Layer-by-Layer Structured Nanofiber Membranes with Photoinduced

ARTICLE pubs.acs.org/JPCC

Layer-by-Layer Structured Nanofiber Membranes with Photoinduced Self-Cleaning Functions Dong Wang,†,‡ Ning Liu,‡ Weilin Xu,† and Gang Sun*,‡ †

Key Laboratory of Green Processing and Functional Textile Materials, Ministry of Education, Wuhan Textile University, Wuhan, 430073, China ‡ Fiber and Polymer Science, University of California, Davis, California 95616, United States ABSTRACT: A series of organic chemicals capable of generating radicals under photoexposure, such as 2-anthraquinone sulfonate (2-AQS), 2,6-AQS, and 2,7-AQS, have been layerby-layer (LBL) assembled onto the hydrophilic poly(vinyl alcohol-co-ethylene) (PVA-co-PE) nanofiber composite membranes. The composite nanofiber membranes assembled with AQS chemicals have demonstrated excellent photoinduced selfcleaning properties by decolorizing colorants and killing bacteria. Scanning electron microscopy (SEM) images showed that the nanofibrous membranes maintained the well-defined fiber morphology and large specific surface area after LBL assembly. The effects of AQS types, the compositions of PVA-co-PE, as well as the number of AQS layers on the decolorization property were studied. The PVA-co-PE (44% ethylene unit) nanofiber membrane assembled with one layer of 2,6-AQS was able to provide expected photoinduced self-cleaning properties, and the system could be a promising candidate of self-cleaning protective clothing materials.

1. INTRODUCTION The increasing environmental concerns for polluted water and air, as well as the growing demands for protecting human beings from various hazards including pathogenic microorganisms, toxic chemicals, and pollutants, have inspired considerable research interests in recent years.1
5 Due to the characteristics of high porosity, large surface area, high permeability, low basis weight, and small fiber diameter, polymeric nanofiber membranes have been recognized as ideal candidates for filter media to physically block the aerosols and particles with sizes above 0.3 μm, offering opportunities for applications in filtration and protective clothing.6
9 However, the absence of active functional groups on the nanofiber membranes makes them fail to neutralize or break down hazardous pollutants, as well as eliminate the propagation of the blocked alive microorganisms. As a result, the effect of the nanofiber membranes against pollutants and microorganism has been largely compromised, since desorbed hazardous substances could cause the secondary contamination.5,10,11 Therefore, there is an urgent need to develop nanofiber membranes that are capable of providing powerful self-cleaning functions against hazardous pollutants in an environmentally friendly manner. The photocatalytic substances capable of generating highly reactive superoxide and hydroxyl radicals have become the materials of interests for functionalization of nanofiber membranes, because these reactive oxygen species (ROS) can decompose the hazardous chemicals and inactivate bacteria under the sunlight illumination.12 Photocatalytic metal oxide nanoscale particles, like anatase titanium dioxide, magnesium oxide, and r 2011 American Chemical Society

tin oxide are commonly used.13 There have been some efforts in incorporating these photocatalytic nanoparticles into various polymeric nanofibers.14
17 The mixing of the nanoparticles with polymer matrixes, however, often leads to poor mechanical properties of nanofiber composites and decreased photocatalytic efficiency due to the shielding effect of active particles by surrounding polymer matrixes. The direct exposure of nanoparticles on surfaces of nanofibers is at the expense of reducing surface area of nanofibers and worsening the permeability as well as appearance. Organic photocatalytic compounds have been considered as an alternative for overcoming the above challenges.18
21 In this study, we demonstrate a novel photoinduced selfcleaning layer-by-layer structured nanofiber membrane. The hydrophilic poly(vinyl alcohol-co-ethylene) (PVA-co-PE) nanofiber membranes were prepared with a previously invented method by us as the substrates.22 Negatively charged photocatalytic compounds of 2-anthraquinone sulfonate (2-AQS), 2,6AQS, and 2,7-AQS were selected and adsorbed onto surfaces of PVA-co-PE polymer nanofibers in the form of an ultrathin conformal coating, using layer-by-layer (LBL) deposition with positively charged polyethyleneimine (PEI) molecules. The effects of the types of photocatalytic compounds, the ethylene contents in PVA-co-PE polymers, as well as the structure of Received: January 15, 2011 Revised: February 20, 2011 Published: March 17, 2011 6825

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The Journal of Physical Chemistry C LBL films on the degradation of representing pollutants, dichloroindophenol and Direct Red 23, and on the antibacterial efficiency against Escherichia coli (E. coli) were investigated.

