Sensors - Hierarchical Nanostructures for Energy Devices - page 245

As a further step, researchers have developed hierarchical nanostructures
for sensing applications in order to retain a higher surface-to-volume ratio.
Hierarchical nanostructures are termed as the higher dimensional struc-
tures that are composed of low dimensional nano building blocks such as
nanoparticles, nanowires, nanorods, nanotubes and nanosheets, etc. 5 With
hierarchical nanostructures, almost all of the surface is exposed to the en-
vironment, meaning an extremely high surface-to-volume ratio, so that a
slight change in analyte concentration can induce a great change in output
signal (high sensitivity/resolution). Such structures also facilitate the dif-
fusion and mass transport of chemical species such as molecules and ions in
bulk analyte samples to the sensor's surface, so that sensors do not dem-
onstrate diffusion-limited behavior. 5 Therefore, target molecules quickly
obtain access to the surface leading to fast changes in the property of
sensing materials (fast response/recovery).
In this chapter, sensors that use hierarchical nanostructures are sum-
marized and discussed in terms of gas sensors, biosensors and surface
enhanced Raman spectroscopy (SERS) sensors. The classification of sensors
is dependent on the kind of target and transducing mechanism. The targets
are limited to gas phase molecules such as ethanol, methanol, acetone,
liquid petroleum gas (LPG), etc in gas sensors whereas biosensors employ a
special type of liquid analyte that plays an important role biologically or
biochemically. Proteins, RNA/DNAs, cells and tissues may become other
types of target substances for biosensors. However, the classification of SERS
sensors is based on the transducing mechanism, especially using the optical
technique. The targets are not limited in SERS sensors, since they use
spontaneous adsorption of molecules to the surface, where the Raman sig-
nal is greatly enhanced. Nevertheless, SERS sensors are of interest in this
chapter, since hierarchical nanostructures are a very promising platform for
SERS, which will be discussed later. Firstly, the hierarchy and morphology
suitable for sensing applications are summarized and discussed. Then, the
preparation of hierarchical nanostructures for sensors is briefly presented in
terms of the material type, which has its own application. The various sen-
sors that use hierarchical nanostructures are summarized and discussed in
terms of sensor performance. One will see the sensing performance sig-
nificantly increases compared to dense and compact counterparts. Finally,
concluding remarks and outlook are provided.

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11.2 Hierarchy and Morphology Suitable for Sensors
Individual nano building blocks are assembled in such a way that more than
two kinds of dimensionality are found, resulting in hierarchical nano-
structures. Building blocks may or may not have the same dimensionality,
shape and scale. Therefore, hierarchical nanostructures usually demonstrate
regular patterns on the surface, which plays an important role in sensing
applications. Many types of hierarchical nanostructures have been reported
depending on hierarchy and morphology as shown in Table 11.1. The nano