Nanofibers functionalized with surfactants to Eliminate SARS-CoV-2 and other airborne pathogens

The recent SARS-CoV-2 pandemic brought to light the difficulty in controlling the pathogenic bioaerosols present in the air. So, several studies have sought efficient and mainly sustainable technologies to develop new filtering media and new biocidal and virucidal agents. Filter media composed of nanofibers stand out for having high collection efficiencies and high permeability. For this reason, they have been widely used in filters for indoor environments and in face masks. Combined with nanofibers or conventional filtering media, the addition of quaternary ammonium surfactants to provide biocidal action proves to be an ecologically sustainable alternative. Thus, the present work reviews these filtering mechanisms, their applications, and perspectives for novel uses of these technologies in engineering and materials science.


INTRODUCTION
Today, people spend around 90% of their time indoors, turning themselves into an "indoor generation" (EU, 2003). In this context, airborne diseases become an important issue, especially for crowded places like hospitals, universities, or public transport (BALAGNA et al., 2020). Even in outdoor areas, the possibility of virus spread remains.
One evidence is the event on 26 of September at the Rose Garden of the White House. At least eleven people were infected with the new coronavirus, including the President of the United States (MANDAVILLI; TULLY, 2020). So, protection against air contamination is crucial, improving the indoor infrastructure of buildings to preserve patients, health workers, caregivers, and people in general.
In the SARS pandemic of 2003, two of the Metropole Hotel guests in Kowloon, Hong Kong, spread the SARS-CoV-1 virus to 23 other people, from the 9th floor to the 7th floor of the building. Those people recently infected traveled to other countries spreading the virus worldwide (HANAGE, 2017). At the start of the SARS-CoV-2 pandemic, various cruise ships with thousands of people on board imposed the passengers' isolation in their cabins, like the Diamond Princess cruise ship in Yokohama . Their objective was to limit the contact and proceed with adequate hygiene methods. But, even with the isolation, many were still being infected with the new coronavirus. Morawaska & Cao (2020) suggested that the ventilation system was responsible for the virus' continuous spread between the cabins (MORAWSKA; . The evidence of airborne transmission of diseases, proposed since the SARS pandemic in (YU et al., 2004, has become more robust in the recent pandemic (GREENHALGH et al., 2021). Studies show that SARS-CoV-2 can remain in aerosol particles for longer than 3 hours ( VAN DOREMALEN et al., 2020). It was also detected in the air and ventilation systems of houses (SHANKAR et al., 2022), hospitals, nursing homes, and exhaustion of ferryboats (MOUCHTOURI et al., 2020).
One of the most applied methods for removing aerosols from the air is the filtration process (ALIABADI, 2017). In the cases mentioned previously, adequate air filters can minimize the disease's spread, catching and killing the virions traveling through ventilation systems. Recently, air filtration with antiviral activity has received more attention, preventing virus spread through the air. The benefits are the rapid inactivation, minimizing the number of active virions blowing off the filter media (PYANKOV et al., 2008).
New strains of SARS-CoV-2 have been surging since the beginning of the COVID-19 pandemic. Variants like the United Kingdom strain (IACOBUCCI, 2021), the South Africa strain (TEGALLY et al., 2021), the Brazilian strain (FARIA et al., 2021), the Indian strain (ALAI et al., 2021), and the later omicron strain (XU et al., 2022) has been motive of preoccupation. New strains showed stronger interactions with the binding receptors of the host cell, resulting in higher infectivity (DASH et al., 2021). They spread worldwide, reinflating the pandemic by causing new infection waves (RENDANA; IDRIS, 2021). So, the necessity for materials that can eliminate such bioaerosols during the traveling process is urgent.

