Electrochemical immunosensor for botulinum neurotoxin type-E using covalently ordered graphene nanosheets modified electrodes and gold nanoparticles-enzyme conjugate

Graphene-based biosensors in milk analysis: A review of recent developments

Cow's milk, an excellent source of fat, protein, amino acids, vitamins and minerals, is currently one of the most consumed products worldwide. Contaminations originating from diverse sources, such as biological, chemical, and physical, cause dairy product quality problems and thus dairy-related disorders, raising public health issues. For this reason, legal authorities have deemed it necessary to classify certain contaminations in commercial milk and keep them within particular limitations; therefore, it is urgent to develop next-generation detection systems that can accurately identify just the contaminants of concern to human health. This review presents a detailed investigation of biosensors based on graphene and its derivatives, which offer superior sensitivity and selectivity, by classifying the contaminants under the headings biological, chemical, and physical, in cow's milk according to their sources. We reviewed the current status of graphene-based biosensor (GBs) technology for milk or dairy analysis, highlighting its strengths and weaknesses with the help of comparative studies, tables, and charts, and we put forward a novel perspective to handle future challenges.

Facile detection of botulinum neurotoxin using LSPR nanosensor based on Langmuir-Blodgett films of gold nanoparticles

In this exploratory study, Langmuir-Blodgett (LB) films of gold nanoparticles (Au NPs) were utilized for the first time to detect botulinum neurotoxin (BoNT) based on localized surface plasmon resonance (LSPR), acting as biosensors. Monolayers of Au NPs were initially transferred onto a transparent polymer substrate using the LB technique. This substrate was then used as the base material for subsequent depositions of capping ligands, and eventually, the BoNT at different concentrations. Upon each deposition, LSPR signals were recorded employing UV-Vis spectroscopy. As a result, it was demonstrated that the LB films transferred at a surface pressure of 35 mN m-1 were the optimal choice, capable of detecting BoNT at a concentration as low as 1 pg ml-1. Furthermore, it was discovered that the formation of Au NP clusters reduced the sensing capacity of the LB films. This sensor offers advantages such as easy fabrication and a quick detection process that utilizes visible light.

Silver nanoparticles in electrochemical immunosensing and the emergence of silver-gold galvanic exchange detection

Nanoparticle-based electrochemical immunosensors demonstrate high sensitivity toward biomarker detection due to the large surface area of the nanoparticles and their ability to amplify the signal of the target molecule. Additionally, they have a fast response time, relatively lower cost, and can be easily miniaturized for point-of-care applications. Among noble metals, silver nanoparticles (AgNPs) have been extensively used in electrochemical sensors due to their unique properties, such as catalytic activity and excellent electrical conductivity. This Feature Article describes six approaches for incorporating AgNPs in electrochemical platforms, featuring the most recent developments in the silver-gold galvanic exchange-based detection strategy. With a few exceptions, many of these detection methods use AgNP oxidation into Ag+ ions, followed by electrodeposition of Ag+ ions onto the working electrode as zero-valent Ag metal and a final stripping step using a voltammetric technique. Combining these steps provides desirable low detection limits and good sensitivity for various biomarkers. A few other methods involved the reduction of Ag+ ions and depositing them as Ag metal onto the electrode using a reagent mixture so that the striping analysis could be performed. Typically, this reagent mixture includes Ag+ ions, a reducing agent, or an enzyme substrate. Besides, AgNPs have also been directly used to modify the surface of electrodes to facilitate kinetically favored redox-mediated electrochemical reactions. In addition to Ag detection methods, this report will also provide recent examples to illustrate how the size and shape of AgNPs impact the detection limits and sensitivity of an electrochemical assay. Finally, we discuss recent developments in lab-on-a-chip type immunosensors designed explicitly for Ag-based metalloimmunoassay detection, and we envision that this article will provide a comprehensive summary of the operational principles and new insights into such immunoassay systems.

Progress in Electrochemical Immunosensors with Alkaline Phosphatase as the Signal Label

Electrochemical immunosensors have shown great potential in clinical diagnosis, food safety, environmental protection, and other fields. The feasible and innovative combination of enzyme catalysis and other signal-amplified elements has yielded exciting progress in the development of electrochemical immunosensors. Alkaline phosphatase (ALP) is one of the most popularly used enzyme reporters in bioassays. It has been widely utilized to design electrochemical immunosensors owing to its significant advantages (e.g., high catalytic activity, high turnover number, and excellent substrate specificity). In this work, we summarized the achievements of electrochemical immunosensors with ALP as the signal reporter. We mainly focused on detection principles and signal amplification strategies and briefly discussed the challenges regarding how to further improve the performance of ALP-based immunoassays.

