Synthesis of molecularly imprinted polymers using a functionalized initiator for chiral-selective recognition of propranolol

We present a new concept of synthesis for preparation of molecularly imprinted polymers using a functionalized initiator to replace the traditional functional monomer. Using propranolol as a model template, a carboxyl-functionalized radical initiator was demonstrated to lead to high-selectivity polymer particles prepared in a standard precipitation polymerization system. When a single enantiomer of propranolol was used as template, the imprinted polymer particles exhibited clear chiral selectivity in an equilibrium binding experiment. Unlike the previous molecular imprinting systems where the active free radicals can be distant from the template-functional monomer complex, the method reported in this work makes sure that the actual radical polymerization takes place in the vicinity of the template-associated functional groups. The success of using functional initiator to synthesize molecularly imprinted polymers brings in new possibilities to improve the functional performance of molecularly imprinted synthetic receptors.     

Binding site characteristics of 17beta-estradiol imprinted polymers

The variety of applications utilizing molecularly imprinted polymers (MIPs) requires synthetic strategies yielding different MIP formats including films, irregular particles, or spheres, along with precise knowledge on the specific material characteristics, such as binding capacity and binding efficiency of these materials. In response to this demand, MIPs are prepared in different formats by variation of the polymerization methodology. It is commonly agreed that micro- and sub-microspheres are particularly advantageous MIP formats, due to their monodispersity and facile synthesis procedures in contrast to conventional imprinted polymers prepared by bulk polymerization. However, the differences in actual rebinding characteristics of different MIP formats based on molecular interactions under a variety of binding/rebinding conditions have not been studied in detail to date. Consequently, the present work details an analytical strategy generically applicable to MIP systems for rebinding studies including equilibrium binding, non-equilibrium binding, and release experiments enabling more profound understanding on the molecular interactions between the imprinted materials and the template molecules. In this study, three MIP formats were considered for the same template molecule, 17beta-estradiol: irregularly shaped particulate polymers prepared by bulk polymerization and grinding, microspheres, and sub-microspheres. The latter two formats were synthesized via precipitation polymerization using different processing strategies. The morphologies and porosities of the resulting imprinted materials were characterized by scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis, respectively. The obtained results indicate that microspheres prepared by precipitation polymerization provide superior rebinding properties during equilibrium binding in contrast to bulk polymers and sub-microspheres, and that the rebinding properties are different during equilibrium binding versus non-equilibrium binding. The median binding affinity constant determined during non-equilibrium rebinding is higher than the values obtained from equilibrium rebinding. Furthermore, the binding site distribution appears more homogeneous thief derived from non-equilibrium rebinding, as reflected in a heterogeneity index of m=0.725. Moreover, it is hypothesized that the specific interactions between template and monomers are related to the porosity of the imprinted polymers, which implies that the amount of binding sites and the pore sized distribution of the imprinted materials are a critical factor in achieving the desired MIP performance in various analytical applications. The BET results indicate that particles prepared with lower cross-linker-to-template ratio have a reduced surface area. Furthermore, it can be expected that there are less specific binding sites available at particles with reduced surface area and pore volume given similar distribution of the binding sites, as confirmed by the equilibrium binding isotherm studies. The pore size distribution results reveal that control of the pore size in the range of 100-180 A is essential to obtain the desired retention properties and Gaussian peak shape during HPLC analysis of small molecules.     

Molecularly imprinted polymer films for reflectometric interference spectroscopic sensors

Reflectometric interference spectroscopic measurements were performed on molecularly imprinted polymer (MIP) films with the herbicide atrazine as the template molecule. A conventional imprinting protocol was used relying on non-covalent interactions between the functional monomers and the template. The MIPs were deposited on glass transducers by two different methods: spin-coating followed by in situ polymerization of thin films of monomers containing a sacrificial polymeric porogen, and autoassembly of MIP nanoparticles with the aid of an associative linear polymer. Reproducible results were obtained upon measurements of atrazine solutions in toluene with both films. Atrazine concentrations as low as 1.7 ppm could be detected with the autoassembled particle film. No or very little analyte adsorption was observed onto non-imprinted control films made by spin-coating and by particle assembly, respectively. We believe that these MIP layers in combination with the general reflectrometric transduction scheme could be a versatile sensing tool for the detection of environmentally important and other analytes.     

