A heterobifunctional PEG crosslinking agent. The free thiol group readily reacts with metal surfaces such as gold or silver. Alternatively, it binds to other thiol specific groups like maleimides, vinylsulfones etc. The protected hydrazide may then be liberated by treating the Boc group with acid. The hydrazine group is used to conjugate carbonyl compounds like aldehydes or ketones with the formation of hydrolysable and pH sensitive hydrazone links.