3D Conformal Modification of Electrospun Silk Nanofibers with

Nov 22, 2016 - 28, Xianning West Road, Xi,an,. Shaanxi 710049, P.R. China. ∥. State Key Laboratory for Diagnosis and Treatment of Infectious Disease...
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3D Conformal Modification of Electrospun Silk Nanofibers with Nanoscaled ZnO Deposition for Enhanced Photocatalytic Activity Guoxu Zhao,†,‡ Yijun Zhang,§ Le Zhang,§ Zuo-Guang Ye,§,∇ Wei Ren,§ Feng Xu,†,‡ ShuQi Wang,∥,⊥,# Ming Liu,§ and Xiaohui Zhang*,†,‡ †

The Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Bioinspired Engineering and Biomechanics Center (BEBC) and §Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an, Shaanxi 710049, P.R. China ∥ State Key Laboratory for Diagnosis and Treatment of Infectious Diseases, First Affiliated Hospital, College of Medicine, ⊥ Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, and #Institute for Translational Medicine, Zhejiang University, No. 866 Yuhangtang Road, Hangzhou, Zhejiang Province 310003, P.R. China ∇ Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, B.C., V5A 1S6, Canada ‡

ABSTRACT: Nanostructured metal oxide materials have drawn great attention because of their enhanced semiconducting, electrical, optical, sensing, and chemical catalyzing properties. The application of metal oxides in biomedicine has recently emerged as a promising field, especially in the format of organic/ metal oxide composites. However, the existing methods for fabricating organic/metal oxide materials revealed limitations on the precise control over the deposition of metal oxides and the maintenance of organic nanostructures. Here, we developed an approach for the fabrication of composite materials by depositing metal oxides on the nanostructured organic templates through atomic layer deposition (ALD). With this method, we fabricated a series of silk fibroin/ZnO composites at varied deposition temperatures. The results demonstrated that the ZnO layer had a 3D conformality and hexagonal wurtzite structure, and the deposition thickness was well controlled. The photocatalytic activity of silk/ZnO composites was confirmed by the photodegradation of Rh−B under UV exposure, and the efficiency was found to be temperature dependent. These results demonstrated the successful integration of organic materials with metal oxides through an easy and controllable approach for the development of multifunctional organic/metal oxide biomaterials. KEYWORDS: electrospun materials, atomic layer deposition (ALD), nanofibers, silk fibroin, ZnO

1. INTRODUCTION

nanostructured organic/metal oxide materials could be simplified by using organic materials as templates, as the fabrication of 3D nanostructured organic architectures (e.g., nanofibers,16 nanopatterns17,18) is much easier and more versatile. Therefore, it is of great significance to develop a suitable method to fabricate composites that can benefit from organic templates for biocompatibility and flexibility, and nanostructured metal oxides for versatile functionalities. So far, a variety of methods have been explored for the fabrication of nanostructured organic/metal oxide composites. In particular, depositing metal oxide on the surface of nanostructured organic templates via surface deposition

Metal oxides (e.g., TiO2, ZnO, Al2O3) have attracted great attention for their unique semiconducting, electrical, optical, sensing, and chemical-catalyzing properties endowed by their surface defects.1−4 Nanostructured metal oxide materials with expanded chemical reactive surface3,5 have found even broader applications in photocatalysis,6,7 electronics,8 sensing,9 energy storage,10 and more recently in biomedicine (e.g., biosensing,11 bioimaging,12 and antibiosis13,14). However, the synthetic onecomponent metal oxides (e.g., nanoparticles, nanowires) showed limitations in biomedical application due to the lack of biological properties (e.g., biodegradation, biocompatibility) and the difficulty in handling materials at the nanoscale. Combining organic materials with metal oxides has demonstrated great potential in improving the biocompatibility and flexibility of the composites,15 which significantly extended their applications in biomedical fields such as antibacterial dressings and flexible biosensors.7 In particular, the fabrication of © XXXX American Chemical Society

Special Issue: Multiscale Biological Materials and Systems: Integration of Experiment, Modeling, and Theory Received: September 14, 2016 Accepted: November 22, 2016

