Chem. Mater. 2003, 15, 4703-4704
Molecularly Imprinted Polymer Membranes with Photoregulated Template Binding Norihiko Minoura,*,† Kazuto Idei,§ Alexandre Rachkov,†,‡ Hirotaka Uzawa,† 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, Japan Science and Technology Corporation (JST), and Graduate School of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan Received July 8, 2003 Revised Manuscript Received October 21, 2003 Here, we present a preparation of molecularly imprinted polymer membranes with photoregulated ability to interact reversibly with a predetermined compound. Molecular imprinting is an efficient method of mimicking the specificity of molecular recognition ability by biological structures such as antibodies. Preparation of molecularly imprinted polymers (MIPs) relies on the presence of a template (imprint molecule) during polymerization. Chemically and mechanically stable MIPs able to recognize specific substances can be used as a stationary phase in chromatography or solid-phase extraction, as sensitive elements for biomimetic sensors, and also as artificial catalysts.1-5 MIPs are usually synthesized as bulk materials, which are crushed to small particles for further use. Preparation of molecularly imprinted polymer membranes have been described also.6-9 Isomerization of photoresponsive chromophores incorporated into a polymer leads to changes in physicochemical properties of the latter. These changes can be used to implement various photonic devices, such as photoswitches. Several years ago a preparation of imprinted merocyanine copolymer membranes was reported.7 As far as we know, little progress has been made in this direction, probably because merocyanine is not a very suitable monomer for preparation of reversible photoregulated materials. The rate of conversion of merocyanine to spiropyran depends on solvent polarity and on the presence of other compounds.10,11 Photoisomerization of merocyanine is accompanied by its fatigue and by the appearance of several degradation * Corresponding author. Tel.: +81-29-861-2987. Fax: +81-29-8553833. E-mail:
[email protected]. † AIST. ‡ JST. § Nihon University. (1) Haupt, K.; Mosbach, K. Chem. Rev. 2000, 100, 2495. (2) Andersson, L. I. J. Chromatogr., B 2000, 745, 3. (3) Sellergren, B. Angew. Chem., Int. Ed. 2000, 39, 1031. (4) Nicholls, I. A.; Andersson, H. S. In Molecularly Imprinted Polymers: Man-Made Mimics of Antibodies and Their Applications in Analytical Chemistry; Sellergren, B., Ed.; Elsevier: Amsterdam, 2001; Chapter 3. (5) Wulff, G. Chem. Rev. 2002, 102, 1. (6) Mathew-Krotz, J.; Shea, K. J. Am. Chem. Soc. 1996, 118, 8154. (7) Marx-Tibbon, S.; Willner, I. J. Chem. Soc., Chem. Commun. 1994, 1261. (8) Piletsky, S. A.; Panasyuk, T. L.; Piletskaya, E. V.; Nicholls, I. A.; Ulbricht, M. J. Membr. Sci. 1999, 157, 263. (9) Duffy, D. J.; Das, K.; Hsu, S. L.; Penelle, J.; Rotello, V. M.; Stidham, H. D. J. Am. Chem. Soc. 2002, 124, 8290.
