Immunosensor array platforms based on self-assembled dithiols for the electrochemical detection of tetrodotoxins in puffer fish

A smartphone-based fluorescent biosensor with metal-organic framework biocomposites and cotton swabs for the rapid determination of tetrodotoxin in seafood

BACKGROUND

Tetrodotoxin (TTX) is a potent neurovirulent marine biotoxin that is present in puffer fish and certain marine animals. It is capable of causing severe neurotoxic symptoms and even death when consumed through contaminated seafood. Due to its high toxicity, developing an effective assay for TTX determination in seafood has significant benefits for food safety and human health. Currently, it remains challenging to achieve on-site determination of TTX in seafood. To facilitate mass on-site assays, more affordable technologies utilizing accessible equipment that require no skilled personnel are needed.

RESULTS

A smartphone-based portable fluorescent biosensor is proposed for TTX determination by using metal-organic framework (MOF) biocomposites and cotton swabs. Oriented antibody (Ab)-decorated and fluorescent quantum dot (QD)-loaded MOF biocomposites (QD@MOF*Ab) are rapidly synthesized for binding targets and fluorescent responses by utilizing the tunability of zinc-based MOF. Moreover, facile Ab-immobilized household cotton swabs are utilized as TTX capture tools. TTX forms sandwich immune complexes with QD@MOF*Ab probes, achieving signal amplification. These probes are excited by a portable device to generate bright fluorescent signals, which can be detected by the naked eye, and TTX quantitative results are obtained using a smartphone. When observed with the naked eye, the limit of detection (LOD) is 0.4 ng/mL, while intelligent quantitation presents an LOD of 0.13 ng/mL at logarithmic concentrations of 0.2-400 ng/mL.

SIGNIFICANCE

This biosensor is convenient to use, and an easy-to-operate analysis is completed within 15 min, thus demonstrating excellent performance in terms of detection speed and portability. Furthermore, it successfully determines TTX contents in puffer fish and clam samples, demonstrating its potential for monitoring seafood. Herein, this work provides a favorable rapid sensing platform that is easily portable.

Automated Patch Clamp for the Detection of Tetrodotoxin in Pufferfish Samples

Tetrodotoxin (TTX) is a marine toxin responsible for many intoxications around the world. Its presence in some pufferfish species and, as recently reported, in shellfish, poses a serious health concern. Although TTX is not routinely monitored, there is a need for fast, sensitive, reliable, and simple methods for its detection and quantification. In this work, we describe the use of an automated patch clamp (APC) system with Neuro-2a cells for the determination of TTX contents in pufferfish samples. The cells showed an IC50 of 6.4 nM for TTX and were not affected by the presence of muscle, skin, liver, and gonad tissues of a Sphoeroides pachygaster specimen (TTX-free) when analysed at 10 mg/mL. The LOD achieved with this technique was 0.05 mg TTX equiv./kg, which is far below the Japanese regulatory limit of 2 mg TTX equiv./kg. The APC system was applied to the analysis of extracts of a Lagocephalus sceleratus specimen, showing TTX contents that followed the trend of gonads > liver > skin > muscle. The APC system, providing an in vitro toxicological approach, offers the advantages of being sensitive, rapid, and reliable for the detection of TTX-like compounds in seafood.

A label-free ratiometric fluorescent aptasensor based on a peroxidase-mimetic multifunctional ZrFe-MOF for the determination of tetrodotoxin

A Fe/Zr bimetal-organic framework (ZrFe-MOF) is utilized to establish a ratiometric fluorescent aptasensor for the determination of tetrodotoxin (TTX). The multifunctional ZrFe-MOF possesses inherent fluorescence at 445 nm wavelength, peroxidase-mimetic activity, and specific recognition and adsorption capabilities for aptamers, owing to its organic ligand, and Fe and Zr nodes. The peroxidation of o-phenylenediamine (OPD) substrate generates fluorescent 2,3-diaminophenazine (OPDox) at 555 nm wavelength, thus quenching the inherent fluorescence of ZrFe-MOF because of the fluorescence resonance energy transfer (FRET) effect. TTX aptamers, which are absorbed on the material surface without immobilization or fluorescent labeling, inhibit the peroxidase-mimetic activity of ZrFe-MOF. It causes the decreased OPDox fluorescence at 555 nm wavelength and the inverse restoration of ZrFe-MOF fluorescence at 445 nm wavelength. With TTX, the aptamers specifically bind to TTX, triggering rigid complex release from ZrFe-MOF surface and reactivating its peroxidase-mimetic activity. Consequently, the two fluorescence signals exhibit opposite changes. Employing this ratiometric strategy, the determination of TTX is achieved with a detection limit of 0.027 ng/mL and a linear range of 0.05-500 ng/mL. This aptasensor also successfully determines TTX concentrations in puffer fish and clam samples, demonstrating its promising application for monitoring trace TTX in food safety.

