A 3D graphene-based biosensor as an early microcystin-LR screening tool in sources of drinking water supply

Nanobioengineered surface comprising carbon based materials for advanced biosensing and biomedical application

Carbon-based nanomaterials (CNMs) are at the cutting edge of materials science. Due to their distinctive architectures, substantial surface area, favourable biocompatibility, and reactivity to internal and/or external chemico-physical stimuli, carbon-based nanomaterials are becoming more and more significant in a wide range of applications. Numerous research has been conducted and still is going on to investigate the potential uses of carbon-based hybrid materials for diverse applications such as biosensing, bioimaging, smart drug delivery with the potential for theranostic or combinatorial therapies etc. This review is mainly focused on the classifications and synthesis of various types of CNMs and their electroanalytical application for development of efficient and ultra-sensitive electrochemical biosensors for the point of care diagnosis of fatal and severe diseases at their very initial stage. This review is mainly focused on the classification, synthesis and application of carbon-based material for biosensing applications. The integration of various types of CNMs with nanomaterials, enzymes, redox mediators and biomarkers have been used discussed in development of smart biosensing platform. We have also made an effort to discuss the future prospects for these CNMs in the biosensing area as well as the most recent advancements and applications which will be quite useful for the researchers working across the globe working specially in biosensors field.

Electrochemical biosensor using SnO2 colloidal quantum wire for monitoring the interaction of microcystin antigen-antibody

Electrochemical sensors that incorporate immunoassay principles have the ability to monitor dynamic processes of antigen-antibody interactions in real time. In this study, a gold electrode was modified with tin dioxide colloidal quantum wire (SnO2 QWs) and then coated with the leucine/arginine subtype microcystin (MC-LR) antibody. The active site of SnO2 QWs that was not bound by MC-LR antibody was then passivated with bovine serum protein (BSA). When the MC-LR antigen binds specifically to the antibodies on the electrode's surface, it triggers electrochemical reactions and generates electrical signals at specific voltage conditions. The SnO2 QW exhibits excellent electron transport ability, and its ability to form a loose and porous microstructure on the gold electrode surface, which is conducive to the receptor function of the biosensor. The results show a high affinity between the MC-LR antigen and antibody, ranging from 1 pg/mL to 10 ng/mL of MC-LR antigen concentration. The kinetic characteristics of the immune reaction between MC-LR antigen and antibody were elucidated, obtaining a binding constant of 1.399 × 1011 M-1 and a dissociation constant of 7.147 pM, demonstrating the potential of electrochemical biosensing technology in biomolecular interactions.

Allelopathic Inhibition and Mechanism of Quercetin on Microcystis aeruginosa

The utilization of allelochemicals to inhibit algal overgrowth is a promising approach for controlling harmful algal blooms (HABs). Quercetin has been found to have an allelopathic effect on algae. However, its responsive mechanism needs to be better understood. In the present study, the inhibitory effects of different quercetin concentrations on M. aeruginosa were evaluated, and the inhibition mechanisms were explored. The results demonstrated that quercetin significantly inhibited M. aeruginosa growth, and the inhibitory effect was concentration-dependent. The inhibition rate of 40 mg L^-1 quercetin on algal density reached 90.79% after 96 h treatment. The concentration of chlorophyll-a (chl-a) in treatment groups with quercetin concentrations of 10, 20, and 40 mg L^-1 decreased by 59.74%, 74.77%, and 80.66% at 96 h, respectively. Furthermore, quercetin affects photosynthesis and damages the cell membrane, respiratory system, and enzyme system. All photosynthetic fluorescence parameters, including the maximum photochemical quantum yield (F_v/F_m), the actual photochemical quantum yield (YII), the maximum relative electron transfer rate (rETR_max), and light use efficiency (α), exhibited a downtrend after exposure. After treatment with 20 mg L^-1 quercetin, the nucleic acid and protein content in the algal solution increased, and the respiration rate of algae decreased significantly. Additionally, superoxide dismutase (SOD) activities significantly increased as a response to oxidative stress. In comparison, the activities of ribulose 1,5-biphosphate carboxylase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) decreased significantly. These results revealed that quercetin could inhibit M. aeruginosa by affecting its photosynthesis, respiration, cell membrane, and enzymic system. These results are promising for controlling M. aeruginosa effectively.

