Introduction of Multilayered Dual-Signal Nanotags into a Colorimetric-Fluorescent Coenhanced Immunochromatographic Assay for Ultrasensitive and Flexible Monitoring of SARS-CoV-2

Colorimetric and ECL dual-mode aptasensor for smartphone-based onsite sensitive detection of aflatoxin B1 in combination with ZnO@MWCNTs/g-C3N4 nanosheets and CuO@CuPt nanocomposites

The development of dual-mode strategies with superior sensitivity and accuracy have garnered increasing attention for researchers in Aflatoxin B1 (AFB1) analysis. Herein, a colorimetric-electrochemiluminescence (ECL) dual-mode biosensor was constructed for onsite and ultrasensitive determination of AFB1. The multi-wall carbon nanotubes (MWCNTs) were integrated with the ZnO metal organic frameworks (MOFs) to accelerate the electron transfer and boost the ECL intensity of g-C3N4 nanoemitters. Through the aptamer-based DNA sandwich assay, the CuO@CuPt nanocomposites were introduced onto the electrode and acted as the dual functional signal nanoprobes. Due to the good spectrum overlap between the CuO@CuPt nanoprobes and g-C3N4 nanosheets, ECL signal could be efficiently quenched. Additionally, the CuO@CuPt nanoprobes show superior catalytic properties towards the TMB and H2O2 colorimetric reactions, and an obvious color alteration from colorless to blue can be observed using the smartphone. Under optimized conditions, a sensitive and accurate dual-mode analysis of the AFB1 was accomplished with the colorimetric detection limit of 3.26 fg/mL and ECL detection limit of 0.971 fg/mL (S/N = 3). This study combines innovative nanomaterial properties of ZnO@MWCNTs, g-C3N4 and CuO@CuPt for ultrasensitive dual-mode detection, which offers new opportunities for the innovative engineering of the dual-mode sensors and demonstrates significant potential in food safety analysis.

Janus plasmonic-aggregation induced emission nanobeads as high-performance colorimetric-fluorescent probe of immunochromatographic assay for the ultrasensitive detection of staphylococcal enterotoxin B in milk

Herein, a colorimetric-fluorescent hybrid bifunctional nanobead with Janus structure (J-cf-HBN) was synthesized via one-pot microemulsification. Oleylamine-coated AuNPs and aggregation-induced emission luminogens (AIEgens) were suggested as building blocks to obtain high-performance colorimetric-fluorescent signals. The as-prepared J-cf-HBNs were used as a signal amplification probe to construct an immunochromatographic assay (J-cf-HBNs-ICA) platform for the ultrasensitive detection of staphylococcal enterotoxin B (SEB) in milk samples. Owing to the rational spatial distribution of AuNPs and AIEgens, the J-cf-HBNs present a highly retained photoluminescence and enhanced colorimetric signals. Combined with a pair of highly affinitive anti-SEB antibodies, the J-cf-HBN-ICA platform enabled the fast naked-eye visualization and fluorescent quantitative detection of SEB in various milk matrices. Given the advantages of the dual-mode high-performance J-cf-HBNs, the proposed strip achieved a high sensitivity for SEB qualitative determination with a visual limit of detection (LOD) of 1.56 ng mL-1 and exhibited ultrasensitivity for SEB quantitative detection with a LOD of 0.09 ng mL-1, which is 139-fold lower than that of ELISA using same antibodies. In conclusion, this work provides new insights into the construction of multimode immunochromatographic methods for food safety detection in the field.

Optical lateral flow assays in early diagnosis of SARS-CoV-2 infection

So far, the 2019 novel coronavirus (COVID-19) is spreading widely worldwide. The early diagnosis of infection by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is essential to provide timely treatment and prevent its further spread. Lateral flow assays (LFAs) have the advantages of rapid detection, simple operation, low cost, ease of mass production, and no need for special devices and professional operators, which make them suitable for self-testing at home. This review focuses on the early diagnosis of SARS-CoV-2 infection based on optical LFAs including colorimetric, fluorescent (FL), chemiluminescent (CL), and surface-enhanced Raman scattering (SERS) LFAs for the detection of SARS-CoV-2 antigens and nucleic acids. The types of recognition components, detection modes used for antigen detection, labels employed in different optical LFAs, and strategies to improve the detection sensitivity of LFAs were reviewed. Meanwhile, LFAs coupled with different nucleic acid amplification techniques and CRISPR-Cas systems for the detection of SARS-CoV-2 nucleic acids were summarized. We hope this review provides research mentalities for developing highly sensitive LFAs that can be used in home self-testing for the early diagnosis of SARS-CoV-2 infection.

