Colorimetric aptasensing of ochratoxin A using Au@Fe3O4 nanoparticles as signal indicator and magnetic separator

Enhanced sensitivity in electrochemical detection of ochratoxin A within food samples using ferrocene- and aptamer-tethered gold nanoparticles on disposable electrodes

Ensuring food security is crucial for public health, and the presence of mycotoxins, produced by fungi in improperly stored processed or unprocessed food, poses a significant threat. This research introduces a novel approach - a disposable aptasensing platform designed for the detection of ochratoxin A (OTA). The platform employs gold-nanostructured screen-printed carbon electrodes functionalized with a ferrocene derivative, serving as an integrated faradaic transducing system, and an anti-OTA aptamer as a bioreceptor site. Detection relies on the ferrocene electrochemical signal changes induced by the aptamer folding in the presence of the target molecule. Remarkably sensitive, the platform detects OTA within the range of 0.5 to 70 ng mL-1 and a detection limit of 11 pg mL-1. This limit is approximately 200 times below the levels stipulated by the European Commission for agricultural commodities. Notably, the sensing device exhibits efficacy in detecting OTA in complex media, such as roasted coffee beans and wine, without the need for sample pretreatment, yielding accurate recoveries. Furthermore, while label-free electrochemical aptasensors have proliferated, this study addresses a gap in understanding the binding mechanisms of some aptasensors. To enhance the experimental findings, a theoretical study was conducted to underscore the specificity of the anti-OTA aptamer as a donor for OTA detection. The molecular docking technique was employed to unveil the key binding region of the aptamer, providing valuable insights into the aptasensor specificity.

Metallic nanostructure-based aptasensors for robust detection of proteins

There is a significant need for fast, cost-effective, and highly sensitive protein target detection, particularly in the fields of food, environmental monitoring, and healthcare. The integration of high-affinity aptamers with metal-based nanomaterials has played a crucial role in advancing the development of innovative aptasensors tailored for the precise detection of specific proteins. Aptamers offer several advantages over commonly used molecular recognition methods, such as antibodies. Recently, a variety of metal-based aptasensors have been established. These metallic nanomaterials encompass noble metal nanoparticles, metal oxides, metal-carbon nanotubes, carbon quantum dots, graphene-conjugated metallic nanostructures, as well as their nanocomposites, metal-organic frameworks (MOFs), and MXenes. In general, these materials provide enhanced sensitivity through signal amplification and transduction mechanisms. This review primarily focuses on the advancement of aptasensors based on metallic materials for the highly sensitive detection of protein targets, including enzymes and growth factors. Additionally, it sheds light on the challenges encountered in this field and outlines future prospects. We firmly believe that this review will offer a comprehensive overview and fresh insights into metallic nanomaterials-based aptasensors and their capabilities, paving the way for the development of innovative point-of-care (POC) diagnostic devices.

Magnetically actuated microfluidic chip combined with a G-quadruplex DNAzyme-based fluorescent/colorimetric sensor for the dual-mode detection of ochratoxin A in wheat

In this work, a portable fluorescent/colorimetric sensor based on G-quadruplex DNAzyme was constructed to achieve rapid and dual-mode detection of ochratoxin A (OTA) in wheat. OTA aptamers coupled with magnetic beads (MBs) can self-assemble with two segments of DNA and hemin to form a G-quadruplex DNAzyme structure that can catalyze the oxidation of Amplex Red (ADHP) with H2O2, making the solution red and producing strong fluorescence in solution. However, in the presence of OTA, the structure of the G-quadruplex DNAzyme was damaged, resulting in reduced catalytic activity. According to the principle of detection, a magnet-controlled chip integrating the reaction, washing, and detection was designed in this study. Shuttling the MB-DNAzyme probes onto a magnetically controlled chip considerably reduced the background signal and improved the detection efficiency and sensitivity. In addition, a portable fluorescence and colorimetric detection platform was built for on-site OTA detection.

Recent progress of MOF-functionalized nanocomposites: From structure to properties

Metal-organic frameworks (MOFs) are novel crystalline porous materials assembled from metal ions and organic ligands. The adaptability of their design and the fine-tuning of the pore structures make them stand out in porous materials. Furthermore, by integrating MOF guest functional materials with other hosts, the novel composites have synergistic benefits in numerous fields such as batteries, supercapacitors, catalysis, gas storage and separation, sensors, and drug delivery. This article starts by examining the structural relationship between the host and guest materials, providing a comprehensive overview of the research advancements in various types of MOF-functionalized composites reported to date. The review focuses specifically on four types of spatial structures, including MOFs being (1) embedded in nanopores, (2) immobilized on surface, (3) coated as shells and (4) assembled into hybrids. In addition, specific design ideas for these four MOF-based composites are presented. Some of them involve in situ synthesis method, solvothermal method, etc. The specific properties and applications of these materials are also mentioned. Finally, a brief summary of the advantages of these four types of MOF composites is given. Hopefully, this article will help researchers in the design of MOF composite structures.

Rapid colorimetric sensor for ultrasensitive and highly selective detection of Fumonisin B1 in cereal based on laccase-mimicking activity of silver phosphate nanoparticles

Ag3PO4 nanoparticles (NPs) was prepared through a facile coprecipitation method, and was first found to have excellent laccase-mimicking catalytic activity. The study confirms that Fumonisin B1 (FB1) can effectively hinder the production of superoxide anion (O2-) between Ag3PO4 NPs and dissolved oxygen, and further inhibit laccase-mimicking activity of Ag3PO4 NPs. Thus, a novel rapid colorimetric sensor for FB1 analysis in cereal was first established using laccase-mimicking activity as sensing signal. The absorbance variation of sensing solution is directly related to the amount of FB1, and the color change is further combined with smartphone for quantitively analysis of FB1. The limit of detection (LOD) of the sensor is determined as low as 1.73 μg·L-1, which is far lower than the maximum residue limits (MRLs) of FB1 set by European Commission and US Food and Drug Administration (FDA). The average recovery of 87.8-104.5% for FB1 detection was obtained in cereal.

