Photoelectrochemical sensitive detection of insulin based on CdS/polydopamine co-sensitized WO3 nanorod and signal amplification of carbon nanotubes@polydopamine

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

A sensitive label-free biosensor based on Ag2S-sensitived Bi2WO6/BiOBr heterojunction for photoelectrochemical immunoassay of prostate specific antigen

Prostate cancer is one of the most common cancers in the world, and its early diagnosis can effectively reduce mortality. A new label-free photoelectrochemical (PEC) immunosensor on the basis of Bi₂WO₆/BiOBr nanocomposite materials has been successfully prepared for the test of prostate-specific antigen (PSA) in human serum in this work. The Ag₂S-sensitized Bi₂WO₆/BiOBr heterojunction was used as a photosensitive material, which effectively improved the photocurrent response. On Bi₂WO₆/BiOBr surface, dopamine immobilized PSA antibody by self-polymerizing to form polydopamine membrane. Antigen and antibody are specifically combined to achieve quantitative detection of PSA according to the current changes at different concentrations of antigen. Under the optimal experimental conditions, the PEC immunosensor has an ideal linear relationship between 1 pg/mL - 50 ng/mL, and the detection limit is 0.084 pg/mL. In addition, the prepared immunosensor has good stability, reproducibility and selectivity, providing a new method for the detection of PSA in actual sample analysis.

Semiconductor quantum dots in photoelectrochemical sensors from fabrication to biosensing applications

Semiconductor quantum dots (QDs) are a promising class of nanomaterials for developing new photoelectrodes and photoelectrochemistry systems for energy storage, transfer, and biosensing applications. These materials have unique electronic and photophysical properties and can be used as optical nanoprobes in displays, biosensors, imaging, optoelectronics, energy storage and energy harvesting. Researchers have recently been exploring the use of QDs in photoelectrochemical (PEC) sensors, which involve exciting a QD-interfaced photoactive material with a flashlight source and generating a photoelectrical current as an output signal. The simple surface properties of QDs also make them suitable for addressing issues related to sensitivity, miniaturization, and cost-effectiveness. This technology has the potential to replace current laboratory practices and equipment, such as spectrophotometers, used for testing sample absorption and emission. Semiconductor QD-based PEC sensors offer simple, fast, and easily miniaturized sensors for analyzing a variety of analytes. This review summarizes the various strategies for interfacing QD nanoarchitectures for PEC sensing, as well as their signal amplification. PEC sensing devices, particularly those used for the detection of disease biomarkers, biomolecules (glucose, dopamine), drugs, and various pathogens, have the potential to revolutionize the biomedical field. This review discusses the advantages of semiconductor QD-based PEC biosensors and their fabrication methods, with a focus on disease diagnostics and the detection of various biomolecules. Finally, the review provides prospects and considerations for QD-based photoelectrochemical sensor systems in terms of their sensitivity, speed, and portability for biomedical applications.

2D carbon materials based photoelectrochemical biosensors for detection of cancer antigens

Cancer is a leading cause of death globally and early diagnosis is of paramount importance for identifying appropriate treatment pathways to improve cancer patient survival. However, conventional methods for cancer detection such as biopsy, CT scan, magnetic resonance imaging, endoscopy, X-ray and ultrasound are limited and not efficient for early cancer detection. Advancements in molecular technology have enabled the identification of various cancer biomarkers for diagnosis and prognosis of the deadly disease. The detection of these biomarkers can be done by biosensors. Biosensors are less time consuming compared to conventional methods and has the potential to detect cancer at an earlier stage. Compared to conventional biosensors, photoelectrochemical (PEC) biosensors have improved selectivity and sensitivity and is a suitable tool for detecting cancer agents. Recently, 2D carbon materials have gained interest as a PEC sensing platform due to their high surface area and ease of surface modifications for improved electrical transfer and attachment of biorecognition elements. This review will focus on the development of 2D carbon nanomaterials as electrode platform in PEC biosensors for the detection of cancer biomarkers. The working principles, biorecognition strategies and key parameters that influence the performance of the biosensors will be critically discussed. In addition, the potential application of PEC biosensor in clinical settings will also be explored, providing insights into the future perspective and challenges of exploiting PEC biosensors for cancer diagnosis.

