Nanobiosensors for the Detection of Novel Coronavirus 2019-nCoV and Other Pandemic/Epidemic Respiratory Viruses: A Review

The coronavirus disease 2019 (COVID-19) pandemic is considered a public health emergency of international concern. The 2019 novel coronavirus (2019-nCoV) or severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that caused this pandemic has spread rapidly to over 200 countries, and has drastically affected public health and the economies of states at unprecedented levels. In this context, efforts around the world are focusing on solving this problem in several directions of research, by: (i) exploring the origin and evolution of the phylogeny of the SARS-CoV-2 viral genome; (ii) developing nanobiosensors that could be highly effective in detecting the new coronavirus; (iii) finding effective treatments for COVID-19; and (iv) working on vaccine development. In this paper, an overview of the progress made in the development of nanobiosensors for the detection of human coronaviruses (SARS-CoV, SARS-CoV-2, and Middle East respiratory syndrome coronavirus (MERS-CoV) is presented, along with specific techniques for modifying the surface of nanobiosensors. The newest detection methods of the influenza virus responsible for acute respiratory syndrome were compared with conventional methods, highlighting the newest trends in diagnostics, applications, and challenges of SARS-CoV-2 (COVID-19 causative virus) nanobiosensors.


Introduction
As a result of the coronavirus disease 2019 (COVID-19) pandemic, most research efforts around the world are focusing on solving this pressing problem, firstly, by developing ultrafast detection of the virus and isolating infected people, and secondly, finding and developing effective treatments, including vaccines specific to this new disease.

MERS-CoV
Sensors 2020, 20, x FOR PEER REVIEW 4 of 43 HCoV, as well as other types of viruses, including 229E, OC43, and NL63, generally infect the human upper respiratory tract [61], as presented in Table 1. According to its genomic structure, coronavirus is classified into four broad categories: alpha, beta, gamma, and delta [8]. Schematic trees of human coronaviruses (HCoVs) created from all four gen-groups are illustrated in Figure 2. Alpha and beta coronaviruses infect only mammals, usually causing respiratory symptoms in humans and gastroenteritis in other animals [8,63,66]. Alpha-CoVs includes human coronaviruses, such as HCoV-229E and HCoV-NL63, and bat coronaviruses. Different types of beta-CoV may infect a wide range of mammals, such as mice and humans. These types include SARS-CoV, HCoV-OC43, HCoV-HKU1, and MERS-CoV, murine coronavirus (MHV), and bovine coronavirus (B-CoV). Gamma-CoVs are specific to birds, except for beluga whale coronavirus. Delta-CoVs were discovered in 2012 with several subunits (HKU11, HKU12, and HKU13) [61]. Coronaviruses are positive-sense RNA viruses that belong to the Coronaviridae family of the nidovirales order, and the coronviridae subfamily [8,61,66,67].
The structural protein components of SARS-CoV-2 are spike (S) glycoprotein, small envelope (E), matrix (M) protein, nucleocapsid (N) protein, and several other accessory proteins. The spike (S) glycoprotein is critical for the host cell adhesion. The SARS-CoV-2 spike (S) protein binds with high affinity of the host cell receptor of SARS-CoV, the host cell receptor known as angiotensin-converting enzyme 2 (ACE 2) [9,68,69]. The M and E proteins are essential for virus assembly. The M protein is responsible for the transmembrane transport of nutrients and the formation of envelope, and the N and E proteins, and other several accessory proteins, obstruct the host immune response or have other unknown functions. S and N proteins are among the most valuable antigen biomarkers for diagnosing coronavirus disease 2019, similar to many detection methods for diagnosing SARS based on S and N proteins [20]. The schematic diagram of structural protein components of SARS-CoV-2 and H1N1 influenza is presented in Figure 3. Alpha-CoVs includes human coronaviruses, such as HCoV-229E and HCoV-NL63, and bat coronaviruses. Different types of beta-CoV may infect a wide range of mammals, such as mice and humans. These types include SARS-CoV, HCoV-OC43, HCoV-HKU1, and MERS-CoV, murine coronavirus (MHV), and bovine coronavirus (B-CoV). Gamma-CoVs are specific to birds, except for beluga whale coronavirus. Delta-CoVs were discovered in 2012 with several subunits (HKU11, HKU12, and HKU13) [61]. Coronaviruses are positive-sense RNA viruses that belong to the Coronaviridae family of the nidovirales order, and the coronviridae subfamily [8,61,66,67].
The structural protein components of SARS-CoV-2 are spike (S) glycoprotein, small envelope (E), matrix (M) protein, nucleocapsid (N) protein, and several other accessory proteins. The spike (S) glycoprotein is critical for the host cell adhesion. The SARS-CoV-2 spike (S) protein binds with high affinity of the host cell receptor of SARS-CoV, the host cell receptor known as angiotensin-converting enzyme 2 (ACE 2) [9,68,69]. The M and E proteins are essential for virus assembly. The M protein is responsible for the transmembrane transport of nutrients and the formation of envelope, and the N and E proteins, and other several accessory proteins, obstruct the host immune response or have other unknown functions. S and N proteins are among the most valuable antigen biomarkers for diagnosing coronavirus disease 2019, similar to many detection methods for diagnosing SARS based on S and N proteins [20]. The schematic diagram of structural protein components of SARS-CoV-2 and H1N1 influenza is presented in Figure 3. These proteins can be essential bioindicators that can be used to enhance the sensitivity and selectivity of nanobiosensors.
To detect the COVID-19 causative virus, the coronavirus shows spike protein immunogens [71]. The generation of immunoglobulins by the immune system increases the detection mechanism. Accordingly, immunoglobulins are important for the detection and possible treatment of COVID-19 [72]. The mechanism of coronavirus infection and replication cycle is very important for choosing the right detection method applications and laboratory tests.
According to Acter et al. [73], the mechanism by which coronavirus infection occurs, and its mode of replication/transcription (Figure 4), are as follows: (i) people with SARS-CoV-2 virus through the environment by an intermediate host; (ii) in the first phase, coronavirus connects to the alveolar cells in the lungs through the ACE 2 enzyme using spike (a special surface of glycoprotein), and in the second phase, it enters in the host cell; then, the virus detaches and the RNA genome penetrates the cell cytoplasm, attaches to the ribosomes of the host cell and undergoes translation of viral polymerase; (iii) RNA replication and transcription: nonstructural proteins combine and form RdRp, which represents a multi-protein replicase-transcriptase complex (RTC), RNA polymerase RdRp, and finally RdRp synthesizes positive-sense genomic RNA as descending viruses by replication and subgenomic transcripts; (iv) the host's ribosomes translate the messenger RNA into the structural These proteins can be essential bioindicators that can be used to enhance the sensitivity and selectivity of nanobiosensors.
To detect the COVID-19 causative virus, the coronavirus shows spike protein immunogens [71]. The generation of immunoglobulins by the immune system increases the detection mechanism. Accordingly, immunoglobulins are important for the detection and possible treatment of COVID-19 [72]. The mechanism of coronavirus infection and replication cycle is very important for choosing the right detection method applications and laboratory tests.
According to Acter et al. [73], the mechanism by which coronavirus infection occurs, and its mode of replication/transcription (Figure 4), are as follows: (i) people with SARS-CoV-2 virus through the environment by an intermediate host; (ii) in the first phase, coronavirus connects to the alveolar cells in the lungs through the ACE 2 enzyme using spike (a special surface of glycoprotein), and in the second phase, it enters in the host cell; then, the virus detaches and the RNA genome penetrates the cell cytoplasm, attaches to the ribosomes of the host cell and undergoes translation of viral polymerase; (iii) RNA replication and transcription: nonstructural proteins combine and form RdRp, which represents a multi-protein replicase-transcriptase complex (RTC), RNA polymerase RdRp, and finally RdRp synthesizes positive-sense genomic RNA as descending viruses by replication and subgenomic transcripts; (iv) the host's ribosomes translate the messenger RNA into the structural proteins, the viral structural proteins (S, E, and M) combine with nucleoplasmin by protein-protein interactions, resulting in viral formation, which finally is released from the host cell.
Sensors 2020, 20, x FOR PEER REVIEW 7 of 43 proteins, the viral structural proteins (S, E, and M) combine with nucleoplasmin by protein-protein interactions, resulting in viral formation, which finally is released from the host cell.  (Table 1) and have different infection mechanisms. For comparison, the mechanism by which infection with the influenza virus occurs and replicates has the following main stages: (i) binding influenza virus with cell receptors (adsorption); (ii) penetration of the influenza virus into the cell (endocytosis); (iii) fusion with endosome (nuclear entry); (iv) transcription, transition, and replication of viral genome segments; and (v) the release of free nucleocapsid in the cytoplasm (fusion sites) [70].
Based on similar aforementioned symptoms (fever, cough, shortness of breath, or difficulty breathing) of contagious respiratory diseases, it is difficult to differentiate the respiratory viruses that caused these epidemic/pandemic diseases. As a result, it is necessary to test the effects of these diseases by different methods [6,60,[74][75][76][77][78][79], firstly to confirm the diagnosis, and secondly to detect the viral nucleic acid, specific viral proteins,, or virions of human coronaviruses (HCoV), the newest and most deadly one, the 2019-nCoV (SARS-CoV-2) viruses.

