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Tuning Molecular Recognition in Water-Soluble Nanogels
FULL PAPER
trile result in an increase of the CMC value.[29,28,31] Prelimi-
nary experiments showed that the addition of Tween 80 was
not having any significant effect on either the reaction rate
or the imprinting factor. Reactions carried out with SDS,
with its anionic residues, were showing the highest catalytic
activity in terms of reaction rate, but the imprinting efficien-
cy, indicated by the ratio between the apparent rate con-
stants for the imprinted (MIP) and non-imprinted nanogels
(NIP) was drastically reduced. Evidence in the literature in-
dicates that SDS can absorb onto the surface of the nano-
gels and, with its anionic side chains, can weaken the bind-
ing affinity of the polymer matrix towards the substrate,
acting as a competitor in the formation of the hydrogen
bond.[28] The most significant effect was obtained with
Tween 20 and this was studied in greater detail.
Kinetic experiments were carried out at fixed substrate
concentration, using 0.02 mgmLÀ1 of MIP AS230 and NIP
AS231, with a concentration of Tween 20 varying from 0.1
to 0.75%. The data are shown in Figure 4b. The graph
shows that for the imprinted polymer AS230, the initial
rates increase with increasing surfactant concentration up to
0.5%, after which the rates drop down. This can be ex-
plained by a propensity towards micelle formation that can
prevent positive interactions between substrate and poly-
mers. An interesting result is that although a similar trend is
observed with the non-imprinted polymer AS231, the effect
on the particles is not as significant. When these results are
compared with the ones obtained with Tween 80, it clearly
indicates an effect due to the presence of the longer alkyl
chains, which when absorbed on the surface on the polymer
do not favor the interaction with the substrate.
threefold increase in rate acceleration between the imprint-
ed polymer and the background reaction. The data therefore
suggests that the measured imprinting effect can be maxi-
mized by the addition of a surfactant. It was decided to
study this phenomenon more in detail by carrying out not
only a full kinetic study but also a detailed morphological
analysis of the materials.
Comparison of the kinetic data for MIP AS230 and NIP
AS231 obtained without surfactant (Figure 4a) and with
Tween 20 (0.5%, Figure 4c) under the same conditions and
using initial rates corrected for the background reaction, in-
dicate a clear difference, especially when compared with the
kinetic data obtained without surfactant. The data provided
evidence of a real effect of the surfactant on the catalytic ac-
tivity of MIP AS230. The experimental data for both MIP
AS230 and NIP AS231 with Tween 20 could be fitted to the
single site saturation equation. The values of the kinetic pa-
rameters obtained are Vmax =1.75ꢂ10À6 (S.E. Æ1.83ꢂ10À7)
mminÀ1 and KM =1.39ꢂ10À3 (S.E. Æ2.16ꢂ10À4) m for the im-
printed polymer and Vmax =4.71ꢂ10À7 (S.E. Æ7.0ꢂ10À8)
mminÀ1 and KM =1.83ꢂ10À3 (S.E. Æ3.69ꢂ10À4) m for the
non-imprinted polymer. Two significant observations can be
made by comparing these sets of data with the ones ob-
tained before: 1) MIP AS230 displays an apparent higher
catalytic activity when Tween 20 is added to the buffer solu-
tion; 2) the difference in rate accelerations between imprint-
ed and non-imprinted polymer, an indication of imprinting
effect, appears to be higher in the presence of Tween 20. To
verify that the curvature in the vi versus [S] plot for MIP
AS230 is a true indication of enzyme-like behavior and not
a result of mass transfer and diffusion effects, the linear de-
pendence of vi from catalyst concentration at a fixed sub-
strate concentration needs to be verified. This was per-
formed at two substrate concentrations, 1 mm and 0.5 mm,
chosen within the concentration range investigated. The
data shown in Figure 4d) can be fitted to a linear plot, as
would be expected if the saturation was the result of a true
enzyme mimic. This result is very important and provides
confirmation that the application of the imprinting approach
to nanogels leads to catalysts showing saturation behavior
and following the Michaelis–Menten kinetic model, as true
enzyme mimics. In addition, these nanogels are also water-
soluble and able to maintain their recognition properties in
aqueous phases.
A full characterization was carried out using MIP AS230
and NIP AS231 at 0.02 mgmLÀ1 of polymer concentration,
at pH 9.4 with CH3CN (10%) and Tween 20 (0.5%). The
data in Figure 3 shows a clear difference in initial rates be-
tween the imprinted and non-imprinted polymer. The initial
rates, measured at a substrate concentration of 1 mm, show a
To obtain a meaningful comparison of the data acquired
for the imprinted and non-imprinted nanogels, the kinetic
parameters for the catalysts need to be determined in an ac-
curate way by measuring the concentration of pyridine resi-
dues in each polymer preparation, under the same experi-
mental conditions. This was obtained by titrating the pyri-
dine moieties using an acid–base reaction. The polymers
were treated with HCl to fully protonate the pyridine
groups, which were subsequently back-titrated with NaOH
using a pH meter.
~
Figure 3. Plot of vi,vs substrate concentration for MIP AS230 ( ), NIP
~
*
AS231 ( ) and the background reaction without any catalysts ( ). Reac-
tion were carried out at pH 9.4, carbonate buffer, CH3CN (10%) and
with Tween 20 (0.5%). Initial rates are here shown uncorrected for the
background reaction.
The data in Figure 5show the titration results for the non-
imprinted nanogel AS231 with and without Tween 20, in
which the different inflection points can be observed. The
Chem. Eur. J. 2011, 17, 11052 – 11059
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