Ratiometric Detection of Ochratoxin A Using a Regenerable COF-Au-MB-Apt Signal Probe on a Thermal-Regulated Sensor Module

An ultrasensitive homogeneous electrochemical strategy for ochratoxin a sensing based on nanoscale PCN-224@MB@Apt

In this reasearch, a homogeneous electrochemical sensor based on PCN-224@MB@Apt was fabricated for the ultrasensitive determination of ochratoxin A (OTA). Firstly, nanoscale PCN-224 were synthesized as the nanocarrier to embed the signal probe of methylene blue (MB). Then, the OTA aptamer (Apt) was added and connected to PCN-224@MB via the Zr-O-P bond between Zr metal sites of PCN-224 and phosphate group of Apt as the biogate. When OTA exists, the Apt would preferentially bind with OTA and fall off from PCN-224@MB, leading to the release of MB and generation of differential pulse voltammetry (DPV) response. The DPV response of MB was linearly correlated with the amount of OTA. The optimized PCN-224@MB@Apt sensor showed outstanding detection performance towards OTA with a low detection limit of 2.6 × 10-5 ng/mL (S/N = 3) and wide linear range (10-4-10 ng/mL). Meanwhile, the fabricated homogeneous electrochemical sensor exhibited splendid stability, reproducibility, and specificity. To assess the practical applicability, the PCN-224@MB@Apt sensor was applied to detect OTA in real corn samples and desirable recovery rates varying from 83.2 % to 109.6 % were obtained.

Flower-like biomimetic enzyme for rapid and sensitive detection of zearalenone in vegetable oil deodorizer distillate

In order to achieve high quality production of vitamin E and plant sterols, it is necessary to conduct rapid and accurate detection of fungal toxins in their production raw material (vegetable oil deodorizer distillate, VODD). In this study, the flower-like biomimetic enzyme of silver-doped ZnO was synthesized through wet chemical method and in-situ reduction method. Based on above work, a flower-like biomimetic enzyme modified glass carbon electrode was fabricated, and its excellent detection capability against fungal toxins zearalenone was confirmed through electrochemical analysis. The detection limit was 8 ng mL-1, with a linear range of 40 ng mL-1-25 μg mL-1. Simultaneously, the biomimetic enzyme sensor takes only 10 min from preparation to completion of detection, and the RSD between the 7 repeated test results was only 0.612 %. After seven days of storage, the current response value remains 91.5 % of the initial value. In practical applications, the recovery rate of zearalenone in VODD using this sensor ranged from 98.1 % to 102.08 %, yielding satisfactory results. Therefore, the novel flower-like biomimetic enzyme represents an ideal choice for developing zearalenone sensors and holds promising prospects for wide application in fungal toxins analysis.

Cu-doped ZiF-8 and calcined UIO-66-based biomimetic enzyme electrochemical sensor for rapid and sensitive detection of Zearalenone in vegetable oil

In order to ensure the safety of the raw material vegetable oil deodorizer distillate (VODD) using in vitamin E and phytosterols production process, it is necessary to rapidly and sensitively detect the content of Zearalenone (ZEN) mycotoxins. In this study, the catalytic activity of ZIF-8 was enhanced by adding copper, and the synergistic effect of calcined UIO-66 and Cu-ZIF-8 was used to improve the electrical response of the sensor to ZEN, improve the conductivity and promote electron transfer, so as to enhance the recognition ability of ZEN. Cu-ZiF-8@CN-UIO-66 biomimetic enzyme was prepared and characterized using SEM, EDS, XPS, XRD and FT-IR. The linear range of the biosensor for ZEN detection was 10 ng mL-1-25 μg mL-1, and the detection limit was as low as 0.6 ng mL-1. The experiment result indicated that the prepared composite nano-biomimetic enzyme was an ideal choice for the detection of ZEN in VODD and showed a broad application prospect in mycotoxin analysis.

