Thanks to a variety of extraordinary electrical, chemical, magnetic, and antimicrobial properties as well as an extensive surface area/volume ratio, nanomaterials offer a diverse range of application in relevance to the COVID-19 pandemic. Figure 1 illustrates the biomedical applications of nanomaterials addressing different medicinal objectives such as control, prevention, diagnosis, and treatment.
Within the course of the past decade, thanks to recent technological advances, nanohybrids have been realized in new dimensionalities (0D–3D) and new compositions (especially involving the biological materials), Figure 2. Therefore, the definition of such advanced materials should be updated to include a more comprehensive view on the interplay of the constituting phases and their configuration. As a solid, ubiquitous definition, nanohybrids are multiphasic materials that show unique properties. These properties are tailored by the size (dimension), composition, and arrangement of the nanofillers. The latter could be notably influenced by the structure and characteristics of the encapsulating medium (material). In this study, we review biomedical applications of antiviral nanohybrids in relevance to COVID-19 pandemic and other viral crises in several classes of control, prevention, diagnosis, and treatment.
2.1. Nanohybrid Materials Used for Control/Prevention of Viral Transmission
SARS-COV-2 is highly enduring and may attach on the surface of objects or persistently exist in the air as aerosols. According to various reports, coronavirus can survive and stay infectious for a sufficiently long period of time raising potential danger for spreading the virus from one place to another and especially for crowded places such as aircraft cabin, restaurants, and shopping centers
[11]. In this regard, the main strategy to hamper the virus person-to-person transmission is wearing the protective equipment such as masks, gloves and protective clothing,
Figure 3a. The conventional masks include the respirator masks (N95 and P2) and the surgical (face) masks. Respirator masks are particularly made to protect individuals against high-risk medical conditions in terms of presence of viruses and bacteria. Differently, surgical masks are meant to provide a lower level of such protection
[12].
Figure 3. (
a) airborne transmission of infectious aerosol particles is notably reduced by wearing masks. Reproduced with permission
[13], Copyright 2020, Science. (
b) Operation mechanisms of fibrous face masks depending on the size of the filtrate. (
c) size (length) scale of different particles available in nature and the respective filtration means including surgical masks, N95 respirators, and electrospun fibrous filters. Reproduced with permission
[14], Copyright 2021, Elsevier.
Nanohybrid materials can be implemented as the building blocks of the above mentioned personal protective equipment (PPE) to offer hydrophobicity and antimicrobial function, while maintaining breathability. The hydrophobicity of PPE can lead to formation of an effective barrier against deposition of the airborne droplets originating from cough or sneeze
[11]. At the same time, Cu and Ag nanoparticles, for instance, can be applied on the surface of the PPE products to build up an antimicrobial barrier preventing the rapid growth of coronavirus on the surface. Moreover, the nanostructured, hybrid PPE materials, i.e., those implementing nanofibers alongside microfibers, can decrease the breathing resistance, thereby providing a comfortable wearing experience
[11].
Recently, nanofiber based nanohybrids have been developed to improve the filtering performance of PPE against pathogens
[15]. In this regard, Ag nanoparticles, CuO, iodine (I), titanium oxide (TiO
2) with antimicrobial properties can confer the masks surfaces with a disinfection capacity for viruses
[15]. The high surface to volume ratio of nanofibers can also enhance the capturing efficiency and provide some surface area- related phenomena including ion exchange and catalysis
[16]. For instance, the nanofibers functionalized with chemicals and nucleating agents (e.g., o-iodosobenzoic acid (IBA) and β-cyclodextrin (β-CD)) can decompose or deactivate the contaminant, thereby minimizing the possibility of virus and pathogen inhale
[17]. As a more sophisticated class of protective masks, filtering face piece respirator (FFPR) shows an improved performance in terms of air flow resistance and bacterial filtration efficiency. Such merits originate from co-presence of partially gelled sub-micron polypropylene (PP), PP nanofibers, and a hydrophilic bactericidal layer that can disinfect pathogens efficiently
[18].
