A series of differently sized gold nanoparticles stabilized by bidentate calixarene ligands, prepared by the Brust–Schiffrin two-phase synthesis, were characterized by XPS, in combination with TEM. The experimental data show that the particular conditions in the synthesis reaction allow to obtain subnanometric gold nanoparticles. Such nanoparticles with controlled and reduced size are the best candidate for anchoring on a Si surface, through specific and reversible supramolecular interactions, in view of the development of new hybrid nanomaterials. XPS measurements allow to separate the Au atoms in two subsets, on the basis of their clearly distinct Au 4f binding energy: central and surface atoms bound to S. The S 2p spectra present a main component due to thiolate–Au bond, compatible with the structural ‘staple motif’ reported in the literature. Because very small nanoparticles have a larger percentage of surface atoms, the experimental S/Au and Au–S/Aucentral ratios are an indirect evaluation of the cluster nuclearity, likely to be assigned to Au11 or Au13, both compatible with the observed diameters.
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