A novel visible-light driven photoelectrochemical immunosensor based on multi-amplification strategy for ultrasensitive detection of microcystin-LR

Nucleic acid aptasensor with magnetically induced self-assembly for the detection of EpCAM glycoprotein

A "turn-on" aptasensor for label-free and cell-free EpCAM detection was constructed by employing magnetic α-Fe2O3/Fe3O4@Au nanocomposites as a matrix for signal amplification and double-stranded complex (SH-DNA/Apt probes) immobilization through Au-S binding. α-Fe2O3/Fe3O4@Au could be efficiently assembled into uniform and stable self-assembly films via magnetic-induced self-assembly technique on a magnetic glassy carbon electrode (MGCE). The effectiveness of the platform for EpCAM detection was confirmed through differential pulse voltammetry (DPV). Under optimized conditions, the platform exhibited excellent specificity for EpCAM, and a strong linear correlation was observed between the current and the logarithm of EpCAM protein concentration in the range 1 pg/mL-1000 pg/mL (R2 = 0.9964), with a limit of detection (LOD) of 0.27 pg/mL. Furthermore, the developed platform demonstrated good stability during a 14-day storage test, with fluctuations remaining below 93.33% of the initial current value. Promising results were obtained when detecting EpCAM in spiked serum samples, suggesting its potential as a point-of-care (POC) testing.

Photoelectrochemical sensors based on paper and their emerging applications in point-of-care testing

Point-of-care testing (POCT) technology is urgently required owing to the prevalence of the Internet of Things and portable electronics. In light of the attractive properties of low background and high sensitivity caused by the complete separation of excitation source and detection signal, the paper-based photoelectrochemical (PEC) sensors, featured with fast in analysis, disposable and environmental-friendly have become one of the most promising strategies in POCT. Therefore, in this review, the latest advances and principal issues in the design and fabrication of portable paper-based PEC sensors for POCT are systematically discussed. Primarily, the flexible electronic devices that can be constructed by paper and the reasons why they can be used in PEC sensors are expounded. Afterwards, the photosensitive materials involved in paper-based PEC sensor and the signal amplification strategies are emphatically introduced. Subsequently, the application of paper-based PEC sensors in medical diagnosis, environmental monitoring and food safety are further discussed. Finally, the main opportunities and challenges of paper-based PEC sensing platforms for POCT are briefly summarized. It provides a distinct perspective for researchers to construct paper-based PEC sensors with portable and cost-effective, hoping to enlighten the fast development of POCT soon after, as well as benefit human society.

Dual II-scheme nanosheet-like Bi2S3/Bi2O3/Ag2S heterostructures for ultrasensitive PEC aptasensing of aflatoxin B1 coupled with catalytic signal amplification by dendritic nanorod-like Au@Pd@Pt nanozyme

As one of the most toxic chemical substances, aflatoxin B1 (AFB1) has a strong carcinogenic effect even at a trace level in human and animal, which severely threatens human health and even causes cancers. Therefore, ultrasensitive detection of AFB1 is of significant importance. For such analysis, dual II-scheme sheet-like Bi₂S₃/Bi₂O₃/Ag₂S heterostructures were prepared by the in-situ growth method, which exhibited high separation efficiency for the electron-hole (e^--h⁺) pairs, prominent stability, and high photoactivity. Moreover, the dendritic nanorod-like Au@Pd@Pt (Au@Pd@Pt DNRs) nanozyme was homely synthesized, whose peroxidase-like activity was scrupulously investigated by catalytical oxidation of diaminobenzidine (DAB) in the presence of H₂O₂. Integration by the aptasensing strategy, a photoelectrochemical (PEC) "signal-on" aptasensor was prepared, which exhibited a broader linear range of 0.5 pg mL^-1-100 ng mL^-1 with a lower limit of detection (LOD = 0.09 pg mL^-1, S/N = 3). This work provides a feasible strategy to develop advanced PEC biosensors for actual analysis of environmental pollutants.

