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

Aptasensor based on entropy-driven catalytic amplification system for the sensitive detection of acetamiprid in Chinese herbal medicine

The traditional method of acetamiprid residue detection is difficult to operate, time-consuming, laborious and requires high professional knowledge of the detection personnel, which cannot meet the requirement of on-field rapid detection. For this reason, a colorimetric aptasensor based on an entropy-catalyzed amplification system was developed for the ultrasensitive and selective determination of acetamiprid. In the absence of acetamiprid, the aptamer and cDNA form a double-stranded structure. The formed hemin/G-quadruplex mimicking DNAzyme can catalyze the substrate ABTS to generate the colored ion ABTS- with the help of H2O2, and the solution turns blue-green. On the contrary, the presence of acetamiprid triggers the release of cDNA, which in turn initiates the entropy-driven system, resulting in the inability to form DNAzyme and therefore no blue-green color production in the solution. The quantity of acetamiprid determines the color. Under the optimal experimental conditions, the method showed a linear correlation (R2 = 0.9837) for the detection of acetamiprid in the concentration range of 0.1-100 ng/mL, with a limit of detection of 0.06 ng/mL. The developed method was used for the determination of acetamiprid in spiked Coix lacryma and Bitter almond, with recoveries in the range of 90.3-110.3%. The proposed enzyme-free and label-free assay can be developed into a simple, sensitive and rapid detection platform.

A magnetic bead-based dual-aptamer sandwich assay for quantitative detection of ciprofloxacin using CRISPR/Cas12a

Ciprofloxacin (CIP) is a broad-spectrum fluoroquinolone antibiotic, and its excessive residues in food and water sources pose potential risks to human health. Therefore, there is a need for a rapid and convenient method for its accurate quantification. The clustered regularly interspaced short palindromic repeat (CRISPR)/Cas12a system has gained extensive application in signal detection and amplification due to the trans-cleavage activity of Cas12a. In this study, we devised a novel magnetic bead-based dual sandwich aptamer coupled with a CRISPR/Cas12a system for the precise quantification of CIP in milk, river water, and honey. Through the incorporation of a magnetic bead-based dual aptamer sandwich approach, the concentration of CIP in the samples was pre-enriched. Additionally, by optimizing the Fluorescence-Quencher (F-Q) probe concentration, detection aptamer (APTd) concentration, and assay duration, the limit of blank (LOB) of the system was determined as 362 nM, while the limit of detection (LOD) was determined as 403 nM. This enabled the accurate quantification of CIP within the linear range of 0.5 μM to 0.2 mM with high specificity. Moreover, the performance of this detection method was comparable to that of high-performance liquid chromatography (HPLC) in river water, milk, and honey samples.

Enzyme-Assisted Solid-Phase Microextraction Coupled with a DNA Nanowalker for Dual-Amplified Detection of Chloramphenicol in Animal-Derived Food Products

Chloramphenicol (CAP), an aminoalcohol antibiotic, exerts its action on bacterial ribosomes, thereby obstructing protein synthesis. However, the use of CAP in husbandry may lead to its excessive accumulation in animal-derived food products. This presents potential risks to consumer health. This study developed a novel dual-amplification fluorescence detection method by integrating enzyme-assisted solid-phase microextraction (SPME) with a Fe3O4@Au NP-based DNA nanowalker for the detection of CAP in food. The combination of a quartz rod-based SPME biosensor and DNA nanowalker effectively eliminated matrix interference, enabling the conversion of CAP and enhancement of detection signals through two cyclic amplification processes. The strategy demonstrated high sensitivity with a limit of detection of 28.1 aM as well as a wide linear range from 0.1 fM to 1 nM (with R2 > 0.99). This method also demonstrates robust stability and accuracy in detecting trace amounts of CAP in both authentic and prepared positive samples.

