Compared with the traditional cyanine dyes (Cy5), both the maximum absorption and emission wavelengths of new Cy5 dyes with aldehyde group substituted at central position of the conjugated chain slightly blue shift, and the Stokes Shift is enlarged evidently to more than 28 nm, and largest up to 48 nm in water. The fluorescence quantum yield is rather low without obvious change in different solvents. It is found that the fluorescence of dye is sensitive to viscosity of the solvents, and the emission intensity of the dye can be enhanced largely by ascending of environmental viscosity with well linear relationship between lg I_f and lg (viscosity) (F?rster-Hoffmann equation). It is living cell membrane permeable for the dye 4 to map cellular microenvironmental viscosity, which demonstrates the potential application for dye 4 to investigate the intracellular microviscosity change.

Key words: cyanine dyes, Cy5, viscosity, fluorescent sensor, cell imaging