Evaluation of Toxicity Equivalency Factors of Tetrodotoxin Analogues with a Neuro-2a Cell-Based Assay and Application to Puffer Fish from Greece

Tetrodotoxin (TTX) is a potent marine neurotoxin involved in poisoning cases, especially through the consumption of puffer fish. Knowledge of the toxicity equivalency factors (TEFs) of TTX analogues is crucial in monitoring programs to estimate the toxicity of samples analyzed with instrumental analysis methods. In this work, TTX analogues were isolated from the liver of a Lagocephalus sceleratus individual caught on South Crete coasts. A cell-based assay (CBA) for TTXs was optimized and applied to the establishment of the TEFs of 5,11-dideoxyTTX, 11-norTTX-6(S)-ol, 11-deoxyTTX and 5,6,11-trideoxyTTX. Results showed that all TTX analogues were less toxic than the parent TTX, their TEFs being in the range of 0.75-0.011. Then, different tissues of three Lagocephalus sceleratus individuals were analyzed with CBA and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The obtained TEFs were applied to the TTX analogues' concentrations obtained by LC-MS/MS analysis, providing an indication of the overall toxicity of the sample. Information about the TEFs of TTX analogues is valuable for food safety control, allowing the estimation of the risk of fish products to consumers.

Detection of Ciguatoxins and Tetrodotoxins in Seafood with Biosensors and Other Smart Bioanalytical Systems

The emergence of marine toxins such as ciguatoxins (CTXs) and tetrodotoxins (TTXs) in non-endemic regions may pose a serious food safety threat and public health concern if proper control measures are not applied. This article provides an overview of the main biorecognition molecules used for the detection of CTXs and TTXs and the different assay configurations and transduction strategies explored in the development of biosensors and other biotechnological tools for these marine toxins. The advantages and limitations of the systems based on cells, receptors, antibodies, and aptamers are described, and new challenges in marine toxin detection are identified. The validation of these smart bioanalytical systems through analysis of samples and comparison with other techniques is also rationally discussed. These tools have already been demonstrated to be useful in the detection and quantification of CTXs and TTXs, and are, therefore, highly promising for their implementation in research activities and monitoring programs.

Tetrodotoxin and Its Analogues in Food: Recent Updates on Sample Preparation and Analytical Methods Since 2012

Tetrodotoxin (TTX), found in various organisms including pufferfish, is an extremely potent marine toxin responsible for numerous food poisoning accidents. Due to its serious toxicity and public health threat, detecting TTX and its analogues in diverse food matrices with a simple, fast, efficient method has become a worldwide concern. This review summarizes the advances in sample preparation and analytical methods for the determination of TTX and its analogues, focusing on the latest development over the past five years. Current state-of-the-art technologies, such as solid-phase microextraction, online technology, novel injection technology, two-dimensional liquid chromatography, high-resolution mass spectrometry, newly developed lateral flow immunochromatographic strips, immunosensors, dual-mode aptasensors, and nanomaterials-based approaches, are thoroughly discussed. The advantages and limitations of different techniques, critical comments, and future perspectives are also proposed. This review is expected to provide rewarding insights to the future development and broad application of pretreatment and detection methods for TTX and its analogues.

An Updated Review of Tetrodotoxin and Its Peculiarities

Tetrodotoxin (TTX) is a crystalline, weakly basic, colorless organic substance and is one of the most potent marine toxins known. Although TTX was first isolated from pufferfish, it has been found in numerous other marine organisms and a few terrestrial species. Moreover, tetrodotoxication is still an important health problem today, as TTX has no known antidote. TTX poisonings were most commonly reported from Japan, Thailand, and China, but today the risk of TTX poisoning is spreading around the world. Recent studies have shown that TTX-containing fish are being found in other regions of the Pacific and in the Indian Ocean, as well as the Mediterranean Sea. This review aims to summarize pertinent information available to date on the structure, origin, distribution, mechanism of action of TTX and analytical methods used for the detection of TTX, as well as on TTX-containing organisms, symptoms of TTX poisoning, and incidence worldwide.

