Solvatochromic spiropyran - a facile method for visualized, sensitive and selective response of lead (Pb2+) ions in aqueous solution

1′-(4′-carboxybutyl) −3′, 3′-dimethyl-6-nitro-8-hydroxylspiro [chromene-2, 2′-indoline] (SPC) is a visual, selective, and sensitive detector to lead (Pb²⁺) ions. The presence of 8 -hydroxyl in pyran ring makes SPC have a remarkable solvatochromism in polar solvents (SPC can isomerize to the ring-opening body (Mcc) spontaneously under visible light conditions), ascribed to the formation of intramolecular hydrogen bonds. The color and spectrum of the Mcc solution could be changed instantly and significantly by addition of Pb²⁺, and Mcc showed excellent selectivity to Pb²⁺ in the simulated lead–zinc ore leachate. The complex Mcc - Pb²⁺, with a stoichiometry ratio of 1:1, can stably exist under visible light conditions, which is attributed to the interaction between Pb²⁺ and the deprotonated carboxy (–COO^—), phenolic oxygen atom and 8-hydroxyl of the Mcc, and the whole process does not need UV irradiation, avoiding the serious photo-degradation of spiropyran derivatives. The low limit of detection (0.61 μM) and the established test strips indicate that SPC has the potential to be applied in the quantitative detection of Pb²⁺ in real samples.