Advances in particulate matter filtration: Materials, performance, and application

Abstract

Air-borne pollutants in particulate matter (PM) form, produced either physically during industrial processes or certain biological routes, have posed a great threat to human health. Particularly during the current COVID-19 pandemic, effective filtration of the virus is an urgent matter worldwide. In this review, we first introduce some fundamentals about PM, including its source and classification, filtration mechanisms, and evaluation parameters. Advanced filtration materials and their functions are then summarized, among which polymers and MOFs are discussed in detail together with their antibacterial performance. The discussion on the application is divided into end-of-pipe treatment and source control. Finally, we conclude this review with our prospective view on future research in this area.

Graphical abstract

1. Introduction

With the rapid development of the economy and industrialization, particulate matter (PM) has become a major source of air pollutants and posed a serious threat to the future environment and human health [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. It can form a grey yellow and sunless haze obscuring the sky, causing regional climate changes by influencing solar and infrared radiation in the atmosphere, contaminating rivers and lakes, and damaging forests' ecology environment, and agricultural systems [[10], [11], [12], [13]]. According to the World Health Organization (WHO) global air quality guidelines, about 4–9 million people die from ambient air pollution annually [14]. Besides, exposure to air pollution may increase the incidence and mortality from a greater number of diseases than those currently thought, such as Alzheimer's and other neurological diseases [14,15]. Evidence is mounting of causal relationships between PM_2.5 exposure and all-cause mortality, as well as chronic obstructive pulmonary disease, acute lower respiratory infections, ischemic heart disease, stroke, and lung cancer [14]. Particularly in recent years, the coronavirus disease 2019 (COVID-19) pandemic has witnessed severe respiratory illness in most parts of the world [16]. Hereafter, due to people detecting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral RNA provoking the COVID-19 from the surface of PM aerosol in public places, it is now understood that the virus spread in the form of aerosol [[17], [18], [19], [20]].

There are two different strategies to eliminate or reduce the risks caused by PM. One is the so-called end-of-pipe treatment, which handles the risk close to the end-users through masks for personal protection or filtration in public premises (e.g., indoor air purifier and central air conditioning and ventilation system) [3,21]. The other approach is source control namely energy conservation and emission reduction, which manages the risk at the PM generation site [22]. Source control can be further divided into two aspects: one is to optimize the industrial technology and improve resource quality and cleanliness. It is a long-term process that requires continuous effort from scientists, engineers, as well as law enforcement [12]. The other one is to install dust removal facilities to reduce emissions at the source of particulate matter generation, including bag filter, electrostatic precipitator (ESP), gasoline particulate filter (GPF), and so on [[23], [24], [25]].

Here, we summarize recent advances in PM filtration technology from the materials to the performance and application outlook. First, the review introduces some basic concepts involved in PM filtration, such as classification and source of PM, classic filtration mechanism, and the key evaluation parameters of PM filtration. Second, we analyze different advanced filtration materials including polymers, porous metal–organic frameworks (MOFs), carbon-based materials, and some others. Third, some interesting additional functions that can enhance filtration effect of these materials are introduced. For example, antibacterial property and special wettability are discussed in relation to PM filtration. Fourth, potential applications are discussed following the above-mentioned end-of-pipe treatment and source control strategies. Finally, a conclusion will be made, and future prospective views proposed based on the summarized current research achievements and challenges of PM filtration.

2. Basic theory

2.1. PM

Particulate matter is a complex mixture and the main component of air pollution [26,27]. Generally, it contains organic and elemental carbon, inorganic matter such as NO₃ ⁻, SO₄ ²⁻, Cl⁻, SiO₂, and heavy metals, and is found as in form of tiny liquid droplets and small suspended solid particles [28]. Based on the differences of chemical components, PM can be categorized as non-oily or oily particles, bioaerosol (contains bacteria or viruses) and non-biological aerosol [29,30]. NaCl particle and dioctyl phthalate (DOP) aerosol are most commonly used for the non-oily and oily model pollutant, respectively [21,30]. Based on surface wettability and physical states, PM can be classified as wetting liquid droplets, nonwetting liquid droplets, and solid particles [31]. Based on the aerodynamic diameter, PM can be classified as coarse particles (>2.5 μm in size) such as PM₁₀, fine particles (≤2.5 μm in size) such as PM_2.5 and PM_0.3, and ultrafine particles (≤0.1 μm in size) such as PM_0.1 [32]. In particular, it is worth noting the size classification of PM. For instance, since notorious PM_2.5 has a small diameter of no greater than 2.5 μm, they can penetrate into the alveoli and blood circulatory system and cause an increased health risk, especially when they carry bacteria or viruses with them [[33], [34], [35], [36], [37]]. Currently, many countries have set up PM_2.5 and PM₁₀ monitoring systems, and relevant standards and protection products are available based on aerodynamic features of PM [[38], [39], [40], [41], [42]]. Accordingly, it will be more convenient to follow the classification based on aerodynamic diameters in this review article.

PM sources can be categorized as natural and human activities (Fig. 1 ). The natural sources include re-suspended road or soil dust, sandstorm, volcanic eruption, sea spray, naturally ignited grassland, and forest fire [26,27,34]. However, the sources of PM are predominantly through anthropogenic activities, such as incomplete fossil fuel-related energy combustion, industrial emissions, mining and quarrying activities, agriculture production, construction dust, household heating, and vehicle exhaust [4,12,20,43]. On the basis of the source and formation mechanism of PM, source control and end-of-pipe treatment have been proposed. End-of-pipe treatment can only control air quality in a confined space. On the contrary, source control such as improving resource quality and using industrial dust removal facilities represent more fundamental measures to eradicate pollution [22].

Fig. 1.

A schematic illustrating the sources of PM.

2.2. Mechanisms of PM filtration

2.2.1. Passive trap

According to the classical single-fiber filtration theories, the passive trap of PM, also known as mechanical capture mechanism, includes interception, inertial impaction, diffusion, and gravity (Fig. 2 a). This approach is the primary mechanism and is used in most of the current commercial filters [3,44,45].

Fig. 2.

(a) Schematic the major mechanisms of PM filtration: interception, inertial impaction, diffusion, gravity, and electrostatic attraction. Reprinted with permission from Ref. [45]. Copyright (2007) Elsevier. (b–d) Photograph of the in-situ capture of (b) wetting liquid droplets, (c) nonwetting liquid droplets, and (d) solid particles. Insets: Schematic showing the formation of three PM aerosols on PI nanofibers, respectively. Reprinted with permission from Ref. [31]. Copyright (2018) American Chemical Society.

The passive trap mechanism is relevant to particulate diameter, interception and inertial impaction play a key role when diameter greater than 0.3 μm, while diffusion is the dominant mechanism when the diameter is smaller than 0.1 μm [46]. Interception, or sieving, takes place when the particle diameter is more than filter pore size or the particulate does not have enough inertia to separate from the air streamline [45]. When the large particles have sufficient inertia and momentum to break away from fluid streamlines and directly impact the filter surface fibers, inertial impaction will occur [45]. Diffusion of small particles (<0.1 μm) is known as Brownian motion, which will lead to small particles deviating from air streamline to adhere on fibers [[44], [45], [46], [47]]. Lastly, PM could be passively trapped in the filter by gravity when airflow is perpendicular to the ground [47].

In addition, filtration performance is also closely related to the material structure, packing density (i.e., porosity), thickness, face velocity, etc. Packing density is an important geometric parameter of purification material. A higher packing density results in lower porosity and smaller pore size. Thus, the smaller pores form the stronger interception effect and a better filtration performance [48]. Increasing the thickness of the filtration medium also raises filtration performance via decreasing aerosol penetration [48]. Though improving packing density and thickness both enhance the performance, Leung et al. found that packing density has more a prominent effect on most penetrating particle size than the thickness [49]. As for face velocity, Leung et al. compared filtration performance over particles size range from 50 to 480 nm with respect to face velocity from 5 to 10 cm s⁻¹. The experimental results show that the reduction in filtration efficiency becomes greater at smaller particle sizes. For particulate larger than 300 nm in size, the filtration efficiency remains virtually unchanged when the face velocity was increased [49]. This is because increasing the face velocity equates to reducing the residence time of particles in the filter medium, thereby reducing the chance of small particles colliding with fibers through diffusion [49]. Since diffusion is the primary capture mechanism for particulates smaller than 100 nm, the filtration efficiency of these particles is mostly affected [49].

Despite the good establishment of single fiber model, the dynamic trap and evolution processes of three nanoscale aerosols (i.e., wetting liquid droplets, nonwetting liquid droplets, and solid particles) are not yet fully understood [31]. Cui's group has systematically investigated the in-situ capture and evolution of nanoscale aerosols on polyimide (PI) nanofibers (Fig. 2b–d). They found both wetting liquid droplets with a small contact angle (CA) and nonwetting liquid droplets with a large CA can coalesce into bigger droplets. The difference is the former can move on PI nanofiber and form axisymmetric conformations, while the latter cannot [31]. Solid particles neither move along the surface of nanofibers nor coalesce, they will ultimately form dendritic structures via weak van der Waals forces [31].

2.2.2. Proactive capture

For small particulates in the range of 0.05–0.5 μm, which are too large for diffusion and yet much small to have enough momentum for inertial impaction, capturing by air filter becomes most challenging [45,50]. In such a case, making use of chemical forces and/or electrical forces through the proactive capture route is clearly a better strategy [44]. Electrostatic attraction between PM and filter enables the PM that is diverted from the airflow streamline to be attached to the fiber surface [45] (Fig. 2a). Surgical mask and N95 facemask use such principle by preparing charged melt-blown nonwovens as the filter materials to capture PM including bioaerosol.

