Xianzhi Guo

Narrowly Dispersed Hydrophilic Molecularly Imprinted Polymer Nanoparticles for Efficient Molecular Recognition in Real Aqueous Samples Including River Water, Milk, and Bovine Serum

Molecular imprinting has proven to be a versatile approach to the preparation of synthetic receptors with tailor-made recognition sites. The ultimate goal of molecular imprinting is to generate molecularly imprinted polymers (MIPs) with affinity and specificity approaching those of the biological receptors so that they can eventually replace such biological entities in real applications. However, previously developed MIPs that target small organic molecules are normally only compatible with organic solvents, and they mostly fail to show specific template binding in aqueous solutions (whereas peptideor protein-imprinted polymers are intrinsically water-compatible), which significantly limits their practical application in such areas as molecularly imprinted sorbent assays and biomimetic sensors. Despite some progress made in the development of MIPs applicable for analyte detection in relatively simple aqueous media, such as pure water, surfactant-containing water, aqueous buffer solutions (mostly containing an organic solvent), and beer or solutions that mimic alcoholic beverages, the design of MIPs directly capable of specifically recognizing the targeted small organic molecules in real biological samples remains a formidable challenge owing to the complex nature of the sample matrices. Herein, we report the efficient synthesis of narrowly dispersed hydrophilic MIP nanoparticles with excellent specific molecular-recognition ability in real aqueous solutions, including river water and biological samples (both diluted and undiluted milk and bovine serum). Reversible addition–fragmentation chain-transfer (RAFT) precipitation polymerization (RAFTPP) mediated by hydrophilic macromolecular chain-transfer agents (macro-CTAs) provided for the first time narrowly dispersed highly cross-linked MIP (or polymer) nanoparticles with surface-grafted hydrophilic polymer brushes in a facile one-pot approach (Figure 1). The hydrophilic polymer brushes on the MIP nanoparticles not only significantly improved their surface hydrophilicity and led to their water compatibility, but they also acted as a protective layer to prevent proteins in the biological samples from accumulating on the nanoparticle surface and blocking the imprinting sites and thus enabled the MIP nanoparticles to function properly in such complex matrices. Although studies on the use of MIPs in biological media for specific molecular recognition have been disclosed, either diluted biological solutions (containing 40 vol% of a mixture of ethanol/water (1:1 v/v) and a phosphate buffer) were used or the targeted molecule was a highly water soluble 26 amino acid peptide (i.e., the bee toxin melittin). In this context, there have been many reports on the MIP-based solid-phase extraction of target analytes in real matrices; however, the selectivity of the MIPs in this case is controlled by the choice of solvents in the extraction procedure. The direct use of MIPs in real matrices is more difficult, as the conditions used are mainly fixed by the nature of the sample. To our knowledge, we report herein the first successful example of the synthesis of MIP nanoparticles that can be used directly in undiluted biological samples for the efficient specific recognition of small organic molecules. This finding is a major breakthrough for molecular-imprinting technology, since it opens the door to the facile preparation of nanoscale MIPs (with good dispersion and outstanding performance in real aqueous samples) that are very attractive synthetic substitutes for biological receptors in bioanalytical applications and many other fields. Figure 1. Preparation and characterization of MIP nanoparticles. a) Chemical structures of the RAFT agents used: hydrophilic macroCTAs and CDB. b) Schematic protocol for the one-pot preparation by RAFTPP of MIP nanoparticles compatible with real aqueous solutions. c) Characterization of MIP nanoparticles (Mn,NMR of PHEMA brushes: 4800) by SEM and DLS.

An efficient approach to obtaining water-compatible and stimuli-responsive molecularly imprinted polymers by the facile surface-grafting of functional polymer brushes via RAFT polymerization

A new and efficient approach to obtaining molecularly imprinted polymers (MIPs) with both pure water-compatible (i.e., applicable in the pure aqueous environments) and stimuli-responsive binding properties is described, whose proof-of-principle is demonstrated by the facile modification of the preformed MIP microspheres via surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAAm). The presence of poly(NIPAAm) (PNIPAAm) brushes on the obtained MIP microspheres was confirmed by FT-IR as well as the water dispersion and static contact angle experiments, and some quantitative information including the molecular weights and polydispersities of the grafted polymer brushes, the thickness of the polymer brush layers, and their grafting densities was provided. In addition, the binding properties of the ungrafted and grafted MIPs/NIPs in both methanol/water (4/1, v/v) and pure water solutions were also investigated. The introduction of PNIPAAm brushes onto the MIP microspheres has proven to significantly improve their surface hydrophilicity and impart stimuli-responsive properties to them, leading to their pure water-compatible and thermo-responsive binding properties. The application of the facile surface-grafting approach, together with the versatility of RAFT polymerization and the availability of many different functional monomers, makes the present methodology a general and promising way to prepare water-compatible and stimuli-responsive MIPs for a wide range of templates.

