In this review paper, we discuss various gas sensors based on technique and sensing materials used in there fabrication. Various sensors are designed making use of salient features of carbon nanotubes and its electrical, mechanical, and electromechanical properties. Effect of using nano-composites on sensitivity and selectivity of gas sensor have been studied.

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Carbon Nanotubes Based Sensor for Detection
of Traces of Gas Molecules- A Review
1
Arvind Kumar, 2
Jaspreet Kaur Rajput, 3
Sukhvir Kaur
1,
Department of ECE,UIET Panjab University, Chandigarh 160025
2,
Dr. B.R. Ambedkar National Institute of Technology, Jalandhar 144011
Abstract- In this review paper, we discuss various gas sensors
based on technique and sensing materials used in there
fabrication. Various sensors are designed making use
of salient features of carbon nanotubes and its electrical,
mechanical, and electromechanical properties. Effect of using
nano-composites on sensitivity and selectivity of gas sensor
have been studied.
Index Terms – Carbon Nanotubes (CNTs), Electrochemical
sensor (E-nose), sensors.
I. INTRODUCTION
Sensors are the devices which convert any form of energy
or any physical parameters into electrical
signals/measurable signals. Pressure sensor, gas sensor,
humidity sensor, chemical sensors are some few examples
of sensors available in the market. Gas sensors are the
devices that are used to detect gas molecules in atmosphere
which may cause harm to our health.
There are more than one hundred types of military and
civilian explosives and around twenty commonly used
drugs. A number of characteristics can be used for the
detection of gas molecules [1]:
˗ Geometry (the presence a metallic detonator can be
detected using image shape analysis),
˗ Material density (explosive material is denser
than most organic materials),
˗ Elemental composition (e.g. vapor emission
analysis can be used to detect them),
˗ Vapor emissions (e.g. nitrogen or its compounds can be
detected in a vapor sample).
A. Detection methods
Vapor detection methods are used to measure traces of
characteristic volatile compounds that evaporate from the
explosive and other gas molecules [1]. However, the
concentration of explosive vapors inside the sensors is
many orders of magnitude lower than the pressure of
saturated vapor on the explosives surface.
Vapors and traces are commonly detected by means of:
- Electronic/chemical sensors,
- Optical sensors,
- Biosensors..
A new type of pressure sensor propose approaches for
improving sensitivity, selectivity and size. The most
powerful pressure sensor contains microelectronics
circuits, which enable to install a digital pressure
gauge just in sensor and software control starting of
different electronic regulations in according to the
measured values. Special pressure sensors present sensors
for explosive environment.
B. Electrochemical sensor (E-nose)
While some commercial products for detecting trace
energetic chemicals including explosives are now
available, the need of highly sensitive and fast detection
methods is continuously increasing and the research into
alternative technologies continues at a pace. This is a
result of the fact that detecting trace chemicals in a
complex environment still remains a significant
technological challenge because of the extremely low
concentrations of the chemicals in solutions or low partial
pressures in the air. For example, the partial pressure of
2,4,6-trinitrotoluene (TNT) is a few parts per billion
(ppb) at room temperature and in the subpart per
trillion range in the air above a buried mine. Nowadays,
electronic nose has become a powerful tool to detect even
the traces of explosive materials [2].
To address this challenge, various detection methods with
potential to achieve low limit of detection (LOD) are
being investigated for trace chemical detection, such as
electro-chemical sensors, biosensor, fluorescence and
Raman-based optical
methods, mass spectrometry (MS), ion mobility
spectrometry (IMS), and sensors based on nano and
microfabrication technologies.
