Int. Journal of Applied Sciences and Engineering Research, Vol. 4, Issue 4, 2015
© 2015 by the authors – Licensee IJASER- Under Creative Commons License 3.0
Research article
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ISSN 2277 – 9442
Performance analysis of Zinc oxide based alcohol sensors
Ruchika, Ashok Kumar
Department of Electronics and Communication Engineering, Ambala College of Engineering and Applied
Research- Ambala, Haryana, India
DOI: 10.6088.ijaser.04042
Abstract: Sensor technology is one of the most significant technology for the future with a constantly
growing number of applications, ranging from toxic gas detection, manufacturing process monitoring to
medical diagnosis and health monitoring. Among the different existing sensor technologies, the
semiconductor sensors are most attractive for their high sensitivity, small size and light-weight construction.
Additionally, they are cheap, rugged and simple in operation which makes them suitable for a large number
of applications. The n-type metal oxides like SnO2, ZnO, Fe2O3, WO3 are generally used as sensing
material for semiconductor sensors. Fortunately, such semiconductor sensors have the ability to detect a very
trace amount of volatile organic compounds. The main goal of the present work was to study the long term
stability of gas sensors, which are capable of detecting trace amounts of ethanol vapours. In this paper,
different types of doped and undoped ZnO nanoparticles have been studied and compared for sensing the
concentration of alcohol.
Keywords: Semiconductor gas sensors; zinc oxide based alcohol sensors.
1 Introduction
The atmospheric air contains several kinds of chemical compounds, natural and artificial, some of which
are very important to the life while many others are dangerous more or less. Among different kinds of
monitoring methods, semiconductor-based sensors are being used for many applications due to their low
price, robustness, and simple measurement electronics. Semiconductor gas sensors are solid-state sensors
whose sensing component is made up of mostly semiconductor metal oxide. Materials such as tin oxide
(SnO2), zinc oxide (ZnO), titanium oxide (TiO2) and tungsten oxide (WO3) have been used by most
researchers (Bajpai Ritu, 2012). The report on a ZnO-based thin film gas sensor gave rise to extraordinary
development and commercialization of a host of semiconducting oxide for the recognition of a variety of
gases over a wide range of composition. Sensors are devices that convert physical or chemical quantities
into electrical signals that are convenient to be detected. A gas sensor must possess at least two functions,
that is, to recognize a particular gas and to transduce the gas recognition into a measurable sensing signals.
The gas recognition is done through surface chemical processes due to gas– solid interactions. These
interactions may be of the form of adsorption, or chemical reactions. The transducer function of a gas
sensor depends on the sensor material itself. The transduction modes employed are due to the change of
thermal, mass, electrical or optical properties. Nevertheless, most gas sensors give an electrical output,
measuring the change of resistance or capacitance. Generally the change of electric field of the sensor is
monitored as a function of the target gas concentration. Semiconducting II-VI metal oxide, especially tin
dioxide (SnO2) and zinc oxide (ZnO) have gained attention of numerous researchers and scientists
interested in gas sensing under atmospheric conditions.
Along with the enlargement of economy, more and more people have cars and more and more cars appear
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*Corresponding author (e-mail [email protected])
Received on January 2014; Published on August, 2015
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Performance analysis of Zinc oxide based alcohol sensors
on the roads. Many drivers ignore the danger about driving after drinking. A problem comes out: It killed so
many people all around the world. In 2009, 10,839 people (Bochenkov V.E, 2004) were killed in traffic
accidents which were related to drunk driving, accounting for nearly one-third (32%) of all traffic related
deaths in the United States. Every day, almost 30 people in the United States die in motor vehicle crashes
that involve an alcohol-impaired driver. It becomes very noteworthy to prevent it, and make certain devices
that can predict the alcohol content in driver's body. Gas sensors are sensitive to alcohol, H2, CO. It gives
out different signals in different concentration. It is possible to determine alcohol concentration in driver's
breath with the help of an alcohol gas sensor.Ethanol gas sensors are widely used for breath analyzer of
drivers which aid to reduce the number of roads accidents, monitoring of fermentation, foodstuffs
conserving and other processes in chemical industries. In recent years, ZnO nanostructures have been
considered as possible candidate for the fabrication of ethanol gas sensors due to their high surface area,
low cost and high sensitivity. Ethanol is explosively utilized for beverages, industrial and scientific sectors.
