Methyl-PEG₄-thiol is a methyl- and sulfhydryl-terminated compound that contains a 4-unit polyethylene glycol (PEG) spacer and is used to modify surfaces such as quantum dots, monolayers and magnetic particles.

• PEG groups are flexible, non-immunogenic and hydrophilic
• Terminal monodentate thiol reacts spontaneously with gold surfaces through dative binding
• Terminal methyl group coats quantum dot and other surfaces to reduce nonspecific binding in assays

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 sulfo-NHS.