Tungsten-based Nanomaterials in the Biomedical Field: A Bibliometric Analysis of Research Progress and Prospects

Tungsten-based nanomaterials (TNMs) with diverse nanostructures and unique physicochemical properties have been widely applied in the biomedical field. Although various reviews have described the application of TNMs in specific biomedical fields, there are still no comprehensive studies that summarize and analyze research trends of the field as a whole. To identify and further promote the development of biomedical TNMs, a bibliometric analysis method is used to analyze all relevant literature on this topic. First, general bibliometric distributions of the dataset by year, country, institute, referenced source, and research hotspots are recognized. Next, a comprehensive review of the subjectively recognized research hotspots in various biomedical fields, including biological sensing, anticancer treatments, antibacterials, and toxicity evaluation, is provided. Finally, the prospects and challenges of TNMs are discussed to provide a new perspective for further promoting their development in biomedical research.

A "signal-on" photoelectrochemical sensor for human epidermal growth factor receptor 2 detection based on Y6/CdS organic-inorganic heterojunction

A "signal-on" photoelectrochemical (PEC) immunosensor was successfully constructed for determination of human epidermal growth factor receptor 2 (HER2) based on organic-inorganic heterojunction Y6/CdS as photoactive material. Compared with single organic semiconductor, Y6, Y6/CdS exhibited higher photoelectric conversion efficiency due to the formation of heterojunction. In the presence of HER2, sandwich immune structure was formed between HER2 aptamer and anti-HER2 antibody (Ab) by specific recognition. The polydopamine (PDA) nanoparticles were used for signal amplification to enhance photocurrent intensity as PDA can act as electron donor to eliminate holes and promote electron-hole pairs separation. The developed PEC sensor displayed a wide detection range of 5-1000 pg mL^-1 and a low detection limit of 2.2 pg mL^-1 for HER2 (S/N = 3). The sensor was successfully used for the detection of HER2 in serum with recoveries between 94.8 and 104% and relative standard deviations (RSDs) in the range of 1.2-4.3%. Furthermore, the designed immunosensor possessed good stability, selectivity, and reproducibility, which can find potential clinical applications for disease diagnosis. A "signal-on" photoelectrochemical sensor was reported for human epidermal growth factor receptor 2 detection based on Y6/CdS organic-inorganic heterojunction.

Z-scheme Bi2O3/CuBi2O4 heterojunction enabled sensitive photoelectrochemical detection of aflatoxin B1 for health care, the environment, and food

Designing a photoelectrochemical (PEC) biosensor with preponderant sensitivity and anti-interference is a challenge for detecting small molecules in real samples with complex matrices. To this end, the Bi₂O₃/CuBi₂O₄ was synthesized in one step to enhance visible light's absorption ability, transferring the interfacial carrier's efficiency, a high-active Z-scheme heterojunction, and a photocathode biosensor was proposed. For the first time, we used the density functional theory to verify a Z-scheme transfer pathway of photogenerated electrons in Bi₂O₃/CuBi₂O₄ and the energy band structure of Bi₂O₃ and CuBi₂O₄, respectively. Bi₂O₃/CuBi₂O₄-based PEC biosensor was developed for competive immunoassay of small molecular, aflatoxin B1 (AFB1) as an example, resulting in a low detection limit of 297.4 fg/mL and a linear range of 1.4 pg/mL-280 ng/mL in urine, water, peanut, and wheat samples. Using spiked experiments, the satisfied repeatability, reproducibility, stability, and specificity of the Bi₂O₃/CuBi₂O₄-based PEC biosensor indicated a promise for application in health care, the environment, and food.

