Layer-by-Layer Surface Molecular Imprinting on Polyacrylonitrile

Jun 3, 2015 - *E-mail: [email protected]. ... to build imprinting sites that are more accessible to template molecules compared ... The SMI-LbL ...
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Layer-by-Layer Surface Molecular Imprinting on Polyacrylonitrile Nanofiber Mats Yuxuan Liu, Bing Cao, Peng Jia, Junhu An, Chao Luo, Lijing Ma, Jiao Chang, and Kai Pan* Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China 100029 ABSTRACT: Surface molecular imprinting in layer-by-layer (SMI-LbL) film is known as a facile and effective strategy to build imprinting sites that are more accessible to template molecules compared with molecular imprinting in polymers. Herein, we accomplished the formation of SMI-LbL film on electrospun nanofibers for the first time. The SMI-LbL nanofibers were prepared by a template-induced LbL process on the polyacrylonitrile (PAN) nanofiber substrates, followed by postinfiltrating and photo-cross-linking of photosensitive agent 4,4′-diazostilbene-2,2′-disulfonic acid disodium salt (DAS). The obtained nanofiber mat maintained the nanofibrous structure and showed rapid absorption and extraction of template molecules of meso-tetra(4-carboxyphenyl)−porphine (Por). The binding capacity of Por reached 2.1 mg/g when 3.5 bilayers were deposited on the nanofibers. After six cycles of extraction and reabsorption, the binding capacity of Por remained at 83%. Moreover, the absorption results of the targeted templated molecule of Por and the control molecule of Fast Green, which had a very similar chemical structure and charge status to Por, indicated the specific absorption for template molecule of Por. Thus, a surface molecular imprinted nanofiber mat with high selectivity of the templated molecule has been demonstrated.

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INTRODUCTION Molecular imprinting, as an effective and promising technique to create template-defined binding sites in polymers, has been widely used to fabricate molecular-imprinted polymers (MIP) that are applicable to sensors, detectors, and catalysts.1−4 In a typical imprinting process, functional monomers and template molecules are first preorganized by covalent or noncovalent bonding. After copolymerization with cross-linker and removal of the template, the MIP is fabricated with special cavities, which are complementary to the templated molecule. The advantages of molecular imprinting lie in its easy preparation, thermal, and chemical stability, and highly specific recognition. But MIPs still suffer from problems caused by the deeply buried binding sites, such as poor site-accessibility and low binding capacity. To overcome this limitation, many efforts have been made to address the above issues. A possible solution is to build the binding sites on the surface or near the surface to reduce the diffusion distance and make the sites more accessible.5,6 Layer-by-layer (LbL) assembly is a versatile assembly technique based upon the alternate adsorption of oppositely charged polyions onto a surface.7 As a powerful method to build layered nanostructures with tailored functionalities,8,9 it has been used to build nanoscale films on surfaces or interfaces of various substrates.10,11 Taking the advantages of both surface imprinting and LbL assembly, Zhang et al.12 first introduced the SMI-LbL technique in 2007. In a typical SMI-LbL process, the imprinting complex is first formed in solution, followed by a subsequent LbL procedure of imprinted multilayers and further interlayer cross-linking. With imprinted sites located near the surface, the SMI-LbL thin films reveal rapid binding kinetics and high reproducibility.13 Thereafter, unconventional LbL © XXXX American Chemical Society

imprinting methods have also been explored and successful applications of SMI-LbL devices have been demonstrated in various fields including selective filtration, biorecognition, and drug delivery.14−16 The combination of surface molecular imprinting and nanomaterials, such as nanoparticles and nanofibers, is expected to overcome the problems of conventional MIPs.17,18 However, solutions of nanoparticles are thermodynamically unstable and easy to aggregate due to high surface energies. Nanofiber materials, featuring excellent flexibility, high surface area, easy preparation, and low cost, have drawn increased attention.17,19 Electrospinning is an efficient technique for producing long continuous ultrafine fibers, which are intensively applied as supporting materials in the fabrication of advanced intelligent devices due to their flexibility, complex pore structure, and high surface area.20,21 Both the diameter of fibers and the thickness of obtained nanofiber mats are adjustable by changing the electrospinning parameters. Various polymer materials can be used for electrospinning to fabricate uniform fibers with wellcontrolled structure with hollow,22 multichannel,23 and porous24 morphologies, and so on.25 The obtained electrospun nanofiber mats could be applied as catalysts, selective separators, tissue engineering scaffolds, and so on.26−29 However, the application of nanofiber mats for the construction of SMI-LbL thin films has not been reported yet. In this paper, we first employ a nanofiber mat for an SMILbL thin film. A flexible surface-imprinted nanofiber mat with Received: March 10, 2015 Revised: May 24, 2015

