Gold nanorods etching-based plasmonic immunoassay for qualitative and quantitative detection of aflatoxin M1 in milk

Gold nanocubes etching enhanced light scattering immunoassay for highly sensitive detection of Staphylococcus aureus enterotoxin A

An innovative light scattering immunoassay was developed using an AuNPs etching strategy. Three types of anisotropic gold nanoparticles, including gold nanocubes, nanorods, and nanoflowers with distinct morphologies, were utilized to investigate how these morphological differences affect the sensitivity of light scattering signal transduction. Based on theoretical insights into light scattering and electromagnetic fields, gold nanocubes were identified as the optimal probes for enhancing light scattering signal transduction and were employed to construct an immunoassay for detecting staphylococcal enterotoxin A (SEA). The developed immunoassay achieved ultrahigh sensitivity for SEA detection in milk samples, with a detection limit of 10.39 pg mL-1, which is 190 times lower than that of conventional ELISA. The proposed immunoassay was validated across ten food samples, demonstrating high accuracy and robustness. Given these promising results, we believe this method has significant potential for screening trace levels of SEA in food products.

Integrated gold nanorods-based dual-signal platform for accurate total antioxidant capacity assessment in food samples

Accurate assessment of Total Antioxidant Capacity (TAC) in food is crucial for evaluating nutritional quality and potential health benefits. This study aims to enhance the sensitivity and reliability of TAC detection through a dual-signal method, combining colorimetric and photothermal signals. Gold nanorods (AuNRs) were utilized to establish a dual-signal method duo to the colorimetric and photothermal properties. Fenton reaction can etch the AuNRs from the tips, as a result, a blue shift in the longitudinal LSPR absorption peak was obtained, leading to significant changes in color and photothermal effects, facilitating discrimination through both visual observation and thermometer measurements. In the presence of antioxidants, the Fenton reaction was suppressed or inhibited, protecting the AuNRs from etching. The colorimetric and photothermal signals were therefore positively correlated with TAC levels, enabling dual-signal detection of TAC. The linear range of AA was 4-100 μM in both colorimetry and photothermal modes, with detection limits of 1.60 μM and 1.38 μM, respectively. This dual-signal approach achieves low detection limits, enhancing precision and sensitivity. The method thus has the potential to act as a promising candidate for TAC detection in food samples, contributing to improved food quality and safety assessment.

A Clickase-Mediated Immunoassay Based on Nanopore and Bionic Signal Labels for Ultrasensitive, Portable, and On-Site Detection of Ricin

It is of particular importance to develop an effective method that possesses several merits simultaneously of rapid, ultrasensitive, portable, and on-site detection potential for food safety detection. Herein, we propose a clickase-mediated immunoassay based on nanopore and bionic signal labels for the detection of ricin. The introduction of Cu/Cys clickase and nanopore simultaneously effectively addressed the inherent limitations of natural enzymes and colorimetric signal output, respectively. Using this method, bionic signal labels can be easily formed through DNA and Gram-positive bacterial cell wall terminal peptide fragments (labeled by alkynyl and azide, respectively) and vancomycin. Translocation of the D-P@vancomycin through the nanopore generated highly specific oscillation current traces. This method showed a great on-site detection potential and superior analytical performance owing to the combination of the specificity of antibodies, high CuAAC click reaction catalytic efficiency of clickase, ultrasensitivity of the nanopore, and high signal resolution of D-P@vancomycin. Moreover, the practical applicability of the established method was also verified, achieving a limit of detection (LOD) down to 200.9 ag/mL with a wide linear relationship under the optimized conditions. In conclusion, this method is promising for rapid, portable, ultrasensitive, and on-site food safety detection.