AIRBORNE PATHOGENS SPREAD MECHANISM
The new Coronavirus (SARS-CoV-2) has 60 to 140 nm in diameter and spikes about 9 to 12 nm (ZHU, N. et al., 2020). It possesses improvements on its spikes, making possible better affixation of the virions in carrier particles (LEUNG; SUN, 2020). After being expelled from an infected person, its virions can be aerosolized by droplets or attach to solid particles (LEUNG; SUN, 2020). It was expected that the particles with an attached virion are too heavy to remain in the air and then descend to the ground (STADNYTSKYI et al., 2020). However, after being expelled by the body, droplets start to evaporate their liquid portion and shrink (BOUROUIBA et al., 2014;HOWARD et al., 2021;NICAS et al., 2005). Some become smaller, being more influenced by the airflow than gravity, making the droplet travel long distances (AYDIN et al., 2020;MORAWSKA et al., 2009), reaching distances greater than 3 m from its source (FENG et al., 2020). Some viruses can remain infective even after hours, especially in higher relative humidity (OSWIN et al., 2021).
During the flight, the airborne virions can be transferred to ambient aerosols. The excellent ability to attach makes it possible for the virions to adhere to small solid particles, resulting in aerosols in the proportion of the virion size (60 to 140 nm). Minor combined aerosols (i.e., virion and particle) will reach longer distances (MORAWSKA; . This phenomenon is similar to fine particulate pollution suspended in the air. As finer as the particles, they can travel more distance (LEUNG; SUN, 2020), is already detected in the air near infected people (SHANKAR et al., 2022). This effect explains how the SARS-CoV-2 spread, primarily indoor , and can remain in the air for periods longer than 60 min, as shown studies with MERS coronavirus (PYANKOV et al., 2018).
Both aerosols and larger droplets can carry viruses (BOUROUIBA, 2020;GRALTON et al., 2011). During talking, breathing, and coughing, infected people, can generate a significant amount of submicron particles, including many viruses (RENGASAMY et al., 2010). Studies show that 87% of exhaled particles of patients infected with influenza were minor than 1000 nm in diameter (FABIAN et al., 2008). The number of particles emitted is so high that another person's inhalation is unavoidable, even when wearing surgical masks (CHENG et al., 2021). Particle size and concentration vary from one study to another and are summarized in table 1. Observations have found that aerosol particles with less than 5 μm of diameter contain more viruses than larger droplets (FENNELLY, 2020;. A possible explanation is that smaller aerosols are produced in the lower respiratory tract that has a higher viral load during infections (BAKE et al., 2019;). Unfortunately, particulate matter minor than 2.5 μm of diameter (PM2.5) can penetrate the respiratory system, reaching the alveoli and even the cardiovascular system (CHENG et al., 2021;NEUPANE et al., 2019).

FACE MASKS
Face masks are an item used for personal protection, and they have been used for medical purposes since the 17 th century (GOH et al., 2020). In 1900 their use was diffused, aiming to reduce nasal and oral bacteria in surgical procedures (BELKIN, 1996).
In the last decade, the use of masks on prevention of bioaerosol gained attention again, Ebola (OSTERHOLM et al., 2015). With the recent pandemic of SARS-CoV-2, the mask usage has become essential, leading researchers to expand studies on the field (GANCZAK et al., 2021;IPPOLITO et al., 2020;SELVARANJAN et al., 2021;TABATABAEIZADEH, 2021). The area's cutting-edge technology developed new face masks with lyophilized CRISPR sensors capable of detecting pathogens as SARS-CoV-2 in situ (NGUYEN et al., 2021).
During a pandemic, health care workers' protection relies on personal protective equipment (LEPELLETIER et al., 2020), and facemasks of high performance such as N95 and FFP2 models have already proved their efficacy against the SARS-CoV-2 and other pathogens (CHENG et al., 2021;GRINSHPUN et al., 2009;LINDSLEY et al., 2021). Unfortunately, this material is insufficient to attend to the high world demand and needs to be directed to health care workers (IPPOLITO et al., 2020;THAPER et al., 2021).
The use of masks becomes a security symbol in a pandemic (GOH et al., 2020) but can be a false sense of protection. Shortages in supply force the population to use more accessible masks, such as surgical and cloth masks. Household materials are unsuitable for retaining particles, and cloth masks were not designed for respiratory safety (RENGASAMY et al., 2010). They have a wide range of filtration efficiency, varying from one material to another (MORAIS et al., 2021). The utilization and moisture retention caused by the cloth materials' physical properties can be potential factors that increase infection rates (MACINTYRE et al., 2015). Washing cloth masks also stretch the fabric, altering the pore size and consequently decreasing the filtering efficiency of the material (NEUPANE et al., 2019). Even high-performance masks during cleaning procedures can lose their capability to retain particles, becoming unsuitable to use (OU et al., 2020).
Especially in cloth masks, small aerosols can penetrate through the mat pores as projectiles or carry by the airflow, specially droplets in the range between 0.3 to 2 µm (KÄHLER; HAIN, 2020). Droplets with enough momentum can surpass the barrier of the fabric pores (as described in figure 1). Shear stress and surface tension can force the droplet to squeeze through the interfiber spaces (AYDIN et al., 2020), and they may reach the respiratory tract (LÖNDAHL et al., 2007). Viruses carried by the liquid particle can remain viable on the surface mask, remaining on the retentate portion of the droplet (OSTERHOLM et al., 2015).
Even though the efficiency of masks has pros and cons, the general usage by the population has already proved to diminish virus transmission (BROOKS et al., 2020;CHIRIZZI et al., 2021;CHU et al., 2020;TABATABAEIZADEH, 2021). For example, tests with cloth masks against particles emitted by diesel combustion (ranging between 30 -500 nm) obtained efficiencies between 15 to 57 % (SHAKYA et al., 2017), so still more effective than no masks at all. A possible way to improve the filtration efficiency of cloth masks is to use multilayers, imposing additional barriers to the penetration of the particles (AYDIN et al., 2020;CRILLEY et al., 2021;KONDA et al., 2020;NICOSIA et al., 2015). But cloth masks still not being suitable for respiratory protection. The goal is to functionalize cloth materials with a biocidal material that can remain on the fiber surface and eliminate the pathogen (i.e., viruses, bacteria, and fungi) without being dangerous to the wearer (DE ALMEIDA et al., 2021).

