A University of California, Los Angeles research team led by Robert Candler of the Sensors and Technology Laboratory is investigating the use of porous silicon resonant sensors to detect chemical vapors. That sounds great and all, but why should people care?

The continual progress of technology is obviously extremely beneficial and has solved many of the world’s problems, but not without creating some along the way as well. One such example is the rise of warfare capabilities, or more specifically, chemical warfare. With this ever increasing chemical threat, the need to prevent and defend against such danger is more important than ever.

Chemical warfare often comes in the form of chemical vapors because of their difficulty to detect and ability to rapidly spread through the air. Resonators are one promising technology for detecting chemical vapors due to their high sensitivity and amenability to miniaturization. The resonant frequency of a resonator is determined by its stiffness and mass; thus, when gas molecules become attached to it, the mass will change and so will the resonant frequency. By detecting changes in resonant frequency, the detection of chemical vapors is possible. One continuing goal is to reduce the size of resonators -- the smaller the resonator, the more sensitive it is, which is especially important for detection of low concentrations of chemicals. As resonators are decreased in size, small molecules seem relatively larger, making it possible to detect smaller molecules and easier to detect all molecules in general.

However, there is one obvious problem with scaling down—smaller resonators have less area for molecules to hit. So while a nano-sized resonator is more sensitive to molecules that attach to it, its small area means that there is lower possibility of the molecules hitting the resonator in the first place. No matter how sensitive a resonator is, it won’t detect anything if molecules don’t hit it. The challenge, then, is to essentially find the best of both worlds.

Candler’s research team has taken on this challenge by using porous materials, or more specifically porous silicon, for their resonators. Many current resonators are made of non-porous materials. The advantage of porous materials is that they are sponge-like and can 'soak up' more molecules compared to non-porous materials. The advantage of using porous silicon is that additional coating processes, which can damage fragile resonators, are not necessary. In essence, the same sensitivity can be achieved with larger resonators. This allows for nano-sized sensitivity and, at the same time, a larger area for molecules to hit. Since the effectiveness of a resonant sensor depends on both those factors, Candler’s research team has been able to achieve better results with their porous silicon resonant sensors than current sensors.

The project is still in the research phase and will face many obstacles on its way to commercialization. For example, fabrication will be a key obstacle as decisions will have to be made about which parts and how much of the resonator should be made of porous materials, taking into account both performance and durability. However, if ultimately commercialized, this improved detection of chemical vapors will be very useful for security, military, and environmental purposes. Prime targets for chemical attacks, such as airports, will be better equipped to prevent and respond to such attacks. Military locations and personnel definitely face the possibility of a chemical attack, as well. Detection of chemical vapors is also essential for determining air quality. Another byproduct of technology has been, and always will be, pollution, making it necessary to be able to detect harmful chemicals in the air we breathe in. With technology too often used to harm, it is always comforting to see a case where technology is being used to protect human lives.

