Aptamer-mediated colorimetric method for rapid and sensitive detection of chloramphenicol in food

A facile electrochemical aptasensor for chloramphenicol detection based on synergistically photosensitization enhanced by SYBR Green I and MoS2

This study reports the development of a photocatalytic electrochemical aptasensor for the purpose of detecting chloramphenicol (CAP) antibiotic residues in water by utilizing SYBR Green I (SG) and chemically exfoliated MoS2 (ce-MoS2) as synergistically signal-amplification platforms. The Au nanoparticles (AuNPs) were electrodeposited onto the surface of an indium tin oxide (ITO) electrode. After that, the thiolate-modified cDNA, also known as capture DNA, was combined with the aptamer. Subsequently, photosensitized SG molecules and ce-MoS2 nanomaterial were inserted into the groove of the resultant double-stranded DNA (dsDNA). The activation of the photocatalytic process upon exposure to light resulted in the generation of singlet oxygen. The singlet oxygen effectively split the dsDNA, resulting in significant enhancement in the current of [Fe(CN)6]3-/4-. When the CAP was present, both SG molecules and ce-MoS2 broke away from the dsDNA, which turned off the photosensitization response, leading to significant reduction in the current of [Fe(CN)6]3-/4-. Under the optimal conditions, the aptasensor exhibited a linear relationship between the current of [Fe(CN)6]3-/4- with logarithmic concentrations of CAP from 20 to 1000 nM, with a detection of limit (3σ) of 3.391 nM. The aptasensor also demonstrated good selectivity towards CAP in the presence of interfering antibiotics, such as tetracycline, streptomycin, levofloxacin, ciprofloxacin, and sulfadimethoxine. Additionally, the results obtained from the analysis of natural water samples using the proposed aptasensor were consistent with the findings acquired through the use of a liquid chromatograph-mass spectrometer. Therefore, with its simplicity and high selectivity, this aptasensor can potentially detect alternative antibiotics in environmental water samples by replacing the aptamers based on photosensitization.

An aptasensor for chloramphenicol determination based on dual signal output of photoelectrochemistry and colorimetry

In the present work, we developed an aptasensor to determine chloramphenicol (CAP) based on the dual signal output of photoelectrochemistry (PEC) and colorimetry. The Fe3+-doped porous tungsten trioxide was prepared by sol-gel method and coated on the ITO conductive glass to form ITO/p-W(Fe)O3. After assembling the captured DNA (cDNA) and the aptamer of CAP (apt) successively, the constructed ITO/p-W(Fe)O3-cDNA/apt aptasensor exhibited excellent photocurrent response under visible light irradiation in the presence of glucose, which provided the feasibility for PEC measurement with high sensitivity. In the presence of CAP, the apt left the ITO/p-W(Fe)O3 surface and AuNPs linked on the probe DNA would be assembled on it, which led to the decrease of photocurrent. Thanks to the oxidase-mimic catalytic performance of AuNPs and the recycling catalytic hydrolysis by exonuclease I, the measurement signal of the aptasensor could be amplified significantly, and the photocurrent decrease of the aptasensor was linearly related to the concentration of CAP in the range of 1.0 pM-10.0 nM and low detection limit was 0.36 pM. Meanwhile, the H2O2 produced from catalytic oxidation of glucose could oxidize TMB to blue oxTMB under HRP catalysis, which absorbance at 652 nm was linearly related to the concentration of CAP in the range of 5.0 pM-10.0 nM and low detection limit was 1.72 pM. Therefore, an aptasensor that determine CAP in real samples was successfully constructed with good precision of the relative standard deviation less than 5.7 % for PEC method and 7.3 % for colorimetric method, which can meet the analysis needs in different scenarios.

Advances in magnetic analytical extraction techniques for detecting antibiotic residues in edible samples

The widespread use of antibiotics in agricultural and animal husbandry to treat bacterial illnesses has resulted in a rise in antibiotic-resistant bacteria. These bacteria can grow when antibiotic residues are present in food items, especially in edible animal products. As a result, it is crucial to monitor and regulate the amounts of antibiotics in food. Magnetic analytical extractions (MAEs) have emerged as a potential approach for extracting antibiotic residues from food using magnetic nanoparticles (MNPs). Recent improvements in MAEs have resulted in the emergence of novel MNPs with better selectivity and sensitivity for the extraction of antibiotic residues from food samples. Consequently, this review paper addresses current developments in MAE for extracting antibiotic residues from edible samples. It also provides a critical analysis of contemporary MAE practices. The current issues and potential future developments in this field are also discussed, thereby providing a framework for future study paths.

Tuning the shell thickness of N-doped interconnected hollow carbon sphere for the electrochemical sensing of antibiotic drug chloramphenicol

N-doped hollow carbon spheres (NHCSs) with different shell thicknesses are constructed using various amounts of SiO2 precursor. An interconnected framework with diminished wall thickness ensures an efficient and continuous electron transport which helps to enhance the performance of NHCS. Improvement of the electrocatalytic performance was shown in the determination of antibiotic drug chloramphenicol (CAP) due to the unique hollow thin shell morphology, ample defect sites, accessible surface area, higher surface-to-volume ratio and an synergistic effect. Boosted electrocatalytic activity of 1.5 N-doped HCS (1.5 NHCS) was applied to detect CAP with a linear range and detection limit of 1-1150 µM and 0.098 µM (n = 3), respectively, with superior storage stability and considerable sensitivity. These results suggest that the proposed work can be successfully applied to the determination of CAP in milk and water samples.