RETENTION MECHANISMS ONTO FIBER FILTER MEDIA
The principal mechanism of collecting aerosols by face masks and air filters is the retention onto micro or nanofibers. Nanofibers are a class of materials extensively used in the last decade as air filter media due to the low-pressure drops (SAMBAER et al., 2012;SUNDARRAJAN et al., 2014). Its channels are commonly sinuous and interconnected, granting low air resistance and high filtration efficiency, essential for air filtering . Many industries employ these materials in their air filters to clean large amounts of air 78, even in medical applications (CHANG et al., 2020;KRAVTSOV et al., 2000). Commercial filters typically use microfibers with low air resistance and limited fine particle removal efficiency (WANG et al., 2016). It can be a problem since small particles frequently carry compounds due to their large surface area (DIEME et al., 2012;KENDALL et al., 2004).
Particles and aerosols can be classified accordingly with the Particulate Matter (PM) diameter range, being PM 0.1 (< 0.1 µm), or ultrafine; PM 2.5 (0.1 -2.5 µm), or fine; and PM 10 (2.5 -10 µm) or course (KADAM et al., 2016). Since the SARS-CoV-2 has an average diameter ranging from 60 to 140 nm, it can be defined as ultrafine (PM 0.1) or fine (PM 2.5) particles. Common air filtering methods can effectively remove PM 2.5 (XIONG et al., 2017). However, the typical sieving process does not collect such particles onto filters. Fibers in the nanometric scale exhibit properties that enhance filtration efficiency, such as high surface energy and enhanced surface reactivity (MATULEVICIUS et al., 2014;THAKUR, 2014;XIA et al., 2018). The PM's removal process by filters involves a dynamic adsorption and desorption process, and stronger interactions increase the filtering efficiency (ZHU, M. et al., 2020). The particles retention occurs due to three distinct mechanisms: the flow hydrodynamics that passes through a single fiber; small particulates stochastic movement; and electrostatic mechanism caused by a charge in the particles or fibers. The retention mechanisms in the filtration (figure 2) can be classified according to as: Particles are removed from the streamline and collide onto a retention site.
Direct interception: particles that find a retention site along the fluid trajectory.
Inertial impaction: particles with mass greater than the carrier fluid have difficulties following the streamlines. Their trajectory "thrown" them away into a retention site.
Diffusion: small particles do not follow a fluid streamline but diffuse across the fiber mat.
In this process, they may reach a retention site. Brownian motion is higher in smaller particles and decreases with the increment in the fluid velocity.
Gravity: at low velocities, particles with a different density than the fluid can deviate from the streamlines to a retention site. Fluids with higher viscosity and velocity reduce the gravitational effect, while heavier particles are more subjected to it.
Electrostatic Attraction: particles that possess dipoles or different charges from the filter fibers can be attracted to their surfaces.
Hydrodynamic Effects: due to the non-uniformity of the flow field, nonspherical particles tend to migrate to the outside of a fluid streamline and may enter in a retention site.
One or another mechanism can dominate over the others, depending on the particle size (KRAVTSOV et al., 2000). Aerosols in the range of 1 μm to 10 μm are more influenced by hydrodynamic effect (ballistic energy) or gravity forces. In sizes from 100 nm to 1 μm, the predominant mechanism becomes diffusion caused by Brownian motion.
Diffusion is the primary filtration phenomenon for the SARS family due to its tiny diameter (DAS, O. et al., 2020). The mechanical capture by interception is also a relevant effect (LEUNG; SUN, 2020). Nanometer-sized particles (less than 100 nm) can easily slide through the pores on the mat, so electrostatic attraction predominates over other phenomena. Low mass particles are attracted to the fibers and then bounded by the electric field (KONDA et al., 2020).