Advances in immunoassay-based strategies for mycotoxin detection in food: From single-mode immunosensors to dual-mode immunosensors

Mycotoxin contamination in foods and other goods has become a broad issue owing to serious toxicity, tremendous threat to public safety, and terrible loss of resources. Herein, it is necessary to develop simple, sensitive, inexpensive, and rapid platforms for the detection of mycotoxins. Currently, the limitation of instrumental and chemical methods cannot be massively applied in practice. Immunoassays are considered one of the best candidates for toxin detection due to their simplicity, rapidness, and cost-effectiveness. Especially, the field of dual-mode immunosensors and corresponding assays is rapidly developing as an advanced and intersected technology. So, this review summarized the types and detection principles of single-mode immunosensors including optical and electrical immunosensors in recent years, then focused on developing dual-mode immunosensors including integrated immunosensors and combined immunosensors to detect mycotoxins, as well as the combination of dual-mode immunosensors with a portable device for point-of-care test. The remaining challenges were discussed with the aim of stimulating future development of dual-mode immunosensors to accelerate the transformation of scientific laboratory technologies into easy-to-operate and rapid detection platforms.

Enhanced cathodic electrochemiluminescent microcystin-LR aptasensor based on surface plasmon resonance of Bi nanoparticles

Sensitive testing for Microcystins-LR (MC-LR) is needed because of its serious environmental and human health hazards. In this work, a new type of surface plasmon resonance (SPR) enhanced cathodic electrochemiluminescence (ECL) aptasensing platform was designed in which boron and nitrogen co-doped graphene quantum dots (BN-GQDs) were used as the luminary and bismuth nanoparticles (Bi NPs) were used as the SPR source. SPR effect of non-precious metal Bi NPs can induce and enhance ECL signal of BN-GQDs because the fluorescence spectrum of BN-GQDs overlaps well with the ultraviolet-visible absorption spectrum of Bi NPs. On this basis, a sensitive sensing system based on the Bi NPs and BN-GQDs was established for MC-LR detection. The results showed that the ECL sensing signal obtained was linear with the negative logarithm of the target MC-LR concentration in the range of 0.01-5000 pM, and the detection limit was 0.003 pM. In addition, the sensor had high stability and good reproducibility, which can be applied to the detection of MC-LR in actual samples. The method had good specificity and can not be disturbed by its homolog, which can be used for sensitive and reliable detection of complex samples.

Photoelectrochemical aptasensors for detection of viruses

Photoelectrochemistry (PEC) is a dynamic discipline studying the effect of light on photoelectrode or photosensitive material, and the conversion from solar energy into electrical power. The basic PEC process refers to the oxidation or reduction reactions between electrochemical active species in solution and photoactive materials that occurred at the electrode/electrolyte interface during illumination. In recent years, the PEC biosensing approaches have also been developed by the combination of the PEC technique with bioanalysis, where the interaction between biological recognition element and analyte influences a photocurrent signal. This involves the charge and energy transfer of PEC reaction between electron donor/acceptor and photoactive material upon light irradiation. Coupling the advantages of PEC bioanalysis and aptamers has provided new concepts for highly selective and sensitive biosensors development, applicable in human health monitoring and environmental protection. In a typical assay, a photoactive material converts the affinity binding properties of aptamers into a detectable electrical signal, presenting an innovative method for probing numerous aptamer-analyte interactions. Using different aptamer probes aiming for specific purposes, more sensing strategies with rational design and exquisite signaling mechanisms have been proposed. This review concentrated on the current topic of PEC aptasensors that are used for the detection of viruses. The prospects in this area are also discussed.

Graphical abstract

Immunoassay technology: Research progress in microcystin-LR detection in water samples

Increasing global warming and eutrophication have led to frequent outbreaks of cyanobacteria blooms in freshwater. Cyanobacteria blooms cause the death of aquatic and terrestrial organisms and have attracted considerable attention since the 19th century. Microcystin-LR (MC-LR) is one of the most typical cyanobacterial toxins. Therefore, the fast, sensitive, and accurate determination of MC-LR plays an important role in the health of humans and animals. Immunoassay refers to a method that uses the principle of immunology to determine the content of the tested substance in a sample using the tested substance as an antigen or antibody. In analytical applications, the immunoassay technology could use the specific recognition of antibodies for MC-LR detection. In this review, we firstly highlight the immunoassay detection of MC-LR over the past two decades, including classical enzyme-link immunosorbent assay (ELISA), modern immunoassay with optical signal, and modern immunoassay with electrical signal. Among these detection methods, the water environment was used as the main detection system. The advantages and disadvantages of the different detection methods were compared and analyzed, and the principles and applications of immunoassays in water samples were elaborated. Furthermore, the current challenges and developmental trends in immunoassay were systematically introduced to enhance MC-LR detection performance, and some critical points were given to deal with current challenges. This review provides novel insight into MC-LR detection based on immunoassay method.

