Composition-Based Separation of Pt-Fe₃O₄ Hybrid Nanoparticles by Thermal Field-Flow Fractionation

Colloidal hybrid nanoparticles integrate two or more material domains into a single construct to achieve multifunctionality and synergistic properties. The synthesis of these complex multicomponent hybrid nanoparticles often yields size and composition distributions that subsequently impact observed properties and complicate structure-function studies. Analyses of nanoparticle systems such as these that contain multiple simultaneously occurring distributions are highly challenging. In this work, we introduce thermal field-flow fractionation (ThFFF) as an analytical method to separate and characterize iron oxide (Fe₃O₄), platinum (Pt), and multicomponent platinum-iron oxide (Pt-Fe₃O₄) hybrid nanoparticles by composition. The transport of matter by a temperature gradient or thermal diffusion is exploited in the ThFFF separation and characterization of inorganic nanoparticles. Thermal diffusion coefficient (D_T) values are determined from measured ThFFF retention times and translational diffusion coefficients. The D_T for pure Fe₃O₄ and pure Pt are found to bracket those determined for hybrid Pt-Fe₃O₄ nanoparticles, demonstrating a proportionality between D_T and composition. Furthermore, composition distributions can be determined since measured D_T values vary as a function of retention time. The correlation of D_T-derived cumulative distribution plots with TEM-EDS results confirmed the compositional polydispersity within hybrid nanoparticle populations. This composition-based fractionation of multicomponent inorganic nanoparticles provides insights into the fundamental forces governing thermal diffusion and highlights the ability of this technique for complex nanoparticle separations and analyses.