Comparative study of colloidal gold and quantum dots as labels for multiplex screening tests for multi-mycotoxin detection

Array-optimized artificial olfactory sensor enabling cost-effective and non-destructive detection of mycotoxin-contaminated maize

The MobileNetV3-based improved sine-cosine algorithm (ISCA-MobileNetV3) was combined with an artificial olfactory sensor (AOS) to address the redundancy in olfactory arrays, thereby achieving low-cost and high-precision detection of mycotoxin-contaminated maize. Specifically, volatile organic compounds of maize interacted with unoptimized AOS containing eight porphyrins and eight dye-attached nanocomposites to obtain the scent fingerprints for constructing the initial data set. The optimal decision model was MobileNetV3, with more than 98.5% classification accuracy, and its output training loss would be input into the optimizer ISCA. Remarkably, the number of olfactory arrays was reduced from 16 to 6 by ISCA-MobileNetV3 with about a 1% decrease in classification accuracy. Additionally, the developed system showed that each online evaluation was less than one second on average, demonstrating outstanding real-time performance for ensuring food safety. Therefore, AOS combined with ISCA-MobileNetV3 will encourage the development of an affordable and on-site platform for maize quality detection.

Understanding Antibiotic Detection with Fluorescence Quantum Dots: A Review

The utilization of fluorescent quantum dots (FL QDs) has gained significant traction in the realm of antibiotic detection, owing to their exceptional FL properties and versatility. Various types of QDs have been tailored to exhibit superior FL characteristics, employing diverse capping agents such as metals, surfactants, polymers, and biomass to protect and stabilize their surfaces. In their evolution, FL QDs have demonstrated both "turn-off" and "turn-on" mechanisms in response to the presence of analytes, offering promising avenues for biosensing applications. This review article provides a comprehensive overview of the recent advancements in antibiotic detection utilizing FL QDs as biosensors. It encompasses an extensive examination of different types of FL QDs, including carbon, metal, and core-shell QDs, deployed for the detection of antibiotics. Furthermore, the synthesis methods employed for the fabrication of various FL QDs are elucidated, shedding light on the diverse approaches adopted in their preparation. Moreover, this review delves into the intricate sensing mechanisms underlying FL QDs-based antibiotic detection. Various mechanisms, such as photoinduced electron transfer, electron transfer, charge transfer, Forster resonance energy transfer, static quenching, dynamic quenching, inner filter effect, hydrogen bonding, and aggregation-induced emission, are discussed in detail. These mechanisms provide a robust scientific rationale for the detection of antibiotics using FL QDs, showcasing their potential for sensitive and selective sensing applications. Finally, the review addresses current challenges and offers perspectives on the future improvement of FL QDs in sensing applications. Insights into overcoming existing limitations and harnessing emerging technologies are provided, charting a course for the continued advancement of FL QDs-based biosensing platforms in the field of antibiotic detection.

A novel core-shell up-conversion nanoparticles immunochromatographic assay for the detection of deoxynivalenol in cereals

Deoxynivalenol (DON) toxin is a type B group of trichothecene mycotoxins mainly originating from specific Fusarium fungi, seriously harming human and livestock health. Herein, a novel core-shell up-conversion nanoparticles immunochromatographic assay (CS-UCNPs-ICA) was developed for deoxynivalenol based on the competitive reaction principle. By exploiting the fluorescence intensity of the T and C lines of CS-UCNPs-ICA, the concentrations of DON were obtained sensitively and precisely under optimized conditions in 5 min with a detection limit of 0.1 ng/mL. The CS-UCNPs-ICA strips only specifically detect DON and its derivatives (3-Ac-DON and 15-Ac-DON), with no cross-reaction with other mycotoxins. The low CV values illustrated a modest intra- and inter-assay variation, confirming the superior precision of this method. In the spiked experiment, the mean recoveries of corn and wheat ranged from 94.74% to 100.90% and 96.21%-104.81%, respectively. Furthermore, the approach generated results that were in good agreement with data from HPLC and ELISA analyses of naturally contaminated feed and cereals, confirming that the significant advantages of proposed strips were their high practicality, rapidness, and simplicity. Therefore, the CS-UCNPs-ICA strips platform serves as a promising candidate for developing new approaches for rapid testing or high throughput screening from DON in food products.

Development of a fluorescent multiplexed lateral flow immunoassay for the simultaneous detection of crustacean allergen tropomyosin, sarcoplasmic calcium binding protein and egg allergen ovalbumin in different matrices and commercial foods

A quantum dot (QD) based multiplexed lateral flow immunoassay (xLFIA) for the simultaneous detection of egg allergen ovalbumin, crustacean allergen tropomyosin (TM) and sarcoplasmic calcium binding protein (SCP) was developed in this study. QD-labeled rabbit anti-ovalbumin, SCP and TM antibodies were applied as fluorescent detection probes. The chromatography system was optimized to reduce the mutual interference of different test lines. Visual and instrumental detection limits of the xLFIA were 0.1 and 0.05 μg/mL for SCP, both 0.05 μg/mL for ovalbumin and both 0.5 μg/mL for TM. As low as 0.10 % crab powder, 0.01 % egg white powder and 0.05 % shrimp powder could be detected in all three model foods using xLFIA. Besides, the xLFIA detection results of 23 of 28 commercial foods were consistent with ingredient labels. These findings indicate that the developed xLFIA is a practical tool for point-of-care detection of egg and crustacean allergens in processed and commercial foods.

