A Nano-Protein Functionalized Hierarchical Composite Air-filter

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A Nano-Protein Functionalized Hierarchical Composite Air-filter Xin Fan, Yu Wang, Lushi Kong, Xuewei Fu, Min Zheng, Tian Liu, Wei-Hong Zhong, and Siyi Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01827 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Sustainable Chemistry & Engineering

A Nano-Protein Functionalized Hierarchical Composite Air-filter Xin Fan†,‡,§, Yu Wang*§, Lushi Kongǁ, Xuewei Fu§, Min Zheng§, Tian Liu§, Wei-Hong Zhong*§, Siyi Pan*†,‡

† College of Food Science and Technology, Huazhong Agricultural University, No. 1 Shizishan Road, Wuhan, Hubei, 430070, PR China ‡ Key Laboratory of Environment Correlative Dietology (Huazhong Agricultural University), Ministry of Education, No. 1 Shizishan Road, Wuhan, Hubei, 430070, PR China § School of Mechanical and Materials Engineering, Washington State University, 100 Dairy Road, Pullman, WA, 99164, USA ǁ College of Materials Science and Engineering, Beijing University of Chemical Technology, North Third Ring Road, Beijing, 100029, PR China

Corresponding Authors: Yu Wang ([email protected]) Tel: +1 5095927208 Wei-Hong Zhong ([email protected]) Tel: +1 5093395483 Siyi Pan ([email protected]) Tel: +86 13554029828

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ABSTRACT Building nanostructured active materials and rational porous structures in air-filters will be significant; to realize high filtration efficiency and low normalized pressure drop. Nanofabrics by electrospinning can lead to large active surface areas, but it has been challenging to control the porous structures to reduce the normalized pressure drop, in particular for thick fabrics. To address this issue, here, we report a protein-functionalized composite air-filter with hierarchical structures. This composite is made of bacterial nanocellulose coated by protein nanoparticles and microcellulose fibers from wood pulp. The protein-functionalized nanocellulose cannot only help expose the functional groups of protein for trapping pollutants, but also act as a binder to reinforce the composite fabrics. At the same time, the long microcellulose fibers form large pores for reducing normalized pressure drop and improve mechanical properties. Via adjusting the component ratios, we demonstrate a high-performance protein/nanocellulose/microcellulose composite air-filter with high filtration efficiency of above 99.5% for PM1-2.5, but extremely low normalized pressure drop of 0.194 kPa/g that is only about 1% of that for protein nanofabrics by electrospinning. This study brings about a cost-effective strategy based on protein-functionlized hierarchical composite fabrics for fabrication of advanced green and sustainable air-filter.

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Table of Contents/Abstract Graphic

KEYWORDS: Hierarchical structure, Protein-functionlized materials Air filtration, Particulate matter, normalized pressure drop

INTRODUCTION Air pollution has been a serious health hazard due to excessive human activities such as incineration, power generation, industry, automobile exhaust and so forth 1. Particulate Matter (PM) is one type of prevalent air pollutants, which can cause serious health problems to people 2-4. The composition of PM can be extremely complicated and it is usually composed of sulfate (SO22- and SO42-), nitrates (NO3-), mineral dust, black carbon and so on5. It may further form blends with organic and inorganic particles suspended in the air. PM can be categorized into different levels based on the particulate size, such as PM2.5 which refers to the particulate matter with size smaller than 2.5 µm6. Therefore, a high-efficiency air-filter should be able to capture PM with different sizes effectively. Synthetic plastics, such as polypropylene (PP), glass fibers and polyethylene (PE)7, are used as the raw materials for commercial air-filter. These conventional air-filter work primarily based on size-based mechanisms, such as inertial impaction, diffusion, sieving and interception8-9. These mechanisms are facing challenges in removing small pollutants (e.g. nanoparticles) because of the lack of active functional groups. Recently, high-efficiency air-filters based on nanofabrics by electrospinning have attracted intensive attention. For example, different types of polymers, including polyimide (PI), polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), polystyrene (PS), polyvinyl alcohol (PVA) and polypropylene (PP)

1, 10

were fabricated into nanofabrics by electrospinning for air filtration application. 3



