Amphiphilic ligand-doped liquid crystal-based detection of Hg2+ ions on polyimide surface with alkyl pendent groups

In this study, amphiphilic thiosemicarbazone was used to align liquid crystal (4-cyano-4′-pentylbiphenyl) in a homeotropic way on polyimide containing alkyl pendent groups (AHDPI), which was coated on a glass slide. The amphiphilic ligands 2-(4-(dodecyloxy)benzylid-ene)hydrazine-1-carbothioamide (DT) and 2-(1-dodecyl-2-oxoindolin-3-ylidene)hydrazine-1-carbothioamide (IT) were doped with liquid crystal (LC) to develop LC-based sensors to detect Hg²⁺ ions in water. The selective interaction of carbothioamide with Hg²⁺ ions triggered the orientation transition of LC from homeotropic to parallel alignment and gave dark to bright optical signal at the LC/aqueous interphase. Self-immobilization of thiosemicarbazone-based ligands on AHDPI-coated glass slide can be used to detect Hg²⁺ ions with high sensitivity. The limit of detection with DT and IT was found to be 0.5 and 0.25 μmol L⁻¹, respectively. Density functional studies were carried out to study the interaction of the thiosemicarbazone ligands with mercuric ions, resulting in highly negative binding energies of −1.55 and −2.06 eV for DT and IT with Hg²⁺ ions, respectively. The chemical and thermal stability (up to 268 °C) of the AHDPI coated on glass slide made it reusable at least twice for sensor fabrication. This provides a quicker and cheaper alternative to traditional methods of sensor fabrication. © 2024 Society of Chemical Industry.