Improvement of the sensitivity of lateral flow systems for detecting mycotoxins: Up-to-date strategies and future perspectives

Mycotoxins are dangerous human and animal health-threatening secondary fungal metabolites that can be found in various food and agricultural products. Several countries have established regulations to restrict their presence in food and agricultural products destined for human and animal consumption. Consequently, the need to develop highly sensitive and smart detection systems was recognized worldwide. Lateral flow assay possesses the advantages of easy operation, rapidity, stability, accuracy, and specificity, and it plays an important role in the detection of mycotoxins. Nevertheless, strategies to comprehensively improve the sensitivity of lateral flow assay to mycotoxins in food have rarely been highlighted and discussed. In this article, a comprehensive overview was presented on the application of lateral flow assay in mycotoxin detection in food samples by highlighting the principle of lateral flow assay, presenting a detailed discussion on various analytical performance-improvement strategies, such as the development of high-affinity recognition reagents, immunogen immobilization methods, and signal amplification. Additionally, a detailed discussion on the various signal analyzers and interpretation approaches was provided. Finally, current hurdles and future perspectives on the application of lateral flow assay in the detection of mycotoxins were discussed.

High Bioaffinity Controllable Assembly Nanocarrier UiO-66-NH2@Quantum Dot-Based Immunochromatographic Assay for Simultaneous Detection of Five Mycotoxins in Cereals and Feed

Herein, the UiO-66-NH2@quantum dot (NU66@QD) was synthesized with excellent fluorescence intensity and biocompatibility, which was used to develop a multiple immunochromatographic assay (ICA) for the detection of aflatoxin B1 (AFB1), fumonisin B1 (FB1), deoxynivalenol (DON), T-2 toxins (T-2), and zearalenone (ZEN) in cereals and feed. Five monoclonal antibodies and NU66@QD were efficiently labeled by a one-step mixed method to form a multiple detection probe. The limits of detection of the proposed NU66@QD-ICA for AFB1/FB1/DON/T-2/ZEN were 0.04/0.28/0.25/0.09/0.08 μg/kg. The recoveries ranged from 82.83-117.44%, with the coefficient of variation from 2.88-11.80%. A parallel analysis in 35 naturally contaminated cereal and feed samples was confirmed by LC-MS/MS, and the results showed a good correlation (R2 > 0.9), indicating the practical reliability of the multiple NU66@QD-ICA. Overall, the introduction of the novel nanomaterial NU66@QD provides a highly sensitive and efficient multiplex detection strategy for the development of ICA.

Recent Advances in Quantum Dot-Based Lateral Flow Immunoassays for the Rapid, Point-of-Care Diagnosis of COVID-19

The COVID-19 pandemic has spurred demand for efficient and rapid diagnostic tools that can be deployed at point of care to quickly identify infected individuals. Existing detection methods are time consuming and they lack sensitivity. Point-of-care testing (POCT) has emerged as a promising alternative due to its user-friendliness, rapidity, and high specificity and sensitivity. Such tests can be conveniently conducted at the patient's bedside. Immunodiagnostic methods that offer the rapid identification of positive cases are urgently required. Quantum dots (QDs), known for their multimodal properties, have shown potential in terms of combating or inhibiting the COVID-19 virus. When coupled with specific antibodies, QDs enable the highly sensitive detection of viral antigens in patient samples. Conventional lateral flow immunoassays (LFAs) have been widely used for diagnostic testing due to their simplicity, low cost, and portability. However, they often lack the sensitivity required to accurately detect low viral loads. Quantum dot (QD)-based lateral flow immunoassays have emerged as a promising alternative, offering significant advancements in sensitivity and specificity. Moreover, the lateral flow immunoassay (LFIA) method, which fulfils POCT standards, has gained popularity in diagnosing COVID-19. This review focuses on recent advancements in QD-based LFIA for rapid POCT COVID-19 diagnosis. Strategies to enhance sensitivity using QDs are explored, and the underlying principles of LFIA are elucidated. The benefits of using the QD-based LFIA as a POCT method are highlighted, and its published performance in COVID-19 diagnostics is examined. Overall, the integration of quantum dots with LFIA holds immense promise in terms of revolutionizing COVID-19 detection, treatment, and prevention, offering a convenient and effective approach to combat the pandemic.

T-2 and HT-2 Toxins: Toxicity, Occurrence and Analysis: A Review

One of the major classes of mycotoxins posing serious hazards to humans and animals and potentially causing severe economic impact to the cereal industry are the trichothecenes, produced by many fungal genera. As such, indicative limits for the sum of T-2 and HT-2 were introduced in the European Union in 2013 and discussions are ongoing as to the establishment of maximum levels. This review provides a concise assessment of the existing understanding concerning the toxicological effects of T-2 and HT-2 in humans and animals, their biosynthetic pathways, occurrence, impact of climate change on their production and an evaluation of the analytical methods applied to their detection. This study highlights that the ecology of F. sporotrichioides and F. langsethiae as well as the influence of interacting environmental factors on their growth and activation of biosynthetic genes are still not fully understood. Predictive models of Fusarium growth and subsequent mycotoxin production would be beneficial in predicting the risk of contamination and thus aid early mitigation. With the likelihood of regulatory maximum limits being introduced, increased surveillance using rapid, on-site tests in addition to confirmatory methods will be required. allowing the industry to be proactive rather than reactive.