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Scheme 1. Chemical Structures of the Chemicals DCIP and Direct Red 23

2. EXPERIMENTAL SECTION Materials. Anthraquinone-2-sulfonic acid sodium salt (2AQS), PVA-co-PE (ethylene contents of 27, 32, and 44 mol %, respectively), PEI (50 wt % in H2O) solution, dichloroindophenol sodium (DCIP), and Direct Red 23 were purchased from Sigma-Aldrich (Milwaukee, WI). Anthraquinone-2,6-disulfonic acid, disodium salt (2,6-AQS) and anthraquinone-2,7-disulfonic acid, disodium salt (2,7-AQS) were purchased from Acros Chemical (Pittsburgh, PA). Cellulose acetate butyrate (CAB; butyryl content 35
39%) was purchased from the Eastman Chemical (Kingsport, TN). All aqueous solutions were prepared using deionized water. Preparation of PVA-co-PE Nanofibers Membranes. The PVA-co-PE nanofibers were prepared according to a previously published procedure.22 Typically, mixtures of CAB/ PVA-co-PE with a blend ratio of 80/20 were gravimetrically fed into a Leistritz corotating twin-screw (18 mm) extruder (model MIC 18/GL 30D, Nurnberg, Germany) at a feed rate of 12 g/min. Barrel temperature profiles were 150, 175, 190, 220, 235, and 240 °C. The blends were extruded and hotdrawn by a take-up device with a drawn ratio of 25 (the area of cross section of the die to that of the extrudates) and cooled to room temperature. The PVA-co-PE nanofibers in form of continuous yarns were prepared by the extraction of CAB/ PVA-co-PE composite fibers for 24 h with acetone as a solvent to remove the CAB matrix. The prepared PVA-co-PE nanofibers were made into suspension and coated onto the polypropylene (PP) melt-blown nonwovens as composite membranes, which were used as the major substrates in following LBL assembling. LBL Self-Assembly of Photocatalytic Compounds. The PVA-co-PE composite nanofiber membranes were soaked into the 3 N sodium hydroxide solutions for 30 min at room temperature for surface activation. The photoactive compounds, 2-AQS, 2,6-AQS, and 2,7-AQS, were assembled on the activated PVA-co-PE nanofiber membranes by alternate adsorptions of positively charged PEI and negatively charged AQS compounds with the following details. The concentrations of the PEI and 2-AQS, 2,6-AQS, or 2,7-AQS solutions were 1 g/L, and the pH value was controlled at 3.0. The NaOHactivated PVA-co-PE nanofiber membranes were immersed in the PEI solution for 20 min. The membranes were then rinsed twice with deionized water for 2 min each. The rinsed membranes were subsequently immersed in 2-AQS, 2,6-AQS, or 2,7-AQS solutions, respectively, for another 20 min, followed by the same rinsing steps to obtain the one bilayer assembly structured film on PVA-co-PE nanofiber membranes. The electrostatic adsorption and rinsing steps were repeated one more time to make the two bilayers assembly structured PVAco-PE nanofiber membranes. Characterization. The morphologies of PVA-co-PE with ethylene contents of 27, 32, and 44 mol % nanofiber membranes before and after one bilayer and two bilayers assembly of PEI and photoactive AQS compounds were examined using a FEI XL-30 SFEG scanning electron microscope (SEM). The LBL structured PVA-co-PE nanofiber membranes were cut into swatches of 2 in. by 2 in. Two swatches were immersed in 10 mL of 10
4 M

Figure 1. SEM images of original PVA-co-PE nanofiber membranes containing (a) 27, (b) 32, and (c) 44 mol % ethylene units.

solutions of DCIP or Direct Red 23 and then exposed under UVA (365 nm) irradiation with light intensity of 1.3
2.0 mW/ cm2 for certain durations. The chemical structures of DCIP and Direct Red 23 are shown in Scheme 1. The changes in the concentration of DCIP or Direct Red 23 solutions were recorded with a UV
vis spectrophotometer (Evolution 600, Thermo, U.S.A.). The degradation efficiencies were obtained by normalizing the concentration to the original one. The exposed DCIP solutions were characterized with a Waters e2695 liquid chromatography(LC) system, equipped with a Waters 2998 photodiode array (PDA) detector and Waters Micromass ZQ (ESI-MS) for chemical structure analysis. Antibacterial Test. The antibacterial properties of the PVAco-PE nanofiber membranes assembled with AQS were examined against E. coli (K-12, a Gram-negative bacterium), according to a modified AATCC 100 test method. PVA-co-PE nanofiber membranes without AQS were used as control. Two swatches of the control or the PVA-co-PE nanofiber membranes assembled with AQS were placed in a sterilized container. An amount of 1.0 mL of an aqueous suspension containing E. coli was dropped onto the surfaces of the nanofiber membranes. The control and samples with bacteria suspensions 6826

