Development of a two-step immunochromatographic assay for microcystin-LR based on fluorescent microspheres

Cost-effective microfluidic flow cytometry for precise and gentle cell sorting

Microfluidic flow cytometry (MFCM) is considered to be an effective substitute for traditional flow cytometry, because of its advantages in terms of higher integration, smaller device size, lower cost, and higher cell sorting activity. However, MFCM still faces challenges in balancing parameters such as sorting throughput, viability, sorting efficiency, and cost. Here, we demonstrate a cost-effective and high-performance microfluidic cytometry cell sorting system, along with a customized microfluidic chip that integrates hydrodynamic focusing, droplet encapsulation, and sorting for precise cell manipulation. An innovative photon incremental counting-based fluorescence detection method is proposed, which requires only one-fiftieth of the data compared to traditional methods. This significantly simplifies the structure of the system and substantially reduces costs. The system exhibits detection recoveries exceeding 95% across sample solution flow rates ranging from 10 to 80 μL min-1. Moreover, it accurately achieves individual droplet deflections at a droplet generation frequency of 1600 Hz. Ultimately, our cell sorting system offers an impressive sorting efficiency of 90.7% and a high cell viability of 94.3% when operating at a droplet generation frequency of 1316 Hz, highlighting its accuracy and gentleness throughout the entire process. Our work will enhance advances in the life sciences, thereby creating a boom in great applications in single-cell cloning, single-cell analysis, drug screening, etc.

Multivalent bifunctional nanobody to enhance the sensitivity of direct competitive chemiluminescence immunoassay for the detection of microcystin LR in lake water

Microcystin-LR (MC-LR), a toxic cyanobacterial toxin in freshwater, poses significant health and ecological risks due to its ability to induce cell apoptosis and liver damage. Sensitive detection of MC-LR is crucial for public health and water safety. In this work, we engineered a multivalent bifunctional nanobody (A2.3-C4-SBP) by fusing the anti-MC-LR nanobody gene (A2.3) with self-assembling peptides (C4) and a streptavidin-binding peptide (SBP). A2.3-C4-SBP was directionally immobilized on the ELISA microplate via streptavidin-mediated to develop a multivalent bifunctional nanobody-based chemiluminescent immunoassay (MBN-CLIA) for MC-LR detection in lake water. The IC50 of the A2.3-C4-SBP heptamer based CLIA was 5.80 ng/mL, and the LOD (IC10) was 0.33 ng/mL, which were 9.51-fold and 1.82-fold lower, respectively, than those of the A2.3-SBP monomer based CLIA. Additionally, the IC50 and LOD were 1.26-fold and 1.82-fold lower, respectively, than those of the A2.3-C4-SBP heptamer without streptavidin-mediated directional immobilization. In summary, this work developed a sensitive, rapid and simple immunoassay for the detection of MC-LR in lake water based on multivalent bifunctional nanobodies. Furthermore, the proposed combined strategy of nanobody multimerization and directed immobilization is simple to operate and has great potential to improve the sensitivity and signal amplification of various immunoassays.

Highly sensitive time-resolved fluorescent microspheres lateral flow immunoassay for the quantitative detection of triadimefon and its metabolite residues in fruits and vegetables

A general one-step lateral flow immunochromatographic assay (LFIA) for the quantitative detection of triadimefon (TDF) and triadimenol (TDN) in fruit and vegetable samples was developed using time-resolved fluorescence microspheres (TRFM) as labels. A specific anti-triadimefon monoclonal antibody (mAb) was conjugated with TRFM to fabricate LFIA test strips. A time-resolved fluorometer as an LFIA reader was applied to obtain quantitative results and assess risk ranges for the LFIA test strips. Under the optimized experimental conditions, the limits of detection (LODs) in buffer/cucumbers/tomatoes/oranges were 0.046 ng/mL, 0.135 µg/kg, 1.047 µg/kg, and 5.811 µg/kg, respectively, which are ca. 1000 times lower than that of colloidal gold-labeled strips. The recovery in cucumber/tomato/orange samples was 109.4-116.7%, 87.7-110.9%, and 88.0-111.9%, respectively, indicating that the test strips had good reliability. Coupled with the easily customizable pretreatment procedures for various samples, the LFIA results were obtained within 18 min without the need for professional personnel or complicated equipment. TRFM-LFIA for TDF and TDN also shows remarkable specificity and precision. The test strips were also low-cost, portable, and convenient to use. These results indicate the test strips could be utilized as a novel strategy for on-site detection of TDF and TDN, which has the potential to expand and detect other pesticide or insecticide residues in food.