Dramatic fluorescence enhancement of PCN-224 and its application in "turn off" immunoassay for sensitive detection of E. coli O157:H7 in milk

In this study, a type of luminescent porous coordination network-224 (PCN-224) in alkaline conditions was synthesized with the dramatic fluorescence enhancement by 20.4 times, which was explained by the fact that the decrease of Zr4+ content in alkaline conditions resulted in the partial recovery of the electron cloud density of 4,4',4'',4'''-(Porphine-5,10,15,20-tetrayl) tetrakis(benzoic acid) (TCPP). Given the large overlap between the excitation spectrum of PCN-224 and the absorption band of Ag nanoparticles (Ag NPs), the coating of the Ag layer on PCN-224 triggered the fluorescence quenching effect, which was applied to "turn off" fluorescence immunoassay for sensitive detection of Escherichia coli O157:H7 (E. coli O157:H7) in milk. The proposed immunoassay reached a low limit of detection (LOD) of 3.3 × 102 CFU mL-1, 29.7 times more sensitive than the conventional ELISA. It will provide a novel alternative strategy for sensitively detecting pathogenic bacteria in the field of food safety.

Fiber Optic LSPR Sensing AFM1 in Milk with Enhanced Sensitivity by the Hot Spot Effect Based on Nanogap Construction

The detection of the amount of aflatoxin M1 (AFM1) in milk is crucial for food safety. Here, we utilize a fiber optic (FO) localized surface plasmon resonance (LSPR) biosensor by constructing gold nanoparticle (AuNP) multimers, in which the nanogaps amplified the LSPR signal by the hot spot effect, and achieved a highly sensitive detection of f AFM1. Through the optimization of parameter conditions for the fabrication of the sensor and detection system, a high performance result from the FO LSPR biosensor was obtained, and the method for AFM1 detection was established, with a wide detection range of 0.05-100 ng/mL and a low limit of detection (LOD) of 0.04 ng/mL, and it has been successfully validated with the actual sample milk. Therefore, it is a good strategy to fabricate highly sensitive FO LSPR sensors for detecting AFM1 by constructing AuNP multimers, and this approach is suitable for developing other biosensors.

Plasmonic nanoparticle etching-based optical sensors: current status and future prospects

Plasmonic nanoparticles are an emerging tool for developing label-free multicolorimetric sensors for biosensing and chemosensing applications. The color absorbed by nanoparticles from visible light is influenced by their size, shape, orientation, and interparticle distance. Differently sized and shaped gold and silver nanoparticles exhibit a wide range of colors, aiding in the development of label-free sensors. Etching is the process of oxidizing nanoparticles, which alters their aspect ratio, shape, plasmonic peak, and outward appearance. It is typically used to create sensitive sensing platforms. Through etching, analytes could be detected in a simple, sensitive, and selective manner. The multicolor readout of nanoparticle etching-based multicolorimetric sensors can overcome the limitations of conventional colorimetric assays and improve the accuracy of visual inspection. This review discusses different approaches for target sensing using nanoparticle etching strategies like direct etching, enzyme-mediated etching, chemical reaction-driven etching, and anti-etching-based sensors and their mechanisms. In the future, etching strategies could be modified into portable sensing devices to detect a variety of analytes, which will aid in the development of on-time, in situ, and point-of-care sensing systems.

Overview-gold nanoparticles-based sensitive nanosensors in mycotoxins detection

Food-borne mycotoxins is one of the food safety concerns in the world. At present, nanosensors are widely used in the detection and analysis of mycotoxins due to their high specificity and sensitivity. In nanosensor-based mycotoxindetections, the sensitivity is mainly improved from two aspects. On the one hand, based on the principle of immune response, antigens and antibodies can be modified and developed. Such as single-domain heavy chain antibodies, aptamers, peptides, and antigen mimotopes. On the other hand, improvements and innovations have been made on signal amplification materials, including gold nanoparticles (AuNPs), quantum dots, and graphene, etc. Among them, gold nanoparticles can not only be used as a signal amplification material, but also can be used as carriers for identification elements, which can be used for signal amplification in detection. In this article, we systematically summarized the emerging strategies for enhancing the detection sensitivity of traditional gold nanoparticles-based nanosensors, in terms of recognition elements and signal amplification. Representative examples were selected to illustrate the potential mechanism of each strategy in enhancing the colorimetric signal intensity of AuNP and its potential application in biosensing. Finally, our review suggested the challenges and future prospects of gold particles in detection of mycotoxins.