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Scheme 2. Proposed Mechanism of Photoinduced Self-Cleaning Layer-by-Layer Structured Nanofiber Membranes

were illuminated under UVA irradiation with light intensity of 1.3
2.0 mW/cm2 in 365 nm wavelength for 1 h. After exposure, the inoculated controls or the PVA-co-PE nanofiber membranes assembled with AQS were placed into 100 mL of distilled water. The mixture was vigorously shaken for 1 min. Then 100 μL of microbial suspension was taken out from the container and diluted to 10, 102, and 103 times in sequence.

Finally, 100 μL of the microbial suspension and the three diluted solutions were placed onto four zones of a nutrient agar plate and incubated at 37 °C for 18 h. The reduction of bacteria was calculated according to the following equation: reduction of bacteria ð%Þ ¼ 6827

A
B  100 A

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Figure 3. Effect of ethylene units in PVA-co-PE nanofiber membranes on photodecomposition efficiency of DCIP (one bilayer 2,6-AQS assembled).

Figure 4. Effect of bilayers on photodecomposition efficiency of DCIP (2,6-AQS assembled on PVA-co-PE nanofiber membrane (44 mol % ethylene unit).

and AQS assembled PVA-co-PE nanofiber membranes, respectively.

3. RESULTS AND DISCUSSION Morphologies of Original PVA-co-PE Nanofiber Membranes. Photoactive functions on the materials rely on the

Figure 2. Photodecomposition efficiency of DCIP by one bilayer 2-AQS, 2,6-AQS, or 2,7-AQS assembled PVA-co-PE nanofiber membranes with (a) 27, (b) 32, and (c) 44 mol % ethylene.

where A and B are the surviving cells (colony forming unit mL
1) on the agar plates corresponding to the control

amount of the agents on surfaces. Nanofibrous media provides a huge surface area to volume ratio of the substrate, which is ideal for immobilization of a large quantity of the photoactive AQS compounds and achievement of high photocatalysis efficiency. Additionally, it is reported that the hydrophilicity of nanofibers can enhance the interface contacts between hazardous chemicals or bacteria and nanofiber membranes, as well as improve water/ air perspiration through nanofiber membranes.12,23 PVA-co-PE polymers, hydrophilic thermoplastic copolymers of ethylene and vinyl alcohol, were selected, while maintaining the excellent 6828

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Figure 5. LC
MS analysis of degraded DCIP after UVA exposure for 3 h with (a) 2,6-AQS, one bilayer assembled PVA-co-PE (44 mol % ethylene unit) nanofiber membrane, (b) 2,6-AQS, two bilayers assembled PVA-co-PE (44 mol % ethylene unit) nanofiber membrane, and (c) original DCIP solution.

solvent resistance in water without requiring cross-linking. PVAco-PE polymers containing 27%, 32%, and 44% molar ratio of ethylene units, respectively, were fabricated into the nanofibers. The prepared PVA-co-PE nanofibers were dispersed and coated onto PP melt-blown nonwovens to make composite membranes. The morphologies of composite membranes of the PVA-co-PE with 27, 32, and 44 mol % ethylene units on the PP nonwovens were characterized with SEM and are presented in Figure 1a
c. It was found that the PVA-co-PE nanofibers maintained the welldefined nanofiber but randomly distributed morphologies in

membranes. Compared with the nanofibers made from PVA-coPE containing 32 and 44 mol % ethylene units, PVA-co-PE nanofibers with 27 mol % ethylene were more continuous, uniform, and the surfaces were smoother. Nanofibers made from PVA-co-PE containing 44 mol % ethylene units were interconnected and in quite rough surfaces, which might be caused by imperfect coalescence of dispersed PVA-co-PE spheres in CAB matrix. Effect of AQS Compounds on Decomposition Efficiency. The AQS can be excited to the singlet state under UV light 6829