A facile dual-mode aptasensor based on AuNPs@MIL-101 nanohybrids for ultrasensitive fluorescence and surface-enhanced Raman spectroscopy detection of tetrodotoxin

The development of ultrasensitive, reliable, and facile detection technologies for trace tetrodotoxin (TTX) is challenging. We presented a facile dual-mode aptamer-based biosensor (aptasensor) for ultrasensitive fluorescence and surface-enhanced Raman spectroscopy (SERS) detection of TTX by using gold nanoparticles (AuNPs)-embedded metal-organic framework (MOF) nanohybrids (AuNPs@MIL-101) because of their superior properties. A TTX-specific aptamer labelled with fluorescence and Raman reporter cyanine-3 (Cy3-aptamer) was selected as the recognition element and signal probe. Without immobilisation processing steps, Cy3-aptamers were effectively adsorbed onto the surface of AuNPs@MIL-101, thereby generating both fluorescence quenching and SERS enhancement. The preferential binding of TTX towards the Cy3-aptamer triggered the release of rigid Cy3-aptamer-TTX complexes from the AuNPs@MIL-101 surface, which resulted in recovered fluorescence signals and weakened SERS signals. Switched fluorescence and SERS intensities exhibited excellent linear relationships with logarithms of TTX concentrations of 0.01-300 ng/mL, and ultrahigh detection sensitivities of 6 and 8 pg/mL, respectively, were obtained. Furthermore, two quantitative detection approaches for TTX-spiked puffer fish and clam samples obtained satisfactory spiked recoveries and coefficient of variation (CV) values. Notably, the dual-mode aptasensor also successfully determined natural TTX-contaminated samples, showing excellent practical applications. The results indicated that this dual-mode measurement not only was ultrasensitive and simple but also markedly boosted analysis reliability and precision. This study is the first to propose a dual-mechanism AuNPs@MIL-101-based aptasensor for detection of trace TTX and provides a favourable pathway for developing multimode sensing platforms for various applications.

Application of selected biosensor techniques in clinical diagnostics

INTRODUCTION

Examination of disease biomarkers mostly performed on crude materials, such as serum, meets some obstacles, resulting from sample complexity and the wide range of concentrations and sizes of the components. Techniques currently used in clinical diagnostics are usually time-consuming and expensive. The more sensitive and portable devices are needed for early diagnostics. Chemical sensors are devices that convert chemical information into parameters suitable for fast and precise processing and measurement.

AREA COVERED

We review the use of biosensors and their possible application in early diagnostics of some diseases like cancer or viral infections. We focus on different types of biorecognition and some technical modifications, lowering the limit of detection potentially attractive to medical practitioners.

EXPERT OPINION

Among the new diagnostic strategies, the use of biosensors is of increasing interest. In these techniques, the capture ligand interacts with the analyte of interest. Measuring interactions between partners in real time by surface plasmon resonance yields valuable information about kinetics and affinity in a short time and without labels. Importantly, the tendency in such techniques is to make biosensor devices smaller and the test results apparent with the naked eye, so they can be used in point-of-care medicine.

Highly sensitive electrochemical detection of the marine toxins okadaic acid and domoic acid with carbon black modified screen printed electrodes

A novel electrochemical immunosensor for the detection of the important marine biotoxins domoic acid (DA) and okadaic acid (OA) was developed. The sensors used carbon black modified screen-printed electrodes (CB-SPE) obtained using a high-throughput method. The electrochemical performance and stability of CB modified SPEs and bare carbon SPEs (c-SPEs) were compared using cyclic voltammetry and electrochemical impedance spectroscopy. CB-SPEs showed improved long-term (at least six months) stability and electro-catalytic properties compared with c-SPEs. The CB-SPEs were bio-functionalized with DA or OA protein-conjugates and used to develop two indirect competitive immunosensors using differential pulse voltammetry (DPV). The DPV signals obtained for the OA and DA immunosensors fitted well to four-parameter dose-response curves (R² > 0.98) and showed excellent LODs (LOD = 1.7 ng mL^-1 for DA in buffer; LOD = 1.9 ng mL^-1 for DA in mussel extract; LOD = 0.15 ng mL^-1 for OA in buffer; LOD = 0.18 ng mL^-1 for OA in mussel extract). No significant interference of the naturally co-occurring marine toxins saxitoxin, tetrodotoxin and OA was detected for the DA immunosensor. Similarly, for the OA immunosensor saxitoxin, tetrodotoxin and DA did not cross-react and very limited interference was observed for the dinophysis toxins DTX-1, DTX-2 and DTX-3 (OA congeners). Moreover, both immunosensors remained stable after at least 25 days of storage at 4 °C. This work demonstrates the potential of affordable, mass-produced nanomaterial-modified SPEs for marine toxin detection in shellfish.

