Gels and microgels for nanotechnological applications

In recent years, "smart" materials have been the focus of considerable interest, from both fundamental and applied perspectives. Polymer gels are within this category; they respond to specific environmental stimuli by changing their size. Thus, the internal structure, the refractive index, and the mechanical properties of the polymer network change. They are considered super absorbent materials, as they can absorb solvent up to several hundred times their own weight. They respond rapidly to local environmental variations, an important fact in device miniaturization and microsensor developments. As size changes are accompanied by changes in internal dimensions, microgels have found application as carriers of therapeutic drugs and as diagnostic agents. They have also been used as microreactors, optically active materials, for template synthesis of nanoparticles or fabrication of artificial muscle. In this paper we review a set of application based on the special features associated to this systems. Basic concepts on the physical-chemistry of gel swelling is first described, followed by different applications covering drug delivery, composite materials using polymer gels to modulate optical or magnetic and electrical properties, molecular imprinting, gel-based biosensors and polymer sensors and actuators used in the field of artificial muscles.

Molecularly Imprinted Polymers and Controlled/Living Radical Polymerization

Molecularly imprinted polymers (MIPs) are tailor-made biomimetic receptors that are obtained by polymerization in the presence of molecular templates. They contain binding sites for target molecules with affinities and specificities on a par with those of natural receptors such as antibodies, hormone receptors, or enzymes. A great majority of the literature in the field describes materials based on polymers obtained by free radical polymerization. In order to solve general problems associated with MIPs, in particular their heterogeneity in terms of inner morphology and distribution of binding site affinities, it has been suggested to use modern methods of controlled/living radical polymerization for their synthesis. This also facilitates their generation in the form of nanomaterials, nanocomposites, and thin films, a strong recent trend in the field. The present paper reviews recent advances in the molecular imprinting area, with special emphasis on the use of controlled polymerization methods, their benefits, and current limitations.

Monoclonal behavior of molecularly imprinted polymer nanoparticles in capillary electrochromatography

A new approach based on miniemulsion polymerization is demonstrated for synthesis of molecularly imprinted nanoparticles (MIP-NP; 30-150 nm) with "monoclonal" binding behavior. The performance of the MIP nanoparticles is characterized with partial filling capillary electrochromatography, for the analysis of rac-propranolol, where (S)-propranolol is used as a template. In contrast to previous HPLC and CEC methods based on the use of MIPs, there is no apparent tailing for the enantiomer peaks, and baseline separation with 25,000-60,000 plate number is achieved. These effects are attributed to reduction of the MIP site heterogeneity by means of peripheral location of the core cross-linked NP and to MIP-binding sites with the same ordered radial orientation. This new MIP approach is based on the substitution of the functional monomers with a surfactant monomer, sodium N-undecenoyl glycinate (SUG) for improved inclusion in the MIP-NP structure and to the use of a miniemulsion in the MIP-NP synthesis. The feasibility of working primarily with aqueous electrolytes (10 mM phosphate with a 20% acetonitrile at pH 7) is attributable to the micellar character of the MIP-NPs, provided by the inclusion of the SUG monomers in the structure. To our knowledge this is the first example of "monoclonal" MIP-NPs incorporated in CEC separations of drug enantiomers.

In situ synthesis of molecularly imprinted polymers on glass microspheres in a column

A facile method to fabricate molecularly imprinted polymers (MIPs) on glass microspheres in a column was developed. The column was prepacked with glass microspheres, and then the prepolymerization mixture was injected into the interstitial volume of the column. The polymerization took place in situ and the column could be directly used for high-performance liquid chromatography after the template had been removed. The template consumption was reduced greatly because the prepolymerization mixture just filled the interstitial volume between the glass microspheres in the column. The MIPs obtained exhibited better kinetic properties, higher efficiency, and low back pressure of the column. Emodin imprinted polymers were prepared by this method and were used for solid-phase extraction.

Recent advances on noncovalent molecular imprints for affinity separations

Molecularly imprinted polymers (MIPs) are tailor-made synthetic materials capable of selectively rebinding a target analyte, or a group of structurally related compounds based on a combination of recognition mechanisms including size, shape, and functionality. Among the advantageous properties of MIPs are the achievable specific affinity, the relative ease of preparation, and their mechanical and chemical robustness, which renders them ideal materials for applications as stationary phase (e. g., affinity chromatography or SPE), or as antibody mimics (e. g., biomimetic assays). Here, we review recent advancements on the application of MIPs in affinity separations and biomimetic assays, which have focused on the synthesis of size- and shape-uniform particles facilitating reproducibility, improved binding site accessibility, and enhanced affinity. While MIPs certainly offer promising potential as selective separation phase in a variety of applications, deeper understanding of the fundamental interactions governing imprinting, and rational understanding of the imprinting mechanism has yet to be achieved for providing rational guidelines in deliberately designing next-generation MIP materials.

