An unplugged and quantitative foam based immunochromatographic assay for Escherichia coli O157:H7 using nanozymes to catalyze hydrogen peroxide decomposition reaction

A dual-mode biosensing platform based on polydopamine-modified FeCoMOF/Co3O4 nanoenzyme for sensitive detection of Escherichia coli O157:H7

The sensitive and specific detection of Escherichia coli O157:H7 (E. coli O157:H7) plays a crucial role for human health in the food field. Here, polydopamine (PDA)-based FeCoMOF/Co3O4 nanoenzymes (FeCoMOF/Co3O4@PDA) were successfully synthesized with three enzyme-like activities of catalase-like, superoxide dismutase-like and peroxidase-like. The PDA layer not only provided an adhesion surface for protein immobilization, but also significantly increased peroxidase-like activity of FeCoMOF/Co3O4@PDA. FeCoMOF/Co3O4@PDA could be directly coupled with antibody as probe for the E. coli O157:H7 detection. Accordingly, the improved Enzyme-Linked Immunosorbent Assay (ELISA) with a dual detection platform of colorimetric-intelligent sensing was developed. Its detection range was as wide as 101-108 CFU mL-1, and the detection limit was as low as 2 CFU mL-1. In addition, FeCoMOF/Co3O4@PDA-ELISA was further used for E. coli O157:H7 detection in actual samples. Thus, this work demonstrated that the proposed FeCoMOF/Co3O4@PDA-ELISA showed excellent potential for analytical testing of E. coli O157:H7.

Overview of Gas-Generating-Reaction-Based Immunoassays

Point-of-care (POC) immunoassays have become convincing alternatives to traditional immunosensing methods for the sensitive and real-time detection of targets. Immunoassays based on gas-generating reactions were recently developed and have been used in various fields due to their advantages, such as rapid measurement, direct reading, simple operation, and low cost. Enzymes or nanoparticles modified with antibodies can effectively catalyze gas-generating reactions and convert immunorecognition events into gas pressure signals, which can be easily recorded by multifunctional portable devices. This article summarizes the advances in gas-generating-reaction-based immunoassays, according to different types of signal output systems, including distance-based readout, pressure differential, visualized detection, and thermal measurement. The review mainly focuses on the role of photothermal materials and the working principle of immunoassays. In addition, the challenges and prospects for the future development of gas-generating-reaction-based immunoassays are briefly discussed.

New application of bimetallic Ag/Pt nanoplates in a colorimetric biosensor for specific detection of E. coli in water

A fast and sensitive aptasensor was developed using nanoplates with peroxidase activity as a novel approach. E. coli detection is described using a silver/platinum nanoplate (Ag/Pt NPL) that interacts with an oligonucleotide aptamer as a bioreceptor. The size of the Ag/Pt NPLs was about 42 nm according to the FE-SEM images. The EDS result indicates that a thin layer of Pt ions was coated on the surface of the Ag NPLs. This nanobiosensor has the ability to specifically bind to E. coli, increasing the peroxidase activity of the apt-Ag/Pt NPL. Finally, the blue color of the solution in the contaminated water samples was increased in the presence of 3,3',5,5'-tetramethylbenzidine (TMB) as a substrate and H2O2. The assay can be completed in 30 min and the presence of E. coli levels can be distinguished with the naked eye. The absorbance at 652 nm is proportional to pathogen concentration from 10 to 108 CFU·mL-1, with a detection limit of 10 CFU·mL-1. The percent recovery for the water samples spiked with E. coli is 95%. The developed assay should serve as a general platform for detecting other pathogenic bacteria which affect water and food quality. The proposed E. coli detection strategy has appealing characteristics such as high sensitivity, simple operation, short testing time, and low cost.

Post-Assay Chemical Enhancement for Highly Sensitive Lateral Flow Immunoassays: A Critical Review

Lateral flow immunoassay (LFIA) has found a broad application for testing in point-of-care (POC) settings. LFIA is performed using test strips-fully integrated multimembrane assemblies containing all reagents for assay performance. Migration of liquid sample along the test strip initiates the formation of labeled immunocomplexes, which are detected visually or instrumentally. The tradeoff of LFIA's rapidity and user-friendliness is its relatively low sensitivity (high limit of detection), which restricts its applicability for detecting low-abundant targets. An increase in LFIA's sensitivity has attracted many efforts and is often considered one of the primary directions in developing immunochemical POC assays. Post-assay enhancements based on chemical reactions facilitate high sensitivity. In this critical review, we explain the performance of post-assay chemical enhancements, discuss their advantages, limitations, compared limit of detection (LOD) improvements, and required time for the enhancement procedures. We raise concerns about the performance of enhanced LFIA and discuss the bottlenecks in the existing experiments. Finally, we suggest the experimental workflow for step-by-step development and validation of enhanced LFIA. This review summarizes the state-of-art of LFIA with chemical enhancement, offers ways to overcome existing limitations, and discusses future outlooks for highly sensitive testing in POC conditions.

