Organic zeolites from a diolefinic monomer

Article abstract of DOI:10.1021/ja0745658

Submicrometer particles of diethyl p-phenylenediacrylate (EPA) with tunable molecular adsorption characteristics were produced by solid-state photopolymerization in the presence of template molecules. EPA monomer particles were produced by rapid expansion of supercritical solutions (RESS), and deposited directly onto surface acoustic wave (SAW) resonators. The EPA particles were photopolymerized directly on the SAW devices in the presence of molecular templates, and dynamic sorption isotherms of C₁- through C₉-alkanes were studied to characterize the particle-vapor interaction. The mass increase due to vapor uptake into the particulate coatings was measured by monitoring the SAW resonance frequency during vapor sorption. The vapor selectivity and molecular porosity of the particulate coatings were studied in situ on the piezoelectric substrate by measuring sorption isotherms. A gradual exclusion of smaller alkane molecules from the molecularly imprinted particulate coatings was observed with decreasing template molecule size. The observed selective and reversible adsorption of alkane analytes with different molecular sizes suggest that these imprinted polymers may be categorized as organic analogues of zeolites.

Full text of DOI:10.1021/ja0745658

Published on Web 10/19/2007
Organic Zeolites from a Diolefinic Monomer
Ozge Guney-Altay, Dmitry Pestov, and Gary Tepper*
Contribution from the Department of Mechanical Engineering, Virginia Commonwealth
UniVersity, Richmond, Virginia 23284-3015
Received June 21, 2007; E-mail: gctepper@vcu.edu
Abstract: Submicrometer particles of diethyl p-phenylenediacrylate (EPA) with tunable molecular adsorption
characteristics were produced by solid-state photopolymerization in the presence of template molecules.
EPA monomer particles were produced by rapid expansion of supercritical solutions (RESS), and deposited
directly onto surface acoustic wave (SAW) resonators. The EPA particles were photopolymerized directly
on the SAW devices in the presence of molecular templates, and dynamic sorption isotherms of C1- through
C9-alkanes were studied to characterize the particle-vapor interaction. The mass increase due to vapor
uptake into the particulate coatings was measured by monitoring the SAW resonance frequency during
vapor sorption. The vapor selectivity and molecular porosity of the particulate coatings were studied in situ
on the piezoelectric substrate by measuring sorption isotherms. A gradual exclusion of smaller alkane
molecules from the molecularly imprinted particulate coatings was observed with decreasing template
molecule size. The observed selective and reversible adsorption of alkane analytes with different molecular
sizes suggest that these imprinted polymers may be categorized as organic analogues of zeolites.
Introduction
solids mimicking the physicochemical behavior of zeolites with
respect to organic sorbate species.3 The “zeolites” label was
Zeolites are inorganic microporous crystalline solids, which
have well-defined network structures made up of silicon,
aluminum, and oxygen. The tunnels and cages within the zeolite
framework allow molecules small enough to pass through to
be adsorbed while larger molecules are rejected. For this reason
zeolites are also referred to as molecular sieves. Because of their
precise and uniform molecular porosity and very high internal
surface areas, zeolites are uniquely suited for a variety of
applications, including petrochemical cracking, ion exchange
(water softening and purification), and separation and removal
of gases and solvents.1,2
used to mark permanent porosity of these new materials, which
is a characteristic of true zeolites.12 The adsorption in zeolites
is fully reversible and purely physical in nature. The structure
of the zeolite stays intact during adsorption and its regeneration,
which usually involves heating and purging with a carrier gas.
The “organic” label was used to stress the hydrophobic nature
of the interior pore surface12 that can therefore selectively absorb
nonpolar organic compounds. This is contradictory to the unique
characteristic that distinguishes zeolites from other commercially
available adsorbents: that is the highly polar surface within the
pores is the main driving force for adsorption in zeolites.
