Chemiresistors containing poly(aromatic amine) films have been invented to satisfy a need for simple, inexpensive sensors for real-time measurement of parts-per-million and possibly lower concentrations of acidic and basic gases in air. These sensors could also be used to measure concentrations of acids and bases in aqueous solutions. Likely applications could include measuring concentrations of HF, HCl, HBr, and HI near incinerators in which halogenated hydrocarbons are burned; measuring concentrations of HF in and near semiconductor-processing plants; and measuring concentrations of HCl from rocket-engine exhausts. In these applications, the chemiresistors could supplant larger, heavier, more-expensive instruments that are difficult to use, do not respond in real time, and are often insufficiently sensitive.
Chemiresistors have been described in a number of previous articles in NASA Tech Briefs. A chemiresistor comprises a pair of metal electrodes in contact with a thin, electrically conductive polymeric film on an electrically insulating substrate. The electrical resistivity of the film increases or decreases reversibly when the film is exposed to certain chemicals. In a chemiresistor of the present type, the film is made of a poly(aromatic amine), the resistivity of which decreases upon exposure to acids in both the vapor and aqueous phases. In addition, the color of a poly(aromatic amine) varies with the acidity of its environment; thus, in principle, one could deposit a poly(aromatic amine) on the end of an optical fiber and monitor the concentration of acidic gas by measuring the optical absorption spectrum from the opposite end of the fiber.
Examples of suitable poly(aromatic amine)s include the following polymers in all of the various oxidation states in which they can exist: (1) polyaniline; (2) such derivatives of polyaniline as poly(ethylaniline), poly(butylaniline), and poly(orthotoluidine); and (3) the electroactive copolymers of polyaniline with its derivatives. Protonation (doping) or deprotonation (de-doping) of the - N= sites in these polymers leads to characteristic conductivity-versus-concentration curves that can be calibrated to obtain reliable, instantaneous readings of acid/base concentrations.
A sensor of this type could be made in a variety of ways. For example, a poly(aromatic amine) film could be cast from solution on an insulating substrate to which electrodes were previously attached (see figure). Instead of a pure poly(aromatic amine), the film could consist of a poly(aromatic amine) sensory component dispersed in a nonsensory matrix of poly(methyl methacrylate), poly(vinyl chloride), or polystyrene. An array of sensors, wherein the electrode spacings, film thicknesses, and or doping levels of the sensors would differ, could be fabricated for use quantitating various acids and bases in various concentration ranges.
The doping level of a poly(aromatic amine) film can be adjusted, during fabrication, by any of a variety of chemical or electrochemical treatments. For example, a film initially in the emeraldine base (low-conductivity, blue) state can be protonated to an emeraldine salt (high-conductivity, green) state by equilibrating the film with an aqueous protonic acid. For another example, a polymeric dopant can be incorporated during polymerization or during a post-polymerization treatment.
Of the poly(aromatic amine)s, polyaniline is especially attractive, not only because it has suitable physical and chemical properties, but also because its monomer is relatively inexpensive; thus, it should be possible to manufacture relatively inexpensive (even disposable) sensors for deployment over large areas and for personal monitoring badges.
This work was done by Guang-Way Jang of Gumbs Associates, Inc., for Stennis Space Center.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Ronald W. Gumbs
Gumbs Associates, Inc.
11 Harts Lane
East Brunswick, NJ 08816
Refer to SSC-00069