Preparation of azobenzene-containing polymer membranes that function in photoregulated molecular recognition

Article abstract of DOI:10.1021/ma040010a

Selective, stable, molecularly imprinted polymers having intrinsic photoresponsive properties were synthesized for the purpose of photoregulated binding of a predetermined ligand. Highly cross-linked, free-standing polymer membranes were synthesized from

Full text of DOI:10.1021/ma040010a

Macromolecules 2004, 37, 9571-9576
9571
Preparation of Azobenzene-Containing Polymer Membranes That
Function in Photoregulated Molecular Recognition
Norihiko Minoura,*,† Kazuto Idei,‡ Alexandre Rachkov,† Young-Woo Choi,§
Masayo Ogiso,† and Kiyomi Matsuda‡
Research Center of Advanced Bionics, National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, Graduate
School of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575,
Japan, and Department of Environmental Science and Engineering, Kwangju Institute of Science
Technology, 1 Oryong-dong, Puk-gu, Kwangju, 500-712, Korea
Received January 14, 2004; Revised Manuscript Received September 1, 2004
ABSTRACT: Selective, stable, molecularly imprinted polymers having intrinsic photoresponsive properties
were synthesized for the purpose of photoregulated binding of a predetermined ligand. Highly cross-
linked, free-standing polymer membranes were synthesized from an optimized mixture of two cross-
linkerssethylene glycol dimethacrylate and tetraethylene glycol diacrylate. For synthesizing molecularly
imprinted polymers, p-phenylazoacrylanilide (PhAAAn) was used as a new photoresponsive functional
monomer. A study of the kinetics of photoisomerization of PhAAAn within the polymer membranes before
and after treatment with various media showed the excellent functional stability of the membranes.
Polymer membranes synthesized in the presence of the template dansylamide possess selective sites for
recognizing dansylamide, and the affinity of these sites can be reversibly changed by illumination with
ultraviolet or visible light.
Introduction
ration of materials whose recognition ability can be
regulated by illumination. A preparation of imprinted
merocyanine copolymer membranes has been described.13
To our knowledge, little progress has been made in
developing photosensitive MIPs based on merocyanine,
probably because merocyanine is not a very suitable
monomer for preparation of reversibly photoregulated
materials. That is, the rate of conversion of merocyanine
to spiropyran varies depending on solvent polarity and
on the presence of other compounds. For example, the
rate constant for this process in benzene or toluene can
be 1-2 orders higher that in DMSO or DMF.14,15
Photoisomerization of merocyanine is accompanied by
its fatigue and by the appearance of several degradation
products. The number of alternating irradiation cycles
sufficient to reduce the merocyanine absorbance to one-
half its initial intensity (half-life cycle) was only four in
nonpolar toluene, and addition of benzophenone induced
a considerable acceleration of fatigue in both polar and
nonpolar solvents.16
In contrast, the photoisomerization of azobenzene
does not strongly depend on solvent polarity. For
example, the quantum yield for cis-to-trans photo-
isomerization of azobenzene in various solvents was
about 0.4 in all solvents examined.17 There is no known
evidence for emission from the excited states of azoben-
zene in either its cis or trans form, so the photochemical
process is entirely efficient.9 Photoisomerization of
azobenzene residues within the recognition sites of
imprinted polymers would lead to changes in distribu-
tion of electron density within these sites and in their
geometrical parameters and, consequently, their recog-
nition ability. However, photoinduced isomerization
would influence the process of dansylamide recognition
only when the energy of isomerization is comparable to
or higher than that of the interaction of dansylamide
with azobenzene residues in the recognition sites.
Otherwise, photoregulation of the recognition would be
Molecular imprinting is an efficient method for mim-
icking the high selectivity of biological structures, such
as antibodies, in molecular recognition. The essence of
the method is formation of specific binding sites for a
predetermined ligand. The preparation of molecularly
imprinted polymers (MIPs) relies on the presence of a
template (imprinting molecule) during polymerization.
