Soft assembly of active and surfactant-analogue nanoparticles at the mesoscale

• Applicants: Dr. Irene Morales Casero; Professor Dr. Sebastian Polarz
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 573944842

Controlling a superstructure's periodicity and symmetry is a crucial challenge for exploiting nanoparticle-based materials. Self-assembly is the most favorable method for achieving superlattices from colloidal particles, being the particle size, shape, and surface functionalization are crucial factors to tailor their formation. If the colloids can be described as persistent objects moving only by Brownian motion (without external triggers), the structure formation can be generally explained by thermodynamics and volume exclusion. Nevertheless, assembly processes in concentrated dispersions are hardly studied when the particles display an additional non-Brownian movement. In the latter case, a connection between the fields of 'particle-based materials' and 'active colloids' emerges. In addition, physical energy from light, magnetic or electric fields, or chemical energy can be supplied to dynamize particle assembly. The current project will deliver new insights into the self-assembly properties of active colloids. We build on our preliminary work on particle-based amphiphiles, consisting of an iron oxide core and an organosilica shell modified in a Janus-type fashion, keeping the size below 100 nm. At this size, they can be manipulated by weak external forces/magnetic field strength << 1T, and they are mainly in the superparamagnetic regime, so the magnetic moments are randomly oriented due to thermal fluctuations in the absence of an applied field. We will analyze the Janus particles' assembly behavior driven by their amphiphilicity and under different magnetic field conditions. Not only will we vary the strength of the magnetic field systematically, but we will also apply oscillating magnetic fields. Depending on the frequency, we expect two effects that could critically affect how the Janus particles assemble. At low frequencies, the reorientation of the magnetic moment in the magnetite core exerts torque. It induces rotation if the particles possess a non-spherical shape, being one dedicated goal in the synthetic work packages of the project. At high frequencies, heat is generated, enhancing Brownian motion. A magneto-optical set-up (Cotton-Mouton effect) will be constructed and combined with light-scattering detectors to investigate the resulting non-equilibrium self-assembly processes. From the emergence of an optical anisotropy, we can conclude about the particle orientation processes and correlate it to aggregation phenomena deduced from dynamic light scattering. We will deliver rod-shaped Janus nanoparticles varying in aspect ratio and resembling the design of molecular surfactants, with distinct hydrophilic and hydrophobic domains precisely aligned concerning the magnetic moment of the Fe3O4 core. The project combines expertise in the physics of iron oxide-based systems and particle magnetization dynamics (Morales) with the chemical synthesis of perfect model particles (Polarz).

• DFG Programme: Research Grants