Sensors zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA and Acruarors B, 10 (1993) 149-153 149 Conductivity of polystyrene film upon exposure to nitrogen dioxide: a novel NO2 sensor W. H. Christensena*, D. N. Sinha”** and S. F. Agnewb “ Electronic Materials and Device Research MEE-11, Mail Stop 0429, bInorganic and Structural Chemistry Group INC-14, Mail Stop C345, Los Alamos National Laboratory, Los Alamos, NM 87545 (USA) (Received January 6, 1992; accepted July 27, 1992) Abstract A novel NO2 sensing device has been developed based on a reversible change in d.c. conductivity in an otherwise insulating film of polystyrene upon exposure to NO, gas. The device consists of a polystyrene film-coated interdigitated electrode, for which several orders of magnitude increase in conductivity occurs when an NO,/N, mixture, I:10 v/v, comesinto contact with the film. This rapid (2 min) change in conductivity is reversible and is not associated with any irreversible decomposition of the film. The conductivity of the film is believed to be due to the self-ionization of N,O,, the form of NO, within the film, to NOfN03-, with the appropriate anodic and cathodic electrochemistry; it is therefore extremely selective for NOz. Intrduction Nitrogen dioxide is a very common pollutant in urban smog, and is a rigidly controlled substance in the industrial workplace as well. Many sensors for detec- tion of NO, vapor have appeared in the marketplace, with most commonly used sensors (e.g., Interscan, Inc. or Mine Safety Appliances, Inc. NOZ sensors) being based on oxidation of NOz to nitric acid, and determin- ing a change in acidity or conductivity of the resultant solution or exposed indicator. Another type of commer- cial NO, sensor (manufactured by Thermoelectron, Inc.) relies on the luminescence of NO in a discharge to measure all nitrogen oxides. Other sensors [I, 21 that have been developed, but not commercially exploited, depend on the interaction of NO* with a heated semi- conductor surface, such as InSnO, or SnO,, thereby modifying the current-voltage characteristic of that semiconductor. More recently, phthalocyanine [3] and polypyrrole [4] films have also been used to sense NO2 effectively at room temperature, albeit non-selectively. We have discovered a novel NO2 sensor that operates on an entirely new principle, one that has not as yet been exploited in any existing devices. The sensor de- scribed here uses the self-ionization of N,O, within a polystyrene film to transport charge between electrodes where the oxidation and reduction of the N204 is *Present address: Calscience Environmental Laboratories, Inc., 111631 Seaboard Circle, Stanton, CA 90680, USA. **Author to whom correspondence should be addressed. 09254005/93/$6.00 proceeding. The unexpected appearance of ionic con- ductivity in a non-polar matrix occurs because of the unusual nature of N204 chemistry in the condensed phase, since it is non-polar in its molecular form, but undergoes ready self-ionization to NO+NO, -. Thus, the sensor is in principle extremely specific to NO*. In this paper, we report our preliminary observations on this novel NOZ sensor. Experimental Polystyrene films were cast onto interdigitited elec- trodes by simply dipping them into a l-2% solution formed by dissolving small pieces of l/4” polystyrene rod in benzene. The molecular weight of this material was not determined. Polystyrene was added to th& solution until its viscosity allowed a thin film to form on the interdigitated electrode that had been dipped into the solution. A single dip in the solution resulted in a 0.35 pm thick coating, as measured with a profilometer, with subsequent coats adding a corresponding thickness to the layer. Typically, two coatings were used, which resulted in a film approximately 0.85 pm thick. Residual benzene within the film was evident, and as the film dried, in many cases it tended to delaminate from the electrode. Oven drying the film only made the delamina- tion worse, and so all measurements were performed on fresh films within several hours of their being formed. The electrodes were purchased from Microsensor Systems, Inc. (part no. 302) and a diagram of one is @ 1993 - Elsevier Sequoia. All rights reserved