These nanofabrics showed much improved filtration performance mainly due to the high surface area of the nanofibers. Recently, some researchers applied protein-based nanofibers by electrospinning in preparing air-filters, such as silk protein11, silk fibroin12, soy protein9, 13 and whey proetin13. These protein-based nanofilters can capture various types of pollutants via the large surface area and abundant functional groups on the nanofiber surface9, which can deliver strong interactions with different pollutants15-16. These natural protein nanofabrics showed very high removal efficiency for both particulate matter and toxic chemicals. Nanofabrics-based air-filters can notably improve the interactions of filter/pollutants, and thus increase removing efficiency. However, there are challenges as well. For example, the pore size of the nanofabrics is usually very small and it is very difficult to control the porous structures. This becomes even more challenging when thick nanofabrics are required for high-capacity air-filters. Therefore, it is critical to develop new methods to simultaneously realize nanostructured functional protein and controllable porous structures for the filters. In this study, we report a composite air-filter with hierarchical structures including protein functionalized bacteria nanocellulose (BC) as the active filler and microcellulose as the structural frame. The long-term goal of this study is to fabricate high-performance nanocomposite air-filter capable of solving the trade-off between efficiency or capability and normalized pressure drop. The design strategy is illustrated in Figure 1. The idea is to functionalize the BC by whey protein concentrate (WPC) in order to achieve both functional groups on the surface and high surface area. The diverse functional groups from about 20 kinds of amino acids can interact with PM via various types of interactions17. The BC synthesized by Komagataeibacter xylinus is a type of nanocellulose with a high surface area and porous network structures18. By taking the advantages of surface charges of both BC and WPC, WPC protein particles can be absorbed by the surface of BC, resulting in protein functionalized BC particles. In the meantime, the nano-structured protein can be used as the functional additive and also reinforcement agent. The cellulose microfibers from wood pulp (WP) are employed as a structure frame to provide good mechanical properties, and rational porous structures with low pressure drop. As a result, a unique protein/cellulose composite with hierarchical structures is fabricated for high-performance air-filtration application. It is 4


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noted that all the raw materials are derived from abundant natural resources. Therefore, it is a green air-filter made of biomaterials. For example, WPC is a by-product of milk, nanocellulose is produced from Komagataeibacter xylinus, and the microcellulose is derived from wood pulp. These three raw materials are extremely abundant and biodegradable 19-20.

Figure 1. Illustration of the fabrication strategy for the protein-functionalized composite air-filter that is reinforced and functionalized by natural proteins. (For details, see the text).

EXPERIMENTAL SECTION Materials. Whey protein concentrate was obtained from Shanghai Yuanye Bio-technology Co., Ltd. Wood pulp was purchased from Shanghai Yingjia Industrial Development Co., Ltd. Bacterial cellulose was prepared in our lab18. Acetic acid (99.9% purity) was obtained from J.T. Baker (Center Valley, PA). Preparation of D-WPC/WP, BC/WP and D-WPC@BC/WP samples. The WPC solution was prepared in a mixture solvent of acetic acid and distilled water with a weight ratio of 5



80:20 at 80 °C for 2h. A homogeneous yellow denatured WPC (D-WPC) solution with solid content of 1% was achieved 21. Then, BC was added in WPC solution and stirred 2h at room temperature to form protein-functionalized BC. Wood pulp was then added into the above D-WPC@BC mixture and the mixture was further stirred to get a homogeneous mixture. The mixture was dried on a thermal platform for 12h at 50 °C and finally D-WPC@BC/WP composite samples were obtained. By the optimization of experimental conditions, the weight concentration of BC, D-WPC and WP were 15 wt%, 20 wt% and 65 wt%, respectively. The preparation of the control sample of D-WPC/WP sample without nano bacterial cellulose followed the same process for D-WPC@BC/WP composite samples as introduced above. The weight concentrations of D-WPC and WP were 25 wt% and 75 wt%. The same procedure was adopted for BC/WP control sample. The weight concentrations of BC and WP were 20 wt% and 80 wt%. Characterization. Circular dichroism (CD, JASCO J-1500) spectropolarimeter was utilized to investigate the secondary structure of the protein at far-UV (190-250 nm) regions22. Zeta potential of BC and D-WPC was determined by a dynamic laser light scattering (Zetasizer Nano, Malvern Instruments)23. Fourier transform infrared (FTIR, Thermofisher iS10) spectrophotometer was used to investigate the functional groups of D-WPC, BC and D-WPC@BC, and the interactions between BC and D-WPC. The morphological characteristics of the BC, WPC, WP and their composites were investigated by scanning electron microscopy (SEM, FEI SEM Quanta 200F). The mechanical properties of the D-WPC/WP and D-WPC@BC/WP samples were tested by a universal testing machine (Instron, 5565A) with a tensile rate of 5 mm/min24. Air-Filtration Testing. The PMs with particle sizes ranging from 0.01 to 5 µm were produced by burning joss sticks. The concentrations of PM were diluted in a glass bottle to the level which can be measured by a particle counter (CEM, DT-9881). The airflow with a velocity of 4 cm/s was applied to test the filtration performance and pressure drop, which was measured by a manometer (UEi, EM201-B). Air-filtration testing was performed with a circular air-filter sample with a diameter of 37 mm, which was placed in a homemade sample holder. The removal efficiency (η) was determined as 6


