Electrospun Hydrophobic Nanofiber Films from Biodegradable Zein and Curcumin with Improved Tensile Strength for Air Filtration

Zein as a natural protein had been widely used in various fields due to its biodegradability and biocompatibility. However, its sensitivity to humidity and poor mechanical performance limited its application in practice. In this study, zein-based composite nanofibers loaded with curcumin were prepared by electrospinning assisted with polyvinyl alcohol (PVA). The filtration efficiency of the modified nanofibers to the particles with diameters larger than 0.5 μm was all above 98%. The loaded curcumin interacted with protein molecular chains to form a network structure within tightly connected nanofibers, which exhibited excellent moisture resistance and good adhesion to cellulose paper towels used as air filter substrates. Meanwhile, its tensile strength had also been enhanced vastly to 0.72 MPa compared with the initial tensile strength of 0.21 MPa. This study provides a new electrospinning strategy for the preparation of zein-based composite nanofibers that can be widely utilized in air filtration with high moisture resistance.


Introduction
With the rapid development of industrialization, the environmental problems caused by it are threatening human life and health, one of which is atmospheric particulate pollution [1]. Particulate matter (PM) can be divided into PM 2.5 and PM 10 according to particle size, representing particles less than 2.5 and 10 μm in diameter, respectively [2]. The and the receiving device [9]. The electric field formed by the high voltage power supply causes the spinning solution overcome the surface tension, form a jet, and gather on the surface of the receiving device to form nanofibers [10]. It has been reported that the electrospun nanofabrics as air filtration materials could effectively intercept fine particles and bacteria [11]. Fe 3 O 4 /polyacrylonitrile (PAN) magnetic nanofibers (MNFs) fabricated by electrospinning method were used for the remediation of phenol wastewater achieved the removal efficiency of phenol to 85% in the first round use, and the filtration efficiency of phenol still maintained at 52% after 5 cycles [12]. Researchers fabricated PAN/Ag composite nanofibers scaffold for their possible antimicrobial effect. The results showed the capability of PAN/Ag nanofibers scaffold to inhibit both Gram-positive (Bacillus cereus) and Gram-negative (Escherichia coli) bacterial growth [13].
Nowadays, traditional filtering materials used commercially are mainly petroleum-based polymers such as polyethylene and polypropylene, which are difficult to degrade and can cause secondary pollution after disposal, further polluting the environment [14]. Consequently, an increasing number of researchers turn their attention to renewable green raw materials [15][16][17], among which, protein as a natural compound has attracted much attention [18][19][20]. Zein, which is the main storage protein of maize seeds, has long been a subject of research for scientific interest as well as industrial applications. There are a lot of non-polar amino acids stored in zein, which make it a hydrophobic plant protein [21]. Its internal protein molecular chain has a multi-level structure, and denaturation can expose its internal hidden functional groups, interacting with air pollutants to achieve the effect of air filtration [22]. In our previous work, the hydrophobic and multifunctional zein nanofibers as efficient air filters had been prepared by electrospinning with the assistance of polyvinyl alcohol and poly(ethylene oxide), followed by cross-linking with glutaraldehyde. The prepared nanofibers exhibited a high removal rate of 0.1-10 μm particles [15,19,22]. However, the performance of pure zein nanofibers prepared by electrospinning is still insufficient for air filtration, especially for moisture resistance and limited strength.
Curcumin is a natural polyphenol compound with low molecular weight, which exists in the rhizome of turmeric [23]. The structure of curcumin consists of two aromatic rings with methoxyl and hydroxyl groups at ortho, connected by a 7-carbon chain consisting of α, β-unsaturated β-diketone [24]. The molecular configuration of curcumin can exist in the tautomeric forming keto and enol due to intramolecular hydrogen atoms transfer at the β-diketone chain of curcumin. Curcumin has been proved to have health benefits, such as antioxidant [25], anti-inflammatory [26] and anti-cancer [27] properties, and has a great application prospect.
In this work, by introducing curcumin with good compatibility with zein matrix as modifier, the composite nanofibers were prepared by electrospinning. It was found that the introduction of curcumin not only improved the filtration performance of nanofibers, but also improved the moisture resistance and mechanical properties of nanofibers. Thus, the weakness of mechanical properties and moisture sensitivity of zein-based materials was improved.

