Colorimetric Environmental Sensors for Amine and Alkylating Fumigant Using Nicotinic and Isonicotinic Squaramide Ligands

Multianalyte squaramide ligands have been utilized to investigate the real‐time detection of biogenic amines, aliphatic amines, and toxic alkylating agricultural fumigants colorimetrically in solution and the gas phase. Here, the careful design of the ligand was found to show an instant (a few seconds) color change in the presence of aliphatic primary amines such as methyl amine and, hence, has been effectively utilized for identifying the freshness of the food samples, where a filter paper‐based test‐kit was found to sense the presence of biogenic amines in less than a minute. The ligands were also found to exhibit visual sensing ability for methyl iodide, a carcinogenic agricultural fumigant that has been effectively detected colorimetrically through the nucleophilic substitution reaction of the pyridinic ligand, assisted by basic media. Compared to the traditionally known ligands, the squaramide in this study offers the sensing of gaseous amines and toxic fumigants, eliminating the need for long incubation times or temperatures at the ppm level by producing colored compounds of deprotonated squaramide and highly delocalized azo squaramide. The study favors using simple ligands, where positional changes in the pyridinic nitrogen of two ligands determine the sensing ability, also indicated by the computational study.