NANO TECHNOLOGY IN CHEMICAL SENSORS

Abstract

Chemical sensor is a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal. There has been a strong demand for producing highly selective, sensitive, responsive, and cost effective sensors. Nanostructures, such as nanowires and nanotubes offer new and unique sensing opportunities. The new nanowires architecture, with high surface-to-volume ratio, makes possible the conducting polymers an ultrafast detection of chemical at low concentrations. Similarly Carbon nanotubes (CNTs) have many distinct properties that may be exploited to develop next generation of sensors. This manuscript reviews the distinct properties of nanomaterials. The main focus of this review is to highlight the development in the area of nanotechnology based sensors for real-world applications.

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NANO TECHNOLOGY IN CHEMICAL SENSORS
Tapankumar Mehta,*Neha Patni, Shibu.G.Pillai
Department of Chemical Engineering, Institute of Technology, Nirma University,S. G.
Highway, Ahmedabad-382481, Gujarat, INDIA
neha.patni@nirmauni.ac.in
Abstract
Chemical sensor is a device that transforms chemical information, ranging from the
concentration of a specific sample component to total composition analysis, into an
analytically useful signal. There has been a strong demand for producing highly
selective, sensitive, responsive, and cost effective sensors. Nanostructures, such as
nanowires and nanotubes offer new and unique sensing opportunities. The new
nanowires architecture, with high surface-to-volume ratio, makes possible the
conducting polymers an ultrafast detection of chemical at low concentrations.
Similarly Carbon nanotubes (CNTs) have many distinct properties that may be
exploited to develop next generation of sensors. This manuscript reviews the distinct
properties of nanomaterials. The main focus of this review is to highlight the
development in the area of nanotechnology based sensors for real-world applications.
1. Introduction
Demand for information in every aspect of day-to-day life has grown tremendously
since our civilization is becoming more technologically advanced. Sensors and
sensing technology play a crucial role in the process of information gathering. The
need for chemical sensing has increased in the last decade in the newer application
areas, viz., healthcare-genetics, diagnostics, and drug discovery, environmental and
industrial monitoring, quality control, etc.
1
Chemical sensors are defined as the
analytical devices that convert chemical information (composition, presence of a
particular element or ion, concentration, chemical activity, partial pressure, etc.) of
targeted analyte into a proportionate measurable signal. Chemical sensors usually
contain two basic components connected in series. A chemical (molecular)
recognition system (receptor) and a physicochemical transducer. In the majority of
chemical sensors, the receptor interacts with analyte molecules. Among the
interaction processes, the most important for chemical sensors are adsorption, ion

exchange and liquid-liquid extraction. Primarily these phenomena act at the interface
between analyte and receptor surface. As a result, its physical properties are changed
in such a way that the appending transducer can gain an electrical signal.
2
2 Nano technology based chemical sensors
Recently, nanomaterials in different structured forms such as nanowires, nanorods,
nanobelts, nanorings, and nanocrystals are used, as they provide remarkable high
surface area and interfacial properties compared to their counterpart at macroscopic
size scale, are increasingly attractive for different types of sensing devices. The
microstructure, namely high ratio of surface area-to-volume, very small grain and
pore sizes, and shape of metal/metal oxide particles, are important for sensing
properties, which gives better sensitivity and reproducibility. Similarly, the diameters
of nanostructured materials are comparable to the size of chemical species, which
sense and intuitively represent excellent primary transducers for producing signals.
3
These characteristics of the nanostructured materials and nanocrystals suggest that
devices based on these could revolutionize many aspects of sensing and detection of
chemical species.
2.1 Nanoparticle sensors
Semiconductor metal oxide (SMO) materials such as SnO
2
, ZnO, TiO
2
, WO
3
, etc.
have certain specific advantages such as higher robustness, up to 10 years of life, less
sensitive to environmental moisture and temperature, simple interface electronics,
faster response and recovery time, etc. The sensing properties of SMOs are highly
influenced by a number of parameters including phase composition, structure, type of
dopants, morphology, grain size, etc. The nanoparticles are the most studied materials
due to certain specific advantages such as ease of preparation, high surface-to-volume

ratio, scope of developing large variety of materials with very good control on the
particle size, availability of greater number of active centres, also shown in table 1.
Table 1: Researches in Sensors based on nanoparticle
Study
Result
Reference
Electrical response of SnO
2
particles
with nanosized SnO
2
and SnO
2
-Pd
to LPG
drastic increase in the sensitivity
of the sensor fabricated using
nanosized SnO
2
Sarala
Devi
4
Nanocrystalline ZnO-based thick
films for NH
3
higher sensitivity, relatively fast
response and recovery time (35 s)
Sarla
Devi
5
The effect of particle size on the
sensitivity of CuO-doped SnO
2
SnO
2
(CuO) is more sensitive to
NO as compared to pure SnO
2
Zhang
6
Comparative studies on H
2
sensing
properties of TiO
2
and mixed
samples of SnO
2
and TiO
2.
Samples with higher surface
areas were more sensitive to H
2
in the presence of oxygen.
Akbar
7
The effects of various metal
additives, viz., Au, Ag, Co,V etc on
the gas-sensing performances of
TiO
2
nanocrystals.
Au was found to be the most
attractive promoter for the CO
sensing properties.
Ruiz
8
Nanocomposites of metals and
mixed metal oxides in a copolymer
matrix of aniline-formaldehyde.
The films of aluminum-doped
Fe
2
O
3
showed high sensitivity,
towards polar toxic gases such as
NH
3
, CO, HCl, HCN etc.
Kumar
9
2.2. Nanowire based sensors
Amongst various nanostructures, inorganic quasi one-dimensional (Q1D) systems
show promising sensing capabilities for gas molecules and biological species. Based
on the mechanism of detection, various Q1D systems have been configured as
chemical sensors for detection of toxic and flammable gases such as NO
2
, CO, NH
3
,
ethanol, etc as shown in Table 2.
Table 2: Research in Nanowire based sensors
Study
Result
Reference

