Electrochemical and piezoelectric monitoring of taurine via electropolymerized molecularly imprinted films

A molecularly imprinted electrochemical quartz crystal microbalance (EQCM) sensor is fabricated here for taurine, a β ‐amino acid significant for functioning of almost all vital organs. The polymeric film of l ‐methionine was electrochemically deposited on gold‐coated EQCM electrode. Experimental parameters were optimized for controlling the performance of molecularly imprinted polymer (MIP)‐modified sensor such as ratio of monomer and template, number of electropolymerization cycles, mass deposited in each cycle, and pH. Thus, fabricated MIP‐EQCM sensor was successfully applied for estimation of taurine in solutions with varying matrices, such as aqueous, human blood plasma, milk from cow, buffalo, and milk powder. Under optimized parameters, response of MIP sensor to taurine was linearly proportional to its concentration with limit of detection as 0.12μM. Hence, a highly sensitive and selective piezoelectric sensor for taurine has been reported here via imprinting approach.     

Molecular imprinting technology: challenges and prospects for the future

Molecular imprinting is a technique for the fabrication of biomimetic polymeric recognition sites or “plastic antibodies/receptors” which is attracting rapidly increasing interest. By this technology, recognition matrices can be prepared which possess high substrate selectivity and specificity. In the development of this technology, several applications have been foreseen in which imprinted materials may be exchanged for natural recognition elements. Thus, molecularly imprinted polymers have been used as antibody/receptor binding mimics in immunoassay-type analyses, as enzyme mimics in catalytic applications and as recognition matrices in biosensors. The best developed application area for imprinted materials, though, has been as stationary phases for chromatography, in general, and chiral chromatography, in particular. This review seeks to highlight some of the more intriguing advantages of the technique as well as pointing out some of the difficulties encountered. The prospects for future development will also be considered. Chirality 10:195–209, 1998. © 1998 Wiley-Liss, Inc.     

An epitope‐imprinted piezoelectric diagnostic tool for Neisseria meningitidis detection

Neisseria meningitidis, a human‐specific bacterial pathogen causes bacterial meningitis by invading the meninges (outer lining) of central nervous system. It is the polysaccharide present on the bacterial capsid that distinguishes various serogroups of N. meningitidis and can be utilized as antigens to elicit immune response. A computational approach identified candidate T‐cell epitopes from outer membrane proteins Por B of N. meningitidis (MC58): (²⁷³KGLVDDADI²⁸² in loop VII and ¹⁷⁰GRHNSESYH¹⁷⁹ in loop IV) present on the exposed surface of immunogenic loops of class 3 outer membrane proteins allele of N. meningitidis. One of them, KGLVDDADI is used here for designing a diagnostic tool via molecularly imprinted piezoelectric sensor (molecularly imprinted polymer‐quartz crystal microbalance) for N. meningitidis strain MC58. Methacrylic acid, ethylene glycol dimethacrylate and azoisobutyronitrile were used as functional monomer, cross‐linker and initiator, respectively. The epitope can be simultaneously bound to methacrylic acid and fitted into the shape‐selective cavities. On extraction of epitope sequence from thus grafted polymeric film, shape‐selective and sensitive sites were generated on electrochemical quartz crystal microbalance crystal, ie, known as epitope imprinted polymers. Imprinting was characterized by atomic force microscopy images. The epitope‐imprinted sensor was able to selectively bind N. meningitidis proteins present in blood serum of patients suffering from brain fever. Thus, fabricated sensor can be used as a diagnostic tool for meningitis disease.     

