Selective analysis of the okadaic acid group in shellfish samples using fluorous derivatization coupled with liquid chromatography-tandem mass spectrometry

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.

Efficient capture and highly sensitive analysis of okadaic acid by three-dimensional covalent organic frameworks with hydroxyl surface engineering

A novel three-dimensional covalent organic framework (3D-COF) with content-tunable and active hydroxyl groups (OH) on the pore walls was developed and adopted for the high-performance capture of okadaic acid (OA) marine toxins. Using pore-surface engineering, the integration of linear building blocks (4,4'-diamino-3,3'-biphenyldiol, BD(OH)2 and benzidine, BD) with the 3D structural building block backbone (4,4',4'',4'''-methane-tetrayltetrabenzaldehyde, TFPM) was achieved. By adjusting the ratio of BD(OH)2, functional multicomponent-COFs [OH]x-BD-TFPM COFs (X = 25%) were synthesized, which offered ideal access to convert a conventional COF into a functional platform with multiple-mode interactions of hydrophobic and hydrophilic groups for OA capture. [OH]x-BD-TFPM was characterized using SEM, XRD, FT-IR, and BET. The adsorption features and analytical performance of OA were screened and evaluated. Optimization of dispersive solid-phase extraction using [OH]25-BD-TFPM was accomplished, and the method was verified for sensitive quantitative detection of OA in clam and mussel samples. Coupled with LC-MS/MS, the resultant [OH]25-BD-TFPM COF demonstrated the ability to analyze OA, and the limit of detection for OA in shellfish was determined to be 0.005 μg/kg. A significant improvement in trace OA detection was observed compared to previously reported SPE materials without adjustable hydrophilic interactions. The recoveries of OA in the fortified clam and mussel samples were in the ranges of 93.9‒105.1% and 96.7‒110.2%, respectively. This study highlights that OH-group surface engineering in channel walls is a facile and powerful strategy for developing functional 3D-COFs with multiple interactions for high-performance target capture.

Single-stranded DNA binding protein coupled aptasensor with carbon-gold nanoparticle amplification for marine toxins detection assisted by a miniaturized absorbance reader

Okadaic acid (OA), one of the most widely distributed marine toxins worldwide poses a severe threat to human health. Previous sensing methods for OA detection are usually based on antigen-antibody binding mechanism. However, the drawbacks of antibodies especially the enzyme-labeled antibodies, such as the harsh storage condition and high cost, lead to significant challenges to OA detection in biological samples. To overcome these limitations, a single-stranded DNA binding protein (SSB) coupled aptasensor was developed for OA detection. SSB was incubated on the microplate as a substitute for conventional OA-protein conjugations. Carbon-gold nanoparticles were synthesized and labeled with horseradish peroxidase and thiol-modified aptamers to obtain a capture probe (CGNs@HRP-Apt) instead of the enzyme-labeled antibody for signal amplification. OA and SSB competed to bind with limited aptamers on CGNs@HRP-Apt probes followed by colorimetric assay to obtain the optical signals correlated to OA concentration. To achieve on-site detection, a miniaturized and multichannel absorbance reader (Smart-plate reader) was self-designed with full automation for OA detection. Utilizing the SSB coupled aptasensor and the Smart-plate reader, our approach enables cost-effective and on-site OA sensing with a detection range of 2.5-80 ppb and an ultra-low limit of detection of 0.68 ppb. Moreover, novel OA detection kits based on the SSB coupled aptasensor were prepared which can effectively reduce the cost by 15 times lower than that of commercial ELISA kits. Therefore, the developed platform provides a favorable and promising avenue for marine toxin detection in aquaculture and food safety.

