In this work, a new benzothiazole azo dye sensor (BTS) was synthesized, and its cation binding affnity was studied using the colorimetric method, UV–vis, and 1H NMR spectral data. The results revealed that the sensor BTS exhibits a remarkable tendency for Pb2+ ion to perform spontaneous visual color change from blue (BTS) to pink (BTS + Pb2+), without any color change in the aqueous solutions of other cations such as Hg2+, Cu2+, Al3+, Ni2+, Cd2+, Ag+ , Ba2+, K+, Co2+, Mg2+, Na+, Ca2+, Fe2+, and Fe3+ ions. The observed selectivity could be due to the formation of the complex (BTS + Pb2+), which led to a blue shift from 586 nm (BTS) to 514 nm (BTS + Pb2+) in the UV spectrum. The job’s plot provided the stoichiometry ratio of the complex (BTS + Pb2+) to be 1:1. The limit of detection (LOD) of BTS for Pb2+ ion sensing was obtained at 0.67 µM. Additionally, the binding constant for BTS toward Pb 2+ ion was studied using the Benesi-Hildebrand equation. As a result of the BTS test paper strips investigations, it was found that the synthesized sensor BTS could be used as a rapid colorimetric chemosensor for the detection of the Pb2+ ions in the distilled, tap, and sea waters.