ZnO nanowire field effect in
chemical sensor for detection of
NO
2
and NH
3
at room temperature
Ammonia sensing observed to
switch from oxidizing to
reducing when temperature was
increased from 300K to 500K.
Fan
10
ZnO nanowires by electrochemical
deposition in alumina membranes as
well as ZnO nanotubes.
Nanowires after impregnation
with Pt show excellent
sensitivity to H
2
and low
concentration of ethanol vapour
Rao
11
Gas sensors from In
2
O
3
nanowires
and investigated their gas-sensing
properties.
Excellent performance, good
selectivity, very short response
time to dilute C
2
H
5
OH
Xiangfeng
1
2
Nanostructured titania pad arrays in
form of sponge-like structure
consisting of interconnected
nanoscale wires and walls.
Operate at lower temperature,
show fast response time and
superior sensitivity as oxygen
sensors.
Zuruzi
13
Chemiresistor-type gas sensor by
the deposition of V
2
O
5
nanofibres
from aqueous suspension onto
silicon substrate.
Extremely sensitive for 1-
butyalamine and moderate
sensitivity for ammonia.
Rible
14
Composite nanostructures of
polymer (PVP) and metal-oxide
nanofibres/nanowires of MoO
3
/WO
3
Used as NO
2
gas-sensing
elements, higher sensitivity,
faster response and lower gas
detection limits
Sawicka
15
Sensors of GaN nanowires
decorated by gold nanoparticles and
their exposure to Ar, N
2
gas.
Exhibit chemically selective
response to Ar and slightly
greater response to N
2
.
Dobrokhot
ov
16
Arrays of Pd nanowire on to the top
step edges of pyrolytic graphite
Fast response to H
2
gas
Atsbar
17
Si nanowire film used as a sensor for
glucose detection in aqueous
solution.
Show high sensitivity, good
reproducibility and long term
stability.
Shao
18
Grown metal-catalysed silicon
nanowires in between silicon
electrodes studied for vapours of
HCl or NH
3
at reduced pressure.
Nanowires showed increase in
conductance while exposed to
HCl vapour and decrease in
conductance to NH
3
vapour.
Kamins
19
2.3 Nanotube based sensor
A single-walled carbon nanotube (SW-CNT) is a nano scale tube 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-meters sized CNTs

have novel electronic properties and can be used as electronic wire between two metal
electrodes and the conductance between the electrodes can be measured, shown in
Table 3. Since the nanotube electronic property is a strong function of its atomic
structure, mechanical deformations or chemical doping can induce strong changes in
conductance, detected by electron current signals, and these properties make CNTs
extremely small sensors sensitive to their chemical and mechanical environments.
Table 3: Research in Nanotubes based Sensors
Study
Result
Reference
Single-walled carbon nanotube
(SWCNT) sensor for exposure to
gaseous molecules such as NO
2
and NH
3
Increase in the conductivity after
exposing the SWCNT to NO
2
, while
conductivity decreases upon
exposure to 1 per cent NH
3
vapour
Kong
20
Composite films of SWCNT
mesh doped with alkanethiol
monolayer protected gold clusters
(MPC).
The detection limit of NO
2
gas was
improved 9.6-fold compared with
pure SWCNT sensors.
Young
21
New hybrid SWCNTs/SnO
2
gas
sensors by adding SWCNTs into
SnO
2
substrate.
Higher sensitivity for the detection
of NO
2
in flowing air or N
2
Wei
22
Microacoustic sensors, wherein
SWCNTs were embedded in
cadmium arachidate (CdA)
amphiphilic matrix.
Sensitivity to ethanol, ethyalacetate,
and toluene was up to two times
higher than that of unembedded CdA
devices.
Penza
23
Conclusion
The unique and fascinating properties of nanostructured materials have triggered
tremendous motivation among researchers to explore the possibilities of using these in
various shapes and sizes for sensing applications. Nowadays, the ability to control the
particle size and morphology of nanoparticles is of crucial importance from the
fundamental and industrial points of view, as promised by nanomaterials of different
categories (metal oxide, self-assembled structures, carbon nanotubes, etc).
Characteristically high surface-to-volume ratio of nanostructured materials leads to
increased reactivity and catalytic properties which are particularly very attractive for
their chemical sensor applications.

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