Portable sensing system based on electrochemical impedance spectroscopy for the simultaneous quantification of free and total microcystin-LR in freshwaters

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.

Acute Aquatic Toxicity to Zebrafish and Bioaccumulation in Marine Mussels of Antimony Tin Oxide Nanoparticles

Antimony tin oxide (Sb₂O₅/SnO₂) is effective in the absorption of infrared radiation for applications, such as skylights. As a nanoparticle (NP), it can be incorporated into films or sheets providing infrared radiation attenuation while allowing for a transparent final product. The acute toxicity exerted by commercial Sb₂O₅/SnO₂ (ATO) NPs was studied in adults and embryos of zebrafish (Danio rerio). Our results suggest that these NPs do not induce an acute toxicity in zebrafish, either adults or embryos. However, some sub-lethal parameters were altered: heart rate and spontaneous movements. Finally, the possible bioaccumulation of these NPs in the aquacultured marine mussel Mytilus sp. was studied. A quantitative analysis was performed using single particle inductively coupled plasma mass spectrometry (sp-ICP-MS). The results indicated that, despite being scarce (2.31 × 10⁶ ± 9.05 × 10⁵ NPs/g), there is some accumulation of the ATO NPs in the mussel. In conclusion, commercial ATO NPs seem to be quite innocuous to aquatic organisms; however, the fact that some of the developmental parameters in zebrafish embryos are altered should be considered for further investigation. More in-depth analysis of these NPs transformations in the digestive tract of humans is needed to assess whether their accumulation in mussels presents an actual risk to humans.

A novel portable label-free electrochemical immunosensor for ultrasensitive detection of Aeromonas salmonicida in aquaculture seawater

Infectious diseases caused by Aeromonas salmonicida (A. salmonicida) have a huge impact and produce significant losses in aquaculture and fish farming. Fish pathogen early detection is a critical step for the rapid identification and prevention of these problems. This work presents a novel portable label-free ultrasensitive electrochemical immunosensor for A. salmonicida detection in seawater. It consists of a fluidic integrated electrochemical-cell-chip (ECC) with independent chambers enclosing three electrochemical cells (ECs). Anti-A. salmonicida (AbSalm) antibodies were covalently attached to the gold surface of the microfabricated electrodes and were used for the sensitive detection of A. salmonicida. The antibody-antigen immunoreaction was studied by enzyme-linked immunosorbent assay (ELISA), and the surface functionalization was characterized by using quartz crystal microbalance (QCM), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). The performance of the developed immunosensor, in terms of sensitivity, repeatability, and specificity, was also studied. The linear working range varied between 1 and 10⁷ CFU mL^-1, with a limit of detection (LOD) as low as 1 CFU mL^-1. The suitability of the immunosensor for real sample detection was successfully demonstrated via recovery studies performed in spiked seawater samples. The proposed technology supports the use of low-cost and portable instrumentation that concedes the ultrasensitive, simple, and fast quantification of the A. salmonicida. To the best of our knowledge, this is the first portable sensing system for the detection of A. salmonicida in seawater samples, which provides a promising online monitoring platform for the detection of this bacterium in aquaculture facilities.

Recent Advancements in Electrochemical Biosensors for Monitoring the Water Quality

The release of chemicals and microorganisms from various sources, such as industry, agriculture, animal farming, wastewater treatment plants, and flooding, into water systems have caused water pollution in several parts of our world, endangering aquatic ecosystems and individual health. World Health Organization (WHO) has introduced strict standards for the maximum concentration limits for nutrients and chemicals in drinking water, surface water, and groundwater. It is crucial to have rapid, sensitive, and reliable analytical detection systems to monitor the pollution level regularly and meet the standard limit. Electrochemical biosensors are advantageous analytical devices or tools that convert a bio-signal by biorecognition elements into a significant electrical response. Thanks to the micro/nano fabrication techniques, electrochemical biosensors for sensitive, continuous, and real-time detection have attracted increasing attention among researchers and users worldwide. These devices take advantage of easy operation, portability, and rapid response. They can also be miniaturized, have a long-life span and a quick response time, and possess high sensitivity and selectivity and can be considered as portable biosensing assays. They are of special importance due to their great advantages such as affordability, simplicity, portability, and ability to detect at on-site. This review paper is concerned with the basic concepts of electrochemical biosensors and their applications in various water quality monitoring, such as inorganic chemicals, nutrients, microorganisms' pollution, and organic pollutants, especially for developing real-time/online detection systems. The basic concepts of electrochemical biosensors, different surface modification techniques, bio-recognition elements (BRE), detection methods, and specific real-time water quality monitoring applications are reviewed thoroughly in this article.