The greatest advantage of organic zeolites over traditional
zeolites is the greater control over the chemical nature of the
accessible surface that is provided by organic components. There
exists a huge multitude of organic building units and structural
patterns into which they can be assembled to develop new and
improved zeolites. Current efforts in the synthesis of organic
zeolites has focused on glassy or semicrystalline polymers,
crystalline organic materials, and metal-organic frameworks.3,13
In particular, crystalline metal-organic frameworks that have
been produced exhibit impressive levels of gas uptake.14,15
Some types of zeolites occur naturally as minerals and are
extensively mined in many parts of the world. Naturally
occurring zeolites are the oldest known family of microporous
materials. About 50 natural zeolites have been identified to date.3
Others are synthetic and are produced commercially for specific
uses. The interest in the synthesis of zeolitic materials started
in the 1950s and was stimulated mostly by their efficacy as
catalysts in the petrochemical industry.4 Synthetic zeolites have
a high impact on today’s oil industry.5
In recent years, many researchers have tried to create organic
analogues of zeolites.6-11 Organic zeolites are non-silicious
(7) Hodge, K.; Prodpran, T.; Shenogina, N. B.; Nazarenko, S. J. Polym. Sci.,
Part B: Polym. Phys. 2001, 39, 2519-2538.
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28, 359-376.
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D. Chem. Mater. 2001, 13, 1506-1511.
(2) Breck, D. W. Zeolite molecular sieVes: structure, chemistry, and use;
Wiley: New York, 1973; pp 1-2.
(9) Nossov, A. V.; Soldatov, D. V.; Ripmeester, J. A. J. Am. Chem. Soc. 2001,
123, 3563-3568.
(3) Soldatov, D. V.; Ripmeester, J. A. In Nanoporous Materials IV; Elsevier
Science BV: Amsterdam, 2005; Vol. 156, pp 37-54.
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591-593.
(4) Rabo, J. A.; Schoonover, M. W. Appl. Catal., A 2001, 222, 261-275.
(5) Szostak, R. Molecular SieVes: Principles of Synthesis and Identification,
2nd ed.; Blackie Academic and Professional: London, 1998; p 464.
(6) Gorbatchuk, V. V.; Mironov, N. A.; Solomonov, B. N.; Habicher, W. D.
Biomacromolecules 2004, 5, 1615-1623.
(11) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich,
N. Nature 1993, 366, 324-327.
(12) Soldatov, D. V.; Enright, G. D.; Ripmeester, J. A. Cryst. Growth Des. 2004,
4, 1185-1194.
(13) Budd, P. M. Science 2007, 316, 210-211.
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J. AM. CHEM. SOC. 2007, 129, 13957-13962
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A R T I C L E S
Guney-Altay et al.
Scheme 1
The variable pore dimensions and the large pore volume of
traditional zeolites make them ideal candidates for sensor design
with effective molecular size discrimination and high sensitiv-
ity.16 For this very purpose, several strategies, including dip-
coating of zeolite/silica sol suspension,17 chemical attachment
of zeolite crystals via molecular coupling,18 embedding zeolite
crystals into polymers,19 covering with glazes,20 and extruding
into composite monoliths,21 have been explored for thin-film
formation. In order to enhance control of molecular selectivity
in sensors, the pore sizes of the zeolite crystals in these films
were controlled through in situ cation exchange.16 The main
concern in the above preparations is the potential blockage of
the zeolitic pores and/or the formation of nondesirable additional
affinity introduced by the matrix in the composites.16
Experimental Section
Materials and Equipment. HPLC-grade pentane, hexane, heptane,
octane, isooctane, nonane, decane, and 1-butanol were obtained from
Fisher Scientific. Methane (99.9995%), ethane (99.9995%), propane
(99.99%), butane (99.99%) were obtained from BOC Gases. HPLC/
spectrophotometric grade ethanol (dry) was obtained from Sigma-
Aldrich. DI water was used for monomer synthesis and imprinting.
Chlorodifluoromethane (Freon 22) was obtained from DuPont. The
Diethyl p-phenylenediacrylate (EPA) was synthesized using a procedure
described below. Nitrogen (Grade 5.5) was obtained from National
Welders. SAW 250 MHz resonators were from Microsensors Systems,
Inc. For polymerization of EPA particles, a UV lamp from American
Ultraviolet was used. This lamp has 100-W power and emits UV within
the range of 325-382 nm.
Diethyl p-Phenylenediacrylate (EPA). The monomer used in this
study, EPA, was synthesized using the method described by Grinev et
al. with some modifications.25 Sodium ethoxide (14.9 mL of 21%; 0.04
mol) in ethanol was added to 8.97 g (0.04 mol) of triethyl phospho-
noacetate in 30 mL of dry DMF, and the mixture was stirred for 1 h.
