Inanimate surfaces and disinfection methods

Summary

Viral epidemics develop due to the emergence of new variants of infectious viruses. The lack of effective antiviral treatments for new viral infections, coupled with the rapid spread of the infection in the community, often results in huge human and financial losses. Viral transmission can occur through close person-to-person contact or contact with a contaminated surface. Therefore, careful disinfection or sanitization is essential to reduce viral spread. There are numerous disinfectant/sanitizing agents/biocidal agents that can inactivate viruses, but their effectiveness depends on many factors such as agent concentration, reaction time, temperature and organic load. Advances in nanotechnology are of great importance in the inactivation of viruses and in the control of the COVID-19 epidemic.

Key words/phrases: disinfection, nanotechnology, COVID-19

1. Introduction

Viral transmissions and infections have historically posed serious threats to human health and well-being and led to widespread socioeconomic disruptions. A century ago, in 1918, the “Spanish flu” pandemic caused a worldwide catastrophe with more than 50 million deaths and 500 million infections [33]. One of the country worst affected by the Ebola epidemic in West Africa in 2014, Liberia’s gross domestic product (GDP) growth decreased from 8.7% in 2013 to 0.7% in 2014 [66]. The threat of a global virus pandemic could lead to greater catastrophes than at any time in human history, as viruses can spread around the world at unprecedented rates due to easier global connections and transportation, and the ease and intensity of human mobility today. Today, however, we see that a similar pandemic has undoubtedly led to more disastrous results [7]. The novel coronavirus, which causes the COVID-19 disease (SARS-CoV-2 or formerly HCoV-19), first reported by China in late 2019, has resulted in an estimated more than six million deaths at the time of this writing. The pandemic is still not over, and new cases and new deaths are reported every day. While governments and states can influence the rates and range of outbreaks [2], individuals can have more important roles in limiting the spread of viruses in public and healthcare settings [47]. Human-to-human transmission of common influenza viruses and coronaviruses can occur through self-inoculation of mucous membranes in the nose, mouth, or eyes by touching contaminated dry surfaces as well as virus-laden bodily fluids [37]. Depending on the surface type and environmental conditions, viruses can remain on inanimate surfaces for 5 minutes or less to more than 28 days [22]. The use of sanitizing agents for personal care and surface disinfection is of great importance in limiting viral transmissions by inactivating viruses before they have a chance to enter the human body.

In this training unit, we provide information on the various types of sanitizing agents used in commercially available formulations with scientifically proven virucidal properties to inactivate viruses in suspension and on surfaces. We also provide information on the antiviral disinfection use of nanotechnological materials, one of the promising new developments that have been shown to inactivate viruses but have not yet reached widespread commercial use.

2. Disinfectants against viruses and general working principles

2.1. Viruses and infectivity

Viruses typically consist of a viral capsid containing nucleic acids (Fig. 1). The nucleic acid serves as template information for replication, while the capsid and its associated proteins function both to protect the nucleic acid and to bind to host cell receptors [29].

Figure 1. Types of viruses.

Viruses cannot reproduce and increase in number outside of a host cell. However, they can often survive for a long time in this condition [67]. When they encounter a suitable host cell, they will become infected and enter the host cell and hijack the cellular machinery for its own replication (Fig. 2). Viruses can infect cells, including bacterial cells, and cause a number of common diseases. This situation is exacerbated by the lack of effective treatment against many of the viruses.

Corona viruses, on the other hand, are enveloped and single-stranded RNA viruses, that is, their genetic material consists of an RNA strand and each viral particle is wrapped in a protein envelope (Fig. 3). All viruses basically follow the same path in infecting their hosts. A virus that invades a cell replicates itself using some components of that cell, and then its copies infect other cells. However, RNA viruses have a different feature. These viruses cannot correct errors that occur during RNA replication because they do not have the error correction mechanisms that cells typically use when copying DNA.

Figure 2. The four stages of the viral infection of a cell in the model.

