Electrospun Silk Fibroin/Cyclodextrin Nanofibers for Multifunctional Air Filtration

Highlights

What are the main findings?

• Electrospun nanofibers composed of silk fibroin, poly(ethylene oxide), and cyclodextrin (SF/PEO/CD) were developed as multifunctional air filters.
• The best multifunctional filtration performance of SF/PEO/CD, achieving PM_2.5 capture efficiencies greater than 90% and a VOC removal efficiency of 50%, surpassed that of conventional melt-blown PP.

What is the implication of the main findings?

• Electrospun SF/PEO/CD nanofibers consist of bio-based and biodegradable components, along with efficient multifunctional filtration (PM_2.5 and VOC), which make them promising materials for eco-friendly air filters.

Abstract

Particulate matter (PM) and volatile organic compounds (VOCs) are major air pollutants that can cause significant risks to public health. To mitigate exposure, fibrous filters have been widely utilized for air purification. In this study, we developed electrospun silk fibroin/poly (ethylene oxide)/cyclodextrin (SF/PEO/CD) nanofibers as multifunctional air filters capable of efficiently reducing PM_2.5 and degrading VOCs. The resulting SF/PEO/10CD demonstrated the best multifunctional filtration performance, achieving PM_2.5 capture efficiencies of 91.3% with a minimal pressure drop of 4 Pa and VOC removal efficiency of 50%. These characteristics highlight the potential of the SF/PEO/10CD nanofiber with effective, multifunctional properties and environmental benefits for sustainable air filtration application.

1. Introduction

Presently, air pollutants, including fine particulate matter with diameters smaller than 2.5 μm (PM_2.5), toxic gases, and volatile organic compounds (VOCs), represent some of the most significant global threats [1,2,3]. These ambient pollutants are inhaled and have been linked to various health issues. Several studies report that exposure to PM_2.5 is strongly linked to respiratory illness, pulmonary failure, asthma, coronary artery disease, Alzheimer’s disease, and multiple forms of cancer [2,4,5]. In order to mitigate the risk of individuals being exposed to harmful air pollution, fibrous air filtration systems have been extensively employed. These systems are renowned for their superior efficiency, high surface-to-volume ratio, high porosity, and cost-effectiveness [1,6,7]. However, existing fibrous air filters often have micron-scale dimensions with multiple fabrication layers, resulting in bulkiness, inevitably high airflow resistance, and elevated energy consumption [1,6]. Furthermore, conventional air filters are predominantly manufactured from synthetic materials, such as polypropylene (PP) filters and high-efficiency particle air (HEPA) filters, which are generally non-biodegradable. Consequently, these materials have the potential to persist in the environment for extended durations, posing a serious threat to ecological systems. To mitigate these issues, the creation of biodegradable nanofibrous air filters composed of natural and sustainable materials has been investigated. Natural polymers such as cellulose, chitosan, zein, and silk fibroin nanofibers can efficiently capture PM_2.5 with efficiencies greater than 90% [8]. Among natural polymers, silk fibroin (SF) has received significant attention owing to its high mechanical strength, biocompatibility, ease of processing, and wide availability [9,10,11,12]. Several research studies have indicated that SF nanofibers generated through electrospinning can effectively capture PM_2.5 with low pressure drops. For instance, Wang et al. [13] prepared SF utilizing a formic acid solution. The electrospun SF nanofibers exhibited diameters ranging between 100 and 480 nm and demonstrated the capability to remove PM_2.5 by up to 98% with a pressure drop of 98 Pa. Gao et al. [9] mixed poly(ethylene oxide) (PEO) with SF solution in order to increase viscosity, thereby facilitating the electrospinning process. The obtained SF/PEO nanofiber revealed a PM_2.5 removal efficiency of 99% with a pressure drop of 75 Pa. In another investigation, Min et al. [14] conducted the electrospinning of silk and PEO on an aluminum mesh substrate. They found that the PM_2.5 performance of the silk/PEO filter was comparable to that of the HEPA filter, but the silk/PEO showed a 50% reduction in pressure.

