Lipoic acid-PEG-COOH (LA-PEG-COOH) is a heterofunctional linkes for surface modification. Lipoic acid, also known as thioctic acid, is an important bioactive molecule participating in various biological processes. PEG functionalized lipoic acid can be used to bind to metallic particle or film surface with its -S-S-bond. Lipoic acid group has high affinity to metallic surfaces and have been widely used for gold nanoparticles and quantum dots surfaces. PEGylated lipoic acid is water soluble and can be used directly in aqueous buffer.