Structural analysis of nanosystems: Solid Sorbitan esters Nanoparticles (SSN) as a case study

Innovative approaches in nanotechnology can provide drug delivery systems with a high potential in different fields. To avoid trial and error assays as a main driving force governing new designs and, furthermore, to develop successful nanosystem optimization strategies, it is of the greatest importance to develop specific characterisation techniques beyond conventional determinations of size, zeta potential and morphology. However, the application of techniques able to determine some key characteristics, such as nanostructure (i.e., solid structure vs vesicular), and the way in which the reorganization of components takes place on these structures has been scarcely explored. The present work has been devoted to provide some insights about the potential offered by some NMR techniques to those scientists working on nanotechnological approaches. For this purpose, we selected our nanosystems based on sorbitan monooleate as a case study. We used ¹H NMR methods, including a recently proposed method relying in the well-known Saturation Transfer Difference (STD) experiment for the observation of 'invisible signals' in large aggregates (Invisible State STD or ISSTD). Overall, these techniques revealed the presence in these nanosystems of a gradient of flexibility from an internal rigid core towards a more flexible region located on their surface, as well as the absence of water content in both regions. Such structure, corresponding to a solid nanostructure rather than a vesicular one, can explain some of the interesting properties previously observed for these innovative nanosystems, such as their high stability, and allows us to refer to these nanosystems with the term "Solid Sorbitan esters Nanoparticles" (SSN). On the basis of the valuable information provided by the mentioned characterisation techniques, it is our understanding that they could facilitate the future design of new drug delivery nanosystems as well as the improvement of existing ones and/or the development of new applications for classical drug delivery concepts.