Rechargeable Photoactive Silk-Derived Nanofibrous Membranes for

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Rechargeable Photoactive Silk-Derived Nanofibrous Membranes for Degradation of Reactive Red 195 Shixiong Yi, Yushan Zou, Sheng Sun, Fangyin Dai, Yang Si, and Gang Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04646 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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ACS Sustainable Chemistry & Engineering

Rechargeable Photoactive Silk-Derived Nanofibrous Membranes for Degradation of Reactive Red 195 Shixiong Yi† Yushan Zou,† Sheng Sun,† Fangyin Dai,† Yang Si,*,‡ Gang Sun*,‡ †College of Textile and Garment & State Key Laboratory of Silkworm Genome Biology & College of Biotechnology, Southwest University, Chongqing, 400715, P.R. China, and ‡ Fiber and Polymer Science, University of California, Davis, CA 95616, USA. Full mailing address of all the authors: Shixiong Yi† Yushan Zou,† Sheng Sun,† Fangyin Dai,† † College of Textile and Garment & State Key Laboratory of Silkworm Genome Biology & College of Biotechnology, Southwest University, No. 2 Tiansheng Road, Beibei District, Chongqing 400715, P.R. China Phone: 86-23-68251228 Yang Si,*,‡ Gang Sun*,‡ Fiber and Polymer Science, Division of Textiles and Clothing, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA. Phone: 530-752-0840

Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].

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ABSTRACT Constructing nanosized photoactive membrane materials would facilitate the pretreatment of dyeing wastewater and reducing the environmental pollution. However, preparation of such membrane materials remains tremendously challenging. In this work, we fabricate the silk-derived nanofibrous membranes modified with 3,3',4,4'benzophenone tetracarboxylic dianhydride (BDSNM) that could yield reactive oxygen species (ROS) driven under UV light irradiation. The premise of this study is that BDSNM can store photoactive activity at UV light and release the ROS under dark conditions. The resultant BDSNM exhibited the extra-fine fiber diameter (129 nm), larger surface area (13.8 m2 g−1), superhydrophilicity, fast ROS production, good activity storing capacity and good degradation capacity for reactive red 195 ( ＞ 99.9999%) within 30 min. The effective synthesis of such economic and fascinating BDSNM may pave a way for fabrication of the photoactive membranes for dye degradation. KEYWORDS: Rechargeable, Photoactive, Silk, Membranes, Dye, Degradation


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Introduction Dyeing effluent from the textile manufacturing is a main source for environmental pollution1-4. These non-biodegradable dyes are toxic to the water plants and animals, which are considered to carcinogenic or teratogenic for human health5-8. Thus, wastewater discharged from textile dyeing should be pretreated before releasing into the natural environment9-12. In recent years, Fenton process has attracted more interesting for degradation of dyes. The produced •OH radicals are a powerful oxidant, which could degrade the organic matter into water, carbon dioxide, and inorganic compounds by oxidation reactions13-16. In our previous report, 3,3',4,4'-benzophenone tetracarboxylic dianhydride modified poly (vinyl alcohol-co-ethylene) nanofibrous membranes was prepared for killing the bacteria under UV irradiation17. The rechargeable nanofibrous membranes could produce reactive oxygen species (ROS) and store the photoactivity activity at light, and release •OH radical and H2O2 to provide biocidal functions at the dark, which exhibited good bactericidal and virucidal efficacy. However, the application of the similar nanofibrous membranes on degradation of dyes has seldom been reported. Moreover, these poly (vinyl alcohol-co-ethylene) as synthetic polymers are not biodegradable, the discarding of the functional polymers will lead to serious environmental worry18-21. Biomasses such as protein fiber are obtainable and low-cost resources22-25. For environmental protection, it is extremely desirable to create the green nanofibrous material for degradation of azo dyes at a large scale. The silk as protein fibers have the advantage of slow degradation, good biocompatibility and

mechanical

property, etc26-29. The silk fibroin molecule has reactive groups at side chain30-33. The grafting of functional material on the silk can be carried out through chemical bonding34-35. We expected that the silk fibroin nanofibers can obtain excellent



