Characterization of the Ligand Capping of Hydrophobic CdSe–ZnS Quantum Dots Using NMR Spectroscopy

We have combined a few advanced solution phase NMR spectroscopy techniques, namely, H-1, P-31, heteronuclear single quantum coherence (HSQC), and diffusion ordered spectroscopy (DOSY), to probe the composition of the organic capping layer on colloidal CdSe-ZnS core-shell quantum dots grown via the "hot injection" route. Combining solution phase P-31 and H-1 NMR with DOSY, we are able to distinguish between free ligands and those coordinated on the QD surfaces. Furthermore, when those NMR measurements are complemented with matrix-assisted laser desorption ionization (MALDI) and FTIR data, we find that the organic shell of the as-prepared QDs consists of a mixture of tri-noctylphosphine oxide (TOPO), tri-n-octylphosphine (TOP), alkyl amine, and alkyl phosphonic acid (L-and X-type ligands); the latter molecules are usually added during growth at a rather small concentration to improve the quality of the prepared nanocrystals. However, NMR data collected from QD dispersions subjected to two or three rounds of purification reveal that the organic shell composition (of purified QDs) is essentially dominated by monomeric or oligomeric n-hexylphosphonic acid, along with small fractions of surface-coordinated or hydrogen-bonded 1hexadecyl amine and TOP/TOPO. This is true even though large excesses of TOP and TOPO surfactants are used during QD growth. This proves that n-hexylphosphonic acid (HPA) exhibits substantially higher coordinating affinity to the QD surfaces, compared to other phosphorus-containing surfactants such as TOP and TOPO. Finally, we test the utilitys of DOSY NMR to provide accurate data on the translational diffusion coefficient (and hydrodynamic radius) of QDs, as well as freely diffusing ligands in a sample. This proves that DOSY is a highly effective characterization technique for such small colloids and organic surfactants where DLS reaches its limit.