Coating of gold nanoparticles by thermosensitive poly(N-isopropylacrylamide) end-capped by biotin

Gold nanoparticles (NPs) were prepared by reduction of HAuCl₄ in aqueous solution and stabilized by poly(N-isopropylacrylamide) (PNIPAM). PNIPAM was prepared by two distinct routes: (i) conventional free-radical polymerization leading to polymer without any reactive end-group, and (ii) Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization with 2-dodecylsulfanylthiocarbonylsulfanyl-2-methyl propionic acid (DMP) as a RAFT agent. PNIPAM with low polydispersity was then end-capped by an α-carboxylic acid and an ω-trithiocarbonate that was converted into an ω-thiol upon hydrolysis. This hetero-telechelic polymer was analyzed by mass spectroscopy, size exclusion chromatography (SEC) and ¹H NMR. Even without thiol end-group, known for chemisorption onto gold, PNIPAM was effective in stabilizing gold NPs (∼1-5 nm). The thermosensitivity of PNIPAM at the surface of gold NPs was, however, dependent on the molecular weight of the chains. Finally, the α-carboxyl end-group of PNIPAM was used to anchor biotin, which is indeed known for complexation with avidin, which is a possible strategy for the coated gold NPs to be involved as building blocks in supramolecular assemblies. TEM and UV-vis spectroscopy were used to characterize the gold nanoparticles.