QDs-based fluorescent lateral flow assays for Point-of-care testing of insulin

Point-of-care testing (POCT) can be performed near the site of the patient to achieve results in a few minutes. Different POCT devices are available in the market, such as microfluidic chips and paper-based lateral flow assays (LFAs). The paper-based LFAs have certain advantages, such as being cheap and disposable, able to detect a wide range of biomolecules, and the fluid flows through them via capillary action eliminating the need for external forces. The LFAs can be optimized for the sensitive and rapid detection of biomolecules. In this study, paper-based fluorescent LFAs platforms using aptamers as the biorecognition molecules were developed for the POCT of insulin. Various parameters were optimized such as concentrations of aptamers, the type of reporter molecules, the volume of sample, and the assay time to quantify insulin levels using a standard LFA reader. The fluorescent LFAs exhibited a linear detection range of 0.1-4 ng.mL-1 with a limit of detection (LOD) 0.1 ng.mL-1. The developed LFAs will help to achieve insulin measurement in a few minutes and will be easy to perform by end-users without the requirement of sophisticated instruments, laboratory set-up, and trained personnel. The developed device will be useful for the measurement of insulin levels in biological samples without the need for pretreatment, reducing the overall cost and time of testing. Moreover, the POCT device were fabricated using paper which is a low-cost (approximately AUD 2 per strip) option and is disposable.Clinical Relevance- POCT monitoring of insulin can facilitate both disease diagnosis and management. The developed LFAs have the capability of rapidly testing insulin concentration within several minutes. It will benefit both patients for at-home daily insulin monitoring and clinicians for hospital rapid insulin testing.

Site-specific labeling of antibodies with quantum dots could promote to retain the antigen binding capacity of antibodies

A major concern with antibody labeling is the decreased antigen affinity binding capacity of antibodies, owing mainly to the randomly oriented binding of the marker. Herein, a universal approach for site-specific photocrosslinking of quantum dots (QDs) to the Fc-terminal of antibodies was investigated utilizing antibody Fc-terminal affinity proteins. Results showed that the QDs only bound to the heavy chain of the antibody. Further comparative tests confirmed that the site-specific directed labeling approach maximizes the retention of the antigen-binding capacity of the natural antibody. Compared with the commonly employed random orientation labeling approach, the directional labeling approach allows the labeled antibody showed 6 times greater binding affinity to antigen. QDs-labeled monoclonal antibodies were applied to fluorescent immunochromatographic test strips for the detection of shrimp tropomyosin (TM). The established procedure has a detection limit of 0.054 μg/mL. Thus, the site-specific labeling approach significantly improves the antigen binding capacity of the labeled antibody.

Recent advances in simultaneous detection strategies for multi-mycotoxins in foods

Mycotoxin contamination has become a challenge in the field of food safety testing, given the increasing emphasis on food safety in recent years. Mycotoxins are widely distributed, in heavily polluted areas. Food contamination with these toxins is difficult to prevent and control. Mycotoxins, as are small-molecule toxic metabolites produced by several species belonging to the genera Aspergillus, Fusarium, and Penicillium growing in food. They are considered teratogenic, carcinogenic, and mutagenic to humans and animals. Food systems are often simultaneously contaminated with multiple mycotoxins. Due to the additive or synergistic toxicological effects caused by the co-existence of multiple mycotoxins, their individual detection requires reliable, accurate, and high-throughput techniques. Currently available, methods for the detection of multiple mycotoxins are mainly based on chromatography, spectroscopy (colorimetry, fluorescence, and surface-enhanced Raman scattering), and electrochemistry. This review provides a comprehensive overview of advances in the multiple detection methods of mycotoxins during the recent 5 years. The principles and features of these techniques are described. The practical applications and challenges associated with assays for multiple detection methods of mycotoxins are summarized. The potential for future development and application is discussed in an effort, to provide standards of references for further research.

The evolution of multiplex detection of mycotoxins using immunoassay platform technologies

Mycotoxins present serious threats not only for public health, but also for the economy and environment. The problems become more complex and serious due to co-contamination of multiple hazardous mycotoxins in commodities and environment. To mitigate against this issue, accurate, affordable, and rapid multiplex detection methods are required. This review presents an overview of emerging rapid immuno-based multiplex methods capable of detecting mycotoxins present in agricultural products and feed ingredients published within the past five years. The scientific principles, advantages, disadvantages, and assay performance of these rapid multiplex immunoassays, including lateral flow, fluorescence polarization, chemiluminescence, surface plasmon resonance, surface enhanced Raman scattering, electrochemical sensor, and nanoarray are discussed. From the recent literature landscape, it is predicted that the future trend of the detection methods for multiple mycotoxins will rely on the advance of various sensor technologies, a variety of enhancing and reporting signals based on nanomaterials, rapid and effective sample preparation, and capacity for quantitative analysis.

Microfluidics-Based Time-Resolved Fluorescence Immunoassay for the On-Site Detection of Aflatoxins B1 Zearalenone and Deoxynivalenol in Cereals

The primary pollutants in cereal products are aflatoxins B1 (AFB1), zearalenone (ZEN), and deoxynivalenol (DON). In this study, anti-AFB1 MAb (4C9), anti-ZEN MAb (2A3), and anti-DON MAb (1F10) were developed and used in time-resolved fluorescence immunoassay microfluidics to determine AFB1, ZEN, and DON in agricultural products. The linear range for AFB1, ZEN, and DON were 0.05~2.2 μg/kg, 1.45~375.75 μg/kg, and 11.1~124.2 μg/kg, respectively. In maize, the recoveries of AFB1/ZEN/DON were 92~101%, 102~105%, and 103~108%, respectively. High-performance liquid chromatography and the proposed approach had a good correlation. Time-resolved fluorescence immunoassay microfluidics is a highly efficient and sensitive field detection method for fungal toxins in agricultural products.

