Dramatically Enhanced Immunochromatographic Assay Using Cascade Signal Amplification for Ultrasensitive Detection of Escherichia coli O157:H7 in Milk

Anti-fouling array paper-based device for rapid and accurate discrimination of foodborne pathogens in real samples

Foodborne pathogens contamination can cause serious food safety incidents. Early and rapid detection of foodborne pathogens is very important. Paper-based devices have been applied for on-site rapid detection of foodborne pathogens due to their cheapness and portability. However, the paper-based foodborne pathogen detection devices for real samples often receive interference, resulting in inaccurate detection results. In this study, bovine serum albumin (BSA) was modified on the surface of nitrocellulose membrane to construct anti-fouling sensing interface. Through array design, the anti-fouling array paper-based device (APAD) with multiple detection channels was constructed. The naked eye cut-off limits of the APAD were 102 colony forming units (cfu)/mL, 102 cfu/mL, and 103 cfu/mL for Staphylococcus aureus in orange juice, Escherichia coli O157: H7 in milk and orange juice, and Escherichia coli O157:H7 in soy sauce, respectively. The above results indicate that the APAD has good practical application potential in food safety.

Utilizing the Thermostability of Nanozymes for Joule Heating to Remove Background Peroxidase Activities in Lateral Flow Assays

Lateral flow assays (LFAs) are essential for point-of-care testing. The use of peroxidase-mimicking nanozymes as catalytic labels is an actively developing direction in LFA, primarily focused on enhancing sensitivity. However, endogenous peroxidases, naturally present in various samples, can interfere with nanozyme signal amplification, leading to a high background signal and making visual detection more challenging. The issue of endogenous peroxidases is particularly significant for LFAs as wash-free biosensors. In this study, we showcase the remarkable thermostability of nanozymes in contrast to enzymes, applied to the analytically relevant use of lateral flow assays for the detection of aflatoxin B1. By employing Joule heating in a portable battery-powered device, the test strips were rapidly heated to 75-80 °C after completing the conventional LFA process. This heating caused thermal denaturation of endogenous peroxidases without affecting the Au@Pt nanozymes. As a result, substrate oxidation on the test strip was carried out solely by the Au@Pt nanozymes, which reduced background noise and improved the limit of detection by a factor of 3.5 compared to the assay without heating.

A gold nanoparticle-based colorimetric strategy for DNA detection: principles and novel approaches

The development of nanotechnology has led to the rapid growth of many different fields, including sensors. Bulky and complex sensor systems are gradually being replaced by streamlined sensor devices with advantages in size, simplicity, cost-effectiveness, and fast response, allowing qualitative detection of target analyte on-site application for clinical diagnosis. Significantly, since the COVID-19 pandemic, research on developing test kits for detecting biological molecules has grown rapidly, with an increasing number of publications. The number of studies developing colorimetric sensors based on gold nanoparticles (AuNPs) has increased continuously over the years, demonstrating the potential application of this material. The surface plasmon resonance (SPR) effect and high biocompatibility of AuNPs make them different from many other metal nanomaterials. In addition, the peroxidase activity properties of AuNPs have also received much attention in colorimetric sensors. In this review, the colorimetric sensors developed based on the AuNP material platform for DNA detection will be discussed in detail. Among them, the commonly used synthesis methods of AuNPs based on their applications and the primary mechanism of AuNP-based colorimetric sensors for DNA detection will be discussed. In addition, AuNP-based colorimetric applications in POCT for pathogenic bacteria and viruses are also mentioned in this review to provide a broader perspective on the potential and developmental direction of AuNP-based colorimetric sensors. Another aspect this review provides is development strategies that allow simple readout using the naked eye, a spectrophotometer, or a smartphone camera, which present many opportunities for integration into other electronic devices.

Rational design to improve the detection sensitivity of sandwich-type lateral flow immunoassays

This study presents two strategies to improve the detection sensitivity of the gold nanoparticle (Au NP)-based sandwich-type lateral flow immunoassay (LFIA) using the typical human chorionic gonadotropin (hCG) LFIA as the model. One efficient way is to adjust the size of Au NPs used as the label in the LFIA. Four kinds of Au NPs with different sizes (80, 110, 130, and 160 nm) were synthesized using a seed-mediated growth method, and the influence of nanoparticle size on the detection sensitivity of hCG was investigated. The results revealed that 110 nm Au NPs offered the highest detection sensitivity. Under the optimized conditions, the limit of detection (LOD) for hCG protein using Au NPs with a diameter around 110 nm as the signal probe was 5 mIU mL-1. Another powerful strategy discovered accidentally involves the introduction of an additional glass cellulose pad on the MAX-line label. The MAX-line label, which features an adhesive backing and a marked line indicating the maximum sample volume, is assembled on the test strip covering the sample pad and conjugate pad. We found that the introduction of an additional glass cellulose pad with a length of 6 mm on the MAX-line label could achieve a visual LOD of 1 mIU mL-1 for the detection of hCG. We assume that the improvement in detection sensitivity may be attributed to the increased sample carrying capacity and the extended flow time of gold nanoprobes, as the gold probes may diffuse from the conjugate pad into the enhancement pad upon rehydration. The developed LFIA also exhibited good reproducibility and stability. We believe that the developed strategies, particularly the approach of adding an enhancement pad on the MAX-line label, will serve as a general method applicable to other types of LFIAs to improve the sensitivity of target analytes.