Apart from a high antimicrobial filtration efficiency, nanofibers can also optimize the wearing experience and offer a fitter mask choice through better facial seal. As a (nano)hybrid system, meltblown fibers and spunbond fibers can be combined with nanofibers through the electrospinning technology to form a basis for a particular type of respirators
[14]. The nanofiber-based filtering facepiece respirators (FFR) can effectively capture viruses or pathogens via different mechanisms such as Brownian diffusion, because of their higher surface to volume ratio compared to that seen in the conventional masks. Moreover, the nanofiber FFR can be made thinner than N95 FFR, thus with higher air permeability and better breathability, leading to enhanced wearing experience
[19]. The main filtration mechanisms of fiber-based face masks for three classes of particles in terms of size are shown in
Figure 3b. The size range of the particles discriminated by electrospun fiber filters compared to conventional surgical masks and respirators is illustrated in
Figure 3c. The face masks are designed to efficiently block macro, micro, and nanoparticles through interception, inertial impaction, and diffusion, respectively
[12]. Particularly, inertial impaction matters most, given the size range of viruses and bacteria.
The particles smaller than 300 nm, hardly collide with the pore walls and are readily bombarded by the surrounding air molecules. To capture these particles, electrospun nanohybrid nanofibers have been proved to be very efficient. Xianhua et al.
[20] prepared a nanohybrid nanofiber mask based on Ag nanoparticles reinforced polyvinyl alcohol (PVA) nanofibers through electrospinning. The as-formed nanohybrid nanofibers were deposited on an activated carbon nonwoven fabric as the substrate. As verified in this study, the nanohybrid protective mask is able to show an excellent performance in terms of filterability, moisture permeability, and breathability. Such characteristics depend mainly on the Ag nanofiller amount, so that with the increase of filling factor, the nanohybrid nanofiber layer turns thicker and thus lowers the air permeability. On the other hand, under same conditions, the moisture permeability increases. Zhang et al.
[21] devised an antiviral nanofibrous membrane composed of vitamin k incorporated poly(vinyl alcohol-co-ethylene) that could show intensive photoactivity, thereby generating ROS under solar and UVA light. As a result of such a performance, the nanofiber membrane was able to inactivate >99.9% of viruses and thus offer a promising capacity for development of PPE and face masks.
Apart from the use of PPEs, vaccination has ever been considered as a successful prevention strategy. In this regard, nanomaterials as efficient carriers can transport the vaccine into proper cellular colonies and subcellular areas. Given the size and dimensionality, viruses are in fact living nanomaterials and the Live Attenuated Vaccines (LAVs), Inactivated Vaccines (IVs) and viral vectors inspired from viruses are nanotechnologies. Performing in the same size scale as viruses, nanoparticles can play a crucial role in the development of vaccines and thus immunoengineering
[22]. Man-made and natural nanoparticles resemble viruses in terms of structure, while biotechnology, nanochemistry, and chemical biology synergistically enable creation of advanced vaccines that will be incorporated therein.
DNA vaccine carriers: DNA vaccine’s working mechanism is based on inclusion of a protein antigen’s gene in a recombinant eukaryotic expression vector (e.g., a plasmid) and its subsequent introduction into the body to generate the exogenous antigen, driving antigen-specific immune reactions against the disease
[23]. Versus traditional vaccines, these vaccines imitate natural infections and provoke antibody secretion as well as the cell mediated immune reactions. Therefore, they hold promise for treatment of chronic viral infection and cancer. Furthermore, the DNA vaccine assures reliable gene expression in transfected host cells, thus producing enough quantity of antigen. Facile manipulation of genetic sequences through recombinant methods, large scale production and simple storage and transport (no need to cold-chain), the potential of induction of particular antigen and immunoregulatory proteins, and most importantly proven safety in animal/human clinical tests are of other advantages of the DNA vaccines
[24][25][26]. However, such vaccines show insufficient immunogenicity and due to enzymatic (nuclease) degradation and subsequent elimination through the reticular endothelial system, their in vivo half-life is quite short (minutes)
[27]. Additionally, naked DNA must be able to defeat several cellular and subcellular hurdles such as the plasma membrane to induce protein expression
[28]. Therefore, efficient transportation of DNA vaccine specifically to the desired cells in different organs and lymphoid tissues and assuring reliable, optimum gene transfection efficiency along with induction of the maturation of the dendritic cells and presentation of antigen are the main objectives towards the creation of DNA vaccines that could be properly met by introduction of new, advanced biomaterial based carriers
[29].
Different kinds of man-made nanocarriers such as polymeric nanoparticles and cationic liposomes have been synthesized for the DNA vaccine delivery through cell membranes
[30]. For instance, Zhao et al.
