On the surface structure of 1,3-dithiol-protected gold nanoparticles interpreted by the size effect of IR absorption properties

• Dihydrolipoic acid is employed to prepare 1,3-dithiol-protected gold nanoparticles.
• A bidentate ligation is prone to make the nanoparticle size large unexpectedly.
• This is due to thermal lability of dithiolate desorption from the gold surface.
• The IR absorption due to the hydrogen-bonded carboxylic acid shows size dependence.
• The existence of intraparticle coupling indicates that of bent SAuS binding modes.

Inspired by the monothiol-bridging staple structure typically observed for the ligand-protected Au25 or Au38 clusters, a bidentate molecule, (R)-dihydrolipoic acid (R-DHLA), is employed to synthesize dithiol-protected gold nanoparticles. The obtained nanoparticles are separated by gel electrophoresis; the most abundant compound has the mean core diameter of 2.47 nm, but a very small cluster compound with the diameter of 0.83 nm is also produced. We find for the first time that the IR absorption assigned to the hydrogen-bonded carboxylic acid in the surface ligands shows size dependence. This can be explained in terms of a bonding manner whether the hydrogen-bond formation occurs between nanoparticles or within a single nanoparticle. The existence of intraparticle coupling between ligands also implies that the bent SAuS is likely to form on the surface structures.

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