Molecularly imprinted polymers and room temperature ionic liquids: Impact of template on polymer morphology

Article abstract of DOI:10.1071/CH06284

Molecularly imprinted polymers (MIPs) were generated for trans-aconitic acid 1 and cocaine 2 in a variety of porogens (CH₃CN, CHCl ₃, [bmim][BF₄], and [bmim][PF₆]). MIP synthesis in either [bmim][BF₄] or [bmim][PF₆] resulted in significant acceleration of polymerization rates and, in the case of low temperature polymerizations, reactions were complete in less than 2 h, while no product was observed in the corresponding volatile organic carbon (VOC) porogen. In all instances, MIPs generated in [bmim][BF₄] or [bmim][PF ₆] returned imprinting selectivities (I values) on par with or better than the corresponding MIP generated in VOCs. Imprinting values ranged between I ≤ 1 and 2.9, with rebinding limited to 1 h. MIP synthesis conducted at low temperature (5°C) afforded the highest I values. Scanning electron microscopy examination of MIP morphology highlighted an unexpected template effect with MIP structure varying between discrete nanoparticles and robust monoliths. This template?monomer interaction was also observed in the rates of polymerizations with differences noted in reaction times for 1 and 2 MIPs, thus providing indirect conformation of our previously proposed use of molecular modelling?nuclear magnetic resonance titrations (the MM-NMR method) in the design phase of MIP generation. In addition, considerable batch-to-batch rebinding selectivities were observed. CSIRO 2007.

Full text of DOI:10.1071/CH06284

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2007, 60, 51–56
Molecularly Imprinted Polymers and Room Temperature
Ionic Liquids: Impact of Template on Polymer Morphology
Katherine Booker,A Michael C. Bowyer,B Chris J. Lennard,C
Clovia I. Holdsworth,A and Adam McCluskeyA,D
AChemistry Building, School of Environmental & Life Sciences, University of Newcastle,
Callaghan NSW 2308, Australia.
BDiscipline of Applied Sciences, Central Coast Campus, University of Newcastle,
Ourimbah NSW 2258, Australia.
CForensic Services, Australian Federal Police Services Centre, Weston ACT 2611, Australia.
DCorresponding author. Email: Adam.McCluskey@newcastle.edu.au
Molecularly imprinted polymers (MIPs) were generated for trans-aconitic acid 1 and cocaine 2 in a variety of porogens
(CH3CN, CHCl3, [bmim][BF4], and [bmim][PF6]). MIP synthesis in either [bmim][BF4] or [bmim][PF6] resulted in
significant acceleration of polymerization rates and, in the case of low temperature polymerizations, reactions were
complete in less than 2 h, while no product was observed in the corresponding volatile organic carbon (VOC) porogen. In
all instances, MIPs generated in [bmim][BF4] or [bmim][PF6] returned imprinting selectivities (I values) on par with or
better than the corresponding MIP generated in VOCs. Imprinting values ranged between I = 1 and 2.9, with rebinding
limited to 1 h. MIP synthesis conducted at low temperature (5◦C) afforded the highest I values.
Scanning electron microscopy examination of MIP morphology highlighted an unexpected template effect with MIP
structure varying between discrete nanoparticles and robust monoliths. This template–monomer interaction was also
observed in the rates of polymerizations with differences noted in reaction times for 1 and 2 MIPs, thus providing indirect
conformation of our previously proposed use of molecular modelling–nuclear magnetic resonance titrations (the MM-
NMR method) in the design phase of MIP generation. In addition, considerable batch-to-batch rebinding selectivities were
observed.
Manuscript received: 8 August 2006.
Final version: 15 November 2006.
Introduction
target molecules, for example, flavour contaminants in wine and
selected illicit drugs[5–9] in a wide range of domestic settings.
Pivotal to our approach is the rapid development of specific
recognition elements that have the ability to respond (generate a
signal) in the presence of a chemical vapor.[10] Most transduction
methods are already well developed but the design of a suitable
chemical recognition element remains a challenge to date. Tech-
niques that have been widely used to impart recognition to sens-
ing devices for trace levels of explosives and narcotics are based
on the interactions of the substrate with biological molecules
(such as host–guest and antigen–antibody interactions),[4,5]
which are unstable and prone to saturation and decomposition.
Over the past five years, we have developed considerable
expertise in the field of molecular imprinting (a ‘biomimetic’
process) and have applied the technology to the extraction, detec-
tion, and measurement of specific chemical components.[5–9]
Specific recognition sites created during the molecular imprint-
ing process hold considerable potential as highly specific chem-
ical recognition elements. Our group,[5–9] and others,[10–12] have
expended considerable effort in developing rational approaches
to such elements. The application of the molecular modelling–
nuclear magnetic resonance titration (MM-NMR) approach by
us has resulted in the rapid and rational design of a wide variety
of molecularly imprinted polymers (MIPs). Notably, we have
The illegal importation of illicit drugs is a growing concern
for the Australian Federal Police and Australian Customs ser-
vices and is placing an increasing demand on already stretched
resources at major Australian border entry points. While canine
detection and intelligence led policing has been highly success-
ful, bothhaveconsiderabledrawbacks, thelatterhasconsiderable
resource implications and the former is limited by the expense
associated with the handling and training of dogs, their nar-
row attention span, and the limited amount of reliable scientific
information obtained.[1,2] While some instrumental trace-level
detection methods are commercially available for use (gas
chromatography with chemiluminescence, electron capture, or
surface acoustic waves detectors and, of particular note, ion
mobility spectrometers, and biosensors) and are continually
improving, they generally still suffer from selectivity and sensi-
tivity problems, limited mobility/tracking ability, high level of
intrusiveness, high rate of false positive results, short shelf-lives
(e.g., immunosensors), and high cost.[1,3,4] These existing tech-
nologies also often require expert training in their use and data
analysis, and may be too complex and specialized to allow a lay
jury to fully comprehend the significance of the data generated.
Over the past decade, our group has been developing method-
ologies that have the potential to allow passive sensing of
© CSIRO 2007
10.1071/CH06284
0004-9425/07/010051

