Intelligent Nanobiosensors for SARS-CoV-2 Diagnosis: Comparison
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Please note this is a comparison between Version 2 by Vivi Li and Version 1 by Kshitij RB Singh.

The novel coronavirus appeared to be a milder infection initially, but the unexpected outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), commonly called COVID-19, was transmitted all over the world in late 2019 and caused a pandemic. Human health has been disastrously affected by SARS-CoV-2, which is still evolving and causing more serious concerns, leading to the innumerable loss of lives. Smart and intelligent nanomaterial-enabled biosensors (nanobiosensors) have already proven their utility for the diagnosis of several viral infections, as various detection strategies based on nanobiosensor devices are already present, and several other methods are also being investigated by researchers for the determination of SARS-CoV-2 disease.

  • nanobiosensors
  • smart and intelligent nanomaterials
  • nanomaterials
  • SARS-CoV-2
  • COVID-19
  • biosensors

1. Introduction

In Wuhan, China, on 31 December, an unprecedented respirational disease became apparent and spread rapidly throughout the world. Subsequently, it became evident that this was a new form of coronavirus that had occurred previously, and the World Health Organization (WHO) termed it as the 2019 novel coronavirus (nCoV-2019) [1]. Afterward, the International Committee on Taxonomy of Viruses named it severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 belongs to the Coronaviridae and Nidovirales families, which have a single-stranded ribonucleic acid (ssRNA) genome analogous to Middle East respiratory syndrome (MERS-CoV) and SARS-CoV, and alpha (α), beta (β), gamma (γ), and delta (δ) coronavirus are subgroups of this family based on genetic properties. Moreover, SARS-CoV-2 comprises the matrix (M), spike (S), nucleocapsid (N), and envelope (E) structural proteins, and these proteins can be employed as the analyte of interest for the determination of SARS-CoV-2 [2,3][2][3].
The devastating pandemic situation has led to approximately 540 million affirmed cases, and beyond 6.33 million fatalities all over the world as of 24 June 2022; therefore, compared to preceding pandemics, SARS-CoV-2 is considerably more infectious, and it shows the utmost contagious efficiency between all the members of the Coronaviridae family [5,6][4][5]. Unfortunately, this dreadful condition can continue for an indeterminate duration. Early SARS-CoV-2 symptoms include fever, sore throat, discomfort, loss of taste and/or smell, cough, and respiratory distress. It mainly spreads through the exposure of a normal person to an infected person within the vicinity of 1 m, as while coughing or sneezing, the produced germ particles keep dangling in the atmosphere or may be enclosed in a suspended substance, permitting their transmission through the air [7,8][6][7]. Apart from causing health issues, this disease has caused an economic crisis globally. The serious obstacles related to SARS-CoV-2 involve its prevention and prompt diagnosis. Further, presently, no authorized medicines are accessible to combat the SARS-CoV-2 virus [7,9][6][8]. Several strategies and investigations have been proposed to prevent further transmission of the SARS-CoV-2 virus, for instance, quarantine, social distancing, the use of face masks, timely detection, and reliable treatment. Many diagnostic methods are accessible for a SARS-CoV-2 assay, and various are under progress [10,11][9][10]. Currently, the most popular molecular technique for the ongoing epidemic is reverse transcription–polymerase chain reaction (RT-PCR), which is based on single-stranded nucleic acid amplification tests; this may be performed via brush biopsies or nasal swabs and may give results within 4–6 h. It is a precise method that needs a small amount of RNA. RT-PCR sensitivity and specificity depend on the primer design and assay design; moreover, it is dependent on the proficiency of the individual conducting the analysis as the analysis is automated in most cases. Although these techniques are more remarkable, there are still a few limitations, such as the time taken, the high cost, its unsuitability for rapid detection, and laborious nature [12,13][11][12]. Similarly, reverse transcription–loop-mediated isothermal amplification (RT-LAMP) and clustered regularly interspaced short palindromic repeats (CRISPR) testing are used for rapid and point-of-care detection. Moreover, the lateral flow immunoassay (LFA) and enzyme-linked immunosorbent assay (ELISA) have emerged with great potential for point-of-care testing [5,14][4][13]. Notwithstanding that the efforts have never come to an end to create quick and exceptional techniques, there is still an absolute necessity for a fast, practical, cheap, and easy-to-use testing approach for diagnosing SARS-CoV-2 quickly and accurately. The most recent development in nanotechnology may provide considerable support for diagnostic and treatment methods in this period of pandemic diseases. Nanotechnology also has an essential role in designing and developing biosensors [15][14].
Fortunately, biosensors can be an excellent alternative to the traditional tools for quick point-of-care detection, and may be regarded as an analytical tool for determining the SARS-CoV-2 virus. Moreover, a biosensor (analytical tool) is comprised of biological components that can detect the existence of a target ligand and produce a signal which is proportional to the concentration of the analyte. The three main components of the biosensor are the bioreceptor, transducer, and amplifier. The bioreceptor is a biomolecule that catalyzes or binds a specific reaction for a particular analyte to produce a detectable signal, and the transducer converts the recognition event into a measurable signal that can be stored, manipulated, analyzed, and displayed. The amplifier simply amplifies the converted biological signal to an electrical signal [16,17][15][16]. The sensitivity and efficiency of the biosensor may be enhanced and developed by using nanomaterials for their fabrication [18][17]. The nanomaterial size is between 1–100 nm, which is intended to exhibit new characteristics compared to materials without nanoscale features, in particular, better conductivity, and unique thermal, optical, and chemical properties. The application of nanomaterials in the healthcare sector for rapid point-of-care diagnostics, drug delivery, and bio-imaging has resulted in significant interest. Due to their enhanced volume-to-surface ratio and magnetic and electrical properties, the nanostructures of metal and metal oxide (gold, silver, copper, cupric oxide, etc.), carbon materials, quantum dots, and polymers have been used extensively for the fabrication of biosensors [19][18]. Therefore, the nanomaterial-based biosensor comprises several advantages because of the nanomaterial’s remarkable physicochemical properties, quantum-size effect, and high adsorption. Nanomaterial-based biosensing has been successfully implemented to detect numerous infections, for example, HIV, influenza virus, and hepatitis B virus, so it also has great potential to determine SARS-CoV-2 [2]. This entreviewy emphasizes the application and function of various nanomaterial-based biosensing techniques for SARS-CoV-2 because of the urge to develop early diagnostics devices. Consequently, metal-based, carbon-based, quantum dot-based, and polymer-based nanobiosensors have played a crucial part in determining the SARS-CoV-2 virus. Accordingly, this studentry focuses on nanomaterial-based biosensors, challenges, limitations, and prospects.

