Self-assembling LEGO sets under pressure

From drug delivery to the development of responsive materials, self-assembled supramolecular structures have many applications. Understanding their organisation and reaction to external parameters is key. In a new study, researchers explore the effect of high pressures taking full advantage of ILL’s leadership in this domain.

The self-assembly of small molecules into ordered, large (from a few nanometres to micrometres) supramolecular structures represent a fascinating field of research. These structures have a variety of forms and behaviours. They are also highly sensitive to environmental stimuli, such as temperature, pressure, light, pH, or the presence of specific molecules. This sensitivity can be used to control them and it forms the basis of a number of applications, from drug delivery to the development of responsive materials. Indeed, understanding how these complexes organise themselves and react to external parameters is key to being able to control them – and better use them.

In a now published study, researchers explore the effect of pressure. The challenging high-pressure neutron experiments performed take full advantage of the ILL’s leadership in this domain, in both equipment and expertise. The publication appeared in the Emerging Investigator Themed Collection of the journal PCCP (Physical Chemistry Chemical Physics), following an invitation to ILL scientist Leonardo Chiappisi.

2 Neutron experiments under high pressure

In this work, researchers investigated the effect of pressure on the supramolecular assemblies of cyclodextrin/surfactant complexes (see figure 1). To this end, they used the small-angle neutron scattering (SANS) technique in order to probe structures in the 1-100 nanometres scale. The experiments were performed on instrument D33 at the ILL, under varying pressures up to 1800 bar.

“High-pressure in soft matter is a growing field of interest, but it is associated with significant technical challenges that make these experiments rather difficult to perform,” explains Leonardo Chiappisi, who adds: “The ILL is a leading institute in high pressure. We benefit not only from excellent equipment but also from the expertise built over the years, and the strong cooperation between different groups”.

Small-angle neutron scattering (SANS) explores the structure of substances on length scales ranging from 1 nanometre to close to 1 micron. In a SANS experiment, a beam of neutrons is directed at a sample. The neutrons are elastically scattered by nuclear interaction with the nuclei in the sample. SANS measures the deviation at small angles (from much less than one degree to several degrees) of the neutron beam due to structures of such sizes in the sample.

The experiments were performed placing the samples inside a high-pressure cell designed by the ILL’s Advanced Neutron Environment Service (SANE). It is made in hardened copper–beryllium, featuringfully neutron-transparent sapphire windows. The Partnership for Soft Condensed Matter (PSCM) provided essential laboratory support.

As pressure increased, the shape of the obtained neutron scattering curve remained roughly unchanged (figure 2). There was, however, a noticeable increase in the intensity of the peaks. This hints that while the overall structure of the supramolecular aggregates remained stable, high pressure markedly enhanced rigidity, resulting in a clear increase in order within the supramolecular architecture.

Based on these results, researchers hypothesise that, somewhere between 250 and 1000 bar, pressure induces a change in the crystalline structure of the assemblies, triggering a molecular reorganisation towards a more rigid structure. Future studies might confirm or disprove this hypothesis, and other methods will be needed to provide more accurate measurements of rigidity – for example small-angle X-ray scatteringor neutron spin-echo measurements.

In Chiappisi’s words, these results not only “hold promise for deepening our understanding of the supramolecular assembly of cyclodextrin inclusion complexes and their implications across diverse applications, including drug delivery and material science” but, in the broader context, they “demonstrate that high-pressure SANS is a valuable to better understand the mechanisms of supramolecular assembly processes aiding in the design of more robust and functional systems.” And he concludes: “Despite the inherent challenges associated with high-pressure experiments, we are convinced that these preliminary findings clearly underscore the significance of such studies.”