Molecular imprinting of nitrophenol and hydroxybenzoic acid isomers: effect of molecular structure and acidity on imprinting

Three nitrophenol isomer-imprinted polymers were prepared under the same conditions using 4-vinylpyridine as a functional monomer. Different recognition capacities for template molecules were observed for the three polymers. Another imprinting system with stronger acidity than nitrophenol isomers, 2-hydroxybenzoic acid (salicylic acid) and 4-hydroxybenzoic acid, was imprinted using 4-vinylpyridine or acrylamide as functional monomer respectively. Both 4-hydroxybenzoic acid-imprinted polymers using the two monomers showed recognition ability for the template molecule. However, when acrylamide was chosen as functional monomer, the salicylic acid-imprinted polymer showed very weak recognition for the template molecule, whereas strong recognition ability of the resultant polymer for salicylic acid was observed with 4-vinylpyridine as functional monomer. It seems that the structure and acidity of template molecules is responsible for the difference in recognition, by influencing the formation and strength of interaction between template molecule and functional monomer during the imprinting process. An understanding of the mechanism of molecular imprinting and molecular recognition of MIPs will help to predict the selectivity of MIPs on the basis of template molecule properties.     

New biomedical devices with selective peptide recognition properties. Part 1: Characterization and cytotoxicity of molecularly imprinted polymers

Molecular imprinting is a technique for the synthesis of polymers capable to bind target molecules selectively. The imprinting of large proteins, such as cell adhesion proteins or cell receptors, opens the way to important and innovative biomedical applications. However, such molecules can incur into important conformational changes during the preparation of the imprinted polymer impairing the specificity of the recognition cavities. The "epitope approach" can overcome this limit by adopting, as template, a short peptide sequence representative of an accessible fragment of a larger protein. The resulting imprinted polymer can recognize both the template and the whole molecule thanks to the specific cavities for the epitope. In this work two molecularly imprinted polymer formulations (a macroporous monolith and nanospheres) were obtained using the protected peptide Z-Thr-Ala-Ala-OMe, as template, and Z-Thr-Ile-Leu-OMe, as analogue for the selectivity evaluation, methacrylic acid, as functional monomer, and trimethylolpropane trimethacrylate and pentaerythritol triacrylate (PETRA), as cross-linkers. Polymers were synthesized by precipitation polymerization and characterized by standard techniques. Polymerization and rebinding solutions were analyzed by high performance liquid chromatography. The highly cross-linked polymers retained about 70% of the total template amount, against (20% for the less cross-linked ones). The extracted template amount and the rebinding capacity decreased with the cross-linking degree, while the selectivity showed the opposite behaviour. The PETRA cross-linked polymers showed the best recognition (MIP 2-, alpha=1.71) and selectivity (MIP 2+, alpha'=5.58) capabilities. The cytotoxicity tests showed normal adhesion and proliferation of fibroblasts cultured in the medium that was put in contact with the imprinted polymers.     

Some new developments and challenges in non-covalent molecular imprinting technology

The technique of molecular imprinting allows the formation of specific recognition and catalytic sites in macromolecules via the use of templates. Molecularly imprinted polymers have been applied in an increasing number of applications where molecular binding events are of interest. These include the use of molecularly imprinted polymers as tailor-made separation materials, antibody and receptor binding site mimics in recognition and assay systems, enzyme mimics for catalytic applications and as recognition elements in biosensors. The stability and low cost of molecularly imprinted polymers make them advantageous for use in analysis as well as in industrial-scale production and application.     

A computational approach to study functional monomer‐protein molecular interactions to optimize protein molecular imprinting

Molecular imprinting has become a promising approach for synthesis of polymeric materials having binding sites with a predetermined selectivity for a given analyte, the so‐called molecularly imprinted polymers (MIPs), which can be used as artificial receptors in various application fields. Realization of binding sites in a MIP involves the formation of prepolymerization complexes between a template molecule and monomers, their subsequent polymerization, and the removal of the template. It is believed that the strength of the monomer‐template interactions in the prepolymerization mixture influences directly on the quality of the binding sites in a MIP and consequently on its performance. In this study, a computational approach allowing the rational selection of an appropriate monomer for building a MIP capable of selectively rebinding macromolecular analytes has been developed. Molecular docking combined with quantum chemical calculations was used for modeling and comparing molecular interactions among a model macromolecular template, immunoglobulin G (IgG), and 1 of 3 electropolymerizable functional monomers: m‐phenylenediamine (mPD), dopamine, and 3,4‐ethylenedioxythiophene, as well as to predict the probable arrangement of multiple monomers around the protein. It was revealed that mPD was arranged more uniformly around IgG participating in multiple H‐bond interactions with its polar residues and, therefore, could be considered as more advantageous for synthesis of a MIP for IgG recognition (IgG‐MIP). These theoretical predictions were verified by the experimental results and found to be in good agreement showing higher binding affinity of the mPD‐based IgG‐MIP toward IgG as compared with the IgG‐MIPs generated from the other 2 monomers.     

