A robust, water-soluble and low cytotoxic fluorescent probe for sulfide anion achieved through incorporation of betaine

Sulfide anions are generated not only from industrial processes but also in biosystems. Hence, robust fluorescent sensors for sulfide which are fast-responding, water soluble and biocompatible are highly desirable. Herein, we report the first betaine-based fluorescent sensor, which features excellent water solubility, low cytotoxicity and short response time. This sensor is based on the chelator/Cu(II) approach so as to achieve fast sensing, in which tris(2-aminoethyl)amine is selected as the Cu(II) chelator since this tripodal chelator can strongly bind with Cu2+. Two sulfobetaine groups, which are utilized to achieve water solubility and biocompatibility, are incorporated onto the sensor. The fluorophore–chelator–Cu(II) complex functions as the sulfide sensor which exhibits very low fluorescence, and sulfide anion can extract Cu2+ from the sensor since sulfide anion can form very stable CuS with Cu(II), resulting in fluorescence enhancement. With two betaine groups and a tripodal chelator in its structure, the sensor exhibits excellent water solubility and low cytotoxicity, and is capable of permeating the cell membrane and realizing sulfide anion imaging in live cells. The sensor displays high selectivity and low detection limit of 1.1 μM, and can be used for sulfide monitoring in such real sample as running water.