Chemical Sensing with Microarrays

A means of detecting a broad range of toxic and hazardous chemicals with parts per trillion (10 -9 ) to parts per million (10 -6 ) sensitivity is required in a variety of applications. Detection approaches are needed that will not be compromised by variable concentrations of environmental interferences. Recognition that a material of interest is present above a particular threshold level must be accomplished quickly (in seconds), and with minimum or no false negatives or false positives. Research efforts are focused in three areas, transduction materials development, signal response processing, and device design. NIST has developed capabilities to test these devices at trace concentration ranges. These are beginning to push into the part per trillion levels. Response algorithms have been developed that successfully identify analytes in the presence of interfering background levels up to 10,000 higher in concentration. Vapors from the following have been used as interfering species to date, Diesel fuel, gasoline, WD-40, Orange Fantastic, Clorox bleach, Windex, ZEP Perimeter Floor Stripper, and water vapor. The approach is to couple chemically functional thin films with MEMS structures to develop microsensor arrays for real time chemical sensing in areas as diverse as automotive exhaust gas speciation, planetary exploration, and detection of toxic chemicals. Chemical microsensors are based on NIST-developed, and patented, ‘microhotplate' arrays formed by silicon micromachining, and similar devices such as differential scanning calorimeters. The sensors are fabricated by depositing on MEMS structures, thin films of materials that undergo or produce effects that can be measured electronically. Arrays of such devices provide analytical data that permit identification and quantification of chemical species in gaseous mixtures. High sensitivity for the solid-state devices (demonstrated at 10 nanomole/mole for both methanol in air and sarin in air) has been produced using nano-crystalline sensing materials. Long test runs (10-100 hours) have also demonstrated the operational stability of the technology. New microheater-based components such as preconcentrators and separators are being developed for integration with the miniature sensors, to further enhance sensitivity and selectivity.

• Demonstrated detection of organic phosphorus and sulfur compounds at the ppb level and above in the presence of interfering species whose concentration are significantly higher than analyte concentrations.
• Integrated nanostructured semi-conducting oxide micro-shells with MEMS micro-hotplate platforms and demonstrated enhance detection sensitivity.

• Extend sensing sensitivity to the picogram/gram range.