Recent Advances in Recognition Receptors for Electrochemical Biosensing of Mycotoxins-A Review

Mycotoxins are naturally occurring toxic secondary metabolites produced by fungi in cereals and foodstuffs during the stages of cultivation and storage. Electrochemical biosensing has emerged as a rapid, efficient, and economical approach for the detection and quantification of mycotoxins in different sample media. An electrochemical biosensor consists of two main units, a recognition receptor and a signal transducer. Natural or artificial antibodies, aptamers, molecularly imprinted polymers (MIP), peptides, and DNAzymes have been extensively employed as selective recognition receptors for the electrochemical biosensing of mycotoxins. This article affords a detailed discussion of the recent advances and future prospects of various types of recognition receptors exploited in the electrochemical biosensing of mycotoxins.

Simultaneous determination of BoNT/A and /E using an electrochemical sandwich immunoassay based on the nanomagnetic immunosensing platform

Developing new ultrasensitive assays for the detection of the presence, and determination of the serotype of the most poisonous material known i.e. botulinum neurotoxin (BoNT) is vital to human health and the wellbeing of the surrounding environment. Here, an electrochemical sandwich immunoassay with high sensitivity is adopted to achieve simultaneous determination of BoNT serotypes A and E based on polystyrene@polydopamine/Cd²⁺ and Ag nanoparticles acting as monoclonal antibody labels. Two well-separated peaks with strong electrochemical signals are generated by the labels, allowing for the simultaneous detection of two analytes existing on the electrode. To obtain well-oriented polyclonal antibodies immobilization, boronic acid is directly attached to the magnetic core/metal-organic framework (MOF) shell nanoagent surfaces without the requirement of a long and flexible spacer. Accordingly, it is possible to directly detect the metal ion labels through square wave voltammetry without the metal pre-concentration step. This results in distinct and well-defined voltammetric peaks, pertaining to each sandwich-type immunocomplexes. The limits of detection of BoNT/A and BoNT/E analyses were found to be 0.04 and 0.16 pg mL^-1 with the linear dynamic ranges of 0.1-1000 and 0.5-1000 pg mL^-1, respectively. Based on the obtained results, this immunosensor has the wide linear ranges, while also exhibiting low limits of detection along with good stability and reproducibility.

Nanoarchitectonics of graphene based sensors for food safety monitoring

Since the desire for the real-time food quality monitoring, plenty of research effort has been made to develop novel tools and to offer extremely efficient detection of food contaminants. Unique electrical, mechanical, and thermal properties make graphene an important material in the field of sensor research. The material can be manufactured into flakes, sheets, films and with its oxidized derivatives could be almost used for a limitless set of application. Herein, current graphene-based sensors for food quality monitoring, novel designs, sensing mechanisms and elements of sensor systems and potential challenges will be outlined and discussed.

Recent Developments in Botulinum Neurotoxins Detection

Botulinum neurotoxins (BoNTs) are produced as protein complexes by bacteria of the genus Clostridium that are Gram-positive, anaerobic and spore forming (Clostridium botulinum, C. butyricum, C. baratii and C. argentinense spp.). BoNTs show a high immunological and genetic diversity. Therefore, fast, precise, and more reliable detection methods are still required to monitor outbreaks and ensure surveillance of botulism. The botulinum toxin field also comprises therapeutic uses, basic research studies and biodefense issues. This review presents currently available detection methods, and new methods offering the potential of enhanced precision and reproducibility. While the immunological methods offer a range of benefits, such as rapid analysis time, reproducibility and high sensitivity, their implementation is subject to the availability of suitable tools and reagents, such as specific antibodies. Currently, the mass spectrometry approach is the most sensitive in vitro method for a rapid detection of active or inactive forms of BoNTs. However, these methods require inter-laboratory validation before they can be more widely implemented in reference laboratories. In addition, these surrogate in vitro models also require full validation before they can be used as replacement bioassays of potency. Cell-based assays using neuronal cells in culture recapitulate all functional steps of toxin activity, but are still at various stages of development; they are not yet sufficiently robust, due to high batch-to-batch cell variability. Cell-based assays have a strong potential to replace the mouse bioassay (MBA) in terms of BoNT potency determination in pharmaceutical formulations; they can also help to identify suitable inhibitors while reducing the number of animals used. However, the development of safe countermeasures still requires the use of in vivo studies to complement in vitro immunological or cell-based approaches.

