Ald-CH2-PEG5-t-butyl ester is a PEG derivative containing aldehyde and a t-butyl ester. Aldehyde groups are likely to undergo nucleophile substitution, reduction and oxidation reactions. The t-butyl ester can be removed under acidic conditions and can further be reacted. The hydrophilic PEG linker increases the compound's water solubility in aqueous media. The hydrophilic properties of the compound are improved with longer PEG chains.