2. Nanobiosensors for SARS-CoV-2 Diagnosis

Countless efforts have been made to develop several nanomaterial-based biosensors to identify various diseases such as the hepatitis C virus, porcine epidemic diarrhea, human immune deficiency virus, and SARS-CoV-2 virus in the last several years [2,52][2][19]. The nanomaterial-based biosensor has been the focus because of its exclusive detection properties, which allow the sensing of analytes such as aptamers, nucleic acids, and proteins. Nanomaterials, namely carbon-based, polymer-based, quantum-dot, and metal-based nanomaterials, are frequently intended for biosensing applications to diagnose SARS-CoV-2 infection through various modes of detection (Table 1)—such as viral sequence, antibody, or antigen—as they have already exhibited their potential for detecting other viruses. Thus, nanomaterials and their composites have unique properties such as excellent adsorption, quantum-size effects, and great surface-to-volume ratios, which play a vital role in refining the performance of biosensors [53][20].
Table 1.
 Characteristics of nanobiosensors for detection of SARS-CoV-2 virus.
S. No. Type of Sensor Core Nanomaterials Biomarker Limit of Detection Detection Time Ref.
1 Plasmonic-effect-based colorimetric biosensing Gold NPs Thiol-modified antisense oligonucleotides (ASOs) specific for nucleocapsid phosphoprotein (N-gene) of SARS-CoV-2 0.18 ng/µL 10 min [54][21]
2 Combined effect of plasmonic photothermal

