key: cord-0766421-y06t6nd3
authors: Xiong, Si-Wei; Zou, Qian; Wang, Ze-Gang; Qin, Jun; Liu, Yang; Wei, Nan-Jun; Jiang, Meng-ying; Gai, Jing-Gang
title: Temperature-adjustable F-carbon nanofiber/carbon fiber nanocomposite fibrous masks with excellent comfortability and anti-pathogen functionality
date: 2021-12-15
journal: Chem Eng J
DOI: 10.1016/j.cej.2021.134160
sha: d464d09a07c8adbc0ec3530a6d14b22556956750
doc_id: 766421
cord_uid: y06t6nd3

Wearing surgical masks remains the most effective protective measure against COVID-19 before mass vaccination, but insufficient comfortability and low antibacterial/antiviral activities accelerate the replacement frequency of surgical masks, resulting in large amounts of medical waste. To solve this problem, we report new nanofiber membrane masks with outstanding comfortability and anti-pathogen functionality prepared using fluorinated carbon nanofibers/carbon fiber (F-CNFs/CF). This was used to replace commercial polypropylene (PP) nonwovens as the core layer of face masks. The through-plane and in-plane thermal conductivity of commercial PP nonwovens were only 0.12 and 0.20 W/m K, but the F-CNFs/CF nanofiber membranes reached 0.62 and 5.23 W/m K, which represent enhancements of 380% and 2523%, respectively. The surface temperature of the PP surgical masks was 23.9 ℃ when the wearing time was 15 min, while the F-CNFs/CF nanocomposite fibrous masks reached 27.3 ℃, displaying stronger heat dissipation. Moreover, the F-CNFs/CF nanofiber membranes displayed excellent electrical conductivity and produced a high-temperature layer that killed viruses and bacteria in the masks. The surface temperature of the F-CNFs/CF nanocomposite fibrous masks reached 69.2 ℃ after being connected to a portable power source for 60 s. Their antibacterial rates were 97.9% and 98.6% against E. coli and S. aureus, respectively, after being connected to a portable power source for 30 min.

COVID-19 was caused by a novel infectious virus [1, 2] . This pathogen was renamed as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by the professional organization [3] [4] [5] . By the end of May 2021, more than 160 million people were infected with COVID-19, resulting in more than 3.4 million deaths [6] .

Vaccination is a critical measure for controlling COVID-19 [7] , but before mass vaccination is achieved, wearing personal respiratory protection equipment remains the most effective measure for reducing the risk of viral infection [8, 9] .

Surgical masks have a high performance and low cost and are the most frequently used face masks during COVID-19 [10, 11] . People must wear face masks for increasingly longer periods. Front-line workers, such as doctors, nurses, customs quarantine staff, airport staff, etc., are often in close contact with patients that carry the virus; therefore, they must wear face masks all day for several days each week.

However, the comfortability of surgical masks is poor. In hot weather, most people feel uncomfortably hot after wearing surgical masks for a long time [8] . To relieve this uncomfortable feeling, people will often touch or take off their face masks, which increases the risk of infection [12] . The poor thermal conductivity of surgical masks is one of the main causes of uncomfortable feelings for users. Polypropylene (PP) melt nonwovens are the most important component in the surgical masks, which have a large number of micropores and randomly distributed monofilaments [13] [14] [15] . Similar to most polymers, PP is a poor conductor of heat, so the thermal conductivity of surgical masks is also low [16, 17] . In addition, the thermal insulation of surgical masks is also insufficient in cold weather. People living in cold regions wear a scarf to replace face masks, but scarfs do not effectively intercept bacteria or viruses.

Another issue is that large amounts of medical waste are produced by surgical masks [18, 19] . It has been estimated that a monthly use of 129 billion face masks is necessary to protect humans worldwide, and this number continues to increase [20] .

Surgical masks effectively intercept bacteria and viruses, but they do not possess antibacterial or antiviral activity. Bacteria and viruses adhere to surgical masks, which threatens people's health. To maintain a clean respiratory environment, people must frequently replace face masks, resulting in large amounts of medical plastic wastes.