A comprehensive review of the application of Zr-based metal-organic frameworks for electrochemical sensors and biosensors

A family of inorganic-organic hybrid crystalline materials made up of organic ligands and metal cations or clusters is known as metal-organic frameworks (MOFs). Because of their unique stability, intriguing characteristics, and structural diversity, zirconium-based MOFs (Zr-MOFs) are regarded as one of the most interesting families of MOF materials for real-world applications. Zr-MOFs that have the ligands, metal nodes, and guest molecules enclosed show distinct electrochemical reactions. They can successfully and sensitively identify a wide range of substances, which is important for both environmental preservation and human health. The rational design and synthesis of Zr-MOF electrochemical sensors and biosensors, as well as their applications in the detection of drugs, biomarkers, pesticides, food additives, hydrogen peroxide, and other materials, are the main topics of this comprehensive review. We also touch on the current issues and potential future paths for Zr-MOF electrochemical sensor research.

Ultrasensitive detection strategy for CAP by molecularity imprinted SERS sensor based on multiple synergistic enhancement of SiO2@AuAg with MOFs@Au signal carrier

Chloramphenicol (CAP) residue in food can cause great harm to human health, it is important to develop a rapid and sensitive method to detect CAP. Here, molecularly imprinted polymer (MIP) was combined with metal-organic frameworks@Au (MOFs@Au) collaborative construction surface-enhanced Raman spectroscopy (SERS) based aptasensor for CAP ultrasensitive detection. MOFs@Au first carried the Raman signal molecule toluidine blue (TB) and aptamer to form MOFs@Au@TB@Apt. In addition, rMIP (CAP was removed) was dropped onto the uniform three-dimensional (3D) SERS substrate SiO2@AuAg to form SiO2@AuAg@rMIP. In the presence of target CAP, it could be specifically captured with rMIP by covalent interaction and was recognised by the aptamer. During this time, SiO2@AuAg@rMIP@CAP could selectively connect MOFs@Au@TB@Apt to realise synergistic enhance the Raman signal. Based on this principle, the proposed SERS aptasensor exhibits excellent sensitivity with a detection limit of 7.59×10-13 M for CAP, providing a new strategy for trace detection in food.

Dual-Track Multifunctional Bimetallic Metal-Organic Frameworks for Antibiotic Enrichment and Detection

The improper use and overuse of antibiotics have led to significant burdens and detrimental effects on the environment, food supply, and human health. Herein, a magnetic solid-phase extraction program and an optical immunosensor based on bimetallic Ce/Zr-UiO 66 for the detection of antibiotics are developed. A magnetic Fe3O4@SiO2@Ce/Zr-UiO 66 metal-organic framework (MOF) is prepared to extract and enrich chloramphenicol from fish, wastewater, and urine samples, and a horseradish peroxidase (HRP)-Ce/Zr-UiO 66@bovine serum protein-chloramphenicol probe is used for the sensitive detection of chloramphenicol based on the dual-effect catalysis of Ce and HRP. In this manner, the application of Ce/Zr-UiO 66 in integrating sample pretreatment and antibiotic detection is systematically investigated and the associated mechanisms are explored. It is concluded that Ce/Zr-UiO 66 is a versatile dual-track material exhibiting high enrichment efficiency (6.37 mg g-1) and high sensitivity (limit of detection of 51.3 pg mL-1) for chloramphenicol detection and serving as a multifunctional MOF for safeguarding public health and hygiene.

Carbon-dots encapsulated luminescent metal-organic frameworks@surface molecularly imprinted polymer: A facile fluorescent probe for the determination of chloramphenicol

In this study, carbon dots (CDs)-encapsulated luminescent metal-organic frameworks@surface molecularly imprinted polymer (CDs@MOF@SMIP) was facilely prepared and applied as fluorescent probe for specific identification and sensitive detection of chloramphenicol (CAP) in food. Fluorescent CDs, serving as signal tags, were encapsulated within metal-organic backbones (ZIF-8), yielding luminescent MOF materials (CDs@ZIF-8). The synthesized CDs, CDs@ZIF-8 and CDs@ZIF-8@SMIP were investigated by morphological and structural characterizations (UV-Vis, XRD, FT-IR, BET, TEM). The CDs@ZIF-8@SMIP probe was demonstrated to have remarkable selectivity and sensitivity towards CAP. Its fluorescence decreased linearly with CAP concentration from 0.323 μg L-1 (0.001 μM) to 8075.0 μg L-1 (25.0 μM), featuring a low detection limit of 0.08 μg L-1. The CDs@ZIF-8@SMIP-based fluorescence strategy achieved satisfactory recoveries (95.5 % - 101.0 %) in CAP-spiked commercial foods with RSD < 4.4 % (n = 3). These results indicate that this method can effectively detect trace CAP in food matrices and has broad application prospects.

Iodinated organic molecule as tag for inductively coupled Plasma-mass spectrometry aptamer assays

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) aptamer-based assays using metallic nanostructures or chelates as exogenous tags have gained growing attention in the last decade. We describe here a proof-of-concept study based on the exploitation of a simple organic molecule as a tag, i.e.l-thyroxine carrying four iodine atoms detectable by ICP-MS. A solid-phase assay involving the structure-switching format was deployed for the detection of the small molecule l-tyrosinamide as model target. The overall design involved (i) a reporter agent consisting of a DNA aptamer incorporating a single l-thyroxine label at its end and (ii) a capture agent, which is a partially complementary strand, immobilized on a microplate. Limit of detection in the nanomolar range was reported. The present labeling approach was further developed for the detection of a model protein (α-thrombin), using a sandwich mode, and proved effective in a biological matrix. We believe that the l-thyroxine tagging method could become a simple and robust alternative to commonly used labeling methods for ICP-MS aptamer-based assays.