However, air pollutants comprise not only PM but also VOCs, harmful gases, and bioaerosols. This has driven significant interest in developing multifunctional air filters that integrate nanofibers into active components such as TiO₂, metal–organic frameworks (MOFs), and cyclodextrin (CD) to capture or degrade diverse air pollutants. The incorporation of TiO₂ not only enhances the filter’s ability to capture PM but also enables the photocatalytic degradation of VOCs when exposed to light. For instance, in our previous work, TiO₂(001) facets and Ag-TiO₂(001) facets were synthesized and subsequently electrospun with silk fibroin (SF). The 1%Ag-TiO₂ silk nanofiber demonstrated the best properties for capturing PM_2.5 (99.04 ± 1.70%), with a VOC photocatalytic degradation efficiency of 98.3% and antibacterial properties [15]. MOFs, known for their high surface area and porosity, are highly effective in absorbing gases and capturing PMs via electrostatic interaction [16]. Cyclodextrins (CDs) are cyclic oligosaccharides derived from starch, which have a torus-shaped structure. The internal cavity of CDs is hydrophobic, enabling them to form inclusion complexes with small molecules through host–guest interactions [17,18]. CDs offer both biodegradability and the ability to capture various VOCs. For example, Celebioglu et al. [18] fabricated HPβCD and HPγCD nanofibers to capture VOCs (aniline and benzene). It was found that HPβCD, which has a smaller cavity, encapsulated a higher amount of aniline and benzene compared to HPγCD. Kadam et al. [19] functionalized β-CD on a polyester viscose non-woven air filter and demonstrated that the modified filter could efficiently adsorb various VOCs, including styrene, benzene, and formaldehyde, compared to a commercial facemask. Therefore, CDs provide a unique and environmentally friendly option for enhancing nanofiber-based air filters, promoting sustainable air filtration solutions.

To the best of our knowledge, the use of electrospun silk fibroin combined with cyclodextrin for air filtrations has not been reported. In this study, we focused on the preparation of electrospun nanofibers composed of SF blended with PEO and CD. The multifunctional and eco-friendly air filter obtained (SF/PEO/CD) is designed not only to efficiently capture PM_2.5 and VOCs but also to minimize airflow resistance. The electrospun SF/PEO/CD nanofibers were characterized to assess their surface morphology, chemical composition and thermal and mechanical properties. Consequently, their PM_2.5 and VOC filtration efficiencies were investigated.

2. Materials and Methods

2.1. Materials

Silk cocoons (Bombyx mori, Saraburi yellow strain) were provided by a textile company, Sing Tor Satin Co. Ltd., which received silkworm eggs bred by the Queen Sirikit Department of Sericulture, Kanchanaburi, Thailand. Sodium carbonate (Na₂CO₃) was purchased from Kemaus, New South Wales, Australia. Lithium bromide (LiBr) was purchased from TCI, Tokyo, Japan. Poly(ethylene oxide) or PEO (MW~900,000) and β-cyclodextrin (CD) were purchased from Sigma Aldrich, Massachusetts, USA. Formaldehyde (CH₂O) was purchased from Merck, Darmstadt, Germany. Deionized (DI) water was purchased from Siam Beta, Bangkok, Thailand. Dialysis membrane with a molecular weight cutoff of 3.5 kDa (Spectra/Por) was purchased from Fischer Scientific, Waltham, MA, USA. Plastic mesh (mesh size = 1.5 × 1.5 mm²) was purchased from, Homepro, Bangkok, Thailand. Melt-blown polypropylene (PP) was obtained from a middle filter layer of a face mask (Disposable 3-layer face mask, Gaungdong Danerzheng Medical Equipment Co., Ltd., Puning, China) purchased from A.T. homemart.