NANOFIBERS IN FILTRATION PROCESS
Nanofibers are versatile materials that can be successfully used as high-efficiency filtration membranes, capable of capturing organic and inorganic particulate materials, including viruses and microorganisms (e.g. fungi, bacteria).

Manufacture:
There are many techniques to produce micro and nanofibers, such as meltspinning and laser spinning (TOMISAWA et al., 2017). Electrospinning stands out among those because of its versatility and the applicability of different materials, generating fibers with controllable morphology . The process is described in figure 3. Spider-net structures ranged between 9-28 nm and 7-15 nm in height and width, respectively. To test the performance of nanofibers against nanoparticles, Leung & Sun (2020) used sodium chloride aerosols ranging between 50-500 nm to simulate SARS-CoV-2.

SURFACTANTS AND THEIR BIOCIDAL ACTIVITIES
Surfactants are common substances found in a series of different applications. It is present in various products such as toothpaste, mouthwash, shampoos, and detergents (HOWETT et al., 1999). Typically, surfactants are not harmful to the skin and mucous (PIRET et al., 2002) and may exhibit microbiocidal activity (FALK, 2019).
The word "surfactant" is an abbreviation of "surface active agent" because they are molecules able to diminish liquids' surface tension. Surfactants are a class of amphiphilic organic substances, having a hydrophobic group in one part of the molecule and a hydrophilic group in the other, usually called tails and heads, respectively (DAVIDOVITS, 2019). Surfactants can modify particle-surface interactions and provide a steric barrier to contact. Adsorption of their molecules can alter a series of interfacial properties such as van der Waals forces, electrostatic attraction and, hydrophobicity (FREE, 2016). The intracellular matrix of bacteria is destabilized by the ion exchange of QAC with Ca 2+ and Mg 2+ from the cytoplasmatic membrane (TAVAKOLIAN et al., 2020). The intracellular material is leaked, leading to proteins and nucleic acid degradation. As a consequence, the cell suffers lysis and death (MCDONNELL, 2007). Fungi and yeasts can also be affected by cationic surfactants' antimicrobial activity (BONVILA et al., 2008), reducing the number of fungal spores (KOZIRÓG et al., 2019).
Surfactants act by dispersing the fungi aggregates, influencing their surface structure, and changing the fugal surface's physicochemical properties of the adsorption process (HAMZAH et al., 2018). The hydrophobic portion of the surfactant coats the fungal surface, also hydrophobic. The other part of the surfactant molecule is exposed to the ambient, giving hydrophilic properties to the fungus outer layer (WU; PENDLETON, 2001). This effect, combined with the electrostatic adsorption phenomenon at the water/membrane interface, disrupts the fungi surface structure , causing its death.
Some surfactants also present antiviral properties, and their presence decreases viruses' survivability (CHATTOPADHYAY et al., 2002). The inactivation of viruses under surfactants was already studied with HIV (human immunodeficiency virus) (SOUSA et al., 2019), Ebola (CHEPURNOV et al., 2003), and H1N1 flu (NUMATA et al., 2020), for example. They have shown a good inhibition effect, acting against enveloped and nonenveloped viruses. The mechanism involves the denaturation of proteins in the viral capsid and dissociates the viral envelope (SOUSA et al., 2019).
Surfactants can also denature and unfold monomeric and subunit proteins (HOWETT et al., 1999) and form micelles around the capsid or membrane, enclosing the virus (PRAMOD et al., 2020). They also diminish viruses' sorption and consequently increase their mobility (CHATTOPADHYAY et al., 2002).
Surfactants also have a role in the recent pandemic and can be a good option against the SARS-CoV-2 virus. The inactivation by surfactants occurs through damage in their spikes (STURMAN et al., 1990). Surfactants can also act in situ by reaching cells and extracellular fluids interfering in one or more steps of viral replication (PRAMOD et al., 2020). The human body has natural surfactants working on defense against the new coronavirus-2019. Our systems produce a protein surfactants monolayer to control the interface air-epithelium of lung alveoli (LETH-LARSEN et al., 2007). The objective is to reduce surface tension at the end of expiration, avoiding the lung alveoli's collapse (HAWGOOD;CLEMENTS, 1990). SARS-CoV-2 causes the inhibition of those natural surfactants (TAKANO, 2020), exposing the alveoli during the inhalation-expiration process.
The study performed by de Almeida et al. (2020) aimed to evaluate the filtration efficiency of cellulose acetate with cetyl pyridinium bromide, a cationic surfactant (and a quaternary ammonia salt). The authors found about 99 % efficiency for small particles for low surface air velocity, ranging from 7 to 300 nm. They also suggest that these nanofibers present biocide action (ALMEIDA et al., 2020). Another study conducted by Jeong & co-workers (2007) prepared polyurethane cationomer nanofibers (PUC), using a mixture of QAC on its non-woven mats for antimicrobial nanofilter applications. They tested the sample's antimicrobial activity with low content of quaternary ammonium compounds against S. aureus and E. coli. The results showed a reduction of colonies in 99.9% after 24 h incubation (JEONG et al., 2007). They also observed that the fiber diameter of electrospun PUC decreases with increasing the quaternary ammonium group content.  produced a nanofiber mat of PVA (polyvinyl alcohol), functionalizing with a quaternary ammonium salt and zwitterionic sulfopropylbetaine.
They observed that only 0.5% of surfactant applied to the fibers could achieve 99.9% antimicrobial activity against S. aureus and E. coli . It was much lower than the amount used in a previous research group study using cotton textiles (ZHANG, S. et al., 2018). This result is attributed to the higher surface area of PVA mats, and the bond between the quaternary ammonium salt and the PVA fiber is covalent instead of physical. The stronger bond avoids a loss of biocide agents and improves its durability. A different study also used sulfopropilbetaine (SSPB) for biocidal purposes.
They functionalized cotton fabrics, observing that the addition of SSPB also improves the fabrics' mechanical resistance (CHEN et al., 2011).
The presence of QAC in a membrane can also change the surface properties for other purposes. Researchers have functionalized a PVDF membrane with polydopamine and polyethylenimine to synthesize an outer layer of hydrophilic nanoparticles in situ.
Then, the immobilized QAC on the silica layer produces an antibacterial layer. They observed that after adding QAC, the membrane gains biocidal activity. It also prevents biofouling, the adhesion of microorganisms on its surface, avoiding bacterial and fungal growth. They also tested the system's durability to wastewater treatment, obtaining 85% elimination for Gram-positive and Gram-negative bacteria after washing the filter for 9 hours .