Optical biosensors for diagnosis of COVID-19: nanomaterial-enabled particle strategies for post pandemic era

The COVID-19 pandemic underlines the need for effective strategies for controlling virus spread and ensuring sensitive detection of SARS-CoV-2. This review presents the potential of nanomaterial-enabled optical biosensors for rapid and low-cost detection of SARS-CoV-2 biomarkers, demonstrating a comprehensive analysis including colorimetric, fluorescence, surface-enhanced Raman scattering, and surface plasmon resonance detection methods. Nanomaterials including metal-based nanomaterials, metal-organic frame-based nanoparticles, nanorods, nanoporous materials, nanoshell materials, and magnetic nanoparticles employed in the production of optical biosensors are presented in detail. This review also discusses the detection principles, fabrication methods, nanomaterial synthesis, and their applications for the detection of SARS-CoV-2 in four categories: antibody-based, antigen-based, nucleic acid-based, and aptamer-based biosensors. This critical review includes reports published in the literature between the years 2021 and 2024. In addition, the review offers critical insights into optical nanobiosensors for the diagnosis of COVID-19. The integration of artificial intelligence and machine learning technologies with optical nanomaterial-enabled biosensors is proposed to improve the efficiency of optical diagnostic systems for future pandemic scenarios.

Preparation of dyed polymer microspheres by a physical-chemical dual-binding method and their application in lateral flow immunoassay

Lateral flow immunoassay (LFIA) has shown great competitiveness in point-of-care testing due to its flexibility and simplicity. Dyed polymer microspheres are one of the most widely used marker particles for signal presentation as they are very convenient for visual interpretation, which is one of the most attractive features of LFIA. The color intensity, as the most critical factor, is directly related to the visual effect. In this work, a physical-chemical dual-binding strategy was proposed for the preparation of functionalized dyed microspheres. Bifunctional seed microspheres were synthesized by introducing 4-vinylbenzyl chloride (VBC) into the soap-free emulsion polymerization process, and the effective immobilization of dyes inside and on the surface of the polymer microspheres was achieved by covalent bonding and swelling methods. The microspheres were characterized by SEM, FT-IR spectroscopy and UV-vis spectroscopy. The results showed that the microspheres containing VBC were spherical with an average particle size of 300 nm. When the microspheres were prepared by adding 10 wt% VBC relative to the total monomer, the immobilization amount of 1-[[4-[(dimethylphenyl)azo]dimethyl phenyl]azo]-2-naphthol (Red-27) was increased to 180 mg g-1, which was 1.8 times that of the microspheres without VBC. The resulting nanomaterials were successfully used to establish a lateral flow immunoassay for the detection of COVID-19 virus N protein. The linear response concentration range was 2.64-87.84 COI, and the detection limit was 14.95 COI.

Portable fluorescent lateral flow assay for ultrasensitive point-of-care analysis of acute myocardial infarction related microRNA

Point-of-care quantitative analysis of tracing microRNA disease-biomarkers remains a great challenge in the clinical diagnosis. In this paper, we developed a portable fluorescent lateral flow assay for ultrasensitive quantified detection of acute myocardial infarction related microRNAs in bio-samples. SiO2@DQD (bilayer quantum dots assembly with SiO2 core) based fluorescent lateral flow strip was fabricated as the analysis tool. In order to quantify the tracing microRNA in biosamples, a catalytic hairpin assembly and CRISPR/Cas12a cascade amplification method was performed and combined with the fabricated SiO2@DQD lateral flow strip. Thus, our platform gathered double advantages of portability and ultrasensitive quantification. Based on our strips, target myocardial biomarker microRNA-133a can be detected with a detection limit of 0.32 fM, which was almost 1000-fold sensitive compared with previous reported microRNAs-lateral flow strips. Significantly, this portable fluorescent strip can directly detect microRNAs in serum without any pretreatment and PCR amplification steps. When spiked in serum samples, a recovery of 99.65 %-102.38 % can be obtained. Therefore, our method offers a potential tool for ultrasensitive quantification of diseases related microRNA in the point-of-care diseases diagnosis field.

Plasmonic silver and gold nanoparticles: shape- and structure-modulated plasmonic functionality for point-of-caring sensing, bio-imaging and medical therapy

Silver and gold nanoparticles have found extensive biomedical applications due to their strong localized surface plasmon resonance (LSPR) and intriguing plasmonic properties. This review article focuses on the correlation among particle geometry, plasmon properties and biomedical applications. It discusses how particle shape and size are tailored via controllable synthetic approaches, and how plasmonic properties are tuned by particle shape and size, which are embodied by nanospheres, nanorods, nanocubes, nanocages, nanostars and core-shell composites. This article summarizes the design strategies for the use of silver and gold nanoparticles in plasmon-enhanced fluorescence, surface-enhanced Raman scattering (SERS), electroluminescence, and photoelectrochemistry. It especially discusses how to use plasmonic nanoparticles to construct optical probes including colorimetric, SERS and plasmonic fluorescence probes (labels/reporters). It also demonstrates the employment of Ag and Au nanoparticles in polymer- and paper-based microfluidic devices for point-of-care testing (POCT). In addition, this article highlights how to utilize plasmonic nanoparticles for in vitro and in vivo bio-imaging based on SERS, fluorescence, photoacoustic and dark-field models. Finally, this article shows perspectives in plasmon-enhanced photothermal and photodynamic therapy.

