Current diagnostic and therapeutic approaches for severe acute respiratory syndrome coronavirus-2 (SARS-COV-2) and the role of nanomaterial-based theragnosis in combating the pandemic

1.1 Origin, epidemiology, and global current status of COVID-19

One of the most devastating pandemics since the flu of 1918 [1,2], with its local center in Wuhan, China, was caused by coronavirus in 2019 and designated COVID-19 [3,4]. As per the World Health Organisation’s (WHO) declaration on February 3, 2023, the disease was responsible for causing illnesses in approximately 700 million individuals and leading to 6.8 million fatalities globally. In December of 2019, a cluster of pneumonia cases of mysterious origin was observed in Wuhan, China. Later on January 12, 2020, the sequencing of a novel coronavirus known as severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) found from clustered cases of respiratory contagion was announced and publicized as the etiological agent for an unknown Pneumonia [5]. Due to the lack of rigorous epidemiological investigations, evaluating the risk of this occurrence was apprehensive with great uncertainty, despite a common link to a wet market in Wuhan’s Huanan Seafood Wholesale Market was reported [6]. As a result of the increasing number of fatalities, irrespective of age and the uncertainty encompassing the antidote discovery, the WHO labeled the COVID-19 outbreak a public health emergency of worldwide alarm on January 30, 2020, and a pandemic on March 11, 2020 [7].

The high pathogenicity of human coronavirus (HCoV) has put forward this specific respiratory infection causing pathogen to the spotlight of research community[8]. Pertaining to this concern, coronaviruses are positive-sense, single-stranded, enclosed RNA viruses of 60–140 nm in diameter [9]. Among the six human betacoronaviruses species, four species including HCoV-229E, OC43, NL63, and HKU1 can cause upper respiratory tract infections like the common cold similar to the symptoms caused by rhinoviruses [10]. Other two coronaviruses species include severe acute respiratory syndrome coronavirus (SARS-CoV) and middle east respiratory syndrome coronavirus (MERS-CoV), with fatality rates 9.6 and 35%, respectively [11]. Among the outbreaks of these two viruses, SARS-CoV outbreak of 2002–2004 was believed to have its primary host as bat, mutating to infect the transitional host Civet cats, and finally spreading to humans [12]. The pathogenesis was better understood through host-pathogen interaction, in which case the virus was found to infect and cause a range of respiratory ailments eventually leading to death [13]. Originating in the primary reservoir (horse shoe bats), the virus typically transmitted to humans. Furthermore, it did spread among humans, through infected body fluids including respiratory droplets (saliva, breathing, coughing, sneezing, and talking) [14,15,16].

Benign coronavirus has been reported to be harmless, being the reason for common cold alone. But the species that mutated and transitioned from their natural reservoirs have been declared virulent, inflicting excessive infection and mortality in the affected individuals [17]. Despite the majority of cases being asymptomatic or self-recovering, the disease’s clinical spectrum includes chronic pneumonia with acute respiratory distress syndrome (ARDS), a deadly illness which demands mechanical ventilation and treatment with critical care [18]. This has caused a severe impact on the human population with severe mortality around the globe. The lack of specific regimen for this disease has provoked or steered the research and medical community toward a necessity in identifying solutions that can be taken to stay healthy [18,19]. Real-time mapping has demonstrated that differential transmission prototypes and infectivity are related to alteration in lineages, clades, and strains of COVID-19 virus. This is despite the fact that the community spread studies of SARS-COV-2 exposed implausible transmission patterns between places which are not even connected geographically [13,20].

