Sensors and Actuators B 96 (2003) 261–267
Iridium oxide sensors for acidity and basicity
detection in industrial lubricants
Matthew F. Smiechowski a , Vadim F. Lvovich b,∗
a
Department of Chemical Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
b The Lubrizol Corporation, 29400 Lakeland Blvd., Wickliffe, OH 44092, USA
Received 4 December 2002; accepted 9 June 2003
Abstract
The lubrication and automotive industries are seeking on-line sensors capable of determining the chemical condition and degree of degradation of industrial and automotive lubricants in order to signal the need for an oil change. Levels of acidity and basicity in non-aqueous media
are parameters common to a wide variety of industrial fluids and closely related to rate of the lubricant oxidation breakdown. Therefore, development of chemical devices capable of continuous monitoring of these parameters remains an important goal of industrial sensor research.
Chronopotentiometric (CP) sensors based on solid-state reversible oxide films of iridium demonstrate a number of advantages for
detection of acidity and basicity levels in non-aqueous industrial lubricants over other types of acidity sensors, such as glass electrode and
ion-selective field effect transistors. Iridium oxide sensors fabricated by different methods and having various geometric configurations
(macro-scale versus microelectromechanical systems (MEMS)) were compared. The sensors demonstrated linear responses to changes in
oil acidity and basicity that were in agreement with proposed electrochemical mechanisms. Studies of long term stability and durability of
the metal oxide sensors in the oil solutions illustrated that improvement to the sensor fabrication process is needed.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Iridium oxide; Potentiometry; Acidity; Lubricant; Non-aqueous system
1. Introduction
The useful life of engine oil depends on a number of
factors—base oil formulation, type and amount of oil additives, engine size and vehicle operating conditions [1].
Combinations of base oil and of oil additives improve
chemical and physical properties, performance and long
term stability of engine oil. During its life, oil undergoes
substantial chemical changes due to oxidative degradation
and contamination by water, ethylene glycol, fuel, soot, and
wear metals. Degradation of industrial lubricant is a result
of its exposure to high temperature and the presence of
nitrogen oxides, alcohols, glycol, water, and air [2]. Chemically active oil additives interact with engine oil contaminants and oxidative by-products of oil degradation (high
aldehydes, ketones, and carboxylic acids) to make them
innocuous. Such additives include dispersants, detergents
(surfactants), oxidation inhibitors, and antiwear agents.
In every-day practice, used lubricants are changed after a
given service time or mileage without prior testing. Auto-
∗ Corresponding author. Tel.: +1-440-347-2123; fax: +1-440-347-4482.
E-mail address: [email protected] (V.F. Lvovich).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0925-4005(03)00542-2
motive and lubricant industries are seeking on-line sensors
capable of monitoring oil condition. An oil-condition sensor could signal the need for an oil change when the oil’s
condition warrants it, reducing the likelihood for costly repairs due to overdue maintenance and equally eliminating
unnecessary maintenance. A significant obstacle in the development of an on-line sensor is the large variety of oil
formulations. It is then necessary to look for common traits
in degradation of a variety of industrial fluids, such as water
leak [3] or oxidative breakdown, to use in the sensor development. Oxidative degradation is related to a decrease in total base number (TBN) and an increase in total acid number
(TAN) values of the oil. The decrease in the TBN is caused
by the degradation of antioxidants, dispersants and detergents from the oil system thereby decreasing the basicity,
while the TAN increase is related to the continued chemical
oxidation of the oil. The change in additive concentration
and rising oxidation result in an overall loss of performance
and eventual failure of the oil. These changes have been
shown to affect the electrochemical reactivity of the oil [2].
Development of on-line sensors capable of accurate monitoring of changes in TAN and TBN is an important step in
making engines more efficient and safe. ASTM methods for
the determination of TAN and TBN (D2896-98, D4739-96)
262
M.F. Smiechowski, V.F. Lvovich / Sensors and Actuators B 96 (2003) 261–267
by off-line titration are complicated, time consuming, unstable, and user and protocol dependent.
