DBCO-PEG5-NHS Ester is an amine-reactive building block which reacts specifically and efficiently with primary amines (e.g. the side chain of lysine residues or aminosilane-coated surfaces) at pH 7-9 to form a covalent bond. The polyethylene glycol (PEG) spacer arm is hydrophilic and imparts water solubility which is transferred to the labelled molecule, therefore, reducing aggregation of labelled proteins in the solution. It also provides a long, flexible connection that minimises steric hindrance in reactions with complementary azide-containing molecules.