[31] designed a nanocarrier composed of poly (lactic-co-glycolic) acid (PLGA) coated with chitosan (CS) to deliver the DNA vaccine against Newcastle disease virus (NDV). As stated by the authors, the chitosan coated PLGA nanocarrier of the DNA vaccine assures a higher cellular, humoral, and mucosal immune response in a safer and efficient manner. Farris et al.
[32] encapsulated DNA vaccine/chitosan nanoparticle within a zein (ZN) microparticle and in fact devised a hybrid-dual particulate carrier to govern the DNA vaccine delivery. Routinely, the DNA vaccines that are delivered orally target the mucosa (i.e., intestinal mucosa) to cause the mucosal immunity. However, when the DNA vaccines pass through the gastrointestinal tract, it would be readily disintegrated at a low pH, and by endogenous nucleases and gastric enzymes. The designed hybrid-dual particulate carrier shows a more controlled DNA release behaviour in simulated gastric fluid (SGF) as compared to DNA/CS nanoparticles. Additionally, the external ZN matrix degrades at specific areas and thereby leads to successful mediated transfection of the DNA/CS nanoparticles
in vitro. Eventually, the hybrid-dual particulate carrier encoding GFP could drive the production of anti-GFP IgA antibodies, verifying in vivo transfection and expression. Layek et al.
[33] developed a DNA vaccine nanocarrier made of mannosylated phenylalanine grafted chitosan (Man-CS-Phe) as a possible immunotherapeutic route for chronic hepatitis B. Within such cationic micelles, plasmid DNA is condensed as nanosized polyplexes. As a result, the complexed DNA is shielded against enzymatic (nuclease) degradation.
mRNA vaccine carriers: the mRNA vaccines have also found notable application with respect to prevention of viral infections. As currently seen in the case of the COVID-19 mRNA vaccines, e.g., the one produced by Moderna, lipid nanoparticles are crucial in effective protection against enzymatic (ribonuclease) degradation and transportation of mRNA to cells
[34]. Other than lipid nanoparticles, several new nanotechnologies have been also devised for the sake of delivery of mRNA vaccines,
Figure 4. Nanomaterial based technologies such as dendrimers, cationic nanoemulsions, polysaccharide particles, or liposomes have been also created to raise the stability of mRNA vaccines and optimize their delivery mode
[35].
Figure 4. The as-yet studied mRNA vaccine nanocarriers. Reproduced with permission
[36].
Elia et al.
[37] designed an mRNA vaccine holding lipid nanoparticle system. In this mRNA nanocarrier, the lipid nanoparticles enclosed SARS-CoV-2 human Fc-conjugated receptor-binding domain (RBD-hFc). After intramuscular administration of the mRNA nanocarrier, a notable humoral reaction, a Th1-biased cellular behaviour in BALB/c mice, as well as a large extent of neutralizing antibodies were recorded. This strategy, i.e., encapsulating mRNA vaccine into lipid nanoparticles has been also applied in relation to other types of viruses. For instance, Zhuang et al.
[38] incorporated mRNA vaccine (coupled with in vitro transcription (IVT)) into cationic lipid nanoparticle as an H1N1 influenza virus vaccine nanocarrier and compared its gene delivery efficiency with that of mannose-conjugated lipid nanoparticle. As they reported, the latter nanocarrier was more successful in this respect than its counterpart both in vivo and
in vitro. The most optimum system, IVT-mRNA-n3 loaded mannose-lipid nanoparticle was proved to be efficiently applicable for immunization of C57BL/6 mice against H1N1 influenza virus.
Subunit vaccine carriers: Subunit vaccines comprise very limited structural components of SARS-CoV-2, able to induce protective immune reactions in the body, upon administration alongside molecular supports, thereby raising immunogenicity
[22]. The subunit vaccines can be made via incorporation of viral proteins in protein cages, virus-like particles (VLPs), and synthetic nanomaterials, performing as delivery carriers and/or adjuvants
[39][40][41]. For instance, influenza protein haemagglutinin incorporated liposome is the basis of the influenza virus vaccine Crucell (Janssen, Johnson & Johnson)
[42]. The subunit vaccine nanocarriers provide antigen multivalency and allow optimum concurrent delivery of adjuvant and antigen to secondary lymphoid organs
[43]. Furthermore, thanks to their specific size scale, they enable lymphatic trafficking and favourable uptake by the antigen presenting cells (APCs). They induce depot effects for stable immune stimulus and ease the antigen cross presentation, that allows the extracellular antigens to be presented via the MHC-I pathway for CD8
+ T cell engagement
[44].
