Photo-induced copper mediated copolymerization of activated-ester methacrylate polymers and their use as reactive precursors to prepare multi-dentate ligands for the water transfer of inorganic nanoparticles†
Multi-dentate ligands enabling a quick and high yield water transfer of inorganic nanocrystals (NCs) via a ligand exchange process is of great interest to promote the applications of NCs in the biomedical field. Here, we describe a facile two-step protocol for the synthesis of polymeric multi-dentate ligands that are suitable for the water transfer of hydrophobic colloidal inorganic nanocrystals (NCs). This protocol first exploits the photo-induced atom transfer radical (photo-ATRP) copolymerization of ester-activated N-succinimidyl methacrylate and oligoethylene glycol methyl ether methacrylate. A high monomer conversion rate along with a fair control over the polymerization is confirmed by size exclusion chromatography and nuclear magnetic resonance spectroscopy. In the second step, the activated carboxyl moieties of the copolymers are reacted with nucleophilic agents such as 2-aminoethylphosphonic acid or histamine dihydrochloride via a post-polymerization reaction to generate phosphonic- or amino-based multi-dentate ligands, respectively. As shown here, polymers comprising poly-phosphonic acid moieties are suitable as multi-dentate ligands for water transfer of multiple varieties of NCs with distinct compositions including iron oxide nanoparticles, CdSe@CdS quantum dots (QDs) and up-converting nanoparticles (UCNPs). Meanwhile the polymers containing histamine groups are also able to strongly coordinate to the surface of semiconductor QDs, thus enabling their water transfer. Notably, the NCs exhibit long-term stability in physiological media (saline) upon water transfer, while their size, shape, magnetic properties, and optical properties were also maintained. The UCNPs could be imaged when excited under an infrared laser while the QDs show a bright fluorescence signal under UV irradiation. QDs coated with a poly-phosphonic acid-based ligand resulted in a more homogeneous coating as demonstrated by the narrow band on gel electrophoresis, along with a higher quantum yield (QY ∼48%) in comparison with the polyimidazole-based ones (QY ∼31%). The aqueous IONPs instead were proven to provide a transversal relaxation making them useful as contrast agents in magnetic resonance imaging. The water transfer procedure is straightforward thanks to the full solubility of the amphiphilic polymer in the NC chloroform solution. This enables the right interaction between the anchoring moieties on the polymer chains and the surface of NCs, thus replacing the surfactant molecules. The gram scale production of the polymer together with the very simple steps of the water transfer protocol enables a quick translation of the protocol for large scale production of aqueous stabilized nanoparticles.