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DOI: 10.1021/acsbiomaterials.6b00548 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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ACS Biomaterials Science & Engineering techniques (e.g., chemical vapor deposition (CVD),19 physical vapor deposition (PVD),9 sol−gel,19 and atomic layer deposition (ALD)20) has exhibited excellent capability in depositing metal oxide layer with large surface area and controllable thickness. However, there exist several challenges associated with these methods. For instance, the high operating temperature of CVD (usually >400 °C) is not compatible with organic materials,21 PVD is not feasible to fabricate 3D conformal layer,9 and the sol−gel method involves wet condition that may dissolve organic templates.22 Therefore, there is still an unmet need for a suitable technique that could create functional metal oxide layer on the surface of organic nanostructured architectures. Recently, ALD has exhibited great potential in addressing these challenges. The sequential and self-limiting surface reactions in ALD procedure can ensure sufficient diffusion of precursors into a 3D nanostructured architecture and deposition of a monolayer in one ALD cycle, thus providing the deposited metal oxide layer with 3D conformality and precise thickness.20 In addition, by selecting proper ALD precursors, the ALD procedure can be realized at significantly reduced temperatures that are compatible with organic materials.23 Therefore, ALD is a desirable technique for the fabrication of functional organic/metal oxide composite biomaterials.7,24,25 Natural polymers, as one of the major organic biomaterials, have been widely applied for biomedical engineering because of their good biocompatibility, bioactivity, and biodegradation. However, most of them lack thermal stability,26,27 and they cannot be used as substrates for the deposition of functional metal oxide layer through ALD. Silk fibroin, as a natural protein extracted from natural silk, not only possesses excellent biocompatibility, controllable biodegradation, versatile processability and unique mechanical properties,28 but also exhibits excellent thermal stability (starts being denatured at around 250 °C) attributed to its enrichment of β-sheet structure.29 Moreover, silk fibroin has been successfully processed into various nano- and microstructured materials through an allaqueous process,30 and it has been widely used in tissue engineering31 and drug delivery.32 Thus, silk fibroin-based biomaterials have demonstrated to be an ideal organic substrate for the deposition of metal oxide layers through ALD to investigate the deposition process and parameters, and eventually develop multifunctional composite biomaterials. In this study, we employed electrospinning technique to prepare nanofibrous silk biomaterials that served as an organic template, and selected zinc oxide (ZnO) as a model metal oxide considering its wide applications in biomedicine (e.g., antibiosis,33 biosensing34). The effects of deposition temperature on the deposition efficiency was evaluated and optimized in terms of crystal structure of ZnO layer and the functionality of silk/ZnO composites. Our results demonstrated that a 3D conformal ZnO layer had been successfully deposited on the surface of silk nanofibrous materials through ALD without causing significant damage on the silk nanofibers. Such an approach provides a platform for employing 3D nanostructured organic systems as templates to develop organic/metal oxide composite biomaterials.

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Sigma-Aldrich. Deionized water with a resistance of 18.2 MΩ was prepared by Millipore Milli-Q water purification system. Preparation of Regenerated Silk Aqueous Solution. B. mori silk fibroin aqueous solution was prepared as previously described.31,35 Briefly, silk yarn was boiled for 30 min in 0.02 M sodium carbonate solution and rinsed thoroughly with deionized water to extract the glue-like sericin proteins. The extracted silk fibroin was then dissolved in 9.3 M LiBr solution at 60 °C for 4 h. The solution was then dialyzed against deionized water using a Slide-a-Lyzer dialysis cassette (MWCO 3500, Pierce) for 72 h at room temperature to remove the salt. The dialysate was centrifuged for 20 min at 4 °C twice to remove impurities and aggregates that formed during dialysis. Electrospinning of Silk Nanofibers. The electrospinning precursor solution was prepared by gently blending 5.0% (w/v) PEO (900 000 g/mol) solution with 8.0% (w/v) silk fibroin aqueous solution with a volume ratio of 1:4. The homogeneous and bubble-free solution was loaded into a 10 mL syringe (inside diameter 1.5 cm) fitted with a 12-G spinneret, and a syringe pump was used to pump the solution at a constant flow rate of 0.8 mL/h. The needle was connected to the positive output of a high voltage supply set at 10 kV. A slowly rotating drum collector (diameter 15 cm, width 20 cm) was used to collect silk nanofibers. The distance between the spinneret and the surface of the collector was about 15 cm, and the temperature and humidity was 30 °C and 30%, respectively. The as-spun silk materials was treated with 100% methanol for 15 min to induce a β-sheet conformational transition, and then dried in air. The PEO was removed by leaching the electrospun silk materials in deionized water for 72 h. Preparation of ZnO Deposition Layer by ALD. ALD was employed to grow a ZnO layer upon electrospun silk nanofibers to obtain silk/ZnO composite materials. Diethyl zinc (DEZ 99.9999%) and deionized water were used as precursors. A quasi-static growth mode was used to enhance the diffusion of the zinc precursors into the network of silk nanofibers. In this mode, an interrupt valve placed between the reaction chamber and the pump was switched off for 6 s after the DEZ or H2O precursors were injected into the reaction chamber, which facilitated the DEZ and H2O to sufficiently diffuse into the gap of the silk nanofibrous templates. In this study, 300 growth cycles were selected for ZnO layer deposition using ALD, and the operating temperatures were set at 60, 100, 150, and 200 °C, respectively, to obtain temperature-dependent samples. The silk/ZnO materials were calcined at 400 °C in air for 2 h to remove the silk nanofiber core to obtain pure ZnO materials for characterization. Material Characterization. The morphologies of the electrospun silk nanofibers, silk/ZnO and ZnO materials were examined by a field emission scanning electron microscopy (FSEM; Quanta 250FEG, FEI) after Au coating. The average diameter of composite nanofibers was calculated by measuring 500 fibers using SEM images. An energy dispersive X-ray Spectroscopy (EDS) equipped on a SEM system was used to determine the element distribution and chemical composition of ZnO deposited silk nanofibrous networks. The crystallinity of the silk/ZnO materials was characterized by an X-ray diffraction (XRD; X’Pert PROPW 3, PANalytical) within the 2θ range 10−80° with a Cu Kα radiation. The grain size was estimated by the Scherrer Equation: D = (kλ/βDcos θ), where D is the grain size, k is a constant (0.94), λ is the wavelength of the X-ray radiation (0.154 nm for Cu Kα radiation), βD is the full width at half-maximum of peak, and θ is the peak position. The fluorescence spectrum of silk/ZnO materials was measured using a spectrofluorometer (QuantaMasterTM40) using square specimens (side length 1 cm) with excitation at 325 nm. The morphology and crystal structure of the ZnO materials after removing the silk core was characterized using transmission electron microscope (TEM; JEM-2100, JEOL) and selected area electron diffraction (SAED), respectively. The TEM specimens were prepared by sonicating calcined ZnO materials in ethanol, a droplet of which was then dried on TEM grids. Mechanical Characterization. The mechanical properties of silk/ ZnO materials were characterized by using a Universal Testing Machine (MTS criterion model 43) with a 50 N load cell and crosshead speed of 10 mm/min at wet condition. The samples were

