Sensors - Hierarchical Nanostructures for Energy Devices - page 256

11.4.2 Biosensors
Electrochemical biosensors that use hierarchical nanostructures are the
focus of this section. Chemical or biochemical reactions in a sample solution
produce or consume ions or electrons, which induces a variation in electrical
properties in sensing materials. In biosensors, there are several main
quantities to evaluate sensor performance. The sensitivity is a value of the
electrode response with respect to analyte concentration, meaning an in-
tensity of response generated due to detection of a target molecule. The
linear range is the section of concentration in which current output shows
linear behavior with respect to target concentration. Furthermore, response
time means the time to reach the steady-state of current output. The limit of
detection (LOD) is the minimum amount of target analyte for a discernible
current output signal. It is one of the key parameters in evaluating sensor
performance.
Biosensors using hierarchical nanostructures are summarized in
Table 11.3 in terms of material type, hierarchy and morphology, target
biomolecules, and performances such as linear range, sensitivity, and re-
sponse time. In hierarchical biosensors, a reaction can occur easily between
the analyte and receptor because the amount of receptor is abundant on the
sensing materials.
The 2-3 wall-like Cu-NiO nanostructure was prepared by calcination of a
Cu-Ni(OH) 2 precursor at 400 1C for 2 h. 28 The nanostructure produced was
used to modify a glassy carbon electrode (GCE) for the implementation of
non-enzymatic amperometric biosensors. The sensing performance was
compared with a sensor possessing a porous NiO/GCE electrode. As a result,
the hierarchical nanostructure sensor did not show Michaelis-Menton kin-
etics. It was noted that the electron transfer in the oxidation of glucose was
improved because of an increased electrocatalytic active area due to the
hierarchical nanostructure. The sensitivity was 76.36 mAcm 2 (mM) 1 on a
linear range of 0.5 mM to 5 mM and the LOD was 0.5 mM with a fast response
time below 5 s. Chen et al. 29 prepared a hierarchical nano-composite
of CuO nanoplates attached to the surface of TiO 2 nanotubes by electro-
spinning. Fluorine-doped tin oxide (FTO) glass was covered with TiO 2
nanotubes decorated with CuO nanoplates for nonenzymatic ampero-
metric sensors for glucose detection. The sensor demonstrated a sensitivity
of 1321 mAcm 2 (mM) 1 on a linear range of 0.01 to 2 mM, a response time
of 10 s and an LOD of 390 nM at the potential voltage of 0.7 V. This was a
large improvement on the CuO/TiO 2 nanotube arrays without a hierarchical
nanostructure 30 that showed a sensitivity of 79.79 mAcm 2 (mM) 1 and a
detection limit of 1 mM. Si et al. 31 grew 2-3 ivy-like hierarchical nano-
structured TiO 2 by a solvothermal method using a multi-walled carbon
nanotube as a template. The subsequent calcination removed the carbon
nanotubes producing hierarchical structures as shown in Figure 11.5(a)
and (b). It was used to make a glucose sensor, which had a linear range up to
1.5 mM with a sensitivity of 9.9 mAcm 2 (mM) 1 , an LOD of 1.29 mM and a

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