Construction of a molecular imprinting catalyst using target analogue template and its application for an amperometric fructosylamine sensor

Molecular imprinting technology is becoming a versatile tool for preparing tailor-made molecular recognition elements. However, inherent problems of the molecular imprinting technology include the availability and preparation of template molecules. We recently reported artificial enzyme sensors for fructosylamines constructed by imprinting with fructosyl valine (Fru-val), a model compound for HbA1c (Anal. Lett., 2003). However, because the availability of Fru-val is limited, we attempted to construct a Fru-val-oxidizing molecularly imprinted catalyst (MIC) utilizing the analogue molecule methyl valine (m-val) as template molecule. An electrode employing the m-val-imprinted polymer showed 1.2-fold higher sensitivity toward Fru-val compared with the control polymer-employing electrode. We also used the positively charged functional monomer allylamine as functional monomer in order to increase the selectivity of the MIC toward Fru-val. The selectivity of the electrode immobilizing the allylamine-containing polymer showed 1.7-fold higher response toward Fru-val than toward Fru--lys. By combining the use of both allylamine as the functional monomer and m-val as the template molecule, an even better MIC-immobilized electrode was produced with a Fru-val selectivity comparable to that constructed by imprinting with Fru-val.     

Design and evaluation of thin and flexible theophylline imprinted polymer membrane materials

The aim of this work was to produce a thin, flexible and diffusion able molecularly imprinted polymeric matrix with good template accessibility. Membranes were prepared using a non‐covalent molecular imprinting approach and their physical characteristics and binding capabilities investigated. Two materials were used, a poly(tri‐ethyleneglycol dimethyacrylate‐co‐methyl methacrylate‐co‐methacrylic acid) copolymer containing 14% cross‐linker and a monomer (g) to porogen (ml) ratio of 1:0.5 (A), and a blend of poly(TEGMA‐co‐MAA) and polyurethane (B). The polyurethane was added to improve membrane flexiblity and stability. The polymers were characterized using AFM, SEM and nitrogen adsorption, whilst binding was evaluated using batch‐rebinding studies. For all membranes the specific surface area was low (<10 m²/g). MIP (A) films were shown to bind specifically at low concentrations but specific binding was masked by non‐specific interactions at elevated concentrations. Selectivity studies confirmed specificity at low concentrations. K_D approximations confirmed a difference in the population of binding sites within NIP and MIP films. The data also indicated that at low concentrations the ligand‐occupied binding site population approached homogeneity. Scanning electron microscopy images of membrane (B) revealed a complex multi‐layered system, however these membranes did not demonstrate specificity for the template. The results described here demonstrate how the fundamental parameters of a non‐covalent molecularly imprinted system can be successfully modified in order to generate flexible and physically tolerant molecularly imprinted thin films. Copyright © 2009 John Wiley & Sons, Ltd.     

Biosensor for the determination of sorbitol based on molecularly imprinted electrosynthesized polymers

Despite the increasing number of applications of biosensors in many fields, the construction of a steady biosensor remains still challenging. The high selectivity and stability of molecularly imprinted polymers for the template molecule make them ideal alternatives as recognition elements for sensors. In this work, the fabrication and characterization of biosensor based on molecularly imprinted electrosynthesized polymers is reported as the first case of imprinting sorbitol. A relevant molecularly imprinted film is prepared by o-phenylenediamine (o-PD) using the electrochemical method. Quartz crystal microbalance is employed as a sensitive apparatus of biosensor for the determination of sorbitol. An equation is deduced to characterize the interaction between molecularly imprinted films and the template. A linear relationship between the frequency shift and the concentration of analyte in the range of 1-15 mM was found. The detection limit is about 1mM.     

