Biomedical Engineering Reference
In-Depth Information
linking agents in porogenic solvents. These template-monomer complexes became integrated
into a macroporous network. Subsequently the template molecules were removed result from
the chemical cleavage of the supporting covalent bonds, leaving cavities with complementary
size, shape and functional group arrangement to the template molecule. Covalently imprinted
polymers are characterized by high yields of stable binding sites, thermodynamically rather
homogeneous binding nature, and reduced non-specific adsorption [79]. However, the slow
rebinding kinetics resulting from the high steric requirements imposed by the well defined
binding site geometry and the difficulty of the cleavability of the bonds under relatively mild
conditions limit the application of the covalently imprinting approach.
Mosbach [80, 81] established the most common strategy to circumvent disadvantages
associated with the covalent imprinting by exploiting non-covalent interaction, such as Van
der Waals forces, dipole-dipole bonds, hydrophobic interaction, ion pair interaction, and
hydrogen bonds, between the functional monomers and the template to position the
monomers in a particular orientation with respect to the template molecule prior to
polymerization. Various polymerizable functional monomer has been use for non-covalent
molecular imprinting [82, 83]. Following polymerization and template removal, the
functional groups within the imprinted polymers can subsequently recognize and bind the
target analyte using the same non-covalent interactions. Nevertheless, non-covalent
imprinting is not particularly successful for template molecules that do not possess
appropriate functional groups. Another limitation of the non-covalent imprinting is the
relatively poor yield of high-affinity binding sites. Typically, less than 15% of the loaded
template produces functional binding sites, indicating that most of the imprinted cavities are
irreversibly lost, by shrinking, on template elution. Despite these disadvantages, non-covalent
imprinting is considered to be very flexible in terms of the possible functional monomers and
target molecules.
In order to combine the superiority of both covalently and non-covalently imprinting
approach, hybrid techniques have been developed, which also be named semi-covalent and
sacrificial-spacer methods [84, 85]. These approaches exploit covalent template-monomer
complexes in the imprinting step, but entirely non-covalent interactions for rebinding. For
example, Whitcombe et al. prepared a cholesterol imprinted polymers using a carbonate
derivative as sacrificial linker motif [64]. So, this semicovalent strategy not only allows the
creation of homogeneous binding sites with covalent imprinting, but also takes advantage of
the fast rebinding rate of noncovalent adsorption.
Besides various interactions which are possible between the template and the molecular
imprinted polymer receptor, the microenvironment surrounding the binding site can have a
important role in determining how effective the molecular imprinted polymer will be in
recognizing the target molecule. Hart et al. studied the effect of ionic strength on the
template-receptor recognition where ionic interaction was the dominant force [22]. In this
researching, a number of buffer solutions were utilized to observe the effect of ionic strength
and buffer composition on the binding capacity of the molecular imprinted polymers. It was
found that all buffers used resulted in a decrease in binding capacity of the receptor.
Moreover, a 60mM KCl solution instead of a buffer showed the largest inhibition for the
binding capacity. It was therefore concluded that the ionic strength of the binding solution,
not the buffer composition itself, plays a important role in determining the effectiveness of
the molecular imprinted polymers. It was pointed out that the inhibition of binding capacity in
this case is most likely resulted from the change of macromolecule conformation or the
