A competitive microcystin-LR immunosensor based on Au NPs@metal-organic framework (MIL-101)

Current trends in colorimetric biosensors using nanozymes for detecting biotoxins (bacterial food toxins, mycotoxins, and marine toxins)

Biotoxins, predominantly bacterial food toxins, mycotoxins, and marine toxins, have emerged as major threats in the fields of seafood, other foods, feeds, and medicine. They have potential teratogenic, mutagenic, and carcinogenic effects on humans, occasionally triggering high morbidity and mortality. One of the apparent concerns relates to the increasing consumption of fast food resulting in the demand for processed food without adequate consideration of the toxins they may contain. Therefore, developing improved methods for detecting biotoxins is of paramount significance. Nanozymes, a type of nanomaterials exhibiting enzyme-like activity, are increasingly being recognized as viable alternatives to natural enzymes owing to their benefits, such as customizable design, controlled catalytic performance, excellent biocompatibility, and superior stability. The remarkable catalytic activity of nanozymes has led to their broad utilization in the development of colorimetric biosensors. This has emerged as a potent and efficient approach for rapid detection, enabling the creation of innovative colorimetric sensing methodologies through the integration of nanozymes with colorimetric sensors. In this review, recent development in nanozyme research and their application in colorimetric biosensing of biotoxins are examined with an emphasis on their characteristics and performance. The study particularly focuses on the peroxidase (POD) activity, oxidase (OXD) activity, superoxide dismutase (SOD), and catalase (CAT) activity of nanozymes in colorimetric biosensors. Ultimately, the challenges and future prospects of these assays are explored.

Cost-effective screen-printed carbon electrode biosensors for rapid detection of microcystin-LR in surface waters for early warning of harmful algal blooms

Microcystins (MCs) are toxins produced by cyanobacteria commonly found in harmful algal blooms (HABs). Due to their toxicity to humans and other organisms, the World Health Organization (WHO) sets a guideline of 1 μg L-1 for microcystin-leucine-arginine (MC-LR) in drinking water. However, current analytical techniques for the detection of MC-LR such as liquid chromatography-mass spectrometry (LC-MS) and ELISA are costly, bulky, time-consuming, and mostly conducted in a laboratory, requiring highly trained personnel. An analytical method that can be used in the field for rapid determination is essential. In this study, an anti-MC-LR/MC-LR/cysteamine-coated screen-printed carbon electrode (SPCE) biosensor was newly developed to detect MC-LR, bioelectrochemically, in water. The functionalization of the electrode surface was confirmed with surface characterization methods. The sensor performance was evaluated by electrochemical impedance spectroscopy (EIS), obtaining a linear working range of MC-LR concentrations between 0.1 and 100 μg L-1 with a limit of detection (LOD) of 0.69 ng L-1. Natural water samples experiencing HABs were collected and analyzed using the developed biosensor, demonstrating the excellent performance of the biosensor with a relative standard deviation (RSD) of 0.65%. The interference tests showed minimal error and RSD values against other common MCs and possible coexisting ions found in water. The biosensor showed acceptable functionality with a shelf life of up to 12 weeks. Overall, the anti-MC-LR/MC-LR/cysteamine/SPCE biosensors can be an innovative solution with characteristics that allow for in situ, low-cost, and easy-to-use capabilities which are essential for developing an overarching and integrated "smart" environmental management system.

Review: Synthesis of metal organic framework-based composites for application as immunosensors in food safety

Ensuring food safety continues to be one of the major global challenges. For effective food safety monitoring, fast, sensitive, portable, and efficient food safety detection strategies must be devised. Metal organic frameworks (MOFs) are porous crystalline materials that have attracted attention for use in high-performance sensors for food safety detection owing to their advantages such as high porosity, large specific surface area, adjustable structure, and easy surface functional modification. Immunoassay strategies based on antigen-antibody specific binding are one of the important means for accurate and rapid detection of trace contaminants in food. Emerging MOFs and their composites with excellent properties are being synthesized, providing new ideas for immunoassays. This article summarizes the synthesis strategies of MOFs and MOF-based composites and their applications in the immunoassays of food contaminants. The challenges and prospects of the preparation and immunoassay applications of MOF-based composites are also presented. The findings of this study will contribute to the development and application of novel MOF-based composites with excellent properties and provide insights into advanced and efficient strategies for developing immunoassays.