A solution of 2.68 g (0.02 mol) of terephthalaldehyde in 12 mL of dry
DMF was added dropwise with continuous stirring for 0.5 h, and the
reaction mixture was left overnight. EPA was precipitated by adding
water 3:1 by volume to the reaction solution. After 0.5 h the precipitate
was filtered and rinsed by DI water and then ethanol. The resulting
white powderlike crystals were obtained with 68% yield. After
recrystallization from ethanol, the melting point of the EPA was 97 (
1 °C (96 °C lit.25).
Preparation of EPA Particles. EPA was dissolved in supercritical
Freon 22 at 340 atm and 120 °C. The solution was expanded through
a capillary nozzle and sprayed directly onto SAW devices and glass
microscope slides. The EPA particles obtained with this technique were
0.3-1 µm in diameter. The coated glass slides were used to investigate
the particle size and morphology. The coated SAW devices were used
to evaluate the selective vapor sorption property. A more detailed
description of the apparatus and experimental procedure is provided
in previous publications.23,26
Photopolymerization. The EPA particles were polymerized in the
solid state directly on the various deposition substrates (i.e., SAW
transducers, microscope slides). The EPA-coated substrates were placed
in a sealed chamber filled with air. In order to facilitate polymerization,
the samples were irradiated by the UV lamp through an optical filter
(fused silica window by Mellis Griot) for 15 min from a distance of
22 cm. The optimization of these conditions was published pre-
viously.23 The topochemical polymerization reaction of EPA is
shown in Scheme 1.
In this contribution, we present the preparation and charac-
terization of organic analogues of zeolites. The molecular sieving
capability of diethyl p-phenylenediacrylate (EPA) submicrome-
ter polymer particles is demonstrated by the sorption behavior
of alkanes of different sizes. These tunable submicrometer EPA
particles are prepared by rapid expansion of supercritical
solutions (RESS), and the molecular size and shape selectivity
is controlled by the introduction of a vapor template molecule
during a solid-state photopolymerization reaction. Previously,
we showed that when diolefinic monomers are polymerized in
the presence of a template alkane vapor, the final polymer
exhibits a molecular porosity that is related to the size of the
template molecule.22,23 Recently, these molecularly imprinted
polymers were reported to be stable up to 3.8 years.24 We now
report on controlling and tuning the molecular porosity by using
water and alcohols of various sizes. Dynamic sorption isotherms
of alkane analytes were studied to characterize EPA particulate
coatings on SAW devices. Although the molecular porosity was
created using template molecules from a different chemical
family, these coatings exhibited reversible sorption properties
when tested against alkane analytes. A gradual exclusion of
smaller analyte molecules from the molecularly imprinted
particulate coatings is observed as the size of the template
molecule is reduced. The observed reversible adsorption of
alkane analytes with different molecular sizes suggests that these
imprinted polymers may be categorized as organic zeolites. The
high molecular porosity and controllable pore size, coupled with
the versatility of organic synthesis, promise that these materials
may be the initial step toward the preparation of key-lock
structures that may perform like enzymes.
(14) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem. Soc. 2006,
128, 3494-3495.
(15) Lin, X.; Jia, J. H.; Zhao, X. B.; Thomas, K. M.; Blake, A. J.; Walker, G.
S.; Champness, N. R.; Hubberstey, P.; Schroder, M. Angew. Chem., Int.
Ed. 2006, 45, 7358-7364.
(16) Yan, Y. A.; Bein, T. J. Am. Chem. Soc. 1995, 117, 9990-9994.
(17) Bein, T.; Brown, K.; Frye, G. C.; Brinker, C. J. J. Am. Chem. Soc. 1989,
111, 7640-7641.
Preparation of Organic Zeolites via Molecular Imprinting. The
EPA coatings were photopolymerized in the presence of template vapors
directly on the various deposition substrates (i.e., SAW transducers,
microscope slides). The EPA-coated substrates were placed in a sealed
chamber filled with air saturated with the selected template vapor. The
chamber was kept in the dark for 40 min to allow EPA particles to
absorb template vapors before polymerization. The monomer particles
were polymerized in the presence of the template vapor as described
in the section above. Upon photopolymerization, we obtained an
(18) Yan, Y. G.; Bein, T. Chem. Mater. 1992, 4, 975-977.
