Nanoparticles that break the rules
Janus particles, named after the two-faced Roman god Janus because they have two different hemispheres of functionalisation, are finding interesting applications in drug delivery and magnetism. Scientists launched the EU-funded project 'Janus nanoparticles as novel ferroelectric materials' (JANUS FERROELECTRICS) to investigate the possibility of creating new types of electrically responsive materials based on Janus particles. The team focused on gold NPs covered with a monolayer of thiol-terminated molecules. The chemistry is among the simplest for studying monolayer organisation on spherical NPs, and nano-structured monolayers on gold NPs have recently led to important discoveries. Using a variety of advanced experimental techniques to analyse the systems, researchers discovered a rich inherent complexity. In addition to Janus particles, scientists observed striped monolayers, randomly mixed monolayers or even a mixture of the two. They also observed unusual solubility and spontaneous aggregation or self-assembly in some of the systems created to study ferroelectricity. This became the new focus of the project. A common principle in chemistry is that 'like dissolves like'. It refers to the general idea that the solubility of a molecule can be predicted by assessing the structural similarity between the molecule and the solvent. This applies to surface monolayers with different chemistries as well, to the extent that the interactions typically reflect the broad-featured chemistries together. JANUS FERROELECTRICS discovered an important exception to this rule. Namely, in the case of an organic coating that forms a monolayer on a colloid surface, a single chemistry can become dominant with respect to the particle's solubility. It appears to be dependent on surface curvature. Scientists studied a variety of systems. Importantly, they have delivered new and improved techniques for characterisation of organic coatings on colloids. These include computational methods to study atomic-level interface interactions, imaging techniques, and additional computational methods to model and predict the images obtained experimentally. The project has pushed the frontiers with materials and methods for developing new, smart materials with novel wetting properties. It puts the EU at an advantage in nanotechnology and nanomaterials development.