Reusing Waste Coffee Grounds as Electrode Materials: Recent Advances and Future Opportunities

Coffee industry produces more than eight million tons of waste coffee grounds (WCG) annually. These WCG contain caffeine, tannins, and polyphenols and can be of great environmental concern if not properly disposed of. On the other hand, components of WCG are mainly macromolecular cellulose and lignocellulose, which can be utilized as cheap carbon precursors. Accordingly, various forms of carbon materials have been reportedly synthesized from WCG, including activated carbon, mesoporous carbon, carbon nanosheets, carbon nanotubes, graphene sheet fibers (i.e., graphenated carbon nanotubes), and particle-like graphene. Upcycling of various biomass and/or waste into value-added functional materials is of growing significance to offer more sustainable solutions and enable circular economy. In this context, this review offers timely insight on the recent advances of WCG derived carbon as value-added electrode materials. As electrodes, they have shown to possess excellent electrochemical properties and found applications in capacitor/supercapacitor, batteries, electrochemical sensors, owing to their low cost, high electrical conductivity, polarization, and chemical stability. Collectively, these efforts could represent an environmentally friendly and circular economy approach, which could not only help solve the food waste issue, but also generate high performance carbon-based materials for many electrochemical applications.

Capture and detection of Escherichia coli with graphene aerogels

Some pathogenic bacteria may cause serious food poisoning as well as catastrophic infections. Thus, it is critical to identify bacteria using simple, quick, and sensitive methods. Herein, we fabricate a graphene aerogel-based biosensing system to capture and detect Escherichia coli (E. coli) with high specificity and sensitivity. A graphene aerogel is prepared by a one-step hydrothermal synthesis method without any reducing reagent. With the help of E. coli antibodies and the graphene foam with a porous structure, E. coli can be captured using the detection substrate with high specificity and selectivity. The electrical resistance and electrochemical impedance spectroscopy (EIS) results of the graphene aerogel foam changed with high sensitivity during E. coli adhesion. Moreover, the resistance change of the graphene device can still be observed when the E. coli concentration was as low as 10 cfu mL^-1, while there is no obvious resistance change in the use of Staphylococcus aureus. The subsequent EIS test also found that the charge transfer resistance (R_ct) of the detection substrate gradually increased during the E. coli capture process. This nanoelectronic biosensor is simple, quick, safe, and very sensitive, and it may be used as a high-throughput platform for pathogenic bacterial detection, bacterial research, and antimicrobial drug screening.

Sensitive Identification of Microcystin-LR via a Reagent-Free and Reusable Electrochemical Biosensor Using a Methylene Blue-Labeled Aptamer

We report a methylene blue (MB)-modified electrochemical aptamer (E-AB) sensor for determining microcystin-LR (MC-LR). The signal transduction of the sensor was based on changes in conformation and position of MB induced by the binding between MC-LR and the modified aptamer probe. In the absence of MC-LR, an aptamer probe was considered partially folded. After combining aptamer and MC-LR, the configuration of the aptamer probe changed and facilitated the electron transfer between MB and the electrode surface. As a result, an increased current response was observed. We optimized the parameters and evaluated the electrochemical performance of the sensor using square wave voltammetry (SWV). MC-LR was measured from 1.0 to 750.0 ng/L with a detection limit of 0.53 ng/L. The reliability of the method was verified by the determination of MC-LR in environmental real samples, such as pond water and tap water. Moreover, we demonstrated that this reagent-less biosensor could be regenerated and reused after rinsing with deionized water with good accuracy and reproducibility. As a reusable and regenerable E-AB sensor, this rapid, reagent-free, and sensitive sensing platform will facilitate routine monitoring of MC-LR in actual samples.

Ultrasensitive label-free electrochemical biosensor for detecting linear microcystin-LR using degrading enzyme MlrB as recognition element

A label-free electrochemical biosensor was firstly constructed to detect linear microcystin-LR (L-MC-LR) with high sensitivity. Degradation enzyme MlrB was used as recognition element for specific recognition of L-MC-LR. The electrode was modified with -COOH functionalized multi-walled carbon nanotube to increase the specific surface area and improve the conductivity, which was then applied to immobilize MlrB. The electrochemical signal was changed with the reaction between MlrB and L-MC-LR, which was recorded by using square wave voltammetry. The electrochemical biosensor showed superior sensitivity, with a dynamic range of 1 pg/mL to 100 ng/mL and a detection limit of 0.127 pg/mL. Moreover, the fabricated electrochemical biosensor exhibited excellent specificity toward L-MC-LR in real water samples. The concentrations of spiked L-MC-LR were 0.100, 5.00, 50.0 ng/mL, and the recovery rates were 95.0-104% with relative standard deviation (RSD) of 0.900-2.30% and 74.0-93.0% with RSD of 2.30-3.50% in lake water and tap water, respectively. Furthermore, the selectivity, reproducibility, and stability demonstrated the potential of degradation enzymes as recognition element in detection of cyanotoxins.

