Biomedical Engineering Reference
In-Depth Information
hindrance of template-receptor interaction caused by the high salt concentration. It is possible
that the variation of buffer composition and ionic strength will have a large effect on the
binding systems, even when ionic interaction is not the dominant recognition factor. Just
because proteins can adopt various conformations relying on the environment, it is extremely
difficult to make a quantitative comparison between molecular imprinted polymers that have
been prepared and tested under different conditions.
In addition, the different structure and composition of molecular imprinted polymers,
which are usually conduced by different initial concentration of template and/or functional
monomer in the prepared process, can have a important effect on their ability to recognize the
target molecule [86]. This is most likely related to the fact that high template/functional
monomer concentration during preparation will result in a larger number of available binding
sites. However, it is not an advisable choice to increase the amount of template in order to
gain larger adsorption capacity, because the template protein may be deeply buried in the
inner of the molecular imprinted polymers. It is suggested that there exists some optimum
distance between functional groups on the synthetic receptor which will correspond to a
maximal binding capacity. Study of lysozyme imprinted silica beads indicated that the
amount of lysozyme adsorbed onto the beads depended on the composition of functional
monomers used during synthesis of the particles [87]. The determination of the zeta-potential
of lysozyme molecules and the imprinted silica beads showed a coincidence between the zeta-
potential of the template and that of the imprinted polymer beads at the value where the
maximal amount of specifically adsorbed lysozyme was observed. It seemed that zeta
potential matching may be an important factor in the design of synthetic receptors.
Type and concentration of the cross-linking agent is also a critical factor in creating
synthetic receptors with high affinity for their target molecules. When synthesizing
molecularly imprinted polymers, ethylene glycol dimethacrylate has been the most commonly
used cross linking agent. The effect of other cross linking agents on the recognition by
molecular imprinted polymers is still foggy. It is notable that ethylene glycol dimethacrylate
is structurally related to polyethylene glycol, which has long been used to provide resistance
to non-specific protein binding. Lin et al. [88] investigated the importance of the choice of
cross-linking agents in molecular imprinted polymers. In addition, the isothermal titration of
monomers to protein templates, including lysozyme, ribonuclease A and myoglobin, was
studied to screen the functional monomer for molecular imprinted polymers. Four different
cross-linking monomers, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate,
polyethylene glycol 400 dimethacrylate, and polyethylene glycol 600 dimethacrylate, were
first used to synthesize protein imprinted polymers. The chemical structures of the cross-
linking monomers suggest that the size of the protein template may be correlated with the
optimal number of ethylene glycol-repeat units in the cross-linking monomers. Comparing the
poly(ethylene glycol dimethacrylate) or poly(tetraethylene glycol dimethacrylate) protein-
imprinted polymers, it is found that the non-specific hydrophobic binding appears to be the
cause of the high binding to short polyethylene glycol-repeating units, especially since non-
imprinted films bind lysozyme equally well, if not better than, “imprinted” films. It was
interesting to note that more effective protein binding and imprinting was obtained with a
cross-linking agent containing more ethylene glycol repeat units—even though poly(ethylene
glycol) is often used to prevent protein binding. It may be that the greater flexibility of the
longer cross linking agent is important in allowing the polymeric matrix to conform to the
protein molecule. Moreover, microcalorimetry of the interaction between monomers and
