Enhancing thermal and chemical sensitivity of polydiacetylene colorimetric sensors: The opposite effect of zinc oxide nanoparticles

This contribution presents our continuing effort to develop polydiacetylene (PDA)/zinc(II) ion (Zn2+)/zinc oxide (ZnO) nanocomposite for colorimetric sensing. In our previous studies, PDAs with monocarboxylic headgroup have been used to fabricate the nanocomposites. The incorporation of Zn2+/ZnO enhances overall interactions, resulting in reversible thermochromism with color-transition temperature (T-CT) higher than that of the original PDA. Here, we extend this concept to the system of 3-(pentacosa-10,12-diynamido) benzoic acid (PCDA-mBzA) monomer constituting aromatic headgroup. Pure poly(PCDA-mBzA) assemblies exhibit reversible thermochromism with T-CT similar to 90 degrees C. Interestingly, the fabrication of poly(PCDA-mBzA)/Zn2+/ZnO nanocomposites using 5, 10 and 20 wt.% of ZnO nanoparticles results in a systematic decrease of T-CT to about 80, 70 and 60 degrees C, respectively while the reversible thermochromism remains. The observed effect of ZnO nanoparticles is opposite to our previous studies. Structural analysis by utilizing infrared spectroscopy and x-ray diffraction reveals that the Zn2+ ions intercalate the bilayer structure of poly(PCDA-mBzA). The intercalation process perturbs local organization of aromatic headgroups, reducing the strength of overall interactions. Colorimetric sensors in solution and thin film coated on nylon membrane with enhanced sensitivity are fabricated to detect cationic surfactant and organic solvent at various concentration ranges.