Photoelectrochemical sandwich immunoassay of CYFRA21-1 based on In2O3/WO3 type-II heterojunction and CdS quantum dots-polydopamine nanospheres labeling

Using an In₂O₃/WO₃ type-II heterojunction modified fluorine-doped tin oxide (FTO) electrode as the photoanode and CdS quantum dots (QDs)-polydopamine nanospheres (PDA NSs) as the secondary antibody (Ab₂) label, the photoelectrochemistry (PEC) sandwich immunosensing of the lung cancer marker CYFRA21-1 was studied. WO₃ nanoplates were prepared by a hydrothermal method, In₂O₃ nanoporous spheres were prepared by a hydrothermal method followed by calcination, and the In₂O₃/WO₃ type-II heterojunction with high PEC activity was prepared by ultrasonic mixing and cast-coating. PDA NSs with a high surface area can be loaded with abundant Ab₂ molecules and many CdS QDs with an energy level well matched with the heterojunction, so the photocurrent signal can be amplified by the formation of a sandwich immunostructure. Through the simulation experiments of photoelectrode-modified chitosan films of varying thickness, the effective transport distance of photogenerated charges is preliminarily discussed. Under the optimized conditions, the photocurrent was linear with the common logarithm of CYFRA21-1 concentration from 100 fg mL^-1 to 50 ng mL^-1, with a limit of detection of 56 fg mL^-1 (S/N = 3). The immunoassay of CYFRA21-1 in human serum samples gave satisfactory recovery results.

Bioinspired Synthesis of ZnO@polydopamine/Au for Label-Free Photoelectrochemical Immunoassay of Amyloid-β Protein

Amyloid-β protein (Aβ) is an important biomarker and plays a key role in the early stage of Alzheimer's disease (AD). Here, an ultrasensitive photoelectrochemical (PEC) sensor based on ZnO@polydopamine/Au nanocomposites was constructed for quantitative detection of Aβ. In this sensing system, the ZnO nanorod array decorated with PDA films and gold nanoparticles (Au NPs) have excellent visible-light activity. The PDA film was used as a sensitizer for charge separation, and it also was used for antibody binding. Moreover, Au NPs were loaded on the surface of PDA film by in situ deposition, which further improved the charge transfer efficiency and the PEC activity in visible light due to the localized surface plasmon resonance effect of Au NPs. Therefore, in ZnO@polydopamine/Au nanocomposites, a significantly enhanced photocurrent response was obtained on this photoelectrode, which provides a good and reliable signal for early detection of AD. Under the optimized conditions, the PEC immunosensor displayed a wide linear range from 1 pg/mL to 100 ng/mL and a low detection limit of 0.26 pg/mL. In addition, this PEC immunosensor also presented good selectivity, stability, and reproducibility. This work may provide a promising point-of-care testing method toward advanced PEC immunoassays for AD biomarkers.

Polydopamine-based nanomaterials and their potentials in advanced drug delivery and therapy

Polydopamine (PDA) has shown great potentials in biomedical fields due largely to its unique physicochemical properties, including high photothermal transfer efficiency, excellent drug binding capacity, versatile adhesion ability, sensitive pH responsibility and great biocompatibility and biodegradability. These properties confer PDA-based nanoparticles the potentials either as the drug carriers for advanced drug delivery or as the bioactive agents for photothermal therapy, imaging and biosensing. This review aims to provide a comprehensive understanding of PDA, its polymerization mechanisms and the potentials of PDA-based nano-systems in treating various diseases, including cancer, diabetes, inflammation, bacterial infection and Parkinson's disease. In addition, the concerns of PDA in biomedical use are also discussed.

Nanobioconjugates for Signal Amplification in Electrochemical Biosensing

Nanobioconjugates are hybrid materials that result from the coalescence of biomolecules and nanomaterials. They have emerged as a strategy to amplify the signal response in the biosensor field with the potential to enhance the sensitivity and detection limits of analytical assays. This critical review collects a myriad of strategies for the development of nanobioconjugates based on the conjugation of proteins, antibodies, carbohydrates, and DNA/RNA with noble metals, quantum dots, carbon- and magnetic-based nanomaterials, polymers, and complexes. It first discusses nanobioconjugates assembly and characterization to focus on the strategies to amplify a biorecognition event in biosensing, including molecular-, enzymatic-, and electroactive complex-based approaches. It provides some examples, current challenges, and future perspectives of nanobioconjugates for the amplification of signals in electrochemical biosensing.

