A catalytic and positively thermosensitive molecularly imprinted polymer

Article abstract of DOI:10.1002/adfm.201001906

A catalytic and positively thermosensitive molecularly imprinted polymer is reported. This unique imprinted polymer was composed of 4-nitrophenyl phosphate-imprinted networks that exhibited a thermosensitive interpolymer interaction between poly(2-trifluo

Full text of DOI:10.1002/adfm.201001906

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A Catalytic and Positively Thermosensitive Molecularly
Imprinted Polymer
Songjun Li, Yi Ge,* and Anthony P. F. Turner*
highly specific molecular recognition
A catalytic and positively thermosensitive molecularly imprinted polymer is
reported. This unique imprinted polymer was composed of 4-nitrophenyl
phosphate-imprinted networks that exhibited a thermosensitive interpol-
ymer interaction between poly(2-trifluoromethylacrylic acid) (PTFMA) and
poly(1-vinylimidazole) (PVI), which contains catalytically active sites. At a
relatively low temperature (such as 20 °C), this imprinted polymer did not
demonstrate significant catalytic activity for the hydrolysis of 4-nitrophenyl
acetate due to the interpolymer complexation between PVI and PTFMA,
which blocked access to the active sites of PVI and caused shrinking of the
polymer. Conversely, at higher temperatures (such as 40 °C), this polymer
showed significant catalytic activity resulting from the dissociation of the
interpolymer complexes between PVI and PTFMA, which facilitated access
to the active sites of PVI and inflated the polymer. Unlike previously reported
poly(N-isopropylacrylamide)-based molecularly imprinted polymers, which
demonstrated decreased molecular recognition and catalytic activity with
increased temperatures, i.e., negatively thermosensitive molecular recogni-
tion and catalysis abilities, this imprinted polymer exploits the unique inter-
polymer interaction between PVI and PTFMA, enabling the reversed thermal
responsiveness.
and catalysis. Significant applications of
molecular imprinting include separa-
[
4,5]
[6]
tions,
solid phase extraction, adsor-
[
7]
[8]
[9]
ption, catalysis, and sensing. During
the molecular imprinting process, the
template and functional monomers are
first allowed to form a self-assembled
architecture where the functional mono-
mers are regularly positioned around
the template. Polymerization is then per-
formed to fix this self-organized architec-
ture in place, followed by removal of the
imprinted template from the polymeric
network, which thereby leaves behind
binding sites stereochemically comple-
mentary to the template. For the prepa-
ration of catalytic MIPs, the templates
usually used are the transition state or its
analogue (transition state analogue; TSA),
generating active sites for the activation
[
10,11]
of substrate.
The arrangement of
the binding sites constitutes an induced
molecular memory, which makes the pre-
pared polymer capable of recognizing the
imprint species and of catalyzing the spe-
1
. Introduction
cific substrate of TSA. Thus, host-guest interactions within the
molecular imprinting system are usually comparable to typical
biosystems, such as antibody–antigen, receptor–ligand and
enzyme–substrate.
Smart imprinting systems are at the forefront of MIP tech-
nology, exhibiting stimulus-responsive recognition and/or cata-
lytic ability. One typical example is poly(N-isopropylacrylamide)
Molecular imprinting has attracted considerable attention in
recent years because of its importance in the preparation of
antibody-like polymers.[
1–3]
This ‘from-key-to-lock’ technology
generates molecularly imprinted polymers (MIP) capable of
(
PNIPAm)-based MIPs, which demonstrate thermosensitive
Dr. S. Li
molecular recognition and/or catalytic activity resulting from
the thermal phase transition of PNIPAm.
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education
College of Chemistry
[12,13]
The hydropho-
bicity of PNIPAm increases with increasing temperatures.
Thus, at higher temperatures, the PNIPAm networks shrink in
water and the accessibility of analyte to the imprinted networks
decreases, leading to lower molecular recognition and catalytic
activity, i.e., the PNIPAm-based MIPs demonstrate negative
thermal responsiveness. However, such negatively thermosen-
sitive molecular recognition and/or catalytic activity generally
does not have a high potential for practical application, simply
because of the low activity and responsive kinetics at low tem-
peratures. Furthermore, it is difficult to identify the polymer’s
hydrophilic/hydrophobic properties during the reaction. Thus,
an alternative approach is advocated to create a new generation
of smart MIPs.