HVAC'S & VENTILATION SYSTEMS
The recent pandemic has cast doubt on the reliability of ventilation and HVAC systems due to the mechanisms of the disease's spread through airborne droplets. Air conditioning could transfer respiratory droplets containing the virus to persons standing against the airflow direction . Those droplets can also travel inside the ventilation systems, disseminating the virus by being reinserted in the room, as already proved by modeling systems (SHAO et al., 2021). There are proposed three routes of contamination (CORREIA et al., 2020): • Through air recirculation in the buildings; • Through the ventilation exhaustion; • Through air confinement, without renewing the ambient with fresh air.
There are many suggestions to avoid those problems, such as improving ventilation rates, adopting natural ventilation and, personalized ventilation and exhaustion systems for micro-environments (QIAN; ZHENG, 2018). The pivotal advice is to increase outdoor air in ventilation systems and keep the system running day and night  There are other roles that ventilation systems can play against pathogens. When the airflow passage is blocked, there is a risk of increasing the airborne pathogens' concentration and growth, increasing the chances of airborne pathogens' transmissibility (GILKESON et al., 2013). Ventilation systems can withdraw respiratory droplets from the room three times faster than natural ventilation (LYNCH; GORING, 2020), removing exhaled virus-laden air and diminishing the virus concentration (CORREIA et al., 2020;. They are also effective in eliminating airborne bacterial and fungal spores (KUJUNDZIC et al., 2006). Air cleaners also reduce aerosolized virus concentration, especially when adopted alongside natural ventilation .
A possible improvement to ventilation, heating, and HVAC systems is air filters. HEPA's implementation can help control the spreading in facilities with common ducts (SHAKOOR et al., 2015). Air viral particles ' extraction was already observed in an airplane with HEPA in its ventilation systems (MAZUMDAR; CHEN, 2009). Air recirculation also reduces infection risk in areas sharing the same central HVAC system (CORREIA et al., 2020).

CONCLUSION
Due to the growing number of studies under development in recent years, we believe that soon there will be a range of innovative products composed of specialized materials as filter media. Incorporating biocidal and virucidal compounds in commercial filters/textiles and filtering nanofibers is an effective alternative to keep indoor environments safe from nanoparticles and pathogenic organisms. Surfactants are ecofriendly compounds, unlike the metallic nanoparticles widely used today. Thus, being a compound not harmful to human health, its use will occur through its incorporation into microcapsules with controlled release in textiles and nanofibers for masks and various articles for health professionals. Also, considering the control of indoor air pollution, its addition in solution for producing electrospinning nanofibers has already proven effective in inactivating several pathogens, such as viruses and bacteria.

ACKNOWLEDGMENTS
The authors would like to acknowledge the Coordination for the Improvement of