Nanozymes-Mediated Lateral Flow Assays for the Detection of Pathogenic Microorganisms and Toxins: A Review from Synthesis to Application

In today's context, there is an increasing awareness among individuals regarding the importance of healthy and safe food consumption. Consequently, there is a growing demand for food products that are safeguarded against the detrimental effects of pathogens and harmful microbial metabolites. Actually, these organisms and their associated toxins pose a significant risk to food safety and are recognized as a critical threat to human health because of their capacity to induce foodborne infections and intoxications. Consequently, in order to address such challenges, it is imperative to enhance recognizing systems comprising bio/nanosensors for detections, which are trustworthy, quick, beneficial and economical. The advent of digital color imaging technology has led to the gradual establishment of lateral flow assays (LFAs) as one of the most significant sensors for point-of-care applications. Unlike colloidal gold nanoparticles (AuNPs), nanozymes offer enhanced color intensity through target-induced precise enrichment of nanozymes at the test line. Additionally, they amplify the color signal by facilitating the catalytic oxidation of colorless substrates into colored products. This dual functionality presents significant potential for the development of well-organized LFAs. In light of this, significant attempts are dedicated to the development of nanozyme-based LFAs. This review aims to outline recent advancements in the synthesis and design of nanozymes with varying compositions that exhibit distinct activities, as well as the structure and employment of nanozyme-based LFAs for the detection of pathogenic microorganisms and their associated toxins. Furthermore, the existing challenges and prospective development directions are outlined to assist readers in advancing the nanozyme-based LFAs performance.

SERS-Based Immunochromatographic Assay for Sensitive Detection of Escherichia coli O157:H7 Using a Novel WS2-AuDTNB Nanotag

E. coli O157:H7 contamination in food and the environment poses a serious threat to human health. Rapid and sensitive identification of foodborne pathogens remains challenging. Here, we prepared tungsten disulfide (WS2)-Au nanocomposites coupled with the Raman signal molecule 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) and antibodies to replace the conventional colloidal gold nanoparticles and applied SERS-active nanotags in the SERS-ICA method for highly sensitive detection of E. coli O157:H7. The large surface area and numerous effective SERS hotspots of WS2-Au nanotags provide superior SERS signals. Under optimized conditions, this ICA achieves the quantitative detection of E. coli O157:H7 in a broad linear range of 8 × 102-8 × 107 CFU/mL and at a low detection limit of 175 CFU/mL. In addition, the test strip indicates high specificity for E. coli O157:H7 identification, favorable reproducibility, and shows good accuracy in the detection of actual food samples, such as milk and pork. The proposed assay can be used for rapid qualitative and quantitative detection of E. coli O157:H7 and has great potential for field application.

Sensitive Colorimetric Lateral Flow Assays Enabled by Platinum-Group Metal Nanoparticles with Peroxidase-Like Activities

The last several decades have witnessed the success and popularity of colorimetric lateral flow assay (CLFA) in point-of-care testing. Driven by increasing demand, great efforts have been directed toward enhancing the detection sensitivity of CLFA. Recently, platinum-group metal nanoparticles (PGM NPs) with peroxidase-like activities have emerged as a type of promising colorimetric labels for enhancing the sensitivity of CLFA. By incorporating a simple and rapid post-treatment process, the PGM NP-based CLFAs are orders of magnitude more sensitive than conventional gold nanoparticle-based CLFAs. In this perspective, the study begins with introducing the design, synthesis, and characterization of PGM NPs with peroxidase-like activities. The current techniques for surface modification of PGM NPs are then discussed, followed by operation and optimization of PGM NP-based CLFAs. Afterward, opinions are provided on the social impact of PGM NP-based CLFAs. Lastly, this perspective is concluded with an outlook of future research directions in this emerging field, where the challenges and opportunities are discussed.

Review of Detection Limits for Various Techniques for Bacterial Detection in Food Samples

Foodborne illnesses can be infectious and dangerous, and most of them are caused by bacteria. Some common food-related bacteria species exist widely in nature and pose a serious threat to both humans and animals; they can cause poisoning, diseases, disabilities and even death. Rapid, reliable and cost-effective methods for bacterial detection are of paramount importance in food safety and environmental monitoring. Polymerase chain reaction (PCR), lateral flow immunochromatographic assay (LFIA) and electrochemical methods have been widely used in food safety and environmental monitoring. In this paper, the recent developments (2013-2023) covering PCR, LFIA and electrochemical methods for various bacterial species (Salmonella, Listeria, Campylobacter, Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli)), considering different food sample types, analytical performances and the reported limit of detection (LOD), are discussed. It was found that the bacteria species and food sample type contributed significantly to the analytical performance and LOD. Detection via LFIA has a higher average LOD (24 CFU/mL) than detection via electrochemical methods (12 CFU/mL) and PCR (6 CFU/mL). Salmonella and E. coli in the Pseudomonadota domain usually have low LODs. LODs are usually lower for detection in fish and eggs. Gold and iron nanoparticles were the most studied in the reported articles for LFIA, and average LODs were 26 CFU/mL and 12 CFU/mL, respectively. The electrochemical method revealed that the average LOD was highest for cyclic voltammetry (CV) at 18 CFU/mL, followed by electrochemical impedance spectroscopy (EIS) at 12 CFU/mL and differential pulse voltammetry (DPV) at 8 CFU/mL. LOD usually decreases when the sample number increases until it remains unchanged. Exponential relations (R2 > 0.95) between LODs of Listeria in milk via LFIA and via the electrochemical method with sample numbers have been obtained. Finally, the review discusses challenges and future perspectives (including the role of nanomaterials/advanced materials) to improve analytical performance for bacterial detection.

Colorimetric and Reverse Fluorescence Dual-Signal Readout Immunochromatographic Assay for the Sensitive Determination of Sibutramine

Later flow immunochromatographic assay has been widely used in clinical, environmental, and other diagnostic applications owing to its high sensitivity and throughput. However, most immunoassays operate in the "turn-off" mode for detecting targets of low molecular weight. The signal intensity decreases as the analyte concentration increases, which poses a challenge for achieving ultrasensitive detection at low concentrations and is counterintuitive to new users. In this work, a fluorometric immunochromatographic assay (FICA) is developed to simultaneously read "turn-on" fluorescent and "turn-off" colorimetric signals, where ZnCdSe/ZnS quantum dots act as fluorescence donors and gold nanoparticles (AuNPs) act as quenchers. The fluorescent signal (excitation/emission wavelengths of 365/525 nm) is positively correlated with analytes' concentration. Taking sibutramine (SBT) as the analysis target, the visual limit of detection for SBT reached 3.9 ng/mL, and the limit of Quantitation was 5.0 ng/mg in spiked samples. The developed FICA achieves a high sensitivity in SBT detection, which is much lower than that of the colloidal gold-based immunochromatographic assay. This dual-function detection mode has great potential to be used as a rapid on-site semiquantitative method, providing an alternative mode for the determination of low levels of target analytes.