Dual-Functional Tetrahedron Multivalent Aptamer Assisted Amplification-Free CRISPR/Cas12a Assay for Sensitive Detection of Salmonella

Salmonellosis continues to impose a significant economic burden globally. Rapid and sensitive detection of Salmonella is crucial to preventing the outbreaks of foodborne illnesses, yet it remains a formidable challenge. Herein, a dual-functional tetrahedron multivalent aptamer assisted amplification-free CRISPR/Cas12a assay was developed for Salmonella detection. In the system, the aptamer was programmatically assembled on the tetrahedral DNA nanostructure to fabricate a multivalent aptamer (TDN-multiApt), which displayed a 3.5-fold enhanced avidity over the monovalent aptamer and possessed four CRISPR/Cas12a targeting fragments to amplify signal. Therefore, TDN-multiApt could directly activate Cas12a to achieve the second signal amplification without any nucleic acid amplification. By virtue of the synergism of high avidity and cascaded signal amplifications, the proposed method allowed the ultrasensitive detection of Salmonella as low as 7 cfu mL-1. Meanwhile, this novel platform also exhibited excellent specificity against target bacteria and performed well in the detection of various samples, indicating its potential application in real samples.

Not Only Graphene Two-Dimensional Nanomaterials: Recent Trends in Electrochemical (Bio)sensing Area for Biomedical and Healthcare Applications

Two-dimensional (2D) nanomaterials (e.g., graphene) have attracted growing attention in the (bio)sensing area and, in particular, for biomedical applications because of their unique mechanical and physicochemical properties, such as their high thermal and electrical conductivity, biocompatibility, and large surface area. Graphene (G) and its derivatives represent the most common 2D nanomaterials applied to electrochemical (bio)sensors for healthcare applications. This review will pay particular attention to other 2D nanomaterials, such as transition metal dichalcogenides (TMDs), metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and MXenes, applied to the electrochemical biomedical (bio)sensing area, considering the literature of the last five years (2018-2022). An overview of 2D nanostructures focusing on the synthetic approach, the integration with electrodic materials, including other nanomaterials, and with different biorecognition elements such as antibodies, nucleic acids, enzymes, and aptamers, will be provided. Next, significant examples of applications in the clinical field will be reported and discussed together with the role of nanomaterials, the type of (bio)sensor, and the adopted electrochemical technique. Finally, challenges related to future developments of these nanomaterials to design portable sensing systems will be shortly discussed.

Multi-aptamer-mediated hairpin allosteric and aptamer-assisted CRISPR system for detection of S. pneumoniae and S. aureus

A novel nucleic acid aptamer nanoprobes-mediated hairpin allosteric and aptamer-assisted CRISPR system for detection of Streptococcus pneumoniae and Staphylococcus aureus is presented. In this fluorescence assay system, utilizing the hairpin allosteric effect caused by the aptamer binding to the target bacteria, the detection of S. pneumoniae is first achieved through changes in fluorescence due to FRET. Subsequently, a Cas12a protein mixture is added to detect S. aureus. The amplified output signal is triggered by two methods to ensure the sensitivity of the method: the synergistic FRET effect is achieved by the assembly of multi-aptamer through the conjugation of streptavidin-biotin, and the trans-cleavage function of CRISPR/Cas 12a. Under the optimized conditions, the proposed hairpin allosteric aptasensor could achieve high sensitivity (a detection limit of 135 cfu/mL) and broad-concentration quantification (dynamic range of 103-107 cfu/mL) of S. pneumoniae. The aptamer-assisted CRISPR system for S. aureus detection showed good linearity (R2 = 0.996) in the concentration range 102-108 cfu/mL, with a detection limit of 39 cfu/mL. No cross-reactivity with other foodborne pathogenic bacteria was observed in both systems. Taking only 55 min, this method of multiple pathogen detection proved to be promising.

Recent Advances in Design and Application of Nanomaterials-Based Colorimetric Biosensors for Agri-food Safety Analysis

A colorimetric sensor detects an analyte by utilizing the optical properties of the sensor unit, such as absorption or reflection, to generate a structural color that serves as the output signal to detect an analyte. Detecting the refractive index of an analyte by recording the color change of the sensor structure on its surface has several advantages, including simple operation, low cost, suitability for onsite analysis, and real-time detection. Colorimetric sensors have drawn much attention owing to their rapidity, simplicity, high sensitivity and selectivity. This Review discusses the use of colorimetric sensors in the food industry, including their applications for detecting food contaminants. The Review also provides insight into the scope of future research in this area.

Lateral Flow Immunoassay Based on Quantum-Dot Nanobeads for Detection of Chloramphenicol in Aquatic Products

Quantum dot nanobeads (QBs) were used as signal source to develop competitive lateral flow immunoassay (LFIA) for the detection of chloramphenicol (CAP). The quantitative detection of CAP was achieved by calculating the total color difference (∆E) values of the test line (T line) using the images of test strips. QB-based LFIA (QBs-LFIA) allowed the effective dynamic linear detection of CAP in the range of 0.1-1.5 ng/mL. The limit of detection (LOD) was 3.0 ng/mL, which was 50 and 667 times lower than those achieved for two different brands of colloidal gold kits. The recoveries of CAP during real-sample detection were 82.82-104.91% at spiked levels of 0.1, 0.7, and 1.5 ng/mL. These results indicate that the developed QBs-LFIA facilitates the sensitive detection of CAP.

