4.3.3. X-Ray Photon Spectroscopy (XPS)

Due to its ability to determine surface elemental composition and by virtue of being non-destructive, XPS has become a key characterization technique for NCM [101]. It can be utilized to provide detailed and qualitative information on chemical elements present on the surface of materials. This is particularly useful for NCM stabilized by polymers and/or surfactants. [102]. Recent advances utilizing special sample cells permits the use of liquid samples and consequently broadens the application of XPS [103], specifically for dispersed NCM.

Due to the nature of NCM, XPS has taken on more significance in the advancement of pharmaceutical nanotechnological applications by being capable of detecting the presence and determining the relative concentrations of elements while also determining the average thickness of surface coatings in NCM. As such, it is a useful technique for analysis of potential adulteration or impurities arising during nanomaterial synthesis and/or handling [89].

Through XPS, Qiu et al. conducted an investigation into the difference in reduction behaviour between nanocrystalline and microcrystalline ceria after Ar<sup>+</sup> bombardment or X-ray irradiation. Despite identical experimental conditions, viz. pellets compacted by uniaxial pressure of 10 MPa and XPS data being recorded before and after bombardment experiments, it was observed that the reduction levels of Ce4+ to Ce3+ were lower in nanocrystalline than microcrystalline ceria [104].

Chow et al. used XPS to ascertain drug encapsulation and demonstrated that most of the drug was entrapped within the cores of NPs, and that the particle composition of the surface was mainly the adsorbed co-stabilizer and polyethylene glycol (PEG) block [105].

Similarly, Dong et al. used XPS to prove the predominant presence of the PEG shell on the surface of paclitaxel loaded methoxy poly(ethylene glycol)-poly(lactide) (mPEG-PLA) NP [106].

In a study to analyze the external surface of a novel controlled release formulation for the anticancer drug paclitaxel (Taxol®) loaded in PLGA nanoparticles and stabilized by α-tocopheryl polyethylene glycol succinate 1000 using XPS, it is determined that the outermost layer was composed by a majority of TPGS 1000 molecules [107].