A brief review of coarse-grained and other computational studies of molecularly imprinted polymers

Molecular imprinting is an established method for the creation of artificial recognition sites in synthetic materials through polymerization and cross-linking in the presence of template molecules. Removal of the templates leaves cavities that are complementary to the template molecules in size, shape, and functionality. In recent years, various theoretical and computational models have been developed as tools to aid in the design of molecularly imprinted polymers (MIPs) or to provide insight into the features that determine MIP performance. These studies can be grouped into two general approaches-screening for possible functional monomers for particular templates and macromolecular models focusing on the structural characterization of the imprinted material. In this review, we pay special attention to coarse-grained models that characterize the functional heterogeneity in imprinted pores, but also cover recent advances in atomistic and first principle studies. We offer a critical assessment of the potential impact of the various models towards improving the state-of-the-art of molecular imprinting.     

Direct detection of analyte binding to single molecularly imprinted polymer particles by confocal Raman spectroscopy

We describe the use of Raman spectroscopy to detect and quantify, for the first time, the presence of the imprinting template in single molecularly imprinted polymer microspheres. The polymers were imprinted with the β-blocking drugs propranolol and atenolol, and precipitation polymerization was used to obtain spherical particles of diameters of 200 nm and 1.5 μm. The size of the Raman laser spot being between 1 μm and a few μm, the nanoparticles were used for bulk detection whereas with micrometer-sized particles, quantitative measurements on single particles were possible. The laser power, and consequently the acquisition times, needed to be adapted as a function of the polymer and template used in order to avoid burning. Analyte quantification from Raman spectra is straightforward by determining the peak height of a typical Raman band of the analyte, and by using a typical polymer peak for normalization. Relatively low detection limits down to 1 μM have been reached for the detection of S-propranolol through bulk measurements on MIP nanoparticles.     

Molecularly imprinted polymer formats for capillary electrochromatography

The research aimed towards the adaptation of molecularly imprinted polymers (MIPs) to the capillary format and the use of these highly selective matrices for capillary electrochromatography (CEC) is reviewed in this article. The MIP is prepared by incorporation of a template molecule into a polymerization protocol. After polymerization and extraction of the template from the resulting polymer a highly selective material with recognition cavities complementary to the template in size, shape and chemical functionality is obtained. MIPs have been used as recognition elements in several different analytical techniques. In combination with CEC a novel separation system with a unique selectivity towards a predetermined target (the template) is achieved. The merge of molecular imprinting technology (MIT) and CEC have introduced several interesting polymer formats, due to the adaptation of the MIP to the miniaturized capillary format. The polymer formats can be classified according to their preparation protocols and appearance into three conceptually different categories, i.e. the monolith, the coating and the nanoparticles. The preparation protocols, characteristics and applications of these formats will be discussed.     

In situ synthesis of molecularly imprinted nanoparticles in porous support membranes using high-viscosity polymerization solvents

There is a growing need in membrane separations for novel membrane materials providing selective retention. Molecularly imprinted polymers (MIPs) are promising candidates for membrane functionalization. In this work, a novel approach is described to prepare composite membrane adsorbers incorporating molecularly imprinted microparticles or nanoparticles into commercially available macroporous filtration membranes. The polymerization is carried out in highly viscous polymerization solvents, and the particles are formed in situ in the pores of the support membrane. MIP particle composite membranes selective for terbutylazine were prepared and characterized by scanning electron microscopy and N? porosimetry. By varying the polymerization solvent microparticles or nanoparticles with diameters ranging from several hundred nanometers to 1 μm could be embedded into the support. The permeability of the membranes was in the range of 1000 to 20,000 Lm?2 hr?1 bar?1. The imprinted composite membranes showed high MIP/NIP (nonimprinted polymer) selectivity for the template in organic media both in equilibrium-rebinding measurements and in filtration experiments. The solid phase extraction of a mixture of the template, its analogs, and a nonrelated compound demonstrated MIP/NIP selectivity and substance selectivity of the new molecularly imprinted membrane. The synthesis technique offers a potential for the cost-effective production of selective membrane adsorbers with high capacity and high throughput.     