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η=

(Cp -Cc) (1) Cp

where Cp and Cc are the air pollutant concentration before and after filtration testing25. The quality factor (QF) was determined as -ln(1-η) (2) Δ Where ∆P is the pressure drop of the air-filter24. QF=

RESULTS AND DISCUSSION Denaturation of Whey Protein Concentrate. To analyze the denaturation of WPC in the mixture solvent, circular dichroism (CD) spectra studies were performed. The CD spectra of WPC and D-WPC were shown in Figure 2 (a). The fractions of α-helix, β-sheet, and random coil were estimated by CD spectropolarimeter26 and the results are shown in Figure 2 (b). The secondary structure of whey protein was monitored by CD spectra before and after the denaturation in AA/DI mixture solvent. The spectra show that the content of α-helix, β-sheet and random coil changed from 70.9%, 6.0% and 23.5% to 36.4%, 36.0% and 27.6%, respectively. The content of α-helix descreased notably, but the content of β-sheet structure increased a lot after the denaturation treatment. This is probably due to the fact that protein chains carry plenty of positive charges when pH is below their isoelectric point. As a result, the structure of α-helix in protein molecule become instable and unfolded due to intermolecular electrostatic repulsion, and turn into more stable β-sheet structure27. Different from α-helix and β-sheet, the content of random coil structure keeps almost constant after the denaturation treatment. Interactions between bacteria nanocellulose (BC) and denatured whey protein (D-WPC). Due to the richness in functional groups, proteins can strongly interact with other materials via different types of interactions, such as hydrophobic interaction, specific chemical interactions and electrostatic interaction etc.23. It has been proved that protein can interact with polysaccharide by strong electrostatic interaction28, and BC is a kind of polysaccharide. Therefore, one can expect some strong charge-charge interactions between the D-WPC and BC. This point is further confirmed by the zeta potential testing of the two samples at different pH values as shown in Figure 2 (c). More specifically, the D-WPC carries positive 7



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charges when pH value is below its isoelectric point. In contrast, BC carries negative charges due to the electron-rich oxygen atoms of polar hydroxyl and ether groups29. The zeta potentials of the mixture of D-WPC@BC at different pH values are also given in Figure 2(c), which shows a notable reduction in potential and so interactions between them. Figure 2 (d) shows the FTIR spectrum of the solid samples of BC, D-WPC and D-WPC@BC. For BC, there are obvious peaks for -OH, -CH2CH, -CO bonds at 3348 cm−1, 2896 cm−1, 1151 cm−1 and 1030 cm−1, respectively. This result is consistent with the reported BC characteristic spectrum18,

30

. The D-WPC showed several important absorption characteristic peaks at

3300cm-1 (-OH bands), 2920 cm-1 (-CH2CH), 1650 (-C=O bands), 1530 cm−1 (amide II -NH bands), 1390 cm−1 (-CN bands) and 1240 cm-1 (amide I -NH bending)19,

31

. For the

D-WPC@BC composite, the intensity of the peaks at 1390 cm-1 (C-N bands) and 1240 cm-1 (N-H bands) was obviously weakened as compared with pure D-WPC, indicating that most of the -CN and -NH groups of D-WPC may react with the -OH groups of BC molecules. Due to the strong interactions, BC nanofibers work as a charged frame to absorb the D-WPC protein chain and form a D-WPC treated BC composite (D-WPC@BC), as illustrated in Figure 2 (e). Overall, the above studies show that the D-WPC can strongly interact with BC nanofibers and form a unique D-WPC@BC composite with protein coating as functional treatment.