Preparation of zein-based nanofibers
1 g PVA was dissolved in 9 g acetic acid solution (acetic acid / DI water = 8:2), followed by heating in a water bath at 90 ℃ for 1 h to prepare 10% PVA solution. Meanwhile, curcumin with different mass fractions (0 wt%, 0.4 wt%,1 wt%, 2 wt%, 3 wt%) was dissolved in 8.5 g of acetic aqueous solution (acetic acid / DI water = 8:2). The prepared solution was stirred at constant temperature (room temperature) for 30 min, then 1.5 g of zein was added and stirred for another 90 min. After the PVA solution and zein solution were prepared (zein / PVA = 3:2), those two solutions were thoroughly mixed at constant temperature and magnetically stirred for 30 min, the electrospinning solution preparation was completed. The spinning solution was injected into a syringe with a volume of 10 mL and a needle diameter of 0.8 mm, and all the bubbles were completely drained. Then the syringe containing spinning liquid was fixed on a microsyringe pump, and the solution was spun into nanofibers by the action of electrostatic field. Nanofibers could be collected on a receiving device equipped with a paper towel by applying a high voltage power of 20 kV and a flow rate of 1.0 mL/h at 25 ± 2 ℃ and 30 ± 1% relative humidity. The distance from the receiver to the needle was 12 cm. Finally, the composite nanofiber films were dried at room temperature for 24 h. The final obtained nanofiber films were yellow and became darker with the increase of curcumin content. The preparation process is shown in Fig. 1.

Characterization
Fourier transform infrared (FTIR) spectral analysis was performed using a Nicolet iN10 MX device (Thermo Fisher Scientific, US) in the wavenumber region of 4000 − 400 cm − 1 with a resolution of 4 cm − 1 . The secondary structure of the protein was counted by PeakFit software based on the FTIR data.
The morphology of the fiber samples was observed using a scanning electron microscope (SEM) (Quanta FEG-250, FEI Co. Ltd., US) operating at an acceleration voltage of 10 kV. All the samples were sputter coated with gold before observing. The diameter of the nanofibers was measured using the Nano Measurer software, according to the SEM images of composite nanofibers. Then the nanofibers average diameter was calculated based on the diameter measurement of no less than 150 fibers in terms of the following equation: Where − d is the average diameter of the nanofiber; d i is the diameter of the i th nanofiber; n is the number of nanofibers in SEM image.
The polluted air samples from burning joss sticks were diluted in a plastic bag to a level that could be measured by an analyzer because the initial concentration of pollutants was too high. The particle counter (CEM, DT-9881) was used to determine the concentrations of PM (different particle sizes from 0.3 to 10 μm) in the polluted air samples. A circular filter sample with a diameter of 37 mm was placed in a custom-made sample holder to perform air-filtration testing. The downstream air of the filter was collected in a clean vacuum gasbag and similar measurements were conducted. The air-filtration test was performed at 25 ℃ and 30% humidity, and the duration of each test was ca. 21 s.
The static water contact angle was determined by contact shape analyzer OCA 35 (Data physics Instruments GmbH, Germany). A water (1 µL) droplet was deposited on the sample surface and the droplet shape was recorded. A CCD video camera and image analysis software were used to determine the contact angle. The experimental results were averaged for at least five measurements made on different areas of the sample surface.
In order to test the water resistance of curcumin composite nanofibers, the composite nanofibers with varied content of curcumin (0%, 1% and 3%) were immersed in distilled water for different periods (0 h, 0.5 and 1 h). After being removed from water, the excess water on the surface was sucked up with filter paper and dried for 12 h in a 60 ℃ oven. Then, the surface morphology was observed by scanning electron microscope.
The mechanical properties of the films were analyzed using a testing machine (MTS systems China Co. Ltd.) in tensile mode according to the ISO 527-1:2012 standard at room temperature with an extension speed of 2 mm/min.

