), cytosine 5'-monophosphate (3), and uridine 5'-monophosphate (4) are imprinted in an acrylamide-acrylamidephenylboronic acid copolymer (5) membrane. The imprinted membranes are assembled on piezoelectric Au quartz crystals or Au electrodes via electropolymerization or on the gate surface of an ISFET device by radical polymerization. The imprinted membranes reveal selectivity toward the imprinted nucleotide, and the association of the respective nucleotides with the recognition sites is transduced by the following: (i) microgravimetric, quartz crystal microbalance (QCM) measurements; (ii) Faradaic impedance analyses, and (iii) potentiometric responses of the ISFET devices. While the microgravimetric QCM measurements reflect the swelling of the polymers upon the association of the nucleotides with the recognition sites, the ISFET response is due to the charging of the polymer membrane as a result of the formation of the nucleotide-boronate complex. The selective detection of the nucleotides may lead to new DNA/RNA sequencing methods. Also, specific recognition sites for beta-D(+)-glucose (6), D(+)-galactose (7), and beta-D(-)-fructose (8) were imprinted in an acrylamide-acrylamidephenylboronic acid copolymer (5) membrane associated with an ISFET device. Selective sensing of the respective monosaccharides is accomplished in the presence of the imprinted membrane-functionalized ISFET devices.

Fibers coated with molecularly imprinted polymers for solid-phase microextraction

The simplicity and flexibility of solid-phase microextraction have been combined with the selectivity of molecularly imprinted polymers (MIPs). Silica fibers were coated reproducible with a 75-microm layer of methacrylate polymer either nonimprinted or imprinted with clenbuterol to compare their extraction characteristics under various conditions. Although the template molecule could be removed effectively from the imprinted polymer, structural analogues of clenbuterol were used for evaluation. The influence of pH on the extractability of brombuterol was investigated. Extraction yields up to approximately 80% were obtained when both types of fibers were used to extract brombuterol from phosphate buffer (pH 7.0). In contrast, yields of about 75 and <5% were obtained when extraction was performed from acetonitrile with imprinted and nonimprinted polymers, respectively, which demonstrates the selectivity of the MIP-coated fiber. Time sorption profiles were measured for the extraction of brombuterol from buffer and acetonitrile at the 10 and 100 ng/mL level with both types of fibers in order to compare extraction characteristics. Equilibrium times of about 30 and 90 min were found for the extraction of brombuterol from acetonitrile and buffer, respectively. The MIP-coated fibers were capable of extracting five structural analogues of clenbuterol from both buffer and acetonitrile, which suggests that the amine alcohol part of these molecules is responsible for interaction with the imprinted polymer. To achieve selective extraction of brombuterol from human urine, MIP-coated fibers were washed with acetonitrile after the extraction. Clean extracts and yields of approximately 45% were obtained, demonstrating the suitability of MIP-coated fibers for the analysis of biological samples.

Molecularly imprinted polymer membranes for substance-selective solid-phase extraction from water by surface photo-grafting polymerization

Hydrophilized polyvinylidene fluoride microfiltration membranes were surface-modified in the presence of a template (terbumeton) in methanol with a graft copolymer of a functional monomer (2-acrylamido-2-methyl-1-propane sulfonic acid, AMPS, methacrylic acid, MAA, or acrylic acid, AA) and a cross-linker (N,N'-methylene-bis-acrylamide) using UV irradiation and benzophenone as photoinitiator. As result, membranes covered with a thin layer of imprinted polymer selective to terbumeton were obtained. Blank membranes were prepared with the same monomer composition, but in the absence of the template. The membranes' capacity to adsorb terbumetone from aqueous solution was evaluated yielding information regarding the effect of polymer synthesis (type and concentration of functional monomer, concentration of cross-linker) on the resulting membranes' recognition properties. UV spectroscopic studies of the interactions with terbumetone revealed that AMPS forms a stronger complex than MAA and AA. In agreement with that finding, imprinting with AMPS gave higher affinities than with MAA and AA. The terbumeton-imprinted membranes showed significantly higher sorption capability to this herbicide than to similar compounds (atrazine, desmetryn, metribuzine). With the novel surface modification technology, the low non-specific binding properties of the hydrophilized microfiltration membrane could successfully be combined with the receptor properties of molecular imprints, yielding substance-specific molecularly imprinted polymer composite membranes. The high affinity of these synthetic affinity membranes to triazine herbicides together with their straightforward and inexpensive preparation provides a good basis for the development of applications of imprinted polymers in separation processes such as solid-phase extraction.

Imprinted chiral stationary phases in high-performance liquid chromatography

Polymers imprinted with chiral templates offer a new generation of tailor-made chiral stationary phases (CSPs) with predictable selectivities. This review summarizes the present state of the art of molecular imprinting to generate tailor-made CSPs and provides an overview of the main factors involved in the manufacturing process that are crucial to the chromatographic performance of the phases.