Flexible PDMS Foams Decorated with Multi-Walled CNTs for Unprecedented Detection of Ultralow Strain and Pressure Coupled with Large Working Range

A route for surface biofunctionalization of nanostructured materials based on the Layer-by-Layer (LbL) electrostatic nano-assembling of oppositely-charged polyelectrolytes covalently linked to the bioreceptors is presented. LbL biofunctionalization allows stability, density, and distribution of bioreceptors available at the transducer surface to be carefully controlled, thus effectively tackling most of the drawbacks of the common biofunctionalization chemistry on nanostructured materials. As a proof-of-concept, nanostuctured porous silicon (PSi) interferometers are biofunctionalized for streptavidin detection in raw saliva via LbL nano-assembling of a poly(allylamine hydrochloride), PAH, and biotinylated poly(methacrylic acid), b-PMAA. High-sensitivity and high-specificity streptavidin detection was achieved, with a detection limit of 600 fM. This represents a 300-fold improvement with respect to best PSi label-free biosensors reported in the current literature, pushing PSi optical biosensors to performance comparable to those of best label-free plasmonic and photonic platforms.