Cy3-PEG-SH (MW 2000) (Cy3-PEG-Thiol) is a near infrared fluorescent labeling reagent that combines Cy3 fluorescent dye , polyethylene glycol (PEG) and sulfhydryl (SH) polyethylene glycol derivatives, and is suitable for molecular coupling. Cy3-PEG-SH (MW 2000) is absorbed at 550nm and has high solubility and end-group substitution rate.