Porous silicon (PSi) has been recognized as an advantageous material for use in optical biosensors due to its large internal surface area, ability to form multilayer optical structures, and compatibility with standard silicon lithographic techniques. We demonstrate an order of magnitude improvement in small molecule detection sensitivity for on-chip PSi ring resonators and photonic crystal nanobeams compared to the same structures fabricated on silicon-on-insulator wafers. Moreover, we demonstrate that PSi optical structures can be exploited for mobile diagnostics by using a smartphone with no additional functional accessories to detect color changes in the PSi that result from molecule capture.
The formation of resonant photonic structures in porous silicon leverages the benefit of high surface area for improved molecular capture that is characteristic of porous materials with the advantage of high detection sensitivity that is a feature of resonant optical devices. This review provides an overview of the biosensing capabilities of a variety of resonant porous silicon photonic structures including microcavities, Bloch surface waves, ring resonators, and annular Bragg resonators. Detection sensitivities > 1000 nm/RIU are achieved for small molecule detection. The challenge of detecting molecules that approach and exceed the pore diameter is also addressed.
A grating coupled porous silicon (pSi) Bloch Surface Wave (BSW) biosensor capable of supporting a surface mode is demonstrated for the real-time detection of both small and large molecules. In contrast to most pSi based sensor platforms that are unable to perform high sensitivity detection of large molecules that do not infiltrate into the porous matrix, the pSi BSW sensor has more than 15% of the field intensity confined to the surface of the structure, allowing for high sensitivity detection of surface-bound large molecules. Angular interrogated reflectance measurements were carried out to benchmark the performance of the pSi BSW against two common pSi sensor platforms, the waveguide and microcavity, after exposing each sensor to two different small molecules and one large molecule in a flow-cell environment. All of the sensors showed comparable sensitivity towards the detection of the small molecules, but the BSW sensor was clearly superior for detection of the large molecules. The experimental results were found to be in good agreement with simulations based on rigorous coupled wave analysis and the transfer matrix method.