Gold nanostructures: synthesis, properties, and neurological applications

Recent advances in technology are expected to increase our current understanding of neuroscience. Nanotechnology and nanomaterials can alter and control neural functionality in both in vitro and in vivo experimental setups. The intersection between neuroscience and nanoscience may generate long-term neural interfaces adapted at the molecular level. Owing to their intrinsic physicochemical characteristics, gold nanostructures (GNSs) have received much attention in neuroscience, especially for combined diagnostic and therapeutic (theragnostic) purposes. GNSs have been successfully employed to stimulate and monitor neurophysiological signals. Hence, GNSs could provide a promising solution for the regeneration and recovery of neural tissue, novel neuroprotective strategies, and integrated implantable materials. This review covers the broad range of neurological applications of GNS-based materials to improve clinical diagnosis and therapy. Sub-topics include neurotoxicity, targeted delivery of therapeutics to the central nervous system (CNS), neurochemical sensing, neuromodulation, neuroimaging, neurotherapy, tissue engineering, and neural regeneration. It focuses on core concepts of GNSs in neurology, to circumvent the limitations and significant obstacles of innovative approaches in neurobiology and neurochemistry, including theragnostics. We will discuss recent advances in the use of GNSs to overcome current bottlenecks and tackle technical and conceptual challenges.

Electroanalytical Overview: Electrochemical Sensing Platforms for Food and Drink Safety

Robust, reliable, and affordable analytical techniques are essential for screening and monitoring food and water safety from contaminants, pathogens, and allergens that might be harmful upon consumption. Recent advances in decentralised, miniaturised, and rapid tests for health and environmental monitoring can provide an alternative solution to the classic laboratory-based analytical techniques currently utilised. Electrochemical biosensors offer a promising option as portable sensing platforms to expedite the transition from laboratory benchtop to on-site analysis. A plethora of electroanalytical sensor platforms have been produced for the detection of small molecules, proteins, and microorganisms vital to ensuring food and drink safety. These utilise various recognition systems, from direct electrochemical redox processes to biological recognition elements such as antibodies, enzymes, and aptamers; however, further exploration needs to be carried out, with many systems requiring validation against standard benchtop laboratory-based techniques to offer increased confidence in the sensing platforms. This short review demonstrates that electroanalytical biosensors already offer a sensitive, fast, and low-cost sensor platform for food and drink safety monitoring. With continued research into the development of these sensors, increased confidence in the safety of food and drink products for manufacturers, policy makers, and end users will result.

Electrochemical aptasensor for Escherichia coli O157:H7 bacteria detection using a nanocomposite of reduced graphene oxide, gold nanoparticles and polyvinyl alcohol

In recent years, public attention has drawn to food safety due to the constant outbreaks of foodborne diseases; subsequently, to control and prevent this group of diseases, early screening of foodborne pathogens has become significant. In this study, a new aptamer-based electrochemical sensor was proposed to detect Escherichia coli O157:H7 (E. coli), one of the most threatening bacterial pathogens, using nanoparticles-modified glassy carbon electrode. Firstly, the electrode was coated with a reduced graphene oxide-poly(vinyl alcohol) and gold nanoparticles nanocomposite (AuNPs/rGO-PVA/GCE) to increase the electrode surface area and consequently raise the sensor sensitivity. Afterwards, to enhance the selectivity of the modified electrode, aptamers were attached to the surface of the prepared electrode. The prepared electrode was characterized using energy-dispersive spectroscopy, field-emission scanning electron microscopy, atomic force microscopy, Fourier-transform infrared spectroscopy, and electrochemical impedance spectroscopy. The relationship of the E. coli concentration and the peak current in the range from 9.2 CFU mL^-1 to 9.2 × 10⁸ CFU mL^-1 was linear, and the limit of detection was calculated as 9.34 CFU mL^-1. The suitability of the proposed sensor for real sample measurements was investigated by recovery studies in tap water, milk, and meat samples. The results showed that the biosensor and traditional culture counting methods are equally sensitive for detecting E. coli.

The Role of Aryldiazonium Chemistry in Designing Electrochemical Aptasensors for the Detection of Food Contaminants

Food safety monitoring assays based on synthetic recognition structures such as aptamers are receiving considerable attention due to their remarkable advantages in terms of their ability to bind to a wide range of target analytes, strong binding affinity, facile manufacturing, and cost-effectiveness. Although aptasensors for food monitoring are still in the development stage, the use of an electrochemical detection route, combined with the wide range of materials available as transducers and the proper immobilization strategy of the aptamer at the transducer surface, can lead to powerful analytical tools. In such a context, employing aryldiazonium salts for the surface derivatization of transducer electrodes serves as a simple, versatile and robust strategy to fine-tune the interface properties and to facilitate the convenient anchoring and stability of the aptamer. By summarizing the most important results disclosed in the last years, this article provides a comprehensive review that emphasizes the contribution of aryldiazonium chemistry in developing electrochemical aptasensors for food safety monitoring.