Molecular Engineering Powered Dual-Readout Point-of-Care Testing for Sensitive Detection of Escherichia coli O157:H7

Escherichia coli O157:H7 (E. coli O157:H7) has become one of the major threats to public health and food safety. However, the culture method as a gold standard for the detection of E. coli O157:H7 requires laborious operations and a long processing time. Herein, we developed a dual-readout aggregation-induced emission nanoparticle-based lateral flow immunoassay (LFIA) for sensitive detection of E. coli O157:H7 to achieve a qualitative and quantitative assay for satisfying the applications under varying scenarios. 2,3-Bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)fumaronitrile (BAPF), an aggregation-induced emission luminogen, was designed to achieve a strong molar extinction coefficient (3.0 × 104 M-1 cm-1) and high quantum yield (33.28%), which was further verified by a large rotation angle and low energy gap. Subsequently, BAPFs were integrated into a nanostructured system to form excellent water-soluble nanoparticles (BAPFNPs) for the detection of E. coli O157:H7 with colorimetric and fluorescent readout. The designed BAPFNPs-based LFIA (BAPFNPs-LFIA) exhibited nearly qualitative ability with gold nanoparticles-LFIA (AuNPs-LFIA) and a 9 times enhancement compared with quantum beads-LFIA (QBs-LFIA) in quantitative aspect. Especially, FL-BAPFNPs-LFIA could detect E. coli O157:H7 earlier than QBs-LFIA and AuNPs-LFIA when samples with low E. coli O157:H7 concentrations were cultured. Overall, the proposed strategy revealed that versatile BAPFNPs have great potential as reporters for dual-readout ability and enhancing detection sensitivity for rapid and accurate pathogenic bacteria assay.

High extinction coefficient material combined with multi-line lateral flow immunoassay strip for ultrasensitive detection of bacteria

Lateral flow immunoassay strips (LFIAs) are a reliable and point-of-care detection method for rapid monitoring of bacteria, but their sensitivity was limited by the low extinction coefficient of colloidal gold nanoparticles (Au NPs) and low capture efficiency of test-line. In this study, polydopamine nanoparticles (PDA NPs) were employed to replace Au NPs, due to their high extinction coefficient. And the amount of test-line was increased to 5 for further improving the efficiency of bacteria capture. Thus, under visual observation, the detection limits of PDA-based LFIAs (102 CFU/mL) were about 2 orders of magnitude lower than Au-based LFIAs (104 CFU/mL). Furthermore, the invisible signal could be collected by Image J and the detection limit can reach 10 CFU/mL. The proposed test strips were successfully applied for the quantitative, accurate, and rapid screening of E. coli in food samples. This study provided a universal approach to enhance the sensitivity of bacteria LFIAs.

Signal Amplification Strategy Design in Nanozyme-Based Biosensors for Highly Sensitive Detection of Trace Biomarkers

Nanozymes show great promise in enhancing disease biomarker sensing by leveraging their physicochemical properties and enzymatic activities. These qualities facilitate signal amplification and matrix effects reduction, thus boosting biomarker sensing performance. In this review, recent studies from the last five years, concentrating on disease biomarker detection improvement through nanozyme-based biosensing are examined. This enhancement primarily involves the modulations of the size, morphology, doping, modification, electromagnetic mechanisms, electron conduction efficiency, and surface plasmon resonance effects of nanozymes for increased sensitivity. In addition, a comprehensive description of the synthesis and tuning strategies employed for nanozymes has been provided. This includes a detailed elucidation of their catalytic mechanisms in alignment with the fundamental principles of enhanced sensing technology, accompanied by the presentation of quantitatively analyzed results. Moreover, the diverse applications of nanozymes in strip sensing, colorimetric sensing, electrochemical sensing, and surface-enhanced Raman scattering have been outlined. Additionally, the limitations, challenges, and corresponding recommendations concerning the application of nanozymes in biosensing have been summarized. Furthermore, insights have been offered into the future development and outlook of nanozymes for biosensing. This review aims to serve not only as a reference for enhancing the sensitivity of nanozyme-based biosensors but also as a catalyst for exploring nanozyme properties and their broader applications in biosensing.

Role of biotechnology in the advancement of biodegradable polymers and functionalized additives for food packaging systems

Biodegradable polymers have shown enormous potential for application in food packaging systems and offer solutions to mitigate the challenges of single-use plastics. Over the past decade, advances in fermentation technology, metabolic engineering of microorganisms, and synthetic biology have enabled the optimization and functionalization of biodegradable polymers for food packaging application. This article provides an overview of the biotechnological approaches/methods used in advancing the production of biopolymers and summarizes the recent developments in the application of functionalized biopolymers for decision-making and quality control. It discusses the current applications and future perspectives of extracellular biopolymers in food systems. Finally, this review highlights the complexities of public acceptance, safety, and government regulations and legislations.

Immunochromatographic enhancement strategy for SARS-CoV-2 detection based on nanotechnology

The global outbreak of coronavirus disease 2019 (COVID-19) has been catastrophic to both human health and social development. Therefore, developing highly reliable and sensitive point-of-care testing (POCT) for detecting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has become a priority. Among all available POCTs, the lateral flow immunoassay (LFIA, also known as immunochromatography) has proved to be effective due to its accuracy, portability, convenience, and speed. In areas with a scarcity of laboratory resources and medical personnel, the LFIA provides an affordable option for the diagnosis of COVID-19. This review offers a comprehensive overview of methods for improving the sensitivity of SARS-CoV-2 detection using immunochromatography based on nanotechnology, sorted according to the different detection targets (antigens, antibodies, and nucleic acids). It also looks into the performance and properties of the various sensitivity enhancement strategies, before delving into the remaining challenges in COVID-19 diagnosis through LFIA. Ultimately, it seeks to provide helpful guidance in selecting an appropriate strategy for SARS-CoV-2 immunochromatographic detection based on nanotechnology.