Oxytocin receptor mimetics prepared by molecular imprinting

Summary Oxytocin receptor mimetics were prepared by molecular imprinting using Z-oxytocin as the template. Comparative binding studies with reference polymers showed that the imprinted polymers recognized both Z-oxytocin and unprotected oxytocin selectively. The dissociation constants were 47 μM and 102 μM, respectively, and the density of binding sites was 12 μmol/g. The synthetic oxytocin receptors were easily prepared, possessed high mechanical and chemical stability, and were reused without loss of selectivity and capacity after regeneration by extraction. Abbreviations: B_max, number of binding sites; CLEAR, Cross-Linked Ethoxylate Acrylate Resin; EDMA, ethylene glycol dimethacrylate; FABMS, fast atom bombardment mass spectrometry; Fmoc, 9-fluorenylmethyloxycarbonyl; HPLC, high-performance liquid chromatography; K_D, dissociation constant; MAA, methacrylic acid; MIP, molecularly imprinted polymer; SPPS, solid-phase peptide synthesis; TRIM, trimethylolpropane trimethacrylate; Z, benzyloxycarbonyl. Abbreviations used for amino acids and the designation of peptides follow the rules of the IUPAC-IUB Commission of Biochemical Nomenclature [J. Biol. Chem., 247 (1972) 977–983]. All amino acids were of thel-configuration.     

Molecular recognition of indole derivatives by polymers imprinted with indole-3-acetic acid: A QSPR study

Three molecularly imprinted polymers (MIPs) were prepared using the phytohormone indole-3-acetic acid (IAA) as a template molecule, 4-vinylpyridine (MIP-1 and MIP-2) or N,N-dimethylaminoethyl methacrylate (MIP-3) as functional monomers, ethylenglycol dimethacrylate as a cross linker and acetonitrile (MIP-1), a methanol–water mixture (MIP-2) or chloroform (MIP-3) as porogens. Retention factors for IAA and 29 indole derivatives were determined by high-performance liquid chromatography, using the molecularly imprinted polymers as stationary phases and acetonitrile as an eluent. High correlations between selectivity factors of above mentioned polymers indicate that their retention mechanisms are basically the same. A quantitative structure–property relationships analysis revealed that the presence of the terminal carboxyl group on the 3-side chain plays an essential role in the binding of the indole derivatives to the polymers. The derivatives without the carboxyl group exhibit a drastically lower affinity toward the polymers. Another factor which favors the binding is electronic density of indole nucleus. Substituents with electro-withdrawing properties enhance the binding, while electro-donating substituents have the opposite effect. The length of the 3-side chain also affects the binding. Indole-3-carboxylic acid having the carboxyl group directly attached to the ring as well as the derivatives whose side chain is longer than that of IAA bind to the polymers with a lower affinity.     

Optimizing the formulation of a myoglobin molecularly imprinted thin-film polymer--formed using a micro-contact imprinting method

Thin-film myoglobin molecularly imprinted polymers have been fabricated using a micro-contact approach. By initially selecting the cross-linker on the basis of it having a minimal recognition for the template and using this as a starting point for functional monomer selection, we have produced myoglobin imprinted polymers with exceptionally high selectivities.

The affinity of the polymers, for myoglobin, when prepared with a variety of different cross-linkers and no functional monomer was evaluated. Of these, tetraethylene glycol dimethacrylate (TEGDMA) exhibited the lowest affinity for the template species. Methyl methacrylate (MMA) was chosen as the functional monomer as when it was used in conjunction with TEGDMA, it exhibited maximum selectivity for the template compared to polymers made with other functional monomers.

With a MMA to TEGDMA ratio of 1 to 3, the myoglobin molecularly imprinted polymer adsorbed 15.03 ± 0.89 × 10⁻¹¹ mole/cm² of template from a 5.68 × 10⁻⁷ M myoglobin solution, compared to 2.58 ± 0.02 × 10⁻¹¹ mole/cm² for a polymer of similar composition, but formed in the absence of a template. Various washing conditions, using alkaline media to remove the template, were investigated. An extraction solvent comprising 2 wt.% SDS and 0.6 wt.% NaOH used at 80 °C for 30 min was shown to give the highest imprinting factor i.e. 5.83 with 72.82% myoglobin removal.