Significance of Biological Receptors
Nanobiosensors offer several benefits that make detection highly effective, including achievable process, unique performance, high sensitivity, fast response, miniaturization, portability, and accuracy [80]. Nanobiosensors are devices in which the transducer [81] is modified to capture the target element, to convert the biological response into electrical signals, and to quickly detect it with high accuracy [82].  (Table 1) and have different infection mechanisms. For comparison, the mechanism by which infection with the influenza virus occurs and replicates has the following main stages: (i) binding influenza virus with cell receptors (adsorption); (ii) penetration of the influenza virus into the cell (endocytosis); (iii) fusion with endosome (nuclear entry); (iv) transcription, transition, and replication of viral genome segments; and (v) the release of free nucleocapsid in the cytoplasm (fusion sites) [70].
Based on similar aforementioned symptoms (fever, cough, shortness of breath, or difficulty breathing) of contagious respiratory diseases, it is difficult to differentiate the respiratory viruses that caused these epidemic/pandemic diseases. As a result, it is necessary to test the effects of these diseases by different methods [6,60,[74][75][76][77][78][79], firstly to confirm the diagnosis, and secondly to detect the viral nucleic acid, specific viral proteins" or virions of human coronaviruses (HCoV), the newest and most deadly one, the 2019-nCoV (SARS-CoV-2) viruses.