MATERIALS AND METHODS

Materials. Silk yarn of Bombyx mori silkworm was purchased from Zhejiang Second Silk Factory. LiBr, Slide-a-Lyzer dialysis cassette (MWCO 3500, Pierce), Poly(ethylene oxide) (PEO, 900,000 MW), Diethyl zinc (DEZ) and Rhodamine-B (Rh−B) were purchased from B

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Figure 1. Schematic illustration of the fabrication of silk/ZnO materials and the morphology of silk and silk/ZnO nanofibers. (A) Schematics of the fabrication of electrospun silk nanofibers and surface modification by ALD. An aqueous silk fibroin solution was used for the fabrication of electrospun silk nanofibers, and diethyl zinc and deionized water were used as precursors for ZnO deposition through ALD. The schematics in the round frame illustrates the layer-by-layer deposition of ZnO on the surface of silk nanofibers. (B) SEM characterizations for (a) electrospun silk nanofibrous and (b) cross-section of silk/ZnO materials.

Figure 2. SEM images of silk/ZnO materials. (A) Morphology of silk/ZnO materials deposited at (a) 60, (b) 100, (c) 150, and (d) 200 °C. (B) The cross-section of silk/ZnO materials deposited at (a) 60, (b) 100, (c) 150, and (d) 200 °C. prepared into rectangular shapes with a gauge length of 12 mm and a width of 4 mm, and soaked in deionized water before testing. At least 5 specimens of each group were tested, and the Young’s modulus was determined from the stress−strain curves. Photocatalytic Activity. An aqueous solution of 0.01% (w/v) Rh−B was used to analyze the photocatalytic activity of ZnO formed by examining the photodegradation of Rh−B. For ZnO/UV group, a 3-mg calcined ZnO powder was added to a 3 mL Rh−B solution in a quartz cuvette (length and width of 1 cm, height of 5 cm). The cuvette was then exposed under UV irradiation at 365 nm, and the photodegradation of the dye was detected by an UV−vis-NIR spectrophotometer with intervals of 10 min. The dye solution without adding ZnO nanotube was defined as a ZnO/control group and the dye solution with ZnO nanotubes but without UV exposure was defined as a control/UV group. Statistical Analysis. Values (at least triplicate) were averaged and expressed as mean standard deviation (SD). Statistical differences were determined by Student’s two-tailed t test. Differences were considered statistically significant at p < 0.05.

shown in Figure 1A, electrospinning and ALD techniques were employed for the preparation of silk/ZnO composite materials. Briefly, the nanofibrous electrospun silk materials prepared from an aqueous silk fibroin solution served as substrates for the deposition of ZnO through ALD using Zn(C2H5)2 and water vapor. Silk nanofibers with smooth and bead-free morphology were obtained by optimizing the parameters including solution concentration, flow rate, voltage and ambient humidity and used for the study (Figure 1Ba). The nanostructured silk materials (fiber diameter of 601 ± 89 nm) provided high surface area, which was desirable for serving as nanostructured organic templates for the formation of nanostructured metal oxide. After ALD process, a ZnO layer was clearly observed as shown in the SEM images (Figure 1Bb), indicating the successful deposition of ZnO on silk nanofibers. These results indicated the feasibility of fabricating a 3D conformal ZnO layer using electrospun silk materials as templates through ALD. The operating temperature has been reported to be critical for the crystallite orientation of the ZnO layer formed using ALD, thus influencing its material properties (e.g., optical and electrical properties).36,37 In this study, we investigated the

