Chemistry Reference
In-Depth Information
(Tanabe et al. 1995), and a metal containing complex (Fujii et al. 1985) have been
used to imprint dicarboxylates, amine, and imides.
Solvent
/
Porogen.
The important role that a solvent serves in the imprinting
process is as a porogen that controls the morphology and porosity of the polymer
matrix. MIP polymerizations are usually carried out at very high concentrations in
which the solvent occupies 33-50% of the reaction volume. Under these conditions,
the growing polymer chains ideally phase separate from the solution phase during
polymerization, leading to a macroporous polymer monolith with surface areas of
100-1000 m
2
/g (Shea et al. 1990; Santora et al. 2001). The solvent accessibilities
of these macroporous monoliths are excellent as evidenced by their use as chromato-
graphic stationary phases (Svec and Frechet 1996; Peters et al. 1999). As noted in the
previous section, the solvent can also accentuate or disrupt the noncovalent
interactions in the prepolymerization complex. This dual role of the solvent in the
imprinting process highlights the complexity of optimizing. The variables are all
highly interdependent and attenuate not only the efficiency of the imprinting
process but also the polymer morphology and structure. Because of this complexity,
multivariate experimental design procedures have recently been applied to optimize
the imprinting process (Davies et al. 2004; Rosengren et al. 2005).
15.3. RECOGNITION PROPERTIES OF MIPs
The most attractive characteristic of MIPs is the ability to tailor their binding selec-
tivities in a manner similar to biological recognition systems. The average recognition
properties of MIPs, however, usually fall short of those of biological recognition
systems. This is understandable considering the relative simplicity of the imprinting
process. However, it is not generally appreciated that the recognition properties of
MIPs are very complex and highly concentration dependent. Thus, under optimized
conditions, MIPs can achieve levels of affinity and selectivity equal or even surpass-
ing those of antibodies (Vlatakis et al. 1993; Andersson et al. 1995). Therefore, an
understanding of the recognition properties of MIPs is vital to the development
and optimization of MIP-based sensors.
15.3.1. Binding Site Heterogeneity
The source of the complex recognition behavior of MIPs arises from their binding site
heterogeneity. Individual MIPs contain an array of different binding sites of varying
size, shape, binding affinities, and binding selectivities (Rampey et al. 2004). In con-
trast, binding sites in synthetic molecular receptors and biological recognition
systems are structurally homogeneous because every binding site possesses the
same affinity and selectivity. The characteristic binding site heterogeneity in MIPs
is depicted in Figure 15.2. The binding sites line the channels of the macroporous
MIP matrix and vary widely in their shape and functional group complementarity
to the template molecule. The result of the structural heterogeneity is that only a