RESEARCH FRONT
52
K. Booker et al.
H3C
N
obtained excellent results even with the more recalcitrant of
templates, namely those lacking in hydrogen-bonding or elec-
trostatic interactions. Notwithstanding our successes, post-MIP
manipulation (to uniform particle size) is laborious and often
destroys the specific cavities that were developed during the
templating process. Precipitation polymerization approaches,
excepting their slow reaction rates and hence extended reac-
tion times and high porogen requirements, are ideally disposed
to circumvent this issue.
O
O
OCH3
OH
O
HO
O
O
O
HO
1
2
Recently, room temperature ionic liquids (RTILs) have been
shown to accelerate polymerization rates.[13] The magnitude
of the propagation rate constant for these reactions has been
shown to increase with increasing mole fraction of the ionic
liquid in the reaction mixture. The unusual solvation proper-
ties displayed by RTILs have been partially attributed to the
maintenance of a supramolecular structure in the liquid phase,
which leads to the formation of discrete polar and non-polar
microenvironments.[14] Enzymes have been reported to exhibit
enhanced thermal stability when immersed in RTIL.[15]
Not surprisingly we believed that this might alleviate the
protracted reaction time associated with traditional approaches
to precipitation polymerizations. Our preliminary findings
using arguably the two most highly evaluated RTILs, 1-butyl-
3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and
1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]-
[PF6]), are reported herein.
Fig. 1. Trans-aconitic acid and cocaine.
Table 1. Polymerization times for 1 and 2
Porogen
CH3CN
Reaction
temperature
[◦C]
Volume
porogen
[mL]
Reaction time [h]
1
2
A
B
5
60
5
5
25
5
25
5
25
5
25
5
25
5
25
5
25
5
—
—
6
—
—
—
—
—
—
6
A
B
B
B
18
B
B
B
B
A
A
CHCl3
—
—
—
—
60
5
18
0.5
2
2
4
0.75
2
2
[bmim][BF4]
[bmim][PF6]
0.75
2
Results and Discussion
60
5
2
8
We have previously detailed the procedure whereby we identified
the most appropriate ratio of template to functional monomer
by our MM-NMR approach.[6–9] Herein we evaluate two tem-
plate systems and the effect of volatile organic compounds
(VOCs) (CH3CN and CHCl3), [bmim][BF4], and [bmim][PF6]
on the rebinding efficacy and polymer morphology relative to
the analogous non-imprinted polymers (NIPs). In this study
our data (not shown) indicated that with trans-aconitic acid
and cocaine (Fig. 1), the most favourable template to monomer
ratio was 2:1 (methacrylic acid to template).[8,9] Ethylene glycol
dimethacrylate (EGDMA) was used as the polymer crosslinker.
In the initial templating process of MIP generation, the
template and monomer interact to form a transient cluster.
As the internal energy of the system increases (e.g., by heat-
ing) the transitory nature of the template–monomer interaction
also increases, and should decrease the strength of the pre-
polymerization cluster to theoretically yield lower imprinting
values. As the temperature of the pre-polymerization mixture
is decreased the stability of the template–monomer cluster
increases. Thus in a low temperature system we anticipate larger
template–monomer interactions being reflected in the ultimate
imprinting value. Indeed the change in ‘cluster nature’should be
evident from NMR analysis.[12] To evaluate the impact of tem-
perature, we conducted polymerizations at 5◦C (photochemical
initiation, azobisisobutyronitrile (AIBN)) and at 60◦C (thermal
initiation, AIBN). Thus in addition to evaluating the porogens’
impact on selectivity as a function of bulk versus precipita-
tion polymerization, we were also keen to evaluate the effect
of temperature.
0.5
2
60
2
25
8
4
ANo reaction. BNot performed.
and precipitation). The outcomes of these reactions are shown
in Table 1.
As can be seen from Table 1 traditional VOCs, i.e., CH3CN
and CHCl3, give no polymerization at low temperature under
either bulk or precipitation conditions, whereas both RTILs,
[bmim][BF4] and [bmim][PF6], examined at 5◦C gave excellent
yields of polymer, and also at 60◦C. Interestingly, although the
monomer-to-template ratio was consistent, and the same func-
tional monomer unit (methacrylic acid) was used in preparing
MIPs from 1 and 2, there is an observable difference in the rates
of reaction observed.At both 5 and 60◦C, polymerization is more
rapid with 2 under bulk conditions (5◦C, 30 versus 45 min; 60◦C,
4 versus 8 h) in [bmim][BF4]. The rate of reaction is reversed
when the reactionsare conducted in [bmim][PF6], butonlyatlow
temperature (5◦C, 45 versus 30 min). Perhaps the most surpris-
ing observation in this sequence is the extended polymerization
intervals required at higher temperatures. At 60◦C the rate of
reaction attains consistency with cocaine MIPs generated twice
as fast, but requiring 4 to 8 h for completion. Obviously, at lower
temperatures the RTILs have a pronounced effect on the stability
and reactivity of the radicals generated during polymerization
and at low temperatures affect a cage-like structure, which is
negated by the decrease in viscosity as the reaction tempera-
ture increases. Notwithstanding this, polymerization still occurs
more rapidly in RTILs than in the traditional VOCs examined.
In essence, two series of studies were conducted; the first
under bulk conditions (5 mL of porogen at 5 and 60◦C) and the
second under precipitation conditions (25 mL porogen at 5 and
60◦C). Each template and porogen combination in turn was sub-
jected to our standard polymerization conditions (for both bulk