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.

Enhanced sensitivity and accuracy via gold nanoparticles based multi-line lateral flow immunoassay strip for Salmonella typhimurium detection in milk and orange juice

Food borne pathogens threaten food safety and affect human health. The lateral flow immunoassays (LFIAs) are widely concerned because of simplicity, low cost and user friendliness, and have broad application prospects in pathogen detection. However, the sensitivity of LFIAs is limited. Herein, multi-line LFIAs are introduced into pathogen detection for the first time. Compared with traditional single-line LFIAs, the overall signal strength of multi-line LFIAs has been significantly improved. It is particularly noteworthy that multi-line LFIAs detection accuracy of 10³ CFU/mL pathogen has been improved by about 55%. The proposed multi-line LFIAs reduce the possibility of judging a positive result as a false negative result. The LFIAs strip was validated in real samples of milk and orange juice. This strategy has great potential for rapid detection of pathogens in real samples, and provides new insights for improving the accuracy and sensitivity of LFIAs strips.

High-throughput, highly sensitive and rapid SERS detection of Escherichia coli O157:H7 using aptamer-modified Au@macroporous silica magnetic photonic microsphere array

The aim of this research was to develop a simple, rapid, sensitive, high-throughput detection method for foodborne Escherichia coli (E. coli) O157:H7 based on the aptamer-modified gold nanoparticles@macroporous magnetic silica photonic microsphere (Au@MMSPM). Such Au@MMSPM array system for E. coli O157:H7 not only integrated sample pretreatment with rapid detection, but also showed highly enhanced effect to develop a highly sensitive SERS assay. The established SERS assay platform gave a wide linear detection range (10-10⁶ CFU/mL) and low limit of detection (2.20 CFU/mL) for E. coli O157:H7. The whole analysis time including sample pretreatment and detection was 110 min. This SERS-based assay platform provided a new high-throughput, highly sensitive and fast detection technology for monitoring E. coli O157:H7 in real samples from the fields of food industry, medicine and environment.

Nanostructured biosensing platforms for the detection of food- and water-borne pathogenic Escherichia coli

Pathogenic bacterial infection is one of the principal causes affecting human health and ecosystems. The accurate identification of bacteria in food and water samples is of significant interests to maintain safety and health for humans. Culture-based tests are practically tedious and may produce false-positive results, while viable but non-culturable microorganisms (NCMs) cannot be retrieved. Thus, it requires fast, reliable, and low-cost detection strategies for on-field analysis and point-of-care (POC) monitoring. The standard detection methods such as nucleic acid analysis (RT-PCR) and enzyme-linked immunosorbent assays (ELISA) are still challenging in POC practice due to their time-consuming (several hours to days) and expensive laboratory operations. The optical (surface plasmon resonance (SPR), fluorescence, and surface-enhanced Raman scattering (SERS)) and electrochemical-based detection of microbes (early stage of infective diseases) have been considered as alternative routes in the emerging world of nanostructured biosensing since they can attain a faster and concurrent screening of several pathogens in real samples. Moreover, optical and electrochemical detection strategies are opening a new route for the ability of detecting pathogens through the integration of cellphones, which is well fitted for POC analysis. This review article covers the current state of sensitive mechanistic approaches for the screening and detection of Escherichia coli O157:H7 (E. coli) pathogens in food and water samples, which can be potentially applied in clinical and environmental monitoring.

Synthesizing Submicron Polyelectrolyte Capsules to Boost Enzyme Immobilization and Enhance Enzyme-Based Immunoassays

Polyelectrolyte capsules (PCs) exhibit attractive superiorities in enzyme immobilization, including providing a capacious microenvironment for enzyme conformational freedom, highly effective mass transfer, and protecting enzymes from the external environment. Herein, we provide the first systemic evaluation of submicron PCs (SPCs, 500 nm) for enzyme immobilization. The catalytic kinetics results show that SPC encapsulation affected the affinities of enzymes and substrates but significantly enhanced their catalytic activity. The stability test indicates that SPC-encapsulated horseradish peroxidase (HRP) exhibits ultrahigh resistance to external harsh conditions and has a longer storage life than that of soluble HRP. The proposed encapsulation strategy enables 7.73-, 2.22-, and 11.66-fold relative activities when working at a pH as low as 3, at a NaCl concentration as high as 500 mM, and at a trypsin concentration as high as 10 mg/mL. We find that SPC encapsulation accelerates the cascade reaction efficiency of HRP and glucose oxidase. Owing to SPCs enhancing the catalytic activity of the loaded enzymes, we established an amplified enzyme-linked immunosorbent assay (ELISA) for the detection of Escherichia coli O157:H7 using HRP-loaded SPCs. The detection sensitivity of SPC-improved ELISA was found to be 280 times greater than that of conventional HRP-based ELISA. Altogether, we provide an elaborate evaluation of 500 nm SPCs on enzyme immobilization and its application in the ultrasensitive detection of foodborne pathogens. This evaluation provides evidence to reveal the potential advantage of SPCs on enzyme immobilization for enzyme-based immunoassays. It has excellent biological activity and strong stability and broadens the application prospect in urine, soy sauce, sewage, and other special samples.

