The high specific surface area of MnO2 NFs prepared a high loading area for QDs, which

amplified the signal and enhanced the sensitivity of the microfluidic biosensor.

5.3.3 Carbon-Based Nanomaterials in a Lab-on-Chip

Carbon-based nanomaterials have received a lot of attention for the modification of

transducer platforms in LOC devices. Carbon-based nanomaterials are abundant and low

in cost materials, which feature excellent chemical and physical properties. Electrodes

prepared or modified with carbon nanomaterials showed an excellent low background

current, broad potential window, high surface area for entrapment of different com­

pounds, renewability, and low cost to incorporate with different substances during fab­

rication. The carbon-based nanomaterials have unique and diverse allotropes like

graphite, diamonds, carbon nanotubes (CNTs), graphene oxide (GO), graphene quantum

dots (GQDs), and fullerene [28]. Table 5.4 lists carbon-based nanomaterials applied in

LOC devices for various applications and detection techniques.

Carbon-based nanomaterials have been extensively applied in the LOC device sensor

for signal enhancement of the modified sensor. Most commonly, carbon-based nano­

materials were combined with other types of nanomaterial to further enhance the

property of the sensor part in the LOC device. Zhang et al. [29] reported the synergetic

effect of carbon-based nanomaterials and AuNPs employed for signal enhancement in

electrochemical LOC for saliva glucose detection. The electrochemical LOC was devel­

oped by integrating three electrodes consisting of WE, CE, and RE on a single chip

through a micro-fabrication process. The Si wafer was pre-cleaned, oxidized with wet

atmosphere, and undergo a photolithography process to create the microelectrodes de­

sired pattern. The WE of the LOC was modified with single-walled carbon nanotubes

(SWNTs), AuNPs, chitosan, and glucose oxidase (GOx) through the layer-by-layer as­

sembly. The multilayer of SWNTs/AuNPs/chitosan can increase active surface area and

promote direct electron transfer between GOx and the WE. Therefore, high sensitivity

and low LOD of glucose sensor has been developed. This happens because of the high

electrocatalytic properties and high electrical conductivity of the SWNTs and AuNPs. The

developed electrochemical LOC chip for saliva glucose detection exhibits the linearity of

0.017–0.81 mM, and LOD of 5.6 µM, which in the future is able to be applied in a non-

invasive, pain-free, and easy glucose monitoring.

Chand and Neethirajan [30] have developed a microfluidic LOC device integrated with

SPCE electrode for electrochemical detection of norovirus. As shown in Figure 5.4, the

PDMS microfluidic chip was equipped with silica microbeads to pre-concentrate the

sample and the SPCE was modified with graphene-AuNPs composite as the sensor and

norovirus specific aptamer as the recognition element components on the microfluidic LOC

device. The graphene-AuNPs composite offers dual advantages in terms of increasing the

surface area for an aptamer to immobilize, improve electrical conductivity and accelerate

the electron transfer process. Additionally, the modification of SPCE with graphene-AuNPs

composite can be easily done by a simple process such as drop casting, spin casting, or ink-

jet printing. The detection principle of the aptamer norovirus microfluidic LOC device is

based on the interaction of redox-aptamer and norovirus resulting in increasing the im­

pedance thus decreasing the electrochemical signal obtained (Figure 5.4). The differential

pulse voltammetry (DPV) technique has been employed in the norovirus microfluidic LOC

device with a linearity of 100 pM to 3.5 nM and LOD of 100 pM.

The electrochemical microfluidic LOC for the detection of nitrate ions in a soil solution

has been developed by Ali et al. [31]. The graphene foam and titanium nitrate nanofibers

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