Aptamer-modified metal organic frameworks for measurement of food contaminants: a review

The measurement of food contaminants faces a great challenge owing to the increasing demand for safe food, increasing consumption of fast food, and rapidly changing patterns of human consumption. As different types of contaminants in food products can pose different levels of threat to human health, it is desirable to develop specific and rapid methods for their identification and quantification. During the past few years, metal-organic framework (MOF)-based materials have been extensively explored in the development of food safety sensors. MOFs are porous crystalline materials with tunable composition, dynamic porosity, and facile surface functionalization. The construction of high-performance biosensors for a range of applications (e.g., food safety, environmental monitoring, and biochemical diagnostics) can thus be promoted through the synergistic combination of MOFs with aptamers. Accordingly, this review article delineates recent innovations achieved for the aptamer-functionalized MOFs toward the detection of food contaminants. First, we describe the basic concepts involved in the detection of food contaminants in terms of the advantages and disadvantages of the commonly used analytical methods (e.g., DNA-based methods (PCR/real-time PCR/multiplex PCR/digital PCR) and protein-based methods (enzyme-linked immunosorbent assay/immunochromatography assay/immunosensor/mass spectrometry). Afterward, the progress in aptamer-functionalized MOF biosensors is discussed with respect to the sensing mechanisms (e.g., the role of MOFs as signal probes and carriers for loading signal probes) along with their performance evaluation (e.g., in terms of sensitivity). We finally discuss challenges and opportunities associated with the development of aptamer-functionalized MOFs for the measurement of food contaminants.

Rational Truncation of Aptamer for Ultrasensitive Aptasensing of Chloramphenicol: Studies Using Bio-Layer Interferometry

Aptamers are an excellent choice for the selective detection of small molecules. However, the previously reported aptamer for chloramphenicol suffers from low affinity, probably as a result of steric hindrance due to its bulky nature (80 nucleotides) leading to lower sensitivity in analytical assays. The present work was aimed at improving this binding affinity by truncating the aptamer without compromising its stability and three-dimensional folding. Shorter aptamer sequences were designed by systematically removing bases from each or both ends of the original aptamer. Thermodynamic factors were evaluated computationally to provide insight into the stability and folding patterns of the modified aptamers. Binding affinities were evaluated using bio-layer interferometry. Among the eleven sequences generated, one aptamer was selected based on its low dissociation constant, length, and regression of model fitting with association and dissociation curves. The dissociation constant could be lowered by 86.93% by truncating 30 bases from the 3' end of the previously reported aptamer. The selected aptamer was used for the detection of chloramphenicol in honey samples, based on a visible color change upon the aggregation of gold nanospheres caused by aptamer desorption. The detection limit could be reduced 32.87 times (1.673 pg mL^-1) using the modified length aptamer, indicating its improved affinity as well as its suitability in real-sample analysis for the ultrasensitive detection of chloramphenicol.

Fluorescent aptasensor for the ultrasensitive detection of antibiotic residue in food samples based on dumbbell DNA-mediated signal amplification

Sensitive and reliable detection of antibiotics is of great significance for environmental and food safety due to its high risk in trace concentrations. Herein, we developed a fluorescence sensing system for chloramphenicol (CAP) detection based on dumbbell DNA-mediated signal amplification. Two hairpin dimers (2H1 and 2H2) were employed as the building blocks to construct the sensing scaffolds. The CAP-aptamer binding in another hairpin H0 can liberate the trigger DNA, which then activates the cyclic assembly reaction between 2H1 and 2H2. The separation of FAM and BHQ in the formed product of cascaded DNA ladder yields a high fluorescence signal for CAP monitoring. Compared with the monomer hairpin assembly between H1 and H2, the dimer hairpin assembly between 2H1 and 2H2 exhibits enhanced signal amplification efficiency and reduced reaction time. The developed CAP sensor showed a wide linear range from 10 fM to 10 nM with a detection limit of 2 fM. Importantly, this sensing platform has been successfully applied to the determination of CAP in fish, milk, and water samples with satisfactory recovery and accuracy. With the advantages of high sensitivity, mix-and-read pattern, and robustness, our proposed CAP sensor can be used as a simple and routine tool for the detection of trace amounts of antibiotic residues.

A Highly Selective Electrochemical Sensor Based on Molecularly Imprinted Copolymer Functionalized with Arginine for the Detection of Chloramphenicol in Honey

Developing an efficient method for chloramphenicol (CAP) detection is of great significance for food safety. Arginine (Arg) was selected as a functional monomer. Benefiting from its excellent electrochemical performance, which is different from traditional functional monomers, it can be combined with CAP to form a highly selective molecularly imprinted polymer (MIP) material. It overcomes the shortcoming of poor MIP sensitivity faced by traditional functional monomers, and achieves high sensitivity detection without compounding other nanomaterials, greatly reducing the preparation difficulty and cost investment of the sensor. The possible binding sites between CAP and Arg molecules were calculated by molecular electrostatic potential (MEP). A low-cost, non-modified MIP electrochemical sensor was developed for the high-performance detection of CAP. The prepared sensor has a wide linear range from 1 × 10^-12 mol L^-1 to 5 × 10^-4 mol L^-1, achieves a very low concentration CAP detection, and the detection limit is 1.36 × 10^-13 mol L^-1. It also exhibits excellent selectivity, anti-interference, repeatability, and reproducibility. The detection of CAP in actual honey samples was achieved, which has important practical value in food safety.