A novel electrochemical immunosensor based on SnS2/NiCo metal-organic frameworks loaded with gold nanoparticles for cortisol detection

In this work, a label-free electrochemical immunosensor using tin sulfide/nickel cobalt metal-organic frameworks (SnS₂/NiCo MOFs) was established for the sensitive etection of cortisol. First, SnS₂/NiCo MOFs were synthesized by doping SnS₂ with NiCo MOF nanocubes by a hydrothermal method. Then, gold nanoparticles (AuNPs) were grown in situ on SnS₂/NiCo MOFs for electrochemical detection. The use of SnS₂/NiCo MOFs promoted the electron transfer rate of AuNPs and enhanced the electrochemical sensing performance of AuNPs@SnS₂/NiCo MOFs-modified electrodes. The large specific surface area of AuNPs@SnS₂/NiCo MOFs provides more active sites for antibody loading. After the prepared immunosensor was incubated with the target analyte, cortisol, the electron transfer impedance increased and the amperometric response decreased, thus establishing a highly sensitive immunosensing method. The sensor had a linear range of 100 fg/mL to 100 ng/mL and a low detection limit of 29 fg/mL. The sensor showed good accuracy and practicability and could be used for the determination of cortisol in saliva.

Advances in Metal-Organic Frameworks MIL-101(Cr)

MIL-101(Cr) is one of the most well-studied chromium-based metal-organic frameworks, which consists of metal chromium ion and terephthalic acid ligand. It has an ultra-high specific surface area, large pore size, good thermal/chemical/water stability, and contains unsaturated Lewis acid sites in its structure. Due to the physicochemical properties and structural characteristics, MIL-101(Cr) has a wide range of applications in aqueous phase adsorption, gas storage and separation, and catalysis. In this review, the latest synthesis of MIL-101(Cr) and its research progress in adsorption and catalysis are reviewed.

Metal-organic frameworks based hybrid nanocomposites as state-of-the-art analytical tools for electrochemical sensing applications

Metal-organic frameworks (MOFs) are remarkably porous materials that have sparked a lot of interest in recent years because of their fascinating architectures and variety of potential applications. This paper systematically summarizes recent breakthroughs in MOFs and their derivatives with different materials such as, carbon nanotubes, graphene oxides, carbon fibers, enzymes, antibodies and aptamers etc. for enhanced electrochemical sensing applications. Furthermore, an overview part is highlighted, which provides some insights into the future prospects and directions of MOFs and their derivatives in electrochemical sensing, with the goal of overcoming present limitations by pursuing more inventive ways. This overview can perhaps provide some creative ideas for future research on MOF-based materials in this rapidly expanding field.

Core-shell magnetic zeolite imidazolate framework-8 as adsorbent for magnetic solid phase extraction of brucine and strychnine from human urine

Core-shell magnetic zeolite imidazolate framework-8 (Fe3O4@PAA@ZIF-8) was successfully synthesized and first employed as adsorbent of magnetic solid-phase extraction (MSPE) for the determination of brucine and strychnine in human urine sample coupled with high performance liquid chromatography. The as-prepared Fe3O4@PAA@ZIF-8 was characterized by transmission electron microscopy, Fourier-transform infrared spectrometry, X-ray diffraction, vibrating sample magnetometer and zeta potential analysis. Main parameters affecting the MSPE efficiency, including amount of adsorbent, sample solution pH, extraction time, ionic strength, desorption solvent, desorption time and desorption volume were further optimized. Under the optimized conditions, the proposed method provided good linearity (5.0-1000.0 μg L-1) with determination coefficients between 1.0000 and 0.9998, low limits of detection in the range of 1.1-1.2 μg L-1, and excellent reproducibility with relative standard deviations of less than 7.7%. The intra-day and inter-day precision were 1.5-3.2% and 2.1-7.2%, respectively. Satisfactory spiked recoveries were between97.2% and 115.4% with the relative standard deviations less than 6.3%. The results demonstrated that Fe3O4@PAA@ZIF-8 composite was a promising magnetic adsorbent for the preconcentration of brucine and strychnine in human urine sample.

