Label-Free Immunoassay for Sensitive and Rapid Detection of the SARS-CoV-2 Antigen Based on Functionalized Magnetic Nanobeads with Chemiluminescence and Immunoactivity

Hydrogen-Bonded Organic Framework Nanozyme with Multi-Enzyme Activity for Chemiluminescence Sensing of Acetylcholinesterase and Screening Its Inhibitors

Strong and persistent chemiluminescence (CL) is essential for enhancing the detection accuracy and reproducibility of CL-based analytical methods. In this study, we explored the synergistic effects of amino groups present on the surface of hydrogen-bonded organic framework (HOF-PyTTA, where PyTTA is denoted as 4,4',4″,4‴-(pyrene-1,3,6,8-tetrayl)tetraaniline) materials and N-(4-aminobutyl-N-ethylisoluminol) (ABEI) for reducing gold nanoparticles (AuNPs) on the surface of HOFs. Additionally, we utilized the substantial specific surface area and abundant amino groups of HOFs to sequester Co2+ ions, resulting in the synthesis of HAACo material. The resulting HAACo exhibited remarkable peroxidase, oxidase, and catalase mimetic activities, enabling the luminol-H2O2 chemiluminescence system to maintain a glow-type CL phenomenon for more than 1 h. Subsequently, we developed a CL point-of-care testing (POCT) sensor that integrated the CL characteristics of HAACo with smartphone technology and 3D printing for the determination of acetylcholinesterase (AChE) activity in serum samples, as well as the screening for AChE inhibitors. The sensor demonstrated a linear detection range for AChE activity from 0.001 to 40 mU mL-1, with a detection limit of 0.00057 mU mL-1. The calculated IC50 for the AChE inhibitor tacrine was found to be 21.9 nmol L-1, indicating good selectivity and stability of the sensor. This work not only expands the applications of glow-type CL in biosensing but also enriches the utilization of HOF materials in analytical chemistry, paving the way for the development of multifunctional HOF-based materials for future applications.

The significance of antigen-antibody-binding avidity in clinical diagnosis

Immunoglobulin G (IgG) and immunoglobulin M (IgM) testing are commonly used to determine infection status. Typically, the detection of IgM indicates an acute or recent infection, while the presence of IgG alone suggests a chronic or past infection. However, relying solely on IgG and IgM antibody positivity may not be sufficient to differentiate acute from chronic infections. This limitation arises from several factors. The prolonged presence of IgM can complicate diagnostic interpretations, and false positive IgM results often arise from antibody cross-reactivity with various antigens. Additionally, IgM may remain undetectable in prematurely collected samples or in individuals who are immunocompromised, further complicating accurate diagnosis. As a result, additional diagnostic tools are required to confirm infection status. Avidity is a measure of the strength of the binding between an antigen and antibody. Avidity-based assays have been developed for various infectious agents, including toxoplasma, cytomegalovirus (CMV), SARS-CoV-2, and avian influenza, and are promising tools in clinical diagnostics. By measuring the strength of antibody binding, they offer critical insights into the maturity of the immune response. These assays are instrumental in distinguishing between acute and chronic or past infections, monitoring disease progression, and guiding treatment decisions. The development of automated platforms has optimized the testing process by enhancing efficiency and minimizing the risk of manual errors. Additionally, the recent advent of real-time biosensor immunoassays, including the label-free immunoassays (LFIA), has further amplified the capabilities of these assays. These advances have expanded the clinical applications of avidity-based assays, making them useful tools for the diagnosis and management of various infectious diseases. This review is structured around several key aspects of IgG avidity in clinical diagnosis, including: (i) a detailed exposition of the IgG affinity maturation process; (ii) a thorough discussion of the IgG avidity assays, including the recently emerged biosensor-based approaches; and (iii) an examination of the applications of IgG avidity in clinical diagnosis. This review is intended to contribute toward the development of enhanced diagnostic tools through critical assessment of the present landscape of avidity-based testing, which allows us to identify the existing knowledge gaps and highlight areas for future investigation.

Chemiluminescent Fe3O4@Nickel-Cobalt Double Hydroxide Magnetic Core-Shell Nanomaterial as an Analytical Interface for Label-Free CYFRA21-1 Immunosensing

Cytokeratin 19 fragment (CYFRA21-1) is considered to be a potential marker for the diagnosis and of classification of lung cancer. It is highly desired to develop rapid and highly sensitive label-free chemiluminescence (CL) immunosensors for the detection of CYFRA21-1. In this work, magnetic core-shell nanocomposite Fe3O4@nickel-cobalt double hydroxide (Fe3O4@DH) was used for the first time as a carrier to synthesize N-(4-aminobutyl)-N-ethylisoluminol (ABEI) and gold and silver nanocluster (AuAgNC) bifunctionalized magnetic nanomaterials Fe3O4@DH/AuAgNCs-ABEI. The resulting Fe3O4@DH/AuAgNCs-ABEI demonstrated excellent CL performance, attributed to the exceptional catalytic capability of nickel-cobalt double hydroxide (NiCo-DH) and AuAgNCs, as well as the large specific surface area of NiCo-DH for AuAgNCs and ABEI enrichment. Moreover, Fe3O4@DH/AuAgNCs-ABEI exhibited superior stability compared to Fe3O4-based CL-functionalized magnetic nanomaterials, owing to the presence of NiCo-DH shells. On this basis, a label-free CL immunosensor using CYFRA21-1 antibody-modified Fe3O4@DH/AuAgNCs-ABEI as an analytical interface was constructed for the detection of CYFRA21-1 in the range from 1.0 × 10-13 to 1.0 × 10-8 g/mL. The detection limit of this CL immunosensor was 47.7 fg/mL, 4 orders of magnitude lower than existing CL methods. The CL immunosensor was able to accurately detect the concentration of CYFRA21-1 in serum samples, as evidenced by ELISA results. More importantly, it not only distinguished well between healthy persons and lung cancer patients (90.0% sensitivity and 90.0% specificity), but also effectively distinguished between patients with non-small cell lung cancer and small cell lung cancer (80.0% sensitivity and 86.7% specificity).