Electrochemical Biosensors for Tracing Cyanotoxins in Food and Environmental Matrices

The adoption of electrochemical principles to realize on-field analytical tools for detecting pollutants represents a great possibility for food safety and environmental applications. With respect to the existing transduction mechanisms, i.e., colorimetric, fluorescence, piezoelectric etc., electrochemical mechanisms offer the tremendous advantage of being easily miniaturized, connected with low cost (commercially available) readers and unaffected by the color/turbidity of real matrices. In particular, their versatility represents a powerful approach for detecting traces of emerging pollutants such as cyanotoxins. The combination of electrochemical platforms with nanomaterials, synthetic receptors and microfabrication makes electroanalysis a strong starting point towards decentralized monitoring of toxins in diverse matrices. This review gives an overview of the electrochemical biosensors that have been developed to detect four common cyanotoxins, namely microcystin-LR, anatoxin-a, saxitoxin and cylindrospermopsin. The manuscript provides the readers a quick guide to understand the main electrochemical platforms that have been realized so far, and the presence of a comprehensive table provides a perspective at a glance.

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.

Advances in Electrochemical Impedance Spectroscopy Detection of Endocrine Disruptors

Endocrine disruptors (EDs) are contaminants that may mimic or interfere with the body's hormones, hampering the normal functions of the endocrine system in humans and animals. These substances, either natural or man-made, are involved in development, breeding, and immunity, causing a wide range of diseases and disorders. The traditional detection methods such as enzyme linked immunosorbent assay (ELISA) and chromatography are still the golden techniques for EDs detection due to their high sensitivity, robustness, and accuracy. Nevertheless, they have the disadvantage of being expensive and time-consuming, requiring bulky equipment or skilled personnel. On the other hand, early stage detection of EDs on-the-field requires portable devices fulfilling the Affordable, Sensitive, Specific, User-friendly, Rapid and Robust, Equipment free, Deliverable to end users (ASSURED) norms. Electrochemical impedance spectroscopy (EIS)-based sensors can be easily implemented in fully automated, sample-to-answer devices by integrating electrodes in microfluidic chips. The latest achievements on EIS-based sensors are discussed and critically assessed.

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.

Vertically Aligned Graphene Prepared by Photonic Annealing for Ultrasensitive Biosensors

Graphene exhibits excellent physical, electronic, and chemical properties that are highly desirable for biosensing applications. However, most graphene biosensors are based on graphene lying flat on a substrate and therefore do not utilize its maximum specific surface area for ultrasensitive detection. Herein, we report the novel use of photonic annealing on a flexographically printed graphene-ethyl cellulose composite to produce vertically aligned graphene (VAG) biosensors for ultrasensitive detection of algal toxins in drinking water. These VAG structures, which maximized the specific surface area of graphene, were formed by partial removal of the polymeric binder upon applying intense pulsed light on the printed graphene. A label-free and low-cost VAG biosensor based on a non-faradaic electrochemical impedance spectroscopy technique was fabricated. The biosensor exhibited a limit of detection of 1.2 ng/L for microcystin-LR in local tap water. Such an ultrasensitive VAG biosensor is suitable for low-cost mass production using an integrated roll-to-roll flexographic printing with rapid photonic annealing technique.

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.

Two-Dimensional Layered Nanomaterial-Based Electrochemical Biosensors for Detecting Microbial Toxins

Toxin detection is an important issue in numerous fields, such as agriculture/food safety, environmental monitoring, and homeland security. During the past two decades, nanotechnology has been extensively used to develop various biosensors for achieving fast, sensitive, selective and on-site analysis of toxins. In particular, the two dimensional layered (2D) nanomaterials (such as graphene and transition metal dichalcogenides (TMDs)) and their nanocomposites have been employed as label and/or biosensing transducers to construct electrochemical biosensors for cost-effective detection of toxins with high sensitivity and specificity. This is because the 2D nanomaterials have good electrical conductivity and a large surface area with plenty of active groups for conjugating 2D nanomaterials with the antibodies and/or aptamers of the targeted toxins. Herein, we summarize recent developments in the application of 2D nanomaterial-based electrochemical biosensors for detecting toxins with a particular focus on microbial toxins including bacterial toxins, fungal toxins and algal toxins. The integration of 2D nanomaterials with some existing antibody/aptamer technologies into electrochemical biosensors has led to an unprecedented impact on improving the assaying performance of microbial toxins, and has shown great promise in public health and environmental protection.

