Web Release Date: January 18,
Pulsed Field Gradient NMR Studies of Polymer Adsorption on Colloidal CdSe Quantum Dots
and
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, M5S 3H6 Ontario, Canada, The Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China, Department of Chemical and Physical Sciences, University of Toronto Mississauga, 3359 Mississauga Road, Mississauga, L5L 1C6 Ontario, Canada, and Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong
Received: August 28, 2007
In Final Form: November 1, 2007
Abstract:
Pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments have been used to examine ligand exchange between poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMA) (Mn = 12 000, Mw/Mn = 1.20, Nn = 78) and trioctylphosphine oxide (TOPO) bound to the surface of CdSe/TOPO quantum dots (QDs). We show that PFG 1H NMR can quantify the displacement of TOPO by PDMA through its ability to differentiate signals due to TOPO bound to the QDs versus those from TOPO molecules free in solution. For CdSe QDs with a band edge absorption maximum at 558 nm (diameter 2.7 nm by transmission electron microscopy), we determined that, at saturation, 8 polymer chains on average displace greater than 90% of the surface TOPO groups. At partial saturation, with an average of 6 polymer chains/QD, each TOPO displaced requires 28 DMA repeat units. Assuming that one Me2N- group binds to a surface Cd2+ for each TOPO displaced, we infer that only about 3% of the DMA units are directly bound to the surface. The remaining groups are present as loops or tails that protrude into the solvent and increase the hydrodynamic diameter of the particles.
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