Argon Cluster Sputtering Reveals Internal Chemical Distribution In Submicron Polymeric Particles

The ability to measure the internal chemical distribution of particles is of significant benefit to the development and testing of products relevant to the pharmaceutical, agrichemical, and food industries. These include nanomedicine particle carriers, such as those used in anticancer drugs, vaccines, and viral carriers for gene therapies. In this work, we carefully investigated the sputtering behavior of spherical organic particles using an argon cluster beam and scanning electron microscopy (SEM). Based on these observations, we then developed a geometrical model to interpret useful chemical depth profiles of polymeric particles by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) in conjunction with argon cluster sputtering. We found that change in the shape of isolated particles may be modeled in a simple manner by assuming a constant sputtering yield across the particle surface. Furthermore, packed particles undergo an interesting geometry transition during the sputtering process, which can primarily be explained and modeled by a shadowing effect from other particles in the vicinity. We successfully applied this model to analyze the structure and chemistry of nonideal polymeric core-shell particles. We show that with some knowledge of the materials' sputtering yields, the particle diameter can approximately be determined by using XPS or calibrated SIMS and that the fractional volume of the core in the particle can be measured. Furthermore, the analysis provides a radial distribution of the core material within the particle. This distribution is in accord with transmission electron microscopy data available on the same materials. The radial distribution demonstrates that the cores are randomly positioned within the particles but avoid a region close to the surface. This work demonstrates that it is possible to use a simple model to describe the shape change of organic spherical particles during argon cluster sputtering and interpret chemical measurements by XPS and SIMS methods.