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RESULTS AND DISCUSSION In this work, we introduced a novel approach for the conformal modification of electrospun silk nanofibers with nanoscaled ZnO deposition to enhance their photocatalytic activities. As C

DOI: 10.1021/acsbiomaterials.6b00548 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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Figure 3. EDS results, chemical composition and XRD patterns of silk/ZnO materials. (A) EDS results of silk/ZnO materials, where a−d show the element distribution of C, O, Zn, and N, respectively. (B) Chemical composition of silk/ZnO materials fabricated at different temperatures. (C) XRD patterns of silk/ZnO materials fabricated at different temperatures, the vertical lines on X-axis indicate the JCPDS data of PDF card 36−1451 for ZnO.

60 °C, further indicating that the temperature higher than 100 °C is required for an efficient ZnO deposition through ALD. To investigate the influence of ALD operating temperature on the crystal structure of the ZnO layer formed, we further examined the XRD patterns of all study groups (Figure 3C). The results showed that the silk nanofibers have a characteristic XRD pattern with a broad peak around 22°. After ZnO deposition, the intensity of silk diffraction peak significantly decreased for the 100, 150, and 200 °C groups, and the standard peaks of ZnO (hexagonal wurtzite, PDF card 36− 1451) emerged, indicating that the ZnO layer deposited on silk nanofibers had crystallinity with a calculated grain size of about 17 nm as estimated by the Scherrer equation. In addition, the intensity of standard ZnO peaks for the 60 °C group was significantly lower than those operated at higher temperatures, further confirming the inefficiency of ZnO deposition at 60 °C, which is consistent with the SEM (Figure 2) and EDS (Figure 3B) results. To further study the morphology and crystallinity of the deposited ZnO layer, we performed TEM on the ZnO layer after calcining the silk nanofibers. We observed that the ZnO layer was well preserved after calcining (Figure 4A) and the deposited ZnO layer in the network had a hollow structure and a uniform thickness (Figure 4B, C), which was most attributed to the layer-by-layer self-limiting growth of ZnO layer through the ALD process. The thickness of the ZnO layer was determined to be about 30 nm, indicating a growth rate of about 1 Å/cycle during the 300 ALD cycles. Since such a growth rate was consistent with the thickness of ZnO monolayer that was deposited in one ALD cycle, the thicknesses of ZnO layer can be precisely controlled by the number of ALD cycles. Combined with the 3D confomality of ZnO layer deposited by ALD, the excellent controllability on layer thickness and uniformity can provide precise control over the structure and functionality of metal oxide-deposited biomaterials. To examine the crystal structure and symmetry of the ZnO layer, we performed high resolution TEM imaging and selected area electron diffraction (SAED) (Figure 4D−F).

effect of operating temperature on the structure and properties of the ZnO layer deposited on the silk nanofibers, where relatively low temperatures of 60, 100, 150, and 200 °C were selected to ensure the integrity of the silk fibroin nanofibers. As shown in Figure 2A, the fiber diameters of the silk/ZnO materials fabricated at 60, 100, 150, and 200 °C were 622 ± 67, 656 ± 74, 672 ± 75, and 667 ± 82 nm, respectively. This result indicated that the thickness of ZnO layers formed through ALD were approximately 11, 28, 31, and 33 nm, respectively. Theoretically, one ALD growth cycle produces one monolayer of film, thus approximate a 30 nm thickness of ZnO deposition layer should be obtained through a 300-cycle ALD process in our study.20,38 Thus, an operating temperature higher than 100 °C is necessary to ensure ZnO deposition efficiency. In addition, the thickness of ZnO deposition was found to be consistent throughout the depth of the silk nanofibrous material as shown in the cross-section of SEM images (Figure 2B), indicating a 3D conformal coating of ZnO upon the organic nanofibers. This is most attributed to the self-limiting mechanism of ALD film growth, which makes it possible to control the film growth at atomic/molecular scale surrounding the material surface. Thus, a conformal deposition of ZnO layer on silk nanofibrous substrates with excellent step coverage, uniform thickness and smooth surface could be achieved through ALD operating at 100, 150, and 200 °C. To examine the existence of ZnO within the silk/ZnO materials, we performed EDS and found that the distribution of Zn was highly overlapped with the main components of silk fibroin (i.e., carbon (C), oxygen (O), and nitrogen (N)), indicating that the deposited layer is ZnO (Figure 3A). We also analyzed the chemical composition of each group and found the emerging of Zn in experimental group compared to control, while the percentage of C, O and N decreased significantly, further confirming the successful deposition of ZnO on the silk nanofibers (Figure 3B). In addition, the silk/ZnO materials fabricated at 100 °C, 150 and 200 °C showed a significant increase in the percentage of Zn compared to that fabricated at D

