Fiber optic sensors for long-term unattended monitoring of marine pH and carbon dioxide (CO2) are being developed in this project. The sensing element is a crosslinked polymer that swells with increasing hydrogen ion concentration, causing a reflecting surface to move. The sensor includes two optical fibers, one conducting light to the reflector and the other collecting reflected light. As the reflector moves, the fraction of light collected by the second fiber changes. Therefore, changes in pH can be related to changes in intensity via a calibration curve. The sensing concept can be extended to C02 by placing the pH sensor in contact with a solution of bicarbonate and covering the liquid with a hydrophobic membrane permeable to C02. Because the sensing element is a crosslinked polymer that does not need to be directly photoexcited, the sensors are intrinsically rugged and will hold calibration for extended periods. They can be implemented in the near infrared and take advantage of low cost components developed for fiber optic communications. The sensor design allows complete optical isolation of the sample from the optical measurement. Phase I research objectives are (1) to prepare a fiber optic sensor that responds to pH over the range of 7.5 to 8.5 and can resolve pH differences of 0.01, (2) to experimentally measure the dependence of sensor response on temperature and ionic strength, (3) to develop a highly elastic polymer sensing element that does not show any hysteresis in response to pH, and (4) to evaluate sensor stability. Phase II objectives will be (1) to build a portable instrument for remote, long term unattended pH monitoring, (2) to evaluate long term in-situ response of the sensor system for at least 6 months, and (3) to incorporate the pH sensitive polymer into a sensor that responds to C02 partial pressure.Anticipated Results/Potential Commercial Applications as described by the awardee:The marine research community constitutes a small but significant market for sensors that could be used for remote, unattended monitoring of pH and C02 in seawater. A much larger market for the planned sensors may exist for process control and other applications where ruggedness and calibration stability would give these sensors an advantage over electrochemical sensors.