According to the WHO’s Weekly Epidemiological Update on the survey of COVID-19 pathogenicity, along with other relevant infectious disease information, over 4.8 million fresh cases and 39,000 deaths were announced globally over the duration of January and February months of the year 2023. This has demonstrated a reduction in the number of reported new cases and deaths (at 76% of new cases and 66% of deaths, respectively) compared to the preceding 28 days. At the end of February 2023, a total of approximately 758 million confirmed infections and 6.8 million fatalities had been documented throughout the world [21]. A glimpse at the global status of SARS-COV-2 infection among the affected countries governed by WHO, as on March 22, 2023, Europe had 274,391,717 confirmed cases, 282,646 new cases, and 2,203,052 fatalities. Western Pacific was reported to be affected with statistics showing 201,913,013 confirmed cases, 165,547 new cases, and 408,070 fatalities. Whereas, in Americas, 191,185,511 confirmed cases, 189,265 new cases, and 2,939,388 fatalities have been reported. In this regard, Southeast Asia reported 60,784,561 confirmed cases, 8,336 new cases, and 803,971 fatalities. As the continent considered to be least affected, Africa did report 9,509,869 confirmed cases, 114 new cases, and 175,315 fatalities. The worst affected countries include the United States, China, and India as per the number of cases affected. In the United States alone, 103,436,829 confirmed cases and 1,127,152 deaths were reported on September 21, 2023. As on the same date in China, 99,309,232 confirmed cases and 121,679 fatalities were reported, whereas 44,998,162 confirmed cases and 532,030 deaths were reported in India. As per the number of deaths, Brazil ranks the second with 704,659 deaths after the United States, whereas India ranks the third [22].

Even though personal cleanliness, maintaining social distance, regular hand washing, and avoiding contact with infected individuals are recommended practices, there are evidences suggesting that adequate food, nutrition, and other lifestyle factors increase the immune strength and minimize susceptibility to infectious illnesses. Hence, this review provides detailed insights into the medical management of COVID-19 and the role of nanotechnology in improving conventional treatment modalities for the disease. It also emphasizes the importance of understanding the underlying health conditions (comorbidities) in the context of COVID-19 and the role of biomarkers in disease management. Additionally, it discusses the extensive research and clinical trials aimed at finding effective treatments for COVID-19, which range from antiviral medications to therapies that modulate the immune system, as well as the development of vaccines. Moreover, updated information of currently employed vaccines has been listed. Moreover, the review highlights the diagnostic advancements made possible by nanotechnology in the fight against COVID-19. These innovations include NanoBeads, Protein Aptamer Sensors, and magnetic levitation, which can enhance the accuracy and efficiency of COVID-19 diagnosis. It also explores therapeutic approaches that employ nanotechnology-based drugs to treat COVID-19.

1.2 Comorbid conditions and their relation with COVID-19

Comorbidity is defined as the epidemiologic phenomenon of a population or an individual exhibiting the simultaneous occurrence of two or more disorders or conditions [23]. Understanding the co-occurrence is critical and beneficial in developing effective treatment protocols. The COVID-19 pathogen, the SARS-CoV-2, has rapidly expanded and infected over 180 countries [3]. As the novel coronavirus continually develops, it is only possible to hypothesize as to who will become infected. COVID-19 is a relatively newer and understudied disease, limiting the availability of such data on infection symptoms [24]. Studies have demonstrated that the general symptoms vary from acute common cold [25] to chronic lung infections such as bronchitis, acute respiratory syndrome, and pneumonia, as well as various organ failures, systemic dysfunctions, and eventual death [26,27].

According to the current statistics and clinical expertise, irrespective of age, individuals with substantial pre-existing clinical disorders are at a significant risk of contracting COVID-19, particularly those obtaining long-term medical treatment [3]. However, a meta-analysis of middle-aged and elderly COVID-infected individuals revealed that the geriatric population is more susceptible and pose a significant death rate. This could be attributed to the alterations in the structure and muscular atrophy of the lungs, which can result in changes and a reduction in physiological processes such as lung reserve, airway clearance, and immune barrier functions [28].

In a retrospective review of 1590 laboratory-confirmed Chinese hospitalized patients, 25% were found to have at least one comorbid condition. This statistical information was derived by evaluating the number of documented co-morbidities in connection to composite outcomes such as Intensive Care Units, ventilation, and mortality. Included among these conditions were hypertension and diabetes. The most prevalent comorbidity found was that of hypertension with a percentage of around 17. Additionally, smoking, diabetes, and cancer were found to increase the probability toward a life-threatening stage. The number of comorbidities was found to be directly proportional in reaching the composite endpoints [29]. In yet another research done in India on COVID patients, the same prevalence of hypertension was seen, followed by diabetes, bronchial asthma, renal, and heart illnesses, in the order as so mentioned. Additionally, it was noticed that males were more prone in comparison to females in acquiring symptomatology complexes. Males have a higher prevalence of hypertension than females, which could explain why this pattern of death happened across gender [30].

