Femtogram-level detection of Clostridium botulinum neurotoxin type A by sandwich immunoassay using nanoporous substrate and ultra-bright fluorescent suprananoparticles

► Engineered substrates provide constructive interference at the surface. ► Dye-doped nanoparticle labeled antibody provides high fluorescence per antibody. ► Nanoparticle labeled antibody with NPO substrates provides diffusive scattering. ► Substrates and nanoparticles lead to femtogram-level detection of BoNT/A in an hour.