Evidence of the Grafting Mechanisms of Diazonium Salts on Gold Nanostructures

Gold nanostructures (GNS) were chemically functionalized using four different diazonium salts: benzene-diazonium-tetrafluoroborate (DS), 4-decylbenzene-diazonium-tetrafluoroborate (DS-C10H21), 4-carboxybenzene-diazonium-tetrafluoroborate (DS-COOH), and 4-(aminoethyl)-benzene-diazonium-tetrafluoroborate (DS-(CH2)2NH2). Effective chemical grafting on GNS was shown by surface-enhanced Raman spectroscopy (SERS); aromatic ring deformations in the range of 1570–1591 cm–1 are of particular interest. The very strong band observed around 1075 cm–1, related to CH in-plane bending for mono- and para-substituted benzenes (coupled with ring-N stretching mode), provided further irrefutable evidence of the grafting. SERS enhancement of these two bands ascertains the perpendicular orientation of the aromatic rings on the GNS. X-ray photoelectron spectroscopy (XPS) analyses of chemically grafted flat gold surfaces suggest azophenyl radical pathways when using DS, DS-(CH2)2-NH2, or DS-C10H21. It was shown that coating at the interface is the result of a Au–N covalent bond; growth of the layers is via N═N. These XPS results agree with those provided by SERS without excluding the aryl radical pathways. For DS-COOH, the results provided by SERS, XPS, and density functional theory calculations show (i) effective chemical grafting of the GNS via a covalent bond between gold and carboxylate forms and (ii) growth via multilayers in the meta position between aromatic rings through either N═N or C–C bonds.