Ethanol is a hypnotic gas having toxic nature. Heavy exposure and consumption of alcoholic beverages,
particularly by smokers, increase the risk of cancer of the upper respiratory and digestive tracks. Alcoholic
cirrhosis leads to liver cancer. Amongst the women, the chances of breast cancer increase with alcoholic
consumption or exposure. Those working on ethanol synthesis have great chances of being victims of
respiratory and digestive track cancer. So there is a great demand and emerging challenges for monitoring
ethanol gas at trace level. In the following sections, a survey is done on zinc oxide based alcohol sensors
and different parameters like sensitivity, selectivity, response time, etc. are compared.
2. Literature survey
Different methods for sensing alcohol using zinc oxide based sensors are studied. Firstly, Hongsith et al.
proposed ZnO nanobelts on copper tube by radio frequency (RF) sputtering. The ethanol sensing properties
of ZnO nanobelts were observed from the resistance change under ethanol vapor atmosphere at ethanol
concentrations of 50, 500 and 1000 ppm and at temperature of 200–290°C. After that Chou et al.,
presented the ZnO:Al thin films by RF magnetron sputtering on Si substrate using Pt as interdigitated
electrodes. The structure was characterized by XRD and SEM analyses, and the ethanol vapor gas sensing
as well as electrical properties have been investigated and resulted in high sensitivity and fast recovery.
Patil et al., Analyzed thick films of pure zinc oxide by the screen printing technique. Pure zinc oxide was
almost insensitive to ethanol. Thick films of Al2O3 (1 wt%) doped ZnO were observed to be highly
sensitive to ethanol vapours at 300°C. Later on Santhaveesuk et al., proposed the sensors based on
Ti∞Zn1-∞O tetrapods and investigated the ethanol sensing properties. The FE-SEM, HRTEM, SAED, XRD,
and RS results suggested that Ti∞Zn1-∞O alloy was formed with a slightly decrease of c-axis lattice
parameter. Hongsith et al., discussed the sensors based on ZnO tetrapods and platinum impregnated ZnO
tetrapods were fabricated and investigated for ethanol sensing properties. It was found that the sensitivities
of platinum impregnated ZnO tetrapod sensors were higher than that of pure ZnO tetrapod sensors. Pandya
et al., proposed an ethanol sensor incorporating nanostructured zinc oxide film and silicon micromachining.
A salient feature of the sensor was its lower operating temperature which had been achieved due to the use
of nanostructured material as sensing layer. Shyju et al., discussed gas sensors based on semiconducting
metal oxide nanostructures that were expected to exhibit better sensing properties like sensitivity and
selectivity, than gas sensors based on other materials. Wang et al., proposed the nanostructures and
gas-sensing properties of Zinc oxide nanosheets prepared by hydrothermal method. In particular, needlelike
Zinc oxide was also prepared for comparison. Bajpai et al., analyzed Alcohol sensors using gallium nitride
(GaN) nanowires (NWs) functionalized with zinc oxide (ZnO) nanoparticles. El-Sayed et al., analyzed
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Performance analysis of Zinc oxide based alcohol sensors
Nanocrystalline sensors having the general formula ZnO + x wt% CeO2, where x = 0, 2, 4 and 6 were
prepared by chemical precipitation method and sintered at 400, 600 and 800oC for 2h in static air
atmosphere. . ZnO thick films were prepared from these nanoparticles by dip coating method. Muthuraja et
al., presented zinc oxide nanoparticles by a suitable soft chemical method. The sensor was found to operate
with maximum efficiency. Liao et al., proposed ethanol sensor using the commercial 0.18 μm
complementary metal oxide semiconductor (CMOS) process. The sensor consists of a sensitive film, a
heater and interdigitated electrodes. Deore et al., discussed the thick films of undoped and Al2O3 - doped
ZnO by screen printing technique. AR grade Zinc Oxide powder (99.9% pure) was mixed
mechnochemically with different wt. %( 0.5, 1 and 3) of Aluminium Chloride (Hexahydrate) (AlCl3.6H2O)
in Acetone medium to obtain Al2O3 - ZnO composite material. Saxena et al., proposed undoped and
aluminium (Al)-doped zinc oxide (ZnO) nanorods by electrochemical route. The synthesized materials was
characterized by X-ray diffraction, UV–visible spectrometer and scanning electron microscope and resulted
in higher sensitivity, improved response time but there was no improvement in the recovery time of the
sensor. A survey on zinc oxide based alcohol sensors is carried out and their different parameters like
sensitivity, stability, response time, etc. are studied and compared.