Structure and interaction
To illustrate the structure, FTIR analysis was performed, and the spectra are shown in Fig. 2 A. Characteristic peaks at 1659 cm − 1 (zein amide I band), 1533 cm − 1 (zein amide II band) and 1260 cm − 1 (zein amide III band) corresponded to the stretching vibration of C = O in -CONH (I), the N-H Fig. 1 The schematic for preparation of the electrospun nanofiber films protein aggregation process [22]. The amount of β-sheet in the secondary structure of nanofibers decreased firstly after the addition of curcumin from Table 1; Fig. 2 C, which might indicate that no aggregates in the electrospun fibers. The peak at 1649 cm − 1 was attributed to the random coil [32], and its variation trend was similar to that of β-turn structure from Table 1. This might be because the addition of curcumin made the network structure of composite nanofibers denser, resulting in the increase of intermolecular forces, and promoting the formation of β-turn structure. The interaction between curcumin and protein affected the secondary conformation of protein, and the increase of β-turn was also beneficial to the mechanical properties of the material. bending vibration (II), the C-N stretching vibration and the N-H bending vibration (III), respectively. The peak presented at 1629 cm − 1 was assigned as the mixed peak of vibration of C = C and C = O; while peak at 1602 cm − 1 was determined as stretching vibrations of benzene ring; peak at 1429 cm − 1 was olefinic C-H bending vibration, peak at 1281 cm − 1 was aromatic C-O stretching vibrations [28]. The stretching vibration peak of the ketone group (1509 cm − 1 ) and the deformation peak of -NH (1028 cm − 1 ) were shifted to some extent when the curcumin content increased, indicating that curcumin interacted with the C = O and C-N groups in the protein subunit structure. There was a wider peak near 3300 cm − 1 in all the composite nanofibers, which was related to O-H stretching vibration of zein and curcumin. With the increase of curcumin content, all the peak patterns changed around 3300 cm − 1 , implying that there was hydrogen bond between zein and curcumin. It was proved that polyphenols could bind to some proteins through hydrogen bonding [28,29].