Synthesis of silica-encapsulated tetraphenylethylene with aggregation-induced electrochemiluminescence resonance energy transfer for sensitively sensing microcystin-LR

The reported organic electrochemiluminescence (ECL) luminophors for the detection of various markers often suffer from intermolecular π-π stacking-induced luminophore quenching. Herein, we demonstrate one-pot synthesis of a new aggregation-induced electrochemiluminescence (AIECL) emitter (i.e., TPE@SiO2/rGO composite) for sensitive measurement of microcystin-leucine arginine (MC-LR). The TPE@SiO2/rGO composite is constructed by embedding the silica-encapsuled 1,1,2,2-tetra(4-carboxylphenyl)ethylene (TPE) in the reduced graphene oxide. In comparison with the monomer TPE, this composite exhibit high luminescence efficiency and strong ECL emission, because the AIECL phenomenon triggered by the spatial confinement effect in the SiO2 cage induces the restriction of the internal motion and vibration of molecules. Notably, this composite has distinct advantages of easy preparation, simple functionalization, and stable luminescence. Especially, the TPE@SiO2/rGO-based ECL-RET system exhibits a high quenching efficiency (ΦET) of 69.7%. When target MC-LR is present, it triggers DNA strand displacement reaction (SDR), inducing the quenching of the ECL signal of TPE@SiO2/rGO composite due to ECL resonance energy transfer between TPE@SiO2/rGO composite and methylene blue (MB). The proposed biosensor enables highly sensitive, low-cost, and robust measurement of MC-LR with a large dynamic range of 7 orders of magnitude and a detection limit of 3.78 fg/mL, and it displays excellent detection performance in complex biological matrices, holding potential applications in food safety and water monitoring.

Direct count of fluorescent microspheres in a microfluidic chip based on the capillary electrophoresis method

Fluorescent microspheres (FMs) are tiny particles with special functions that are widely employed in biological research. Counting of microscale FMs is a great challenge by capillary electrophoresis. Herein we developed a method to count 2 μm FMs based on a microfluidic chip with a gradual change in inner size. Such a microfluidic chip can inhibit sample blocking at the inlet of the capillary. The results showed that FMs migrated in the wide part of the microchannel side by side, and then passed through the narrow part one by one. There was a linear relationship between the number of peaks in the electropherogram and concentration of FMs if they were running in the microchannel for more than 20 min. A high separation voltage may lead to aggregation of FMs in the microchannels, and about 2 × 10⁴ FMs can be counted within 30 min by this microfluidic chip.

SERS immuno- and apta-assays in biosensing/bio-detection: Performance comparison, clinical applications, challenges