Significance of Biological Receptors
Nanobiosensors offer several benefits that make detection highly effective, including achievable process, unique performance, high sensitivity, fast response, miniaturization, portability, and accuracy [80]. Nanobiosensors are devices in which the transducer [81] is modified to capture the target element, to convert the biological response into electrical signals, and to quickly detect it with high accuracy [82].
The biological responses can be measured by the determination of the suitable bioreceptors, such as nucleic acids, antigens, DNA probe, peptide, whole cell, micro-organism, and tissue [21,83]. These receptors are easily recognizable, highly sensitive, and able to detect specific bioanalyte. Different types of bioreceptors have been explored to detect the viruses, such as nucleic acids (NA), immunoaffinity and protein in several types of nanobiosensors based on electrochemical, impedance, quartz crystal microbalance, and optical and surface plasmon resonance [25].
The target molecules in the case of respiratory viruses that cause pandemics are viral proteins (antigen, antibody), viral particles, viral nucleic acid, etc. [31,84,85]. The principal structural proteins in novel coronavirus 2019-nCoV that may be suitable targets for viral detection are spike (S) protein, membrane (M) protein, envelope (E) protein, and nucleocapsid (N) protein, and, in the case of other contagious respiratory diseases that have caused pandemics, such as swine flu, the targets are influenza virus M1 protein [86,87], or hemagglutinin (HA) and RNA glycoproteins and neuraminidase (NA) [70], as illustrated in Figure 3. The target molecule attaches to the bioreceptor [88] to detect a biological molecule by a particular reaction. Then, the transducer with integrated nanostructures converts the detection into an electrical signal determined by the detector [89]. The schematic diagram of different analytes, bioreceptors for biorecognition elements, transducers with integrated nanomaterials used for biosensing, as parts of a typical nanobiosensor for respiratory viruses, are presented in Figure 5. The biological responses can be measured by the determination of the suitable bioreceptors, such as nucleic acids, antigens, DNA probe, peptide, whole cell, micro-organism, and tissue [21,83]. These receptors are easily recognizable, highly sensitive, and able to detect specific bioanalyte. Different types of bioreceptors have been explored to detect the viruses, such as nucleic acids (NA), immunoaffinity and protein in several types of nanobiosensors based on electrochemical, impedance, quartz crystal microbalance, and optical and surface plasmon resonance [25].
The target molecules in the case of respiratory viruses that cause pandemics are viral proteins (antigen, antibody), viral particles, viral nucleic acid, etc. [31,84,85]. The principal structural proteins in novel coronavirus 2019-nCoV that may be suitable targets for viral detection are spike (S) protein, membrane (M) protein, envelope (E) protein, and nucleocapsid (N) protein, and, in the case of other contagious respiratory diseases that have caused pandemics, such as swine flu, the targets are influenza virus M1 protein [86,87], or hemagglutinin (HA) and RNA glycoproteins and neuraminidase (NA) [70], as illustrated in Figure 3. The target molecule attaches to the bioreceptor [88] to detect a biological molecule by a particular reaction. Then, the transducer with integrated nanostructures converts the detection into an electrical signal determined by the detector [89]. The schematic diagram of different analytes, bioreceptors for biorecognition elements, transducers with integrated nanomaterials used for biosensing, as parts of a typical nanobiosensor for respiratory viruses, are presented in Figure 5. Moreover, an overview of different biological samples, bioreceptors for biorecognition elements, and transducers with integrated nanostructures that are used in literature as parts of typical nanobiosensors for respiratory viruses are presented in Figure 5.
Nanobiosensors used for the detection of SARS or MERS coronaviruses, can be classified based on the biological molecule of viral target (nucleic acids, antigens, or antibodies) into nucleic acidbased biosensor, antigen-based biosensor, and antibody-based biosensor ( Figure 6). Moreover, an overview of different biological samples, bioreceptors for biorecognition elements, and transducers with integrated nanostructures that are used in literature as parts of typical nanobiosensors for respiratory viruses are presented in Figure 5.
Nanobiosensors used for the detection of SARS or MERS coronaviruses, can be classified based on the biological molecule of viral target (nucleic acids, antigens, or antibodies) into nucleic acid-based biosensor, antigen-based biosensor, and antibody-based biosensor ( Figure 6). Biomarkers play a critical role in the fabrication of nanobiosensors for immediate detection of human coronaviruses, including 2019-nCoV. Layqah et al. [62] used spike protein S1 as a biomarker for the detection of highly pathogenic Middle East respiratory syndrome coronavirus (MERS-CoV). As a detection method, they used an efficient, single-step, sensitive, and accurate square wave voltammetry (SWV) with a limit of detection of 0.4 pg/mL. To prevent pandemics, the analysis of viral affinity for human or avian sialoglycan with high sensitivity at high speed is crucial.
With the use of nanomaterials and nanotechnology, ultrasensitive biosensors for the detection of antigens are developed. In various studies of patients with COVID-19, biomarkers have shown significant response by using testing samples from healthy and infected patients. Table 2 includes the results of some studied biomarkers in medical clinics. The clinical results included in Table 2 were obtained after 452 patients diagnosed with COVID-19 [90]. A major issue needs to be taken into consideration when using biomarkers for COVID-19. For instance, these biomarkers must be user-friendly to ensure the safety of the professional who is testing them from transmitting the virus. Biomarkers play a critical role in the fabrication of nanobiosensors for immediate detection of human coronaviruses, including 2019-nCoV. Layqah et al. [62] used spike protein S1 as a biomarker for the detection of highly pathogenic Middle East respiratory syndrome coronavirus (MERS-CoV). As a detection method, they used an efficient, single-step, sensitive, and accurate square wave voltammetry (SWV) with a limit of detection of 0.4 pg/mL. To prevent pandemics, the analysis of viral affinity for human or avian sialoglycan with high sensitivity at high speed is crucial.
With the use of nanomaterials and nanotechnology, ultrasensitive biosensors for the detection of antigens are developed. In various studies of patients with COVID-19, biomarkers have shown significant response by using testing samples from healthy and infected patients. Table 2 includes the results of some studied biomarkers in medical clinics. The clinical results included in Table 2 were obtained after 452 patients diagnosed with COVID-19 [90]. A major issue needs to be taken into consideration when using biomarkers for COVID-19. For instance, these biomarkers must be user-friendly to ensure the safety of the professional who is testing them from transmitting the virus.
The mechanism of surface detection can be varied based on the interaction process between the bioreceptor and the analyte of interest. Figure 7 shows different assay formats used for the detection of small biomolecules [94].
Sensors 2020, 20, x FOR PEER REVIEW 10 of 43 The mechanism of surface detection can be varied based on the interaction process between the bioreceptor and the analyte of interest. Figure 7 shows different assay formats used for the detection of small biomolecules [94]. In a direct assay, target analyte attaches to the antibody (recognition element) immobilized on the sensor surface; (b) in a competitive assay, the analyte competes with its conjugate to attach to the antibody; (c) in a binding inhibition assay, the analyte conjugate is the one immobilized on the sensing surface [94]. (Licensed CC BY 4.0).
Label-free nanobiosensors are based on the direct interaction with the target analyte, where the detection is achieved with the biological recognition element immobilized on the surface of the nanobiosensor (Figure 7a). In case of competitive ( Figure 7b) and binding inhibition assay ( Figure  7c), an intermediary part between the analyte and the biological recognition element immobilized on the surface of the nanobiosensor is required to increase the detection ability. In case of the competitive assay, the sensing area is coated with the recognition element, whereas the analyte and its conjugated equivalent compete to attach into a finite number of binding sites on the sensing surface. However, in case of inhibition detection assay, there is a reverse process, i.e., the analyte conjugate is immobilized on the sensing surface, while the recognition element is connected to solution of the analyte [95]. Consequently, no matter what type of assay format is chosen, it is important to determine the immobilization phase of the exposed sensing area in the structure of the nanobiosensor. Hence, the surface modification and functionalization should be a selective process that supports the binding and interaction of the analyte of interest.

Nanobiosensors for the Detection of Human Coronavirus (2019-nCoV, SARS/MERS-CoV) and Influenza Viruses
Nanobiosensors are important tools for efficient detection of severe acute respiratory syndrome and other diseases that cause pandemics [96][97][98][99][100][101]. For virus detection, depending on the detection mode and measurable properties, nanosensors can be classified into different classes, such as electrical, chemical, electrochemical, optical, piezoelectric [102], thermal [103,104], magnetic [105], and biological detection [57]. In the following sections, a brief review of the nanobiosensor detection mechanisms of epidemic/pandemic respiratory viruses, such as avian influenza virus (AIV) subtype  Label-free nanobiosensors are based on the direct interaction with the target analyte, where the detection is achieved with the biological recognition element immobilized on the surface of the nanobiosensor (Figure 7a). In case of competitive ( Figure 7b) and binding inhibition assay (Figure 7c), an intermediary part between the analyte and the biological recognition element immobilized on the surface of the nanobiosensor is required to increase the detection ability. In case of the competitive assay, the sensing area is coated with the recognition element, whereas the analyte and its conjugated equivalent compete to attach into a finite number of binding sites on the sensing surface. However, in case of inhibition detection assay, there is a reverse process, i.e., the analyte conjugate is immobilized on the sensing surface, while the recognition element is connected to solution of the analyte [95]. Consequently, no matter what type of assay format is chosen, it is important to determine the immobilization phase of the exposed sensing area in the structure of the nanobiosensor. Hence, the surface modification and functionalization should be a selective process that supports the binding and interaction of the analyte of interest.

Nanobiosensors for the Detection of Human Coronavirus (2019-nCoV, SARS/MERS-CoV) and Influenza Viruses
Nanobiosensors are important tools for efficient detection of severe acute respiratory syndrome and other diseases that cause pandemics [96][97][98][99][100][101]. For virus detection, depending on the detection mode and measurable properties, nanosensors can be classified into different classes, such as electrical, chemical, electrochemical, optical, piezoelectric [102], thermal [103,104], magnetic [105], and biological detection [57]. In the following sections, a brief review of the nanobiosensor detection mechanisms of epidemic/pandemic respiratory viruses, such as avian influenza virus (AIV) subtype H5N1

