The COVID-19 pandemic highlighted the critical need for effective face masks to curb airborne virus transmission. Global demand often exceeded supply, leading to widespread use of makeshift masks, many made from readily available materials like cotton. However, cotton's filtration efficiency is limited. SARS-CoV-2, approximately 97 nm in diameter, transmits through respiratory droplets produced during breathing, speaking, sneezing, and coughing. Even asymptomatic individuals contribute significantly to transmission. While N95 and surgical masks offer superior protection, their availability, cost, reusability, and comfort are often limiting factors. This research addresses this gap by developing a novel face covering that combines the comfort and breathability of cotton with significantly enhanced filtration capabilities. The study hypothesizes that amorphous mesoporous silica, known for its high affinity for proteins, can efficiently capture bioaerosols, including SARS-CoV-2, via adsorption to the virus's spike proteins. The chosen material, cotton, is environmentally friendly, sustainable, renewable, biodegradable, and readily available, making it an ideal base for modification.

Existing literature demonstrates the limitations of homemade cotton face coverings, often exhibiting filtration efficiencies significantly lower than surgical masks. Studies have explored various mask improvements, including functionalized surfaces to enhance pathogen capture and inactivation. These modifications incorporate antimicrobial agents, metal-based particles, photocatalytic and thermal treatments, and superhydrophilic/hydrophobic materials. Silica's established use in protein purification and liquid chromatography highlights its high affinity for proteins, a property this research leverages. Coronaviruses possess spike proteins, and the hypothesis that silica's interaction with these proteins can facilitate efficient bioaerosol capture forms the foundation of this study.

The researchers synthesized amorphous mesoporous spherical silica with a diameter of ~50 μm and pore sizes of 1 nm. A quaternary amine (QA) ligand was added to the silica surface to enhance protein binding. The QA-functionalized silica was then bonded to a cotton substrate. The particle size was chosen as a precaution to prevent silica inhalation. The study compared the filtration efficiency of this silica-coated cotton material with a blank cotton substrate and a commercially available cotton face covering. Aerosolized proteins (cytochrome c, myoglobin, ubiquitin, BSA) and inactivated SARS-CoV-2 were used in filtration efficiency tests. A portable ultrasonic nebulizer generated the aerosols, and particle size distribution was measured using laser diffraction. Filter efficiency was determined by comparing pre- and post-mask aerosol concentrations using mass spectrometry for proteins and a lateral flow assay for SARS-CoV-2. Breathability was assessed by measuring the pressure drop across the materials at various airflow rates. A separate experiment using only silica (with and without heat treatment) investigated the mechanism of protein capture. Zeta potential measurements assessed the surface charge of silica variants. SEM imaging characterized the silica-coated substrate’s morphology. Statistical analysis included two-tailed Student’s t-tests and one-way ANOVA with Tukey’s HSD post-hoc tests.

The silica-coated cotton material demonstrated significantly higher filtration efficiency compared to both the blank cotton and the commercially available cotton face covering for all tested aerosolized proteins and inactivated SARS-CoV-2. For aerosolized proteins, the silica-coated material achieved filter efficiencies above 90%, while the blank control and commercial cotton mask exhibited lower efficiencies (Figure 2). Specifically, the silica-coated material showed a 95 ± 2% filter efficiency for Cytochrome C and 94.4 ± 0.4% for SARS-CoV-2 (Table 1 and Figure 4). The blank control showed efficiencies of 78 ± 5% and 84.3 ± 3.6% respectively, while the commercial cotton mask registered just 48 ± 3% and 80.2 ± 2.1%, respectively. The experiment using silica alone indicated that surface chemistry, rather than solely physical capture, is critical for efficient protein adsorption, with QA-functionalized silica showing significantly higher capture efficiency than the heat-treated variant. Breathability tests (Figure 5) revealed that the addition of silica to the cotton substrate resulted in only a slight (6.2%) increase in pressure drop compared to the blank control, and even remained below the commercially available cotton mask (Table 2). The pressure change curves for all three materials were broadly similar at increasing airflow rates, implying similar breathability. The study noted a consistent particle size distribution of about 0.3 µm for all aerosolized solutions. The mean pore diameter of the non-functionalized silica was measured at 11.8 nm. The QA-functionalized silica exhibited a positive zeta potential of +15.6 mV in water.

The findings strongly support the hypothesis that silica-coated cotton provides a significantly improved filtration efficiency for bioaerosols, including SARS-CoV-2, while maintaining acceptable breathability. The superior performance of the silica-coated material is attributed to silica's high affinity for proteins, particularly through electrostatic interactions with the viral spike proteins. The high filtration efficiency combined with comparable breathability suggests a viable, environmentally friendly, and effective alternative to existing face coverings. The observation that surface chemistry plays a dominant role in protein capture implies further optimization potential through ligand selection or surface modification. This research contributes significantly to the development of improved personal protective equipment.

This study successfully demonstrated a novel approach to enhance the filtration efficiency of cotton face coverings using protein-adsorbing silica particles. The resulting material showed a substantial improvement in SARS-CoV-2 filtration while maintaining breathability comparable to standard cotton masks. Future research could explore different silica modifications, substrate materials, and optimization of the coating process to further improve performance and scalability.

The study used inactivated SARS-CoV-2, limiting the direct translation of findings to live virus scenarios. Further research is needed to confirm these results with live virus. The breathability tests were conducted under controlled laboratory conditions, and real-world breathability may vary due to factors like fit and environmental conditions. The sample size for some experiments was relatively small, warranting further investigation with a larger sample size. The study focused on specific proteins and SARS-CoV-2; additional testing with other viruses or biological particles could broaden the generalizability of the findings.