Layer-By-Layer Biofunctionalization As a Novel Route for High-Sensitivity and High-Specificity Label-Free Affinity Biosensing with Nanostructured Materials

Nanostructured materials hold the promise to revolutionize the label-free biosensing of analytes at concentrations down to those required for clinical applications, beyond what microstructured materials did so far, by leveraging the deeper interaction between materials and analytes with comparable size. On the other hand, when the characteristic dimension of the materials gets down to the nanoscale, the biofunctionalization chemistry commonly used to promote the binding of bioreceptors and enable, in turn, the specific detection of target analytes becomes less effective. Here, we propose an effective and robust route for the surface biofunctionalization of nanostructured materials based on the layer-by-layer electrostatic nano-assembling of oppositely-charged polyelectrolytes, which were engineered with bioreceptors covalently linked to the polymer chain. LbL biofunctionalization allows stability, density, and distribution of bioreceptors available on the transducer surface to be carefully controlled, thus solving most of the drawbacks of the common biofunctionalization chemistry on nanostructured materials. The proof-of-concept demonstration of advantages of LbL biofunctionalization is given on nanostuctured porous silicon (PSi) interferometers, which are biofunctionalized for the affinity detection of streptavidin in raw saliva via LbL nano-assembling of a positively-charged poly(allylamine hydrochloride) (PAH) with a negatively-charged biotinylated poly(methacrylic acid) (b-PMAA). LbL-biofunctionalizion of PSi interferometers results to be very effective and highly robust, enabling high-sensitivity and high-specificity detection of streptavidin with a detection limit of 600 fM. This represents a 100000-fold improvement with respect to control PSi interferometers biofunctionalized using common silane-based chemistry and 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. [1] Layer-by-layer biofunctionalization of nanostructured porous silicon for high-sensitivity and high-selectivity label-free affinity biosensing, S Mariani, V Robbiano, LM Strambini, A Debrassi, G Egri, L Dähne, G. Barillaro, Nature Communications 9 (1), 5256 (2018)