Chemistry Reference
In-Depth Information
coordination, and hydrophobic interactions that are effective in water and can bind
peptides (Asanuma et al. 2000; Hart and Shea 2001), carbohydrates (Wulff and
Schauhoff 1991; Striegler 2003), and proteins (Bossi et al. 2001; Takeuchi et al.
2007). Perhaps the most notable of these is the report of a peptide imprinted
polymer system that can effectively distinguish peptides in water that differ by one
single amino acid (Nishino et al. 2006).
Binding Kinetics.
Another important consideration in adapting MIPs for sensing
applications is the optimization of their binding kinetics. MIP binding processes are
often quite slow in reaching equilibrium and are on the order of minutes to hours
(Norell et al. 1998; Chen et al. 1999). This is further complicated by the binding
site heterogeneity of MIPs, which leads to a wide variation in the rebinding kinetics
for different binding sites within an MIP (Khasawneh et al. 2001; Garcia-Calzon and
Diaz-Garcia 2007). Unfortunately, the more desirable high-affinity sites show the
slowest binding kinetics (Sellergren 1989). The binding kinetics are also highly
solvent, concentration, and temperature dependent. One strategy that has been used
to circumvent this is the implementation of MIPs in sensing systems that do not
need to reach equilibrium such as continuous flow systems.
A related consideration is the slow leaching of the template molecule from the
imprinted polymer matrix (Ellwanger et al. 2001). The majority of the template
used in the imprinting process is readily removed from the polymer matrix during
the washing step. However, a small fraction of template remains encapsulated
in the polymer matrix and is slowly released into solution. Under normal conditions,
the leaching of the template from the polymer does not impact the binding properties.
However, at low analyte concentrations that are common to sensing applications, the
concentration of template leaching from the matrix becomes competitive with the
analyte concentration. This limits the use of MIPs in trace analysis. One solution is
to imprint a structural analog to the analyte of interest (Andersson et al. 1997;
Kim and Guiochon 2005b). Alternatively, the binding of the analyte can be moni-
tored indirectly using a competitive radioligand or spectroscopically labeled ligand
assays (Vlatakis et al. 1993; Haupt et al. 1998).
Low Average Affinities and Capacities.
Despite improvements in the imprint-
ing process and better understanding of the optimal binding conditions for MIPs, the
low binding affinities and selectivities of MIPs are still a critical problem for sensing
applications. The problem of the template slowly leaching from the matrix, in parti-
cular, does not allow the analysis using MIPs to be easily carried out at the submicro-
molar concentrations where MIPs show the highest selectivities. A number
of strategies have been applied to improve the binding properties of MIPs. These
can be divided into pre- and postpolymerization strategies. The prepolymerization
strategies have focused on improving the efficiency of the imprinting process. One
of the most popular strategies has been to develop functional monomers that form
much stronger noncovalent interactions. Examples of these “stoichiometric imprint-
ing” monomers include the carboxylate and phosphate binding amidine functional
monomers (Wulff and Knorr 2001), an ampicillin binding oxyanion receptor-based