2.2. Nanohybrid Materials Used for Biomedical Waste Management
As the pandemic spreads all over the world, a large number of PPE is being consumed every day. Therefore, management of biomedical wastes is crucial to avoid an extra pressure on the environment and spread of the virus
[45]. The biomedical waste differs notably from the normal waste considering the likely presence of pathogens on its surface that could lead to further contamination and release of viruses and bacteria
[45].
The contaminated healthcare wastes are typically discarded in landfills, thus potentially might engender pollution of surface, drinking and ground water resources, in case the landfill has been improperly designed. The healthcare wastes can be chemically treated by chemical disinfectants, thereby further contaminating the environment via release of hazardous chemicals into the environment. As another option, waste incineration has been largely taken into practice. Yet, incomplete incineration could potentially pollute the air and produce ash remainder. The other conventional methods in management of biomedical waste include mechanical methods involving granulation, crushing, pulverization, grinding, shredding, agitation, and mixing. These approaches do not necessarily eliminate the pathogen related contaminations, but lower the waste mass thus ease its subsequent processing or disposal. Chemical disinfection, e.g., via implementation of chlorine compounds, is another popular approach to inactivate the pathogens in medical waste, and also to oxidize harmful chemical materials. Microwave radiation and its resulting generated heat can also potentially be utilized to treat medical waste. Thanks to detrimental effect of gamma rays on pathogen’s DNA, gamma irradiation caused by radioactive isotopes of cobalt, shows a high potential for sterilization of waste. However, the shadowing effect is challenging and the waste surfaces directly exposed to the radiation source become more sterile than those located on the shaded side
[45].
Advantageous over the mentioned management techniques in terms of cost-efficiency, sunlight based photocatalysis has been developed for both solid and liquid wastes
[46]. This approach is drawing more attention for environmental remediation, considering the demand to obtain the highest degradation efficiency of contaminants possible under affordable conditions in terms of temperature and pressure. The main driving force in this treatment is the near-UV light (whose wavelength varies from 400 nm down to 300 nm) that could be even replaced by sunlight to render the technique more economical. As shown in
Figure 5a, sunlight driven photocatalysis in the presence of a nanophotocatalyst enables release of hydroxyl radicals (OH•). Having unpaired electrons, OH• notably oxidizes the resistant organics (e.g., pathogens)
[47]. Owing to the abundance of low cost, efficient photocatalysts, photocatalysis is of the most renowned dissociation process for organic pollutants. This type of pollutants is easily decomposed to water and CO
2 in a liquid medium under the influence of the photocatalytic activity of a semiconductor metal oxide nanoparticle, e.g., TiO
2 [48].
Figure 5. (
a) Schematic illustrates the solar light driven photocatalysis (
Ehv and
Eg are the solar light photon energy and band gap energy, respectively, and, CB and VB are the conduction and valence band, respectively. R and D are the electron acceptor and electron donor, respectively. Reproduced with permission
[49], Copyright 2017, Elsevier. (
b) The Ag-CuFe
2O
4@WO
3 nanohybrid nanoparticles kill
E. coli within the course of a 60 h incubation period upon exposure to UV light. The density of the bacteria colonies imaged in the absence of the nanohybrid nanoparticles (
c), and in the presence of CuFe
2O
4@WO
3 (
d) and Ag-CuFe
2O
4@WO
3 nanoparticles (
e). Reproduced with permission
[50].
Despite several advantages of the photocatalysis process for waste management including low energy consumption, eco-friendliness with insignificant chemical input, production of no harmful byproducts (in general least amount of secondary waste is produced), mild operational conditions (temperature and pressure), and versatility in terms of operating medium (solid, liquid, and gas), the applicability of the process is limited due to charge separation, interfacial charge transfer and charge carrier recombination
[10][51][52][53]. One promising solution for such challenges and also extending applicability of the nanophotocatalysts is their hybridization with other supplementary, supportive components. For instance, Cu-deposited TiO
2 photocatalysts have been proposed to offer biocidal activity and cooperative effect of photocatalysis and lethality of copper
[54]. Such a nanohybrid system was able to deactivate
E. coli bacteria when irradiated with very weak UV light. It has been claimed that the photocatalysis process leads to destruction of the external membrane in the cell envelope, thereby allowing the Cu ions to get into the cytoplasmic membrane. The copper ions damage the cytoplasmic membrane and engender the cell’s disintegration.