Ultrasensitive electrochemical detection of ochratoxin A based on signal amplification by one-pot synthesized flower-like PEDOT-AuNFs supported on a graphene oxide sponge

To enhance the sensitivity of an aptasensor, a novel strategy was designed to develop an electrochemical aptasensor based on poly(3,4-ethylenedioxy thiophene)-gold nanoflower (PEDOT-AuNF) composites supported on a three-dimensional graphene oxide sponge (GOS). GOS with a three-dimensional sponge-like porous structure, exhibiting excellent electrical conductivity and a large surface area, provided the first amplification of the electrochemical signal for ochratoxin A (OTA) detection. PEDOT-AuNFs, synthesized by an ionic liquid-assisted one-pot method, presented a peculiar hierarchical flower-like structure, a high electroactive surface area, and more binding sites for immobilizing the aptamer molecules by the Au-S bonds. When PEDOT-AuNFs were supported on the surface of GOS by the interaction of the π-π packing between PEDOT and graphene oxide, a synergistic effect was produced to provide the second amplification for the aptasensor. PEDOT-AuNFs/GOS provided an ultrasensitive detection technique by multiple signal amplification for the electrochemical sensing of OTA. Consequently, this strategy not only endowed the aptasensor with high sensitivity but also needed no complicated signal amplification. The electrochemical sensor was fabricated successfully on a glassy carbon electrode to detect OTA with a linear response in the range of 0.01-20 ng L^-1 and a limit of detection of 4.9 pg L^-1. Moreover, it displayed good specificity, reproducibility and stability. The utilization of the proposed aptasensor for the quantitative determination of OTA in wine indicates that it can find promising applications in detecting OTA and even other mycotoxins in foodstuffs.

Photocatalysis as an advanced reduction process (ARP): The reduction of 4-nitrophenol using titania nanotubes-ferrite nanocomposites

TiO₂ photocatalysis is an advanced process, employed worldwide for the oxidation of organic compounds, that leads to significant technological applications in the fields of health and environment. The use of the photocatalytic approach in reduction reactions seems very promising and can open new horizons for green chemistry synthesis. For this purpose, titanium dioxide nanotubes (TNTs) were developed in autoclave conditions using TiO₂ P25 as a precursor material. Based on these nanotubular substrates, TiO₂/CoFe₂O₄ (TCF) nanocomposites were further obtained by wet impregnation method. The materials were thoroughly characterized and their structural, textural, vibrational, optoelectronic and magnetic properties were determined. The composite materials combine absorbance in the visible optical range and high BET surface area values (˜100 m²/g), showing extremely high yield in the photocatalytic reduction of 4-nitrophenol (4-NP), exceeding 94% within short illumination time (only 35 min). The developed nanocomposites were successfully reused in consecutive photocatalytic experiments and were easily removed from the reaction medium using magnets. Both remarkable recycling ability and high-performance stability in the photocatalytic reduction of nitrophenol were observed, thus justifying the significant economic potential and industrial perspectives for this advanced reduction process.

An aptamer based fluorometric microcystin-LR assay using DNA strand-based competitive displacement

The high-affinity region of a truncated aptamer was applied to the development of a sensitive method for the determination of microcystin-LR (MC-LR) using competitive displacement and molecular beacons. In this assay, the fluorophore and quencher labelled complementary sequences of the aptamer are hybridized with the truncated aptamer to form a fluorophore-quencher pair. In the presence of MC-LR, the aptamer duplex dissociates, and the fluorophore-quencher pair is separated. This turn leads to an increase in the yellow fluorescence which is best measured at excitation/emission wavelengths of 555/580 nm. One of the truncated aptamers showed a 50-fold increase in the affinity (0.93 nM) compared to the wild type aptamer (50 nM). The truncated sequence shows considerable cross-reactivity with L congeners but none with other congeners. The assay works in 0.5 to 200 nM MC-LR concentration range. It was applied to spiked tap water samples and gave recoveries around 95 ± 5%. Graphical abstract Schematic representation of a method for determination of microcystin-LR via fluorescence that is induced by competitive displacement of complementary DNA strands in a truncated dsDNA aptamer.

