Synthesis of silica nanoparticles covered with silver beads.

Procedures for producing silica nanoparticles suitable for further amino functionalization and subsequent decoration with silica beads were investigated in a comparative way. Several methods, one based on tetrapropylammonium hydroxide, the classical Stöber synthesis, and two with amino acids (either lysine or arginine) as catalysts were employed and followed by means of DLS, SAXS, and TEM. The amino acid methods proved to be by far the most satisfactory ones, yielding highly spherical and monodisperse nanoparticles with a tunable size range of 15-100 nm. The surface of the particles could be functionalized with propylamine, which enabled to obtain positive surface charge at low pH and to tune the zeta potential by the pH in the range of +/- 40 mV. Finally, the modified particles were used to reduce silver (I) ions at high pH, leading to the formation of very small silver beads covering the silica surface and yielding a nanocomposite with a "raspberry" structure. Interestingly, this could be achieved without using any complementary reducing agent besides the particles themselves, thereby opening a very simple path to the formation of composite metal containing colloidal systems.