(PPT) and localized-surface-plasmon resonance (LSPR) biosensing		 Gold NPs SARS-CoV-2 nucleic acid/thiol-cDNA 0.22 15–20 min [55][22]
3 LSPR biosensing Silver NPs SARS-CoV-2 spike RBD protein 0.83 pM 20 min [56][23]
4 Lateral flow immunoassay based on fluorescence biosensing Lanthanide-doped polystyrene NPs Protein - 10 min [57][24]
5 Electrochemical biosensing Cobalt functionalized TiO2 SARS-CoV-2 spike RBD protein ~0.7 nM 30 s [8][7]
6 Electrochemical biosensing Graphene oxide RNA of SARS-CoV-2 200 copies/mL - [58][25]
7 Nanoresonator biosensing Graphene SARS-CoV-2 spike S1 antigen 10 copies per test - [59][26]
8 Field-effect transistor-based amperometric biosensing Graphene SARS-CoV-2 spike protein S1 1.6 × 10 1 pfu/mL in spike culture

2.42 × 10 2 copies/mL in clinical samples		 5–8 min [1]
9 Electrochemical biosensing Carbon nanotube SARS-CoV-2 spike S1 antigen 4.12 fg/mL 2–3 min [60][27]
10 Electrochemical biosensing Quantum dots RNA of SARS-CoV-2 - - [61][28]
11 Electrochemical biosensing Polymer nanomaterials SARS-CoV-2 nucleoprotein 15 fM - [62][29]
12 Electrochemical biosensing Polymer nanomaterials SARS-CoV-2 spike protein subunit S1 15 fM in buffered saline and 64 fM in nasopharyngeal - [63][30]
13 Circle-to-circle-amplification-based optomagnetic biosensing Iron oxide NPs DNA of SARS-CoV-2 0.4 fM ~100 min [64][31]
14 LSPR biosensing Gold NPs SARS-CoV-2 nucleocapsid (N) protein 150 ng/mL 5 min [65][32]