Sensitive fluorescence ELISA with streptavidin scaffolded DNA tetrads for the detection of Escherichia coli O157:H7

Escherichia coli O157:H7 poses a threat to humans. Traditional ELISA is not a sensitive method for the detection of E. coli O157:H7. Here, an efficient method was designed for improving the load capacity of alkaline phosphatase (ALP) with streptavidin scaffolded DNA tetrad (SS-DNAt). With more ALP, more ascorbic acid 2-phosphate was catalyzed to ascorbic acid that was used to synthesize fluorescence poly adenine-thymine-templated copper nanoclusters. Based on SS-DNAt, fluorescence ELISA was successfully proposed for improving the sensitivity for detection of E. coli O157:H7 in milk samples. The method showed a linear range of 104 to 106 cfu/mL. The limit of detection of fluorescence ELISA was 3.75 × 103 cfu/mL and 6.16-fold better than that of traditional ELISA. The recovery of the fluorescence ELISA was 86.7 to 93.6% with the coefficient of variation of 5.6 to 10.5% in milk. This method could be used to detect hazardous material in food.

Plasmonic colorimetric immunosensor based on Poly-HRP and AuNS etching for tri-modal readout of small molecule

It was urgent to improve the intuitive, portable, sensitive and multi-modal detection method for small molecules. In this study, a tri-modal readout of plasmonic colorimetric immunosensor (PCIS) for small molecule (zearalenone, ZEN, as an example) had been established based on the Poly-HRP amplification and gold nanostars (AuNS) etching. The immobilized Poly-HRP from the competitive immunoassay was used to catalyze iodide (I^-) into iodine (I₂), which could prevent the AuNS etching by I^-. With the increasing of ZEN, the AuNS etching was enhanced, and the localized surface plasmon resonance (LSPR) peak of AuNS showed stronger blue shift, which resulted in the color changing from deep blue (no-etching) to blue violet (half-etching) and finally to shiny red (all-etching). The results of PCIS could be selectively obtained by the tri-modal readout: (1) naked eye (LOD of 0.10 ng/mL), (2) smartphone (LOD of 0.07 ng/mL) and (3) UV-spectrum (LOD of 0.04 ng/mL). The proposed PCIS had performed well in the sensitivity, specificity, accuracy and reliability. In addition, the harmless reagents were used in the overall process to further guarantee the environmental friendliness. Therefore, the PCIS might provide a novel and green avenue for the tri-modal readout of ZEN via the intuitive naked eye, portable smartphone and accurate UV-spectrum, which hold great potential for small molecule monitoring.

Simultaneous Dual-Sensing Platform Based on Aptamer-Functionalized DNA Hydrogels for Visual and Fluorescence Detection of Chloramphenicol and Aflatoxin M1

In this study, a chloramphenicol and aflatoxin M1 aptamer-functionalized DNA hydrogel was designed for the simultaneous detection of chloramphenicol and aflatoxin M1 for the first time. The acrydite-modified chloramphenicol aptamer sequence was used to synthesize the DNA hydrogel and for visual detection of chloramphenicol depending on the gel-to-sol transition of the target-responsive DNA hydrogel. The DNA hydrogel formulation was set as follows: 60% of each linear polyacrylamide-DNA conjugate and 40% of acrylamide and chloramphenicol aptamer/DNA strand-1 at a molar ratio of 1:1, and the lowest concentration of chloramphenicol leading to gel dissociation was 1.0 nM at 25 °C. Furthermore, the formalized aptamer-functionalized DNA hydrogel was used to detect aflatoxin M1 by measuring the recovery of the fluorescence signal that was quenched when the FAM-labeled aflatoxin M1 aptamer and BHQ1-labeled DNA strand-2 were hybridized to form a double-stranded DNA in the network of hydrogel. The detection platform was successfully applied to the detection of chloramphenicol and aflatoxin M1, both in aqueous solution and in milk. The aptamer-functionalized DNA hydrogel had detection (LOD) and quantification limits (LOQ) for aflatoxin M1 as 1.7 and 5.2 nM, respectively. Using two aptamer sequences with high affinity and specificity, the dual-sensing platform based on the DNA hydrogel achieved higher selectivity for chloramphenicol and aflatoxin M1, which demonstrated its potential as a reliable simultaneous detection platform against two different targets for monitoring food safety.

