Nanoparticles can be defined as lumps of solid matter dispersed in a liquid or gas, with at least one characteristic dimension smaller than 100 nm. Their shape is very often spherical, especially when they are made of amorphous material such as silica or polystyrene, due to the minimization of surface/interface energy. These spherical objects have largely served as model systems for physical chemists to study the behavior of atoms: compact stacking, crystallization, phase transition, etc. Today, they are somewhat less common than in the past. Today, they are somewhat outdated, as the variety of stackings to which they can lay claim is very limited.
To go further, we need to imagine objects with more complex shapes, in the same way that molecules increase the possibilities of combination tenfold compared to simple atoms. This concept has given rise to "colloidal molecules", which can be defined as robust aggregates of spherical particles of controlled size and shape. For example, a tetrapod consisting of a central sphere surrounded by four other spheres of different chemical nature and pointing towards the four vertices of a tetrahedron could advantageously mimic a methane molecule.
The aim of this seminar is to describe the strategy developed by the speaker and his collaborators to fabricate this type of object with a silica core and polystyrene peripheral nodules, using a seeded emulsion polymerization technique. The idea is first to produce spherical, perfectly calibrated silica seeds and modify their surface to make them partially hydrophobic. These seeds are then introduced into an emulsion polymerization reactor, and the latex particles nucleate and grow on the surface of the seeds. Their number and size are controlled by the size of the silica seeds, their concentration and the monomer conversion rate. It has been shown that, in the first instants of the reaction, the number of nuclei is very large, then reduces and stabilizes thanks to a coalescence mechanism. Subsequently, the exemplary regularity of the objects created is controlled by interaction forces, presumably electrostatic, which force the nodules to position themselves at equal distances from one another. The parallel with CSEPR(Valence Shell Electron Pair Repulsion) theory, which explains the geometry of molecules, is therefore particularly relevant. Cryo-electron tomography has been used extensively to study the symmetry of objects, while large-scale statistical studies have shown that morphological yields of over 80% are possible, particularly for the most symmetrical objects, the Platonic solids: tetrahedra (tetrapods), octahedra (hexapods) and icosahedra (dodecapods). The next step is to study the assembly of these objects, and in particular their optical properties.
This work was carried out by Stéphane Reculusa, Adeline Perro, David Nguyen and Anthony Désert in collaboration with Serge Ravaine, Elodie Bourgeat-Lami, Murielle Lansalot, Olivier Spalla, Antoine Thill, Olivier Lambert, Jean-Christophe Taveau.