Several competitive technologies exist for acidity determination of a non-aqueous solution. Infrared (IR) spectroscopy
provides a complete picture of the chemical system; however, it has several drawbacks for developing an individual
sensor. Species related to the solution acidity/basicity need
to be identified and the concentration of these species must
be correlated to pH, TAN or TBN. This correlation must
then be proven correct and comprehensive for the selected
species. Further complicating this issue, IR data analysis
techniques quickly become mathematically intensive requiring computer systems for data analysis. This coupled with
the detection system leads to large capital costs for a single
sensor. The light absorbent nature of oil systems and baseline shifts from compositional changes add difficulty to
making accurate IR measurements [4]. Glass pH electrodes
have been widely used because of their sensitivity, selectivity and long lifetime [5]. However, these electrodes also
have several disadvantages when applied in hydrocarbon
solutions, such as slow response, low stability in aggressive
media, requirement of a high input impedance pH meter, and
brittleness. This makes glass electrodes difficult to employ
in hydrocarbon systems [6,7]. Ion-selective field effect transistor (ISFET) sensors, such as the Si3 N4 type and the Ta2 O5
type, have been shown to have fast response times and nearNernstian response to pH [6]. However, these sensors have
the disadvantages of low stability at temperatures higher
than 105 ◦ C, sensitivity to light, and significant signal drift
due to external electromagnetic fields and thermal noise [8].
Versatile, sensitive, inexpensive electrochemical methods
demonstrated a remarkable potential for direct analysis and
monitoring of chemistry in highly resistive non-aqueous media, including oil degradation [9–12]. Potentiometric testing using metal oxide sensors is a well-established tool for
the detection of acidity and basicity in both aqueous and
non-aqueous solutions. Of the developed metal oxide sensors, the Sb2 O3 electrode is the most widely used, especially
in acid–base titrations, although the drift of the output potential makes it unsuitable for accurate pH measurements [7].
Palladium oxide electrodes have been developed for measuring the pH of aggressive systems such as whole blood, and
have demonstrated a long lifetime [13,14]. However, the recent studies of the pH sensitivity, working pH range, ion and
redox interference, and hysteresis effects of several potentiometric metal oxide sensors showed that the most promising
performance was achieved from IrO2 -based sensors [15,16].
Based on this conclusion, it was initially decided to use iridium oxide (IrO2 ) as working electrode material for sensor
testing in non-aqueous systems. The evaluated sensors were
fabricated as both conventional (macro-scale) and miniaturized (microelectromechanical systems (MEMS)) devices.
The application of microelectromechanical systems technology for the fabrication of individual sensors and sensor
arrays offers several distinct advantages over conventional
(macro) methods of sensor fabrication, such as well-defined
geometry, small size, enhanced sensitivity, compactness, reduction in cost and packaging compatibility. All of these
advantages of utilizing MEMS demonstrated the validity of
exploring the technology when compared to conventional
size electrochemical sensors.
2. Experiment
2.1. The sensors
The first type of chronopotentiometric (CP) sensor (here
defined as “Melt iridium oxide sensor”) consisted of a working electrode and a reference/counter electrode. The working electrode supplied by SensIrOx Inc. (Columbus, OH), a
0.25 mm diameter iridium wire, was oxidized in a Li2 CO3
melt producing a 20 ␮m Lix IrOy film on the surface of the
wire [16]. Studies by SensIrOx Inc. show that the value of
“x” in this formula was 0.86, and the value for “y” was approximately 2. A more detailed compositional analysis is
reported to be underway [16]. The second style of conventional iridium oxide sensor (here defined as “Sputter iridium
oxide sensor”) was produced at the Electronic Design Center
of Case Western Reserve University (Cleveland, OH). The
working electrode was made by sputtering 2000 Å of iridium
through argon gas onto a 0.25 mm titanium wire, then thermally oxidizing it at 700 ◦ C. Each IrOx wire was imbedded
into a cylindrical Teflon body along with a 1 mm diameter
silver wire, acting as the counter and reference electrode.
Both wires extended from the Teflon body by approximately
7 mm. This resulted in each IrOx sensor having a working
surface area of approximately 5.6 mm2 (π̇ (0.125 mm)2 + π̇
0.25 mm × 7 mm). The reference/counter electrode was further prepared by oxidizing silver wire in an aqueous acetate
buffer solution at a potential of +1 V (versus an Ag/AgCl
reference electrode) for 10 min.