Two-Dimensional Nanostructures for Electrochemical Biosensor

Current advancements in the development of functional nanomaterials and precisely designed nanostructures have created new opportunities for the fabrication of practical biosensors for field analysis. Two-dimensional (2D) and three-dimensional (3D) nanomaterials provide unique hierarchical structures, high surface area, and layered configurations with multiple length scales and porosity, and the possibility to create functionalities for targeted recognition at their surface. Such hierarchical structures offer prospects to tune the characteristics of materials-e.g., the electronic properties, performance, and mechanical flexibility-and they provide additional functions such as structural color, organized morphological features, and the ability to recognize and respond to external stimuli. Combining these unique features of the different types of nanostructures and using them as support for bimolecular assemblies can provide biosensing platforms with targeted recognition and transduction properties, and increased robustness, sensitivity, and selectivity for detection of a variety of analytes that can positively impact many fields. Herein, we first provide an overview of the recently developed 2D nanostructures focusing on the characteristics that are most relevant for the design of practical biosensors. Then, we discuss the integration of these materials with bio-elements such as bacteriophages, antibodies, nucleic acids, enzymes, and proteins, and we provide examples of applications in the environmental, food, and clinical fields. We conclude with a discussion of the manufacturing challenges of these devices and opportunities for the future development and exploration of these nanomaterials to design field-deployable biosensors.

Building Tailored Interfaces through Covalent Coupling Reactions at Layers Grafted from Aryldiazonium Salts

Aryldiazonium ions are widely used reagents for surface modification. Attractive aspects of their use include wide substrate compatibility (ranging from plastics to carbons to metals and metal oxides), formation of stable covalent bonding to the substrate, simplicity of modification methods that are compatible with organic and aqueous solvents, and the commercial availability of many aniline precursors with a straightforward conversion to the active reagent. Importantly, the strong bonding of the modifying layer to the surface makes the method ideally suited to further on-surface (postfunctionalization) chemistry. After an initial grafting from a suitable aryldiazonium ion to give an anchor layer, a target species can be coupled to the layer, hugely expanding the range of species that can be immobilized. This strategy has been widely employed to prepare materials for numerous applications including chemical sensors, biosensors, catalysis, optoelectronics, composite materials, and energy conversion and storage. In this Review our goal is first to summarize how a target species with a particular functional group may be covalently coupled to an appropriate anchor layer. We then review applications of the resulting materials.

The electrochemical detection of bioterrorism agents: a review of the detection, diagnostics, and implementation of sensors in biosafety programs for Class A bioweapons

During the past year, disease has shown us the iron grip it can hold over a population of people. Health systems can be overwhelmed, economies can be brought into recession, and many people can be harmed or killed. When weaponized, diseases can be manipulated to create a detriment to health while becoming an economic burden on any society. It is consequently prudent that easy detection of bioweapons is available to governments for protecting their people. Electrochemical sensing displays many distinct advantages, such as its low limit of detection, low cost to run, rapid generation of results, and in many instances portability. We therefore present a wide array of electrochemical sensing platforms currently being fabricated, a brief summary of Class A bioweapons, and the potential future of bioweapon detection and biosafety.

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.

An Impedance Based Electrochemical Immunosensor for Aflatoxin B1 Monitoring in Pistachio Matrices

Aflatoxins are highly toxic fungal secondary metabolites that often contaminate food and feed commodities. An electrochemical immunosensor for the determination of aflatoxin B1 (AFB1) was fabricated by immobilizing monoclonal AFB1 antibodies onto a screen-printed gold electrode that was modified with carbo-methyldextran by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride/N-hydroxysuccinimide cross-linking. An electrochemical interfacial modelling of biomolecular recognition was suggested and reasonably interpreted. Impedance technology was employed for the quantitative determination of AFB1. The limit of detection concentration of AFB1 for standard solutions and spiked pistachio samples was 0.5 ng/mL and 1 ng/mL, respectively. The immunosensor was able to successfully determine AFB1 concentrations in the range of 4.56–50.86 ng/mL in unknown pistachio samples. Comparative chromatographic analysis revealed that AFB1 concentrations that were higher than 345 ng/mL were not within the immunosensor’s upper limits of detection. Selectivity studies against Ochratoxin A and Aflatoxin M1 demonstrated that the proposed AFB1 immunosensor was able to differentiate between these other fungal mycotoxins. The novel electrochemical immunosensor approach has the potential for rapid sample screening in a portable, disposable format, thus contributing to the requirement for effective prevention and the control of aflatoxin B1 in pistachios.

Tailoring the performance of electrochemical biosensors based on carbon nanomaterials via aryldiazonium electrografting

Carbon nanomaterials (CNs) offer some of the most valuable properties for electrochemical biosensing applications, such as good electrical conductivity, wide electrochemical stability, high specific surface area, and biocompatibility. Regardless the envisioned sensing application, endowing CNs with specific functions through controlled chemical functionalization is fundamental for promoting the specific binding of the analyte. As a versatile and straightforward method of surface functionalization, aryldiazonium chemistry have been successfully used to accommodate in a stable and reproducible way different functionalities, while the electrochemical route has become the favourite choice since the deposition conditions can be readily controlled and adapted to the substrate. In particular, the modification of CNs by electrochemical reduction of aryl diazonium salts is established as a powerful tool which allows tailoring the chemical and electronic properties of the sensing platform. By outlining the stimulating results disclosed in the last years, this article provides not only a comprehensively review, but also a rational assessment on contribution of aryldiazonium electrografting in developing CNs-based electrochemical biosensors. Furthermore, some of the emerging challenges to be surpassed to effectively implement this methodology for in vivo and point of care analysis are also highlighted.