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Figure 2. Denaturation of whey protein and D-WPC/BC interaction studies. (a) Circular dichroism of whey protein concentrate (WPC) and denatured whey protein concentrate (D-WPC). (b) Secondary structure fractions of WPC and D-WPC. (c) Zeta potential of bacterial cellulose (BC), D-WPC and D-WPC@BC with a weight ratio of 45/55 at different pH values. (d) FTIR spectra of BC, D-WPC and D-WPC@BC solid samples. (e) Illustration of the interaction between D-WPC and BC.

Morphology and mechanical properties of D-WPC@BC/WP composite filter. The morphology of WPC, BC and WP were first studied and the SEM images are shown in Figure 3(a), (b) and (e), respectively. For the pristine WPC powder, it is irregular 9



micro-spheres with size from ca. 5 µm to ca. 100 µm (see Figure 3(a) and Figure S1). While, as shown in Figure 3 (b) and Figure S2, BC is a kind of nanofabric with nanofiber diameter of 50 - 80 nm32. After the denaturation by acid treatment and the combination of D-WPC and BC, D-WPC was successfully coated onto the BC nanofabric and lots of D-WPC nanoparticles with size of ca. 120 nm can be found on the surface as displayed in Figure 3 (c) and Figure S3. The nanoparticle-coating of protein onto the BC surface can help to expose functional groups of protein for trapping air pollutants, which will be discussed later. This SEM image further confirms that BC can absorb lots of D-WPC chains and help to form nanostructured D-WPC due to the strong interactions between BC and D-WPC as illustrated in Figure 3 (d). The above results indicate a very simple way to fabricate nanostructured protein materials via compositing with naturally derived nanofabrics, such as the BC nanofabric. This finding is very significant for protein-based air-filters. Firstly, it is a new method to fabricate nanostructured proteins. It is well-known that nano-proteins are usually fabricated by traditional electrospinning method. Secondly, this method is more scalable and cost-effective as compared with electrospinning. Nanostructured protein is highly desired for air-filters as it can provide more active sites to capture air pollutants33. Although we have successfully achieved nanostructures in the D-WPC@BC composite, this nanocomposite cannot be directly used as air-filter due to the poor porous structures, which will generate a high normalized pressure drop. Therefore, in this study, it is employed as a kind of functional filler that is dispersed into a porous frame built by micro-cellulose of wood pulp (WP), another bio-derived and abundant natural material. Similar to a paper towel, long WP microfibers can form a loose fabric with large pores as shown in Figure 3 (e) and Figure S4 in Supporting information, which will help to reduce normalized pressure drop. At the same time, the WP fabric will provide a good mechanical support for the final composite air-filter, which will be discussed later. Figure 3 (f) shows the digital photo of the final D-WPC@BC/WP composite filter. Indeed, as shown by the SEM images (see Figure 3 (g) and Figure S3), there are lots of D-WPC@BC particles inside the composite filter. At the same time, the surface of even WP fibers was coated by some protein nanoparticles as well (see the insert SEM in Figure 3 (g)). Based on these SEM images, the hierarchical 10


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microstructures of the resultant D-WPC@BC/WP composite filter is illustrated by Figure 3 (h). In this composite, at nano-scale, the D-WPC forms nanoparticles and nanocoating onto BC nanofabric. At micro-scale, long WP microfibers form a loose micro-fabric with big pores (10 – 200 µm) and good mechanical flexibility and strength as shown later.

Figure 3. Morphological studies of D-WPC@BC/WP composite air-filter. (a) - (c), SEM images of whey protein concentrate (WPC), bacterial cellulose (BC) and D-WPC@BC composite. (d) Schematic of the structures for D-WPC@BC. (e) SEM images of wood pulp (WP) fabric. (f) Photograph of D-WPC@BC/WP composite air-filter sample. (g) SEM images of the D-WPC@BC/WP. (h) Schematic of the hierarchical microstructures of the D-WPC@BC/WP composite air-filter.

