A novel approach for ultrasensitive refractive index sensing and label-free optical biosensing with porous silicon interferometers

Porous silicon (PSi) is a promising nanostructured material for optical/interferometric (bio)sensing characterized by a huge specific surface (hundreds of m2/g) and low-cost preparation (~ 0.01$ per 8''silicon wafer) but lacking in sensitivity for refractive index (r.i.) sensing (DLs ~ 10-5 RIU) and biosensing (DLs ~ µM) . In this study we report on the development of a novel interferometric technique (i.e. Interferogram Average over Wavelength reflectance spectroscopy, IAW-RS) for ultrasensitive and label-free (bio)sensing with PSi interferometers. The IAW-RS is based on the calculation of spectral interferograms by subtraction (intensity, wavelength by wavelength) of the reflection spectrum acquired after accumulation/binding of (bio)molecules inside the PSi nanopores from a reference reflection spectrum recorded in buffer. The IAW signal is calculated as average value from the obtained interferograms . Using the IAW-RS in r.i.variation measurements with NaCl/KCl aqueous solutions, a r.i. variation of 10-7 RIU was detected exploiting also the electrical double layer-induced ion surface accumulation of Na+/K+ ions inside the oxidized PSi nanopores. Moreover employing the IAW-RS for the monitoring of BSA nonspecific adsorption inside the oxidized PSi nanopores (model for biosensing) a DL=20 pM was reached. For biosensing applications, an anti-TNF? aptamer was covalently immobilized inside the oxidized PSi nanopores. The IAW-RS pushed the selective TNF? detection down to a DL of 200 pM . The reported results prove that the IAW-RS significantly reduces the DLs (~2-4 orders of magnitude) in r.i. sensing and label-free biosensing with PSi interferometric transducers close to the most sensitive optical and label-free transduction techniques, e.g., Surface Plasmon Resonance.