A ratiometric electrochemical aptasensor for Ochratoxin A detection based on electroactive Cu-MOF and DNA conjugates resembling the structure of Bidens pilosa

BACKGROUND

Ochratoxin A (OTA) represents a naturally occurring mycotoxin with a serious hazard to the health of individuals because of carcinogenic and teratogenic properties. To date, various analytical methods have been developed for the detection of OTA, among which aptamer-based electrochemical sensing has attracted significant attention due to its rapidity and high sensitivity. As a subtype of aptamer-based electrochemical sensing, ratiometric electrochemical methods further exhibit excellent anti-interference capability. However, their analytical performance remains limited by the labor-intensive and resource-consuming modification of electroactive signal molecules, as well as the restricted specific surface area of the electrodes.

RESULT

Here, we develop a ratiometric electrochemical aptasensor functionalized with Bidens pilosa-like DNA-gold structures and copper-based metal-organic frameworks (Cu-MOFs) for OTA detection. Cu-MOFs served as a substrate for electrode modification, performing two key roles: 1) providing a large surface area for aptamer immobilization, and 2) generating one current signal. Double-stranded DNA-gold nanoparticles (dsDNA-AuNPs) were assembled through Au-S bonding. The dsDNA-AuNPs conjugates, structurally resembling Bidens pilosa, could load more dsDNA and connect to Cu-MOFs via π-π stacking. When OTA was present, the aptamer-OTA complex was stripped from the aptasensor, reducing the amount of Fc-Apt, thus decreasing the corresponding Fc current (IFc). Simultaneously, the decreased interfacial resistance caused an increase in the Cu-MOF current (ICu), providing the decreased IFc/ICu ratio as a quantitative indicator. The aptasensor exhibited a linear detection range from 0.01 ng mL-1 to 300 ng mL-1, with a detection limit of 0.002 ng mL-1 for OTA.

SIGNIFICANCE

The developed electrochemical ratiometric aptasensor demonstrated high reproducibility and stability, and it was successfully applied to maize sample analysis, underscoring its practical applicability. Moreover, it provides a promising strategy for the application of Cu-MOF-based electrochemical aptasensors. Furthermore, the modification procedures of the developed aptasensor were simplified by preparing dsDNA-AuNPs in solution rather than assembling them step-by-step on the electrode surface.

Construction of a Self-Enhanced Electrochemiluminescent Sensor Based on Tandem Signal Amplification and a Self-Luminescent Lanthanide Covalent-Organic Polymer for Ochratoxin A Assay

Ochratoxin A (OTA) is a type of mycotoxin found in various contaminated foods, and it is highly toxic to the livers and kidneys of humans. Herein, we develop a self-enhanced electrochemiluminescent (ECL) sensor based on tandem signal amplification and a self-luminescent europium covalent-organic polymer (Eu-COPTMT-BPA) for OTA assay. Eu-COPTMT-BPA is a self-enhanced ECL emitter that is obtained by using 2,4,6-trimethyl-1,3,5-triazine (TMT) and 2,2'-bipyridine-5,5'-dicarbaldehyde (BPA) as the first and second ligands to form a highly conjugated structure for sensitizing the Eu3+ luminescence. In Eu-COPTMT-BPA, BPA can adjust the energy gap between the triplet ligand and Eu3+excited state to achieve rapid and effective energy transfer for the generation of an enhanced ECL signal. When target OTA is present, it binds with the aptamer segment of the hairpin aptamer probe (HAP) to form a tight structure. The exposed sequence of HAP subsequently binds with the hairpin probe 1/single-stranded DNA 1 (HP1/S1) hybrid to release OTA and trigger (S1). The released S1 can be recognized by a magnetic bead (MB)-capture probe conjugate to trigger a hybridization chain reaction (HCR) between Fc-labeled HP2 and Fc-labeled HP3, leading to the formation of long double-stranded DNA (dsDNA) nanowires on the MB surface and the accumulation of abundant Fc that can quench the ECL intensity of the Eu-COPTMT-BPA/TPrA system. This ECL sensor exhibits good stability, excellent specificity, and high sensitivity, with a detection limit (LOD) of 0.47 fg/mL and a linear range of 1 pg/mL-10 ng/mL, and it can quantitatively measure OTA in wine and coffee.

