applications (Yu et al. 2014). MOX display high sensitivity towards chemical
environment changes, and these have many physical, chemical and electronic
properties. Doped and well-structured MOX (mainly ZnO and SnO 2 ) have been
used for commercially available stable solid-state chemical sensors with low cost of
production, and these sensors have been utilized for highly sensitive gas detection
applications. The basic mechanism of MOX-based gas sensors involves charge
transfer between analyte molecules and surface complexes (O
2À ,O
À , H
+ , OH
À ),
leading to electrical conductivity change. This process demands energy of activation,
hence, classical MOX sensors only work at high temperatures (>200
C) (Francia
et al. 2009). MOX nanomaterials have properties like stability, distinct electrochemical activity, high capacity for adsorption and large surface area, which are significant for electrochemical sensor fabrication. Features like particle size, morphology,
functionality on the surface, and surface area, determine the analytical performance
of MOX nanomaterials (Zhang and Gao 2019). Semiconductor MOX nanostructures
like ZnO, In 2 O 3 , TiO 2 , NiO, WO 3 and SnO 2 have been explored for preparation of
resistive gas sensors for the detection of volatile organic compounds (VOCs) and
toxic pollutant gases. The principle of operation of such sensors involves change in
resistance on variation of the molecules of test gas on the surface of electrode. For
improvement in the LOD and sensitivity, numerous research activities have been
performed for designing hierarchical MOX nanostructures (Shimizu et al. 2001).
1-D MOX nanostructures have also shown great potential for electrochemical
detection of environmental contaminants.
Nanosensors fabricated from pure and composite MOX nanomaterials like ZnO,
SnO 2 , TiO 2 , etc., have been well explored for environmental applications. Tin
oxides (SnO 2 ) NPs have been the most applied materials for gas sensing
applications. For e.g., Khoang et al., reported highly selective and sensitive detection
of ethanol using hierarchical SnO 2 /ZnO nanostructures (Khoang et al. 2012). Apart
from gas sensing, ultra-trace detection of heavy metal ions was reported with SnO 2
combined with rGO, the analysis was done in drink water samples (Maduraiveeran
and Jin 2017).
Zinc oxide (ZnO) in a n-type semiconductor material, and ZnO nanostructures
have numerous advantages over bulk ZnO, like excellent properties for electron
transfer, low cost, eco-friendly, cost-effective, high ratio of surface to volume, and
ability to be synthesized with many morphologies like nanorods, nanowires,
nanotubes, nanoflakes, etc., making them suitable for sensing of a large variety of
analytes (Napi et al. 2019). ZnO nanomaterials are suitable materials for gas sensing
applications due to high conductivity, biocompatibility, and chemical/thermal stability resistance to oxidation., etc. Zhang et al. reported the fabrication of 3D flowerlike ZnO nanostructures and discussed their excellent gas sensing properties for
n-butanol due to their large surface area and higher number of surface active sites
(Zhang et al. 2012). ZnO nanostructures have also been used for other sensing
application. For instance, Kumar et al. (2019) prepared low cost pH nanosensor
for pH measurements in water, using interdigited electrodes (IDEs) synthesized
using hydrothermally grown ZnO nanorods (NRs).
5 Development of Environmental Nanosensors for Detection Monitoring. . .
111
environment changes, and these have many physical, chemical and electronic
properties. Doped and well-structured MOX (mainly ZnO and SnO 2 ) have been
used for commercially available stable solid-state chemical sensors with low cost of
production, and these sensors have been utilized for highly sensitive gas detection
applications. The basic mechanism of MOX-based gas sensors involves charge
transfer between analyte molecules and surface complexes (O
2À ,O
À , H
+ , OH
À ),
leading to electrical conductivity change. This process demands energy of activation,
hence, classical MOX sensors only work at high temperatures (>200
C) (Francia
et al. 2009). MOX nanomaterials have properties like stability, distinct electrochemical activity, high capacity for adsorption and large surface area, which are significant for electrochemical sensor fabrication. Features like particle size, morphology,
functionality on the surface, and surface area, determine the analytical performance
of MOX nanomaterials (Zhang and Gao 2019). Semiconductor MOX nanostructures
like ZnO, In 2 O 3 , TiO 2 , NiO, WO 3 and SnO 2 have been explored for preparation of
resistive gas sensors for the detection of volatile organic compounds (VOCs) and
toxic pollutant gases. The principle of operation of such sensors involves change in
resistance on variation of the molecules of test gas on the surface of electrode. For
improvement in the LOD and sensitivity, numerous research activities have been
performed for designing hierarchical MOX nanostructures (Shimizu et al. 2001).
1-D MOX nanostructures have also shown great potential for electrochemical
detection of environmental contaminants.
Nanosensors fabricated from pure and composite MOX nanomaterials like ZnO,
SnO 2 , TiO 2 , etc., have been well explored for environmental applications. Tin
oxides (SnO 2 ) NPs have been the most applied materials for gas sensing
applications. For e.g., Khoang et al., reported highly selective and sensitive detection
of ethanol using hierarchical SnO 2 /ZnO nanostructures (Khoang et al. 2012). Apart
from gas sensing, ultra-trace detection of heavy metal ions was reported with SnO 2
combined with rGO, the analysis was done in drink water samples (Maduraiveeran
and Jin 2017).
Zinc oxide (ZnO) in a n-type semiconductor material, and ZnO nanostructures
have numerous advantages over bulk ZnO, like excellent properties for electron
transfer, low cost, eco-friendly, cost-effective, high ratio of surface to volume, and
ability to be synthesized with many morphologies like nanorods, nanowires,
nanotubes, nanoflakes, etc., making them suitable for sensing of a large variety of
analytes (Napi et al. 2019). ZnO nanomaterials are suitable materials for gas sensing
applications due to high conductivity, biocompatibility, and chemical/thermal stability resistance to oxidation., etc. Zhang et al. reported the fabrication of 3D flowerlike ZnO nanostructures and discussed their excellent gas sensing properties for
n-butanol due to their large surface area and higher number of surface active sites
(Zhang et al. 2012). ZnO nanostructures have also been used for other sensing
application. For instance, Kumar et al. (2019) prepared low cost pH nanosensor
for pH measurements in water, using interdigited electrodes (IDEs) synthesized
using hydrothermally grown ZnO nanorods (NRs).
5 Development of Environmental Nanosensors for Detection Monitoring. . .
111