The MEMS version of the potentiometric iridium
oxide-based sensor (here defined as “MEMS iridium oxide
sensor”) was produced at the Electronics Design Center of
Case Western Reserve University (Cleveland, OH). Positive
photoresist (#S1818, Shiphey Co., Newton, MA) was patterned with UV light to design the electrodes on an alumina
substrate. At first a layer of titanium (100 Å) was deposited
as an adhesion layer. The working electrode was fabricated
by sputtering iridium through argon gas to form a metal surface 2000 Å thick. The iridium was then oxidized at 700 ◦ C.
The second electrode was fabricated by sputtering 2000 Å
of gold onto the substrate and then electrochemically depositing silver on to the gold reference/counter electrode.
The silver coated reference/counter electrode was then
electrochemically oxidized in an aqueous acetate buffer solution for approximately 2 min at a potential of +1 V. The
final surface area for both electrodes on this sensor is approximately 2.85 mm2 (0.5 mm × 5.7 mm). The completed
sensor was soldered to an eight-pin connector with indium
#4 (Indium Corp. of America, Utica, NY). The sensor
M.F. Smiechowski, V.F. Lvovich / Sensors and Actuators B 96 (2003) 261–267
assembly was then inserted in a second eight-pin connector
and soldered to shielded copper wire using lead–tin solder
and mounted in a Teflon cylinder with a stainless steel body
in the same fashion as the conventional IrOx sensors.
2.2. Experimental methods
Sensors of each configuration were initially tested in standard aqueous solutions of pH 4, 7, and 10 (VWR Scientific Products) at 20 ◦ C to confirm the functionality of the
sensors and to determine a base line response to acidity.
For studies of acidity and basicity levels of industrial lubricants the sensors were immersed in four different sets of
diesel oil drain samples at 80 ◦ C, a temperature characteristic of an engine environment. Potentiometric data was first
recorded as the value after 5 min of continuous readings using a Keithley 617 Programmable Electrometer (Keithley
Inc., Cleveland, OH). The testing of the sensors’ long term
stability was performed using fresh diesel oil maintained at
80 ◦ C. Ten-minute long potentiometric measurements were
made every hour for 24 h using a CHI 660A Electrochemical
Workstation (CH Instruments, Austin, TX). Readings were
taken at 300, 400 and 500 s during the 10 min run. Elevated
temperatures were maintained by immersing a glass jar of
the sample into a thermostated oil bath (E-100 Immersion
Thermostat, Lauda, Lauda-Königshofen, Germany).
Differential capacitance values were determined between
−2.5 and +2.5 V (versus silver oxide reference electrode)
at 50 mV incremental steps using a Voltalab 40 potentiostat (Radiometer Inc., Westlake, OH). The measurements
in aqueous pH standards were taken at 25 ◦ C and 1 Hz,
while the measurements in oils were conducted at 80 ◦ C and
10 mHz.
Studies of the electrochemical interactions of the electrodes using electrochemical impedance spectroscopy (EIS)
were performed on the conventional style sensors only using the Voltalab 40 potentiostat. The data was collected at
80 ◦ C in oil media with a frequency sweep from 100 kHz
400
263
to 100 mHz and 500 mV ac amplitude. The sensors’ functionalities were analyzed by determining their respective
impedance response to individual model components that
closely resemble typical products of oxidation breakdown of industrial lubricant. Individual samples were prepared by adding methyl isobutyl ketone (MIBK), an ester
(di-2-ethylhexyl adipate), and a tall oil fatty acid (carboxylic
acid) to base oil at a 1 M concentration. In addition the EIS
studies for both sensors in a diesel oil drain samples (same
samples as the potentiometry testing) was performed using
the above testing specifications.
3. Results and discussion
3.1. pH testing
The results of potentiometric tests in aqueous solutions
are shown in Fig. 1. and Table 1. In each case it was seen that
the signal from the sensors moved in a negative direction
as the basicity increased. Comparison of the sensors across
the range of pHs tested showed a consistent linearity as
expected. Relating each sensor response to the change in
pH, the MEMS sensor gave a sub-Nernstian response to
the change in pH, while the Melt IrOx sensor response was
super-Nerstian (∼80 mV per pH unit). This deviation from
Nerstian response is seen as a common difficulty with using
metal oxide sensors [17].