Functional polymers in photoelectrochemical biosensing

Photoelectrochemical (PEC) analysis is a detection technique that has gained a wide attention in sensing applications. PEC presents the advantages of high sensitivity, low background signal, simple equipment and easy miniaturization. In PEC detection, light is used as an excitation source while current or voltage is measured as the output detection signal. The ability to couple the PEC process with specific bioreceptors gives PEC biosensing a unique advantage of being both selective and sensitive. The growing interest in PEC bioanalysis has resulted in essential progress in its analytical performance and biodetection applications. Functional polymers have different applications in the development of novel PEC biosensing platforms. Recently, the interest in polymer-based photoactive materials has emerged as they are efficient and less toxic alternatives to certain kinds of inorganic semiconductors and sensitizers. Moreover, molecularly imprinted polymers are a class of synthetic bioreceptors that are increasingly used in PEC bioanalytics. In this review, we will provide an overview on functional polymer-based PEC biosensing approaches. Novel classes of polymers as photoactive materials are reviewed and selected applications are described. Furthermore, molecularly imprinted polymers in the development of smart and sensitive PEC bioanalytical strategies are discussed.

Polydopamine-PEG-Folic Acid Conjugate Film Engineered TiO2 Nanotube Arrays for Photoelectrochemical Sensing of Folate Binding Protein

Serum-soluble folate binding protein (FBP) is an important tumor marker, and the development of a simple biosensing method is highly needed. In this work, a photoelectrochemical (PEC) biosensor for the detection of FBP was proposed based on the construction of an antifouling interface and the unique ligand-protein recognition. The PEC sensing platform was prepared by the biomimetic polydopamine (PDA) coating on TiO₂ nanotubes arrays (NTAs). A significant PEC enhancement effect was obtained due to the macroporous structures. Excellent antifouling performance was achieved by conjugation of amino-group-terminated 8-arm poly(ethylene glycol) (PEG). The incorporation of folic acid (FA) retains the antifouling property and shows recognition abilities toward FBP. The fabricated PEC biosensor shows good analytical performance. The combination of ligand-protein recognition and a PEC antifouling interface provides a good consideration for the development of FBP biosensors.

Photoelectrochemical competitive immunosensor for 17β-estradiol detection based on ZnIn2S4@NH2-MIL-125(Ti) amplified by PDA NS/Mn:ZnCdS

A competitive-type PEC immunosensor for 17β-estradiol (E₂) detection was successfully fabricated using ZnIn₂S₄@NH₂-MIL-125(Ti) composite as matrix. The excellent PEC behavior of ZnIn₂S₄@NH₂-MIL-125(Ti) composite could be attributed to that the Ti⁴⁺-Ti³⁺ intervalence cycles in the titanium oxo-cluster of NH₂-MIL-125(Ti) as well as the matching energy level between ZnIn₂S₄ and NH₂-MIL-125(Ti) promote the migration and separation of photocarrier. Besides, polydopamine (PDA) with abundant amino- and quinone-groups was selected to further improve the PEC signals and capture antibody, which implement through the covalent bonding of PDA and BSA-E₂ or carboxyl-group functionalized Mn:ZnCdS QDs in the competitive-type strategy. Concretely, the quinone functional groups in PDA film was applied to immobilize BSA-E₂ through Michael reactions, and the PDA nanosphere loaded Mn:ZnCdS quantum dot (PDA NS/Mn:ZnCdS QDs) was used as antibodies' labels to amplify PEC signals. After PDA NS/Mn:ZnCdS-anti-E₂ immobilized on the modified electrode, a remarkable increase of photocurrent signal was observed owing to the specific bonding of antigen and antibody. Based on the competitive binding of PDA NS/Mn:ZnCdS-anti-E₂ with either free E₂ or bovine serum albumin (BSA)-E₂ causing the change of the photocurrent signal, the standard sample free E₂ could be accuracy detect. Under optimal conditions, the competitive-type PEC immunosensor exhibited the linear range from 0.0005 ng/mL to 20 ng/mL and a limit detection of 0.3 pg/mL (S/N = 3). Meanwhile, the acceptable stability, selectivity and reproducibility of the proposed PEC immunosensing platform indicating the promising detection of small molecular environmental pollutants.