(19) Hennepe, H.; Boswerger, W. B. F.; Bargeman, D.; Mulder, M. H. V.;
Smolders, C. A. J. Membr. Sci. 1994, 89, 185-196.
(20) Geus, E. R.; Schooman, J.; van Bekkum, H. Synthesis, Characterization
and NoVel Applications of Molecular SieVe Materials; Materials Research
Society Pittsburgh: PA, 1993; p 304.
(21) Byrne, J. W.; Chen, J. M.; Speronello, B. K. Catal. Today 1992, 13, 33-
42.
(22) Pestov, D.; Levit, N.; Colby, D.; Tepper, G. Presented at the 220th National
Meeting of the American Chemical Society, Washington, DC, August 2000;
Paper PMSE-293.
(23) Pestov, D.; Levit, N.; Maniscalco, V.; Deveney, B.; Tepper, G. Anal. Chim.
Acta 2004, 504, 31-35.
(24) Pestov, D.; Guney-Altay, O.; Levit, N.; Tepper, G. Sens. Actuators, B 2007,
126, 557-561.
(25) Grinev, G. V.; Dombrovskii, V. A.; Yanovskaya, L. A. IzV. Akad. Nauk
SSSR, Ser. Khim. 1972, 635-637.
(26) Guney-Altay, O.; Levit, N.; Pestov, D.; Tepper, G. J. Supercrit. Fluids
2006, 37, 229-241.
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Organic Zeolites from a Diolefinic Monomer
A R T I C L E S
Figure 1. Dynamic sorption testing station for the characterization of coating porosity. *Gas analytes: C1-C4-alkanes. **Liquid analytes: C5-C9-alkanes.
inclusion compound in an atmosphere of the template sorbate. The
coatings were then flushed with dry nitrogen for 20 min, and the
inclusion compound converts into the microporous form by means of
template sorbate removal.
diameters are maintained at less than 1 µm is a crucial step in
the preparation of organic zeolites with controlled molecular
porosities. The optimization of RESS processing conditions
yields particle sizes within the desired range.26 Previously, we
have shown that the unmicronized EPA monomer crystals
undergo mechanical disintegration upon polymerization.26 Stress
release during the photopolymerization results in the formation
of large cracks in the direction of chain growth. The main
concern in the preparation of organic zeolites from EPA is the
possible destruction of the molecular porosity during crystal
disintegration caused by photopolymerization. The desired
mechanical stability during polymerization can be achieved by
micronization of the EPA monomer via RESS.26
Photopolymerization. EPA is a diolefinic monomer and can
go through a four-center polymerization reaction in the solid
state (see Scheme 1) and produce a linear polymer. The
polymerization reaction is initiated by exposing the solid
monomeric crystalline substance to UV radiation and converting
it directly to the polymer without the use of external photoini-
tiators. Polymerization proceeds by the direct rearrangement of
molecules from the monomer to the polymer without internal
diffusion.28,29 Photopolymerization of diolefinic monomers in
the crystalline state has been studied extensively.30 In general,
topochemical polymerization is highly sensitive to the crystal
structure of monomers. We have previously studied the pho-
topolymerization of micronized EPA using FTIR spectro-
metry.23 The kinetics for various samples was evaluated at
different exposure times by measuring IR absorption of the
double bond at 1630 cm-1. It was concluded that 15 min of
UV exposure was sufficient for complete polymerization of these
particles.
Characterization of Coating Porosity via Dynamic Sorption
Isotherms. An experiment designed to test the ability of these structures
to exchange guest molecules with the environment was performed in
order to confirm zeolitic behavior of the prepared particles.3 The
reversible sorption behavior of imprinted EPA coatings was evaluated
by measuring the mass uptake of these particles when tested against
C1-C9-alkane analytes using SAW resonators. For each analyte six
different concentrations and for each concentration 10 sorption-
desorption cycles were tested using a custom-built system. The system
depicted in Figure 1 is the modified version of the system that was
described previously27 and is capable of introducing gas-phase analytes
as well as liquid. Analyte vapors were generated by means of
evaporation of liquid alkanes (C5-C9) at a temperature of 10 (
0.1 °C, or by directly introducing the gas-phase analytes (C1-C4) from
the cylinders through a regulator and a moisture trap (zeolite 3A).