DOI: 10.1021/acsbiomaterials.6b00548 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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properties of silk substrates when the ALD deposition was operated at temperatures lower than 150 °C. However, the tensile strength and Young’s modulus was significantly reinforced when deposition was operated at 200 °C, which may be due to the thermally induced denaturation of protein in silk nanofibers at 200 °C. The appearance of silk/ZnO composite was also found changing from white to pale yellow after the ALD procedure at 200 °C. Therefore, the silk/ZnO materials fabricated by ALD showed excellent flexibility and elasticity, indicating that ALD is a desirable approach for the surface modification of nanostructured silk substrates. The silk/ ZnO composite fabricated through ALD demonstrated superior characteristics for wound dressing applications. The fluorescence pattern of the deposited layer has found to be related to its structural quality, vacancy defects and thus photic functionalities.36 Therefore, the photoluminescence (PL) spectrum was performed for the silk/ZnO composite using a spectrofluorometer with an excitation of 325 nm (Figure 5C). The excitation wavelength was selected to detect the near-band-edge emission (∼380 nm) and broad defectrelated emission (centered at ∼500 nm) of ZnO materials.39,40 The results were normalized by the intensity of each group at an emission wavelength of 200 nm for comparison. The electrospun silk materials showed a typical PL spectrum pattern of at the excitation of 325 nm with a series of descending peaks from 400 to 550 nm. Compared to the silk nanofibers, the fluorescence intensity of silk/ZnO materials was significantly enhanced with a rapid rising beginning from 370 nm in all the silk/ZnO groups, indicating the existence of near band edge emission of ZnO (around 384 nm).41 In addition, the peak intensity around 470 nm was significantly enhanced for the silk/ZnO materials prepared at 100 and 150 °C by ALD, indicating the existence of defect-related bands of ZnO (450− 500 nm). The results indicated that the ZnO deposition layer obtained through ALD had high structural quality (edge PL in all groups) and reactivity (obvious defect-related PL in 100 and 150 °C groups). In addition, the PL spectrum of silk/ZnO materials fabricated at 100 °C presented the most intensive edge PL and defect-related PL, indicating that 100 °C might be the optimal temperature for the fabrication of silk/ZnO composites through ALD. To further verify the functionality of the ZnO layer deposited through ALD, we examined the photocatalytic activity of ZnO layer by detecting the degradation of Rh−B solution upon UV irradiation. The degradation on Rh−B was visually observed by the fading of pink color in the Rh−B solution and detected by UV−vis spectrophotometer. To avoid the interference of Rh−B adsorption to silk nanofibers for the photocatalytic measurements, pure ZnO layer materials obtained by calcining the silk/

Figure 4. TEM Characterizations on ZnO layer. (A) Cross-section of ZnO layer (100 °C). (B, C) TEM images of ZnO layer at low magnification show hollow structure and uniform thickness. (D, E) TEM images of ZnO layer at high magnification show ZnO grains; (F) SAED pattern of the deposited ZnO layer.

We found that the ZnO layer was nanocrystalline with a grain size of 17.7 ± 1.8 nm, which was consistent with the result obtained by the XRD (17 nm), and the SAED results also confirmed that the ZnO layer had a pure polycrystalline hexagonal wurtzite phase with different grain orientations. Therefore, ALD is an ideal technique to produce ZnO layers with excellent nanostructures and crystallinity on electrospun silk nanofibers, which are important for the functionalities of ZnO layer. Electrospun silk materials have shown great potentials for applications such as tissue engineering and wound dressings, which are closely relied on their excellent mechanical strength and flexibility in wet environment. To study the influence of ZnO layer and ALD operating temperature on the mechanical properties of silk/ZnO materials, we performed tensile tests on the composite materials (Figure 5A, B). The results showed that there was no significant difference for the tensile strength and Young’s moduli among the control, 60, 100, and 150 °C groups, indicating no significant influence on the mechanical

Figure 5. Mechanical and optical characterizations on silk/ZnO materials. (A) Tensile curves of silk/ZnO materials. (B) Young’s moduli of silk/ZnO materials. (C) PL spectra of silk/ZnO materials. E

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Figure 6. Photocatalytic activity of ZnO layer. (A) Comparison of the Rh−B solutions in ZnO/control, control/UV, and ZnO/UV groups. (B) UV− vis spectra of the Rh−B solutions with the photocatalysis of ZnO layer at UV exposure time of every 20 min. (C) Degradation rate (C/C0) of Rh−B in solutions photocatalyzed by ZnO nanotubes.

