Biomedical Engineering Reference
In-Depth Information
Whitcombe and coworkers [66] described a promising approach to prepare homogeneous
molecular imprints where a combination of covalent imprinting and non-covalent rebinding
techniques was developed. The template was prepared by covalently linking the target peptide
with a functional molecule. Upon polymerization and chemical cleavage, the functional group
was left in the cavities with precise spatial arrangement, and the essential void space
facilitating removal and rebinding of the target peptide.
The creation of thin molecular films on suitable substrates, such as gold, mica and
carbon, is a commonly used approach to surface imprinting. Various ways were used to
produce such molecular imprinted films. For example, organic/inorganic hybrid thin films for
protein recognition were prepared by the liquid-phase deposition (LPD) where pepsin was
used as a template protein and titanium oxide was deposited on gold substrates in the
presence of pepsin-poly-L-lysine complexes, just as shown in Figure 4 [67]. Surface plasmon
resonance (SPR) signals on the deposited films were measured to examine the binding
behaviors toward proteins. Matsunaga et al. [68] also prepared molecular imprinted polymer
films which were covalently conjugated on gold-coated SPR sensor chips. It was found that
the presence of salt in the pre-polymerization mixtures derived higher selectivity and the
optimization of salt concentrations in the re-binding buffers could make the selective binding
prominent due to the decrease of non-specific binding. Turner et al. [69] studied the crucial
role that monolayer constituency and transfer play in the formation and preservation of
template protein on multi-component lipid monolayers with both protein attracting and
protein repelling elements. All the results indicated that the controllable formation of imprints
was aided by a balance between promotion of the protein-monolayer interaction through
electrostatics and hindrance through steric effects. Upon transfer to a hydrophobic substrate,
these imprinted lipids were immobilized, thus “locking in” their structure (Figure 5).
For the thin film imprinting technique above mentioned, one major issue is controlling
the molecular imprinted polymer film thickness, as it should be sufficiently thin for rapid
mass transfer. This is especially important in sensor applications, where high sensitivity and
short response time are desired. Using atom transfer radical polymerization, Wei et al. [70]
reported the successful preparation of ultra thin (<10 nm), surface-confined, molecular
imprinted polymer films on model gold substrates. It is found that the template removal from
the imprinted films appeared to be 100% efficient. In addition, because the controllable nature
of atom transfer radical polymerization allows the growth of uniform molecular imprinted
films with adjustable thicknesses, it is possible to tailor the resulting materials to have high
capacities by growing thicker films or high binding efficiencies by growing thinner films.
Using single- and dual-site Langmuir adsorption models, further researching [71] on the
adsorption kinetics of the molecular imprinted film indicated that the adsorption could be well
described by any of the four isotherm models: Langmuir, dual-site Langmuir, Freundlich, or
Langmuir-Freundlich. The relatively high heterogeneity index values regressed using the
Freundlich and Langmuir-Freundlich isotherms suggested the formation of fairly
homogeneous molecular imprinted polymer films, although Scatchard analysis revealed that
binding site heterogeneity did exist.