Insight into molecular imprinting in precipitation polymerization systems using solution NMR and dynamic light scattering

Molecular imprinting is a powerful synthetic technique for generating template-defined binding sites in cross-linked polymers. One scientific challenge in molecular imprinting research is to understand the intermolecular interactions leading to molecular complexation and the process of binding site formation during polymerization. In this work, we present a novel method for studying the molecular imprinting process in precipitation polymerization systems. This method employs solution (1) H NMR and dynamic light scattering (DLS) to investigate the association of template molecules with colloidal particles and the dynamic process of particle growth. Under precipitation polymerization conditions, the colloidal particles formed did not interfere with NMR signals from the soluble components, allowing unreacted monomers and free template to be easily quantified. To examine the process of particle nucleation and growth, DLS was used to measure the hydrodynamic particle size at different reaction times. To corroborate the interpretation of the NMR and DLS results, imprinted nanoparticles were collected at different reaction times and their binding characteristics were evaluated using radioligand-binding analysis. Our experimental results provide new insights into the molecular imprinting process that will be useful in the development of new imprinted nanoparticles.     

Substitution of antibodies and receptors with molecularly imprinted polymers in enzyme-linked and fluorescent assays

A new technique for coating microtitre plates with molecularly imprinted polymers (MIP), specific for low-molecular weight analytes (epinephrine, atrazine) and proteins is presented. Oxidative polymerization was performed in the presence of template; monomers: 3-aminophenylboronic acid (APBA), 3-thiopheneboronic acid (TBA) and aniline were polymerized in water and the polymers were grafted onto the polystyrene surface of the microplates. It was found that this process results in the creation of synthetic materials with antibody-like binding properties. It was shown that the MIP-coated microplates are particularly useful for assay development. The high stability of the polymers and good reproducibility of the measurements make MIP coating an attractive alternative to conventional antibodies or receptors used in enzyme linked immunosorbent assay (ELISA).     

Rational design and synthesis of molecularly imprinted polymers (MIP) for purifying tylosin by seeded precipitation polymerization

A useful approach was proposed to easily synthesize molecularly imprinted adsorbent for the purification of tylosin from broth. Firstly, by molecular simulation based on density functional theory, methacrylic acid was chosen as a functional monomer by comparing the binding energy. Second, a novel method of polymerization based on precipitation polymerization with added seeds was used in water-mixed solvent for the preparation of water-compatible micron-sized MIP. Its static adsorption capacity for tylosin in aqueous solution was estimated to be 106.5 mg/g with the highest imprinting factor (IF) of 3.6. The selectivity coefficient (α) of tylosin to desmycosin was 3.3. The antibiotic in fermentation broth could be purified by means of molecularly imprinted solid phase extraction (MISPE), which allows MIP to be used for the purification of tylosin from a complex sample.     

Selective molecular adsorption using electrospun nanofiber affinity membranes

Molecularly imprinted nanoparticles were encapsulated into polymer nanofibers with a simple electrospinning method. The composite nanofibers form non-woven mats that can be used as affinity membrane to greatly simplify solid phase extraction of drug residues in analytical samples. Upward 100% of propranolol-imprinted nanoparticles can be easily encapsulated into poly(ethylene terephthalate) nanofibers, ensuring the composite materials to have a high specific binding capacity. As confirmed by radioligand binding analysis, the specific binding sites in the composite materials remain easily accessible and are chiral-selective. Using the new composite nanofiber mats as solid phase extraction materials, trace amount of propranolol (1 ng mL(-1)) in tap water can be easily detected after a simple sample preparation. As validated in this study, there is no problem of template leakage from the composite nanofibers. Without the solid phase extraction, the existence of propranolol residues in water cannot be confirmed with even tandem HPLC-MS/MS analysis.     

Synthesis and evaluation of uniformly sized nalidixic acid-imprinted nanospheres based on precipitation polymerization method for analytical and biomedical applications

For the first time in this work, uniform molecularly imprinted polymer (MIP) nanoparticles were prepared using nalidixic acid as a template. The MIP nanoparticles were successfully synthesized by precipitation polymerization applying methacrylic acid (MAA) as a functional monomer and trimethylolpropane trimethacrylate (TRIM) as a cross-linking monomer at different mole ratios. The morphology, binding, recognition, selectivity, and in vitro release behaviors of obtained particles were studied. The produced polymers were characterized by Fourier transform infrared spectroscopy and differential scanning calorimetric. Furthermore, their morphology was analyzed accurately by scanning electron microscopy, photon correlation spectroscopy, and Brunauer-Emmett-Teller analysis. The nanospheres and microspheres with mean diameter values of 94 nm, 256 nm, and 1.2 μm were obtained using nalidixic acid-MAA-TRIM various mole ratios. Among the MIPs, the product with nalidixic acid-MAA-TRIM mole ratio of 1:12:12 established nanospheres with the lowest polydispersity index (0.003), an average pore diameter (12 nm), and the highest specific surface area (280 m(2) g(-1)) and selectivity factor (10.4). Results from binding experiments demonstrated that the imprinted nanospheres with a 94-nm mean diameter and a binding capacity of 28 mg of nalidixic acid per gram of polymer had higher specific affinity to nalidixic acid in contrast with the other imprinted nanospheres, microspheres, and nonimprinted particles. However, the binding performance of imprinted nanospheres in human serum was estimated using high-performance liquid chromatography analysis (binding approximately 98% of nalidixic acid). In addition, release experiments proved to be successful in the controlled release of nalidixic acid during a long period. The 20% of loaded nalidixic acid was released from the imprinted nanospheres within the first 20 h, whereas the remaining 80% was released in the after 120 h. The nalidixic acid release kinetics from the MIPs was highly affected by properties of the particles.     

