The Electronic Tongue (E-tongue 2) 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. The device 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

Buehler, Martin

NASA Technical Reports Server

22 NASA Tech Briefs, August 2007
gles, thereby imparting some perpendicu-
lar polarization. Unfortunately, the corru-
gation-fabrication process increases the
overall nonuniformity of a large QWIP
array. The proposed scheme is an alterna-
tive to the use of surface corrugations.
For a given QWIP, the metal and the
size and shape of the antenna would be
chosen so that the combination of the
antenna and the adjacent surface dielec-
tric layer of the QWIP would support
surface plasmon states at wavelengths of
interest. The interface between a dielec-
tric and a metal can support a surface
electromagnetic wave if the permittivity
of the metal, expressed as a complex
number, has a negative real component.
The distribution of amplitude in a plas-
mon peaks at the metal/dielectric inter-
face and decays exponentially with dis-
tance from the interface into the metal
or the dielectric.
In cases relevant to the proposal, the
polarization states of the electric field
are elliptical, characterized by major
axes parallel to the interface on the
metal side and perpendicular to the in-
terface in the dielectric side. The contri-
bution of the surface-plasmon effect to
perpendicular polarization would be
augmented by the contribution of
strong perpendicular-polarization com-
ponents of the near field of the antenna.
Presumably, designs could also be opti-
mized to obtain resonant or broadband
antenna structures to maximize cou-
pling of light from free space into per-
pendicularly polarized plasmon modes.
This work was done by John Hong of Caltech
for NASA’s Jet Propulsion Laboratory. 
In accordance with Public Law 96-517, the
contractor has elected to retain title to this inven-
tion. Inquiries concerning rights for its commer-
cial use should be addressed to:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: iaoffice@jpl.nasa.gov
Refer to NPO-43681, volume and number of
this NASA Tech Briefs issue, and the page
number.
Elliptical
Polarization
Plasmon Electric-Field
Pattern and/or Near-Field Radiation Pattern
Interior
QWIP
Layers
Incident
Light Polarization of
Incident Light
Antenna
Dielectric
Surface
QWIP Layer
A Surface Plasmon with elliptical polarization
would form under the antenna. The perpendicu-
lar component of the elliptical polarization
would enable absorption of the incident light.
Electronic Tongue Containing Redox and Conductivity Sensors
Progress has been made toward long-lived sensors for monitoring water quality.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Electronic tongue 2 (E-tongue 2) rep-
resents the second generation of the ap-
paratus 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 chemi-
cal quality of water in a variety of natu-
ral, industrial, and laboratory settings,
and detecting micro-organisms indi-
rectly by measuring microbially influ-
enced corrosion.
E-tongue 2 includes a heater, a temper-
ature 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 configu-
ration on an alumina substrate 1 mm
thick and 1.3 in. (3.3 cm) in diameter (see
Figure 1). The fabrication process in-
cludes 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
3.3 cm
Temperature
Sensor
Heater
Array of Ion-Specific Electrodes
Array of
Galvanic
Cells
Oxidation/Reduction
(Redox) Sensor Pair
Electrical-
Conductivity
Sensor
Figure 1. The Layout of E-Tongue 2 is nearly identical to the layout of E-tongue 1, depicted in an ear-
lier NASA Tech Briefs article.
https://ntrs.nasa.gov/search.jsp?R=20100002876 2019-08-30T08:47:49+00:00Z
NASA Tech Briefs, August 2007 23
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 elec-
trodes of all the cells in the array are in-
tegral parts of a grounded common con-
ductor on the front surface of the
substrate. The use of this common con-
ductor helps to minimize the number of
wires and pins and facilitates miniatur-
ization of the array. The common con-
ductor 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 gen-
eration of hydrogen and oxygen. The
planar configuration also facilitates the
reservation of some cells and sensors as
spares. The cleaning and sparing are in-
tended to enable extension of the life of
the E-tongue. Yet other advantages of
the planar configuration are that it facil-
itates incorporation of the E-tongue into
a flow-through cell and is amenable to
microscopic observations.
The progression from E-tongue 1 to
E-tongue 2 has involved improvements
in design, operation, interpretation of
sensor readings, and understanding of
the underlying chemical and physical
mechanisms, all directed toward devel-
opment of large arrays of robust, long-
lived sensors for monitoring water qual-
ity. 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 in-
cludes a working electrode, a reference
electrode, and a counter electrode.
However, whereas the electrodes in a
traditional cell have a pencillike config-
uration, 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 specifi-
cally for use in operating the E-tongue.
In operation, the currents flowing to
and from the electrodes of the electro-
chemical cells and an adjacent electrical-
conductivity sensor on the E-tongue are
measured as the voltages applied to the
electrodes are scanned through appro-
priate ranges, called detection windows.
The measurement data are then
processed and interpreted to detect and
quantitate ions in solution. The detec-
tion windows are bounded by elec-
trolytic generation of hydrogen at nega-
tive potentials and oxygen at positive
potentials. In the case of the redox sen-
sors 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
(818) 354-2240
E-mail: iaoffice@jpl.nasa.gov
Refer to NPO-30862, volume and number
of this NASA Tech Briefs issue, and the
page number.
Counter 
Electrode
Reference 
Electrode
Working 
Electrode 
Figure 2. One of the Redox Sensors is shown
here magnified. The working and counter elec-
trodes are made of a platinum/gold alloy; the
reference electrode is made of silver/palladium
alloy. The inner diameter of the counter elec-
trode is 2.75 mm.


Electronic Tongue Containing Redox and Conductivity Sensors

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