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<sup>+</sup> and K<sup>+</sup> ions in water as the model) onto a negatively charged nanostructured surface (e.g., thermally growth SiO<sub>2</sub>)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 m<sup>2</sup> per gram) and low preparation cost (less than $0.01 per 8 in. silicon wafer), though they have shown poor resolution (<i>R</i>) and detection limit (DL) (on the order of 10<sup>–4</sup>–10<sup>–5</sup> RIU) compared to other plasmonic and photonic platforms (<i>R</i> and DL on the order of 10<sup>–7</sup>–10<sup>–8</sup> 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<sup>–4</sup> 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<sup>–4</sup> 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<sup>–7</sup> 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.