Abstract
Botulinum is a deadly bacterial toxin that causes neuroparalytic disease. However, appropriate tools to detect trace toxic proteins are scarce. This study presents a bead-based diffusometric technique for the rapid, simple, and quantitative detection of biological toxins. Functionalized particles called nano-immunosensors were fabricated by forming sandwiched immunocomplexes comprising Au nanoparticles (AuNPs), toxic proteins, and antibodies on fluorescent probe particles. Particle diffusivity tended to decline with increasing concentration of the target proteins. Calibration curves of purified botulinum toxins (0.01-500 ng/mL) were obtained from whole milk and bovine serum, and results suggested that measurement was independent of the background matrix. The activity of botulinum toxin was evaluated by coating synaptosomal-associated protein 25 (SNAP-25) on fluorescent probe particles. AuNP-conjugated antibodies attached to the probe particles when SNAP-25 proteins were cleaved by active botulinum. Thus, toxicity could be detected from slight changes in diffusivity. A short measurement time of 2 min and a limit of detection of 10 pg/mL were achieved. The nano-immunosensors demonstrated rapid biosensing capability and met the demands of onsite screening for food safety, medical instrument hygiene, and cosmetic surgery products.