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The objective of this study is to characterize the pH sensing performance of carbon nanotubebased thin¯lms and compare them to their non-carbon nanotube-based counterparts. A layerby-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¯lm fabrication. In particular, polyaniline is doped with di®erent counter ions such as hydrochloric acid (HCl) and methane sulfonic acid (MeSA) for fabricating four di®erent thin¯lm sample sets. The as fabricated¯lms are subjected to various pH bu®er solutions ranging from pH 1 to 13 while their electrical properties are simultaneously measured using two di®erent techniques. First, time-domain bulk¯lm resistance measurements have been conducted, and the¯ndings show that all four types of¯lms exhibit pH sensitivity. Their bulk¯lm resistances increase in tandem with increasing pH. Second, frequency-domain electrical impedance spectroscopy (EIS) has also been conducted when the¯lms are exposed to di®erent pH bu®ers. The recorded EIS spectra have been¯t to a proposed equivalent circuit model consisting of resistors, capacitors and a constant-phase element. The results suggest that the MeSA-based¯lms exhibit linear sensitivity, whereas the HCl-based¯lms exhibit a bilinear sensitivity in the time-domain case. Both HCl-and MeSA-based¯lms 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¯lms also exhibit an inverse bilinear sensitivity to pH bu®er solutions.

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Our results show that the storage elastic modulus as a function of time increases at a higher rate for the cement paste mixed at higher versus lower mixing intensity. Hence, higher mixing appears to be enhancing thixotropy. Using calorimetry analysis we find that higher mixing decreases the setting time and enhances the peak of the heat flow. By analyzing the nanoparticles present in the suspending fluid of the cement paste, we show, in accordance with literature, that an appropriate combination of mixing energy and super-plasticizer dosage promotes hydration by scratching hydrates from the surface of cement particles, stabilizing them in the suspending fluid and hence generating additional nucleation surfaces. These results open the door for the design of printing heads including high-shear micro mixers allowing for a faster liquid-to-solid transition of the printable material.