Aptamer selection via versatile microfluidic platforms and their diverse applications

Aptamers are synthetic oligonucleotides that bind with high affinity and specificity to various targets, making them invaluable for diagnostics, therapeutics, and biosensing. Microfluidic platforms can improve the efficiency and scalability of aptamer selection, especially through advancements in systematic evolution of ligands by exponential enrichment (SELEX) methods. Microfluidic SELEX methods are less time-consuming and labor-intensive and include critical steps like library preparation, binding, partitioning, and amplification. This review examines the contributions of microfluidic technology to SELEX-based aptamer identification, with alternative methods like conditional SELEX, in vivo-like SELEX and Non-SELEX for selecting aptamers and also discusses critical SELEX steps over the past decade. This work also examined the integrated microfluidic systems for SELEX, highlighting innovations such as conditional SELEX and in vivo-like SELEX. These advancements provide potential solutions to existing challenges in aptamer selection using conventional SELEX, especially concerning biological samples. A trend toward non-SELEX methods was also reviewed and discussed, wherein nucleic acid amplification was eliminated to improve aptamer selection. Microfluidic platforms have demonstrated versatility not only in aptamer selection but also in various detection applications; they allow for precise control of liquid flow and have been essential in the advancement of therapeutic aptamers, facilitating accurate screening, enhancing drug delivery systems, and enabling targeted therapeutic interventions. Although advances in microfluidic technology are expected to enhance aptamer-based diagnostics, therapeutics, and biosensing, challenges still persist, especially in up-scaling microfluidic systems for various clinical applications. The advantages and limitations of integrating microfluidic platforms with aptamer development are further addressed, emphasizing areas for future research. We also present a perspective on the future of microfluidic systems and aptamer technologies, highlighting their increasing significance in healthcare and diagnostics.

SERS aptasensor for simultaneous detection of ochratoxin A and zearalenone utilizing a rigid enhanced substrate (ITO/AuNPs/GO) combined with Au@AgNPs

The contamination of mycotoxins poses a serious threat to global food security, hence the urgent need for simultaneous detection of multiple mycotoxins. Herein, two SERS nanoprobes were synthesized by embedded SERS tags (4-mercaptopyridine, 4MPy; 4-mercaptobenzonitrile, TBN) into the Au and Ag core-shell structure, and each was coupled with the aptamers specific to ochratoxin A (OTA) and zearalenone (ZEN). Meanwhile, a rigid enhanced substrate Indium tin oxide glass/AuNPs/Graphene oxide (ITO/AuNPs/GO) was combined with aptamer functionalized Au@AgNPs via π-π stacking interactions between the aptamer and GO to construct a surface-enhanced Raman spectroscopy (SERS) aptasensor, thereby inducing a SERS enhancement effect for the effective and swift simultaneous detection of both OTA and ZEN. The presence of OTA and ZEN caused signal probes dissociation, resulting in an inverse correlation between Raman signal intensity (1005 cm-1 and 2227 cm-1) and the concentrations of OTA and ZEN, respectively. The SERS aptasensor exhibited wide linear detection ranges of 0.001-20 ng/mL for OTA and 0.1-100 ng/mL for ZEN, with low detection limits (LOD) of 0.94 pg/mL for OTA and 59 pg/mL for ZEN. Furthermore, the developed SERS aptasensor demonstrated feasible applicability in the detection of OTA and ZEN in maize, showcasing its substantial potential for practical implementation.

In-situ nanozyme catalytic amplification coupled with a universal antibody orientation strategy based electrochemical immunosensor for AD-related biomarker

