Ligand dynamics in spin-labeled au nanoparticles

Variable temperature electron paramagnetic resonance (EPR) spectra of spin-labeled Au nanoparticles in toluene solution were recorded at X-, Q- and W-bands. The chain length of the nitroxide-based spin-labeled ligand and the surrounding alkanethiol ligands was systematically varied. The spectra were analyzed using the NLSL program (Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson., Ser. A 1996, 120, 155) which simulates slow motion spectra using the stochastic Lionville approach. The simulation results show that the rotational diffusion parameters for particles (similar to 2.6 nm An core diameter) are dominated by the local motion of the ligand. The length of the spin label has the biggest effect on the nitroxide dynamics followed by the chain length of the surrounding ligands. The results are consistent with the loose packing of ligands on the nanoparticle surface. The packing density of ligands further decreases with the increasing distance from the Au surface. The Arrhenius analysis of the dynamic parameters obtained at different temperatures showed an activation energy for rotational diffusion of similar to 2.5-4 kcal/mol, regardless of the nature of the spin label and the surrounding ligand. This result is also consistent with the very open, disordered structure of the organic ligands on the nanoparticle surface in toluene.