Coronaviruses are the group of viruses with the longest genome, with 30,000 bases, among the RNA viruses. These pathogens, which lack the ability to correct errors during replication, increase the probability of making mistakes as the amount of base they copy increases. Therefore, every mistake brings with it a new mutation. Some of these mutations may also confer new properties on the virus, such as the ability to infect new cell types, or even new strains. A coronavirus consists of four structural proteins: nucleocapsid, envelope, membrane, and rod-like protrusions (thorns). Since these protrusions are called “corona”, which means crown in Latin, these viruses are called coronavirus (crown virus). The nucleocapsid contains the genetic material in a spherical structure formed by envelope and membrane proteins. The spiny protrusions identify the cells that the virus can infect and attach to the receptors in the cells.

Surfaces, including our hands, play an important role in the spread of viruses. Viruses such as poliovirus and bacteriophage show a much higher survivability when transferred by direct contact of surfaces, as opposed to viruses containing droplet aerosolization or dust. Just 5 seconds of hand and face contact is sufficient to transfer a significant portion of the virus, and viruses can then spread by touching the nasal mucosa or the conjunctiva of the eye. The chance of spread is directly related to the viral survival time on the surface, which differs significantly between different viruses. A very recent study reported that the Covid-19 coronavirus (SARS-CoV-2) can persist the longest on propylene plastic surfaces and stainless steel, and live viruses are present up to 72 hours after initial application, although at a greatly reduced viral titer [60].

Figure 3: Illustration of SARS-CoV-2 virus

2.2. Factors affecting disinfectant efficacy

The main measure of the disinfectant’s effectiveness is the reduction in the infectivity of the virus. Virus infectivity measurement is typically performed by carrier tests and suspension tests. The main parameters affecting the effectiveness of disinfectants against viruses include contact time, concentration of disinfectant and related virus.

In addition, disinfection efficiency may also be affected by environmental factors. If disinfection requires chemical reactions, such as formaldehyde, to occur, the disinfection rate will be higher at higher temperatures. In cold weather, some disinfectants may be ineffective as the disinfection rate will be extremely low. Humidity is another factor that can affect the penetration of the disinfectant into the virus. For reactions such as aldehyde disinfectants, a change in pH will also affect disinfectant effectiveness.

2.3. Factors influencing virus susceptibility

Viruses that affect inactivation by disinfection have certain unique properties. Here there are three main types of viruses with different structures, classified according to the increasing difficulty of chemical disinfectant inactivation: enveloped viruses, large non-enveloped viruses, and small non-enveloped viruses. Although there are exceptions, larger viruses are generally more sensitive to disinfectants [32]. All of the few disinfectant solutions tested are effective against the enveloped viruses Herpes Simplex Virus and Human Immunodeficiency Virus (HIV) type 1, but less effective against the small non-enveloped human coxsackie virus. Enveloped viruses contain a lipid envelope that is essential for infection, and thus interfering with the envelope can potentially reduce virus transmission. Lipophilic disinfectants can often be used to inactivate enveloped viruses. In contrast, non-enveloped viruses use a protein coat for infection, and thus inactivation often requires denaturation of unnecessary viral capsid proteins or essential replicative proteins [36]. Disinfectants that degrade proteins, such as glutaraldehyde or sodium hypochlorite, can be effective in inactivating non-enveloped viruses [32]. Electron microscopy studies show extensive structural damage to the phage, including sodium hypochlorite inactivating bacteriophage PAO1 and damage to capsid proteins. However, since viruses such as polio only maintain infectivity with RNA, the disinfectant may need to penetrate to destroy nucleic acids [32]. While the enveloped virus influenza H1N1 can be inactivated by all disinfectants tested [16], small non-enveloped noroviruses are much more difficult to inactivate and few commonly available disinfectants cannot adequately reduce infection [56].

Viruses also resist disinfection because of the cellular materials with which the viruses are associated. Viruses are normally dependent on host cells for replication, so they are often found in association with materials such as cell debris, soil, and aerosolized droplets. These are called viral aggregation protective factors and they can both reduce the penetration of the disinfectant to the virus and reduce the interaction and activity of the disinfectant agents. This has a great effect on disinfectants and requires a much higher concentration for effective disinfection. Disinfection is often associated with and dependent on cleaning processes, as primarily removal of organic material impurities may allow for a better disinfection process [14]. In addition, viruses can accumulate in the environment when exposed to disinfectants, making it difficult for disinfectants to penetrate and reach viruses [15].

