N-(m-PEG4)-N'-(PEG4-acid)-Cy5 is a near infrared (NIR) fluorescent Cy 5 labeled PEG derivative containing free terminal carboxyl acid. Terminal carboxylic acid can react with primary amine groups in the presence of of activators (e.g. EDC, or DCC) to form a stable amide bond. The hydophilic PEG spacer increases solubility in aqueous media. Cy5 labeled PEG derivative can be easily traced from its blue color and strong fluorescence.