1. Introduction
Particulate matter (PM) pollution is a major risk that threatens human health in many regions of the world (WHO 2021). Atmospheric PM can be a carrier of bio-aerosols such as viruses, bacteria, and fungi [
1,
2], along with attached volatile organic compounds (VOCs). In particular, PM
1.0 (i.e., particulate matter smaller than 1 micrometer) has gained much interest due to its ability to penetrate deep into the human lungs and interact with lung or bronchial cells. Inhalation of PM can cause heart disease, lung cancer, chronic and acute respiratory disease such as asthma, and infections due to carried viruses and bacteria [
3]. Recent studies have reported that the main pathway of COVID-19 infection is the inhalation of airborne respiratory droplets and aerosols [
4,
5]. Therefore, protection against fine particulate matter and its associated pollutants has become the focus of recent research [
6,
7].
Electrospun nanofiber filters have been widely used in air cleaning applications as they can be woven into a systematic structure having a large specific surface area and high porosity that enable the effective capture of PM with low flow resistance [
6,
8,
9]. Electrospun nanofibers have been fabricated from various polymers, including polyacrylonitrile (PAN), nylon, and polyvinylidene fluoride [
10,
11]. Among them, PAN is the most commonly used polymer due to its high dipole moment and excellent mechanical stability [
12,
13]. To meet the requirements for high-performance PM filtration, electrospun nanofibers can be modified by several structural and chemical methods [
14], with one of the most effective being the combination of two or more materials with complementary functions [
11]. For instance, a core–shell structured nanofiber filter composed of triphenyl phosphate core and nylon-6 shell has been shown to exhibit an excellent PM
2.5 filtration efficiency and flame resistance property [
15]. In addition, nanofibers incorporating metal oxide additives have recently gained increasing research interest [
14]. Metal oxide additives can endow the nanofiber with various beneficial properties, including an increased specific surface area and, hence, an increased probability of PM impacting the fiber. Additionally, metal oxides create a rough fiber surface that can prevent particles from disentangling, thus resulting in enhanced filtration [
6]. Furthermore, the metal oxides can impart additional functions such as photocatalytic activity [
16] and antibacterial activity [
17] to the fibers. For instance, the incorporation of TiO
2 nanoparticles into polysulfone (PSU) formed nanoprotrusions on the fiber surface with micro- and nanoscale roughness that improved the filtration performance of the composite PSU/TiO
2 fibers [
18].
Zinc oxide (ZnO) is a multifunctional semiconductor oxide with excellent properties such as wide band gap (3.37 eV), high UV absorbance, non-toxicity, and excellent chemical stability [
19]. In addition, as the safety of ZnO is acknowledged by the U.S. Food and Drug Administration (21CFR182.8991) [
20], it is commonly used as an antibacterial agent for increasing the shelf-life of various products, and in textiles that come into direct contact with the human body [
21]. Furthermore, ZnO has been reported to have excellent photocatalytic properties and antibacterial activity. For instance, Ghanbari et al. investigated the photocatalytic activity of ZnO nanorods, grown on the surface of alumina microfiber for liquid-phase and gas-phase decompositions of methylene blue and toluene, respectively. The results showed that ZnO nanorods grown on alumina could decompose the organics completely under UV irradiation within 10–20 min of light exposure [
22]. The antibacterial activity of PAN nanofiber impregnated with ZnO nanoparticles has been investigated, and the PAN/ZnO composite was shown to have superior antibacterial activity against both
E. coli and
S. aureus [
17].