Hypertension, obesity, and diabetes mellitus were the most frequent comorbidities among COVID-19 patients with fatal endpoints, according to another meta-analysis study on the connection between comorbidities and fatal endpoints among 14 countries around the globe. Even though cancer, chronic renal disease, and chronic heart failure were independently related to death in these individuals, obesity was not reported as a factor in the associated mortality [31]. However, the precise processes through which pre-existing diseases influence the vulnerability of the illness and its severity are unknown. Inflammatory and hormonal pathways, and also social variables such as living in a populated or regimented environment, are hypothesized to have a role [32].

2.1 Biomarkers of COVID-19

A biomarker is a trait which is used to track changes in regular or dysfunctional biological processes, or in pharmacological responses to a treatment which provide objective values during the progression or course of a disease [33,34]. Depending on the severity of a disorder, the symptoms or clinical presentation can provide clinicians with a method for precise categorizing of patients as mild, severe, and being critical, thereby predicting the outcome and mortality on the basis of spectrum of the ailment. This can enable early treatment [35]. In addition, this can assist in identification of populations at high risk, justifying therapeutics, assessing treatment response, developing criteria for hospital ICU admission, and discharge [36].

Most of these biomarkers fall into four main groups: immunological and inflammatory host immune response indicators, hematological markers/coagulation factors, organ damage markers (cardiac enzymes, liver markers, and renal function markers), and general response markers (electrolytes). The commonly employed biomarkers for COVID-19 with its identification levels in human body are listed in Table 1.

2.2 Clinical management of COVID-19

In the months following the emergence of the new coronavirus, pharmaceutical corporations and researchers of all sizes endeavoured to combat the pandemic. This comprised of innovative regimens and medicaments to treat COVID-19, such as vaccines based on existing antiviral medications, expediting development of new drug, plasma treatment, and cell-based and monoclonal antibody convalescent therapeutics [63]. During the epidemic, the FDA’s programs enabling doctor’s access to experimental treatments were indispensable. The expanded access and emergency use authorization (EUA) initiatives enabled quick implementation of prospective experimental medicines and investigational drugs with growing evidence [64].

However, considering the current global health emergency, the absence of verified clinical data on COVID-19 therapeutic agents, as well as the time, expense, and high attrition rate associated with drug development indicate that it is high time to identify a promising medicine. Due to the fact that drug research is costly, time-consuming, and has a high failure rate, time remains critical and of ultimate importance. Significant interest in improvising the existing medications and accelerating the production of vaccines can enable the quick identification of therapeutic candidates. More than 30 categories of substances, coming under conventional pharmaceuticals, environmental and traditional remedies, have been deemed via research to date as being potentially effective against COVID-19 where several of these agents have undergone rapid clinical testing and have proven tentative effectiveness against COVID-19 [65]. As of February 2023, there were 8,921 on-going clinical trials, 714 mapped medicines, 3,329 finished studies, and 199 completed vaccination studies [66].

As the introduction of mutant SARS-CoV-2 strains rendered certain vaccinations less-effective and also limited the worldwide availability of COVID-19 vaccines, it supported the rationale for exuberating efforts to discover potential therapeutic approaches; this includes antivirals such as remdesivir, chloroquine (CQ), Kaletra, favipiravir, and hydroxychloroquine (HCQ) along with immunosuppressive drugs such as tocilizumab, and tyrosine kinase inhibitors such as mastinib. These drugs have been used to treat hepatitis C and malaria and were used as monoclonal antibodies for rheumatoid arthritis, HIV treatment, and kinase inhibitors for mast cell tumors in animals [67,68].

3.1 Metal NPs

Unique optical and electrical features (localized surface plasmon resonance [LSPR]) of metals, particularly noble metals such as gold, silver, and copper, have been considered in the development of metal NPs [131] to detect viral cells in biosensing applications including disease marker detection, photocousting imaging, and near-infrared thermal ablation [132]. This is due to the LSPR property for tunable electromangnetic light absorption and scattering wavelength in the visible region [133], in which the change in the LSPR extinction maxima of metal NPs is dependent directly on the refractive index of encircling media and the range of NP aggregation, both of which are crucial for NPs to be used in biological applications [134].

