Carboxy-PEG₁₂-thiol, is a carboxyl- and sulphhydryl-terminated compound that contains a 12-unit polyethylene glycol (PEG) spacer and is used to modify surfaces such as quantum dots, monolayers and magnetic particles.

• Provides carboxylate "handles" for protein immobilisation via EDC crosslinking
• PEG groups are flexible, non-immunogenic and hydrophilic
• Terminal monodentate thiol reacts with gold surfaces through dative bonding
• Terminal carboxylic acid group allows conjugation to amine-containing affinity ligands

The use of MT(PEG)₄ with CT(PEG)₁₂ in surface modification can form a hydrophilic "lawn" of methyl ether-terminated PEGs with periodic exposed carboxylic acid-containing PEGs. The exposed carboxylic acid groups can be coupled to affinity ligands using the carbodiimide coupling reaction with EDC and sulpho-NHS.