A highly selective fluorescent chemosensor for Hg2+ based on a squaraine-bis(rhodamine-B) derivative: Part II

Herein, we report an effective strategy based on coordination-induced signaling by introducing a rhodamine B group linkage into squarainediamine dyads as a reversible switch. The optimized design, synthesis and application of a new optical squarainebis(rhodamine-B) chemosensor (SRB) as an offon fluorescent probe for the detection of Hg2+ ions were investigated. SRB exhibited high selectivity toward Hg2+ in the presence of various metal ions, such as Al3+, Ag+, Co2+, Cs+, Cu2+, Fe3+, K+, Li+, Mg2+, Na+, Ni2+, Pb2+, and Zn2+, and the resulting complex [SRB-Hg2+] was investigated using UVvis and fluorescence spectroscopy in acetonitrile (CH3CN). The offon fluorescence and color signal change of the probe are based on a Hg2+-triggered domino reaction that employs the open-ring form of rhodamine spirolactam to regain the conjugated system of the rhodamine skeleton. The mechanism for the opening of the rhodamine spirolactam ring induced by Hg2+ binding and the 1:1 stoichiometric structure of SRB and Hg2+ were confirmed using a Job's plot estimation, optical titration and FT-IR. Subsequently, a SRB-Hg2+ complex chemosensor was employed to detect CN- in the presence of different anions, such as Br-, CH3COO- or AcO-, Cl-, ClO4-, F-, HPO4-, HSO4-, I-, N-3(-), NO3-, PF6- and SCN-, in acetonitrile. In addition, this sensor exhibited highly selective and sensitive recognition of cyanide ions upon the addition of Hg2+ with a color change back to colorless in the same solution. Finally, SRB was successfully applied with the PEGDMA polymer to sense Hg2+ ions, which was analyzed using fluorescence confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images.