NEAs are fabricated by anodizing RF-sputter deposited Al films on a substrate-of-choice, resulting in billions/cm2 tailored with controllable pore diameters. Each nanopore is partially filled via electrochemical plating with an appropriate working electrode material, such as Au, Pt or Ni. A thin W film is beneath the nanopore template for both processing assistance and to provide a backside electrical contact in parallel to every nanoelectrode. In the case that the NEA is recessed within the porous alumina, a thin conducting film can be added to the topside to function as a counter/pseudo-reference electrode. In some situations, the counter electrode can also serve as an ion collector, preconcentrating ionic species near the pore openings prior to beginning an electrochemical measurement.
Using such NEA sensors, we will demonstrate trace detection levels for both Cl and I ionic species in water, and compare the limits-of-detection with conventional electrodes.
In addition, we explore the possibility of using NEA sensors to detect halogens in the gas phase. By manipulating the NEA structure, the monitoring capability may be extended to gas phase detection of Cl2 or I2. Such gas-phase detection is possible by utilizing the natural humidity in air condensing on the nanopore walls above the recessed working electrodes. Indirect sensing of such gases occurs after they dissolve in the pore water to form electrochemically detectable halogen species. Notably, the electrochemical measurement is achieved with simple, condensed humidity as the sole ionically conducting phase, without the need for a supporting electrolyte. This is because the high resistivity of water† is mitigated by the short ionic path length: the net solution resistance is small because of the sub-micron distance separating the NEA electrodes from the counter/pseudo-reference electrode sputtered upon the surface of the opposite (open) end of the pore.
†The conductivity of humidity levels we consider in this study range from 10-6 S/m (humidity from DI water) to 10-3-10-4 S/m (conductivity range of natural humidity across the USA as monitored by the National Atmospheric Deposition Program’s National Trend Network).
This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.