Gold NPs (AuNPs) are the most common NPs used in coronavirus diagnosis or detection due to their unique optical properties, stability, and biocompatible properties [135,136]. Kim et al. created and utilized a simple colorimetric hybridization technique to detect SARS-CoV, which was based on the synthesis of dsDNA from viral ssRNA by the interaction of viral ssRNA with citrate-coated AuNPs, therefore stabilizing the particle [137]. AuNPs with modified surface properties by functionalizing with biomolecules are used for effective detection of COVID-19 variants without cross reactivity including MERS-CoV, HCoV-HKU1, HCoV-HKU4, SARS-CoV, HCoV-229E, and HCoV-OC43. AuNPs conjugated with streptavidin were used for an RT-LAMP (Reverse Transcriptase Loop-mediated isothermal amplification) assay which in turn was then combined with a vertical flow visualization strip (RT-LAMPVF), for detecting the nucleic acid of COVID-19 variant (MERS-COV). Later, the viral RNA made by RT LAMP was labeled with biotin and fluorescein isothiocyanate (FITC) to make the labeled amplicons, which could bind to streptavidin-functionalized AuNPs to make a complex that changed color when an anti-FITC antibody coated on the detection strip demonstrated it. Within 35 min, the creation of the complex on the strip was obvious to the naked eye. This technique was found to produce a high specificity and effective detection limit of 10 copies of viral RNA/μL. Similarly, a study on using Plasmonic NP study SARS-CoV-2’s N gene (nucleocapsid phosphoprotein) was colorimetrically identified by Thiol modified antisense oligonucleotides functionalized AuNPs [138].

3.2 MNPs

For the specific identifications of SARS-COV-2, the principle of magnetic efficiency of nanometals is currently being employed to prepare super-paramagnetic NPs (SMNPs) conjugated with amplified viral DNAs and to identify them with silica-coated fluorescent NP-based signalling probes via hybridization assay. Recently, scientists have used silica-coated SMNPs in PCR-based assays in order to increase the specific selection of the target cDNA of SARS-COV during the process, thereby allowing the identification of the target cDNA with a limit of detection (LOD) of around 2 × 10³ copies in a time span of 6 h [139].

Nanotechnological approaches, such as protein corona detector panels and magnetic levitation, have also been highlighted by Mahmoudi in his study as having great promise for identifying chronic patients during the early phases of the COVID-19 infection [140]. According to the study, a biomolecular corona is generated in which the introduction of NPs happens in a human environment (e.g., blood plasma) and can lead to instantaneous association with numerous biomolecules, such as proteins, which in turn forms an outer covering of NPs. This can assist in distinguishing the protein corona biomolecule from the others. As a result of the increased affinity of other proteins and the recruitment of additional particles onto the NPs that have previously been included, this method is still susceptible to a number of lacks in correlation. As a consequence, disease-specific protein corona biomolecules were created, the molecules in which the protein corona sensor array technology can define the plasma protein and biomolecule patterns that indicate catastrophic COVID-19 infection at its earliest stages [140]. And also colorimetric nanotechnologies, such as optoelectronic nose [141] and plasmonic NP (AuNPs) technologies [142], can be enhanced or updated for identification of the virus in susceptible individuals at an earliest time point, based on the fingerprint protein corona biomolecular pattern.

A typical MagLev device consists of a pair of permanent magnets with identical poles positioned opposite to one another along the gravity vector. Serving this purpose better, superparamagnetic iron oxide NPs were able to circumvent the instability of proteins in the paramagnetic solution and thereby levitate plasma proteins [140]. A heterogeneity mapping study revealed that the MagLev system cannot only separate corona-coated NPs but also examine the homogeneity/heterogeneity of the protein corona and aid in rapid screening of the homogeneity of corona-coated NPs prior to quantitative analysis of the disease [143].

3.3 QDs

Conventional luminous NPs known as QDs are gaining popularity because of their unique photonic and electrical properties. These properties include a substantial quantum yield, anti-photo bleaching action, broad absorbance range, tunable emission wave length, and excellent stability. In addition, it is projected that these QDs will contribute to the advancement of virus detectors and antiviral medications through their enhanced broad-spectrum action, point-of-care (POC) diagnostics, and profitable manufacturing [144–146].

