Ultrasensitive lateral flow immunoassay of phycotoxin microcystin-LR in seafood based on magnetic particles and peroxidase signal amplification

Post-Assay Chemical Enhancement for Highly Sensitive Lateral Flow Immunoassays: A Critical Review

Lateral flow immunoassay (LFIA) has found a broad application for testing in point-of-care (POC) settings. LFIA is performed using test strips-fully integrated multimembrane assemblies containing all reagents for assay performance. Migration of liquid sample along the test strip initiates the formation of labeled immunocomplexes, which are detected visually or instrumentally. The tradeoff of LFIA's rapidity and user-friendliness is its relatively low sensitivity (high limit of detection), which restricts its applicability for detecting low-abundant targets. An increase in LFIA's sensitivity has attracted many efforts and is often considered one of the primary directions in developing immunochemical POC assays. Post-assay enhancements based on chemical reactions facilitate high sensitivity. In this critical review, we explain the performance of post-assay chemical enhancements, discuss their advantages, limitations, compared limit of detection (LOD) improvements, and required time for the enhancement procedures. We raise concerns about the performance of enhanced LFIA and discuss the bottlenecks in the existing experiments. Finally, we suggest the experimental workflow for step-by-step development and validation of enhanced LFIA. This review summarizes the state-of-art of LFIA with chemical enhancement, offers ways to overcome existing limitations, and discusses future outlooks for highly sensitive testing in POC conditions.

DNA controllable peroxidase-like activity of Ti3C2 nanosheets for colorimetric detection of microcystin-LR

The peroxidase-like activity of Ti₃C₂ nanosheets (Ti₃C₂ NSs) was evaluated by catalytic oxidation of colorless o-phenylenediamine (OPD) into orange-yellow 2,3-diaminophenazine (DAP) with the aid of H₂O₂. The catalytic behavior followed the typical Michaelis-Menten kinetics. Systematic studies about the catalytic activity of Ti₃C₂ NSs including cytochrome C (Cyt C) electron transfer experiments, radical capture experiments, and fluorescence analysis were conducted, revealing that the catalytic mechanism of Ti₃C₂ NSs was attributed to nanozyme-accelerated electron transfer between substrates and nanozyme-promoted generation of active species (superoxide anion free radical (·O₂^-) and holes (h⁺)). Single-stranded DNA (ssDNA) inhibited the peroxidase-like activity of Ti₃C₂ NSs, and the reduced catalytic activity was ascribed to DNA-hindered substrate accessibility to nanozyme surface. Based on the DNA controllable peroxidase-mimicking activity of Ti₃C₂ NSs, taking microcystin-LR (MC-LR) aptamer as an example, a label-free colorimetric aptasensor was proposed for the sensitive detection of MC-LR. The colorimetric aptasensor showed a wide linear range (0.01-60 ng mL^-1), low limit of detection (6.5 pg mL^-1), and high selectivity. The practicality of the colorimetric aptasensor was demonstrated by detecting different levels of MC-LR in spiked real water samples; satisfactory recoveries (97.2-102.1%) and low relative standard deviations (1.16-3.72%) were obtained.

Сomparison of Au, Au-Pt, and Au-Ag nanoparticles as markers for immunochromatographic determination of nonylphenol

Gold spherical nanoparticles, gold-platinum nanoflowers, and gold-silver nanostars were obtained and compared as labels for immunochromatographic analysis. The nanoparticles were synthesized by chemical reduction from various precursors and then conjugated with staphylococcal protein A to be used in indirect immunochromatographic determination of nonylphenol. The results obtained were evaluated in terms of analytical characteristics and R2 value, as well as the color intensity of the test band. According to the comparison results, it was revealed that the R2 value varied from 0.82 for the gold-silver nanostars to 0.96 for the spherical gold nanoparticles. The working range of determined concentrations was from 2 to 100 μg/mL for unspherical and from 2 to 50 μg/mL – for spherical markers used; the analysis time was 20 min.

Triple immunochromatographic test system for detection of priority aquatic toxins in water and fish

Aquatic toxins are a group of toxic compounds produced by several types of freshwater and marine algae and cyanobacteria and transported through the food chains of water bodies. Potential contamination of aquaculture products (raw and processed fish and seafood) with aquatic toxins requires the use of efficient screening methods for their control. In this study, a multiplex immunochromatographic test system for the simultaneous detection of three aquatic toxins-phycotoxins domoic acid (DA) and okadaic acid (OA), and cyanotoxin microcystin-LR (MC-LR)-is for the first time developed. For this, a competitive indirect immunochromatographic analysis (ICA) based on gold-labeled secondary antibodies was carried out. The LODs/cutoffs/working ranges of the ICA were 0.05/0.3/0.07-0.29, 1.3/100/3.2-58.2, and 0.1/2.0/0.2-1.1 ng/mL for MC-LR, DA, and OA, respectively. The assay duration was 18 min. The developed test system was used to analyze water samples from natural sources (salt and fresh water) and fish samples. For sample preparation of water, simple dilution with a buffer was proposed; for fish samples, methanol-water extraction was utilized. It was demonstrated that the triple LFIA specifically detected target aquatic toxins with recoveries of 85.0-121.5%. The developed multiplex LFIA can be considered a promising analytical solution for the rapid, easy, and sensitive control of water and food safety.

Double Immunochromatographic Test System for Sensitive Detection of Phycotoxins Domoic Acid and Okadaic Acid in Seawater and Seafood

In this investigation, a double immunochromatographic analysis (ICA) of two relevant phycotoxins, domoic acid (DA) and okadaic acid (OA), was developed for the first time. The ICA was performed in the indirect competitive format using gold nanoparticles conjugated with anti-species antibodies. Under optimal conditions, the instrumental detection limits/cutoffs for simultaneous detection of DA and OA were 1.2/100 and 0.1/2.5 ng/mL, respectively. The time of the assay was 18 min. The ICA was applied to test seawater and a large panel of seafood, including mussels, tiger shrimps, octopuses, whelks, crabs, and scallops. The proposed simple sample preparation method for seafood takes only 20 min. For seawater, a dilution by buffer was implemented. The assay recoveries varied from 80.8% to 124.5%. The competitive potential of the proposed technique as a tool to control natural water and seafood samples is determined by its simplicity, rapidity, and sensitivity.

Rapid detection of phycotoxin domoic acid in seawater and seafood based on the developed lateral flow immunoassay

A lateral flow immunoassay (LFIA) of phycotoxin domoic acid (DA) contaminating seawater and marine organisms was developed in this investigation. Nine clones of monoclonal antibodies against DA were produced and characterized. The test system was implemented in the indirect competitive format, where gold nanoparticles as a marker were conjugated with secondary antibodies. The developed test system allows for the detection of DA with a cutoff of 60 ng mL^-1 and an instrumental detection limit of 1.4 ng mL^-1 within 15 min. The LFIA was applied to detect DA in seawater, mussels, shrimps, and octopuses. A simple method of seafood sample preparation was proposed. The entire analytical cycle, from obtaining a sample to the estimation of final results, takes only 30 min. The assay recoveries ranged from 88.5% to 124%. The developed analytical method is a promising solution for rapid on-site monitoring of marine toxicants in water and food throughout the farm-to-fork chain.