I2/I--mediated fluorescence quenching of an Ag+-doped gold nanocluster-based immunoassay for sensitive detection of Escherichia coli O157:H7 in milk

Escherichia coli O157:H7 is a type of hazardous bacteria in the field of food safety. A sensitive and effective method is urgently needed to detect it, avoiding enormous harm for the human health. In this study, we synthesized stable Ag⁺-doped gold nanoclusters (Ag-AuNC) with a fluorescence intensity 4.8 times stronger than that of AuNC. It was further demonstrated that Ag⁰ existing in the AuNC core and a fraction of Ag⁺ anchored on the AuNC shell eliminated the surface defects and improved the luminescent properties of AuNC. A combination of I₂ and I^- was used to quench fluorescence-enhanced Ag-AuNC, which was first applied in ELISA for detecting E. coli O157:H7 to improve the sensitivity. In the presence of E. coli O157:H7, the biotinylated anti-E. coli O157:H7 mAb and streptavidin-alkaline phosphatase would be immobilized and catalyze l-ascorbic acid 2-phosphate sesquimagnesium salt hydrate to produce ascorbic acid. After addition of KIO₃, I₂/I^- were generated. The I₂ could trigger oxidative etching of Ag-AuNC and I^- could combine with Ag⁺ to decrease the Ag⁺ concentration of Ag-AuNC, which resulted in fluorescence quenching of Ag-AuNC. Under optimal conditions, the linear range of I₂/I^--mediated fluorescence quenching of Ag-AuNC-based immunoassay for detecting E. coli O157:H7 was 3.3 × 10³ to 10⁶ cfu/mL, with a detection limit of 9.2 × 10² cfu/mL, 10.7-fold lower than that of the traditional ELISA. The proposed immunoassay exhibits excellent sensitivity, specificity, recovery, and accuracy, which is useful for quantitative detection of E. coli O157:H7 in food safety.

An aptamer-exonuclease III (Exo III)-assisted amplification-based lateral flow assay for sensitive detection of Escherichia coli O157:H7 in milk

Escherichia coli O157:H7 (E. coli O157:H7), one of the most widespread foodborne pathogens, can cause a series of diseases and even lead to death. In this study, a highly sensitive method was developed by combining aptamer-exonuclease III (Exo III)-assisted amplification with lateral flow assay (LFA) based on gold nanoparticles (AuNP). The compound of single-stranded (ss) DNA-anti-E. coli O157:H7 aptamer (ssDNA-aptamer) was formed by hybridization between designed target ssDNA and aptamer. When E. coli O157:H7 was present, target bacteria were bound with the aptamer, and the free target ssDNA was hybridized with the probes of the designed hairpin (HP) structure. Exo III digests the 3' double-stranded blunt end of the complex and releases the enzyme product. Because the remaining sequence of the HP of the designed enzyme product was the same as the target ssDNA sequence, the target ssDNA could be amplified. Finally, the enhanced target ssDNA was combined with AuNP-LFA to achieve visual detection of E. coli O157:H7. The quantitative ability of this platform for E. coli O157:H7 was 7.6 × 10¹ cfu/mL in pure culture, and the detection limit in milk was 8.35 × 10² cfu/mL. This LFA was highly specific to E. coli O157:H7, and the time for detection of E. coli O157:H7 in milk was 4 h. Hence, this system has important application prospects in the detection of pathogenic bacteria in dairy products.

Gas-propelled biosensors for quantitative analysis

Gas-propelled biosensors display a simple gas-based signal amplification with quantitative detection features based on the target recognition event in combination with gas propulsion. Due to the liquid-gas conversion, the gas not only pushes the ink bar forward in the microchannel, but also serves as the power to propel the micromotors in the liquid. Thus, this continuous motion leads to a shift in distances which is associated with the target amount. Therefore, gas-propelled biosensors provide a visual quantification based on distance or speed signals without the need for expensive instruments. In this review, we focus on current developments in gas-propelled biosensors for quantitative analysis. First, we list the types of gas utilized as actuators in biosensors. Second, we review the representative gas-propelled biosensors, including the propulsion mechanisms and fabrication methods. Moreover, gas-propelled quantification based on distance and speed is summarized. Finally, we cover applications and provide a future perspective of gas-propelled biosensors.