The mechanical properties of the D-WPC/WP, BC/WP and D-WPC@BC/WP samples were further studied by tensile testing. Mechanical properties are important for practical application since air-filters work with compression pressure especially when the flow rate is very high24, 34. For the D-WPC@BC/WP composite, it was found that the combination of D-WPC and BC can significantly improve not only the mechanical properties, but also the structure uniformity as shown in Figure 4. In specific, the tensile test of D-WPC/WP, BC/WP 11



and D-WPC@BC/WP samples were compared in Figure 4 (a) - (b). It clearly shows that the D-WPC@BC/WP sample has much higher tensile strength and modulus than those of D-WPC/WP and BC/WP samples. The tensile strength of the D-WPC@BC/WP sample is ca. 0.4 MPa, about two times of that for D-WPC/WP and four times of that for BC/WP. Similar improvement was achieved for modulus as well as shown in Figure 4 (b). At the same time, the D-WPC@BC/WP composite shows excellent mechanical flexibility as demonstrated in Figure 4 (c). Basically, one can bend, roll, fold and unfold it without breaking the composite.

Figure 4. Studies on the mechanical properties and structural uniformity of D-WPC/WP and D-WPC@BC/WP. (a) Stress-strain curve comparison for D-WPC/WP, BC/WP and D-WPC@BC/WP samples. (b) Tensile strength and modulus comparisons for D-WPC/WP, BC/WP and D-WPC@BC/WP samples. (c) Digital photos showing the mechanical flexibility of D-WPC@BC/WP composite under different deformation; (d) and (e), Schematic of the structure evolution and the photos of the top and the back surface for D-WPC/WP and D-WPC@BC/WP composite, respectively. 12


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The above mechanical properties suggest that the D-WPC@BC nanocomposite plays a critical role in controlling the microstructures of the composite filter. In fact, there is a huge difference in the structure uniformity as shown by the digital photos in Figure 4 (d) and (e). More specifically, for the D-WPC/WP composite as shown by Figure 4 (d), most of the D-WPC concentrated on the top surface of the composite (i.e., the surface facing the open air during drying process), resulting in yellow surface on the top but white surface on the back. In contrast, the D-WPC@BC/WP composite in Figure 4 (e) showed very uniform and consistent color on both top and back surface. This result indicates that BC nanofabrics can help to anchor D-WPC chains and give rise to uniform distribution of D-WPC inside the composite filter. To help understand the above result, the structure evolution for both D-WPC/WP and D-WPC@BC/WP composites is illustrated in Figure 4 (d) and (e). In both cases, there is a capillary force during the process of evaporation, which tends to drive the migration of D-WPC to the top surface35-36. For the D-WPC/WP sample, since the D-WPC is basically free inside the slurry of WP, it will finally concentrate on the top surface. In contrast, for the D-WPC@BC/WP sample, there are some strong electrostatic interactions between BC and D-WPC as shown in Figure 2. This will notably reduce the migration of D-WPC and lead to a uniform distribution of D-WPC inside the composite sample. Moreover, the D-WPC@BC may act as a binding agent to notably improve the mechanical properties as illustrated in Figure 4 (e). Specifically, as shown by the SEM images (see Figure 3 (e)), D-WPC can also coat the surface of micro-cellulose fiber. Therefore, D-WPC actually plays the role of binder that connects the BC nanofabric and WP microfabric.

Air-filtration performance. In addition to the above contribution to structural uniformity and mechanical properties, the D-WPC functionalized BC nanofabric (D-WPC@BC) also notably improves the filtration performance as shown below. This study is focused on the filtration capability of the composite air-filter for PM with different sizes. Therefore, we are particularly interested in the removing efficiency for each set of PM with size from 0.3 to 5 µm. To do that, the pollutant concentration was measured by a particle counter (CEM, DT-9880, Shenzhen) with particle counting mode and the unit is count/L. The PM removal efficiency for the D-WPC/WP, BC/WP and D-WPC@BC/WP samples were investigated, and 13



