DBCO-PEG24-NHS ester is a click chemistry PEG reagent containing NHS ester that is able to react specifically and efficiently with primary amines (e.g. the side chain of lysine residues or aminosilane-coated surfaces) at neutral or slightly basic condition to form a covalent bond. The hydrophilic PEG spacer arm improves water solubility and provides a long and flexible connection that minimizes steric hindrance involved with ligation. DBCO is commonly used for copper-free Click Chemistry reactions.