Comparison of water-soluble squaraine and norsquaraine as fluorescent material for biomedical applications

Spectral and photophysical properties of norsquaraine dye Nor-Sq were investigated and compared with those for 3-carboxypentyl substituted squaraine Sq in aqueous buffer solutions, organic solvents and under interactions with proteins in order to obtain fluorescent labels for proteins with improved properties. Both dyes absorb and fluoresce in the same spectral region, but Sq has higher the molar absorptivity and is much more sensitive to polarity and viscosity of the medium, as well as to the presence of proteins. The absence of a substituent at the nitrogen atom in norsquaraine molecule provides the pH-sensitivity of Nor-Sq in the range of 9.0-11.4. As a result, Sq is more preferable for using in the most of biomedical applications, where interaction with proteins increases the brightness of the dye due to changes of polarity and/or viscosity of environment, while Nor-Sq is applicable for pH monitoring and in assays, where signal should not depend on changes in dye molecule environment.