A Quantum Dot Nanobiosensor for Rapid Detection of Botulinum Neurotoxin Serotype E

Botulinum neurotoxins (BoNTs) are potent toxins produced by Clostridium bacteria that are responsible for the illness botulism and are listed as bioterrorism agents. BoNT serotype E (BoNT/E) is one of four BoNT serotypes that cause human botulism and is the second most frequent cause of foodborne botulism. Rapid detection and discrimination of BoNT serotypes implicated in human disease are critical for ensuring timely treatment of patients and identifying sources of toxins, but there have been few reported detection methods for BoNT/E and even fewer methods usable for BoNT serotyping. We report a nanobiosensor based on Förster resonance energy transfer (FRET) between semiconductor nanocrystals (quantum dots, QDs) and dark quencher-labeled peptide probes to detect biologically active BoNT/E in aqueous media. The peptide probes contain a specific cleavage site for active BoNT/E. QD photoluminescence, which changes intensity due to FRET when the peptide probe is cleaved, was used to indicate toxin presence and quantity. The detection of a BoNT/E light chain (LcE) and holotoxin was observed within 3 h. The limits of detection were 0.02 and 2 ng/mL for LcE and holotoxin, respectively. The nanobiosensor shows good specificity toward the target in tests with nontarget BoNT serotypes. The high sensitivity, simple operation, short detection time, and ability to be used in parallel with probes developed for other BoNT serotypes indicate that the nanobiosensor will be useful for rapid BoNT/E detection and serotype discrimination in food analysis.

A simple and fast protocol for the synthesis of 2-amino-4-(4-formylphenyl)-4H-chromene-3-carbonitrile to develop an optical immunoassay for the quantification of botulinum neurotoxin type F

In this study, a novel optical immunoassay platform using (S)-2-amino-4-(4-formylphenyl)-4H-chromene-3-carbonitrile, which was synthesized by an ultra-sonication method, as an optical probe.

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.

Advances in Molecular Diagnostic Approaches for Biothreat Agents

The advancement in Molecular techniques has been implicated in the development of sophisticated, high-end diagnostic platform and point-of-care (POC) devices for the detection of biothreat agents. Different molecular and immunological approaches such as Immunochromatographic and lateral flow assays, Enzyme-linked Immunosorbent assays (ELISA), Biosensors, Isothermal amplification assays, Nucleic acid amplification tests (NAATs), Next Generation Sequencers (NGS), Microarrays and Microfluidics have been used for a long time as detection strategies of the biothreat agents. In addition, several point of care (POC) devices have been approved by FDA and commercialized in markets. The high-end molecular platforms like NGS and Microarray are time-consuming, costly, and produce huge amount of data. Therefore, the future prospects of molecular based technique should focus on developing quick, user-friendly, cost-effective and portable devices against biological attacks and surveillance programs.

Click Chemistry: A Versatile Method for Tuning the Composition of Mixed Organic Layers Obtained by Reduction of Diazonium Cations

Postfunctionalization of glassy carbon electrodes previously modified by reduction of 4-azidobenzenediazonium was exploited to conveniently synthesize controlled mixed organic layers. Huisgen 1,3-dipolar cycloaddition was used to anchor functional entities to azide platform. By this way, ((4-ethynylphenyl)carbamoyl)ferrocene (ϕ-Fc) was coimmobilized with a set of acetylene derivatives: 1-ethynyl-4-nitrobenzene (ϕ-NO₂), 4-ethynylaniline (ϕ-NH₂) or ethylnylbenzene (ϕ). The composition of the resulting organic layers was tuned by adjusting the acetylene derivatives ratio in the postfunctionalization binary solution. Electronic properties of the substituents beared by the aromatic rings were found to have a strong impact on the cycloaddition kinetics toward the confined azide moieties. From this study, rules to prepare finely tuned bifunctional organic layers can be anticipated.

