Novel dual-emissive fluorescent immunoassay for synchronous monitoring of okadaic acid and saxitoxin in shellfish

A graphene oxide-assisted protein immobilization paper-tip immunosensor with smartphone and naked eye readout for the detection of okadaic acid

BACKGROUND

Okadaic acid (OA), as a diarrhetic shellfish poisoning, can increase the risk of acute carcinogenic or teratogenic effects for the ingestion of OA contaminated shellfish. At present, much effort has been made to graft immunoassay onto a paper substrate to make paper-based sensors for rapid and simple detection of shellfish toxin. However, the complicated washing steps and low protein fixation efficiency on the paper substrate need to be further addressed.

RESULTS

A novel paper-tip immunosensor for detecting OA was developed combined with smartphone and naked eye readout. The trapezoid paper tip was consisted of quantitative and qualitative detection zones. To improve the OA antigen immobilization efficiency on the paper substrate, graphene oxide (GO)-assisted protein immobilization method was introduced. Meanwhile, Au nanoparticles composite probe combined with the lateral flow washing was developed to simplify the washing step. The OA antigen-immobilized zone, as the detection zone Ⅰ, was used for quantitative assay by smartphone imaging. The paper-tip front, as the detection zone Ⅱ, which could qualitatively differentiate OA pollution level within 45 min using the naked eye. The competitive immunoassay on the paper tip exhibited a wide linear range for detecting OA (0.02-50 ng∙mL-1) with low detection limit of 0.02 ng∙mL-1. The recovery of OA in spiked shellfish samples was in the range of 90.3 %-113.%.

SIGNIFICANCE

These results demonstrated that the proposed paper-tip immunosensor could provide a simple, low-cost and high-sensitivity test for OA detection without the need for additional large-scale equipment or expertise. We anticipate that this paper-tip immunosensor will be a flexible and versatile tool for on-site detecting the pollution of marine products.

Toward Sensitive and Reliable Immunoassays of Marine Biotoxins: From Rational Design to Food Analysis

Marine biotoxins are metabolites produced by algae that can accumulate in shellfish or fish and enter organisms through the food chain, posing a serious threat to biological health. Therefore, accurate and rapid detection is an urgent requirement for food safety. Although various detection methods, including the mouse bioassay, liquid chromatography-mass spectrometry, and cell detection methods, and protein phosphatase inhibition assays have been developed in the past decades, the current detection methods cannot fully meet these demands. Among these methods, the outstanding immunoassay virtues of high sensitivity, reliability, and low cost are highly advantageous for marine biotoxin detection in complex samples. In this work, we review the recent 5-year progress in marine biotoxin immunodetection technologies such as optical immunoassays, electrochemical immunoassays, and piezoelectric immunoassays. With the assistance of immunoassays, the detection of food-related marine biotoxins can be implemented for ensuring public health and preventing food poisoning. In addition, the immunodetection technique platforms including lateral flow chips and microfluidic chips are also discussed. We carefully investigate the advantages and disadvantages for each immunoassay, which are compared to demonstrate the guidance for selecting appropriate immunoassays and platforms for the detection of marine biotoxins. It is expected that this review will provide insights for the further development of immunoassays and promote the rapid progress and successful translation of advanced immunoassays with food safety detection.

Development of magnetic fluorescence aptasensor for sensitive detection of saxitoxin based on Fe3O4@Au-Pt nanozymes

In this work, on the basis of Fe3O4@Au-Pt nanozymes (MAP NZs) and aptamer recognition, a magnetic fluorescent aptasensor (MFA) was developed for sensitive and accurate detection of saxitoxin (STX). With the bridge of STX aptamer (AptSTX) and complementary DNA (cDNA), AptSTX decorated MAP NZs (MAP/Apt) and cDNA modified green quantum dots (cDNA@g-QDs) were connected to form MAP/Apt-cDNA@g-QDs complex. As STX behaves a strong binding ability towards AptSTX, it will compete with cDNA and hybridize with Apt to release cDNA@g-QDs. With the addition of TMB, MAP will catalyze TMB to the oxidized TMB (ox-TMB), thereby quenching the fluorescence of g-QDs due to the inner filter effect. Based on this finding, the quantitative relationship between the change in fluorescence of gQDs and STX concentration was explored with a limit of detection (LOD, S/N = 3) of 0.6 nM. An internal standard signal of oxTMB was adopted and reduced the fluctuation of fluorescence signal output. Besides, the fluorescence probe can selectively recognize and detect STX among five marine toxins. Eventually, the MFA method behaved good performance in detecting seafood samples with recoveries of 82.0 % ∼ 102.6 % as well as coefficient of variations (CV) of 7.2 % ∼ 10.3 %. Therefore, the method with internal signal is hopeful to be a potential candidate for sensitive and accurate detection of STX in seafood.