Electrochemical sensors provide some selectivity but
suffer from limited sensitivity and require mobile
electrolytes, which may cause stability issues and delayed
response time. In addition, electrodes can be easily fouled,
and interfering problems may occur as some interferents
are electrochemically active. Nanomaterials, e.g., carbon
nanotubes (CNTs) and nanowires, have been employed to
construct electrochemical sensors for explosive detections
[3]. CNTs applications include CNTs field-effect
transistors or resistors and CNTs modified glassy carbon
electrodes. More recently, CNTs have been employed in
explosive sensors by exploiting passive functions rather
than transduction, such as enhanced Raman scattering and
preconcentration of target samples. LODs down to the ppb
level have been achieved for TNT detection by exploiting
the extremely large surface-to-volume ratio. Despite high
sensitivity, significant challenges such as low selectivity,
low signal-to-noise ratio (SNR), long recovery times,
interference, and device fabrication difficulties still remain
to be addressed [4-6].
C. CNTs as sensing material
Detection of trace explosives is still a challenging task
because of the extremely low vapor concentrations.
Current explosive/gas sensors are mainly categorized into
two modes of operation; chemical type operating by gas
adsorption and physical type using ionization method.
Chemical type conductivity-based explosive detectors are
bulky, they require high operating temperatures and have
slow response time. Moreover most of them are capable of

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detecting only single type fumes of explosive materials
due to their low selectivity. While on other hand
ionization-based sensors have better selectivity and
response time, but their huge size becomes its limitation.
Both chemical and physical type gas detectors are using
semiconductor materials as their sensing elements. With
the discovery of nanomaterials, different types of sensing
elements have been investigated to produce sensors which
are smaller in size, one of which is carbon nanotubes
(CNTs). Development of high performance sensor is
now focused towards CNT- based sensors because of
their inherent properties such as small size, large surface
area and high electrical conductivity. Sensors designed
using CNTs are smaller in size, have lower power
consumption, high sensitivity and better selectivity
compared to existing semiconductor based gas sensors.
CNT- based gas sensors operate at room temperature
which will result in safer environment. Researches
investigates the structural and electrical characterization
of carbon nanotubes array as
suitability and an effective sensing element in the
ionization-based sensor. The atomic structure of SWCNT
can be formed by wrapping a stripe of single atomic
layer of graphite sheet along a certain direction and this
direction determines the diameter and chirality of the
nanotubes. Experimental and theoretical studies have
found that these nano-meter sized CNTs have novel
electronic properties that is these can be metallic or
semiconducting, depending on their radius or chiralities.
CNTs possess unique set of electrical and mechanical
properties which make them useful in variety of
applications [7]:
• 100 times stronger than steel and 1/6th
the
weight of steel (Tensile strength value, 63
GPa, exceeds that of any reported value for
any type of material. Applications for very light-weight,
high-strength cables and composites, where the carbon
nanotubes are the load-carrying element.)
• Electrical conductivity as high as copper,
thermal conductivity as high as diamond.
• Average diameter of 1.2 – 1.4 nm
(10000 times smaller than a human hair).
CNTs are categorized as conductor, semiconductor or
insulator based on their structure (patterns in which
graphene sheets are folded). Carbon nanotubes
labeled as armchair show electrical properties similar to
metals. When potential is applied to ends of CNTs
current start flowing even they have conductivity better
than copper [8].
II. CNTs AS A SENSOR
Seong jeen kim presented CNT based sensor for detection
of gas molecules, in this CNTs were used with silane
hybrid thin film deposited by spray coating on
substrate. It has shown great potential in sensitivity,
operation at room temperature, small size. In this various
silane binders are used to improve selectivity of the
sensor, response property to different gas will be
changed if the binder in the solution is different. Here
comparison between MTMS and TEOS has been
observed. With the improvement in technology to reduce
cost and improve sensitivity, metal oxides are being used
for doping CNTs in order to improve selectivity [9].
Chatchawal wongchoosuk come with an idea of
electronic nose based on carbon nanotubes doped with
SnO2 used as a gas sensor. MOS based sensor generally
exhibit relatively poor selectivity, in this E- nose various
processing elements are used. Features extraction plays a
main role for pre-processing steps, these help to enhance
performance through more stable data representation. The
doping of CNTs improves selectivity and sensitivity of
MOS SnO2 gas
sensor as CNT provide large surface area that increase
gas reaction at the metal oxide.