In general, photocatalysis can provoke degradation of simple compounds (proteins and DNA), thereby imposing an inhibitory effect on viruses and bacteria
[55]. The photocatalytic detrimental impact takes place at two levels
[56]: (1) Photo-inactivation ending up with a disinfectant effect, and (2) Decomposition of viral cells bringing about a sterilizing effect. Despite validation of effectiveness of the photocatalytic systems for virus inactivation via various laboratory experiments involving several types of microorganisms, the antiviral mechanism is still to be understood
[57][58][59]. The whole process seems to initiate with adsorption of the virus on the photocatalyst surface, followed by an oxidative radical attack on the capsid protein and on virus binding sites, aka, direct attack—redox type. As explained in
[60], the inactivation process of viruses is governed by the presence of radicals and particularly reactive oxygen species (ROS) such as °OH, •O
2−, HO
2•, and H
2O
2 that are available in the bulk phase and are independent of the catalyst. As similarly mentioned for the
E. coli bacteria, the decomposition process continues with destruction of the cell wall and the cytoplasmic membrane, induced by the generation of ROS that can result in the release of cellular content, cell lysis and eventually total mineralization of the organism
[56].
Given the same inactivation mechanism of viruses and bacteria, the photocatalytic nanohybrids with effective antibacterial performance can potentially be applied for disinfection (virus removal) of biomedical wastes. For instance, CdO-MgO
[61], CeO
2/CdO
[62], PbS-CdO
[63], CdO-NiO
[64], CdO-ZnO
[65], NiO-CdO
[66], ZnO/MgO
[67], CuO-MgO
[68] nanohybrids are some examples for the photocatalysts able to show antimicrobial performance. In this regard, Sayadi et al.
[50] devised a photocatalytic nanohybrid composed of tungsten trioxide (WO
3) hybridized with Ag doped CuFe
2O
4 that could offer a photocatalytic activity when exposed to UV/visible light and be recovered from water efficiently thanks to the excellent photocatalytic and magnetic properties of the mentioned components, respectively. Due to co-existence of Ag, this nanohybrid photocatalyst was able to inactivate the
E. coli bacteria when UV irradiated and after 12 h incubation,
Figure 5b–e. At the interface of the WO
3 and CuFe
2O
4 phases (core-shell structure), the electrons supplied by Ag are captured by the O
2 molecules, and consequently generate reactive species including OOH◦, O
2, and HOH that optimally inactivate the bacteria.
2.3. Nanohybrid Materials Used for Early Diagnosis of Virus Infection
To accurately diagnose the presence of viruses, advanced, reliable technologies able to detect viruses with outstanding sensitivity are now needed more than ever. The conventional PCR-based diagnosis technology, that is routinely employed for virus detection necessitates operators and are costly in terms of equipment, challenging its implementation in the areas poor in resources
[69][70]. Additionally, the technologies relying on antigen–antibody reactions, such as immunochromatography, are not sufficiently sensitive
[71]. Therefore, it is indeed questionable to target accurate detection of viruses to prevent their spread based on the techniques that are susceptible to false negatives and false positives
[72].
Thanks to the expansive specific surface area of nanomaterials, the detection technologies employing nanomaterials can potentially achieve precise analyte-specific detection signals with superior sensitivity to even trace amounts of viruses
[73][74]. In this regard, virus detection can be optically carried out by provoking a fluorescence-raising effect on virus when the adjacent plasmonic nanoparticles create a localized surface plasmon resonance (LSPR) effect
[75]. The as-emerging optical properties of the nanoparticles highly depend on their interparticle spacing
[76]. In addition to plasmonic nanoparticles, superconductive nanomaterials can be also employed in development of highly sensitive electrochemical biosensors for virus detection
[77][78].