Due to the persistence and high contagiousness of SARS-CoV-2, most countries classify waste face masks as infectious, requiring their incineration at high temperatures, followed by landfilling of the residual ash [21, 22] ; however, these treatment modalities intensify the greenhouse effect and release potentially dangerous compounds [23] . Zhong et al. [24] adopted a laser method to create few-layered graphene onto a surgical mask. The introduction of graphene formed a hightemperature layer that killed the virus by photothermal conversion. Li et al. [25] first fabricated polyvinyl alcohol, poly (ethylene oxide), and cellulose nanofiber masks, followed by the deposition of a mixture of nitrogen-doped TiO2 and TiO2. The masks reached 100% bacteria disinfection under irradiation in natural sunlight for 10 min.

Sio et al. [26] outlined how to realize non-disposable and highly comfortable respirators capable of light-triggered self-disinfection by combining bioactive nanofiber properties with stimuli-responsive nanomaterials. Karmacharya et al. [27] discussed the manufacturing, practical applications, and anti-COVID-19 performance of novel masks. Melayil et al. [28] investigated interactions of droplets over the most used surgical masks and studied the wetting signature, adhesion, and impact dynamics of water droplets and microbe-laden droplets on both sides of the mask. However, these reported masks are difficult to solve the above problems simultaneously.

In this work, we report a multifunctional face mask with excellent comfortability and anti-pathogen functionality, composed of carbon fibers (CF) and carbon nanofibers (CNFs). As shown in Figure 1 , the preparation process included three steps: (1) CFs with a clean surface were prepared by acid washing and alkali washing;

(2) The CFs, CNFs, and chitosan hybrid suspension were prepared by solution blending; (3) the hybrid suspension was filtered and dried to afford F-CNFs/CF nanofiber membranes. Then, 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS) chains were grafted onto the nanofiber membranes to prepare hydrophobic F-CNFs/CF nanofiber membranes. The F-CNFs/CF nanofiber membranes exhibited a better heat-dissipation efficiency than commercial PP nonwovens, providing users with a more comfortable breathing environment. The F-CNFs/CF nanofiber membranes also formed a high-temperature layer that killed viruses and bacteria in the masks when connected to a portable power source. 

First, the continuous CF was cut off in short fibers of 2 mm length by using a fixed length cutter. Second, the short CF was slowly added to a 10 vol% NaOH aqueous solution. After immersing in NaOH aqueous solution for 1 h, the short CF was rinsed with deionized water several times to completely remove the residual alkali liquor on the surface of the CF. Finally, the short CF was added to the 20 vol% nitric acid solution and soaked at 37°C for 1 h, and then the short CF was also rinsed with deionized water several times to obtain the short CF with a surface clean [29] .

CNFs was added to the 40 vol% nitric acid solution, and then the mixture solution was heated to 60 ℃. After immersing for 2 h, the CNFs were separated from the acid solution and rinse with deionized water several times [30] . Subsequently, the CNFs with a clean surface were added into 100 mL of chitosan solution; the concentration of chitosan solution was 0.5 wt%. Then, the solution was stirred continuously for 12 h to obtain the CNFs/chitosan hybrid solution (Figure 1a ). Finally, a certain amount of short CF was added to the CNFs/chitosan hybrid solution. After ultrasonic dispersion for 12 h, the CNFs/CF hybrid solution was filtrated, washed with deionized water, and dried to prepare the CNFs/CF nanofiber membranes (Figure 1b) . Chitosan was used as adhesive to enhance the bonding strength between fibers in the CNFs/CF nanofiber membranes. The dosage of short CF in CNFs/CF nanofiber membranes was a constant value of 1 g; the dosages of CNFs were 0.05, 0.1, 0.15, and 0.2 g, respectively. The thickness of CF, F-CNFs/CF-1, F-CNFs/CF-2, F-CNFs/CF-3, and F-CNFs/CF-4 nanofiber membranes were shown in Figure S1 .

