gap, work function, carrier mobility, etc.) of channel materials govern the performance of the sensor. 1-dimensional (1-D) nanomaterials like SWCNTs, silicon
nanowires (SiNWs), metal oxide nanowires (MONWs), and 2-D nanomaterials
have been used successfully as FET channel materials (Mao et al. 2017). The lack
of specificity for a particular target molecule, limits the applications of pristine
nanowires. To overcome this problem, researchers have developed surface
functionalized and hybrid gas sensors, for e.g., Chen et al. (2009) developed an
electronic nose hybrid nanosensor composed of SWCNT and nanowires of In 2 O 3 ,
ZnO, SnO 2 and used this for sensing ethanol, NO 2 and H 2 gases. Choi et al.
functionalized SiNWs with palladium to design FET-based hydrogen gas sensor
(Choi et al. 2015). Besides, nanowires-based FET sensors have also been applied in
the field of biomedical sensor applications (Ambhorkar et al. 2018). Metal oxides
nanomaterials used for conducting channel like SiO 2 , ZnO, SnO 2 and TiO 2 are
highly reactive and hence, can act as suitable materials for surface functionalization.
The oxide surfaces can be modified by different self-assembled monolayers (SAMs).
For instance, oxide surfaces are provided with covalent bonds by carbonyl, phosphoric acid and silanol groups. Properties of SAM layer modified surface depend on
the functional groups present on them, for e.g., amine-terminated SAMs show
selectivity for H
+ ions. Surface functionalization of SiNW FETs with receptors
like antigens, biotin and amine-terminated silanol has made them applicable for
efficient detection of bio-substances in aqueous medium samples.
Nanomaterials with 2-D layered structures, like graphene and reduced graphene
oxide (RGO) possess unique electronic properties and large ratio of surface area to
volume, thus making them suitable materials for FET sensors. For gas sensing
applications, the target gases and 2-D nanomaterials interact with physical adsorption, and the 2-D nanomaterials undergo enhancement or reduction in the conductance, depending on the type of both the gas species (electron acceptor oxidizing
gases like NO 2 /electron donor reducing gases like NH 3 ) and the semiconductor
(n-type/p-type) (Mao et al. 2017). The selectivity/sensitivity of the gas sensors can
be improved by nanoparticles (NPs like WO 3 , Ag, SnO 2 and Pt) deposition on the
channel materials. For e.g., NPs and 2-D nanomaterials can be used to form hybrid
structures for sensitive and selective gas sensing applications. The gases interact
directly with the coated NPs, leading to the conductivity change of the hybrids, while
the 2-D materials work as underlying conducting channel. Besides gas sensing, 2-D
semiconductor nanomaterials with tunable and appropriate band gap can also be
used favourably for water sensor and biosensor applications (Mao et al. 2017).
Electrochemical nanosensors can also be modified for detection of proteins.
Conjugation of immunoassays with nanosensors can be used to develop
immunosensors. Electrochemical sensing techniques have been used for the detection of a large variety of analytes for environmental applications.
5 Development of Environmental Nanosensors for Detection Monitoring. . .
103
nanowires (SiNWs), metal oxide nanowires (MONWs), and 2-D nanomaterials
have been used successfully as FET channel materials (Mao et al. 2017). The lack
of specificity for a particular target molecule, limits the applications of pristine
nanowires. To overcome this problem, researchers have developed surface
functionalized and hybrid gas sensors, for e.g., Chen et al. (2009) developed an
electronic nose hybrid nanosensor composed of SWCNT and nanowires of In 2 O 3 ,
ZnO, SnO 2 and used this for sensing ethanol, NO 2 and H 2 gases. Choi et al.
functionalized SiNWs with palladium to design FET-based hydrogen gas sensor
(Choi et al. 2015). Besides, nanowires-based FET sensors have also been applied in
the field of biomedical sensor applications (Ambhorkar et al. 2018). Metal oxides
nanomaterials used for conducting channel like SiO 2 , ZnO, SnO 2 and TiO 2 are
highly reactive and hence, can act as suitable materials for surface functionalization.
The oxide surfaces can be modified by different self-assembled monolayers (SAMs).
For instance, oxide surfaces are provided with covalent bonds by carbonyl, phosphoric acid and silanol groups. Properties of SAM layer modified surface depend on
the functional groups present on them, for e.g., amine-terminated SAMs show
selectivity for H
+ ions. Surface functionalization of SiNW FETs with receptors
like antigens, biotin and amine-terminated silanol has made them applicable for
efficient detection of bio-substances in aqueous medium samples.
Nanomaterials with 2-D layered structures, like graphene and reduced graphene
oxide (RGO) possess unique electronic properties and large ratio of surface area to
volume, thus making them suitable materials for FET sensors. For gas sensing
applications, the target gases and 2-D nanomaterials interact with physical adsorption, and the 2-D nanomaterials undergo enhancement or reduction in the conductance, depending on the type of both the gas species (electron acceptor oxidizing
gases like NO 2 /electron donor reducing gases like NH 3 ) and the semiconductor
(n-type/p-type) (Mao et al. 2017). The selectivity/sensitivity of the gas sensors can
be improved by nanoparticles (NPs like WO 3 , Ag, SnO 2 and Pt) deposition on the
channel materials. For e.g., NPs and 2-D nanomaterials can be used to form hybrid
structures for sensitive and selective gas sensing applications. The gases interact
directly with the coated NPs, leading to the conductivity change of the hybrids, while
the 2-D materials work as underlying conducting channel. Besides gas sensing, 2-D
semiconductor nanomaterials with tunable and appropriate band gap can also be
used favourably for water sensor and biosensor applications (Mao et al. 2017).
Electrochemical nanosensors can also be modified for detection of proteins.
Conjugation of immunoassays with nanosensors can be used to develop
immunosensors. Electrochemical sensing techniques have been used for the detection of a large variety of analytes for environmental applications.
5 Development of Environmental Nanosensors for Detection Monitoring. . .
103