Electrical force is predominant when the PM is charged or the external electric field is applied to filtration material [44,[50], [51], [52]]. There is an advantage of this proactive capture method since there is no pressure drop with improved filtration efficiency as other methods do [51,52]. For instance, Xie et al. [52] made the PM negatively charged by the ionizer and applied a positive voltage to the carbonized cellulose aerogel. As a result, the electric field force enabled the PM_2.5 to adsorb on the aerogel efficiently (>99.91%), and this process had no pressure drop. On the contrary, if the traditional filtration materials seek to improve the filtration performance by increasing the thickness or packing density and reducing the fiber diameter, the pressure drop will be significantly increased [48,49].

Chemical forces are derived between advanced filtration materials with high dipole moment and the surface of PM with high-polarity functional groups such as C–N, C–O, –SO₃H, and –NO₃ [53]. There is a stronger dipole–dipole and induced-dipole intermolecular interaction between advanced filtration materials and PM, resulting in PM adhesion via electrostatic attraction [44,50]. For example, Cui et al. [53] regulated PM adhesion through surface chemical control strategies. They firstly compared the dipole moments of polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), polystyrene (PS), polyvinyl alcohol (PVA), and polypropylene (PP) repeating units to be 3.6, 2.3, 0.7, 1.2, and 0.6 D, respectively (Fig. 3 a). Further performance test results showed that PAN with the highest dipole moment owned the best filtration efficiency [53] (Fig. 3b). There are many different sizes of contaminants in the air and among them, the virus is smaller than dust, bacteria, pollen, etc. [29] The size of the COVID-19 virus only ranges from 60 to 140 nm [54]. Since small-sized aerosols are more likely to penetrate material pores, it is a great challenge to filtrate nanoscale virus aerosol. Inspired by particle adhesion that could be controlled by surface chemistry groups of membranes, Yu et al. [55] studied polarity-dominated virus capture filtering facepiece respirators. Due to the highly polar thin-film (TF) coated on electrospun PAN nanofibrous membrane (TFPNM), TFPNM always maintained N97-grade filtration efficiency against tiny Coxsackie B4 virus (CV-B4, 27–30 nm) up to 24 filtration cycles [55] (Fig. 3c). As shown in Fig. 3d, after filtering the CV-B4 viral aerosols for 24 h, fine particles with sizes less than 100 nm attached to fibers could be clearly visible. Such a facile polar-dominated approach will guide to achieving more effective and safe personal healthcare.

Fig. 3.

(a) Molecular model and corresponding dipole moments of the repeating units of PAN, PVP, PS, PVA, and PP. (b) Removal efficiency comparison of different transparent air filters under the same conditions. Reprinted with permission from Ref. [53]. Copyright (2015) Springer Nature. (c) The filtration performance evolution of TFPNMs and MEO-brand N95 respirators against CV-B4 virus. (d) SEM images of TFPNMs after filtering CV-B4 virus for 24 h. Reprinted with permission from Ref. [55]. Copyright (2021) John Wiley and Sons.

2.3. The evaluation parameters of PM filtration materials

2.3.1. Filtration efficiency (E_PM)

The filtration efficiency is defined as:

$$E_{\text{PM}}=\frac{C_0-C_1}{C_0}\times 100\%$$ (1)

where C ₀ and C ₁ are the concentration of PM before and after the filter, respectively [47,49]. It includes both number concentration (m⁻³) and mass concentration (μg m⁻³). The E _PM is one of the most key evaluation parameters for PM filters and is related to the material structure, thickness, porosity, and airflow velocity [49].

2.3.2. Pressure drop (ΔP)

The pressure drop is given as:

$$\Delta P=P_0-P_1$$ (2)

where P ₀ (Pa) and P ₁ (Pa) are, respectively, the pressures before and after the PM filtration. Pressure drop is often used to evaluate the flow resistance or air permeability of filtration materials [49]. For large filtration installations like building ventilation systems, it represents energy consumption and operating costs [56]. The greater the pressure drop, the larger the fan power consumption. For small filtration device such as face mask, ΔP represents breathability and personal comfort [21,57]. The lower the pressure drop, the smaller the breathing resistance and the more comfortable the user is.

2.3.3. Quality factor (QF)

The quality factor (QF) of filtration is defined as:

$$\text{QF}=\frac{-\ln \left(1-E_{\text{PM}}\right)}{\Delta P}$$ (3)

where E _PM and ΔP refer to filtration efficiency and pressure drop, respectively [49]. From equation (3), we can see that QF will become larger when E _PM increases or ΔP decreases. However, there always exists a balance or rather contradiction in traditional air filters: when E _PM improves, ΔP also increases [49]. QF is adopted to reflect the overall performance of a filtration system or material. A larger value means that the material has a good filtration performance.

Airflow rate plays an important role in filter performance. Since E _PM tends to decrease and ΔP increases when air flow rate increases, the effect of airflow velocity is expected to be a complex relationship. To directly incorporate the effect airflow velocity, Choi et al. proposed a modified QF (m–QF) [4,58]:

$$\text{m}-\text{QF}=\frac{-\ln \left(1-E_{\text{PM}}\right)V}{\Delta P}$$ (4)

3. Advanced filtration materials

In recent years, many types of advanced filtration materials, such as synthetic and natural polymers, MOFs, and carbon-based materials have been proposed for PM capture. Their key characteristics for PM filtration are discussed in the following sections.

3.1. Polymers

3.1.1. Synthetic polymers

Owing to low cost and easiness to be modified, petroleum-based synthetic polymers including polyethylene (PE) and PP are often used for preparing conventional commercial air filters [59,60]. Later, many different kinds of synthetic polymers, such as PI [61,62], PS [63], polyurethane (PU) [64], PAN [53,65,66], PVA [67], polyvinyl chloride (PVC) [68], PVP [69], polylactic acid (PLA) [70], polyacrylic acid (PAA) [71], and polyvinylidene fluoride (PVDF) [65,66,72,73] were synthesized into nanofibers by electrospinning in order to produce advanced filtration materials for air purification purpose. Table 1 shows the PM_2.5 filtration performance of air filtration materials mainly composed of common synthetic polymers.

Table 1.

Common synthetic polymer for PM_2.5 filtration.

Materials	Particle size (μm)	EPM (%)	ΔP (Pa)	QF (Pa−1)	Ref.
PI	2.5	99.97	73	0.1072	[61]
PS	2.5	99.99	145	0.15	[63]
PU	2.5	99.73	28	0.211	[64]
PAN	2.5	96.12	133	0.024	[53]
PVA	2.5	96.70	178	0.019	[67]
PVP	2.5	95	101	0.029	[74]
PAA	2.5	99.6	146.3	0.034	[71]
PVDF	2.5	98.16	30	0.120	[72]

With the development of fiber fabrication techniques, electrospinning has become one of the most well-established and cost-effective techniques for producing micro-nano fibers. For example, Buivydiene et al. [75] utilized polyamide (PA) 6/6 as the precursor solution. They combined solution electrospinning and melt electrospinning to obtain nano/sub-micrometer and micrometer multi-layered fibers filter. They found melt/solution electrospun fiber mats have higher porosity (up to 94.78%), lower ΔP (15.92–50.17 Pa), higher E _PM (from 91.3% of PM_1.0 to 98.5% of PM₁₀), and higher QF (0.068–0.085 Pa^-1) than the ones by other single-step techniques. However, there are still some challenges in filtering finer particulate PM_0.3. To overcome the challenges, Lou et al. [76] adopted a structural optimization strategy to form grooves on the fibers. As shown in Fig. 4 a and b, the surface of polyethersulfone (PES)/PAN electrospun nanofiber membranes (ENMs) possesses a grooved structure after chloroform treatment due to partial PES dissolution. These groove structures improve surface roughness and contact area, which will form backflow when the air passes through and increase the collision and diffusion of the particulates. The groove structure can be adjusted by manipulating the PES content. When the mass ratio of the PES component reaches 15% in PES/PAN membrane, the prepared air filter presented excellent performance with E _PM up to 99.54 ± 0.11% and ΔP as low as 133.9 ± 1.8 Pa for PM_0.3.

Fig. 4.

(a–b) SEM photographs before and after chloroform treated PES/PAN ENMs. Reprinted with permission from Ref. [76]. Copyright (2021) IOP Publishing. (c–d) SEM images of 2D NF-net. (e) TEM images of the 2D networks and triangular junctions. Reprinted with permission from Ref. [77]. Copyright (2020) John Wiley and Sons. (f–h) SEM images of TPU nanofiber/net. Reprinted with permission from Ref. [78]. Copyright (2021) American Chemical Society.

On the other hand, Ding's group [77] adopts a morphology optimization strategy inspired by spider web to solve the filtration problem of fine particles PM_0.3. In the first step, PVDF and dodecyl trimethyl ammonium bromide (DTAB) were prepared into precursor solutions. The spider-web-inspired network generator (SWING) technique was applied to produce 2D nanostructured fibrous networks (NF-nets), which can apply to self-sustained electrostatic air filters (Fig. 4c–e). The 2D SWING NF-nets are primarily derived from droplet ejection and deformation/phase separation, self-charging piezoelectric β-phase PVDF via aeolian vibration. These unique properties and 2D net topography enable the SWING air filters to exhibit excellent self-sustained long-range electrostatic adhesion. As a result, high filtration efficiency with E _PM > 99.995% for PM_0.3, low air resistance with ΔP < 88.5 Pa namely only < 0.09% of atmosphere pressure, and high transparency > 82% were realized. Enlightened by Ding's work, Chen and Zhang et al. [78] developed a novel electrospinning/netting technique for the filtration of ultrafine PM_0.1. As the addition of Li ⁺ ions enhanced the solution conductivity and rapid phase separation, they could controllably fabricate of thermoplastic polyurethane (TPU) nanofiber/net structure (Fig. 4f–h). With the content of Li⁺ ions incorporated in the electrospun solution increased, the filtration efficiency first increased and then decreased. When the Li⁺ ions concentration is 20 g L⁻¹ and the airflow velocity is 1.0 L min⁻¹, the TPU nanofiber/net-based filter displayed a high E _PM of 97.08% for PM_0.1, a low ΔP of 58 Pa, and a good QF of 0.061 Pa⁻¹ with only 6 g m⁻² base weigh.