Ultrasensitive detection of ineradicable and harmful antibiotic chloramphenicol residue in soil, water, and food samples

Chloramphenicol (CAP) is a harmful antibiotic that inevitably enters our food chain through natural or manmade means. Its ineradicable residue pollutes soils and water, accumulates in plants and animal products, and eventually affects human health. An ultrasensitive method for detecting and monitoring CAP is therefore urgently required. Herein, we report an ultrafast extraction and amperometry detection method based on a graphite-sulfate-modified electrode for detecting CAP in soil, water, and food samples. The graphite sulfate is prepared by the oxidation method and its structural properties are comprehensively investigated. The developed sensor electrode showed a wider linear range of 0.3-32.0 μg kg^-1 and an ultralow detection limit of 0.1 μg kg^-1, both of which meet the European Commission Reg 1871/2019 reference points for action. The method works well with both meat and plant samples, achieving CAP recoveries ranging from 90.8 to 99.1% even at low concentrations. Moreover, the sensor electrode shows more than 95% selectivity toward CAP detection in the soil, water, and food matrices. The developed method exhibits good repeatability and reproducibility in the analysis of real samples.

Aptamer-based NanoBioSensors for seafood safety

Chemical and biological contaminants are of primary concern in ensuring seafood safety. Rapid detection of such contaminants is needed to keep us safe from being affected. For over three decades, immunoassay (IA) technology has been used for the detection of contaminants in seafood products. However, limitations inherent to antibody generation against small molecular targets that cannot elicit an immune response, along with the instability of antibodies under ambient conditions greatly limit their wider application for developing robust detection and monitoring tools, particularly for non-biomedical applications. As an alternative, aptamer-based biosensors (aptasensors) have emerged as a powerful yet robust analytical tool for the detection of a wide range of analytes. Due to the high specificity of aptamers in recognising targets ranging from small molecules to large proteins and even whole cells, these have been suggested to be viable molecular recognition elements (MREs) in the development of new diagnostic and biosensing tools for detecting a wide range of contaminants including heavy metals, antibiotics, pesticides, pathogens and biotoxins. In this review, we discuss the recent progress made in the field of aptasensors for detection of contaminants in seafood products with a view of effectively managing their potential human health hazards. A critical outlook is also provided to facilitate translation of aptasensors from academic laboratories to the mainstream seafood industry and consumer applications.

A facile aptamer-based sensing strategy for dopamine detection through the fluorescence energy transfer between dye and single-wall carbon nanohorns

Dopamine (DBA) as an important biomarker, plays a crucial role in disease diagnosis. In this study, we have developed a fast and simple aptamer-based fluorescence strategy which used single-wall carbon nanohorns (SWCNHs) as a quencher for dopamine detection. SWCNHs were negatively charged after pretreated, which improved its dispersion in solution. 5-carboxy-fluorescein (FAM) was used to label dopamine aptamer. In the absence of dopamine, FAM-modified aptamer could be absorbed onto the SWCNHs surface due to π-π interaction, resulting in the fluorescence intensity decreased. Dopamine could specifically bind with FAM-DNA to form G-quadruplex, which could not be absorbed onto the surface of SWCNHs. Hence, the fluorescence of FAM-DNA recovered, and the fluorescent intensity as a function of different concentrations of dopamine was measured. We obtained a detection limit of 5 μM for this detection system with a linear detection range of 0.02-2.20 mM. Furthermore, the feasibility of the innovative detection system has been verified by detecting dopamine in spiked serum samples.

A competitive colorimetric aptasensor for simple and sensitive detection of kanamycin based on terminal deoxynucleotidyl transferase-mediated signal amplification strategy

We developed a rapid and sensitive colorimetric biosensor based on competitive recognition between kanamycin (KAN), magnetic beads-kanamycin (MBs-KAN) and aptamer and terminal deoxynucleotidyl transferase (TdT)-mediated signal amplification strategy. In the absence of KAN, aptamers recognize MBs-KAN. TdT can amplify oligonucleotides to the 3'-OH ends of aptamers, with biotin-dUTP being embedded in the long single stranded DNA (ssDNA). Then the assay produced visual readout due to the horseradish peroxidase (HRP)-catalyzed color change of the substrate after the linkage between biotin and streptavidin (SA)-HRP. In the presence of KAN, however, aptamers tend to bind free KAN rather than MBs-KAN. In this case, aptamers are isolated by magnetic separation, resulting in the failure of signal amplification and catalytic signals. This competitive colorimetric sensor showed excellent selectivity toward KAN with the limit of detection (LOD) as low as 9 pM. And recovery values were between 93.8 and 107.8% when spiked KAN in milk and honey samples.

High-Throughput Aptamer Microarrays for Fluorescent Detection of Multiple Organophosphorus Pesticides in Food

A novel high-throughput aptamer microarray fluorescent method based on thioflavin T (ThT) was established for the sensitive detection of phoxim, parathion, fensulfothion, and isocarbophos. In this work, the aptamers in binding buffer tended to have the antiparallel G-quadruplex structure, which can bind ThT and release its potential fluorescence signal. However, when the organophosphorus pesticides (OPs) were present, partial aptamers preferred to bind them, forcing the displacement of ThT from the G-quadruplex and resulting in the significant decrease in fluorescence signal. Under optimal experimental conditions (12T spacer, 300 nM aptamer, and 80 μM ThT), the OP aptamer microarray has low limits of detection of 25.4 ng/mL for phoxim, 12.0 ng/mL for parathion, 7.7 ng/mL for fensulfothion, and 9.9 ng/mL for isocarbophos. The accuracy and reliability of the method is further verified by testing the recovery rate of OPs spiked in two different complicated sample matrices (pears and radishes). It is worth mentioning that not only the developed aptamer microarray technology has low sensitivity and a broad spectrum, but it also allows for high-throughput and rapid analysis of a variety OPs, which overcomes some of the shortcomings of other OP detection methods.

