PH-Sensing Characteristics of Multi-Walled Carbon Nanotube Assembled across Transparent Electrodes with Dielectrophoresis

Chemical and biological processes are reliant on the pH value of the elements involved in the reaction. Transparent pH sensors with a large detection area are desirable in transparent electronics and microfluidic analytical platforms. This work reports the fabrication of pH sensors using the dielectrophoresis (DEP) method. Carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs) were assembled across indium tin oxide (ITO) electrodes separated with a gap width of $50 ~\mu \text{m}$. The DEP process was assisted with airflow and thermal annealing to increase the stability of the aligned MWCNT connections and reduce the variation in sensor resistance. The fabricated pH sensors were exposed to analytes with different pH values. The sensors showed high sensitivity between 170% (pH=4) to 2000% (pH=10) and response times between 4 to 6 s. The long-term performance and repeatability of the sensors were improved, where the sensors can recover their initial resistance after each testing cycle.