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Scheme 1. (a) Schematic Representation of the Construction of an SMI-LbL Nanofiber Mat. (b) Illustration of the Formation of the SMI-LbL Structure on a Single Nanofiber

rate and the spinneret-to-collector distance were set as 1.0 mL/ h and 10 cm, respectively. Preparation and Cross-Linking of PAH−Por/PAA Multilayers. The concentrations of the PAA, PAH, and Por aqueous solutions were 1 mg/mL, and the pH value was controlled at 6.0. DAS aqueous solution was prepared at a concentration of 5 mg/mL at a pH of 3.8. The formation of the PAH−Por complex was completed by slowly dropping Por solution into PAH solution under ultrasonication, resulting in a final molar ratio of 5:1. The construction process of the molecularly imprinted nanofiber (MIN) is illustrated in Scheme 1a. In detail, the PAN nanofiber mat was first immersed into sodium hydroxide solution (10 wt %) at 60 °C for 2 h to hydrolyze the surface, which is essential for the subsequent LbL deposition. The LbL multilayers were assembled on the negatively charged nanofiber mat by alternate adsorptions of PAH−Por and PAA for 30 min each. Between each step, the nanofiber mat was rinsed thoroughly with deionized water three times. By repeating this process in a cyclic manner, a multilayer of (PAH−Por/ PAA)n was obtained, where n represents the cycle number of the bilayer. For instance, (PAH−Por/PAA)1.5 consists of PAH−Por/PAA/PAH−Por. After the deposition of (PAH−Por/PAA)n multilayers, the nanofiber mat was soaked in DAS solution for 2 h, followed by UV irradiation with a 400 W high-pressure mercury lamp with an intensity of 2.5 mW cm−2 at a distance of 20 cm. As a control, the reference nonimprinted nanofiber (NIN) mat was prepared and processed in a similar way without introducing the template molecule. Removal and Rebinding of Por. The template molecules are removed by immersing the nanofiber mat into an aqueous solution of sodium hydroxide (pH = 12.5) for 30 min in a shaker. The rebinding of Por was completed by soaking the nanofiber mat in Por solution (1 mg/mL) followed by washing with deionized water. The efficiency of this procedure was checked by immersing the nanofiber mat in sodium hydroxide (pH = 12.5) for another 1 h to release the Por molecules, which were detected by UV spectra. The featured absorption peak of

high binding capacity, fast binding kinetics, and high reproducibility has been demonstrated. In detail, the positively charged composite of poly(allylamine hydrochloride) (PAH) and Por (PAH−Por) and the negatively charged poly(acrylic acid) (PAA) are employed to build the LbL film; DAS is chosen for the photo-cross-link step. Characterization of these procedures on nanofiber mats is studied by attenuated total reflections Fourier-transform infrared spectroscopy (ATRFTIR) and scanning electron microscopy (SEM). Template removal is completed by the deprotonation of PAH by immersing the nanofiber into sodium hydroxide solutions. The rebinding experiment of Por or Fast Green is monitored by UV−visible spectra. We have obtained a surface molecular imprinted LbL film on nanofibers for the first time to our knowledge.