Surgical masks have excellent hydrophobicity, which can prevent the droplets or aerogels with viruses from the outside to enter our respiratory tract and lungs, reducing the risk of infection. However, the CNFs/CF nanofiber membranes exhibit high hydrophilicity, they cannot be used to intercept and filtrate droplets and aerogels with the virus. To address this problem, we grafted the FAS on the CNFs/CF nanofiber membranes for improving their hydrophobicity. The preparation process of hydrophobic F-CNFs/CF nanofiber membranes was as follows: First, an amount of 88 g ethanol and 1.5 g hydrochloric acid were mixed in 10 mL deionized water, followed by the addition of 0.5 g FAS solution to prepare the FAS modification solution [31] . Subsequently, the prepared CNFs/CF nanofiber membrane was soaked in FAS modification solution for 5 min. Finally, the wet samples were dried at 80 ℃ to obtain F-CNFs/CF nanofiber membranes. Antibacterial rate (%) = 1 − 2

where OD refers to the optical density of the bacteria solution at 600 nm, which was measured by an ST-360 micro-plate reader (Kehua Experimental System Co.

Ltd., Shanghai, China); OD0, OD1, and OD2 represent the optical densities of the blank, control group, and experimental group, respectively. For each sample, three individual measurements were performed, and the average was reported as the final antibacterial rate. Surgical masks with excellent hydrophobicity can prevent the virus in droplets from entering the respiratory tract and lungs, thus reducing the infection risk. Both CF and CNFs used in this work were hydrophilic; thus, FAS chains were grafted onto the CNFs/CF nanofiber membranes to prepare hydrophobic F-CNFs/CF nanofiber membranes. Figure 2a presents the FITR spectra of the CNFs before and after acid washing. Four significant peaks appeared for both pure CNFs and acid-washed CNFs.

The wide peak around 3626-3116 cm -1 was assigned to the -OH stretching from an acid group [33] . The two peaks at 1638 and 1577 cm -1 were attributed to conjugated aromatic C=C and quinoid C=O bonds, respectively [34] . The peak located at 1102 cm -1 corresponds to C-O ether stretching [34] . After washing by acid solution, no new absorption peaks were detected, but the -OH absorption peak was stronger than in the pure CNFs spectrum. Figure 2b provides the FITR spectra of pure CF and CF membrane. In the FITR spectra of pure CF, the peak at 3440 cm -1 is related to the tensile vibration of -OH groups [35] . The peaks at 2918 and 2850 cm -1 were attributed to the -CH2 asymmetric and symmetric stretching vibrations, respectively [36, 37] .

The peak at 1645 cm -1 corresponds to the C=C tensile vibration [38] . The peak at 1078 cm -1 was assigned to the C-O in the carboxyl group [36] . Compared with pure CF, two new absorption peaks at 1544 and 1413 cm -1 in the CF membrane correspond to amide II and the N-H absorption peak of glucosamine [39] . These peaks were derived from chitosan. Remarkably, no new absorption bands were detected in the CNFs/CF nanofiber membranes, and they appear to be the overlapping of the pure CF and CF membrane spectra. Figure 2c presents the FITR spectra of F-CNFs/CF nanofiber membranes. After processing in FAS solution, two strong peaks appeared at 1203 and 1144 cm -1 in the F-CNFs/CF spectrum, which are attributed to the C-F3

vibrations and Si-O-Si asymmetric stretching vibrations, respectively [40] . XPS was used to analyze the surface states and components of the as-prepared samples. In the full-scan XPS spectrum (Figure 2d ), only C and O elements were found in the curves of pure CF and F-CNFs/CF-4 nanofiber membranes; however, after processing by FAS solution, an intense peak appeared at 687.8 eV in the F-CNFs/CF-4 nanofiber membranes, which corresponded to F 1s. Figure 2e is an enlarged view of the red box region in Figure 2d , which confirmed the existence of Si atoms. Unexpectedly, Si atoms were detected in all three samples; thus, Si cannot be used to prove that FAS chains existed in the F-CNFs/CF-4 nanofiber membranes.

Fitting of the C 1s core-level spectra of the three samples was conducted by using Casa XPS analysis software. In the C 1s core-level spectra of pure CF (Figure 1f ), the C 1s peak was fitted to four peaks at 284.6, 285.5, 286.3, and 287.8 eV, which were attributed to C-C/C=C, C-OH, C=O, and O-C=O groups, respectively. These groups are consistent with pure CF in the C 1s core-level spectra of the F-CNFs/CF-4 nanofiber membranes ( Figure 2g ). As shown in Figure 2h , after processing in FAS solution, two new characteristic peaks at 291.4 and 293.6 eV appeared in the curve of the F-CNFs/CF-4 nanofiber membranes, which correspond to C-F and F-C-F [41] . Subsequently, water droplets were dropped onto the surface of F-CNFs/CF nanofiber membranes to assess their hydrophobicity, using the unmodified CNFs/CF nanofiber membranes as a control group. they could not cover all pores ( Figure S4a ). As the CNFs dosage increased, the larger pores were completely covered, and many ultrafine microporous structures formed ( Figure S4b-S4d) . Thereby, the filtration efficiency of the F-CNFs/CF nanofiber membranes gradually improved upon increasing the CNFs dosage.