In recent years, the photocatalytic effect was also introduced in the field of PM removal. For instance, Zhang and his coworkers [79] used PVP and tetra butyl titanate (TBOT) as main components for electrospinning precursor solutions. The precursor solution was used to prepare TBOT/PVP nanofibrous membrane (NFM) through the electrospinning technique. After which, the TBOT/PVP NFM was calcined in the tube furnace to obtain TiO₂/carbon NFM. As shown in Fig. 5 , the TiO₂/carbon NFM under UV–vis light irradiation displayed an enhanced E _PM for fine particles PM_1.0 and PM_0.3 (i.e., E _PM of PM_1.0 increased from 98.26% to 99.62% and E _PM of PM_0.3 increased from 61.59% to 92.98%). This is due to the multilevel electrostatic fields composed of the Schottky-junction interfacial charge-polarization field induced by the TiO₂ NPs on carbon NFs and the photoinduced charge-polarization field resulting from the time kinetics difference between the charge separation and recombination processes of TiO₂ NPs. Meanwhile, the as-captured PM on the TiO₂/carbon NFM can be in-situ photocatalytic decomposition, and some carbonaceous PM could be decomposed into useful fuels such as CH₄ and CO by multi-step photochemical reaction.

Fig. 5.

(a–b) E_PM (%) of fine PM before and after UV–vis light irradiation for (a) pure carbon NFM (blue bar) and (b) TiO₂/carbon NFM (red bar). (c) Schematic of space-charge polarizations induced multilevel electrostatic fields in the interface of metal/semiconductor Schottky-junction of TiO₂/carbon NF. Reprinted with permission from Ref. [79]. Copyright (2020) Elsevier.

3.1.2. Natural polymers

Unlike synthetic polymers, natural polymers refer to raw materials derived from primary substances extracted from the biomass that are either naturally produced or synthesized polymers created by biomass process [43]. Natural polymers used for PM removal mainly include cellulose, zein, silk fibroin, chitosan (CS), soy protein isolate. Because most of them possess excellent biocompatibility, good biodegradability, and are environmentally friendly, a growing number of research works have been focused on natural polymers and their derivatives in recent years.

Along this line, Xiong et al. [80] and Xie et al. [52] fabricated hierarchically porous structures of corrugated paper frame cellulose nanofibril (CNF) air filter and freestanding molybdenum disulfide (MoS₂)@carbonized cellulose aerogel air purifier. The former could be optimized via altering preparation parameters like the CNFs size, freezing temperature, concentration, base weight, etc. At the CNF concentration is 0.05 wt%, it could self-assemble to well-dispersed fibril networks, and the as-obtained CNF/corrugated paper air filters showed a high E _PM of 94.6% for PM_0.3 removal, relatively low ΔP of 174.2 Pa, and QF of 0.0168 Pa⁻¹. In the latter example, sheet MoS₂ was vertically grown on the carbonized cellulose aerogel (CCA) surface via a hydrothermal method. The high specific surface area and the strong surface charge accumulation have created a strong electrostatic force between the air filter and PM particulates under an applied electrostatic field. Accordingly, the filtration efficiency of PM_2.5 and PM₁₀ was over 99.91% and 99.95%, respectively. After 5 cycles, the removal efficiency was still maintained at 97.80%, and the regeneration of the filter can be simply realized by washing using deionized water.

In addition, Tian et al. [81] found natural proteins, such as soy protein, gelatin, and zein that contained abundant functional groups, which might promote strong interactions with PM and had huge potential in PM filtration. However, among them, only zein-based nanofibers show a high PM filtration performance and a better moisture resistance. The other natural proteins are quite susceptible to moisture, leading to the decay of electrostatic forces, which limits their application in air purification. For these reasons, They [82] synthesized zein nanofibers with good mechanical properties and moisture resistance via electrospinning with the assistance of polyvinyl alcohol, followed by cross-linking with glutaraldehyde (Fig. 6 a). The optimized nanofibers exhibited 97.3% filtration efficiency of PM_0.3, and the filtration efficiency was over 98% for particulates size larger than 0.5 μm.

Fig. 6.

(a) A schematic illustrating the preparation of hydrophobic cross-linking zein nanofibers. Reprinted with permission from Ref. [82]. Copyright (2020) Elsevier. (b–c) SEM images of (b) PU and (c) PU@ZIF film. (d) XRD patterns of ZIF-8, PU, and PU@ZIF-8. (e) The contact angle photo of PU@ZIF containing different concentrations of ZIF-8. Reprinted with permission from Ref. [99]. Copyright (2021) American Chemical Society. (f–g) The (f) PM removal efficiency (%) and (g) quality factor (Pa⁻¹) of isomeric MAFs/cotton. Reprinted with permission from Ref. [100]. Copyright (2021) Elsevier.

Silk fibroin and chitosan are popular natural polymer materials. The former is mainly obtained from Bombyx mori silkworm cocoons, the latter is a saccharide that is primarily extracted from the hard outer skeleton of shellfish, shrimp, and other crustacean shells. They are all biocompatible, biodegradable, and environmentally friendly. Xie et al. [83] developed a solvent welding electrospun strategy to cross-link lyophilized silk nanofiber (SNF) 3D networks by phenethyl alcohol (PEA), fabricating low density and water-insoluble silk fibroin nanofibrous aerogels (SNFAs), which could be applied in air filtration and oil/water separation simultaneously. Hu et al. [84] obtained natural SNFs through exfoliation from silkworm silks fibers by a mechanical disintegration method. Subsequently, adhesive PVA solution was mixed with the SNF suspension to solidify the junctions between SNFs through their rich inherent hydroxyl groups. A 3D nanofibrous aerogel was obtained after freeze-drying the suspension. The as-prepared SNF-based aerogels have controllable and stable 3D structures, ultrahigh porosity, good PM filtration efficiency (about 98% for PM₁₀), and ultralow thermal conductivity (0.0263 W m⁻¹ K⁻¹), which may be used for advanced filtration materials and ultralight heat preservation materials. Due to some characteristics of chitosan including strong polarity, lightweight, antibacterial properties, non-toxic and biodegradable, Choi et al. [85] fabricated a biodegradable, moisture-resistant, and highly breathable fibrous mask filter. This mask displayed a comparable filtration efficiency of 98.3% for PM_2.5 as the commercial N95 mask. In addition, it is very breathable (ΔP ∼ 59 Pa) and better in humidity management compared to the traditional N95 mask.

3.2. Porous metal–organic frameworks

Porous metal–organic frameworks (MOFs), constructed from metal ions or clusters and organic ligands via the coordination bonding, have attracted an increasing amount of attention owing to their ultrahigh porosity, controllable pore size, high thermal and chemical stability, and easiness to be functionalized [[86], [87], [88], [89], [90], [91], [92], [93]]. Up to now, researchers have explored many methods for MOF preparation, including solvothermal, seeded growth, microwave-assisted deposition, freeze-drying, dip-coating, electrochemical synthesis [[94], [95], [96], [97], [98]]. Yet it is worth noting that MOFs could only be applied to PM filtration after being loaded on certain substrates, such as the electrospun polymer nanofibrous membranes, cotton, and graphene aerogel.

Wang et al. [99], using PU as the substrate, prepared PU@Zeolitic imidazolate frameworks (ZIF)-15 fiber membrane by direct blending and electrospinning method. As shown in Fig. 6b and c, the surface of the PU fibers appears rougher after loading ZIF. X-ray diffraction (XRD) in Fig. 6d confirms the presence of ZIF in the PU/ZIF samples. The authors have also characterized the hydrophobicity of samples by measuring the water contact angle (WCA). As shown in Fig. 6e, due to the increase in surface roughness, the contact angle increased after loading ZIF onto PU, indicating the water resistance has significantly improved. The prepared PU@ZIF-15 nanofiber membranes exhibited excellent mechanical performance (tensile strength 10.38 MPa), water repellency (WCA ∼ 128.63°), high E _PM (97.99 ± 2.0%) of PM_2.5. At the same time, due to the good mechanical properties and water resistance, the membrane also exhibited superior stability and reusability with water washing.

Jhung et al. [100,101] carried out two studies on MOFs coated on the cotton substrate. In one of the studies, ionic salts (ISs) such as CaCl₂ and LiCl were loaded onto highly porous MOFs (UiO-67 or MIL-101) by the impregnation method [101]. Subsequently, the modified MOFs were coated onto a cotton substrate. E _PM and QF were found in the order of CaCl₂(20)@MIL-101/cotton > LiCl(20)@MIL-101/cotton > CaCl₂(20)@UiO-67/cotton > LiCl(20)@UiO-67/cotton. The reason is that the MIL-101 has a higher porosity than UiO-67, and CaCl₂(Ca²⁺) has a larger permanent charge separation compared to LiCl (Li⁺). Since permanent charge separation of ionic salts increase the electrostatic interactions with PMs, the high porosity ISs@MOFs display the highest QF of 0.085 Pa⁻¹. In the other research by this group of authors, three isomeric MOFs, namely MAF-5, MAF-6, and MAF-32, were coated on a cotton substrate to investigate the influence of voidage on PM removal efficiency [100]. As displayed in Fig. 6f and g, the E _PM and QF linearly increased with improving voidage of the isomeric MAFs on cotton.

