Optical Sensor Based On Porous Silicon

Porous Si (PSi) is an attractive inorganic porous material due to its high surface area, convenient surface chemistry, and optical signal transduction capability. However, the optical response of porous Si to analyst is not specific. Any kind of analyst entering the pore of porous Si will lead to changes in the average refractive index and produce an optical response. Surface modification of porous Si is an alternative way to achieving high selectivity of this kind of optical sensor. In this paper, porous silicon was prepared by anodic electrochemical etching of bulk silicon. Different surface modification was studied. And gas sensors and biosensors based on PSi were also studied.(1) It is summarized that the current research status of porous silicon. Preparation methods, formation mechanism and optical properties of porous silicon were investigated. The development of porous silicon as sensing material was summarized based on different structures, such as Fabry-Perot, double-layer PSi, rugate filter and microcavity.(2) An ammonia gas sensor chip was prepared by coating an electrochemically-etched porous Si rugate filter with a chitosan film that is crosslinked by glycidoxypropyltrimethoxysilane (GPTMS). The bromothylmol blue (BTB), a pH indicator, was loaded in the film as ammonia-sensing molecules. White light reflected from the porous Si has a narrow bandwidth spectrum with a peak at 610 nm. Monitoring reflective optical intensity at the peak position allows for direct, real-time observation of changes in the concentration of ammonia gas in air samples. The reflective optical intensity decreased linearly with increasing concentrations of ammonia gas over the range of 0-100 ppm. The lowest detection limit was 0.5ppm for ammonia gas. At optimum conditions, the full response time of the ammonia gas sensor was less than 15s. Therefore, the simple sensor design has potential application in miniaturized optical measurement for online ammonia gas detection.(3) A porous silicon sensor array for volatile organic compounds (VOC) detection was constructed by modifying different kinds of room temperature ionic liquids (ILs) on PDMS patterned porous silicon chip. Responding pattern could be generated by exploiting the structural diversity of ILs, which shows different molecular interaction with VOC. Classification or identification of different organic vapours could be achieved by mathematical methods such as principal component analysis (PCA). At optimum conditions, the full response time of the ammonia gas sensor was less than 30s. The recovery time of the ammonia gas sensor was less than 60s.(4) Porous Si/chitosan hydrogel based bio-sensors was constructed. A double optical reflective layer could be formed with the combination of chitosan hydrogel layer and porous silicon layer. The swelling of the chitosan hydrogel layer could induce optical thickness change of the combined layer, which could be monitored by Fourier transform reflective interference spectroscopy(FT-RIFS), pH-responsive phase transition behaviors of the porous Si/chitosan in different ionic strength was studied, and the results demonstrated that the ionic strength could significantly affect the swelling behavior of chitosan hydrogel. We also studied glucose sensing on chitosan hydrogel/porous Si chip. In the presence of glucose oxidase, it was found that a positive correlation between the concentration of the glucose and the shift of the optical thickness in a range of 0.5%-4% This result indicated that the hydrogel/porous silicon chip could be potentially applied in glucose monitor system.