Dual-Enzyme-Based Signal-Amplified Aptasensor for Zearalenone Detection by Using CRISPR-Cas12a and Nt.AlwI

Zearalenone (ZEN) is harmful to animals and human beings, so it is very important to develop a rapid and sensitive method for the detection of ZEN. In this paper, we proposed a novel ZEN-monitoring method using two aptamers as recognition elements and EnGen LbaCas12a and Nt.AlwI nicking endonuclease as signal amplifiers. When ZEN was present, it bound to the aptamer Z0 and, Z1 was released into solution. The solution was then separated and the Nt.AlwI enzyme was added in order to form a nicking-enzyme cycle, thereby producing large amounts of the ssDNA Z3 for 30 min. The Z3 formed a CRISPR-Cas12a-Z3 complex with CRISPR-Cas12a, activated the trans-cleavage ability of Cas12a, cleaved the Quenched Reporter for 20 min, and underwent fluorescence recovery. The aptasensor was able to sensitively detect ZEN in the linear range of 1–1000 pg/mL, with a detection limit as low as 0.213 pg/mL. The detection time lasted for 2 h. Additionally, this detection technology can also be used to monitor other hazards.

An entropy driven catalytic reaction powered DNA motor for simultaneous detection of ochratoxin A and chloramphenicol in food

An entropy driven catalytic reaction powered DNA motor was proposed for simultaneous detection of ochratoxin A (OTA) and chloramphenicol (CAP) in food. The dumbbell hairpin structure was formed by the two aptamers of OTA and CAP. The dumbbell hairpin can be opened by the interaction of OTA and CAP with their aptamers. The tails of the end of dumbbell hairpin sequence can induce the entropy driven catalytic reactions on the AuNPs, causing the sustained releasing of the fluorophore labeled DNA sequences. The recovery of fluorescent intensities can be used for quantitative detection of OTA and CAP. The limit of detection reached 2 pM for OTA and 6 pM for CAP respectively, which was great improved by entropy driven amplification of the self-powdered DNA motor. This strategy is simple and sensitive and only needs one-step operation. It exhibits promising potentiality in food quality control and food security supervision.

Rapid detection and prediction of chloramphenicol in food employing label-free HAu/Ag NFs-SERS sensor coupled multivariate calibration

Considering growing food safety issues, hollow Au/Ag nano-flower (HAu/Ag NFs) nanosensor has been synthesized for label-free and ultrasensitive detection of chloramphenicol (CP) via integrating the surface-enhanced Raman scattering (SERS) and multivariate calibration. As the anisotropic plasmonic nanomaterials, HAu/Ag NFs had numerous nano-chink on their surface, which offered huge hotspots for analytes. CP generated a strong SERS signal while adsorbed on the surface of HAu/Ag NFs and noted excellent linearity with 1st derivative-competitive adaptive reweighted sampling-partial least squares (CARS-PLS) in the range of 0.0001-1000 µg/mL among the four applied multivariate calibrations. Additionally, CARS-PLS generated the lowest prediction error (RMSEP) of 0.089 and 0.123 µg/mL for milk and water samples, respectively, and any CARS-PLS model could be used for both samples according to T-test results (P > 0.05). The intra- and interday recovery for both samples were in the range of 92.62-96.74% with CV < 10%, suggested the proposed method has excellent accuracy and precision.

Aptamer-Based Fluorescent Biosensor for the Rapid and Sensitive Detection of Allergens in Food Matrices

Food allergies have seriously affected the life quality of some people and even endangered their lives. At present, there is still no effective cure for food allergies. Avoiding the intake of allergenic food is still the most effective way to prevent allergic diseases. Therefore, it is necessary to develop rapid, accurate, sensitive, and reliable analysis methods to detect food allergens from different sources. Aptamers are oligonucleotide sequences that can bind to a variety of targets with high specificity and selectivity, and they are often combined with different transduction technologies, thereby constructing various types of aptamer sensors. In recent years, with the development of technology and the application of new materials, the sensitivity, portability, and cost of fluorescence sensing technology have been greatly improved. Therefore, aptamer-based fluorescence sensing technology has been widely developed and applied in the specific recognition of food allergens. In this paper, the classification of major allergens and their characteristics in animal and plant foods were comprehensively reviewed, and the preparation principles and practical applications of aptamer-based fluorescence biosensors are summarized. In addition, we hope that this article can provide some strategies for the rapid and sensitive detection of allergens in food matrices.

Aptamer-based diagnostic and therapeutic approaches in animals: Current potential and challenges

Fast and precise diagnosis of infectious and non-infectious animal diseases and their targeted treatments are of utmost importance for their clinical management. The existing biochemical, serological and molecular methods of disease diagnosis need improvement in their specificity, sensitivity and cost and, are generally not amenable for being used as points-of-care (POC) device. Further, with dramatic changes in environment and farm management practices, one should also arm ourselves and prepare for emerging and re-emerging animal diseases such as cancer, prion diseases, COVID-19, influenza etc. Aptamer – oligonucleotide or short peptides that can specifically bind to target molecules – have increasingly become popular in developing biosensors for sensitive detection of analytes, pathogens (bacteria, virus, fungus, prions), drug residues, toxins and, cancerous cells. They have also been proven successful in the cellular delivery of drugs and targeted therapy of infectious diseases and physiological disorders. However, the in vivo application of aptamer-mediated biosensing and therapy in animals has been limited. This paper reviews the existing reports on the application of aptamer-based biosensors and targeted therapy in animals. It also dissects the various modifications to aptamers that were found to be successful in in vivo application of the aptamers in diagnostics and therapeutics. Finally, it also highlights major challenges and future directions in the application of aptamers in the field of veterinary medicine.

