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Research Interests: Optical Sensors for Environmental Monitoring

My research group is dedicated to the design, characterization and implementation of chemical sensor technology. Although most of our efforts focus on biomedical applications of such devices, recent student interests have initiated projects oriented toward environmental measurements. Examples include sensors for monitoring sulfur dioxide in smoke stack emissions and self-powered oxygen sensors suitable for remote, long-term environmental monitoring.

Optical sensors for sulfur dioxide are being developed based on fluorescence quenching of immobilized indicator dyes. The trick is to identify an indicator molecule that is effectively quenched by sulfur dioxide but is not affected by atmospheric oxygen. A series of potential dyes has been surveyed and several candidates have been identified for further evaluation. The corresponding chemical sensors are constructed by immobilizing these dyes in a silicone matrix which is illuminated by a modulated LED source. Luminescence is detected by lock-in amplification to provide a high signal-to-noise ratio. Gaseous samples flow past the indicator layer and changes in luminescence are recorded. The magnitude of fluorescence quenching, corresponding to a decrease in emitted light intensity, is a function of sulfur dioxide level in the sample. Issues of selectivity, response time, sensitivity, and stability are critical design parameters to be characterized and optimized. Eventually, measurements in actual smoke stacks are planned.

We are also developing an optical sensor for oxygen based on fluorescence quenching of an immobilized ruthenium complex. The novel feature of this device is a self-powered light source. In our design, radio-luminescence (RL) supplies the radiant energy to excite the ruthenium complex. The RL source is composed of a beta-emitting nuclide (either tritium or promethium) and a solid-state phosphor (similar to those used in television screens). Energy from a beta particle is absorbed by the phosphor which subsequently releases this energy in the form of light. We have found that this light can be used effectively for building optical sensors. In fact, RL sources are extremely stable and possess essentially zero noise. These features result in oxygen sensors with minimal external power requirements and excellent long-term stability. In addition, the resulting devices are compact and rugged which make them ideally suited for environmental applications. Possible measurements of enviromental interest include oxygen depth profiling in rivers, ponds, lakes, municiple water supplies, etc. and monitoring changes in ambient oxygen levels in either gaseous or aqueous samples as a function of time.