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EXPERIMENTAL SECTION Materials. PAN (Mw = 150000 g/mol), PAA (Mw = 240000 g/mol), 25% aqueous solution, PAH (Mw = 15000 g/mol), Por, and Fast Green FCF were purchased from Sigma-Aldrich. DAS was provided by Tokyo Chemical Industry (TCI). N,Ndimethylformamide (DMF), sodium hydroxide, and hydrogen chloride of analytical reagent grade were obtained from Beijing Chemical Co. All chemicals were used without further purification. Deionized water was used for all experiments. The pH of the polyelectrolyte solution was adjusted with dilute HCl or NaOH to the desired value. Characterization. The surface morphology and structures of the nanofiber mat were studied using a Hitachi S-4700 SEM. UV−visible spectra were obtained on a Hitachi U-3900 spectrophotometer. ATR-FTIR spectra were acquired with an ATR-FTIR spectrometer (PerkinElmer, Spectrum RX-I). Electrospinning of PAN Nanofiber Mat. PAN was dissolved in DMF and stirred at 60 °C for 12 h to obtain a homogeneous solution with a weight ratio of 8%. The electrospinning of PAN solution was carried out at ambient temperature using a syringe with a needle having an inner diameter of 0.7 mm at an applied voltage of 15 kV. The feeding B

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Figure 1. (a) The UV−vis spectra of the Por aqueous solution and the PAH−Por complex. (b) The zeta potentials of PAA, PAH, and PAH−Por solutions.

and the process was confirmed by ATR-FTIR. As seen in Figure 2, after hydrolyzation, new peaks occurred at 1673,

Por was at 414.5 nm. The nanofiber mat after removal of templated Por was then dried to a constant weight under vacuum, and the weight increase increment (W) was calculated using the following equation: m − m0 W (%) = c × 100% m0 (1) where m0 and mc are the weights of original and modified nanofibers after LbL deposition, correspondingly. The rebinding of the control molecule of Fast Green was carried out after removal of templated Por by immersing the nanofiber mat into a Fast Green solution (aq, 1 mg/mL), after which it was thoroughly washed with deionized water three times.

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Figure 2. ATR-FTIR spectra of PAN nanofiber mat (a) before and (b) after hydrolyzation.

RESULTS AND DISCUSSION Formation of the PAH−Por Complex. In this work, Por was selected as the template molecule of the MIN because of its specific planar molecular geometry and multiple negative charges, which should be favorable for the formation of imprinted sites, and these advantages make it very popular in SMI-LbL studies.14,30 The PAH/PAA LbL system was employed because it is a widely studied system with applications in desalination, sensors, and selective ion permeability.31−33 At a pH value of 6, the positively charged weak polyelectrolyte PAH could form the imprinting complex (PAH−Por) with negatively charged Por by electrostatic attraction. Figure 1a shows the UV−vis spectra of Por and the PAH− Por complex. The absorption at 414.5 nm is the characteristic peak of the Soret band of the Por. Compared with the absorption of the aqueous solution of Por, the peak for the PAH−Por complex is blue-shifted by around 10 nm. This result confirmed the formation of the PAH−Por complex. We expected the PAH−Por complex to be positively charged, so the molar ratio of PAH and Por was 5:1. The zeta potential of PAH and PAH−Por (Figure 1b) proved our expectation. This complex of PAH−Por then electrostatically interacted with PAA to construct (PAH−Por/PAA) multilayers during the LbL process. Surface Activation of the PAN Nanofiber Mat. Nanofibrous material provides a huge surface-area-to-volume ratio of the substrate, which is ideal for immobilization of a large quantity of the imprinted cavities and achievement of high imprinting efficiency. Therefore, the PAN nanofiber mat, with its advantages of commercial availability as textile material and simple electrospinning process, was selected as the substrate of the LbL film. Before LbL deposition, the PAN nanofiber mat was immersed in NaOH solution to generate the COO− group,