Air permeability is one of the key indexes used to evaluate face masks. If the air permeability is poor, the mask will provide large respiratory resistance and have no practical application value. As shown in Figure 3e , the air permeability of commercial PP nonwovens was 172.1 mm/s, while that of the homemade CF membrane reached 374.8 mm/s. Compared with commercial PP nonwovens, the CF membrane had a larger pore size and wider pore size distribution; therefore, the resistance was lower when airflow passed through the CF membrane, resulting in higher air permeability.

After the CNFs were introduced, the air permeability of the F-CNFs/CF nanofiber membranes decreased slowly upon increasing the filling content. When the filling content of CNFs was 0.2 g, the air permeability of the F-CNFs/CF-4 nanofiber membranes was 150.3 mm/s, which was slightly lower than the commercial PP nonwovens. Because the average pore size of the F-CNFs/CF-4 nanofiber membranes was only 0.94 µm, they created a higher resistance to airflow, resulting in a decrease in the air permeability; however, the F-CNFs/CF-4 nanofiber membranes kept their abundant ultrafine micropores at high loadings; thus, their air permeability was maintained at an ideal level.

To evaluate the reusability of F-CNFs/CF nanofiber membranes, we measured their PM2.5 filtration efficiency after five cycles. Before every cycle, the F-CNFs/CF nanofiber membrane was immersed in deionized water, ultrasonically cleaned for 30 minutes, and then dried at a low temperature. As shown in Figure 3f To investigate the heat-dissipation difference of commercial PP nonwovens, CF membrane, and F-CNFs/CF-4 nanofiber membranes in practical applications, the surface temperatures of the three samples were measured after different heating times.

As shown in Figure S6a , three samples of the same size were simultaneously placed on an 80 ℃ heating table. Figure S6b shows infrared thermal images of the three samples under different testing times. Compared with the other two samples, the color map of the surface temperature of the commercial PP nonwovens was significantly different. Their surface temperature was in the low-temperature zone. We utilized IRSoft software to analyze the surface temperature distribution of the three samples and calculate their average surface temperature. As shown in Figure S6c , the surface temperatures of commercial PP nonwovens were much lower than those of the homemade CF membrane and F-CNFs/CF nanofiber membranes. When the test time was 60 s, the surface temperatures of commercial PP nonwovens, CF membrane, and F-CNFs/CF-4 nanofiber membranes were 46.7, 61.9, and 65.4 ℃, showing that F-CNFs/CF exhibited the best heat-dissipation efficiency. nanocomposite fibrous mask, which is similar to surgical masks and is composed of three parts: an inner layer, a core layer, and an external layer. The inner layer is also called the hygroscopic layer and is made of medical gauze or hydrophilic nonwovens.