Eu3+-Doped Anionic Zinc-Based Organic Framework Ratio Fluorescence Sensing Platform: Supersensitive Visual Identification of Prescription Drugs

Advanced techniques for both environmental and biological prescription drug monitoring are of ongoing interest. In this work, a fluorescent sensor based on an Eu3+-doped anionic zinc-based metal-organic framework (Eu3+@Zn-MOF) was constructed for rapid visual analysis of the prescription drug molecule demecycline (DEM), achieving both high sensitivity and selectivity. The ligand 2-amino-[1,1'-biphenyl]-4,4'-dicarboxylic acid (bpdc-NH2) not only provides stable cyan fluorescence (467 nm) for the framework through intramolecular charge transfer of bpdc-NH2 infinitesimal disturbanced by Zn2+ but also chelates Eu3+, resulting in red (617 nm) fluorescence. Through the synergy of photoinduced electron transfer and the antenna effect, a bidirectional response to DEM is achieved, enabling concentration quantification. The Eu3+@Zn-MOF platform exhibits a wide linear range (0.25-2.5 μM) to DEM and a detection limit (LOD) of 10.9 nM. Further, we integrated the DEM sensing platform into a paper-based system and utilized a smartphone for the visual detection of DEM in water samples and milk products, demonstrating the potential for large-scale, low-cost utilization of the technology.

Recent Advances in Aptasensors for Rapid Pesticide Residues Detection

Pesticides are applied widely to increase agricultural output and quality, however, this practice results in residual issues that not only harm the environment but also put people and animals' lives and health at risk. As a result, it is critical to find pesticide residues in a variety of sources, including crops, water supplies, and soil. Aptamers are more flexible in their synthesis and modification, have a high level of specificity, are inexpensive, and have good stability compared to conventional detection methods. They have therefore attracted a lot of interest in the industry. This study reviews the most recent aptasensor advancements in the detection of pesticide residues. Firstly, aptamers specifically binding to many pesticides are summarized. Secondly, the combination of aptasensors with colorimetric, fluorescent, surface enhanced Raman spectroscopy (SERS), resonance Light Scattering (RLS), chemiluminescence (CL), electrochemical, and electrochemiluminescence (ECL) technologies are systematically introduced, and their advantages and disadvantages are expounded. Importantly, the aptasensors for the detection of various pesticides (organochlorine, organophosphorus, neonicotinoids, carbamates, and pyrethroids) that have been developed so far are systematically analyzed and discussed. Finally, the furture prospects and challenges of the aptasensors are highlighted. It is expected to offer suggestions for the later creation of novel, highly effective and sensitive aptasensors for the detection of pesticide residues.

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.

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.

Ultrasensitive Aptasensor for Microcystin-LR Detection in Food Samples Based on Target-Activated Assembly of Y-Shaped Hairpin Probes

As a kind of algal toxin, microcystin-LR (MC-LR) causes a tremendous treat to food safety and the detection of trace levels of MC-LR is highly desirable. Herein, we developed an ultrasensitive aptasensor for MC-LR detection based on target-activated assembly of Y-shaped hairpins. The aptamer-target recognition initiates the assembly step between two Y-shaped hairpin probes through toehold-mediated DNA replacement. One of the hairpins was modified with FAM and BHQ. Through cyclic assembly reactions, a high fluorescence signal can be observed in the product. The detection limit is 0.2 pM for MC-LR detection. In addition, the biosensor is robust and has been successfully explored to assess the MC-LR concentrations in real fish and water samples with satisfactory recovery rates and good accuracy. The signal amplification can be gained through the cyclic Y-shaped hairpin assembly, which offers a simple, ultrasensitive, and reliable method for MC-LR monitoring in food samples.

Enzyme-modulated photothermal immunoassay of chloramphenicol residues in milk and egg using a self-calibrated thermal imager

Highly sensitive and accurate detection of chloramphenicol is of paramount importance for food safety. Herein, an enzyme-modulated photothermal immunosensor that uses a self-calibrated thermal imaging system (SCTIS) as signal read-out was developed for detecting chloramphenicol. In this immunosensor, alkaline phosphatase was used as a modulator of the photothermal conversion. It could hydrolyze the substrate into ascorbic acid, thereby reducing oxidized 3,3',5,5'-tetramethylbenzidine, which exhibited a near-infrared laser-driven photothermal effect. For precise temperature measurement, the SCTIS was designed by using the temperature compensation of a ceramic chip to enable real-time self-calibration of the temperature. This SCTIS-based immunosensor could detect chloramphenicol with a LOD of 9 pg/mL in 2 h, and relative standard derivations from 3.95% to 13.58%. The average recoveries in milk and egg samples ranged from 76% to 114%. This versatile sensing strategy can detect various targets by altering recognition elements, thus has wide applicability in food safety testing and monitoring.