Analyte concentration adjustments were made by controlled dilutions
with research grade nitrogen by means of mass flow meters. The analyte
gas is then delivered to the unit that is equipped with a pair of SAW
devices at a constant temperature of 30 °C. The pair of SAW devices
includes one reference device (labeled “R” in the Figure 1) and one
coated device (labeled “W” in the Figure 1), and the difference
frequency is measured. The reference device is uncoated and is exposed
to pure nitrogen at the same flow rate and temperature as for the analyte
stream directed to the coated device. The frequency difference versus
time data is recorded for increasing concentrations of each analyte gas.
Results and Discussion
Preparation of EPA Particles. Submicrometer-sized EPA
particles were prepared via RESS. This technique consists of
dissolution of the monomer in supercritical Freon 22 followed
by expansion of this solution through a capillary nozzle. Upon
expansion, the solvent (Freon 22) goes directly into the gas
phase, and the micronized monomer precipitates are deposited
on the target substrates. The main advantage of this technique
is that the monomer particles are formed free of residual solvent
contamination. Micronization of EPA such that the particle
Preparation of Organic Zeolites via Molecular Imprinting.
Preparation of organic zeolites is very similar to molecular
imprinting.31-34 The monomer compound is polymerized in the
(28) Hasegawa, M.; Suzuki, F.; Nakanishi, H.; Suzuki, Y. J. Polym. Sci., Polym.
Lett. Ed. 1968, 6, 293-297.
(29) Hasegawa, M. AdV. Polym. Sci. 1982, 42, 1-49.
(30) Hasegawa, M. AdV. Phys. Org. Chem. 1995, 30, 117-171.
(31) Mosbach, K. Trends Biochem. Sci. 1994, 19, 9-14.
(32) Ye, L.; Mosbach, K. Mater. Res. Soc. Symp. Proc. 2002, 723, 51-59.
(27) Levit, N.; Pestov, D.; Tepper, G. Sens. Actuators, B 2002, 82, 241-249.
9
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Figure 3. XRD scans of EPA polymer. The vapor used for the lower scan
is butanol.
able to prepare cavities of different molecular sizes suggests
that the porosity does not depend on the self-rearrangement of
the building blocks, but rather, on templating.
Obviously, there is a balance between rigidity of the polymer
and its ability to be imprinted. Rigidity is needed in order to
preserve the cavities upon removal of the template molecule.
However, if the polymer is highly rigid, then it will not conform
around the template molecule. We have found that EPA polymer
maintains this balance by testing EPA particulate coatings
imprinted to various template molecules against C1-C9-alkane
analytes. For each imprinted polymer tested, reversible sorption
experiments revealed that there was sufficient amount of vapor
sorption for analytes of proper molecular sizessufficient enough
to be easily detected by SAW devices. This result also suggests
that the imprinting procedure used to prepare organic zeolites
was an efficient process and that enough cavities were created
for selective absorption of analytes for each template molecule
tested. The template molecules used in this study are heptane,
butanol, ethanol, and water. For template molecules with
hydrogen-bonding ability, the imprinting efficiency was sig-
nificantly higher.
Figure 2. EPA particles (a) photopolymerized without template, (b)
photopolymerized in the presence of water, (c) photopolymerized in the
presence of butanol.
We have also studied the morphology and the structure of
the imprinted vs nonimprinted EPA polymers. Figure 2 shows
SEM images of the nonimprinted, butanol-imprinted, and water-
imprinted EPA polymers. The nonimprinted polymer seems to
exhibit crystalline features, but both of the imprinted polymers
look amorphous. This observation was confirmed by XRD
(Figure 3). Crystallinity is not necessary for microporosity, and
amorphous porous materials, such as activated carbons, are
widely used in industry.13 As such, all of these polymers
exhibited high stability. They were all able to reversibly absorb
analyte molecules and maintained their morphologies. Recently,
we have reported that the nonimprinted and butanol-imprinted
EPA polymer coatings were stable for up to 3.8 years.24
Characterization of Coating Porosity via Dynamic Sorp-
tion Isotherms. Zeolitic sorption implies the presence of
significant and permanently available micropore space in a
material. Any experiment designed to confirm zeolitic behavior
presence of a template molecule. Upon template extraction,
cavities complementing the size and shape of the employed
template molecule are obtained. Preparation of another polymer
with the same technique, but without the introduction of the
template molecule, and comparison of their sorption properties
can be used to evaluate the porosity of the organic zeolite.
