Application of ZnO thick films for detection of NO microconcentrations
S. Krutovertsev, O. Ivanova, A. Tarasova, L. Krutovertseva
JSC “Ecological sensors and systems”, Zelenograd, Moscow, 124460, p/b 146, Russia,
Transparent conductive oxides can be used not only in optoelectronic, solar cells,
microelectronic but they are interesting in sensor application for gas analysis. This work is
development of problem solution of early non-invasive diagnostics.
Exhaled nitric oxide is
widely accepted as a non-invasive marker of airway inflammation for research. Informed
therapy decision can be made based on an objective non-invasive method by measuring of NO in
exhaled air. A method of nonoxidizing detection of NO in breath is developed as an alternative
to the expensive chemiluminescence technique.
The comparative characteristics of sensors, developed on the ZnO and WO
3
films, were
investigated for NO detection in air. The sensitive element consists of an alumina substrate
measured 2x0.5x0.2 mm. One side of the substrate is covered with a gas sensitive film made by
thick-film technology; the other side of the substrate bears a film heater made of platinum paste.
The film heater is at the same time a thermal resistor in the sensor. The sensitive layers were
formed by two methods: the thermal sputtering in vacuum and thick film technology. In the
former, WO
3
films were thermally sputtered in vacuum. The thin films of 0.1 - 0.3 μm were
obtained at evaporation temperature of 1100
0
C and pressure in chamber 1,33 x 10
-8
bar. In the
latter, WO
3
and ZnO films were formed by thick film technology based on pastes. Thick-film
pastes were prepared by adding of substance nanopowder in organic vehicle. After mixing a
powder with a vehicle, paste was milled to homogenize the mixture. Then WO
3
and ZnO thick
films screen printed on substrates, dried and annealed.
The investigations were carried out under the sensor thermal stabilization conditions in
the range 50 - 400
0
C with the constant values of the heater resistance and applied power. The
nitric oxide concentrations were measured in a range of 1-200 ppb. Interaction products of
sensitive layer with gas remain on the sensor surface if the sensitive layer temperature is too
small. Surface is not able to be recovered, that interrupts the sorption of gas in the future
measurements. But if the surface temperature is very high, an analyzed gas is not able to be
adsorbed on the sensor surface. Therefore, sensitive layer working temperature has to be
optimal. Besides, sensor’s resistance changing in the presence of detecting gas has to be top. The
optimal working temperature for detection of NO was 150
0
C. The sensor signal stabilization
and repeatability are insufficient at such low temperature. Taking into account this fact there is
necessity to keep a special temperature conditions for sensor working regime. A temperature
impulse mode effect on a sensitive layer was used for the model gas mixtures and breath
analysis. The time cycle was equal to 180 seconds.
A monitoring device for measuring NO concentration has been made on the base of
preliminary investigations. It is complex system with of sample preparation block, measuring
block and special software. The mathematical method of sensor signals processing in presence of
other exhaled air background gases has been applied. Sensors based on ZnO had high
repeatability of gas sensitivity and resistance’s stability. Sensors showed high sensitivity to
presence of NO micro concentrations at 10 ppb level in air, response time of sensors does not
exceed 10 seconds. The influence of other gases present in exhaled air at 150
0
C is minimal and
can be estimated when the signals of sensors are mathematically processed. For this purpose the
Generalized Regression based on the Radial Basis Networks was used.
Test measurements of nitric oxide concentrations in nasal exhaled air of healthy
volunteers and patients with bronchial asthma and allergic rhinitis have been performed using a
method of nonoxidizing detection with developed device.
PS2 12
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