pH-Dependent gold nanoparticle self-organization on functionalized Si/SiO2surfaces

The self-organization of citrate- and acrylate-stabilized gold nanoparticles\nonto SiO2/hydroxyl-, amino- and nitro-terminated surfaces was investigated\nas a function of pH. Bare clean Si/SiO2 substrates were used as the\nSiO2/hydroxyl-terminated surfaces and self-assembled monolayers (SAM)\nof (3-aminopropyl)trimethoxysilane (APTMS) and 3-( 4-nitrophenoxy)propyltrimethoxysilane\n( NPPTMS) on Si/SiO2 were employed as the amino- and nitro-terminated\nsurfaces, respectively. All the surfaces were fully characterized\nby contact angle, atomic force microscopy (AFM), ellipsometry and\nX-ray photoelectron spectroscopy (XPS). Citrate- and acrylate-stabilized\ngold nanoparticle stability was also investigated as a function of\npH by UV-visible absorption spectroscopy and Z-potentiometry. The\ngold nanoparticle surface coverage of the substrates was independently\nestimated by AFM and XPS. The results show that colloid deposition\non bare SiO2/OH surfaces and on NPPTMS monolayers is negligible with\nthe exception of acrylate-stabilized gold nanoparticles which were\nfound to be immobilized on nitro-terminated surfaces at pH lower\nthan 3.5. Nevertheless, APTMS monolayers interact strongly with citrate-\nand acrylate-stabilized gold nanoparticles exhibiting a dependence\nof the surface coverage from the pH of the colloidal solution.