3.4 Electrochemical aptamer sensors

A QD-conjugated RNA aptamer particular to the SARS-COV N protein has revealed greater sensitivity for coronavirus detection. In the respective study, authors employed QD-605 with maximum emission at 605 nm to achieve an exceptional detection limit of 0.1 pg/mL SARS-COV N protein mounted on a glass chip. The authors suggest that an optical QDs-based RNA aptamer chip may be able to get around the limitations of other techniques because it is sensitive, specific, easy to use, and can monitor one spot [147]. In another study for detecting N‐gene of SARS‐COV‐2, a surface plasmon resonance aptasensor was constructed where N‐gene‐targeted aptamer was attached on thiol‐altered niobium carbide MXene QD bioplatform. This aptasensor thereby exhibited an LOD of 4.9 pg/mL for N gene through a concentration range of 0.05–100 ng/mL [148]. In addition, photoelectric aptasensors were developed for quantifiable detection of RBD SARS-COV-2. The design included a modified ITO electrode with chitosan/cadmium sulfide (CdS)–graphitic carbon nitride (gC3N4) nanocomposite (gC3N4 and CdS) with immobilized amine-terminal aptamer probes. Results indicated that the aptasensor may be utilized to quantify Sars-Cov-2 RBD concentrations between 0.5 and 32.0 nM, with 0.12 nM LOD [149].

3.5 QD-conjugated chiral plasmonic NPs

Another nanohybrid structure with optical resonances that have crucial role in viral detection is chiral plasmonic NPs integrated with QDs. This include far-field coupling and near-field processes, as well as enhanced chiroptical characteristics [150]. In nanostructures, cumulative oscillations of free electrons produce plasmonic phenomena that enable the nanoscale confinement of light, which in turn can enhance the chiroptical interactions [151]. A sensitive chiro-immunosensor, conjugated QDs with chiral gold (Au) nanohybrids, was developed on the view of achieving low values of LOD. Based on self-assembly techniques, an asymmetric plasmonic chiral nanostructure hybrid will broaden the spectrum of circular dichroism reaction to achieve an exclusive plasmonic resonant association with the energized state of QD for LOD. The developed probe was originally utilized for the highly sensitive picomolar level detection of avian influenza A (H5N1) virus. Thus, the applicability of this sensing system was also examined on other viral cultures, including avian influenza A (H4N6), poultry adenovirus, and also coronavirus in respective blood/serum samples [150].

Chiral zirconium NPs assembled with l(+)‐ascorbic acid are another example for conjugated plasmonic NPs which is predominantly employed for SARS-COV with LOD of 79.15 EID/50 μL. In this technique, in addition to self-assembly and circular dichroism, these QDs might be conjugated with COV antibodies like bronchitis virus (IBV) to generate an immune link in the vicinity of anti-IBV antibody coupled magneto-plasmonic NPs and a tagged analyte. This in turn produced high sensitivity optical detection for COVs with LOD about 79.15 EID/50 μL [152]. As it has been demonstrated that exciton–plasmon interactions may influence chirality, employing integrated nanostructures as an approach for improving the responsiveness of optical (nano)sensors seems to be a promising idea.

3.6 QDs‐based Förster resonance energy transfer

A recent study explores a highly sensitive biosensing approach using QD-Förster resonance energy transfer, relying on the resonance energy transfer patterns between distinct partners to pick out the inhibitors of SARS-COV-2. The study focuses on the development of a versatile imaging probe involving the spike receptor binding domain of SARS-COV-2 conjugated to fluorescent QDs. This probe is designed to monitor the binding of the spike protein to the host cell’s ACE2 receptor, which is the initial step in SARS-COV-2 infection. The probe can undergo energy transfer quenching while interacting with ACE2-conjugated AuNPs, allowing for the real-time monitoring of this binding event in solution. The study demonstrates that neutralizing antibodies and recombinant human ACE2 effectively block this quenching, indicating a specific binding interaction [153]. For the identification of SARS-COV-2 RNA, a ligand exchange-based CdTe QDs–DNA (Cadmium telluride QDs) nanobiosensor was developed. This nanosensor might be utilized for the quick detection of RNA from SARS-COV-2 in actual samples with results equivalent to RT-PCR with high selectivity and sensitivity of LOD with 2.52 × 10⁻⁹ mol L⁻¹ [154].