Surface Enhanced Raman Spectroscopy is increasingly used as a sensitive bioanalytical tool for detection of variety of analytes ranging from viruses and bacteria to cancer biomarkers and toxins, etc. This comprehensive review describes principles of operation and compares the performance of immunoassays and aptamer assays with Surface Enhanced Raman scattering (SERS) detection to each other and to some other bioassay methods, including ELISA and fluorescence assays. Both immuno- and aptamer-based assays are categorized into assay on solid substrates, assays with magnetic nanoparticles and assays in laminar flow or/and strip assays. The best performing and recent examples of assays in each category are described in the text and illustrated in the figures. The average performance, particularly, limit of detection (LOD) for each of those methods reflected in 9 tables of the manuscript and average LODs are calculated and compared. We found out that, on average, there is some advantage in terms of LOD for SERS immunoassays (0.5 pM median LOD of 88 papers) vs SERS aptamer-based assays (1.7 pM median LOD of 51 papers). We also tabulated and analyzed the clinical performance of SERS immune and aptamer assays, where selectivity, specificity, and accuracy are reported, we summarized the best examples. We also reviewed challenges to SERS bioassay performance and real-life application, including non-specific protein binding, nanoparticle aggregation, limited nanotag stability, sometimes, relatively long time to results, etc. The proposed solutions to those challenges are also discussed in the review. Overall, this review may be interesting not only to bioanalytical chemist, but to medical and life science researchers who are interested in improvement of bioanalyte detection and diagnostics.

AIEgens: Next Generation Signaling Source for Immunoassays?

Luminogens with aggregation-induced emission (AIEgens) properties have numerous broad applications in fields of chemical and biological analyses due to their exceptional photostability, excellent signal reliability, high quantum yield, and large Stokes' shift. In particular, AIEgens also bring new blood for immunoassay. Since publication of the first 2004 paper, AIEgens-based immunoassays have received significant attention because of their high sensitivity, specificity, accuracy, and reliability. However, until now, there have been no comprehensive literature reviews focused on the evolving field of AIEgens-based immunoassays. Thus, we have extensively reviewed AIEgens-based immunoassays from their basic working principles to specific applications. We focus on several fundamental elements of AIEgens-based immunoassays, including the typical structures of AIEgens, emission mechanism of AIEgens probes, function of AIEgens in immunoassays, and platform of AIEgens-based immunoassays. Then, the representative applications of AIEgens-based immunoassays in food safety, medical diagnostics, and environmental monitoring are explored. Thus, proposals on how to further improve the AIEgens-based immunoassay performance are also discussed, as well as future challenges and perspectives, aiming to provide brief and valid guidelines for choosing suitable AIEgens-based immunoassays according to specific application requirements.

ATP-Responsive Strand Displacement Coupling with DNA Origami/AuNPs Strategy for the Determination of Microcystin-LR Using Surface-Enhanced Raman Spectroscopy

The DNA origami-mediated self-assembly strategy has emerged as a powerful tool in surface-enhanced Raman spectroscopy (SERS). However, these self-assembly approaches typically do not possess high detection specificity. Herein, a novel strategy based on adenosine triphosphate (ATP)-responsive strand displacement (ARSD) coupling with DNA origami/AuNPs for SERS analysis of microcystin-LR (MC-LR) is presented. In the presence of MC-LR and ATP molecules, nucleic acid sensing structures fabricated with anti-MC-LR aptamer (T1) and ATP aptamer (T2) were triggered to release the remaining ATP. In addition, DNA origami-assisted assembly results in the formation of homogeneous plasmonic nanostructures for Raman enhancement via strong plasmonic coupling. After the binding in the gaps of functionalized DNA origami/AuNPs, the Raman shift of the ATP molecules becomes detectable, leading to increased SERS intensity in 734 cm^-1. A linear response to MC-LR was obtained in the concentration range of 1.56-50 μg·L^-1, and the limit of detection (LOD) was 0.29 μg·L^-1. Combined with the solid-phase extraction sample pretreatment for extraction and 10-fold concentration, this proposed method was successfully used to detect MC-LR type in real lake-water samples with good recoveries of 98.4-116% and relative standard deviations of 1.9-6.7%. Furthermore, for the detection of MC-LR in contaminated lake-water samples, the results of the developed method and ultrahigh-performance liquid chromatography-tandem mass spectrometry were found to be in agreement with relative errors between -12 and 2.4%. The proposed strategy provides a sensitive recognition and signal amplification platform for trace MC-LR analysis as well as innovative nucleic acid sensing structures for toxin analysis more generally.

