Massively Parallel Chemiresistor Arrays for Volatile Organic Vapor Detection
|Chemiresistor on Interdigitated Electrode Array|
Motivation and Operational Concept
The detection of VOCs is a key national security concern: detection of the proliferation of weapons of mass destruction (WMD). Particular combinations of chemical precursors, solvents, and byproducts signal the production of nuclear, chemical, and biological WMD. Massively parallel chemiresistor arrays are being evaluated for WMD compounds, including various multicomponent VOC mixtures. In addition, environmental monitoring and remediation applications, as well as industrial waste minimization, require the development of effective, inexpensive VOC µ-labs; key species, many of which are potential interferants for WMD detection and must therefore be included in any case, are being addressed.
The low cost of the chemiresistor results from its structure: two interdigitated, lithographically defined metal "combs" on a planar substrate provides a simple means to monitor thin-film resistance (Figure 1). The sensitive materials, instrumentation, and the computational hardware for Pattern Recognition are inexpensive as well. The potential for high sensitivity is a consequence of the ease of adapting the chemiresistor structure. Using modest lithographic technology, our sensor has one hundred interdigitated fingers separated by several-µm gaps. These structures enable measurement of very high resistivity thin films with an ordinary ohmmeter, a key to high sensitivity and low cost. High-resistivity capability also allows the use of thinner films (10s of nm), minimizing response time.
A Sandia-invented pattern-recognition technique based on human visual perception is well suited for handling arbitrary sensor responses (e.g. nonlinear and non-additive responses to mixture component concentrations). Using SAW sensor systems, we recently demonstrated the ability to distinguish and quantify small sets of VOCs and simple VOC mixtures using small sensor arrays; we also demonstrated the ability to automatically, without user threshold selection or adjustment, correctly reject (rather than misidentify) chemicals not in the calibration database. Many popular analysis techniques have difficulties with unexpected chemicals not in the database, and often incorrectly identify them as an expected species. The " massively parallel " array is the key to extending our analysis abilities to handle a wide variety of mixtures over wide concentration ranges.
Sensing films are easily obtained without sacrificing the chemical diversity requisite for effective Pattern Recognition: we are using thin films of many different "cataloged" polymers, inexpensive commercial materials that have been well characterized for their affinity to different classes of materials. Polymer films are rendered electrically conductive by adding a few percent by weight of dispersed conductive particles, such as colloidal silver or graphite, prior to film deposition. Swelling of the film as a consequence of the VOC sorption changes particle-particle spacing, thereby increasing film resistance. For individual-chip chemiresistors, a different polymer is spin-coated onto each chip. For the monolithic n x n device array, we are developing computer controlled deposition techniques to apply each polymer to a selected area using an area-selective dispensing technique.
Last modified: August 23, 1999
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