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The Journal of Physical Chemistry C irradiation and then intersystem-crossed efficiently to triplet states for diaryl ketone, which can easily abstract a hydrogen atom from a weak C
H bond or other hydrogen donor to form a ketyl radical.18 The reaction of ketyl radicals with oxygen can then produce active superoxide, peroxide, and hydroxyl radicals which are capable of degrading pollutants and inactivating bacteria. The mechanism of the photoassisted self-cleaning functions of AQS LBL assembled PVA-co-PE nanofiber membranes was proposed and is indicated in Scheme 2. The decomposition efficiency of DCIP by PVA-co-PE nanofiber membranes assembled with one bilayer of PEI and 2-AQS, 2,6-AQS, or 2,7AQS compounds was performed with the UV
vis spectrometer. The concentration ratio of the DCIP solution immersed with PVA-co-PE nanofiber membranes assembled with 2-AQS, 2,6AQS, or 2,7-AQS compounds to that of the original DCIP solution, C/C0, was derived to indicate the decomposition efficiency. The lower the ratio is, the higher decomposition efficiency is. Figure 2a
c displays the decomposition efficiency of DCIP as a function of the UVA exposure time for the same PVA-co-PE nanofiber membrane (containing 27 mol % ethylene units) assembled with three photocatalytic compounds, 2-AQS, 2,6AQS, or 2,7-AQS, respectively. Under UVA exposure, all three AQS compound assembled PVA-co-PE nanofiber membranes could decompose the DCIP. With extending the exposure time, the decomposition ratio increased. The PVA-co-PE nanofiber membranes assembled with 2,6-AQS exhibited faster decomposition rate and higher efficiency than those of 2-AQS and 2,7AQS. The 2,6-AQS on the PVA-co-PE nanofiber membrane could decompose about 77% of DCIP within 0.5 h and 98% after 2 h of UVA exposure, while the 2-AQS assembled nanofiber membrane would take 1 and 2.5 h to reach the DCIP decomposition efficiencies of 79% and 97%, respectively. The 2,7-AQS assembled nanofiber membrane was least efficient in decomposing DCIP. This suggests that the decomposition efficiency was primarily determined by the photocatalytic activity of AQS compounds. The photoreactivity order of the three compounds on the nanofiber membrane is 2,6-AQS > 2-AQS > 2,7-AQS. Effect of Ethylene Unit in PVA-co-PE. PVA-co-PE nanofiber membranes containing ethylene units of 27, 32, and 44 mol % were used, and one bilayer of PEI and 2,6-AQS were assembled onto them. The effect of the composition of PVA-co-PE on the decomposition efficiency was studied, as shown in Figure 3. The PVA-co-PE nanofiber membranes containing 44 mol % ethylene unit had the slightly higher decomposition efficiency, followed by those with 27 and 32 mol % ethylene content, which might be explained by the larger surface area due to the rough and highly porous structures of PVA-co-PE nanofiber membrane having highest (44 mol %) ethylene units as observed in Figure 1c. Effect of Bilayers on Decomposition Efficiency. The photoactive functions of the membrane materials depend on the amount of photoactive agents on surfaces, which is also related to bilayers of PEI and AQS compound. Figure 4 compares the effect of two bilayers versus one bilayer of PEI and 2,6-AQS assembled on PVA-co-PE nanofiber membranes with 44 mol % ethylene content. Surprisingly, the membrane assembled with one bilayer of PEI and 2,6-AQS had much higher decomposition efficiency of DCIP than the membrane with two bilayers did. In fact, in this LBL assembling process, formation of the bilayer of PEI and 2,6-AQS on the surface could induce additional interactions between different electrolytes. One of the interactions is that free PEI molecules in the solution for the second LBL assembly compete with the PEI immobilized on the PVA-co-PE

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Figure 6. Photodecomposition efficiency of Direct Red 23 by one bilayer 2,6-AQS assembled PVA-co-PE nanofiber membrane (44 mol % ethylene unit).

membranes, leading to electrostatic adsorption of 2,6-AQS compounds on the PVA-co-PE membranes. The competition resulted in that the repeated assembly process did not increase the loading amount of the 2,6-AQS compounds, but reduced them, lowering the loading of the AQS compounds and decomposition efficiency of the DCIP. DCIP decomposed products were analyzed by an LC
MS system. Figure 5 shows the LC chromatograms of the original DCIP solution, the DCIP solution with one bilayer assembled PVA-co-PE nanofiber membranes and the DCIP solution with two bilayers assembled membranes after 3 h of UVA exposure. It was evident that the peak of original DCIP showing at 8.215 min significantly decreased due to the decomposition by AQS compounds adsorbed on PVA-co-PE nanofiber membranes, and two small new peaks at 9.385 and 13.846 min appeared. The MS spectra illustrated the structures of the two elutes as shown in Figure 5. The photogenerated active superoxide ions and radicals from AQS compounds cleaved one and two chlorine atoms from the DCIP molecules, corresponding to the peaks at 13.846 and 9.385 min, respectively. Decomposition of Direct Red 23. The LBL assembled 2,6AQS on the PVA-co-PE nanofiber membranes (44 mol % ethylene) were able to degrade other chemicals. Figure 6 shows the results of decolorization of Direct Red 23 solution. The membrane was immersed in the dye solution. After 1 h of UVA exposure, 90% of the Direct Red 23 was decomposed, while the Direct Red 23 solution without assembled PVA-co-PE nanofiber membranes remained unchanged. Antibacterial Property. Biological self-cleaning properties of the 2,6-AQS by LBL assembled on PVA-co-PE nanofiber membranes were examined against E. coli according to a modified AATCC 100 method as shown in Figure 7. The membranes without and with assembly of 2,6-AQS compounds were inoculated with E. coli bacteria solution with concentration of 105
106 CFU/mL and then irradiated under UVA light (365 nm) for 1 h. No viable colony of the bacteria was found on the agar plate for the membranes assembled with 2,6-AQS compounds, whereas proliferated colonies of E. coli were observed for the nanofiber membranes without being 2,6-AQS assembled. The reduction rate of the E. coli bacteria were in the range of 99.999
99.9999%, 6830

