Phosphonic and sulfonic acid-functionalized gold nanoparticles: a solid-state NMR study

Gold nanoparticles capped with 11-mercaptoundecanylphosphonic acid (MUP) and sodium 10-mercaptodecanesulfonic acid (MDS) were characterized by a range of techniques which included solid-state 31P and 13C NMR spectroscopies. Despite similar core sizes and alkyl chain lengths, the conformational and dynamic properties of the capping ligands are very different for the two types of nanoparticles. Whereas MDS produces disordered monolayers on planar gold surfaces, the MDS-capped nanoparticles show a high degree of chain order with the onset of reversible chain disordering occurring just above room temperature. The alkyl chains of MUP adsorbed on the gold nanoparticles are more ordered and motionally restricted than the unbound solid surfactant due to strong intramonolayer and interparticle hydrogen bonds. This conformational order is thermally stable, and disordering only occurs upon decomposition and desorption of MUP from the gold core. Solid-state 31P NMR has been demonstrated to be a sensitive probe of the interactions of the PO3H2 terminal groups.