3. Metal oxide semiconductor based gas sensors
3.1 Brief description of metal oxide gas sensors
In chemiresistors, metal oxides are typically used as gas-sensing materials, which change their electrical
resistance in existence of oxidizing or reducing gases. The physically small size of MOS based gas sensors
makes them attractive for compact devices (Santhaveesuk Theerapong, 2010). In addition, they operate in
real time, which is desirable for alarm giving devices like LPG leakage alarm. They are also typically less
expensive and more suitable than a comparable instrument for similar used. Since 1960s, much
technological effort has been made in order to improve the sensitivity, selectivity, stability, as well as the
response and recovery time, which are the key parameters for the performance and, hence, for the
applicability of the respective sensor element, for example, in multi-component analysis or in the selective
detection of a single compound in a complex and changing background.
3.2 Structure of the sensing layer
The typical metal-oxide gas sensor element consists of the following parts:
1. Sensitive layer
2. Substrate
3. Electrodes
4. Heater
Today, most of the commercial metal-oxide gas sensors are manufactured by screen printing on small and
thin ceramic substrates (Santhaveesuk Theerapong, 2010). The advantage of this preparation technique is
that the thick films of metal oxide semiconductor can be deposited in batch processing, leading to small
deviations of characteristics for different sensor elements. Although this fabrication technology is
well-established, it possesses a number of drawbacks and needs to be improved. Primarily, the drawbacks
are connected with the necessity to keep the thick metal oxide film at high temperature. Due to this reason
the power consumption of screen-printed sensors can be as high as 1 W, which makes them unable to be
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Performance analysis of Zinc oxide based alcohol sensors
used in battery-driven devices. Another technological problem is the proper mounting of the overall hot
ceramic plate to ensure the good thermal isolation between the sensor element and housing. These problems
have promoted the development of substrate technology and strong research in preparation of the sensitive
layer. One promising solution is the integration of a sensing layer in standard microelectronic processing,
which overcomes the difficulties of the screen-printed sensors. In this case, an oxide layer is deposited onto
a thin dielectric membrane of low thermal conductivity, which provides good thermal isolation between the
substrate and the heated area on the membrane. Such a construction allows the power consumption to be
kept at very low levels. Moreover, the total size of single sensor elements is reduced, so that a minimal
distance between electrodes lying in the µm range can be achieved.
3.3 Principle of operation
A wide range of metal oxide based materials like SnO2, ZnO, Fe2O3, Ga2O3, WO3, In2O3 etc. have been
studied for their gas sensing ability during several decades. For example, SnO2 is an oxygen deficient oxide
and therefore is an n-type semiconductor. It crystallizes in the rutile structure and is by far the most studied
and most successful semiconductor material for gas sensors. These sensors frequently change their
resistance by more than a factor of 100 upon exposure to a trace of reducing gases like hydrogen, methane,
ethanol, carbon monoxide and propane. Incidentally, the free electrons of n-type semiconductors like SnO2
are trapped by oxygen from the ambient by its e- affinity. Oxygen adsorbed on the surface of the grains
extracts an e- to ionize into O- or O2- species, which increases the resistance of the sensor coating (Saxena
Kanchan, 2010). The modified energy band at the grain boundary due to absorbed oxygen species is shown
in figure 1. Upon exposure to a reducing gas, the adsorbed oxygen species, being exceedingly metastable,
oxidize the reducing gas, releasing the trapped electron and consequently lowering the resistance. The
amount of resistance change is proportional to the concentration of the reducing gas in the ambient, which
is supposed to be the dominant sensing mechanism of surface conductive gas sensors like SnO2. Though,
the concentration of charged oxygen species is limited to less than 1% of the total number of surface states.