photoactive properties and be used for degradation of azo dyes. In this work, we construct the photo rechargeable silk fibroin nanofibrous membranes (BDSNM) by modifying the membranes with 3,3',4,4'-benzophenone tetracarboxylic dianhydride that can efficiently yield the ROS. The BDSNM can store the photoactive activity and provide photoactive functions under dark conditions. The resultant BDSNM exhibited the extra-fine diameter (129 nm), larger surface area (13.8 m2 g−1), fast ROS production, good activity storing capacity and good degradation capacity for reactive red 195 ( ＞ 99.9999%) within 30 min. We believe that these membranes prepared could serve as the photoactive membranes for degradation of dyes. EXPERIMENTAL SECTION Materials and Reagents. Silkworm cocoon was provided by Southwest University (Chongqing, China). They were pretreated with 0.5 wt % Na2CO3 aqueous solution to remove sericin. The anionic azo dyes (reactive red 195) was supplied by Chongqing Qiuhong chemicals Co., China. The molecular structure of dye was presented in Scheme 2. 3,3',4,4'benzophenone tetracarboxylic dianhydride (BPTCD), polyphosphoric acid (PPA), tetrahydrofuran (THF), potassium iodide, ammonium molybdate tetrahydrate, dioxane, potassium hydrogen phthalate, p-Nitrosodimethylaniline (p-NDA), formic acid and all other chemicals were purchased from Chongqing Taixin Chemical Company (Chongqing, China). Fabrication of Nanofibrous Membranes. The silk was put in the CaCl2/CH3CH2OH/H2O solvent systems for further dialyzing36. After dialysis, it was firstly lyophilized to produce the silk fibroin sponges. Then, the sponges were dissolved in the formic acid to obtain the silk fibroin spinning solution36-37. Finally, silk fibroin nanofibrous membranes (SNM) were collected as shown in Scheme 1a. The experimental temperature was 25°C. The humidity was 45%.


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Fabrication of photoactive BDSNM. The BPTCD and PPA were dissolved in dioxane to obtain the mixed solution. The BDSNM was obtained by immersing SNM in the solution. After adequate reaction, the resulted BDSNM were washed and dried. The BPTCD modified silk fabrics (BD-Silk fabrics) were prepared under the same modified condition. Measurement of ROS. A schematic illustration of photoreaction system is presented in Figure 1. The generated ROS was measured dependence on the detection of OH• and H2O217. The concentration of OH• was determined with p-NDA used as a scavenger for quenching the •OH radicals. In this test, the samples were placed into p-NDA (50 mM) at a given time under UV irradiation or dark conditions. The amount of residual p-NDA was measured at λmax = 440 nm. Moreover, the amount of H2O2 was measured by using a similar method. The samples were placed into water at a given time under UV irradiation or dark conditions. Afterwards, the mixed solution containing 1 ml of the sample solution, reagent I (66 g L-1 potassium iodide，2 g L-1 sodium hydroxide and 0.2 g L-1 ammonium molybdate tetrahydrate) and reagent II (20 g L-1 potassium hydrogen phthalate) was fully mixed and stirred. Finally, the amount of H2O2 was determined at λmax = 351 nm. Degradation procedure. 0.05 g of BDSNM was put in the 50 mL solutions containing 50 mg L−1 reactive red 195. The temperature was kept at 25 ℃. The solution was exposed to the irradiation of UV 365 nm lamp. The test was carried out using the light intensity of 6.0 mW cm−2. At given time, the dye solution was extracted. Following this, the concentration change of dyes was measure and observed at λmax = 523 nm, and the UV-visible spectra were recorded between 190 and 800 nm using a scanning UV-visible spectrophotometer



(UV-2401, Shimadzu Company, Japan). Pseudo-first-order kinetic model. To further systematically investigate the degradation kinetics of BDSNM, reactive red 195 solutions in different initial concentrations were carried out. The concentration of dyes was measured and compared at a given time intervals. The pseudo-first-order kinetic model was used to fit the dye degradation process. The kinetic equation is displayed as follows38.

