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:

  1. The gel layers of the electrodes would be deposited on the substrate by screen printing.
  2. 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.
  3. The polymer layers of the electrodes would be deposited by screen printing in the same manner as that of the gel layers.
  4. The polymer layers would be doped in the same manner as that of the gel layers.
  5. 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

4800 Oak Grove Drive

Pasadena, CA 91109

(818) 354-2240

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

Topics:
Body structures Ceramics Fabrication Ferrous metals Frames Iron Manufacturing processes Metals Physical Sciences Polymers Production Sensors and actuators Water
This Brief includes a Technical Support Package (TSP).
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Integrated Arrays of Ion-Sensitive Electrodes

(reference NPO-20700) is currently available for download from the TSP library.

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NASA Tech Briefs Magazine

This article first appeared in the May, 2003 issue of NASA Tech Briefs Magazine (Vol. 27 No. 5).

Read more articles from the archives here.

Overview

The document outlines a novel approach to water quality monitoring using an array of ion-sensitive electrodes (ISEs) developed at the Jet Propulsion Laboratory (JPL) under NASA's contract. The primary motivation for this work is to determine the ionic contaminants in water, particularly in environments like the International Space Station, where water quality is critical for astronaut health and mission success.

The innovation lies in the fabrication process of the multi-sensor array, which utilizes a maskless technique that eliminates the need for traditional photolithography, micro/nano pipetting, or ink jetting of dopant molecules. This method enhances manufacturability, reduces costs, and increases reliability, making the sensors more accessible for widespread use.

The technical disclosure is structured into several sections. The first section describes the novelty of the work, emphasizing the improvements over prior art in sensor manufacturing. The second section outlines the problem that prompted the development of the sensors: the need for effective water quality assessment in space. The solution proposed involves using an array of ISEs to measure various ionic contaminants present in the water.

The detailed description explains the construction of the sensor array. It begins with the formation of the first seven layers using conventional hybrid microelectronic co-fired ceramic techniques. A gel layer is then screen printed and electrostatically doped with an ionophore, followed by another polymer layer that is similarly treated. Finally, a protective layer is applied over the electrodes. The sensor array is integrated with microelectronics that measure the ISE potentials selectively through multiplexers. These potentials are converted using analog-to-digital converters and transmitted serially to a laptop computer for analysis, allowing for the identification of the type and amount of ionic contamination in the water.

The document also includes references to previous works related to gas sensor test chips and results from the Space Shuttle STS-95 Electronic Nose Experiment, indicating a foundation of research that supports the current development. Additionally, it clarifies that any mention of specific commercial products does not imply endorsement by the U.S. Government or JPL.

Overall, this document presents a significant advancement in sensor technology for water quality monitoring, with potential applications extending beyond space missions to various environmental and public health contexts.