The Dengue virus has been similarly detected using nanohybrid optical sensors. In this regard, the optical sensors composed of graphene oxide (GO) have drawn interest because of their fascinating characteristics such as high density of functional groups, thinness, negligible mass, extensive specific area, a structure providing high π-conjugation, and optimum robustness
[79][80][81]. The presence of a large number of functional groups on GO, however, could be challenging with respect to its electron transfer ability, given that the functional groups can potentially disrupt conductive zones and even the largely oxidized GO is extremely poor in terms of electrical conductivity
[82]. To resolve this issue, yet maintaining beneficial properties, GO is typically reduced and its oxidized functional groups are discarded, ending up with formation of reduced graphene oxide (rGO), that is an electrically conductive material, can be stored for a longer period without agglomeration, and is more chemical resistant (endures in the organic solvents)
[83][84]. Additionally, the residual oxygen bearing functional groups of rGO can be employed for the purpose of chemical functionalization and creation of nanohybrid materials. As an example, rGO can be further functionalized using primary amines (-NH
2) to get hydrophilized and to allow adhesion and binding of analytes, e.g., viruses
[85][86]. In this relevance, globular dendrimer of polyamidoamine (PAMAM) has been coupled with rGO to realize a highly sensitive detection platform. PAMAM dendrimers are notably useful for various sensing purposes, thanks to their efficient transporting behaviour for bioactive agents and their non-toxicity
[87][88]. Omar et al.
[89] devised an optical sensor, performing based on the SPR effect, for detection of the dengue virus. The nanohybrid sensor was composed of monoclonal antibody decorated dithiobis (succinimidyl undecanoate, DSU)/amine-functionalized rGO–PAMAM. The as-developed sensor was able to accurately record the alterations of the SPR angle, for instance, for the virus concentrations as low as 0.08 pM in 8 min. Interestingly, the sensor behaved selectively, and the sensitivity to other types of proteins was lower than that to the virus. Highly conductive substrates made of graphene have been hybridized with Au to form a sensing platform (overlaying a Au screen printed electrode) for electrochemical detection of influenza virus. The graphene-Au nanohybrid biosensor performs based on quantification of neuraminidase (N) activity
[90].
Despite the mentioned merits, nanomaterial-based virus detectors suffer from lack of stability and reliability of the detection signal
[91]. Additionally, high sensitivity imposes a high concern of producing nonspecific signals. To prevent generation of nonspecific signals, thereby enabling a more reliable and stable detection process, two different signals can be based
[92]. Therefore, highly sensitive and reliable nanomaterial-based biosensors should be developed that generate two signals. To achieve such a feature and develop a dual-signal virus detector, the nanomaterials able to efficiently generate and enhance electrochemical and optical signals, such as gold nanoparticles (AuNPs) are of paramount importance
[93]. In this relevance, Takemura et al.
[94] have designed a novel technique for detection of the Influenza virus based on the electrochemical and optical signals. The detection performance tightly depends on the virus concentration. The virus detector is in fact a nanohybrid system that comprises of: (1) plasmonic (Au) nanoparticles (AuNP) to reinforce the fluorescence signal of CdSeTeS quantum dots (QDs) through the LSPR effect (noteworthy, the virus could bound onto the plasmonic nanoparticles via antibody (Ab)), (2) magnetic nanoparticles (MNP) to separate the analyte stuck within the nanohybrid structure, and (3) carbon nanotubes (CNT), meant to act as a matrix to integrate all the components coupled with the antibodies of the Influenza virus. The mentioned QDs suspending in the solution were responsible for generation of the optical signal, particularly in close proximity of the Au nanoparticles, thanks to their long fluorescence lifetime, and were able to release Cd ions at low pHs and thereby produce an electrochemical signal,
Figure 6a. In more precise words, when the QD–Ab/virus/Ab–AuNP–MNP–CNT nanoassembly is dissolved at a low pH, the Cd ions are released from the QDs, whose concentration is quantified electrochemically by the AuNP–MNP–CNT-deposited carbon electrode.
Figure 6. (
a) Schematic illustration of the optical/electrochemical detection process of the Influenza virus based on implementation of plasmonic nanocomposites and quantum dots. Reproduced with permission
[94], Copyright 2021, American Chemical Society. The improvement of the PCR efficiency (nano-PCR based on pan-type primers) through inclusion of GO-AuNPs at different concentrations for the purpose of quantification of FMDV O-type (
b) and FMDV A-type (
c). Reproduced with permission
[95].
As mentioned earlier, despite popularity of PCR as a detection technique for viruses, it suffers from poor specificity, insufficient sensitivity, and generation of false positive results. To address such shortcomings, metal and carbon nanomaterials are included to raise the quality of performance of PCR assays. Kim et al.