In a separate work, Mao et al. [102] constructed a high-performance PM filter by decorating ZIF-67 uniformly on a 3D porous network of reduced graphene oxide aerogel (rGA) substrate. Due to the continuous 3D networks, high specific surface area, and numerous active sites, the ZIF-67/rGA possessed a filtration efficiency of more than 98.1% in 100 h static test, dynamic filtration of PM_2.5 > 97.8%, and PM₁₀ > 98.2%. What's more, the ZIF-67/rGA after PM capture could be converted to Co₃O₄ loaded on nitrogen-doped rGA (Co₃O₄/N-rGA) by in-situ carbonization and activation in a pyrolysis process. The obtained Co₃O₄/N-rGA exhibited unprecedented electrochemical performance and had exceeded the Co₃O₄/r-GA obtained by converting ZIF-67/r-GA without PM loading. Therefore, the as-prepared material can be used not only for PM removal but also for high-performance energy storage and conversion after the pyrolysis conversion.

3.3. Carbon-based materials

Carbon-based materials have been widely used in many fields. Among them, activated carbon is often applied to air filtration to remove SO_x, NO_x, volatile organic compounds (VOCs), and other exhaust gases, owing to their numerous active adsorption sites, high specific surface area, and porosity [[103], [104], [105]]. In recent years, other carbon-based materials, such as carbon nanotubes (CNTs) and graphene oxide (GO) have emerged as strong candidates for a wide range of applications including PM filtration.

Zhong's group [106] proposed a novel strategy by in situ growing one-dimensional (1D) CNTs on a porous SiC membrane substrate via chemical vapor deposition (CVD) without introducing a transition or sacrificial layer. Three steric configurations involving spiral CNTs-coated SiC membrane (S-CNTs/SiC), interweaving CNTs/SiC membrane (I-CNTs/SiC) and vertical CNTs/SiC membrane (V-CNTs/SiC) were controllably prepared. Among these, the S-CNTs/SiC exhibited excellent performance with gas permeance of 450 m³ m⁻² h⁻¹  kPa⁻¹ and ultrafine PM filtration efficiency of 99.48%.

GO is derived from graphene with rich surface oxygen-containing functional groups, large specific surface area, low density, good mechanical properties, and electrical conductivity. GO can be used either directly for air filters or indirectly through decorating onto other filter materials. Lai's group [51] fabricated graphene aerogel (GA) via solvothermal reaction and freeze-drying. Subsequently, a novel charged graphene aerogel filter (CGAF) was created by applying an external electrical field through an inserted conductor. Since the CGAF carries electrical charges, it has achieved a high filtration efficiency while maintaining a low pressure drop. The filtration efficiency is above 99.9% for non-oily PM_2.5 and more than 99.6% for oily PM_2.5 even under a very high concentration (>10,000 μg m⁻³). Furthermore, it has outstanding reusability, flame-retardancy, and stability in harsh environments. After 10 times of washing or 5 min of burning, the E _PM remained greater than 99%. Chen et al. [107] prepared the GO functionalized PVDF NFMs. 2D GO nanosheets were introduced into PVDF NFMs via blending-electrospinning of GO/PVDF as well as GO@PVDF dip-coating to prepare a different composite NFMs air filter. Due to the functionalization of GO, the as-prepared samples displayed good mechanical strength, reusability, and enhanced filtration performance. The PM_2.5 removal efficiency of NFMs is as follows: GO/PVDF NFMs (99.31%) > GO@PVDF NFMs (95.41%) > pristine PVDF NFMs (93.74%).

3.4. Other materials

Other materials, such as inorganic nanofibers and metal nanowires, have also been used for PM removal applications. Inorganic compounds possess prominent thermal and chemical stability, which makes them particularly useful for harsh environment applications [[108], [109], [110]]. Various types of oxides, carbides, nitrides have been explored, including SiO₂ [111], Al₂O₃ [112], SiC [[113], [114], [115]], and Si₃N₄ [116].

As an example, Li et al. [116] synthesized a highly porous Si₃N₄ nanofiber sponge (NFS) air filter via CVD. Its fibers aligned along the airflow direction, which has resulted in stagnation of PM under the friction force when they sliding along the fiber direction. Such structure decreases the pore blocking and pressure drop. As a result, the as-prepared Si₃N₄ NFS showed a very high filtration efficiency of 99.97% for PM_2.5, a low pressure drop at 340 Pa (only <0.33% of atmospheric pressure), and a high airflow rate of 8.72 m s⁻¹ when tested at 1000 °C.

In 2017, Ko et al. [117] first exhibited the use of an Ag nanowire percolation network to capture PM_2.5. The air filter exhibited high filtration efficiency (>99.99%) for PM_2.5 under low working voltage condition (<5 V), and is reusable and transparent. Recently, Liu et al. [118] fabricated metallic copper nanowire foams using electrodeposition, freeze-drying, and sintering processes. The foams are mechanically sturdy and possess a large specific surface area and low density (∼2 to 30% of the bulk metals). The foams displayed a filtration efficiency of over 96.6% for PM_0.3, and are easy to clean for reuse.

4. Additional functions

With the continuous development of air filters towards practical applications, more and more additional functions have been suggested to satisfy the multifunctional requirements in practical problems encountered by the industry. Considerations for high thermal stability, antibacterial activity, and special wettability are often among these additional functions.

4.1. Thermal stability

Thermal stability refers to the ability to maintain performance under high temperatures. It is very important in purifying some high-temperature waste exhaust, such as industrial emissions (140–300 °C) and vehicle exhaust (30–80 °C) [119,120].

PI is one of the most commonly used heat-resisting polymer fibers with excellent mechanical properties, prominent chemical and thermal stability [121,122]. Xie et al. [123] fabricated a wrinkled porous structured PI nanofiber air filter using PAN as the template via electrospinning and subsequent thermal-induced phase separation. The filter showed outstanding properties, with the PM_0.3 filtration efficiency of 95.55%, the pressure drop of 38.52 Pa, and the quality factor of 0.0808 Pa⁻¹ at 280 °C. Qiao et al. [124] prepared a PI nanofiber (PINF) aerogel using water as the dispersant and triethylamine (TEA) as the binder (Fig. 7 a). Ice templating and thermal imidization were applied to generate the aerogel. According to the thermogravimetric analysis (Fig. 7b and c), the resultant PINF aerogel could withstand a high temperature of 300 °C while maintaining a high removal efficiency for PM_2.5 (99.83%). Lu et al. [125] prepared a hybrid air filter using PI through electrospinning and subsequently multiple hydrogen bonding self-assembly. In their work, octa(amino-propylsilsesquioxane) (POSS–NH₂) was utilized to activate the PI nanofiber surface, and then amino-functionalized zeolitic imidazolate framework-8 (NH₂-ZIF-8) nanocrystals were anchored on the active fiber surface by hydrogen bonding. The as-prepared PI-POSS@ZIF hybrid filter exhibited a high PM_0.3 filtration efficiency of 99.28% (Fig. 7d, e), a low pressure drop of 49.21 Pa, a high quality factor of 0.1002 Pa⁻¹ (2.74 times than PI filter of 0.0366 Pa⁻¹) at 280 °C.

Fig. 7.

(a) The schematics of preparation processes of PINF aerogel. (b) TG and DTG curves of PINF aerogel sample. (c) The variations in the storage modulus (E′), loss modulus (E′′), and damping ratio of the sample at 25–300 °C. Reprinted with permission from Ref. [124]. Copyright (2021) Elsevier. (d–e) The removal efficiency of (d) PI filter, (e) PI-POSS@ZIF filter at 25 °C and 280 °C, respectively. Reprinted with permission from Ref. [125]. Copyright (2021) Elsevier. (f) TGA curves of corresponding samples (g) E_PM and ΔP of the as-prepared sample after different temperature treatment. Reprinted with permission from Ref. [127]. Copyright (2020) Elsevier.

Other novel composite nanofibrous membranes were also exploited for use in a high-temperature environment. For example, Zhang et al. [126] used spinnable PAN and high-performance poly(m-phenylene isophthalamide) (PMIA) to prepare high-temperature resistant PMIA/PAN composite nanofibers membrane by optimizing the mass ratio of the two-component solutions. After a series of heating tests at 140–220 °C, the filtration efficiency of the PMIA/PAN was maintained at over 99% for PM_0.3 at all times. Furthermore, Yang et al. [127] developed a novel multifunctional composite nanofiber membrane by introducing BaTiO₃ during the spinning of PU and polysulfonamide (PSA) fibers. As a result of the cooperation of PU, PSA, and BaTiO₃, the BaTiO₃@PU/PSA nanofiber membrane achieved a high E _PM of 99.99% for fine particles PM_2.5 with a low ΔP of 39.4 ± 0.2 Pa. As is shown in Fig. 7f and g, its physical stability and E _PM were well preserved under heating up to 300 °C.

4.2. Antibacterial property

Faced with several major pandemics, for example, the highly infectious COVID-19, the Middle East respiratory syndrome (MERS), and the severe acute respiratory syndrome (SARS) [128], a lot of attention has been paid to the filtration of bioaerosols and the antibacterial performance of air filters. Here, we summarize three groups of antimicrobial materials, viz., nano-metallic materials, photocatalysts, and other materials.