Graphene-based nano composites and their applications. A review

The purpose of the current review article is to present a comprehensive understanding regarding pros and cons of graphene related nanocomposites and to find ways in order to improve the performance of nanocomposites with new designs. Nanomaterials including GR are employed in industrial applications such as supercapacitors, biosensors, solar cells, and corrosion studies. The present article has been prepared in three main categories. In the first part, graphene types have been presented, as pristine graphene, graphene oxide and reduced graphene oxide. In the second part, nanocomposites with many graphene, inorganic and polymeric materials such as polymer/GR, activated carbon/GR, metal oxide/GR, metal/graphene and carbon fibre/GR have been investigated in more detail. In the third part, the focus in on the industrial applications of GR nanocomposite, including super capacitors, biosensors, solar cells, and corrosion protection studies.

Recent Advances on Electrochemical Sensors for the Detection of Organic Disinfection Byproducts in Water

Irreversible organ damage or even death frequently occurs when humans or animals unknowingly drink contaminated water. Therefore, in many countries drinking water is disinfected to ensure removal of harmful pathogens from drinking water. If upstream water treatment prior to disinfection is not adequate, disinfection byproducts (DBPs) can be formed. DBPs can exist as wide variety of compounds, but up until now, only several typical compounds have drinking water standards attributed to them. However, it is apparent that the range of DBPs present in water can comprise hundreds of compounds, some of which are at high enough concentrations to be toxic or potentially carcinogenic. Hence, it becomes increasingly significant and urgent to develop an accessible, affordable, and durable sensing platform for a broader range and more sensitive detection of DBPs. Compared with well-established laboratory detection techniques, electrochemical sensing has been identified as a promising alternative that will provide rapid, affordable, and sensitive DBP monitoring in remote water sources. Therefore, this Review covers current state-of-the-art development (within the past decade) in electrochemical sensing to detect organic DBPs in water, which covered three major aspects: (1) recognition mechanism, (2) electrodes with signal amplification, and (3) signal read-out techniques. Moreover, comprehensive quality assessments on electrochemical biosensors, including linear detection range, limit of detection (LoD) and recovery, have also been summarized.

A Comprehensive Review: Development of Electrochemical Biosensors for Detection of Cyanotoxins in Freshwater

Cyanobacteria harmful algal blooms are increasing in frequency and cyanotoxins have become an environmental and public concern in the U.S. and worldwide. In this Review, the majority of reported studies and developments of electrochemical affinity biosensors for cyanotoxins are critically reviewed and discussed. Essential background information about cyanobacterial toxins and electrochemical biosensors is combined with the rapidly moving development of electrochemical biosensors for these toxins. Current issues and future challenges for the development of useful electrochemical biosensors for cyanotoxin detection that meet the demands for applications in field freshwater samples are discussed. The major aspects of the entire review article in a prescribed sequence include (i) the state-of-the-art knowledge of the toxicity of cyanotoxins, (ii) important harmful algal bloom events, (iii) advisories, guidelines, and regulations, (iv) conventional analytical methods for determination of cyanotoxins, (v) electrochemical transduction, (vi) recognition receptors, (vii) reported electrochemical biosensors for cyanotoxins, (viii) summary of analytical performance, and (ix) recent advances and future trends. Discussion includes electrochemical techniques and devices, biomolecules with high affinity, numerous array designs, various detection approaches, and research strategies in tailoring the properties of the transducer-biomolecule interface. Scientific and engineering aspects are presented in depth. This review aims to serve as a valuable source to scientists and engineers entering the interdisciplinary field of electrochemical biosensors for detection of cyanotoxins in freshwaters.

Electrochemical Biosensing of Algal Toxins in Water: The Current State-of-the-Art

Due to increasing stringency of water legislation and extreme consequences that failure to detect some contaminants in water can involve, there has been a strong interest in developing electrochemical biosensors for algal toxin detection during the past decade, evidenced by literature increasing from 2 journal papers pre-2009 to 24 between 2009 and 2018. In this context, this review has summarized recent progress of successful algal toxin detection in water using electrochemical biosensing techniques. Satisfactory detection recoveries using real environmental water samples and good sensor repeatability and reproducibility have been achieved, along with some excellent limit-of-detection (LOD) reported. Recent electrochemical biosensor literature in algal toxin detection is compared and discussed to cover three major design components: (1) biorecognition elements, (2) electrochemical read-out techniques, and (3) sensor electrodes and signal amplification strategy. The recent development of electrochemical biosensors has provided one more step further toward quick in situ detection of algal toxins in the contamination point of the water source. In the end, we have also critically reviewed the current challenges and research opportunities regarding electrochemical biosensors for algal toxin detection that need to be addressed before they attain commercial viability.