Previously, we have reported the selective sorption of
heptane-imprinted EPA with respect to its nonimprinted coun-
terpart that were prepared by a procedure similar to the one
described above.23 It was found that EPA polymer prepared in
the presence of heptane vapor has preferential affinity to alkanes
with molecular size equal to or smaller than that of a C7-alkane.23
There are polymers and crystalline organic compounds with
intrinsic angstrom-sized cavities that have previously been
reported in the literature.3,8,35 In our case, the fact that we are
(33) Bossi, A.; Bonini, F.; Turner, A. P. F.; Piletsky, S. A. Biosens. Bioelectron.
2007, 22, 1131-1137.
(34) Dickert, F. L.; Hayden, O. Fresenius J. Anal. Chem. 1999, 364, 506-511.
(35) Miller, S. A.; Kim, E.; Gray, D. H.; Gin, D. L. Angew. Chem., Int. Ed.
1999, 38, 3022-3026.
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Organic Zeolites from a Diolefinic Monomer
A R T I C L E S
Figure 5. Frequency shift, which is proportional to mass uptake, as a
function of (9) ethane concentration for ethanol-imprinted EPA polymer,
(0) ethane concentration for nonimprinted EPA polymer, (b) heptane
concentration for ethanol-imprinted EPA polymer, and (O) heptane
concentration for nonimprinted EPA polymer.
functional groups and their sorption is based on Van der Waals
forces, which leads to a fast sorption-desorption process.
For each concentration of analyte molecules tested, data for
10 reversible sorption cycles were collected. A typical adsorp-
tion-desorption cycle is depicted in Figure 4a. The amplitude
of the sorption cycles is proportional to analyte concentration
and increases with increasing concentration. The amplitudes for
the 10 consecutive cycles are calculated and plotted as a function
of analyte concentration. Representative data are given in Figure
4b.
Figure 4. (a) Reversible sorption data for ethanol-imprinted EPA polymer
tested against heptane at a concentration of 2836 ppm. (b) Frequency shift,
which is proportional to mass uptake, as a function of heptane concentration
for ethanol-imprinted EPA polymer.
SAW devices are mass-sensitive tools, and the difference in
frequency during the sorption cycle and the baseline in Figure
4a is an indication of mass uptake by the imprinted coating.36
Accordingly, Figure 4b represents the adsorption isotherm of
heptane. The frequency increase and decrease that correspond
respectively to the adsorption and desorption of the analyte
molecule for each cycle are equal in magnitude. This may
suggest that the analyte adsorbed is flushed out during desorption
and that the imprinted EPA polymer is fully regenerated.
The sensitivity of the device to any given analyte is measured
by the slope of the isotherm and will increase with increasing
mass uptake. A comparison of the sensitivity of the SAW
devices coated with imprinted vs nonimprinted EPA polymer
revealed that the imprinted polymers preferentially adsorb
analyte molecules with sizes that are equal to or smaller than
the size of the template. This behavior is presented in Figure 5
with two sets of representative data collected for SAW devices
coated with ethanol imprinted vs nonimprinted EPA polymer.
In the first data set, both the ethanol-imprinted polymer
(represented by solid circles) and the nonimprinted polymer
(represented by hollow circles) are tested against heptane at
various concentrations. Because the analyte molecule, heptane,
is larger than the template molecule, ethanol, a significant
difference between the sensitivities of these devices to the
heptane analyte was not observed. In the second data set, both
the ethanol-imprinted polymer (represented by solid squares)
of an organic or metal-organic material should test the ability
of these structures to exchange guest molecules with the
environment.3 It was discovered that SAW and other acoustic
wave devices can be used as mass-sensitive tools for the
detection of even small mass changes in polymer coatings.36-40
In this study, the reversible sorption behavior of the imprinted
EPA polymers was evaluated by measuring the mass uptake of
these particles when tested against analyte molecules of various
sizes and concentrations using SAW resonators.