Versatile Polydopamine Platforms: Synthesis and Promising Applications for Surface Modification and Advanced Nanomedicine

As a mussel-inspired material, polydopamine (PDA), possesses many properties, such as a simple preparation process, good biocompatibility, strong adhesive property, easy functionalization, outstanding photothermal conversion efficiency, and strong quenching effect. PDA has attracted increasingly considerable attention because it provides a simple and versatile approach to functionalize material surfaces for obtaining a variety of multifunctional nanomaterials. In this review, recent significant research developments of PDA including its synthesis and polymerization mechanism, physicochemical properties, different nano/microstructures, and diverse applications are summarized and discussed. For the sections of its applications in surface modification and biomedicine, we mainly highlight the achievements in the past few years (2016-2019). The remaining challenges and future perspectives of PDA-based nanoplatforms are discussed rationally at the end. This timely and overall review should be desirable for a wide range of scientists and facilitate further development of surface coating methods and the production of PDA-based materials.

A novel fluorescence method for the highly sensitive detection of T4 polynucleotide kinase based on polydopamine nanotubes

A novel fluorescence method has been developed for the detection of T4 PNK using FRET between dye-labeled ssDNA and PDANTs.

Optimization adsorption of norfloxacin onto polydopamine microspheres from aqueous solution: Kinetic, equilibrium and adsorption mechanism studies

Polydopamine microspheres (PDMPs) synthesized by a facile solution oxidation method were adopted as a potential adsorbent for the removal of Norfloxacin (NOR) from aqueous solution. The morphologies and properties of PDMPs were characterized using TEM, SEM, FTIR and pH_PZC. Parameters effects such as contact time, initial pH, initial concentration and ionic strength on the adsorption capacity of NOR onto PDMPs were studied. To maximize NOR removal from liquid phase, Box-Behnken experimental design (BBD) combined with response surface modeling (RSM) was employed based on the 17 preliminary experiments at 308 K. Optimum contact time, initial NOR concentration and initial pH value were found to be 97 min, 303 mg·L^-1 and 6.6, respectively, the corresponding NOR removal capacity was found to be 307 mg·g^-1. Batch adsorption experiments under the optimal conditions were conducted to investigate kinetics, thermodynamics and adsorption isotherm. Kinetic analysis confirmed that the kinetic data were well described by Pseudo-second order model. The experimental equilibrium data were well fitted by Langmuir, Redlich-Peterson, Koble-Corrigan and Dubinin-Radushkevich models. Thermodynamic parameters such as Gibbs free energy, enthalpy and entropy were calculated and the results indicated that the NOR adsorption onto PDMPs was spontaneous and endothermic. The adsorption process may be attributed to the electrostatic interaction, the formation of hydrogen bonds or π-π stacking interactions among the polydopamine (PDA) and NOR molecule.

Quenching Electrochemiluminescence Immunosensor Based on Resonance Energy Transfer between Ruthenium (II) Complex Incorporated in the UiO-67 Metal-Organic Framework and Gold Nanoparticles for Insulin Detection

This work describes a sandwich-type electrochemiluminescence (ECL) strategy for insulin detection by using Ru(bpy)₃²⁺ as the luminophore which was encapsulated in the UiO-67 metal-organic framework (UiO-67/Ru(bpy)₃²⁺). Because UiO-67 possesses the characteristics of large specific surface area and porosity, more Ru(bpy)₃²⁺ could be loaded onto its surface and holes, thus greatly improving the ECL efficiency. Furthermore, the ECL resonance energy transfer (ECL-RET) could occur between UiO-67/Ru(bpy)₃²⁺ (ECL donor) and Au@SiO₂ nanoparticles (ECL acceptor), resulting in a conspicuously decreased ECL response. The ECL spectrum of UiO-67/Ru(bpy)₃²⁺ which exhibited strong ECL intensity has suitable spectral overlap with the absorption spectrum of Au@SiO₂, which further proved the occurrence of the ECL-RET action. The ECL intensity decreased with the increase of the concentration of insulin. In addition, the sandwich-type ECL immunosensor was applied to insulin detection, and the ECL decrease efficiency was found to be logarithmically related to the concentration of the insulin antigen in the range of 0.0025 to 50 ng mL^-1 with the limit of detection of 0.001 ng mL^-1. Meanwhile, this work provides an important reference for the application of metal-organic frameworks in the ECL and ECL-RET study and also exhibits potential capability in the detection of other hormones.