Ratiometric fluorescent aptasensor for convenient detection of ochratoxin A in beer and orange juice

Based on the principle of fluorescence resonance energy transfer (FRET), a simple ratiometric fluorescent aptasensor for convenient detection of ochratoxin A (OTA), a Group IIB carcinogen secreted by some fungi, was established. Initially, the anti-OTA aptamer with a quadruplex structure was flanked with FAM and BHQ1, and its partially complementary DNA (cDNA) was tagged with Cy3. In the absence of OTA, this aptamer hybridized with the cDNA strand forming a DNA duplex structure, in which BHQ1 was adjacent to Cy3 and distant from FAM. Due to the FRET principle, the fluorescence intensity emitted by Cy3 (FCy3) was quenched by BHQ1, and the fluorescence intensity emitted by FAM (FFAM) recovered. In the presence of OTA, the prepared aptamer preferred to bind with OTA instead of cDNA, forming an aptamer-OTA complex structure in which BHQ1 was adjacent to FAM and distant from Cy3. As a result, FFAM was quenched and FCy3 was restored. OTA can be accurately detected via the determination of the FCy3/FFAM ratio value. Under optimal conditions, this ratiometric fluorescent aptasensor offers excellent OTA detection in the range of 0.6 nmol L-1-5 μmol L-1, with a limit of detection (LOD) of 0.3 nmol L-1. This ratiometric aptasensor showed the advantages of easy operation, accuracy and sensitive analysis. Good specificity of this aptasensor was demonstrated. This ratiometric aptasensor could be used for the detection of OTA in real samples, e.g. beer and orange juice, showing its promising application potential.

A Colorimetric Aptasensor for Ochratoxin A Detection Based on Tetramethylrhodamine Charge Effect-Assisted Silver Enhancement

A novel colorimetric aptasensor based on charge effect-assisted silver enhancement was developed to detect ochratoxin A (OTA). To achieve this objective, gold nanoparticles (AuNPs), which can catalyze silver reduction and deposition, were used as the carrier of the aptamers tagged with a positively charged tetramethylrhodamine (TAMRA). Due to the mutual attraction of positive and negative charges, the TAMRA attracted and retained the silver lactate around the AuNPs. Thus, the chance of AuNP-catalyzed silver reduction was increased. The charge effect-assisted silver enhancement was verified by tagging different base pair length aptamers with TAMRA. Under optimized conditions, the as-prepared OTA aptasensor had a working range of 1 × 10²-1 × 10⁶ pg mL^-1. The detection limit was as low as 28.18 pg mL^-1. Moreover, the proposed aptasensor has been successfully applied to determine OTA in actual samples with satisfactory results.

Colorimetric-photothermal-magnetic three-in-one lateral flow immunoassay for two formats of biogenic amines sensitive and reliable quantification

Rapid, simple, sensitive and reliable approaches for biogenic amines quantification in various food samples are essential to food safety. Lateral flow immunoassay (LFIA) has been wildly utilized in point-of-care testing (POCT) owing to its advantage of flexibility and feasibility. Here, we reported a Fe₃O₄@Au nanoparticles (NPs) (Fe₃O₄@AuNPs) based multimodal readout LFIA for rapid putrescine (Put) and histamine (His) quantification with a LOD down to 10 and 10 ng/mL in naked eye mode, 2.31 and 4.39 ng/mL in photothermal mode, 0.17 and 0.31 ng/mL in magnetic mode, respectively. Such multi-mode assay has been successfully used to detect Biogenic amines (BAs) in raw aquatic foods, including fish, prawns, beef, and pork, with overall recoveries ranging from 93.68 to 109.34%. Meanwhile, it is easily expanded to detect other typical BAs with high sensitivity by simply replacing antibodies. In view of the multi-signal reading, two quantitative formats, and high sensitivity, it may greatly widen the application of lateral flow detection in food safety.

A peroxidase-like magneto-gold nanozyme AuNC@Fe3O4 with photothermal effect for induced cell apoptosis of hepatocellular carcinoma cells in vitro

Hepatocellular carcinoma (HCC) is one of the most commonly diagnosed and malignant cancers worldwide. Conventional therapy strategies may not completely eradicate the tumor and may cause side effects during treatment. Nano-catalytic therapy, as a novel strategy, has attracted a great deal of attention. This study aimed to synthesize a multifunctional magneto-gold nanozyme AuNC@Fe₃O₄ and evaluate its anti-cancer potential in HepG2 cells in vitro. The characteristics of AuNC@Fe₃O₄ were assessed using a transmission electron microscope, dynamic light scattering, and energy-dispersive X-ray. The photothermal performance and peroxidase (POD)-like activity of AuNC@Fe₃O₄ were detected, using thermal camera and ultraviolet-visible spectrophotometer, respectively. The anti-cancer potential of AuNC@Fe₃O₄ was examined using cell counting kit-8, live/dead cell staining, and apoptosis analysis. Further research on HepG2 cells included the detection of intracellular reactive oxygen species (ROS) and lysosomal impairment. We observed that the AuNC@Fe₃O₄ had a small size, good photothermal conversion efficiency and high POD-like activity, and also inhibited cell proliferation and enhanced cell apoptotic ability in HepG2 cells. Furthermore, the AuNC@Fe₃O₄ enhanced ROS production and lysosomal impairment via the synergistic effect of photothermal and nano-catalytic therapies, which induced cell death or apoptosis. Thus, the magneto-gold nanozyme AuNC@Fe₃O₄ may offer a potential anti-cancer strategy for HCC.

