Carboxy-PEG4-sulfonic acid（Cat No.:I016087）is a heterobifunctional polyethylene glycol bearing a terminal carboxyl group and a permanently anionic sulfonic acid, separated by a short PEG4 spacer. The carboxyl handle couples efficiently with primary amines using EDC/NHS chemistry to form stable amide bonds, enabling covalent attachment to proteins, peptides, and surfaces. The sulfonate provides high water solubility, antifouling behavior, and strong electrostatic interactions with cationic materials, aiding charge tuning and colloidal stability. PEG4 imparts compact, flexible spacing and low nonspecific adsorption. Ideal for nanoparticle functionalization, layer-by-layer assembly, hydrogel modification, and ionically addressable biomaterials.