2.2. Extraction of Silk Fibroin (SF)

Silk fibroin (SF) was extracted following the protocol of Kaplan et al. [20]. First, silk cocoons (5 g) were cut into small pieces and then boiled in Na₂CO₃ (0.02 M) at 100 °C for 1 h to remove sericin. The degummed silk was washed with DI water several times and dried at ambient temperature for 24 h. Next, the degummed silk was dissolved in 9.3 M LiBr solution (33 wt%) at 60 °C for 4 h to obtain silk fibroin (SF). Then, the SF solution was dialyzed against DI water for 48 h using a dialysis membrane. During dialysis, DI water was changed every 12 h to purifying SF. After that, the SF was centrifuged, and the resulting 4 wt% SF solution was stored at 4 °C in the fridge before use.

2.3. Electrospinning of SF and SF/CD

4 wt% of silk fibroin (SF) (10 mL) was mixed with 0.6 g of PEO and stirred at room temperature for 10 min. Next, 5 and 10 wt% of CD (denoted as 5CD and 10CD, respectively) were added to the SF solution to prepare SF/PEO/CD solutions. To fabricate SF nanofibers, the solution was placed in a 5 mL plastic syringe. A 20 mm stainless steel needle (18G) was used to inject the solution. The distance between the syringe tip and the plastic mesh attached aluminum sheet collector was 20 cm. During the electrospinning process, the constant injection rate and applied voltage were fixed at 1.2 mL/h and 12 kV, respectively. Finally, the obtained nanofibers were dried in a vacuum oven set at 60 °C overnight.

2.4. Characterization

The morphology of electrospun SF/PEO and SF/PEO/CD nanofibers was investigated using a scanning electron microscope (SEM, FEI-NOVA NanoSEM 450). The average diameter of the electrospun nanofibers was determined by using ImageJ (Java 1.8.0_172) software. The thickness of the fiber was measured using a thickness gauge (digimatic thickness gauge, Mitutoyo). The chemical functional groups of the nanofibers were determined using a Fourier transform infrared (FTIR) spectrometer (Thermo Scientific Nicolet 6700). The chemical state information for the composite nanofibers was characterized using X-ray photoelectron spectroscopy (XPS, Kratos, Axis Ultra DLD). The thermal properties of the samples were investigated using differential scanning calorimetry (DSC) measurements (Mettler Toledo DSC 3) with a heating rate of 10 °C/min, scanning from 25 °C to 300 °C under a N₂ atmosphere. The mechanical properties of nanofibers and melt-blown PP were determined using a universal testing machine (Hounsfield H10 KM) (grip to grip = 25 mm, test speed = 5 mm/min, pre-load = 0.1 MPa, and load cell = 50 N). The nitrogen adsorption isothermal analysis was performed using a gas adsorption instrument (Micromeritics 3Flex Surface).

2.5. PM_2.5 Removal Efficiency

The effectiveness of SF nanofibers in removing PM_2.5 was evaluated using a custom-built system (Figure 1), as described in our previous works [15,21]. The PM removal testing chamber was made from an acrylic box with a volume of 3.5 L. The SF nanofiber samples (effective area = 5 cm²) were placed into a filter holder. On one side of the testing box, Buddhist incense was burned to generate PM_2.5 particles (at a concentration of approximately 2500 μg/cm³) and circulated throughout the chamber by DC electric fans (Sunon, model MB40201V1-0000-A99, 12 V_DC). The airflow rate through the filter was 1.0 L/s·m². The concentration of PM_2.5 was dynamically monitored by digital PM_2.5 air quality detector tester meters (ExGizmo, 5Vdc) over a 20-minute monitoring duration, with real-time data recorded every 30 s The pressure drop (∆P) was measured using a differential pressure meter (Testo 510 pressure manometer). The experiments were carried out at room temperature with 80% relative humidity. The PM_2.5 removal efficiency (η), pressure drop (∆P), and quality factor (QF) were calculated by using Equations (1), (2), and (3), respectively.