A colorimetric aptasensor based on gold nanoparticles for detection of microbial toxins: an alternative approach to conventional methods

Frequent contamination of foods with microbial toxins produced by microorganisms such as bacteria, fungi, and algae represents an increasing public health problem that requires the development of quick and easy tools to detect them at trace levels. Recently, it has been found that colorimetric detection methods may replace traditional methods in the field because of their ease of use, quick response, ease of manufacture, low cost, and naked-eye visibility. Therefore, it is suitable for fieldwork, especially for work in remote areas of the world. However, the development of colorimetric detection methods with low detection limits is a challenge that limits their wide applicability in the detection of food contaminants. To address these challenges, nanomaterial-based transduction systems are used to construct colorimetric biosensors. For example, gold nanoparticles (AuNPs) provide an excellent platform for the development of colorimetric biosensors because they offer the advantages of easy synthesis, biocompatibility, advanced surface functionality, and adjustable physicochemical properties. The selectivity of the colorimetric biosensor can be achieved by the combination of aptamers and gold nanoparticles, which provides an unprecedented opportunity to detect microbial toxins. Compared to antibodies, aptamers have significant advantages in the analysis of microbial toxins due to their smaller size, higher binding affinity, reproducible chemical synthesis and modification, stability, and specificity. Two colorimetric mechanisms for the detection of microbial toxins based on AuNPs have been described. First, sensors that use the localized surface plasmon resonance (LSPR) phenomenon of gold nanoparticles can exhibit very strong colors in the visible range because of changes caused by aggregation or disaggregation. Second, the detection mechanism of AuNPs is based on their enzyme mimetic properties and it is possible to construct a colorimetric biosensor based on the 3,3',5,5'-tetramethylbenzidine/Hydrogen peroxide, TMB/H₂O₂ reaction to detect microbial toxins. Therefore, this review summarizes the recent applications of AuNP-based colorimetric aptasensors for detecting microbial toxins, including bacterial toxins, fungal toxins, and algal toxins focusing on selectivity, sensitivity, and practicality. Finally, the most important current challenges in this field and future research opportunities are discussed.

Detection of ochratoxin A by fluorescence sensing based on mesoporous materials

We developed a new ochratoxin A (OTA) aptamer biosensor to promptly detect OTA in food. Mesoporous silica nanoparticles (MSN) were used as carriers, and aptamers were used as recognition probes and gating molecules. The fluorescent dye Rhodamine 6G (Rh6G) was loaded into mesoporous silica, and through electrostatic contact, the OTA aptamer was adsorbed on amino-modified mesoporous silica. The fluorescent dye released from the mesopore in the presence of OTA because of the conformational change induced in the aptamer by the target. The amount of ochratoxin was determined by measuring the fluorescence intensity. Our findings revealed a positive relationship between the fluorescence intensity and OTA concentration, with a limit of detection of 0.28 ng mL-1, and the detection range was 0.05-200 ng mL-1. The recovery rate was 80.7%-110.8% in real samples. The proposed approach is suitable for the quantification of other toxins.

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.

Nano-engineered materials for sensing food pollutants: Technological advancements and safety issues

Food spoilage and safety are key concerns of the modern food sector. Among them, several types of polluting agents are the prime grounds of food deterioration. In this context, nanotechnology-based measures are setting new frontiers to strengthen food applications. Herein, we summarize the nanotechnological dimension of the food industry for both processing and packaging applications. Active bioseparation, smart delivery, nanoencapsulation, nutraceuticals, and nanosensors for biological detection are a few emerging topics of nanobiotechnology in the food sector. The development of functional foods is another milestone set by food nanotechnology by building the link between humans and diet. However, the establishment of optimal intake, product formulations, and delivery matrices, the discovery of beneficial compounds are a few of the key challenges that need to be addressed. Nanotechnology provides effective solutions for the aforementioned problem giving various novel nanomaterials and methodologies. Various nanodelivery systems have been designed, e.g., cochleate, liposomes, multiple emulsions, and polysaccharide-protein coacervates. However, their real applications in food sciences are very limited. This review also provides the status and outlook of nanotechnological systems for future food applications.

Nanomaterial-based aptamer biosensors for ochratoxin A detection: a review

Ochratoxin A (OTA) is a widely distributed mycotoxin that often contaminates food, grains and animal feed. It poses a serious threat to human health because of its high toxicity and persistence. Therefore, the development of an inexpensive, highly sensitive, accurate and rapid method for OTA detection is imperative. In recent years, various nanomaterials used in the establishment of aptasensors have attracted great attention due to their large surface-to-volume ratio, good stability and facile preparation. This review summarizes the development of nanomaterial-based aptasensors for OTA determination and sample treatment over the past 5 years. The nanomaterials used in OTA aptasensors include metal, carbon, luminescent, magnetic and other nanomaterials. Finally, the limitations and future challenges in the development of nanomaterial-based OTA aptasensors are reviewed and discussed.