The external layer of the mask is a hydrophobic layer, which is usually made of a thinner PP nonwoven. The core layer is the most critical component used to cut off and filter dust, bacteria, and viruses. In this paper, the core layer of the mask was F-CNFs/CF-4 nanofiber membranes. We recorded the temperatures and thermal images of the F-CNFs/CF nanocomposite fibrous masks at different times using an IR thermal imager, and the surgical masks were measured as the control group. As shown in Figure 5b , the inset image on the right bottom of each picture is the statistical result of the zone temperature. At different test times, the surface temperature and color maps of F-CNFs/CF nanocomposite fibrous masks displayed higher temperatures than the commercial PP surgical masks. As shown in Figure Spraying 75% ethanol or disinfectant is the primary antiviral method for surgical masks, but this also destroys the fibrous structure, leading to decreased filtration efficiency [43, 44] . Another method utilizes high temperatures to kill SARS-CoV-2, which hardly affects the fiber structure [45] . Abraham et al. [46] reported that SARS-CoV-2 can be killed by heating within 20 min at 60 ℃, 5 min at 65 ℃, and only 3 min at 75 ℃. The F-CNFs/CF nanofiber membranes are made of carbon nanofibers and carbon fibers with excellent electrical conductivity, and they could form a hightemperature layer to kill the virus by connecting them to a power source. As shown in Figure 6a , the electrical resistance of the F-CNFs/CF-4 nanofiber membranes was 1.7 Ω, as measured by a multimeter. To achieve more precise results, the resistance of F-CNFs/CF nanofiber membranes was measured by voltammetry by manually applying different voltages to obtain the corresponding current of the tested samples. We then performed a linear fit to the obtained data to determine the resistance of the tested samples. As shown in Figure 6b , the resistance of the homemade CF membrane was As shown in Figure 6d , the F-CNFs/CF-4 nanofiber membrane with the highest conductivity was used as the core layer of the F-CNFs/CF nanocomposite fibrous masks, and a portable mobile power source was employed to provide electrical energy to the face masks. The output voltage of the mobile power source was 5 V, and the output current was 2 A. Figure 6e shows the infrared thermal images of the F- The electrothermal conversion in F-CNFs/CF nanocomposite fibrous masks can also provide heat for people living in cold areas. As shown in Figure 8a , the F-CNFs/CF nanocomposite fibrous masks were placed in a low-temperature test box at -11 ℃ after being connected to a portable power bank. Then, we employed an infrared thermal imaging instrument to record the surface temperature of the F-CNFs/CF nanocomposite fibrous masks at different times, and the results are shown in Figures   8b and 8c . As shown in Figure 8c , the red area in the infrared thermal images of the F-CNFs/CF nanocomposite fibrous masks gradually shrank over time, indicating that the surface temperature of the masks was gradually reduced. The statistical analysis demonstrates that the surface temperature of the F-CNFs/CF nanocomposite fibrous masks was 37.9 ℃ after 10 min, which is higher than the temperature of the human body. The surface temperature of the masks decreased to 35.6 ℃ at 30 min, and the surface temperature remained at 34.8 ℃ for 60 min.

Since different areas have different ambient temperatures in the cold season, the heating temperature of the F-CNFs/CF nanocomposite fibrous masks need to be adjustable depending on the practical requirements. As shown in Figure 8d , a rheostat was connected in series between the F-CNFs/CF nanocomposite fibrous masks and the portable power bank to adjust the supplied heat. Figure 8e presents the surface temperatures of the F-CNFs/CF nanocomposite fibrous masks connecting resistors with different resistance values. The surface temperature of the F-CNFs/CF nanocomposite fibrous mask was 69.2 ℃ without a resistor connected in series. As the resistance gradually increased from 1 Ω to 5 Ω, the surface temperature of the face mask decreased correspondingly. When the resistance of the resistor was 5 Ω, the surface temperature of the face mask was 21.5 ℃. According to actual needs, the user can control the heating temperature of the F-CNFs/CF nanocomposite fibrous mask within a specific range by connecting a resistor with different resistance values. Figure 8f provides the infrared thermal images of the F-CNFs/CF nanocomposite fibrous mask connected to different resistors. The surface temperature data in Figure   8e was calculated by the infrared thermal imaging of the F-CNFs/CF nanocomposite fibrous masks. The recent reports on the preparation strategy and performance comparison of functional masks were shown in Table S2 .

In this work, we designed a new face mask with excellent comfort and antipathogen functions to cope with the COVID-19 pandemic. Its core layer was composed of F-CNFs/CF nanofiber membranes. Compared to traditional surgical masks, the F-CNFs/CF nanocomposite fibrous mask exhibited better heat dissipation efficiency. The F-CNFs/CF nanofibers membranes with a high conductivity converted electrical energy into heat to form a high-temperature layer. When connected to mobile power for 1 min, the surface temperature of the F-CNFs/CF nanocomposite fibrous masks reached 69.2 ℃. This formed a high-temperature layer that denatured the secondary structure of proteins in SARS-CoV-2, ultimately killing the virus.

Additionally, the high-temperature layer also significantly inhibited bacteria, and the antibacterial rates reached 97.9% and 98.6% against E. coli and S. aureus when connected to a mobile power source for 30 min.

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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: