Using Chemical Kinetic Effects for Understanding and Developing Chemical Sensors

M. C. Wheeler, R. E. Cavicchi, G. E. Poirier, and S. Semancik

Chemical Science and Technology Laboratory

While conductometric gas sensing has been widely studied, the mechanisms are not well understood (particularly for modified oxides). For example, what magnitude of conductance change occurs for a given coverage of adsorbed analyte? We are using specially constructed arrays of the microhotplate devices as surface science tools to answer this and other interesting questions. To this end, we recently constructed an ultra-high vacuum system with a differentially pumped mass spectrometer used for monitoring desorbing species. Rather than using one or several microhotplates as in the previously demonstrated NIST sensing platforms, 340 microhotplates have been replicated into an array of simultaneously controllable elements (Figure 1). This approach is an improvement over using macrosamples because the array provides a large enough surface area to measure the desorption signal while maintaining the rapid thermal characteristics of the microhotplate structures. We have made Temperature Programmed Desorption (TPD) measurements of CO on a Pt covered microarray as a prototype system for developing our experimental techniques, and are working on similar measurements on tin-oxide sensor materials.

Carbon monoxide detection provides an excellent example of how chemical kinetics can impact and even be exploited in chemical sensing. While adsorption kinetics can be related to conductance vs. temperature behavior, many sensing applications involve reactive processes. To increase sensitivity, certain oxide-based sensor films are modified by surface-dispersed catalytic materials such as Pt. Interesting oscillatory behavior has been observed in CO oxidation on Pt, and we have observed similar oscillations in film conductance when sensing CO in air mixtures. When held at a fixed temperature in a CO/air mixture, the film conductance oscillates between a low conductance (oxidized) and a high conductance (reduced) state (Figure 2). The frequency of the oscillations is highly sensitive to CO concentration (Figure 3), and in fact, is a nearly linear function. This highly sensitive relationship may have utility in sensor applications. Because there is much interest in developing sensors based on arrays of devices, it is important to know to what extent "cross-talk", or interactions with proximal devices, might affect measurements by one device. We have used a microsensor array in the oscillatory sensing mode to quantify the range of cross-talk effects between microsensor elements as input to sensor array design and operation.