Fabrication of graphene film composite electrochemical biosensor as a pre-screening algal toxin detection tool in the event of water contamination

In this work, we fabricated a novel graphene film composite biosensor for microcystin-LR detection as an alternative to time-consuming, expensive, non-portable and often skills-demanding conventional methods of analysis involved in water quality monitoring and assessment. Excellent linear correlation (R² = 0.99) of the electron-transfer resistance was achieved over a wide range of microcystin-LR (MC-LR) concentration, i.e. 0.005–10 μg/L. As-prepared graphene film composite biosensors can specifically detect MC-LR with remarkable sensitivity and detection limit (2.3 ng/L) much lower than the World Health Organization (WHO) provisional guideline limit of microcystin-LR concentration (i.e. 1 μg/L) in different water sources. Their great potential can be attributed to large active surface area of graphene film and efficient charge transfer process enabled by their high conductivity. Developed graphene film composite biosensors were also successfully applied to determination of MC-LR in several environmental water samples with high detection recovery, which offers a promising possibility of large-scale manufacture of sensor tips due to their macroscopic free-standing nature, the scalable fabrication route and easily tunable size.

A photoelectrochemical aptasensor constructed with core-shell CuS-TiO2 heterostructure for detection of microcystin-LR

In this work, a CuS-TiO₂ heterojunction composite was prepared by dispersedly depositing CuS nanoparticles on TiO₂ nanospheres surface with a hydrothermal method, and was then used to construct a photoelectrochemical (PEC) aptasensor for sensitive detection of microcystin-LR (MC-LR) in aquatic environment. The energy bands of CuS nanoparticles and spherical anatase TiO₂ were well matched, which enhanced the photo-to-current conversion efficiency. The composite exhibited the enhanced visible light absorption, the improved separation of photo-generated charges, and the reduced self-aggregation of CuS nanoparticles, leading to the enhanced photocurrent response. The PEC aptasensor was constructed by immobilizing CuS-TiO₂ composite on ITO electrode with chitosan film that further covalently bound aminated aptamer. After the target, microcystin-LR (MC-LR) as an analyte model, was captured by the aptamer on the aptasensor, it could be oxidized by the photo-generated hole to impede the electron-hole recombination and further amplify the photocurrent. The PEC aptasensor showed superior analytical performance for MC-LR with a linear range of 5.0 × 10^-5 nM to 250 nM and a detection limit of 2.0 × 10^-5 nM. The detection results with the aptasensor for practical water samples indicated its promising application in environmental monitoring.

Practical Application of Aptamer-Based Biosensors in Detection of Low Molecular Weight Pollutants in Water Sources

Water pollution has become one of the leading causes of human health problems. Low molecular weight pollutants, even at trace concentrations in water sources, have aroused global attention due to their toxicity after long-time exposure. There is an increased demand for appropriate methods to detect these pollutants in aquatic systems. Aptamers, single-stranded DNA or RNA, have high affinity and specificity to each of their target molecule, similar to antigen-antibody interaction. Aptamers can be selected using a method called Systematic Evolution of Ligands by EXponential enrichment (SELEX). Recent years we have witnessed great progress in developing aptamer selection and aptamer-based sensors for low molecular weight pollutants in water sources, such as tap water, seawater, lake water, river water, as well as wastewater and its effluents. This review provides an overview of aptamer-based methods as a novel approach for detecting low molecular weight pollutants in water sources.

Recent Progress in Biosensors for Environmental Monitoring: A Review

The environmental monitoring has been one of the priorities at the European and global scale due to the close relationship between the environmental pollution and the human health/socioeconomic development. In this field, the biosensors have been widely employed as cost-effective, fast, in situ, and real-time analytical techniques. The need of portable, rapid, and smart biosensing devices explains the recent development of biosensors with new transduction materials, obtained from nanotechnology, and for multiplexed pollutant detection, involving multidisciplinary experts. This review article provides an update on recent progress in biosensors for the monitoring of air, water, and soil pollutants in real conditions such as pesticides, potentially toxic elements, and small organic molecules including toxins and endocrine disrupting chemicals.

Recent advances in nanomaterials for water protection and monitoring

Nanomaterials (NMs) for adsorption, catalysis, separation, and disinfection are scrutinized. NMs-based sensor technologies and environmental transformations of NMs are highlighted.