Interferogram Average over Wavelength Reflectance Spectroscopy: A Novel Interferometric Technique For Ultrasensitive Label-Free Biosensing with Porous Silicon

Porous silicon (PSi) is a nanostructured material increasingly exploited for both refractometric and (bio)sensing applications, though currently suffering of restricted real applications due to insufficient sensitivity [1]. Here we report on development, characterization, and application (to both refractometry and biosensing) of a novel ultrasensitive technique for label-free discrimination of bulk and surface refractive index changes (namely, Interferogram Average over Wavelength –IAW– reflectance spectroscopy) using PSi interferometer. The IAW reflectance spectroscopy relies on the calculation of the average value over wavelength of spectral interferograms, namely IAW value. Interferograms are calculated by subtraction (intensity, wavelength by wavelength) of the reflection spectrum, acquired after infiltration of the analyte within the nanopores of the PSi interferometer, from a reference reflection spectrum. As to refractometric applications, a minimum bulk refraction index variation of 10-6 RIU was experimentally measured using NaCl aqueous solutions, with a theoretical detection limit (DL) of 10-8 RIU. As proof-of-concept for label-free biosensing applications, the IAW spectroscopy was used to demonstrate detection of BSA adsorption on PSi surface down to a DL of 20 pM [1] while for a “real” biosensing application, concentrations of TNFα (a protein biomarker of inflammation and sepsis) were experimentally monitored through an aptasensor, reaching a DL of 200 pM. Both these results represent a 1000-fold improvement with respect to the commonly used FFT reflectance spectroscopy [2]. In conclusion, the IAW reflectance spectroscopy envisages bringing PSi optical (bio)sensors at the forefront of ultrasensitive label-free biosensing techniques, with application for point-of-care clinical analysis where low analyte concentrations are required to be detected. Reference [1] Mariani, S., Strambini, L. M., Barillaro, G., Anal. Chem. 88, 8502–8509 (2016). [2] Mariani, S., Pino, L., Strambini, L. M., Tedeschi, L., Barillaro, G., ACS Sens., 1 (12), 1471–1479 (2016).