Central China Normal University
Wuhan 430079, China
Dr. S. Li, Dr. Y. Ge, Prof. A. P. F. Turner
Cranfield Health
Vincent Building
Cranfield University
Cranfield, Bedfordshire, MK43 0AL, UK
E-mails: y.ge@cranfield.ac.uk; a.p.turner@cranfield.ac.uk
Prof. A. P. F. Turner
Biosensors & Bioelectronics Centre
IFM, Linköping University
SE-58183, Linköping, Sweden
E-mail: anthony.turner@liu.se
DOI: 10.1002/adfm.201001906
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without PTFMA in their polymeric networks. NIP-R was the
thermosensitive non-imprinted polymer that contained the same
amount of PTFMA as MIP-R. In order to investigate the catalytic
specificity, 4-nitrophenyl butyrate (NPB), the structural analogue
of NPA, was selected as the control. The purpose of this study
is to realize the intriguing concept of developing a catalytic and
positively thermosensitive molecularly imprinted polymer.
2
. Results and Discussion
2
.1. Rationally Optimized Interactions Within Imprinted
Polymers
It is well known that molecular self-assembly plays an important
role in the pre-determination of the properties of an imprinted
polymer. An excessive amount of monomer over the template
would cause spatial and steric mismatch due to the excessive
binding sites that distribute randomly throughout the polymer.
Conversely, an underestimated amount of monomer will deliver
a polymer with an insufficient quantity of active binding sites.
Thus, only stoichiometric amounts of monomer can result in an
Scheme 1. Proposed mechanism for the positively thermosensitive
catalysis by MIP-R.
[18]
optimized imprinted polymer. In order to ensure a complete
blockage and release of these active binding sites (i.e., imidazole
groups), the final polymer should also contain stoichiometric
TFMA–VI interaction. Too low or too high an amount of TFMA
would not result in the complete complexation and dissocia-
tion of PTFMA and PVI. Thus, dual stoichiometric interactions
between the template, monomer, and TFMA are needed to
achieve an optimal smart imprinted polymer. With these points
To the best of our knowledge, this paper represents the first
report of a catalytic and positively thermosensitive molecularly
imprinted polymer (i.e., MIP-R) (Scheme 1). In order to pre-
pare the MIP-R, the hydrolysis of 4-nitrophenyl acetate (NPA)
was selected as a model reaction, as it is compatible with water
and has a well-proven TSA (i.e., 4-nitroph-
[14]
enyl phosphate; NPP) (cf. Scheme 2).
NPP
was imprinted in a unique polymeric matrix
made up of poly(2-trifluoromethylacrylic acid)
PTFMA) and poly(1-vinylimidazole) (PVI),[
15]
(
which contains hydrolytically active sites in the
form of imidazole groups. The interpolymer
complexation and dissociation[
16,17]
between
PVI and PTFMA in response to a tempera-
ture change can control the accessibility of
analyte to the active binding sites (Scheme 1).
At relatively low temperatures, the interpolymer
complexation between PVI and PTFMA blocked
access to the active sites of PVI and caused
shrinking of the polymer, thus inhibiting catal-
ysis. In contrast, at relatively high temperatures,
the dissociation of the interpolymer complexes
facilitated access to the active sites of PVI and
inflated the polymer, enabling the catalysis. In this
way, positively thermosensitive catalysis could be
achieved by using this unique “smart” imprinted
polymer. For comparison, three control polymers,
namely MIP, NIP, and NIP-R were also prepared
under comparable conditions (see Experimental
Section). MIP and NIP were the conventional
imprinted and non-imprinted polymers, respec-
tively, with the same composition as MIP-R (the
mlecularly imprinted responsive polymer) but
Scheme 2. Schematic representation of the preparation of MIP-R.
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T a b le 1 . Synthesis composition of imprinted and non-imprinted
polymers.
2
1
0
.95
.95
.95
Composition
NPP
MIP-S
MIP
NIP-R
/
NIP
/
0.11g
0.11g
(0.5 mmol)
(0.5 mmol)
VI
78. 6mg
78. 6mg
78.6mg
78. 6mg
Shift ing
(
(
0.835 mmol)
(0.835 mmol)
(0.835 mmol) (0.835 mmol)
TFMA
DVB
137. 9mg
/
137. 9mg
/
0.985 mmol)
(0.985 mmol)
1.42 mL
1.42 mL
1.42 mL (8.0
mmol)
1.42 mL
(
8.0 mmol)
(8.0 mmol)
(8.0 mmol)
ABCHCN
0.6 g
0.6 g
0.6 g
0.6 g
(2.46 mmol)
(2.46 mmol)
(2.46 mmol) (2.46 mmol)
-
0.05
2
20
260
300
340
380
420
2
.2. FTIR Spectra and SEM Morphology
Wavelength (nm)
In order to ascertain the imprinting behavior, FTIR spectra
of the prepared MIP-R, NPP, and the MIP-R precursor
(i.e., the MIP-R system where the imprinted NPP had not
yet been removed from the polymer matrix) were obtained
Figure 1. The change of UV spectra as a function of VI/NPP molar ratio,
−1
(
(
In which, NPP (7.5 mmol L ; 10 μL/titration) was titrated into VI
0.5 mmol L ; 2.5 mL)
−1
(
3
1
Figure 3). Three characteristic peaks (3200–3700, 2800–
in hand, Figures 1 and 2 present the UV spectra used to monitor
these interactions as functions of the VI/NPP and TFMA/VI
ratios. Using a proportional composition as the control and
initial baseline (Table 1), NPP and TFMA were titrated into VI,
respectively. Both titrations caused shifts in the UV spectra.