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the removal efficiency for PM0.3, PM0.3-0.5, PM0.5-1.0, PM1.0-2.5 and PM2.5-5 is compared in Figure 5 (a) - (b). There are several points worthy of discussion here. Firstly, the removal efficiency of D-WPC@BC/WP is notably higher than both D-WPC/WP and BC/WP for all the sizes of the PM tested in this study. This result indicates that the combination of D-WPC and BC with nanostructured D-WPC plays an important role in absorbing the pollutants. At the same time, BC can help to achieve uniform D-WPC distribution inside the composite filter (see Figure 4). In contrast, the BC/WP sample shows the lowest removal efficiency among the three samples. This result indicates that BC itself cannot help to improve filtration performance and further proves that protein is the key for the filtration performance. Secondly, for all the three types of samples, the removal efficiency increases obviously with the increasing of the number of layers as shown in Figure 5 (b). For instance, the removal efficiency for PM2.5-5 of D-WPC@BC/WP sample was increased from 85.4% to 99.9% by adding the number of layers from one to three. The D-WPC/WP with triple layers shows a PM1-2.5 removal efficiency of 92.2% and a PM2.5-5 removal efficiency of 95.0%. While the BC/WP composite still shows the lowest removal efficiency for all the PM (eg. 76.7% for PM1-2.5 and 83.1% for PM2.5-5). More significantly, D-WPC@BC/WP composite with triple layers can notably improve the removal efficiency for espically small PM, such as PM0.5-1 and PM0.3. More specifically, the removal efficiency for PM0.3 is increased from 45% to 88%, about 200% improvement. It is well-known that the filtration of small particles is more challenging as compared with big particles due to the fast diffusion in filters6, 9, 33,

37-38

.

Therefore, filtration of these small particles is highly dependent on the interactions between the filter material and small particles, and the diffusion time for the small particles in the filter. The studies as shown above further confirm that the nanostructured protein in the D-WPC@BC/WP and its absolute amount inside the composite filter are critical factors affecting the filtration performance for particularly small PM.

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Figure 5. Filtration performance. The removal efficiency of D-WPC/WP, BC/WP and D-WPC@BC/WP with (a) single layer and (b) triple layers; (c) The pressure drop and PM1.0-2.5 removal efficiency of D-WPC@BC/WP and nanofabrics1, 24, 33; (d) The normalized pressure drop of D-WPC/WP, BC/WP, D-WPC@BC/WP and protein nanofabric24; (e) The quality factor of D-WPC/WP, BC/WP and D-WPC@BC/WP samples with triple layers.

Table 1. Comparison of normalized pressure drop of air-filters Flow rate

Mass

Pressure drop

Normalized pressure Ref.

(cm/s)

(g)

(Pa)

drop (kPa/g) our

4

0.4750

92

0.194 work

4

0.0105

180

17.142

[24]

4

0.0039

ca.170

43.590

[33]

4

0.0039

ca.165

42.308

[6]

4

0.0030

ca.100

33.333

[9]

5.33

0.0108

23

2.129

[40]

15



5.31

0.0035

98

28.000

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[11]

As shown above, one can easily improve the filtration efficiency by increasing the number of filter layers, that is, the absolute amount of active materials. However, there is cost by doing that. Usually, low pressure drop or low normalized pressure drop is desired for an air-filter, which can help to save the energy and improve the user experience1. Therefore, maintaining high removal efficiency but low pressure drop is the key for a high-performance air-filter39. For the D-WPC@BC/WP composite filter, the pressure drop increased by about 2 times, that is, from ca. 30 Pa to 92 Pa, when one triples the layers (See Figure S5). This is only slightly higher than 75 Pa for D-WPC/WP sample. This value is much lower than other studies with the similar testing condition by a flow rate of 4 cm/s. For example, the pressure drop values and the efficiency for Zein protein nanofabrics, gelatin/paper towel and polyacrylonitrile (PAN)-85 are 180 Pa24, 138 Pa33 and 133 Pa1, respectively, shown in Figure 5 (c). And It seems that D-WPC@BC/WP sample shows higher pressure drop than D-WPC/WP does.