Fundamentals of Biosensors and Detection Methods

Biosensors have a great impact on our society to enhance the life quality, playing an important role in the development of Point-of-Care (POC) technologies for rapid diagnostics, and monitoring of disease progression. COVID-19 rapid antigen tests, home pregnancy tests, and glucose monitoring sensors represent three examples of successful biosensor POC devices. Biosensors have extensively been used in applications related to the control of diseases, food quality and safety, and environment quality. They can provide great specificity and portability at significantly reduced costs. In this chapter are described the fundamentals of biosensors including the working principles, general configurations, performance factors, and their classifications according to the type of bioreceptors and transducers. It is also briefly illustrated the general strategies applied to immobilize biorecognition elements on the transducer surface for the construction of biosensors. Moreover, the principal detection methods used in biosensors are described, giving special emphasis on optical, electrochemical, and mass-based methods. Finally, the challenges for biosensing in real applications are addressed at the end of this chapter.

Label-free electrochemical microfluidic biosensors: futuristic point-of-care analytical devices for monitoring diseases

The integration of microfluidics with electrochemical analysis has resulted in the development of single miniaturized detection systems, which allows the precise control of sample volume with multianalyte detection capability in a cost- and time-effective manner. Microfluidic electrochemical sensing devices (MESDs) can potentially serve as precise sensing and monitoring systems for the detection of molecular markers in various detrimental diseases. MESDs offer several advantages, including (i) automated sample preparation and detection, (ii) low sample and reagent requirement, (iii) detection of multianalyte in a single run, (iv) multiplex analysis in a single integrated device, and (v) portability with simplicity in application and disposability. Label-free MESDs can serve an affordable real-time detection with a simple analysis in a short processing time, providing point-of-care diagnosis/detection possibilities in precision medicine, and environmental analysis. In the current review, we elaborate on label-free microfluidic biosensors, provide comprehensive insights into electrochemical detection techniques, and discuss the principles of label-free microfluidic-based sensing approaches.

On-chip-based electrochemical biosensor for the sensitive and label-free detection of Cryptosporidium

Cryptosporidium, an intestinal protozoan pathogen, is one of the leading causes of death in children and diarrhea in healthy adults. Detection of Cryptosporidium has become a high priority to prevent potential outbreaks. In this paper, a simple, easy to fabricate, and cost-effective on-chip-based electrochemical biosensor has been developed for the sensitive and label-free detection of Cryptosporidium oocysts in water samples. The sensor was fabricated using standard lithography using a mask with a 3-electrode design and modified by self-assembling a hybrid of a thiolated protein/G and the specific anti-Cryptosporidium monoclonal antibodies (IgG3). The electrochemical impedance spectroscopy (EIS) was employed to quantitate C. parvum in the range of 0 to 300 oocysts, with a detection limit of approximately 20 oocysts/5 µL. The high sensitivity and specificity of the developed label-free electrochemical biosensor suggest that this novel platform is a significant step towards the development of fast, real-time, inexpensive and label-free sensing tool for early warning and immediate on-site detection of C. parvum oocysts in water samples, as compared to the traditional methods (such as PCR and microscopy). Furthermore, under optimized conditions, this label-free biosensor can be extended to detect other analytes and biomarkers for environmental and biomedical analyses.

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.

Aptamer-Target Recognition-Promoted Ratiometric Electrochemical Strategy for Evaluating the Microcystin-LR Residue in Fish without Interferences

