Nanoporous-carbon adsorbers for chemical microsensors.

Abstract

Chemical microsensors rely on partitioning of airborne chemicals into films to collect and measure trace quantities of hazardous vapors. Polymer sensor coatings used today are typically slow to respond and difficult to apply reproducibly. The objective of this project was to produce a durable sensor coating material based on graphitic nanoporous-carbon (NPC), a new material first studied at Sandia, for collection and detection of volatile organic compounds (VOC), toxic industrial chemicals (TIC), chemical warfare agents (CWA) and nuclear processing precursors (NPP). Preliminary studies using NPC films on exploratory surface-acoustic-wave (SAW) devices and as a µChemLab membrane preconcentrator suggested that NPC may outperform existing, irreproducible coatings for SAW sensor and µChemLab preconcentrator applications. Success of this project will provide a strategic advantage to the development of a robust, manufacturable, highly-sensitive chemical microsensor for public health, industrial, and national security needs. We use pulsed-laser deposition to grow NPC films at room-temperature with negligible residual stress, and hence, can be deposited onto nearly any substrate material to any thickness. Controlled deposition yields reproducible NPC density, morphology, and porosity, without any discernable variation in surface chemistry. NPC coatings > 20 µm thick with density < 5% that of graphite have been demonstrated. NPC can be "doped" with nearly any metal during growth to provide further enhancements in analyte detection and selectivity.Optimized NPC-coated SAW devices were compared directly to commonly-used polymercoated SAWs for sensitivity to a variety of VOC, TIC, CWA and NPP. In every analyte, NPC outperforms each polymer coating by multiple orders-of-magnitude in detection sensitivity, with improvements ranging from 10 3 to 10 8 times greater detection sensitivity! NPC-coated SAW sensors appear capable of detecting most analytes tested to concentrations below parts-perbillion. In addition, the graphitic nature of NPC enables thermal stability > 600 C, several hundred degrees higher than the polymers. This superior thermal stability will enable higherTemperature preconcentrator operation, as well as greatly prolonged device reliability, since polymers tend to degrade with time and repeated thermal cycling.iii Acknowledgements