ZnO composites at 400 °C for 2 h were utilized. To investigate the photocatalytic activity of the ZnO layer, we tested ZnO/UV (Rh−B solution with ZnO and 365 nm UV irradiation) and two controls groups, ZnO/control (Rh−B solution with ZnO but no UV irradiation) and control/UV (Rh−B solution with 365 nm UV irradiation but no ZnO). The UV−vis absorption of the Rh−B solutions was detected every 10 min (Figure 6). We found that the color of the Rh−B solution in ZnO/UV group faded significantly after UV irradiation for 5 h, whereas the two controls showed no significant color changes (Figure 6A). The dynamic results showed that the intensity of the peak centered around 554 nm in ZnO/UV group steadily decreased along with the UV irradiation time (Figure 6B). To quantitatively characterize the photocatalytic activity, we calculated the change on Rh−B concentration (C/C0) and represented as a function of UV irradiation time (Figure 6C). As shown in Figure 6C, the Rh−B has been significantly photodegraded in the ZnO/UV group (reduced for 86% in 5 h) compared to the control/UV and ZnO/UV groups, further confirming the photocatalytic activity of the ZnO layer deposited by ALD. It has been widely demonstrated that the photocatalytic activities of ZnO and other metal oxides (e.g., TiO2) provide the capacity to produce reactive oxygen species (ROS) under UV exposure.33,42,43 By integrating such photocatalytic materials with UV irradiation, the concentration of ROS in the solution can be raised faster and kept at a higher level than using UV only. The silk/ZnO composites fabricated through ALD exhibited excellent photocatalytic activity attributed to the extremely expanded surface area as well as the uniform and well-controlled nanostructure of the ZnO layer, and thus holding great potential in water purification and antibacterial dressing applications. In addition, the excellent biocompatibility31,44 and flexibility of the silk/ZnO materials fabricated through ALD makes it even more ideal for antibacterial dressing applications.

conferred the composite with great advantages in potential biomedical applications. Finally, the photocatalytic activity of ZnO layer was visually demonstrated by the fading of the Rh−B solution. As electrospun silk materials have been widely used for biomedical applications (e.g., tissue engineering, drug delivery and wound dressing) due to their excellent biocompatibility and biological properties, the deposition of metal oxide layer on electrospun silk through ALD has created a new platform to develop novel multifunctional materials for biomedical applications such as biosensing, bioimaging, and antibacterial dressings.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Feng Xu: 0000-0003-4351-0222 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (81401270, 51472199, 11534015), Natural Science Basic Research Plan in Shaanxi Province of China (2015JM3108), and the Fundamental Research Funds for the Central Universities. X.Z. was partially supported by the China Young 1000-Talent Program. S.W. acknowledges the support from the National Key Research and Development Program (2016YFC1101302). Z.G.Y. acknowledges the support from the "Qian-Ren" Program of the Chinese Government and the Natural Sciences and Engineering Research Council of Canada.

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REFERENCES

(1) Yu, X.; Marks, T. J.; Facchetti, A. Metal oxides for optoelectronic applications. Nat. Mater. 2016, 15 (4), 383−96. (2) Jackson, S. D.; Hargreaves, J. S. Metal Oxide Catalysis; Wiley: New York, 2009; Vol. 1. (3) Brown, G. E.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Metal oxide surfaces and their interactions with aqueous solutions and microbial organisms. Chem. Rev. 1999, 99 (1), 77−174. (4) Roy, P.; Berger, S.; Schmuki, P. TiO2 nanotubes: synthesis and applications. Angew. Chem., Int. Ed. 2011, 50 (13), 2904−39. (5) Zhang, X.; Qin, J.; Xue, Y.; Yu, P.; Zhang, B.; Wang, L.; Liu, R. Effect of aspect ratio and surface defects on the photocatalytic activity of ZnO nanorods. Sci. Rep. 2014, 4, 4596.

CONCLUSIONS In this study, we have successfully demonstrated the feasibility of fabricating silk/ZnO composite materials by combing electrospinning for the silk nanofibrous material preparation and ALD for the deposition of ZnO layers on the electrospun silk nanofibers. The ALD operating temperature was optimized to realize efficient deposition of ZnO on the silk nanofibers. The ZnO layer fabricated exhibited excellent uniformity, 3D conformality and hexagonal wurtzite structure, which were attributed to the layer-by-layer deposition of ZnO through ALD. Furthermore, the excellent flexibility of silk/ZnO materials as demonstrated by mechanical characterization has F