First, complexes between functional monomers and a
template are formed in solution, and then these com-
plexes are fixed by polymerization with a high degree
of cross-linking. Subsequent removal of the template
leaves cavities having size, shape, and arrangement of
functional groups that are complementary to those of
the desired ligand. Chemically and mechanically stable
MIPs able to recognize specific substances can be used
as the stationary phase in chromatography or solid-
phase extractions, as sensitive elements in biomimetic
sensors, and as artificial catalysts.1-7 Usually MIPs are
prepared as a polymer monolith, which can be crushed
to small particles; preparation of molecularly imprinted
polymer films or membranes has also been reported.5-7
Stimulus-sensitive materials, especially those with
light-affected properties, have been the subject of nu-
merous investigations.8-12 Isomerization of photore-
sponsive chromophores incorporated into polymers leads
to changes in their refractive index, dielectric constant,
oxidation/reduction potential, and other physicochemical
properties. Such polymers can be applied in various
photonic devices, such as erasable optical memory media
and photoswitches. Introduction of a photoresponsive
monomer into stable, selective MIPs can lead to prepa-
* Corresponding author: Phone: +81-29-861-2987. Fax: +81-
29-855-3833. E-mail: n.minoura@aist.go.jp
† National Institute of Advanced Industrial Science and Tech-
nology (AIST).
‡ Nihon University.
§ Kwangju Institute of Science Technology.
10.1021/ma040010a CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/11/2004

9572 Minoura et al.
Macromolecules, Vol. 37, No. 25, 2004
of the membranes was determined by spacers and was about
80 µm. The resulting membranes were washed several times
with acetonitrile until DA could no longer be detected in the
supernatant. Control (nonimprinted) polymers were synthe-
sized under the same conditions but in the absence of the
template.
impossible. The energy difference between the ground
state of the trans or “E” form and the cis or “Z” form of
azobenzene is about 50 kJ mol-1 8
. This value corre-
sponds to an association constant of 108-109 M-1 and
is comparable to or greater than published data on the
affinity of known MIP binding sites.18 The distance
between the 4 and 4′ positions is about 9.0 Å in trans-
azobenzene and 5.5 Å in cis-azobenzene, and the dipole
moment increases from 0.5 to 3.1 D on going from the
trans to cis isomers. These data show the potential for
preparing photoregulated MIPs based on azobenzene
and formed the basis for our investigations.
Spectroscopic Measurements. A membrane (9 mm × 25
mm) was inserted into a screw-capped quartz cell (optical path
length 1 cm), which had four transparent sides and contained
3 mL of acetonitrile. The mechanical properties of the prepared
membranes allow them to stand upright inside the cell. UV-
vis spectra were obtained at 25 °C in a V-530 spectrophoto-
meter (Jasco, Tokyo, Japan). Isomerization was induced by
irradiation with a UI-501C mercury lamp (500 W, Ushio,
Osaka, Japan). The UV light was isolated using a UV-D35
band-pass filter (Toshiba, Tokyo, Japan). The visible light was
obtained with a Y-43 cutoff filter (Toshiba). To reduce the
intensity of irradiation, a TND-30% filter (Toshiba) was used.
For photoisomerization and its monitoring, both the beam of
illuminating light from the mercury lamp and the light beam
of the spectrophotometer were perpendicular to the plane of
the tested membrane.
For monitoring of DA adsorption by membranes in rebinding
experiments, after the removal of template DA by intensive
washing with acetonitrile, a piece of membrane was inserted
into a screw-capped spectrophotometric quartz cell containing
3 mL of 10 µM DA solution in acetonitrile. The solution
concentration of DA was measured at 251 nm with the light
beam of the spectrophotometer parallel to the plane of the
tested membrane. The samples were first incubated in the
dark; after equilibration was achieved, the cell was exposed
to UV irradiation; and after the next equilibration, the cell
was exposed to visible light irradiation.