Rapid field determination of SARS-CoV-2 by a colorimetric and fluorescent dual-functional lateral flow immunoassay biosensor

The rapid and accurate diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at the early stage of virus infection can effectively prevent the spread of the virus and control the epidemic. Here, a colorimetric and fluorescent dual-functional lateral flow immunoassay (LFIA) biosensor was developed for the rapid and sensitive detection of spike 1 (S1) protein of SARS-CoV-2. A novel dual-functional immune label was fabricated by coating a single-layer shell formed by mixing 20 nm Au nanoparticles (Au NPs) and quantum dots (QDs) on SiO₂ core to produce strong colorimetric and fluorescence signals and ensure good monodispersity and high stability. The colorimetric signal was used for visual detection and rapid screening of suspected SARS-CoV-2 infection on sites. The fluorescence signal was utilized for sensitive and quantitative detection of virus infection at the early stage. The detection limits of detecting S1 protein via colorimetric and fluorescence functions of the biosensor were 1 and 0.033 ng/mL, respectively. Furthermore, we evaluated the performance of the biosensor for analyzing real samples. The novel biosensor developed herein had good repeatability, specificity and accuracy, which showed great potential as a tool for rapidly detecting SARS-CoV-2.

Quantum dot microsphere-based immunochromatography test strip enabled sensitive and quantitative on-site detections for multiple mycotoxins in grains

In this work, we design a sensitive and quantitative on-site detecting solution for Aflatoxin B1 (AFB1), Ochratoxin A (OTA) and Zearalenone (ZEN) as often found in moldy grains and harmful to human health. Using quantum dot microsphere-based immunochromatography test strip, the proposed method can sensitively detect AFB1, OTA and ZEN in low detection limits of 0.01 ng/mL, 0.2 ng/mL and 0.032 ng/mL, and quantitatively measure their concentrations from 0.01 ng/mL to 1 ng/mL, from 0.2 ng/mL to 200 ng/mL and from 0.032 ng/mL to 32 ng/mL in high accuracy and good selectivity. More importantly, these multiple mycotoxin detections only relying on simple manual operations and portable handheld test strip reader can be finished on site within 45 min. Therefore, the proposed method is a promising solution supporting sensitive and quantitative on-site detections for multiple mycotoxins.

Recent advances in immunoassays and biosensors for mycotoxins detection in feedstuffs and foods

Mycotoxins are secondary metabolites produced by fungus. Many mycotoxin species are highly toxic and are frequently found in cereals and feedstuffs. So, powerful detection methods are vital and effective ways to prevent feed contamination. Traditional detection methods can no longer meet the needs of massive, real-time, simple, and fast mycotoxin monitoring. Rapid detection methods based on advanced material and sensor technology are the future trend. In this review, we highlight recent progress of mycotoxin rapid detection strategies in feedstuffs and foods, especially for simultaneous multiplex mycotoxin determination. Immunoassays, biosensors, and the prominent roles of nanomaterials are introduced. The principles of different types of recognition and signal transduction are explained, and the merits and pitfalls of these methods are compared. Furthermore, limitations and challenges of existing rapid sensing strategies and perspectives of future research are discussed.

One-step time-resolved fluorescence microsphere immunochromatographic test strip for quantitative and simultaneous detection of DON and ZEN

Deoxynivalenol (DON) and zearalenone (ZEN) are mycotoxins that contaminate a wide range of grains and crops. In this study, a one-step time-resolved single-channel immunochromatographic test strip based on europium ion polystyrene fluorescence microspheres was first developed for sensitive and quantitative detection of DON and ZEN. The concentration of the artificial antigen and the mass ratio of the monoclonal antibody to fluorescent microspheres for conjugation were optimized to simplify the sample addition process during immunochromatographic assay and improve the on-site detection efficiency. The limits of detection (LOD) of the single-channel immunochromatographic test strip for DON and ZEN detection were 0.17 and 0.54 μg/L, respectively. Meanwhile, the dual-channel immunochromatographic test strip was designed to simultaneously detect DON and ZEN, with LODs of 0.24 and 0.69 μg/L achieved for DON and ZEN, respectively. The developed test strips also yielded recovery results consistent with that obtained by LC-MS/MS for DON and ZEN detection in real samples of wheat and corn flour, confirming the practicability and reliability of the test strip. The developed immunochromatographic test strips realize quick and sensitive detection of DON and ZEN, exhibiting potential for broad applications in the point-of-care testing platform of multiple mycotoxins in agricultural products. Graphic abstract.

