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The detection of rosiglitazone (RSG) in food is of great importance since the excessive intake of RSG could cause adverse effects on the human body. Although liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry are the preliminary methods for the detection of hazardous materials in food, they are not suitable for point-of-care or on-site detection. Herein, a time-based readout (TBR) device with an application software (APP) controlled by a smart phone was developed for the sensitive and selective immunoassay of RSG. The homemade TBR device was based on a two-electrode system, where the immune molecule-modified glassy carbon electrode was used as the bioanode, and Prussian blue-modified FTO was used as the cathode. By using Au-modified octahedral Cu

The abnormal expression of microRNA (miRNA) can affect the RNA transcription and protein translation, leading to tumor progression and metastasis. Currently, the accurate detection of aberrant expression of miRNA, particularly using a portable detection system, remains a great challenge. Herein, a novel dual-mode biosensor with high sensitivity and robustness for miR-21 detection was developed based on the cis-cleavage and trans-cleavage activities of Cas12a. miRNA can be combined with hairpin DNA-horseradish peroxidase anchored on a CdS/g-C3N4/B-TiO2 photoelectrode, thus the nonenzymatic amplification was triggered to form numerous HRP-modified double-stranded DNA (HRP-dsDNA). Then, HRP-dsDNA can be specifically recognized and efficiently cis-cleaved by Cas12a nucleases to detach HRP from the substrate, while the remaining HRP on HRP-dsDNA can catalyze 4-chloro-1-naphthol (4-CN) to form biocatalytic precipitation (BCP) on the surface of the photoelectrode, and thus the photocurrent can be changed. Meanwhile, the trans-cleavage ability of Cas12a was activated, and nonspecifically degrade the FQ-reporter and a significant fluorescence signal can be generated. Such two different kinds of signals with independent transmission paths can mutually support to improve the performance of the detection platform. Besides, a portable device was constructed for the point-of-care (POC) detection of miR-21. Moreover, the dual-mode detection platform can be easily expanded for the specific detection of other types of biomarkers by changing the sequence of hairpin DNA, thereby promoting the establishment of POC detection for early cancer diagnosis.

Nanozymes are commonly used in the construction of immunosensors, yet they are generally susceptible to pH condition, which greatly hindered their practical use. To break the limitation of pH conditions, polyethyleneimine-coated Prussian blue nanocubes (PBNCs@PEI) were synthesized as the pH-switchable nanozyme, which can show peroxidase-like and catalase-like activity in acidic and alkaline condition, respectively. Besides, the modification of PEI can largely improve the catalytic activity of PBNCs. Herein, the pH-switchable catalytic property of PBNCs@PEI was used to construct the dual-mode immunosensor for the detection of illegal additive, rosiglitazone. In acidic condition, PBNCs@PEI showed excellent peroxidase-like activity, which can trigger the colorimetric reaction of Au nanostars with TMB

Many kinds of mycotoxins can be found in cereals and feeds, thus increasing food safety requires effective and simultaneous detection of multitoxins. Herein, a flexible and three-dimensional (3D) f...

Colorimetric or fluorescent biosensors based on mimic enzymes have come into the spotlight in virtue of their visual detection. In traditional visual sensors, fluorescent-changing or color-changing substances should be introduced for the catalytic reaction with mimic enzymes. Herein, a mimic enzyme (Au@Fe-MIL-88B) with self-triggered fluorescent property was prepared. By incorporating Au nanoparticles (Au NPs) in Fe-MIL-88B, a higher peroxidase activity of Au@Fe-MIL-88B was monitored due to the synergistic effect between Au NPs and Fe-MIL-88B. Besides, Au NPs can change the valence of Fe ion in metal organic framework (MOF), thus lower background fluorescence was discovered, but the addition of H

In this study, a versatile dual-modal readout immunoassay platform was achieved for sensitive and broad-spectrum detection of ochratoxins based on the photocurrent response of flexible CdS/ZnO nanorod arrays/reduced graphene oxide and the localized surface plasmon resonance (LSPR) peak shift of Au nanobipyramids (Au NBPs). By using nanoliposomes as the vehicle to carry the secondary antibody and encapsulate horseradish peroxidase (HRP), the photocurrent change and the peak shift can be greatly amplified. The reaction mechanism was investigated in detail, indicating that HRP can trigger enzymatic bioetching in the presence of H2O2. In the photoelectrochemical detection, the oxidized HRP can etch CdS on the photoelectrode, resulting in the photocurrent change, while in the colorimetric detection, HRP can oxidize H2O2 to produce hydroxyl radicals that can etch Au NBPs to form multiple color changes and LSPR shifts. Compared with the common single-modal immunoassay for ochratoxins, such dual-modal immunoassay is more precise and reliable, owing to the completely independent signal conversion and transmission mechanism. Therefore, we hope that this accurate, simple, and visualized strategy may create a new avenue and provide innovative inspiration for food analysis.