An fluorescence resonance energy transfer sensing platform based on signal amplification strategy of hybridization chain reaction and triplex DNA for the detection of Chloramphenicol in milk

The use of chloramphenicol (CAP) in food had been strictly regulated or banned in many countries. Herein, an enzyme-free fluorescence resonance energy transfer (FRET) strategy was established for sensitive, rapid and specific detection of CAP in milk, which was based on triplex DNA and hybridization chain reaction amplification. CAP can specifically bind to the aptamer and release the trigger sequence, causing HCR to efficiently prime and forming triplex DNA, hence the FRET pairs (FAM and TAMRA) were close enough to cause fluorescent decreases. Consequently, CAP can be quantitatively detected by measuring the fluorescence reduction at 520 nm, and the reliability of the method was confirmed by enzyme-linked immunosorbent assay. The limit of CAP detection for 1.2 pg·mL-1, and the average recoveries of milk samples were 97.5%-106%, and the relative standard deviation were 3.9%-5.3%. Thus, this method has a wide range of potential applications in CAP detection.

Detection of chloramphenicol with an aptamer-based colorimetric assay: critical evaluation of specific and unspecific binding of analyte molecules

A chloramphenicol (CAP)-binding aptamer of 80 nucleotides (nt) was reported in 2011. In 2014, it was truncated to 40 nt and has since been used by most researchers, although a careful binding study is still lacking. In this work, binding assays using isothermal titration calorimetry and various DNA-staining dyes were performed. By comparing the truncated aptamer with three control sequences, no specific binding of CAP was observed in each case. The secondary structures of the original and truncated aptamers were analyzed, and it was shown that the likelihood of the truncated aptamer to retain the same binding mechanism as the original sequence is low. We further examined gold nanoparticle (AuNP)-based label-free colorimetric assays. By quantifying the extinction ratio at 620 nm over that at 520 nm, a similar color response was observed regardless of the sequence of DNA, suggesting the color change mainly reflected other events such as the adsorption of CAP by the AuNPs, instead of aptamer binding to CAP. Salt-induced aggregation experiments suggested direct adsorption of CAP on AuNPs. CAP only weakly inhibited DNA adsorption by AuNPs but did not displace pre-adsorbed DNA. Therefore, CAP adsorption by AuNPs needs to be considered when designing related sensors, for example, by using non-aptamer sequences as controls. This work calls for careful confirmation of aptamer binding and control experiments for designing aptamer and AuNP-based biosensors.

Detection of toxin B of Clostridium difficile based on immunomagnetic separation and aptamer-mediated colorimetric assay

Clostridium difficile can cause antibiotic-associated diarrhoea or pseudo-membranous colitis in humans and animals. Currently, the various methods such as microbiological culture, cytotoxic assay, ELISA and polymerase chain reaction have been used to detect Clostridium difficile infection (CDI). These conventional methods, however, require long detection time and professional staff. The paper is to describe a simple strategy which employs immunomagnetic separation and aptamer-mediated colorimetric assay for the detection of toxin B of C. difficile (TcdB) in the stool samples. HRP-labelled aptamer against TcdB selected by SELEX was firstly captured on the surface of magnetic beads (MB) by DNA hybridization with a complementary strand. In the presence of TcdB, aptamer specifically recognized and bound TcdB, disturbing the DNA hybridization and causing the release of HRP-aptamer from MB. This reduced the catalytic capacity of HRP and consequently the absorption intensity. As there was a relationship between the decrease in the absorption intensity and target concentration, a quantitative analysis of TcdB can be accomplished by the measurement of the absorption intensity. Under the optimal conditions, the assay system is able to detect TcdB at a concentration down to 5 ng ml^-1 . Moreover the method had specificity of 97% and sensitivity of 66% and the system remained excellent stability within 4 weeks. The proposed method is a valuable screening procedure for CDI and can be extended readily to detection of other clinically important pathogens.

Dual Triple Helix-Aptamer Probes for Mix-and-Read Detecting Antibiotics in Fish and Milk

Antibiotic abuse in agricultural products leads to serious food safety issues. To this end, we proposed a mix-and-read and enzyme-free amplified assay for antibiotics based on a dual triple helix-aptamer probe, potentially applicable for on-site monitoring of antibiotic residues. A dual triple helix-aptamer probe can leverage the response toward target molecules without enzyme-based amplification, rendering it sensitive and robust for profiling target molecules. The proposed assay allowed mix-and-read detection of chloramphenicol with a detection limit of 0.18 nM. Besides, it accommodated for specifically resolving chloramphenicol among other antibiotics. Chloramphenicol residual in aquatic products in fish and milk can be precisely determined. Thus, the aptamer probe deems to enrich the toolbox for managing antibiotic use.

Directly profiling intact Staphylococcus aureus in water and foods via enzymatic cleavage aptasensor

Staphylococcus aureus (S. aureus) causes serious food-borne diseases, and tools able to directly profile intact S. aureus would greatly facilitate food safety and public health. Herein, we proposed a biosensing platform for culture-independent and separation-free profiling S. aureus, thus allow us to directly detect intact S. aureus in complex samples. The binding protection effect of aptamer-cell complex was introduced to construct the aptasensor, and it allowed to eliminate the optimization of aptamer probe sequences. The proposed aptasensor, terms enzymatic cleavage aptasensor could achieve a sensitive (a detection limit of 64 CFU/mL) and broad-concentration quantification (dynamic range 10²-10⁷ CFU/mL) of S. aureus. Furthermore, it could specifically identify intact S. aureus in complex samples, and the quantifying of S. aureus was achieved in tap water, milk and porker with high precision. Therefore, enzymatic cleavage aptasensor could be a good candidate for on-site biosensing platform of S. aureus, as well as other pathogens by replacing the aptamer sequences.