Detecting Biothreat Agents: From Current Diagnostics to Developing Sensor Technologies

Although a fundamental understanding of the pathogenicity of most biothreat agents has been elucidated and available treatments have increased substantially over the past decades, they still represent a significant public health threat in this age of (bio)terrorism, indiscriminate warfare, pollution, climate change, unchecked population growth, and globalization. The key step to almost all prevention, protection, prophylaxis, post-exposure treatment, and mitigation of any bioagent is early detection. Here, we review available methods for detecting bioagents including pathogenic bacteria and viruses along with their toxins. An introduction placing this subject in the historical context of previous naturally occurring outbreaks and efforts to weaponize selected agents is first provided along with definitions and relevant considerations. An overview of the detection technologies that find use in this endeavor along with how they provide data or transduce signal within a sensing configuration follows. Current "gold" standards for biothreat detection/diagnostics along with a listing of relevant FDA approved in vitro diagnostic devices is then discussed to provide an overview of the current state of the art. Given the 2014 outbreak of Ebola virus in Western Africa and the recent 2016 spread of Zika virus in the Americas, discussion of what constitutes a public health emergency and how new in vitro diagnostic devices are authorized for emergency use in the U.S. are also included. The majority of the Review is then subdivided around the sensing of bacterial, viral, and toxin biothreats with each including an overview of the major agents in that class, a detailed cross-section of different sensing methods in development based on assay format or analytical technique, and some discussion of related microfluidic lab-on-a-chip/point-of-care devices. Finally, an outlook is given on how this field will develop from the perspective of the biosensing technology itself and the new emerging threats they may face.

Development of immunodetection system for botulinum neurotoxin serotype E

Background & objectives

Botulism, a potentially fatal paralytic illness, is caused by the botulinum neurotoxins (BoNTs) secreted by Clostridium botulinum. It is an obligate anaerobic, Gram-positive, spore-forming bacterium. BoNTs are classified into seven serotypes based on the serological properties. Among these seven serotypes, A, B, E and, rarely, F are responsible for human botulism. The present study was undertaken to develop an enzyme-linked immunosorbent assay (ELISA)-based detection system for the detection of BoNT/E.

Methods

The synthetic gene coding the light chain of BoNT serotype E (BoNT/E LC) was constructed using the polymerase chain reaction primer overlapping method, cloned into pQE30UA vector and then transformed into Escherichia coli M15 host cells. Recombinant protein expression was optimized using different concentrations of isopropyl-β-D-1-thiogalactopyranoside (IPTG), different temperature and the rBoNT/E LC protein was purified in native conditions using affinity column chromatography. The purified recombinant protein was checked by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and further confirmed by western blot and matrix-assisted laser desorption ionization-tandem time-of-flight (MALDI-TOF). Polyclonal antibodies were generated against rBoNT/E LC using Freund's adjuvant in BALB/c mice and rabbit. Sandwich ELISA was optimized for the detection of rBoNT/E LC and native crude BoNT/E, and food matrix interference was tested. The developed antibodies were further evaluated for their specificity/cross-reactivity with BoNT serotypes and other bacterial toxins.

Results

BoNT/E LC was successfully cloned, and the maximum expression was achieved in 16 h of post-induction using 0.5 mM IPTG concentration at 25°C. Polyclonal antibodies were generated in BALB/c mice and rabbit and the antibody titre was raised up to 128,000 after the 2^nd booster dose. The developed polyclonal antibodies were highly specific and sensitive with a detection limit about 50 ng/ml for rBoNT/E LC and 2.5×10^[3] MLD₅₀ of native crude BoNT/E at a dilution of 1:3000 of mouse (capturing) and rabbit (revealing) antibodies. Further, different liquid, semisolid and solid food matrices were tested, and rBoNT/E LC was detected in almost all food samples, but different levels of interference were detected in different food matrices.

Interpretation & conclusions

There is no immune detection system available commercially in India to detect botulism. The developed system might be useful for the detection of botulinum toxin in food and clinical samples. Further work is in progress.

Progress on nanostructured electrochemical sensors and their recognition elements for detection of mycotoxins: A review

Nanomaterial-embedded sensors have been developed and applied to monitor various targets. Mycotoxins are fungal secondary metabolites that can exert carcinogenic, mutagenic, teratogenic, immunotoxic, and estrogenic effects on humans and animals. Consequently, the need for the proper regulation on foodstuff and feed materials has been recognized from times long past. This review provides an overview of recent developments in electrochemical sensors and biosensors employed for the detection of mycotoxins. Basic aspects of the toxicity of mycotoxins and the implications of their detection are comprehensively discussed. Furthermore, the development of different molecular recognition elements and nanomaterials required for the detection of mycotoxins (such as portable biosensing systems for point-of-care analysis) is described. The current capabilities, limitations, and future challenges in mycotoxin detection and analysis are also addressed.