Ultrasensitive photoelectrochemical microcystin-LR immunosensor using carboxyl-functionalized graphene oxide enhanced gold nanoclusters for signal amplification

An ultrasensitive photoelectrochemical (PEC) immunosensor based on gold nanoclusters (AuNCs) with 11-mercaptoundecanoic acid (MUA) ligands was fabricated for determination of microcystin-LR (MC-LR). The PEC immunosensor was developed by loading the monoclonal MC-LR antibody (Ab) to the MUA-AuNCs modified gold electrodes. After different measurement conditions being optimized, silver nanoparticles (AgNPs), gold nanorods (AuNRs), graphene oxide (GO) and carboxyl-functionalized graphene oxide (cGO) were introduced into MUA-AuNCs to enhance the sensing properties. The experimental result revealed that the sensitivity of PEC immunosensors was enhanced by both their photoelectrochemical properties and antibody loading properties with dependent relationship, which was different from the enhancement strategy of PEC sensors based on redox reactions. Among different hybrid nanocomposites, MUA-AuNCs/cGO not only improved the photoelectrochemical properties, but also loaded more antibodies for sensing, which resulted in best sensing performance. Thus, a universal method was proposed to enhance the sensing performance of PEC immunosensors based on impedance changes. Finally, MUA-AuNCs/cGO based PEC immunosensors exhibited a wide linear range of 0.001 nM-1000 nM with low detection limit of 0.011 pM (S/N = 3) for MC-LR determination. Meanwhile, the designed PEC immunosensors showed high selectivity, reproducibility and specificity, which provided the promising applications in aquatic environment.

Recent developments in the methods of quantitative analysis of microcystins

Cyanotoxins are produced by the toxic cyanobacterial species present in algal blooms formed in water bodies due to nutrient over-enrichment by human influences and natural environmental conditions. Extensive studies are available on the most widely encountered cyanotoxins, microcystins (MCs) in fresh and brackish water bodies. MC contaminated water poses severe risks to human health, environmental sustainability, and aquatic life. Therefore, commonly occurring MCs should be monitored. Occasionally, detection and quantification of these toxins are difficult due to the unavailability of pure standards. Enzymatic, immunological assays, and analytical techniques like protein phosphatase inhibition assay, enzyme-linked immunosorbent assay, high-performance liquid chromatography, liquid chromatography-mass spectrometry, and biosensors are used for their detection and quantification. There is no single method for the detection of all the different types of MCs; therefore, various techniques are often combined to yield reliable results. Biosensor development offered a problem-solving approach in the detection of MCs due to their high accuracy, sensitivity, rapid response, and portability. In this review, an endeavor has been made to uncover emerging techniques used for the detection and quantification of the MCs.

A novel photoelectrochemical sensor based on tailoring printable mesoscopic chip for fast and real-time phospholipids oxidation detection

The detection of phospholipids oxidation is important for meat control and disease prevention. In this paper, a photoelectrochemical sensor based on printable mesoscopic chip (PMC) for fast and real-time monitoring phospholipids oxidation was designed and fabricated. TiO2, ZrO2 and carbon films of PMC were screen-printed onto the FTO glass layer by layer. The PMC and the feasibility for determination of phospholipids oxidation were investigated by scanning electron microscope (SEM), UV-vis spectroscopy, cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS), etc. The short circuit current (Jsc) was used as a signal current, which would decrease if phospholipids in PMC were undergoing oxidation for the change of electrical properties. Compared with other methods, phospholipids in PMC did not require pretreatment, and the process was nondestructive and real-time. Meanwhile, this method showed high sensitivity and good selectivity. The fabricating process of PMC is simple, and the costs are low, relatively.

Multifunctional Peroxidase-Encapsulated Nanoliposomes: Bioetching-Induced Photoelectrometric and Colorimetric Immunoassay for Broad-Spectrum Detection of Ochratoxins

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.