Morphology and air filtration performance
FTIR spectrum could provide effective information for studying the secondary structure of protein. It was generally acknowledged that the absorption peaks of different secondary structures such as α-helix, β-sheet, β-turn and random coil appear at different positions for the amide I band [20]. The peak at 1659 cm − 1 was distributed to the α-helix structure [30], and the band at 1620 cm − 1 was assigned to the β-sheet structure [31], which was thought to be related to the  10 μm. In general, nanofibers without curcumin exhibited lowest removal efficiency for all particle sizes. This might be because the interactions between pollutants and nanofibers might not be enough [15]. The filtration efficiency of larger particle size was higher than that of smaller particle size. For particles with diameters larger than 0.5 μm, the filtration efficiency of nanofibers increased with the increase of curcumin content when curcumin content was less than 1%. Nevertheless, when the curcumin content was higher than 1%, the filtration efficiency tended to be stable with the increase of curcumin. For particles with small diameter (such as 0.3 μm), the filtration efficiency of nanofibers generally increased with the increase of curcumin content, but decreased slightly when curcumin content was 2%, mainly because the particle size was small and the pore size formed between nanofibers was larger than the particle size. In addition to the traditional filter system of diffusion, intercept and electrostatic deposition, hydrophobic groups such as glycine and charged groups such as lysine on proteins could facilitate adsorption of very small pollutants by interacting with pollutant particles, which might include chemical bonding, hydrogen bonding, electrostatic interaction, and hydrophobic interaction [22,33]. Curcumin, as a kind of phenolic compound, was a small molecular compound, that could occupy the space between protein molecules and form hydrogen bonds with the amino and hydroxyl groups of zein and polyvinyl alcohol molecules, resulting the structure of nanofibers more compact [34]. It was difficult for larger particulate pollutants in the air to pass through, enhancing filtration efficiency. It was generally accepted that for a given ventilation rate of air filtration, the pressure on the upper surface of the sample was the highest, and the pressure decreased with increasing sample depth (i.e., the distance from the upper surface). The pressure difference between the upper surface and bottom surface of the nanofiber films curcumin appeared as smooth cylindrical shape without microspheres and distributed uniformly with a fine diameter before filtration. With the increase of curcumin, the diameters of the nanofibers increased slightly. It could be noted that when curcumin content was above 1%, the increase of diameter was more obvious than below 1%. When curcumin content was 3%, the diameter of nanofibers increased to 1.17 μm compared with 0.65 μm of 0% curcumin and 0.80 μm of 1% curcumin. This might be due to the hydrogen bond formed between zein and curcumin, which elevated the viscosity of the spinning solution. Generally speaking, the increase of solution viscosity would result in the increase of nanofiber diameters. In order to investigate the effect of curcumin on filtration efficiency of composite nanofibers, filtration tests were carried out on systems with various curcumin content. After the air filtration test, a large number of pollution particles were absorbed on the surface of the nanofibers, and the particles closely bonded on the surface of the fiber to form a layer, which also showed a strong binding force with the fiber. The fibers were noticeably thicker, for example, when the content of curcumin was 0.4%, the average diameter of the nanofibers was 0.78 μm before filtration compared with 1.48 μm after filtration. With the increase of curcumin content, growing numbers of pollutant particles precipitated on the surface of nanofibers. When the content of curcumin increased to 2%, nanofibers contaminated part of the shadow area was larger, more intensive, indicating that there was a better bonding force between nanofibers and particulate matter with the increase of curcumin, which could capture more pollutants and pollution particles and form dense layers on the surface of nanofibers. When curcumin content was 3%, the contact angle slightly decreased to 99.5°. Nanofiber diameters were an important factor affecting the hydrophilicity of materials. The smaller the diameter of the nanofiber would result in the larger specific surface area. Nevertheless, there was a tendency that solid surfaces had adsorption properties to reduce surface energy. Therefore, the larger the specific surface area led to the stronger ability of absorbing water[36].

Surface morphology of nanofibers after swelling
The SEM images of composite nanofibers with different curcumin contents under gradient immersion time are presented in Fig. 5. It could be seen that when the nanofibers with varied curcumin content were not soaked, the surface distribution was uniform and the morphology was well in Fig. 5ABC. After soaking the nanofibers in distilled water, the surface of the nanofibers with 0% curcumin swelled after soaking for 0.5 h, and the fiber structure almost disappeared after soaking for 1 h, mainly because the polyvinyl alcohol and zein molecular chain contained a large amount of hydrophilic polar groups. While with the increase of curcumin, the composite nanofibers were swelling, but not been completely destroyed under different soaking time. It has been shown that curcumin could improve the moisture resistance of nanofibers to some extent, which was mainly due to the strong hydrogen bonding interactions between curcumin and matrix could stabilize the matrix under high humid conditions. determined the pressure drop. There was also a correlation between pressure drop and filtration efficiency. Meanwhile, the zigzag path formed by randomly arranged fibers in the nanofiber film could improve the filtration efficiency of particle pollutants [35]. Figure 4 shows the static contact angle curve of composite nanofiber films with various curcumin contents and the surface morphology of nanofiber films after the contact angle test. The contact angle values of nanofiber films first increased and then decreased with the increase of curcumin. For neat zein/PVA nanofiber film without curcumin, the contact angle of the nanofiber film was 96.3°, and the surface of the nanofiber film was transparent after the contact angle test. The nanofiber morphology was completely destroyed by SEM observation as shown in Fig. 4 C.