Formation of Homogeneous Epinephrine-Melanin Solutions to Fabricate Electrodes for Enhanced Photoelectrochemical Biosensing

The development of a simple but effective surface modification method is very important for the construction of biosensing interfaces. In this work, a postsynthetic water-soluble epinephrine-melanin (EPM) prepared from the self-polymerization of epinephrine has been demonstrated as an alternative of the widely used in situ formed polydopamine (PDA) for the surface coating of TiO₂ nanoparticles and the construction of a photoelectrochemical (PEC) biosensing interface. In contrast to the formation of insoluble aggregates in solution for dopamine, a homogeneous solution was obtained for epinephrine after the self-polymerization. The use of EPM as a postsynthetic material enables the surface coating of TiO₂ with the simple drop-casting method. Compared with the widely used dip-coating method for in situ PDA modification, the developed drop-casting method based on the use of water-soluble postsynthetic EPM saves more time, avoids the waste of bulk solution, and undoubtedly decreases the batch-to-batch inconsistencies. The simple coating of commercially available TiO₂ nanoparticles with EPM greatly enhances the PEC performance due to the charge transfer property of EPM. The application of EPM in the construction of the PEC biosensing interface was demonstrated by the immobilization of a model biorecognition element (prostate specific antigen (PSA) antibody) onto EPM modified indium tin oxide (ITO) photoanode. Sensitive detection of PSA with high selectivity and stability was obtained on the basis of the biological recognition ability of PSA antibody. This work may renew the use of postsynthetic melanin-like biopolymers in other fields.

PVP-coated gold nanoparticles for the selective determination of ochratoxin A via quenching fluorescence of the free aptamer

This paper describes an aptamer/gold nanoparticle-based assay for ochratoxin A (OTA) detection. This assay is based on the use of an aptamer labeled with carboxyfluorescein (FAM) at its 5'-end and gold nanoparticles (AuNPs) that act as quenchers of fluorescence. When OTA is absent in the system, the fluorescently labeled aptamers are adsorbed on the surface of AuNPs. The fluorescence signal of the fluorescein-labeled OTA aptamer generated is quenched by the fluorescence resonance energy transfer effect of AuNPs. When OTA is present in the system, the fluorescently labeled aptamer binds to OTA and forms a folded structure, which can resist the adsorption of AuNPs. Thus, the fluorescent signal can be retained. The detection limit of this sensing platform is 5 nM, and the linear detection range is 10-1000 nM (R2 = 0.994). The procedure was validated by the quantitation of OTA in spiked ginger powder samples and were found to be free of interference by the sample matrix. The recoveries and the relative standard deviation varied from 89.0% to 117.8% and from 1.9% to 6.3%, respectively.

Fluorometric aptamer assay for ochratoxin A based on the use of single walled carbon nanohorns and exonuclease III-aided amplification

The authors describe an aptamer based assay for the food mycotoxin ochratoxin A (OTA). It is based on the use of exonuclease III (Exo III) which assists in signal amplification, and of single-walled carbon nanohorns (SWCNHs) which act as quenchers of fluorescence. The detection scheme employs a hairpin probe (HP) and a signal probe (SP) labeled with carboxyfluorescein (FAM) at its 5'-end. The fluorescence of intact SPs (best measured at excitation/emission wavelengths of 495/518 nm) is quenched by SWCNHs. The HP contains the OTA-specific aptamer sequence and is partially complementary to the SP. After addition of OTA, the aptamer binds OTA and thus exposes a single-stranded sequence that can hybridize with the SP. Exo III digests the SP to liberate the free fluorophore labels. The damaged SPs no longer are adsorbed by the SWCNHs so that fluorescence is no longer quenched. The method has a detection range that is linear from 10 nM to 1000 nM (with a correlation coefficient of 0.997). The limit of detection (LOD), calculated on the basis of a signal to noise ratio of 3, is 4.2 nM. The procedure was validated by the quantitation of OTA in spiked real samples and were found to be free of interference by the sample matrix. Recoveries ranged from 93.8 to 113.0% in beer and from 92.0 to115.9% in red wine. Graphical abstract After adding ochratoxin A (OTA), the aptamer region in hairpin probe (HP) combined with OTA and thus exposed a single-stranded sequence to hybridize with signal probe (SP). Exonuclease III (Exo III) digested SP to liberate the free fluorophore (FAM).