Development of a fluorescent immunochromatographic assay based on quantum dots for the detection of fleroxacin

Fleroxacin (FLE) is a broad-spectrum fluoroquinolone antibiotic widely used in animal husbandry, veterinary medicine and aquaculture. Eating animal-derived foods with FLE residues can cause allergies, poisoning or drug resistance. The water-soluble QDs (CdSe/ZnS) and anti-FLE monoclonal antibody (mAb) were used to prepare a fluorescent probe by the method of N-(3-dimethylaminopropyl)-N'-ethylcarbodimide hydrochloride (EDC) activation. The fluorescent probe was characterized by dynamic light scattering (DLS). The better bioactivity and stability of the fluorescent probe was obtained under the pH value of 8.0, the molecule molar ratio of EDC (1 : 2000) and anti-FLE monoclonal antibodies (1 : 10). The control line (C line) and test line (T line) of a nitrocellulose (NC) filter membrane were sprayed with SPA (0.05 mg mL-1) and FLE-OVA (1.4 mg mL-1) solutions with optimal concentration, respectively. A novel method of fluorescent immunochromatographic assay based on quantum dots (QDs-ICA) in this work exhibited good accuracy, reproductivity and excellent specificity under the optimal experimental conditions. Compared with the traditional method for the visual detection of FLE, the developed QDs-ICA can successfully determine FLE residues in pork meat with a better cut-off value of 2.5 ng mL-1. The QDs-ICA could be adapted for the rapid preliminary detection of FLE residues in pork meat for the first time.

Development and Optimization of a Rapid Colorimetric Membrane Immunoassay for Porphyromonas gingivalis

Porphyromonas gingivalis (P. gingivalis) is a major bacterial pathogen that causes periodontitis, a chronic inflammatory disease of tissues around the teeth. Periodontitis is known to be related to other diseases, such as oral cancer, Alzheimer's disease, and rheumatism. Thus, a precise and sensitive test to detect P. gingivalis is necessary for the early diagnosis of periodontitis. The objective of this study was to optimize a rapid visual detection system for P. gingivalis. First, we performed a visual membrane immunoassay using 3,3',5,5'-tetramethylbenzidine (TMB; blue) and coating and detection antibodies that could bind to the host laboratory strain, ATCC 33277. Antibodies against the P. gingivalis surface adhesion molecules RgpB (arginine proteinase) and Kgp (lysine proteinase) were determined to be the most specific coating and detection antibodies, respectively. Using these two selected antibodies, the streptavidin-horseradish peroxidase (HRP) reaction was performed using a nitrocellulose membrane and visualized with a detection range of 10³-10⁵ bacterial cells/ml following incubation for 15 min. These selected conditions were applied to test other oral bacteria, and the results showed that P. gingivalis could be detected without crossreactivity to other bacteria, including Streptococcus mutans and Escherichia fergusonii. Furthermore, three clinical strains of P. gingivalis, KCOM 2880, KCOM 2803, and KCOM 3190, were also recognized using this optimized enzyme immunoassay (EIA) system. To conclude, we established optimized conditions for P. gingivalis detection with specificity, accuracy, and sensitivity. These results could be utilized to manufacture economical and rapid detection kits for P. gingivalis.

Immunoassays for rapid mycotoxin detection: state of the art

The widespread presence of mycotoxins in nature not only poses a huge health risk to people in terms of food but also causes incalculable losses to the agricultural economy. As a rapidly developing technology in recent years, the mycotoxin immunoassay technology has approached or even surpassed the traditional chromatography technology in some aspects. Using this approach, the lateral flow immunoassay (LFIA) has attracted the interest of researchers due to its user-friendly operation, short time consumption, little interference, low cost, and ability to process a large number of samples at the same time. This paper provides an overview of the immunogens commonly used for mycotoxins, the development of antibodies, and the use of gold nanoparticles, quantum dots, carbon nanoparticles, enzymes, and fluorescent microsphere labeling materials for the construction of LFIAs to improve detection sensitivity. The analytical performance, detection substrates, detection limits or detection ranges of LFIA for mycotoxins have been listed in recent years. Finally, we describe the future outlook for the field, predicting that portable mobile detection devices and simultaneous quantitative detection of multiple mycotoxins is one of the important directions for future development.

Comparative Study of Gold and Carbon Nanoparticles in Nucleic Acid Lateral Flow Assay

A lateral flow assay (LFA) is a paper-based, point-of-need test designed to detect a specific analyte in complex samples in low-resource settings. Although LFA has been successfully used in different applications, its use is still limited when high sensitivity is required, especially in the diagnosis of an early-stage condition. The limit of detection (LOD) is clearly related to the signal-generating system used to achieve the visual readout, in many cases involving nanoparticles coupled to a biomolecule, which, when combined, provides sensitivity and specificity, respectively. While colloidal gold is currently the most-used label, other detection systems are being developed. Carbon nanoparticles (CNPs) demonstrate outstanding features to improve the sensitivity of this technology by producing an increased contrast in the paper background. Based on the necessity of sensitivity improvement, the aim of this work is a comparative study, in terms of analytical performance, between commercial streptavidin gold nanoparticles (streptAv-AuNPs) and avidin carbon nanoparticles (Av-CNPs) in a nucleic acid lateral flow assay. The visual LOD of the method was calculated by serial dilution of the DNA template, ranging from 0.0 to 7 pg μL-1/1.5 × 104 CFU mL-1). The LFA achieved visual detection of as low as 2.2 × 10-2 pg μL-1 using Av-CNPs and 8.4 × 10-2 pg μL-1 using streptAv-AuNPs. These LODs could be obtained without the assistance of any instrumentation. The results demonstrate that CNPs showed an increased sensitivity, achieving the nanomolar range even by visual inspection. Furthermore, CNPs are the cheapest labels, and the suspensions are very stable and easy to modify.