Several methods can be used to prepare various forms
of photoresponsive materials (for example, mono-, bi-,
or multilayer structures such as Langmuir-Blodgett
films, vesicles, or polymer membranes), which can be
broadly classed into two approaches: (1) noncovalent
modification (doping) of conventional materials, e.g.,
formation of poly(vinyl chloride) membranes in the
presence of an azobenzene derivative, and (2) synthesis
of polymers containing a covalently attached chromo-
phore. Doping procedures are much simpler than mak-
ing covalently bound derivatives, but depending on the
solvent composition and other working conditions, doped
membranes demonstrate rather limited stability and
reproducibility. Therefore, polymers with pendant pho-
toresponsive chromophores are usually much more
attractive. Thus, the aim of the current work was to
prepare molecularly imprinted polymer membranes
containing an azobenzene chromophore and study the
possibilities for regulating their properties by light.
FTIR spectra were measured on an FT/IR-620 spectrometer
(Jasco, Tokyo, Japan) at room temperature. The samples were
prepared as KBr pellets.
Experimental Section
Materials. Dansylamide (DA), dansyl-L-leucine (DL), N,N-
dimethyl-1-naphthylamine (DMN), 4-phenylazoaniline (4-ami-
noazobenzene), tetraethylene glycol diacrylate (TEGDA), and
acryloyl chloride were purchased from TCI (Tokyo, Japan).
Ethylene glycol dimethacrylate (EGDMA), methacrylic acid
(MAA), 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile) (AB-
MDV), and acetonitrile (HPLC grade) were obtained from
Wako (Osaka, Japan).
Results and Discussion
To develop materials possessing photoresponsive prop-
erties, we synthesized a polymerizable derivative of
azobenzene, p-phenylazoacrylanilide (PhAAAn), by con-
densation of acryloyl chloride with 4-phenylazoaniline.
The progress of synthesis and purification of the mono-
mer was monitored by thin-layer chromatography. The
molecular mass and composition of PhAAAn were
confirmed by FTIR, mass spectrometry, and elemental
analysis; we also studied its photochromic properties.19
For the formation of defined recognition sites within
MIPs, the structural integrity of the monomer-template
assemblies must be preserved during polymerization to
allow the functional groups of the polymer to be fixed
in space in a stable arrangement that is complementary
to the template. This is achieved by using a high degree
of cross-linking. Usually the amount of cross-linker in
the polymerization mixture must be more than 50% of
the total quantity of monomer used for synthesis of
MIPs. However, a polymer matrix must not only contain
the binding sites in a stable form, but also be porous
enough for easy access by the template (or other
analytes) to these sites. Porosity is achieved by carrying
out the polymerization in the presence of a solvent (a
porogen).1-4,21 Therefore, we started by investigating the
polymerizability of PhAAAn in mixtures with various
concentrations of PhAAAn and the cross-linking agent
TEGDA. The copolymerization diagram and the values
of rPhAAAn ) 0.84 and rTEGDA ) 0.99 showed a similar
level of reactivity of both PhAAAn and TEGDA, which
means that the compounds do not tend to form homo-
polymer blocks. The product rPhAAAn‚rTEGDA ) 0.84
confirms that the copolymerization behavior is close to
the ideal type that leads to a truly random copolymer,
Synthesis of p-Phenylazoacrylanilide. p-Phenylazo-
acrylanilide (PhAAAn) was prepared by condensation of acryl-
oyl chloride with 4-phenylazoaniline, as described earlier.19
Polymerization. Polymerizability of PhAAAn was inves-
tigated in mixtures in which one-half the volume consisted of
acetonitrile and the other half of mixtures of monomers. These
mixtures contained from 1 to 10 mol % of PhAAAn and 90 to
99 mol % of the cross-linking agent TEGDA. After degassing