COVID-19 is known to be very contagious and there are many ways of transmission. Recent studies have shown that SARS-CoV-2 spreads mainly through micro-droplets spread from person to person or by touching contaminated surfaces (Fig. 4) [58]. SARS-CoV-2 is known to have the ability to remain in aerosol form for more than 3 hours. It is also stated that depending on the surface, the human corona virus can persist for up to 9 days and at temperatures above 30 °C. In this context, the use of personal protective equipment (PPE), disinfectant and disinfectant is extremely important. The World Health Organization (WHO) recommends the use of physical and chemical factors to reduce contamination through disinfection of surfaces, especially on frequently touched surfaces such as doorknobs, tables, chairs, railings and keys, as well as mask use and hygiene personal care procedures. Different disinfecting agents have been described in the literature, including sodium hypochlorite, hydrogen peroxide, alcohols, soaps/surfactants, etc. [31].

Figure 4. Examples of virus spreading points.

3. Commercially available virucidal sanitizing agents

3.1. Alcohols

Alcohols, particularly isopropyl alcohol (also known as isopropanol and propan-2-ol) and ethyl alcohol (ethanol), can inactivate a broad spectrum of bacterial, fungal and viral activity (Figs. 5-6). These active ingredients play an important role in the healthcare industry for skin antisepsis and disinfection of small medical instruments. Although it has been shown to be effective at destroying infectious microorganisms, alcohols are not sporicidal [44] and are often combined with other major biocidal actives to increase disinfection efficacy.

Potent biocidal agents destroy viruses and bacteria by various mechanisms, such as disrupting cell structure and coagulating and/or denaturing proteins in microorganisms. Although few studies have been conducted to fully understand the biocidal activity of alcohol, it is generally believed that alcohols disrupt cell membranes and denature proteins in general [4]. Viruses and many other microorganisms are generally susceptible to this mode of action. Previous studies have reported that the incorporation of water into the biocidal system increases the effectiveness of alcohol, as water facilitates faster denaturing of proteins [44]. Additionally, the addition of water significantly increases the effectiveness of alcohols as it delays the evaporation of the alcohol and increases its exposure to viruses and bacteria.

However, the virucidal activity of alcohol is highly dependent on the concentration of active substances and the type of test viruses. The effectiveness of alcohols in inactivating viruses largely depends on the surface properties of the microorganism. Non-enveloped viruses are generally known to be more resistant to disinfectants than enveloped viruses, including alcohols.

While alcohols were effective at destroying some types of viruses, other disinfectants such as quaternary ammonium compounds (QAC), glutaraldehyde, and hydrogen peroxide quickly overshadowed its performance [44]. Therefore, disinfectants whose main active ingredients are alcohol are not generally used to disinfect critical equipment or environment in healthcare settings [44]. The use of isopropyl alcohol is also limited, as it only inactivates lipid viruses. This greatly reduces alcohol’s abilities as a broader use disinfectant. Because alcohols are flammable liquids, large amounts of alcohol will increase their risks and hazards as a disinfectant. The flash point of the higher concentration alcohol solution is lower than that of the lower concentration [4]. In addition, prolonged and repeated use of alcohol compromises the integrity of materials such as plastics and paints. Materials that are constantly exposed to alcohol may experience color change, cracking and swelling due to the effects of alcohol. Another challenge with alcohol use is that it evaporates quickly when exposed to air, thus reducing the time of contact with the virus. Maximum disinfection is difficult to achieve unless instruments have been immersed in the bath for some time.

Figure 5. Factors affecting the effectiveness of alcohol-based hand sanitizers against SARS-CoV-2.

