MIP Sensors on the Way to Real-World Applications - Designing Receptors for the Next Generation of Biosensors - page 164

5 Commercially Available MIPs for Analytical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
6 Summary and Outlook ...................................................................... 184
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
1 Molecular Imprinting
Life in general is based on a wide range of molecules working in a predefined
ordered fashion ensuring survival. For functioning properly, it is imperative that
these molecules interact with one another highly selectively in molecular recogni-
tion processes. Usually recognition takes place through non-covalent interactions in
all inter- and intracellular events, e.g., binding of antibodies to antigens, interaction
of receptors with hormones, and enzymatic catalysis. The excellent fit between
natural binding partners has triggered the wish in researchers to fabricate artificial
recognition materials [ 1 ] with bio-analogous binding properties. A both highly
feasible and appreciably straightforward synthetic approach to achieve this is
given by molecular imprinting. Here substrate-selective binding sites in man-
made polymers are generated by means of a template-assisted method. Generally,
the strategy can be defined as “the construction of ligand selective recognition sites
in synthetic polymers where a template (atom, ion, molecule, complex or a molec-
ular, ionic or macromolecular assembly, including microorganisms) is employed in
order to facilitate recognition site formation during the covalent assembly of the
bulk phase by a polymerization or polycondensation process, with subsequent
removal of some or all of the template being necessary for recognition to occur in
the spaces vacated by the templating species” [ 2 ]. In the early 1970s, related
research approaches were extended from silica-based matrices to synthetic organic
polymers. Since then numerous ideas and techniques have been developed in the
field. Main credit for this “New Era” goes to three individuals [ 3 ], namely Wulff
[ 4 , 5 ], Mosbach [ 6 ], and Shea [ 7 , 8 ] as a consequence of their respective pioneering
work. Initially named host-guest polymerization, now the term “molecularly
imprinted polymer (MIP)” is common. Interestingly, it was not used until 1993
and Mosbach himself has preferred “molecularly imprinted absorbents (MIA)” [ 9 ].
The general synthetic strategy for molecular imprinting is summarized in Fig. 1 .
Usually, monomer(s) and a template compound that share complementary
functional groups self-organize with one another forming association complexes.
After adding suitable cross-linking monomers, the mixture is polymerized to
stabilize the spatial arrangement of the preformed aggregates, which rigidly fixes
it in the polymeric matrix. Extraction of template species from the polymer leaves
behind cavities with complementary size, shape, and chemical functionality. Ide-
ally, these cavities are highly selective and reversibly bind either the template itself
or a closely related compound.
Generally speaking, molecular imprinting can be achieved in two ways, namely
covalently and non-covalently. However, the non-covalent approach can be further
classified into two categories, one based on hydrogen bonds to prearrange
monomers and templates, and the other one relying on weaker types of interaction