The experimental setup shown in Figure 1 was designed to
test the ability of the imprinted EPA polymer coatings on SAW
devices to reversibly absorb analyte molecules to confirm
zeolitic behavior. EPA polymer imprinted to hydroxyl-contain-
ing molecules such as water, ethanol, n-butanol along with
n-heptane, were tested against C1-C9-alkane analytes. Alkanes
were chosen as analytes because they do not have polar
(36) Ballantine, D. S., Jr.; White, R. M.; Martin, S. J.; Ricco, A. J.; Zellers, E.
T.; Frye, G. C.; Wohltjen, H. Acoustic WaVe Sensors. Theory, Design, and
Physico-Chemical Applications; Academic Press: San Diego, 1996; p 436.
(37) Dickert, F. L.; Bauer, P. AdV. Mater. 1991, 3, 436-438.
(38) Yan, Y. G.; Bein, T. J. Phys. Chem. 1992, 96, 9387-9393.
(39) Grate, J. W. Chem. ReV. 2000, 100, 2627-2647.
(40) Avramov, I. D.; Kurosawa, S.; Rapp, M.; Krawczak, P.; Radeva, E. I. IEEE
Trans. MicrowaVe Theory Tech. 2001, 49, 827-837.
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The selective adsorption of analyte molecules that have equal
or smaller sizes than the template is clearly revealed in Figure
6. For example, the SAW device imprinted to butanol shows
the highest affinity to the C5-alkane since these two molecules
have similar molecular sizes. The sensitivity difference between
the “small” C5 and the “big” C9 is on the order of 400%. The
relative sensitivity increase observed with heptane-imprinted
coating as the molecular size of the analyte decreases from
nonane to pentane is about 20%, which is plotted on the right
ordinate for comparison. The sensitivity improvement observed
in hydroxyl-containing template molecules may be attributed
to intermolecular hydrogen bonding between these molecules
and EPA during photopolymerization. The stronger bonding
between the template molecule and the polymer yields a higher
degree of molecular porosity.
Conclusions
Figure 6. Relative sensitivity of polymers to C1-C9 n-alkane analytes,
which are prepared by templated polymerization and characterized by mass
loading response. The templates used for imprinting are H2O, EtOH, BuOH,
and heptane. The corresponding data are labeled accordingly.
In summary, the polymerization of micronized EPA particles
in the presence of water and alcohol vapors result in polymers
with molecular size porosity corresponding to the template
vapors. The obtained material has amorphous structure with
randomly distributed pores. Accordingly, they do not show any
peaks in the XRD scans.
and the nonimprinted polymer (represented by hollow squares)
are tested against ethane at various concentrations. Because the
analyte molecule, ethane, is smaller than the template molecule,
ethanol, a significant difference between the sensitivities of these
devices is observed.
The results of the systematic analysis of the isotherms for
EPA polymer coatings imprinted to water, ethanol, n-butanol,
and n-heptane tested against C1-C9-alkane analytes are depicted
in Figure 6. Each data point in Figure 6 is the ratio of the slope
of the corresponding analyte’s isotherm for the imprinted EPA
polymer to that of the nonimprinted. Therefore, they represent
the sensitivity of the SAW devices with imprinted coatings
relative to the device with the nonimprinted coating. This
representation provides a comparison relative to a common
reference. They also isolate the response based on the molecular
porosity prepared by templated polymerization, eliminating the
effect of the bulk polymer.
In addition, the described technique allows preparing poly-
mers with zeolitic properties and hydrophobicity. The observed
reversible adsorption of alkane analytes with different molecular
sizes suggests that these imprinted polymers may be categorized
as organic zeolites. We believe that this supramolecular-
controlled templated polymerization and hydrophobicity offers
an intriguing potential for the design of macromolecular systems
with enzymelike catalytic activity.
Acknowledgment. This work was supported in part by grants
from the National Science Foundation and the U.S. Department
of Energy.
JA0745658
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13962 J. AM. CHEM. SOC. VOL. 129, NO. 45, 2007