Given the significance of food safety, it is highly urgent to develop a sensitive yet reliable sensor for the practical analysis of algal toxins. As most of the developed sensors are disturbed by interfering substances and the target toxin is detected in a single-signal manner based on the immunoassay technology. Herein, we developed an aptamer-based dual-signal ratiometric electrochemical sensor for the sensitive and accurate analysis of microcystin-LR (MC-LR), using it as a proof-of-concept analyte. Methylene blue-tagged ssDNA (MB-ssDNA) was immobilized at the gold electrode surface accompanied with the absence of ferrocene-tagged ssDNA (Fc-ssDNA), resulting in a high differential pulse voltammetry (DPV) current of MB and a low DPV current of Fc. The recognition of MB-ssDNA by MC-LR stimulated the formation of MC-LR@MB-ssDNA, which induced the removal of MB-ssDNA from the electrode and the exposure of SH-ssDNA, enabling Fc-ssDNA to be captured at the electrode surface via nucleic acid hybridization. In comparison with MC-LR deficiency, the DPV signal of MB dropped along with an improved DPV signal of Fc, contributing to the ratiometric detection of MC-LR, with the limit of detection down to 0.0015 nM. Furthermore, this ratiometric electrochemical sensor was successfully explored to assess the bioaccumulated amount of MC-LR in the liver and meat of fish. The aptamer-based ratiometric strategy to develop an electrochemical MC-LR assay will offer a promising avenue to develop high-performance sensors, and the sensor will find more useful application in MC-LR-related aquatic product safety studies.

Improving Biosensors by the Use of Different Nanomaterials: Case Study with Microcystins as Target Analytes

The eutrophication of lakes and rivers without adequate rainfall leads to excessive growth of cyanobacterial harmful algal blooms (CyanoHABs) that produce toxicants, green tides, and unpleasant odors. The rapid growth of CyanoHABs owing to global warming, climate change, and the development of rainforests and dams without considering the environmental concern towards lakes and rivers is a serious issue. Humans and livestock consuming the toxicant-contaminated water that originated from CyanoHABs suffer severe health problems. Among the various toxicants produced by CyanoHABs, microcystins (MCs) are the most harmful. Excess accumulation of MC within living organisms can result in liver failure and hepatocirrhosis, eventually leading to death. Therefore, it is essential to precisely detect MCs in water samples. To date, the liquid chromatography-mass spectrometry (LC-MS) and enzyme-linked immunosorbent assay (ELISA) have been the standard methods for the detection of MC and provide precise results with high reliability. However, these methods require heavy instruments and complicated operation steps that could hamper the portability and field-readiness of the detection system. Therefore, in order for this goal to be achieved, the biosensor has been attracted to a powerful alternative for MC detection. Thus far, several types of MC biosensor have been proposed to detect MC in freshwater sample. The introduction of material is a useful option in order to improve the biosensor performance and construct new types of biosensors. Introducing nanomaterials to the biosensor interface provides new phenomena or enhances the sensitivity. In recent times, different types of nanomaterials, such as metallic, carbon-based, and transition metal dichalcogenide-based nanomaterials, have been developed and used to fabricate biosensors for MC detection. This study reviews the recent advancements in different nanomaterial-based MC biosensors.

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.

Machine Learning Enhances the Performance of Bioreceptor-Free Biosensors

Since their inception, biosensors have frequently employed simple regression models to calculate analyte composition based on the biosensor's signal magnitude. Traditionally, bioreceptors provide excellent sensitivity and specificity to the biosensor. Increasingly, however, bioreceptor-free biosensors have been developed for a wide range of applications. Without a bioreceptor, maintaining strong specificity and a low limit of detection have become the major challenge. Machine learning (ML) has been introduced to improve the performance of these biosensors, effectively replacing the bioreceptor with modeling to gain specificity. Here, we present how ML has been used to enhance the performance of these bioreceptor-free biosensors. Particularly, we discuss how ML has been used for imaging, Enose and Etongue, and surface-enhanced Raman spectroscopy (SERS) biosensors. Notably, principal component analysis (PCA) combined with support vector machine (SVM) and various artificial neural network (ANN) algorithms have shown outstanding performance in a variety of tasks. We anticipate that ML will continue to improve the performance of bioreceptor-free biosensors, especially with the prospects of sharing trained models and cloud computing for mobile computation. To facilitate this, the biosensing community would benefit from increased contributions to open-access data repositories for biosensor data.