Impedimetric Microcystin-LR Aptasensor Prepared with Sulfonated Poly(2,5-dimethoxyaniline)–Silver Nanocomposite

This paper presents a novel impedimetric aptasensor for cyanobacterial microcystin-LR (L, l-leucine; R, l-arginine) (MC-LR) containing a 5′ thiolated 60-mer DNA aptamer (i.e., 5′-SH-(CH2)6GGCGCCAAACAGGACCACCATGACAATTACCCATACCACCTCATTATGCCCCATCT CCGC-3′). A nanocomposite electrode platform comprising biocompatible poly(2,5-dimethoxyaniline) (PDMA)-poly(vinylsulfonate) (PVS) and silver nanoparticle (Ag0) on a glassy carbon electrode (GCE), i.e., (GCE/PDMA–PVS–Ag0) was used in the biosensor development. Small-angle X-ray scattering (SAXS) spectroscopic analysis revealed that the PDMA–PVS–Ag0 nanocomposites were polydispersed and contained embedded Ag0. Electrochemical impedance spectroscopy (EIS) responses of the aptasensor gave a dynamic linear range (DLR) and limit of detection (LOD) values of 0.01–0.1 ng L−1 MC-LR and 0.003 ng L−1 MC-LR, respectively. The cross-reactivity studies, which was validated with enzyme-linked immunosorbent assay (ELISA), showed that the aptasensor possesses excellent selectivity for MC-LR.

Advances in nanomaterial-based electrochemical biosensors for the detection of microbial toxins, pathogenic bacteria in food matrices

There is a growing demand to protect food products against the hazard of microbes and their toxins. To satisfy such goals, it is important to develop highly sensitive, reliable, sophisticated, rapid, and cost-effective sensing techniques such as electrochemical sensors/biosensors. Although diverse forms of nanomaterials (NMs)-based electrochemical sensing methods have been introduced in markets, the reliability of commercial products is yet insufficient to meet the practical goal. In this review, we focused on: 1) sources of pathogenic microbes and their toxins; 2) possible routes of their entrainment in food, and 3) current development of NM-based biosensors to realize real-time detection of the target analytes. At last, future prospects and challenges in this research field are discussed.

Nanobiochar paper based electrochemical immunosensor for fast and ultrasensitive detection of microcystin-LR

A portable, cheap and sensitive paper type electrochemical immunosensor was developed with conductive nanobiochar paper as the conductive layer and utilized for sensitive detection of microcystin-LR (MCLR) toxin in water. The paper immunosensor was constructed by coating of highly conductive and dispersible nanobiochar particle (nBC) and anti-MCLR antibody on the filter paper via dipping-drying method. The presence of MCLR could be specifically quantified amperometrically by the nBC-paper immunosensor with the response time of less than 5 min, and the lowest detection limit of 17 pM (0.017 μg/L) was achieved. Moreover, the proposed immunosensor exhibited high selectivity, reproducibility and storage stability, and was also used for environmental water detection with satisfactory recovery. The successful fabrication of low cost and ubiquitous biochar based paper type electrochemical immunosensing system would have significant value for the development of highly cost-effective electrochemical device.

Morphology-maintaining synthesis of copper hydroxy phosphate@metal-organic framework composite for extraction and determination of trace mercury in rice

A novel copper hydroxy phosphate@MOF composite DMP-Cu decorated by 2, 5-dimercapto-1, 3, 4-thiadiazol was facilely prepared and characterized. A dispersive SPE strategy using DMP-Cu as adsorbent combined with atomic fluorescence spectroscopy was developed for the selective capture of trace total mercury in rice sample. The adsorption mechanism showed that the Hg²⁺ removal process was fitted with pseudo second-order kinetics and the Langmuir adsorption model. The adsorbent was easy to be regenerated and the maximum adsorption capacity for the removal of Hg²⁺ was 249.5 mg g^-1 at the optimal pH of 4. X-ray photoelectron spectroscopy and Raman spectra verified the selective and strong interaction between Hg²⁺ and thiol/nitrogen-containing functional groups of DMTZ on DMP-Cu. The trace total mercury in rice samples was determined with detection limit of 0.0125 ng mL^-1 and relative standard deviation below 6%. The high recoveries were obtained in range of 98.8-109% for the spiked rice samples.