1568, and 1405 cm−1, which are assigned to the CO stretching vibration and the asymmetric stretching and symmetric stretching vibrations of COO−, respectively.34 The characteristic peak of the CN group at 2242 cm−1 decreased a little, which also verified the hydrolyzation of the PAN nanofibers. In addition, the hydrolyzation procedure had little effect on the morphology of the PAN nanofibers. Layer Formation on PAN Nanofiber Mat. The LbL imprinting procedure on a quartz substrate has been wellstudied.30 However, in this work, these procedures cannot be monitored by UV−vis spectra because the PAN nanofiber mat is not optically transparent. Instead, the ATR-FTIR spectrometer was employed to monitor the layer formation on the nanofiber mat. As shown in Figure 3a, comparing the spectra of PAN− COO− with PAN−(PAH−POR/PAA)1.5, the new observed peak at 1625 cm−1 is assigned to the asymmetric stretching vibration of NH3+,35 and the two COO− group (1400 and 1560 cm−1) peaks are slightly red-shifted due to the formation of the ionic bond of COO−NH3+. The height of these three peaks increased gradually with the deposition of the PAH−Por/PAA bilayer, suggesting an increased amount of PAH−Por/PAA in the LbL deposition. Meanwhile, Figure 4 displays the changes in fiber diameter when different layers of PAH−Por and PAA were assembled on the surface of the nanofiber. The diameter of the single nanofiber increased from 163.4 to 262.6 nm with multipolyelectrolyte bilayers coated on. We can see clearly that the nanofibrous structure was maintained after the LbL deposition. This is very important because the nanofibrous structure gives this system a larger surface area. During the LbL assembly process, any weight change of the PAN nanofiber mat is supposed to be attributed to the assembled LbL thin film. To further investigate this process, the C

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Figure 3. ATR-FTIR spectra of (a) LbL buildup on nanofiber; (b) PAN−(PAH−Por/PAA)3.5 before and after UV cross-linking.

with 1.5−5.5 bilayers of PAH−Por/PAA are 3.96, 9.78, 19.33, 35.15, and 57.09%, respectively, indicating that the weight of the PAH−Por/PAA bilayers kept increasing during the LbL process. Exponential growth behavior was observed rather than linear growth, and the result is consistent with the conclusion of Schönhoff:36 when the pH value is between 4.5 and 6, the PAH and PAA adsorbed per layer reach a maximum, resulting in an exponential layer growth of the LbL film. Also the result exhibited a similar growth trend compared with the results of ATR-FTIR spectra and SEM images. Cross-linking plays a crucial rule in molecular imprinting. It not only serves to stabilize the imprinting sites but also improves the fatigue-resistance of the polymer matrix. In order to stabilize the multilayer structure and facilitate the subsequent extraction and rebinding of Por, the photo-cross-linkable, bifunctional molecule DAS was employed to postinfiltrate into the multilayer for further UV cross-linking (Scheme 1b). Under UV irradiation, the azido groups in DAS will quickly decompose into highly reactive nitrene intermediates with released nitrogen. The nitrene intermediates can then insert themselves into the C−H or C−C bonds and form the interlayer and intralayer photo-cross-linking.37 This procedure is confirmed by ATR-FTIR spectra. As seen in Figure 3b, the peak at 2113 cm−1, which was assigned to the NNN asymmetric stretching vibration vanished after UV irradiation.38 SEM images of cross-linked nanofiber were also obtained in order to get a better understanding of the cross-linking process (Figure 4d). The morphology of the nanofibers changed significantly after photo-cross-linking, which might prove that the PAH−Por/PAA multilayers became compact after photocross-linking. These results indicated that covalently crosslinked three-dimensional multilayers with higher rigidity were obtained on the nanofiber mat. Reproducible Extraction and Recognition of the Surface Imprinted Multilayers. After photo-cross-linking,

Figure 4. SEM images of (a) PAN−(PAH−POR/PAA)1.5, (b) PAN− (PAH−POR/PAA)2.5, (c) PAN−(PAH−POR/PAA)3.5, and (d) after photo-cross-linking.

weight increase increment [W(%)] is shown in Figure 5. The weight increase increment of the nanofiber mats assembled

Figure 5. Weight increased after various PAH-Por/PAA bilayers deposition.

Figure 6. Adsorption kinetics curves of Por (a) in different (PAH−Por/PAA) bilayers. (b) In different bulk concentrations of Por solution using PAN−(PAH−Por/PAA)3.5. (c) Extraction kinetics of Por in a basic solution (pH = 12.5) using PAN−(PAH−Por/PAA)3.5. D

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Figure 7. (a) Por extraction and reloading cycle of PAN−(PAH−Por/PAA)3.5 compared to PAN−(PAH/PAA)3.5. (b) Por and Fast Green extraction and reloading cycle of PAN−(PAH−Por/PAA)3.5..