η = (C₁ − C₂)/C₁

(1)

∆P = P₂ − P₁

(2)

QF = − ln(1 − η)/∆P

(3)

where C₁ and C₂ (μg/m³) are the concentrations of PM_2.5 before and after filtration, and P₁ and P₂ (Pa) are the air pressure before and after filtration.

2.6. VOC Removal Efficiency

The VOC removal efficiency of electrospun nanofibers was assessed using formaldehyde as the target VOC. A filter with an exposure area of 3 × 3 cm² was positioned in a 25 L VOC testing chamber. Subsequently, formaldehyde (1000 ppm) was injected into the chamber through a rubber septum. The formaldehyde concentration before and after absorption was monitored by the Extech formaldehyde monitor (FM3000). The data was collected in real time at 30 min intervals. The VOC removal efficiency (E) of nanofibrous filters was calculated by using Equation (4), where C₀ and C_t are the initial and final concentration of formaldehyde (in ppm), respectively.

E = (C₀ − C_t)/C₀ × 100

(4)

3. Results and Discussion

3.1. Morphological Structure of Electrospun SF Nanofibers

The electrospun SF/PEO, SF/PEO/5CD, and SF/PEO/10CD mats were successfully fabricated with a comparable thickness of 0.13–0.14 mm (as depicted in Table 1 and Figure S1). The morphology of electrospun SF fibers was observed using SEM. As shown in Figure 2a, the resulting non-woven SF/PEO mat exhibited continuous and smooth fibers, with an average fiber diameter of 325.57 ± 55.44 nm. Upon the incorporation of CD at concentrations of 5 and 10 wt% into the SF/PEO, the changes in fiber morphology were observed. The electrospun SF/PEO/CD fibers maintained a smooth appearance, with the fiber diameter increasing to 415.22 ± 49.45 and 505.54 ± 29.69 nm for 5 wt% CD and 10 wt% CD, respectively (Figure 2b,c). The increase in fiber diameter can be attributed to the addition of CD, which raises the viscosity of the polymer solution, increases resistance to flow, and reduces the stretching of the fiber. Consequently, a higher amount of CD filler results in the formation of a thicker fiber diameter [22]. Figure 2d shows the morphology of melt-blown non-woven polypropylene (PP), which is commonly used as a filter medium in face masks. The average fiber diameter of the melt-blown PP is in the micron range (1.796 ± 98.12 μm), which is significantly larger than that of the electrospun SF fibers.

To explore the surface area and pore size of electrospun SF, nitrogen adsorption isotherm analysis was performed (Figures S2 and S3). The specific surface area, calculated by the Brunauer–Emmett–Teller (BET) equation [23], and average pore size are presented in Table 1. The N₂ adsorption isotherm of all electrospun SF was a type V isotherm, which represented mesoporous materials, with average pore size of 8.08–6.56 nm. It was observed that SF/PEO exhibited the largest surface area of 8.30 m²/g, followed by SF/PEO/5CD and SF/PEO/10CD, respectively. The reduction in surface area of SF/PEO/CD could be associated with a larger fiber diameter, which reduces the surface area of the electrospun fiber [24,25].

Table 1. Fiber diameter and thickness of SF-based nanofibers and melt-blown PP.

Samples	Average Fiber Diameter (nm)	Thickness (mm)	Surface Area (m2/g)	Pore Size (nm)
SF/PEO	325.57 ± 55.44	0.13 ± 0.01	8.30	8.08
SF/PEO/5CD	415.22 ± 49.45	0.14 ± 0.02	6.51	7.51
SF/PEO/10CD	528.16 ± 6.38	0.14 ± 0.02	4.51	6.56
Melt-blown PP	1796.92 ± 98.12	0.22 ± 0.01	1.0 [26]	33,400 [27]

Table 1. Fiber diameter and thickness of SF-based nanofibers and melt-blown PP.