"Three-in-One" Multifunctional Nanohybrids with Colorimetric Magnetic Catalytic Activities to Enhance Immunochromatographic Diagnosis

Colorimetric lateral flow immunoassay (LFIA) with gold nanoparticles (AuNPs) as signal reporters has been widely used in point-of-care testing. Nonetheless, the potential of traditional AuNP-based LFIA for the early diagnosis of disease is often compromised by limited sensitivity due to the insufficient colorimetric signal brightness of AuNPs. Herein, we develop a "three-in-one" multifunctional catalytic colorimetric nanohybrid (Fe₃O₄@MOF@Pt) composed of Fe₃O₄ nanoparticles, MIL-100(Fe), and platinum (Pt) nanoparticles. Fe₃O₄@MOF@Pt displays enhanced colorimetric signal brightness, fast magnetic response, and ultrahigh peroxidase-mimicking activity, which are beneficial to the enhancement of the sensitivity of LFIA by coupling with magnetic separation and catalytic amplification. When integrated with the dual-antibody sandwich LFIA platform, the developed Fe₃O₄@MOF@Pt can achieve an ultrasensitive immunochromatographic assay of procalcitonin with a sensitivity of 0.5 pg mL^-1, which is approximately 2280-fold higher than that of conventional AuNP-based LFIA and superior to previously published immunoassays. Therefore, this work suggests that the proposed catalytic colorimetric nanohybrid can act as promising signal reporters to enable ultrasensitive immunochromatographic disease diagnostics.

Regulation Mechanism of ssDNA Aptamer in Nanozymes and Application of Nanozyme-Based Aptasensors in Food Safety

Food safety issues are a worldwide concern. Pathogens, toxins, pesticides, veterinary drugs, heavy metals, and illegal additives are frequently reported to contaminate food and pose a serious threat to human health. Conventional detection methods have difficulties fulfilling the requirements for food development in a modern society. Therefore, novel rapid detection methods are urgently needed for on-site and rapid screening of massive food samples. Due to the extraordinary properties of nanozymes and aptamers, biosensors composed of both of them provide considerable advantages in analytical performances, including sensitivity, specificity, repeatability, and accuracy. They are considered a promising complementary detection method on top of conventional ones for the rapid and accurate detection of food contaminants. In recent years, we have witnessed a flourishing of analytical strategies based on aptamers and nanozymes for the detection of food contaminants, especially novel detection models based on the regulation by single-stranded DNA (ssDNA) of nanozyme activity. However, the applications of nanozyme-based aptasensors in food safety are seldom reviewed. Thus, this paper aims to provide a comprehensive review on nanozyme-based aptasensors in food safety, which are arranged according to the different interaction modes of ssDNA and nanozymes: aptasensors based on nanozyme activity either inhibited or enhanced by ssDNA, nanozymes as signal tags, and other methods. Before introducing the nanozyme-based aptasensors, the regulation by ssDNA of nanozyme activity via diverse factors is discussed systematically for precisely tailoring nanozyme activity in biosensors. Furthermore, current challenges are emphasized, and future perspectives are discussed.

No-wash point-of-care biosensing assay for rapid and sensitive detection of aflatoxin B1

In many cases of in-situ or point-of-care testing (POCT), the single pursuit of good detection performance cannot meet the testing requirements, and thus no-wash testing has become one of the most effective methods to develop sustainable biosensing assay, providing more convenient operation procedures and shorting the detection time. Herein, a disposable POC biosensing assay was prepared based on the RGB color detector software on the smartphone and the peroxide-like activity of gold nanoparticles (Au NPs) for aflatoxin B1 (AFB1) sensitive detection. Using syringe filters for a simple physical separation procedure can easily realize washing free detection, which is superior to most biosensing assays with cumbersome detection procedures. The syringe filters with 200 nm pore diameter could only pass through small Au NPs (30 nm) while the large-sized SiO₂ nanoparticles (300 nm) was blocked on the membrane. In this work, Au NPs utilized their inherent peroxidase-like activity to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ to ox-TMB under acidic conditions, which results in blue color in aqueous solution. The color change due to different antigen concentrations was quantitatively determined by measuring the color intensity with a smartphone and the RGB color detector. By measuring the color intensity, it was known that the linear concentration range of the biosensing assay was 100 fg mL^-1 to 50 ng mL^-1, and the detection limit of AFB1 was 33 fg mL^-1 (S/N = 3). Additionally, the designed biosensing assay exhibited excellent selectivity, storage stability and reproducibility. More importantly, the innovation of detecting and analyzing technology is the outstanding advantage of the biosensing assay, providing a more flexible and convenient strategy for some other small molecule analysis.

Aptamer-based biosensors and application in tumor theranostics

An aptamer is a short oligonucleotide chain that can specifically recognize targeting analytes. Due to its high specificity, low cost, and good biocompatibility, aptamers as the targeting elements of biosensors have been applied widely in non-invasive tumor imaging and treatment in situ to replace traditional methods. In this review, we will summarize recent advances in using aptamer-based biosensors in tumor diagnosis. After a brief introduction of the advantage of aptamers compared with enzyme sensors and immune sensors, the different sensing designs and mechanisms based on 3 signal transduction modes will be reviewed to cover different kinds of analytical methods, including: electrochemistry analysis, colorimetry analysis, and fluorescence analysis. Finally, the prospective advantages of aptamer-based biosensors in tumor theranostics and post-treatment monitoring are also evaluated in this review.

