Engineering Porous Silicon Photonic Structures towards Fast and Reliable Optical Biosensing

Abstract

Porous silicon, a nanostructured material formed by electrochemical etching of a silicon substrate, is an ideal candidate for constructing optical biosensors due to its large internal surface area, straightforward fabrication, and tunable optical properties that can be exploited to form numerous photonic structures. A major challenge for porous silicon biosensors is its reactive surface that is highly susceptible to oxidation and corrosion in an aqueous environment. In DNA sensing applications, porous silicon corrosion can mask the DNA binding signal as the dissolution of porous silicon is accelerated by the negative charges on the phosphate backbone of the DNA molecules. This corrosion process can be mitigated through surface passivation of porous silicon and the use of charge neutral peptide nucleic acid molecules as capturing probes for DNA targets. Complete mitigation can be achieved by additionally introducing Mg2+ ions to shield the negative charges on the DNA targets. Another key challenge facing porous silicon biosensors is the inefficient analyte transport through nanopores, which can be as slow as a few molecules per pore per second for molecules whose size approaches that of the pore opening. An open-ended porous silicon membrane is demonstrated to overcome the mass transport challenge by allowing analytes to flow through the pores in microfluidic-based assays. The flow-through approach for biosensing using porous silicon membranes enables a 6-fold improvement in sensor response time compared to closed-ended, flow-over porous silicon sensors when detecting high molecular weight analytes (e.g., streptavidin). For small analytes, little to no sensor performance improvement is observed as the closed-ended porous silicon films do not suffer significant mass transport challenges with these molecules. Experimental results and finite element method simulations also indicate that the flow-through scheme enables more reasonable response times for the detection of dilute analytes and reduces the volume of solution required for analysis. Overall, the improvement of surface stabilization and analyte transport efficiency in porous silicon photonic structures opens the door to a fast and reliable optical biosensing platform.