4.2.1. Nano-metallic materials

Metallic materials are commonly used for antibacterial purposes, involving silver (Ag), copper (Cu), titanium (Ti), aluminum (Al), zinc (Zn). Among them, Ag nanoparticles (Ag NPs) are the most frequently used because of their outstanding and broad-spectrum antibacterial property. The antibacterial mechanism of Ag NPs is to denature the phospholipids and proteins of microorganism's cell wall, destroy the integrity of the cells and inhibit bacterial growth [129]. For instance, Ju et al. [130] anchored Ag NPs on polyamide-6 electrospun nanofibers through hydrogen-bond to obtain bumpy nanorough structure surface as well as the strong antibacterial ability and antiviral property. Constructing the bumpy rough surface is an effective strategy to facilitate particles stagnation, thereby enhancing filtration performance (E _PM ∼ 99.99% and ΔP ∼ 31 Pa for PM_2.5). Due to the existence of Ag NPs, the as-prepared membrane exhibited excellent antibacterial ability against Escherichia coli and Staphylococcus aureus and good antiviral property against Porcine Deltacoronavirus (PDCoV) which is a positive-sense RNA virus and typical Coronaviruses. Xiao et al. [131] prepared nanoscale Ag NPs via the liquid phase reduction method and used them to generate tree-like Ag NPs/PVDF nanofibers by electrospinning the particle-containing solution. The Ag NPs/PVDF nanofibrous membranes were kept at a high level (99.95–99.97%) of filtration efficiency, with the bacterial reduction rates (BR) greater than 99.6% value for E. coli (E. coli) and S. aureus (S. aureus). Fan et al. [132] fabricated a novel tug-of-war-inspired bio-based multifunctional air filter with antimicrobial activity. They first anchored Ag NPs on paper towel (PT) microfibers (Ag@PT) by in situ reduction. Subsequently, aligned zein nanofibers (zNFs) were prepared as the upper layer by electrospinning protein Pickering emulsion (a stable emulsion, here formulated with cellulose nanofibril suspension and zein solution) [132,133]. Ultimately, the zNFs-Ag@PT air filter was obtained. Candida albicans (yeast), Micrococcus luteus (Gram-positive bacteria), and E. coli (Gram-negative bacteria) were selected to characterize broad-spectrum antimicrobial activity. The results showed that the inhibition zones in the zNFs-Ag@PT plate were clearly visible and demonstrated excellent antibacterial properties. The diameters of the inhibition zones were about 2.18 cm for M. luteus, 1.85 cm for E. coli, and 1.70 cm for C. albicans, respectively for the 3 bacteria (Fig. 8 a). Furthermore, the filter also showed excellent particle filtration performance (99.30% for PM_0.3).

Fig. 8.

(a) Photographs of inhibition zone of zNFs-Ag@PT. (From left to right: M. luteus, E. coli, and C. albicans). Reprinted with permission from Ref. [132]. Copyright (2021) American Chemical Society. (b) The capture efficiency (%) of PET/Al-180 for different sizes of charged E. coli. (c) The inactivation efficiency (%) against E. coli and S. epidermidis for PET/Al samples under various Al deposition times. (d) Photographs of recultivated E. coli and S. epidermidis colonies were sampled from raw PET, PET/Al-15, and PET/Al-180 filters, respectively. Reprinted with permission from Ref. [134]. Copyright (2018) Elsevier. (e) Antibacterial tests against for E. coli a) control group, b) Cu₂O/MOF-801@PVDF MNF; Antibacterial tests against S. aureus c) control group, d) Cu₂O/MOF-801@PVDF MNF. Insets: corresponding SEM images. Reprinted with permission from Ref. [135]. Copyright (2021) Royal Society of Chemistry. (f) Inhibition zone test of samples against a) E. coli and b) S. aureus. Reprinted with permission from Ref. [136]. Copyright (2021) Springer Nature.

A washable antibacterial polyester/aluminum (PET/Al) air filter was prepared by Choi et al. [134] During the test, a voltage of −10 kV and +10 kV was applied to the ionizer and PET/Al, respectively. The average capture efficiency of charged E. coli cells (Gram-negative bacteria) significantly improved to about 99.99% via electrostatic forces (Fig. 8b). As the Al deposition time added from 5 min to 180 min, the bacterial inactivation efficiency improved to 96.9 ± 0.59% against Staphylococcus epidermidis (Gram-positive bacteria) and 94.8 ± 1.18% against E. coli (Gram-negative bacteria) (Fig. 8c and d). On the one hand, this is because the enhanced nano-rough surface of PET/Al could impede bacteria preliminary adhesion, and consequently lead to cell apoptosis. On the other hand, in addition to the physical mechanism, chemical reactive oxygen species (ROS) generated from Al oxides may also play a key role in disrupting the cell walls.

In a separate effort, Jang et al. [135] prepared PVDF multifunctional nanofiber (MNF) containing well-dispersed MOF-801 and Cu₂O NPs via a facile electrospinning process. Due to the physical blockage and cell wall destruction by Cu₂O NPs, the MOF-801@PVDF MNFs possessed good antimicrobial efficacy against Gram-positive (S. aureus) and Gram-negative (E. coli) bacteria (Fig. 8e). In the same year, Victor et al. [136] adopted titanium nanotubes (TNT) as the main antibacterial active ingredient to synthesis electrospun PVDF nanofiber. It showed the highest bacterial filtration efficiency of 99.88% when electrospinning time was 3 h and TNT filler was 15 wt%. From the inhibition zone tests in Fig. 8f, the increasing concentration of TNT enhanced the antibacterial activity. This is because TNT can bind with the negatively charged bacterial cell wall and rupture the cell wall.

4.2.2. Photocatalysts

Photocatalytic effect can be utilized for antibacterial applications, in which strong germicidal ROS are produced upon light irradiation on a semiconductor surface. The following section will explain ZnO, TiO₂-based, MOF-based materials, and so on.

Zhong's group [137] hydrothermally synthesized 1D ZnO nanorods (NRs) on the surface of a 3D porous expanded polytetrafluoroethylene (ePTFE) matrix. The prepared air filters have outstanding performance with filtration efficiency greater than 99.9999%, sterilization rate against both Gram-negative bacterium (E. coli) and Gram-positive bacterium (S. aureus) higher than 99.0%. They suggested two possible antimicrobial mechanisms. One is that Zn²⁺ ions bring about the breakage of the bacterium cell wall. The other mechanism is that ROS such as hydroxyl radicals (·OH) and/or hydrogen peroxide (H₂O₂), originated from photocatalytic actions of ZnO NRs, inhibit bacterial growth. Zhu et al. [138] fabricated nitrogen-doped TiO₂ (N-TiO₂) and TiO₂ mixture to prepare photocatalytic antibacterial masks. Since nitrogen doping into TiO₂ has increased the light absorption range into the visible light range, more bactericidal free radicals of HO· and ·O²⁻ will be generated (Fig. 9 a). The as-prepared N-TiO₂/TiO₂ masks revealed a 100% bacteria sterilization against E. coli K-12 strain MG1655 and S. aureus strain HG003 under either 0.1 Sun simulator irradiation (200–2500 nm, 106 W m⁻²) or natural sunlight for 10 min (Fig. 9b). Jung et al. [139] adopted a visible-light-activated (VLA) dye sensitization strategy, combining TiO₂ NPs with a hydrophobic molecule, 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES), and a visible-light sensitizer, the crystal violet (CV) organic dye to obtain TiO₂@PFOTES-CV air filter. The CV dye has strengthened the visible light photocatalytic activity, and PFOTES formed a hydrophobic barrier to protect CV. With such a design, the TiO₂@PFOTES-CV filter possessed strong photocatalytic antibacterial property under visible light with an inactivation rate of ∼99.98% and blocking efficiency of ∼99.9% against various bacterium bioaerosols. Sun et al. [140] reported a daylight-driven photo-induced antimicrobial nanofibrous membrane (RNM) that is green, bioprotective, rechargeable, and antiviral. The various ROS generated under light irradiation, including ·OH, superoxide (·O²⁻), and H₂O₂, could cause damage to DNA, RNA, lipids, and proteins, ultimately causing the apoptosis of bacteria and viruses. The RNM showed excellent overall performance indices, such as above 99% removal rate for fine particulates, over 99.9999% germicidal rate, and more than 99.999% virucidal efficacy.

Fig. 9.

(a) The sterilization mechanism of N–TiO₂ photocatalytic antibacterial masks under visible light irradiation. (b) Photocatalytic rejuvenation tests of prepared masks with and without sunlight irradiation. Reprinted with permission from Ref. [138]. Copyright (2021) American Chemical Society. (c) Photocatalytic inactivation efficiency and PM removal efficiency of MOFilter. Reprinted with permission from Ref. [141]. Copyright (2019) Springer Nature.

With regard to MOF-based materials, Wang's group [141] synthesized a zinc-imidazolate MOF ZIF-8, which has excellent photocatalytic bactericidal activity and PM removal efficiency. The inactivation efficiency was over 99.9999% against E. coli in saline within simulated solar irradiation for 2 h and 97% PM removal efficiency (Fig. 9c). Through low-temperature electron paramagnetic resonance (EPR) spectra and valence band X-ray photoelectron spectra (VB-XPS) analyses, the authors believe that the mechanism begins with the photoelectron production through the ligand to metal charge transfer (LMCT) process. Then the generated ROS (H₂O₂ and ·O₂ ⁻) contribute to photocatalytic antibacterial action.

