Electronic tongue 2 (E-tongue 2) represents the second generation of the apparatus described in "Electronic Tongue for Quantitation of Contaminants in Water" (NPO-30601), NASA Tech Briefs, Vol. 28, No. 2 (February 2004), page 31. To recapitulate: The previously reported apparatus, now retrospectively denoted E-tongue 1, is an assembly of sensors for measuring concentrations of metal ions and possibly other contaminants in water. Potential uses for electronic tongues include monitoring the chemical quality of water in a variety of natural, industrial, and laboratory settings, and detecting micro-organisms indirectly by measuring microbially influenced corrosion.
E-tongue 2 includes a heater, a temperature sensor, an oxidation/reduction (redox) sensor pair, an electrical sensor, an array of eight galvanic cells, and eight ion-specific electrodes. These devices are formed in a substantially planar configuration on an alumina substrate 1 mm thick and 1.3 in. (3.3 cm) in diameter (see Figure 1). The fabrication process includes screen printing of the components of the aforementioned devices on the front side of the substrate, laser drilling of via holes for electrical contacts with wires screen-printed on the back side of the substrate, rendering the vias conductive by drawing screen-print material through them using partial vacuum on one side, adding pins for connection to external circuitry, and firing the printed substrate in air at high temperature.
An important aspect of the planar configuration is that the counter electrodes of all the cells in the array are integral parts of a grounded common conductor on the front surface of the substrate. The use of this common conductor helps to minimize the number of wires and pins and facilitates miniaturization of the array. The common conductor also serves as a ground plane that suppresses electric-field interference among the cells in the array.
The planarity of the array facilitates cleaning of the array by a number of techniques, including wiping, exposure to ultraviolet light, and electrolytic generation of hydrogen and oxygen. The planar configuration also facilitates the reservation of some cells and sensors as spares. The cleaning and sparing are intended to enable extension of the life of the E-tongue. Yet other advantages of the planar configuration are that it facilitates incorporation of the E-tongue into a flow-through cell and is amenable to microscopic observations.
The progression from E-tongue 1 to Etongue 2 has involved improvements in design, operation, interpretation of sensor readings, and understanding of the underlying chemical and physical mechanisms, all directed toward development of large arrays of robust, long-lived sensors for monitoring water quality. There has been particular emphasis on the redox sensors, which are based on a traditional electrochemical cell used for making pH and other ionic-content measurements. Such a cell includes a working electrode, a reference electrode, and a counter electrode. However, whereas the electrodes in a traditional cell have a pencillike configuration, the electrodes in each redox sensor of the E-tongue are planar and concentric (see Figure 2) and their counter electrodes are integral parts of the ground plane.
Excitation and readout circuitry has been designed and constructed specifically for use in operating the E-tongue. In operation, the currents flowing to and from the electrodes of the electrochemical cells and an adjacent electrical- conductivity sensor on the E-tongue are measured as the voltages applied to the electrodes are scanned through appropriate ranges, called detection windows. The measurement data are then processed and interpreted to detect and quantitate ions in solution. The detection windows are bounded by electrolytic generation of hydrogen at negative potentials and oxygen at positive potentials. In the case of the redox sensors of E-nose 2, the window is between –2 and +1 V.
This work was done by Martin Buehler of Caltech for NASA’s Jet Propulsion Laboratory. 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:
Innovative Technology Assets Management
JPL Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-30862, volume and number of this NASA Tech Briefs issue, and the page number.