Organophosphorus pesticides (OPs) can inhibit the activity of acetylcholinesterase (AChE) to induce neurological diseases. It is significant to exploit a rapid and sensitive strategy to monitor OPs. Here, a metal-organic framework (MOF) acted as a carrier to encapsulate AuNCs, which can limit the molecular motion of AuNCs, trigger the aggregation-induced emission (AIE) effect, and exhibit a strong fluorescence with a fluorescence lifetime and quantum yield of 6.83 μs and 4.63%, respectively. Then, the marriage of fluorescence and colorimetric signals was realized on the basis of the dual function of the enzymolysis product from AChE and choline oxidase (CHO) on AuNCs@ZIF-8. First, it can decompose ZIF-8 to weaken the restraint on AuNCs, and thus the fluorescence receded. Second, it can be used as a substrate for the peroxidase mimics of the released AuNCs to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) and a visible blue appeared. Thus, on the basis of the inhibition of AChE activity by OPs, a fluorescence-colorimetric dual-signal biosensor was established. In addition, colorimetric paper strips were exploited to realize a visual semiquantitative detection, and a smartphone APP was developed to make the visualization results more precise and realize real-time supervision of pesticide contamination.

The convenient and effective detection of toxins is urgently demanded for food security and human health. Herein, based on the catalytic activity of mimetic peroxidase from the Cu

A broad-spectrum photoelectrochemical (PEC) immunoassay was established to detect ochratoxins by using the CdS nanorod arrays (CdS NRs)/Ag2S on FTO as the photoelectrode and the CuO@secondary antibody (Ab2) label as the signal amplification. Due to the advantages of broad-spectrum antibody, such immunosensor exhibits high sensitivity to detect a low concentration of 0.67 ng/L ochratoxin A (OTA), 0.85 ng/L ochratoxin B (OTB) and 0.46 ng/L ochratoxin C (OTC), respectively. Especially, the orthogonal test was used to simulate an actual mixture containing OTA, OTB and OTC, then a multivariable linear regression model was applied to analyze the relationship of the total response with each substance. Finally, the detection of unknown mixture containing different ochratoxins was achieved by the standard addition. Such immunosensor also provided a credible selectivity and stability, suggesting its potential to detect ochratoxins or other similar compounds in food and environment.

Microcystins, the lethal cyanotoxins from Microcystis aeruginosa, can inhibit the activity of protein phosphatase and promote liver tumors. Herein, a dual-modal split-type immunosensor was constructed to detect microcystin-LR (MC-LR), based on the photocurrent change of CdS/ZnO hollow nanorod arrays (HNRs) and the blue shift of the surface plasmon resonance peak from Au nanobipyramids@Ag. By using mesoporous silica nanospheres as the carrier to immobilize secondary antibody and DNA primer, a hybridization chain reaction was adopted to capture alkaline phosphatase, while its catalytic reaction product, ascorbic acid, exhibited dual functions. The detailed mechanism was investigated, showing that ascorbic acid can not only act as the electron donor to capture the holes in CdS/ZnO-HNRs, leading to the increase photocurrent, but also as the reductant to form silver shells on Au nanobipyramids, generating multiply vivid color variations and blue shifts. Compared with the traditional photoelectrochemical immunosensor or colorimetric method for MC-LR, a more accurate and reliable result can be obtained, due to different mechanisms and independent signal transduction. Therefore, this work can not only propose a new dual-modal immunosensor for MC-LR detection but also provide innovative inspiration for constructing sensitive, accurate, and visual analysis for toxins.

The detection of glucose is important in clinic diagnosis, while the traditional methods for glucose sensing rely heavily on glucose oxidase (GOx). Herein, with the easy preparation, stability as well as selectivity, a polyvinyl borate (PVB) hydrogel was used for the non-GOx and portable detection of glucose. In the colorimetric channel, the hydrogel can be specifically decomposed in the presence of glucose, thus the electric resistance can be changed, causing significant color change of the electrochromic Prussian blue (PB) film in a very short time of 3 s. A linear relationship between the grayscale value of PB and glucose concentration was established in the range between 0.1 and 10.0 mM with the limit of detection (LOD) of 0.02 mM. In addition, the alkaline phosphatase (ALP) in the PVB gel was released to react with L -ascorbyl-2-phosphate, thus the product of ascorbic acid can reduce HAuCl4 to form the aggregation-induced emission enhancement (AIEE) of Au nanoclusters. Two separate ranges with glucose concentrations of 0.1–1.0 mM and 1.0–10.0 mM, and also a LOD of 0.018 mM were achieved. In particular, a mobile APP was designed to read the grayscale values of PB, making the sensor suitable for the on-site detection. Overall, this work may show a great potential in low-cost, stable and portable glucose detection.