Ensuring seafood safe to spoon: a brief review of biosensors for marine biotoxin monitoring

With harmful algal blooms, marine food poisoning caused by marine biotoxins frequently occurs and is life-threatening if severe. However, the conventional detection methods of marine toxins have a few limitations: low sensitivity and high-cost. Therefore, it is necessary to establish a fast and sensitive on-site detection method for real seafood sample. Biosensors based on aptamers, antibodies, and cells have been applied in marine toxins monitoring. This review presents the classification and toxic effects of marine toxins, and recent biosensor for marine toxin detection. In addition, we have compared the superiority and limitation of these biosensors. Finally, challenges and opportunities of biosensors in food safety detection were discussed. Considering the excellent results achieved by the aptasensor in the field of detection, it seems ready to be put into practical applications.

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.

Tetrodotoxin and saxitoxin in two native species of puffer fish, Sphoeroides marmoratus and Lagocephalus lagocephalus, from NE Atlantic Ocean (Madeira Island, Portugal)

The presence in EU waters of invasive tetrodotoxin (TTX) -harbouring puffer fishes has been receiving increasingly attention due to potential new threats posed by this potent neurotoxin. The present study investigates the occurrence of tetrodotoxin, saxitoxin (STX), and their analogues in two native puffer fish species from the NE Atlantic. High TTX content was detected by LC-MS/MS in several tissues of the Guinean puffer Sphoeroides marmoratus from Madeira Island (Portugal), reaching concentrations as high as 15 mg TTX kg^-1 in the digestive tract of a male specimen and 7.4 mg TTX kg^-1 in gonads of a female specimen. Several TTX analogues were also detected, including the 4-epi-TTX, 4,9-Anhydro-TTX, 5- 11- deoxyTTX and 6,11-dideoxyTTX. Although at low levels, STX was detected in liver of the Oceanic puffer Lagocephalus lagocephalus. Trace levels of decarbamoylsaxitoxin (dcSTX) were also observed in L. lagocephalus. This study reports the presence of TTX and STX in native fish from EU waters, highlighting the need for a proper understating of the origin, distribution and fate of these toxins in NE Atlantic.

Advances in electrochemical immunosensors

Immunosensors are compact tools on which antibody and antigen interactions are formed. The specific interaction between antibody and antigen is detected by using a transducer and an electrical signal is measured. This specific interaction between these molecules makes immunosensor very attractive for several applications in different fields. Electrochemical immunosensors are successful devices in selective and sensitive detection of several analytes. Electrochemical transducing methods such as voltammetric, potentiometric, conductometric or impedimetric have been utilized in different applications due to their excellent properties such as being low-cost, sensitivity and simplicity. In this chapter, the fundamentals of electrochemical immunosensors are summarized and different applications in food, environmental and clinical analyses are investigated and discussed.

Development and validation of a maleimide-based enzyme-linked immunosorbent assay for the detection of tetrodotoxin in oysters and mussels

The recent detection of tetrodotoxins (TTXs) in puffer fish and shellfish in Europe highlights the necessity to monitor the levels of TTXs in seafood by rapid, specific, sensitive and reliable methods in order to protect human consumers. A previous immunoassay for TTX detection in puffer fish, based on the use of self-assembled monolayers (SAMs) for the immobilization of TTX on maleimide plates (mELISA), has been modified and adapted to the analysis of oyster and mussel samples. Changing dithiol for cysteamine-based SAMs enabled reductions in the assay time and cost, while maintaining the sensitivity of the assay. The mELISA showed high selectivity for TTX since the antibody did not cross-react with co-occurring paralytic shellfish poisoning (PSP) toxins and no interferences were observed from arginine (Arg). Moreover, TTX-coated maleimide plates stored for 3 months at -20°C and 4°C were stable, thus when pre-prepared, the time to perform the assay is reduced. When analyzing shellfish samples, matrix effects and toxin recovery values strongly depended on the shellfish type and the sample treatment. Blank oyster extracts could be directly analyzed without solid-phase extraction (SPE) clean-up, whereas blank mussel extracts showed strong matrix effects and SPE and subsequent solvent evaporation were required for removal. However, the SPE clean-up and evaporation resulted in toxin loss. Toxin recovery values were taken as correction factors (CFs) and were applied to the quantification of TTX contents in the analysis of naturally-contaminated shellfish samples by mELISA. The lowest effective limits of detection (eLODs) were about 20 and 50µg/kg for oyster extracts without and with SPE clean-up, respectively, and about 30µg/kg for mussel extracts with both protocols, all of them substantially below the eLOD attained in the previous mELISA for puffer fish (230µg/kg). Analysis of naturally-contaminated samples by mELISA and comparison with LC-MS/MS quantifications demonstrated the viability of the approach. This mELISA is a selective and sensitive tool for the rapid detection of TTX in oyster and mussel samples showing promise to be implemented in routine monitoring programs to protect human health.