Dual Gold Nanoparticle/Chemiluminescent Immunoassay for Sensitive Detection of Multiple Analytes

Multiple antibiotics and mycotoxins usually simultaneously exist in foods, which poses a serious threat to human health. How to detect them in one test with high sensitivity and fidelity is challenging. In this study, we develop a dual readout lateral flow immunodetection platform that can quantitatively detect five kinds of antibiotics and five kinds of mycotoxins within one sample. The platform is composed of a chip and a portable readout instrument where gold nanoparticle (AuNP)-based and chemiluminescence immunoassays could be performed to reach a maximum throughput of 220 analytes in one setting. For a rapid screen, qualitative analysis by detecting the color change of the deposited AuNPs on the chip could be realized. For quantitative results, chemiluminescence imaging and analysis can be completed within 15 min. Apart from the high throughput and high efficiency, this platform has a high detection sensitivity. For instance, the limit of detection (LOD) for thiamphenicol (a representative antibiotic) and fumonisins B1 (a representative mycotoxin) is 8 times and 40 times lower than those of the previously reported methods, respectively. Thus, this dual readout immunodetection platform is promising as a universal device for rapid and quantitative detection of multiple analytes with high throughput, high sensitivity, and high fidelity.

Immunoassay technology: Research progress in microcystin-LR detection in water samples

Increasing global warming and eutrophication have led to frequent outbreaks of cyanobacteria blooms in freshwater. Cyanobacteria blooms cause the death of aquatic and terrestrial organisms and have attracted considerable attention since the 19th century. Microcystin-LR (MC-LR) is one of the most typical cyanobacterial toxins. Therefore, the fast, sensitive, and accurate determination of MC-LR plays an important role in the health of humans and animals. Immunoassay refers to a method that uses the principle of immunology to determine the content of the tested substance in a sample using the tested substance as an antigen or antibody. In analytical applications, the immunoassay technology could use the specific recognition of antibodies for MC-LR detection. In this review, we firstly highlight the immunoassay detection of MC-LR over the past two decades, including classical enzyme-link immunosorbent assay (ELISA), modern immunoassay with optical signal, and modern immunoassay with electrical signal. Among these detection methods, the water environment was used as the main detection system. The advantages and disadvantages of the different detection methods were compared and analyzed, and the principles and applications of immunoassays in water samples were elaborated. Furthermore, the current challenges and developmental trends in immunoassay were systematically introduced to enhance MC-LR detection performance, and some critical points were given to deal with current challenges. This review provides novel insight into MC-LR detection based on immunoassay method.

An Overview for the Nanoparticles-Based Quantitative Lateral Flow Assay

The development of the lateral flow assay (LFA) has received much attention in both academia and industry because of their broad applications to food safety, environmental monitoring, clinical diagnosis, and so forth. The user friendliness, low cost, and easy operation are the most attractive advantages of the LFA. In recent years, quantitative detection has become another focus of LFA development. Here, the most recent studies of quantitative LFAs are reviewed. First, the principles and corresponding formats of quantitative LFAs are introduced. In the biomaterial and nanomaterial sections, the detection, capture, and signal amplification biomolecules and the optical, fluorescent, luminescent, and magnetic labels used in LFAs are described. The invention of dedicated strip readers has drawn further interest in exploiting the better performance of LFAs. Therefore, next, the development of dedicated reader devices is described and the usefulness and specifications of these devices for LFAs are discussed. Finally, the applications of LFAs in the detection of metal ions, biotoxins, pathogenic microorganisms, veterinary drugs, and pesticides in the fields of food safety and environmental health and the detection of nucleic acids, biomarkers, and viruses in clinical analyses are summarized.