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Figure 7. Antibacterial test of one bilayer 2,6-AQS assembled PVAco-PE nanofiber membrane having 44 mol % ethylene unit after UVA exposure for 1 h.

Figure 9. SEM images of one bilayer 2,6-AQS assembled PVA-co-PE nanofiber membranes with (a) 27, (b) 32, and (c) 44 mol % ethylene units after 1 h of UVA exposure.

Figure 8. SEM images of one bilayer 2,6-AQS assembled PVA-co-PE nanofiber membranes with (a) 27, (b) 32, and (c) 44 mol % ethylene units.

indicating the AQS assembled PVA-co-PE nanofiber membranes exhibited excellent photoinduced biological self-cleaning properties. Morphological Changes of PVA-co-PE Nanofiber Membranes. Practically speaking, there are two concerns associated with the functionalization of the nanofiber membranes. One is whether the introduction of the active substances to the nanofiber membranes could lead to loss of the surface area. The other one is whether the sufficient photoinduced self-cleaning properties could damage the supporting nanofiber materials. Figure 8 reveals the morphologies of the PVA-co-PE nanofiber membranes assembled with one bilayer of PEI and 2,6-AQS compounds. Compared with the Figure 1, there was no significant difference observed, which suggested the LBL assembly with AQS compounds could form the conformal coating on the nanofibers without compromising the surface area. The SEM images of PVA-co-PE nanofiber membranes assembled with one bilayer of PEI and 2,6-AQS compounds after 1 h of being UVA light irradiated are displayed in Figure 9. It could be found that although the AQS assembled PVA-co-PE nanofiber membranes demonstrated excellent photoinduced self-cleaning activities, the surface morphology was well-maintained.

4. CONCLUSIONS Photoactive 2-AQS, 2,6-AQS, or 2,7-AQS compounds were LBL assembled onto surfaces of hydrophilic PVA-co-PE nanofiber membranes. The assembly of AQS compounds had no influence on the well-defined nanofibrous morphology of PVAco-PE nanofiber membranes. The photoinduced activity of the AQS compounds played an important role in determining the decomposition efficiency of a colorant, DCIP, under UVA (365 nm) irradiation. When a model PVA-co-PE nanofiber membrane system containing 44 mol % ethylene unit and 2,6AQS was used in photoinduced self-cleaning tests, higher decomposition efficiency against DCIP was attained. The increase in the number of bilayers had the adverse effect on the decomposition efficiency. The same membrane system also exhibited excellent photoinduced self-cleaning functions against Direct Red 23 and the bacterium of E. coli. ’ AUTHOR INFORMATION Corresponding Author

*Phone: (530) 752-0840. Fax: (530) 752-7584. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors are grateful for the financial support from Defense Threat Reduction Agency (HDTRA1-08-1-0005). D. Wang is thankful to the National Nature Science Foundation (51003083). ’ REFERENCES (1) Likodimos, V.; Dionysiou, D. D.; Falaras, P. Rev. Enviorn. Sci. Biotechnol. 2010, 9, 87. (2) Hong, K. H.; Sun, G. Polym. Eng. Sci. 2007, 10, 1751. (3) Nanoscience and Nanotechnology for Chemical and Biological Defense; Nagarajan, R., Zukas, W., Hatton, T. A., Lee, S., Eds.; American Chemical Society: Washington, DC, 2009. (4) Chen, L. Next generation of electrospun textiles for chemical and biological protection and air filtration. Ph.D. Thesis, Massachusetts Institute of Technology, 2009. (5) Dong, H.; Wang, D.; Sun, G.; Hinestroza, J. P. Chem. Mater. 2008, 20, 6627. 6831

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