Development and fabrication of carbon nanotube (CNT) based pH sensor

The development, fabrication and characterization of single-walled carbon nanotubes (SWCNTs) based pH sensor using aligned SWCNT were reported. The SWCNT alignment is defined by a single carbon nanotube aligned between the fabricated electrodes. This research involves the study of SWCNTs dispersion, alignment of SWCNT between microgap electrodes and characterization on the effect of change in the pH level on the impedance, conductance and capacitance of the aligned SWCNT. In the SWCNT dispersion study, the SWCNTs were dispersed in Isopropyl Alcohol (IPA), Dichloromethane (DCM), Acetone and Triton-X 100. It was found that SWCNT disperse best in the IPA solution because the dispersed SWCNTs have remained dispersed which can be observed from the clear solution even after 14 days as compared to DCM, acetone and Triton-X 100. On the other hand, the SWCNTs in DCM, acetone and Triton-X 100 have shown a thick mass of coagulated SWCNT after 14 days of dispersion. A chrome mask which consists of 6 groups with different gap measurement was designed. Each group has 5 different designs to facilitate the SWCNT alignment. After that, the devices were fabricated using gold material as electrode to increase the electrical conductivity and permittivity of the device. The SWCNT was then aligned on the fabricated devices using AC dielectrophoresis method. The AC dielectrophoresis method involved control in the voltage and frequency to increase the chance of SWCNT alignment between the microgap. The devices were brought to electrical characterization before and after SWCNT alignment to compare the effect on the device capacitance. It was found that the capacitance before SWCNT alignment is higher than after SWCNT alignment of the device. Before SWCNT alignment, the dielectric of the capacitive device is air which is a better insulator than SWCNT that is a semiconductor material. This phenomenon is due to the fact that dielectric decrease electric field and capacitance is inversely proportional to electric field. On the other hand, the device was tested for its impedance using pH buffer solutions. As pH value was decreased, impedance has also decreased. The hydrogen ions were found to bind to the carboxyl group of the SWCNT creating positive holes in the SWCNT hence increasing its conductivity. As a conclusion, this research successfully demonstrated the process to design, fabricate and characterize the SWCNT based sensor.