2.1. Metal and Metal Oxides-Based Nanobiosensors

Metal nanoparticles have created enormous potential for treating and diagnosing pathogenic viruses, as nanoparticles, namely copper (Cu), Gold (Au), iron (Fe), and silver (Ag), display excellent antibacterial and antiviral activity, contrary to other nanoparticle types [7,66][6][33]. NP biosensors comprise a biological substrate or immobilized ligand, which undergoes conversion with the occurrence of an analyte. The analyte induces modification in the substrate composition, thus imparting a change in the surroundings of the NPs, resulting in changes in the aggregation status of the NPs and leading to detectable color changes [67][34]. Gold nanoparticles (AuNPs) have recently garnered consideration by virtue of their unique optical, physical, catalytical, biocompatible, and electrical properties; furthermore, the biosensing of virus infection is mostly performed by gold nanoparticles [68,69][35][36]. NPs could conjugate with receptors to form aggregates. AuNPs couple with antibodies specific to the SARS-CoV-2 antigens, and in the presence of antigens, AuNPs aggregate. The aggregation of AuNPs leads to the optical absorption spectrum, which undergoes a red shift (larger wavelength) and the solution changes to a blue spectrum [70][37]. Therefore, various gold-nanoparticle-based biosensors can determine the SARS-CoV-2 infection via the colorimetric method. For instance, Moitra, et al. conducted a colorimetric bioassay with gold-nanoparticle-capped thiol-modified antisense oligonucleotides (ASOs) to determine SARS-CoV-2 by considering nucleocapsid phosphoprotein (N-Gene) as an analyte of interest. The methodology uses a comprehensive targeting method facilitated by four of the ASO sequences (which are selected based on their relative binding energies with the target sequence and binding disruption energies) covering two regions of the target gene sequence (N-gene) simultaneously. Additionally, with the occurrence of the target RNA sequence of SARS-CoV-2, the thiolated ASO-capped AuNPs agglomerate and a modification in its SPR is demonstrated; this is further augmented with the addition of RNaseH (which cleaves the RNA strand from the RNA–DNA hybrid), leading to the visual naked-eye detection of AuNPs. The identification is made through surface plasmon resonance with a red shift of approx. 40 nm in the spectra of absorbance. Further, this same method was carried out for MERS-CoV viral RNA determination, but no alteration in absorbance was observed [54][21]. Additionally, an unusual effort was made with gold nanoparticles to identify the disease and bacteria in laboratory and medical samples. Gasbarri, et al. illustrated the antiviral activity of the sulfonated compound against SARS-CoV-2 by using gold nanoparticles coated with decanesulfonic acid in the micromolar and nanomolar range. Previously, sulfonated compounds were shown to be active against the heparan sulfates (HS) virus [71][38]. Further, Li, et al. fabricated a gold-nanoparticle-based LFIA strip that can concurrently diagnose IgM and IgG antibodies for SARS-CoV-2, which might be employed to quickly screen asymptomatic and symptomatic patients of SARS-CoV-2 in laboratories and hospitals in real-time, POC diagnostics. Additionally, a combined IgG-IgM antibody test kit for SARS-CoV-2 has numerous advantages; for example, it saves time, is easy to carry out, additional equipment is not required, and it requires little training [72][39]. Further, a single line at the control (C) validates the authenticity of the test; thus, this line is a must during analysis, and G and M, respectively, represent IgG and IgM. Further, the single line at G and M represents previous and recent infection with SARS-CoV-2, respectively. In addition, lines at both G and M represent SARS-CoV-2 infection, and the absence of lines in both places represents no infection or early infection due to antibodies not being available in traceable amounts [72][39]. Qui, et al. reported a two-dimensional gold-nanoisland (AuNIs)-based nucleic acid sensor for detecting SARS-CoV-2. A dual-functional plasmonic biosensor was successfully fabricated and incorporated with localized surface plasmon resonance (LSPR) and a plasmonic photothermal (PPT) effect as transduction principles for sensing. This biosensor was fabricated using a 2D AuNIs chip following the self-assembly of thermally de-wetted Au nanofilm on the BK7 glass surface. The Au-film was firstly prepared via magnetron-sputtering. The LSPR and PPT can be excited at two different wavelengths through two different rent angles of incidence, which considerably amplifies the sensitivity, reliability, and sensing stability. Therefore, the suggested biosensor can provide an easy and reliable diagnosis platform; moreover, it alleviates the pressure of PCR-based tests. The proposed LSPR biosensor has superb sensitivity for detecting SARS-CoV-2 nucleic acid, with a detection limit of 0.22 pM [55][22]. Similarly, silver nanoparticles (AgNPs) are taken into consideration as one of the potential antiviral agents that can act against the lethal virus. AgNPs can be incorporated to the viral genome, obstructing the interaction and activity of numerous virus and cellular factors responsible for repetition and resulting in the prohibition of viral replication. By evaluating various properties such as shape, size, and surface charge, the interaction of AgNPs with viruses is enhanced [73][40]. AgNP-based biosensors could be employed in diagnostics and in the sensing of viral infection. Due to their catalytic, photonic, electronic, optical, and clinical applications, AgNPs are crucial in combating the SARS-CoV-2 virus [55,74][22][41]. Yang, et al. developed an ACE2-functionalized silver-nanotriangle (AgNT)-based LSPR sensor for detecting