Dual-mode photoelectrochemical/electrochemical sensor based on Z-scheme AgBr/AgI-Ag-CNTs and aptamer structure switch for the determination of kanamycin

A "signal-on" dual-mode aptasensor based on photoelectrochemical (PEC) and electrochemical (EC) signals was established for kanamycin (Kana) assay by using a novel Z-scheme AgBr/AgI-Ag-CNTs composite as sensing platform, an aptamer structure switch, and K₃[Fe(CN)₆] as photoelectron acceptor and electrochemical signal indicator. The aptamer structure switch was designed to obtain a "signal-off" state, which included an extended Kana aptamer (APT), one immobilized probe (P1), and one blocking probe (P2) covalently linked with graphdiyne oxide (GDYO) nanosheets. P1, P2, and aptamer formed the double helix structure, which resulted in the inhibited photocurrent intensity because of the weak conductivity of double helix layer and serious electrostatic repulsion of GDYO towards K₃[Fe(CN)₆]. In the presence of Kana, APT specifically bound to the target and dissociated from P1 and P2, and thus, a "signal-on" state was initiated by releasing P2-GDYO from the platform. Based on the sensing platform and the aptamer structure switch, the dual-mode aptasensor realized the linear determination ranges of 1.0 pM-2.0 μM with a detection limit (LOD) of 0.4 pM (for PEC method) and 10 pM-5.0 μM with a LOD of 5 pM (for EC method). The aptasensor displayed good application potential for Kana test in real samples.

Accelerated CRISPR/Cas12a-based small molecule detection using bivalent aptamer

CRISPR/Cas holds great promise for biosensing applications, however, restricted to nucleic acid targets. Here, we broaden the sensing target of CRISPR/Cas to small molecules via integrating a bivalent aptamer as a recognition component. Using adenosine 5'-triphosphate (ATP) as a model molecule, we found that a bivalent aptamer we selected could shorten the binding time between the aptamer and ATP from 30 min to 3 min, thus dramatically accelerating the detection of ATP. The accelerated bivalent aptamer binding to ATP was mainly ascribed to the extended conformation of the aptamer, which was stabilized through linking with a 5 T bases connector on specific loops of the monovalent aptamer. To facilitate on-site detection, we integrated lateral flow assay (LFA) with the CRISPR/Cas sensing strategy (termed BA-CASLFA) to serve as a visual readout of the presence of ATP. In addition, in the CASLFA platform, due to the unique characteristics of LFA, the thermal step of Cas12a inactivation can be omitted. The BA-CASLFA could output a colorimetric "TURN ON" signal for ATP within 26 min, which could be easily discriminated by the naked eye and sensitively quantified by the portable reader. Furthermore, we showed the versatility of BA-CASLFA for detecting kanamycin using a kanamycin bivalent aptamer obtained through the same design as the ATP bivalent aptamer. Therefore, this strategy is amenable to serve as a general sensing strategy for small molecular targets. The above work opened a new way in developing CRISPR-based on-site sensors for clinic diagnosis, food safety, and environmental analysis.

Triple-Helix Molecular Switch Triggered Cleavage Effect of DNAzyme for Ultrasensitive Electrochemical Detection of Chloramphenicol