ln

C0  kt  const (1) Ct

Where C0 and Ct are the concentrations of reactive red 195 at initial and given time, k and t are the rate and time. Reusability Measurements To measure the reusability of the BDSNM, the used membranes were fully washed and dried. Subsequently, the rechargeable BDSNM were reused by the same process. Characterization. Surface morphologies of SNM, BDSNM and silk fiber were obtained by using a Philips XL30 field-emission SEM system. Fourier transform infrared spectra (FT-IR) of SNM and BDSNM were collected by using a Nicolet 8700 spectrometer. Dynamic water permeation capacity of BDSNM and BD-Silk fabrics were observed and analyzed with a Dino-Lite device (AnMo Electronics Corporation, Taiwan). The surface area measurement of SNM and BDSNM was performed by an ASAP 2020 analyzer. Electron spin resonance (ESR) signals of •OH radical for BDSNM and BD-Silk fabrics were measured on a Bruker ESP 300E spectrometer. RESULTS AND DISCUSSION Morphology and Structure.


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The BDSNM is generally evaluated by the following standards: (1) large surface area structure, (2) the membranes can be used for degradation of dyes, (3) the degradation capacity of membranes should be rechargeable, (4) good store and release capacity for photoactive activity. The first criteria were obtained by electrospinning technique36. In order to meet the other three criteria, our molecular design of membrane material is based on benzophenones. The degradation capacity of silk fibroin membrane came from the produced ROS. Firstly, the SEM images of SNM before and after reacting with BPTCD are shown in Figure 2a-b. The diameter distribution is demonstrated in Figure 2c-d. SEM images indicated that the BDSNM possesses stable structure with 129 nm diameters, which is obviously smaller than that of silk fibers (about 11 μm), shown in Figure 2a (insert). Moreover, the SNM and BDSNM have the surface areas of 14.25 m2 g-1 and 13.80 m2 g-1, which are obviously larger than that of silk fibers. The intrinsic hydrophilicity of the regenerated silk protein fibers could be improved by the unique microporous and nanofibrous membrane architecture and incorporated functional chemicals. Water permeation results of the BDSNM and BDSilk fabrics are obtained in Figure 3a. The BDSNM exhibits good superhydrophilicity with 2 second infiltration, which is obviously faster than that of BD-Silk fabrics (9 seconds). Moreover, the FT-IR spectra of SNM and BDSNM was measured and compared in Figure 3b, respectively. There is a new peak at 1586 cm−1, corresponding to the carboxylate ions. The appearance of vibrational absorbance at 1722 cm−1 represented the formation of ester bonds between BPTCD and SNM39-40. Photoactivity of BDSNM. The principle of photoactive and storable capacity is displayed in Figure 4a-b, based on our reports17. The initial reaction depends on the capacity of the photoexcited



singlet benzophenone group to convert to the triplet excited state (3BDSNM*) by the intersystem crossing (ISC). Once formed, 3BDSNM* could abstract the hydrogen atom to form the hydroquinone radical (BDSNMH•). The BDSNMH• was caught by oxygen molecules under aerobic conditions, forming various possible ROS and others. At the same time, if the BDSNMH• is not completely quenched by oxygen, a competing reaction could happen, leading to structure rearrangement and second hydrogen abstraction. Then, the metastable structures of BDSNMH• could form, which could store the activity and still react with oxygen to produce ROS even under the dark conditions17. Figure 4c-d reveals that the generation of OH• and H2O2 from BDSNM and BDSilk fabrics was observed at light and paused without the light exposure. It is worth noting that the ROS was continuously increased under irradiation. Compared with BDSilk fabrics, the BDSNMs exhibited robust photoactive activity. Moreover, the generated •OH radicals for BDSNM and BD-Silk fabrics were detected by DMPOtrapping ESR technique. Figure 4e-f indicated that the intensity peaks of 1:2:2:1, attributed to the signs of DMPO-•OH adducts, were obtained both under light irradiation and under the dark, and the intensity of peaks enhanced with increasing reaction time41. This result demonstrated that the formation of •OH radicals from BDSNM and BD-Silk fabrics. Significantly, the signals of BDSNM were stronger than that of BD-Silk fabrics under the same conditions because of the smaller diameters and larger surface area of BDSNM. Degradation performance analysis. The effect of experimental conditions, such as amount of BDSNM, temperature and irradiation intensity on degradation of reactive red 195, were intensively examined. As predicted, Figure 5a shows that the degradation degree of reactive dye 195 was