Precautions for seafood consumers: An updated review of toxicity, bioaccumulation, and rapid detection methods of marine biotoxins

Seafood products are globally consumed, and there is an increasing demand for the quality and safety of these products among consumers. Some seafoods are easily contaminated by marine biotoxins in natural environments or cultured farming processes. When humans ingest different toxins accumulated in seafood, they may exhibit different poisoning symptoms. According to the investigations, marine toxins produced by harmful algal blooms and various other marine organisms mainly accumulate in the body organs such as liver and digestive tract of seafood animals. Several regions around the world have reported incidents of seafood poisoning by biotoxins, posing a threat to human health. Thus, most countries have legislated to specify the permissible levels of these biotoxins in seafood. Therefore, it is necessary for seafood producers and suppliers to conduct necessary testing of toxins in seafood before and after harvesting to prohibit excessive toxins containing seafood from entering the market, which therefore can reduce the occurrence of seafood poisoning incidents. In recent years, some technologies which can quickly, conveniently, and sensitively detect biological toxins in seafood, have been developed and validated, these technologies have the potential to help seafood producers, suppliers and regulatory authorities. This article reviews the seafood toxins sources and types, mechanism of action and bioaccumulation of marine toxins, as well as legislation and rapid detection technologies for biotoxins in seafood for official and fishermen supervision.

NIR-responsive photoelectrochemical sensing platform for the simultaneous determination of tetrodotoxin and okadaic acid in Nassariidae

Simultaneous detection of tetrodotoxin (TTX) and okadaic acid (OA) is important for seafood safety. In this work, a novel paper electrode-based near-infrared (NIR) light-responsive photoelectrochemical (PEC) immunosensor was constructed using Ag2S quantum dots (QDs) and NaYF4: Yb, Er upconversion nanoparticles (UCNPs) matched with BiOI for the simultaneous detection of TTX and OA in aquatic products. A low-cost, easily prepared gold nanoparticle-functionalized paper-based screen-printed electrode with six channels was designed to immobilize OA and Ab1 of TTX. Correspondingly, PEC signal immunoprobes (BiOI@UCNPs-Ab and Ab2-Ag2S QDs) with NIR-light response were introduced to construct competitive-type and sandwich-type PEC immunosensors for OA and TTX, respectively. Under optimal conditions, the linear ranges for TTX and OA were 0.001-100 and 0.001-80 ng mL-1, respectively, and the detection limits were 5 and 7 pg mL-1, respectively. The proposed sensor was successfully used for the simultaneous analysis of TTX and OA in Nassariidae samples.

A microfluidic immunosensor based on magnetic separation for rapid detection of okadaic acid in marine shellfish

Okadaic acid (OA) is a marine biotoxin that accumulates in seafood and can cause diarrheic shellfish poisoning if consumed. Accordingly, many countries have established regulatory limits for the content of OA in shellfish. At present, methods used for the detection of marine toxins are time-consuming and labor-intensive. In order to realize rapid, simple, and accurate detection of OA, we developed a novel microfluidic immunosensor based on magnetic beads modified with a highly specific and sensitive monoclonal antibody (mAb) against OA that is used in conjunction with smartphone imaging to realize the rapid detection of OA in shellfish. The method achieves on-site detection results within 1 h with an IC₅₀ value of 3.30 ng/mL for OA and a limit of detection (LOD) of 0.49 ng/mL. In addition, the analysis of real samples showed that the recoveries for spiked shellfish samples ranged from 84.91% to 95.18%, and the results were confirmed by indirect competitive enzyme-linked immunosorbent assay (icELISA), indicating that the method has good accuracy and precision. Furthermore, the results are reported in a specially designed smartphone app. The microfluidic immunosensor has the advantages of simple operation, rapid detection, and high sensitivity, providing a reliable technical solution for detecting OA residues in shellfish.

Sensitive detection of the okadaic acid marine toxin in shellfish by Au@Pt NPs/horseradish peroxidase dual catalysis immunoassay

Based on the catalysis enhancement strategy of Au@Pt nanoparticles (Au@Pt NPs) and horseradish peroxidase (HRP) related to the TMB-H₂O₂ indicator, a sensitive colorimetric immunoassay was established for trace okadaic acid (OA) detection. The anti-OA monoclonal antibody (McAb) with a high K_aff constant was prepared and modified on Au@Pt NPs. Through grafting the HRP conjugated goat anti-mouse IgG antibody (IgG) on Au@Pt/McAb, bifunctional composites with Au@Pt-Ab and HRP were prepared and adopted. Characteristics including morphology, specificity and catalytic performance were evaluated. Under the optimal conditions, the sensitivity of the resultant enzyme immunoassay was significantly improved, and a low limit of detection (LOD) of OA was achieved at 0.04 ng mL^-1 (equivalent to 0.6 μg kg^-1 in mussel tissue), which was better than that of most HRP or Au/HRP enzyme-linked immunosorbent assays. When applied to fortified shellfish samples (e.g. oysters, mussels and clams), the recoveries ranging from 98.3 ± 2.3% to 106.0 ± 9.0% were acceptable and comparable with those of the LC-MS method. Acceptable precision was achieved with a variation coefficient (CV) of 2.3-8.4%. The method provides a promising alternative for the highly sensitive detection of the OA marine toxin at trace levels.