Backscattered electron emission after proton impact on gold nanoparticles with and without polymer shell coating

Abstract

This work aims at measuring experimentally proton induced secondary electron energy spectra after interaction with gold nano particles (GNPs) and polymer-coated GNPs. Backscattered electron energy spectra were collected over a 0 to 1000 eV energy range using a retarding field analyzer (RFA). This paper presents the spectra obtained for proton beam energies of 0.5 and 2 MeV and diameter 2.5 and 3.8 nm GNPs. The spectra were also measured for 3.8 nm GNPs after 5 and 10 MeV proton irradiations. GNPs were deposited on a 100 nm carbon film. Each experimental spectrum was compared with dedicated simulations based on existing numerical models used in the TRAX and Geant4 Monte Carlo codes.For 100 nm carbon target, good agreement between experimental, TRAX and Geant4 simulation results can be observed.For 3.8 nm GNPs, the TRAX simulations reproduce with good agreement the electron energy spectra produced after 0.5, 2, 5 and 10 MeV proton irradiations, while Geant4 spectra display a lower secondary electron yield at low energy (<600 eV) for all the studied energies. This underestimation can mostly be explained by the 790 eV threshold applied in the condensed history model used by Geant4 which impacts the secondary electron energy distribution. Results obtained for carbon and gold targets highlight the impact of the secondary electron production threshold for proton ionization process considered in condensed history models.The experimental results demonstrate that the single interaction approach used in TRAX is adapted to reproduce secondary electron emission from GNPs. On the other hand, the standard electron generation threshold implement in G4BetheBlochModel and G4BraggModel condensedhistory models used in Geant4 is not adapted to reproduce low energy electron emission in gold targets.Finally, the results highlight that the GNP coating leads to a decrease of the electron yield and mostly affects low energy electrons (<500 eV) emitted from GNPs.

Gold Nanoparticles as a Potent Radiosensitizer: A Transdisciplinary Approach from Physics to Patient

“…While the grafting of increasing amounts of PEG molecules confers a series of advantages in terms of in vivo biodistribution, it also decreases the physical and chemical enhancement effectiveness. Low-energy electrons emitted following the interaction between IR and GNPs can be scavenged by a large organic layer at the GNP surface resulting in a decrease in electron yield as observed by different groups [63,64]. Moreover, chemical enhancement through catalytic ROS production can be reduced as a function of coating thickness as reported by Gilles et al [80].…”

“…Spaas et al [63] have measured an average 5.2% loss of radiosensitization enhancement per nanometer of PEG coating after 200 kVp irradiation. Thereby, the coating effects have to be taken into account in simulation codes like in [64], especially for charged particle irradiation for which the effect will be more significant due to the main production of short-range electrons. Finally, the cell exposure to IR generates a succession of different responses that are not limited to physics, as discussed previously.…”

Systematic quantification of nanoscopic dose enhancement of gold nanoparticles in ion beams

“…In contrast, maximum DEF is not affected for the two larger sizes and the global DEF changes only slightly. Dedicated analysis on the influence of the coating can be found in Haume et al (2018) and Hespeels et al (2019b). Also the cellular localization is more relevant for very small NPs since only nearby structures are affected, but a high enhancement could be achieved in case of successful nuclear targeting.…”

Theoretical derivation and benchmarking of cross sections for low-energy electron transport in gold

“…Cleaning of the sample increases the yield by nearly a factor of two. More experimental results for a wider range of energies would be necessary to validate the experimental data, as it is very sensitive to the experimental set-up, in particular to the impurities on the surface and surface charging effects, as, for example, shown by Hespeels et al and Gomati et al [23,31,32] (Fig. 13).…”