Magnetic properties of gold nanoparticles: A room-temperature quantum e ffect
Résumé
Gold nanoparticles elicit a huge research activity in view of
their applications in diagnostics,[1, 2] therapy,[3] drug or gene delivery,[
4] sensing[5, 6, 7] and imaging.[8] Gold nanoparticles also display
interesting catalytic[9, 10] and optical[11, 12, 13, 14] properties.
This Communication focuses on the least understood and so
far unused property of gold: its becoming magnetic when prepared
in the form of nanoparticles. All these desirable properties,
bound together in one nanometric piece of matter, possibly
self-organized thanks to its ligands, make functionalized
gold nanoparticles a treasurable entity for nanosciences. The
ex nihilo magnetic properties of functionalized gold (and other
diamagnetic metals, such a Ag or Cu) nanoparticles, that is,
their ferromagnetic-like behavior, are well documented,
though still poorly understood.[15] This unexpected property
opens new perspectives in materials science, in particular for
the design of metamaterials. One may also envisage applications
in information storage and processing: nanometric magnetic
particles with no obvious temperature limitation and possibly
self-organizing are currently much sought-after by the
computer industry and developing a room-temperature magnetic semiconductor is paramount for the realization of spintronics
technologies.
Herein, we wish to present the results of our own investigations
into the magnetic properties of functionalized gold nanoparticles.
We have made attempts at understanding this magnetic
behavior using both traditional techniques (e.g. superconducting
quantum interference device, SQUID, magnetometry)
and other methods less common in this field, such as zerofield
197Au NMR (nuclear magnetic resonance) and SANS (smallangle
neutron scattering). We also directly probed the local
magnetic field at the surface of gold nanoparticles using paramagnetic
TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] radicals
and ESR (electron spin resonance) spectrometry. Surprisingly,
none of these experiments provided a clearer picture in
fine. These “negative” results led us to pondering whether or
not the explanation could be elsewhere. Our hypothesis is that
the magnetism of gold (and possibly other metals) could very
well originate in self-sustained persistent currents. We shall
demonstrate hereafter that this hypothesis is indeed very plausible
and would actually reconcile all of the experimental data
reported to date.Striking results are often obtained when SQUID magnetometry
is performed on functionalized Au nanoparticles, such as
dodecanethiol-coated ones. Rather than being diamagnetic, as
expected, the nanoparticles can be found to be para- or ferromagnetic
at room temperature and above. When hysteresis is
observed, the magnetization curve looks like that of a soft ferromagnet
and exhibits a remnant magnetization MR and a coercive
field HC, though both are rather weak. These parameters
have been observed to have values that vary by orders of
magnitude from sample to sample[15] (see Figure ESI-1 of the
Supporting Information). Very often, the magnetization does
not saturate. Diamagnetic samples are more diamagnetic than
the bulk metal. Also, the magnetic observables show little dependence
on temperature between 2 and 400 K. The measurements
reported so far have been performed by totally independent
groups, on systems that were synthesized using
known chemical procedures. Figure 1 compares the magnetization
of bulk gold with that of two diamagnetic samples of
gold nanoparticles. It can be seen that nanoparticles have
a much larger absolute diamagnetic susceptibility than massive
gold.
Figure 2 compares two samples of gold nanoparticles, exhibiting
a paramagnetic behavior and a ferromagnetic-like one.
There is a weak but clear hysteresis, and the magnetization
does not really saturate even at high field values.