Electrochemical Nanobiosensors
Electrochemical nanobiosensors have been applied in several fields [25,26,29,57,106,107]. An electrochemical nanobiosensor is a molecular sensing device that couples a biological recognition event with an electrode transducer to produce a useful electrical signal [108]. Electrochemical nanosensors contain electrodes, for which the semiconductors properties, dielectric properties, and charge distribution are important factors.
Surface modification of the nanostructures or nanomaterials is required to modify the functional layer in order to obtain definite selectivity for a given sensing surface. The various studies have been performed to enhance the performance of nanobiosensors by incorporating nanomaterials in the surface to increase the volume-to-surface area ratio and improve the selectivity of the surface. As shown in Figure 5, nanomaterials have a great impact when used in biosensing applications. Most of the nanobiosensors used for detection of pathogen and viruses are based on electrochemical transducers, i.e., amperometric, potentiometric, or impedance transducers [109]. The mechanism of the nanobiosensor is based on the change in the electrical conductance or resistance through the nanostructures or nanomaterials, when the target element attaches to the electrode surface. When the electron consumption or production occurs in a biological reaction on the electrode surface, an electrochemical signal is generated. The selectivity leads to the ability to measure only one chemical in the presence of many other chemicals in real time with high detection accuracy. The general mechanism of sensing is based on the chemisorption of molecules that induce a change in conductance due to chemical sensitization.
Bioreceptors must be integrated effectively into the right transducer to detect the presence of any virus within the target sample. For the detection of different types of coronaviruses, most researchers used electrochemical detection methods, such as field effect transistor (FET) [69,110,111], bioelectric recognition assay (BERA) [111], electrochemical impedance spectroscopy (EIS) [78,[112][113][114][115][116][117], amperometry [27,118,119], cyclic voltammetry [53,62,87,108,[120][121][122], or conductometry [55,123]. Influenza is an acute infectious disease caused by a coated ribonucleic acid (RNA), which contains viruses belonging to the Orthomyxoviridae family [80]. Influenza has four types A, B, C, and D, which differ in the structure of the virus (ion channel, matrix, and membrane protein). Individual differences could help detect individual subtypes of influenza virus [80][81][82]. Influenza A viruses are divided into subtypes on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). For example, the most studied is the influenza A virus subtype H1N1 (A/H1N1), which was responsible for the 2009 swine flu pandemic, as well as the 1918 flu pandemic, known as the Spanish pandemic flu, which caused millions of deaths, are still a threat to people in many countries.
Avian influenza virus (AIV) H5N1, H7N7, and H7N9, which are subtypes of influenza A virus, were studied by many researchers [115][116][117] through EIS, amperometry [27], cyclic voltammetry [120], etc. The biological response was transferred to the detector via a transducer integrated or functionalized with nanostructures, using different types of nanomaterials, such as magnetic iron oxide nanobeads [115]. A second category of respiratory viruses that caused epidemics or pandemics are coronaviruses, including SARS-CoV, MERS-CoV, and the novel coronavirus 2019-nCoV, which is the most deadly one. For these aforementioned reasons, many researchers studied respiratory viruses specific to seasonal influenza through different types of electrochemical methods, along with the biosensors designed for respiratory coronavirus detection that causes COVID-19 disease. These studies are summarized in Table 3, which shows a comparison of electrochemical detection methods of influenza viruses, human coronaviruses (including recently developed COVID-19 causative virus) and their parameters. Table 3 shows that impedance biosensors are the most sensitive compared to other types of biosensors, such as optical or mass sensitive, and are ideal for detecting specific proteins or DNA strains, or for monitoring the environment [86,142]. Electrochemical impedance spectroscopy (EIS) is less destructive for measuring the interactions between biological samples, in comparison with other electrochemical methods, such as differential pulse voltammetry (DPV), cyclic voltammetry (CV), etc. [143].    Many research groups studied the M1 protein of influenza A virus using covalently bound antibodies and a nano-scale boron-doped diamond surface sensor [86,113]. Nidzworski et al. [113] developed a rapid-response, ultrasensitive biosensor for influenza virus detection using an antibody modified boron-doped diamond, and obtained a limit of detection of 1 fg/mLa and a linear range of 1-100 fg/mL. Similarly, Siuzdak et al. [86] obtained LOD: 5 × 10 −14 g/mL. Both groups of researchers used saliva as biological samples from the tested subjects, which is easy to collect and analyze by EIS-based biosensors.
To analyze the saliva samples and detect the nCovid-19 spike antigen (nCovid-19Ag), Mahari et al. [141] used an in-house built printed circuit board for fast detection of nCovid-19 antigen (nCovid-19Ag). The sensor was manufactured using a fluorine-doped tin oxide (FTO) electrode with gold nanoparticles (AuNPs) and immobilized with nCovid-19 (nCovid-19Ab) monoclonal antibody to measure the change in electrical conductivity. They used different detection methods, such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV), and obtained very good sensitivity, as follows: the limit of detection was 10 fM of nCovid-19 Ag and the linear range was 1 fM to 1 µM in standard buffer [141]. Veerapandian and co-workers [121] developed an electrochemical biosensor based on chronoamperometry and differential pulse voltammetry (DPV). Graphene oxide nanosheets and immobilized H5N1 and H1N1 antibodies were used to examine the detection and response of the nanobiosensor. The limit of detection was measured to be 9.4 pM for H1N1 and 8.3 for H5N1. The use of antibodies revealed an important interaction with the corresponding antigen. The biosensor showed an increase in all necessary parameters, such as sensitivity, speed, and simplicity at a lower cost.
Various pathogenic viruses, including SARS-CoV-2 spike proteins can be used as reliable markers for the presence of the infection and virus replication. For direct and ultra-rapid detection of SARS-CoV-2 (3 min and 1 fg/mL concentration) S1 spike protein antigen from green monkey kidney cell culture samples [4], the bioelectric recognition assay (BERA) was used. Over the years, various electrochemical nanobiosensors have been used to detect human influenza A virus (such as H1N1) using electrochemical impedance spectroscopy (EIS) [78], chronoamperometry [27,118,126], voltammetry [135], or conductometry [55].

FET-Based Electrochemical Nanobiosensor
Field-effect transistor (FET)-based biosensors have several advantageous and properties compared to other techniques. These biosensors can be highly sensitive and provide instant measurements with very low concentration of the bioanalytes [144,145]. FET-based biosensors can be utilized in a number of applications, particularly in medical care, point-of-care testing, and diagnosis [146][147][148].
Graphene is an advanced nanomaterial that consists of a two-dimensional layer of carbon atoms [149]. This material can be used as an active sensing surface due to its excellent electrical conductivity, high carrier mobility, simplicity of surface functionalization, and large surface area [150]. Therefore, to optimize the sensitivity of detection, the integration of graphene-based material can be the best candidate for FET biosensors.
To prevent an influenza pandemic, it is necessary to differentiate with high accuracy, speed and sensitivity the viruses of pandemic potentials on the basis of the viral affinity for sialoglycan [110]. In this case, and also for highly sensitive detection of biological substances, such as DNA and proteins (like lectin), the graphene-based field effect transistor (G-FET) method was used. For rapid detection, high stability and sensitivity of biological targets, such as the H5N1 influenza virus gene, and the bio-FET method, as a promising platform for label-free detection via a flow-through strategy, was used [111]. For instance, high sensitivity was reported by Ishikawa et al. [45]. Ono et al. [110] and Chan et al. [111] reported that Limit of Detection (LOD) of influenza viruses are in the picomolar range, i.e., 130 pM and 50 pM, respectively. For influenza detection, Ono et al. [110] used a single-layer hexagonal carbon networks (grapheme FET) for selective detection of lectins, while Chan et al. [111] used reduced graphene oxide (rGO) films on Si/SiO 2 substrate for increasing the sensitivity of Bio-FET sensor for H5N1 influenza virus gene detection in a flowing environment.
Seo et al. [69] developed a FET biosensor for the COVID-19 causative virus, which contains a sheet of graphene-based material functionalized by attaching antibodies alongside the SARS-CoV-2 spike antibody (Figure 8a). The SARS-CoV-2 spike antibody is used as a bioreceptor because it is a transmembrane protein, immunogenic, and has the selectivity to detect SARS-CoV-2. If the SARS-CoV-2 antigen protein is exposed, an electrical response is obtained because of its binding to the antibody. The results of the electrical characterization of this biosensor showed an effective response to the COVID-19 detection by integrating a graphene-based FET biosensor. Figure 8c shows the real-time detection of 1 fg/mL of SARS-CoV-2 spike protein compared to that without SARS-CoV-2 spike protein which did not reveal any response as the concentration of the samples was changed. In addition, Figure 8 reveals the selectivity and sensitivity of the biosensor to differentiate the SARS-CoV-2 antigen protein from others such as MERS-CoV, leading to the surface ability to bind particularly with the selected SARS-CoV-2 antigen. The results of this biosensor showed a remarkable detection of the virus, which could be used in the future as it is, or modified for testing other diseases.