Hence, it is unlikely that a change in less than 1% of surface coverage causes a factor of 100 change in the
total resistance. Therefore, the surface barrier at the intergranular contact must play an significant role in
achieving high sensitivity of the sensors. The intergranular contact comprising of the space charge layer,
exhausted of electrons is usually more resistive than the bulk. Electrons must overcome the intergranular
contact barrier in order to cross from one grain to another for conduction. In this case, the sensor resistance
(R) can be written as
(1)
Where Ro is a constant and Eb is the barrier height. It can be shown that Eb is proportional to the square of
the coverage and consequently the conductivity has an exponential dependence on the square of the
coverage. The sensitivity of a sensor is calculated from either of the following relations:
(2)
(3)
Where, RT is the sensor resistance in air at a particular temperature and RGAS is the sensor resistance in gas
at the same temperature.
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Performance analysis of Zinc oxide based alcohol sensors
Figure 1: Energy band at grain boundary for polycrystalline SnO2
3.4 Factors affecting the sensitivity of metal-oxide gas sensor materials
The requirements for each gas sensor depend on the particular application. It is not necessary to have
material with a detection limit of one molecule if the sensor is designed to work in the 1-10% concentration
range. Nevertheless, materials with high sensitivity and low detection limit always attract the interest of
scientists and engineers. Size and shapes affects the sensitivity of metal oxide sensors. Also doping of metal
oxides with different dopants increases the sensitivity level of the sensors.
3.4.1 Size and shape effects
Since during the formation of a space charge layer the carrier concentration in volume is decreased only in
thickness L, there are three types of conductance mechanisms that are illustrated in figure 2,can be realized
[19]. For large crystallites the grain size D > 2L, and the conductance of the film is limited by Schottky
barriers at grain boundaries. In this case, the sensitivity is practically independent of D. When grain size is
comparable to 2L (D = 2L) every conducting channel in the necks between grains becomes small enough to
influence the total conductivity. Since the amount of necks is much larger the grain contacts, they manage
the conductivity of the material and define the size-dependence of gas sensitivity. If D < 2L, every grain is
fully involved in the space charge layer, and the electron transport is affected by the charge on the particles'
surfaces. Another prospective approach is to affect the sensitivity by changing the microstructure and
porosity.
Figure 2: Three mechanisms of conductance in metal-oxide gas-sensitive materials. The shaded part shows
the space charge region (high resistivity), while the un-shaded part shows the core region (low resistivity),
(a) D >> 2L, grain boundary control; (b) D ≥ 2L, neck control; (c) D < 2L, grain control
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Performance analysis of Zinc oxide based alcohol sensors
3.4.2 Doping
The sensitivity of metal-oxide gas sensors can be considerably improved by dispersing a low concentration
of additives, like Pd, Pt, Au, Ag, Cu, Co, and F (Shyju G. J, 2012) on oxide surface or in its volume.
Though doping has been used for a long time now in preparation of viable gas sensors, the working
principle of additive-modified metal-oxide materials is still not completely understood. There are two
general schemes of the gas sensing mechanism that are depicted in figure 3. In the chemical scheme (figure
3(a)) the reaction occurs at the oxide surface. The role of the additive nanoparticles is considered within a
spillover process which increases the metal-oxide surface coverage of the gas, involved in the sensing
scheme. In the electronic mechanism (figure 3(b)) the reaction provides the dopant atoms, and the oxide
material has to transduce the electrochemical changes into a detectable output signal. Furthermore the
introduction of additives may lead to the formation of new donor or acceptor energy states or influence the
grain size and growth mechanism. The chemical scheme is regularly considered in the case of catalytic
additives. This reaction results to the re-injection of the localized electrons back to the bulk, thus increasing
the conductivity of the material.