The saturation kinetics of template binding to the thin-film MIP were examined and found to display a simple two-phase profile typical of non-cooperative binding. A Scatchard binding plot showed the dissociation constant (K_d) for the specific binding phase to be 3.4 × 10⁻⁷ M and the binding site capacity to be 7.24 × 10⁻¹¹ mole/cm². For the non-specific binding phase, K_d was found to be 1.355 × 10⁻⁵ M and the binding site capacity was determined as 9.62 × 10⁻¹⁰ mole/cm².

Selectivity experiments were carried out in both single protein and binary protein systems all using a total protein concentration of 5.68 × 10⁻⁷ M. The molar ratio of adsorbed myoglobin to IgG, HSA and hemoglobin was found to 115.5, 230.9 and 2.5, respectively. While, in binary competition systems, myoglobin selectivity to IgG, HSA and hemoglobin was, respectively, 94.18, 98.21 and 61.09%. Rebinding in natural biological matrices, i.e. human serum or urine, showed the imprinted films to have significantly greater uptake than non-imprinted films. Re-binding in undiluted urine was found to be a facile process, with the imprinting factor, i.e. the ratio of MIP to NIP binding, being determined as 37.4.     

Stoichiometric noncovalent interaction in molecular imprinting

In this review article the function of the binding site monomers in the molecular imprinting procedure is discussed. Especially, new developments towards stoichiometric noncovalent interactions are highlighted. In stoichiometric noncovalent interactions template and binding site monomer in an 1:1 molar ratio are nearly completely bound to each other. This is only possible if the association constants are considerably high (K _ass < 900 M^–1). Using this type of interaction in molecular imprinting no excess of binding sites is necessary and binding sites are only located inside the imprinted cavity. Since all cavities can be reloaded these polymers show high capacity (e.g., for preparative application) and are especially suited for the synthesis of catalytically active imprinted polymers. Discussed are binding site interactions based on amidines (and guanidines), multiple hydrogen bonding, charge-transfer interactions, and host–guest inclusion. The systematic investigation of the underlying binding reaction is described in detail. With low-molecular weight model substances the thermodynamics of the association can be conveniently investigated, e.g., by NMR spectroscopy.     

Face-to-face porphyrin moieties assembled with spacing for pyrazine recognition in molecularly imprinted polymers

A strategy for arranging two porphyrin moieties in a face-to-face fashion in polymeric material was demonstrated by molecular imprinting, whereby porphyrin Zn(II) complex monomers were cross-linked with ethylene glycol dimethacrylate in the presence of pyrazine or 1,5-naphthyridine as a template molecule. In chromatographic studies using the resultant imprinted polymers as stationary phase, both the polymers showed selectivity for the original template molecule, suggesting that two zinc porphyrin moieties were immobilized in the face-to-face fashion, and were center-aligned for pyrazine recognition and offset-arranged for 1,5-naphthyridine recognition. The imprinted polymer with porphyrin moieties also showed a decrease in its fluorescence intensity in response to the concentration of the target molecule, suggesting the potential utility as sensing material.     

Design and synthesis of molecularly imprinted polypyrrole based on nanoreactor SBA-15 for recognition of ascorbic acid