SARS-CoV-2 spike RBD with a detection limit of 0.83 pM. The fabricated biosensor consists of four steps: the fabrication of AgNT, the immobilization of ACE2 on AgNT, BSA blocking, and then, the detection of the virus or RBD protein. The developed sensor shows high specificity and sensitivity to CoV NL63 and SARS-CoV-2 [56][23]. Ghisetti, et al. reported on the POCT STANDARD Q COVID-19 Ag for rapid SARS-CoV-2 nucleoprotein detection in NP swabs, with the result appearing in less than 30 min; this was applied to 330 patients of two different populations from phase 1 and 3 of the pandemic. The main advantage of POCT for antigen testing is that it requires limited technical skill and infrastructure; furthermore, it is certainly preferable in point-of-care centers for mass screening of the SARS-CoV-2 virus. The study reported that 70.6% of the positive RT-PCR samples were not false-positive results, which were identified via STANDARD Q COVID-19 Ag [75][42]. Teengam, et al. reported AgNP paper-based colorimetric DNA sensors for detecting Middle East respiratory syndrome (MERS). MERS-CoV DNA was effortlessly determined using a cationic pyrrolidinyl peptide nucleic acid (acpcPNA) probe. The probe was positively charged with a lysine moiety present at the carboxyl terminus so it could interact with the negatively charged targeted DNA or AgNPs. In the case of DNA binding, it prevents aggregation without any color change, while in the case of AgNP binding, it leads to NP aggregation with a significant color change. This fabricated paper-based colorimetric DNA sensor could be another approach for easy, quick, sensitive, and selective DNA detection, and therefore, the fabricated sensor can serve as a POC diagnostic tool for viral DNA detection. In the same way, SARS-CoV-2 DNA may be detected by AgNPs [76,77][43][44]. One more work by Chen, et al. developed a simple and rapid lateral flow immunoassay based on lanthanide-doped nanoparticles (LNPs) to diagnose anti-SARS-CoV-2 IgG in serum samples within 10 min. This biosensing platform was fabricated through lanthanide-doped polystyrene NPs, developed using the mini-emulsion polymerization method. By using rabbit IgG (R-IgG) following a EDC/NHS chemical reaction and mouse anti-human IgG antibody (MH-IgG), the surface modification of LNPs was performed. LNPs have demonstrated several optical properties, as they possess remarkable electronic configurations along with high and sharp emission bands, which are extensively used for sensitive biosensing applications [57][24]. Likewise, various metal oxides were recently turned into significant materials for identifying several viral diseases, including influenza, ebola, and current COVID-19 [78,79][45][46]. Ishikawa, et al. reported that In2O3 nanowires were grown on Si/SiO2 substrate via the laser ablation CVD method to detect the SARS-CoV-2 N protein. The fabricated FET biosensor could detect N protein at subnanomolar concentrations, with a response time of ~10 min [80][47]. Similarly, transition metal oxide is extensively reviewed for its antiviral nature due to its stability, physiochemical properties, cost-effectiveness, and reasonable stability rate. Vadlamani, et al. developed a cobalt-functionalized TiO2-nanotube-(Co-TNTs)-based electrochemical sensor for detecting the SARS-CoV-2 spike-receptor-binding domain protein with a detection limit of 0.7 nM. The TNTs were fabricated via one-step electrochemical anodization of the Ti sheet and Co functionalization was performed using the wet ion-exchange method [8][7]. Neal, et al. synthesized two sliver-modified cerium oxide nanoparticles by using the base-mediated forced hydrolysis method for SARS-CoV-2, as the cerium oxide nanomaterial has demonstrated considerable biomedical efficiency [81][48]. The SARS-CoV-2 virus spreads through surfaces (such as handles, gas pumps, doors, and elevator buttons), and remains on the surface for up to 1 week; thus, surface transmission can be inhibited by using a coating. Coating the surface with cuprous oxide (Cu2O) inactivates 99.9% of SARS-CoV-2 [82][49]. For example, Hosseini, et al. fabricated cupric oxide (CuO), which diminishes infection from SARS-CoV-2. The porous film creates a good connection between the microbe and the CuO solid because of the highly sintered area. A thin coating of CuO, developed via the thermal oxidation of Cu2O, succeeded via sintering. The inactivity of the virus due to the coating of CuO was examined using Vero E6 cells [83][50]. It has become known that the SARS-CoV-2 virus is mainly transmitted from an infected person’s nose or mouth in small liquid particles when they cough, speak, sneeze, and breathe; these pass-through air and are small enough to remain airborne for hours. Therefore, there have been several reviews reported on breath analysis due to its potential for the screening, treatment, and diagnosis of SARS-CoV-2. Moreover, it could be used at home, in central facilities, and in POC, which lowers the burden on hospitals [84,85][51][52]. Thus, Haick, et al., in 2022, reported a AuNP-based sensor for screening and detecting the SARS-CoV-2 virus directly from exhaled breath. Recent studies have suggested that the microenvironments emitting volatile organic compounds were created by the viral agents of SARS-CoV-2. The various sensing layers developed by the link of AuNPs to organic ligands shrink and bulge upon exposure to VOCs; this causes changes in the electric resistance, which can be measured digitally. The proposed device needs frequent washing with alcohol and the sensor needs to be flushed with air after every use. The VOCs can target breath diagnostics without being invasive to patients. The study cohort included 14 symptomatic negative cases, 47 asymptomatic controls, and 41 confirmed cases of the SARS-CoV-2 virus [86][53].