To enable sustained antibacterial and filtration efficacy in dark conditions, Li et al. [142] added ZIF-8 to the melt blowing-electrospinning process to prepare bead-on-string hierarchical micro/nanofibrous structured photodynamic-type composite membranes. First, they prepared the polypropylene/poly-ɛ-caprolactone (PP/PCL) melt-blown membranes (denoted as PPCL) by the melt-blown method. Second, PPCL membranes were immersed in the polydopamine (PDA) solution to obtain PPCL@PDA melt-blown membrane. Third, chemical grafting bis-benzophenone-type 4, 4′-terephthaloyl diphthalic anhydride (TDPA) photosensitizer and epigallocatechin gallate (EGCG) antibacterial agent to obtain PPCL@PDA/TAEG. Finally, the mixed solution containing PCL and ZIF-8 was electrospun over PPCL@PDA/TAEG membranes to prepare PPCL@PDA/TAEG/PCL/ZIF8 composite membranes. The as-prepared bead-on-string structured composite membranes showed excellent filtration performance. The PM_2.5 filtration efficiency is over 99.99%, and the antimicrobial rate against S. aureus and E. coli is 99% and 95%, respectively, in the daytime and nighttime. Because the composite membranes can generate ROS under both light and dark conditions, and the structure is stable. After seven light–dark cycles, the PPCL@PDA/TAEG/PCL/ZIF8 composite membranes still maintained the original charging capacity of 89.9% for ·OH and 65.1% for H₂O₂.

4.2.3. Others

Other types of antibacterial materials include chitosan, zwitterionic amphiphiles, organic biocides such as N-halamine compounds, triclosan, and quaternary ammonium salts [143,144]. Chitosan is a naturally available polysaccharide extracted from chitin, and it can inhibit the growth of bacteria. Liu et al. [145] used chitosan as the main antibacterial ingredient to prepare transparent multilayered nanofibrous poly(methylmethacrylate)/polydimethylsiloxane (PMMA/PDMS) antibacterial air filters. The filter has displayed a high antibacterial efficiency of 96.5% for E. coli and 95.2% for S. aureus with a high filtration efficiency of 98.23 for PM_2.5. Zhang et al. [146] employed chitosan and N-halamine as antimicrobial active substances. They prepared a multilayer PVA/P(ADMH-NVF) nanofibrous PM filter consisting of N-Vinylformamide (NVF), 3-allyl-5,5-dimethylhydantoin (ADMH), and PVA. Through inhibition zones test, they discovered that the PVA/P(ADMH-NVF) filter exhibited superb antibacterial properties against Gram-negative bacteria E. coli and Gram-positive bacteria S. aureus. Kumar et al. [147] first prepared a novel antibacterial electrospun nanofibrous material containing sulfobetaine (SB)-type zwitterionic amphiphiles. Their results suggest that filter inhibited above 99.9% the growth of both Gram-negative bacterium Klebsiella pneumoniae (ATCC 4352) and Gram-positive bacterium S. aureus (ATCC 6538) with a high filtration efficiency (>99.9%). In another attempt, N-halamine and quaternary ammonia salt (QAS) groups through covalent integration were introduced into nanofiber membranes by Ding's group [148]. The resulting membranes realized excellent bactericidal efficiency (>99.9999%) against E. coli and S. aureus., superior virucidal efficiency (>99.999%) against E. coli phage, and high bacteria filtration efficiency (99.77%).

4.3. Special wettability

Special wettability usually refers to extreme wetting states such as superhydrophobicity and superhydrophilicity [149]. Superhydrophobicity is manifested through a liquid droplet that exhibits water contact angle greater than 150° from a macroscopic view, while superhydrophilicity refers to the case that water contact angle is less than 5°. Wetting state impacts the performance of filter materials as will be elaborated below.

4.3.1. Superhydrophobicity

Superhydrophobicity endows filtration materials with some advantages including self-cleaning, resistance to moisture, reuse after washing, and improved water vapor transmission. Inspired by self-cleaning natural adsorptive leaves, Kang et al. [150] coated nanoporous organic networks (NONs) on nanostructured polyurethane acrylate (N-PUA) films (denoted as N-PUA-NONs). As shown in Fig. 10 a, from N-PUA film to N-PUA-NON-2 film, the surface of the film became slightly bumpy and the WCA from 75° increased to 150°. As a result, the hydrophilic N-PUA film was converted to a superhydrophobic surface with enhanced capture performance toward PM and washing recyclability. Li et al. [151,152] exploited conjugated microporous polymer bearing aminopyridine moiety (A-CMPs) and thiophene-based conjugated microporous polymer (T-CMP). As depicted in Fig. 10b and c, the WCA of A-CMPs aerogels (A-CMPAs) reached 155°, which has severe advantages in antifouling, and filtrating particulates (99.307% for PM_0.3) under high humidity air (relative humidity (RH): 89 ± 3%). Similarly, as displayed in Fig. 10d and e, the WCA of T-CMP-1 and T-CMP-2 were 154 and 152°, respectively, and their filtration efficiency in a moist environment (RH: 90 ± 5%) was all in the high end of 99–100% range for PM_0.3, PM_2.5, and PM₁₀. Liu et al. [153] utilized methyltrimethoxysilane (MTMS) for silanization hydrophobic modification of cellulose nanofibers (CNFs). Subsequently, the samples were freeze-dried to produce a porous aerogel air filter with excellent superhydrophobicity (WCA = 154.2°). A high removal efficiency above 99% was reported for the size greater than 0.5 μm of particulate matter (Fig. 10f and g).

Fig. 10.

(a) SEM images and corresponding WCA images of N-PUA and N-PUA-NON-2. Reprinted with permission from Ref. [150]. Copyright (2019) American Chemical Society. (b) Simulated demonstration device for the PM capture of the A-CMPAs filter under high humidity conditions. Inset demonstrates the photograph of the water contact angle of A-CMPAs, and photo of A-CMPAs filter. (c) The schematic illustration of A-CMPAs for PM filtration in high humidity conditions. Reprinted with permission from Ref. [151]. Copyright (2021) Elsevier. (d) PM removal efficiency of T-CMP-1 filter under various times (RH: 90 ± 5%). (e) The images of water/oil contact angle for T-CMP. Reprinted with permission from Ref. [152]. Copyright (2021) American Chemical Society. (f) The digital images of the water contact angle of the surface and cross section. (g) The filtration efficiency of samples. Reprinted with permission from Ref. [153]. Copyright (2021) American Chemical Society.

4.3.2. Superhydrophilicity

Hydrophilic polar functional groups on polymeric materials can proactively capture ultrafine particulate matter through chemical interactions [58,154]. Since a large amount of water vapor exhaled from the mouth and nose during breathing may condense and block the membrane pores, further increase ΔP and affect the wearer's thermal-physiological comfort [155,156]. Therefore, in recent years, gradient structures, from hydrophobic to superhydrophilic state, have been explored for their high moisture-vapor transmission rate (MVTR). For example, Ding et al. [155] designed a hydrophobic to superhydrophilic gradient structure by adopting electrospun superhydrophilic PAN/SiO₂ nanofibers as moisture-vapor transfer vectors and hydrophobic PVDF nanofibers as water-repellent components (Fig. 11 a). The resultant membranes displayed a steady filtration efficiency of 99.99% for PM_2.5, a low pressure drop of 86 Pa, and a high MVTR of 13,612 g m⁻² d⁻¹. Wang et al. [156] developed a new type of polymer/MOF-derived multilayer membranes, which consisted of a superhydrophilic PAN fiber outer layer with multiscale surface roughness (direct exposure to polluted air), a hydrophilic composite intermediate layer containing alternating layers of PAN nanofibers and PAN-ZIF-8 microfibers, and the hydrophobic PS fiber mat as an inner layer (Fig. 11b). The resultant hydrophilic to hydrophobic gradient structure possessed good moisture absorption capability and high MVTR of 10,560 g m⁻² d⁻¹ owing to strong capillary force and push–pull effect. More importantly, benefiting from the large specific surface area, hierarchical porous structure, and high porosity as well as the multiscale surface roughness of the fibers, the membrane exhibited a high E _PM of 99.973% for PM_0.3 at a low ΔP of 80.1 Pa. Fig. 11c demonstrates the apparent water contact angles of superhydrophilic side in different alkali treatment times. In addition, a TiO₂/H₂O₂ photocatalysis approach was adopted to improve hydrophilicity by Li et al. [157], as is shown in Fig. 11d; the contact angle for this non-woven fabric (NWF) sharply decreased from 112° at the pristine state to nearly 0° after the surface TiO₂/H₂O₂ modification.

Fig. 11.

(a) Schematics illustrate a molecular model of main components and the fabrication process of gradient composite membranes. Reprinted with permission from Ref. [155]. Copyright (2017) John Wiley and Sons. (b) The schematic of preparing gradient multilayer composite membranes. (c) Superhydrophilic side apparent water contact angles of multilayer membranes in different alkali treatment times. Reprinted with permission from Ref. [156]. Copyright (2020) Elsevier. (d) WCA of the pristine and modified NWF. Reprinted with permission from Ref. [157]. Copyright (2020) Elsevier.