Enzyme-assisted metal nanoparticles etching based plasmonic ELISA: Progress and insights

The unique size and shape tunable localized surface plasmon resonance (LSPR) properties of the noble metal nanoparticle have been extensively exploited to realize a variety of enzyme-based optical biosensors. Although approaches like metal film deposition, nanoparticle aggregation, and synthesis & growth of metal nanoparticles are quite useful, metal nanoparticle etching-based biosensors offer greater sensitivity, selectivity, and stability against various environmental factors which makes this strategy easy to use for field applications. This review discusses the current state-of-art of plasmonic nanoparticle etching-based enzyme-linked immunosorbent assay (ELISA) realized for visual detection of various analytes. The naked eye detection, i.e. without any optical readout device, is the additional advantage of this sensing approach that reduces the analysis cost significantly making it feasible under resource-constrained settings. This review paper provides deeper insights into biocatalytic etching mechanisms of various plasmonic nanoparticles resulting in vivid color change as a function of analyte concentration. Although nanoparticle etching-based ELISA has huge potential, steps need to be taken to realize a point-of-care (POC) nanodiagnostic before its translation to a commercial technique or product that can be achieved in near future by integrating it with microfluidics technology and other technological avenues.

Application of Nanomaterials for Coping with Mycotoxin Contamination in Food Safety: From Detection to Control

Mycotoxins, which are toxic secondary metabolites produced by fungi, are harmful to humans. Mycotoxin-induced contamination has drawn attention worldwide. Consequently, the development of reliable and sensitive detection methods and high-efficiency control strategies for mycotoxins is important to safeguard food industry safety and public health. With the rapid development of nanotechnology, many novel nanomaterials that provide tremendous opportunities for greatly improving the detection and control performance of mycotoxins because of their unique properties have emerged. This review comprehensively summarizes recent trends in the application of nanomaterials for detecting mycotoxins (fluorescence, colorimetric, surface-enhanced Raman scattering, electrochemical, and point-of-care testing) and controlling mycotoxins (inhibition of fungal growth, mycotoxin absorption, and degradation). These detection methods possess the advantages of high sensitivity and selectivity, operational simplicity, and rapidity. With research attention on the control of mycotoxins and the gradual excavation of the properties of nanomaterials, nanomaterials are also employed for the inhibition of fungal growth, mycotoxin absorption, and mycotoxin degradation, and impressive controlling effects are obtained. This review is expected to provide the readers insight into this state-of-the-art area and a reference to design nanomaterials-based schemes for the detection and control of mycotoxins.

Surfactant stabilized gold nanomaterials for environmental sensing applications - A review

Surfactant stabilized Gold (Au) nanomaterials (NMs) have been documented extensively in recent years for numerous sensing applications in the academic literature. Despite the crucial role these surfactants play in the sensing applications, the comprehensive reviews that highlights the fundamentals associated with these assemblies and impact of these surfactants on the properties and sensing mechanisms are still quite scare. This review is an attempt in organizing the vast literature associated with this domain by providing critical insights into the fundamentals, preparation methodologies and sensing mechanisms of these surfactant stabilized Au NMs. For the simplification, the surfactants are divided into the typical and advanced surfactants and the Au NMs are classified into Au nanoparticles (NPs) and Au nanoclusters (NCs) depending upon the complexity in structure and size of the NMs respectively. The preparative methodologies are also elaborated for enhancing the understanding of the readers regarding such assemblies. The case studies regarding surfactant stabilized Au NMs were further divided into colorimetric sensors, surface plasmonic resonance (SPR) based sensors, luminescence-based sensors, and electrochemical/electrical sensors depending upon the property utilized by the sensor for the sensing of an analyte. Future perspectives are also discussed in detail for the researchers looking for further progress in that particular research domain.

