(471h) A Novel Aerosol Based Process for Controlled Nanoparticle Clusters Via the Breath Figure Technique
Colloidal nanoparticles are of great interest and are used in various disciplines such as materials science, chemistry, physics and engineering because of their unique properties which are not found in their bulk counterparts. Recently, various synthetic strategies have been employed to create secondary structures of colloidal nanoparticles for the development of advanced materials. These so called colloidal nanoparticle clusters not only allow one to combine the properties of the individual nanoparticles but also allow to take advantage of the interaction between particles resulting in new properties. A notable example is the assembly of noble metal nanoparticles into secondary structures inducing near field coupling of surface plasmon between adjacent particles producing new optical properties. This research describes a novel and cost effective approach to form controlled clusters of nanoparticles by assembling them in the pores of breath figures by using an aerosol mist of the nanoparticles. The breath figure technique (based on a water templating mechanism) can produce a mono-dispersed porous polymer film through the evaporation of a polymer solution in a volatile organic solvent under constant humid conditions. We hypothesized that if we used an aerosol mist carrying nanoparticles instead of humid air, we could place the nanoparticles specifically in the pores of the breath figures. We synthesized breath figures by casting polymer films under humid aerosol mists containing colloidal silica. These particles stayed in the condensed water droplets and as the droplets evaporated, they assembled only inside the pores of the breath figures.
The concentration of the nanoparticles in the solution to be aerosolized governs the amount of nanoparticles which get deposited in the pores of the breath figures. Thus the size of the nanoparticle cluster can be controlled. Also, the pore size of the breath figures can be tuned by adjusting parameters such as the polymer concentration, flow rate of the mist and solvent etc. Polystyrene breath figures were fabricated by spin coating a solution of polystyrene in the presence of an aerosol mist of silica nanoparticles (100 nm diameter). The colloidal silica got deposited inside the pores of these polystyrene breath figure films and by varying the pore size and the colloidal silica concentration in the aerosol mist, clusters of a variety of sizes were obtained. The nanoparticle cluster loaded porous polymer films can be used as such or the polymer film can be dissolved in an organic solvent and the clusters can be retrieved. In the case of colloidal silica, once the breath figures containing clusters were formed, silanol bonds were formed between the individual silica nanoparticles. Then the polymer was dissolved by using an organic solvent and the controlled clusters were extracted. These results were confirmed by scanning electron microscopy. The mechanism of the formation of these clusters will be explained. Thus a new strategy to obtain controlled nanoparticle clusters is found which can be easily scaled up and does not require any specialized equipment (the spin coating can be avoided if thin films are not required). This work was supported by the National Science Foundation.