[95] enhanced the capability of PCR using a nanocomposite made of GO sheets (as the matrix) and AuNPs (~15 nm) to enable sensitive diagnosis of the foot-and-mouth disease virus (FMDV), which leads to a drastically infectious and fatal viral disease for the animals with cloven-hoof such as pigs, and thus negatively impacts on the swine industry. As validated by the authors, the limit of detection (LoD) of real-time PCR upgraded by GO-AuNPs raised up to ~1000 times. The improvement in the PCR detection efficiency was determined via comparison of DNA amplification potential in the presence (at various concentrations) and absence of GO-AuNPs. As shown in
Figure 6b, the optimal concentration was proved to be ~10 µg/mL, particularly, 20 µg/mL of GO-AuNPs led to a notable decline in signals. It is known that GO-AuNPs can electrostatically attach to single-stranded DNA (ssDNA) in a selective manner
[96]. As a result, the quality of PCR performance is notably enhanced when acting as a single-stranded DNA binding protein (SSB), optimizing the interaction between templates and primers while DNA replicates
in vivo. On the other hand, larger GO-AuNPs concentrations could potentially impede the PCR reaction, due to their tendency to bind to double-stranded DNA (dsDNA), rather than to ssDNA.
2.4. Nanohybrid Materials Used for Viral Infection Therapy
Nanohybrids benefitting from a therapeutic agent coupled with a supportive organic/inorganic nanomaterial have shown promising applicability for treatment of various viral infections caused by Influenza, Hepatitis C, and SARS-CoV-2.
Influenza A virus (IAV) is accounted as an important infectious pathogen in relation to humans and animals and frequently engenders epidemics and epizootics. Annually, influenza infects around 100–500 million people, thereof almost 500,000 die
[97][98]. The exclusive structure of this virus, with respect to genome fragmentation (the genome comprises eight negatively charged segments of single-stranded RNA) and glycoproteins diversity, is the main ground for its global spread. As a consequence of gene recombination and antigenic shift, new variants of the Influenza virus emerge and bring about pandemics and epidemics
[99]. To address the global challenge caused by the spread of Influenza, only a limited variety of drugs with selective performance such as neuraminidase inhibitors (e.g., oseltamivir) and adamantane drugs are available that could even drive the formation of resistant IAV strains and side effects
[100]. Therefore, there is a demand for development of new therapeutics that can deactivate the influenza virus in an effective manner. In this regard, the nucleic acid fragments that can identify the target nucleic acids selectively, thereby minimizing adverse side effects in comparison to non-specific traditional drugs, are promising candidates. Despite such merits, these therapeutics have not found extensive application mainly due to their poor serum stability and their challenging diffusion into cells. To address the former bottleneck, i.e., stabilization of oligonucleotides, chemical treatment has been proved to be efficient
[101]. On the other hand, liposomes, virus vectors, cationic polymers, transporting peptides, among others can act as carriers to ease and improve delivery of oligonucleotides
[102][103][104]. However, even the most successful delivery approach is not efficient enough and might impose toxicity. To advance this research area and to meet the need to effective delivery systems, Levina et al.
[99] proposed a new platform able to deliver nucleic acid fragments (ODN) into eukaryotic cells based on a nanocomposite material. The nanocomposite comprised of ODN loaded polylysine that was non-covalently immobilized onto TiO
2 nanoparticles. The selected nanoparticles are biocompatible and non-toxic and enable the ODN delivery via the cell membrane
[105][106]. Moreover, they stabilize and protect the ODN against intracellular enzymes
[107]. Upon entry into cells, the carried oligonucleotides are released from the nanocomposites in the cytoplasm or penetrate into the nuclei and bind to the RNA molecules
[108]. As verified via in vitro tests, the nanocomposites induce no toxicity reaction, readily enter into the eukaryotic cells, and significantly deactivate 3 IAV subtypes, e.g., hazardous H5N1 avian influenza.
Hepatitis C virus (HCV) is another important virus affecting more than 58 million people worldwide and resulting in severe liver disease
[109]. Thanks to further comprehension of the biology of HCV as well as identification of antiviral targeting of vital functions of the virus, the relevant treatments are being advanced and becoming more efficient
[110]. In this regard, the NS3 and NS5A proteases and NS5B polymerase are regarded as important targets for the creation of direct-acting antiviral drugs
[111]. The standard therapy for the HCV genotype 4a comprises ledipasvir, ombitasvir, and sofosbuvir with a response rate of over 90%
[112]. Although this treatment has shown considerable promises, its efficacy is below optimum and undesired consequences such as photosensitivity, pruritus, rash, anemia, etc.