DOI: 10.1021/acsbiomaterials.6b00548 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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ACS Biomaterials Science & Engineering (6) Zhang, Z.; Shao, C.; Li, X.; Wang, C.; Zhang, M.; Liu, Y. Electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with enhanced photocatalytic activity. ACS Appl. Mater. Interfaces 2010, 2 (10), 2915−23. (7) Kayaci, F.; Ozgit-Akgun, C.; Donmez, I.; Biyikli, N.; Uyar, T. Polymer-inorganic core-shell nanofibers by electrospinning and atomic layer deposition: flexible nylon-ZnO core-shell nanofiber mats and their photocatalytic activity. ACS Appl. Mater. Interfaces 2012, 4 (11), 6185−94. (8) Archana, P. S.; Gupta, A.; Yusoff, M. M.; Jose, R. Tungsten doped titanium dioxide nanowires for high efficiency dye-sensitized solar cells. Phys. Chem. Chem. Phys. 2014, 16 (16), 7448−54. (9) Choi, S.-H.; Ankonina, G.; Youn, D.-Y.; Oh, S.-G.; Hong, J.-M.; Rothschild, A.; Kim, I.-D. Hollow ZnO nanofibers fabricated using electrospun polymer templates and their electronic transport properties. ACS Nano 2009, 3 (9), 2623−2631. (10) Yang, Y.; Wang, H.; Zhou, Q.; Kong, M.; Ye, H.; Yang, G. Improved lithium storage properties of electrospun TiO2 with tunable morphology: from porous anatase to necklace rutile. Nanoscale 2013, 5 (21), 10267−74. (11) Xia, L.; Song, J.; Xu, R.; Liu, D.; Dong, B.; Xu, L.; Song, H. Zinc oxide inverse opal electrodes modified by glucose oxidase for electrochemical and photoelectrochemical biosensor. Biosens. Bioelectron. 2014, 59, 350−7. (12) Xiong, H. M. ZnO nanoparticles applied to bioimaging and drug delivery. Adv. Mater. 2013, 25 (37), 5329−35. (13) Zhang, N.; Zhang, Y.; Xu, Y. J. Recent progress on graphenebased photocatalysts: current status and future perspectives. Nanoscale 2012, 4 (19), 5792−813. (14) Hwang, S. H.; Song, J.; Jung, Y.; Kweon, O. Y.; Song, H.; Jang, J. Electrospun ZnO/TiO2 composite nanofibers as a bactericidal agent. Chem. Commun. 2011, 47 (32), 9164−6. (15) Sarkar, S.; Guibal, E.; Quignard, F.; SenGupta, A. K. Polymersupported metals and metal oxide nanoparticles: synthesis, characterization, and applications. J. Nanopart. Res. 2012, 14 (2), 715. (16) Sun, B.; Long, Y. Z.; Zhang, H. D.; Li, M. M.; Duvail, J. L.; Jiang, X. Y.; Yin, H. L. Advances in three-dimensional nanofibrous macrostructures via electrospinning. Prog. Polym. Sci. 2014, 39 (5), 862−890. (17) Chen, T.; Amin, I.; Jordan, R. Patterned polymer brushes. Chem. Soc. Rev. 2012, 41 (8), 3280−96. (18) Suresh, V.; Huang, M. S.; Srinivasan, M. P.; Guan, C.; Fan, H. J.; Krishnamoorthy, S. Robust, High-Density Zinc Oxide Nanoarrays by Nanoimprint Lithography-Assisted Area-Selective Atomic Layer Deposition. J. Phys. Chem. C 2012, 116 (44), 23729−23734. (19) Ichinose, I.; Senzu, H.; Kunitake, T. A surface sol-gel process of TiO2 and other metal oxide films with molecular precision. Chem. Mater. 1997, 9 (6), 1296−1298. (20) George, S. M. Atomic layer deposition: an overview. Chem. Rev. 2010, 110 (1), 111−31. (21) Pearson, H. Handbook of Chemical Vapor Deposition; Noyes: Park Ridge, NJ, 1992. (22) Niederberger, M. Nonaqueous sol-gel routes to metal oxide nanoparticles. Acc. Chem. Res. 2007, 40 (9), 793−800. (23) Kim, H.; Lee, J.; Sohn, S.; Jung, D. Low-Temperature Process for Atomic Layer Chemical Vapor Deposition of an Al2O3 Passivation Layer for Organic Photovoltaic Cells. J. Nanosci. Nanotechnol. 2016, 16 (5), 5285−90. (24) Peng, Q.; Sun, X. Y.; Spagnola, J. C.; Saquing, C.; Khan, S. A.; Spontak, R. J.; Parsons, G. N. Bi-directional Kirkendall effect in coaxial microtube nanolaminate assemblies fabricated by atomic layer deposition. ACS Nano 2009, 3 (3), 546−54. (25) Kim, G. M.; Lee, S. M.; Michler, G. H.; Roggendorf, H.; Gosele, U.; Knez, M. Nanostructured pure anatase titania tubes replicated from electrospun polymer fiber templates by atomic layer deposition. Chem. Mater. 2008, 20 (9), 3085−3091. (26) Goswami, T. H.; Maiti, M. M. Thermal stability of cured gelatin−trimethylolphenol blends: effect of plasticizer. Polym. Degrad. Stab. 1998, 62 (3), 447−454.

