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. PEG-NHS derivatives are active esters and useful reagents to pegylate proteines, peptides or any other surfaces with a free primary amino group to form stable amide bonds. The Reaction requires a neutral to alkaline pH (7 to 8.5) to proceed. However, hydrolysis competes with aminolysis of the active ester and also increases with higher pH. Thus, working with excess PEG-NHS ester is necessary to obtain high pegylation yields. The PEG butyric acid NHS derivative has a C4 chain directly attached to PEG. It is one of the most stable PEG NHS esters known with a hydrolysis half-life of over 20 min at pH 8.