Development and Characterization of Propranolol Selective Molecular Imprinted Polymer Composite Electrospun Nanofiber Membrane

Propranolol (PPL) imprinted microspheres (MIP) were successfully prepared via oil/water polymerization using a methyl methacrylate (MMA) monomer, PLL template, and divinylbenzene (DVB) cross-linker and favorably incorporated in a Eudragit-RS100 nanofiber membrane. A non-PPL imprinted polymer (NIP), without a template, was used as a control. The morphology and particle size of the beads were investigated using scanning electron microscopy. The results revealed that both MIP and NIP had a spherical shape with a micron size of approximately 50–100 μm depending on the amounts of DVB and PPL used. NIP2 (MMA/DVB, 75:2.5) and MIP8 (PPL/MMA/DVB, 0.8:75:2.5) were selected for reloading of PPL, and the result indicated that increasing the ratio of PPL to polymer beads resulted in increase PPL reloading (>80%). A total of 10–50% NIP2 or MIP8 was incorporated into a 40% (w/v) Eudragit-RS100 fiber membrane using an electrospinning technique. PPL could be bound to the 50% MIP8 composite fiber membrane with a higher extent and at a higher rate than the control (NIP2). Furthermore, the MIP8 composite fiber membrane showed higher selectivity to PPL than the other β-blockers (atenolol, metoprolol, and timolol). Thus, the MIP8 composite fiber membrane can be further developed for various applications in pharmaceutical and other affinity separation fields.Key words: membrane, molecularly imprinted, propranolol, selective molecular imprinting     

Synthesis and evaluation of a selective molecularly imprinted polymer for the contraceptive drug levonorgestrel

A molecularly imprinted polymer (MIP) has been prepared using levonorgestrel (LEV) as template. The polymer was synthesised in a non-covalent approach using methacrylic acid (MAA) as functional monomer and ethylene glycol dimethacrylate (EGDMA) as cross-linking monomer via a free radical polymerization. An equivalent blank polymer was also synthesised in the absence of the template compound. Batch adsorption experiments were used to evaluate the binding affinity of the imprinted polymer. After packing MIP into a stainless steel column (150 mm x 4.6 mm i.d.), retention and elution of the template and related compounds were evaluated by high-performance liquid chromatography (HPLC). This LEV imprinted polymer was further applied for selective solid phase extraction (SPE) of LEV from human serum. It was confirmed that the binding ability of the prepared MIP for LEV was essentially sufficient in the presence of other compounds coexisting in serum sample. Therefore, as a selective and efficient solid phase material, LEV imprinted polymer has a high potential application in analysis of this steroidal hormone in clinical purposes.     

Reading Biochips by Raman and Surface-Enhanced Raman Spectroscopies

Biochips are a rapidly increasing research field, driven by the versatility of sensing devices and the importance of their applications. The regular approaches for creating biochips and for reading them suffer from some limitations, motivating development of miniature biochips and label-free formats. To push forward these challenges, we have chosen to combine the methods of printing of droplets of synthetic receptors by pipettes or nanofountain pens with detection by Raman spectroscopy or its surface-assisted plasmon variant, namely, surface-enhanced Raman spectroscopy (SERS). The selected receptors included molecularly imprinted polymers (MIPs), produced by polymerization of functional and cross-linking monomers around a molecular template, the β-blocking drug propranolol. The measured Raman and SERS spectra of the MIP constituents enabled identification of the template presence and consequently chemical imaging of individual and multiple dots in an array. This concept, combining nanolithography techniques with SERS paves the road toward miniaturized arrayed MIP sensors with label-free, specific and quantitative molecular recognition.     

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.     

