EVALUATING THE PH SENSITIVITY OF CARBON NANOTUBE-POLYANILINE THIN FILMS WITH DIFFERENT DOPANTS
Abstract
The objective of this study is to characterize the pH sensing performance of carbon nanotube-based thin films and compare them to their non-carbon nanotube-based counterparts. A layer-by-layer technique is employed for fabricating the nanocomposites, and pH sensitivity is encoded by incorporating polyaniline (PANI) by itself or with single-walled carbon nanotubes during film fabrication. In particular, polyaniline is doped with different counter ions such as hydrochloric acid (HCl) and methane sulfonic acid (MeSA) for fabricating four different thin film sample sets. The as fabricated films are subjected to various pH buffer solutions ranging from pH 1 to 13 while their electrical properties are simultaneously measured using two different techniques. First, time-domain bulk film resistance measurements have been conducted, and the findings show that all four types of films exhibit pH sensitivity. Their bulk film resistances increase in tandem with increasing pH. Second, frequency-domain electrical impedance spectroscopy (EIS) has also been conducted when the films are exposed to different pH buffers. The recorded EIS spectra have been fit to a proposed equivalent circuit model consisting of resistors, capacitors and a constant-phase element. The results suggest that the MeSA-based films exhibit linear sensitivity, whereas the HCl-based films exhibit a bilinear sensitivity in the time-domain case. Both HCl- and MeSA-based films exhibit a bilinear pH response in the frequency domain. The equivalent circuit has also revealed that the equivalent parallel capacitor and the constant-phase element of the HCl-and MeSA-doped films also exhibit an inverse bilinear sensitivity to pH buffer solutions.