Highly sensitive detection of okadaic acid in seawater by magnetic solid-phase extraction based on low-cost metal/nitrogen-doped carbon nanotubes

Algae toxins pose a severe threat to human health all over the world. In this study, magnetic metal/nitrogen-doped carbon nanotubes (M-NCNTs) were facilely synthesized based on one-step carbonization and applied for magnetic solid-phase extraction of okadaic acid (OA) from seawater followed by high performance liquid chromatographic tandem mass spectrometry (HPLC-MS/MS) analyses. Differences in the physicochemical properties of the three prepared materials (Fe/Co/Ni-NCNTs) were investigated to confirm the best extraction material. Among them, Ni-NCNTs demonstrated a faster extraction rate (10 min) and higher adsorption capacity (223.5 mg g^-1), mainly due to the higher specific surface area, suitable pore structure and more abundant pyridine nitrogen ring. Under the optimal conditions, the calibration curve was linear over the range (1.0-800.0 pg mL^-1) with good determination coefficients (R) of 0.9992. The limit of detection (LOD) obtained in multiple replicates was 0.4 pg mL^-1. Three seawater samples were measured by the developed method, 12.3 pg mL^-1 of OA was detected with a satisfying recovery (88.6%-106.7%) and acceptable repeatability (RSD ≤ 4.8%, n = 6). The results demonstrate that M-NCNTs materials are a promising candidate for magnetic solid-phase extraction. Benefiting from its high extraction and interference resistance, the established analytical method is expected to be extended to detect other marine environmental pollutions.

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.

Superhydrophobic fluorinated microspheres for fluorous affinity chromatography

Fluorous affinity chromatography has received growing attention in separation and purification of fluoro compounds, but the wettability of the fluorinated stationary phases is seldom noticed. Here, we construct a series of micro-sized fluorine-containing microspheres by solvothermal precipitation polymerization. The fluorinated microspheres could be obtained with narrow size distribution at even high monomer loading of 15 wt%. Through alternating fluoro monomer, both the particle size and the wettability of the microsphere array could be tuned. Among them, the poly(divinylbenzene -dodecafluoroheptyl methacrylate), P(DVB-DFHMA), microsphere (6.1 μm) arrays displays superhydrophobicity with 153.2° water contact angle. The P(DVB-DFHMA) fluorinated microspheres (7.58% fluorine content) can be packed into steel-less columns as stationary phase for high-performance liquid chromatography. The retention mechanism of the fluorinated column is proven to be the specific fluorine-fluorine interaction. Compared to the commercial C18 silica column, the fluorinated column can completely separate fluorine-containing compounds under high water content mobile phase, including small fluoro molecules and fluoro macromolecules, at much lower back pressure by fluorous affinity.

Quantification of Casein in Baked Food Products by Selective Analysis of Phosphorylated Peptides Using Fluorous Derivatization with Liquid Chromatography-Tandem Mass Spectrometry Method

Casein is one of the allergen proteins present in milk. Therefore, a quantification method for the selective analysis of casein using fluorous derivatization with LC-tandem mass spectrometry (LC-MS/MS) was developed. After two allergen proteins (α_S1-casein and β-casein) extracted from baked sugar cookies were tryptic digested, the obtained phosphorylated peptides were selectively derivatized by β-elimination with Ba(NO₃)₂ under basic condition and Michael addition with perfluoroalkylthiol (1H,1H,2H,2H-perfluorooctanethiol, PFOT). In this study, YKVPQLEIVPN(pSer)AQQR (104-119 fragment from α_S1-casein) and FQ(pSer)EEQQQTEDELQDK (33-48 fragment from β-casein) obtained by tryptic digestion were selected as target peptides. The phosphorylated serine residue in each peptide was converted to a perfluoroalkyl group by derivatization. The obtained fluorous-derivatized peptides were analyzed by LC-MS/MS, to which a fluorous LC column was connected. Therefore, it was possible to analyze casein without being affected by the matrix components in the baked food sample. When the present method was applied to cookies with arbitrary amounts of α_S1-casein and β-casein, the obtained quantification values were in good agreement with the arbitrary amounts spiked. The quantification limits of α_S1- and β-casein in cookie analysis were 246 and 152 ng/g, respectively. Hence, this method can be used to analyze trace amounts of allergen proteins present in the baked food.

A robust colorimetric aptasensor for the label-free detection of marine toxins based on tyrosine-capped gold nanoparticles

Tyrosine-capped AuNPs decrease the interaction between targets and AuNPs, presenting better robustness than traditional Cit-AuNPs. Utilizing the superior features, a label-free aptasensor is developed for marine toxin detection for the first time.