Simultaneous Monitoring of Ammonia and Moisture Using a Single Fiber Optoelectrode as a Transducer

The lack of selectivity is one of the problems of chemical sensor technologies in real world applications. In an effort to develop chemical sensors of improved selectivity, we integrated electrochemistry and optical spectroscopy into one single transducer. An electrically conductive polyaniline (PANi) membrane has been coated on the end of an optical fiber to form a fiber optoelectrode. Two electrodes were glued on the side of the optical fiber for measuring the membrane's electrical resistance. At the same time, a light beam was delivered to the membrane coated on the end of the optical fiber using a 1 x 2 optical fiber coupler. The light reflected back from the PANi membrane was delivered with the second arm of the optical fiber coupler to an optical fiber compatible spectrometer. This makes it possible to simultaneously monitor optical spectroscopic and electrical resistance signals from a single fiber optoelectrode transducer. The responses of the fiber optoelectrode to trace ammonia and water vapor in air samples have been investigated. It was found that the electrical resistance of the fiber optoelectrode changes when exposed to ammonia or water vapor. However, the fiber optoelectrode showed optical spectrometric response to trace ammonia in air, but did not show optical spectrometric response to water vapor in air in the tested concentration range (relative humidity from <1% to 83%). The nature of electrical and optical spectrometric responses of this fiber optoelectrode gives an opportunity to design a sensor for simultaneously monitoring both trace ammonia and water vapor in air.