Detailed positron annihilation lifetime spectroscopic investigation of atrazine imprinted polymers grafted onto PE/PP non‐woven fabrics

This study presents the preparation of molecularly imprinted matrices by using radiation‐induced grafting technique onto polyethylene/polypropylene (PE/PP) non‐woven fabrics. Atrazine imprinted polymers were grafted onto PE/PP non‐woven fabrics through the use of methacrylic acid (MAA) and ethylene glycol dimethylacrylate (EGDMA) as the functional monomer and crosslinking agent, respectively. Grafted MIPs were characterized by attenuated total reflectance Fourier transform infra‐red spectroscopy (ATR‐FTIR), X‐ray photoelectron spectroscopy (XPS), elemental analysis, scanning electron microscopy (SEM), and positron annihilation lifetime spectroscopy (PALS). The average diameter of free volume holes was determined as 0.612 nm which correlates very well with the size of template molecule atrazine, 0.512 nm. Binding behaviors were investigated against various factors, such as concentration of template molecule, pH, and contact time. Furthermore, the specific selectivity of grafted MIP on non‐woven fabric was studied by using other common triazine compounds, such as simazine and metribuzine which show structural similarities to atrazine. The specific binding values for atrazine, simazine, and metribuzine were determined as 40%, 2.5%, and 1.5%, respectively.     

Efficient one-pot synthesis of molecularly imprinted silica nanospheres embedded carbon dots for fluorescent dopamine optosensing

A new type of eco-friendly molecularly imprinted polymer (MIP) was synthesized through an efficient one-pot room-temperature sol-gel polymerization and applied as a molecular recognition element to construct dopamine (DA) fluorescence (FL) optosensor. Highly luminescent carbon dots (CDs) were firstly synthesized via a one-step reaction in organosilane, and their surface were anchored with MIP matrix (CDs@MIP). The resulting composite of a synergetic combination of CDs with MIP showed high photostability and template selectivity. Moreover, the composite allowed a highly sensitive determination of DA via FL intensity decreasing when removal of the original templates. The new MIP-based DA sensing protocol was applied to detect DA concentration in aqueous solution, the relative FL intensity of CDs@MIP decreased linearly with the increasing DA in the concentration range of 25-500nM with a detection limit (3σ) of 1.7nM. Furthermore, the proposed method was successfully intended for the determination of trace DA in human urine samples without the interference of other molecules and ions.     

Molecularly imprinted monolithic stationary phases for liquid chromatographic separation of tryptophan andN-CBZ-phenylalanine enantiomers

Monolithic molecularly imprinted columns were designed and prepared by anin-situ thermal-initiated copolymerization technique for rapid separation of tryptophan andN-CBZ-phenylalanine enantiomers. The influence of polymerization conditions and separation conditions on the specific molecular recognition ability for enantiomers and diastereomers was investigated. The specious molecular recognition was found to be dependent on the stereo structures and the arrangement of functional groups of the imprinted molecule and the cavities in the molecularly imprinted polymer (MIP). Moreover, hydrogen bonding interactions and hydrophobic interactions played an important role in the retention and separation. Compared to conventional MIP preparation procedures, the present method is very simple, and its macroporous structure has excellent separation properties.     

Surface molecular imprinting by atom transfer radical polymerization

Results are presented that demonstrate the successful preparation of ultrathin (< 10 nm), surface-confined, molecularly imprinted polymer (MIP) films on model gold substrates using atom transfer radical polymerization (ATRP). 2-Vinylpyridine (2Vpy) was investigated as the functional monomer, and ethylene glycol dimethacrylate (EGDMA) was the cross-linking monomer. Fluorescently labeled N,N'-didansyl-L-cystine and N,N'-didansyl-L-lysine were used as the template molecules to form the MIPs. Spectroscopic and ellipsometric results are presented that follow film formation and growth rates. Results are also presented from fluorescence experiments used to quantify and compare the adsorption capacities of MIP surface films and nonimprinted (NIP) control films. MIP films exhibited higher binding capacities than the control NIP films at all solution concentrations of N,N'-didansyl-L-cystine and N,N'-didansyl-L-lysine. Furthermore, template removal from these imprinted films appears to be 100% efficient. Selectivity studies showed that the MIPs display some cross-reactivity between these two molecules; nevertheless, MIPs prepared against one template showed selectivity for that template. A selectivity coefficient of 1.13 was achieved for MIP surfaces prepared against N,N'-didansyl-L-lysine; a value of 1.51 was observed for MIP surfaces prepared against N,N'-didansyl-L-cystine.