Both the shifts achieved maximal values when the titrated NPP
and TFMA reached a critical amount (corresponding to the
−
1
200, ∼ 1750 cm ) and one broad fingerprint band (1000–
−
1
600 cm ) appeared in the spectra of both MIP-R and its
precursor. These characteristic peaks can be attributed to
the stretching vibration of O–H/N–H, C–H, and C=O.
The fingerprint band may arise from C–N and C–C bonds
and their rotation. For comparison, we also included the
spectra of the control polymers in Figure 3. The MIP-R
precursor, MIP-R and NIP-R demonstrated an absorption
[
19,20]
1.67 mol/mol VI/NPP ratio and 1.18 mol/mol TFMA/VI ratio).
Beyond the critical values, no further shift in the UV spectra was
observed except for increasing absorbency. It is therefore clear
that the VI–NPP and VI–TFMA interactions are saturated by the
stoichiometric titrations. Based on the original NPP (0.5 mmol),
−
1
peak at ∼ 1750 cm , which is closely associated with the
stretching vibration of C=O in PFTMA, but MIP and NIP
did not. This result suggests that these thermosensitive
polymers did have PTFMA in their polymeric networks. As
further noted, the MIP-R precursor showed the character-
istic peaks of both NIP-R and NPP, implying the complicated
composition of the MIP-R precursor. After washing, the spec-
trum of the MIP-R precursor became comparable to NIP-R.
This result strongly suggests that the molecular imprinting
of NPP did occur during the preparation, as expected.
Figure 4 shows the SEM images of these synthetic poly-
mers. These polymers appeared spherical in morphology and
were about 1.5–2 μm in diameter. Compared with MIP and
NIP, both MIP-R and NIP-R had somewhat uneven surfaces.
This observation probably results from the PTFMA contained,
which is present in the polymeric networks and therefore par-
tially affected the surface of both MIP-R and NIP-R. Thus, the
successful preparation of both thermosensitive and traditional
polymers provides a convenient framework for straightforward
and comparative studies.
0.835 mmol VI and 0.985 mmol TFMA were therefore used to
prepare the thermosensitive MIP-R and its control polymers. In
this context, further studies were performed as shown below.
1
0
0
.18
.78
.38
Shift ing
-
0.02
2
.3. Specific Interaction Between Polymers and Analyte
2
55
265
275
Wavelength (nm)
Figure 2. The change of UV spectra as a function of TFMA/VI molar
ratio, (In which, TFMA (0.2 mol L ; 50 μL/titration) was titrated into VI
0.03 mol L ; 2.5 mL)
285
295
305
315
Figure 5 displays the temperature-programmed desorption
(
TPD)profiles,withthepurposeofcharacterizingtheinteraction
[
21,22]
−1
between these prepared polymers and analyte.
and MIP both demonstrated significant separation abilities
MIP-R
−1
(
1
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Figure 4. SEM images of the prepared polymers (a: NIP; b: NIP-R;
c: MIP; d: MIP-R)
in Figure 6, MIP-R and NIP-R both demonstrated significant
dependencies on temperature in comparison with the conven-
tional MIP and NIP, as expected. The R of MIP-R and NIP-R
c
both increased with increased temperature. A dramatic increase
Figure 3. FTIR spectra of the prepared polymers.
for NPA and NPB, but NIP and NIP-R did not. This result
indicates that the interaction between both the imprinted
polymers (i.e., MIP-R and MIP) and NPA is highly specific
and capable of separation of NPA and NPB. In conjunction
with the characterization of molecular imprinting, this result
reflects again a consequence of the NPA-specific imprint.
Since MIP-R and MIP both contained imprinted structures
in their polymeric networks, such separation ability is to be
expected.
2
.4. Thermosensitive Interaction Behavior
Dynamic light scattering (DLS) was used to evaluate the inter-
[
23,24]
polymer interaction within the prepared polymers.