RESEARCH FRONT
MIPs and RTILs: Template and Polymer Morphology
53
Table 2. Rebinding results (after 1 h) for MIPs generated from 1 and 2 in porogens: CH3CN, CHCl3, [bmim][BF4],
and [bmim][PF6]
Porogen
Reaction temp.
[◦C]
Volume
[mL]
I of 1A
I of 2A
Batch 2B
Batch 1B
Ave.
CH3CN
CHCl3
60
60
25
5
25
5
25
5
25
5
0.98 (1.6)B
—
—
1.2C
1.1
1.2
1.1
1.3
1
1
1
2.2
1.6
—
2.9
1.3
1.7
2.8
1.2
2.1
1.2
1.2
1.2
2.3
2.0
1.2
1.5
2.0
1.2
1.6
1.1
1.1
1.7
1.9
—
[bmim][BF4]
5
60
5
2.3
1.2 (2.3)B
1.8 (2.2)B
2.7
[bmim][PF6]
2.3
2.5
1.5 (1.8)B
1.0 (1.4)B
25
5
25
60
AI = BMIP/BNIP
.
B24 h rebinding time. CI = 3.26 (from McCluskey et al.,[8] T:M ratio 1:2) and 1 h rebinding.
The inability to form polymers in CH3CN at low tempera-
ture could have been caused by the presence of significant free
radicals capable of inhibiting polymer chain growth brought
about by photochemically induced homolytic cleavage of the
C–H bond.[16] Conversely, chloroform is a recognized chain-
transfer agent in free radical polymerization reactions, its C–Cl
bond can be easily cleaved to generate free radicals. While chain
transfer to these solvents is also expected at 60◦C, the effect is
more pronounced at lower temperature (5◦C), where diffusion
is hindered. Hence chain propagation can be very slow, and free
radicals generated by the solvents abound.
With the required polymers in hand, we set about examining
theirabilitytorebindtheoriginaltemplate.Asourobjectiveisthe
development of a convenient test for the detection of illicit drugs,
we restricted rebinding times to 1 h. These data are presented in
Table 2.
a decrease in I[bmim][BF ] and I[bmim][PF ] under thermal initiation
4 6
conditions. This is most notable with MIP[bmim][PF ] at 60◦C,
6
which gives no selective rebinding after 1 h.
With cocaine-based MIPs our initial results were extremely
disappointing and led us to question the utility of RTIL-based
porogens in MIP preparation. In previous work, we had observed
an ICHCl = 1.17 for a cocaine MIP (and ICHCl 3.26;[8] with
3
3
template/monomer ratios of 1:2), but we obtained a maximal
rebinding from RTIL-based MIPs of I[bmim][BF ] 2.17. Indeed the
4
other data presented are strongly suggestive of no specific bind-
ing being obtained, with I values ranging from 1 (no specificity)
to 1.60 (modest specificity). Perplexed, we generated a second
batch of cocaine MIPs using both RTILs and CHCl3, and noted
an across the board improvement in I values on an equal footing
with ICHCl 2.86 and I[bmim][BF ] 2.83. Again both RTILs dis-
3
4
played divergent outcomes with the maximal I[bmim][BF ] 2.83
4
First, examination of the data obtained with trans-aconitic
acid templated MIPs clearly reveals that MIPsRTIL are signif-
observed at low temperature and precipitation polymerization
conditions, whereas the maximal I[bmim][PF ] 2.27 was observed
6
icantly superior to the corresponding MIPCH CN. Rebinding
under thermal precipitation conditions. We are thus not yet able
to predict which RTIL and/or combination of polymerization
