A large array of chemical sensors with responses interpreted using a pattern-recognition algorithm can discriminate a variety of chemicals. Chemiresistors, which change resistance when exposed to a gas or vapor, are low-cost devices that can be easily implemented in a sensor array. These devices consist of interdigitated electrodes coated with a conducting polymer film. We are examining a number of polymer/conductive particle combinations as chemiresistor materials.
Sandia National Laboratories' Microsensors S&T department is developing a flexible chemical sensor "micro-lab" for detecting volatile organic compounds (VOCs) with high chemical selectivity based on large (~ 25 - 50 sensors) arrays of sorption-based resistors (chemiresistors). This µ-lab will comprise a "massively parallel" microsensor array using inexpensive, easily-fabricated polymer-coated planar interdigitated resistors; polymers are rendered conductive through admixture of conductive colloidal particles. Sensor-array resistance variations due to VOC exposure can be monitored inexpensively and in real time. The response of each sensor is rapid, reversible, and repeatable; 10s of polymers with distinct sorptive characteristics are under study. Limits of detection should reach ppb levels. Our approach is analogous to spectroscopy: the rich "spectrum" of resistor responses enables high chemical selectivity through multivariate analyses without separations; sensor redundancy provides improved system robustness.
The chemiresistor is a small, simple, sensitive, rugged microsensor with low power requirements capable of detecting chemical vapors in air, soil, or water. As such, it is ideal for incorporation in a badge-type device to protect first-responders from chemical exposure, or for serving as an environmental monitor for contamination (e.g., municipal water supplies). Chemical detection with the chemiresistor is possible through thin electrically-conductive polymer films that swell in the presence of volatile organic chemicals in the vapor phase; chemical concentration is indicated by the degree of swelling as measured through a change in electrical resistance across the film. Because the swelling of the polymer is reversible, the chemiresistor resets when the chemical disappears from the environment. Therefore, it can be used repeatedly without component replacement. An array of these miniature, low power devices has been used to detect multiple chemical contaminants. The chemiresistor has no moving parts and only requires simple DC circuitry to read electrical resistance.
Because of its small size and low power consumption, the chemiresistor’s principal impact will be in a miniature, unobtrusive chemical sensing badge that can be worn by first responders. Other potential applications include facilities protection and contamination assessment.
Several chemiresistor prototype systems have been developed to address specific applications. The following illustrates the versatility of the technology:
Fig. 1. Four-polymer chemiresistor array and temperature control system in stainless steel geologic probe with GORE-TEX® membrane for monitoring chemical concentrations in groundwater and/or soil.
Fig. 2. Conceptual picture and diagram of a chemiresistor as a first-responder badge for CW agent or TIC detection.
A general rule of thumb is that the chemiresistor can detect as low as 1/1000th of the saturated vapor pressure of a chemical of interest. Detection limits can then be improved up to 100 times through the combination of a preconcentrator with the chemiresistor.
Mid Term (2-3 years)
For additional information or questions, please email us at MGA@sandia.gov