Monolithic molecularly imprinted polymers, preparation

D. Djozan and T. Baheri, Preparation and evaluation of solid-phase microextraction fibers based on monolithic molecularly imprinted polymers for selective extraction of diacetyl-morphine and analogous compounds, / Chromatogr. A, 1166 (1-2) 16-23,2007. 

Molecularly imprinted polymers with a variety of shapes have also been prepared by polymerizing monoliths in molds. This in situ preparation of MIPs was utilized for filling of capillaries [20], columns [21], and membranes [22, 23]. Each specific particle geometry however needs optimization of the respective polymerization conditions while maintaining the correct conditions for successful imprinting. It would be advantageous to separate these two processes, e.g., to prepare a molecularly imprinted material in one step, which then can be processed in a mold process in a separate step to result the desired shape. 

The prepared molecularly imprinted polymer is then subjected to a work-up scheme. If the polymer is made as a solid monolith, a particle fragmentation is normally performed by grinding and sieving, where particles with an average size of 10-25 pm are collected. 

However, it is still unclear how the efficiencies and resolutions of MIP monoliths compare to other methods of capillary fabrication using molecularly imprinted polymers. Future studies that directly compare the chromatographic results obtained using the same imprinting system under different preparation techniques (i.e., packed columns, coated thin films, immobilized particles, and monoliths) have... 

Figure 5 Illustration of the preparation procedure for capillary columns with molecularly imprinted polymer monolith. (1) Polymerization mixture is prepared and plastic tubing is placed on both ends of a capillary derivatized with methacryloxy propyltrimethoxysilane. (2) The capillary is filled with the mixture using a syringe. (3) The capillary is sealed by placing clips on the plastic tubings. The polymerization is performed under an UV source (350 nm) at —20°C for 80 min. (4) The polymerization reaction is interrupted by flushing remaining monomer, radical initiator, and imprint molecule out of the capillary. (5) The capillary column is then ready for CEC. (Reproduced with permission from Ref. 38.)...

Although the first report on boronic acids was published in 1862, boronate affinity materials have not been extensively investigated until recently. In recent years, various boronate-functionalized materials, " such as macroporous monoliths, " " nanoparticles, and mesoporous materials, have been developed into important tools for the facile selective extraction of cis-diol-containing compounds. With these matrices, several important materials with teamed boronate affinity and boronate avidity as well as boronate affinity-based molecularly imprinted polymers have been prepared. 

SPE is an exhaustive and almost solvent-free sample preparation technique. Typically, a tube is filled with a sorbent, which can be porous particles or a polymerized monolith. Various interactions are used to extract analytes from complex samples. Many of the commercially available SPE systems are for single use, but some, like RAM (restricted access materials) and MIP (molecular imprinted polymers), are typically obtained as reusable extraction devices. As will be discussed in detail, the extraction sorbents mainly function as normal phase, reversed phase, cation exchange, anion exchange, or a combination of these. 

Molecular imprinting has recently attracted considerable attention as an approach to the preparation of polymers containing recognition sites with predetermined selectivity. The history and specifics of the imprinting technique pioneered by Wulff in the 1970s have been detailed in several excellent review articles [122-124]. Imprinted monoliths have also received attention as stationary phases for capillary electrochromatography. 

The performance of the molecularly imprinted monolith in terms of molecular recognition and flow-through properties depends on several factors, especially the density and the porosity of the polymer. In order to obtain a monolith with high selectivity and high permeability, some preparation conditions must be optimised, in particular the composition of the prepolymerisation mixture including the amount of template, the type and amount of functional monomer, crosslinker, porogenic solvent and the initiator, and the polymerisation conditions such as initiation process and polymerisation time. 

The molecular imprinting strategy can be applied for the recognition of different kinds of templates from small organic molecules to biomacromolecules as proteins. Some examples of separations investigated with MIP monoliths in CEC and LC are shown in Table 2. The influence of the imprinted monolithic phase preparation procedure and of the separation conditions on the selectivity and chromatographic efficiency have been widely studied [154, 157, 161, 166, 167, 192]. The performance of imprinted monoliths as chromatographic stationary phase has also been compared to that of the traditional bulk polymer packed column [149, 160]. It was shown that the monolithic phases yielded faster analyses and improved chiral separations. 

The bulk polymeric format, characterised by highly cross-linked monolithic materials, is still widely used for the preparation of enzyme mimic despite some of its evident drawbacks. This polymerisation method is well known and described in detail in the literature and has often be considered the first choice when developing molecular imprinted catalysts for new reactions. The bulk polymer section is presented in three subsections related to the main topics covered hydrolytic reactions, carbon-carbon bond forming reactions and functional groups interconversion. 

The early attempts at fabricating molecularly imprinted capillary monoliths adapted the procedure set forth by Frechet and Svec [4] for the in situ preparation of non-MIP macroporous polymer rods for FC separation. In this procedure, porogenic solvents cyclohexanol and dodecanol (80 20 v/v) were used with a methacrylate-based polymer system to produce porous monoliths. When this system was applied to the fabrication of molecularly imprinted monoliths for CEC, the polymers obtained were sufficiently porous but resulted in poor enantiomeric separations [36]. It is thought that the polar-protic nature of the porogens used may have inhibited the formation of well-defined imprints. Polar-protic solvents such as these are often poor porogens for the noncovalent imprinting approach because they interfere... 

---