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

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

Immunochromatographic assays for ultrasensitive and high specific determination of enrofloxacin in milk, eggs, honey, and chicken meat

Enrofloxacin, a veterinary antibiotic that persists in food, poses a risk to human health. Here, a monoclonal antibody against enrofloxacin, 1H12, was prepared based on the hapten ENR-1, and showed excellent sensitivity with a 50% inhibitory concentration (IC₅₀) of 0.03 ng/mL. Using this antibody, 2 lateral-flow immunochromatographic assays were developed for determination of enrofloxacin in egg, milk, honey, and chicken meat samples. The detection ranges (IC₂₀-IC₈₀) were 0.16-0.82 ng/g, 0.24-1.8 ng/g, 0.25-3.6 ng/g, and 0.61-3.9 ng/g by colloidal gold-immunochromatographic sensor (CG-ICS) analysis, and 0.022-0.42 ng/g, 0.054-0.42 ng/g, 0.069-1.4 ng/g, and 0.19-2.2 ng/g by Eu-fluorescence-immunochromatographic sensor (EF-ICS) analysis. The intraassay and interassay recovery rates were 88.9 to 108.5% with coefficients of variation of 1.3 to 7.0% by CG-ICS analysis, and 88.6 to 113.6% with coefficients of variation of 1.3 to 8.1% by EF-ICS analysis. Thus, our newly developed ICS are sensitive and reliable, providing an option for rapid quantitative detection of enrofloxacin in food samples.

Flexible Plasmonic Biosensors for Healthcare Monitoring: Progress and Prospects

The noble metal nanoparticle has been widely utilized as a plasmonic unit to enhance biosensors, by leveraging its electric and/or optical properties. Integrated with the "flexible" feature, it further enables opportunities in developing healthcare products in a conformal and adaptive fashion, such as wrist pulse tracers, body temperature trackers, blood glucose monitors, etc. In this work, we present a holistic review of the recent advance of flexible plasmonic biosensors for the healthcare sector. The technical spectrum broadly covers the design and selection of a flexible substrate, the process to integrate flexible and plasmonic units, the exploration of different types of flexible plasmonic biosensors to monitor human temperature, blood glucose, ions, gas, and motion indicators, as well as their applications for surface-enhanced Raman scattering (SERS) and colorimetric detections. Their fundamental working principles and structural innovations are scoped and summarized. The challenges and prospects are articulated regarding the critical importance for continued progress of flexible plasmonic biosensors to improve living quality.

A novel electrochemical aptasensor based on layer-by-layer assembly of DNA-Au@Ag conjugates for rapid detection of aflatoxin M1 in milk samples

Aflatoxin M₁ (AFM₁) is a common toxin in dairy products that causes acute and chronic human health disorders. Thus, the development of a rapid and accurate AFM₁ detection method is of vital importance for food safety monitoring. This work was to develop a novel electrochemical aptasensor for sensitive and specific determination of AFM_1. The dendritic-like nanostructure was formed on the gold electrode surface by layer-by-layer assembly of gold-silver core-shell nanoparticles modified with DNA conjugates. In the presence of AFM₁, the specific recognition between AFM₁ and Apt caused the disassociation of the DNA controlled dual Au@Ag conjugates from the surface of the electrode, causing less methylene blue to bind to the surface and weakening the electrochemical signal. The more AFM₁ there is, the weaker the electrochemical signal. Transmission electron microscope results showed that the successfully synthesized Au@Ag nanoparticles exhibited a core-shell structure with Au as core and Ag as shell, and their average diameter was about 30 nm. Under optimal conditions, the electrochemical aptasensor showed a wide detection ranging from 0.05 ng mL^-1 to 200 ng mL^-1, and a low detection limit of 0.02 ng mL^-1. Moreover, the proposed strategy has been successfully applied to the detection of AFM₁ in cow, goat, and sheep milk samples with satisfactory recoveries ranging from 91.10% to 104.05%. This work can provide a novel rapid detection method for AFM₁, and also provide a new sensing platform for the detection of other toxins.

