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CHEM. RES. CHINESE U. 2006, 22( 3 ) , 390-393

Molecular Imprinting Fibrous Membranes of Poly ( acrylonitrileco-acrylic acid) Prepared by Electrospinning* CHE Ai-ful , YANG Yun-feng' , WAN Ling-shu' , WU Jian' * and XU Zhi-kang' 1, Institute of Polymer Science, 2. Department of Chemistry,Zhejiung University, Hangzhou 310027, P. R. China Received Dec. 12, 2005

Keywords Molecular imprinting; Electmspinning ; Fibrous membrane; Crystallization; Surface; Poly( acrylonitrileco-acrylic acid) Article ID 1005-9040(2006)-03-39044

Introduction Over the past few decades, molecular imprinting has been described as a technology for preparing " molecular doors" which can be matched to " template keys". It has been found to be a simple and effective approach to introduce specific recognition sites into synthetic polymers, namely, to create molecular imprinting p o l y ~ n e r s " ~ ' .Remarkable features such as stability, ease of preparation and low cost, have made molecular imprinting polymers particularly attractive in chemical sensors, catalysis, drug delivery, and dedicated separations. Practical applications of molecular imprinting polymers require accessible sites, fast mass transfer, and quick binding. However, present techniques used to prepare molecular imprinting polymers most often result in materials exhibiting a high affinity and selectivity but a low capacity and poor site accessibility for the target molecules. It is also very difficult to remove the imprinted molecules located in these molecular imprinting polymers because the highly cmss-linked structures do not allow the templates to move freely. To some extent, combining molecular imprinting technology with membrane separation"-lol and surface can overcome the shortcomings, such as mass transfer limitations and non-quantitative recovery of the template molecules seen for imprinted materials fabricated by conventional bulk methods. In that case, it appears to us that molecular imprinting polymers with high surface area to volume ratios are particularly desirable for largescale applications. Electrospun nano and ultrafine fibrous membranes are the most suitable materials due

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to advantages such as: ( 1 ) large specific surfaces, providing relatively high imprinting sites per unit mass ; ( 2 ) fine porous structures, resulting in the accessibility of imprinting sites and low diffusion resistance necessary for high efficiency; and ( 3 ) easy recoverability from practical operation or applicability for continuous usages. Therefore, in this work, we prepared a unique kind of imprinted material-molecularly imprinted fibrous membranes of poly ( acrylonitrile-co-acrylic acid ) fabricated by means of an electrospinning process.

Experimental 1 Materials and Copolymer Synthesis All the reagents used were of analytical grade. Acrylonitrile, acrylic acid and N , N-dimethyl formamide ( DMF) were pudied by vacuum distillation before use. Theophylline ( THO ) was purchased from Sigma, and was used without further purification. Copolymerization of acrylonitrile and acrylic acid was performed in our laboratory via a water-phase precipitation copolymerization process as previously with a molar ratio of 85 to 15 between the two monomers. The viscosity average molecular weight of the resulting p l y ( acrylonitrile-co-acrylic acid ) ( PANCAA ) is 18. 8 x lo4 g/mol and the molar fraction of acrylic acid in the copolymer determined by NMR is 16.5%. 2 Fabrication of Electrospun Fibrous Membranes with Molecular Imprinting PANCAA ( 6.0% , mass fraction ) and THO ( 0 , 1.5% , or 2.9% , mass fraction) were dissolved in DMF at 60 T with gentle stimng for 15 h to form homogeneous solutions. After air bubbles were removed

Suppofied by the National Natural Science Foundation of China( No. 50473038). To whom correspondence should be addressed. E-mail: mz k@ ipsm. zju. edu. cn; jianwu0 zju. edu. cn

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completely, the solution was placed in a plastic syringe ( 5 0 mL) bearing a flat metal needle(i. d. : 1.2 mm) that was connected with a high voltage power supply ( DW-1303-1 AC , Tianjin Dongwen High-voltage Power Supply Company, China). A grounded ‘counter electrode was connected to an aluminium foil collector. Typically, electrospinning was performed at 10 kV with a distance of 150 mm between the needle tip and the collector. The flow rate of the solution was controlled by a syringe pump ( WZ-5OC2, Medical Instrument Co. , Ltd. , Zhejiang University, China) and maintained at 2.0 m V h from the needle outlet. It usually took 2 h to obtain a sufficiently thick membrane that could be detached from the aluminum foil collector. The membrane on the aluminium foil was dried under vacuum at 60 “c before it was detached. Subsequently, a 0. 1% aqueous acetic acid( AcOH) solution was used to wash the membranes continuously until the template THO molecules were removed completely. Then, a large excess of water was used to wash the residual AcOH, and the molecular imprinting membranes (MIMs) were dried at 60 9: in a vacuum oven till a constant weight was obtained.

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then fabricated into fibrous membranes by means of an electrospinning process. With this technology, it is possible to produce continuous ultrafine fibers deposited on the surface of a conductive collector as a randomoriented fibrous membrane with diameters ranging from nanometer to Owing to the high surface area to volume ratios of ultrafine fibers, as schematically represented in Fig. 1 , it is expected that with the electrospinning technology in combination with the molecular imprinting technology-porous MIMs can be generated-which will be able to induce the nucleation and growth of template crystals in the imprinted cavities of the membrane surface under certain conditions.

