N-(m-PEG4)-N'-(PEG3-Mal)-Cy5 is a cyanine5 dye derivative containing a PEG4 chain and a PEG3-Mal chain. The cyanine5 has an excitation/emission wavelength of 646/662 nm. The maleimide group can react with thiol groups from pH 6.5 to 7.5. The hydrophilic PEG linkers increase the water solubility of the compound in aqueous media. The water solubility properties of the PEG linker are enhanced with longer PEG chains.