Nanoarray Device for Detection of Gas Phase I2

Abstract

We report on a nanoelectrode array sensor for the detection of I2 (iodine gas) at levels as low as near 0,02 ppm within tens of seconds of exposure. The sensor is constructed on a free-standing anodic aluminum oxide wafer with nanopores of 120 nm diameter and 50 um length. On each wafer surface, continuous metal films of tens of nm thickness are deposited: on one surface, a gold film is deposited such that it covers the nanopores, and, on the opposite surface, a platinum film is deposited such that the nanopores remain open and their volumes in contact with the environment. The gold film serves as the working electrode and the platinum film serves as the auxiliary and pseudo-reference electrode. The nanopores are filled with a pH 9 buffer aqueous solution to create an array of electrochemical cells in parallel, and ionic conductivity is maintained for environments with relative humidity levels greater than about 30%. The detection mechanism is the electrochemical oxidation of anionic species of I2 that form from the dissolution and subsequent hydrolysis of gas phase I2 in the pH 9 buffer electrolyte: the gas phase I2 dissolves in the buffer in proportion to Henry's Law constant and then the dissolved I2 is hydrolyzed to several species, including the anionic species iodide and tri-iodide. The detection of I2 is achieved by measuring oxidizing current during the cyclic voltammetry technique bounded between -0,2 and 0,5 V vs the Pt pseudoreference electrode. The hydrolysis mechanism of I2 in the pH 9 buffer enables the buffer to function as a concentrator for the anionic species of I2. When the sensor is exposed to a gas stream containing I2, practically continuous accumulation of the electrochemically detectable anionic species of I2 is possible, with a saturation point where the pH 9 buffer contains > 100 ppm iodide anion. For context, the sensor can detect iodide anions in pH 9 buffer at levels as low as 0,01 ppm. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.