A photoelectrochemical immunosensor was developed for the sensitive detection of microcystin-LR (MC-LR) by using the Au nanoclusters as the substrate and silica-functionalized DNAzyme concatamers as the label carrier. The branched TiO 2 nanorods (B-TiO 2 NRs) decorated with CdS nanoparticles were modified on FTO and acted as the photoelectrode, while the bioelectrode was prepared by in-situ electrodepositing Au nanoclusters on dopamine-modified glassy carbon electrode (GCE) to immobilize antigen. Then, silica nanospheres with excellent monodispersity were used to conjugate the secondary antibody and G-quadruplex/hemin, which can accelerate the oxidation of 4-chloro-1-naphthol (4-CN) with H 2 O 2 to yield the biocatalytic precipitation (BCP) on the electrode. Thus, the photoelectrocatalytic activity of the CdS/B-TiO 2 NRs photoelectrode can be greatly retarded. By taking the advantages of surface effect of Au nanoclusters, DNA amplification and high photoelectrocatalytic activity, the proposed photoelectrochemical immunosensor can detect MC-LR in a wide range of 0.001–100 μg/L with a detection limit of 0.7 ng/L. In addition, the acceptable stability and selectivity suggested its possible application in the detection of MC-LR in water samples.

The machine learning based on image matching is presented to construct a broad-specificity immunosensor for the detection of multiple ochratoxins. During the immunoreaction, ascorbic acid 2-phosphate (AAP) is catalyzed by the immobilized alkaline phosphatase to form ascorbic acid (AA), which can initiate the following reactions. First, silver ions (Ag+) can be reduced by AA to form silver coating on Au nanobipyramids (Au NBPs), changing the diameter of Au NBPs and the color of the solution. Second, AA can act as a sacrificial reagent to enhance the photoelectrochemical (PEC) current of CdS. Third, Ce4+ can be reduced to form Ce3+ and an intense fluorescence was discovered. Thus, a three-dimensional signal including colorimetry, photoelectrochemistry and fluorescence is built and then transformed to color signals for image matching. Results show that this method can predict multiple ochratoxins, suggesting that machine learning has great potential in combining multi-signal detection and multi-target detection by immunosensors.

The convenient and effective detection of toxins is urgently demanded for food security and human health. Herein, based on the catalytic activity of mimetic peroxidase from the Cu2O@Fe(OH)3 yolk-shell nanocages, a dual-modal multi-colorimetric and ratiometric fluorescence immunosensor for the sensitive detection of ochratoxin A (OTA) was successfully developed. For the multi-colorimetric detection, H2O2 can be effectively decomposed by Cu2O@Fe(OH)3 to form ·OH groups, thus Au nanorods (Au NRs) can be etched to exhibit vivid color variations and localized surface plasmon resonance (LSPR) shifts. For the ratiometric fluorescence detection, o-phenylenediamine was oxidized by Cu2O@Fe(OH)3 to form 2,3-diaminophenazine (DAP) in the presence of H2O2. Interestingly, the exogenous fluorescence signal source of carbon dots can be quenched by DAP via inner filter effect, while a new emission peak at 563 nm can be discovered, forming a ratiometric fluorescence signal. Due to the independent signals and mutual confirmation, the performance of the dual-modal immunosensor for the detection of OTA was significantly improved, where a broad linear range from 1 ng/L to 10 μg/L with a detection limit of 0.56 ng/L (S/N = 3) was achieved. The sensing strategy was also used to monitor OTA in millet and lake water samples with a satisfied performance.