Molecularly imprinted beads by surface imprinting

Molecular imprinting is a state-of-the-art technique for imparting molecular recognition properties to a synthetic polymeric matrix. Conventionally, the technique is easily carried out using bulk imprinting, where molecularly imprinted polymers (MIPs) are prepared in large chunks and post-treatment processes like grinding and sieving are then required. However, this strategy tends to produce sharp-edged, irregular MIP bits with a limited scope of direct application. In addition, due to the creation of binding sites within the polymeric bulk, the issue of the hindrance of adsorbate diffusion (especially in the case of macromolecules) during template rebinding makes the MIPs prepared through this approach unsuitable for practical applications. Thus over the years, many efforts to address the limitations of conventional molecular imprinting techniques have resulted in new imprinting methodologies. Systems like suspension and precipitation polymerization, where MIPs with tunable morphologies can be prepared, have been developed. Additionally, strategies like surface imprinting have also been employed. Ultimately, both of these approaches have been combined to prepare regularly shaped surface-imprinted MIP beads. Such an approach incorporates the advantages of both methodologies at the same time. Given their desirable physical morphologies and favorable adsorption kinetics, MIPs prepared in this manner show significant promise for industrial applications. Therefore, they will be the main focus of this review.

Molecularly imprinted microspheres and nanospheres for di(2-ethylhexyl)phthalate prepared by precipitation polymerization

Molecularly imprinted microspheres (MIMs, >3 microm) and nanospheres (MINs, approximately 450 nm) for the environmental endocrine disruptor di(2-ethylhexyl)phthalate (DEHP) were prepared by a precipitation polymerization (PP) procedure. The effect of the dispersive solvents acetonitrile (ACN) and cyclohexane (CH), the cross-linkers ethylene glycol dimethacrylate (EDMA) and trimethylpropane trimethacrylate (TRIM), and the template on particle size and morphology of polymers was investigated in detail by scanning electron microscopy (SEM) and BET adsorption isotherm determination. When used as HPLC stationary phase, the microspheres exhibited strong affinity for the template DEHP with an imprint factor (IF) higher than 8.0 in ACN/water (60:40, v/v) as mobile phase. Furthermore, baseline separation of DEHP from benzyl butyl phthalate (BBP) and dibutyl phthalate (DBP) could be achieved. In contrast, no or only poor separation could be observed with non-imprinted polymeric polymers (NIPs) or imprinted bulk polymers (MIB), respectively. Similarly, the obtained MINs exhibited an imprinting effect in pure ACN, i.e. the bond amount of DEHP was significantly higher compared to NIPs, as was shown in rebinding experiments. Besides their use as an HPLC stationary phase, MIMs might further be applicable for SPE sample cleanup, while MINs could be used as a recognition layer on sensor surfaces.

Molecularly imprinted polymer particles: synthetic receptors for future medicine

Molecular imprinting is a relatively new and rapidly evolving technique used to create synthetic receptors; it also possesses great potential in a number of applications in the life sciences. Traditionally, molecularly imprinted polymers are prepared by bulk polymerization, followed by crushing and sieving to obtain polymer beads. However, several methods can be used to synthesize polymer micro- and nano-particles directly, thereby avoiding the time- and labor-consuming process of crush sieving. Possible applications are foreseen in enhanced drug loading, controlled drug delivery and drug targeting. This review describes the different methods of synthesis of molecularly imprinted micro- and nano-particles and discusses how these methods challenge the outstanding issues that molecular imprinting is facing today, thereby facilitating biomedical applications in the future.

Encapsulation and selective recognition of molecularly imprinted theophylline and 17beta-estradiol nanoparticles within electrospun polymer nanofibers

Molecularly imprinted nanoparticles are cross-linked polymer colloids containing tailor-made molecular recognition sites. In this study, molecularly imprinted nanoparticles were easily encapsulated within polymer nanofibers using an electrospinning technique to produce a new type of molecular recognition material. Poly(ethylene terephthalate) (PET) was used as the supporting nanofibers matrix to encapsulate theophylline and 17beta-estradiol imprinted nanoparticles. The composite nanofibers had an average diameter of 150-300 nm, depending on the content of molecularly imprinted nanoparticles. For the theophylline and 17beta-estradiol imprinted polymers, an optimal loading of molecularly imprinted nanoparticles was 25-37.5 wt % based on PET. The composite nanofibers prepared under these conditions had a well-defined morphology and displayed the best selective target recognition. Our approach of electrospinning-for-molecularly imprinted nanoparticles-encapsulation has unique advantages and opens new application opportunities for molecularly imprinted nanoparticles and electrospun nanofibers.

