Detection of paralytic shellfish poisoning (PSP) toxins using a colorimetric immunosensing tool for food safety control

From 1985 to 2018, 35 % of shellfish poisoning cases worldwide were due to paralytic shellfish poisoning (PSP). The main representative toxin congener of the PSP toxin family is saxitoxin (STX), a highly toxic neurotoxin capable of blocking nerve impulse transmission and causing death by respiratory arrest, representing a danger to the environment and human health. The presence of PSP toxins in shellfish is currently detected through instrumental analysis methods, which are costly, or more rarely with the mouse bioassay, which is ethically criticized. Therefore, the development of new approaches for their detection is desired. This study presents the development of a colorimetric immunoassay in which STX has been immobilized on magnetic beads (MBs) for the subsequent competition between free STX present in the sample and immobilized STX for the binding site of an anti-STX antibody, using a secondary antibody labeled with horseradish peroxidase to reveal the signal. The immunoassay exhibits a dose-dependent behavior with an IC₅₀ for STX of 10.9 ng mL^-1. The system shows low matrix effects with a loading capacity of 250 mg of shellfish flesh mL^-1 and, therefore, an effective limit of detection (LOD) of 17.2 µg kg⁻¹. The detection of STX and gonyautoxin-2 & gonyautoxin-3 in shellfish samples is also demonstrated. Furthermore, the covalent immobilization of STX on MBs is stable and allows the reusability of the system up to four times. This immunosensing system has been proved to be successful in the screening and quantification of PSP toxin contents in shellfish samples from Spain and Oman.