Porous Silicon-Based Optical Chemical Sensors

Gas and vapor sensing is an important task in the industrial field, both for production and safety, but this item is relevant also in social ground: from soil extraction to terrorism surveillance, there is always a volatile substance to be monitored and quantified. There are many transducing methods to detect gaseous substances and the most common is the so-called electrical nose: an array of gas sensitive semiconductors, such as ZnO, WO3, or other composite oxides, generates a current/voltage change on exposure to pure or mixed substances, and complex software analyzes the sensor response and quantifies the atmosphere composition. Even if it is a very diffuse and powerful device, the electronic nose and, in general, all the electrical gas sensors, suffers from several limitations. First, the oxide semiconductors are sensitive only at high temperature, so these devices are power consuming; moreover, the most severe limit is that electric power and high temperature cannot be used at all for monitoring explosive or flammable substances. From this point of view, optical sensors easily overcome these difficulties: light is contactless and can be generated with very low consumption (it is the case of light emitting diodes [LED]). In addition, optical sensors do not require electric contacts that may cause explosions or fire during sample preparation. Moreover, optical sensing can be remote, assuring total safety for instruments and operators. Therefore, in the presence of a harsh environment, optical sensing is by far the best technique for in situ monitoring of hazardous compounds.