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gradually increased with increasing reaction time using different amount of BDSNM. The more BDSNM caused faster degradation rate. In addition, Figure 5b indicates that the reaction rate was decreased with increasing NaCl concentration for the aggregation of the dye molecules. Generally, the aggregation of dyes in aqueous solution is affected by concentrations of dyes and electrolytes. It was considered that the π-electron repulsive forces between dye molecules is smaller than that of hydrophobic associated forces in the presence of salt. The increased NaCl concentration could weaken the repulsive force between dyes, leading to the increased aggregation degree of dyes. The aggregation of dyes hindered the reaction between dyes and •OH radicals42-43. Figure 5c indicated that the degradation rate was faster with increasing light irradiation intensity. On one hand, the aggregation equilibrium of the dye was broken under the stronger irradiation power. The amount of dye aggregation units was decreased for their weak stability in the water43. On the other hand, the stronger light could improve the photolysis of H2O2 in the aqueous solution because this process was mainly dependent on the irradiation intensity. Thus, the stronger light power can significantly improve the degradation of dyes. The absorbance at 400-800 nm was used to measure the decoloration process of dyes, which usually represents the n→π* transition of the azo and hydrazine parts. The absorbance at 200-400 nm was used to measure the degradation process of aromatic parts of dyes, which usually represented the n→π* transition of benzene and naphthalene ring44. It was shown from Figure 5d that two characteristic peaks at 296 nm and 523 nm decreased gradually with prolong irradiation time for reactive red 195, indicating that both chromophores and aromatic rings of dyes was completely destructed after degradation within 30 min. As displayed in Figure 5e, the color of reactive red 195 became colorless. The BDSNM was proved to be an efficient



photoactive material for degradation of reactive red 195. The degradation tests were performed with different concentrations of reactive red 195. Figure 5f demonstrated that the degradation degree of dyes was gradually increased with increasing time. Higher initial concentration of the dye caused longer reaction time. Figure 5g and Table 1 show that the degradation process of reactive red 195 could be described by using a pseudo-first-order kinetic equation. The k values were decreased with increasing concentration of reactive red 195. The results demonstrated that the dye concentration had an important influence on the degradation process under the same conditions. Rechargeable capability of BDSNM. The photoactive cycle was carried out by a process of excitation, hydrogen abstraction and quenching in presence of oxygen. When the BDSNMH• is not completely quenched in presence of oxygen, it will rearrange to other metastable structures for storage of the activities. The metastable transient photoproduct called as a light-absorbing transient (LAT) structure for nanofibrous membranes. The oxygensensitive LAT can be easily quenched in the presence of oxygen, reversed to benzophenone and regenerating of photoactive activity17. The detailed mechanism of rechargeable reactions was displayed in our previous study17. To evaluate the ROS rechargeable performances, we treated the BDSNM under irradiation for 1 hour and examine the releasing capacity depending on amount of OH• and H2O2. As seen in Figure 6a-b, the BDSNM rapidly released ROS in the first stage, and then the amount of OH• and H2O2 attained the equilibrium. The BDSNM have the good releasing performance by amounts of 981 μg g−1 OH• and 207 μg g−1 H2O2. The degradation capacity of charged BDSNM under dark conditions is shown in Figure 6c. The characteristic absorption peaks of reactive red 195 at 296 nm and 523


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nm was decreased with prolong reaction time, indicating the destruction of the dye molecular structure. The decolored process was finished within 30 min. The charged BDSNM exhibited the good degradation capacity under dark conditions. To examine the economic cost, it is very significant to discuss the reusability. The recharging measurement of BDSNM was carried out as shown in Figure 6d. For each test, the membranes were irradiated and completely quenched. No significant change in recharging capacity of the BDSNM was observed after 5 cyclic tests. The reuse capacity of dye degradation was shown in Figure 6e. The degradation capacity of BDSNM for reactive red 195 remained after 5 cycles. It was observed from Figure 6f that the SEM image of BDSNM used after 5 cycles without obvious breaking of structure. The results suggested that the BDSNMs could enhance its practical application of the degradation and decoloration for ionic dyes from textiles wastewater.