A serious issue that is usually encountered during the incorporation of NPs into nanofibers is the ready agglomeration of the NPs, which leads to their inhomogeneous dispersion in organic solvents and, hence, poor distribution in the nanofiber structure [
23]. In this case, the effect of the NPs cannot be fully released due to the shielding effect of the outer particles upon the inner particles of the agglomerate [
24], leading to a negative effect on the pressure drop due to the significant impacts on fiber porosity and interconnectivity. Therefore, it is critical to prevent agglomeration in order to make the best use of the NPs [
25]. In this respect, an effective approach is the surface modification of NPs by reaction with silane coupling agents [
23]. The modification aims to enhance the interfacial interaction between polymers and inorganic NPs. Silanes are silicon-based materials that contain both inorganic and organic functional groups in the same molecule, therefore they can function as a connection bridge between organic and inorganic materials [
26]. For example, the modification of ZnO NPs has been performed using 3-methacryloxypropyltrimethoxysilane (MPTMS) to improve their dispersion in polystyrene. The results indicated a homogeneous, monodisperse distribution of nanosized ZnO NPs within the composite, which resulted in enhanced thermal stability and UV absorption capacity compared to those of the pure polymer [
27].
Following on from these past studies, the present work is aimed at achieving a good distribution with minimum agglomeration of the ZnO NPs in the nanofiber via the pre-modification with MPTMS as the coupling agent. The effects of this modification upon the fiber morphology and upon the PM filtration performance, photocatalytic activity, and antibacterial activity are investigated.
2. Materials and Methods
2.1. Materials
Polyacrylonitrile (PAN, Mw = 150,000) was purchased from Sigma-Aldrich, Co. (St. Louis, MO, USA). Zinc oxide nanoparticles (ZnO NPs, 99% purity) were supplied by Nanjing XFNANO Materials Tech Co. (Beijing, China). The commercial silane coupling agent, 3-methacryloxypropyltrimethoxysilane (MPTMS, 97% purity), and Methylene Blue (MB, 1% wt/vol aqueous solution) were procured from Alfa Aesar Chemical Co. (Seoul, Korea). Dimethylformamide (DMF, 99.8% purity) and potassium chloride (KCl, >99% purity) were obtained from Daejung Chemicals & Metals Co. (Seoul, Korea). Ethyl alcohol (99.9%, anhydrous) was supplied by Samchun Pure Chemical Co., LTD (Seoul, Korea). All reagents were used as received without further purification.
2.2. Modification of ZnO NPs with MPTMS
The modification of ZnO NPs was performed via reaction with the silane coupling agent MPTMS, as shown schematically in
Figure 1. First, 0.5, 1, and 2 g of silane were each added dropwise to 100 mL of ethanol, and the mixtures were stirred for 1 h to ensure full hydrolysis of the silane molecules. The amount of silane coupling agents used to modify ZnO NPs was selected based on the amounts reported in previous studies [
23,
25]. Subsequently, 1 g of pristine ZnO NPs (denoted as Z) was added to each of the three solutions, and stirring was continued for an additional 5 h. After that, the suspensions were subjected to sonication for 90 min to improve ZnO NP dispersion. The solution was then centrifuged, and the particles were separated from the liquid by decantation. The modified particles were then washed three times with ethanol to remove any excess silane, and oven dried at 90 °C for 8 h. All modification steps were performed in closed vials or glass beakers covered with aluminum foil and sealed tightly with plastic wraps to prevent the contamination of the surrounding atmosphere with chemicals. Thus, three types of ZnO NP were produced and are designated hereafter as Z(0.5S), Z(1S), and Z(2S), respectively. These were subsequently used as additives to PAN in the nanofiber preparation process.
2.3. Preparation of Electrospinning Solutions
Based on the results of our previous work [
12], two ZnO concentrations in the nanofibers were selected for the present study, namely: 9 wt% and 12 wt%. As detailed in
Table 1, a total of eight 10 mL test solutions were prepared, in which the solid content (including PAN and ZnO NPs) was held constant at 10 wt%. In each case, the selected amount of PAN was added to 9.54 mL of DMF, and the mixtures were stirred for 4 h. After complete dissolution of the PAN, the ZnO NPs were added and stirring was continued for an additional 20 h. Subsequently, the solutions were well dispersed in a sonic bath for 4 h to ensure homogeneity.