A pretreatment-free and eco-friendly rapid detection for mycotoxins in edible oils based on magnetic separation technique

Pretreatment steps of current rapid detection methods for mycotoxins in edible oils not only restrict detection efficiency, but also produce organic waste liquid to pollute environment. In this work, a pretreatment-free and eco-friendly rapid detection method for edible oil is established. This proposed method does not require pretreatment operation, and automated quantitative detection could be achieved by directly adding oil samples. According to polarity of target molecules, the content of surfactant in reaction solutions could be adjusted to achieve the quantitative detection of AFB1 in peanut oil and ZEN in corn oil. The recoveries are between 96.5%-110.7% with standard deviation <10.4%, and the limit of detection is 0.17 μg/kg for AFB1 and 4.91 μg/kg for ZEN. This method realizes full automation of the whole chain detection, i.e. sample in-result out, and is suitable for the on-site detection of batches of edible oils samples.

Trends and Perspectives in Biosensing and Diagnosis

Biosensors are attractive tools for detecting molecules and small particles, as they can produce rapid, sensitive, and specific signals [...].

Photoluminescence-based biosensor for the detection of antibodies against SARS-CoV-2 virus proteins by ZnO tetrapod structure integrated within microfluidic system

This paper reports on development of an optical biosensor for the detection of antibodies against SARS-CoV-2 virus proteins in blood serum. ZnO nanotetrapods with high surface area and stable room temperature photoluminescence (PL) were selected as transducers. Structure and optical properties of the ZnO tetrapods have been studied by XRD, SEM and Raman spectroscopy. Crystallinity, dimensions and emission peaks of the ZnO tetrapods were determined. The ZnO tetrapods were fixed on glass chip. Silanization of ZnO tetrapods surface resulted in forming of functional surface groups suitable for the immobilization of bioselective layer. Two types of recombinant proteins (rS and rN) have been used to form bioselective layer on the surface of the ZnO tetrapods. Flow through microfluidic system, integrated with optical system, has been used for the determination of antibodies against SARS-CoV-2 virus proteins present in blood samples. The SARS-CoV-2 probes, prepared in PBS solution, have been injected into the measurement chamber with a constant pumping speed. Steady-state photoluminescence spectra and photoluminescence kinetics have been studied before and after injection of the probes. The biosensor signal has been tested to anti-SARS-CoV-2 antibodies in the range of 0.001 nM-1 nM. Control measurements have been performed with blood serum of healthy person. ZnO-SARS-CoV-2-rS and ZnO-SARS-CoV-2-rN biosensors showed high stability and sensitivity to anti-SARS-CoV-2 antibodies in the range of 0.025-0.5 nM (LOD 0.01 nM) and 0.3-1 nM (LOD 0.3 nM), respectively. Gibbs free energy of interaction between ZnO/SARS-CoV-2-rS and ZnO/SARS-CoV-2-rN bioselective layers with anti-SARS-CoV-2 antibodies showed -35.5 and -21.4 kJ/mol, respectively. Average detection time of biosensor integrated within microfluidic system was 15-20 min. The detection time and pumping speed (50 μL/min) were optimized to make detection faster. The developed system and ZnO-SARS-CoV-2-rS nanostructures have good potential for detection of anti-SARS-CoV-2 antibodies from patient's probes.

Ultrasensitive PD-L1-Expressing Exosome Immunosensors Based on a Chemiluminescent Nickel-Cobalt Hydroxide Nanoflower for Diagnosis and Classification of Lung Adenocarcinoma

Programmed death ligand-1 (PD-L1)-expressing exosomes are considered a potential marker for diagnosis and classification of lung adenocarcinoma (LUAD). There is an urgent need to develop highly sensitive and accurate chemiluminescence (CL) immunosensors for the detection of PD-L1-expressing exosomes. Herein, N-(4-aminobutyl)-N-ethylisopropanol-functionalized nickel-cobalt hydroxide (NiCo-DH-AA) with a hollow nanoflower structure as a highly efficient CL nanoprobe was synthesized using gold nanoparticles as a "bridge". The resulting NiCo-DH-AA exhibited a strong and stable CL emission, which was ascribed to the exceptional catalytic capability and large specific surface area of NiCo-DH, along with the capacity of AuNPs to facilitate free radical generation. On this basis, an ultrasensitive sandwich CL immunosensor for the detection of PD-L1-expressing exosomes was constructed by using PD-L1 antibody-modified NiCo-DH-AA as an effective signal probe and rabbit anti-CD63 protein polyclonal antibody-modified carboxylated magnetic bead as a capture platform. The immunosensor demonstrated outstanding analytical performance with a wide detection range of 4.75 × 103-4.75 × 108 particles/mL and a low detection limit of 7.76 × 102 particles/mL, which was over 2 orders of magnitude lower than the reported CL method for detecting PD-L1-expressing exosomes. Importantly, it was able to differentiate well not only between healthy persons and LUAD patients (100% specificity and 87.5% sensitivity) but also between patients with minimally invasive adenocarcinoma and invasive adenocarcinoma (92.3% specificity and 52.6% sensitivity). Therefore, this study not only presents an ultrasensitive and accurate diagnostic method for LUAD but also offers a novel, simple, and noninvasive approach for the classification of LUAD.

