Development of a lifetime-based fiber-optic imaging sensor to study water transport in thin Nafion^TM membranes

Fiber-optic chemical sensors (FOCS's) are useful for making remote, in-situ, and microscale measurements. Many intensity-based and lifetime-based FOCS's have been developed for a wide range of properties and analytes. Most of these sensors give only a single-point measurement however, recently a few intensity based imaging FOCS's have been described. We have developed intensity-based fiber-optic imaging systems to measure the transport of water in thin Nafion^TM membranes and to monitor the development of pH gradients at the surface of an electrode during electrolysis of water. However, intensity-based measurements are difficult to calibrate because of the dependence of luminescence intensity on many interfering factors including dye concentration and varying excitation intensity. As a result, we are developing lifetime-based fiber-optic imaging sensors for a variety of applications. At this point we have measured a lifetime image across a sol-gel crack using a fiber-optic image guide to carry the excitation light to the sample and the resulting luminescence image to an ICCD. Currently, we are testing an oxygen imaging FOCS to capture lifetime-based images of at least two different lifetimes. This paper describes the single-point, lifetime-based sensors we have developed as precursors to fiber-optic imaging chemical sensors, the intensity-based imaging studies of water transport in thin NafionTM membranes and the development of pH gradients at electrode surfaces. It also discusses the instrumental system and methods used to collect lifetime images of sol-gel cracks with a fiber-optic, and the preliminary results of our imaging oxygen sensor.