(TiN NF) composite immobilized with nitrate reductase (NaR) enzyme has been used as

the sensor platform in nitrate detection. In this work, the electrochemical microfluidic

LOC was developed on the Si wafer where the Au was patterned as the CE, Ag/AgCl

as the RE, and the graphene-TiN NF composite as the WE. The graphene-TiN NF com­

posite was embedded into the channel using the liquid phase photopolymerization

technique to produce flow through a microfluidic electrochemical sensor. The combina­

tion of the graphene and TiN NF composite creates three-dimensional structure nano­

materials, which increases the electrochemically active surface area and provides high

loading capacity for the NaR enzyme immobilization. In the detection of nitrate ions, the

graphene-TiN NF composite porous structure allows the sample analytes to flow through

and react with immobilized NaR enzymes which resulted in catalytic reduction. This

reaction led to increases in the amperometric current response. The fabricated electro­

chemical microfluidic LOC can detect the nitrite ions in agricultural soil solution samples

in a linear range of 0.01 to 442 mg/L and LOD of 0.01 mg/L.

A similar research group reported on the electrochemical microfluidic immunosensor

LOC for breast cancer biomarker detection [32]. This time, the combination of porous

graphene and carbon-doped titanium dioxide nanofibers (nTiO2) composite immobilized

with functionalized anti-ErbB2 were used as the electrochemical WE. The combination of

the excellent properties nTiO2 embedded into the porous structure of graphene, resulted

in a larger electrochemical surface area and high charge transfer resistance in the elec­

trochemical breast cancer biomarker detection. In this LOC device, the sensor was con­

nected to two types of detection techniques, which are impedance and DPV techniques.

The impedance value increased and voltammetric peak current values decreased with the

presence of high concentration ErbB2 antibodies. This happened because more ErbB2

antibodies bound to the WE thus increased the thickness of the insulating layer. The

impedance and voltammetric electrochemical performance of the microfluidic im­

munosensor LOC display high sensitivities of 0.585 µA/µMcm² and 43.7 kΩ/µMcm²,

respectively, for wide linearity concentration of 1.0 fM to 0.1 µM for impedance technique

and 0.1 pM to 0.1 µM for voltammetric technique.

The reliable properties of carbon-based materials have made it a possible nanoma­

terial to be applied in the detection of SARS-CoV-2 (COVID-19) surface spike protein

S1. Recently, Zamzami et al. [35] fabricated a high selectivity and sensitive carbon

nanotubes field-effect transistor (CNT/FET) sensor that allows digital detection of the

SARS-CoV-2 S1 in saliva samples. As shown in Figure 5.5, the sensor was fabricated

on a Si/SiO2 surface by deposited single-wall carbon nanotube (SWCNTs) and

FIGURE 5.4

The schematic of the microfluidic electrochemical LOC aptasensor for norovirus detection and the electro­

chemical signal obtained in the absence and presence of norovirus. Adapted with permission [ 30]. Copyright

(2017) Elsevier.

Nanomaterials and Lab-on-a-Chip Technologies

87