Simultaneous and sensitive detection of SARS-CoV-2 proteins spike and nucleocapsid based on long-range SERS biosensor

BACKGROUND

Early diagnosis of SARS-CoV-2 infection is still critical to control COVID-19 outbreak. Traditional polymerase chain reaction, enzyme-linked immunosorbent assay or lateral flow immunoassay performed poorly on detection times, sample preparation process and accuracy. Surface-enhanced Raman scattering (SERS)-based detection has emerged as a powerful analytical technique, which overcomes the above limitations. However, due to the near-field effect of traditional substrate, it is difficult to monitor the binding event of aptamers with proteins. It is obvious that a novel SERS substrate thatsupportedextended and stronger electromagnetic fields was required to hold long-range effects and allow for binding event testing.

RESULTS

Driven by this challenge, we reported a long-range SERS-active substrate, which was built by inserting bowtie nanoaperture arrays in a refractive-index-symmetric environment and Au mirror surfaces, for SARS-CoV-2 protein binding event detection. Then, a double-π structure aptasensor was simply designed through the hybridization of spike (S) and nucleocapsid (N) proteins aptamers, and a corresponding complementary strand. This kind of double-π structure would dissociate when targets proteins S and N existed and led to the SERS responses decreased, which established the detection basis of our system. What's more, due to two Raman labels were involved, both proteins S and N can be sensed simultaneously. Our proposed method showed improved sensitivity with a low limit of detection for multiplex detection (1.6 × 10-16 g/mL for protein S and 1.0 × 10-16 g/mL for protein N) over a wide concentration range.

SIGNIFICANCE

This represents the first long-range SERS apatasensor platform for detection of S and N proteins simultaneously. Our method showed high sensitivity, selectivity, reproducibility, stability and remarkable recoveries in human in saliva and serum samples, which is particularly important for the early diagnostics of COVID as well as for future unknown coronavirus.

Advances in nanobiosensors during the COVID-19 pandemic and future perspectives for the post-COVID era

The unprecedented threat of the highly contagious virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes exponentially increased infections of coronavirus disease 2019 (COVID-19), highlights the weak spots of the current diagnostic toolbox. In the midst of catastrophe, nanobiosensors offer a new opportunity as an alternative tool to fill a gap among molecular tests, rapid antigen tests, and serological tests. Nanobiosensors surpass the potential of antigen tests because of their enhanced sensitivity, thus enabling us to see antigens as stable and easy-to-access targets. During the first three years of the COVID-19 pandemic, a substantial number of studies have reported nanobiosensors for the detection of SARS-CoV-2 antigens. The number of articles on nanobiosensors and SARS-CoV-2 exceeds the amount of nanobiosensor research on detecting previous infectious diseases, from influenza to SARS-CoV and MERS-CoV. This unprecedented publishing pace also implies the significance of SARS-CoV-2 and the present pandemic. In this review, 158 studies reporting nanobiosensors for detecting SARS-CoV-2 antigens are collected to discuss the current challenges of nanobiosensors using the criteria of point-of-care (POC) diagnostics along with COVID-specific issues. These advances and lessons during the pandemic pave the way for preparing for the post-COVID era and potential upcoming infectious diseases.

Carbon Nanostructured Immunosensing of Anti-SARS-CoV-2 S-Protein Antibodies

The rampant spread and death rate of the recent coronavirus pandemic related to the SARS-CoV-2 respiratory virus have underscored the critical need for affordable, portable virus diagnostics, particularly in resource-limited settings. Moreover, efficient and timely monitoring of vaccine efficacy is needed to prevent future widespread infections. Electrochemical immunosensing poses an effective alternative to conventional molecular spectroscopic approaches, offering rapid, cost-effective, sensitive, and portable electroanalysis of disease biomarkers and antibodies; however, efforts to improve binding efficiency and sensitivity are still being investigated. Graphene quantum dots (GQDs) in particular have shown promise in improving device sensitivity. This study reports the development of a GQD-functionalized point-of-contamination device leveraging the selective interactions between SARS-CoV-2-specific Spike (S) Protein receptor binding domain (RBD) antigens and IgG anti-SARS-CoV-2-specific S-protein antibodies at screen-printed carbon electrode (SPCE) surfaces. The immunocomplexes formed at the GQD surfaces result in the interruption of the redox reactions that take place in the presence of a redox probe, decreasing the current response. Increased active surface area, conductivity, and binding via EDC/NHS chemistry were achieved due to the nanomaterial inclusion, with 5 nm, blue luminescent GQDs offering the best results. GQD concentration, EDC/NHS ratio, and RBD S-protein incubation time and concentration were optimized for the biosensor, and inter- and intra-screen-printed carbon electrode detection was investigated by calibration studies on multiple and single electrodes. The single electrode used for the entire calibration provided the best results. The label-free immunosensor was able to selectively detect anti-SARS-CoV-2 IgG antibodies between 0.5 and 100 ng/mL in the presence of IgM and other coronavirus antibodies with an excellent regression of 0.9599. A LOD of 2.028 ng/mL was found, offering comparable findings to the literature-reported values. The detection sensitivity of the sensor is further compared to non-specific IgM antibodies. The developed GQD immunosensor was compared to other low-oxygen content carbon nanomaterials, namely (i) carbon quantum dot (CQD), (ii) electrochemically reduced graphene oxide, and (iii) carbon black-functionalized devices. The findings suggest that improved electron transfer kinetics and increased active surface area of the CNs, along with surface oxygen content, aid in the detection of anti-SARS-CoV-2 IgG antibodies. The novel immunosensor suggests a possible application toward monitoring of IgG antibody production in SARS-CoV-2-vaccinated patients to study immune responses, vaccine efficacy, and lifetime to meet the demands for POC analysis in resource-limited settings.