[113] could potentially emerge. Additionally, the rising number of patients who suffer from dangerous liver diseases with no proper treatment choice further highlights the importance of development of suitable therapeutic approaches. Challenging fulfilment of this objective, viral RNA is rapidly mutated and thereby creation of state-of-the art anti-HCV drugs are hampered. One promising solution for treatment of the HCV infections is indeed the use of plant derived natural compounds that show relevant antiviral performance
[114]. Furthermore, such compounds exhibit hepatoprotective effect due to their natural substances including caffeine, naringenin, silymarin, and epigallocatechin-3-gallate (EGCG)
[115][116]. As an example, for the plant-derived antiviral compound, turmeric curcumin, aka,
curcuma longa, has shown promising therapeutic efficiency with no particular side effects
[117]. Curcumin offers antiviral effect against hepatitis B virus
[118], influenza, human herpes, and HIV viruses
[119]. As reported by Pecheur
[120], curcumin can also deactivate HCV through challenging the viral adhesion and fusing to hepatocytes and inhibiting intercellular transmission by damaging the membrane’s structure. Nevertheless, curcumin is poorly soluble and hardly penetrated into cells. One solution to overcome such problems, is the use of polymeric nanoparticles as a carrier for curcumin, that assures its steady release to infected cells and its bioavailability, and hampers its degradation, thereby enhancing its therapeutic capacity
[121]. In this regard, biocompatible polymers originating from nature are highly demanded. For instance, chitosan, a derivative of chitin, is a natural biopolymer (polysaccharide) and after cellulose is ranked the second most available polysaccharide in nature
[122]. Chitosan shows immunogenicity, and anticancer, antimicrobial, and anti-inflammatory effects
[123]. Chitosan nanoparticles are non-toxic at low dosages (30 µg/mL) and offer an anticancer effect at high dosages (>100 µg/mL). These promising potentials encourage its implementation as a drug carrier
[124][125]. In this regard, Ramana et al.
[126] employed chitosan nanoparticles as a carrier for antiretroviral agents such as saquinavir, that is a protease inhibitor, to deactivate HIV. By this strategy, cell targeting efficiency raised up to 92% in comparison with that of the soluble drug alone. In another study, Loutfy et al.
[127] studied the antiviral efficiency of curcumin-chitosan nanocomposite against HCV-4
in silico, involving the hepatoblastoma cells. According to their results, the nanocomposite system is able to notably reduce the HCV core protein expression, as verified by the Western blot assay. Such reduction was much superior in the case of the nanocomposite as compared to the controls including curcumin and chitosan nanoparticles, implying its extraordinary antiviral effect via blocking the penetration or replication of virus. Though, the achieved results clearly demonstrate the applicability of the nanocomposite against viral infection, the anti-HCV effect of the nanocomposite in the replicating system and its therapeutic index need to be evaluated. Moreover, the nanocomposite carrier should be also precisely tested
in vivo.
In addition to the aforementioned drug-based antiviral therapies, photodynamic therapy (PDT) is an efficient strategy to deactivate viruses including SARS CoV-2. In this approach, the target cells are attacked when photosensitive agents, aka, photosensitizers (PSs), are excited by light irradiation and thereby produce reactive oxygen species (ROS) that are fatal to cells
[1]. In fact, ROS seriously damages the nucleic acids and proteins of virus
[128].
Figure 7a shows the mechanism of generation of ROS by PSs. Majority of PSs in their ground (i.e., singlet) state possess 2 electrons, spinning oppositely in a molecular orbit with the most optimum energetic level. Upon light absorption, one of these electrons jump to an orbit with a higher energy state. As a result, the PS becomes extremely unstable and starts to emit the extra energy in the form of heat and/or fluorescence. As a second scenario, the PS after excitation might experience an intersystem crossing, thereby forming a triplet state with higher stability where one electron inversely spins. To revert to the ground state, the PS should either lose energy without emitting radiation or passing the extra energy on molecular oxygen (O
2), ending up with generation of
1O
2 (Type II process)
[129]. PS can also react with an organic substance within a cellular medium, whereby it receives an electron or a hydrogen atom and converts to a radical (Type I process). The as-reduced PS is subsequently autoxidized and releases a superoxide anion radical (
O2.−). The generated radical undergoes one-electron reduction thereby forming hydrogen peroxide (H2O2), that is subjected to one-electron reduction, leading to creation of a highly oxidative hydroxyl radical (HO•). It is thought that majority of PSs perform through Type II process to generate ROS [130].