2.2. Carbon-Based Nanobiosensor

Carbon-based nanomaterials, namely carbon nanotubes, graphene, and fullerene, are fascinating for antimicrobial application. They are favorable materials for combating SARS-CoV-2 viruses via various mechanisms because of their exceptional electrical conductivity, optical and biocompatible performance, etc. [87,88][54][55].

2.2.1. Graphene

Graphene is an intriguing two-dimensional (2D) carbon material in which each carbon in graphene is sp2 hybridized, and the carbon atoms are covalently linked in a honeycomb lattice. Furthermore, it has attracted much consideration owing to its promising antiviral and antimicrobial applications. However, the interactions among graphene derivatives and viruses are still disputable. Graphene materials are highly suitable for biosensing to detect viruses due to their unique characteristics and properties such as electrical and thermal conductivity, light weight, chemical stability, high optical transparency, their function as a zero-band-gap semiconductor, non-toxicity, and mechanical stability. Additionally, the large surface-area-to-volume ratio of graphene is advantageous for drug carrier applications, but it can also play a vital role in biosensing applications [89,90,91][56][57][58]. In biosensors, graphene-based nanomaterials are employed as transducers that convert the interactions among the target molecule and the receptor molecules into measurable signals [92][59]. For this to occur, molecules such as ssDNA, enzymes, and antibodies must be attached to the transducer surface. The most common approach for enzyme immobilization onto graphene, graphene oxide, and reduced graphene is physisorption, while ssDNA and antibodies are most commonly immobilized using EDC/NHS chemistry [93][60]. In several studies, graphene-based biosensing devices have already been reported to screen various infectious diseases, including SARS-CoV-2. Two-dimensional-material-based biosensors can be classified as optical-based devices, mainly relying on sensing via SPR, surface-enhanced Raman scattering (SERS), and fluorescence and electrode-based devices, including field-effect transistors (FET) and electrochemical biosensors [94][61]. Graphene-based FETs have mainly been significant due to their low noise detection, rapid estimation, even in the presence of small amounts of the analyte, and ultra-sensitivity [95][62]. For instance, by utilizing the properties of graphene, Seo, et al. fabricated a FET-based biosensor with a LOD of 1.6 × 101 pfu/mL, and 2.24 × 102 copies/mL in the case of SARS-CoV-2 spiked culture and clinical samples. The FET-based biosensing device was developed by coating graphene surface with a specific antibody of the SARS-CoV-2 spike protein. The fabricated graphene-based biosensor could detect the SARS-CoV-2 virus because the functionalized graphene-based biosensor provides rapid, highly responsive, and simple detection [1]. Huang, et al. [96][63] developed ultrasensitive (low limit of detection of 1.6 pg/mL) and selective graphene@chitosan@Au nanoparticles (G@CS@Au NPs) and G@Ag NPs@CS-based sandwich-type biosensors (immunosensor) for diagnosing avian influenza virus H7. In this sandwich-type immunosensor, H7-monoclonal antibodies (MAbs) are attached with G@CS@Au NPs, and H7-polyclonal antibodies (PAbs) are attached with G@Ag NPs@CS; this fabrication of a diagnostic approach for the detection of the virus led to the development of biosensors for virus determination. Further, Palmieri, et al. demonstrated that graphene could be one of the prime materials for combating the SARS-CoV-2 virus by interacting with the surface of the virus, its genetic material, and its proteins via diffusion of these materials to inhibit functions such as genome replication and protein synthesis, which damages the integrity of the virus [97][64]. Layqah, et al. developed an electrochemical immunosensor using a carbon electrode with gold nanoparticles to detect the MERS-CoV virus [98][65]. Yusof, et al. showed that graphene is an appropriate material to fight the spread of the SARS-CoV-2 virus, as biomass-derived graphene has enormous potential to be implemented as a sterilizer and can be used in face masks to inhibit any droplets from entering our body [99][66].

