Nanoparticles can be used to increase the effects of hadrontherapy exactly where they are needed: in the cancer cells. This is so, because they can be coated with organic molecules which selectively target the cancer cells, going into them in much larger proportion than in the healthy cells. Then, when the body is irradiated with an ion beam, the interaction between the beam and the nanoparticles can produce an increase in the number of secondary species (electrons and free radicals) which damage the cell genetic material, increasing the cancer cell-killing ability of the treatment.

Among the materials which are being investigated for these radioenhancing nanoparticles, cerium oxides (ceria) are one of the most promising. It has been tested experimentally that ceria nanoparticles can, on the one hand, radiosensitise cancer cells while, on the other hand, radioprotect healthy cells.

To understand these complex radiosensitisation and radioprotection mechanisms is very challenging, but it is clear that a first step in this direction is to understand how charged particles (particularly electrons, which are abundantly produced by radiation) interact elastically and, mainly, inelastically with cerium oxides.

In our recent publication in the journal Physical Chemistry Chemical Physics, we have investigated from first principles the electronic excitation spectrum of both phases of ceria (CeO2 and Ce2O3), i.e., its Energy-Loss Function (ELF), by means of linear-response time-dependent density functional theory (TDDFT) calculations. For CeO2, material for which there are optical measurements, we found excellent agreement with experiment when local field effects are included in the calculations, while we also predicted the ELF of the more unknown Ce2O3.

These ELFs have been used to obtain the inelastic scattering probabilities of electrons in these materials, which, together with elastic scattering probabilities from the Mott theory, have provided the input for precise Monte Carlo simulations of the electron transport in ceria. These simulations have been used to try to reproduce the reflection electron energy loss spectrum experimentally determined for ceria (REELS, a very well-known electron analysis and spectroscopic technique). We found that the REELS spectrum can be fairly well reproduced by simulations which, moreover, allow interpreting the origin of the different features of the spectrum, which come from different electronic excitations of both CeO2 and Ce2O3. As it was suspected in the experimental work, the ceria material commonly used turns out to be a mix of these two phases of cerium oxide.

We expect that these calculations and simulations will help in the future to understand the physical mechanisms of ceria nanoparticle radiosensitisation in hadrontherapy.

The open access article, which has been showcased on the PCCP back cover, can be downloaded, free of charge, in the following link:

https://pubs.rsc.org/en/content/articlelanding/2021/CP/D1CP01810H

or through the Publications tab.

Image reproduced by permission of the authors from Phys. Chem. Chem. Phys., 2021, 23, 19173, DOI: 10.1039/D1CP01810H

Pablo gave the invited talk Electronic interactions of swift ions and their secondary electrons in biologically relevant materials at ISIAC 2021, talking about recent works which can be found on the Publications tab.

Nanoparticles are objects (typically nearly spherical) formed by hundreds or thousands of atoms, having diameters of a few, tens or hundreds of nanometers, hence their name. They can be made up of different metals or of other materials, like oxides. Nanoparticles are the subject of intensive research since several decades, as they present very peculiar characteristics, both for the fundamental science and for many applications.

Fundamentally, they are objects which are not bulk materials, but neither individual atoms, and thus they present very interesting and unexpected properties, as for example “magic numbers”: nanoparticles made up of specific numbers of atoms are much more stable than others (even being different by just one atom), which reflects the interplay of fundamental interactions arising from geometry or the closure of electronic shells.

From the applied point of view, nanoparticles are being used for numerous technological developments, from catalysis in chemistry, components of nanoelectronic devices or as enhancers for cancer radiotherapy. Of course, many of these applications require the deposition of the nanoparticles on a surface, and this apparently simple, but really complex process, can affect the properties and function of the nanoparticle. Therefore, the deposition process has to be well understood.

With my former MSc student Yannick Fortouna (https://www.bioremia.eu/.../our.../esr-1-yannick-fortouna) and our colleagues at MBN Research Center (Frankfurt, Germany), we did a computational study of the deposition of sodium nanoparticles on magnesium oxide substrates by means of molecular dynamics simulations. This simulation technique allows obtaining a lot of information which otherwise could be difficult to get from experiments, such as the change in nanoparticle shape and structure as a function of landing velocity, the heating of the nanoparticle upon collision and the heat exchange with the substrate, etc. All these aspects have been studied in the work which was just published yesterday in the journal Theoretical Chemistry Accounts, and that you can access, if you are interested, through the following link:

https://rdcu.be/cmkKR

or through the Publications tab.

The image shows some examples of simulated trajectories of the nanoparticle deposition, in which different regimes can be observed as the energy is increased, such as soft landing without melting (b-c), harder landing with impact-induced melting and droplet formation followed by recrystallization (d-e), and multifragmentation of the nanoparticle due to a very energetic impact (f). Image taken from the mentioned work, first published in Theoretical Chemistry Accounts, volume 140, page 84, 2021, by Springer Nature.