Polymer-coated nanoparticles by adsorption of hydrophobically modified poly(N,N-dimethylacrylamide)

We prepared a reactive random copolymer of N-acryloxysuccinimide and N,N-dimethylacrylamide (DMA) by reversible addition-fragmentation chain transfer polymerization, with Mn ≈ 50k and 23 mol % reactive NAS groups. This copolymer was subsequently modified with hydrophobic (dodecyl) and fluorescent (pyrene, PY; phenanthrene, PHE; or anthracene, AN) side groups, to obtain fluorescent amphiphilic polymers with the same backbone and different substituents. These polymers were adsorbed onto model (ca. 130 nm diameter) poly(butyl methacrylate) nanoparticles, and the size and structure of the adsorbed layer were evaluated using a combination of fluorescence techniques and light scattering. The total diameter increases very fast with polymer concentration up to ca. 140 nm, and then more slowly to 154 nm, stabilizing at this value which corresponds to a polymer shell thickness of ca. 12 nm. In order to evaluate the distribution of hydrophobic groups on the adsorbed polymer layer, we used Förster resonance energy transfer between PHE- and AN-labeled poly(DMA) chains. The obtained concentration profile of the adsorbed polymer corresponds to a coated particle radius which is only slightly smaller than the hydrodynamic radius measured in the same conditions, indicating that the dyes are not located at the particle interface but mostly distributed across the adsorbed layer. Finally, we observed that hydrophobically modified PHE-labeled poly(DMA) chains adsorbed to the nanoparticles were very efficiently displaced by identical hydrophobically modified chains with five times their molecular weight (Mn ≈ 250k) but labeled with PY.