Organic nanoparticles are small particles made of aggregated molecules or polymers. These materials are of broad interest owing to ease of fabrication and wide range of aggregated structures that can be achieved. The morphology of the aggregated molecules/polymers are not easily accessible in annealed thin films, providing a useful platform for fundamental photophysical studies. They are also of interest for applications in photovoltaics, where their small size shortens the distance changes need to travel in order to be extracted.
They are typically synthesised using one of two methods: miniemulsion and reprecipitation. In the miniemulsion method, an aqueous solution containing surfactant and an organic solvent containing the molecule or polymer units are stirred together to make a microemulsion with large droplets of oil in water. Sonification of this macroemulsion forms a miniemulsion and subsequent purification steps remove excess surfactant to leave a concentrated aqueous dispersion.
The reprecipitation method differs from the miniemulsion method in that it doesn’t require surfactant. The molecules or polymer are dissolved in a favourable solvent – typically THF or chloroform and this is injected into water or ethanol respectively. The solvent combinations are chosen as they are partially but not fully miscible in each other. After the rapid injection of the solution containing the molecule or polymer into the “bad” solvent, the solvents can rapidly inter-diffuse and the nanoparticles or small aggregates of molecules begin to form. The “good” solvent is then evaporated to leave the concentrated nanoparticle solution.