Aptasensors for mycotoxins in foods: Recent advances and future trends

Mycotoxin contamination in foods has posed serious threat to public health and raised worldwide concern. The development of simple, rapid, facile, and cost-effective methods for mycotoxin detection is of urgent need. Aptamer-based sensors, abbreviated as aptasensors, with excellent recognition capacity to a wide variety of mycotoxins have attracted ever-increasing interest of researchers because of their simple fabrication, rapid response, high sensitivity, low cost, and easy adaptability for in situ measurement. The past few decades have witnessed the rapid advances of aptasensors for mycotoxin detection in foods. Therefore, this review first summarizes the reported aptamer sequences specific for mycotoxins. Then, the recent 5-year advancements in various newly developed aptasensors, which, according to the signal output mode, are divided into electrochemical, optical and photoelectrochemical categories, for mycotoxin detection are comprehensively discussed. A special attention is taken on their strengths and limitations in real-world application. Finally, the current challenges and future perspectives for developing novel highly reliable aptasensors for mycotoxin detection are highlighted, which is expected to provide powerful references for their thorough research and extended applications. Owing to their unique advantages, aptasensors display a fascinating prospect in food field for safety inspection and risk assessment.

Colorimetric detection of chlorpyrifos in peach based on cobalt-graphene nanohybrid with excellent oxidase-like activity and reusability

Cobalt nanocrystal has been widely used as nano-enzyme for sensing and catalysis due to its high stability and low cost, but poor catalytic activity limits its applications in bioanalysis. The study reports one strategy for synthesis of cobalt-graphene nanohybrid. Histidine-functionalized graphene quantum dot (His-GQD) was bound to graphene sheet via π-π stacking and then combined with cobalt ions in the presence of cetyltrimethylammonium chloride to form stable complex and finally reduced under nitrogen to obtain Co-His-GQD-G. The as-synthesized nanohybrid offers well-defined three-dimensional structure and quasi-superparamagnetism. The cobalt nanoparticles were well dispersed on graphene sheets. The unique structure improves oxidase-like activity of Co-His-GQD-G. Further, Co-His-GQD-G was used as the nanozyme for colorimetric detection of chlorpyrifos. Co-His-GQD-G catalyzes oxidization of 3,3',5,5'-tetramethylbenzidine into blue product. Thiocholine produced by hydrolysis of acetylthiocholine under catalysis of acetylcholinesterase inhibits catalytic activity of Co-His-GQD-G and leads to a reduced oxidization rate. Chlorpyrifos inhibits activity of acetylcholinesterase and brings an enhanced absorbance of blue product. The absorbance at 652 nm linearly increases with increasing chlorpyrifos concentration in the range of 2-20 ng mL^-1 with detection limit of 0.57 ng mL^-1 (S/N = 3). The method was successfully applied in determination of chlorpyrifos in peach by preparing Co-His-GQD-G magnetic gel sheet.

Two-color, ultra-sensitive fluorescent strategy for Ochratoxin A detection based on hybridization chain reaction and DNA tweezers

A two-color fluorescent DNA tweezers was developed for ultrasensitive detection of Ochratoxin A (OTA) based on hairpin-locked aptamer and hybridization chain reaction (HCR) amplification strategy. OTA can bind with hairpin-locked aptamer and then trigger the HCR reaction to produce a long double-strand DNA. The side-chains of the long duplex can separately hybridize with the two locker sequences of DNA tweezer, causing the opening of DNA tweezer and the recovery of two-color fluorescent signals. It shows a good linear range from 0.02 to 0.8 ppb with limit of detection of 0.006 ppb for FAM and 0.014 ppb for Cy5, which is beyond the requirement of actual application. In addition, the two-color fluorescent strategy can greatly reduce the false positive rate. It shows excellent performance for detection of OTA in practical food sample.

Advances in gold nanoparticles for mycotoxin analysis

Mycotoxins are toxic secondary metabolites naturally produced by fungi. They can cause various kinds of acute and chronic diseases in both humans and animals since food usually contains trace amounts of mycotoxins. Thus, it is important to develop a rapid and sensitive technique for mycotoxin detection. Except for the original and classical enzyme-linked immunosorbent assays (ELISA), a series of biosensors has been developed to analyze mycotoxins in food in the last decade with the advantages of rapid analysis, simplicity, portability, reproducibility, stability, accuracy, and low cost. Nanomaterials have been incorporated into biosensors for the purpose of achieving better analytical performance in terms of limit of detection, linear range, analytical stability, low production cost, etc. Gold nanoparticles (AuNPs) are one of the most extensively studied and commonly used nanomaterials, which can be employed as an immobilization carrier, signal amplifier, mediator and mimic enzyme label. This paper aims to present an extensive overview of the recent progress in AuNPs in mycotoxin detection through ELISA and biosensors. The details of the detection methods and their application principles are described, and current challenges and future prospects are discussed as well.

Electrochemical/visual dual-readout aptasensor for Ochratoxin A detection integrated into a miniaturized paper-based analytical device

Development of portable, sensitive and reliable devices for Ochratoxin A (OTA) detection is highly demanded, especially for resource-limited regions. Herein, a novel paper-based analytical device (PAD) is designed through wax printing and screen-printed technologies, which integrates sample flowing, electrode modification, cleaning and electrochemical (EC)/colorimetric signal output. To greatly enhance the detection sensitivity, we synthesized a chitosan functionalized MoS₂-Au@Pt (Ch-MoS₂-Au@Pt) via electrostatic self-assembly, and used it to immobilize the label aptamer (apta₂) for signal regulation and amplification. Concretely, with the addition of analytes, the Ch-MoS₂-Au@Pt-apta₂ could be combined on the sensing interface by specific biorecognition and catalyzed reduction of H₂O₂, resulting in a remarkable EC response. Meanwhile, the released hydroxyl radicals (·OH) flowed to the visualization zone and promoted the oxidation of 3,3',5,5'-tetramethylbenzidine for colorimetric detection. Consequently, the dual-mode PAD achieved acceptable prediction and accurate analysis in the range of 0.1-200 ng mL^-1 and 1 × 10^-4-200 ng mL^-1 by matching the visual and EC signal intensity, respectively. Compared with traditional single-mode sensor for OTA, the proposed dual-mode aptasensor featuring independent signal conversion and readout, not only avoided the false-positive signal associated with detection condition and operation, but also enlarged the detection ranges and improved the sensitivity. Furthermore, the consistency of EC/colorimetric assay was validated in real OTA samples. Overall, this work provided a portable, cost-effective, sensitive and visualized aptasensor platform, which could be extended to various other mycotoxins in the field of food safety.