Pressure drop is determined by mainly two factors: the porous structures

and the total amount of materials. Therefore, given the mass for all the samples (0.4014 g for D-WPC/WP, 0.3748 g for BC/WP, and 0.4750 g for D-WPC@BC/WP), there is no big difference in the normalized pressure drop as shown in Figure 5 (d). To remove the effect from the amount of materials, we proposed the concept of normalized pressure drop. We define a normalized pressure drop (NPD) as the pressure drop per unit mass and show the equation as follow: NPD = ∆P/m

(3)

where ∆P is the pressure drop with a unit of Pa, m is the mass with a unit of g. Therefore, the normalized pressure drop NPD will take the unit of Pa/g. From Figure 5 (d), one can find that the NPD value for D-WPC@BC/WP acutally is very close to the other two samples, and it is ca. 0.194 kPa/g. This indicates that the introduction of D-WPC@BC didn’t change the porous structures of WP micro-fabric. At the same time, the NPD value also didn’t change by increasing the numbers of layers. This is reasonable as the porous structures of the whole composite filter are mainly controlled by the porous structures of the individual layer. 16


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Another big advantange of the D-WPC@BC/WP composite filter as compared with protein nanofabrics by electrospinning is the extremely low normalized pressure drop. As one can see from Figure 5 (d), the NPD value of protein nanofabrics cacluated from our previous studies24 is ca. 17.142 kPa/g. This value is about 100 times of that for D-WPC@BC/WP composite filter. We compared the normalized pressure drop of variety air-filter shown in Table 1. From this table, one can find that the normalized pressure drop for the composite air-filter is the lowest (ca. 0.194 kPa/g), which is one of the primary advantages for our composite air-filter as compared with traditional fabric filters made via electro-spinning. In addition to the advantages in sustainability and bio-degradability, the capacity of the composite air-filters can be easily improved by thick filters, which will be challenging for nano-fabrics based air-filters due to the low fabrication efficiency and high flow-resistance. The advantage in filtration performance can be further proved by another critical parameter, quality factor (QF), which takes both efficiency and pressure drop into consideration10-11, 40. The QF results are shown in Figure 5 (e). In particular, the QF of D-WPC@BC/WP composite filter with triple layers is much higher than those of D-WPC/WP and BC/WP especially for PM with size above 0.5 µm. The higher QF value means the filter can remove pollutants with a high efficiency but a low pressure drop, which is highly desired by a high-performance air-filter.

CONCLUSION In summary, a novel all-biomass-derived protein/cellulose composite air-filter with hierarchical structures for high filtration efficiency but extremely low normalized pressure drop is achieved. Instead of electrospinning that is usually employed to fabricate nanofibers, a very facile method to producing nanostructured proteins by coating denatured protein onto bacterial nanocellulose (BC) through electrostatic interaction is developed. It is found that the nano-protein treated BC (D-WPC@BC) can work as both functional agent to absorb pollutants and reinforcing agent to notably improve the mechanical properties of its composite with micro-cellulosefibers of WP (D-WPC@BC/WP). Via a rational combination of nano-protein and micro-cellulosefibers of WP, we succussfully achieved good mehcanical properties, high fitlration eficiency and extremely low normalized pressure drop for the 17



D-WPC@BC/WP composite air-filter. With similar filtration efficiency to that of protein nanofabrics that were prepared by electrospinning in our previous studies, the specific flow resistance defined as the result of pressure drop divided by mass for the composite air-filter is only about 1% of that of protein nanofabrics. Moreover, all of the materials are environment-friendly and derived from sustainable and abundant natural materails. In short, this study does not only bring a green and sustainable materials solution, but also provides a new fabrication strategy for achieving advanced composite air-filters to realize low normalized pressure drop but high removing efficiency.

ASSOCIATED CONTENT Supporting Information SEM images of WPC, BC, D-WOC@BC and WP; the pressure drop of D-WPC/WP, BC/WP and D-WPC@BC/WP samples.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (31571847), Science and Technology Innovation Program of Hubei (2015ABA035) and supported partially by USDA NIFA 201567021-22911. The authors appreciate the School of Biological Sciences Franceschi Microscopy & Imaging Center (FMIC), Washington State University for providing the ﬁeld emission electron microscope.

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For Table of Contents Use Only

Synopsis: An advanced green and sustainable air-filter was achieved by a cost-effective strategy based on protein-functionlized hierarchical composite fabrics.

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A Nano-Protein Functionalized Hierarchical Composite Air-filter

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