by sonication for 1 min in a bath-type ultrasonicator and
adding ABMDV as a free radical initiator, the polymerization
mixtures in screw-capped glass reaction vials were purged with
nitrogen for 1 min. Polymerization was carried out at 35 °C
overnight. The resulting polymers were crushed into vials and
washed with several portions of acetonitrile until PhAAAn
could no longer be detected in the washing solutions. The
quantity of unreacted PhAAAn in the washing solutions was
determined spectrophotometrically at 353 nm. After washing,
the polymer samples were dried until their weights were
constant. The quantity of TEGDA included in the polymer was
determined by subtracting the incorporated quantity of PhAAAn
from the constant weight of the polymer sample. The copo-
lymerization reactivity ratios for PhAAAn and TEGDA were
calculated according to method 6 of Tu¨do´s et al.20
For preparation of imprinted polymers, DA as the template,
PhAAAn as the functional monomer, and EGDMA and TEGDA
as the cross-linkers were dissolved in acetonitrile in a screw-
capped glass reaction vial. After degassing, adding ABMDV,
and purging with nitrogen as described above, the polymeri-
zation mixtures were poured between two glass plates. Po-
lymerization was carried out at 35 °C overnight. The thickness

Macromolecules, Vol. 37, No. 25, 2004
Azobenzene-Containing Polymer Membranes 9573
Table 1. Dependence of Photoisomerization Rate
Constant on PhAAAn Concentration within Polymer
Membranes and on Monomer/Solvent (M/S) Ratio during
Polymerization
PhAAAn,
mol % and
trans-to-cis
cis-to-trans
(M/S ratio, v/v)
rate constant, s-1
rate constant, s-1
0.05 (5/5)
0.1 (5/5)
0.2 (5/5)
0.1 (6/4)
0.1 (7/3)
0.052
0.039
0.026
0.033
0.029
0.032
0.022
0.018
0.019
0.018
whose composition depends primarily on monomer
concentration in the polymerization mixture. These
properties of the monomers create favorable conditions
for the formation of specific recognition sites within
MIPs.
Conventional MIPs are hard and fragile. However,
free-standing MIP membranes require a combination
of some flexibility, necessary for their handling, and
hardness, to ensure stability of their recognition sites.
We explored several cross-linking agents and their
mixtures for use in preparing PhAAAn-containing mem-
branes. EGDMA or TEGDA, used as the only cross-
linker, led to formation of fragile, brittle membranes.
However, mechanically stable, but flexible, membranes
were obtained when EGDMA and TEGDA were mixed
at molar ratios from 5:5 (stress to breakage 22.9 MPa;
strain to breakage 4.7%) to 3:7 (stress to breakage 24.1
MPa; strain to breakage 10.2%).
The high level of cross-linking of the synthesized
polymers was confirmed by FTIR spectrometry. Whereas
the spectrum of monomeric TEGDA has IR bands near
1200 and 1280 cm-1 (typical of the acrylate CdC group)
as well as bands at 987 (dCH wag) and 811 cm-1 (dCH2
twist),22 these bands are barely evident in the FTIR
spectra of the polymers based on this cross-linker. This
means that the quantity of unreacted double bonds is
very low. Therefore, our polymers are indeed highly
cross-linked.
Upon UV irradiation of these membranes, PhAAAn
undergoes trans-to-cis isomerization. Upon visible light
irradiation, cis-to-trans isomerization occurs. Corre-
spondingly, the shapes, intensities, and positions of the
absorption bands change. The UV-vis spectra of
PhAAAn (trans configuration) show an intense band
centered at around 350 nm and a weak one at around
440 nm. These correspond to the π f π* and n f π*
electronic transitions of the azobenzene chromophore.
When the content of PhAAAn in the membranes was
more than 0.3 mol %, its absorbance at around 350 nm
was too strong for accurate measurement (>1.5 o.u.).
Therefore, we studied the process of photoisomerization
first with membranes having relatively low PhAAAn
content.