Figure 3 Chemical (a) and electronic (b) sensitization schemes in metal-doped SnO2, gas sensor [20]
4. Zinc oxide as a volatile organic compound sensor
4.1 Importance of Ethanol Sensor
Detection of alcohol concentration in the brain is important for safety on the road as well as in the
workplace. Expediently, blood alcohol concentration (BAC), defined as the percentage of alcohol in the
blood, is used to assess the alcohol level in the brain tissue as a measure of impairment from alcohol
poisoning (Sinha Writwik. Sen A, 2006). Most people demonstrate the measurable mental impairment at
about 0.05 % BAC. Above this level, the ability to operate an auto-mobile deteriorates progressively with
increased blood alcohol level. For the average person, unconsciousness results at about a BAC of 0.4 %.
Above 0.5 % BAC, basic body functions such as breathing or the beating action of the heart can be
depressed that causes death. Major symptoms occurring in a person who is intoxicated by alcohol are listed
in table 1.
Table 1: Various human symptoms according to the BAC level
Blood Alcohol Concentration(BAC)
Symptoms
>0.05 %
Measurable mental impairment
>0.10 %
Unsteady gait
>0.15 %
Slurred speech
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Performance analysis of Zinc oxide based alcohol sensors
>0.40 %
Unconsciousness
>0.50 %
Difficulty in breathing, heart failure,
death
Another challenging need of alcohol detection is in the area of automatic control of fermentation processes,
especially when alcoholic fermentation needs to be avoided. For example the fabrication of beer yeast,
where the ethanol concentration in the solution must be maintained <0.1 %. The sensor tracks the ethanol
level in the reactor to determine the modification of the reaction parameters in the reactor before it reaches
the threshold value.Ttherefore, ethanol sensors have been extensively studied due to their crucial important
in the biomedical applications particularly in breath analysis and also in the chemical and food industries.
ZnO is a II-VI compound semiconductor whose ionicity resides at the borderline between covalent and
ionic semiconductor. Zinc oxide usually appears as a white powder, nearly insoluble in water. The powder
is widely used as an additive into numerous materials and products including plastics, ceramics, glass,
cement, rubber (e.g., car tires), lubricants, paints, ointments, adhesives, sealants, pigments, foods (source of
Zn nutrient), batteries, ferrites, fire retardants, first aid tapes, etc. ZnO is present in the earth's crust as the
mineral zincite; however, most ZnO used commercially is produced synthetically. Nanostructured ZnO
materials have received broad attention due to their distinguished performance in electronics, optics and
photonics. From the 1960s, synthesis of ZnO thin films has been an active field because of their
applications as sensors, transducers and catalysts. ZnO is a key technological material (Sinha Writwik. Sen
A, 2006). The lack of a centre of symmetry in wurtzite, combined with large electromechanical coupling,
results in strong piezoelectric and pyroelectric properties and the consequent use of ZnO in mechanical
actuators and piezoelectric sensors. In addition, ZnO is a wide band-gap (3.37 eV) compound
semiconductor that is suitable for short wavelength optoelectronic applications. ZnO is transparent to
visible light and can be made highly conductive by doping. The change of conductivity at the present of
reducing gases makes them suitable material for gas sensors. A variety of ZnO based gas sensors are
reported in literature, which can detect CO CH4, NO, NO2, C2H5OH etc. Such sensors use different types of
designs like thin film structure, circular pellets, powder pressed in bar shape etc. The ethanol sensing
property of ZnO creates a great scope for developing a low concentration ethanol sensor.
4.2 Zinc oxide having Wurtzite structure
In ambient condition thermodynamically stable structure of ZnO is wurtzite structure. Wurtzite zinc oxide
has a hexagonal structure with lattice parameters a = 0.3296 and c = 0.52065 nm (Wang Ping, 2012). The
structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally
coordinated O2− and Zn2+ ions, stacked alternately along the c axis (Figure 4). The tetrahedral coordination
in ZnO results in non-centre symmetric structure. Another important characteristic of ZnO is polar surfaces.
Zinc oxide is the thoroughly studied post-transition-metal oxide for gas-sensing applications next to tin
oxide. A relative study shows that, 10 % of gas-sensing applications use ZnO based sensing materials. Point
defects on ZnO surfaces are extremely important in gas sensing as they produce very large changes in the
surface conductivity. The changes occur at the surface of the grains as a result of charge transfer and band
bending caused by the adsorbates. The dominant defects identified in these films are O vacancies. Heating
the films to high temperatures generally creates these vacancies. ZnO has been reported both in intrinsic
form as well as in doped form for various gas sensors applications. It is one of the most promising materials
for sensors due to its chemical stability, low cost and good flexibility in fabrication. ZnO has sensitivity
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Performance analysis of Zinc oxide based alcohol sensors
towards ethanol vapour and hydrogen.