conditions will afford the most responsive MIP.
3
values, I, increase from ICH CN 0.98 (no specificity relative
3
to NIP) to a maximum of I[bmim][BF ] 2.7. At low temper-
4
atures, which should favour template monomer interaction,
Scrutiny of the scanning electron microscopy (SEM) images
inFig. 2showsstriking, template-dependantcontrastsinpolymer
morphology.With traditionalVOCs there is an obvious transition
from precipitation micro-polymer spheres (Fig. 2a) to a mono-
lithic structure (Fig. 2b). While it is conceivable that the change
from MIPCH CN-1 to MIPCHCl -2 is the sole rational for a change
MIP[bmim][BF ] exhibits maximal rebinding (I[bmim][BF ] 2.3)
4
4
under bulk polymerizations conditions, not under precipitation
conditions. Whereas with MIP[bmim][PF ], maximal I[bmim][PF ]
6
6
2.5 is obtained under precipitation conditions, but also at low
temperature. While at first glance the MIP[bmim][BF ] result
4
3
3
appears counterintuitive (precipitation polymerization generates
discrete particles of uniform size while bulk polymerization
yields a monolith that requires post-imprinting manipulation
(grinding and sieving to < 38 × 10−6 m)), this is not the case
with trans-aconitic acid MIPRTIL. Both precipitation and bulk
polymerization approaches give nanometer-sized polymer par-
of this magnitude, our findings with MIPRTILs suggest that this
is not the case. Rather, it strongly suggests that the imprinting
template influences the polymerization process and is a major
determinant of the type of polymer material isolated. In addition,
different RTILs affect template-induced polymer morphology
changes in different manners. With MIP[bmim][BF ]-2, a greater
4
ticles (Figs 2b–2i). In this instance the higher I[bmim][BF ] values
degree of monolithicity compared with MIP[bmim][BF ]-1 is evi-
4
4
are presumably a result of a tighter template–monomer cluster as
a function of the higher reaction concentration (all other things
being equal). To further explore these findings we also evaluated
this MIP series after 24 h rebinding (the values in parentheses
within Table 2) and note that when diffusion and contact times
increaseitistheprecipitation-basedMIPsthatexhibitthehighest
I values.This is presumably an artifact of accessing well-defined
cavities within the polymeric matrix, and a slight difference in
surface morphology (Fig. 2). Porogen viscosity undoubtedly also
affects the outcome of rebinding studies and there is, uniformly,
dent, and differences in surface structure and particle sizing is
also noted when comparing the corresponding SEM images, e.g.,
Fig. 2c versus 2l, 2d versus 2m, and 2e versus 2n. Similar but
less distinct trends are evident with MIP[bmim][PF ]-2 compared
6
with MIP[bmim][PF ]-1 with the major variations appearing to be
6
a difference in particle size, with most MIP[bmim][PF ] examined
6
showing an apparent hybrid of monoliths and discrete particles.
SEM was not able to provide suitable resolution to deter-
mine a statistical distribution of each (monolith versus polymer
sphere).

