Plastic Antibodies - Designing Receptors for the Next Generation of Biosensors - page 114

Fig. 5 Transmission electron microscopy (TEM)
image of catalytic imprinted nanogels with
aldolase type I activity (reproduced with
permission from [ 55 ])

(CUOOH) by 600,000 times. However, the difference in catalytic behavior of
imprinted and non-imprinted particles was not impressive. Recently Resmini and
coauthors produced MIP NPs able to catalyze a Kemp elimination reaction, for
which no natural enzymes are currently known to exist [ 90 ]. They imprinted an
indolic structure as TSA using 4-vinylpyridine as a basic functional monomer. MIP
NPs were completely soluble in water and exhibited an optimum activity at pH 9.4.
The same group also synthesized MIP NPs capable of catalyzing a C-C bond
formation reaction, and in particular a cross-aldol reaction between 4-nitroben-
zaldeyde and acetone, thus mimicking the enamine-based mechanism of the natural
aldolase type I enzymes [ 55 ]. In this case a covalent imprinting approach was
exploited, bonding a diketone TSA to a polymerizable proline derivative as func-
tional monomer. By using high-dilution radical polymerization,

the authors

obtained 20 nm MIP NPs with a very low polydispersity (Fig. 5 ).
The MIP NPs were not inhibited by product, exhibiting 20-fold higher catalytic
activity compared to the non-imprinted NPs and a good enantioselectivity (62%
enantiomeric excess). These results confirm that MIP NPs could be used when there
are no suitable enzymes or whenever natural molecules have inadequate stability or
high price.

3.4 Sensing Applications

Given their robustness and entirely synthetic nature, MIP nanoparticles are particu-
larly suitable for application in sensors and assays. Reimhult and coauthors pro-
duced a quartz crystal microbalance (QCM) sensor by coating its surface with MIP
NPs imprinted with R -or S -propranolol [ 63 ]. MIP NPs of 130 nm diameter were
synthesized by precipitation polymerization, then dispersed in a solution of poly
(ethylene terephthalate) and eventually spin-coated onto the surface of the QCM
crystal. However it seems this treatment altered the accessibility of MIP binding
sites, adversely affecting the polymer recognition properties. Rather than using MIP