Controlled synthesis of water-compatible molecularly imprinted polymer microspheres with ultrathin hydrophilic polymer shells via surface-initiated reversible addition-fragmentation chain transfer polymerization

The first controlled synthesis of pure water-compatible molecularly imprinted polymer (MIP) microspheres with ultrathin hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) shells (including both PHEMA brushes and lightly crosslinked PHEMA hydrogel layer) via surface-initiated reversible addition-fragmentation chain transfer (RAFT) polymerization is described. The presence of ultrathin hydrophilic polymer shells on the MIP microspheres was confirmed by SEM, FT-IR, fluorescent labeling treatment, contact angle studies, and water dispersion stability test, and some quantitative information including the thickness of the grafted hydrophilic polymer layers as well as the molecular weights and polydispersities of the grafted polymer brushes and their grafting densities was provided. The facile surface-grafting of both PHEMA brushes and PHEMA hydrogel layer on the MIP microspheres proved to be highly efficient for improving their surface hydrophilicity and suppressing the hydrophobically driven nonspecific interactions between the MIPs and template molecules, leading to MIPs with pure water-compatible binding properties. The findings presented here not only prove the general applicability of the controlled hydrophilic polymer brushes-grafting approach in obtaining pure water-compatible MIPs, but also largely extend the scope of this versatile surface-grafting approach through the controlled surface-grafting of hydrophilic polymer hydrogel layer onto the MIPs. Moreover, the significant effect of the chain length of the grafted polymer brushes and the presence of crosslinking in the grafted polymer shells on the surface hydrophilicity and water-compatibility of the MIP microspheres was also demonstrated for the first time, which is of great importance for the rational design of water-compatible MIPs by using this controlled surface-grafting approach.

Comparative study of the molecularly imprinted polymers prepared by reversible addition–fragmentation chain transfer “bulk” polymerization and traditional radical “bulk” polymerization

Bisphenol A (BPA) and propranolol‐imprinted polymers have been prepared via both reversible addition–fragmentation chain transfer “bulk” polymerization (RAFTBP) and traditional radical “bulk” polymerization (TRBP) under similar reaction conditions, and their equilibrium binding properties were compared in detail for the first time. The chemical compositions, specific surface areas, equilibrium bindings, and selectivity of the obtained molecularly imprinted polymers (MIPs) were systematically characterized. The experimental results showed that the MIPs with molecular imprinting effects and quite fast binding kinetics could be readily prepared via RAFTBP, but they did not show improved template binding properties in comparison with those prepared via TRBP, which is in sharp contrast to many previous reports. This could be attributed to the heavily interrupted equilibrium between the dormant species and active radicals in the RAFT mechanism because of the occurrence of fast gelation during RAFTBP. The findings presented here strongly demonstrates that the application of controlled radical polymerizations (CRPs) in molecular imprinting does not always benefit the binding properties of the resultant MIPs, which is of significant importance for the rational use of CRPs in generating MIPs with improved properties. Copyright

Ambient temperature synthesis of narrow or monodisperse, highly cross-linked, and “living” polymer microspheres by atom transfer radical precipitation polymerization

A facile ambient temperature atom transfer radical precipitation polymerization (ATRPP) approach is developed for the efficient one-pot synthesis of narrow or monodisperse, highly cross-linked, and “living” polymer microspheres under mild reaction conditions. The simple introduction of an atom transfer radical polymerization (ATRP) mechanism into a precipitation polymerization system, together with the rational use of polar alcoholic solvents, allows the direct ambient temperature preparation of uniform “living” polymer microspheres with their number-average diameters ranging from 0.36–1.95 μm and their particle size distributions being typically less than 1.01. The polymerization parameters (including the monomer loading, polymerization time, and kind of alcoholic solvent) proved to have a pronounced influence on the yields and morphologies of the polymer microspheres, which makes it very convenient to tailor the particle sizes by tuning the polymerization conditions. The general applicability of ambient temperature ATRPP was demonstrated by its successful application in a range of alcoholic solvents as well as its versatility in the synthesis of a series of uniform copolymer microspheres of different monovinyl functional monomers (4-vinylpyridine, glycidyl methacrylate, methyl methacrylate, and 2-hydroxyethyl methacrylate) with ethylene glycol dimethacrylate. In addition, the “livingness” of the resulting polymer microspheres was confirmed by their direct grafting of hydrophilic polymer brushes via surface-initiated ambient temperature ATRP, leading to advanced functional polymer microspheres with significantly improved surface hydrophilicity.