Novel impedimetric sensing strategy for detecting ochratoxin A based on NH2-MIL-101(Fe) metal-organic framework doped with cobalt phthalocyanine nanoparticles

Iron-based metal-organic framework, NH₂-MIL-101(Fe), was doped with different dosages of cobalt phthalocyanine nanoparticles (CoPc) to synthesize a series of NH₂-MIL-101(Fe)@CoPc nanocomposites. The NH₂-MIL-101(Fe)@CoPc nanocomposites were then employed to construct novel impedimetric aptasensors for the detection of ochratoxin A (OTA). Combining the intrinsic advantages of NH₂-MIL-101(Fe) (highly porous structure and excellently electrochemical activity) and CoPc (good physiochemical stability and strong bioaffinity), the NH₂-MIL-101(Fe)@CoPc nanocomposites show promising properties, which are beneficial for immobilizing OTA-targeted aptamer strands. Amongst, the developed impedimetric aptasensor based on NH₂-MIL-101(Fe)@CoPc_6:1, prepared using the mass ratio of NH₂-MIL-101(Fe):CoPc of 6:1, exhibits the best amplified electrochemical signal and highest sensitivity for detecting OTA. The detection limitation is 0.063 fg·mL^-1 within the OTA concentration of 0.0001-100 pg·mL^-1, accompanying with high selectivity, good reproducibility and stability, acceptable regenerability, and wide applicability in diverse real samples. Consequently, the proposed sensing strategy can be applied for detecting OTA to cope with food safety.

Spectrofluorimetric study of the complexation of ochratoxin A and Cu2+: Towards the hybrid fluorimetric sensor and visual detection of ochratoxin A in wheat flour samples from farm to fork

In this study, the ochratoxin A (OTA) interaction with some metal cations exhibits a quenching effect of the Cu²⁺, Co²⁺, and Ni²⁺ in the OTA intrinsic fluorescence intensity. The property of the OTA fluorescence change in complexation with the Cu²⁺ has been applied in developing a label-free and selective fluorimetric sensor for fast detection of the OTA trace amounts in wheat flour samples. The achieved color difference map (CDM) based on the spectral profile provides the technical support for the rapid visual sensing of OTA in the wheat flour samples. The feasibility of the hybrid fluorimetric sensor and visual OTA detection in wheat flour samples has been confirmed with the high-performance liquid chromatography (HPLC) method as a standard method. The calculated LOD (0.4 ng g^-1) and LOQ (1.2 ng g^-1) values of the proposed method are much lower than the maximum permissible limit of OTA value reported by the European Union (5 ng g^-1).

A novel colorimetric immunoassay for sensitive monitoring of ochratoxin A based on an enzyme-controlled citrate-iron(iii) chelating system

Schematic illustration of an enzyme-controlled citrate-iron(iii) chelating system-based colorimetric immunoassay for sensitive determination of ochratoxin A.

Advances in Colorimetric Strategies for Mycotoxins Detection: Toward Rapid Industrial Monitoring

Mycotoxins contamination is a global public health concern. Therefore, highly sensitive and selective techniques are needed for their on-site monitoring. Several approaches are conceivable for mycotoxins analysis, among which colorimetric methods are the most attractive for commercialization purposes thanks to their visual read-out, easy operation, cost-effectiveness, and rapid response. This review covers the latest achievements in the last five years for the development of colorimetric methods specific to mycotoxins analysis, with a particular emphasis on their potential for large-scale applications in food industries. Gathering all types of (bio)receptors, main colorimetric methods are critically discussed, including enzyme-linked assays, lateral flow-assays, microfluidic devices, and homogenous in-solution strategies. This special focus on colorimetry as a versatile transduction method for mycotoxins analysis is comprehensively reviewed for the first time.

An Electrochemical Aptasensor for Pb2+ Detection Based on Metal-Organic-Framework-Derived Hybrid Carbon

A new double-shelled carbon nanocages material was synthesized and developed an aptasensor for determining Pb²⁺ in aqueous solution. Herein, nanoporous carbon materials derived from core–shell zeolitic imidazolate frameworks (ZIFs) demonstrated excellent electrochemical activity, stability, and high specificity surface area, consequently resulting in the strong binding with aptamers. The aptamer strands would be induced to form G-quadruplex structure when Pb²⁺ was introduced. Under optimal conditions, the aptasensor exhibited a good linear relationship of Pb²⁺ concentration ranging from 0.1 to 10 μg L⁻¹ with the detection limits of 0.096 μg L⁻¹. The feasibility was proved by detecting Pb²⁺ in spiked water samples and polluted soil digestion solution. The proposed aptasensor showed excellent selectivity and reproducibility, indicating promising applications in environmental monitoring.

A competitive fluorescence assay based on free-complementary DNA for ochratoxin A detection

An ultrasensitive, rapid, and specific method for Ochratoxin A (OTA) detection was designed using complementary sequence to aptamer as a target of molecular beacon (MB). The designed loop structure of the MB has the same sequence as the aptamer with a complementary DNA (cDNA) which translates the level of the target into a measurable response. The presence of the target holds aptamer at the corresponding amount and the additional cDNAs are consumed by unbound aptamers which avails free cDNAs that resulting in fluorescence rising due to unfolding of MBs. Under the optimized conditions, the fluorescence intensity increased linearly with OTA concentration over the range of 10 pg mL^-1-1 µg mL^-1 with the detection limit of 0.247 pg mL^-1. The application of this assay in wheat sample in comparison with HPLC-MS/MS method, demonstrated that the new assay could be a potential sensing platform for OTA detection.