Multi-modal nanoprobe-enabled biosensing platforms: a critical review

Nanomaterials show great potential for applications in biosensing due to their unique physical, chemical, and biological properties. However, the single-modal signal sensing mechanism greatly limits the development of single-modal nanoprobes and their related sensors. Multi-modal nanoprobes can realize the output of fluorescence, colorimetric, electrochemical, and magnetic signals through composite nanomaterials, which can effectively compensate for the defects of single-modal nanoprobes. Following the multi-modal nanoprobes, multi-modal biosensors break through the performance limitation of the current single-modal signal and realize multi-modal signal reading. Herein, the current status and classification of multi-modal nanoprobes are provided. Moreover, the multi-modal signal sensing mechanisms and the working principle of multi-modal biosensing platforms are discussed in detail. We also focus on the applications in pharmaceutical detection, food and environmental fields. Finally, we highlight this field's challenges and development prospects to create potential enlightenment.

A nanogap-enhanced SERS nanotag-based lateral flow assay for ultrasensitive and simultaneous monitoring of SARS-CoV-2 S and NP antigens

A lateral flow assay (LFA) strip based on dual 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB)-encoded satellite Fe3O4@Au (Mag@Au) SERS tags with nanogap is reported for  ultrasensitive and simultaneous diagnosis of two SARS-CoV-2 functional proteins. Composed of Fe3O4 core, satellite gold shell with nanogaps, and double-layer DTNB, the Mag@Au nanoparticles with an average size of 238 nm were designed as multifunctional tags to efficiently enrich the target SARS-CoV-2 protein from complex samples, significantly enhancing the SERS signal of the LFA strip and provide quantitative SERS detection of analyte on test lines. The developed dual DTNB-encoded satellite Mag@Au-based LFA allowed simultaneous quantification of spike (S) protein and nucleocapsid (NP) protein with detection limits of 23 pg mL-1 and 2 pg mL-1, respectively, lower than commercial ELISA kits and reported SERS-LFA detection system-based Au NPs and Fe3O4@3 nm Au MNPs. This magnetic SERS-LFA also showed high performance of multi-variant strain detection and further distinguished clinical samples of Omicron variant infection, demonstrating the potential of in situ detection of respiratory virus diseases.

Recent advances in SERS-based immunochromatographic assay for pathogenic microorganism diagnosis: A review

Infectious diseases caused by bacteria, viruses, fungi, and other pathogenic microorganisms are among the most harmful public health problems in the world, causing tens of millions of deaths and incalculable economic losses every year. The establishment of rapid, simple, and highly sensitive diagnostic methods for pathogenic microorganisms is important for the prevention and control of infectious diseases, guidance of timely treatment, and the reduction of public safety risks. Lateral flow immunoassay (LFA) based on the colorimetric signal of colloidal gold is the most popular point-of-care testing technology at present, but it is limited by poor sensitivity and low throughput and hardly meets the needs of the highly sensitive screening of pathogenic microorganisms. In recent years, the combination of surface-enhanced Raman scattering (SERS) and LFA technology has developed into a novel analytical platform with high sensitivity and multiple detection capabilities and has shown great advantages in the detection of pathogenic microorganisms and infectious diseases. This review summarizes the working principle, design ideas, and application of the existing SERS-based LFA methods in pathogenic microorganism detection and further introduces the effect of new technologies such as Raman signal encoding, magnetic enrichment, novel membrane nanotags, and integrated Raman reading equipment on the performance of SERS-LFA. Finally, the main challenges and the future direction of development in this field of SERS-LFA are discussed.

High-Performance Fluorescent Microspheres Based on Fluorescence Resonance Energy Transfer Mode for Lateral Flow Immunoassays

The label with a large Stokes shift and strong fluorescence properties could improve the sensitivity of the lateral flow immunoassay (LFIA). Herein, two aggregation-induced emission (AIE) luminogens with spectral overlap were encapsulated in polymers by using the microemulsion method as a label, and the construction of a fluorescence resonance energy transfer mode was further verified via theoretical calculation and spectral analysis. Satisfactorily, the doped AIE polymer microspheres (DAIEPMs) exhibited a large Stokes shift of 285 nm and a 10.8-fold fluorescence enhancement compared to those of the AIEPMs loaded with acceptors. Benefiting from the excellent optical performance, DAIEPMs were applied to the LFIA for sensitive detection of chlorothalonil, which is an organochlorine pesticide. The limit of detection of the proposed DAIEPMs-LFIA was 1.2 pg/mL, which was 4.8-fold and 11.6-fold lower than those of quantum dot bead LFIA and gold nanoparticle LFIA, respectively. This work provides a new strategy to improve the optical properties of fluorescent materials and construct a sensitive and reliable detection platform.