3.7 QD nanobeads (QBs)

A portable smartphone imaging stage that automates quantitative QD barcode immunoassay and interacts utilizing an internally developed data dashboard was developed for the quantitative assay of SARS-COV-2. Here, a database and dashboard were used to illustrate real-time reporting of test results. The principle of the technique comprises (a) previously coded QD microbeads to identify target antibodies in human blood serum, so as to identify distinct antibody targets; several colors of QD barcoded microbeads were constructed; (b) hand-held instrument to stimulate and photograph the fluorescent microbead; and (c) an application that transmits the data to a controlling facility. The technology was certainly found to be highly sensitive and specific in being reported with an LOD of 1.99 pM for nucleocapsid coated microbeads and 0.11 pM for S1-RBD coated microbeads [155]. Also, lateral flow immuno assay (LFIA) with POC treatment is gaining popularity attributed to its simplicity, ease, speed, plus economic convenience for qualitative analysis [156]. Particularly, detecting SARS-COV-2 infection using colloidal gold NP-based LFIA (AuNP-LFIA) has undergone rapid advancement. In a recent study, a QB-based LFIA (QB-LFIA) for detecting total SARS-COV-2 antibodies in human serum was created. The QB-LFIA makes use of immunoassay construction including two antigens. During the assay process, SARS-COV-2 spike protein conjugated with QB is employed as a detecting probe to bind with specific antibodies in virtue of determining their serum levels. After being prepared, QB-LFIA was approximately an order of magnitude more sensitive than AuNP-LFIA [157].

4.1 Inhibition of viral attachment and entry into cell

A powerful approach for medication development and therapy is by preventing the process through which the virus attaches to the ACE2 receptor or restrict endocytosis [158]. CQ is one of the medications that are regularly evaluated for this purpose [159]. CQ has been initially found to hinder the NP endocytosis. As structurally SARS-COV-2 is comparable to certain NPs, it has been reported that CQ can prevent the endocytosis of SARS-CoV-2 virus particles [160].

This occurs in the following manner:

1. CQ-induced inhibition of phosphatidylinositol-binding clathrin assembly protein (PICALM), which in turn prevents endocytosis-mediated absorption of NPs. Generally, in the endocytosis pathway, PICALM is an accessory protein that, together with clarithrin, facilitates endocytosis.

2. encasing the molecule inside polymeric NPs like the often used poly lactic acid [161].

The ability of 1.6 nm cationic carbon dots (CDs) produced from curcumin to prevent the invasion of a coronavirus model, porcine epidemic diarrhea virus (PEDV), has been reported in a recent study [162]. At 125 μg/mL, the inhibition efficiency was over 50%, preventing viral entrance at an early stage. The blockage is probably thought to be brought about by electrostatic reactions among the anionic PEDV and the cationic CDs, which negate the effective charge on the virus particles and produced viral aggregation, together with this, the CDs also prevented the formation of reactive oxygen species (ROS), and thereby minimized cell death [162,163]. In cases of respiratory viruses, curcumin-conjugated AgNPs have been reported to possess the ability of viral inhibition and entry [163].

AuNPs have been found to be directly linked with the blockage of cell entry apart from the viral aggregation function. When considering NPs, AuNPs are less toxic than AgNPs. Huang et al., demonstrated the inhibitory activity of AuNPs in his recent study, in which he found a homologous protein that mimics the structural identity with one of the viral protein that is essential for cell fusion and invasion [164]. Pregnancy-induced hypertension (PIH), a peptide that mimics the structure of HR2, was discovered by him. Hence, this peptide can interact with HR1 of virus and prevent the creation of 6HB (six-helix bundle), which is supposed to draw the viral encapsulation within the cell. Through preventing 6HB, the process of cell fusion and subsequent infection are effectively inhibited. Gold nanorods coated with PIH displayed 10 times more suppressive activity at the optimum dose, completely blocking cell fusion [164].

In addition to AuNPs, other biocompatible and less toxic NPs with antiviral action such as SiNPs and selenium NPs (SeNPs) are effectively utilized. Engineered NPs including porous SiNPs break down to form non-toxic silicic acid and are favoured for their antiviral impacts due to their extraordinary biocompatibility and biodegradability properties. These particles, when functionalized separately or conjugated to different moieties (mesoporous SiNPs), act as scavengers of enveloped infection particles and prevent cellular invasion [165,166]. SeNPs remain prominent example for biocompatible NPs against SARS-COV-2 with antiviral efficiency by blocking viral entry when administrated [167]. In a recent study using the anti-inflammatory drug Ebselen, an organic Se species, it was found that the drug can block coronavirus by covalently binding to the virion through cell membranes, when administrated at a concentration of 10 μm. However, when administrated at high concentrations, it was found to be toxic; therefore, nanoselenium or low toxicity selenium for their biocompatibility are considered for their antiviral efficacy in the fight against viral infections [167].