An in-situ nanozyme signal tag combined with a DNA-mediated universal antibody-oriented strategy was proposed to establish a high-performance immunosensing platform for Alzheimer's disease (AD)-related biomarker detection. Briefly, a Zr-based metal-organic framework (MOF) with peroxidase (POD)-like activity was synthesized to encapsulating the electroactive molecule methylene blue (MB), and subsequently modified with a layer of gold nanoparticles on its surface. This led to the creation of double POD-like activity nanozymes surrounding the MB molecule to form a nanozyme signal tag. A large number of hydroxyl radicals were generated by the nanozyme signal tag with the help of H2O2, which catalyzed MB molecules in situ to achieve efficient signal amplification. Subsequently, a DNA-aptamer-mediated universal antibody-oriented strategy was proposed to enhance the binding efficiency for the antigen (target). Meanwhile, a poly adenine was incorporated at the end of the aptamer, facilitating binding to the gold electrode and providing anti-fouling properties due to the hydrophilicity of the phosphate group. Under optimal conditions, this platform was successfully employed for highly sensitive detection of AD-associated tau protein and BACE1, achieving limits of detection with concentrations of 3.34 fg/mL and 1.67 fg/mL, respectively. It is worth mentioning that in the tau immunosensing mode, 20 clinical samples from volunteers of varying ages were analyzed, revealing significantly higher tau expression levels in the blood samples of elderly volunteers compared to young volunteers. This suggests that the developed strategy holds great promise for early AD diagnosis.

Self-designed portable dual-mode fluorescence device with custom python-based analysis software for rapid detection via dual-color FRET aptasensor with IoT capabilities

An innovative aptasensor incorporating MoS2-modified bicolor quantum dots and a portable spectrometer, designed for the simultaneous detection of ochratoxin A (OTA) and aflatoxin B1 (AFB1) in corn was developed. Carbon dots and CdZnTe quantum dots were as nano-donors to label OTA and AFB1 aptamers, respectively. These labeled aptamers were subsequently attached to MoS2 receptors, enabling fluorescence resonance energy transfer (FRET). With targets, the labeled aptamers detached from the nano-donors, thereby disrupting the FRET process and resulting in fluorescence recovery. Furthermore, a portable dual-mode fluorescence detection system, complemented with customized python-based analysis software, was developed to facilitate rapid and convenient detection using this dual-color FRET aptasensor. The developed host program is connected to the spectrometer and transmits data to the cloud, enabling the device to have Internet of Things (IoT) characteristics. Connected to the cloud, this IoT-enabled device offers convenient and reliable fungal toxin detection for food safety.

Host-Guest Interaction Mediated Perovskite@Metal-Organic Framework Z-Scheme Heterojunction Enabled Paper-Based Photoelectrochemical Sensing

Exploring the high-performance photoelectronic properties of perovskite quantum dots (QDs) is desirable for paper-based photoelectrochemical (PEC) sensing;however, challenges remain in improving their stability and fundamental performance. Herein, a novel Z-scheme heterostructure with host-guest interaction by the confinement of CH3NH3PbBr3 QDs within Cu3(BTC)2 metal-organic framework (MOF) crystal (MAPbBr3@Cu3(BTC)2) is successfully constructed on the paper-based PEC device for ultrasensitive detection of Ochratoxin A (OTA), with the assistance of the exciton-plasmon interaction (EPI) effect. The host-guest interaction is estabilished by encapsulating MAPbBr3 QDs as guests within Cu3(BTC)2 MOF as a host, which prevents MAPbBr3 QDs from being damaged in the polar system, offering access to long-term stability with high-performance PEC properties. Benefiting from the precise alignment of energy levels, the photogenerated charge carriers can migrate according to the Z-scheme charge-transfer pathway under the driving force of the internal electric field, achieving a high photoelectric conversion efficiency. Upon OTA recognition, the EPI effect is activated to modulate the exciton response in MAPbBr3 QDs by accelerating radiative decay, finally achieving sensitive OTA sensing with a detection limit of 0.017 pg mL-1. We believe this work renders new insight into designing host-guest Z-scheme heterojunctions in constructing the paper-based PEC sensing platforms for environmental monitoring.

A film-like SERS aptasensor for sensitive detection of patulin based on GO@Au nanosheets

Patulin (PAT) commonly contaminates fruits, posing a significant risk to human health. Therefore, a highly effective and sensitive approach in identifying PAT is warranted. Herein, a SERS aptasensor was constructed based on a two-dimensional film-like structure. GO@Au nanosheets modified with SH-cDNA were employed as capture probes, while core-shell Au@Ag nanoparticles modified with 4-MBA and SH-Apt were utilized as signal probes. Through the interaction between capture probes and signal probes, adjustable hotspots were formed, yielding a significant Raman signal. During sensing, the GO@Au-cDNA competitively attached to Au@AgNPs@MBA-Apt, resulting in an inverse relationship between PAT levels and SERS intensity. The acquired results exhibited linear responses to PAT within the range of 1-70 ng/mL, with a calculated limit of detection of 0.46 ng/mL. In addition, the SERS aptasensor exhibited satisfactory recoveries in apple samples, which aligned closely with HPLC. With high sensitivity and specificity, this method holds significant potential for PAT detection.