Figure 4: The Wurtzite Structure Model of ZnO
4.3 Performance analysis on the detection of alcohols
Metal oxides are broadly used for detection of ethanol. Ethanol sensors are being enhanced and the
problems of sensitivity, selectivity, and stability are being addressed. Some of the most recent papers on
detection of alcohols are presented in Table 2. The metal oxide materials for alcohol detection are SnO2,
ZnO, WO3, TiO2 and Zr2O3. The analysis is carried out at temperatures above 400°C; therefore one
important task is to decrease the operating temperature. Beside the conductivity measurements for sensor
detection of alcohol vapors by metal oxides, the oxidation process can also be used. Oxidation of ethanol
may lead to formation of acetaldehyde. Oxidation of aldehyde is accompanied by photoluminescence,
which is used for detection of alcohol. The sensitivity and its dependence on alcohol vapour, flow rate and
temperature as well as selectivity and lifetime were investigated by luminescence studies.The highest
sensitivity was demonstrated on materials with the lowest ZnO nanorod diameter (100 nm). The authors
suggested the reason is the higher surface-to-volume ratio, which promotes the adsorption of oxygen and
higher concentration of lattice oxygen vacancies. Equation 4 represents the reaction between ethanol and
oxygen ions:
(4)
Some of the most recent papers on detection of alcohols are presented in Table 2. The most widely used
metal oxide materials for alcohol detection are doped and undoped ZnO.
Table 2: Some Recent Work on Detection of Alcohol
Material
Methods Used For Syntheses
Analyte
Conc. (ppm)
Temp. (°C)
Ref.
ZnO
Tetrapods
Oxidation Reaction Technique
Ethanol
50-1000
300, 400
[3]
ZnO, Al
RF Magnetron Sputtering
Ethanol
400
250
[4]
Al2O3 Doped
Screen Printing Technique
Ethanol
1000
300
[5]
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Performance analysis of Zinc oxide based alcohol sensors
ZnO
ZnO, TiO2
Thermal Oxidation Method
Ethanol
50-1000
300
[6]
ZnO
Nanobelts
RF Sputtering
Ethanol
50, 500, 1000
220
[7]
ZnO, Ni
Thermal Evaporation
Ethanol
10
30-100
[8]
IZO
Electron Beam Deposition
Technique
Ethanol
100
200
[9]
ZnO, CeO2
Chemical Precipitation Method
Ethanol
100
400-800
[12]
ZnO, Al
Soft Chemical Method
Ethanol
50
350
[13]
ZnO
Sol-Gel Method
Ethanol
0-250
350
[14]
Al2O3 Doped
ZnO
Screen Printing Technique
Ethanol
500
400
[15]
5. Conclusions and future scope
The considered applications and particular features of metal-oxide gas sensors allow us to formulate some
general trends in this actively developing field of science. There has been an increase in the number of
different dopants, particularly various metal and metal oxide nanoparticles and substrate materials which
are aimed at increasing the sensitivity and selectivity of metal-oxide gas sensors. Much effort is being made
to extend the working temperature range of metal-oxide gas sensors and lower the optimal working
temperature. The goal of these investigations is to decrease the power consumption of sensor elements. The
analysis of available papers on sensor applications of metal-oxide nanostructures enables the formulation of
a number of tasks that will draw the main efforts of scientists and engineers working on gas sensing
nanomaterials in near future. Detailed theoretical and experimental investigations are needed for deep
understanding of possible physical and chemical processes in real systems, involving metal oxides, dopants,
analytes and various other molecules, such as water, which may be present in the atmosphere in target
applications. This understanding can help in solving the problems with sensitivity, selectivity, and stability
of sensor materials. From a practical point of view, the problem of sensor stability deserves more attention
than it has now. Higher stability will decrease the frequency of or completely eliminate the need for
verification and re-calibration of sensors. Another important problem is the development of simpler sensor
materials and cheaper preparation methods.
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