Development of Gold Nanoparticles Decorated Molecularly Imprinted–Based Plasmonic Sensor for the Detection of Aflatoxin M1 in Milk Samples

Aflatoxins are a group of extremely toxic and carcinogenic substances generated by the mold of the genus Aspergillus that contaminate agricultural products. When dairy cows ingest aflatoxin B1 (AFB1)−contaminated feeds, it is metabolized and transformed in the liver into a carcinogenic major form of aflatoxin M1 (AFM1), which is eliminated through the milk. The detection of AFM1 in milk is very important to be able to guarantee food safety and quality. In recent years, sensors have emerged as a quick, low–cost, and reliable platform for the detection of aflatoxins. Plasmonic sensors with molecularly imprinted polymers (MIPs) can be interesting alternatives for the determination of AFM1. In this work, we designed a molecularly–imprinted–based plasmonic sensor to directly detect lower amounts of AFM1 in raw milk samples. For this purpose, we prepared gold–nanoparticle–(AuNP)−integrated polymer nanofilm on a gold plasmonic sensor chip coated with allyl mercaptan. N−methacryloyl−l−phenylalanine (MAPA) was chosen as a functional monomer. The MIP nanofilm was prepared using the light–initiated polymerization of MAPA and ethylene glycol dimethacrylate in the presence of AFM1 as a template molecule. The developed method enabled the detection of AFM1 with a detection limit of 0.4 pg/mL and demonstrated good linearity (0.0003 ng/mL–20.0 ng/mL) under optimized experimental conditions. The AFM1 determination was performed in random dairy farmer milk samples. Using the analogous mycotoxins, it was also demonstrated that the plasmonic sensor platforms were specific to the detection of AFM1.

A bivalent binding aptamer-cDNA on MoS2 nanosheets based fluorescent aptasensor for detection of aflatoxin M1

To ensure the safety of dairy products, especially milk, and consequently protect human health, accurate and simple analytical techniques are highly necessary to determine the low concentration of aflatoxin M₁ (AFM₁) as an important carcinogen. Herein, a novel, accurate and simple fluorescent aptasensor was designed for selective detection of AFM₁ based on bivalent binding aptamer-cDNA (BBA-cDNA) structure. Moreover, MoS₂ nanosheets (MoS₂ NSs) were used as the fluorescent quencher and FAM-labeled complementary strand of aptamer (FAM-CS) was applied as a fluorescent probe. In this study, we achieved a new result. Unlike previous studies, in this work, the BBA-cDNA structure was not disassembled in the presence of the target. Therefore, as the AFM1 concentration increased, more targets were attached to the BBA-cDNA structure and as a result, the BBA-cDNA structure/AFM1 could not be placed on the surface of MoS2 NSs, leading to the more fluorescent intensity detection. Under optimized conditions, the developed fluorescent analytical method revealed great selectivity toward AFM₁ with a limit of detection (LOD) of 0.5 nM and a linear range from 0.7 to 10 nM. This fabricated aptasensor indicated excellent analytical performance for AFM₁ detection in milk samples with LOD of 0.1 nM. Overall, the proposed approach could provide an effective basis for small molecule analysis to guarantee food and human safety using appropriate aptamer sequences.

LSPR-based colorimetric biosensing for food quality and safety

Ensuring consistently high quality and safety is paramount to food producers and consumers alike. Wet chemistry and microbiological methods provide accurate results, but those methods are not conducive to rapid, onsite testing needs. Hence, many efforts have focused on rapid testing for food quality and safety, including the development of various biosensors. Herein, we focus on a group of biosensors, which provide visually recognizable colorimetric signals within minutes and can be used onsite. Although there are different ways to achieve visual color-change signals, we restrict our focus on sensors that exploit the localized surface plasmon resonance (LSPR) phenomenon of metal nanoparticles, primarily gold and silver nanoparticles. The typical approach in the design of LSPR biosensors is to conjugate biorecognition ligands on the surface of metal nanoparticles and allow the ligands to specifically recognize and bind the target analyte. This ligand-target binding reaction leads to a change in color of the test sample and a concomitant shift in the ultraviolet-visual absorption peak. Various designs applying this and other signal generation schemes are reviewed with an emphasis on those applied for evaluating factors that compromise the quality and safety of food and agricultural products. The LSPR-based colorimetric biosensing platform is a promising technology for enhancing food quality and safety. Aided by the advances in nanotechnology, this sensing technique lends itself easily for further development on field-deployable platforms such as smartphones for onsite and end-user applications.