Recent advances in aptasensors for mycotoxin detection: On the surface and in the colloid

Mycotoxins are a great potential threat to human health, and the progress in the development of mycotoxin detection methods is of an escalating importance with the increasing emphasis on food safety. Aptamer, performing the same function as antibody in specific binding with targets, exhibits profound potential in biosensing since its debut in 1990. Recent years have witnessed the rapid development of aptasensors for mycotoxin detection with the achievement of ultralow limit of detection and high sensitivity in the lab. However, there is still no officially approved aptasensing methods in mycotoxin detection application. In order to provide researchers with inspirations in the design and development of aptasensors for mycotoxin detection, we divide these aptasensors into two types, namely "on the surface" and "in the colloid", according to the location where the key sensing reaction occurs. We also systematically review aptasensors reported in the past 5 years under the abovementioned criterion of classification, and compare the advantages and disadvantages of each kind of aptasensors. Finally, we discuss prospective directions in the development of aptasensors for mycotoxin detection. This paper will offer insight and motivation to practitioners working on the research and practical application of aptasensors in the detection of mycotoxins and other substances.

A novel electrochemical aptasensor for ochratoxin a sensing in spiked food using strand-displacement polymerase reaction

Herein, an aptasensor is presented for electrochemical determination of ochratoxin A (OTA) based on nontarget-triggered production of rolling circular amplification (RCA). The surface of gold electrode is modified with thiolated complementary strand of aptamer (CS) as both capture probe and primer and OTA aptamer (Apt) as both sensing molecule and padlock probe (PLP). Following the addition of OTA, Apt/OTA conjugate is formed and detached from the electrode surface. Therefore, no RCA is produced after incubation of the modified electrode with T4 DNA ligase and phi29 DNA polymerase and a sharp current signal occurs. The analytical response ranged from 30 pM to 120 nM with detection limit of 5 pM. The designed aptasensor showed superior analytical performance in comparison with other approaches for OTA detection. Also, the approach exhibited good performance for OTA determination in spiked grape juice samples. The technique presented in this study, can be applied to develop sensors for detecting different toxins by replacing the relevant aptamers and complementary strands.

Hairpin DNA assisted dual-ratiometric electrochemical aptasensor with high reliability and anti-interference ability for simultaneous detection of aflatoxin B1 and ochratoxin A

The simultaneous detection of multiple mycotoxins in grains is significant due to the enhanced toxicity induced by their synergistic effects. In this work, a dual-ratiometric electrochemical aptasensing strategy for the simultaneous detection of aflatoxin B1 (AFB1) and ochratoxin A (OTA) was developed. Here, an anthraquinone-2-carboxylic acid (AQ)-labelled complementary DNA (cDNA) was used to provide separate and specific binding sites to assemble the ferrocene-labelled AFB1 aptamer (Fc-Apt1) and methylene blue-labelled OTA aptamer (MB-Apt2). The target-induced current ratios of I_Fc/I_AQ and I_MB/I_AQ were then used to quantitatively relate to AFB1 and OTA, respectively. Following this principle, two types of aptasensors involving the hairpin DNA (hDNA) and linear single-stranded DNA (ssDNA) as the cDNA were fabricated for performance comparisons. The results revealed that hairpin DNA with a rigid 2D structure can greatly improve the assembly and recognition efficiency of the sensing interface, which makes the hDNA-based aptasensor possess high sensitivity, reliability and anti-interference ability. The hDNA-based aptasensor exhibited a detection range of 10-3000 pg mL^-1 for AFB1 and 30-10000 pg mL^-1 for OTA, respectively, with no observable cross-reactivity. Furthermore, the aptasensor was applied to analyze corn and wheat samples, and the reliability was validated by HPLC-MS/MS. Our work has presented a novel way for fabricating a high-performance aptasensor for simultaneous detection of multiple mycotoxins.

Progress of Iron-Based Nanozymes for Antitumor Therapy

Artificial nanoscale enzyme-mimics (nanozymes) are promising functional alternatives to natural enzymes and have aroused great interest due to their inherent in vivo stability, affordability, and high catalytic ability. Iron-based nanozymes are one of the most investigated synthetic nanomaterials with versatile enzyme-like catalytic properties and have demonstrated remarkable relevance to a variety of biomedical applications, especially biocatalytic therapy against tumor indications. Nevertheless, despite the recent advances in biology and nanotechnology, the therapeutic performance of iron-based nanozymes in vivo is still limited by technical issues such as low catalytic efficiency and lack of tumor specificity. In this mini review, we briefly summarized the representative studies of iron-based nanozymes, while special emphasis was placed on the current challenges and future direction regarding the therapeutic implementation of iron-based nanozymes for the development of advanced tumor therapies with improved availability and biosafety.