Although alcohol’s capabilities are limited, it is still widely used in a variety of active disinfectant procedures. It is imperative to note that the role of alcohol as a disinfectant along with its other properties is still irreplaceable. Alcohols are often used in hospital as an effective disinfectant for thermometers, non-critical instruments, and non-invasive probes [44]. Non-critical surfaces of reusable medical instruments are also disinfected with alcohol. Another advantage of using alcohol as a disinfectant is that it is user-friendly. Alcohol solutions are non-staining, evaporate quickly, have low toxicity compared to other forms of disinfectant, and have a mild, acceptable odor. These features are critical in healthcare environments as they contribute to the efficiency and necessary sanitization of the system.

Figure 6. The antiviral mechanism of action of alcohol against enveloped viruses.

3.2. Surfactants

Surfactants are amphiphilic moieties that have both hydrophilic and lipophilic segments, further classified as cationic, anionic, nonionic, and zwitterionic surfactants. They are active ingredients often found in household disinfectants and detergents and have been shown to be able to inactivate viruses. Enveloped viruses such as the coronavirus family, which includes SARS-CoV-1, MERS, and the novel SARS-CoV-2 viruses, are therefore susceptible to these surfactants. However, some surfactants do not rely on dissolution of their lipid envelope to inactivate viruses.

3.2.1. Cationic surfactants (Quaternary ammonium compounds)

Quaternary ammonium compounds (QACs) form the main mass of cationic surfactants and inactivate viruses mostly by dissolving and breaking down their lipid envelope or membrane. It is reported that they retain virucidal activity better in hard water and also in the presence of anionic residues [38]. QACs are attractive because they are relatively non-toxic, colorless and odorless [19]. They are well known for inactivating enveloped viruses, but their virucidal activity is dependent on concentration, exposure time, and temperature. Effective disinfection with surfactants using QACs is best achieved using warm water and longer reaction times [30]. An advantage of using QAC-based disinfectants is their relatively high tolerance to the presence of contaminating organic matter. This is because their ability to inactivate viruses is generally not reduced by the presence of organic matter, as is seen with other common disinfectants such as alcohol and chlorine-based disinfectants.

3.2.2. Vaccines for cancer

They are some common anionic surfactants found in personal care products such as soap, shampoo, toothpaste and detergents [49].

3.2.3. Vaccines for cancer

Non-ionic surfactants are commonly used as emulsifiers. These non-ionic surfactants inactivate viruses by dissolving the viral envelope and cleaving the nucleocapsid. Zwitterionic surfactants are molecules that carry both cationic and anionic charges, but generally neutrally charged [63]. Researchers have suggested that the mechanism of disinfection by zwitterionic detergent is via viral degradation rather than solubilizing surface proteins [8]. This special ability, which inactivates viruses but preserves the biological activity of surface antigens, allows the use of zwitterionic detergent during the development of vaccines.

3.3. Oxidizing agents

Disinfectants such as sodium hypochlorite, hydrogen peroxide, and peracetic acid use their oxidizing abilities to inactivate viruses. Strong oxidizing agents are among the most effective disinfectants for small non-enveloped viruses that are difficult to disinfect, such as noroviruses [17].

3.3.1. Sodium hypochlorite

Sodium hypochlorite, the active ingredient in household bleach, is a powerful oxidizing agent. It dissolves in water to form hypochlorous acid, which can be reduced to form water and chloride anion [13]. The effectiveness of disinfection decreases with increase in pH, probably due to the decreasing proportion of the hypochlorous acid moiety present [3]. Sodium hypochlorite is fast acting and effective at low concentrations. Its effect was found to be proportional to its concentration and contact time. Sodium hypochlorite can be used for non-enveloped viruses that are difficult to disinfect, such as noroviruses.

3.3.2. Sodium dichloroisocyanurate

Compared to sodium hypochlorite, sodium dichloroisocyanurate has longer lasting disinfectant activity, is more tolerant of the presence of organic material, and has a higher overall disinfectant effectiveness.