New application of Mn-doped ZnS quantum dots: phosphorescent sensor for the rapid screening of chloramphenicol and tetracycline residues

In this work, a new application of Mn-doped ZnS quantum dots (Mn-ZnS QDs) was developed to screen chloramphenicol (CAP) and tetracycline (TC) residues simply and rapidly. Mn-ZnS QDs synthesized by a hydration method and modified by l-cysteine for better stability emit phosphorescence at 583 nm with the excitation wavelength at 289 nm. Based on the overlap of the Mn-ZnS QDs excitation spectra and CAP or TCs ultraviolet spectra, the excited light of the Mn-ZnS QDs was partially absorbed by CAP or TCs owing to the inner-filter effect (IFE), leading to a decrease in the phosphorescence intensity. The phosphorescence intensities of the samples prepared by mixing different TCs and CAP were in good agreement with the expected results from adding a single antibiotic sample. Therefore, the total molar concentrations of CAP and TCs could be screened based on the linear equation of a single standard substance. Represented by tetracycline (TC), as a member of the tetracycline family, under optimized conditions, showed a good linear relational concentration range over 4 orders of magnitude from 50 to 1.5 × 105 nM with a limit of detection (LOD; S/N ratio = 3) down to 8.6 nM. The phosphorescent sensor was also used to detect total TCs in actual samples successfully. The evaluations of the recovery rate and selectivity were good. These results demonstrated that the presented phosphorescent sensor can be a simple and rapid screening platform for CAP and TCs.

Bimetallic organic framework-based aptamer sensors: a new platform for fluorescence detection of chloramphenicol

A fluorescence method for the quantitative detection of chloramphenicol (CAP) has been developed using phosphate and fluorescent dye 6-carboxy-x-rhodamine (ROX) double-labeled aptamers of CAP and the bimetallic organic framework nanomaterial Cu/UiO-66. Cu/UiO-66 was prepared by coordinate bonding of metal organic framework (MOF) nanomaterial UiO-66 with copper ions. Cu/UiO-66 contains a large number of metal defect sites, which can be combined with phosphate-modified nucleic acid aptamers through strong coordination between phosphate and zirconium to form "fluorescence turn-on" sensors. In the absence of CAP, all single-stranded aptamers were adsorbed on the surface of Cu/UiO-66 through π-π stacking between single-stranded DNA and Cu/UiO-66, which brings the ROX fluorophores and Cu/UiO-66 into close proximity. The ROX fluorescence of aptamers was then quenched by Cu/UiO-66 through photoinduced electron transfer (PET). In the presence of CAP, however, CAP reacted with nucleic acid aptamers to form a special spatial structure, in which the ROX fluorophores were far away from the MOF surface via a change in the spatial structure of the aptamers, and the fluorescence of ROX was able to be recovered. The quantitative detection of CAP can be achieved by measuring the fluorescence signal of ROX using synchronous scanning fluorescence spectrometry. Under optimum conditions, the fluorescence intensities of ROX exhibit a good linear dependence on the concentration of CAP in the range of 0.2-10 nmol/L, with a detection limit of 0.09 nmol/L. The method has advantages of high sensitivity, good selectivity, and a low limit of detection. Graphical abstract.

Aptamer-based fluorometric determination of chloramphenicol by controlling the activity of hemin as a peroxidase mimetic

A method for the aptamer-based determination of chloramphenicol (CAP) was developed by exploiting the peroxidase mimicking activity of hemin. The method includes two hemin-modified DNA probes termed P1 and P2. P1, which was modified at its 5' end with one hemin monomer, contains the CAP-binding sequence. The hybridization between P1 and P2 brings the two hemin monomers in close proximity, resulting in the formation of a hemin dimer with low peroxidase mimicking activity. The duplex structure was dehybridized in the presence of CAP. The formed hemin monomer featured a strong peroxidase mimicking activity and catalyzed the conversion of non-fluorescent tyramine into fluorescent dityramine by hydrogen peroxide. Fluorescence (with an excitation/emission maxima at 320 and 410 nm, respectively) increased linearly in the 0.1 ng mL-1 to 10 ng mL-1 CAP concentration range. The detection limit based on the 3σ/k criterion reached 0.07 ng mL-1. The proposed assay was successfully employed for CAP detection in (spiked) honey samples with recoveries of 94.3-117.2%. Given its high sensitivity and good stability, this method shows potential in providing a platform for antibiotic detection.

Real-time detection of foodborne bacterial viability using a colorimetric bienzyme system in food and drinking water

Foodborne bacterial infection poses a serious threat to human health. As most diseases are caused by living bacteria, real-time assessment of bacterial viability is vitally important to the public health sector. Herein, we developed a simple and novel colorimetric assay based on the Glucose oxidase (GOD)/Horseradish peroxidase (HRP) bienzyme system for real-time monitoring of bacterial viability in food and drinking water. This bienzyme system is free of any chemical synthesis and only requires 3 sample handling steps. The color response is easily observable with the naked eye or recordable with a smartphone for precise determination of bacterial viability. The proposed strategy was validated with various bacteria both Gram-positive and Gram-negative, indicating its capability for broad-spectrum bacteria viability detection. Therefore, the proposed strategy shows promise for rapid and reliable quality control in food and drinking water.