2.3. Quantum-Dot-Based Nanobiosensors

Quantum dots (QDs) were first ascertained around the beginning of the 1980s. Quantum dots are inorganic semiconductor materials conjugated with high fluorescent probes, usually in the range of 1 to 10 nm with a tunable optical wavelength. Furthermore, quantum dots are often cited as artificial atoms [124][67]. Quantum dots have captured enormous attention in nanotechnology, biotechnology, and bioimaging applications, with high sensitivity, selectivity, and rapidity because of their unique optical and electronic properties [125,126][68][69]. Quantum dots have been considered promising for sensing and treating SARS-CoV-2 and other viral infections, as their low toxicity and high surface performance make them supreme amongst other nanomaterials [127][70]. Quantum-dot-based nanomaterials, for example, fluorescent, graphene, and carbon quantum dots, have also attracted considerable attention for developing an efficacious biosensor for detecting various viral infections due to their unique properties [128][71]. Boroushaki, et al. fabricated a carbon electrochemical sensor to fabricate a screen-printed electrode reinforced by boron nitride quantum dots/a flower-like gold nanostructure. The developed electrochemical biosensor has the potential for the sensitive and accurate diagnosis of SARS-CoV-2 in laboratory samples [61][28]. Further, Zheng, et al. developed a fluorescent quantum dot coupled with the B-cell epitopes of SARS-CoV-2 to determine SARS-CoV-2 with a detection limit of 100 pM. B-cell-epitope-based quantum dot biosensors can vigorously separate coronavirus diseases compared to the conventional ELISA. Fluorescent quantum dots have the potential to detect antigens and antibodies quickly [129][72]. Graphene quantum dots (GQDs) are nanoscale structures of graphene with a size of less than 100 nm, which show outstanding properties such as exceptional biocompatibility; electronic, optical, and quantum confinement; low toxicity; high water solubility; and multicolored emission [130,131][73][74]. Owing to the aforementioned properties, graphene quantum dots have potential in biomolecule detection, drug delivery, and the biological imaging of numerous viral infections, and are promising for nanotechnological development [132][75]. Graphene quantum dots can be synthesized either via bottom-up or top-down methods [133][76]. Song, et al. synthesized N-doped graphene quantum dots (NGQDs) via a microwave-assisted hydrothermal reaction for the transfection of several genes, as the graphene quantum dots provide a promising platform for drug delivery [134][77]. Ray, et al. synthesized human cathelicidin LL-37 and neutrophil α-defensin HNP1 (a human host defense peptide) attached to GQDs. GQDs inhibit the delta variant’s virus by accessing host cells and prohibiting the spike protein RBD from binding with ACE2 due to the GQDs’ unique properties [135][78]. Li., et al. fabricated a magnetic relaxation switch (MRSw) to detect and eradicate the antigen S protein of SARS-CoV-2 based on GQDs. This approach gives quick, safe, and extremely sensitive identification of the SARS-CoV-2 virus [136][79]. Carbon quantum dots (CQDs), also called carbon dots, are carbon-based nanomaterials that are small nanoparticles with an average size below 10 nm. CQDs have shown tremendous potential in various applications such as photothermal therapy, drug delivery, bioimaging, and biosensing, due to their state-of-the-art properties for example, conductivity, optical detection, high chemical stability, outstanding biocompatibility, and low cost. Furthermore, with brilliant accuracy, they have demonstrated the incredible capability to detect and exterminate several pathogenic viruses, including the SARS-CoV-2 [137][80]. Loczechin, et al. developed carbon-based quantum dots for the potential diagnosis of human coronavirus (HCoV-229E), and boronic acid improved CQDs [138][81]. Thus, QDs can play a significant role in determining SARS-CoV-2, but are not explored fully in terms of SARS-CoV-2 diagnosis [139,140][82][83]. Hence, researchers focusing on the applications of various QDs, such as CQDs and GQDs, can explore their utility for determining contagious viral infections such as SARS-CoV-2 and MERS-CoV.

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