Abstract
Porous silicon has been established as an excellent sensing platform for the optical detection of hazardous chemicals and biomolecular interactions such as DNA hybridization, antigen/antibody binding, and enzymatic reactions. Its porous nature provides a high surface area within a small volume, which can be easily controlled by changing the pore sizes. As the porosity and consequently the refractive index of an etched porous silicon layer depends on the electrochemial etching conditions photonic crystals composed of multilayered porous silicon films with well-resolved and narrow optical reflectivity features can easily be obtained. The prominent optical response of the photonic crystal decreases the detection limit and therefore increases the sensitivity of porous silicon sensors in comparison to sensors utilizing Fabry-Pérot based optical transduction. Development of porous silicon photonic crystal sensors which allow for the detection of analytes by the naked eye using a simple color change or the fabrication of stacked porous silicon photonic crystals showing two distinct optical features which can be utilized for the discrimination of analytes emphasize its high application potential.
Highlights
Optical sensors for the detection and quantification of hazardous chemicals, investigation of biomolecular interactions or studies on cellular systems have been developed for decades and are still a field of extensive research
Sensors 2013, 13 (LSPR) transducers in comparison to interferometric sensors identified the superiority of localized surface plasmon resonance (LSPR) based devices for the analysis of thin analyte and recognition interfaces and emphasized the advantage of interferometric sensors for the investigation of thicker layers [2]
The high potential of porous silicon for fabrication of interferometric sensors originates from its controllable fabrication process resulting in layers with defined porosity, its high surface area, simple surface chemistry, and full compatibility with microprocessing techniques
Summary
Optical sensors for the detection and quantification of hazardous chemicals, investigation of biomolecular interactions or studies on cellular systems have been developed for decades and are still a field of extensive research (reviewed for example in Reference [1]). Changes in the average refractive index of the porous silicon layer caused by infiltration or adsorption of analytes are detected by spectral shifts in the reflectivity spectrum [3,4]. The implementation of more complicated optical structures, e.g., photonic crystals, into porous silicon based sensors improved their sensing capabilities in two ways. The sharp resonant optical response of the photonic crystal makes it much easier to detect small shifts in the reflectivity spectrum leading to detection limits on the femtomolar level. On the other hand photonic crystal sensors allow for the detection of analytes by the naked eye Based on their internal structure photonic crystal solely reflect light at distinct frequencies and appear as a pure color to the eye. Penetration of analytes into the pores cause noticeable color changes in the photonic crystal sensors
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