In solution, nanoparticles (NPs) are normally stabilized with surface ligands, such as thiols, phosphines, and amines. These ligands are unstable in presence of oxygen saturated aqueous and oxygen-rich biological media. N-heterocyclic carbenes (NHCs), on the other hand, are excellent ligands that bind to a variety of metal ions with extremely high binding strength via σ donation. NHCs as ligands are stable under oxygen-rich conditions. However, there are only a few studies of NHC ligands for metal NPs in literature, largely due to synthetic challenges of carbene. In particular their sensitivity as free NHCs to oxygen and water largely limits the application of NHC ligands for metal NPs. We report a general method to prepare NHC-functionalized polymers to modify metal NPs in water using a facile transmetallation. NHC-functionalized polymers can be prepared using simple post-polymerization functionalization of polymers capped with halogen atoms (e.g., Cl and Br) synthesized via atom transfer radical polymerization (ATRP). When reacted with N-methyl imidazole, halogen-ended polymers can be converted to yield imidazolium salt and further transferred into Cu(I)-NHC-capped polymers in presence of a weak base, e.g., K2CO3 and Cu(I)Cl. Cu(I)-NHC-capped polymers are stable in water and can be used to modify metal NPs using transmetallation in any good solvents of polymers. The surface modification of citrate-capped gold NPs has been examined using a ligand exchange method in an emulsion solution. The NHC polymer-capped gold NPs are able to disperse in a series of organic solvents based on the solubility of polymer ligands. We show the successful surface modification of gold NPs using both hydrophobic and hydrophilic polymers, including polystyrene, poly(methyl methacrylate)), poly(n-butyl acrylate)) and poly(2-(2-methoxyethoxy)ethyl methacrylate)) functionalized with carbenes. The stability of NHC capped AuNPs in various conditions such as high slat concentrations (1M NaCl) and hydrophobic solvents has been examined. This method will open the doors to use the carbene functionalized polymers for the surface modification of other metal or metal oxide NPs for potential applications in nanomedicine and nanocatalysis.

NHC-capped polymers for surface functionalization of metal nanoparticles in aqueous solution

256th National Meeting (2018)

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