A self-enhanced electrochemiluminescence array chip for portable label-free detection of SARS-CoV-2 nucleocapsid protein with smartphone

SARS-CoV-2 antigen detection plays a key role in the rapid diagnosis of COVID-19. However, current clinically used immunoassays are often limited by assay throughput, sensitivity, accuracy, and field operating conditions. To address these challenges, we constructed a self-enhanced electrochemiluminescence (ECL) array chip (SE2AC) for highly sensitive and label-free detection of SARS-CoV-2 nucleocapsid protein (N protein) with a facile and portable assay setup. Firstly, the self-enhanced ECL nanomaterials with inherent film-forming properties were synthesized by co-doping Ru(bpy)32+ and polyethyleneimine (PEI) in silica nanoparticles (Ru/PEI@SiO2). Secondly, a resistance-induced potential difference-based single-electrode electrochemical system (SEES) was adapted to serve as the electrode array to facilitate one-step assembly without the need for chip alignment. Thirdly, the chip electrode array was functionalized with the synthesized self-enhanced ECL emitters and captured antibodies. In addition, a portable detection box equipped with a smartphone was 3D-printed to serve as the chip holder and "dark room" for imaging acquisition. The SE2AC performance was validated with N protein with a limit of detection (LOD) of 0.47 pg/mL in the range of 1-10,000 pg/mL. Furthermore, the chip successfully detected the viral antigen residue as low as 1.92 pg/mL from diluted rehabilitation patients' serum samples. This is the first study reporting label-free detection of SARS-Cov-2 N protein based on a self-enhanced ECL immunosensor, which provides a novel facile method for highly sensitive diagnosis of COVID-19 with high throughput, portability, and low cost.

Construction and bioanalytical applications of poly-adenine-mediated gold nanoparticle-based spherical nucleic acids

Owing to the versatile photophysical and chemical properties, spherical nucleic acids (SNAs) have been widely used in biosensing. However, traditional SNAs are formed by self-assembly of thiolated DNA on the surface of a gold nanoparticle (AuNP), where it is challenging to precisely control the orientation and surface density of DNA. As a new SNA, a polyadenine (polyA)-mediated SNA using the high binding affinity of consecutive adenines to AuNPs shows controllable surface density and configuration of DNA, which can be used to improve the performance of a biosensor. Herein, we first introduce the properties of polyA-mediated SNAs and fundamental principles regarding the polyA-AuNP interaction. Then, we provide an overview of current representative synthesis methods of polyA-mediated SNAs and their advantages and disadvantages. After that, we summarize the application of polyA-mediated SNAs in biosensing based on fluorescence and colorimetric methods, followed by discussion and an outlook of future challenges in this field.

Construction of Magnetic Core-Large Mesoporous Satellite Immunosensor for Long-Lasting Chemiluminescence and Highly Sensitive Tumor Marker Determination

Chemiluminescence immunoassay exhibits high sensitivity and signal-to-noise ratio, thus attracting great attention in the early diagnosis and dynamic monitoring of diseases. However, the collection of conventional flash-type chemiluminescence signal (<5 s) relies heavily on automatic sampling and reading instrument. Herein, a novel core-satellite multifunctional chemiluminescence immunosensor is designed for the efficient enrichment and highly sensitive determination of cancer biomarker carcinoembryonic antigen (CEA) with enhanced and long-lasting output signal that can be conveniently recorded by a simple microplate plate reading instrument. Anti-CEA monoclonal antibody 2 (Ab2) modified Fe3 O4 @SiO2 microspheres (Fe3 O4 @SiO2 -Ab2, 370 nm in diameter) are synthesized as the core for selectively capturing and enriching target CEA in solution, and anti-human CEA monoclonal antibody 1 (Ab1) and horseradish peroxidase (HRP) co-immobilized dendritic large-mesoporous silica nanospheres (MSNs-HRP/Ab1, 80 nm in diameter, pore size: 17 nm) are synthesized as the satellite for efficient immunological recognition and signal amplification. The as-designed core-satellite magnetic chemiluminescence immunosensors exhibit a broad linear range of 0.01-20 ng mL-1 and a low detection limit of 3.0 pg mL-1 for the convenient, highly specific, and sensitive determination of CEA in human serum. Such core-satellite chemiluminescence immunosensors are expected to act as a powerful tool for in vitro detection of various biomarkers, overcome the defect of conventional chemiluminescence relying heavily on expensive and bulky automatic instruments and popularize chemiluminescence analysis to primary medical institutions and remote areas.