An advanced ratiometric molecularly imprinted sensor based on metal ion reoxidation for indirect and ultrasensitive glyphosate detection in fruit

An indirect and ultrasensitive ratiometric molecularly imprinted (MIP) sensor, based on metal ion reoxidation, is introduced for glyphosate (GLY) determination in fruit. As high-performance signal amplification substrates, carbon nanotubes (MWCNTs) and gold nanoparticles (AuNPs) are conveniently modified on GCE. The artificial antibody-MIP membrane, presents typical three-dimensional structure to GLY template. Built-in reference methylene blue (MB) is directly electropolymerized on MWCNTs-Au/GCE. Particularly, Cu2+ and GLY interestingly form chelate complex, and the Cu2+ (ICu) in Cu(Ⅱ)-GLY-complex can be reoxidized, and indirectly quantizes GLY. The reference signal (IMB) presents noteworthy stability with different GLY levels, and the ratiometric readout (ICu/IMB) is recognized as a more trustworthy indicator to quantize GLY. Proposed sensor presents broad range as 1.73 ∼ 400 ng/mL, and limit of detection is well found as 0.24 ng/mL (S/N = 3). Finally, as-fabricated method is verified with standard HPLC in real-fruit-sample, and the errors and recovery rates are calculated as 3.4% ∼ 6.7% and 94.4% ∼ 104.6%, respectively.

Competitive immunoassay using enzyme-regulated Fe3O4@COF/Fe3+ fluorescence probe for natural chloramphenicol detection

Accurate and sensitive detection of chloramphenicol (CAP) in natural samples is essential for ensuring human health. Herein, an enzyme-regulated fluorescence sensor using Fe3O4@COF/Fe3+ probe, is developed for CAP determination. Fe3O4@COF, synthesized via hydrothermal method, exhibits dual functions as a magnetic carrier and signal probe. Bovine serum albumin conjugated-chloramphenicol, adsorbed on the surface of Fe3O4@COF, competes with CAP for antibody binding. The antibody interacts with alkaline phosphatase via the biotin-streptavidin system. Meanwhile, ascorbic acid, produced from the enzyme-catalyzed reaction dominated by alkaline phosphatase, effectively restores the fluorescence of Fe3O4@COF that is quenched by Fe3+. After experimental verification and gradual optimization, a logarithmic linear relationship between CAP concentration and fluorescence intensity is established in the range of 2 × 10-4∼10 μg mL-1, with a good limit of detection (9.2 × 10-5 μg mL-1). Proposed method exhibits excellent stability (15 days) and reusability (8 cycles), providing a sensitive and reliable method for accurate CAP detection. The readouts show good agreement with HPLC and recoveries during laboratory and natural CAP analysis.

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

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

Recent advances of ratiometric sensors in food matrices: mycotoxins detection

The public health problem caused by mycotoxins contamination has received a great deal of attention worldwide. Mycotoxins produced by filamentous fungi widely distributed in foodstuffs can cause adverse impacts on humans and livestock, posing serious health threats. Particularly worth mentioning is that mycotoxins can accumulate in organisms and be enriched through the food chain. Improving early trace detection and control from the source is a more desirable approach than the contaminated food disposal process to ensure food safety. Conventional sensors are susceptible to interference from various components in intricate food matrices when detecting trace mycotoxins. The application of ratiometric sensors avoids signal fluctuations, and reduce background influences, which casts new light on developing sensors with superior performance. This work is the first to provide an overview of the recent progress of ratiometric sensors in the detection of mycotoxins in intricate food matrices, and highlight the output types of ratiometric signal with respect to accurate quantitative analysis. The prospects of this field are also included in this paper and are intended to have key ramifications on the development of sensing detection conducive to food safety.