Samples	Average Fiber Diameter (nm)	Thickness (mm)	Surface Area (m2/g)	Pore Size (nm)
SF/PEO	325.57 ± 55.44	0.13 ± 0.01	8.30	8.08
SF/PEO/5CD	415.22 ± 49.45	0.14 ± 0.02	6.51	7.51
SF/PEO/10CD	528.16 ± 6.38	0.14 ± 0.02	4.51	6.56
Melt-blown PP	1796.92 ± 98.12	0.22 ± 0.01	1.0 [26]	33,400 [27]

3.2. Chemical Functionalities of Electrospun SF Nanofibers

The chemical functionalities of SF nanofibers were investigated using FTIR spectroscopy. As shown in Figure 3, the FTIR spectrum of SF/PEO exhibits absorption peaks at 3291, 1625, 1508, and 1095 cm⁻¹ which correspond to N-H stretching, amide I and amide II vibrations of the β-sheet, and C-O-C stretching of the silk fibroin (SF), respectively [9,28]. In addition, absorption peaks at 2878 and 1097 cm⁻¹ are observed in the SF/PEO spectrum, which correspond to C-H stretching and C-O-C stretching of PEO [9]. After mixing CD (5 and 10 wt%) with SF/PEO, the FTIR spectrum of SF/PEO/CD shows an additional peak at 1039 cm⁻¹, which can be assigned to the C-O vibration of the acetal group in CD moiety [29,30]. The FTIR result confirms that SF/PEO/CD was successfully fabricated.

Next, the chemical composition of SF/PEO and SF/PEO/CD was analyzed using X-ray photoelectron microscopy (XPS). The O1s and C1s peaks in the XPS spectrum were deconvoluted by using CasaXPS software coupled with Shirley as the baseline correction. As illustrated in Figure 4a,b, the C1s spectrum of SF/PEO and SF/PEO/CD samples can be fitted into three peaks positioned at 285.0, 286.6, and 288.1 eV, which are associated with sp³ hybridized carbon in C-C/C-H, C-O-C, and C=O bonds, respectively [21,31]. The interaction between SF/PEO and CD can be elucidated by the O1s spectra, as shown in Figure 4c. The O1s spectrum of SF/PEO can be fitted into three peaks centered at 531.5, 532.9, and 533.5 eV, which can be indexed to C=O, C-O, and O-H, respectively [21,31]. Additionally, the peak positions of each component in SF/PEO/CD remain unchanged, indicating that CD does not interact with either SF or PEO. However, the ratio of C-O (CD) to C=O (SF) in the XPS O1s spectrum of SF/PEO/CD is higher than that of SF/PEO, suggesting that the incorporation of CD led to an increase in oxygen-containing groups. This is further supported by the significant rise in the proportion of O-H in the SF/PEO/CD sample (Figure 4d), confirming that CD was successfully integrated into the SF/PEO matrix.

3.3. Thermal Analysis

The thermal properties of electrospun SF/PEO and SF/PEO/CD were assessed through DSC analysis. Figure 5 shows DSC thermograms, and the thermal properties are summarized in

Table 2. Summary of T_m and enthalpy at T_m of SF/PEO and SF/PEO/CD nanofibers.

Samples	Tm of PEO (°C)	Enthalpy at Tm of PEO (J/g)
PEO	67.4	230.0
SF/PEO	54.0	65.1
SF/PEO/5CD	51.3	44.1
SF/PEO/10CD	50.3	16.1

. Pure PEO exhibits a melting temperature (T_m) at 66.8 °C, which is consistent with values reported in the literature [32]. For the SF/PEO sample, only the T_m transition is observed at 54 °C, which is lower than that of pure PEO. Additionally, incorporating 5 and 10 %wt CD into the SF/PEO matrix resulted in a further decrease in the T_m of PEO to 51.3 and 50.3 °C, respectively. The enthalpy at T_m of PEO also significantly decreased to 44.11 and 16.05 J/g. These results suggest that the incorporation of SF and CD into the PEO matrix disrupts the crystalline structure of PEO, leading to a reduction in its melting point [33]. In addition, it should be noted that pure CD exhibits an endothermic peak at 156.7 °C, which could be attributed to the dehydration process rather than the true melting point transition [34].