Portable, Rapid, and Sensitive Time-Resolved Fluorescence Immunochromatography for On-Site Detection of Dexamethasone in Milk and Pork

Dexamethasone (DEX) is widely used because of its anti-inflammatory, anti-endotoxin, anti-shock, and stress-enhancing response activities. It can increase the risk of diabetes and hypertension if it is abused or used improperly. However, there is a lack of sensitive and rapid screening methods for DEX in food. In this study, a time-resolved fluorescent microspheres immunochromatographic assay (TRFM-ICA) integrated with a portable fluorescence reader was developed for the quantitative detection of DEX in milk and pork. The cut-off values of the TRFM-ICA were 0.25 ng/mL and 0.7 µg/kg, respectively. The limits of quantitation (LOQs) were 0.003 ng/mL and 0.062 µg/kg, respectively. The recovery rates were 80.0–106.7%, and 78.6–83.6%, respectively, with the coefficients of variation ranging 6.3–12.5%, and 7.5–10.3%, respectively. A parallel experiment for 20 milk and 10 pork samples with LC-MS/MS was carried out to confirm the performance of the on-site application of the developed TRFM-ICA. The results of the two methods are basically the same; the correlation (R²) was >0.98. The establishment of TRFM-ICA will provide a new sensitive and efficient technical support for the rapid screening of DEX in food.

Cu/Au/Pt trimetallic nanoparticles coated with DNA hydrogel as target-responsive and signal-amplification material for sensitive detection of microcystin-LR

Sensitive and reliable analytical methods for monitoring of microcystin-LR (MC-LR) are urgently necessary due to its great harm to human health and aquatic organisms. In this work, a novel Cu/Au/Pt trimetallic nanoparticles (Cu/Au/Pt TNs)-encapsulated DNA hydrogel was prepared for colorimetric detection of MC-LR. The Cu/Au/Pt TNs were captured and released with precise control by the target-responsive 3D DNA hydrogels, which combined dual advantages of the target responsive DNA hydrogel and Cu/Au/Pt TNs of enhanced peroxidase-like activity. The DNA hydrogel network was constructed by hybridizing MC-LR aptamer with two complementary DNA strands on linear polyacrylamide chains. As long as MC-LR presented, the aptamer competitively binds with the MC-LR, causing the hydrogel to dissolve and release the preloaded Cu/Au/Pt TNs which could catalyze the reaction between H₂O₂ and TMB to produce color changes. In view of this sensitive strategy, this Cu/Au/Pt TNs-encapsulated DNA hydrogel-based colorimetric biosensor can achieve quantitative determination of MC-LR. The results showed that as-proposed colorimetric biosensor could sensitively detect MC-LR with a linear range of 4.0-10000 ng L^-1 and a detection limit of 3.0 ng L^-1. This work proved that the sensor had great potential to be applied in MC-LR detection and also provided the opportunity to develop colorimetric biosensor for other targets using this target-responsive and signal-amplification strategy.

An ultrasensitive electrochemical sensing method for detection of microcystin-LR based on infinity-shaped DNA structure using double aptamer and terminal deoxynucleotidyl transferase

This study develops a novel electrochemical sensing platform for microcystin-LR (MC-LR) detection. This aptasensor comprises the hybridization of double aptamer to its complementary strand (CS) on the surface of electrode and generation of an Infinity-shaped DNA structure in the absence of target by terminal deoxynucleotidyl transferase (TdT). The formation of Infinity-shaped construction leads to the development of an ultrasensitive aptasensor for MC-LR detection. In the presence of MC-LR, double aptamer is dissociated from its CS because of its high affinity for MC-LR and leaves the surface of electrode. Subsequently, no Infinity-shaped structure is formed following the introduction of TdT and a strong current signal is observed. The proposed method was employed for specific detection of MC-LR in the range from 60 pM to 1000 nM with a detection limit of 15 pM. The credibility of the approach was confirmed by detection of MC-LR in real samples like serum and tap water samples. This study provides a new aptasensor for detection of MC-LR as well as other toxin analysis.