Cell-Based Electrochemical Nanobiosensor
The estimation of the number of infected patients in real-time is one of the major challenges in the management of the recent COVID-19 disease. A novel biosensor for faster detection of the SARS-CoV-2 S1 spike protein in 3 min and ultra-sensitive surface, with a detection limit of 1 fg/mL, has been recently developed by Mavrikou et al. [4]. The SARS-CoV-2 spike antibody is used as a bioreceptor because it is a transmembrane protein, immunogenic, and has the selectivity to detect SARS-CoV-2. If the SARS-CoV-2 antigen protein is exposed, an electrical response is obtained because of its binding to the antibody. The results of the electrical characterization of this biosensor showed an effective response to the COVID-19 detection by integrating a graphene-based FET biosensor. Figure 8c shows the real-time detection of 1 fg/mL of SARS-CoV-2 spike protein compared to that without SARS-CoV-2 spike protein which did not reveal any response as the concentration of the samples was changed. In addition, Figure 8 reveals the selectivity and sensitivity of the biosensor to differentiate the SARS-CoV-2 antigen protein from others such as MERS-CoV, leading to the surface ability to bind particularly with the selected SARS-CoV-2 antigen. The results of this biosensor showed a remarkable detection of the virus, which could be used in the future as it is, or modified for testing other diseases.

Cell-Based Electrochemical Nanobiosensor
The estimation of the number of infected patients in real-time is one of the major challenges in the management of the recent COVID-19 disease. A novel biosensor for faster detection of the SARS-CoV-2 S1 spike protein in 3 min and ultra-sensitive surface, with a detection limit of 1 fg/mL, has been recently developed by Mavrikou et al. [4].
Membrane engineering, cell-based assay concept for the determination of biomolecules has been utilized to enhance the attachment of a particular protein to cellular components that were obtained by electro-inserting spike S1 antibody. It has been found that biomolecules binding to the electro-inserted antibodies provided a successful change in electrical measurements of the engineered cell membrane [151,152]. The results have shown significant difference in the properties of electrical behavior, as shown in Figure 9. The developed biosensor has different advantages for the clinical testing, monitoring, and managing the virus. For example, this biosensor has a portable read-out device, easy to handle, and mass screening of SARS-CoV-2 surface antigens. Membrane engineering, cell-based assay concept for the determination of biomolecules has been utilized to enhance the attachment of a particular protein to cellular components that were obtained by electro-inserting spike S1 antibody. It has been found that biomolecules binding to the electroinserted antibodies provided a successful change in electrical measurements of the engineered cell membrane [151,152]. The results have shown significant difference in the properties of electrical behavior, as shown in Figure 9. The developed biosensor has different advantages for the clinical testing, monitoring, and managing the virus. For example, this biosensor has a portable read-out device, easy to handle, and mass screening of SARS-CoV-2 surface antigens.

Optical Nanobiosensors
In recent decades, new photonic devices have looked promising for wide-range applications in the field of nanobiosensor technology for food safety, homeland security, biology, environmental monitoring, and medicine [153][154][155]. Different parameters can be used in detection, such as energy, polarization, absorption, fluorescence, light scattering, amplitude, decay time, and/or phase [156].
The surface plasmon resonance (SPR) transduction of the optical nanosensors can determine the variation in reflective index of the transducer as the target analyte interacts with the biorecognition element on the surface of the sensor [157].
Fluorescence involves the exposure to an external light source to excite the electron transitions in the biomolecules, which then generate luminescence. Hence, this type of optical biosensor requires the integration of fluorochrome molecules to generate light during the interaction with the immobilized biorecognition element [158].
Optical fibers in biosensing applications have received special attention due to their high sensitivity, high performance, and fast response [159,160]. Optical fibers are commonly integrated with surface plasmon resonance or fluorescence in various applications to monitor the changes of the optical properties, such as the wavelengths wave propagation, time, intensity, or polarity of the light to detect the analytes of interest [161]. These properties can be measured to detect the analyte concentration. For example, an amplitude is the most important parameter that is correlated with the concentration of the analyte [162]. Optical fiber biosensors have been widely used for the detection of pathogen, virus and bacteria [161,163,164]. Table 4 shows a comparison of optical detection method of influenza viruses and other coronavirus, including recently developed COVID-19 causative virus nanobiosensors and their properties.