Development of enzyme-free single-step immunoassays for glycocholic acid based on palladium nanoparticle-mediated signal generation

Palladium nanoparticles (PdNPs) are composed mainly of inert noble metals, and their outstanding properties have attracted wide attention. PdNPs are not only capable of mimicking the oxidase-like characteristics of natural bio-enzymes, but they also present a clear black band in the test zone. In this work, the synthesized PdNPs promoted a transformation of colorless tetramethylbenzidine (TMB) to a blue oxidation product of TMB, providing a K_m value of 0.09 mM for TMB, and revealing the good catalytic performance of the synthesized PdNPs. For both signal generation and amplification, PdNPs effectively replaced natural bio-enzymes as a new labeling tag. Thus, the PdNP-based enzyme-free single-step immunoassays were successfully developed for efficient and sensitive detection of glycocholic acid (GCA). Under optimal conditions, a noticeable linear relationship was identified by the enzyme-linked immunosorbent assay (ELISA) over a range of 8-2390 ng/mL, while the visual limit of detection (vLOD) in the constructed lateral flow immunoassay (LFA) was 10 ng/mL for GCA. The recovery rate in spiked urine samples obtained by the ELISA ranged from 84.2 to 117.9%, which was consistent with the results in LFA. The present work demonstrates the potential of PdNPs as labeling matrices in enzyme-free single-step immunoassays.

Recent advances in Surface Plasmon Resonance (SPR) biosensors for food analysis: a review

Food safety is the prime area of concern that builds trust. With the prevailing advancements, it has become facile to ensure safety in almost all aspects. Technology has grown from tedious lab techniques to modern chromatographic techniques and immunoassays, progressed with more precise and rapid sensing through the advent of Biosensors. Biosensors provide an automated technology by presenting superfast, nondestructive and cost-effective detection in food analysis. SPR biosensor is an optical biosensor known for its versatility and has wider applications in food testing and analysis. It has an optical system for excitation and interrogation of surface plasmons, and a biomolecular recognition element to detect and seize the target analyte present in a sample. The optical signal detects the binding analyte, on the recognition element, which results in a change in refractive index at the surface and modifies the surface plasmons' propagation constant. SPR aids in label-free detection of various components such as adulterants, antibiotics, biomolecules, genetically modified foods, pesticides, insecticides, herbicides, microorganisms and microbial toxins in food and assures safety. The distinct advancements of SPR in food analysis have been found and discussed. The review also provides knowledge on the advantages and the key challenges encountered by SPR.

A novel colorimetric aptasensor for ultrasensitive detection of aflatoxin M1 based on the combination of CRISPR-Cas12a, rolling circle amplification and catalytic activity of gold nanoparticles

Colorimetric approaches have received noticeable attention among sensing methods in view of simplicity and watching the color change of sample by the naked eyes. However, developing colorimetric sensing methods which show high sensitivity is still problematic. Herein, based on CRISPR-Cas12a, rolling circle amplification (RCA) and catalytic activity of gold nanoparticles (AuNPs), a colorimetric aptasensor was introduced for highly sensitive detection of aflatoxin M₁ (AFM₁). In the presence of AFM₁, the CRISPR-Cas12a is inactivated and large single-stranded DNA (ssDNA) structures are formed on the surface of AuNPs following the addition of T4 DNA ligase and phi29 DNA polymerase. So, the sample color remains yellow after addition of 4-nitrophenol. However, no huge DNA structure is observed on the surface of AuNPs in the absence of target because of activation of CRISPR-Cas12a and digestion of primer. So, the color of sample switches to colorless. The results indicated that the biosensor had high selectivity toward AFM₁ and the approach achieved a detection limit as low as 0.05 ng/L. In addition, it could sensitively identify AFM₁ in the spiked milk samples. Overall, this approach is highly sensitive and does not require sophisticated equipment. Therefore, it maintains promising potential for other mycotoxins detection in real samples by simply replacing the applied sequences.

