We report here the structure of complexes made of proteins (lysozyme, positively charged) and polyelectrolytes (PSSNa, negatively charged). We stay in conditions where the volume fractions of the components are of the same order and where PSS concentrations correspond to a semidilute regime. The final complexes structure is determined by SANS. We obtain three main types of structures: (i) For a protein excess and for long polyelectrolyte chains, the network preformed by PSS chains still exists but chains are partially shrunk due to cross-linking by lysozyme. Macroscopically, samples are gelled. (ii) For a protein excess and for short polyelectrolyte chains, PSS chains are locally shrunk and do not form a network anymore. Lysozyme and PSS chains are embedded in dense 3-D aggregates that arrange in a fractal network at a larger scale. Macroscopically, samples are liquid. (iii) For a polyelectrolyte excess and whatever the chain length, the internal structure of the lysozyme changes. After an initial strong electrostatic binding, lysozyme is progressively unfolded thanks to a hydrophobic contact with PSS. The two chainlike objects are finally organized in a homogeneous costructure. Macroscopically, samples are liquids.

accessory. Samples were prepared by evaporating a drop of solution on a carbon (Agar) grid.Nonlinear Optical Measurements: Harmonic light scattering measurements were conducted with a Q-switched Nd:YAG (yttrium aluminum garnet) laser emitting pulses of about 40 ns at 1.91 lm. Metal nanoparticles with various shapes and organizations are desired in order to understand nanometer-scale properties, and they are attractive for several applications in the fields of optics, electronics, and magnetism. As far as magnetic storage is concerned, very high storage density requires high magnetic anisotropy to overcome thermal effects and to prevent superparamagnetic behavior, which appears as the size of the magnetic single-domain particles is reduced. Several kinds of magnetic anisotropy can be considered for this application, including: i) magnetocrystalline anisotropy (for example, of CoPt and FePt alloys with tetragonal structures); [1±3] ii) exchange anisotropy of ferromagnetic/antiferromagnetic core±shell particles; [4] and iii) shape anisotropy of elongated magnetic particles such as rods and wires.[5]The chemical synthesis of nanoparticles presents the advantage of simplicity and low cost compared with physical approaches. Moreover, it is now well-established that the structure of fine cobalt particles prepared by physical means under high vacuum is size-dependent: the face-centered cubic (fcc) phase is stabilized for mean diameters below 20 nm. [6,7] In contrast, several examples of hexagonal close-packed (hcp) cobalt nanoparticles prepared by wet-chemical processes have been reported. [8,9] The synthesis of metal nanoparticles with anisotropic shapes by liquid-phase processes is an interesting challenge, because in most cases the isotropic shapes minimize their surface energy in solution.In this context, several solid hosts have been used as templates for the anisotropic growth of metal particles, for example, mesoporous silica [10] and carbon nanotubes.[11] The most COMMUNICATIONS 338

Cobalt and cobalt−nickel nanoparticles were synthesized by reducing mixtures of cobalt and nickel acetates in sodium hydroxide solution in 1,2-propanediol. The particle shape depends strongly on the sodium hydroxide concentration and the Co/Ni composition of the particles. For cobalt-rich content, agglomerated rods, nanowires with a mean diameter of about 8 nm, and platelets were successively observed when [NaOH] was increased in the range 0−0.2 M. For the Co50Ni50 composition and [NaOH] in the range 0.1−0.18 M, nanodumbbells are formed that consist of a central column richer in cobalt capped with two terminal platelets richer in nickel. The shape of the dumbbells strongly depends on the basicity; long dumbbells are obtained for the lowest NaOH concentration, and short dumbbells and diabolos for the highest. To understand the role of the sodium hydroxide concentration and the different reactivities of cobalt and nickel, we analyzed the equilibrium between the Co2+ and Ni2+ ions in solution and the intermediate unreduced solid phase. For the Co80Ni20 composition, we show that increasing the sodium hydroxide amount lowers the Co2+ and Ni2+ ions in solution through the precipitation of the intermediate solid phase, suggesting that the nanowires are obtained with a higher growth rate than the platelets. The analysis of the solid intermediate phases revealed a Co(II) alkoxide and a Ni(II) hydroxy-acetate showing strong differences in the chemistry of the these two ions in basic solutions of 1,2-propanediol. These differences can explain the two well-separated growth steps originating the Co50Ni50 nanodumbbells.