3 Directed Nucleation and Crystal Growth The MIMs were immersed in supersaturated THO solutions at 10 “c , respectively. After 24 h , they were taken out and rinsed thoroughly with a saturated THO solution to remove weakly attached crystals on the surface of MIMs , and then dried in a vacuum oven at room temperature.

4 Characterization Morphologies of the MIMs coated with gold were inspected by means of a field emission scanning electron microscope ( FESEM , Sirion, FEI ) . To confirm that the nucleation is the result of specific recognition to the template by imprinting sites, indeed, X-ray powder diffraction ( XRD) and differential scanning calorimetry (DSC) were used. The XRD patterns were measured on an M18 X-ray diffraction spectrometer( Mac Science Co. , Ltd. ) , the DSC experiments were performed on an STA 409 PC thermal analysis system ( NE’IZSCH Geratebau GmbH) under N, atmosphere at a heating rate of 20 ‘t/min from mom temperature to 400 ‘t .

Results and Discussion PANCAA, which was reported as MIMs synthesized through a conventional phase inversion process by Wang et al. “I and most recently by Ciardelli[si et al. , was synthesized by means of a water-phase precipitation copolymerization process as previously reported[m1 with a molar ratio of 85 to 15 between the two monomers and

Fig.l

Schematic representation for the fabrication of P A N C M T H O fibers and the surface-induced crystallization of THO.

Fig. 2 represents the scanning electron micrographs of the studied fibrous membranes. The results show that the fibrous membranes with a uniform distribution of fiber diameters ranging from 650 to 700 nm can be prepared by means of an electrospinning process. With the addition of THO, the fiber diameter increases slightly due to the interactions between the template molecules and the functional groups (-COOH) . -Compared with that of the blank membrane [ Fig. 2 ( A ) ] , the surfaces of the other two membranes [ Fig. 2 ( B ) and ( C ) ] are covered by slice THO crystals with embedded roots in the fibers. This is because in an electrospinning process, as the solvent is evaporated, most of the small template molecules will move with the solvent and remain on the fibrous membrane surface on which the imprinting sites are assembled. We found that these THO crystals can be easily

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and clearly removed by an AcOH aqueous solution and water as shown in Fig. 2 ( B1) -( C1) without varying the morphology of the fibrous membranes. Apparently, many imprinted cavities might be formed on the mem-

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brane surface, and such a supposition was firmly confirmed by the induced crystallization process subsequently.

Fig.2 SEM photographs of the fibrous membranes electrospun from PANCMTHO mixtures. (A) n(PANCAA)/n(THO) =6.0/0;(B) n(PANCAA)/n(THO) = 6.0/1.5; ( C ) n(PANCAA)/n(THO) =6.0/2.9); ( A1 )-( CI ) after washed with the A c O H solution and water, ( a)-(C2)surface induced crystallization in a supersaturated THO solution a~ 10 93.

After the MIMs were immersed in a supersaturated THO solution for 24 h , crystals grown at the imprint sites were markedly observed as shown in Fig. 2 ( B2)-( C2) ; however, no growth w a s observed on the surface of the blank membrane [ Fig. 2 ( A2) ] simultaneously. With the increase of the molar ratio of PANCAA to THO for the fabrication of the fibrous membranes, the amount of crystals on the membrane surface increased significantly which indicated that the induced crystallization had occurred because of the imprinting effect. Furthermore, XRD and DSC were used to determine the crystal structures and the cause for the induced crystallization. As shown in Fig. 3 , the diffraction pattern of the THO crystals[ Fig. 3 ( b ) ] on the THO-imprinted fibrous membrane surfaces is exactly the same as that of pure THO [ Fig. 3 ( a ) A strong reflection appears at 28 = 12.48" and a relatively weak reflection can be seen at 28 = 25.51" in both of the XRD patterns of pure THO and the THO crystals on the THO-imprinted fibrous membrane surfaces, respectively. After the template was removed, these reflections disappeared. Thus the result is favorable evidence for the fact that the nucleation should be

attributed to the imprinting effect rather than the effect of the residual THO seed crystals. An evident reflection at 28 = 16.48" in the X-ray diffraction patterns of the membranes [ Fig. 3 ( b ) and ( c ) ] could also be seen, which is ascribed to the regular orientation and arrangement of polymer chains in the membranes during the electrospinning process.

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a?/(') Fig. 3

X-ray diffraction pattern of THO ( a ) , THO crystalked on the fibrous membrane(b ) , THOimprinted fibrous membrane hadog been washed with AeOH solution( c).

Fig. 4 gives a set of DSC curves for the studied

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mixture, may lead to the introduction of new separation methods in the pharmaceutical and fine-chemical industries.

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Mg.4 DSC curves of blank membrane( a ) , THO imprinted flbrous membrane having been wssbed with AcOH solution ( b ) , THO crystallized on tbe fibrous membrane( e). membrane.

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