Development of an optical sensor for acidic environments from Pani/LDHs composites

ABSTRACT

Hybrid materials formed between polyaniline and layered double hydroxides of Zn and Al (Pani/ZnAlHDL) were obtained by adding previously prepared polyaniline (Pani) to the reaction medium during the synthesis of the HDLs, including HDLs modified with the anionic surfactants sodium dodecyl sulfate and sodium dodecylbenzenesulfonate. The composites exhibited a blue color characteristic of the emeraldine base phase of Pani. X-ray diffraction (XRD) patterns of these compounds showed a shift of the peaks (003) and (006) of ZnAlHDL to lower 2θ values, indicating the presence of both phases in the composites. Additionally, it was observed that the anionic surfactants also influence the structural characteristics of the composites. Fourier-transform infrared (FTIR) spectra support the XRD results. Composites obtained with surfactants were subjected to colorimetric sensor tests for HCl. After only 5 minutes of exposure to HCl vapors, the materials changed their color from blue to green, indicating the formation of the Pani emeraldine salt phase. This indication was confirmed by UV-Vis spectroscopy, as spectra after acid interaction showed new bands in the 400 nm and 820 nm regions, characteristic of the emeraldine salt, the green-colored structural form of Pani. These results demonstrate the feasibility of using these materials in acid sensing, providing a rapid and effective response.

Keywords:
Polyaniline; layered double hydroxides; composite materials; colorimetric sensor; hydrochloric acid