Supported imprinted nanospheres for the selective recognition of cholesterol

The preparation of innovative polymeric systems using molecular imprinting technology for application in extracorporeal blood purification is described. Membranes based on a methylmethacrylate-co-acrylic acid copolymer, produced through the phase inversion method, were modified introducing into their structure specific binding sites for cholesterol molecule by adding molecularly imprinted nanoparticles in the membrane matrix. Membranes prepared are intended to selectively remove cholesterol from the blood by using interactions at a molecular level, between the membrane/nanoparticles devices and the template, created during the preparation of polymers. Three polymeric systems in form of nanoparticles were prepared differing in the polymerisation solvent (a mixture of acetonitrile and ethanol (1:1) or pure ethanol), and the molar ratio between the functional monomer and the cross-linker (2.3:1 and 1:1). Two out of three of the prepared polymers showed a very good template rebinding capacity both in phosphate buffer solution (pH 6.9) and in ethanol. In particular the nanoparticles rebound 115.4 mg cholesterol/g polymer in buffer solution, and 57 mg cholesterol/g polymer in ethanol. The deposition of the nanoparticles on the surface of the phase inversion membranes produced devices with interesting rebinding performances towards cholesterol in buffer solution: a specific recognition of 14.09 mg cholesterol/g system (membrane and nanoparticles) was detected, indicating maintained binding capacity of supported particles as well.

Efficient preparation and improved sensitivity of molecularly imprinted polymers using room temperature ionic liquids

Synthesis of trans-aconitic acid molecularly imprinted polymers in [bmim][BF4] and [bmim][PF6] under photochemical (5 degrees C, AIBN) and thermal (60 degrees C, AIBN) conditions gave polymer micro-spheres (<200 nm), under bulk and precipitation polymerisation conditions, and higher selectivity indices (100% improvement) relative to the more traditional precipitation polymerisation (CH3CN, high solvent volumes) approach.

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.

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.

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.

Affinity parameters of amino acid derivative binding to molecularly imprinted nanospheres consisting of poly[(ethylene glycol dimethacrylate)-co-(methacrylic acid)]

The binding of L-Boc-phenylalanine anilide (BFA) and L-Boc-phenylalanine (phe) to molecularly imprinted and non-imprinted polymer nanoparticles consisting of poly[(ethylene glycol dimethacrylate)-co-(methacrylic acid)] has been investigated by adsorption experiments and mathematical modeling. The experimental isotherms have been mathematically adapted following the models of Freundlich, Langmuir, Langmuir-Freundlich, Bi-Langmuir, and extended Langmuir. The extended Langmuir model differentiated between specific and nonspecific binding of the ligand to the receptor nanoparticles and rendered excellent fitting of the experimental data. It delivered a thermodynamic and kinetic parameter set on the experimental association curves of L-BFA by L-BFA-imprinted nanospheres in suspension experiments with the equilibrium constant KD= 4.09 +/- 0.69 micromol L(-1) and the kinetic association rate constant Ka= 5.60 mL micromol(-1) min(-1).

Enhanced capacities and selectivities for cholesterol in aqueous media by molecular imprinting: role of novel cross-linkers

Molecularly imprinted polymers are being increasingly investigated as selective sorbents. For the recovery of cholesterol from aqueous media, the utility of the molecularly imprinted polymers has been limited by modest capacities and selectivities, especially when compared with alternative adsorbents reported for the binding of bile acids [Macromolecules 34 (2001) 1548]. This paper describes the use of cholesterol conjugated monomers and cross-linkers, which bind to the template cholesterol molecule by hydrophobic interactions. This leads to enhanced capacities and selectivities during the recovery of cholesterol from aqueous media. The templating effect is clearly seen in the enhanced capacity and selectivity in the retention of cholesterol vis-a-vis stigmasterol and testosterone.

Noncovalent imprinting in the shell of core-shell nanoparticles

Propranolol was imprinted using noncovalent interactions in the shell of core-shell nanoparticles prepared by aqueous emulsion polymerization in the presence and absence of toluene. The imprinted particles were characterized, and their capacity to rebind propranolol from both organic and aqueous media was analyzed. Results showed that the amount of template incorporated into the polymer and the presence of toluene as a "porogenic" agent influenced the ability of the nanoparticles obtained to rebind propranolol. The presence of toluene during imprinting increased rebinding by about 2-fold in buffer and by 3-fold in toluene, compared with similar materials made in the absence of toluene during imprinting. It also influenced the final surface area of the particles. Binding site affinity, assessed by radioligand displacement, was measured as IC50 values of about 1-10 microM. This compares with about 3 microM for bulk polymer made with a similar composition. Finally, to demonstrate the advantages of structured particles for analytical applications a new property, fluorescence, was incorporated into the core of the particles without interfering with the imprinted shell and its ability to rebind propranolol.

Synthesis of molecularly imprinted polymer with 7-chloroethyl-theophylline-immobilized silica gel as template and its molecular recognition function

By reaction of 7-chloroethyl-theophylline with aminopropylsilanized silica gel we synthesized a 7-chloroethyl-theophylline-immobilized silica gel as template molecule and prepared a molecularly imprinted polymer (MIP-Si), which had special recognition sites to 7-chloroethyl-theophylline. A conventional molecularly imprinted polymer (MIP) using 7-chloroethyl-theophylline as template was also prepared for comparison. Binding abilities to 7-chloroethyl-theophylline and its structural analogs revealed that the MIP-Si shows much higher binding speed and much more binding capacity than the MIP does.