Combining hybrid nanoflowers with hybridization chain reaction for highly sensitive detection of SARS-CoV-2 nucleocapsid protein

BACKGROUND

COVID-19 (coronavirus disease 2019) pandemic has had enormous social and economic impacts so far. The nucleocapsid protein (N protein) is highly conserved and is a key antigenic marker for the diagnosis of early SARS-CoV-2 infection.

RESULTS

In this study, the N protein was first captured by an aptamer (Aptamer 58) coupled to magnetic beads (MBs), which in turn were bound to another DNA sequence containing the aptamer (Aptamer 48-Initiator). After adding 5'-biotinylated hairpin DNA Amplifier 1 and Amplifier 2 with cohesive ends for complementary hybridization, the Initiator in the Aptamer 48-Initiator began to trigger the hybridization chain reaction (HCR), generating multiple biotin-labeled DNA concatamers. When incubated with synthetic streptavidin-invertase-Ca3(PO4)2 hybrid nanoflower (SICa), DNA concatamers could specifically bind to SICa through biotin-streptavidin interaction with high affinity. After adding sucrose, invertase in SICa hydrolyzed sucrose to glucose, whose concentration could be directly read with a portable glucometer, and its concentration was positively correlated with the amount of captured N protein. The method is highly sensitive with a detection limit as low as 1 pg/mL.

SIGNIFICANCE

We believe this study provided a practical solution for the early detection of SARS-CoV-2 infection, and offered a new method for detecting other viruses through different target proteins.

Highly sensitive digital detection of SARS-CoV-2 nucleocapsid protein through single-molecule counting

Nucleocapsid protein (NP) is one of the structural proteins of SARS-CoV-2 which is stable, well-conserved, highly immunogenic, and abundantly expressed due to the host's adaptive immune response, making it a promising antigenic biomarker for the early and rapid identification and diagnosis of SARS-CoV-2. Traditional antigen analytical methods with NP as the detection marker often have insufficient sensitivity. To achieve rapid and highly sensitive detection of NP, we constructed a novel single-molecule (digital) fluorescence-linked immunosorbent assay (FLISA) based on streptavidin-modified transparent 96-well microplates. Streptavidin was immobilized on the microplate under optimized conditions with a 15 mM carbonate buffer solution (pH 9.6) as the coating solution, biotinylated antibodies conjugated with streptavidin as capture probes, and carboxylated fluorescent microsphere-conjugated monoclonal antibodies (FMs-mAbs) as fluorescent probes. Individual sandwich immunolabeled complexes of the SARS-CoV-2 diagnostic marker NP were detected and counted though wide-field inverted fluorescence microscopy (1.1 × 1.4 mm2). FLISA had a linear detection range of 0.2 pg/mL to 200 ng/mL and a limit of detection (LOD) of 0.73 fg/mL and 8 fg/mL for NP in phosphate buffer saline and spiked nasal swab samples, respectively. The sensitivity was much higher than commercial antigen detection kits, providing wide detection prospects in future clinical diagnosis, environmental monitoring, and other fields.

N-(4-Aminobutyl)-N-ethylisopropanol and Co2+ Dual-Functionalized Core-Shell Fe3O4@Au/Ag Magnetic Nanomaterials with Strong and Stable Chemiluminescence for a Label-Free Exosome Immunosensor

Recently, magnetic beads (MBs) are moving toward chemiluminescence (CL) functional magnetic nanomaterials with a great potential for constructing label-free immunosensors. However, most of the CL-functionalized MBs suffer from scarce binding sites, easy aggregation, and leakage of CL reagents, which will ultimately affect the analytical performance of immunosensors. Herein, by using core-shell Fe3O4@Au/Ag magnetic nanomaterials as a nanoplatform, a novel N-(4-aminobutyl)-N-ethylisopropanol (ABEI) and Co2+ dual-functionalized magnetic nanomaterial, namely, Fe3O4@Au/Ag/ABEI/Co2+, with strong and stable CL emission was successfully synthesized. Its CL intensity was 36 and 3.5 times higher than that of MB@ABEI-Au/Co2+ and ABEI and Co2+ dual-functionalized chemiluminescent MBs previously reported by our group, respectively. It was found that the excellent CL performance of Fe3O4@Au/Ag/ABEI/Co2+ could be attributed to the enrichment effect of the Au/Ag shell and the synergistic enhance effect of the Au/Ag shell and Co2+. A related CL mechanism has been proposed. Afterward, based on the intense and stable CL emission of Fe3O4@Au/Ag/ABEI/Co2+, a sensitive and effective label-free CL immunosensor for exosome detection was established. It exhibited excellent analytical performance with a wide detection range of 3.1 × 103 to 3.1 × 108 particles/mL and a low detection limit of 2.1 × 103 particles/mL, which were better than the vast majority of the reported CL immunosensors. Moreover, the proposed label-free CL immunosensor was successfully used to detect exosomes in human serum samples and enabled us to distinguish healthy persons and lung cancer patients. It has the potential to be a powerful tool for exosome study and early cancer diagnosis.

