The chemiresistor is a simple type of chemical sensor that relies on the change in conductivity of an organic or inorganic material in response to an analyte. Sensors for organic solvent vapors are required for the detection of leaks, toxic chemicals, explosives, and solvent spills. As part of a system, these sensors need to be highly sensitive to small concentrations of vapors in the ambient air, while consuming minimal power for use in portable or remotely located devices. Such a sensor system must be able to quickly and reproducibly distinguish solvents from the ambient relative humidity - classifying the responses as a particular solvent, relative humidity, a mixture, or an unknown. Sensor arrays with several different chemiresistors can be used to sense and identify a wide variety of solvents.
The objective of this project was to demonstrate that the chemiresistor arrays could be packaged in such a way that they could be submerged in the aqueous phase and measure dissolved volatile organic compounds (VOCs) at low levels.
Chemiresistors are fabricated by selecting polymers closely matched to the chemical to be detected. Polymers for the chemiresistors were selected that showed high sensitivity to chlorinated hydrocarbons such as trichloroethylene (TCE) and dichloroethylene (DCE), aromatics such as xylene, hydrocarbons such as kerosene, BTEX (the aromatic hydrocarbons benzene, toluene, ethylbenzene, and xylene) and iso-octane, and more polar pollutants like methyl tertbutyl ether (MTBE). The polymers were then dissolved, turned into an "ink" by adding up to 40 percent by weight fine carbon powder, and applied to microfabricated platinum (Pt) electrodes by a computer-controlled dispenser. Measurements were taken on a wide range of concentrations and VOCs to establish lower limits of detection and to compare sensor response to headspace vapor versus the immersed response.
The real-time detection of VOCs in water was successfully demonstrated using the chemiresistor technology. The chemiresistors were found to measure the higher concentrations of VOCs quite easily; the rule of thumb is that the best chemiresistor for a particular VOC can measure down to around 0.1 percent of the saturated vapor pressure of the liquid VOC at ambient temperature. Depending on the solubility of the VOC in water, this could be a few ppb in the water. For more soluble VOCs like MTBE, a few ppm would be the limit of detection. The chemiresistor was able to detect contamination in water actually touching the chemiresistor polymer; however, based on slowness of response and long-term instability, it was determined that the chemiresistors work best sensing the headspace vapor of contaminated water. The use of a GORE-TEX® membrane and small dead volume inside the sensor housing means that equilibration between water and the chemiresistors can be fairly rapid. Other compounds detected in water include kerosene, TCE, trans-DCE, and isooctane. This project was completed in FY 2001.
Chemiresistor arrays recently developed at Sandia have the potential to significantly lower the cost of real-time monitoring of VOC contamination in groundwater.