2.4. Nanofiber Fabrication by Electrospinning
An electrospinning technique was used to fabricate the nanofiber filters. The prepared electrospinning solutions were loaded into 10 mL syringes with a needle with an inner diameter of 0.337 mm. The syringe was driven by a syringe pump with a flow rate of 1 mL/h. A high voltage, 20 kV, was applied between the needle and the collector. The collector was a flat aluminum sheet covered with polypropylene substrate with dimensions 10 cm × 10 cm, located 15 cm away from the needle tip. The experimental set-up was enclosed in an isolated chamber measuring 60 cm × 50 cm × 50 cm. The temperature and humidity inside the chamber were controlled at 20–25 °C and 40–50%, respectively. After electrospinning, the nanofiber sheets were left to dry at an ambient condition for 2 days to ensure sufficient evaporation of the solvent.
2.5. Characterization of Nanofiber Filters
The fiber morphology was examined via field emission scanning electron microscopy (FE-SEM, LEOSUPRA 55, Carl Zeis Co., Krefeld, Germany). The SEM images were then analyzed using ImageJ software (ImageJ 1.x, LOCI, University of Wisconsin, Madison, WS, USA). The chemical characterization of the nanofiber filters was performed via energy-dispersive X-ray (EDX) spectroscopy to evaluate the presence and distribution of NPs within the fibers. The surface area and porosity of the fibers were analyzed by the nitrogen adsorption–desorption isotherms obtained at 77 K (BELSORP-mini II, MicrotracBEL, Osaka, Japan).
2.6. Filtration Test
The filtration performance of the nanofiber filters was evaluated using a lab-built set-up. The test aerosol was generated by atomizing a 0.5 mol/L aqueous solution of potassium chloride (KCl), and dried by passing through a silica gel bed before being directed to the filter. A circular filter with a diameter of 5 cm was fixed inside the filter holder. The pressure drop across the filter was measured using a differential pressure manometer (Ulfa Technology Co., Ltd., Seoul, Korea). The number concentration according to particle size was measured before and after passage through the filter using two particle spectrometers (Grimm 11-A, Grimm Co., Ainring, Germany). The face velocity for all filtration tests was 5.3 cm/s. The test particle size of the inlet KCl aerosol was distributed between 0.265 to 1 µm, as shown in
Figure 2. The filtration efficiency (η) was calculated based Equation (1).
where C
up is the particle number concentration in the upstream position and C
down is the number concentration in the downstream. The pressure drop (ΔP) across the filter was measured using an electric differential manometer. The overall performance and quality factor (Q
f) (Equation (2)) of the filter was then determined from Equation (2):
2.7. Photocatalytic Activity Test
The photocatalytic activity of the composite nanofiber filters was investigated by the degradation of methylene blue (MB) which is a highly colored, non-biodegradable, and toxic organic dye. All tests were performed under visible light irradiation (Haloline Eco 64,702,220 V 400 W R7s Osram, Seoul, Korea) using the experimental set-up shown in
Figure 3. First, the fiber photocatalyst (25 mg) was added to MB solution (50 mL, 10 ppm) in a 100 mL beaker. The beaker was then completely covered with aluminum foil and the mixture was stirred in the dark for 30 min to achieve adsorption–desorption equilibrium [
28] between the fiber and MB solution. After that, stirring was continued while the cover was removed and the light source turned on. Samples were then taken every 10 min for a period of 1 h. The samples were analyzed using a UV-Vis spectrophotometer (CARY 300 Bio, Agilent Tech., Santa Clara, CA, USA) at a wavelength of 664 nm.
2.8. Antibacterial Activity Test
The antibacterial activity was evaluated via a disk diffusion method. An agar solution was poured into a sterile culture dish and allowed to solidify. Afterwards, 100 µL of Gram-negative E. coli bacterial suspension was distributed uniformly in the dish. Circular nanofiber films (8 mm in diameter) were cut and placed on the surface of the bacterial suspension using sterile forceps. After overnight incubation at 37 °C, the regions of inhibition were measured using a ruler.