Synthesis of PDA-Mediated Magnetic Bimetallic Nanozyme and Its Application in Immunochromatographic Assay

Immunochromatographic assay (ICA) is widely applied in various fields. However, severe matrix interference and weak signal output present major challenges in achieving accurate and ultrasensitive detection in ICA. Here, a polydopamine (PDA)-mediated magnetic bimetallic nanozyme (Fe₃O₄@PDA@Pd/Pt) with peroxidase-like activity was synthesized and used as a probe in ICA. The magnetic property of Fe₃O₄@PDA@Pd/Pt enabled effective magnetic enrichment of targets, thereby reducing the matrix interference in the sample. PDA coating on the magnetic bimetallic nanozyme was employed as a mediator and a stabilizer. It improved the catalytic ability and stability of the magnetic bimetallic nanozyme by providing more coordination sites for Pd/Pt growth and functional groups (-NH and -OH). In addition, the Pd/Pt bimetallic synergistic effect could further enhance the catalytic ability of the nanozyme. A method was developed by integrating Fe₃O₄, PDA, and Pd/Pt into Fe₃O₄@PDA@Pd/Pt as a probe in ICA. With the proposed method, human chorionic gonadotropin and Escherichia coli O157:H7 were successfully detected to be as low as 0.0094 mIU/mL in human blood serum and 9 × 10¹ CFU/mL in the milk sample, respectively. This method may be readily adapted for accurate and ultrasensitive detection of other biomolecules in various fields.

(Nano)tag-antibody conjugates in rapid tests

Antibodies (Abs) are naturally derived materials with favorable affinity, selectivity, and fast binding kinetics to the respective antigens, which enables their application as promising recognition elements in the development of various types of biosensors/bioassays, especially in rapid tests. These tests are low-cost and easy-to-use biosensing devices with broad applications including medical or veterinary diagnostics, environmental monitoring and industrial usages such as safety and quality analysis in food, providing on-site quick monitoring of various analytes, making it possible to save analysis costs and time. To reach such features, the conjugation of Abs with various nanomaterials (NMs) as tags is necessary, which range from conventional gold nanoparticles to other nanoparticles recently introduced, where magnetic, plasmonic, photoluminescent, or multi-modal properties play a critical role in the overall performance of the analytical device. In this context, to preserve the Ab affinity and provide a rapid response with long-term storage capability, the use of efficient bio-conjugation techniques is critical. Thanks to their prominent role in rapid tests, many studies have been devoted to the design and development of Abs-NMs conjugates with various chemistries including passive adsorption, covalent coupling, and affinity interactions. In this review, we present the state-of-the-art techniques allowing various Ab-NM conjugates with a special focus on the efficiency of the developed probes to be employed in in vitro rapid tests. Challenges and future perspectives on the development of Ab-conjugated nanotags in rapid diagnostic tests are highlighted along with a survey of the progress in commercially available Ab-NM conjugates.

Aptamer-gold nanozyme based competitive lateral flow assay for rapid detection of CA125 in human serum

For several decades, point-of-care technology (POCT) has proven its potential regarding swift and cost-efficient detection of analytes. Lateral flow assay is a highly popular POC technology that needs improvisation to increase its sensitivity, cost effectiveness and quantification so that it becomes more user friendly and affordable technology. In this context, the present study has investigated the use of aptamers and nanozymes together for the first time in developing an Aptamer-nanozyme lateral flow assay (ALFA). The present study uses a specific aptamer for CA125 as capture reagent and peroxidase mimetic gold nanoparticles as label for detection of CA125 in human serum through developed competitive ALFA. The assay was specific and has a limit of detection of 3.71 U/mL. The ALFA test was in house validated for its precision, recovery and showed a significant correlation with established CA125 chemiluminiscent ELISA with P-value<0.0001. In summary, this assay quantitatively detects an analyte by using an aptamer and peroxidase mimetic gold nanoparticles that ensures circumventing the use of antibodies and incorporating enzyme mimetic activity in assay systems.