Highly sensitive detection of microcystin-LR under visible light using a self-powered photoelectrochemical aptasensor based on a CoO/Au/g-C3N4 Z-scheme heterojunction

Based on the unique photoelectrochemical properties of a CoO/Au/g-C₃N₄ Z-scheme heterojunction, a self-powered photoelectrochemical (PEC) aptasensor was constructed for the detection of microcystin-leucine arginine (MC-LR). Z-scheme heterojunctions can promote the separation of a photo-induced electron-hole pair, and the surface plasmonic resonance (SPR) of Au nanoparticles can significantly enhance the adsorption of visible light. Importantly, MC-LR molecules were captured by aptamers initially immobilized on the modified electrode due to their high affinity, and then oxidized by the photogenerated holes, which caused an amplified photocurrent signal, allowing the quantitative analysis of MC-LR by measuring the photocurrent intensity change. This PEC MC-LR aptasensor showed high sensitivity and selectivity within a wide linear response range from 0.1 pM to 10 nM and a detection limit of 0.01 pM. The application of this sensor in the analysis of lake water samples provided accurate results with a relative standard deviation (RSD) of 2.6%-4.2%.

Magnetic nanoparticles for smart electrochemical immunoassays: a review on recent developments

This review (with 129 refs) summarizes the progress in electrochemical immunoassays combined with magnetic particles that was made in the past 5 years. The specifity of antibodies linked to electrochemical transduction (by amperometry, voltammetry, impedimetry or electrochemiluminescence) gains further attractive features by introducing magnetic nanoparticles (MNPs). This enables fairly easy preconcentration of analytes, minimizes matrix effects, and introduces an appropriate label. Following an introduction into the fundamentals of electrochemical immunoassays and on nanomaterials for respective uses, a large chapter addresses method for magnetic capture and preconcentration of analytes. A next chapter discusses commonly used labels such as dots, enzymes, metal and metal oxide nanoparticles and combined clusters. The large field of hybrid nanomaterials for use in such immunoassays is discussed next, with a focus on MNPs composites with various kinds of graphene variants, polydopamine, noble metal nanoparticles or nanotubes. Typical applications address clinical markers (mainly blood and urine parameters), diagnosis of cancer (markers and cells), detection of pathogens (with subsections on viruses and bacteria), and environmental and food contaminants as toxic agents and pesticides. A concluding section summarizes the present status, current challenges, and highlights future trends. Graphical abstract Magnetic nanoparticles (MNP) with antibodies (Ab) capture and preconcentrate analyte from sample (a) and afterwards become magnetically (b) or immunospecifically (c) bound at an electrode. Signal either increases due to the presence of alabel (b) or decreases as the redox probe is blocked (c).

Colorimetric Method for Sensitive Detection of Microcystin-LR Using Surface Copper Nanoparticles of Polydopamine Nanosphere as Turn-On Probe

A novel, facile sensor was further developed for microcystin-LR (MC-LR) determination by visible spectroscopy. Antibody-functionalized SiO₂-coated magnetic nanoparticles (Fe₃O₄@SiO₂) and aptamer-functionalized polydopamine nanospheres decorated with Cu nanoparticles (PDA/CuNPs) recognized specific sites in MC-LR and then the sandwich-type composites were separated magnetically. The Cu in the separated composites was converted to Cu²⁺ ions in solution and turn-on visible absorption was achieved after reaction with bis(cyclohexanone)oxaldihydrazone (BCO) (λ_max = 600 nm). There was a quantitative relationship between the spectral intensity and MC-LR concentration. In addition, under the optimum conditions, the sensor turns out to be a linear relationship from 0.05 to 25 nM, with a limit of detection of 0.05 nM (0.05 μg/L) (S/N = 3) for MC-LR. The sensitivity was dependent on the low background absorption from the off-to-on spectrum and label amplification by the polydopamine (PDA) surface. The sensor had high selectivity, which shows the importance of dual-site recognition by the aptamer and antibody and the highly specific color formed by BCO with Cu²⁺. The bioassay was complete within 150 min, which enabled quick determination. The sensor was successfully used with real spiked samples. These results suggest it has potential applications in visible detection and could be used to detect other microcystin analogs.

Dual-Modal Split-Type Immunosensor for Sensitive Detection of Microcystin-LR: Enzyme-Induced Photoelectrochemistry and Colorimetry

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.

A review on visible-light induced photoelectrochemical sensors based on CdS nanoparticles

Discovering the distinctive photophysical properties of semiconductor nanoparticles (NPs) has made these a popular subject in recent advances in nanotechnology-related analytical methods.