Synthesis, characterization and protein-association of dicyanomethylene squaraine dyes

Abstract We prepared several new dicyanomethylene squaraine dyes derived from indolenine, benzothiazole and benzoselenazole, with different substituent groups. All the obtained dyes display intense and narrow absorption bands and bright fluorescence emission in the Vis/NIR region (665–716 nm) when dissolved in organic solvents, especially for the unsymmetrical dyes containing sulphur and selenium atoms. The use of synthesized squaraine dyes as fluorescent probes for detection of bovine serum albumin (BSA) and human serum albumin (HSA) was explored in Tris-HCl buffer solutions. We observed that all the dyes are non-emissive in buffer solution but become fluorescent in the presence of BSA or HSA proteins, with the emission intensity increasing proportionally to the protein concentration (0.00–1.22 μM). The most pronounced effect was observed for the symmetrical squaraine having two moieties of indolenine.