Optical Nanobiosensors
In recent decades, new photonic devices have looked promising for wide-range applications in the field of nanobiosensor technology for food safety, homeland security, biology, environmental monitoring, and medicine [153][154][155]. Different parameters can be used in detection, such as energy, polarization, absorption, fluorescence, light scattering, amplitude, decay time, and/or phase [156].
The surface plasmon resonance (SPR) transduction of the optical nanosensors can determine the variation in reflective index of the transducer as the target analyte interacts with the biorecognition element on the surface of the sensor [157].
Fluorescence involves the exposure to an external light source to excite the electron transitions in the biomolecules, which then generate luminescence. Hence, this type of optical biosensor requires the integration of fluorochrome molecules to generate light during the interaction with the immobilized biorecognition element [158].
Optical fibers in biosensing applications have received special attention due to their high sensitivity, high performance, and fast response [159,160]. Optical fibers are commonly integrated with surface plasmon resonance or fluorescence in various applications to monitor the changes of the optical properties, such as the wavelengths wave propagation, time, intensity, or polarity of the light to detect the analytes of interest [161]. These properties can be measured to detect the analyte concentration. For example, an amplitude is the most important parameter that is correlated with the concentration of the analyte [162]. Optical fiber biosensors have been widely used for the detection of pathogen, virus and bacteria [161,163,164]. Table 4 shows a comparison of optical detection method of influenza viruses and other coronavirus, including recently developed COVID-19 causative virus nanobiosensors and their properties.    [183]  The nanobiosensors based on surface plasmon resonance [46,170,192,193], and fluorescence principle are the most common sensors because they demonstrated a high potential for optical based detection of viruses, including the SARS-CoV-2 coronavirus [194]. The SPR sensor system is extremely sensitive, has a rapid response time, is label-free and has real-time detection of binding events between biomolecules and the surface, and has the advantage for detecting the changes at nanoscale interface with high accuracy.
For instance, surface plasmon resonance-based biosensor was used by Miodek et al. [87] to immobilize specific anti-PB1-F2 antibody on the surface of the modified gold microelectrode. This type of biosensor has the ability to perform direct and quantitative detection of PB1-F2 protein of influenza A virus in infected cells, leading to the exploration of the capability of the biosensor detection for other viral proteins in infected cells, tissues or biological samples. The successful detection of SARS-CoV antigen via SPR analytical systems with reference (high-throughput, multianalyte imaging SPR analytical system) by directly immobilizing antibodies was achieved by Dafu et al. [195]. With the improved SPR reference system (improved system by eliminating non-specific disturbances and avoiding the inference of refractivity of different solutions at different temperatures), samples collected from patients can be analyzed without pretreatment.
Among the different biosensing techniques, localized surface plasmon resonance (LSPR) biosensing systems are applicable to many pathogenic and viral agents, such as different types of influenza viruses, SARS-CoV, MERS-CoV, and SARS-CoV-2 viruses. Qiu et al. [1] developed a highly sensitive, fast, and reliable dual-functionalized LSPR biosensor for the detection of selected sequences from severe acute respiratory syndrome coronavirus 2 through nucleic acid hybridization.
The concept of double functionality of the plasmonic biosensing was integrated with the plasmonic photothermal (PPT) effect and the LSPR sensing transduction on a single cost-effective gold nanoislands (AuNI) chip. Ultra-sensitive detection was successfully developed by Park et al. [77] with SPR imaging and localized SPR-based optical biosensor for label-free monitoring target molecules of influenza B virus (with a limit of detection up to the 1 pg/mL).
Fluorescence is frequently used to detect viruses (viral protein) and monitor viral infection of cells [196]. The fluorescence-based detection system requires the labeling of the process for obtaining the fluorescence signal, on the principle of the on/off signal between the binding of the target to the fluorescent probe (e.g., fluorescent nanoparticles/proteins, dye-labeled nucleic acid, etc.). Fluorescent labels in biosensors for pathogen detection offer a user-friendly, fast, efficient, and low-cost biosensing systems for pathogen monitoring [197]. For instance, Nguyen et al. [54], developed a fluorescence biosensor based on CdTe quantum dots, for the specific detection of influenza virus H5N1 type (with 3 ng/µL LOD for H5N1). The biosensor included highly luminescent CdTe/CdS quantum dots, antibody, and chromatophores extracted from particular bacteria. This part of the biosensor was then connected to a peripheral part of the biosensor and also connected to the H5N1 antibody to make it ready for the detection of the H5N1 avian influenza virus. The advantage of this fluorescence biosensor consists in the flexibility of the peripheral part to detect any other types of viruses only by replacing the specific antibody to the determined target virus [54]. Waye et al. [180] used also the fluorescent detection of the 3a Protein (3a gene encodes a non-structural viral protein) of SARS-Coronavirus. The chromatin condensation experiments and DNA fragmentation were performed in vitro.
Chemiluminescence is a luminescence technique similar to florescence used in nanobiosensors, i.e., it detects the light emitted by atoms and molecules when the electron relaxes from the excitation level. Although similar in detection mode, chemiluminescence nanobiosensors diverged from fluorescence at one point in time when advanced chemiluminescence array for magnetic separation were developed. In fluorescence, the light is released when the electrons relaxes from higher energy states, while in chemiluminescence, the light is released when the excited electrons in unstable intermediary states relax to produce the final reaction products. Xi et al. [198] developed a chemiluminescence sensor with a detection limit for HBsAg, a marker of the Hepatitis B virus, as low as 0.05 ng/mL, which is ten times lower than the typical ELISA used in hospitals. More recently, to improve the collection efficiency of chemiluminescent emission induced by samples, Nie et al. [199] designed a chemiluminescent optical fiber sensor by using a concave mirror and a coaxial tubular mirror as its bottom and wall, respectively. The limit of detection was as low as 0.31 pg/mL [199], which is about 2 orders of magnitude lower than that obtained by a normal chemiluminescent optical fiber sensor.
In immunofluorescence detection of viruses, nanobiosensors were developed based on the gold nanoparticles (AuNP)-induced quantum dot (QD) fluorescence signal, with a detection limit of 10 PFU/mL [30]. The H1N1 virus-a virus that belongs to the family of influenza A viruses, such as H3N2-has been responsible for the 2009 flu pandemic and continues to be in the attention of scientists searching for rapid and effective detection in real time. For the rapid diagnosis of influenza virus type A, Park et al. [176] reported a surface-enhanced Raman scattering (SERS)-based lateral flow assay (LFA) kit with a detection limit of 1.9 × 10 4 PFU/mL, which is approximately one order of magnitude more sensitive than the LOD obtained from the colorimetric LFA kit.
Colorimetric assays are representative tools that basically identify the target molecules in tested specimens through color changes of an indicator (e.g., nanosized metallic particle and dye molecules) and, because of their fascinating optical properties, plasmonic nanostructures have inspired numerous colorimetric detections of biomolecule for a wide range of applications from pharmaceutical to environmental analyses [200].
The colorimetric test offers the advantages of on-site detection of analytes due to its direct reading, convenient operation and minimum instrumentation requirement. This technique allows visual observation of the presence of biomarkers and measures the absorbance of the colored compounds at a specific wavelength. In the actual context of public health emergency caused by the human coronavirus (HCoV), a simple and fast colorimetric assay for detecting infectious disease (with naked eye and without costly equipment) is still imperative.
Thus, Kim et al. [186] developed a label-free colorimetric assay for MERS-CoV based on an extended form of double-stranded DNA (dsDNA) self-assembly shielded gold nanoparticles (AuNPs) under positive electrolyte (e.g., 0.1 M MgCl 2 ), for the detection of MERS-CoV. This colorimetric test can detect up to 1 pmol/µL of 30 bp MERS-CoV and can be further adapted for convenient on-site detection of other infectious diseases, especially in resource-limited settings.

