Synthesis of hollow Ag-Au bimetallic nanoparticles in polyelectrolyte multilayers

Ag nanoparticles of ~20 nm size and rather uniform size distribution were synthesized in polyelectrolyte multilayers (PEMs) via an ion-exchange/reduction process in two stages (seeding and growth), which were used as sacrificial templates to fabricate Ag-Au bimetallic hollow nanoparticles via galvanic replacement reaction. The reaction process was monitored by UV-vis spectroscopy. The morphology and structure of the nanoparticles were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy, which confirmed the formation of hollow Ag-Au bimetallic nanoparticles. UV-vis absorbance spectroscopy and TEM results indicated that both size and optical properties of the Ag nanoparticles in the PEM can be controlled by manipulating ion content in the PEM and the number of the ion-exchange/reduction cycle, whereas that of Ag-Au bimetallic nanoparticles were dependent on size of the Ag templates and the replacement reaction kinetics. The hollow Ag-Au bimetallic nanoparticles exhibited a significant red shift in the surface plasmon resonance to the near-infrared region. The strategy enables facile preparation of hollow bimetallic nanoparticles in situ in polymer matrixes.