Integrated Arrays of Ion-Sensitive Electrodes

NASA's Jet Propulsion Laboratory, Pasadena, California

The figure depicts an example of proposed compact water-quality sensors that would contain integrated arrays of ion-sensitive electrodes (ISEs). These sensors would serve as electronic "tongues": they would be placed in contact with water and used to "taste" selected dissolved ions (that is, they would be used to measure the concentrations of the ions). The selected ions could be any or all of a variety of organic and inorganic cations and anions that could be regarded as contaminants or analytes, depending on the specific application. In addition, some of the ISEs could be made sensitive to some neutral analytes (see table).

These Ions and Neutral Analytes are ones for which ionophores have been reported and for which ISEs are expected to exhibit (1) linear responses at concentrations up to 1 M and (2) sensitivity to concentrations as low as 10–6 M.

Discrete ISEs are commercially available, but are large, relative to the ISEs in the proposed sensors. In comparison with an array of commercial ISEs, an array of the proposed type would be about ten times as dense; in other words, one could detect a greater variety of ions by use of a sensor of a given size, or, alternatively, the sensor needed to detect a given set of ionic species could be made smaller. In addition, the proposed sensors have been conceptually designed to be amenable to mass production at relatively low cost.

In the example of the figure, the array of ISEs would be formed on a co-fired ceramic substrate strengthened by a Kovar (or equivalent) iron/nickel/cobalt-alloy frame. Each electrode would comprise a gel layer covered by a polymer layer and further covered by a protective layer. Fabrication of the array would include the following sequence of operations:

The gel layers of the electrodes would be deposited on the substrate by screen printing.
By use of an electrolytic doping apparatus, each electrode would be doped electrostatically with a selected ionophore. The basic principle of the doping process would be similar to that of electrophoresis, wherein molecular fragments migrate along a conductive gel channel under the influence of an electric field. The design and mode of operation of the apparatus would be such that the process of doping each electrode would not deplete previously doped electrodes.
The polymer layers of the electrodes would be deposited by screen printing in the same manner as that of the gel layers.
The polymer layers would be doped in the same manner as that of the gel layers.
The protective layers would be screen-printed over the electrodes.

A Unitary Array of 16 ISEs would occupy an area of only about 3.1 cm². An array of 16 discrete ISEs would be much larger.

The sensor would be inserted in a socket on a printed-circuit board that would contain electronic circuitry for processing the ISE outputs. The circuitry would include analog-to-digital converters measuring the ISE potentials. The circuitry would also include digital multiplexers for transmitting the potentials to a computer, which would analyze the potentials to determine the concentrations of ions of the selected species.

This work was done by Martin Buehler and Kimberly Kuhlman of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.

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

Intellectual Property group

JPL

Mail Stop 202-233

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Refer to NPO-20700, volume and number of this NASA Tech Briefs issue, and the page number.