One challenge in medical applications is the strategic transport of the particles to the desired location. Apart from directly injecting the particles at the target site or coating them with organic substances that only bind to certain tissues, another option is to manoeuvre the nanoparticles to the desired position using a magnetic field. Aggregates of the particles are better suited for this than individual nanoparticles because their magnetic moment is greater, which means they can be controlled by smaller magnetic fields. At the same time, thread-like aggregates are advantageous compared to other shapes because the long, thin formations can move better through narrow structures, such as blood vessels.

Finely distributed in a solvent, the nanoparticles can self-assemble into the thread-like chains when conditions are suitable. If there is no spatial boundary acting on the particles, certain parameters such as size, concentration and the composition of the nanoparticles, as well as the strength of the applied magnetic field, determine the self-organization. However, the only method up to now that could directly make the tiny chains visible and track their formation could not function without spatial limitations: For studies involving an electron microscope, samples must be placed on a carrier whose lattice structure provides spatial limitation and is thus able to influence the self-organization of the IONPs.

Neutrons make the chains of nanoparticles visible

Now researchers from Forschungszentrum Jülich, RWTH Aachen University and other German and international research institutions are presenting a new approach that makes the chains and their formation directly visible in the solvent without the need for spatial limitation. To do this, they combined experimental studies using small-angle neutron scattering at the instrument KWS-1 with reverse Monte Carlo simulations. Small-angle scattering enables similarly high-resolution structural analyses to be made as with electron microscopy, but generates data that are not intuitively comprehensible and must be interpreted using elaborate procedures. With the help of simulations, directly recognisable images of the particle chains can now be obtained.

“Reverse Monte Carlo simulation is an established method, but this is the first time we have used it to interpret small-angle scattering data,” reports Nileena Nandakumaran, a PhD student at the Jülich Centre for Neutron Science. Her supervisor, Dr. Mikhail Feygenson, adds: “Visualising IONP aggregates in dispersions in real space offers a novel tool for biomedical researchers. This is because the method enables a comparatively simple, rapid investigation of the behaviour of IONPs in solution in a broad parameter space and an unambiguous extraction of the parameters of the equilibrium structures. It also allows the study of geometrically more complex aggregates of IONPs.”

Text: Angela Wenzik / Forschungszentrum Jülich