5.3.1.3 Mechanical/Acoustic Detection
Nanomechanical sensors are sensitive to mass and these are based on detection of
displacements, mechanical forces and changes in mass. Overall mass of the mechanical device is related to the mass to be determined. Hence, when mechanical sensor’s
mass is reduced to nanoscale, there is enhancement in the detection of mass (Ku et al.
2013). The major obstacle in mechanical nanosensors is the reduction of sensor
sensitivity due to viscous damping (Burg et al. 2007). Cantilevers enclosed with
fluid filled channels can be used for fluid measurements. However sensitivities
achieved for such cantilevers is lower than that obtained in gas phase. Barun et al.
devised resonant cantilever sensing array, which were used against E. coli (Barun
et al. 2009).
Piezoelectric cantilever sensors (PECS) were reported for detection of
S. typhimurium, which showed greater sensitivity than array biosensors and
ELISA (Shih and Shih 2007; Rowe et al. 1999). PECS sensors are types of mass
sensors based on the change in mechanical resonance frequency due to binding of
analyte molecules resulting in a change in mass. PECS can be miniaturized, and
involve electrically driven mechanical resonance and sensing, giving them advantage over other techniques. Target molecules can attach with the receptors coated on
the surface of piezoelectric device (Sakti et al. 1999; Ward and Buttry 1990). This
change in resonant frequency can be monitored and simple electrical measurements
with PECS can be used for fast, label-free and quantitative in situ pathogen detection
(Shih and Shih 2007).
5.3.1.4 Magnetic Transduction
Magnetic properties of some nanomaterials can be explored to design nanosensors
which can work on the principle of magnetic transduction. Nanosensing devices like
magnetic relaxation switching assays-(MRWs) based nanosensors, utilize these
properties of magnetic nanoparticles (MNPs) such as superparamagnetic iron
oxide nanoparticles (SPIOs), and such types of nanosensors can be applied for the
detection environmental contaminants like biological macromolecules, small
organic molecules and heavy metal ions. Upon interaction with the target analyte,
the individual nanomagnetic probes (MNPs) get clustered into larger assemblies,
causing inhomogeneity in the magnetic field. As a result, the surrounding water
protons experience enhancement in the dephasing of spins. The detection of this
subsequent variation in the spin-spin (transverse; T 2 ) relaxation of the water
molecules can be done using techniques like magnetic resonance relaxometry
(Willner and Vikesland 2018). Rational design of MNPs with high T 2 relaxivity
can result in high sensitivity of MRWs-based nanosensors. Also, the specificity of
receptor (oligonucleotides, proteins, small-molecule ligands, etc.) is important for
designing the nanosensor. Mostly iron oxide (Fe 2 O 3 ) NPs are used for these
applications due to their high mass magnetization value (Ms). Doped and core/
shell MNPs have also been developed for improved magnetic properties, as
discussed in detail in Sect. 5.3.2.2.
MRWs operation involves radio frequency (RF), and due to the deep-penetrating
power of RF radiations, MRWs-based sensing can be performed in light104
U. Chakraborty et al.
Nanomechanical sensors are sensitive to mass and these are based on detection of
displacements, mechanical forces and changes in mass. Overall mass of the mechanical device is related to the mass to be determined. Hence, when mechanical sensor’s
mass is reduced to nanoscale, there is enhancement in the detection of mass (Ku et al.
2013). The major obstacle in mechanical nanosensors is the reduction of sensor
sensitivity due to viscous damping (Burg et al. 2007). Cantilevers enclosed with
fluid filled channels can be used for fluid measurements. However sensitivities
achieved for such cantilevers is lower than that obtained in gas phase. Barun et al.
devised resonant cantilever sensing array, which were used against E. coli (Barun
et al. 2009).
Piezoelectric cantilever sensors (PECS) were reported for detection of
S. typhimurium, which showed greater sensitivity than array biosensors and
ELISA (Shih and Shih 2007; Rowe et al. 1999). PECS sensors are types of mass
sensors based on the change in mechanical resonance frequency due to binding of
analyte molecules resulting in a change in mass. PECS can be miniaturized, and
involve electrically driven mechanical resonance and sensing, giving them advantage over other techniques. Target molecules can attach with the receptors coated on
the surface of piezoelectric device (Sakti et al. 1999; Ward and Buttry 1990). This
change in resonant frequency can be monitored and simple electrical measurements
with PECS can be used for fast, label-free and quantitative in situ pathogen detection
(Shih and Shih 2007).
5.3.1.4 Magnetic Transduction
Magnetic properties of some nanomaterials can be explored to design nanosensors
which can work on the principle of magnetic transduction. Nanosensing devices like
magnetic relaxation switching assays-(MRWs) based nanosensors, utilize these
properties of magnetic nanoparticles (MNPs) such as superparamagnetic iron
oxide nanoparticles (SPIOs), and such types of nanosensors can be applied for the
detection environmental contaminants like biological macromolecules, small
organic molecules and heavy metal ions. Upon interaction with the target analyte,
the individual nanomagnetic probes (MNPs) get clustered into larger assemblies,
causing inhomogeneity in the magnetic field. As a result, the surrounding water
protons experience enhancement in the dephasing of spins. The detection of this
subsequent variation in the spin-spin (transverse; T 2 ) relaxation of the water
molecules can be done using techniques like magnetic resonance relaxometry
(Willner and Vikesland 2018). Rational design of MNPs with high T 2 relaxivity
can result in high sensitivity of MRWs-based nanosensors. Also, the specificity of
receptor (oligonucleotides, proteins, small-molecule ligands, etc.) is important for
designing the nanosensor. Mostly iron oxide (Fe 2 O 3 ) NPs are used for these
applications due to their high mass magnetization value (Ms). Doped and core/
shell MNPs have also been developed for improved magnetic properties, as
discussed in detail in Sect. 5.3.2.2.
MRWs operation involves radio frequency (RF), and due to the deep-penetrating
power of RF radiations, MRWs-based sensing can be performed in light104
U. Chakraborty et al.