3.3.3. Hydrogen peroxide

Hydrogen peroxide is a potent broad spectrum inactivation agent. It decomposes to form water, oxygen, and highly reactive hydroxyl free radicals that can degrade or cross-link a wide variety of biomolecules, including proteins, nucleic acids, and lipids. Hydrogen peroxide is also effective against noroviruses, although it usually requires a higher concentration than sodium hypochlorite.

3.4. Peracetic acid

Peracetic acid similarly decomposes to form highly reactive hydroxyl free radicals as well as acetic acid and oxygen [42]. Peracetic acid forms have been developed to provide greater stability and can be dissolved in situ to form the disinfectant solution.

3.5. Halogenated compounds

3.5.1. Povidone iodine

Povidone-iodine is a broad-spectrum virucidal agent. It is used in clinical applications such as sterilizing agents, surgical swabs, scrubs, and ointments for pre- and post-operative skin cleansing, as well as in everyday products such as antiseptic hand washes, mouthwashes, and mouthwashes containing lower iodophor concentrations [12]. Not suitable for use with silicone products such as povidone-iodine silicone catheters, as iodine may cause the material to degrade faster. Although generally safer and more effective at inactivating viruses than many other antiseptic agents, povidone-iodine can, with prolonged use, cause thyroid dysfunction [27] and allergic contact dermatitis requiring careful medical monitoring [61]. The origins of the broad virucidal activities of povidone iodide have not yet been fully elucidated and are likely to occur by more than one mechanism, reducing the likelihood of chance viral mutations conferring resistance. There is evidence that iodine can block the receptors of the virus responsible for binding to the host cell surface [50]. In addition, iodine can prevent the virus from spreading to other uninfected cells by inhibiting the activity of viral enzymes (eg, neuraminidase) necessary for virus release from host cells [12]. For enveloped viruses, it has also been suggested that the virus membrane can be destabilized by the reaction of iodine, membrane fatty acids with unsaturated C=C bonds.

3.5.2. Chlorhexidine digluconate

Chlorhexidine is a broad spectrum cationic bisguanide biocide found in many antiseptic products. An active ingredient in hand washes, mouthwashes and oral gels, disinfectants and preservatives, chlorhexidine generally has low irritability, good persistence on the skin, and rapid bactericidal activity. However, its activity is highly dependent on its formulation, being reduced by the presence of anionic surfactants and phospholipids as well as organic matter, including serum, and is also pH dependent. Compared to bacteria, its virucidal activity is more variable than povidone-iodine, and it is significantly less potent and slower acting. In general, chlorhexidine is ineffective against non-enveloped viruses (polio and adenoviruses), but shows variable potency to inactivate enveloped viruses

3.5.3. Chloroxylenol

Chloroxylenol is a halogenated phenolic type antiseptic. Widely used for household disinfectants, wound cleaning and disinfecting surgical equipment, it is most effective against bacteria, but its virucidal activity is variable. Despite its extensive commercial use for a long time, surprisingly little is known about its mechanism of action against both bacteria and viruses. Chloroxylenol is generally safe for external use in humans, but has been reported to cause irritant contact dermatitis and contact depigmentation [62].

3.6. Aldehydes

3.6.1. Formaldehyde

Formaldehyde is the simplest aldehyde and is a powerful high-level disinfectant with potent viral inactivation properties. Usually sold as an aqueous solution called formalin, it has been used to inactivate viruses for vaccine production [35] and scientific study [35]. As a top-level disinfectant, it can effectively and rapidly inactivate many different types of viruses, both in suspension and on surfaces, by chemically alkylating the amino and sulfhydryl groups of proteins, as well as the amino groups of nucleic acid bases of DNA and RNA [23]. However, due to its high reactivity, its use makes it harmful to health by causing irritation on exposed body surfaces (e.g. skin and eyes) [43], apart from being a mutagenic and suspected carcinogen [52]. As a result, it is subject to strict regulations regarding human exposure as a disinfectant and sterilizing agent in hospitals and healthcare facilities, except for use in a well-ventilated area, and is therefore not used as a household disinfectant.