Graphene oxide-based colorimetric/fluorescence dual-mode immunochromatography assay for simultaneous ultrasensitive detection of respiratory virus and bacteria in complex samples

A point-of-care testing biosensor that supports direct, sensitive, and simultaneous identification of bacteria and virus is still lacking. In this study, an ultrasensitive immunochromatography assay (ICA) with colorimetric/fluorescence dual-signal output was proposed for flexible and accurate detection of respiratory virus and bacteria in complex samples. Colorimetric AuNPs of 16 nm and two layers of quantum dots (QDs) were coated onto the surface of monolayer graphene oxide (GO) layer by layer to form a multilayered dual-signal nanofilm. This material not only can generate strong colorimetric and fluorescence signals for ICA analysis but also can provide larger surface area, better stability, and superior dispersibility than conventional spherical nanomaterials. Two test lines were built onto the ICA strip to simultaneously detect common respiratory virus influenza A and respiratory bacteria Streptococcus pneumoniae. The dual-signal mode of assay greatly broadened the applied range of ICA method, in which the colorimetric mode allows for quick determination of virus/bacteria and the fluorescence mode ensures the highly sensitive and quantitative detection of target pathogens with detection limits down to 891 copies/mL and 17 cells/mL, respectively. The proposed dual-mode ICA can also be applied directly for real biological and environment samples, which suggests its great potential for field application.

Introduction of multilayered quantum dot nanobeads into competitive lateral flow assays for ultrasensitive and quantitative monitoring of pesticides in complex samples

A competitive fluorescent lateral flow assay (CFLFA) is proposed for direct, ultrasensitive, quantitative detection of common pesticides imidacloprid (IMI) and carbendazim (CBZ) in complex food samples by using silica-core multilayered quantum dot nanobeads (SiO2-MQB) as liquid fluorescent tags. The SiO2-MQB nanostructure comprises a 200-nm SiO2 core and a shell of hundreds of carboxylated QDs (excitation/emission maxima ~365/631 nm), and can generate better stability, superior dispersibility, and higher luminescence than traditional fluorescent beads, greatly improving the sensitivity of current LFA methods for pesticides. Moreover, using liquid SiO2-MQB directly instead of via the conjugate pad both simplifies the structure of LFA system and improves the efficiency of immunobinding reactions between nanotags and the targets. Applying these methods, the established CFLFA realized the stable and accurate detection of IMI and CBZ in 12 min, with detection limits down to 1.94 and 14.79 pg/mL, respectively. The SiO2-MQB-CFLFA is practicable for application to real food samples (corn, apple, cucumber, and cabbage), and undoubtedly a promising and low-cost tool for on-site monitoring of trace pesticide residues.

Development of a Fluorescent Immunochromatographic Assay Based on Quantum Dot-Functionalized Two-Dimensional Monolayer Ti3C2 MXene Nanoprobes for the Simultaneous Detection of Influenza A Virus and SARS-CoV-2

Accurate and rapid detection of the influenza A virus (FluA) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can effectively control their spread. We developed a colorimetric and fluorescent dual-functional two-channel immunochromatographic assay (ICA) biosensor to simultaneously detect the above-mentioned viruses. A unique two-dimensional Ti3C2-QD immunoprobe was established by adsorbing dense quantum dots (QDs) onto the light green monostromatic Ti3C2 MXene surface, resulting in light green colorimetric and superior fluorescence signals and guaranteeing high sensitivity, stability, and excellent liquidity for ICA detection. Rapid visual screening for FluA and SARS-CoV-2 infections was applicable via a green colorimetric signal. Sensitive and quantitative detection of viruses in their early stages of infection was performed by using the fluorescence signal. Our proposed Ti3C2-QD-ICA biosensor can simultaneously detect 1 ng/mL or 2.4 pg/mL FluA and 1 ng/mL or 6.2 pg/mL SARS-CoV-2 via its colorimetric or fluorescence signals, respectively, with a short testing time (20 min), good reproducibility, specificity, and accuracy. In addition, this method demonstrated sensitivity higher than that of the conventional AuNP-based ICA method in throat swab samples. Hence, our proposed Ti3C2-QD-ICA method can be potentially applied for the rapid, ultrasensitive, and multiplex detection of respiratory viruses.