Pulsed Field Gradient NMR Studies of Polymer Adsorption on Colloidal CdSe Quantum\nDots

Abstract

Pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments have been used to examine ligand\nexchange between poly(2-(<i>N</i>,<i>N</i>-dimethylamino)ethyl methacrylate) (PDMA) (<i>M</i>_n = 12 000, <i>M</i>_w/<i>M</i>_n = 1.20,\n<i>N</i>_n = 78) and trioctylphosphine oxide (TOPO) bound to the surface of CdSe/TOPO quantum dots (QDs). We\nshow that PFG ¹H NMR can quantify the displacement of TOPO by PDMA through its ability to differentiate\nsignals due to TOPO bound to the QDs versus those from TOPO molecules free in solution. For CdSe QDs\nwith a band edge absorption maximum at 558 nm (diameter 2.7 nm by transmission electron microscopy),\nwe determined that, at saturation, 8 polymer chains on average displace greater than 90% of the surface\nTOPO groups. At partial saturation, with an average of 6 polymer chains/QD, each TOPO displaced requires\n28 DMA repeat units. Assuming that one Me₂N− group binds to a surface Cd²⁺ for each TOPO displaced,\nwe infer that only about 3% of the DMA units are directly bound to the surface. The remaining groups are\npresent as loops or tails that protrude into the solvent and increase the hydrodynamic diameter of the particles.