Porous silicon for ultrasensitive and label-free interferometric (bio)sensing

Porous silicon (PSi) is a nanostructured and interferometric optical transducer for label-free (bio)sensing really appealing for ease of fabrication and low costs, though currently suffering of poor sensitivity (with micromolar DLs in biosensing) commonly ascribed to hindered diffusion of biomolecules (tens of KDa) inside the columnar nanometric pores. In this study the development of a novel interferometric technique (i.e. Interferogram Average over Wavelength - IAW - reflectance spectroscopy) is reported for ultrasensitive and label-free (bio)sensing using nanostructured PSi interferometers. The IAW reflectance spectroscopy is based on the calculation of spectral interferograms by subtraction (intensity, wavelength by wavelength) of the reflection spectrum acquired after diffusion of the (bio)molecules of interest inside the PSi nanopores from a reference reflection spectrum recorded in buffer. The IAW signal, related to the analyte concentration, is calculated as average value from the obtained interferograms. As case of study of refractometric applications, a minimum refractive index variation of 10-7 RIU was measured through IAW reflectance spectroscopy using NaCl aqueous solutions, exploiting Electrical Double Layer-Induced Ion Surface Accumulationof Na+ ions onto the oxidized PSi surface. As proof of concept of biosensing applications, an assay making use of an anti-TNF? aptamer to specifically target TNF?, a peptidic biomarker of sepsis, was optimized onto the PSi surface. The IAW reflectance spectroscopy allowed a minimum concentration of 3.0 nM to be selectively measured, with a theoretical DL of 200 pM. Both the presented results, obtained with IAW reflectance spectroscopy, represent at least a 1000-fold improvement compared to the Fast Fourier Transform (FFT) reflectance spectroscopy commonly used for PSi interferometers and with which only ~ 10-4 RIU variations (in bulk refractive index sensing) and micromolar concentration (in biosensing) can be detected.