The detection of ochratoxins is of great significance for rapid and accurate food safety screening. This work was aimed to design a photoelectrochemical/colorimetric immunosensor for the broad-spectrum detection of ochratoxin A (OTA), ochratoxin B (OTB) and ochratoxin C (OTC). In order to provide a comprehensive and accurate assessment of ochratoxins contamination, the broad spectrum antibodies were used as recognized elements and the bifunctional copper oxide nanoflowers (CuO NFs) were used as dual-signal probe to trigger the response of photoelectrochemical (PEC) and colorimetric detection. In PEC mode, Cu2+ released by CuO NFs can replace Cd2+ in photoactive material to form a new band gap, which accelerated the electron-hole recombination to change the PEC current. In colorimetric mode, Cu2+ efficiently assisted etching of Au nanorods (Au NRs), causing multiply color changes and localized surface plasmon resonance (LSPR) shifts of Au NRs. Two independent modes from different detection mechanisms were used to reflect the concentrations of ochratoxins with more accuracy and reliability. Therefore, this work not only realized the broad-spectrum detection of OTA, OTB and OTC, but also provided a new strategy for the development of accurate, visual and multi-signal immunosensing platform.

Herein, gold nanoparticles (Au NPs) were decorated on magnetic Fe3O4 nanoclusters@polydopamine nanocomposites (Fe3O4@PDA NCs) through a direct and green reduction method. The morphology was investigated via transmission electronic microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Results showed that Fe3O4 nanoclusters were successfully coated with a PDA shell layer of 25 nm thickness that could work as a reducing reagent to form Au NPs on the surface of Fe3O4@PDA NCs and prevent the aggregation of Au NPs as well. More interestingly, the concentration of the Au precursor had a great effect on both the size and the dispersion of Au NPs on the surface of Fe3O4@PDA NCs, which directly affected the catalytic activity of Fe3O4@PDA@Au. The catalytic performance of Fe3O4@PDA@Au was determined by reducing 4-nitrophenol to 4-aminophenol in the presence of excessive NaBH4, and the result showed that Fe3O4@PDA@Au prepared from a 130 μM Au precursor exhibited the best catalytic activity with the reaction rate constant of 39.2 s−1 g−1 and a conversion of >99% in ten minutes. After being recycled and reused ten times, the magnetic catalyst still had a conversion of >95%; this suggested that it might have practical applications in the reduction of nitroaromatic compounds.

A ratiometric immunosensor was built to detect different ochratoxins in a mixture by combining broad-specific antibody and antigen, multidimensional signals, and computer programs. This immunosensor used MoS2 NFs/CdS as the photoelectrode to immobilize antigens and ZnS/Ag2S nanocages to capture the secondary antibody. During the immunoreaction recognition, the photoelectrochemical (PEC) current of MoS2 NFs/CdS and the square wave voltammetric (SWV) current of the dissolved Zn2+ from ZnS can be influenced by the deposition of the ZnS/Ag2S label. The results show that the ratio of PEC to SWV current decreased linearly with the logarithm of ochratoxin concentrations in a range from 1 ng/L to 1 μg/L. The present work provides a possible route to design broad-specific immunosensors and expands the scope of ratiometric biosensors in the detection of a mixture.

Recently, mimic enzymes have obtained particular interest by their high activity, stability, and biocompatibility. In this work, by coupling copper hydroxide nanozyme and G-quadruplex/hemin DNAzyme to form a double-integrated mimic enzyme, a visual, sensitive and selective immunosensor was established to detect microcystin-LR (MC-LR). In this immunoassay, the microplates were modified with core-shell silica/nickel silicate as the substrate to capture MC-LR antigens. Then, Cu(OH)2 nanocages with fine regulation were used as the label to capture the secondary antibody for immunoreaction and the DNA primer for propagation, followed by using hybridization chain reaction to amplify the DNA primer, thus numerous DNAzymes (G-quadruplex/hemin) can be formed on the surface of Cu(OH)2 nanocages with the aid of hemin. Such double-integrated mimic enzyme including Cu(OH)2 nanozymes and DNAzymes showed excellent peroxidase activity for the chromogenic reaction of 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), which realized the visual detection of MC-LR in the range from 0.007 to 75 μg/L with the detection limit of 6 ng/mL, and thus provided the probability for the portable assessment of MC-LR in real sample.

A broad-specific photoelectrochemical (PEC) immunosensor was developed for the simultaneous detection of ochratoxin A, ochratoxin B and ochratoxin C (OTA, OTB, OTC) by using the direct growth of CdS nanorods on FTO as the photoelectrode and Au nanoflowers-modified glass carbon electrode (GCE) as the bioelectrode. The bioelectrode was used to capture antigens and then associate corresponding antibodies, followed by using SiO2@Cu2+ nanocomposites to conjugate the secondary antibody (Ab2) and a DNA strand as the initiator. After the hybridization chain reaction (HCR) and the addition of hemin, numerous DNAzymes (G-quadruplex/hemin) were produced. Due to the similar enzymatic property with horseradish peroxidase (HRP), G-quadruplex/hemin can accelerate the oxidation of 4-chloro-1-naphthol (4-CN) with H2O2 to yield the biocatalytic precipitation (BCP) on the bioelectrode. Then, the bioelectrode was further treated with moderate acid and thus Cu2+ was released, which can decrease the photocurrent of the photoelectrode by the formation of CuS. Due to the advantages of surface effect of Au nanoflowers, DNA amplification and high photoelectrocatalytic activity, the proposed broad-specificity PEC immunosensor can detect OTA, OTB and OTC with a detection limit of 0.02, 0.04 and 0.03 pg/mL, respectively. In addition, the acceptable stability and selectivity suggest its possible application in the detection of OTA, OTB and OTC in water samples.

