DOI: 10.1002/adma.200600984 Magnetism in Polymers with Embedded Gold Nanoparticles** By Jose de la Venta, Andrea Pucci, Enrique Fernández Pinel, Miguel A. García,* Cesar de Julián Fernandez, Patricia Crespo, Paolo Mazzoldi, Giacomo Ruggeri, and Antonio Hernando Nanoparticles (NPs) are interesting systems, as they present different properties from bulk materials with the same com- position. [1–4] It was recently found that dispersed Au NPs capped with thiols exhibit ferromagnetic behavior at room temperature, despite the diamagnetic character of bulk Au. [1,2] These NPs, when embedded in a polyethylene (PE) matrix, [3] preserve the surprising magnetic properties induced at the nanoscale. NPs exhibit a large fraction of surface atoms, which are different to those of the bulk volume mainly because of the broken symmetry at the surface, which alters their elec- tronic configuration and consequently their physical and chemical properties, depending on the electronic state. Al- though bulk materials also have surface atoms, they represent a negligible fraction of the total, so their contribution to the physical properties of the material is insignificant. Moreover, in the case of NPs, as the fraction of surface atoms strongly depends on the particle size, so do their properties. Another interesting feature of the surface atoms is that their properties can be modified by capping the NP with different chemical species, which create bonds that induce a modification of the charge distribution at the NP surface. A surprising example of these surface effects is the observation of magnetism in gold NPs capped with dodecanethiol, in spite of the diamagnetic behavior of the bulk gold, as noted above. [1] When Au NPs are capped with thiols, the interaction between the Au and the S atoms at the end of the alkyl chain modifies the elec- tronic structure of the NP surface, producing 5d localized holes. [5] These holes give rise to localized magnetic moments that are strongly fixed along the bond direction, [6] providing NPs with magnetic properties that are independent of temper- ature in the range 5–300 K. Therefore, only surface atoms contribute to this magnetism, and consequently, as the NP size is reduced (i.e., the fraction of surface atoms increases), its magnetization increases. It is worth noting that both of the pa- rameters governing the appearance of magnetism (the particle size and the interaction with the surrounding medium) strongly depend on the preparation procedure. So far, this magnetism has been observed in thiol-capped Au NPs dispersed in a liquid medium and deposited onto a substrate in air. For these free NPs, the magnetic moments can fluctuate because of NP rotation, hence prohibiting their use for any application requiring permanent magnetic mo- ments. Furthermore, the continuous movement of the NPs prevents their use in creating patterns of stable magnetic fields. These problems could be overcome if the particles were embedded in a solid matrix, creating an ensemble of NPs as well as allowing simplified handling. However, it does not fol- low straightforwardly that the magnetic behavior will be pre- served when the particles are embedded in a matrix. As the magnetism arises from the surface and the particular features of the thiol–Au bond, slight modifications of the environment may significantly alter the magnetic properties of the NPs. Therefore, synthesis methods with an accurate control of both the NP features (size distribution and shape) and the interac- tion with the surrounding media are required. Another important aspect of nanoparticles, which has been thoroughly investigated for Au NPs, is that they show optical, electronic, and catalytic properties that are related to size ef- fects and are highly correlated. In this sense, we have also in- vestigated the correlation between the optical and magnetic properties of polymer-dispersed Au NPs. Dodecanethiol-capped Au NPs embedded in a polymeric matrix were prepared as described in the literature [3,7] and ex- plained in the Experimental section. A bright-field transmis- sion electron microscopy (TEM) image of Au NPs embedded into a PE matrix revealed a regular spherical particle shape with a mean diameter of about 3–4 nm (Fig. 1). Moreover, the Au NPs were well dispersed inside the polymer matrix, due to the dodecanethiol protecting layer, which prevented aggregation of the particles during the film preparation at high temperature. In order to study the surface plasmon resonance (SPR), the most remarkable optical property of metallic NPs, [8] which is strongly correlated to particle size and changes in the elec- tronic structure induced by thiol capping, we measured the UV-vis optical absorption of the NPs. SPR consists of a collec- tive oscillation of the conduction electrons inside the NP with a resonance frequency that for most of the noble metals lies COMMUNICATION Adv. Mater. 2007, 19, 875–877 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 875 – [*] Dr. M. A. Garcia, J. de la Venta, E. Fernández Pinel, Dr. P. Crespo, Prof. A. Hernando Instituto de Magnetismo Aplicado & Depto. Física de Materiales PO Box 155, 28230 Las Rozas, Madrid (Spain) E-mail: miguelag@adif.es Dr. A. Pucci, Prof. G. Ruggeri Dept. of Chemistry and Industrial Chemistry, University of Pisa Via Risorgimento, 35 56126 Pisa (Italy) Dr. C. de Julián Fernandez, Prof. P. Mazzoldi Dipartimento di Fisica “G. Galilei”, Università di Padova Via Marzolo, 8 35131 Padova (Italy) [**] This work has been partially supported by project No. NAN2004- 09125-C07-05 (Spanish Ministry of Education and Science) and the Italian FIRB project MICROPOLYS. The authors thank Prof. Fernan- do Galembeck (Universidade Estadual de Campinas) for the TEM measurements.