2.3. pH measurement using the CNT-based sensors
Since the electrical property of CNTs is sensitive to chemicals,
single CNT and CNT bundled sensors fabricated on silicon substrate
have been developed to monitor the pH value of the analyte solution
[4,20]. It showed that the resistance of the CNT bundles across
the microelectrodes was proportional to the pH value of the analyte
solution applied to the sensor. When the OH group is introduced
to the CNT wall, a peak in the dense of state (DOS) arises
at the Fermi level (EF) and the energy gap (Egap) is significantly reduced.
This is because of the interaction between the oxygen and
carbon atom. OH group can form an acceptor level and enhance
the conductivity of the CNT [21]. The concentration of the OH
group in the solution can be calculated by the pH value. Higher
pH value represents higher concentration of the OH group in the
solution. Therefore, the conductivity of CNT increases with the
appearance of OH group in the solution.
To investigate the pH response of the CNT-based sensor on paper,
different pH values of the analyte solutions were prepared
based on the appropriate mixing of HCl and NaOH. The pH values
of the solution were calibrated by a benchtop pH meter (6175, Jenco
Instruments Inc.). Since the paper-based microfluidic device was
designed for the bio-assay application, pH values of 5, 7, and 9
were selected for the investigation. The CNT-based sensor with
the width of 1.5 mm and length of 12 mm was used in this study
and its two ends were defined as source and drain. Analyte solution