Magneto-Optical Nanobiosensors
Magneto-optic (MO) nanobiosensors present a great interest in the development of ultra-sensitive biosensing application due to their combined magnetic properties and surface plasmonic enhancement that is associated with metal nanoparticles [201][202][203]. MO sensing platforms have exceptional parameters, including label-free biosensing, fast response, and ultra-sensitive detection.
Novel applications of MO nanobiosensors have been successfully reported in the medical field including hyperthermia treatment, magnetic actuation, targeted drug delivery, and the use of magnetic particles [204].
Magnetic nanosensors are obtained by using magnetic beads coated with a ligand, which can be detected by a magnetic field [29]. The SPR transducers in biosensing are highly influenced by the change in the reflective index associated with the binding or the reaction of biomolecules and bioreceptors at the metal surface. The presence of ferromagnetic metals enhance the surface plasmons resonance [205][206][207][208]. Under the applied magnetic field, the ferromagnetic surface can play a role in tuning the SPRs. The properties of MO nanobiosensors are not used only to improve the SPRs, but also to optimize and increase the sensitivity of the sensors. Table 5 presents an assessment of magneto-optical detection methods and their different parameters for the current developed influenza and coronavirus nanobiosensors. Rapid and ultrasensitive detection (5 × 10 −12 g/mL, by human eyes and 44.2 × 10 −15 g/mL, by a microplate reader) of influenza virus type A was determined by Oh et al. [210]. The linear range was in this case from 5 × 10 −15 to 5 × 10 −6 g/mL. They used silica-shelled magnetic nanobeads (MagNBs) and gold nanoparticles with a new platform-based ELISA technology. The new MagLISA detection platform has many advantages, such as advanced sample separation, sensitive reading and anti-interference ability that can reduce the spread of influenza virus and provide immediate clinical treatment.
For the detection of SARS-CoV-2 RdRp coding sequences, Tian et al. [14] used optomagnetic sensing and achieved a sub-femtomolar level detection limit of 0.4 fM. Another advantage of the real-time optomagnetic detection of SARS-CoV-2 RdRp coding, compared with previously reported C2CA-based sensors, is to significantly simplify the operation by eliminating the labor-intensive and time-consuming operation steps that require different reaction temperatures.
Developments in chemiluminescence have led to an improvement in the optical signal of a luminophore near the surface of metal nanoparticles [213][214][215][216]. The electromagnetic field of incident light can be enhanced to accelerate the detection and control the energy transfer. This type of optical configuration has emerged as a potential method for the fabrication of various medical applications due to its high sensitivity, simplicity, and low noise [217]. For example, Lee et al. [209] used a magnetofluoro-immunosensing platform for virus detection (i.e., H1N1 Influenza) using Au/Fe 3 O 4 decorated graphene. The LOD of influenza virus in deionized water was 7.27 fg/mL. The Au/iron oxides decorated graphene was prepared as plasmonic/magnetic graphene, which was used for a target virus separator and a plasmonic substrate. On the other hand, Zhao et al. [212] have used poly(amino ester) with carboxyl groups (PC)-coated magnetic nanoparticle (PCMNPs) to detect SARS-CoV-2 pseudovirus particles. Extracting the SARS-CoV-2 viral RNA, a 10-copy sensitivity and a linear correlation of 10-10 5 copies of SARS-CoV-2 pseudovirus particles were obtained. The major advantage of this structure is its high performance in the extraction procedure to reduce the detection time in the recent diagnosis of COVID-19 causative virus.

Recently Developed COVID-19 Optical Nanobiosensors
Researchers from Empa and ETH Zurich (Zürich, Switzerland) have developed a successful optical sensor to detect COVID-19 virus [218]. This could be utilized to measure the presence of the virus in the surroundings. The sensor would be considered an alternative method to quantify the virus concentration in the air and in real time, particularly in crowded and busy places. Wang et al. [218] have worked on analyzing and minimizing the presence of airborne pollutants such as aerosols and artificially produced nanoparticles. To enhance the detection in a safe, reliable and more sensitive technique, researchers have fabricated the optical nanobiosensor based on localized surface plasmon resonance (LSPR) phenomena. A LSPR biosensor consists of the succinimidyl ester group that was functionalized on the surface and attached to two-dimensional (2D) gold nanoislands with an average size of 40.2 nm, as active sensing surface on a glass substrate (Figure 10a).
As the light polarized the sensing surface, the plasmonic phase varies to offer superior detection sensitivity for bioaerosol concentration [219,220]. The presence of gold nanoparticles on the surface increases the sensitivity as the biomarkers are illuminated. Figure 10b shows the LSPR response of the functionalized surface of the biosensor, which interacts with the sulfhydryl group of 11-mercaptoundecanoic (MUA) to form in situ surface modification.
A mixture of 1-ethyl-3-(dimethyl-aminopropyl) carbodiimide hydrochloride (EDC)/N-hydroxyl succinimide (NHS) (EDC/NHS) was injected to test the phase changes. As a result, the active succinimidyl ester was sensitive and responded to an amino group, forming an amide bond while identifying bioaerosols. To assess the reliability of the novel nanobiosensor detection on the COVID-19 causative virus, several challenges are still required to validate the nanobiosensor sensitivity to the coronavirus. [218] have worked on analyzing and minimizing the presence of airborne pollutants such as aerosols and artificially produced nanoparticles. To enhance the detection in a safe, reliable and more sensitive technique, researchers have fabricated the optical nanobiosensor based on localized surface plasmon resonance (LSPR) phenomena. A LSPR biosensor consists of the succinimidyl ester group that was functionalized on the surface and attached to two-dimensional (2D) gold nanoislands with an average size of 40.2 nm, as active sensing surface on a glass substrate (Figure 10a). Fast and precise detection of the COVID-19 causative virus can greatly promote the management, prevention, and control of an emerging disease [221]. Two different sensing platforms have been developed in a nanobiosensors, including localized surface plasmon resonance, and plasmonic photothermal (PPT) effects Figure 11a  As the light polarized the sensing surface, the plasmonic phase varies to offer superior detection sensitivity for bioaerosol concentration [219,220]. The presence of gold nanoparticles on the surface increases the sensitivity as the biomarkers are illuminated. Figure 10b shows the LSPR response of the functionalized surface of the biosensor, which interacts with the sulfhydryl group of 11mercaptoundecanoic (MUA) to form in situ surface modification.
A mixture of 1-ethyl-3-(dimethyl-aminopropyl) carbodiimide hydrochloride (EDC)/N-hydroxyl succinimide (NHS) (EDC/NHS) was injected to test the phase changes. As a result, the active succinimidyl ester was sensitive and responded to an amino group, forming an amide bond while identifying bioaerosols. To assess the reliability of the novel nanobiosensor detection on the COVID-19 causative virus, several challenges are still required to validate the nanobiosensor sensitivity to the coronavirus.
Fast and precise detection of the COVID-19 causative virus can greatly promote the management, prevention, and control of an emerging disease [221]. Two different sensing platforms have been developed in a nanobiosensors, including localized surface plasmon resonance, and plasmonic photothermal (PPT) effects Figure 11a   LSPR is a perfect candidate for immediate and label-free detection of micro-and nanoscale biomolecules [222,223]. In LSPR, the surface sensitivity is greatly influenced by the local variation in the refractive index and molecular binding [44]. The PPT heat energy effect, known as thermoplasmonic, is confined close to the nanoparticles to enhance the kinetics of hybridization of nucleic acid strands (RdRp-COVID and its cDNA) to prevent the interaction with nonmatching sequences, leading to better detection mechanism [1,[224][225][226]. The developed biosensor provides a promising solution for the detection of COVID-19 causative virus. The combination of optical and thermal techniques leads to the excitation of different wavelengths that highly improve sensitivity, reliability, stability, and rapid diagnostic for SARS-CoV-2 virus.
The plasmonic biosensor consisting of two-dimensional gold nanoislands were self-assembled on the glass surface and then functionalized with corresponding bioreceptors. Figure 11b shows an attachment of the thiolcDNA bioreceptor to the Au nanoisland. The accurate surface functionalization is important to functionalize and to improve the sensitivity of the Au nanoislands LSPR is a perfect candidate for immediate and label-free detection of micro-and nanoscale biomolecules [222,223]. In LSPR, the surface sensitivity is greatly influenced by the local variation in the refractive index and molecular binding [44]. The PPT heat energy effect, known as thermoplasmonic, is confined close to the nanoparticles to enhance the kinetics of hybridization of nucleic acid strands (RdRp-COVID and its cDNA) to prevent the interaction with nonmatching sequences, leading to better detection mechanism [1,[224][225][226]. The developed biosensor provides a promising solution for the detection of COVID-19 causative virus. The combination of optical and thermal techniques leads to the excitation of different wavelengths that highly improve sensitivity, reliability, stability, and rapid diagnostic for SARS-CoV-2 virus.
The plasmonic biosensor consisting of two-dimensional gold nanoislands were self-assembled on the glass surface and then functionalized with corresponding bioreceptors. Figure 11b shows an attachment of the thiolcDNA bioreceptor to the Au nanoisland. The accurate surface functionalization is important to functionalize and to improve the sensitivity of the Au nanoislands sensing surface. For better sensing performance, the thermoplasmonic effect is generated on the same Au surface when irradiated at their plasmonic resonance frequency. The localized PPT heat is capable to detect nucleic acids, to elevate the hybridization kinetics and to facilitate the accurate discrimination of two similar strands. The biosensor with double functionalized capability of sensing model exhibits a high sensitivity toward the selective hybridization recognition of SARS-CoV-2 sequences with a minimum detection limit to the concentration of 0.22 pM, and allows precise detection in a multigene combination. Figure 11c presents the result of LSPR sensing detection with the in situ PPT improvement. This work highlights the importance of plasmonic bionanosensor and its effectivity to the detection of the presence of nucleic acid for various diseases.