3.6.2. Glutaraldehyde

Like formaldehyde, glutaraldehyde (or sometimes also known as glutardialdehyde) is a potent broad-spectrum disinfecting and sterilizing agent that is highly effective against many viruses after short exposure times. Although not suspected to be carcinogenic [54], it is known to cause dermatitis in the eyes, nose, and mouth, and irritation of mucous membranes. For these reasons, it is not used as a household disinfectant. Generally, metals, rubber, plastics, and lens instruments are tolerant to glutaraldehyde, but it is not recommended for use to disinfect non-critical surfaces due to its cost.

3.6.3. Ortho-phthalaldehyde (OPA)

Ortho-phthalaldehyde or 1,2-dicarboxybenzaldehyde is another high-level disinfectant. Like both formaldehyde and glutaraldehyde, its virucidal properties result from reactions that cross-link reactive protein and nucleic acid moieties. OPA has no strong detectable odor and does not irritate the skin, eyes or nasal mucosa [9]. In addition, its excellent material compatibility [1] allows it to be used as a disinfectant in many clinical settings such as endoscopes [45] and urological instruments. However, OPA can turn exposed skin gray and therefore needs to be rinsed with copious amounts of water or used with personal protective equipment (eg gloves and eye protection). For this reason, it is not widely used as a household disinfectant.

4. Nanotechnology

4.1. Nanomaterials for surface decontamination

Nanotechnology offers many opportunities for the development of more efficient and promising disinfectant systems (Fig. 7). The use of nanoparticle-based markers could enable the study of the mechanism by which viruses infect host cells. Today, studies based on nanotechnology for the development of new materials are generally on surfaces with self-cleaning properties [39]. These systems may have antimicrobial activity or slowly release chemical disinfectants, prolonging their duration of action. It may also contribute to the introduction of additional features such as responsive systems that deliver active substances in response to different stimuli, such as photothermal, electrothermal, photocatalytic, and others [10]. It is also known that some metallic nanoparticles have a broad spectrum of action against viruses and other microorganisms [11]. Rai et al. [41] conducted a literature review on the antibacterial, antifungal, and antiviral potential of metallic nanoparticles. According to the results of this study, metallic nanoparticles, especially silver nanoparticles, can be used as a potent and broad-spectrum antiviral agent with or without surface modification. However, the antiviral activity of these nanoparticles is still largely unexplored.

Today, nanotechnology has been a solution to many problems in disinfection applications. Over the past few decades, nanotechnology has emerged as a promising new technology for the synthesis of nanomaterials, which are nanometer-sized particles that exhibit antimicrobial effects due to their high surface area-to-volume ratio and unique chemical and physical properties. Many nanomaterials, such as metal nanoparticles and graphene-based nanosheets, have natural antiviral effects due to their unique physicochemical properties [53]. They generally operate by a common mechanism of action that involves direct interaction with the envelope or capsid proteins of viruses, thereby disrupting structural integrity and inhibiting infectivity. In addition, some nanomaterials may interfere with viral gene replication inside infected cells [20, 28, 18]. Further work is needed on the use of nanotechnology for more efficient disinfectant and sanitizing systems, as well as on achieving self-disinfecting surfaces to increase effectiveness for infection control and health and environmental safety.

Table 1 shows published research and patents on different systems based on nanotechnology for application as disinfectant and disinfectant for viruses.

Figure 7: Schematic representation of SARS-CoV-2 infection and the nanotechnologies tools to prevent and control COVID-19.

The virus entering into cell by the angiotensin-converting enzyme 2 (ACE2) receptor and use the host cell’s machinery to reproduce and contaminate new host cells. Nano-based materials could help in: (i) enhanced the speed and sensitivity of virus detection; (ii) help in the development of more efcient and safer treatment and vaccines and (iii) improve the safety of healthcare workers through the development of nano-based Personal Protective equipment (PPE).

Table 1. Articles and patents in the literature on nanotechnology-based disinfectants and disinfectants.

