Molecular imprinting is a state-of-the-art technique for preparing mimics of natural, biological receptors. Nevertheless, the imprinting of macromolecules like proteins remains a challenge due to their bulkiness and sensitivity to denaturation. In this work, a surface imprinting strategy based on covalently immobilized template molecules was adopted for protein imprinting. Bovine serum albumin (BSA) surface-imprinted submicrometer particles (500-600 nm) with magnetic susceptibility were prepared through a two-stage core-shell miniemulsion polymerization system using methyl methacrylate and ethylene glycol dimethacrylate as functional and cross-linking monomers, respectively. The particles possessed a novel red blood cell-like structure and exhibited a very favorable recognition property toward the template BSA molecules in aqueous medium. In a two-protein system, the particles had shown a very high specific recognition of the template proteins over the nontemplate proteins. The magnetic susceptibility was imparted through the successful encapsulation of Fe3O4 nanoparticles. Their superparamagnetic nature increases their potential applications in the fields such as magnetic bioseparation, cell labeling, and bioimaging. In addition, the importance of template immobilization for successful protein imprinting had also been illustrated to demonstrate the potential of this approach as a general methodology for protein imprinting.

Superparamagnetic ribonuclease A surface-imprinted polymeric particles that can preferentially bind the template protein in an aqueous environment were prepared in this study. Methyl methacrylate and ethylene glycol dimethacrylate were employed as the functional and cross-linker monomers, respectively. Regularly shaped submicrometer (700-800 nm) particles imprinted with ribonuclease A were successfully prepared using redox-initiated miniemulsion polymerization. Nanosized Fe3O4 magnetite was encapsulated in the imprinted particles with good encapsulation efficiency (17.5 wt %) for the incorporation of the superparamagnetic property. Good selectivity toward the template over the control protein in an aqueous environment was demonstrated by the imprinted particles in the batchwise and competitive rebinding tests with the highest template loading, Qmax, of 127.7 mg/g observed in the batch rebinding test. Given the small sizes of the imprinted particles and the presence of the binding sites on the surface, the rebinding process was kinetically favorable despite the sheer bulk of the macromolecules. In the desorption study, it was found that the more hydrophobic solvent was more effective for ribonuclease A desorption from the imprinted particles. This indicated that the hydrophobic effect was probably the main form of interaction responsible for the template rebinding to the imprinted sites in an aqueous media.

One of the major difficulties faced in the molecular imprinting of proteins is the inherently fragile and flexible nature of the protein template which makes it incompatible with most polymerization systems. Miniemulsion polymerization is a possible approach for preparing molecularly imprinted nanoparticles, and in this study, the method of initiation, the high-shear homogenization, and the surfactant used for the polymerization reaction had been considered as possible factors that can denature the template protein, ribonuclease A (RNase A). The conformation of the protein in a miniemulsion was studied using circular dichroism (CD). It was found that redox initiation was more suitable for protein imprinting and that the addition of poly(vinyl alcohol) (PVA) as a co-surfactant had proved to be effective in preserving the template protein structural integrity. On the basis of the results of the study, polymeric nanoparticles imprinted with RNase A were prepared via miniemulsion polymerization using methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) as the functional and cross-linker monomers, respectively, with the conditions of the polymerization system optimized to best preserve the integrity of the protein template. In the subsequent investigation for the recognition properties of the prepared nanoparticles through batch and competitive rebinding tests, the imprinted nanoparticles prepared through the conventional (nonoptimized) miniemulsion polymerization lacked the target specificity as displayed by those prepared under the optimized conditions. This illustrated the importance of protein structural integrity in protein 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.

Molecular imprinting has been considered one of the most promising techniques for the preparation of synthetic receptors. In spite of the ease of the conventional imprinting methodology and its associated success with the imprinting of small molecules, the approach has its limititation for the imprinting of protein macromolecules. This is primarily due to the limited diffusion associated with the bulkiness of the template macromolecules and the incompatibility between the fragile protein template and the imprinting conditions. To resolve these issues for the successful imprinting of proteins, miniemulsion polymerization has been employed for preparing protein surface-imprinted nanoparticles. Ribonuclease A (RNase A), bovine serum albumin (BSA), and lysozyme (Lys) were used as the template proteins while methylmethacrylate and ethylene glycol dimethacrylate were the functional and cross-linking monomers, respectively, to produce particles with sizes of about 40 nm. The RNase A surface-imprinted nanoparticles displayed favorable molecular selectivity and rebinding kinetics even in an aqueous medium. However, such a molecular recognition property was not observed for the BSA- and Lys-imprinted nanoparticles. By studying the template protein−surfactant interaction using circular dichroism, it was found that a certain degree of interaction between the template protein and the micelles is required to maintain the proteins at the particle surface. It was also found that such an interaction should not be too extensive to cause a significant conformational change, or denaturation, in the proteins to ensure the recognition of proteins in their native states. This study therefore defines the important parameters required for the successful application of a simple miniemulsion polymerization strategy for protein−surface imprinting.