Solution NMR techniques for investigating colloidal nanocrystal ligands: A case study on trioctylphosphine oxide at InP quantum dots

Colloidal semiconductor nanocrystals or quantum dots are an important building block in bottom-up nanotechnology. They consist of an inorganic, crystalline core surrounded by a monolayer of organic ligands. As these ligands can be modified or exchanged for others, they provide a convenient way to give the quantum dots functionality. Here, we show that solution NMR is a useful tool to investigate the ligands of colloidal nanocrystals using InP quantum dots with trioctylphospine oxide ligands. Combining H1 –C13 HSQC spectroscopy with pulsed field gradient diffusion NMR, an unequivocal identification of the resonances of the bound ligands is possible. This leads to the determination of the diffusion coefficient of the nanocrystals in solutions and allows to verify capping exchange procedures. By calibrating the surface area of the NMR resonances using a solute of known concentration, the density of ligands at the nanocrystal surface can be quantified. This demonstrates that a dynamic equilibrium exists between bound and free ligands. Analysis of the corresponding adsorption isotherm – determined using H1 NMR – leads to an estimation of the free energy of adsorption and the free energy of ligand–ligand interaction at the nanocrystals surface.