PPE	Name of the product	Application of nanomaterial	Manufacturing company
Masks
1.	Surgical Masks-ESpin Technologies	Use of nanofibres for particles removal	ESpin Technologies, Inc.-USA
2.	Defenser Series-Respirator masks	The facemask has nanoparticles of silver and copper acting as a blend with antimicrobial activity	Nexera Medical-Canada
3.	The Guardian (valve)- reusable	The valve mask has nanoparticles of silver and copper acting as a blend with antimicrobial activity	Nexera Medical-Canada
4.	The Guardian masks- reusable	The valve mask has nanoparticles of silver and copper acting as a blend with antimicrobial activity	Nexera Medical-Canada
5.	MVX Nano Mask	A self-cleaning surgical mask containing titanium and silver zeolite nanoparticles	MVX Prime Ltd
Gloves
1.	Everyday Protect Gloves L	A product containing silver nanoparticles and the active compounds thiabendazole and zinc pyrithione	Mapa Spontex- United Kingdom
2.	PADYCARE®	Product coated with silver nanoparticles with antibacterial effect	TEXAMED® GmbH-Germany
3.	Chlorhexidine wash gloves	A product containing silver nanoparticles and 2% chlorhexidine; the antibacterial effects last many hours after use	GAMA HEALTHCARE LTD.

4.2. Metal nanoparticles

Silver and its salts have a long history of use as an antiseptic and disinfectant, and their broad-spectrum biocidal properties are well known [21]. Silver nanoparticles are the most studied antiviral nanomaterial and it has been shown that bare or coated AgNPs can inhibit a wide variety of viruses [40]. It is difficult for viruses to develop resistance to this type of treatment, making it particularly attractive to those with a high rate of mutation. AgNPs have been found to be effective in both blocking virion entry from the outside of the cell and inhibiting replication inside infected cells. Overall, AgNPs are effective biocides in small doses [55], but their potential toxicity to humans is still under intense debate [26]. Modern methods have enabled the synthesis of AgNPs with well-defined shapes, particle sizes and polydispersity, which are important parameters determining their ultimate biocidal activities, biological fate and toxicity [25].

The virucidal properties of AgNPs are still largely unexplored, but initial reports are encouraging. AgNPs can inhibit viruses by a number of mechanisms, including binding to and interacting with viral surface proteins and denaturing enzymes by reacting with amino, carboxyl, imidazole, and sulfhydryl groups [6].

AgNP-containing products are increasingly appearing on the market, including clothing, dressings, ointments, and food packaging materials, whose biocidal activities are the result of the slow sustained release of silver nanomaterials [6]. However, it should be noted that, like all the disinfecting agents mentioned above, the virucidal activities of AgNPs differ from virus to virus. Furthermore, the amounts, shapes, sizes, and types of silver nanomaterials released depend on their real-world settings and applications, all of which affect their virucidal properties. Therefore, the efficacy of these AgNP-containing products against viruses in real-life environments as well as their toxicity to humans need to be carefully evaluated and studied.

Apart from AgNPs, gold nanoparticles (AuNPs) are also promising virucidal agents. AuNPs synthesized using garlic extract with an average size of 6 nm showed virucidal activity against measles virus by also binding to surface viral receptors and subsequently preventing host cell attachment and infection [34]. However, due to the cost of gold chemical precursors, AuNPs are unlikely to become inexpensive and commercially widely available disinfectant agents.

The use of metal nanomaterials to create self-disinfecting surfaces has gained attention in recent years due to the long-term persistence of viruses on contaminated surfaces. Self-disinfecting surfaces inactivate viruses that come into contact with them in situ, reducing the possibility of virus transmission through human contact with contaminated surfaces. In one design, the self-disinfecting surface was formed with photoactive metal nanocrystals that required visible light stimulation for viral inactivation. These surfaces, fabricated from CuInZn4S6 (CIZS) nanocrystals with band gaps in the visible light range, can absorb visible light and produce active oxidative species that inactivate influenza A virus by oxidizing amino acid residues presented in viral envelope proteins (Fig. 8). While highly virucidal, visible light must be present to guarantee the self-cleaning effect, thus limiting the practicality of the system.

Figure 8. Illustration of virus disinfection using the self-disinfecting surface powered by visible light.

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Test LO 2.2

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