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products.12 In contrast, the photoisomerization of azobenzene does not strongly depend on the solvent polarity. The energy difference between the ground state of transand cis-azobenzene is about 50 kJ mol-1.13 This value corresponds to an association constant of 108-109 M-1 and is comparable to or greater than published data on the affinity of MIP binding sites.14 Photoisomerization of azobenzene is accompanied by changes in the dimensions and dipole moment of the molecule. Therefore, we started a study of photoregulated MIPs based on azobenzene as the chromophore. Photoisomerization of a chromophore located in a binding site should influence the affinity and, probably, the selectivity of MIPs. To develop materials possessing photoresponsive properties, we synthesized a polymerizable derivative of azobenzene, p-phenylazoacrylanilide (PhAAAn). The synthesis and purification of the monomer were monitored by thin-layer chromatography. The molecular mass and composition of PhAAAn were confirmed by Fourier transform infrared spectroscopy, NMR spectrometry, mass spectrometry, and elemental analysis. We also studied its photochromic properties.15,16 Conventional MIPs are hard and fragile. However, MIP membranes should combine some flexibility, necessary for their handling, and hardness, to ensure stability of recognition sites. We explored several cross-linking agents and their mixtures for use in preparing PhAAAncontaining membranes and found that mixtures of ethylene glycol dimethacrylate (EGDMA) and tetraethylene glycol diacrylate (TEGDA) led to the formation of mechanically stable, but flexible, polymer membranes.17 Upon UV irradiation of these membranes, PhAAAn undergoes trans-to-cis isomerization. Upon visible light irradiation, cis-to-trans isomerization occurs.18 Correspondingly, the shape, intensity, and positions of the absorption bands change. The presence of isosbestic (10) Sueishi, Y.; Oncho, M.; Nishimura, N. Bull. Chem. Soc. Jpn. 1985, 58, 2608. (11) Keum, S.-R.; Hur, M.-S.; Kazmaier, P. M.; Buncel, E. Can. J. Chem. 1991, 69, 1940. (12) Sakuragi, M.; Aoki, K.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1990, 63, 74. (13) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5, 403. (14) Sellergren, B. Trends Anal. Sci. 1999, 16, 310. (15) Rachkov, A.; Minoura, N.; Shimizu, T. Opt. Mater. 2002, 21, 307. (16) Selective 1H NMR data for PhAAAn (400 MHz, CDCl3), δ (TMS, ppm): 6.489 (Htrans-CHdCH-CONHR, J ) 1.2 and 16.8 Hz), 6.283 (H2CdCH-CONHR, J ) 10.0 and 16.8 Hz), 5.829 (Hcis-CHdCHCONHR, J ) 1.2 and 10.0 Hz). (17) Half the volume of the polymerization mixture was acetonitrile, and the other half was a mixture of PhAAAn as the functional monomer and EGDMA and TEGDA (at ratios of 3:7 to 5:5) as crosslinking agents. After the mixture was purged with N2 gas, it was incubated at 35 °C, in the presence of 2,2′-azobis (4-methoxy-2,4dimethylvaleronitrile) as a free-radical initiator. The membranes were formed between two glass plates, and their thickness was set by spacers and was about 80 µm. (18) Photoisomerization was carried out by irradiation with a 500-W mercury lamp. The UV light was isolated with a UV-D35 filter. Visible light was obtained with a Y-43 filter. A TND-30% filter was used to reduce the intensity of irradiation. Absorption spectra were obtained at 25 °C in a V-530 UV-visible spectrophotometer. A piece (9 × 25 mm) of PhAAAn-containing membrane was inserted into a screwcapped quartz cell (optical path length 1 cm) containing 3 mL of acetonitrile. The membranes are stiff enough to stand inside the cell. For photoisomerization and its monitoring, the beams of both the irradiation from the mercury lamp and in the spectrophotometer were perpendicular to the plane of the tested membrane.
10.1021/cm0305608 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/13/2003
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Communications
Figure 1. Reversibility of photoisomerization of PhAAAn within a polymer membrane. The polymerization mixture contained 6 mol % PhAAAn, 47 mol % TEGDA, and 47 mol % EGDMA.
Figure 2. Binding of dansylamide (DA) by imprinted and control polymer membranes and binding of dansyl-L-leucine (DL) and N,N-dimethylnaphthylamine (DMN) by imprinted polymer membranes. The polymers were prepared at a PhAAAn-to-DA ratio of 4:1. Concentration of all tested compounds was 10 µM.