Emission Onset Time-Adjustable Chemiluminescent Gold Nanoparticles with Ultrastrong Emission for Smartphone-Based Immunoassay of Severe Acute Respiratory Syndrome Coronavirus 2 Antigen

Recently, our group reported a chemical timer approach to manipulate the onset time of chemiluminescence (CL) emission. However, it is still in the proof-of-concept stage, and its analytical applications have not been explored yet. Nanomaterials have merits of good catalytic effect, large specific surface area, good biocompatibility, and ease of self-assembly, which are ideal for constructing analytical-interfaces for bioassays. Herein, an emission onset time-adjustable chemiluminescent L012-Au/Mn2+ was synthesized for the first time by modifying Mn2+ on the surface of L012-protected gold nanoparticle. By using H2O2 and NaHCO3 as coreactants, L012-Au/Mn2+ could not only generate an ultrastrong and long-time CL emission but also its CL emission onset time could be adjusted by the addition of thiourea, which could effectively eliminate interference from the addition of coreactants, shorten the exposure time, reduce the detection background, and finally achieve high sensitivity CL imaging analysis. On this basis, a label-free CL immunoassay was constructed with a smartphone-based imaging system for high-throughput and sensitive determination of severe acute respiratory syndrome coronavirus 2 nucleocapsid (N) protein. The CL image of the immunoassay with different concentrations of N proteins was captured in one photograph 100 s after the injection of H2O2 with a short exposure time of 0.5 s. The immunoassay showed good linearity over the concentration range of 1 pg/mL to 10 ng/mL with a detection limit of 0.13 pg/mL, which was much lower than the reported CCD imaging detection method. In addition, it showed good selectivity and stability and was successfully applied in serum samples from healthy individuals and COVID-19 rehabilitation patients.

Diagnostics and analysis of SARS-CoV-2: current status, recent advances, challenges and perspectives

The disastrous spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has induced severe public healthcare issues and weakened the global economy significantly. Although SARS-CoV-2 infection is not as fatal as the initial outbreak, many infected victims suffer from long COVID. Therefore, rapid and large-scale testing is critical in managing patients and alleviating its transmission. Herein, we review the recent advances in techniques to detect SARS-CoV-2. The sensing principles are detailed together with their application domains and analytical performances. In addition, the advantages and limits of each method are discussed and analyzed. Besides molecular diagnostics and antigen and antibody tests, we also review neutralizing antibodies and emerging SARS-CoV-2 variants. Further, the characteristics of the mutational locations in the different variants with epidemiological features are summarized. Finally, the challenges and possible strategies are prospected to develop new assays to meet different diagnostic needs. Thus, this comprehensive and systematic review of SARS-CoV-2 detection technologies may provide insightful guidance and direction for developing tools for the diagnosis and analysis of SARS-CoV-2 to support public healthcare and effective long-term pandemic management and control.

A label-free colorimetric 3D paper-based device for ochratoxin A detection using G-quadruplex/hemin DNAzyme with a smartphone readout

The colorimetric sensor usually depends on enzyme-mediated signal amplification to achieve trace analysis of ochratoxin A (OTA) residues in food samples. However, the enzyme labeling and manual addition of reagents steps increased assay time and operation complexity, restricting their application in point-of-care testing (POCT). Herein, we report a label-free colorimetric device integrating a 3D paper-based analytical device and a smartphone as handheld readout for rapid and sensitive detection of OTA. Using vertical-flow design, the paper-based analytical device enables the specific recognition of target and self-assembly of G-quadruplex (G4)/hemin DNAzyme to be performed, then employs DNAzyme for transducing the OTA binding event signal into a colorimetric signal. The design of independent functional units, including biorecognition unit, self-assembly unit and colorimetric units, which can address crowding and disorder of biosensing interfaces and improve the recognition efficiency of aptamer (apta). In addition, we eliminated signal losses and nonuniform coloring by introducing carboxymethyl chitosan (CMCS) to obtain perfectly focused signals on colorimetric unit. On the basis of parameter optimization, the device exhibited a detection range of 0.1-500 ng/mL and a detection limit of 41.9 pg/mL for OTA. Importantly, good results were obtained in spiked real samples, indicating applicability and reliability of developed device.

Single-Molecule Evaluation of the SARS-CoV-2 Nucleocapsid Protein Using Gold Particle-in-a-Frame Nanostructures Enhanced Fluorescent Assay

Ultrasensitive evaluation of low-abundance analytes, particularly with limits approaching a single molecule, is a key challenge in the design of an assay for profiling severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigen. Herein, we report an aptamer claw strategy for directly evaluating the SARS-CoV-2 antigen based on gold particle-in-a-frame nanostructures (Au PIAFs). Au PIAF was used as a metal-enhanced fluorescence material. The assay integrated with a microplate reader achieved a sensitivity of 44 fg·mL-1 in under 3 min and accurately detected the SARS-CoV-2 nucleocapsid protein (N protein) in human saliva samples. When our assay is combined with a single-molecule counting platform, the limit of detection can be as low as 0.84 ag·mL-1. This rapid and ultrasensitive assay holds promise as a tool for screening SARS-CoV-2 and other contagious viruses.

An ultrasensitive aptasensor of SARS-CoV-2 N protein based on ion current rectification with nanopipettes

Since the outbreak of COVID-19 in the world, it has spread rapidly all over the world. Rapid and effective detection methods have been a focus of research. The SARS-CoV-2 N protein (NP) detection methods currently in use focus on specific recognition of antibodies, but the reagents are expensive and difficult to be produced. Here, aptamer-functionalized nanopipettes utilize the unique ion current rectification (ICR) of nanopipette to achieve rapid and highly sensitive detection of trace NP, and can significantly reduce the cost of NP detection. In the presence of NP, the surface charge at the tip of the nanopipette changes, which affects ion transport and changes the degree of rectification. Quantitative detection of NP is achieved through quantitative analysis. Relying on the high sensitivity of nanopipettes to charge fluctuations, this sensor platform achieves excellent sensing performance. The sensor platform exhibited a dynamic working range from 10²-10⁶ pg/mL with a detection limit of 73.204 pg/mL, which showed great potential as a tool for rapidly detecting SARS-CoV-2. As parallel and serial testing are widely used in the clinic to avoid missed diagnosis or misdiagnosis, we hope this platform can play a role in controlling the spread and prevention of COVID-19.