3.2. TAN and TBN testing
TAN and TBN values in non-aqueous systems, such as
oil, generally do not share the same correlation that values
of pH and pOH have in aqueous solutions. The relationship
between TAN and TBN is dependent on factors such as additive type and concentration, and stresses on the lubricant
so therefore it is not going to be consistent across all lubricants. The results of the sensor output to TAN and TBN are
A
300
200
C
mV
100
0
-100
-200
-300
-400
B
-500
3
5
7
pH
9
11
Fig. 1. Experimental data and linear fit for the sensor response to pH in aqueous standard solutions: (A) Melt IrOx ; (B) Sputter IrOx ; (C) MEMS IrOx .
264
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Table 1
Response of iridium oxide sensors to acidity/basicity changes
Signal change per unit
increase pH (mV)
R2 for linear fit to pH
Signal change per unit
increase TAN (mV)
R2 for linear fit to TAN
Signal change per unit
increase TBN (mV)
R2 for linear fit to TBN
Table 2
Rbulk component of EIS testing vs. individual oxidation components
Melt IrOx
Sputter IrOx
MEMS IrOx
−79.3
−52.8
−29.3
0.963
−96.9
0.999
−32.8
0.846
−36.3
0.828
78.1
0.943
46.6
0.802
9.7
0.736
0.842
0.512
TBN
TAN
Melt IrOx ,
Rbulk (G)
Sputter IrOx ,
Rbulk (G)
5.8
3.4
2.2
2.2
1.9
1.6
0.9
2.2
2.2
3.3
4.6
6.3
7.3
8.1
1.24
1.18
0.86
0.83
0.80
0.72
0.69
2.63
2.00
1.60
1.38
1.20
0.93
0.80
3.4. Oil drain series EIS studies
illustrated in Figs. 2 and 3 and Table 1. The Melt IrOx sensor
had the greatest sensitivity to TAN/TBN changes followed
by the Sputter IrOx sensor. For all sensors the potential output was becoming more negative with the increase of the
sample acidity and decrease of the sample basicity. When
compared to the results of the aqueous testing, the sensor
output changed in the opposite direction of non-aqueous systems with respect to increasing basicity.
3.3. Potential of zero charge (PZC) measurements
Differential capacitance was used to determine the PZC
for the conventional sensors in oil (non-aqueous) and aqueous solutions. As the oil aged the PZC shifted in a negative
direction, from +0.5 V (for unaged oil) to approximately
−0.2 V for final drain oil (Fig. 4). This concurs with the
observed potential shift in the potentiometry studies, as the
oils become more acidic with age. The negative shift in the
PZC for the oil tests also indicates that anions (or negative
dipoles) are predominantly attracted to the electrode surface
in aged oil samples [18]. In aqueous solutions the PZC was
becoming more positive with increase in acidity, indicating
that protons are becoming more abundant at the electrode
surface [18].
The results of the EIS analysis of two macro sensors in
various drain oil samples (Table 2) showed that the Sputtered IrOx sensor had a greater sensitivity to changes in TAN
and TBN than the Melt IrOx sensor. As the sample became
more electrically active, in this case through oxidation, the
bulk solution resistance (Rbulk ) feature of a Nyquist plot of
the EIS data diminished. Previous studies [3,12] have shown
that EIS response is also sensitive to contamination by water,
soot and other combustion byproducts. Likewise, additive
packages that contain more electrically active components
will alter the EIS response to oxidation. Therefore it is important to determine how all of these possible interactions
simultaneously affect each other, before relying on EIS as a
diagnostic tool to solely determine the oil condition.
Additional EIS studies were performed in mixtures of
base oil and “model” component solutions by comparing
the high frequency bulk solution resistance for each of the
sensors and solutions. The “model” components were chosen for their similarity to oil oxidation products. Table 3
demonstrates that both sensors responded similarly to base
oil (large molecular weight hydrocarbon). Of the two sensors, only the Melt IrOx sensor shows a significant reduction
in resistance when esters are present in solution, probably
700
A
600
B
mV
500
400
300
200
100
C
0
1
2
3
4
5
TAN
6
7
8
9
Fig. 2. Experimental data and linear fit for the sensor response to TAN in non-aqueous (diesel oil) solutions: (A) Melt IrOx ; (B) Sputter IrOx ; (C) MEMS
IrOx .