Cu/Au/Pt trimetallic nanoparticles coated with DNA hydrogel as target-responsive and signal-amplification material for sensitive detection of microcystin-LR

Sensitive and reliable analytical methods for monitoring of microcystin-LR (MC-LR) are urgently necessary due to its great harm to human health and aquatic organisms. In this work, a novel Cu/Au/Pt trimetallic nanoparticles (Cu/Au/Pt TNs)-encapsulated DNA hydrogel was prepared for colorimetric detection of MC-LR. The Cu/Au/Pt TNs were captured and released with precise control by the target-responsive 3D DNA hydrogels, which combined dual advantages of the target responsive DNA hydrogel and Cu/Au/Pt TNs of enhanced peroxidase-like activity. The DNA hydrogel network was constructed by hybridizing MC-LR aptamer with two complementary DNA strands on linear polyacrylamide chains. As long as MC-LR presented, the aptamer competitively binds with the MC-LR, causing the hydrogel to dissolve and release the preloaded Cu/Au/Pt TNs which could catalyze the reaction between H₂O₂ and TMB to produce color changes. In view of this sensitive strategy, this Cu/Au/Pt TNs-encapsulated DNA hydrogel-based colorimetric biosensor can achieve quantitative determination of MC-LR. The results showed that as-proposed colorimetric biosensor could sensitively detect MC-LR with a linear range of 4.0-10000 ng L^-1 and a detection limit of 3.0 ng L^-1. This work proved that the sensor had great potential to be applied in MC-LR detection and also provided the opportunity to develop colorimetric biosensor for other targets using this target-responsive and signal-amplification strategy.

Biosensing of microcystins in water samples; recent advances

Safety and quality of water are significant matters for agriculture, animals and human health. Microcystins, as secondary metabolite of cyanobacteria (blue-green algae) and cyclic heptapeptide cyanotoxin, are one of the main marine toxins in continental aquatic ecosystems. More than 100 microcystins have been identified, of which MC-LR is the most important type due to its high toxicity and common detection in the environment. Climate change is an impressive factor with effects on cyanobacterial blooms as source of microcystins. The presence of this cyanotoxin in freshwater, drinking water, water reservoir supplies and food (vegetable, fish and shellfish) has created a common phenomenon in eutrophic freshwater ecosystems worldwide. International public health organizations have categorized microcystins as a kind of neurotoxin and carcinogen. There are several conventional methods for detection of microcystins. The limitations of traditional methods have encouraged the development of innovative methods for detection of microcystins. In recent years, the developed sensor techniques, with advantages, such as accuracy, reproducibility, portability and low cost, have attracted considerable attention. This review compares the well-known of biosensor types for detection of microcystins with a summary of their analytical performance.

The Role of Electrochemical Immunosensors in Clinical Analysis

An immunosensor is a kind of affinity biosensor based on interactions between an antigen and specific antigen immobilized on a transducer surface. Immunosensors possess high selectivity and sensitivity due to the specific binding between antibody and corresponding antigen, making them a suitable platform for several applications especially in the medical and bioanalysis fields. Electrochemical immunosensors rely on the measurements of an electrical signal recorded by an electrochemical transducer and can be classed as amperometric, potentiometric, conductometric, or impedimetric depending on the signal type. Among the immunosensors, electrochemical immunosensors have been more perfected due to their simplicity and, especially their ability to be portable, and for in situ or automated detection. This review addresses the potential of immunosensors destined for application in clinical analysis, especially cancer biomarker diagnosis. The emphasis is on the approaches used to fabricate electrochemical immunosensors. A general overview of recent applications of the developed electrochemical immunosensors in the clinical approach is described.