points in the spectra recorded after different irradiation times confirmed that only two absorbing species, namely, the trans and the cis isomers, were present. The photoisomerization of PhAAAn within PhAAAn-containing membranes is reversible and reproducible (Figure 1). Previous experiments showed that the presence of dansylamide (DA) in solutions of PhAAAn decreased the rate of its photoisomerization, probably because of hydrogen bonding and stacking interactions. Therefore, DA was chosen as a template for preparation of MIP membranes. DA has the added advantage that it can be easily monitored by UV-visible or fluorescence spectrometry. It is widely used as a probe of binding sites of enzymes,19,20 in investigations of soluble macrocyclic receptors.21 For synthesis of molecularly imprinted polymer membranes, we dissolved 0.05 mmol of DA as the template, 0.05, 0.1, or 0.2 mmol of PhAAAn as the functional monomer, and EGDMA and TEGDA as cross-linkers (both 1.5 mmol) in acetonitrile. The polymer membranes were prepared as mentioned above.17 Control (nonimprinted) polymers were synthesized under the same conditions but in the absence of DA. To study the ability of the MIP membrane to absorb DA, a piece of membrane was incubated at room temperature in a spectrophotometric quartz cell containing 3 mL of a 10 µM acetonitrile solution of DA.22 When incubated in darkness, imprinted membranes, which were prepared at PhAAAn-to-DA ratios of 2:1 and 4:1, adsorbed DA from solution. This finding can be explained by the necessity of some cooperativity between two or more molecules of PhAAAn in forming complexes with the template molecule in solution, then during polymerization, and finally, in the re-binding experiments. Imprinted membranes prepared at a PhAAAnto-DA ratio of 4:1 were able to adsorb more DA than the control membrane (Figure 2). The capacity of the im-
printed membranes is about 1.2 nmol of DA/cm2 or 0.15 nmol/mm3. After incubation in the dark, the cuvette containing the membrane and DA solution was irradiated with UV light. For the 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 cuvette to visible light caused the concentrations of DA in solution and in the imprinted membrane to return to levels close to those seen before UV irradiation. Neither UV nor visible irradiation changed the level of DA adsorbed by control membranes. Assuming that the level of DA adsorbed by the control membrane represents nonspecific adsorption and that the difference between imprinted and control membranes represents specific adsorption, we can say that more than half the specifically adsorbed DA can be reversibly released and re-adsorbed by changing the wavelength of light used to irradiate the sample. The level of DA adsorbed by imprinted membranes is much higher than the reported level of photoinduced effects seen in azobenzene-containing materials.23,24 Compounds similar to the template dansyl-L-leucine (DL) and N,N-dimethylnaphthylamine (DMN) were used to study the selectivity of imprinted membranes prepared with a PhAAAn-to-DA ratio of 4:1 (Figure 2). Probably because of its larger size and modified structure, molecules of DL cannot interact with the recognition sites; DL adsorption by the membranes is very low and is not affected by UV or visible light irradiation. DMN, which is similar to the template DA but smaller, is adsorbed by the imprinted membranes. The level of this adsorption is reduced by UV and increased by visible light irradiation. However, the scale of these changes is clearly smaller than that for adsorption of the template DA. The observed selectivity is higher than that of merocyanine copolymer membranes, which interacted with various compounds to similar extents.7 In summary, we have synthesized imprinted, photosensitive membranes that possess selective recognition sites for their template and that the affinity of these sites can be changed by illumination with light of different wavelengths.
(19) Chen, R. F.; Kernohan, J. C. J. Biol. Chem. 1967, 242, 5813. (20) Jain, A.; Huang, S. G.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 5057. (21) Schneider, H.-J.; Gu¨ttes, D.; Schneider, U. J. Am. Chem. Soc. 1988, 110, 6449. (22) A quartz cell having four transparent sides was used. To monitor the concentration of DA (at 251 nm), the light beam of the spectrophotometer was parallel to the plane of the tested membrane. For photoisomerization of PhAAAn within the tested membranes, the beam of irradiation from the mercury lamp was perpendicular to the plane of the tested membrane.
CM0305608 (23) Jin, T.; Ali, A. H.; Yazawa, T. Chem. Commun. 2001, 99. (24) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1715.
