| Since 1982 when Dodd and Persaud first introduced the transducer-based chemical sensing, also named artificial nose technology, many similar technologies have been developed, including surface acoustic wave devices, carbon black/polymer composite, and fluorescence technology, etc. They detect chemical vapors by monitoring transducer changes that are caused by the interaction between the transducer and the chemical. These technologies have a relatively good sensitivity (from sub part per billion to lower part per million). However, their response is relatively slow (20 seconds to minutes).; Our system uses a dynamic holographic interferometer to observe changes in a transducer's optical and dimensional characteristics. We place a transducer array at one arm of the interferometer. The beam experiences TIR (total internal reflection) at the surface of the transducer array. The reflective beam is subject to a phase shift at the transducer surface because of TIP, which depends on the surface refractive index of the transducer. While the transducer interacts with chemical vapors, the surface adsorbs the molecules and dissolves them. The surface refractive index changes with the dissolution, so the phase shift of the reflective beam also changes, which can be detected by an interferometer. We call this the surface effect. The diffusion is termed "bulk effect". The Surface effect is much faster than bulk effect. Our technology detects the surface effect of transducers in addition to the bulk effect, which separates our technology from other artificial nose technologies. The response time of our system is 0.2 seconds, which is much faster than competing artificial nose technologies.; The fundamental detection limit of the system is the shot noise. The calculation shows the smallest optical path length change we can detect through the dynamic holographic interferometer is 0.7 femtometer, which corresponds to sub ppb (part per billion) level for some chemical vapors. However, our system has not reached that shot noise limit yet. The sensitivity of our system is currently 50 ppb for ethyl alcohol vapor, which is very close to the best sensitivity that other artificial nose technologies ever achieve. Our measurement time is 5 seconds, which is much shorter than other technologies (minutes). |