Recent Advances in Microfluidic Devices for Contamination Detection and Quality Inspection of Milk

Milk is a necessity for human life. However, it is susceptible to contamination and adulteration. Microfluidic analysis devices have attracted significant attention for the high-throughput quality inspection and contaminant analysis of milk samples in recent years. This review describes the major proposals presented in the literature for the pretreatment, contaminant detection, and quality inspection of milk samples using microfluidic lab-on-a-chip and lab-on-paper platforms in the past five years. The review focuses on the sample separation, sample extraction, and sample preconcentration/amplification steps of the pretreatment process and the determination of aflatoxins, antibiotics, drugs, melamine, and foodborne pathogens in the detection process. Recent proposals for the general quality inspection of milk samples, including the viscosity and presence of adulteration, are also discussed. The review concludes with a brief perspective on the challenges facing the future development of microfluidic devices for the analysis of milk samples in the coming years.

Recent progress in visual methods for aflatoxin detection

Aflatoxins (AFs) contamination in food and agricultural products poses a significant threat to human health. Sensitive and accurate detection of AFs provides a strong guarantee for ensuring food safety. Conventional chromatographic-based or mass spectrum methods, which rely on bulky instrument and skilled personnel, are not suitable for on-site surveillance. By contrast, visual detections which possess the merits of rapidity and sophisticated instrument-free present an excellent potential for the on-site detection of AFs. This review intends to summarize the latest development of visual methods for AFs detection, including paper-based tests, chromogenic reactions, and luminescent methods. Emerging technologies, like nanotechnology, DNAzymes, and aptamers combined with these visual methods are introduced. The basic principles, features, and application advantages of each type of visual methods are discussed. The biggest challenges and perspectives on their future trends are also addressed.

Applications of microcapillary films in bioanalytical techniques

Microcapillary film (MCF) is an extruded plastic film with an array of parallel microcapillaries (30-500 μm) and it has wide potential applications in bioanalytical techniques as a microfluidic platform. With different surface modification strategies, an MCF combines the advantages of its structure and modified chemical properties to realize various bioanalytical functions. In this review, we begin by introducing the manufacturing process of MCFs, common materials used to produce MCFs, surface treatment approaches of inner surfaces, and a signal detection and readout system of the MCF platform. Then, we summarize some typical applications of MCFs, particularly in protein chromatography, Escherichia coli detection for urinary tract infections, prostate-specific antigen detection for prostate cancer and multiplex immunoassays. Finally, future perspectives of MCFs in bioanalytical techniques are discussed.

Determination of trace aflatoxin M1 (AFM1) residue in milk by an immunochromatographic assay based on (PEI/PSS)4 red silica nanoparticles

Aflatoxin M1 (AFM1) residues in milk pose a major threat to human health, so there is an urgent need for a simple, rapid, and sensitive method for the determination of trace AFM1 in milk. In this study, a competitive immunochromatographic assay (ICA), using visual (PEI/PSS)₄ red silica nanoparticles (SiNPs) as signal amplification probes, was used for the highly sensitive detection of AFM1. The (PEI/PSS)₄ red SiNPs were used to label AFM1 monoclonal antibody (mAb) to prepare ICA for the detection of AFM1. After exploring the optimal conditions of mAb and immunoprobe dosage conditions, the lowest visual detection limit (VDL) of AFM1 in phosphate-buffered saline with Tween 20 (PBST, 10 mM, pH 7.4, containing 1% BSA, 3% sucrose, 1% trehalose, and 0.5% Tween 20) can reach 0.1 pg/mL. The intuitive visually visible value of AFM1 in both PBST and milk was 10 pg/mL. The results showed that the immunochromatographic system based on high chroma color (PEI/PSS)₄ red SiNPs has high sensitivity and broad application prospects for the detection of trace AFM1 residues in milk. The high chroma (PEI/PSS)₄ red SiNPs are expected to be a convenient biomarker for improving the sensitivity of immune chromatography bands. Graphical abstract The schematic diagram shows the detection principle. In this work, in the competitive experiment, (PEI/PSS)₄ red SiNPs were selected as visual labeling materials, and the specific antibody combined with the labeled material was selected as an immune probe. The AFM1-BSA antigen coupled with the macromolecular BSA was fixed on the T line of the nitrocellulose (NC) membrane. The AFM1 in sample solution competes with AFM1-BSA for the specific binding site of immune probe. The detection sensitivity of this method for AFM1 is obtained by judging the change of the red signal intensity produced by the positive sample, compared with the color at the T line of the negative sample.