Piezoelectric Nanobiosensors
Piezoelectric quartz crystal microbalance nanobiosensors have gained considerable attention in biological and chemical applications particularly for the detection of influenza viruses due to their simple model, direct recognition, and real-time output [227][228][229]. There are two types of piezoelectric sensors, i.e., bulk wave (BW) and surface acoustic wave (SAW). These biosensors are able to detect the biochemical entities [25,29] and convert the mechanical energy into electricity, which provide the user with a usable energy output in response to a specific measurement input. For example, the piezoelectric material generates mechanical resonance of vibrating crystal at its natural frequency. This frequency is influenced by the external electrical signal. As the analyte of interested is exposed to the sensing material, a reaction will eventually occur and produce a shift in the frequency that causes a change in the electrical measurements. Therefore, the detection can be obtained by utilizing microscaled or nanoscaled sensors.
The probe is excited mechanically to resonate and measure the change in mass and viscoelasticity on the surface by tracing the frequency and modifying a quartz crystal resonator [230]. The benefit of this technique is the capability to detect molecules without labeling. However, associated challenges include the detection mechanism complexity and less precise measurements [231]. These biosensors have been used in a wide range of biological applications to identify the presence of some target biomolecules, such as hormones, bacteria, cells, etc. [232,233]. To detect avian influenza virus (H5N1), Wang et al. [227] fabricated a quartz crystal microbalance (QCM) sensor using a 3-dimensional (3D) nanowell onto the surface of gold (Au). The sensitivity limit of detection (LOD) of the sensor exhibited 2-4 HAU/50 µL, and a detection time of 10 min, leading to faster detection time compared to other techniques (30 min) [234]. Among the mass detection-based biosensors, the piezoelectric immunosensor was developed for the detection of SARS-associated coronavirus (SARS-CoV) [102]. The mass-based biosensors are summarized in Table 6, including the recent developed COVID-19 causative virus nanobiosensors and their properties.

2019-nCoV or SARS-CoV-2
Oral swab samples Nanoparticles Quartz crystal microbalance (QCM) SARS-CoV-2 spike protein - [241] The integration of quartz crystal microbalance (QCM)-based method for the detection of several respiratory viruses such as Influenza, SARS-CoV and 2019-nCoV from oral swab has been developed [102,238,240,241]. This method can be used for the label-free testing in real-time response with high sensitivity. QCM-based nanobiosensors are the most appropriate sensor for exploring flat surface. For the detection of 2019-nCoV, oral swab samples were collected to measure the response of SARS-CoV-2 spike protein [241]. The detection method of QCM-based sensors is achieved based on the interactions between the spike glycoprotein and the surfaces of the sensor, which was able to detect the adsorbed spike proteins and, hence, the sensitivity was high in the range of ng level.
For SARS-CoV detection, the utilization of piezoelectric immunosensor has been determined to be fast, stable, and effective [102]. Albano et al. [240] has explored the effect of paramagnetic nanoparticles with using a piezoelectric quartz crystal nanobiosensor to detection protein biomarkers at pg/mL level. Fast detection was obtained in a one-minute assay with a detection limit of 3.5 ng/mL, leading to high sensitivity and selectivity nanobiosensors. This kind of nanobiosensor is highly desirable to detect viruses with high reliability, to determine other respiratory viruses.

Challenges and Opportunities for COVID-19 Causative Virus Nanosensors
Several challenges in the development of novel nanobiosensors still exist and need to be addressed in the research community and manufacturing industry to have consistent and efficient detection devices. In order to develop a novel nanobiosensor that can overcome the current challenges, different factors need to be taken into account in the manufacturing process of sensors, including techniques for modifying the surface chemistry for immobilization, to improve the detection limit and selectivity for better sensing, and to ensure data manipulation and analysis. Many scientists are focused on improving the sensitivity and detection limit of biosensors using metallic and semiconducting oxides [242].
In addition, multi-tasks nanobiosensors are needed to support future needs for immediate detection. Quantitative comparison of the use of multiple sensors on a chip is important for data management. Furthermore, the development potential of portable and wireless nanobiosensors is very good for diverse applications [20].
For the COVID-19 causative virus, different nanobiosensors have been developed for different applications in medical and environmental fields by academic and industrial sectors. However, no one could guarantee whether nanobiosensors have good diagnostic results to be used on the frontline. Even though the concept of the electrostatic approach to capture the virus is interesting, the selectivity of the surface to detect the presence of the virus has not yet been explored. The major challenges in gapping the development of nanobiosensors from the lab to industry are as follows: (1) Fabrication of simple, easy to manipulate, early diagnosis, on-site, inexpensive, fast detection, and highly sensitive nanobiosensors could have remarkable potential for many applications, including hospitals, clinics, laboratories, schools, shopping malls, airports, and home.
(2) Increase the accuracy of the diagnosis in the fabrication of multitask nanobiosensors is essential for fast detection.
(3) Improve the reliability and reproducibility of nanobiosensors, it is necessary to build and use machine learning-based programs for the signal process, and to obtain, directly, correct and safe readings of the results.

Conclusions
Rapid spread of viruses can be prevented if the virus causing the epidemic is identified early. The COVID-19 causative virus spread across the world and became a critical problem for health care systems internationally. It is important to detect patients suspected of infection quickly and accurately. Urgent solutions are required to better detect and prevent the spread of the virus. Nanobiosensors have the greatest potential for detecting and, thus, preventing the spread of the coronavirus pandemic. Nanomaterials have a great impact when used in biosensing applications because of their unique properties at nanoscale. Thus, the fabrication of nanobiosensors can provide the tools necessary to perform diagnosis in a few seconds, with high precision for mass screening. Many proposed nanobiosensors for COVID-19 causative virus detection are in the pipeline for growth, and are in various stages of development. In this review, an up-to-date overview is provided to discuss the most current contributions of biosensors designed to detect respiratory viruses that cause epidemics and pandemics, and to compare them, in terms of detection mechanisms, significance of biological receptors, and surface modifications; the challenges and trends in the field are also discussed. The structural protein components of influenza viruses and coronaviruses, responsible for the mechanisms by which these respiratory viruses attack and replicate were also taken into account, emphasizing the role, classification, and characteristics of nanobiosensors for human coronavirus detection, especially for the COVID-19 disease pandemic.