Molecular imprinting is an attractive technique for preparing mimics of natural and biological receptors. Nevertheless, molecular imprinting for aqueous systems remains a challenge due to the hydrogen bonding between templates and functional monomers destroyed in the bulk water. The hydrogen bonding between templates and monomers are the most crucial factor governing recognition, particularly in non-covalent molecularly imprinted polymers. Using mesoporous materials for molecular imprinting is an effective approach to overcome this barrier and to remove the limitations of the traditional molecularly imprinted polymers which include incomplete template removal, small binding capacity, slow mass transfer, and irregular materials shape. Here, SBA-15 was used as a mesoporous silica material for synthesis of molecularly imprinted polypyrrole. The pyrrole monomers and template molecules were immobilized onto the SBA-15 hexagonal channels, and then polymerization occurred. The resulting nanocomposites were characterized by Fourier transform infrared (FT-IR) analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. In batch rebinding tests, the imprinted nanocomposites reached saturated adsorption within 100min and exhibited significant specific recognition toward the ascorbic acid (AA) with high adsorption capacity (83.7mgg(-1)). To further illustrate the recognition property of the imprinted nanocomposites, binary competitive and non-competitive adsorption experiments were performed with ascorbic acid, dopamine, paracetamol and epinephrine. The imprinting factors for these compounds in non-competitive adsorption experiments were 3.2, 1.5, 1.4 and 1.3, respectively. The results showed that the imprinted nanocomposites exhibited significant adsorption selectivity for the ascorbic acid against the related compounds.     

Probing the limits of molecular imprinting: strategies with a template of limited size and functionality

A series of polymers molecularly imprinted with the general anaesthetic propofol were synthesized using both semi- and non-covalent approaches. The polymers were evaluated with respect to template rebinding in both aqueous and organic media. In aqueous media, the observed propofol binding in these polymer systems was largely hydrophobic and non-specific in nature. In non-polar solvents such as hexane, electrostatic (hydrogen bonding) interactions dominate resulting in some selectivity. The implication of these results, in conjunction with those obtained using structures of similar size in other studies, is that propofol, a template possessing limited functionality and size, appears to define the lower limit for template size and degree of functionalization that can be used for the creation of ligand-selective recognition sites in molecularly imprinted polymers. Furthermore, studies with alternative ligands indicate that the steric crowding of a ligand's functionality to the polymer contributes to the extent of polymer-ligand recognition.     

Altered product specificity of a cyclodextrin glycosyltransferase by molecular imprinting with cyclomaltododecaose

Cyclodextrin glycosyltransferases (CGTases), members of glycoside hydrolase family 13, catalyze the conversion of amylose to cyclodextrins (CDs), circular α‐(1,4)‐linked glucopyranose oligosaccharides of different ring sizes. The CD containing 12 α‐D‐glucopyranose residues was preferentially synthesized by molecular imprinting of CGTase from Paenibacillus sp. A11 with cyclomaltododecaose (CD₁₂) as the template molecule. The imprinted CGTase was stabilized by cross‐linking of the derivatized protein. A high proportion of CD₁₂ and larger CDs was obtained with the imprinted enzyme in an aqueous medium. The molecular imprinted CGTase showed an increased catalytic efficiency of the CD₁₂‐forming cyclization reaction, while decreased k_cat/K_m values of the reverse ring‐opening reaction were observed. The maximum yield of CD₁₂ was obtained when the imprinted CGTase was reacted with amylose at 40°C for 30 min. Molecular imprinting proved to be an effective means toward increase in the yield of large‐ring CDs of a specific size in the biocatalytic production of these interesting novel host compounds for molecular encapsulations. Copyright © 2010 John Wiley & Sons, Ltd.     

Enzyme-based molecular imprinting with metals

Aromatic monomers with various functional groups were utilized in horseradish peroxidase-catalyzed polymerization reactions with metal ions Cu(II), Ni(II), and Fe(III) as imprinting templates. The approach described combines molecular imprinting with enzymatic free radical coupling. Selectivity in metal ion affinity between the various polymer products was assessed and found to depend on the metal used in the imprinting process using aniline, tyramine, and phenol as monomers. Selectivity in binding metals was found when polymers imprinted with copper, nickel, or iron were screened against the three metals, with preference for the metal used in the imprinting step. A model for the structural features of the putative imprinted polymers is proposed based on electron paramagnetic resonance, NMR, and IR analysis. Specific potential benefits to this imprinting method include reactivity with a wide range of aromatic monomers to provide more diverse options for molecular recognition with the target analyte and thus polymer products with higher selectivity, mild reaction conditions for the enzyme polymerization step to enable imprinting against labile substrates, imprinted polymeric products that contain conjugated backbones that could be suitable for electronics-based biosensor applications, and a potential for combinatorial selection to further enhance specificity.