Structural characterization of organic coated ultrasmall gold nanoparticles by combined SAXS-SANS experiments

Ultrasmall nanoparticles (usNPs) of noble metals like. Ag, Au, Pd, Pt, or Ir with high chemical stability and high specific surface area represent a broad field for applications in heterogeneous catalysis, imaging and biomedicine. Such usNPs are surface-functionalized either by sulfur-carrying ligands (cysteine, glutathione, peptides) or by azide-alkyne click chemistry for larger molecules (DNA oligomers). Due to the small size of the particles, the characterization of the nanoparticle shell is challenging because it cannot be analyzed by HRTEM due to lack of contrast. Only NMR spectroscopy can provide some information, either in the form of DOSY or from spectra of dispersed particles, showing broad NMR peaks. SAXS is very suitable for this type of nanoparticles but the high electron density of the core makes the surrounding shell almost invisible. For neutrons, on the other hand, the contrast is not tied to the atomic number and can depend heavily on isotope. Therefore, one can take advantage of the possibility of contrast variation for the study of the organic coating of the usNPs. A combined approach of SAXS and SANS will provide valuable data for the characterization of the nanoparticles' size, shape and coating details. After all, the state of the ligand shell (hydrated/solvated, expanded, mushroom-like etc) is still unknown, but it will determine any biological response to nanoparticles that are surface-functionalized with receptor ligands.