Evaluation of volume-based flow cytometry as a potential primary method for quantification of bacterial reference material

Bacterial reference materials (RMs) play a crucial role in many analytical processes of microbiological detection. Currently, bacteria are typically counted using the traditional plate-based approach, which results in a higher uncertainty of bacterial RMs unfortunately. Therefore, novel methods are urgently required for the value assignment of RMs in the field of microbiology to derive measurement traceability and accuracy. A potential primary method for microbiological quantification based on flow cytometry (FCM) is described in this study using Escherichia coli O157 (E. coli O157) as an example. The proposed method was applied to determine the number of viable E. coli O157 cells in the RMs with a result of (5.48 ± 0.27) × 10⁸ cells mL^-1, which was in good agreement with the result obtained using the plate-based method (E_n = 0.47). Additionally, this method could be entirely described and understood by equations, and provides formal traceability to the SI for counts of viable bacterial cells, while the associated relative expanded uncertainty (4.93%, k = 2) was significantly lower in comparison to the plate-based method. Therefore, the FCM-based method might be a potential primary method for characterizing bacterial RMs. To our knowledge, this is the first description of FCM as a potential primary method for accurate and traceable quantification of viable bacterial cells with a comprehensive uncertainty statement in microbiological metrology.

Nanosheet antibody mimics based label-free and dual-readout lateral flow immunoassay for Salmonella enteritidis rapid detection

Portable devices for on-site foodborne pathogens detection are urgently desirable. Lateral flow immunoassay (LFIA) provides an efficient strategy for pathogens detection, however, antibody labeling independence and detection reliability, are still challenging. Here, we report the development of a label-free LFIA with dual-readout using glucan-functionalized two-dimensional (2D) transition metal dichalcogenides (TMDs) tungsten disulfide (WS₂) as detection probes for sensitive detection of Salmonella enteritidis (S. enteritidis). In particular, glucan-functionalized WS₂, synthesized via liquid exfoliation, are reliable detection antibody candidates which served as antibody mimics for bacteria capturing. This LFIA has not only eliminated the intricate antibody labeling process and screening of paired antibodies in conventional LFIAs, but also promised dual-readout (colorimetric/Raman) for flexible detection. Under optimized conditions, this LFIA achieves selective detection of S. enteritidis with a low visual detection limit of 10³ CFU/mL and a broad linear range of 10³-10⁸ CFU/mL. Additionally, the LFIA could be successfully applied in drinking water and milk with recoveries of 85%-109%. This work is desirable to expand the application of 2D TMDs in biosensors and offers a brand-new alternative protocol of detection antibodies in foodborne pathogens detection.

Paper-based sensors for bacteria detection

The detection of pathogenic bacteria is essential to prevent and treat infections and to provide food security. Current gold-standard detection techniques, such as culture-based assays and polymerase chain reaction, are time-consuming and require centralized laboratories. Therefore, efforts have focused on developing point-of-care devices that are fast, cheap, portable and do not require specialized training. Paper-based analytical devices meet these criteria and are particularly suitable to deployment in low-resource settings. In this Review, we highlight paper-based analytical devices with substantial point-of-care applicability for bacteria detection and discuss challenges and opportunities for future development.

Tunable Magneto-Plasmonic Nanosensor for Sensitive Detection of Foodborne Pathogens

Frequent outbreaks of food-borne pathogens, particularly E. coli O157:H7, continue to impact human health and the agricultural economy tremendously. The required cell count for this pathogenic strain of E. coli O157:H7 is relatively low and hence it is vital to detect at low colony forming unit (CFU) counts. Available detection methods, though sensitive, fall short in terms of timeliness and often require extensive sample processing. To overcome these limitations, we propose a novel magneto-plasmonic nanosensor (MPnS) by integrating surface plasmon resonance (SPR) properties with spin-spin magnetic relaxation (T2 MR) technology. We engineered MPnS by encapsulating several gold nanoparticles (GNPs) within the polymer-coating of iron oxide nanoparticles (IONPs). First, the polyacrylic acid (PAA)-coated IONPs were synthesized using a solvent precipitation method, then gold chloride solution was used to synthesize GNPs and encapsulate them within the PAA-coatings of IONPs in one step. A magnetic separation technique was used to purify the MPnS and the presence of GNPs within IONPs was characterized using transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and other spectroscopic methods. The synthesized MPnS exhibits MR relaxation properties while possessing amplified optical properties than conventional GNPs. This allows for rapid and ultrasensitive detection of E. coli O157:H7 by SPR, T2 MR, and colorimetric readout. Experiments conducted in simple buffer and in milk as a complex media demonstrated that our MPnS-based assay could detect as low as 10 CFUs of this pathogenic strain of E. coli O157:H7 in minutes with no cross-reactivity. Overall, the formulated MPnS is robust and holds great potential for the ultrasensitive detection of E. coli O157:H7 in a simple and timely fashion. Moreover, this platform is highly customizable and can be used for the detection of other foodborne pathogens.

Recent Advances in the Immunoassays Based on Nanozymes

As a rapid and simple method for the detection of multiple targets, immunoassay has attracted extensive attention due to the merits of high specificity and sensitivity. Notably, enzyme-linked immunosorbent assay (ELISA) is a widely used immunoassay, which can provide high detection sensitivity since the enzyme labels can promote the generation of catalytically amplified readouts. However, the natural enzyme labels usually suffer from low stability, high cost, and difficult storage. Inspired by the advantages of superior and tunable catalytic activities, easy preparation, low cost, and high stability, nanozymes have arisen to replace the natural enzymes in immunoassay; they also possess equivalent sensitivity and selectivity, as well as robustness. Up to now, various kinds of nanozymes, including mimic peroxidase, oxidase, and phosphatase, have been incorporated to construct immunosensors. Herein, the development of immunoassays based on nanozymes with various types of detection signals are highlighted and discussed in detail. Furthermore, the challenges and perspectives of the design of novel nanozymes for widespread applications are discussed.