Previous investigations have showed that natural compounds like green tea catechins inhibit encapsulated viruses. Its hydroxyl, galloyl, and pyrogallol groups on B-ring can alter viral antigen expression or genome replication at various phases of viral entry. Green tea polyphenols, like epicatechin gallates, have been identified as powerful viral entry inhibitors efficient of inhibiting the host’s glycoprotein CD4 interaction with glycoprotein gp120 of HIV-1, thereby preventing viral infections [168]. Similarly, curcumin, a polyphenol isolated from the plant Curcuma longa, has been shown to have antiviral action due to the presence of phenolic hydroxyl groups. Also, ionic gelation was used to encapsulate curcumin in chitosan NPs, which increased its bioavailability after oral administration along with it is in vitro antiviral efficacy in feline immunodeficiency virus-infected cats [162]. High-efficiency (homogeneous and stable with polar groups) anti-PEDV coronavirus delivery systems, consisting of glutathione-capped Ag2S nanoclusters and glutathione-modified zinc-sulfide NPs, were produced via the curcumin pyrolysis procedure [169,170]. When tested against PEDV, natural compounds like curcumin and glycyrrhizin were found to have multisite inhibition mechanisms, including: (a) blocking the entry of virus by altering the viral surface protein morphology, preventing pathogen’s genomic RNA production and replication; (b) reducing ROS production; and (c) stimulating IFN-stimulant genes and downstream of pro-inflammatory cytokines to lower the multiplication of virus. Similarly, the strong interaction of griffithsin (antiviral lectin) with the MERS-COV and SARS-COV-2 glycoprotein regions has also been hypothesized to prevent the cellular entry of viral particles [171,172].

4.2 Blocking the viral replication and proliferation

Rectifiers that slow down the rate at which viral particles reproduce or reduce their infectiousness are at consummate significance, as this will prevent the proliferation of these infectious particles and thereby provide time for response of body’s first-line immune system to function effectively in combating the virus. This will also prevent the infectious particles from generating mutated versions as their replication is blocked. Several viruses with positive-sense ssRNA genetic material, phospholipid envelopes, and proteins resemble SARS-CoV-2. Recent coronavirus research has employed several of these viruses as models, and NP efficacy on these viruses may be important for therapeutic development toward SARS-CoV-2 [160].

A recent study has shown that the transmissible gastroenteritis virus (TGEV), one member among the coronavirus family, is much less infectious in the presence of AgNPs and silver nanowires at a concentration below the toxicity limit. In addition to this, AgNPs have been found to limit apoptosis induced by viral inhabitation. This happens when Ag NPs suppress TGEV-induced Pi-p38 protein production to control p38-MAPK-p53 mitochondrial signalling, which in turn regulates TGEV-induced cell death [173]. Furthermore, Du et al. have reported that AgNP-modified graphene oxide suppresses porcine reproductive and respiratory syndrome virus (PRRSV), a model virus utilized in coronavirus research, with 59.2% inhibitory efficacy. To add, GO-AgNPs nanocomposite treatment was found to boost IFN-α and ISG production, which directly restricted viral growth [174]. In another investigation, Tong et al., synthesized a glycyrrhizic-acid-based CDs with multisite PRRSV suppression of up to five orders of virus titers. The multiple inhibitory mechanisms include viral invasion and replication inhibition, cell IFN stimulation, and ROS generation inhibition [175]. The work has exemplified the undefined potential of CDs in viral inhibition, which could help in the development of novel strategies of NPs conjugated antiviral therapies.

Haam et al. employed porous AuNPs to aim the heme agglutinin (HA) protein on several influenza viruses by exploiting effective goldthiol interconnection (PoGNPs) as it has been one of the most intensively researched viruses due to many worldwide pandemics over the past decades. Because of recurring mutations and growing treatment resistance, influenza A viruses are considered the focus of numerous NP-based medicinal research initiatives. In that particular study, Haam found that PoGNPs suppress the viral infectivity and increased host cell viability to 96.8% from 33.8%. In addition, viruses like H1N1, H3N2, and H9N2 were used to demonstrate the approach’s universal effectiveness [176]. In a recent study, Haag et al. employed electron microscope imaging to visually demonstrate that AuNPs functionalized with sialic acid-terminated glycerol dendrons effectively and inhibited viral multiplication by targeting the viral HA protein [177]. A benzoxamine-monomer-derived CDs have been found to reduce Zika virus, the causative of a 2015 pandemic in South and North America which in turn shares structural similarity with coronavirus [178]. The CDs have been also found inhibitory to other viruses that are structurally similar to coronavirus, dengue, and Japanese encephalitis virus. Also, they have been found to inhibit adeno-associated virus and porcine parvovirus, non-enveloped viruses, which in turn emphasizes the spectrum of antiviral potential of NPs [178]. Alphaviruses, a similar genus of RNA viruses to coronaviruses, were substantially repressed in Vero (B) cells by cellulose nanocrystals treated with tyrosine sulfate mimetic ligands, whereas human cells were found to be unaffected [179].