Application of Multiplexed Aptasensors in Food Contaminants Detection

The existence of contaminants in food poses a serious threat to human health. In recent years, aptamer sensors (aptasensors) have been developed rapidly for the detection of food contaminants because of their high specificity, design flexibility, and high efficiency. However, the development of high-throughput, highly sensitive, on-site, and cost-effective methods for simultaneous detection of food contaminants is still restricted due to multiple signal overlap or mutual interference and cross-reaction between different analytes with similar molecular structures. To overcome these problems, this Review summarizes some effective strategies from the articles published in recent years about multiplexed aptasensors for the simultaneous detection of food contaminants. This work focuses on the application of multiplexed aptasensors to simultaneously detect antibiotics, pathogens, and mycotoxins in food. These aptasensors mainly contain fluorescent aptasensors, electrochemical aptasensors, surface-enhanced Raman scattering-based aptasensors, microfluidic chip aptasensors, and paper-based multiplexed aptasensors. In addition, this Review also covers the application of nucleic acid cycle amplification and nanomaterial amplification strategies to improve the detection sensitivity. Finally, the limitations and challenges in the design of multiplexed aptasensor are also taken into account.

Multiplex surface-enhanced Raman scattering detection of deoxynivalenol and ochratoxin A with a linear polymer affinity agent

A linear, methacrylamide polymer affinity agent was explored to capture two mycotoxins, deoxynivalenol (DON) and ochratoxin A (OTA), for multiplex surface-enhanced Raman scattering (SERS) detection. These mycotoxins are naturally occurring small molecules from fungi that can be dangerous at low concentrations. SERS detection was completed for each polymer-toxin complex at concentrations relevant to current safety regulation by the FDA: 1 ppm for DON and 5 ppb for OTA. Visibly distinguishable vibrational modes were observed in the multiplex spectra that were attributed to each mycotoxin individually, thus, not requiring any additional chemometric analysis. Density functional theory (DFT) was used to model DON and OTA to accurately label the vibrational modes in the experimental spectra as well as provide insight on the binding between both targets and the affinity agent. Fully modeled vibrations of these toxins are novel contributions due to OTA never being modeled and only a few published vibrational modes of DON. DFT guides empirical observations regarding hydrogen bonding at multiple sites of each mycotoxin target molecule through the amine groups on the polymer, confirming the capabilities of a single polymer affinity agent to facilitate multiplex detection of a class of molecules through less-specific interactions than traditional affinity agents.

Ultrasensitive and Robust Point-of-Care Immunoassay for the Detection of Plasmodium falciparum Malaria

Plasmodium falciparum malaria is widespread in the tropical and subtropical regions of the world. There is ongoing effort to eliminate malaria from endemic regions, and sensitive point-of-care (POC) diagnostic tests are required to support this effort. However, current POC tests are not sufficiently sensitive to detect P. falciparum in asymptomatic individuals. After extensive optimization, we have developed a highly sensitive and robust POC test for the detection of P. falciparum infection. The test is based on upconverting nanophosphor-based lateral flow (UCNP-LF) immunoassay. The developed UCNP-LF test was validated using whole blood reference panels containing samples at different parasite densities covering eight strains of P. falciparum from different geographical areas. The limit of detection was compared to a WHO-prequalified rapid diagnostic test (RDT). The UCNP-LF achieved a detection limit of 0.2-2 parasites/μL, depending on the strain, which is 50- to 250-fold improvement in analytical sensitivity over the conventional RDTs. The developed UCNP-LF is highly stable even at 40 °C for at least 5 months. The extensively optimized UCNP-LF assay is as simple as the conventional malaria RDTs and requires 5 μL of whole blood as sample. Results can be read after 20 min from sample addition, with a simple photoluminescence reader. In the absence of a reader device at the testing site, the strips after running the test can be transported and read at a central location with access to a reader. We have found that the test and control line signals are stable for at least 10 months after running the test. The UCNP-LF has potential for diagnostic testing of both symptomatic and asymptomatic individuals.

A competitive immunoassay based on engineered magnetic/fluorescent nanoparticles and biolayer interferometry-based assay for T-2 toxin determination

For the first time a competitive immunoassay was developed by employing T-2 antibody-functionalized magnetite nanoparticles and T-2 toxin-conjugated fluorescent quantum dots (QDs). Free T-2 and the T-2-modified QDs compete for binding to antibody-modified magnetic beads; the magnetic beads collected by magnetic separation were subjected to fluorescence intensity analysis (with excitation/emission wavelengths at 460/616 nm). This competitive immunoassay for T-2 toxin determination was applied both in a microcentrifuge tube and on a 96-well plate. The dynamic range of the immunoassay is 1-100 ng mL^-1, the limit of detection (LOD) is 0.1 ng mL^-1, and determination was completed in about 40 min and 30 min in the microcentrifuge tube and 96-well plate, respectively. Moreover, the biolayer interferometry (BLI) technique was employed for T-2 determination for the first time, in which the conjugate of T-2 toxin and bovine serum albumin (BSA) was immobilized on the sensors before detection. Its average recovery of T-2 toxin from barley sample ranged from 82.00 to 123.33%, and the relative standard deviation (RSD) was between 9.42 and 15.73%. The LOD of the BLI-based assay is 5 ng mL^-1, and it only takes 10 min to finish the determination. Graphical abstract.

Simultaneous detection of zearalenone, citrinin, and ochratoxin A in pepper by capillary zone electrophoresis

In the present study, a simple and fast method for simultaneous detection of zearalenone, citrinin, and ochratoxin A utilising capillary zone electrophoresis with an ultraviolet detector was developed. The optimised approach was validated and applied using pepper samples. The proposed method yielded satisfactory linearity between the signal and the mycotoxin concentration in the range of 1.5-150 μg/kg for zearalenone, 4.5-150 μg/kg for citrinin, and 0.8-150 μg/kg for ochratoxin A. The limits of detection for these mycotoxins ranged from 0.3 to 1.5 μg/kg. The corresponding intra- and inter-day precisions were less than 3.5 % and 4.1 %, respectively. Moreover, the matrix effect was also assessed and the result was compared using the capillary zone electrophoresis and high-performance liquid chromatography methods. The developed approach could be used for simultaneous detection of zearalenone, citrinin, and ochratoxin A in pepper.

