Electronic circuitry has been developed to serve as an interface between an electronic tongue and digital input/output boards in a laptop computer that is used to control the tongue and process its readings. Electronic tongues were described in two prior NASA Tech Briefs articles: “Electronic Tongue for Quantitation of Contaminants in Water” (NPO-30601), Vol. 28, No. 2 (February 2004), page 31; and “Electronic Tongue Containing Redox and Conductivity Sensors” (NPO-30862), Vol. 31, No. 8 (August 2007), page 58. Electronic tongues can be used for a variety of purposes, including evaluating water quality, analyzing biochemicals, analyzing biofilms, and measuring electrical conductivities of soils.
The present electronic tongue and interface circuitry are updated versions of those described in the latter-mentioned prior article. The instrument was designed for use in characterizing biofilms by Prof. D. Newman and Dr. D. Lies at Caltech. To recapitulate: An electronic tongue is a rugged, compact sensor unit that can include a heater, a temperature sensor, a conductivity sensor, and an array of three-electrode electrochemical cells, all on one planar surface of a ceramic substrate. The cells of an electronic tongue are connected to electronic excitation and readout circuits. Among the tasks identified by Prof. D. Newman and Dr. D. Lies that must be performed to characterize biofilms are stimulation of the microbial environment through generation of oxygen and hydrogen, detection of their metabolic products, and visual observation of biofilms. An electronic tongue can provide the needed stimulation while serving as a means of electrochemical detection of metabolic products of a biofilm.
A prototype apparatus for characterizing a biofilm includes an electronic tongue mounted in a flow-through, seeinto chamber. The chamber is mounted on a platform under a microscope that is used to observe the biofilm growing on the electronic tongue. The flowthrough, see-into chamber is made of polycarbonate structural components plus a cover glass. A watertight compartment containing the electrodes is formed by O-ring seals between the upper and lower surfaces of the electronic tongue and the facing surfaces of the chamber. On the top side of the electronic tongue, a spacer establishes the thickness of the flow-through cell as a gap between the electrodes and the cover glass.
In the present version, the electronic tongue contains 12 analog cells: nine oxidation/ reduction (redox) electrochemical cells, an electrical-conductivity cell, and the aforementioned heater and temperature sensor. The interface circuitry (see figure) consists mainly of 12 digital-to-analog converters (DACs) for excitation of the cells, and four analog-to-digital converters (ADCs) for readout from the cells connected to 12 analog cells. Each analog cell is made of two instrument amplifiers, two operational amplifiers and analog filters for reducing signal-to-noise ratios, and control and switching circuits.
The interface circuitry resides on a board mounted immediately below the ceramic substrate and, as in the prior version, is connected to the analog cells via miniature edge connectors on the ceramic substrate. By thus placing the ADCs and DACs near the analog cells, the design helps to minimize pickup of noise and reduce cross-talk on the analog signal lines.
As in the prior version, the control circuitry can be programmed to make the DACs generate the specified excitation waveforms and to make the ADCs acquire the specified response waveform data, and each electrochemical cell can be addressed individually. Depending on the specific application, a given electrochemical cell can be operated in a potentiostatic mode (voltage forced, current measured) or a galvanostatic mode (current forced, voltage measured), or can be made to alternate between the two modes. In one typical application, the main sequence of excitations and responses in a potentiostatic mode is chosen to implement anodic stripping voltammetry or cyclic voltammetry. In another typical application, a working electrode of a cell is operated in a galvanostatic mode at a positive bias for generating oxygen or a negative bias for generating hydrogen.
This work was done by Didier Keymeulen and 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
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Refer to NPO-41365, volume and number of this NASA Tech Briefs issue, and the page number.