  In this work we present the magnetic properties of gold coated FePt
nanoparticles and the study of stable aqueous dispersions of FePt@Au and FePt
synthesized after ligand exchange with mercaptoundecanoic acid. The particle
size determined from TEM micrographs goes from 4 nm for the uncoated
nanoparticles to a maximum of 10 nm for the gold coated ones indicating that
the thickness of the shell ranges from 1 to 3 nm. The magnetic characterization
consists in hysteresis cycles at 10 and 300 K. The results show that, at low
field and room temperature, the magnetic behavior of uncoated and coated
nanoparticles are surprisingly quite similar. Since the gold coated
nanoparticles keep the magnetic properties of FePt and the presence of gold
improves the functionalization of nanoparticles, the system is suitable for
biological application. Mercaptoundecanoic ligand transfer was used to render
water stable nanoparticles in a wide pH range. Transmission electron microscopy
and dynamic light scattering results show the nanoparticles slightly
agglomerate after ligand exchange. Fourier transform infrared spectroscopy
results suggest that thiol bind to the gold atoms of the surface.
