Ald-Ph-PEG4-PFP ester is a PEG derivative containing a benzaldehyde and PFP ester group. The benzaldehdye can react with primary amine groups. The PFP ester can be reacted with primary amine groups and is also less susceptible to undergo hydrolysis compared to NHS ester. The hydrophilic PEG chain increases the water solubility of compounds. Longer PEG chains have better water solubility properties relative to shorter PEG chains.