Using orientational transition of acetone-doped liquid crystal triggered by vanillin as a naked-eye vanillin sensor

Vanillin is an aromatic organic compound that is widely used in the food and cosmetics industries. The development of a simple and rapid method for the naked-eye detection of vanillin is essential for monitoring the vanillin production process. In this study, liquid crystal 5CB (4-cyano-4'-pentylbiphenyl) is doped with acetone and used as a sensing platform to detect vanillin in alkaline solutions with high specificity for the first time. Before the addition of vanillin, the acetone-doped 5CB adopts a homeotropic orientation and appears dark due to the formation of an electrical double layer (EDL) at the 5CB/water interface. In the presence of vanillin, it forms a complex with acetone inside 5CB, disrupting the EDL and the homeotropic orientation of 5CB. The event leads to a dark-to-bright transition which can be used to quantify the concentration of vanillin in a range between 0.1 and 25 mg/L. The limit of detection (LOD) for the vanillin sensor is 0.06 mg/L. As a proof of concept, the sensor is employed to monitor the vanillin concentration during oxidation of coniferyl alcohol. In the presence of 5000 mg/L of H2O2 and 1 mg/L iron tetraamidomacrocyclic ligand (Fe-TAML), 1750 mg/L of coniferyl alcohol can be converted to 393.0 mg/L of vanillin after a 30-min reaction at room temperature. This study demonstrates potential applications of the acetone-doped liquid crystal sensors for monitoring the production of vanillin under various conditions.