Positively-charged hierarchical PEDOT interface with enhanced electrode kinetics for NADH-based biosensors

Poly(ethylenedioxythiophene) (PEDOT) has attracted considerable attention as an advanced electrode material for electrochemical sensors and biosensors, due to its unique electrical and physicochemical properties. Here, we demonstrate the facile preparation of a positively-charged and hierarchical micro-structured PEDOT electrochemical interface with enhanced electrode kinetics for the electrooxidation of NADH. Processable PEDOT colloidal microparticles (PEDOT CMs) were synthesised by template-assisted polymerisation and were then utilised as building blocks for the fabrication of hierarchically-structured electrodes with a larger accessible electroactive surface (2.8 times larger than that of the benchmark PEDOT:PSS) and inter-particle space, thus improving electrode kinetics. The intrinsic positive charge of the PEDOT CMs further facilitated the detection of negatively-charged molecules by electrostatic accumulation. Due to the synergistic effect, these hierarchically-structured PEDOT CMs electrodes exhibited improved NADH electrooxidation at lower potentials and enhanced electrocatalytic activity compared to the compact structure of conventional PEDOT:PSS electrodes. The PEDOT CMs electrodes detected NADH over the range of 20-240 μM, with a sensitivity of 0.0156 μA/μM and a limit of detection of 5.3 μM. Moreover, the PEDOT CMs electrode exhibited a larger peak separation from the interferent ascorbic acid, and improved stability. This enhanced analytical performance for NADH provides a sound basis for further work coupling to a range of NAD-dependent dehydrogenases for applications in biosensing, bio-fuel cells and biocatalysis.

A biosensor based on a graphene nanoribbon/silver nanoparticle/polyphenol oxidase composite matrix on a graphite electrode: application in the analysis of catechol in green tea samples

The present report demonstrates the development of a Gr/GNRs/AgNPs/PPO composite biosensor for the detection of catechol.

Detection and Quantification of Biologically Active Botulinum Neurotoxin Serotypes A and B Using a Förster Resonance Energy Transfer-Based Quantum Dot Nanobiosensor

Botulinum neurotoxin (BoNT) is the most potent toxin known. The ingestion of food contaminated with biologically active BoNT causes foodborne botulism, which can lead to respiratory paralysis, coma, and death after ingestion of as little as 70 μg for a 70 kg human. Because of its lethality and challenges associated with current detection methods, there is an urgent need for highly sensitive rapid screening techniques capable of detecting biologically active BoNT. Here, we describe a Förster resonance energy transfer-based nanobiosensor that uses quantum dots (QDs) and two specific quencher-labeled peptide probes to detect and differentiate two biologically active forms of BoNT, serotypes A and B, which were responsible for 80% of human foodborne botulism cases in the U.S. from 2012 to 2015. Each peptide probe contains an enzymatic cleavage site specific to only one serotype. QDs were selected based on the spectral overlap with the quenchers. In the presence of the target BoNT serotype, the peptide probe is cleaved and the quenching of QD photoluminescence (PL) is reduced, giving a signal that is easily detected by a PL spectrophotometer. This sensor performance was evaluated with light chains of BoNT/A and BoNT/B (LcA and LcB), catalytic domains of the respective serotypes. LcA and LcB were detected in 3 h with limits of detection of 0.2 and 2 ng/mL, respectively. The specificity of the sensor was evaluated, and no cross-reactivity from nontarget serotypes was observed with 2 h of incubation. Because each serotype-specific peptide is conjugated to a QD with a unique emission wavelength, multiple biologically active BoNT serotypes could be detected in one PL spectrum. The sensor was also shown to be responsive to BoNT/A and BoNT/B holotoxins. Good performance of this sensor implies its potential application as a rapid screening method for biologically active BoNT/A and BoNT/B in the laboratory and in the field.

Fabrication of a Novel Highly Sensitive and Selective Immunosensor for Botulinum Neurotoxin Serotype A Based on an Effective Platform of Electrosynthesized Gold Nanodendrites/Chitosan Nanoparticles

In this work, a novel nanocomposite consisting of electrosynthesized gold nanodendrites and chitosan nanoparticles (AuNDs/CSNPs) has been prepared to fabricate an impedimetric immunosensor based on a screen printed carbon electrode (SPCE) for the rapid and sensitive immunoassay of botulinum neurotoxin A (BoNT/A). BoNT/A polyclonal antibody was immobilized on the nanocomposite-modified SPCE for the signal amplification. The structure of the prepared nanocomposite was investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The charge transfer resistance (R_CT) changes were used to detect BoNT/A as the specific immuno-interactions at the immunosensor surface that efficiently limited the electron transfer of Fe(CN)₆^3−/4− as a redox probe at pH = 7.4. A linear relationship was observed between the %∆R_CT and the concentration logarithm of BoNT/A within the range of 0.2 to 230 pg·mL⁻¹ with a detection limit (S/N = 3) of 0.15 pg·mL⁻¹. The practical applicability of the proposed sensor was examined by evaluating the detection of BoNT/A in milk and serum samples with satisfactory recoveries. Therefore, the prepared immunosensor holds great promise for the fast, simple and sensitive detection of BoNT/A in various real samples.