An ultrasensitive non-noble metal colorimetric assay using starch-iodide complexation for Ochratoxin A detection

Colorimetric sandwich-type biosensors that can both provide sensitivity competitive with fluorescence-based approaches, and leverage reagents that are cost-effective, widely available and as safe as possible, are highly sought after. Herein, we demonstrate an alternative highly-sensitive colorimetric method for paper-based sandwich-type biosensing that uses starch-iodide complexation to simplify practical biosensing using ubiquitous reagents. Targeting the mycotoxin ochratoxin A (OTA), a covalently-immobilised OTA antibody on a cellulose surface captures OTA and forms a sandwich with OTA aptamer-conjugated glucose oxidase. Adding the chromogenic reagents at an optimized concentration, a distinct blue color develops within 30 min, offering excellent contrast with the clear/white of the negative sample. With a sampling volume down to just 5 μL, the assay exhibits concentration limits of detection and quantitation of 20 and 320 pg mL^-1, respectively, and a linear range from 10^-1 to 10⁵ ng mL^-1 (R² = 0.997). The method displays excellent selectivity against related mycotoxins, excellent %recovery (95-117%) and robust operation in complex matrices (beer, urine and human serum), with no significant difference versus gold-standard liquid chromatography. Along with its excellent analytical performance, this assay benefits from non-toxic and extremely cheap reagents that can be safely disposed of in the field, and presents an attractive alternative to toxic dyes and nanoparticles.

Gold nanoparticles mediated designing of versatile aptasensor for colorimetric/electrochemical dual-channel detection of aflatoxin B1

This work is aimed to develop of a new class of versatile aptasensor to specifically detect aflatoxin B1 (AFB1) using dual-channel detection method. To achieve this objective, gold nanoparticles (AuNPs) having peroxidase-like activity and capability of promoting silver deposition were used as the versatile label for both colorimetric and electrochemical techniques. First of all, aptamer (apt) modified Fe₃O₄@Au magnetic beads (MBs-apt) and cDNA modified AuNPs (cDNA-AuNPs) were prepared to use as capture probes and signal probes, respectively. Taking advantage of hybridization reaction between apt and cDNA, these two probes were coupled with each other to generate MBs-apt/cDNA-AuNPs bioconjugations. The high affinity between apt and AFB1 made cDNA-AuNPs detached from MBs-apt, and the released signal probes were separated and collected using an external magnetic field and used for both colorimetric and electrochemical detection channels. The dual-channel signals were directly proportional to logarithm of AFB1 concentration within the ranges of 5-200 ng mL^-1 and 0.05-100 ng mL^-1. The detection limit can reach as low as 35 pg mL^-1 and 0.43 pg mL^-1 for colorimetric and electrochemical channel, respectively. Moreover, the proposed aptasensor has been successfully applied to determine AFB1 in corn samples with satisfactory results. This dual-channel detection method can not only improve the detection precision and diversity significantly, but also can reduce the false-negative and-positive rates in food quality monitoring. We believe we have provided a general strategy with the convincing dual-readout mode which possess great promising in all of the aptamer related fields.

A Non-Enzyme and Non-Label Sensitive Fluorescent Aptasensor Based on Simulation-Assisted and Target-Triggered Hairpin Probe Self-Assembly for Ochratoxin a Detection

The monitoring and control of mycotoxins has caused widespread concern due to their adverse effects on human health. In this research, a simple, sensitive and non-label fluorescent aptasensor has been reported for mycotoxin ochratoxin A (OTA) detection based on high selectivity of aptamers and amplification of non-enzyme hybridization chain reaction (HCR). After the introduction of OTA, the aptamer portion of hairpin probe H1 will combine with OTA to form OTA-aptamer complexes. Subsequently, the remainder of the opened H1 will act as an initiator for the HCR between the two hairpin probes, causing H1 and H2 to be sequentially opened and assembled into continuous DNA duplexes embedded with numerous G-quadruplexes, leading to a significant enhancement in fluorescence signal after binding with N-methyl-mesoporphyrin IX (NMM). The proposed sensing strategy can detect OTA with concentration as low as 4.9 pM. Besides, satisfactory results have also been obtained in the tests of actual samples. More importantly, the thermodynamic properties of nucleic acid chains in the monitoring platform were analyzed and the reaction processes and conditions were simulated before carrying out biological experiments, which theoretically proved the feasibility and simplified subsequent experimental operations. Therefore, the proposed method possess a certain application value in terms of monitoring mycotoxins in food samples and improving the quality control of food security.

Colorimetric biosensing of nopaline synthase terminator using Fe3O4@Au and hemin-functionalized reduced graphene oxide

In this paper, we present a simple and label-free colorimetric biosensor for detection of the nopaline synthase (NOS) terminator in genetically modified (GM) plants. The "signal on" colorimetric biosensor was developed using a nanocomposite consisted of gold nanoparticles doped magnetic Fe₃O₄ nanoparticles (Fe₃O₄@Au NP), capture probe DNA (cDNA), and hemin-functionalized reduced graphene oxide nanosheets (H-GN). The nanocomposite was successfully prepared by means of Au-S bonds and the strong π interactions between cDNA and H-GN. The sensing approach is based on the excellent peroxidase-mimicking activity of H-GN and its different electrostatic interactions with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). In presence of the target NOS, the cDNA in the nanocomposite will hybridize with its complementary sequence, and form dsDNA structure. Due to the weak π interactions between dsDNA and H-GN, a portion of H-GN will be released from the surface of Fe₃O₄@Au NPs and transferred into solution. After magnetic separation was performed, the supernatant was incubated with 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂. The released H-GN can catalyze the oxidation reaction of TMB and turn the colorless solution blue. This "signal-on" colorimetric biosensor shows a broad linear range of 0.5-100 nM for the target NOS, with a 0.19 nM detection limit. The application of the biosensor for determination of NOS segments in samples of GM and non-GM tomatoes shows that it can discriminate between GM and non-GM plants. The reliability of the method for samples of NOS-spiked GM tomato suggests satisfactory recoveries in the range of 93.6%-94.2%.