19.4 μg. The phenomenon could be due to the fact that some template molecules in the outermost PAH−Por layer may not be fully enclosed by polyelectrolytes and, hence, fail to form recognition cavities after cross-linking. The indispensable role of electrostatic interactions between binding cavities and template molecules in the LbL imprinting system has been well-studied using Por4+.30 But the spatial effect of the functional group of the template molecules, which is vital for conventional imprinting polymers, was barely studied. Toward this end, as well as to determine the molecular selectivity, the saturated binding cycles of imprinted nanofibers toward different analytes were investigated using Fast Green. Fast Green (Mw = 808.84) was selected because of the similar planar molecular structure, weight, and multiple negative charges compared with Por (Mw = 790.77). As shown in Figure 7b, the resulting average saturated binding capacity for Por is 19.4 μg, which is 20 times that for Fast Green. High molecular selectivity of MIN was proved. The ability of the MIN to distinguish between Por and Fast Green suggests that the binding of Por to the imprinted sites is not only based on ionic interactions. The size and shape of the binding cavity play an important role along with the correct spatial orientation of the templated molecule in the binding sites.

Por was removed from the molecular imprinted film by immersing the substrate into a NaOH aqueous solution. The solution of sodium hydroxide was employed because it could deprotonate PAH and effectively break the electrostatic interactions between Por and PAH. To gain further insight into the origin of the imprinting effects, equilibrium binding and desorption experiments were carried out under different conditions. As seen in Figure 6a, the rebinding event reached equilibrium within 25 min for every tested multilayer on the nanofiber mat, indicating that the molecular imprinted film had a shorter diffusion path compared with the conventional MIPs. The saturated binding capacity of MIN increased with increases of the PAH−Por/PAA bilayer, and the equilibrium time remained almost the same, which showed that the thickness of the LbL bilayer has little effect on the binding equilibrium. The adsorption curves of PAN−(PAH−Por/PAA)3.5 in different concentrations of Por solution is shown in Figure 6b. The saturating adsorption capacity increased with the concentrations of Por. To reduce experimental error, the concentration of 1 mg/mL was applied in the following experiments. The extraction kinetics of PAN−(PAH−Por/PAA)3.5 was also investigated (Figure 6c), and a quick equilibrium was observed within 25 min. The quick binding and extraction equilibrium results confirmed that the SMI-LbL system exhibits excellent binding capability. In a typical imprinting process, the complementary molecular recognition site in the imprinted polymer is the crux of its molecular recognition ability. So the existence of specific recognition sites in the SMI-LbL film was examined, and the investigation was completed by checking the specific rebinding capacity of the MIN in Por solution (1 mg/mL) in comparison to a nonimprinted one. As shown in Figure 7a, the average saturated binding capacities for MIN and NIN were calculated to be 19.4 and 6.0 μg (the weight of every tested MIN and NIN was controlled at 10 ± 0.5 mg), respectively. This result is attributed to the recognition cavities formed in the imprinted layer. Also the experiment proved that the introduction of a molecular template in the LbL process is a key factor in achieving a molecular imprinted nanofiber mat with high binding ability. At the same time, the extraction and reabsorption circulation of MIN is illustrated in Figure 7a to investigate its reproducibility. After six circulations, the binding capacity of Por remains at 83% of the original film in the first extraction process. The high fatigue resistance indicated that MIN showed preferable stability under different conditions. However, in comparison to the original template weight (36.6 μg) released from the first extraction step, the average template capacity in extracting and reloading experiments dropped to

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CONCLUSION The successful preparation of surface molecular imprinted LbL films on PAN nanofibers is presented in this work. This imprinted nanofiber mat shows significant selectivity and reproducibility toward the imprinted template with fast binding and extraction equilibrium. The binding capacity of Por was 2.1 mg/g nanofiber when 3.5 bilayers were deposited on the nanofibers. This method of imprinting on nanofibers is promising due to its simple operation and high efficiency. The electrospinning materials and conditions are adjustable if any functional structure is needed. We expect that the molecularly imprinted nanofibers will find interesting applications in affinity separation, sensing, and diagnostics areas.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-13717502828. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS The project is supported by the National Natural Science Foundation of China (Grant 51373014).

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DOI: 10.1021/acs.jpca.5b02325 J. Phys. Chem. A XXXX, XXX, XXX−XXX