Magnetic properties of gold nanoparticles: a room-temperature quantum effect.

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.