Sensor is fabricated using electron beam evaporation.
Initially Cr/Al layer is deposited over the substrate, sensing
film of thickness around 300 nm is deposited. NiCr layer
was evaporated over substrate backside used as heating
element [11].
A. R. Kermany came forward with an idea of ionization
based gas sensor. In this an array of CNTs are aligned in a
particular fashion for sensing gas molecules. This sensor is
based on fingerprinting the ionization characteristics of
different gases. Every gas has unique breakdown voltage
which is used for gas identification. An array of CNTs is
working as gas sensor; in this CNTs are used as anode and
aluminum as cathode. Both plates are connected to voltage
source, this applied voltage provide energy for ionization
[12-14].
Though they have high selectivity and good response time
but it still need improvement in its large size and high input
voltage required for operation. To improve previously
designed sensors Hoel Guerin fabricated conductance
based gas sensor with the help of CNTs as sensing
material. Semiconducting CNTs exhibits change in
resistance upon adsorption of gas molecules on CNTs
surface. As to improve selective detection of analytes
different metals are used as electrode along with CNTs
each distinct CNT-metal contact behaves differently with
distinct gas molecules. Now this difference in resistance
value could be used as electronic signature to identify the
gas.
Wenzhou Ruan emphasis on Low limit of detection (LOD)
to detect traces of gas molecules deficiently, various
methods is investigated to achieve LOD for trace chemical
detection. Electrochemical sensor suffer from limited
selectivity to overcome this mobile electrolytes are
required and they add up to stability issues and delay in
response time. Microsensors are gaining importance in
trace detection techniques. Microcantilevers and
microcalorimeter are typically used techniques in
microsensing. Microcalorimeter constitute three part a
microsuspended membrane, heater, thermistor and a CNT
film on the surface of membrane. Single crystalline silicon
is used for fabrication of heater and thermistor [16].
Suspended membrane is heated to deflagration temperature
due to high thermal conductivity CNT is also heated.
When it is exposed to gas molecules they get adsorbed on
CNT surface and go through endothermic or exothermic
reactions. This change in temperature is measured by
thermistor signifying detection of gas molecules. In this set
up it
utilizes high surface area and thermal conductivity of

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135 NITTTR, Chandigarh EDIT-2015
carbon nanotubes.
In the above discussion various gas sensors have
been studied for detection of gas molecules, every sensor
comes with its own pros and cons. Along with there
effective output they do have some limitations which are
need to be improved.
III. CONCLUSION
Electrochemical sensors provide some selectivity but
suffer from limited sensitivity and require mobile
electrolytes [17], which may cause stability issues and
delayed response time. In addition, electrodes can be
easily fouled, and interfering problems may occur as
some interferents are electrochemically active.
Nanomaterials, e.g., carbon nanotubes (CNTs) and
nanowires, have been employed to construct
electrochemical sensors for explosive detections. Typical
CNTs applications include CNTs field-effect transistors
or resistors and CNTs modified glassy carbon electrodes.
More recently, CNTs have been employed in explosive
sensors by exploiting passive functions rather than
transduction, such as enhanced Raman scattering and
preconcentration of target samples. LODs down to the
ppb level have been achieved for TNT detection by
exploiting the extremely large surface to volume ratio.
Despite high sensitivity, significant challenges such as
low selectivity, low signal-to-noise ratio (SNR), long
recovery times, interference, and device fabrication
difficulties still remain to be addressed [18].
To further improve selectivity and sensitivity for sensing
gas molecules, functionalization of CNT are required.
Functionalization with particular metal oxide or
nanocomposite can improve selectivity of the sensor.
Use of hydrophobic compounds can protect false alarm
from water vapours. Nano- materials provide large
surface area so they should be used as sensing materials.
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