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Electrical Impedance Response of a thick-film
Anodic Aluminum Oxide Chemical Sensor
By: Taylor Bogan
Abstract
The Anodic Aluminum oxide (AAO) nanoporous humidity sensor was constructed and tested at
Northern Illinois University. The AAO sensor was designed to be a low cost sensor that allowed
repeatable and accurate tests to occur. The sensor was tested with two alcohols including
Isopropyl and Methanol. The compounds heptane and water were used as well. The impedance
response of each test allows one to distinguish the difference amongst the alcohols.
Keywords: sensors, alcohols, Isopropyl, Methanol, AAO, Impedance
1 Introduction
Anodic anodized oxide (AAO) thin film sensor were replicated at Northern Illinois
University by the College of Engineering and Engineering Technology and the Chemistry
department. The sensors were designed to test the capacitance and resistance of different organic
chemicals. The sensor underwent several different tests involving Isopropyl and Methanol. The
tests displayed results involving adsorption and desorption. Adsorption is the process of
molecules, atoms, or ions from the different physical states binding to a surface. Adsorption was
reflected on the chart as a slow or rapid increase and it physically occurred when the organic
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chemical was placed onto the sensor. Desorption is the reverse process of adsorption and occurs
when molecules, particles, or ions are removed from a surface. Desorption was reflected on the
chart as a slow or rapid decrease and it physically occurred when the organic chemical was
evaporated from the sensor.
2. Experimental Procedure
Before testing could be done the Anodic Anodize Oxide (AAO) sensor needed to be replicated at
Northern. This was done by working in a class 100 clean room to avoid any unwanted
contaminates to be mixed into the process of creating the sensor. The starting material was said
to be an alumina substrate that had a thin-film layer of aluminum added to it. The sensor was
then anodized and had an interdigitated capacitor patterned onto the surface of the AAO sensor.
It should be noted that one chip had sixteen separate sensors on it. This is of great benefit
because it allowed multiple sensors to undergo different testing environments at the same.
The purpose of this testing was to see the chemical response in regards to adsorption and
desorption. Adsorption and desorption were reflected on the resistance and capacitance charts
that were collected. Once the data was collected calculations were done to find the value of K.
The K value represents the decay constant. The equation that was used to find K was the
Once the sensor was replicated it was able to
undergo testing with different organic chemicals. It
should be noted that for these experiments the
temperature was assumed to be constant. For several
weeks testing with Isopropyl and Methanol
occurred.
Image of AAOchip
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following:𝐹𝑡 = 𝐹𝑜(1 − 𝑒−𝑘𝑡
). From those K values the standard deviation, maximum value,
minimum value, and average were calculated. With those values one was able to compare the
differences amongst the chemicals. The comparisons of chemicals in respect to the sensor
allowed one to see how the sensor reacted to the different chemicals. It gave insight to see the
different characteristics amongst the chemicals as well.
3. Results
Organic chemicals were placed on the sensor by using an automatic pipet set to 10
microliters. It should be noted that the beginning results did not have an automatic pipet
used. This is of importance because it causes variation amongst the results. The
impedance of the sensor after exposure was measured on an Agilent 4980A LCR,
Inductance-Capacitance-Resistance, meter. Figure 1 shows the adsorption with respect to
resistance of a test done involving isopropyl. Figure 2 shows the desorption rate with
respect to the capacitance of a test done involving isopropyl.
The other chemicals used on the sensor had similar but not exact adsorption and
desorption graphs. For deionized water it had a much slower adsorption and desorption
Figure 1 and 2 showthe adsorptionanddesorptionof Isopropyl
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time then the other chemicals that were placed on the sensor. When heptane was placed
on the sensor the adsorption and desorption occurred extremely fast. The rate of
adsorption and desorption for methane occurred at a medium rate. The rate at which the
adsorption and desorption occurred has to do with the characteristics of each chemical.
Once the graphs were collected calculations were done to find the k values. From
those k values the average, minimum value, maximum value, and standard deviation
were found. The k values and other mentioned calculations for all of the chemicals can be
found below.