Conclusions In conclusion, we have prepared 3,3',4,4'-benzophenone tetracarboxylic dianhydride modified silk fibroin nanofibrous membrane with rechargeable photoactive capacity that could yield reactive oxygen species (ROS) for degradation of reactive red 195 under UV irradiation. The resultant photoactive BDSNM could store the photoactive activity and provide photoactive functions. The BSDNM exhibited the extra-fine diameter (129 nm), larger surface area (13.8 m2 g−1), fast ROS production, good activity storing capacity, good degradation capacity for reactive red 195 ( ＞ 99.9999%) within 30 min and good reusability in 5 cycles. The successful construction of silk membrane with excellent photoactive activity would provide assistances for the further application of photoactive membranes for decoloration of dyes.

AUTHOR INFORMATION Corresponding Authors



*E-mail: [email protected]. *E-mail: [email protected]. ORCID Yang Si: 0000-0001-7209-6206 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.

Acknowledgments This research was supported by National Natural Science Foundation of China (21506173), China Postdoctoral Science Foundation (2015M582503), Natural Science Foundation of Chongqing (cstc2016jcyjA0210) and Chongqing Postdoctoral Science Foundation (Xm2015122).

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Table 1 Results from linear regression of the plots for the pseudo-first-order kinetic equation. Dye (mg L-1)

Rate equation

k (1/min)

R2

25 50 75 100

ln(C0/Ct) = 0.1513t -0.0758 ln(C0/Ct) = 0.1006t -0.0285 ln(C0/Ct) = 0.0729t -0.0183 ln(C0/Ct) = 0.0544t -0.0193

0.1513 ± 0.01 0.1006 ± 0.03 0.0729 ± 0.02 0.0544 ± 0.02

0.9968 0.9968 0.9983 0.9987


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Scheme 1. Scheme of the design, processing, and degradation functions of as-prepared BDSNM.



Scheme 2. Chemical structure of reactive red 195.


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Figure 1. Schematic diagram of photoreaction system.



Figure 2. SEM images of (a) SNM, insets: silk fiber, (b) BDSNM, (c) Diameter distribution of SNM, (d) Diameter distribution of BDSNM.


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Figure 3. (a) Dynamic photographic measurements of water permeation on the surface of BDSNM and BD-Silk fabrics, (b) FT-IR spectra of SNM and BDSNM.



Figure 4. (a) Jablonski diagrams representing the singlet excitation and following ISC to triplet, (b) Proposed mechanism for the photoactive and photo-storable cycles, (c) Amount of OH• versus time (irradiation in white and dark periods in gray), (d) Amount of H2O2 versus time (irradiation in white and dark periods in gray), (e) The ESR signals of the DMPO-•OH adducts for BDSNM, (f) The ESR signals of the DMPO-•OH adducts for BD-Silk fabrics.


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Figure 5. (a) The degradation process of reactive red 195 with different amount of BDSNM, (b) and (c) Effect of NaCl concentration and irradiation intensity on the degradation time, (d) and (e) UV–vis absorption spectra and color change of reactive red 195 at time intervals, (f) The degradation performance of BDSNM towards reactive red 195 at different initial concentration. (g) The kinetic linear fitting curves of reactive red 195 degradation performance of BDSNM.



Figure 6. (a) and (b) Amount of OH• and H2O2 generated by BDSNM under dark conditions, (c) UV–vis absorption spectra of reactive red 195 at time intervals under dark conditions, (d) Rechargeable capability of BDSNM, (e) Reversibility capacity and of BDSNM for 5 cycles, (f) SEM images of BDSNM after 5 cycles.


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For Table of Contents Use Only Synopsis The silk fibroin membrane can store photoactive activity and yield reactive oxygen species, which exhibited good degradation capacity for dyes.


Rechargeable Photoactive Silk-Derived Nanofibrous Membranes for

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