Size-Controlled Nanoparticle Clusters of Narrow Size-Polydispersity Formed Using Multiple Particle Types Through Competitive Stabilizer Desorption to a Liquid-Liquid Interface.

A novel colloidal approach is presented for preparing fully dispersed nanoparticle (NP) assemblies (clusters) of narrow size-polydispersity over a wide range of sizes through irreversible depletion of stabilizing ligands onto a liquid-liquid interface. Unusually, the relative monodispersity of the assemblies continuously improves throughout the process. A detailed kinetics study into the assembly of iron oxide NP clusters shows that the assembly rate decreases with NP concentration, pinpointing the role of the interface in size focusing. A new protocol for identifying initial conditions that enable controlled assembly is described, which allows extension of the approach to multiple NP types, opening up a general route to colloidally processed materials. The process uses cheap materials, it is reproducible, robust, and scaleable, and it allows for selection of both particle and cluster size. In the case of assemblies of magnetic iron oxide NPs, these advantages enable tuning of the magnetic properties of the assemblies for applications such as magnetically targetable MRI-trackable agents in biomedicine.

Monodisperse magnetic nanoparticle assemblies prepared at scale by competitive stabiliser desorption.

We report a scalable and reproducible method to assemble magnetic nanoparticle clusters from oleic acid stabilised iron oxide nanoparticles. By controlling the surface coverage of oleic acid on the nanoparticle surface we have achieved controlled nanoparticle assembly following exposure of the suspension to a substrate layer of cyanopropyl-modified silica which competes for the ligand. The clusters can be formed reproducibly and their final size can be selected over a range covering almost two orders of magnitude. Most unusually, the relative monodispersity of the cluster suspension is improved compared to the starting nanoparticle suspension, and the yield is close to 100%. Interestingly, we find that the kinetics of assembly is not altered by scaling up, which is surprising for a complex process involving molecular transport. Kinetic studies provided mechanistic insight into the process, and suggested general requirements for controlled assembly of other nanoparticle types.