The abuse of chloramphenicol (CAP) in animal-derived products leads to serious food safety problems, so the sensitive and accurate determination of CAP residues has great noteworthiness for public health. Herein, we present a novel electrochemical aptasensor that incorporates a poly(diallyldimethylammonium chloride) functionalized graphene/Ag@Au nanosheets (PDDA-Gr/Ag@Au NSs) composite modified electrode and a DNAzyme signal amplification effect triggered by a triple-helix molecular switch (THMS) for detecting CAP. The PDDA-Gr/Ag@Au NSs composite has the advantages of high surface area, great conductivity, and dispersibility and has successfully improved the electrochemical performance of the electrode. Specific interaction with CAP will cause the signal transduction probe (STP) to be released from the THMS. After that, the DNAzyme will be activated with the help of Pb²⁺ and remove the immobilized signal probe on the electrode surface. The signal change was recorded by square wave voltammetry (SWV) and led to an accurate quantification of CAP. With all these features, the proposed sensing strategy yielded a satisfactory analytical performance with linearity between 1 pM and 1 μM and a limit of detection of 18.6 fM. Furthermore, the aptasensor shows excellent specificity for CAP in the presence of other antibiotics and resists interference with other common metal ions. Importantly, the performance is not diminished when the constructed aptasensor is applied to measuring CAP in milk powder. This THMS-based method is easy to design, and alteration to different targets can be achieved by simply replacing the aptamer sequence in the THMS. Therefore, this method shows significant prospects as a flexible platform for accurate monitoring of antibiotic residues in foodstuffs.

Engineering of 2D artificial nanozyme-based blocking effect-triggered colorimetric sensor for onsite visual assay of residual tetracycline in milk

Accurate and low-cost onsite assay of residual antibiotics in food and agriculture-related matrixes (e.g., milk) is of significant importance for evaluating and controlling food pollution risk. Herein, we employed hybrid Cu-doped-g-C₃N₄ nanozyme to engineer smartphone-assisted onsite visual sensor for reliable and precise reporting the levels of tetracycline (TC) residues in milk through π-π stacking-triggered blocking effect. Benefiting from the synergetic effects of Cu²⁺ and g-C₃N₄ nanosheet, Cu-doped-g-C₃N₄ nanocomposite exhibited an improved peroxidase-like activity, which could effectively catalyze H₂O₂ to oxidate colorless TMB into steel-blue product oxTMB. Interestingly, owing to the blocking effect caused by the π-π stacking interaction between TC tetraphenyl skeleton and Cu-doped-g-C₃N₄ nanozyme, the affinity of Cu-doped-g-C₃N₄ nanocomposite toward the catalytic substrates was remarkably blocked, resulting in a TC concentration-dependent fading of solution color. Using smartphone-assisted detection a simple, low-cost, reliable, and sensitive portable colorimetric sensor-based nanozyme for onsite visual monitoring the residual TC in milk was successfully developed with a detection limit of 86.27 nM. Of particular mention is that this detection limit is comparable to most other reported colorimetric methods and below most official allowable residue thresholds in milk matrixes. This work gave a novel insight to integrate two-dimensional (2D) artificial nanozymes-based π-π stacking-triggered blocking effect with smartphone-assisted detection for developing efficient and low-cost colorimetric point-of-care testing of the risk factors in food and agriculture-related matrixes.

Development of label-free fluorescent biosensor for the detection of kanamycin based on aptamer capped metal-organic framework

The abuse of antibiotics has caused serious threat to human health, so it is of great significance to develop a simple and sensitive method for the detection of trace residues of antibiotics in the environment and food. Herein, a novel label-free fluorescent biosensing platform based on the fluorescence change of aptamers-capped zeolitic imidazolate framework-8 (ZIF-8) @ 2,2',2″,2‴-((ethene-1,1,2,2-tetrayltetrakis (benzene-4,1-diyl)) tetrakis (oxy)) tetraacetic acid (TPE) through ATP-assisted competitive coordination reaction was designed for such an end. ZIF-8@TPE/Aptamer (Apt) emits strong fluorescence at 425 nm in HEPES buffer due to the aggregation induced luminescence properties of TPE molecules in confined state. Once kanamycin was added, the conformation of aptamer capped on the surface of ZIF-8@TPE changes because of the specific recognition of kanamycin with aptamer, leading to the collapse of ZIF-8 and release of TPE, accompanied with a dramatic decrease of fluorescence intensity. Under the optimal conditions, a good correlation was obtained between the fluorescence intensity of ZIF-8@TPE/Apt and the concentration of kanamycin ranging from 10 to 10³ ng/mL with a detection limit of 7.3 ng/mL. The satisfactory analytical performance of the assay for kanamycin detection suggests good prospect for its application in food safety analysis.