3.4. Mechanical Properties

The mechanical properties of SF/PEO, SF/PEO/CD nanofibers, and melt-blown polypropylene (PP) were determined using tensile measurements. It should be noted that melt-blown PP was chosen for comparison to evaluate the performance of our nanofibers against the commercial polymeric air filter commonly used in the middle layer of a face mask. Figure 6 depicts the stress–strain curves of all samples. Their mechanical properties, including tensile strength, Young’s modulus, and elongation at break, are summarized in

Table 3. Summary of the tensile strength, elongation at break, and Young’s modulus of SF/PEO and SF/PEO/CD and melt-blown PP (n = 3, mean ± SD).

Samples	Tensile Strength (MPa)	Elongation at Break (%)	Young’s Modulus (MPa)
SF/PEO	1.40 ± 0.27	21.31 ± 1.66	40.44 ± 1.55
SF/PEO/5CD	0.40 ± 0.01	1.75 ± 0.18	29.86 ± 0.19
SF/PEO/10CD	0.36 ± 0.03	0.71 ± 0.09	44.89 ± 7.74
Melt-blown PP	1.03 ± 0.12	4.23 ± 0.28	34.74 ± 2.29

. The SF/PEO nanofiber demonstrated the highest tensile strength of 1.40 ± 0.27 MPa with the highest elongation at break of 21.31 ± 1.66%. Upon incorporating SF/PEO with CD, the mechanical properties are notably changed. As illustrated in Figure 5, there is a substantial decrease in tensile strength observed in SF/PEO/CD samples, which declines approximately 3.5-folds. In addition, SF/PEO/5CD and SF/PEO/10CD samples show a significant reduction in elongation at break when compared to SF/PEO, suggesting a transition from ductile to brittle characteristics in the SF/PEO/CD nanofibers. The reduction in mechanical strength in SF/PEO/CD could be attributed to the presence of CD, which appears to disrupt the continuity of the polymer phase within the SF/PEO matrix and reduce the polymer crystallinity, as evidenced by the DSC results. This disruption leads to altered interfacial interaction and a decrease in homogeneity in the polymer matrix [35].

It should be noted that the tensile strength of melt-blown PP is slightly lower than that of SF/PEO. According to the literature, the tensile strength of melt-blown PP varies depending on the processing conditions. For example, Pu et al. [27] fabricated melt-blown PP (hot air temperature, 255 °C; die-to-collector distance, 20 cm; screw speed, 60 rpm) and reported a tensile strength of 1.77 MPa, whereas Eticha et al. [36] produced melt-blown PP (hot air temperature, 350 °C; die-to-collector distance, 25 cm; hot air pressure, 3 bar; die temperature, 275 °C; screw speed, 12 rpm), with a tensile strength 0.3 MPa. Peng et al. [37] also reported that melt-blown PP produced with different die-to-collector distances of 20, 25, and 30 cm exhibited tensile strengths of 0.6, 0.3, and 0.15 MPa, respectively. In addition, non-woven fibers have randomly aligned structures, resulting in variations in the fiber network and bonding points of the fibers. These variations can also influence the tensile strength [27].