Hydrophobic analysis
When the content of curcumin increased to 1%, the contact angle of the nanofiber film increased to 110.1°. It could be seen from Fig. 4D and E that the surface of the nanofiber with curcumin added swelled after the contact angle test, and the nanofiber diameter increased, but the fiber morphology was not completely destroyed. It mainly because with the increase of curcumin, the hydrogen bond interactions between curcumin and matrix could be strengthen the water stability of the nanofibers, thus improving the contact angle of the nanofiber film, and making the surface morphology more difficult to be destroyed. The contact angle of the nanofiber film gradually decreased and tended to balance while the content of curcumin was rising. increased, the uniformity of diameter distribution of nanofibers decreased. In the process of tensile test, the finer fibers broke first, and then the larger fibers broke rapidly as a result of stress concentration, and could withstand less tension.
As shown in Fig. 6B, after tensile fracture, the nanofibers at 1% curcumin content became fine as a whole, but their ends became coarser. Because when a certain amount of curcumin was added, the interaction force between molecules increased, and it was not easy to slip between molecular chains. Under the action of external force, the molecular chain was fully extended and the fiber became fine, and the molecular chain curled and bounced back after breaking under the external force, resulting in the coarser end,

Mechanical behaviors
As could be seen from the tensile strength curve of nanofiber films in Fig. 6 A, with the increase of curcumin content, the tensile strength first increased and then decreased. The maximum value of the tensile strength of 0.72 MPa was obtained at 1% curcumin content compared with 0.21 MPa of films without curcumin. It was mainly because when a small amount of curcumin was added, hydrogen bonding interactions would strengthen the network structure, and the tensile strength was improved when the external force was applied. When the content of curcumin was further increased, the diameter of the prepared nanofibers the contaminated part of curcumin loaded nanofibers had a larger shadow area, forming a dense layer and having a better binding force with particle pollutants. The surface morphology of composite nanofibers was maintained under high humidity, indicating that nanofibers had high moisture stability. When the content of curcumin in nanofibers was 1%, the maximum contact angle reached 110.1° and the stability increased. Therefore, these composite nanofibers may become promising materials for combating air pollution. While meeting the current concept of green environmental protection and sustainable development, they have excellent comprehensive performance and wide application prospects.
indicating that the elasticity and toughness of nanofibers had been significantly improved. However, when the tensile strength of the composite nanofibers was low, the diameter of the nanofibers after tensile fracture did not change significantly compared with that before fracture, and there was no fracture elongation phenomenon at the end of the nanofibers, which belonged to brittle fracture.
The digital images of the nanofiber films uncovered from the supporting material (paper towel) before and after adding curcumin were shown in Fig. 6 C. Firstly, it could be seen from the figure that the neat sample which was nanofiber film without curcumin was white, while the sample with curcumin was light yellow. But most importantly, the neat sample could retain its original shape when removed from the supporting material. However, the sample with curcumin tended to stick together and could not keep their original shape. This might be due to the large surface tension of the nanofiber films containing curcumin, on the one hand, it could effectively adsorb pollutants in the air; on the other hand, the surface tension of the filter material was too large to adhesion, and the specific surface area was easy to reduce in the process of use, while the convenience was lost.

Conclusion
In this work, the composite nanofibers loaded with curcumin based on biodegradable zein and PVA with moisture resistance, mechanical properties and high filtration efficiency were successfully prepared by electrospinning. The loaded curcumin could interact with the protein molecular chain to form hydrogen bonds, making the nanofibers tightly connected, forming a network shape, enhancing the mechanical properties and adsorption capacity of pollutant particles. The filtration efficiency was more than 98% for pollutant particles larger than 0.5 μm, and the filtration efficiency was 95.6% for 0.3 μm. After filtering pollutants, Springer Nature or its licensor holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Separation and Purification Technology 250
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