An ultrasensitive photoelectrochemical (PEC) immunoassay based on multiple signal amplification strategy was fabricated for the detection of microcystin-LR (MC-LR). The CdS/TiO2 nanorod arrays (CdS/TiO2 NRAs) modified FTO electrode, which can weaken the self-oxidation by photogenerated holes of CdS nanoparticles, and limit the recombination of electron-hole pairs and broaden optical absorption of TiO2 NRAs, was used as a visible-light driven material to immobilize antigens. Then, Fe3O4 nanoparticles/polydopamine (Fe3O4@PDA) was used as the carrier to load secondary antibody (Ab2) and horseradish peroxidase (HRP), where Fe3O4 nanoparticles and HRP can synergistically accelerate the oxidation of 4-chloro-1-naphthol (4-CN) by H2O2 to produce biocatalytic precipitation (BCP) on the surface of modified electrode. Due to the catalytic activity of Fe3O4 nanoparticles and HRP, the nonproductive absorption of HRP and the steric hindrance by BCP, the photocurrent change was amplified. The proposed PEC immunosensor can detect MC-LR in a range of 0.005–500 μg/L with a detection limit of 0.001 μg/L. Meanwhile, the PEC immunosensor exhibited high sensitivity, good stability, acceptable selectivity and reproducibility, indicating its potential application in environmental monitoring.

A novel multiple amplified enzyme-free immunosensor was developed for competitive immunoassay of microcystin-LR (MC-LR). Classical electrochemical immunosensors usually employ enzymes as biocatalysts to afford amplified signals, but the proteolytic degradation and poor stability are still crucial problem. Herein, monodisperse core-shell mesoporous silica (SiO2@MSN)-functionalized DNAzyme concatamers were synthesized to load hemin and methylene blue (MB) as the mimic enzyme. Firstly, the surface of SiO2@MSN was conjugated with secondary antibody as the recognition of MC-LR antibody and with a DNA strand as the initiator. Two auxiliary DNA strands were then selected for the in-situ propagation to form a double-helix DNA through hybridization chain reaction (HCR), forming numerous DNAzymes (G-quadruplex/hemin) after the addition of hemin. Secondly, MB was inserted into the formed double-helix DNA, and also loaded in the brush-like structure of mesoporous SiO2@MSN. The molecular docking study showed that electrons can transfer more effectively with π-π stack of hemin/G-quadruplex and intercalation of MB/DNA, thus the catalytic ability of DNAzymes can be greatly improved. With the aid of MB, DNAzymes can catalyze the reduction of H2O2 to produce the electrochemical signal. This enzyme-free electrochemical immunosensor can successfully detect MC-LR in a range of 0.5ng/L and 25μg/L with a detection limit of 0.3ng/L. This stable, sensitive and selective nonenzymatic electrochemical immunoassay shows promise for applications in food and environmental monitoring.

A facile, green and in situ synthesis for the immobilization of monodispersed Au nanoparticles (Au NPs) on eggshell membrane (ESM) using the Lagerstroemia speciosa leaves extract (LSE) was reported. The stable fibrous-network structure of the ESM offered great surface area and abundant functional groups to provide an ideal support, while the leaves extract of Lagerstroemia speciosa can act as stabilizing and reducing reagents for Au NPs without capping reagents or heating. The morphology and surface chemical information were characterized via transmission electronic microscopy (TEM), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), x-ray diffraction analysis (XRD) and thermogravimetric analysis (TGA). The catalyst exhibited excellent performance for the reduction of 4-nitrophenol (4-NP), showing that ESM@LSE@Au from a 0.6 mM Au precursor exhibited the best catalytic activity with the reaction rate constant of 29.70 s−1 g−1 and a conversion of over 99.84% in 6 min. Moreover, the catalyst can be efficiently recycled by mild centrifugation with a remained conversion over 92.40% after 10 cycles, suggesting the potential for the reduction of nitroaromatic compounds.