DC Pulse-Powered Microdischarges on Planar Electrodes and Their Use in Vapor and Liquid Phase Chemical Sensing in Ambient Air.

Abstract

This report explores DC pulse-powered planar microdischarges and their possible use in handheld chemical sensing systems. For electrode separations >200µm discharges between two electrodes, take the form of a pulsed arc, providing broadband optical emission and select peaks from chemical species present in the glow. The separate temporal characteristics of the line and background spectra present opportunities to improve detection. This concept is evaluated by isopropropanol vapors (100ppm) using emission from CH fragments. For electrode gaps of <75m, the discharge characteristics are intermediate between arc and glow discharges. These hybrid discharges can be tuned to have the required glow or arc like characteristics. They have low broadband emission, greater longevity, and consume less power than the pulsed arc discharges. It is shown that they can detect 17ppm of acetone without pre-concentration in air ambient. The energy of the microdischarges can be controlled by using a three-electrode scheme, which uses separate electrodes for initiation and powering of the discharge. A strategically located high impedance gate electrode that permits the use of pulses <100V between the source and drain. It consumes 22.5J/discharge. Microdischarges employing doped liquid cathodes are used as tunable optical sources to fluoresce biochemicals in microfluidic systems. A stacked microchip that integrates a microfluidic wavelength-tunable optical source, a biochemical sample reservoir and optical filters is used to realize the system. For DNA fluorescence, a BaCl2 solution is used as cathode, to emit light at 454 and 493nm. For tryptophan fluorescence, the cathode contains Pb(NO3)2 solution to provide 280nm emission. A liquid discharge microchip, which uses a wetted porous cathode, is developed for analysis of water samples. It has an electrode gap of 50µm, an active area of 1x1mm2 and can detect 2ppm of aqueous Cr. Finally, a handheld system based on this technology is developed, which employs a customized sensor chip module (which can accommodate swappable gas and liquid discharge chips) and battery-operated circuit, and a commercially available spectrometer coupled to a PDA. Together these results address many of the key challenges in realizing discharge based chemical sensors and demonstrate their feasibility in handheld systems operating in ambient air.