Interferogram Average over Wavelength (IAW) Reflectance Spectroscopy for Nanostructured Porous Silicon Refractive Index Optical (Bio)Sensors: A Novel Ultrasensitive Technique for Label-Free Biosensing
Authors: Mariani Stefano , Strambini Lucanos M , Barillaro Giuseppe 
Published: February 2020

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 and, in turn, poor limit of detection [1].
Here we report on development, characterization, and application (to both refractometry and biosensing) of a novel ultrasensitive technique for the label-free discrimination of either bulk or surface refractive index changes (namely, Interferogram Average over Wavelength – IAW –reflectance spectroscopy) using nanostructured PSi interferometer. As to refractometric applications, a minimum bulk refraction index variation of 10-6 RIU was experimentally measured using NaCl aqueous solutions, with a theoretical limit of detection of 10-8RIU. As to biosensing applications, a minimum concentration of TNFα, a protein biomarker of inflammation and sepsis, at concentration of 3.0 nM was experimentally monitored, with high selectivity and limit of detection of 200 pM. Both these results represent a 1000-fold improvement with respect to the commonly used fast Fourier Transform reflectance spectroscopy for PSi interferometers.
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 target analyte within the nanopores of the PSi interferometer, from a reference reflection spectrum. This technique allows take simultaneously into account all changes occurring on the reflection spectrum of the interferometer, both in terms of intensity, phase, and frequency variation of the Fabry−Perot fringes that are originated by constructive−destructive interference of light within the interferometer.
As proof-of-concept for refractometric applications, aqueous saline solutions with different NaCl concentrations in the range 0.001%−10% were infiltrated inside the nanopores of the PSi interferometer and the corresponding IAW signals calculated from experimental reflection spectra measured. Refractive index variations for the different NaCl concentrations were estimated thorough the use of Lorentz-Lorenz equation. A sigmoidal trend encompassing the whole range of NaCl concentrations tested was obtained, with a clear discrimination of a minimum bulk refractive index variation of 10-6 RIU with signal-to-noise ratio (S/N) of about 5. The results were statistically validated over 4 replica of the same experiment and are in agreements with the theoretical calculations. As a benchmark, the same experimental set of reflection spectra was used to calculate effective optical thickness (EOT) values using FFT reflection spectroscopy, which is the technique commonly empoyed for PSi interferometers. Discrimination of a minimum bulk refractive index variation of 10-3RIU was achieved using EOT values.
As proof-of-concept for biosensing applications, the proposed IAW reflectance spectroscopy was used to demonstrate reliable detection of BSA unspecific adsorption in PSi interferometers at concentrations in the range from 150 pM to 15 μM, down to 3 orders of magnitude lower than those targeted in the current literature using a PSi interferometers operating in label-free mode without any amplification strategy. Good sample-to-sample reproducibility over the whole range of tested concentrations (%CV = 16%) and good signal-to-noise ratio also at the lowest tested concentration (S/N ≈ 4.6 at 150 pM) were achieved, with a detection limit (DL) of 20 pM (20 femtomoles, 1 mL) [2]. Further, the IAW reflectance spectroscopy was applied to a “real” biosensor through the development of a label-free PSi interferometric aptasensor able to specifically detect tumor necrosis factor alpha (TNFα, a protein biomarker of inflammation and sepsis). A lowest concentration down to 3.0 nM with signal-to-noise ratio (S/N) of 10.6 and DL of 200 pM were achieved. This results represent a 10000-fold improvement with respect to direct (i.e., unamplified) label-free PSi biosensors and pushes PSi biosensors close to the most sensitive optical and label-free transduction techniques, e.g., surface plasmon resonance (SPR) for which a lowest DL of 100 pM in label-free aptasensing is reported [3].
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 in small volume of biological samples.

Reference

[1] Vilensky, R., Bercovici, M., Segal, E. Adv. Funct. Mater. 25, 6725–6732 (2015).
[2] Mariani, S., Strambini, L. M., Barillaro, G., Anal. Chem. 88, 8502–8509 (2016).
[3] Mariani, S., Pino, L., Strambini, L. M., Tedeschi, L., Barillaro, G. ACS Sensors (2016). doi:10.1021/acssensors.6b00634.