Nanozyme-based sensors for detection of food biomarkers: a review

Nanozymes have piqued the curiosity of scientists in recent years because of their ability to demonstrate enzyme-like activity combined with advantages such as high stability, inexpensive availability, robust activity, and tunable properties. These attributes have allowed the successful application of nanozymes in sensing to detect various chemical and biological target analytes, overcoming the shortcomings of conventional detection techniques. In this review, we discuss recent developments of nanozyme-based sensors to detect biomarkers associated with food quality and safety. First, we present a brief introduction to this topic, followed by discussing the different types of sensors used in food biomarker detection. We then highlight recent studies on nanozyme-based sensors to detect food markers such as toxins, pathogens, antibiotics, growth hormones, metal ions, additives, small molecules, and drug residues. In the subsequent section, we discuss the challenges and possible solutions towards the development of nanozyme-based sensors for application in the food industry. Finally, we conclude the review by discussing future perspectives of this field towards successful detection and monitoring of food analytes.

Nanozyme-based cascade SPR signal amplification for immunosensing of nitrated alpha-synuclein

A self-assembled nanozyme of iron porphyrin mediated supramolecular modified gold nanoparticles (FpA) was fabricated to determine nitrated alpha-synuclein as the Tyr 39 residue (nT39 α-Syn) of a potential biomarker for early diagnosis of Parkinson's disease (PD). Mechanically, localized surface plasmon resonance (LSPR) and the mass effect caused by catalytic deposition of the nanozyme contributed to a cascade signal amplification strategy. The sensor allowed a signal amplification and selective nT39 α-Syn bioanalysis with a 1.34-fold enhancement by cascade amplified SPR signal and double specific recognition. The detection limit was 1.78 ng/mL in the detection range of 7-240 ng/mL. Benefiting from the excellent immunosensor, this method can distinguish healthy people and PD patients using actual samples. Overall, this strategy provides a nanozyme-based biosensing platform for the early diagnosis of PD and can be applied to detect other protein biomarkers, such as PD-L1.

A review on recent advances in the applications of composite Fe3O4 magnetic nanoparticles in the food industry

Fe3O4 magnetic nanoparticles (MNPs) have attracted tremendous attention due to their superparamagnetic properties, large specific surface area, high biocompatibility, non-toxicity, large-scale production, and recyclability. More importantly, numerous hydroxyl groups (-OH) on the surface of Fe3O4 MNPs can provide coupling sites for various modifiers, forming versatile nanocomposites for applications in the energy, biomedicine, and environmental fields. With the development of science and technology, the potential of nanotechnology in the food industry has also gradually become prominent. However, the application of composite Fe3O4 MNPs in the food industry has not been systematically summarized. Herein, this article reviews composite Fe3O4 MNPs, including their properties, modifications, and physical functions, as well as their applications in the entire food industry from production to processing, storage, and detection. This review lays a solid foundation for promoting food innovation and improving food quality and safety.

Colorimetric Systems for the Detection of Bacterial Contamination: Strategy and Applications

Bacterial contamination is a public health concern worldwide causing enormous social and economic losses. For early diagnosis and adequate management to prevent or treat pathogen-related illnesses, extensive effort has been put into the development of pathogenic bacterial detection systems. Colorimetric sensing systems have attracted increasing attention due to their simple and single-site operation, rapid signal readout with the naked eye, ability to operate without external instruments, portability, compact design, and low cost. In this article, recent trends and advances in colorimetric systems for the detection and monitoring of bacterial contamination are reviewed. This article focuses on pathogen detection strategies and technologies based on reaction factors that affect the color change for visual readout. Reactions used in each strategy are introduced by dividing them into the following five categories: external pH change-induced pH indicator reactions, intracellular enzyme-catalyzed chromogenic reactions, enzyme-like nanoparticle (NP)-catalyzed substrate reactions, NP aggregation-based reactions, and NP accumulation-based reactions. Some recently developed colorimetric systems are introduced, and their challenges and strategies to improve the sensing performance are discussed.

Novel Dual-Color Immunochromatographic Assay Based on Chrysanthemum-like Au@polydopamine and Colloidal Gold for Simultaneous Sensitive Detection of Paclobutrazol and Carbofuran in Fruits and Vegetables

To ensure food safety and prevent the toxic effects of paclobutrazol (PBZ) and carbofuran (CAR) on humans, a sensitive and rapid method for the detection of PBZ and CAR in fruits and vegetables is required. Herein, a highly sensitive PBZ monoclonal antibody (PBZ mAb) and CAR monoclonal antibody (CAR mAb) with half-inhibitory concentrations (IC₅₀) at 0.77 and 0.82 ng mL⁻¹ were prepared, respectively. We proposed a novel dual-color immunochromatographic assay (ICA) with two test lines (T₁ and T₂) and an independent control line (C) based on chrysanthemum-like Au@Polydopamine (AuNC@PDA) and colloidal gold (AuNPs) for the simultaneous and sensitive detection of PBZ and CAR with naked-eye detection limits of 10 and 5 μg kg⁻¹, respectively. The limits of detection (LOD) for PBZ and CAR were 0.117 and 0.087 μg kg⁻¹ in orange, 0.109 and 0.056 μg kg⁻¹ in grape, and 0.131 and 0.094 μg kg⁻¹ in cabbage mustard, respectively. The average recoveries of PBZ and CAR in orange, grape, and cabbage mustard were 97.86−102.83%, with coefficients of variation from 8.94 to 11.05. The detection results of this method for 30 samples (orange, grapes, and cabbage mustard) agreed well with those of liquid chromatography–tandem mass spectrometry. The novel dual-color ICA was sensitive, rapid, and accurate for the simultaneous detection of PBZ and CAR in real samples.

