4.3.2. Small Angle X-ray Scattering (SAXS)

For nanomaterials, SAXS is usually the best choice because below a certain particle size, PXRD is not sufficiently sensitive and the X-rays are scattered in an amorphous halo.

SAXS can be used to elucidate the PS, PDI, and morphology [86]. With regards to PS, more statistically reliable results are obtained with SAXS when compared to transmission electron microscopy (TEM). SAXS has been used successfully to study the structural changes of platinum (Pt) nanoparticles (NP) with changes in temperature [96]. The size obtained by XRD was different from the corresponding SAXS value at certain temperatures. This is because XRD is susceptible to the size of the long-range order region while SAXS is susceptible to the size of the fluctuation region of electronic density [86]. In these experiments, it was observed that the PS obtained with SAXS were slightly larger than those obtained using TEM. The reason is that Pt NP were coated with polyvinyl pyrrolidone (PVP) and the scattering intensity due to the PVP coating could not be easily removed [96].

Cipolla et al. used to SAXS to determine the PS, shape, and asymmetry of liposomes loaded with NC [97,98]. As part of the study outcomes, SAXS was identified as a useful and versatile technique to study the solid state of NC loaded in liposomes and dispersions, as well as to study the behaviour of drug NC in dispersion as a variable of temperature and pH [98].

SAXS has also found use in the solid-state characterization of doxorubicin sulphate nanocrystal loaded liposomal dispersions [99,100].

It has to be noted that SAXS is a low-resolution technique and in certain cases, further studies by XRD and/or electron diffraction techniques are indispensable for the characterization of NCM.