Chemiluminescent Nanogels as Intensive and Stable Signal Probes for Fast Immunoassay of SARS-CoV-2 Nucleocapsid Protein

It is highly desired to exploit good nanomaterials as nanocarriers for immobilizing chemiluminescence (CL) reagents, catalysts and antibodies to develop signal probes with intensive and stable CL properties for immunoassays. In this work, N-(4-aminobutyl)-N-ethylisoluminol (ABEI) and Co2+ bifunctionalized polymethylacrylic acid nanogels (PMAANGs-ABEI/Co2+) were synthesized via a facile strategy by utilizing carboxyl group-rich PMAANGs as nanocarriers to immobilize ABEI and Co2+. The obtained PMAANGs-ABEI/Co2+ showed extraordinary CL performance. The CL intensity is 2 orders of magnitude higher than that of previously reported ABEI and Cu2+-cysteine complex bifunctionalized gold nanoparticles with high CL efficiency. This was attributed to the excellent catalytic ability of Co2+ and polymethylacrylic acid nanogels, as well as the improved CL catalytic efficiency from a decreased spatial distance between ABEI and the catalyst. The as-prepared nanogels also possess abundant surface reaction sites and good CL stability. On this basis, a sandwich immunoassay for the nucleocapsid protein of SARS-CoV-2 (N protein) was developed by using magnetic bead connected primary antibody as a capture probe and PMAANGs-ABEI/Co2+ connected secondary antibody as a signal probe. The linear range of the proposed method for N protein detection was 3.16-316 ng/mL, and its detection limit was 2.19 ng/mL (S/N = 3). Moreover, the developed immunoassay was performed with a short incubation time of 5 min, which greatly reduced the detection time for N protein. By using corresponding antibodies, the developed strategy might be applied to detect other biomarkers.

Smart Nanostructured Materials for SARS-CoV-2 and Variants Prevention, Biosensing and Vaccination

The coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has raised great concerns about human health globally. At the current stage, prevention and vaccination are still the most efficient ways to slow down the pandemic and to treat SARS-CoV-2 in various aspects. In this review, we summarize current progress and research activities in developing smart nanostructured materials for COVID-19 prevention, sensing, and vaccination. A few established concepts to prevent the spreading of SARS-CoV-2 and the variants of concerns (VOCs) are firstly reviewed, which emphasizes the importance of smart nanostructures in cutting the virus spreading chains. In the second part, we focus our discussion on the development of stimuli-responsive nanostructures for high-performance biosensing and detection of SARS-CoV-2 and VOCs. The use of nanostructures in developing effective and reliable vaccines for SARS-CoV-2 and VOCs will be introduced in the following section. In the conclusion, we summarize the current research focus on smart nanostructured materials for SARS-CoV-2 treatment. Some existing challenges are also provided, which need continuous efforts in creating smart nanostructured materials for coronavirus biosensing, treatment, and vaccination.

Angiotensin-Converting Enzyme 2-Based Biosensing Modalities and Devices for Coronavirus Detection

Rapid and cost-effective diagnostic tests for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are a critical and valuable weapon for the coronavirus disease 2019 (COVID-19) pandemic response. SARS-CoV-2 invasion is primarily mediated by human angiotensin-converting enzyme 2 (hACE2). Recent developments in ACE2-based SARS-CoV-2 detection modalities accentuate the potential of this natural host-virus interaction for developing point-of-care (POC) COVID-19 diagnostic systems. Although research on harnessing ACE2 for SARS-CoV-2 detection is in its infancy, some interesting biosensing devices have been developed, showing the commercial viability of this intriguing new approach. The exquisite performance of the reported ACE2-based COVID-19 biosensors provides opportunities for researchers to develop rapid detection tools suitable for virus detection at points of entry, workplaces, or congregate scenarios in order to effectively implement pandemic control and management plans. However, to be considered as an emerging approach, the rationale for ACE2-based biosensing needs to be critically and comprehensively surveyed and discussed. Herein, we review the recent status of ACE2-based detection methods, the signal transduction principles in ACE2 biosensors and the development trend in the future. We discuss the challenges to development of ACE2-biosensors and delineate prospects for their use, along with recommended solutions and suggestions.

A Rapid SARS-CoV-2 Nucleocapsid Protein Profiling Assay with High Sensitivity Comparable to Nucleic Acid Detection

Existing nucleic acid and antigen profiling methods for COVID-19 diagnosis fail to simultaneously meet the demands in sensitivity and detection speed, hampering them from being a comprehensive way for epidemic prevention and control. Thus, effective screening of COVID-19 requires a simple, fast, and sensitive method. Here, we report a rapid assay for ultrasensitive and highly specific profiling of COVID-19 associated antigen. The assay is based on a binding-induced DNA assembly on a nanoparticle scaffold that acts by fluorescence translation. By binding two aptamers to a target protein, the protein brings the DNA regions into close proximity, forming closed-loop conformation and resulting in the formation of the fluorescence translator. Using this assay, saliva nucleocapsid protein (N protein) has been profiled quantitatively by converting the N protein molecule information into a fluorescence signal. The fluorescence intensity is enhanced with increasing N protein concentration caused by the metal enhanced fluorescence using a simple, specific, and fast profiling assay within 3 min. On this basis, the assay enables a high recognition ratio and a limit of detection down to 150 fg mL-1. It is 1-2 orders of magnitude lower than existing commercial antigen ELISA kits, which is comparative to or superior than the PCR based nucleic acid testing. Owing to its rapidity, ultrasensitivity, as well as easy operation, it holds great promise as a tool for screening of COVID-19 and other epidemics such as monkey pox.