RESEARCH FRONT
54
K. Booker et al.
Porogen
trans-Aconitic acid[9]
Cocaine
VOC
CH3CN
(a)
CHCl3
( j)
[bmim][BF4]
5°C 5 mL
(b)
(k)
[bmim][BF4]
5°C 25 mL
(c)
(l)
[bmim][BF4]
60°C 5 mL
(d)
(m)
[bmim][BF4]
60°C 25 mL
(e)
(n)
Fig. 2. SEM images of MIPs prepared using different porogens, temperatures, and volumes. (a)–(i) trans-aconitic
acid MIPs, (j)–(r) cocaine MIPs.
Conclusions
The effect of template–monomer interaction was also
observed in the rates of polymerizations with differences noted
in reaction times for 1 and 2 MIPs. This provided indirect
conformation of our previously proposed use of MM-NMR
titrations in the design phase of MIP generation.
This work has identified RTILs as potentially viable porogens
for the synthesis of molecularly imprinted polymers with ben-
efits, in this limited study, that include a decrease in reaction
duration, the ability to generate MIPs at low temperature with

RESEARCH FRONT
MIPs and RTILs: Template and Polymer Morphology
55
[bmim][PF6]
5°C 5 mL
(f)
(o)
[bmim][PF6]
5°C 25 mL
(g)
(h)
(i)
(p)
(q)
(r)
[bmim][PF6]
60°C 5 mL
[bmim][BF4]
60°C 25 mL
Fig. 2. Continued
a concomitant increase in imprinting specificity and selectivity,
and in selected instances, control over particle size. Thus RTILs
hold considerable promise to accelerate the synthesis of a wide
variety of MIPs, and in the examples studied here deliver selec-
tivities at least on par with, if not better than, those observed with
traditional VOC-based porogens. Notwithstanding this, a more
expansive range of both RTILs and templates must be evalu-
ated before concluding that MIPRTILs are uniformly better than
MIPVOCs. The effect of template on polymer morphology was
unexpected, but confirms a template–monomer interaction dur-
ing the polymerization process, but it further complicates the
issue of how to predict which RTIL and what conditions are best
suited to the development of highly specific and robust MIPs.
A final note of caution is the observed batch-to-batch variation
of selectivity observed when MIPs are prepared under identical
conditions. MIP design and synthesis have evolved over the past
decade or so, but there are still issues to be resolved before their
synthesis and utility can be described as routine. MIPs and RTILs
are thus kindred cousins, they have immense untapped potential,
but are unable to enter mainstream chemistry while their lack of
predictability remains. We are continuing to evaluate the effect
of template and RTILs on MIP generation and selectivity, and
will report our findings in due course.
Experimental
Cocaine MIPs were prepared with a cocaine base (0.14 mmol,
42.6 mg), methacrylic acid (MAA, 0.28 mmol, 24.1 mg) and
EGDMA (1.4 mmol, 280 mg), following the procedure of
Holdsworth et al.,[8] in 5 mL (bulk condition) and 25 mL (pre-
cipitation condition) of porogen at both 60◦C (thermal initiation)
and 5◦C (photochemical initiation, UV irradiation at 365 nm).
The reaction mixture was degassed with N2 before AIBN
(10 mg) was added. The porogens used were CHCl3 (control),
[bmim][BF4], and [bmim][PF6]. NIPs were prepared using the
same method but without the addition of the cocaine base.

RESEARCH FRONT
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K. Booker et al.
Extraction of the template was achieved by five washings
in methanol (30 mL) until no cocaine peak was registered by
GCMS analysis.
Trans-Aconitic Acid Quantification
Thecolumnwassetataninitialtemperatureof84◦C, followed
by an increase of 8◦C min−1 to 200◦C, hold 2 min, then increased
by 10◦C min−1 to 300◦C and hold 15 min.
Rebinding was carried out using 10–40 mg of polymer sus-
pended in a 3–4 ppm solution of cocaine in CHCl3 for various
time periods. The resulting solution was filtered and analyzed
using GC-MS. Cocaine (retention time 9.33 min) was quanti-
fied using an external calibration method with a linear curve
where R2 = 0.998 at the concentration range of 3–30 ppm.
Theconcentrationofreboundcocainewascalculatedfromthe
difference in solution concentration before and after rebinding.
The total selective binding of the polymer (ꢀB) was calculated
as the difference between the MIP binding and the NIP binding
(BMIP − BNIP). The results are expressed as an imprinting factor
Acknowledgments
TheauthorsthanktheAustralianResearchCouncilandtheAustralianFederal
Police Forensic Services for their generous financial support.
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to a flow rate of 1.00 mL min−1, total flow of 9.0 mL min−1
,
linear velocity of 37.2 cm s−1, and a pressure of 73.0 kPa. The
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Cocaine Quantification
The column was set at an initial temperature of 100◦C
which was raised at 20◦C min−1 over 10 min, reaching a final
temperature of 300◦C.