4. Discussions
It can be seen from the above data collection that there is a variation amongst the averages,
minimum value, maximum value, and standard deviation. Many factors played into the outcome
of the results. For isopropyl there were multiple days-worth of data collected which allowed
Type : Isopropyl
Average: 0.028298
Min: -0.03
Max: 0.4119
Std Dev: 0.083954
Type: Heptane
Average: 0.00685
Min: 0.0003
Max: 0.0134
Std Dev: 0.009263
Type: Methane
Average: 0.009533
Min: -0.001
Max: 0.0336
Std Dev: 0.013283
Type: Dionized water
Average: -0.02498
Min: -0.0549
Max: 0.0134
Std Dev: 0.029747
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more numbers to play into the calculations done. Deionized water had two days-worth of data
while methanol and heptane had one days-worth of testing done. Another factor that played into
the results were that for isopropyl there was not a consistent drop of 10 microliters applied to
each tests. There were approximations done because the automatic pipet had not arrived yet.
In regards to the graphs that were collected amongst the tests they did vary from tests to
tests and chemical to chemical. A very important factor that played into all of the tests conducted
was that the temperature was assumed to be constant. However, in the lab there were days that it
was hotter and days that were colder. The fluctuation amongst the temperature affected how fast
or how slow the chemical went through the stages of adsorption and desorption. There was not a
way to accurately determine the temperature every day that tests were being ran which is why it
was assumed to be constant. If the temperature were to have been accounted for this could
further explain differences amongst the tests ran. Another factor that determined the rate of
adsorption and desorption were the characteristics of the chemicals. Heptane is a known
chemical to have a fast adsorption while deionized water takes longer to undergo adsorption.
5. Conclusions
Anodic anodized oxide (AAO) thin film sensor has shown that it is able to distinguish the
differences amongst organic chemicals. Figure 1 and 2 show that the AAO sensor was able to
detect isopropyl when it was dropped onto the sensor. Based off the data that was collected the
sensor shows that it can distinguish between different organic chemicals. With further testing of
these chemicals in a controlled temperature environment and other organic chemicals more
information can be collected on the sensor. Further testing of the sensor would lead to a better
understanding of how the AAO sensor is able to interact with chemicals and what causes those
specific chemicals.
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6 Acknowledgment
I would like to take the time to thank Martin Kocanda, Doctor Motaleb and the College of
Engineering and Engineering Technology for the opportunity to expand my knowledge in
engineering through the opportunity to do research in the fall 2014 semester.
7 References
[1] Christopher Radzik, G. Martin Kocanda , Michael Haji-Sheikh , David S. Ballantine, “
Electrical Impedance Response of a Thick-Thin film Hybrid Anodic Nanoporous Alumina
Sensor to Methanol Vapors” INTERNATIONAL JOURNAL ON SMART SENSING AND
INTELLIGENT SYSTEMS, VOL. 1, NO. 2, JUNE 2008.
[2] Martin Kocanda, Michael Haji-Sheikh, Christina Johnson, Edward King, David S. Ballantine,
Syaifudin, A. R., S.C. Mukhapadhyay, “Hydrogen Sensing using Impedance Measurements of
Nanocrystalline Palladium.”
[3] Martin Kocanda, Member, IEEE Michael Haji-Sheikh, Member, IEEE and David S.
Ballantine. “Detection of cyclic volatile organic compounds using single-step anodized
nanoporous alumina sensors” IEEE Sensors Journal 2009.
[4] Martin Kocanda, Lakshman Potluri, Michael Haji-Sheikh, David S. Ballantine, Anima Bose,
“Enhanced Hydrogen Sensing Employing Electrodeposited Palladium Nanowires Enclosed in
Anodized Aluminum Oxide Nanopores.”
[5] Martin Kocanda, Michael Haji-Sheikh, David S. Ballantine, “Detection and Discrimination
of Alcohol Vapours Using Single-step Anodised Nanoporous Alumina Sensors.”