A portable kit based on thiol-ene Michael addition for acrylamide detection in thermally processed foods

Accurate, sensitive, and selective analysis of acrylamide generated in thermally processed foods is of great significance for food safety. Herein, a novel acrylamide sensing platform is designed for both sensitive on-site colorimetric analysis and accurate UV-vis spectroscopy quantification, by integrating thiol-ene Michael addition with gold nanoparticles-mediated catalytical oxidation. The Michael addition reaction between acrylamide and glutathione efficiently alleviates glutathione-induced catalytic activity inhibition of gold nanoparticles, evoking the chromogenic reaction of H₂O₂-mediated 3,3',5,5'-tetramethylbenzidine. With increasing the concentration of acrylamide, the oxidation of 3,3',5,5'-tetramethylbenzidine is accelerated, presenting a series of shades from colorless to blue. The sensing platform exhibits excellent detection performance of acrylamide in the range of 0.5-175 μM with a detection limit of 0.16 μM, and is successfully employed in food samples. Especially, a portable assay kit based on the proposed platform is developed for visual determination of acrylamide, opening an avenue for smart sensors of food safety hazards.

Tailored enzymes as next-generation food-packaging tools

Enzymes have the potential for biotransformation in the food industry. Engineering tools can be used to develop tailored enzymes for food-packaging systems that perform well and retain their activity under adverse conditions. Consequently, novel tailored enzymes have been produced to improve or include new and useful characteristics for intelligent food-packaging systems. This review discusses the protein-engineering tools applied to create new functionality in food-packaging enzymes. The challenges in applications and anticipated directions for future developments are also highlighted. The development and discovery of tailored enzymes for smart food packaging is a promising way to ensure safe and high-quality food products.

Prussian blue immunochromatography with portable smartphone-based detection device for zearalenone in cereals

In this study, we developed a prussian blue nanoparticles (PBNPs) immunochromatographic assay (ICA) integrated with smartphone-based detection device for ZEN in cereals. PBNPs, as probe labels, were synthesized with properties of controllable structure, environment friendliness, and high affinities to antibody (Ab). PBNPs-ICA quantitative analysis was performed with a hand-held smartphone-based device coupled with a user-friendly and self-programmed detection App. This integrated strategy demonstrated high sensitivity for ZEN with a cut-off value of 10 μg/kg, a detection limit of 0.12 μg/kg, a quantitation limit of 0.27 μg/kg, and recovery rates of 92.0%-105.0% and 88.0%-98.0% for maize and wheat, respectively. The results of 20 naturally contaminated cereal samples showed good correlation (R²＞0.99) between LC-MS/MS and developed system. Moreover, the stability experiment revealed that PBNPs-ICA maintained high stability and bioactivity against competitive antigen (Ag). The proposed strategy exhibited great potential for the rapid monitoring of mycotoxins or other small molecule hazards.

An immunochromatography strip with peroxidase-mimicking ferric oxyhydroxide nanorods-mediated signal amplification and readout

Immunochromatography testing strips (ICTs) promise to become the point-of-care test format for early diagnosis due to their convenience, low cost, and simplification. However, the insufficient signal intensity and limited sensitivity of this format hamper their application. Herein, we overcame these limitations by integrating rod-like ferric oxyhydroxide (β-FeOOH) nanoparticles with ICTs. By varying the concentration of PEI, a one-pot, mild-temperature hydrolysis method was adapted for the synthesis and morphology regulation of β-FeOOH nanorod. Due to the excellent enzyme-like catalytic activity toward peroxidase substrates (TMB) in the presence of hydrogen peroxide (H₂O₂), the β-FeOOH nanorod in ICTs served as a signal generator and the nanozyme for signal amplification. The proof-of-concept work was performed for the detection of human chorionic gonadotropin (hCG). A two fold improvement of detection sensitivity was achieved compared to the sensitivity of conventional Au NPs-based ICTs. These results show that β-FeOOH-based ICT has a potential application in POCT detection in clinical diagnostics.

Tailoring noble metal nanoparticle designs to enable sensitive lateral flow immunoassay

Lateral flow immunoassay (LFIA) with gold nanoparticles (AuNPs) as signal reporters is a popular point-of-care diagnostic technique. However, given the weak absorbance of traditional 20-40 nm spherical AuNPs, their sensitivity is low, which greatly limits the wide application of AuNP-based LFIA. With the rapid advances in materials science and nanotechnology, the synthesis of noble metal nanoparticles (NMNPs) has enhanced physicochemical properties such as optical, plasmonic, catalytic, and multifunctional activity by simply engineering their physical parameters, including the size, shape, composition, and external structure. Using these engineered NMNPs as an alternative to traditional AuNPs, the sensitivity of LFIA has been significantly improved, thereby greatly expanding the working range and application scenarios of LFIA, particularly in trace analysis. Therefore, in this review, we will focus on the design of engineered NMNPs and their demonstration in improving LFIA. We highlight the strategies available for tailoring NMNP designs, the effect of NMNP engineering on their performance, and the working principle of each engineering design for enhancing LFIA. Finally, current challenges and future improvements in this field are briefly discussed.

Recent advances in enzyme-enhanced immunosensors

Among the products for rapid detection in different fields, enzyme-based immunosensors have received considerable attention. Recently, great efforts have been devoted to enhancing the output signals of enzymes through different strategies that can significantly improve the sensitivity of enzyme-based immunosensors for the need of practical applications. In this manuscript, the significance of enzyme-based signal transduction patterns in immunoassay and the central role of enzymes in achieving precise control of reaction systems are systematically described. In view of the rapid development of this field, we classify these strategies based on the combination of immune recognition and enzyme amplification into three categories, namely enzyme-based enhancement strategies, combination of the catalytic amplification of enzymes with other signal amplification methods, and substrate-based enhancement strategies. The current focus and future direction of enzyme-based immunoassays are also discussed. This article is not exhaustive, but focuses on the latest advances in different signal generation methods based on enzyme-initiated catalytic reactions and their applications in the detection field, which could provide an accessible introduction of enzyme-based immunosensors for the community with a view to further improving its application efficiency.

