Clay minerals such as fluorohectorite (FHt) have been considered as potential drug carrier systems because of their layered structure and excellent ion exchange capabilities. It is thought that the water present between the layers of FHt facilitates the uptake of bio-active molecules in these systems, but details of this interaction are not well understood. This study used quasi-elastic neutron scattering (QENS) on IRIS alongside computational calculations and thermodynamic experiments to measure the diffusion behaviour of water inside this synthetic clay mineral.
Drug loading onto clay minerals has great advantages, such as reducing adverse effects and the frequency of dose administration. By prolonging and/or delaying drug release they can enhance the efficacy of poorly soluble drugs, in comparison to conventional therapies.
Fluorohectorite, FHt, has the structure M𝑥Mg6−𝑥Li𝑥Si8O20F4·H2O, where M represents the ion present between the layers, and x is the degree of substitution. The choice of ion can affect the ability of the clay to limit the transport of molecules such as water and bio-active molecules. Previous studies of the sodium and lithium-containing FHts, NaFHt and LiFHt, have shown that the smaller lithium ion can form an additional, different, cation-water complex between the layers to sodium.
In both LiFHt and NaFHt, the cation can be replaced by the antibacterial agent Ciprofloxacin (CIPRO) C17H18FN3O3. During the process of CIPRO intercalation, some water molecules are displaced. Understanding the change to the behaviour of the water between the layers, and how it depends on the cation present, is crucial to developing the FHt system for drug delivery applications.
To study the behaviour of the interlayer water, the group performed QENS experiments on LiFHt, NaFHt and NiFHt to contrast how interlayer water diffusion is affected by changes to relative humidity and the interlayer cation. Their work revealed significant differences in the water diffusion on changing the cation present. The group found that for Li+ and Na+, this was related to how the cation changes the hydrogen-bond network around the water molecules and in the case of Ni2+, to the formation of hydroxyls.
The researchers extended their study to compare the activation energy of the different decomposition processes observed in the thermal analysis data of pure CIPRO, hydrated LiFHt and CIPRO intercalated in LiFHt. Their results clearly show the displacement of interlayer water molecules by CIPRO.
Based on their comprehensive neutron spectroscopy and thermodynamic study of the adsorption and decomposition of CIPRO, their findings once gain highlight the potential of FHt as a nano-carrier. The study also finds that, in order to optimise the formulation of complexes with longer shelf life, systematic studies such as this are hugely important.
“Quasi-elastic neutron scattering results gave unique information on the distinct motions of the intercalated molecules allowing to verify the mechanism behind the intercalation of ciprofloxacin. Understanding the driving forces for bioactive molecules uptake in clay minerals is of great interest to improve, control and extend their use," explains lead researcher Professor Heloisa Nunes Bordallo; “Coming to perform experiments at ISIS has provided the young researchers a fantastic experience and contributed enormously to results reported in their PhD thesis."
The full paper can be found online at DOI: 10.1016/j.micromeso.2020.110512