Sensitive Immunochromatographic Assay Using Highly Luminescent Quantum Dot Nanobeads as Tracer for the Detection of Cyproheptadine Hydrochloride in Animal-Derived Food

Cyproheptadine hydrochloride (CYP), used as human and veterinary drug, has been used illegally as feed additive for food-producing animals, which could remain in food and jeopardize human health. There is a need for on-site detection of CYP residue in animal-derived food. In this study, a hapten was designed, and a specific monoclonal antibody (mAb) was developed to detect CYP with an IC50 of 1.38 ng/mL and negligible cross-reactivity (CR) for other analogs. Forthermore, a high sensitive immunochromatographic assay (QBs-ICA) was developed using quantum dot nanobeads as reporters. The assay showed the linear detection range (IC20-IC80) of 0.03-0.52 ng/mL, the limit of detection (LOD) and visual detection limit (VDL) reached to 0.01 and 0.625 ng/mL, respectively. Spiked recovery study in pig urine and pork confirmed that the QBs-ICA was applicable for on-site testing. This assay showed better sensitivity and speedy than the reported instrumental analysis and immunoassays.

Comparative Study of Time-Resolved Fluorescent Nanobeads, Quantum Dot Nanobeads and Quantum Dots as Labels in Fluorescence Immunochromatography for Detection of Aflatoxin B1 in Grains

Label selection is an essential procedure for improving the sensitivity of fluorescence immunochromatography assays (FICAs). Under optimum conditions, time-resolved fluorescent nanobeads (TRFN), quantum dots nanobeads (QB) and quantum dots (QD)-based immunochromatography assays (TRFN-FICA, QB-FICA and QD-FICA) were systematically and comprehensively compared for the quantitative detection of aflatoxin B₁ (AFB₁) in six grains (corn, soybeans, sorghum, wheat, rice and oat). All three FICAs can be applied as rapid, cost-effective and convenient qualitative tools for onsite screening of AFB₁; TRFN-FICA exhibits the best performance with the least immune reagent consumption, shortest immunoassay duration and lowest limit of detection (LOD). The LODs for TRFN-FICA, QB-FICA and QD-FICA are 0.04, 0.30 and 0.80 μg kg⁻¹ in six grains, respectively. Recoveries range from 83.64% to 125.61% at fortified concentrations of LOD, 2LOD and 4LOD, with the coefficient of variation less than 10.0%. Analysis of 60 field grain samples by three FICAs is in accordance with that of LC-MS/MS, and TRFN-FICA obtained the best fit. In conclusion, TRFN-FICA is more suitable for quantitative detection of AFB₁ in grains when the above factors are taken into consideration.

A multicolor multiplex lateral flow assay for high-sensitivity analyte detection using persistent luminescent nanophosphors

Incorporating two persistent luminescent nanophosphors (PLNPs), green-emitting SrAl2O4:Eu2+,Dy3+ (SAO) and blue-emitting (Sr0.625Ba0.375)2MgSi2O7:Eu2+,Dy3+ (SBMSO), in a single lateral flow assay (LFA) establishes a luminescence-based, multiplex point-of-need test capable of simultaneously detecting two different analytes in a single sample. The advantages of this system are the high sensitivity and photostability of PLNPs, while only requiring access to minimal hardware and a smartphone for signal detection. The PLNPs were obtained by first wet milling bulk synthesized phosphor powders, followed by fractionation using differential centrifugal sedimentation to obtain monodisperse nanoparticles. A modified Stöber process was then employed to encapsulate the nanoparticles in a water-stable silica shell followed by attaching antibodies to the particles' surfaces using reductive amination chemistry. The resulting PLNPs were incorporated in an LFA to concurrently detect two independent model analytes, prostate-specific antigen (PSA) and human chorionic gonadotropin (hCG). The multicolor-multiplex PLNP-based assays were finally imaged using a smartphone-based imaging system with excellent detection limits (0.1 ng mL-1 of PSA and 1 ng mL-1 of hCG) that are competitive with commercially available LFAs.

Ultrasensitive Detection of Severe Fever with Thrombocytopenia Syndrome Virus Based on Immunofluorescent Carbon Dots/SiO2 Nanosphere-Based Lateral Flow Assay

Sensitive detection of severe fever with thrombocytopenia syndrome virus (SFTSV) by a point-of-care assay is of great significance for promoting clinical diagnosis. In this work, ultrasensitive detection of SFTSV was achieved by using fluorescent carbon dots/SiO2 nanospheres (CSNs) as reporters for a lateral flow assay. The prepared CSNs were resistant to extreme environments and had strong stability. The uniform CSNs with the size of about 200 nm were obtained by differential centrifugation. Their absolute quantum yields in the aqueous and solid phases are 56.3 and 36.6%, respectively. The excellent fluorescent properties of CSNs make the test strips more sensitive and have a longer assay lifetime. Thus, the visual detection limit of the lateral flow test strip based on immunofluorescent CSN (iCSN) was as low as 10 pg/mL SFTSV nucleoprotein. The sensitivity of this assay is 2 orders of magnitude higher than that of the colloidal gold-based lateral flow test strip. Besides, the assay owns good reproducibility and high specificity. Then, iCSN-based lateral flow test strips were evaluated in real samples of human serum of patients with satisfactory results. Furthermore, this assay has a general prospect for other fluorescent immunochromatography applications.