The photoisomerization kinetics of PhAAAn within
the membranes was studied at 25 °C in acetonitrile by
monitoring the absorbance at 353 nm (λmax for the trans
isomer). Kinetic data were fitted to the equation ln [(A0
- Aps)/(At - Aps)] ) kt, where A0, At, and Aps are the
absorbance at times 0 and t and at the photostationary
state, respectively.23,24 Both processes (trans-to-cis and
cis-to-trans photoisomerization) obeyed first-order ki-
netics. The photoisomerization kinetics of PhAAAn
within the membranes depended on the concentration
of this monomer (Table 1). The dependence of the cis-
to-trans photoisomerization on the PhAAAn concentra-
Figure 1. Reversibility of photoisomerization of PhAAAn
within a membrane containing 0.1 mol % PhAAAn.
tion was less pronounced than that of the trans-to-cis
photoisomerization. The process was reversible (Figure
1) at all PhAAAn concentrations tested.
The above results were obtained with membranes
that were polymerized in a mixture consisting of 50 vol
% of monomer mixture (including cross-linkers and
functional monomers) and 50 vol % of solvent. A high
level of cross-linking ensures good chemical and me-
chanical properties of the membranes. On the other
hand, the rather high concentration of solvent, which
plays the role of a pore-forming agent (porogen), results
in the formation of membranes with porous structure.
However, the monomer-to-solvent ratio can be changed
over a rather wide range and is one of the ways to
regulate the membrane’s properties. Therefore, we also
prepared membranes in polymerization mixtures having
6:4 and 7:3 monomer-to-solvent ratios (and 0.1 mol %
PhAAAn) and investigated them (Table 1). An increase
in this ratio results in small decreases in the photo-
isomerization rate. Again, the relative effect on the
trans-to-cis photoisomerization was more pronounced
than that on the cis-to-trans photoisomerization, and
complete reversibility of the process was observed for
all the membranes tested. When we compared a solution
of monomeric PhAAAn with polymer membranes con-
taining nearly the same quantity of PhAAAn, their rates
of photoisomerization were similar as well (data not
shown). These data suggest that the free volume within
the synthesized membranes is probably large enough
for virtually unrestricted azobenzene photoisomeriza-
tion. This feature can be considered an important
advantage of our cross-linked porous polymer mem-
branes, compared with condensed LB films and mem-
branes prepared by spin coating or casting from polymer
solution.8-10
To investigate the chemical stability of the synthe-
sized photoresponsive membranes, we compared the
rates of photoisomerization of PhAAAn within the
membranes in acetonitrile before and after incubating
for 2 h in solvents of various polarities and in acidic and
alkaline aqueous solutions. After the incubations, the
membranes were washed several times in acetonitrile
(or in water and then in acetonitrile). The essentially
identical rate constants we obtained clearly indicate the
excellent functional stability of the synthesized mem-
branes (Table 2).

9574 Minoura et al.
Macromolecules, Vol. 37, No. 25, 2004
Figure 3. Absorbance spectra of a membrane containing 6
mol % PhAAAn. Spectra were recorded before irradiation and
after 0.5, 2, 3, 5, and 20 min of irradiation by visible light
(curves from top to bottom, respectively).
Figure 2. Effect of various compounds on the rate of trans-
to-cis photoisomerization of 20 µM PhAAAn in acetonitrile.
absorbance at about 350 nm, which was monitored in
the previous experiments, was too strong. Therefore, to
study the process of PhAAAn photoisomerization within
these membranes, we monitored the absorbance at 440
nm, which corresponds to a peak for the cis isomer. The
spectra recorded after different irradiation times showed
the presence of an isosbestic point at 412 nm (Figure
3), which confirms that only two absorbing species,
namely, the trans and cis isomers, were present in the
membranes. Under the same conditions of light irradia-
tion as in the previous experiments (Table 1), the
photoisomerization rate within these membranes con-
taining a much higher quantity of PhAAAn was slower
(for example, the rate constants are about 0.006 and
0.002 for cis-to-trans and trans-to-cis photoisomeriza-
tion, respectively). However, photoisomerization con-
tinued to be reversible.
