ABSTRACT
Molecular imprinting is a technique that allows the formation of 3D cavities with tailored recognition properties for target molecules, which can act as molecular template during the co-polymerization of functional and cross-linking monomers [1, 2]. To synthesize molecularly imprinted polymers (MIPs), the functional monomers are chosen after considering their ability to interact with the functional groups on the molecular template. After polymerization, the template is removed from the polymer matrix to create binding sites with shape, size, and functionalities complementary to the molecular template. The resulting imprinted polymers are stable, robust, and resistant to a wide range of pH, solvents, and temperature. Therefore, the behavior of MIPs emulates the interactions established by natural receptors to selectively retain a target molecule (i.e., antibody-antigen), but without the stability limitations associated with biological macromolecules. In addition, synthesis of MIPs is relatively cheap and easy, making these synthetic receptors the first alternative to natural receptors for practical uses. To date, molecular imprinting has been widely recognized as the most promising methodology for the preparation of different tailor-made materials with selective binding property, and its development has been largely driven by practical applications such as separation, binding assays, biomimetic catalysis, and chemical sensing [3]. The most common method to synthesize MIPs is by bulk polymerization because of its simplicity and versatility. This method produces a hard monolith that has to be ground and sieved to give particles in the desired size range. However, the significant loss of materials after particle sizing, the irregular shape of the particles and the wide size distribution make this synthetic method undesirable in many situations. Furthermore, the bulk polymerization method is unsuitable for large-scale preparations because of the difficulty in heat dissipation, leading to poor temperature control during the polymerization process. Therefore, a major part of molecular imprinting research has been focused on developing new polymerization methods to resolve the drawbacks of bulk polymerization. These efforts have demonstrated especially fruitful for preparation of uniform MIP nanoparticles with much improved properties. The recent developments in the area of MIP nanoparticles may offer new solutions to several challenging problems associated
with MIPs when it comes to practical applications. The advantage of nanosized MIP materials is not only the small physical size itself (leading to much faster binding equilibrium due to shorter molecular diffusion path), but also new functions such as instant signal transduction (that is impossible with traditional bulk MIPs). For nanosized MIPs, one can also expect a great increase in the number of accessible sites per unit mass, which is important to gain high catalytic activity if MIPs are designed to accelerate specific chemical reactions. Compared to bulk materials, MIP nanoparticles are much easier to handle and in many situations can be treated as “colloidal molecules”, which allows MIP nanoparticles to be modified or immobilized using existing conjugation chemistry protocols already established for small organic molecules. Indeed, the use of MIP nanoparticles as modular building blocks to construct complex and functional materials and devices is attracting great research interests. In this chapter we review the different synthetic approaches that have been developed to prepare MIP nanoparticles and discuss their feasibility in terms of simplicity, productivity, and impact on molecular imprinting effect. Representative applications of MIP nanoparticles for bioseparation, binding assay, biochemical sensing, catalysis, and controlled delivery will also be overviewed. 5.2 Synthesis of Molecularly Imprinted
NanoparticlesAdvancements in synthetic chemistry, processing techniques, and analytical instrumentation in the past decades have made it possible to develop a whole range of new types of polymer nanoparticles. Using new strategies to gain better control of reaction process, it is now possible to achieve polymer nanoparticles with desired shape, chemical composition and distribution of functional groups in 3D space. For preparation of MIP nanoparticles, different polymerization techniques have been developed, for example the highly flexible and scalable precipitation polymerization and mini-emulsion polymerization, as well as some more specialized methods based on different grafting processes. The MIP nanoparticles reported in the literature have various physical structures, including nanospheres, core-shell particles (magnetic or plasmonic), and different hollow spheres.
