Synthesis and characterization of functionalized ionic liquid-stabilized metal (gold and platinum) nanoparticles and metal nanoparticle/carbon nanotube hybrids.

Carboxylic acid- and amino-functionalized ionic liquids were used as the stabilizer for the systhesis of metal nanoparticles in aqueous solution. Smaller gold nanoparticles (3.5 nm) and platinum nanoparticles (2.5 nm) were prepared with NaBH4 as the reductant. Larger gold nanospheres (23, 42, and 98 nm) were synthesized using different quantities of trisodiumcitrate reductant. The morphology and the surface state of the metal nanoparticles were characterized by high-resolution transmission electron microscopy, UV-visible spectroscopy, and X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy spectra indicated that binding energies of C 1s and N 1s from ionic liquids on the surface of metal nanoparticles shifted negatively compared with that from pure ionic liquids. The mechanism of stabilization is proposed to be due to the interactions between imidazolium ions/functional groups in ionic liquids and metal atoms. Resonance Rayleigh scattering property of the functionalized ionic liquid-stabilized metal nanoparticles was also explored. It was found that amino-functionalized ionic liquid-stabilized gold nanoparticles exhibited lower resonance Rayleigh scattering intensity than trisodiumcitrate stabilized gold nanoparticles, which is expected to decrease the background of the resonance Rayleigh scattering intensity in the determination of various analytes. Moreover, it was found that all the as-prepared metal nanoparticles could be easily assembled on the multiwalled carbon nanotubes, which was confirmed by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. In this case, ionic liquids acted as a linker to connect metal nanoparticles with carbon nanotubes. The imidazolium ring moiety of ionic liquids might interact with the pi-electronic nanotube surface by virtue of cation-pi and/or pi-pi interactions, and the functionalized group moiety of ionic liquids might interact with the metal NPs surface. Finally, it was observed that plantinum nanoparticle/multiwalled carbon nanotube hybrids could be well dispersed in water, which may find future applications in fields such as catalysis, nanoscale electronics, as well as sensors.