Modern medicine increasingly relies on targeted drug delivery, a process during which therapeutic molecules are transported directly to specific organs or even certain cell types. To do this, drugs are often packaged inside biocompatible particles (nanoparticles), commonly composed of different fat molecules.
The efficiency of the delivery process is determined by many characteristics of the particles. These include, for example, their internal and external structure as well as the homogeneity of particle sizes within a given batch. International quality standards require that nanoparticle sizes do not vary by more than 30% to be considered safe for application. Close monitoring of particle size distribution is therefore of fundamental importance throughout their manufacturing,
To monitor nanoparticle size, manufacturers commonly use a technique called asymmetric-flow field-flow fractionation (AF4). In brief, it involves separating particles in solution in such a way that smaller particles move faster than larger ones. AF4 is usually coupled with methods such as ultraviolet light absorbance or light scattering in order to measure the amount of particles in each size group.
A powerful new combination of techniques
To understand nanoparticles even better, researchers also need information about their shape and internal structure. This requires combining AF4 with techniques specifically designed to probe how particles are organised at the nanoscale.
In previous studies, AF4 has been coupled to small-angle X-ray scattering (SAXS) to investigate tiny magnetic particles. However, it had never before been combined with neutron-based techniques such as small-angle neutron scattering (SANS).
This challenge has now been overcome by an international team including scientists from the Leibniz Institute for Polymer Research (Dresden) , Stellenbosch University , Max IV and the Department of Process and Life Science Engineering (Lund) and the Institut Laue-Langevin (ILL), who successfully carried out the world's first AF4-SANS experiment on the ILL's D11 instrument.
Using nanoparticles designed for drug delivery, the team analysed them with an AF4 setup coupled simultaneously to Multi-Angle Light scattering and, for the first time, SANS. Using this powerful combination, the researchers were able to determine not only the dimensions of the particles, but also to test the homogeneity of their internal structure and the potential location of drug molecules with great precision. This combined approach gave the researchers a far more detailed understanding of how the nanoparticles are built.
According to Prof. Dr. Albena Lederer, from the Leibniz Institute of Polymer Research Dresden, this first successful AF4-SANS experiment demonstrates how powerful such combined analytical platforms can be for biomedical research: "In the Polymer Separation Group at Leibniz Institute of Polymer Research Dresden, we have pioneered the coupling of advanced field-flow fractionation techniques with powerful scattering methods, including AF4-SANS and thermal FFF-SAXS. These multidetection approaches allow us to extract complementary, orthogonal information from very small amounts of sample, which is particularly valuable in biomedical research. Looking ahead, we believe that such integrated analytical strategies will be decisive for understanding complex polymer systems and for guiding the design of next-generation polymer-based biomedical applications."
Overcoming experimental challenges
The team also introduced an important improvement to the standard AF4 setup. During AF4 experiments, particles become diluted as they move through the system, which weakens the measurement signal and can lead to very long experiment times.
To overcome this problem, the researchers implemented a method to compensate for this dilution effect, allowing them to obtain reliable measurements much more efficiently. This included focusing the detection of the nanoparticle signal exclusively on the nanoparticle-rich solution, while directing the solvent signal away from the detectors and allowing for signal detection with good statistics.
A new role for neutrons in drug delivery research
The coupling of SANS to AF4 is an extremely important step in the development of nanoparticle characterisation platforms. One major advantage of neutron scattering is its sensitivity to hydrogen and deuterium. By selectively replacing hydrogen atoms with deuterium, researchers can make specific parts of a nanoparticle stand out, revealing structural details that are otherwise difficult to observe with other techniques.
As targeted treatments continue to develop, precise characterisation of drug delivery nanoparticles is becoming increasingly important. The experimental framework established in the study described here is an important contribution to tackling this challenge, and opens new possibilities for the use of neutron scattering in biomedical research.