In
order to allow equilibrium reaching, all samples were kept at
each tested temperature for 10 min before measurement of the
hydrodynamic size of particles. Through a comparison between
the thermosensitive and non-thermosensitive polymers, the
relative change of the hydrodynamic particle size (R ) reflected
c
the contribution of the PVI–PTFMA interaction. As shown
Figure 5. TPD profiles of the prepared polymers.
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5
4
3
0
0
0
MIP-R
MIP
—— NPA
----- NPB
6
4
3
1
0
5
0
5
0
NIP
MIP-R
NIP-R
MIP
MIP-R/MIP
NIP-R/NIP
NIP-R
NIP
20
1
0
0
2
2
27
32
37
42
0
1
2
3
4
5
Ti m e ( h )
o
Te m p e r a t u r e ( C)
Catalytic hydrolysis at 40 o
C
Figure 6. DLS curves of the prepared polymers.
3
2
5
8
MIP-R
MIP
—
— NPA
in the R of MIP-R and NIP-R occurred at circa 27–35 °C. Below
----- NPB
c
this temperature region, MIP-R and NIP-R exhibited a relatively
NIP
small R ; however, above this temperature region, they demon-
strated a dramatic increase. As previously explained, this may
be attributed to the unique interpolymer interaction between
c
NIP-R
21
14
7
MIP
PVI and PTFMA. The small R at relatively low temperatures
c
MIP-R
may be due to the interpolymer complexation between PVI and
PTFMA, which inhibited access of water to the polymer interior
and thereby restricted the polymeric networks (Scheme 1). The
increased R at relatively high temperatures can be attributed
c
NIP-R
NIP
to the dissociation of the interpolymer complexes, which facili-
tated access of water to the polymer interior and thus relaxed
the polymeric networks.
0
0
1
2
3
4
5
2
.5. Switched Catalysis
Ti m e ( h )
Catalytic hydrolysis at 20 o
C
Figure 7 presents the catalysis of the prepared polymers. In
view of the occurrence of self-hydrolysis during the catalytic
process, which can affect the true catalytic activity of test
polymers, the hydrolysis of analyte without any polymer was
deducted from the overall activity of these polymers. For com-
parison, herein two representative temperatures, i.e., 40 and
Figure 7. Catalysis of the prepared polymers at 20 and 40 °C.
2
.6. Switchable Binding Behavior
It is known that the potential to reduce/oxidize a binding
molecule depends on the binding constant. A larger binding
constant requires more energy to overcome the binding force,
thereby causing a larger redox potential. Thus, dynamic des-
orbing cyclic voltammetry (DCV) can provide valuable infor-
mation about the binding behavior of the prepared polymers
2
0 °C (either higher or lower than the thermosensitive region
of MIP-R and NIP-R, i.e., 27–35 °C) were selected for a con-
trastive study. It was observed that MIP-R at 40 °C was com-
parable to MIP and demonstrated significant catalysis for the
hydrolysis of NPA. However, MIP-R at 20 °C more resem-
bled NIP-R and NIP, which demonstrated limited catalytic
ability for the hydrolysis of NPA. MIP-R apparently showed
a thermo-switchable catalysis mechanism. Clearly, this result
can be attributed to the unique interpolymer interaction
within MIP-R. The complexation and dissociation between
PVI and PTFMA regulated the accessibility of reactants to
the imprinted networks, thereby inducing the switched
catalysis.
[
25]
with their template. As shown in Figure 8, the template NPP
that was bound to MIP-R at 20 °C exhibited a reduction peak
at –699.2 mV (Figure 8a). In contrast, at 40 °C, this reduction
peak shifted to a lower potential (i.e., –729.8 mV) (Figure 8b).
MIP-R demonstrated a stronger interaction with the template at
40 °C than at 20 °C.
For comparison, Figure 8c–h also includes the reduc-
tion potentials of NPP from these control polymers. The
1
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result can be attributed to the unique thermo-
sensitive mechanism of the MIP-R networks.
The thermosensitive interaction between
MIP-R and its analyte regulated the access
to molecularly imprinted networks, thereby
making the switched binding feasible.
3
. Conclusions
The concept of a catalytic, positively thermosen-
sitive molecularly imprinted polymer has been
demonstrated for the first time. This imprinted
polymer was composed of a TSA-imprinted
matrix that demonstrated a thermosensitive
complexation and dissociation between poly
(
1-vinylimidazole) and poly(2-trifluorometh-
ylacrylic acid). At a relatively low temperature
such as 20 °C), the polymer did not demon-
(
strate significant catalysis; however, at higher
temperatures it presented significant catalysis.