Nanomaterials for Electrochemical Immunosensing

Electrochemical immunosensors resulting from a combination of the traditional immunoassay approach with modern biosensors and electrochemical analysis constitute a current research hotspot. They exhibit both the high selectivity characteristics of immunoassays and the high sensitivity of electrochemical analysis, along with other merits such as small volume, convenience, low cost, simple preparation, and real-time on-line detection, and have been widely used in the fields of environmental monitoring, medical clinical trials and food analysis. Notably, the rapid development of nanotechnology and the wide application of nanomaterials have provided new opportunities for the development of high-performance electrochemical immunosensors. Various nanomaterials with different properties can effectively solve issues such as the immobilization of biological recognition molecules, enrichment and concentration of trace analytes, and signal detection and amplification to further enhance the stability and sensitivity of the electrochemical immunoassay procedure. This review introduces the working principles and development of electrochemical immunosensors based on different signals, along with new achievements and progress related to electrochemical immunosensors in various fields. The importance of various types of nanomaterials for improving the performance of electrochemical immunosensor is also reviewed to provide a theoretical basis and guidance for the further development and application of nanomaterials in electrochemical immunosensors.

Advances on Aryldiazonium Salt Chemistry Based Interfacial Fabrication for Sensing Applications

Aryldiazonium salts as coupling agents for surface chemistry have evidenced their wide applications for the development of sensors. Combined with advances in nanomaterials, current trends in sensor science and a variety of particular advantages of aryldiazonium salt chemistry in sensing have driven the aryldiazonium salt-based sensing strategies to grow at an astonishing pace. This review focuses on the advances in the use of aryldiazonium salts for modifying interfaces in sensors and biosensors during the past decade. It will first summarize the current methods for modification of interfaces with aryldiazonium salts, and then discuss the sensing applications of aryldiazonium salts modified on different transducers (bulky solid electrodes, nanomaterials modified bulky solid electrodes, and nanoparticles). Finally, the challenges and perspectives that aryldiazonium salt chemistry is facing in sensing applications are critically discussed.

Graphene and its electrochemistry - an update

The electrochemistry of graphene and its derivatives has been extensively researched in recent years. In the aspect of graphene preparation methods, the efficiencies of the top-down electrochemical exfoliation of graphite, the electrochemical reduction of graphene oxide and the electrochemical delamination of CVD grown graphene, are currently on par with conventional procedures. Electrochemical analysis of graphene oxide has revealed an unexpected inherent redox activity with, in some cases, an astonishing chemical reversibility. Furthermore, graphene modified with p-block elements has shown impressive electrocatalytic performances in processes which have been historically dominated by metal-based catalysts. Further progress has also been achieved in the practical usage of graphene in sensing and biosensing applications. This review is an update of our previous article in Chem. Soc. Rev. 2010, 39, 4146-4157, with special focus on the developments over the past two years.

Label-free Electrochemiluminescent Immunosensor for Detection of Prostate Specific Antigen based on Aminated Graphene Quantum Dots and Carboxyl Graphene Quantum Dots

Prostate-specific antigen (PSA) was used as the model, an ultrasensitive label-free electrochemiluminescent immunosensor was developed based on graphene quantum dots. Au/Ag-rGO was sythsized and used as electrode material to load a great deal of graphene quantum dots due to the large surface area and excellent electron conductivity. After aminated graphene quantum dots and acarboxyl graphene quantum dots were modified onto the electrode, the ECL intensity was much high using K2S2O8 as coreactant. Then, antibody of PSA was immobilized on the surface of modified electrode surface through the adsorption of Au/Ag toward proteins, leading to the decrease of the ECL intensity. As proven by ECL spectra test and electrochemical impedance spectroscopy (EIS) analysis, the fabrication process of the immunosensor is successful. Under the optimal conditions, the ECL intensity decreased linearly with the logarithm of PSA concentration in the range of 1 pg/mL ~ 10 ng/mL. The detection limit achieved is 0.29 pg/mL. The immunosensor results were validated through the detection of PSA in serum samples with satisfactory results. Due to excellent stability, high sensitivity, acceptable repeatability and selectivity, the immunosensor has promising applications in disease and drug analysis.

An electrochemical glycan biosensor based on a thionine-bridged multiwalled carbon nanotube/gold nanoparticle composite-modified electrode

A MWCNT/Th/AuNP composite, used to construct an electrochemical biosensor for the mannose assay of living cancer cells, contained thionine as an electron mediator and simplified detection based on enzymatic catalysis for signal amplification.

Layer-by-layer self-assembly of 2D graphene nanosheets, 3D copper oxide nanoflowers and 0D gold nanoparticles for ultrasensitive electrochemical detection of alpha fetoprotein

In this work, a novel and ultrasensitive label-free electrochemical immunosensor was developed for the quantitative detection of alpha fetoprotein (AFP).

Application of three-dimensional flower-like nanomaterials in the fabrication of sandwich-type electrochemical immunosensors

A novel and ultrasensitive sandwich-type electrochemical immunosensor was developed for the quantitative detection of carcinoembryonic antigen (CEA) in this work.