4.3.3. Janus membrane

Janus membrane displays different wettability on each side, which can be used for directional transportation of liquids. Hydrophobic materials can make the exhaled vapor condense into small droplets, and hydrophilic materials can absorb the condensed water. Xu et al. [158] prepared three types of Janus nanofibrous porous membranes containing β-cyclodextrin (β-CD) by electrospinning (Fig. 12 a). Among them, the PAN/β-CD/PCL/ZnO Janus membrane showed the best filtration performance, with a filtration efficiency of up to 99.99% and a pressure drop of 156.5 Pa. More importantly, the water vapor or condensed water droplets could be directionally transported to the outside of the nanofiber mask to reduce the damp feeling of the wearer. The contact angle of the hydrophilic side and hydrophobic side are shown in Fig. 12b. Park et al. [159] reported a simple and economical strategy to prepare a high-performance Janus air filter (JAF) by employing ubiquitous materials such as a sponge or cotton fabric. As displayed in Fig. 12c, the average WCA of the hydrophilic cotton is 0°, while it is 138° on the hydrophobic cotton/CS/PDMS. At the optimized ratio of hydrophilic (75%) to hydrophobic (25%) components, the as-prepared filter was able to maintain a long-term filtration performance of 99.99% even after 30 cycles and a low pressure drop of 152 Pa. Besides, Choi et al. [85] fabricated a highly breathable, biodegradable, wetproof, and high-efficiency mask via applying poly(butylene succinate)-based (PBS-based) microfiber and nanofiber mats integrated into a Janus membrane with chitosan nanowhiskers coating.

Fig. 12.

(a) Schematic diagram of β-CD-containing Janus NFMs prepared through electrospinning. (b) Optical images, and photos of hydrophilic side and hydrophobic side of three types of Janus membranes: PAN/β-CD/PCL/ZnO, PAN/β-CD/11% PCL/2% PCL, and PAN/β-CD/mask, respectively. Reprinted with permission from Ref. [158]. Copyright (2021) American Chemical Society. (c) Photograph of JAF samples. Insets: WCAs of cotton and cotton/CS/PDMS. Reprinted with permission from Ref. [159]. Copyright (2021) Elsevier.

5. Applications

The development of particulate matter filtration technology has made it possible various kinds of air filters to be developed and optimized in recent years. In the review, we will introduce them according to two broad categories: end-of-pipe treatment (personal protection) and source control (engineering facilities).

5.1. End-of-pipe treatment – personal protection

5.1.1. Face mask

Face mask provides significant protection to individuals, especially in a high-risk environment or during an epidemic time [160]. The commonly used fabrics for respiratory masks include cotton with different thread counts, silk, flannel, chiffon, woven, nonwoven, blended fabrics, various synthetic fabrics, and their recombination [[161], [162], [163], [164], [165]]. There are many different types of face masks, among them, medical face masks and N95 respirators are most commonly used during the current COVID-19 pandemic.

Recently, numerous novel functional masks and preparative technology have emerged, such as electrical heating antibacterial mask (Fig. 13 a) [166], photothermal disinfection face mask (Fig. 13b) [167,168], embossed structural facemask via combining 3D printing membranes technology and electrospinning method [169], all-polymer hybrid electret fibrous respirator (Fig. 13c) [170], environmentally friendly PVA-tannic acid (TA) nanofiber composite mask (Fig. 13d) [171]. Furthermore, the researchers also developed a series of real-time respiratory monitoring facial masks. For example, Lee et al. [172] prepared PAN nanofibrous membranes by electrospinning first and then coated conductive MOF (Ni-CAT-1) on the nanofiber via a two-step hydrothermal reaction. The as-prepared hybrid membrane mask realized respiratory monitoring function based on monitoring electrical resistance change with the relative humidity. The mask has also reached a good removal efficiency (>99%) (Fig. 13e). As shown in Fig. 13f, Ding et al. [173] constructed a super-elastic hard carbon aerogel (s-HCA)/zein composite nanofibrils facemask equipped with real-time respiratory monitoring. The prepared facemasks can not only achieve a high filtration efficiency of 99.5%, but also monitor human health by sensitively detecting breath signals. In addition, Wang et al. [174] reported a green dual function gelatin/β-cyclodextrin composite nanofiber respirator, which can filtrate both PM and VOCs. This study provides a solution for the development of green bi-functional masks at low resistance.

Fig. 13.

(a) Infrared image of the electrical heating antibacterial mask under applying a low voltage of 3 V. The inset shows a real snapshot. Reprinted with permission from Ref. [166]. Copyright (2021) American Chemical Society. (b) Digital photograph of photothermal disinfection face mask. Reprinted with permission from Ref. [167]. Copyright (2020) American Chemical Society. (c) The photo of the N95 respirator adopting all-polymer hybrid electret PS/PVDF-2 fibers as the core material. Reprinted with permission from Ref. [170]. Copyright (2020) Elsevier. (d) The exhibition of PVA-TA nanofiber mask. Reprinted with permission from Ref. [171]. Copyright (2021) Elsevier. (e) The schematic diagram of respiratory monitoring and blocking PMs face mask. Reprinted with permission from Ref. [172]. Copyright (2020) American Chemical Society. (f) The schematic illustration of s-HCA/zein nanofibril composite respiratory monitoring facial mask. Reprinted with permission from Ref. [173]. Copyright (2021) Royal Society of Chemistry.

5.1.2. Smart transparent window

Since Cui et al. [53] pioneered the concept of a transparent air filter for windows, the optical transmittance of filters in certain applications has been of great attention. Optical transparency is the ratio of transmitted luminous flux to incident luminous flux. It measures the degree of light transmittance through an air filter. Traditional PM filters do not have to consider light transmission, so they are usually non-transparent. To enable optical transmittance for filters in some applications, Cui et al. [53] prepared electrospun nanofibers window mesh for indoor air purification based on the natural passive ventilation principle. Since then, the high optical transparency to natural light also become a crucial evaluation parameter for the types of air filters that require light transmission [175].

For example, Cui et al. [176] reported direct blow-spinning nanofibers onto window screens, which has 80% optical transparency and over 99% filtration efficiency of PM_2.5 (Fig. 14 a). Ding's group [177] reported a multifunctional self-assembled ultralight nanonetworks air filter with a low thickness (∼350 nm), high transparency (∼95.0%), and high efficiency (>99.6%) for PM_2.5 (Fig. 14b). Lai's group [145] reported an antibacterial, moisture-resistant, and transparent nanofiber superhydrophobic filter, which has achieved more than 96% removal efficiency for PM_2.5 at optical transmittance of 86% (Fig. 14c). In recent years, Huang et al. [178] fabricated a flexible and transparent composite PVA-sodium lignosulfonate (LS) nanofibrous membrane via green electrospinning and eco-friendly thermal crosslinking method. Remarkably, the preparation process of the PVA-LS filter did not need any organic solvent during preparation. The filter has shown excellent performance with a low pressure drop of 24.5 Pa, high transparency of 78%, good filtration efficiency of 99% for PM_2.5 (Fig. 14d). Li et al. [179] developed air-permeable, transparent, anti-smog, and naturally ventilated window screens by electrospinning for PET. As shown in Fig. 14e, the PET nanofiber screen exhibited nearly the same transmittance compared with no window screen. Liu et al. [180] prepared PVDF/Fe₃O₄ composite electrospun nanofiber membrane that exhibits magnetic and electret effects. The prepared window screen could reach around 65% transparency, E _PM of 99.95%, and ΔP of 58.5 Pa for PM_0.3 filtration (Fig. 14f).

Fig. 14.

(a) Real window including metallic window screen and coating of PAN direct blow-spun fibers. Reprinted with permission from Ref. [176]. Copyright (2017) American Chemical Society. (b) Real photograph of free-standing transparent air filter. Reprinted with permission from Ref. [177]. Copyright (2019) American Chemical Society. (c) Photo of PDMS/PMMA-chitosan transparent filters at 86% transparency. Reprinted with permission from Ref. [145]. Copyright (2019) Elsevier. (d) Transmittances (%) of the PVA and PVA-LS nanofibrous membranes. Reprinted with permission from Ref. [178]. Copyright (2021) Elsevier. (e) The contrast of transparency of no window screen a) and PET nanofiber window screen b). Reprinted with permission from Ref. [179]. Copyright (2019) John Wiley and Sons. (f) The photograph of PVDF/Fe₃O₄ nanofiber anti-haze window screen. Reprinted with permission from Ref. [180]. Copyright (2020) John Wiley and Sons. (g) The photo of the air cleaner inlet is covered with the as-produced composite membrane. Reprinted with permission from Ref. [181]. Copyright (2021) Elsevier. (h) The SEM image of the fully crimp morphology of wool-like nanofibers. (i) Mass-produced NWFs and the resulting core materials of air purifiers. Reprinted with permission from Ref. [182]. Copyright (2020) Elsevier.

5.1.3. Indoor air purifier

Indoor air purifier is a common air filtration protection device for home or office use. Chen et al. [181] designed a thermoplastic TPU/PS/PA-6 multilevel structured composite nanofibrous membrane by electrospinning. Due to the combined electrostatic and physical sieving actions, the TPU/PS/PA-6 composite membrane demonstrated a filtration efficiency of more than 99.99%, the pressure drop as low as 54 Pa, and quality factor up to 0.17 Pa^-1. As displayed in Fig. 14g, they also carried out a field test by applying the produced membrane to the air inlet of an air cleaner under a maximum flow rate in a confined 30 m³ cubic chamber according to the GB/T 18,801–2018 test standard. Ding et al. [182] developed ultrafine and self-curly nano-wool felts (NWFs). Owing to the fluffy curly morphology architecture and the electret effect (Fig. 14h), the NWFs exhibited brilliant filtration performance (>99.995% for PM_0.3) and low pressure drop of 55 Pa. As shown in Fig. 14i, large-scale prepared NWFs can be applied to masks and air purifiers as the core filtration materials.