(27) Danielsen, C. C. Difference in thermal stability of type-I and type-II collagen from rat skin. Biochem. J. 1982, 203 (1), 323−6. (28) Zhang, X.; Reagan, M. R.; Kaplan, D. L. Electrospun silk biomaterial scaffolds for regenerative medicine. Adv. Drug Delivery Rev. 2009, 61 (12), 988−1006. (29) Baimark, Y. Morphology and thermal stability of cross-linked silk fibroin microparticles prepared by the water-in-oil emulsion solvent diffusion method. Asia-Pac. J. Chem. Eng. 2012, 7, S112−S117. (30) Jin, H. J.; Fridrikh, S. V.; Rutledge, G. C.; Kaplan, D. L. Electrospinning Bombyx mori silk with poly(ethylene oxide). Biomacromolecules 2002, 3 (6), 1233−9. (31) Zhang, X.; Baughman, C. B.; Kaplan, D. L. In vitro evaluation of electrospun silk fibroin scaffolds for vascular cell growth. Biomaterials 2008, 29 (14), 2217−27. (32) Lammel, A. S.; Hu, X.; Park, S. H.; Kaplan, D. L.; Scheibel, T. R. Controlling silk fibroin particle features for drug delivery. Biomaterials 2010, 31 (16), 4583−91. (33) Padmavathy, N.; Vijayaraghavan, R. Enhanced bioactivity of ZnO nanoparticlesan antimicrobial study. Sci. Technol. Adv. Mater. 2008, 9 (3), 035004. (34) Arya, S. K.; Saha, S.; Ramirez-Vick, J. E.; Gupta, V.; Bhansali, S.; Singh, S. P. Recent advances in ZnO nanostructures and thin films for biosensor applications: review. Anal. Chim. Acta 2012, 737, 1−21. (35) Rockwood, D. N.; Preda, R. C.; Yucel, T.; Wang, X.; Lovett, M. L.; Kaplan, D. L. Materials fabrication from Bombyx mori silk fibroin. Nat. Protoc. 2011, 6 (10), 1612−31. (36) Guziewicz, E.; Kowalik, I. A.; Godlewski, M.; Kopalko, K.; Osinniy, V.; Wójcik, A.; Yatsunenko, S.; Łusakowska, E.; Paszkowicz, W.; Guziewicz, M. Extremely low temperature growth of ZnO by atomic layer deposition. J. Appl. Phys. 2008, 103 (3), 033515. (37) Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L. Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends. J. Appl. Phys. 2013, 113 (2), 021301. (38) Di Mauro, A.; Cantarella, M.; Nicotra, G.; Privitera, V.; Impellizzeri, G. Low temperature atomic layer deposition of ZnO: Applications in photocatalysis. Appl. Catal., B 2016, 196, 68−76. (39) Najafi, M.; Haratizadeh, H.; Ghezellou, M. The Effect of Annealing, Synthesis Temperature and Structure on Photoluminescence Properties of Eu-Doped ZnO Nanorods. Journal of Nanostructures 2015, 5 (2), 129−135. (40) Lu, J.; Xu, C.; Dai, J.; Li, J.; Wang, Y.; Lin, Y.; Li, P. Improved UV photoresponse of ZnO nanorod arrays by resonant coupling with surface plasmons of Al nanoparticles. Nanoscale 2015, 7 (8), 3396− 403. (41) Masuda, S.; Shimizu, T.; Yamato, M.; Okano, T. Cell sheet engineering for heart tissue repair. Adv. Drug Delivery Rev. 2008, 60 (2), 277−85. (42) He, W.; Kim, H. K.; Wamer, W. G.; Melka, D.; Callahan, J. H.; Yin, J. J. Photogenerated charge carriers and reactive oxygen species in ZnO/Au hybrid nanostructures with enhanced photocatalytic and antibacterial activity. J. Am. Chem. Soc. 2014, 136 (2), 750−7. (43) Gupta, K.; Singh, R. P.; Pandey, A.; Pandey, A. Photocatalytic antibacterial performance of TiO2 and Ag-doped TiO2 against S. aureus. P. aeruginosa and E. coli. Beilstein J. Nanotechnol. 2013, 4, 345−51. (44) Koh, L.-D.; Cheng, Y.; Teng, C.-P.; Khin, Y.-W.; Loh, X.-J.; Tee, S.-Y.; Low, M.; Ye, E.; Yu, H.-D.; Zhang, Y.-W.; Han, M.-Y. Structures, mechanical properties and applications of silk fibroin materials. Prog. Polym. Sci. 2015, 46, 86−110.

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DOI: 10.1021/acsbiomaterials.6b00548 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX