COVID-19: New wearable biosensors can be customized to detect pathogens and toxins and alert the wearer


Most people associate the term “wearable” with a fitness tracker, smartwatch, or wireless earbuds. But what if cutting-edge biotechnology were integrated into your clothing, and could warn you when you were exposed to something dangerous?

A team of researchers from the Wyss Institute for Biologically Inspired Engineering at Harvard University and the Massachusetts Institute of Technology has found a way to embed synthetic biology reactions into fabrics, creating wearable biosensors that can be customized to detect pathogens and toxins and alert the wearer.

The team has integrated this technology into standard face masks to detect the presence of the SARS-CoV-2 virus in a patient’s breath. The button-activated mask gives results within 90 minutes at levels of accuracy comparable to standard nucleic acid-based diagnostic tests like polymerase chain reactions (PCR). The achievement is reported in Nature Biotechnology.

“We have essentially shrunk an entire diagnostic laboratory down into a small, synthetic biology-based sensor that works with any face mask, and combines the high accuracy of PCR tests with the speed and low cost of antigen tests,” said co-first author Peter Nguyen, Ph.D., a Research Scientist at the Wyss Institute. “In addition to face masks, our programmable biosensors can be integrated into other garments to provide on-the-go detection of dangerous substances including viruses, bacteria, toxins, and chemical agents.”

Our programmable biosensors can be integrated into other garments to provide on-the-go detection of dangerous substances including viruses, bacteria, toxins, and chemical agents.


Taking cells out of the equation

The SARS-CoV-2 biosensor is the culmination of three years of work on what the team calls their wearable freeze-dried cell-free (wFDCF) technology, which is built upon earlier iterations created in the lab of Wyss Core Faculty member and senior author Jim Collins.

The technique involves extracting and freeze-drying the molecular machinery that cells use to read DNA and produce RNA and proteins. These biological elements are shelf-stable for long periods of time and activating them is simple: just add water. Synthetic genetic circuits can be added to create biosensors that can produce a detectable signal in response of the presence of a target molecule.

These flexible, wearable biosensors can be integrated into fabric to create clothing that can detect pathogens and environmental toxins and alert the wearer via a companion smartphone app. Credit: Wyss Institute at Harvard University

The researchers first applied this technology to diagnostics by integrating it into a tool to address the Zika virus outbreak in 2015. They created biosensors that can detect pathogen-derived RNA molecules and coupled them with a colored or fluorescent indicator protein, then embedded the genetic circuit into paper to create a cheap, accurate, portable diagnostic. Following their success embedding their biosensors into paper, they next set their sights on making them wearable.

“Other groups have created wearables that can sense biomolecules, but those techniques have all required putting living cells into the wearable itself, as if the user were wearing a tiny aquarium. If that aquarium ever broke, then the engineered bugs could leak out onto the wearer, and nobody likes that idea,” said Nguyen. He and his teammates started investigating whether their wFDCF technology could solve this problem, methodically testing it in more than 100 different kinds of fabrics.

Then, the COVID-19 pandemic struck.

Pivoting from wearables to face masks

“We wanted to contribute to the global effort to fight the virus, and we came up with the idea of integrating wFDCF into face masks to detect SARS-CoV-2. The entire project was done under quarantine or strict social distancing starting in May 2020. We worked hard, sometimes bringing non-biological equipment home and assembling devices manually.

It was definitely different from the usual lab infrastructure we’re used to working under, but everything we did has helped us ensure that the sensors would work in real-world pandemic conditions,” said co-first author Luis Soenksen, Ph.D., a Postdoctoral Fellow at the Wyss Institute.

The team called upon every resource they had available to them at the Wyss Institute to create their COVID-19-detecting face masks, including toehold switches developed in Core Faculty member Peng Yin’s lab and SHERLOCK sensors developed in the Collins lab. The final product consists of three different freeze-dried biological reactions that are sequentially activated by the release of water from a reservoir via the single push of a button.

When SARS-CoV-2 particles are present, the wFDCF system cuts a molecular bond that changes the pattern of lines that form in the readout strip, similar to an at-home pregnancy test. Credit: Wyss Institute at Harvard University

The first reaction cuts open the SARS-CoV-2 virus’ membrane to expose its RNA. The second reaction is an amplification step that makes numerous double-stranded copies of the Spike-coding gene from the viral RNA. The final reaction uses CRISPR-based SHERLOCK technology to detect any Spike gene fragments, and in response cut a probe molecule into two smaller pieces that are then reported via a lateral flow assay strip.

Whether or not there are any Spike fragments available to cut depends on whether the patient has SARS-CoV-2 in their breath. This difference is reflected in changes in a simple pattern of lines that appears on the readout portion of the device, similar to an at-home pregnancy test.

The wFDCF face mask is the first SARS-CoV-2 nucleic acid test that achieves high accuracy rates comparable to current gold standard RT-PCR tests while operating fully at room temperature, eliminating the need for heating or cooling instruments and allowing the rapid screening of patient samples outside of labs.

“This work shows that our freeze-dried, cell-free synthetic biology technology can be extended to wearables and harnessed for novel diagnostic applications, including the development of a face mask diagnostic. I am particularly proud of how our team came together during the pandemic to create deployable solutions for addressing some of the world’s testing challenges,” said Collins, Ph.D., who is also the Termeer Professor of Medical Engineering & Science at MIT.

The Wyss Institute’s wearable freeze-dried cell-free (wFDCF) technology can quickly diagnose COVID-19 from virus in patients’ breath, and can also be integrated into clothing to detect a wide variety of pathogens and other dangerous substances. Credit: Wyss Institute at Harvard University

The face mask diagnostic is in some ways the icing on the cake for the team, which had to overcome numerous challenges in order to make their technology truly wearable, including capturing droplets of a liquid substance within a flexible, unobtrusive device and preventing evaporation. The face mask diagnostic omits electronic components in favor of ease of manufacturing and low cost, but integrating more permanent elements into the system opens up a wide range of other possible applications.

In their paper, the researchers demonstrate that a network of fiber optic cables can be integrated into their wFCDF technology to detect fluorescent light generated by the biological reactions, indicating detection of the target molecule with a high level of accuracy. This digital signal can be sent to a smartphone app that allows the wearer to monitor their exposure to a vast array of substances.

“This technology could be incorporated into lab coats for scientists working with hazardous materials or pathogens, scrubs for doctors and nurses, or the uniforms of first responders and military personnel who could be exposed to dangerous pathogens or toxins, such as nerve gas,” said co-author Nina Donghia, a Staff Scientist at the Wyss Institute.

The team is actively searching for manufacturing partners who are interested in helping to enable the mass production of the face mask diagnostic for use during the COVID-19 pandemic, as well as for detecting other biological and environmental hazards.

“This team’s ingenuity and dedication to creating a useful tool to combat a deadly pandemic while working under unprecedented conditions is impressive in and of itself. But even more impressive is that these wearable biosensors can be applied to a wide variety of health threats beyond SARS-CoV-2, and we at the Wyss Institute are eager to collaborate with commercial manufacturers to realize that potential,” said Don Ingber, M.D., Ph.D., the Wyss Institute’s Founding Director. Ingber is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and Boston Children’s Hospital, and Professor of Bioengineering at the Harvard John A. Paulson School of Engineering and Applied Sciences

Additional authors of the paper include Nicolaas M. Angenent-Mari and Helena de Puig from the Wyss Institute and MIT; former Wyss and MIT member Ally Huang who is now at Ampylus; Rose Lee, Shimyn Slomovic, Geoffrey Lansberry, Hani Sallum, Evan Zhao, and James Niemi from the Wyss Institute; and Tommaso Galbersanini from Dreamlux.

This research was supported by the Defense Threat Reduction Agency under grant HDTRA1-14-1-0006, the Paul G. Allen Frontiers Group, the Wyss Institute for Biologically Inspired Engineering, Harvard University, Johnson & Johnson through the J&J Lab Coat of the Future QuickFire Challenge award, CONACyT grant 342369 / 408970, and MIT-692 TATA Center fellowship 2748460.

Point of care diagnostic platforms for COVID-19 diagnosis

Biosensors are the appropriate PoC diagnostic platforms and defined as portable analytical tools developed to deliver care near the patient [82]. Biosensors involve the utilization of biological molecules such as antibodies, proteins, enzymes, aptamers, cells, and nucleic acids as receptor probes. They are generally immobilized on the transducer surface utilizing physisorption or chemisorption.

At the transducer surface, the biochemical interaction events such as affinity reaction between the antigen and antibody, nucleic acid hybridization, protein-protein interactions are measured employing various transduction modalities such as optical, electrochemical, piezoelectric, mechanical, magnetic and thermal [23,46,83].

The three critical components of the biosensor include the selection of (i) bioreceptors for specific detection of biomarkers (ii) transduction mechanism to interpret the biochemical events as readable signal (iii) detector for quantification and further analysis [84]. These have evolved considerably and are generally categorized into handheld diagnostic units and compact benchtop systems [85].

The commercial market of PoC diagnostic platforms are governed by optical and electrochemical biosensors, which include glucometers and pregnancy test strips. In both optical and electrochemical biosensors, nanoparticles are utilized to promote ultrasensitive detection of biomarkers present in the biofluidic samples.

Optical biosensors

Generally, optical biosensors are defined as the analytical devices in which the analyte-bioreceptor interaction event is interpreted in terms of changes in optical measurements such as luminescence, fluorescence, absorbance and reflectance, which correlates to the concentration of the analyte [86].

From the viewpoint of PoC based optical biosensing platforms, lateral flow immunoassay (LFIA) test strips are considered the economical alternative for the instant diagnosis of COVID-19 in remote locations and public health centres [87] (Fig. 4 ). Although LFAs are proven to be the gold standard, more research is focused on promoting high-throughput immune-analyzers for mass screening.

The sensing principle behind LFAs involves detecting analytes (could be an antigen or antibody) with the help of secondary antibodies conjugated with labels such as gold nanoparticles, fluorescent molecules and quantum dots, promoting visual sensing of color changes. The utilization of nanoparticles as labels has gained attention in developing rapid diagnostic test kits for improved diagnosis and treatment [88,89].

Fig. 4
Fig. 4
Lateral flow immunoassay based sensing of COVID-19 viral infection. The patients’ serum sample containing IgG and IgM antibodies is introduced in the sample pad and flows through the strip via capillary action. The release pad contains the gold SARS-CoV-2 antigen conjugate and gold rabbit IgG conjugate. The two test lines are immobilized with anti-human IgG and anti-human IgM, and the control line is immobilized with anti-rabbit IgG. The respective antigen and antibody interactions are measured as red lines with the help of gold bioconjugates. Reproduced with permission from Ref. [98].

Nanoparticles gain much significance in developing PoC diagnostic platforms as they offer biocompatibility, ease in bioconjugation, robust, rapid and ultrasensitive diagnosis with less sample volume [90]. The property of nanoparticles offering enhanced sensitivity with minimal sample volume is considerable for COVID-19 detection.

A limited amount of biosamples such as throat swab, nose swab, saliva sputum and blood of the patient is collected for analysis. Also, nanoparticles exhibit excellent optical and electronic properties such as high chemical reactivity, excellent stability, enhanced sensitivity and large surface area to volume ratio for better interaction [91]. Some of the widely used nanoparticles as biosensor labels include noble metal nanoparticles (gold and silver) [92], magnetic nanoparticles (iron oxide and magnetic beads) and fluorescent nanoparticles (fluorescent molecules and quantum dots) [93].

Noble metal nanoparticle-based biosensing platforms involve applying gold and silver nanoparticles with various geometrical dimensions as labels. These noble metal nanoparticles exhibit excellent optical absorption and emission characteristics, which are entirely dependent on the dielectric properties of the metal, surrounding medium, particle size and shape [[94], [95], [96]].

Generally, gold nanofilms and gold nanoparticles (GNPs), along with receptor binding domains, are employed to develop plasmonic biosensors such as surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) sensors. In localized surface plasmon resonance (LSPR) based sensors, metallic nanoparticles with a size smaller than the wavelength of light show strong dipolar excitations. LSPRs are non-propagating excitations of the conduction electrons of metallic nanoparticles coupled to the electromagnetic field, vital for ultrasensitive detection of biomolecules [97].

A literature report utilizing gold nanoparticles as labels were presented to develop an LFIA test strip to detect SARS-CoV-2 IgG antibody present in the patient serum. The development of the LFIA test strip involves the utilization of (i) nitrocellulose membrane, (ii) labels such as gold, carbon, magnetic, colored latex and polystyrene nanoparticles, and (iii) antibodies. LFIA test strips consist of a sample pad where the sample can enter and interact with the immobilized antibodies and conjugates, followed by removing excess sample using the absorption pad.

The SARS-CoV-2 nucleocapsid protein is immobilized onto the LFA strip and functions as capture receptors, whereas anti-human IgG conjugated gold nanoparticles are utilized as signal reporters. Upon interaction of the analyte with the gold nanoparticles conjugated antibodies, a red-colored line will be produced, indicating the presence of the analyte in the sample. The developed LFA strips achieved rapid detection (10–15 min) in a less sample volume (10 μL of the patient serum) with improved sensitivity of 69.1% [99].

Furthermore, immunochromatographic test strips for the simultaneous detection of IgG and IgM viral antibodies instead of single viral antibodies were investigated to facilitate better sensitivity [100,101]. The study revealed that the sensitivity of the assay was low in the early stage (<7 days) and increased from intermediate (8–14 days) to a later stage (>15 days), thus confirming the release of antibodies after 7 days from the onset of the disease. The efficiency of the strips for COVID-19 diagnosis was blindly proofed with the RT-PCR results.

In the timeline of COVID-19 viral infections, some of the metabolites are upregulated, and few others are downregulated. In this context, teslin paper substrate-based mass spectrometry (PS-MS) was reported to detect these metabolomic biomarkers. It could be considered as a rapid assistive technique for PCR in the early diagnosis and prognosis of the COVID-19 infection [102]. Teslin, a microporous polyolefin silica matrix, offers more prolonged sample activation and retention, thereby minimizing the interferences and provide a better response compared to conventional cellulose substrates.

The statistical analysis of Teslin PS-MS showed a 93.3% correlation with the PCR results. Targeting glycoprotein of COVID-19 enables a faster diagnosis and therapeutics with high sensitivity. A recent study showed that the glycoprotein spikes have a higher affinity to angiotensin-converting enzyme-2 compared to other antibodies [103].

Besides LFA and paper-based substrates, several PoC diagnostic platforms were reported in the literature for COVID-19 diagnosis, which includes optical fiber probes [104,105] and microfluidic chips. PoC diagnosis using optical fiber probes relies on the principle of evanescence [106]. Generally, light propagates through the optical waveguide employing total internal reflection (TIR). When TIR takes place at the core-clad interface, reflected photons generate an electric field in the direction opposite to that of the interface [107].

This decaying electric field generated at the interface is called evanescence, which is exploited for sensing, stating that the analytes should present in the vicinity of the evanescent field. This decaying electric field could be enhanced with various factors, including fiber geometry and (utilization of plasmonic nanoparticles/nanofilms. Towards this end, a plasmonic fiber optic absorbance biosensor (P-FAB) using both labeled and label free immunosensing approaches was proposed for the detection of nucleocapsid (N) proteins in the COVID-19 virus [108]. Herein, a U-bent decladded optical fiber surface is considered to be the sensing region. The immunoassay is realized on the surface of the U-bent portion and the binding of the analyte with the bioreceptor is realized in the form of a decrease in the optical intensity.

In addition to proteins, genetic biomarkers are also employed to detect the viruses through a hybridization reaction [109]. Generally, GNPs absorbs light at the plasmonic region and emits non-radiative relaxation (heat). This dual property is used to develop an LSPR biosensor based on the plasmonic photothermal effect (PPT). Gold nanoislands were fabricated and conjugated with complementary DNA receptors to detect the SARS-CoV-2 sequence through hybridization of nucleic acid sequences.

The heat generated when exposed at the plasmonic region assist in the hybridization reaction [110]. The proposed LSPR biosensor achieved a limit of detection down to 0.22 pM and encouraged selective detection of the specific target sequence in a multi-gene sample. An SPR sensor with gold nanometal films produced utilizing vacuum evaporation was reported for multiplex detection of respiratory viruses. In this study, the gene probes are printed onto the thin gold film and reported enhanced sensitivity through hybridization detection [111].

Other nanomaterials such as fluorescent nanoclusters and quantum dots also exhibit excellent optical properties, thereby improving sensing and disease diagnosis [112,113]. The optical behavior of fluorescent nanomaterials from group II and VI is due to the quantum confinement and increased surface to volume ratio. Furthermore, they demonstrate color-tunable emission, large stoke shift and higher quantum yield [113].

Fluorescent labels, in conjunction with the optical fibers prove to be a simpler means of developing sensor probes. A localized surface plasmon coupled fluorescence-based fiber optic biosensor (LFI-GNPs) was reported for the enhanced detection of COVID-19 nucleocapsid protein (N protein) [114]. In this report, PMMA optical fiber was utilized in which the decladded portion is subjected to immobilization of biorecognition antibodies and labels conjugated with fluorophores.

The sensor showed 0.1 pg/ml sensitivity, which is 104 fold higher than the conventional ELISA test and proved to be a suitable candidate for high throughput sensing. A low-cost microfluidic platform utilizing polycarbonate was developed for multiplexed detection of IgG/IgM/SARS-CoV-2 antigen using fluorescent molecules as labels. The salient features of the microchip are in-built centrifugation, fluorescence immunodetection, display for quantitative analysis and European CE certification [115].

Various other PoC platforms developed using magnetic nanoparticles are discussed in detail as follows: Magnetic nanoparticle-based sensors have shown promising potential in biosensing applications over the decade [116]. Magnetic materials such as iron, cobalt, nickel, and manganese are used to fabricate the nanosized particles. This involves the application of magnetic nanoparticles and magnetic beads as labels to conjugate with the bioreceptors.

These magnetic nanoparticles could substitute the tedious filtration process of biomolecular separation thereby replacing the heavy systems such as centrifugation and filtration units. Besides, they offer low background noise as the biological molecules are non-magnetic. The fast response and relaxation of magnetic nanoparticles open a new paradigm for biomedical diagnostics to improve the separation and purification of biomolecules, enhanced signal amplification, and ultrasensitive signal read-out.

Opto-magnetic sensors, magneto-resistive sensors, giant magnetoresistance, magnetic reed switch are the currently used methodologies for magnetic biosensing applications [113,117]. Magnetic biosensors have been reported for various applications which include detecting cancer biomarker, stem cell, pathogens (bacteria and virus) and toxic-metal ions.

Extraction of the RNA from the virus is the crucial step involved in diagnosing COVID-19 using RT-PCR; however, nucleic acid extraction using the conventional column technique is laborious and time-consuming. In this perspective, amino-modified magnetic nanoparticles with multiple carboxyl groups (MNPs) were reported to promote automated nucleic extraction as there is a strong interaction between nucleic acid and carboxyl groups [118].

The synthesized MNPs were exploited to extract the RNA from pseudovirus samples to detect ORF1ab region through RT-PCR. Interestingly, the MNPs exhibited 100% RNA extraction, easy extraction process in less time (<30 min), reduced the false-negative result by avoiding RNA loss during elution, and compatible with various amplification procedures. A magnetic chemiluminescence immunoassay was reported to detect target specific COVID-19 antibodies (IgG and IgM)using synthetic biotinylated peptides and streptavidin-coated with magnetic nanobeads [118].

Herein, a luminescence is produced upon the interaction of the antibodies with the corresponding peptides coated magnetic nanobeads. The test results proved the excellent specificity offered by the assay and the detection rate for IgG (71.4%) is higher compared to IgM (57.2%). Compared to the commercial ELISA, magnetic chemiluminescence immunoassay showed higher specificity and sensitivity. Pietschmann and his team developed a novel magnetic immunodetection to promote PoC detection of COVID-19 by employing antigen-coated immunofiltration columns (IFC)as shown in Fig. 5 [119].

In the magnetic immunodetection technique, antibodies from the sample is allowed to flow through the precoated IFC, followed by enabling the secondary antibodies to flow. Then, functionalized magnetic nanobead labels are applied, and the IFC is washed to remove any unbound beads. Different concentration of antibodies were retained within the coloumn and further fed into a portable magnetic sensing device. The sensing of antibodies is made possible by exciting the coloumn at 49 KHz of magnetic field using the faraday coils. This magnetic immunoassasy detection provides higher sensitivity, portable, low cost and four fold less time than ELISAtest. This technique has high sensing ability and could be extended for multiplex detection thereby promoting PoC sensing of COVID-19.

Fig. 5
Fig. 5
Magnetic immunodetection technique using immunofiltration columns. The sample containing antibodies is made to flow through IFC followed by the injection of secondary antibodies. Magnetic nanoparticles labels are utilized to retain the antibodies, and the unbound molecules are washed. PoC sensing of COVID-19 is possible with portable magnetic read-out units. Reproduced with permission from Ref. [119].

Electrochemical biosensors

Generally, electrochemical biosensors transcribe biochemical events such as the interaction between the analyte and bioreceptor into a readable electrical signal using electrodes as sensing probes [120]. Various electrochemical biosensors for detecting vital biomarkers such as blood glucose, uric acid, ketones, lactate and deoxyribose nucleic acid (DNA) have been reported [121]. From the viewpoint of application and measurable signal at the output end, electrochemical biosensors are categorized into potentiometric, amperometric and conductometric sensors [122].

In a potentiometric sensor, a possible potential difference is measured between the working and reference electrodes due to the changes in ion concentration produced as a result of biochemical interactions. Amperometric biosensor generates an electric current, which is associated with the electron flow resulting from a redox reaction. The conductometric biosensor measures electrical conductivity, which is influenced by the variations in the ionic strength of a solution. Nanoparticles based on electrochemical biosensors have been developed and implemented for virus detection as they offer portability, cost-effectiveness and high sensitivity [123].

In this context, Tripathy and co-workers [124] developed an electrochemical biosensing technique in which gold nanoparticles deposited on a titanium (Ti) surface through electrodeposition acts as a sensing electrode as it offers stability, simplicity and inertness to severe chemical treatments. Platinum was used as reference and counter electrodes and a polydimethylsiloxane (PDMS) reservoir, which establishes the area of reaction and confinement of the electrodes.

The hybridization of the viral RNA or cDNA with the complementary probe introduced into the reaction chamber results in an electrical signal, including voltage, current or impedance. In another study, a screen-printed carbon electrode was developed for COVID-19 diagnosis based on the potentiostat, and the efficiency of the electrode is compared with fluorine-doped tin oxide (FTO) electrode cast with gold nanoparticles [125].

Here, the monoclonal antibodies were immobilized onto the gold nanoparticles, and the change in electrical conductivity is measured. In this study, gold nanoparticles serve as a catalyst and amplify the electrochemical signal, thereby enhancing the electrical conductivity.

One of the significant variables influencing the replication of the SARS-CoV-2 infection within the host lungs is triggering mitochondrial reactive oxygen species (ROS) [[126], [127], [128], [129]]. In this direction, an electrochemical sensor utilizing multi-wall carbon nanotubes (MWCNTs) was proposed to detect ROS in the sputum sample, which is a vital source of lung epithelium. Earlier literature reports demonstrated that the MWCNTs are sensitive to super-oxidants, such as hydrogen peroxide (H2O2)/ROS [130]. Herein, the MWCNTs are immobilized on the tip of steel needles in three-electrode conformation consisting of working, reference, and counter electrodes [131].

In this three-electrode configuration, the reference electrode should have a stable potential and is used for measuring the working electrode potential, whereas, the counter electrode is used to complete the cell circuit in the electrochemical cell. With the help of cyclic voltammetry, the interaction of the ROS with the MWCNTs was monitored.

The interaction resulted in the generation of cathodic ionic peak current, and in turn, the electric charges are released through the counter electrode. Moreover, the results correlate with the CT scan results of patients with 92% sensitivity and 94% specificity. In contrast, it presented 97% sensitivity and 91% specificity in correlation with the RT-PCR assay, the gold standard for detecting SARS-CoV-2 infections. Hence, this electrochemical sensor could be promoted as a PoC assistive diagnostic technique for early screening and rapid diagnosis in less than 30 s.

A biosensor based on bioelectric recognition assay (BERA) refers to the estimation of biochemical substances based on the precise and selective interaction of these substances with cells immobilized on a matrix presented [132]. A change in electric potential arises upon contact with the target molecule suspended in the gel matrix.

A membrane engineered mammalian cell is attached with SARS-CoV-2 antibodies generate by electro-insertion [133]. A biosensor based on BERA was developed to identify the S1 functional subunit of spike protein present on the surface of the SARS-CoV-2 virus. This S1 subunit is responsible for binding the virus to the angiotensin-converting enzyme-2 (ACE-2) receptor in the host [13]. The interaction of the antibody with the S1 functional subunit results in a change in bioelectric properties. The device was custom fabricated on screen-printed electrodes covered by the PDMS layer attached to the electrode employing an adhesive. The technique provides rapid response, with a better detection limit of 1 fg/ml. Besides, no cross-reactivity was perceived against the SARS-CoV-2 nucleocapsid protein.

Recently, graphene is gaining significance as potential substrates for biosensing applications, mainly due to its excellent electronic conductivity, large surface area and robust in handling. A graphene-based Field Effect Transistor (FET) electrochemical biosensor was reported to detect the SARS-CoV-2 spike protein by immobilizing spike antibody onto the graphene surface using a crosslinker, and this offers excellent specificity over other coronaviruses due to the amino acid sequence diversity of the spike protein [134].

The basic principle in a FET type sensor is based on the potentiometric detection of changes in the charge-density induced at the gate insulator/solution interface. A schematic of a FET-based sensor is shown in Fig. 6 . The changes in surface charge bindings lead to changes in source-drain current measurements. The reported electrochemical biosensor offers excellent sensitivity (~1 fg/ml), the possibility of detection in a wide range of samples including buffer, clinical samples, cultured medium and nasopharyngeal swabs with no cross-reactivity.

A graphene FET based on antibody-antigen interaction for real-time detection of spike protein S1 was investigated [135]. The sensor was fabricated by immobilizing an antibody of SARS-CoV-2 spike S1 subunit protein or ACE-2 on the surface of graphene. There is a change in conductance/resistance associated with graphene FET, which arises due to the binding of S1protein has a slight positive charge with SARS-CoV-2 spike S1 subunit protein or ACE2. This sensor has the advantages of providing rapid results, SARS-CoV-2 spike S1 subunit protein modified FET sensor showed higher sensitivity with a limit of detection of 0.2 pM.

Fig. 6
Fig. 6
Field Effect Transistor (FET) based electrochemical biosensor for COVID-19 detection. Here, graphene is used as a sensing medium and COVID-19 antibody is immobilized on a graphene sheet using 1-pyrenebutyric acid N-hydroxysuccinimide ester as a crosslinker. Reproduced with permission from Ref. [134].

Wearable and smart nanobiosensors

Despite the extensive research, most of the commercialized PoC biosensors are presented as the supportive diagnostic tool due to the challenges in automation, quantitative analysis and interference from complex biological samples. Another major limitation is remote health monitoring, real-time analysis and fall detection of the physiological parameters of the patients.

This could be addressed with the advancements in the field of wearable sensors, nanotechnology and smartphone technology [136,137]. Wearable sensors are receiving attention mainly to promote continuous monitoring, non-invasive measurements and to overcome the laborious sample processing procedures involved in traditional laboratory diagnostic tests [[138], [139], [140]]. Moreover, biological samples such as sweat and tears offer selective detection compared to blood. Thus, wearable sensors offer PoC diagnosis and are efficient in addressing the mass level screening, which is vital in controlling the widespread of the disease.

Seshadri et al., 2020 discussed the integration of wearables and android applications to predict and remotely monitor the alterations in the physiological status of the COVID-19 affected patients before the onset of clinical symptoms [141] (Fig. 7 ). Nowadays, there has been an increasing trend in the usage of smartwatches, including Fitbit, Amazefit, WHOOP and VivaLNK, for assessing the physical wellbeing of the individual.

These wearables employ accelerometers and optical sensors to measure blood pressure, temperature and heart rate [142]. In this context, the usage of smartwatches and wearable devices for measuring clinically significant physiological parameters for COVID-19 diagnosis is proposed. During a viral infection, some of the physiological responses are elevated, including heart rate, core body temperature and sleep duration.

With the increase in the severity of the infection, the person may exhibit lower SpO2 and arrhythmias. Herein, the abovementioned biological parameters such as heart rate, respiration rate, pulse rate, sleep activity, temperature are monitored using the commercial wearables to remotely track COVID-19 infection. This could ensure early identification and promote patient isolation before the onset of the COVID-19 illness [141]. Thus, these healthcare smartwatches are found to be viable for alerting individuals and identifying COVID-19 affected regions remotely with increasing efforts towards data security and handling.

Fig. 7
Fig. 7
Role of wearable technology and smartphone in the continuous monitoring of variations in the physiological parameters during the COVID-19 course of infection (A) Clinical symptoms and timeline of COVID-19 infection and correlating physiological parameters during the course of infection. (B) WHOOP application showing the decrease in physiological metrics and recovery of an individual diagnosed with COVID-19 (C) iPhone screen showing the physiological data measured from a wearable sensor and alerting the individual on his/her health status. Reproduced with permission from Ref. [141].

Apart from existing wearables measuring physiological parameters such as heart rate and other physical activity measurements, more research towards the development of wearables for identifying the vital parameters to detect COVID-19 infection, including respiratory rate, body temperature and respiratory activity in terms of cough frequency, is in progress. John A. Rogers and his group, in collaboration with the US Department of Health and Human Service’s Biomedical Advanced Research and Development (BARDA) and Sonica Health, developed a chest mount patch sensor (Fig. 8 ) for measuring complete respiration related features, heart activity related vital parameters (heart sound, heart rate, and cardiac amplitude) and body temperature.

The patch sensor consists of an accelerometer and a temperature sensor placed in direct contact with the skin at the base of the neck. The initial phase testing of the patch sensor was carried out with 50 subjects, and the results revealed the changes in the respiratory parameters are in correlation, which helped to understand the prognosis of COVID-19 viral infection [143]. Also, the patch sensor is robust, thereby achieving less discomfort to the patient and promoting the application for mass level screening.

Fig. 8
Fig. 8
Chest mount wearable patch sensor for COVID-19 diagnosis along with the vital parameters measured (A) Wireless sensor placed at the neck position in such a way that the sensor is not affected by any movements/disturbances (B) Testing the robustness of the sensor (C) Physiological parameters including heart rate, respiration rate, coughing, temperature and patterns of activity measured using the sensor from a COVID-19 patient. Reproduced with permission from Ref. [143].

During this pandemic, face masks are found to be used by everyone as the foremost protective kit to control the widespread of COVID-19 infection [144]. There is a huge demand for face masks resulting in a global shortage of face masks. However, more prolonged usage of the facemask will have high risk and are more likely to affect themselves due to prolonged COVID-19 exposure. Predicting such risks with a color change in the facemask could be a better solution to control the spread.

Such innovative high demanding face masks in combination with nanotechnology could pave the way for the development of wearable masks for diagnosis and self-protection [145,146]. Nanocoatings with polymer matrix could improve the filtration efficiency and hydrophobicity of the face mask, which could overcome the present concerns of disposable masks, including contamination, wetting, breathability and anti-microbial resistance.

In addition to self-protection, a face mask for sensing application could be vital for early diagnosis. The metal-organic framework coated wearable mask was developed for real-time diagnosis of COVID-19 virus from the simple visual color changes. Herein, the changes in the optical properties of the noble metal nanoparticles are utilized to detect the presence of the virus.

The nanoparticles are doped in a nanoporous matrix such as metal-organic frameworks (MOF) utilizing physisorption or chemisorption. This is followed by coating the nanoparticles doped MOF onto the surface of the mask (Fig. 9 ). When the doped nanoparticles interact with the virus, there will be alterations in the optical properties of the doped nanoparticles, which results in a visible color change.

Fig. 9
Fig. 9
Wearable mask for real-time detection of COVID-19. The mask is coated with nanoparticles doped metal-organic frameworks which undergo visual color changes upon interaction with the virus. Reproduced with permission from Ref. [135].

Moreover, smartphones could be utilized for color analysis and imaging to promote selective and quantitative detection [135]. There are studies reported on the application of MOF based biosensors for the detection of viruses including HIV-1, H1N1 and Zika virus [147]. Despite the ease in detection, more clinical trials are required to commercialize the MOF treated mask for COVID-19 diagnosis.

The integration of smartphones and nanotechnology has opened up smart nanobiosensors which could assist the public in making use of a smartphone as colorimetric, fluorimetric and electrochemical sensors [135,148]. Smartphones are used in two ways for biosensing applications (i) as a detector in interpreting the changes of the biochemical events 149 as a data analytics platform in creating a database, thereby facilitating the eHealth [150].

Herein, we discuss employing smartphones as a detector, ensuring better quantitative analysis in the following section. The progress in the smartphone PoC diagnostic platforms has been reviewed extensively which includes customized android application development, optoelectronics integration, and instrumental interface for imaging and color analysis, thereby ensuring colorimetric, electrochemical, and fluorimetric detection of an analyte in various biofluid samples such as saliva, urine, blood, sweat, and tears [151,152]. In this context, a paper-based plasmonic biosensor combined with a smartphone (Fig. 10 ) was developed for the automatic detection of interleukin −16 using augmented reality which is vital to diagnose COVID-19 [153].

Fig. 10
Fig. 10
Schematic showing the steps involved in the detection of Interleukin-6 (IL-6) using the paper-based smart plasmonic biosensor (a) Immunosensing scheme in which capture Ab (antibody) is immobilized onto the filter paper, and binding of IL-6 to capture Ab is detected using gold nanoparticles conjugated with avidin and biotinylated antibodies (b) Imaging of the colored spot on the filter paper with the help of virtual frame using smartphone (c) Detection screen displaying the automatic identification of the region of interest using augmented reality (d) The result screen indicating the pixel intensity value. Reproduced with permission from Ref. [154].

The smartphone app is utilized for the quantitative colorimetric analysis of the red spots created on the paper surface due to the interaction of gold nanoparticle labels with the cytokines from the sample. Various smartphone interfaced biosensor applications in conjunction with the microfluidic channels and bioassay cartridges to detect viral infections are tabulated in Table 1 .

The smartphones are utilized to detect

(i) colorimetric and fluorimetric changes in which smartphone camera and standalone android application are utilized for imaging and color analysis

(ii) electrochemical alterations are monitored using a smartphone camera and CMOS detectors for further data interpretation and communication [148,151,152].

This is an easy to use technique and finds more scope towards PoC diagnostic platforms for controlling the COVID-19 and future pandemics.

Table 1

Smartphone-based diagnostic platforms for detection of various viruses.

Signal transductionAnalyteSensing substratesBioreceptorsLabelsSensitivityRef
ElectrochemicalCOVID-19 RNAScreen printed carbon electrodeCalixarene functionalized graphene[email protected]3O4 nanocomposites3 aM[155]
Zika virus (ZIKV)Functionalized interdigitated gold micro electrodeZIKV specific envelope protein antibody10 pM[156]
Hepatitis C VirusThree electrode system using PCB1 ng/μL[157]
ColorimetryZika virus (ZIKV)Vial immunosensorPlatinum/gold core shell nanoparticles1 pg/ml[158]
Avian influenzaPDMS microfluidic channelH5 monoclonal antibodiesGold nanoparticles with silver shells8 × 103 EID50/mLa[159]
Influenza APaper based microfluidic system14B11 antibodyHorse radish peroxidase enzyme conjugated 14F10 antibody32 × 10−4HAb[160]
FluorimetryZika virus non-structural protein 1Nitrocellulose membraneQuantum dot microspheres0.15 ng/ml[161]
Influenza virusCYTOP-on-coverslip3.0 × 104 PFU ml−1c[162]
Equine herpes virus 1 (EHV 1)Silicon microfluidic chipsEvagreen dye5.5 × 104 copies/mLd[163]
Note: a-The amount of infectious virus present in the sample is expressed in terms of 50% embryo infectious dose per millilitre. b- HA refers to the measure of haemagglutinin protein content in the virus. c-The number of infective particles in the sample is expressed in terms of plaque-forming units per millilitre. d-The viral load in the sample is expressed as the number of copies per millilitre.

On the whole, the prospects of deploying wearable sensors for the real-time diagnosis of COVID-19 can aid in controlling the spread of the pandemic. The combination of nanotechnology with the smartphone-based detectors could serve for both qualitative and quantitative detection of COVID-19. However, this could offer only an initial screening for viral detection and followed by the recommendation of standard diagnostic protocols for further diagnosis and treatment.

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