4.3 Viricidal NPs

Another method of preventing viral infections in addition to blocking the host cell contact, genetic material replication and proliferation of virus is inactivating or destroying the virus directly. In a sophisticated reversible viral investigation, AuNPs coated with 3-mercaptoethylsulfate (MES) produced an inhibitory concentration at EC₉₀ (nanomolar range). This in turn is identical to the outcome of heparin, a popular antiviral substance that inhibits pathogen and host contact. However, the authors also discovered that replacement of MES with a 2:1 combination of undecanesulfonic acid (MUS) and 1-octanethiol (OT) could elicit permanent/irreversible viral inactivation of several viruses that specifically target humans, including herpes simplex virus, respiratory synctial virus, dengue virus, as well as human papiloma virus, and lentivirus [180]. As the study reported no cytotoxicity during ex vivo and in vivo studies on mice and humans, respectively, and as the broad-spectrum model viruses employed in the study shared structural and functional similarities with the coronavirus, this study emphasizes the potential use of MUS:OTAuNPs in SARS-Cov-2 treatment [160].

In a recent study, it was demonstrated that a functionalized mock virus receptor nanodisc, a self-assembled discoidal membrane covered in an amphipathic membrane scaffold protein, can neutralize the infected influenza virus (H1N1) by specific inhibition of viral surface proteins and thereby produce permanent damage to the viral envelope. As a result of various associations with viral target proteins, conjugating sialic acid onto nanodiscs enhanced their antiviral efficacy. It was reported that the functionalized nanodiscs also prompted the virus’s fusion machinery to self-disrupt its envelope. This approach is promising for in vivo studies due to the biocompatibility of the NPs and the decoy molecules [181]. Gao and colleagues reported the existence of iron oxide NPs with broad-spectrum antiviral activity against flu viruses (H1N1, H5N1, and H7N9). Using the catalytic, enzyme-like, and peroxidase-catalyzing properties of ferromagnetic NP with an average diameter of 200 nm, it was possible to cause lipid peroxidation of the viral envelope, therefore eliminating the viral surface protein. Interestingly when put on facemasks, these iron oxide nanozymes demonstrated broad spectrum antiviral efficacy [182]. The site of activation of various nanoformulated drugs employed for combating COVID-19 has been illustrated in Figure 2.

Figure 2

Site of activation of nanoformulated drugs at different stages of viral life cycle.

Recently, lipid NP (LNP)-based mRNA vaccines are being considered for clinical management for COVID-19. mRNA is a promising therapeutic tool for COVID-19, requiring safe, stable, and targeted delivery systems together with endosomal escape for in vivo use. In this way, LNPs, especially in combination with mRNA vaccines, have made clinical strides, notably in combating COVID-19, marking a milestone for mRNA therapeutics. When comparing with cationic lipids and ionizable lipid conjugated vaccines, LNP-mRNAs excel in the safe delivery of mRNA molecules without degradation and effective facilitation of cellular uptake by cell membrane and functional immune response [183]. Two successful vaccines viz: Pfizer-BioNTech (Comirnaty) vaccine (BNT162b2) and Moderna (mRNA-1273) COVID-19 vaccine are examples of LNP-mRNA vaccines [184]. In a recent study focused on the development of LNP-mRNA vaccine through in vivo studies in mice, researchers were successful in developing an adjuvant lipidoid for LNP-mRNA-based vaccines that could enhance the adjuvanticity of mRNA molecules. This innovation proved effective in improving the safe delivery of mRNA and simultaneous activation of toll-like receptor 7/8-agonistic properties, thereby enhancing the innate immune response [185].