Interferogram average over wavelength spectroscopy: An ultrasensitive technique for biosensing with porous silicon interferometers

Here we report on a high-sensitivity interferometric technique, namely Interferogram Averaged over Wavelength - IAW-reflectance spectroscopy, enabling sub-nanomolar detection (both nonspecific and specific) of proteins to be accomplished in direct (without any amplification) label-free mode using porous silicon (PSi) interferometers. The IAW reflectance spectroscopy is based on both calculation and elaboration of spectral interferograms. Interferograms are obtained by subtracting (intensity, wavelength by wavelength) reflectance spectra acquired on PSi interferometers in buffer solution after protein injection (at different concentrations) from a reference reflectance spectrum acquired in buffer solution before protein injection. The output signal, namely IAW value, is eventually obtained as the average value, calculated over a given wavelength interval, of each interferogram. Using the IAW technique nonspecific adsorption of Bovine Serum Albumin (BSA) down to 0.15 nM (DL=20 pM) and specific detection of Tumor Necrosis Factor (TNFα) down to 3 nM (DL=0.2 nM) was reliably achieved in direct label-free mode using PSi interferometers. Both the results represent a 10000-fold reduction in DL with respect to the current literature on the use of PSi for direct label-free measurements.