M.F. Smiechowski, V.F. Lvovich / Sensors and Actuators B 96 (2003) 261–267
265
700
600
A
500
mV
400
B
300
200
100
C
0
0
1
2
3
4
5
6
7
TBN
Fig. 3. Experimental data and linear fit for the sensor response to TBN in non-aqueous (diesel oil) solutions: (A) Melt IrOx ; (B) Sputter IrOx ; (C) MEMS
IrOx .
1.0E-05
Drain Oil Capacitance (F)
Drain Oil
1.0E-06
1.0E-07
Fresh Oil
1.0E-08
3
2
1
0
-1
-2
-3
Potential(V)
Fig. 4. Illustration of results from differential capacitance testing, the arrow shows the shift of the PZC from a fresh oil sample to a drain oil sample.
due to the lithium-mediated ester oxidation mechanism (see
the discussion later). For the other two tested components
the Sputtered iridium oxide sensor had a larger response to
acids (70% change) and ketones (50% change) than the Melt
iridium oxide sensor. From these results the Melt IrOx sensor should probably have a better sensitivity to initial stages
of oil oxidation, generally characterized by abundance of
esters. The Sputtered IrOx sensor’s EIS response may have
equal or higher sensitivity to later stages of oxidation, characterized by higher presence of carboxylic acids and ketones.
3.5. Long term stability testing
Results of the long term stability testing are illustrated in
Fig. 5. All three sensors demonstrated poor stability over the
course of the testing. Adsorption of material and develop-
ment of non-uniform film thickness with additional “pseudocapacitive transmission lines” [19,20] could be the cause of
this instability. Possible interactions between the electrodes’
surfaces and the sampled solution involved adsorption on
the electrode surface, which is further complicated by other
surface active chemicals present in the oil formulation. Peeling of the oxide layer to the substrate after several cycles of
testing was seen in the case of the MEMS electrodes. The
poor adhesion to the substrate could result in deformation
Table 3
EIS testing of diesel drain series vs. TAN and TBN
Base oil (w/w)
Alone
Ester
Ketone
Carboxylic
acid
Melt IrOx , Rbulk (G)
Sputter IrOx (G)
60
61
43
60
52
30
45
17
266
M.F. Smiechowski, V.F. Lvovich / Sensors and Actuators B 96 (2003) 261–267
0.16
C
0.14
Potential (V)
0.12
0.1
B
0.08
0.06
0.04
0.02
A
0
0
5
10
15
20
25
time (hrs)
Fig. 5. Illustration of long term stability testing: (A) Melt IrOx ; (B) Sputter IrOx ; (C) MEMS IrOx .
of the electrode over the course of the test, leading to poor
stability. Solutions to the difficulties encountered with long
term stability such as coating the electrodes in a protective
layer, or modifying the substrate and adhesion layer materials may be considered for the future work.
3.6. Sensing mechanism of iridium oxide sensors
In an aqueous solution the general reduction reaction occurs between a metal oxide sensor and the solution as follows [16]:
MO + 2H+ + 2e ↔ M + H2 O
(1)
where “M” is the metal of the metal oxide pair, in this case
Ir. The increased sensitivity by the Melt IrOx sensor is due
to the presence of lithium in the crystal lattice through an
increase in the number of transferred electrons. These results
were later confirmed by testing of a macro size Sputtered
IrOx sensor, which gave an almost Nernstian (54 mV per
pH) response to pH.
The difference in response characteristics of the sensor
in aqueous and non-aqueous solutions was probably caused
by different mechanisms of interactions between the sensor
and the solutions. The results of the PZC study indicated
that anions were predominantly attracted to the surface of
the electrode in oxidized oil. This was the opposite of the
aqueous PZC results that showed the predominant adsorption of H+ ions, resulting in the reduction reaction (Eq. (1)).
In non-aqueous systems for the interactions of noble metals
and formic acid the following oxidation reaction mechanism
was reported [21,22] as:
MO2 + RCOOH ↔ [MO2 · RCOO]− + H+
−
[MO2 · RCOO] ↔ MO2 + CO2 + R + e
−
(2)
(3)
or
2[MO2 · RCOO]− ↔ 2MO2 + 2CO2 + R–R + 2e−
(4)
The above mechanism was shown to be applicable to IrOx
electrodes. The IrOx oxide region was also shown to readily
adsorb reaction intermediates [21].