5.2. Source control – engineering facilities

5.2.1. Bag filter

Bag filter is a common dedusting facility, which is usually used to remove the PM with continuous and efficient filtration under the flue gas exhausted from combustion equipment in various industrial sites. Towards this background, Kim et al. [183] produced thermally stabile meta-aramid nanofiber by electrospinning and investigated the influence of fiber diameter on the QF from both experimental and theoretical aspects. They found that the E_PM and ΔP increased with decreasing fiber diameter, and the QF increased slightly as the fiber diameter decreased. In addition, through TGA, XRD, FE-SEM, FT-IR characterizations, as well as tensile strength measurement, they confirmed the excellent thermal resistance of the fibers, along with good chemical and mechanical stability for use as the heat resistant bag filter media. Jung et al. [184] prepared polytetrafluoroethylene (PTFE) NP-coated high-efficiency bag filters with superhydrophobic (WCA ∼ 152°) three-dimensional (3D) microporous structures via air-assisted electrospraying (AAES) technology. This approach does not require the direct use of PTFE emulsion but instead coats PTFE NPs via AAES. Compared to the traditional electrospraying method, airflow resistance in the AAES process significantly enhanced throughput. This is because the former requires both high voltage and long evaporation times, while the AAES can achieve fast evaporation at low voltage by integrating electrospraying with hydraulic air-jet liquid breakup (Fig. 15 a). Furthermore, as shown in Fig. 15b, the original bag filter of the control group had a filtration efficiency of 65.2% for 0.3–0.5 μm particles, while it was improved to 91.3% after coating with PTFE NPs. The study proposed a new way to design high-efficiency bag filters for large-scale industrial processes.

Fig. 15.

(a) The schematic diagram of AAES process. (b) The filtration efficiency (%) of coated bag filter (green line) and control group (black line). Reprinted with permission from Ref. [184]. Copyright (2021) Elsevier. (c) The schematic of an electrically energized filter. (d) Schematic illustrating the hybrid electrostatic filter composed of particulate precharger and bag filter (side view). (e) The schematic diagram of hybrid electrostatic precipitator containing bag filter (top view). Reprinted with permission from Ref. [186]. Copyright (2019) Elsevier. (f) The particulate matter measurement of real automobile exhaust with Si₃N₄ NFS filter. Reprinted with permission from Ref. [116]. Copyright (2021) John Wiley and Sons. (g) Schematic of exhaust regeneration device of gasoline engine. Reprinted with permission from Ref. [195]. Copyright (2020) Elsevier.

5.2.2. Electrostatic precipitator (ESP)

Electrostatic precipitator (ESP) is also a common dust collection device, which is generally used to remove particulate matter by corona discharge in electric power plants and the cement industry. ESP can filtrate ultrafine particles efficiently and produce only a low pressure drop. Its main principle is the use of a high-voltage electric field to charge the particles in the air and then quickly collect the charged particulates through an oppositely charged plate [185]. Jaworek et al. [186] summarized and investigated three various types of hybrid electrostatic filtration systems: (1) electrically energized filter (bag filters assisted with electric field), in which filtration fibers are charged via an external electric field; (2) hybrid electrostatic filters (bag filters with particulate pre-charging), which include electrostatic particle charging system and subsequent fibrous/bag filter; (3) hybrid electrostatic precipitators (electrostatic precipitators combined with bag filters), which comprises a conventional electrostatic precipitator used for coarse particles and a subsequent bag filter for the filtration of fine particles (Fig. 15c–e). Gao et al. [187] studied the effect of the dust layer thickness and resistivity on space electric strength, particle charging and migration at various temperatures. Results indicated that high resistivity and dust layer thickness at a high temperature can adversely affect space electric strength, particle charging and migration. In addition, electrodes (including collecting electrode and discharge electrode) were found to be crucial to the removal efficiency of an electrostatic precipitator. Zhou et al. [188] developed a multi-physical field model by COMSOL Multiphysics software to investigate the impact of six different shapes collecting electrodes on the ionic wind, re-entrainment effect, and collection efficiency. The results indicate that the shape of the collecting plates has a great effect on electric field intensity in the channel and ionic wind induced by corona discharge, which further influences the capability of ESP. Among various shapes, the triangular collecting electrode displayed the highest collection efficiency for particulate matter of different sizes. In the same year, Yook et al. [189] researched the influence of pipe-and-spike discharge electrode shape on the collection efficiency of ESP. Through the near orthogonal array method to optimize geometric parameters, longer wire length and greater distance between wires were found to be able to enhance the collection efficiency when the space of collecting plates and the number of discharge electrode wires were fixed. The highest collection efficiency was obtained when the bending angle of the wire was 160°.

5.2.3. Gasoline particulate filter (GPF)

Gasoline particulate filters (GPF) and diesel particulate filters (DPF) are designed to control the particulate matter emission effectively from direct injection engines in automobiles [190,191]. With the introduction of more and more stringent vehicle exhaust emission regulations in the United States, Europe, China, and other regions, GPF will gradually become a standardized configuration to meet the increasingly stringent relevant standards [192,193]. Since GPF is exposed to high temperature and corrosive harsh environment for a long time, the materials used must be resistant to high temperature, corrosion, and high mechanical strength [193]. Currently, the most commonly used materials in practical applications are SiC and cordierite. In recent years, Li et al. [116] prepared a thermally stable (1000 °C) Si₃N₄ NFS, to be embedded in a device as a filter in order to filtrate particulate matter from vehicle exhaust (Fig. 15f). When the automobile started, the exhaust flow rate was 4.24 m s⁻¹ and the filtration efficiency of various sized PM_2.5/PM₁₀ was greater than 99.89% (Fig. 15g). The material is expected to be applied to GPF in the future. In addition, regeneration performance is an important index for GPF. When the PM is excessively deposited in the GPF, the pressure drop will increase rapidly and the performance of the gasoline engine will also be severely degraded [194]. At this time, active or passive regeneration is generally used to continuously oxidize and burn the deposited PM to restore it. Recently, Meng et al. [194] studied the effects of ash particle size and loading on the particulate emission characteristics during the regeneration process of GPF. Zuo et al. [195] studied the influence of exhaust parameters on the particulate oxidation rate and regeneration performance of catalytic gasoline particulate filter (CGPF) in equilibrium state by the simulation software Fluent. As shown in Fig. 15g, the gasoline engine exhaust regeneration device consists of a catalytic converter and a CGPF. Zuo et al. [196] also conducted a series of research in the CGPF regeneration system with electric heating. They compared and analyzed the regeneration performance differences between the new CGPF and the traditional CGPF. The comparison results showed that the new CGPF with electric heating can significantly improve the regeneration performance with a small increase in pressure drop. These studies provided important references for improving the regeneration performance of GPFs.

6. Conclusions and outlook

Particulate matter, as a serious air pollutant, can not only form the haze that covers the sky and affect the regional environment and climate but also readily carries bacteria or viruses and penetrate deep into the lungs of living organisms [[12], [13], [14]]. Thus, particulate pollution has aroused wide concern from the government and public, and high-efficiency filtration of PM is extremely urgent. In this review, we firstly introduce the basic theory of PM filtration and then summarize advanced filtration materials, various related functions, and effort in application in recent years. Considering that some emerging infectious diseases pandemic and comfortableness of PM filters, antibacterial property, and special wettability are described along with PM filtration efficiency. It is worth mentioning that there have been no reports to cover the progress from application perspectives, and this review has initiated an effort to fill up this gap.

Although great progress has been made on particulate matter filtration, there are still a variety of challenges to overcome. First, classic solution electrospinning technology commonly used in the preparation of nanofiber filtration membranes is not environment friendly and a large amount of toxic organic solvents will be volatilized during the electrospinning process, which may pose a potential threat to physical health and natural environment [197,198]. The use of high-voltage power supplies will cause a large amount of energy consumption, and the electrospinning speed is slow, making it difficult to achieve large-scale rapid production. Second, as disposable masks become a part of our lives, the disposal of large volumes of wasted disposable filter media will become a heavy environmental burden. Third, existing respirators still suffer from insufficient comforts, such as bad breathability, poor thermal comfort, damp sensation, and indentation when worn for long periods. Fourth, in the context of the COVID-19 pandemic, although many related reports on antibacterial, antiviral and its filtration have been introduced in this review, these technologies are not yet mature enough, and there are still many difficulties in practical application.

Therefore, based on the existing development and analysis of the advantages and disadvantages, future research should be focused on the following aspects. First, water-soluble electrospinning and solvent-free melt electrospinning will make a big splash in green and non-toxic production of nanofiber filtration membranes, because they abandon the traditional volatile organic solvents [198]. The multi-needle and needleless electrospinning will accelerate the large-scale rapid production of electrospun nanofiber filtration membranes. Second, more attention will be paid to biodegradable materials and the reusability of filters. Exploring home-based air filter regeneration standardized disinfection methods and tools may be a good way to reduce wasted disposable filter media. Third, not only the filtration efficiency of the respirator, but also the pressure drop and air permeability needs to be considered. The special wettability, high water vapor transmission rate, and good heat dissipation are also critical to the comfort of a personal protective respirator. Fourth, due to highly infectious diseases pandemic such as SARS, MERS, and COVID-19, personal protective equipment which can filtrate bacteria and viruses, as well as possess antibacterial and antiviral properties may become a greatly significant research direction. In particular, the establishment of a more effective and safer standardized method for the simulated filtration test of the COVID-19 is extremely beneficial for the further research of advanced filtration materials when we encounter future pandemics. Finally, multifunctional integrated filtration devices that can remove multiple air pollutants at the same time may be of interest, such as denitrification, desulfurization, VOCs removal. Despite many emerging research advances that have been reviewed here, more efforts should be made into this field in terms of advanced materials and device design to satisfy the diversified demands.

Conflict of interest

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.