Development and comparison of UPLC-ESI-MS and RP-HPLC-VWD methods for determining microcystin-LR

Microcystin-LR (MC-LR) generated by cyanobacteria is a kind of potent hepatotoxin, which poses a considerable threat to human health. In the research field of MC-LR removal, the quantitative analysis in a wide concentration range of samples is inevitable. In this paper, we presented the pseudo united use of an Ultra Performance Liquid Chromatography Mass Spectrometry (UPLC-MS) and High Performance Liquid Chromatography system with a Variable Wavelength Ultraviolet Detector (HPLC-VWD) approach to detect MC-LR. The UPLC-MS system was applied to determine MC-LR in trace concentration because of its high sensitivity. However, it is generally believed that the determination of high concentration samples by UPLC-MS will cause problems such as inaccurate quantification and contamination of ion sources. In consequence, the HPLC-VWD was employed to determine the high concentration of MC-LR. The sensitivity, precision and accuracy of the two methods were compared in detail. The linear ranges of UPLC-MS and HPLC-VWD methods were from 0.08 to 10 μg L⁻¹ and 1 to 5000 μg L⁻¹, respectively. The detection and quantification limits of UPLC-MS were 0.03–0.05 μg L⁻¹ and 0.08 μg L⁻¹, and the corresponding two values of HPLC-VWD were 0.6 and 1.0 μg L⁻¹. The recoveries of UPLC-MS and HPLC-VWD were 88.5–106.7% and 98.7–101.6%, with the relative standard deviations of 3.72–5.45% and 0.38–1.69%, respectively. The potential adsorption properties of MC-LR on filter membranes with diverse materials and pore sizes were evaluated and the negative results were obtained. The detection of MC-LR by UPLC-MS was free from matrix effects. The presented UPLC-MS and HPLC-VWD methods were used to analyze the water samples from Erhai Lake, which is located in Dali, Yunnan, China. The results of UPLC-MS analysis indicated that the MC-LR was only identified in water samples of Shuanglang Bay and Xier River, with concentrations of 0.120 and 0.303 μg L⁻¹, whereas MC-LR was not detected by HPLC-VWD.

UPLC-MS and HPLC-VWD methods were used together for MC-LR determination in a wide concentration range. UPLC-MS can be applied in trace MC-LR determination, whereas HPLC-VWD is more suitable for high concentration range detection.

Microfluidic impedance cytometry for single-cell sensing: Review on electrode configurations

Single-cell analysis has gained considerable attention for disease diagnosis, drug screening, and differentiation monitoring. Compared to the well-established flow cytometry, which uses fluorescent-labeled antibodies, microfluidic impedance cytometry (MIC) offers a simple, label-free, and noninvasive method for counting, classifying, and monitoring cells. Superior features including a small footprint, low reagent consumption, and ease of use have also been reported. The MIC device detects changes in the impedance signal caused by cells passing through the sensing/electric field zone, which can extract information regarding the size, shape, and dielectric properties of these cells. According to recent studies, electrode configuration has a remarkable effect on detection accuracy, sensitivity, and throughput. With the improvement in microfabrication technology, various electrode configurations have been reported for improving detection accuracy and throughput. However, the various electrode configurations of MIC devices have not been reviewed. In this review, the theoretical background of the impedance technique for single-cell analysis is introduced. Then, two-dimensional, three-dimensional, and liquid electrode configurations are discussed separately; their sensing mechanisms, fabrication processes, advantages, disadvantages, and applications are also described in detail. Finally, the current limitations and future perspectives of these electrode configurations are summarized. The main aim of this review is to offer a guide for researchers on the ongoing advancement in electrode configuration designs.

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.

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.

What Electrochemical Biosensors Can Do for Forensic Science? Unique Features and Applications

This article critically discusses the latest advances in the use of voltammetric, amperometric, potentiometric, and impedimetric biosensors for forensic analysis. Highlighted examples that show the advantages of these tools to develop methods capable of detecting very small concentrations of analytes and provide selective determinations through analytical responses, without significant interferences from other components of the samples, are presented and discussed, thus stressing the great versatility and utility of electrochemical biosensors in this growing research field. To illustrate this, the determination of substances with forensic relevance by using electrochemical biosensors reported in the last five years (2015–2019) are reviewed. The different configurations of enzyme or affinity biosensors used to solve analytical problems related to forensic practice, with special attention to applications in complex samples, are considered. Main prospects, challenges to focus, such as the fabrication of devices for rapid analysis of target analytes directly on-site at the crime scene, or their widespread use and successful applications to complex samples of interest in forensic analysis, and future efforts, are also briefly discussed.