Electrical Double Layer-Induced Ion Surface Accumulation for Ultrasensitive Refractive Index Sensing with Nanostructured Porous Silicon Interferometers.

Herein, we provide the first experimental evidence on the use of electrical double layer (EDL)-induced accumulation of charged ions (using both Na+ and K+ ions in water as the model) onto a negatively charged nanostructured surface (e.g., thermally growth SiO2)-Ion Surface Accumulation, ISA-as a means of improving performance of nanostructured porous silicon (PSi) interferometers for optical refractometric applications. Nanostructured PSi interferometers are very promising optical platforms for refractive index sensing due to PSi huge specific surface (hundreds of m2 per gram) and low preparation cost (less than $0.01 per 8 in. silicon wafer), though they have shown poor resolution ( R) and detection limit (DL) (on the order of 10-4-10-5 RIU) compared to other plasmonic and photonic platforms ( R and DL on the order of 10-7-10-8 RIU). This can be ascribed to both low sensitivity and high noise floor of PSi interferometers when bulk refractive index variation of the solution infiltrating the nanopores either approaches or is below 10-4 RIU. Electrical double layer-induced ion surface accumulation (EDL-ISA) on oxidized PSi interferometers allows the interferometer output signal (spectral interferogram) to be impressively amplified at bulk refractive index variation below 10-4 RIU, increasing, in turn, sensitivity up to 2 orders of magnitude and allowing reliable measurement of refractive index variations to be carried out with both DL and R of 10-7 RIU. This represents a 250-fold-improvement (at least) with respect to the state-of-the-art literature on PSi refractometers and pushes PSi interferometer performance to that of state-of-the-art ultrasensitive photonics/plasmonics refractive index platforms.

Femtomole Detection of Proteins Using a Label-Free Nanostructured Porous Silicon Interferometer for Perspective Ultrasensitive Biosensing.

Nanostructured porous silicon (PS) is a promising material for label-free optical detection of biomolecules, though it currently suffers of limited clinical diagnostic applications due to insufficient sensitivity. In this regard, here we introduce an ultrasensitive and robust signal processing strategy for PS biosensors that relies on the calculation of the average value over wavelength of spectral interferograms, namely IAW, obtained on PS interferometer by subtraction (wavelength by wavelength) of reflection spectra acquired after adsorption of biomolecules inside the nanopores from a reference reflection spectrum recorded in acetate buffer. As a case study, we choose to monitor bovine serum albumin (BSA) unspecific adsorption, which has been often employed in the literature as a model for proof-of-concept studies of perspective biosensing applications. The proposed IAW signal processing strategy enables reliable detection of BSA 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 PS interferometer operating in label-free mode without any amplification strategies, with good sample-to-sample reproducibility over the whole range of tested concentrations (%CV = 16% over 5 replicates) and good signal-to-noise ratio also at the lowest tested concentration (S/N ≈ 4.6 at 150 pM). A detection limit (DL) of 20 pM (20 femtomoles, 1 mL) is estimated from the sigmoidal function best fitting (R(2) = 0.989) IAW experimental data over the whole range of tested concentrations. This is the lowest DL that has been reported in the literature since the seminal paper of Sailor and co-workers (1997) on the use of PS interferometer for biosensing, and lowers of 4 orders of magnitude DL attained with label-free PS interferometers using conventional effective optical thickness (EOT) calculation through reflective interferometric Fourier transform spectroscopy. Accordingly, the IAW signal processing strategy envisage bringing PS optical transduction at the forefront of ultrasensitive label-free biosensing techniques, especially for point-of-care clinical analysis where low analyte concentrations have to be detected in a small amount of biological samples.

Sub-parts per million NO2 chemi-transistor sensors based on composite porous silicon/gold nanostructures prepared by metal-assisted etching.

Surface doping of nano/mesostructured materials with metal nanoparticles to promote and optimize chemi-transistor sensing performance represents the most advanced research trend in the field of solid-state chemical sensing. In spite of the promising results emerging from metal-doping of a number of nanostructured semiconductors, its applicability to silicon-based chemi-transistor sensors has been hindered so far by the difficulties in integrating the composite metal-silicon nanostructures using the complementary metal-oxide-semiconductor (CMOS) technology. Here we propose a facile and effective top-down method for the high-yield fabrication of chemi-transistor sensors making use of composite porous silicon/gold nanostructures (cSiAuNs) acting as sensing gate. In particular, we investigate the integration of cSiAuNs synthesized by metal-assisted etching (MAE), using gold nanoparticles (NPs) as catalyst, in solid-state junction-field-effect transistors (JFETs), aimed at the detection of NO2 down to 100 parts per billion (ppb). The chemi-transistor sensors, namely cSiAuJFETs, are CMOS compatible, operate at room temperature, and are reliable, sensitive, and fully recoverable for the detection of NO2 at concentrations between 100 and 500 ppb, up to 48 h of continuous operation.