PEG crosslinking agents for biotinylation. The biotinyl functionality is used to attach avidin or streptavidin containing substrates. Pegylation by binding of biotinyl PEG to avidin and streptavidin is very specific and fast. This linear PEG derivative has a sulfhydryl group (-SH) on one chain end. The free thiol groups react readily with maleimido or iodacetamido functionalities or transition metal surfaces like gold, silver etc. Reaction with other free thiols leads to S-S disulfide bonds which can be cleaved by reducing agents. PEG thiols may also be conjugated by thiol-ene chemistry.