After synthesizing the imprinted membranes and
washing out DA, we investigated the ability of these
membranes to adsorb DA. For this purpose, a piece of
membrane was incubated at room temperature in a
spectrophotometric quartz cell containing 3 mL of 10
µM DA in acetonitrile. When incubated in the dark,
imprinted membranes that were prepared at PhAAAn-
to-DA ratios of 2:1 and 4:1 adsorbed DA from solution
(Figure 4). Imprinted membranes prepared at a PhAAAn-
to-DA ratio of 4:1 adsorbed much more DA than control
membranes or imprinted membranes prepared at lower
PhAAAn-to-DA ratios. This finding can be explained if
some cooperativity between two or more molecules of
PhAAAn is necessary for formation of complexes with
the template molecule in solution, then during polym-
erization, and finally in the rebinding experiments.
After incubation in the dark, the cell containing the
membrane and DA solution was irradiated with UV
light (Figure 4). For imprinted membrane prepared at
a PhAAAn-to-DA ratio of 4:1, UV irradiation caused
previously adsorbed DA to be released. Subsequent
exposure of this cell to visible light caused the concen-
tration of DA in solution (and, consequently, in the
imprinted membrane as well) to return to nearly the
same level as before UV irradiation. The kinetics of this
process completely coincided with the kinetics of photo-
isomerization of the same membranes. Neither UV nor
visible irradiation changed the amount of DA adsorbed
by control membranes or by imprinted membranes
prepared at PhAAAn-to-DA ratios of 2:1 or 1:1. Probably
at these ratios there is not enough PhAAAn for forma-
tion of efficient recognition sites, and their photoizomer-
Table 2. Photoisomerization Rate Constants of PhAAAn
within Polymer Membranes in Acetonitrile before and
after Treatment with Various Media
trans-to-cis
cis-to-trans
treatment
before
Milli Q water
1 M HCl
1 M NaOH
methanol
heptane
rate constant, s-1
rate constant, s-1
0.025
0.026
0.028
0.026
0.025
0.027
0.018
0.019
0.019
0.018
0.019
0.019
Interactions between functional monomers and a
template molecule should be strong enough to form
complexes in solution, which can then be stabilized
during the polymerization process. A reliable method
for preliminary screening of suitable pairs of a func-
tional monomer and a template can be very useful for
efficient synthesis of highly selective MIPs. We expected
that any interactions in solution between our functional
monomer PhAAAn and other compounds would affect
the photoisomerization of PhAAAn. Thus, investigation
of the photoisomerization kinetics of PhAAAn in the
presence of other compounds would allow selecting a
good template for this functional monomer. Compounds
having a rather rigid core structure and a few functional
groups can usually serve as a good template. We tested
several cyclic and heterocyclic compounds, including
steroids, aromatic amino acids, and nucleic acid bases.
The most pronounced decrease in the rate of trans-to-
cis photoisomerization of PhAAAn was observed in the
presence of DA (Figure 2), which probably forms a
combination of hydrogen bonds and stacking interac-
tions with PhAAAn. Therefore, DA was chosen as a
template for the synthesis of MIP membranes. DA has
the added advantage that it can be easily monitored by
UV-vis and fluorescence spectrometry. It is widely used
as a probe of the binding sites of enzymes,25 for
monitoring polymerization reactions,26,27 and in the
development of bio- and chemosensors.28,29
For preparation of MIP membranes, the polymeriza-
tion mixture consisted of 0.05 mmol DA as a template,
0-0.2 mmol PhAAAn as a functional monomer, and
EGDMA and TEGDA (both 1.5 mmol) as cross-linkers,
dissolved in acetonitrile (monomer-to-solvent ratio 1:1).
At the maximum PhAAAn:DA ratio, the concentration
of PhAAAn in these membranes was about 6 mol %,
which is much higher than in the membranes we
studied earlier (Table 1). At this concentration the

Macromolecules, Vol. 37, No. 25, 2004
Azobenzene-Containing Polymer Membranes 9575
Figure 4. Binding of DA by polymer membranes synthesized
with PhAAAn-to-DA ratios of 1:1 to 4:1. DA concentration was
10 µM.