Point-of-care COVID-19 diagnostics powered by lateral flow assay

Since its first discovery in December 2019, the global coronavirus disease 2019 (COVID-19) pandemic caused by the novel coronavirus (SARS-CoV-2) has been posing a serious threat to human life and health. Diagnostic testing is critical for the control and management of the COVID-19 pandemic. In particular, diagnostic testing at the point of care (POC) has been widely accepted as part of the post restriction COVID-19 control strategy. Lateral flow assay (LFA) is a popular POC diagnostic platform that plays an important role in controlling the COVID-19 pandemic in industrialized countries and resource-limited settings. Numerous pioneering studies on the design and development of diverse LFA-based diagnostic technologies for the rapid diagnosis of COVID-19 have been done and reported by researchers. Hundreds of LFA-based diagnostic prototypes have sprung up, some of which have been developed into commercial test kits for the rapid diagnosis of COVID-19. In this review, we summarize the crucial role of rapid diagnostic tests using LFA in targeting SARS-CoV-2-specific RNA, antibodies, antigens, and whole virus. Then, we discuss the design principle and working mechanisms of these available LFA methods, emphasizing their clinical diagnostic efficiency. Ultimately, we elaborate the challenges of current LFA diagnostics for COVID-19 and highlight the need for continuous improvement in rapid diagnostic tests.

Development of Indirect Competitive ELISA and Visualized Multicolor ELISA Based on Gold Nanorods Growth for the Determination of Zearalenone

In this study, a zearalenone (ZEN) hapten was designed and prepared against the mycotoxin ZEN, and the original coating ZEN-ovalbumin (ZEN-OVA) was prepared by conjugation with OVA. Based on the gold nanorods (AuNRs) of uniform size and stable properties synthesized by the seed-mediated method, the indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) and the AuNRs growth-based multicolor ELISA for detecting ZEN toxin were further established. Under the optimal experimental conditions, the coating amount of ZEN-OVA: 0.025 μg/well, antibody (Ab) dilution factor: 32,000 times, blocking solution: 0.5% skimmed milk powder, enzyme-labeled secondary Ab diluted 10,000 times, and a pH of the PBS buffer at 7.4, the sensitivity (IC₅₀) of the established ic-ELISA for ZEN detection reached 0.85 ± 0.04 μg/L, and the limit of detection (IC₁₅) reached 0.22 ± 0.08 μg/L. In the multicolor ELISA based on the growth of AuNRs, as the content of ZEN increased, the mixed solution exhibited a significant color change from brownish red to colorless. ZEN concentration as low as 0.1 μg/L can be detected by the naked eye (brown red to dark gray). This study provided an effective analysis strategy for the rapid screening and accurate monitoring of the ZEN contaminant in foods.

Nanozyme Applications: A Glimpse of Insight in Food Safety

Nanozymes own striking merits, including high enzyme-mimicking activity, good stability, and low cost. Due to the powerful and distinguished functions, nanozymes exhibit widespread applications in the field of biosensing and immunoassay, attracting researchers in various fields to design and engineer nanozymes. Recently, nanozymes have been innovatively used to bridge nanotechnology with analytical techniques to achieve the high sensitivity, specificity, and reproducibility. However, the applications of nanozymes in food applications are seldom reviewed. In this review, we summarize several typical nanozymes and provide a comprehensive description of the history, principles, designs, and applications of nanozyme-based analytical techniques in food contaminants detection. Based on engineering and modification of nanozymes, the food contaminants are classified and then discussed in detail via discriminating the roles of nanozymes in various analytical methods, including fluorescence, colorimetric and electrochemical assay, surface-enhanced Raman scattering, magnetic relaxing sensing, and electrochemiluminescence. Further, representative examples of nanozymes-based methods are highlighted for contaminants analysis and inhibition. Finally, the current challenges and prospects of nanozymes are discussed.

A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications

Since the ferromagnetic (Fe3O4) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymes has been made with deep investigation of diverse nanozymes and rapid development of related nanotechnologies. As promising alternatives for natural enzymes, nanozymes have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymes owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymes. This review digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymes, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymes constituted of metal and metal oxide nanomaterials.

Nucleic Acid Amplification Techniques in Immunoassay: An Integrated Approach with Hybrid Performance

An immunoassay is mostly employed for the direct detection of food contaminants, and a molecular assay for targeting nucleic acids employs amplification techniques for distinguishing genes. The integration of an immunoassay with nucleic acid amplification techniques inherits the direct and rapid performance of an immunoassay and the ultrasensitive merit of a molecular assay. Enthusiastic attention has been attracted in recent years on the utilization of isothermal amplification techniques in an immunoassay, as well as the employment of a lateral flow immunoassay in a molecular assay. Thus, this Review discussed these kinds of approaches from two categories: immuno-nucleic acid amplification (I-NAA) and nucleic acid amplification-immunoassay (NAA-I). The advantages, drawbacks, and future developments were discussed for a comprehensive understanding.

Recent advances in colorimetry/fluorimetry-based dual-modal sensing technologies

Tailored to the increasing demands for sensing technologies, the fabrication of dual-modal sensing technologies through combining two signal transduction channels into one method has been proposed and drawn considerable attention. The integration of two sensing signals not only promotes the analytical efficiency with reduced assumption, but also improves the analytical performances with enlarged detection linear range, enhanced accuracy, and boosted application flexibility. The two top-rated output signals for developing dual-modal sensors are colorimetric and fluorescent signals because of their outstanding merits for point of care applications and real-time sensitive sensing. Given the rapid development of material chemistry and nanotechnology, the recent decade has witnessed great advance in colorimetric/fluorimetric signal based dual-modal sensing technologies. The new sensing strategy leads to a broad avenue for various applications in disease diagnosis, environmental monitoring and food safety because of the complementary and synergistic effects of the two output signals. In this state-of-the-art review, we comprehensively summarize different types of colorimetric/fluorimetric dual-modal sensing methods by highlighting representative research in the last 5 years, digging into their sensing methodologies, particularly the working principles of the signal transduction systems. Then, the challenges and future prospects for boosting further development of this research field are discussed.