Defective Site Modulation Strategy for Preparing Single Atom-Dispersed Catalysts as Superior Chemiluminescent Signal Probes

Single atom-dispersed catalysts (SADCs) with highly exposed active sites can be used as sensitive signal probes because of their superior catalytic efficiency. However, the dispersed atoms tend to aggregate, restricting the loading capacity of metal atoms. Herein, the defective sites on Zr-oxo clusters of metal-organic frameworks (MOFs) UiO-66-NH₂ were modulated by excessive acetic acid and utilized for confining metal atoms with high loading capacity. To verify the feasibility of the designed strategy, the Co element was loaded onto MOFs UiO-66-NH₂ to prepare SADCs with desirable Fenton-like activity. The prepared Co SADCs at a low concentration of 1.0 μg mL^-1 are found to boost chemiluminescent (CL) emission for 3700 times due to the significantly improved Co content of 5.55 wt %. The superior CL enhancement efficiency is ascribed to reactive oxygen species generated by the accelerated decay of H₂O₂. To verify the application potential in CL assay, they were used as signal probes to establish an immunoassay method for carbendazim with a dynamic range of 1.0 pg mL^-1 to 25 ng mL^-1 and a limit of detection of 0.33 pg mL^-1. This defective site modulation strategy paves an avenue for preparing SADCs with a high CL response by improving the loading capacity of metal atoms.

Visual Detection of COVID-19 from Materials Aspect

Abstract

In the recent COVID-19 pandemic, World Health Organization emphasized that early detection is an effective strategy to reduce the spread of SARS-CoV-2 viruses. Several diagnostic methods, such as reverse transcription-polymerase chain reaction (RT-PCR) and lateral flow immunoassay (LFIA), have been applied based on the mechanism of specific recognition and binding of the probes to viruses or viral antigens. Although the remarkable progress, these methods still suffer from inadequate cellular materials or errors in the detection and sampling procedure of nasopharyngeal/oropharyngeal swab collection. Therefore, developing accurate, ultrafast, and visualized detection calls for more advanced materials and technology urgently to fight against the epidemic. In this review, we first summarize the current methodologies for SARS-CoV-2 diagnosis. Then, recent representative examples are introduced based on various output signals (e.g., colorimetric, fluorometric, electronic, acoustic). Finally, we discuss the limitations of the methods and provide our perspectives on priorities for future test development.

Graphical Abstract

A general controllable release amplification strategy of liposomes for single-particle collision electrochemical biosensing

As an important component of the COVID-19 mRNA vaccines, liposomes play a key role in the efficient protection and delivery of mRNA to cells. Herein, due to the controllable release amplification strategy of liposomes, a reliable and robust single-particle collision electrochemical (SPCE) biosensor was constructed for H9N2 avian influenza virus (H9N2 AIV) detection by combining liposome encapsulation-release strategy with immunomagnetic separation. The liposomes modified with biotin and loaded with platinum nanoparticles (Pt NPs) were used as signal probes for the first time. Biotin facilitated the coupling of biomolecules (DNA or antibodies) through the specific reaction of biotin-streptavidin. Each liposome can encapsulate multiple Pt NPs, which were ruptured under the presence of 1 × PBST (phosphate buffer saline with 0.05% Tween-20) within 2 min, and the encapsulated Pt NPs were released for SPCE experiment. The combination of immunomagnetic separation not only improved the anti-interference capabilities but also avoided the agglomeration of Pt NPs, enabling the SPCE biosensor to realize ultrasensitive detection of 18.1 fg/mL H9N2 AIV. Furthermore, the reliable SPCE biosensor was successfully applied in specific detection of H9N2 AIV in complex samples (chicken serum, chicken liver and chicken lung), which promoted the universality of SPCE biosensor and its application prospect in early diagnosis of diseases.

A wearable AIEgen-based lateral flow test strip for rapid detection of SARS-CoV-2 RBD protein and N protein

Accurate and rapid detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is significant for early tracing, isolation, and treatment of infected individuals, which will efficiently prevent large-scale transmission of coronavirus disease 2019 (COVID-19). Here, two kinds of test strips for receptor binding domain (RBD) and N antigens of SARS-CoV-2 are established with high sensitivity and specificity, in which AIE luminogens (AIEgens) are utilized as reporters. Because of the high brightness and resistance to quenching in aqueous solution, the limit of detection can be as low as 6.9 ng/mL for RBD protein and 7.2 ng/mL for N protein. As an antigen collector, an N95 mask equipped with a test strip with an excellent enrichment effect would efficiently simplify the sampling procedures. Compared with a test strip based on Au nanoparticles or fluorescein isothiocyanate (FITC), the AIEgen-based test strip shows high anti-interference capacity in complex biosamples. Therefore, an AIEgen-based test strip assay could be built as a promising platform for emergency use during the pandemic.