3.5. PM_2.5 and VOC Filtration Performance

The PM_2.5 removal efficiencies of SF/PEO, SF/PEO/CD, and melt-blown PP were tested in a sealed custom testing box. As shown in Figure 7a and Table S1, SF/PEO nanofiber exhibited the highest PM_2.5 removal efficiency of 97.16 ± 2.02% with a quality factor (QF) of 0.94 ± 0.06, while incorporating SF/PEO with CD resulted in a slight decrease in removal efficiency, with 92.93 ± 1.13% for SF/PEO/5CD, and 91.30 ± 0.52% for SF/PEO/10CD, respectively. It is known that the PM filtration mechanism of nanofibers is typically attributed to three main size-dependent mechanisms: Brownian diffusion (<0.1 μm), interception (0.1–1 μm), and inertial impaction (1–10 μm) [38,39]. PM_2.5 is primarily removed via inertial impaction, which occurs when particles carried by airflow collide directly with the nanofibers and become trapped on their surface. The greater PM_2.5 removal efficiency of SF/PEO can be described by it having the smallest fiber diameter and highest surface area. In general, the PM_2.5 removal performance improves with a decrease in fiber diameter. This is because a smaller fiber diameter results in a denser nanofiber network and smaller pore sizes. As a result, small particles have a harder time passing through nanofiber media. In addition, according to the BET results (Table 1), SF/PEO has the highest surface area (8.30 m²/g), enabling it to capture a larger amount of PM_2.5, followed by SF/PEO/5CD and SF/PEO/10CD, respectively. This evidence shows the PM_2.5 removal efficiency was also influenced by the surface area of nanofibers. Notably, it is important to highlight that the PM_2.5 filtration performance of all SF nanofibers surpasses that of melt-blown PP. This trend is similar to findings from Ullah et al. [40] and Wu et al., who reported that melt-blown PP exhibits a lower filtration efficiency than electrospun nanofibers due to its lower surface area, lower porosity, larger fiber diameter, and larger pore size in the micron range [41].

Moreover, to determine the reusability of the SF air filter, the SF/PEO was constantly used to filter incense smoke for ten cycles. As shown in Figure S5, after ten cycles of filtration, the PM removal efficiency dropped to 87.9%. The linear equation derived from the ten cycles of filtration data (Figure S5) indicates that the filtration performance of the SF air filter will decline to 50% after approximately 44 cycles, suggesting that the SF filter can be used for a long period. This suggests that SF-based nanofibers could serve as an alternative PM_2.5 filter.

Subsequently, the ability to entrap VOCs was evaluated using formaldehyde vapor. As illustrated in Figure 7b, SF/PEO/CD nanofibers demonstrated a higher amount of formaldehyde entrapment compared to SF/PFO and melt-blown PP. This can be attributed to the formation of an inclusion complex of formaldehyde within the CD cavity. SF/PEO/10CD exhibited the highest VOC removal efficiency, reaching up to 50.4 %, with a maximum pseudo-first-order adsorption rate of 0.142 min⁻¹ (Figure S6), followed by SF/PEO/5CD (40.1%). After the filtration of PM and formaldehyde, the chemical state of SF/PEO/10CD was evaluated using XPS. After filtration, the C1s spectrum of SF/PEO/CD exhibits an increased proportion of C-C and C-O (Figure S7a,b), whereas the proportion of O-H in O1s spectra was substantially decreased (Figure S8a,b). These results indicate that carbon compounds from burned incense and formaldehyde were captured by the SF/PEO/CD nanofiber.

4. Conclusions

In summary, bio-based nanofibrous silk fibroin (SF) combined with PEO and cyclodextrin (CD) was successfully fabricated using electrospinning. Among the tested formulations, SF/PEO/10CD exhibited the highest multifunctional air filtration performance, achieving removal efficiencies of 91.3% for PM_2.5 and 50% for VOCs. This performance surpassed that of conventional melt-blown PP filters. However, some limitations should be acknowledged. The incorporation of CD reduced the mechanical strength of the SF/PEO/CD nanofiber, which may limit durability under high-stress conditions. Although the SF/PEO/CD nanofiber effectively filtered PM_2.5 under high humidity (RH = 80%), the water solubility of PEO poses a challenge for long-term stability in moist environments. Therefore, incorporating a biodegradable and water-repellent layer is recommended for future development. Overall, the multifunctional capabilities of the SF/PEO/10CD nanofiber, along with its filtration efficiency and environmentally friendly composition, make them a promising material for air purification systems such as the middle layer of face masks and portable air purifiers.