Synthesis, photophysical study and BSA association of water-insoluble squaraine dyes

In this work, the synthesis, spectroscopic characterisation, photophysical study and bovine serum albumin association of water-insoluble squaraine dyes are described. The photophysical behaviour of the dyes was investigated using UV–vis and steady-state fluorescence in solution, as well as its association with bovine serum albumin in phosphate buffer solution. No squaraine aggregation was observed in organic solvents, but a J-type aggregation could be observed in phosphate buffer solution. In the absence of protein, the J-aggregation promoted the fluorescence quenching; however, upon addition of protein, the fluorescence intensity increased by a factor of up to 80. Additionally, a linear relationship between the fluorescence intensity and the protein concentration could be observed for the squaraine dyes in a concentration range around 0.02–0.10 μM.

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► Three water-insoluble squaraine dyes based on indoleninium and benzothiazolium moieties were synthesised.
► The dyes presented intense absorption and fluorescence emission in the red region.
► In the phosphate buffer solution and absence of bovine serum albumin the squaraine dyes were almost nonfluorescent.
► The fluorescence intensities of the dyes increased with addition of protein in the buffer/dye solutions.