These results indicate that a catalytic, positively
thermosensitivemolecularlyimprintedpolymer
can be fabricated using this novel design.
4
. Experimental Section
Preparation of Imprinted Polymers: Chemicals
were of analytic grade and used as received from
Sigma-Aldrich, except for 1-vinylimidazole (VI) and
divinylbenzene (DVB), which were washed with
5
wt% sodium hydroxide solution prior to use.
The preparation of these mentioned polymers, as
previously discussed, was based on the optimized
template/monomer and monomer/TFMA ratios
(Table 1). The template–monomer complex was
first formed by adding a stoichiometric amount
of VI into NPP. Subsequently, the complex, DVB,
1,1’-azobis(cyclohexane-1-carbonitrile) (ABCHCN)
and a stoichiometric amount of TFMA were dissolved
in the mixture of acetonitrile/dimethylsulfoxide (each
5.0 mL) to form a homogeneous system. After being
deoxygenated with sonication and nitrogen, the
mixed system was irradiated by ultraviolet light
(365 nm) overnight. The resulting polymers
were crushed roughly and extracted with ethanol
containing 20% acetic acid using a Soxhlet apparatus
for 24 h. The products were profusely washed with
ethanol and dried at room temperature.
SEM and FTIR Analysis: The morphology of these
prepared polymers was studied using a scanning
electron microscopy (FEI XL30 SFEG) (FEI-Philips,
Figure 8. DCV profiles of the template NPP from the prepared polymers. a) MIP-R at 20 °C; b)
MIP-R at 40 °C; c) MIP at 20 °C; d) MIP at 40 °C; e) NIP at 20 °C; f) NIP at 40 °C; g) NIP-R at USA). The acceleration voltage used was 10.0 kV.
0 °C; h) NIP-R at 40 °C)
The infrared spectra were recorded using an Avatar
70 FT-IR apparatus (ThermoNicolet, USA).
TPD: Using device comprising of
chromatograph (TCD) and a data processing system, the test polymer
10 mg) were placed into an online U-shaped quartz tube (4 mm I.D.).
2
3
a
a gas
reduction potential of the NPP that was bound to MIP-R at
(
40 °C was nearly comparable to that was from MIP (–729.8 vs.
−1
After 10 μL of analyte (0.01 μmol mL in acetonitrile) was pre-adsorbed
–
724.5 mV). The reduction potential of the NPP that was bound
by these polymers, the U-shaped tube was heated under a nitrogen flow
to MIP-R at 20 °C became as low as that from NIP-R (–699.2 vs.
690.1 mV). This result strongly suggests that MIP-R did provide
a thermo-switchable interaction with the template. Again, this
−1
−1
(
40 mL min , 0.23 MPa) at 10 °C min from room temperature to
–
the temperature where the analyte desorbed. The desorbing signal was
recorded by the data processing system.
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DLS: The DLS analysis was carried out at a scattering angle of 90o
using a goniometer equipped with a self-rotation unit and a He–Ne laser
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ꢁ
ꢂ
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ꢂ
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Rw−Rd
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c
=
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× 100%
T
NT
Herein, T indicates the thermosensitive polymers and NT represents
1
33, 15.
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evaluated in a batch format using a thermostated apparatus (pH 7.0 PBS
containing circa 5% acetonitrile).[ The initial concentration of analyte
26]
−
1
(
NPA or NPB) was 0.01 μmol mL (totally 10 mL). The solid content of
−
1
the test polymer was 2.5 mg mL in each test. The 4-nitorophenolate
produced was spectrophotometrically monitored at 400 nm. The activity
of these polymers was recorded based on the average value of identical
triple runs. The self-hydrolysis of analyte without polymer was also
performed under comparable conditions and was finally deducted from
the corresponding overall activity.
DCV: Using an electrochemical workstation equipped with a three-
electrode configuration (Pt-working and counter electrodes; Ag/Ag -
reference.) (LK9806) (Lanlike, China), the test polymer (10 mg) that
pre-absorbed circa 1 μmol template was placed into a cuvette encircled
by a diffusion-eliminated sonication apparatus (supporting electrolyte:
[
[
[
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0
.01 mmol mL KCl; 10 mL). The transiently desorbed template was
rapidly scanned by the workstation using up to 20 cycles until a stable
dynamic cyclic voltammogram was achieved (scanning range, from –800
−
1
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The authors would like to thank the Research Directorate-General of
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Marie Curie Actions (No. PIIF-GA-2009–236799). Thanks should also be
expressed to the National Science Foundation of China (No. 21073068).
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