Figure 5. Binding of DA by DA-imprinted and control
polymer membranes and binding of DL and DMN by DA-
imprinted polymer membranes. The polymers were prepared
with a PhAAAn-to-DA ratio of 4:1. The concentration of all
tested compounds was 10 µM.
Chart 1. Structures of Dansylamide (DA),
Dansyl-l-leucine (DL), and
N,N-dimethyl-1-naphthylamine (DMN)
DMN adsorption between control and DA-imprinted
membranes is much smaller than that for DA adsorp-
tion. The level of DMN adsorption was reduced by UV
and increased by visible light. However, the relative
extent of these changes was clearly smaller than for
adsorption and release of DA. The observed level of
selectivity of our membranes is higher than that of
tryptophan-imprinted merocyanine copolymer mem-
branes.13 Those membranes interacted with phenyl-
alanine, which is smaller than the template, and with
a larger substrate, N-(3-carboxypropionyl)-L-phenyl-
alanine-p-nitroanilide, at levels similar to that of the
template.
To estimate the saturation capacity and the associa-
tion constant for DA binding to the MIP membrane, we
incubated the imprinted membranes (prepared at a
PhAAAn-to-DA ratio of 4:1) with various concentrations
of DA (4-100 µM). Scatchard plots were constructed by
plotting the ratio of bound-to-free DA against the bound
DA concentration. Linear least-squares fitting to the
Scatchard plot provided the values of the association
constant Kass ) 4.1 104 M-1 and the saturation capacity
N ) 0.37 µmol g-1 of polymer. The free energy change
for DA binding (-26.3 kJ mol-1) is smaller than for
photoisomerization of free azobenzene. These data are
in accordance with our expectations and with obtained
results showing that photoinduced isomerization of
PhAAAn within DA-imprinted membranes is accompa-
nied by photoregulated adsorption and desorption of DA.
ization cannot influence interactions with DA. Assuming
that the amount of DA adsorbed by the control mem-
brane represents nonspecific adsorption and that the
difference between imprinted and control membranes
represents specific adsorption, we can say that more
than one-half of the specifically adsorbed DA can be
reversibly released and then readsorbed by changing the
wavelength of light used to irradiate the sample.
The level of DA adsorbed by our imprinted mem-
branes is much higher than the reported levels of
photoinduced effects seen in other azobenzene-contain-
ing materials. For example, photoinduced changes in
gas flow through a glass membrane modified with an
azobenzene derivative were only 1-2%.30 In another
work, transmittance by a system containing only the
trans isomer of derivatized azobenzene (98%) was very
close to the transmittance of a cis/trans mixture (100%).31
Compounds having structures similar to that of the
templatesdansyl-L-leucine (DL) and N,N-dimethylnaph-
thylamine (DMN) (Chart 1)swere tested to study the
selectivity of imprinted membranes prepared with a
PhAAAn-to-DA ratio of 4:1. Probably because of its
larger size and modified structure, DL did not interact
with the recognition sites: DL adsorption by the mem-
branes was very low and not affected by UV or visible
irradiation (Figure 5). DMN, which is similar to the
template DA but smaller, is adsorbed by the DA-
imprinted membranes (Figure 5). The difference in
In summary, the present report describes the prepa-
ration of imprinted, photosensitive membranes by using
PhAAAn as a photoresponsive functional monomer, DA
as a template, and mixtures of EGDMA and TEGDA
as cross-linkers. The affinity of DA-specific recognition
sites within these MIP membranes can be reversibly
changed by illumination with UV or visible light. Such
photoresponsive molecularly imprinted materials should
prove to be important for the development of environ-
mentally ‘friendly’, wasteless technologies for separa-

9576 Minoura et al.
Macromolecules, Vol. 37, No. 25, 2004
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