Molecularly imprinted polymers (MIPs) are synthetic polymeric materials that mimic immunosorbents. They are widely used as
sorbents for solid-phase extraction (SPE). The most common way to synthesize them is bulk polymerization because of its simplicity
and versatility. This produces a hard monolith that has to be ground and sieved to obtain particles in the desired size range.
However, the partial loss of the materials as fine dusts; the irregular shape of the particles produced and their wide size
distribution, have led to a search for different polymerization methods to offset the drawbacks of the bulk polymerization
process.
The analysis of organic contaminants in environmental matrices or pharmaceutical compounds in biological fluids (such as plasma
or urine) often requires pretreatment steps because of the low concentration levels of the target analytes and the complexity
of the matrices. Solid-phase extraction (SPE) is the most commonly used technique. However, many interfering compounds might
be coextracted with the target analytes on conventional sorbents.
To overcome this lack of selectivity highly selective sorbents such as immunosorbents (ISs)1,2 and molecularly imprinted polymers (MIPs)3 were developed. They allow extraction, concentration and clean-up in a single step. ISs provide a high degree of molecular
recognition because of the high affinity and the high selectivity of the antigen–antibody interactions. Nevertheless, their
synthesis involves the production of antibodies which is a time-consuming, tedious and expensive process. An alternative technology
that uses MIPs was proposed by chemists; it represents an attractive approach to mimic highly specific antibody binding-sites.4
It involves the copolymerization of functional monomers and cross-linking agents in the presence of an imprint molecule (template),
which leads to the formation of a rigid polymer. The removal of the template results in well-defined cavities within the polymer whose structure and arrangement of functional
groups are complementary to the template. However, the template removal is usually uncompleted. This problem is common to
all MIP formats; its consequences depend on the final use of the sorbent. In the situation of trace analysis, the template
molecules' bleeding gives rise to erroneous results, thus an exhaustive washing of the polymer with a range of solvents should
be performed before use to extract the entire amount of the template molecules
The solvents to be used should promote polymer swelling and disrupt template/polymer interactions. Commonly used solvents
are methanol or acetonitrile with the addition of acidic or basic modifiers; they need to be optimized for each type of polymer.
MIPs have a large number of potential applications but most activities are focused on their use in solid-phase extraction
(SPE). Their use as selective sorbents in SPE was first performed by Sellergren et al.,5 more than 20 years ago, for the extraction of pentamidine from urine. Since this pioneering work, the number of papers dealing
with the application of MIPs in solid-phase extraction (MIP–SPE) is increasing.6,7
Table 1: Comparison of the different polymerization methods. Adapted from reference 48 and reprinted with permission.
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Most of the reported MIPs were prepared by bulk polymerization which results in a monolith that should be ground and sieved
to obtain irregular particles. Other polymerization methods can be performed to directly obtain better defined particles such
as precipitation, suspension and multi-step swelling. The principle of each method and their relative benefits and limitations
are listed in Table 1. This paper discusses different procedures used for preparing MIPs mainly dedicated to SPE.
Principle of MIP–SPE
Figure 1
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The principle of selective extraction on MIPs is similar to the extraction with ISs. In the off-line mode, the MIP particles
are packed into a disposable cartridge between two frits. Figure 1 illustrates the off-line MIP–SPE procedure. After a conditioning
step, the sample is percolated through the MIP and a selective washing step allows the elimination of interfering compounds
retained by non-specific interactions. The target analytes are then eluted by the percolation of a solvent able to disrupt
the selective interactions between the binding sites and the analytes to recover them. The residue obtained can be analysed
either by liquid chromatography (LC) or gas chromatography (GC). In the on-line coupling, the MIP sorbent is slurry packed
into a pre-column which is located in a six-port switching valve8–20 connected to a LC system.
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