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Abstract. Surface plasmon resonance and magnetic characterization have been carried out for two types of thiol-capped gold nanoparticles (NPs) with similar diameters between 2.0 and 2.5 nm and different organic molecules linked to the sulphur atom: dodecanethiol and tiopronin. In addition, Au NPs capped with tetraoctyl ammonium bromide have also been included in the investigation since such capping molecules weakly interact with the gold surface atoms and, therefore, this system can be used as a model for naked gold NPs; such particles presented a bimodal size distribution with diameters around 1.5 and 5 nm. The plasmon resonance is non-existent for tiopronin-capped NPs, whereas a trace of such a feature is observed for NPs covered with dodecanethiol molecules and, a bulk-like feature is measured for NPs capped with tetralkyl ammonium salts. These differences would indicate that the modification of the surface electronic structure of the Au NPs depends on the geometry and selfassembling capabilities of the capping molecules and, on the electric charge transferred between Au and S atoms. Regarding the magnetization, dodecanethiol-capped NPs have a ferromagnetic-like behaviour, while the NPs capped with tiopronin exhibit a paramagnetic behaviour and tetralkyl ammonium-protected NPs are diamagnetic across the studied temperature range; straight chains with a well defined symmetry axis can induce orbital momentum on surface electrons close to the binding atoms. The orbital momentum not only contributes to the magnetization but also to the local anisotropy giving rise to permanent magnetism. Due to the domain structure of the adsorbed molecules, orbital momentum is not induced for tiopronin-capped NPs and the charge transfer only induces a paramagnetic spin component.

In this work the study of oleic acid and oleylamine-capped gold nanoparticles is presented. The structural characterization of the sample shows 6.7 nm gold nanoparticles with a narrow size distribution. The experimental optical absorption spectrum has a maximum at 2.35 eV. The calculated optical absorption spectrum is shifted and narrower than the experimental one, indicating that the oleic acid and oleylamine do not merely passivate the metallic nanoparticles but modify its electronic structure. These gold nanoparticles show in addition a kind of magnetic order similar to other organic passivated gold nanoparticles as thiol-capped gold nanoparticles. Although the magnetic interactions seem to be weaker than in thiol-capped ones, the magnetic behavior looks similar to that, i.e., an invariant temperature dependence of the magnetization from 5 to 300 K and a noticeable coercive field. We analyze the influence of the organic layer bonding the nanoparticles on the magnetic behavior.