Fluorometric lateral flow immunoassay for simultaneous determination of three mycotoxins (aflatoxin B1, zearalenone and deoxynivalenol) using quantum dot microbeads

A fluorometric lateral flow immunoassay (LFA) is described for the simultaneous determination of the mycotoxins aflatoxin B₁ (AFB₁), zearalenone (ZEN) and deoxynivalenol (DON). The method is based on the use of CdSe/SiO₂ quantum dot microbeads (QBs) with a mean diameter of 106 nm. These have strong red luminescence (with excitation/emission peaks at 365/622 nm) which results in enhanced sensitivity. The QBs binding with monoclonal antibodies (mAbs) as the signal probes can react specifically with AFB₁, ZEN and DON, respectively. There is an inverse correlation between the fluorescence signal intensity of test line and the analyte content, which can realize the quantitative analysis of analytes within 15 min. The limits of detection in solution are 10, 80 and 500 pg mL^-1 for AFB₁, ZEN and DON, respectively. Besides, the average recoveries from spiked feed range from 85.5 to 119.0%, and the relative standard deviations are less than 16.4% for both intra- and inter-day assays. The method was used to analyze naturally contaminated feedstuff, and this resulted in a good agreement with data obtained by LC-MS/MS. Graphical abstractSchematic representation of a fluorometric method for the simultaneous determination of three mycotoxins. Quantum dot microbeads (QBs) binding with monoclonal antibodies (mAbs) are signal probes. There is an inverse correlation between the fluorescence intensity of test line and the analyte concentration.

Lateral Flow Immunoassay Based on Polydopamine-Coated Gold Nanoparticles for the Sensitive Detection of Zearalenone in Maize

In this work, polydopamine-coated gold nanoparticles (Au@PDAs) were synthesized by the oxidative self-polymerization of dopamine (DA) on the surface of AuNPs and applied for the first time as a signal-amplification label in lateral flow immunoassays (LFIAs) for the sensitive detection of zearalenone (ZEN) in maize. The PDA layer functioned as a linker between AuNPs and anti-ZEN monoclonal antibody (mAb) to form a probe (Au@PDA-mAb). Compared with AuNPs, Au@PDA had excellent color intensity, colloidal stability, and mAb coupling efficiency. The limit of detection of the Au@PDA-based LFIA (Au@PDA-LFIA) was 7.4 pg/mL, which was 10-fold lower than that of the traditional AuNP-based LFIA (AuNP-LFIA) (76.1 pg/mL). The recoveries of Au@PDA-LFIA were 93.80-111.82%, with the coefficient of variation of 1.08-9.04%. In addition, the reliability of Au@PDA-LFIA was further confirmed by the high-performance liquid chromatography method. Overall, our study showed that PDA coating can chemically modify the surface of AuNPs through a simple method and can thus significantly improve the detection sensitivity of LFIA.

Portable Multiplex Immunochromatographic Assay for Quantitation of Two Typical Algae Toxins Based on Dual-Color Fluorescence Microspheres

A multiplex immunochromatographic assay (ICA) based on dual-color fluorescent microspheres (FMs) as a sensitive label was developed for the first time. Two typical algae toxins, microcystin-LR (MC-LR) and okadaic acid (OA), were chosen as proof-of concept targets to evaluate the feasibility of this ICA format. Commercial red- and green-colored FMs were selected to couple with monoclonal antibodies as fluorescent probes. The use of dual-wavelength FMs as labels guaranteed a lower consumption of material strips, lower sample volume, and shorter reaction time without increasing the length of ICA strips. Under optimal conditions, the multiplex FM-ICA could be completed in 20 min and reached limits of detection for the simultaneous determination of MC-LR and OA in fish samples, which were 0.074 and 2.42 μg/kg, respectively. The developed technique was validated using artificially spiked and naturally contaminated fish samples. Ultra-high-performance liquid chromatography-tandem mass spectrometry was used as confirmatory technique. In summary, this portable ICAs detection mode based on dual-wavelength FMs provided a reliable and sensitive on-site detection of multiple contaminants in food samples, which opens a new field for application of FMs in food safety.

Detection of okadaic acid (OA) and tetrodotoxin (TTX) simultaneously in seafood samples using colloidal gold immunoassay

Tetrodotoxin (TTX) is a neurotoxin mainly responsible for severe neurological illness, and okadaic acid (OA) is another important lipophilic toxin to humans. In this study, we developed a gold strip for simultaneous detection of OA and TTX in real seafood samples. In the assay, the prepared nanoparticles (about 40 nm) was applied to conjugate with specific monoclonal antibodies against OA and TTX, and the resulted mixtures were used to capture its corresponding toxin in test strip. OA and TTX conjugates were coated as two test lines on the nitrocellulose membrane, and goat anti-mouse IgG was used to form the control line, forming three lines on the test strip. The visual detection limits (vLOD) of this immunoassay for OA and TTX were 0.75 and 15 ng/mL, respectively, and no cross reactions were observed in the process of detection. The visual assay for OA and TTX detection could be finished within 10 min. This study might provide a feasible method and good understanding for rapidly simultaneous detection for toxins based on immunoassay.