In order to account for the change in the sensors reaction to changes in acidity in aqueous solutions differently
than non-aqueous solutions, one must regard the differences
in directions of the above two redox reactions. In the case
of an aqueous system the reduction of the metal oxide occurred while in non-aqueous systems the adsorption of a carboxylic acid would be followed by oxidation. The electron
requirements of these reactions accounted for the difference
between the slopes and the directions in sensor response in
non-aqueous and aqueous systems with respect to acidity
and basicity (Figs. 1–3).
Other than surface area and general geometry, the method
of manufacturing of the oxide layer in the iridium electrodes
seems to provide a significant difference. The lithium present
in the oxide layer of the Melt IrOx electrode provided a second reactive site, which could interact with esters present in
the oil formulation through the following reported oxidation
reaction [22]:
Lix IrOy + R1 CH2 COOR2
→ [R1 CHCOOR2 · Lix IrOy ] + e− + H+
(5)
[R1 CHCOOR2 · Lix IrOy ]
→ Lix IrOy + 21 R2 OOCHR1 − R1 CHOOR2
(6)
This oxidation reaction generates additional electrons
through the dimerization of esters, thereby enhancing the
sensor’s sensitivity to these types of oil oxidation products.
4. Conclusions
Iridium oxide potentiometric sensors were chosen out of
several competitive technologies to be used for acidity determination in a non-aqueous industrial lubricant environment. Two versions of the sensor were created: a macro and
M.F. Smiechowski, V.F. Lvovich / Sensors and Actuators B 96 (2003) 261–267
MEMS configurations with iridium oxidized by two different methods. The conventional iridium oxide sensors were
more sensitive to changes in pH than MEMS sensors with
reduction reaction occurring on the surface of working electrode. The Melt IrOx sensor demonstrated enhanced sensitivity attributed to lithium in the crystal lattice adding to the
number of electrons transferred.
Tests of the sensors in diesel oil drains showed a good
correlation between TAN and TBN and the voltage output
of each sensor. Conventional IrOx sensors showed a greater
sensitivity to changes in TAN and TBN than the MEMS
sensors. The large increase in sensitivity of the Melt IrOx
sensor was due to the ability of the Lix IrOy to respond not
only to carboxylic acid but also to esters through a second
surface reaction catalyzed by lithium. This conclusion was
confirmed by the EIS studies of effects of individual oxidation products on the sensors’ outputs. The sputter formed
sensor demonstrated a better response to oxidative degradation of oil due to its higher sensitivity to ketones and
carboxylic acids. The differences in reaction mechanisms
between the iridium oxide and the components of the solution resulted in an opposite direction of response to changes
in basicity in aqueous and non-aqueous systems.
Long term stability tests showed that all three versions
of the sensor were not stable in diesel oil over a 24 h period. Future work in this area may include refinement and
optimization of the sensor design and fabrication to provide
improved stability and better sensor performance.
267
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Matthew F. Smiechowski has been involved in a joint research program
between Case Western Reserve University and the Lubrizol Corporation
since February 2000. In 2002 he received a MS degree from Case Western
Reserve University, his thesis dealt with water contamination of lubricant
systems. He is currently working on his PhD at Case Western Reserve
University under Prof. C.C. Liu. His dissertation focuses on developing
a MEMS sensor array for detection of oil degradation.
Vadim F. Lvovich received his MS degree in chemical engineering
from Moscow University of Chemical Technology in 1992 and his
PhD degree in analytical chemistry from the University of Illinois at
Urbana-Champaign in 1997. He is presently a research scientist in the
Research, Development and Engineering Division of the Lubrizol Corporation. Dr. Lvovich is also an adjunct professor of chemical engineering
at Case Western Reserve University and member of the Applications
Technology Team at the Glennan Microsystems Initiative. Dr. Lvovich
has published a large number of patents and research publications on
sensing techniques for industrial and biochemical fluids. His research
interests include electrochemical engineering, electroanalytical chemistry,
microsensors and microelectronic fabrication.