Spectra of secondary particles generated upon virtual irradiation of gold nanosensitizers: implications for surface modification

Surface modifications of radiosensitizing gold nanoparticles (GNP), in particular coating with polyethylene glycol (PEG), may greatly improve their physico-chemical, pharmacological and theranostic properties. However, GNP coating material can seriously alter the efficacy of radiosensitization. We performed a Monte-Carlo simulation of 17 nm GNP PEGylation (8.5 nm layer thickness) effects on secondary radiation spectra at different energies of primary photons (from similar to 8 keV to 1 MeV). Our calculations revealed a decrease of the amount of low energy electrons (LEE) that leave PEGylated GNP(from 20% to 40% at the photon energy < 500 keV, and from 20% to 0% at energies 500-1000 keV). In all cases, the average energy of the low-energy component of the electron spectrum was smaller for coated GNP. Total energy differed only slightly at photon energy up to similar to 200 keV; at bigger energies the normalized total energy of secondary electrons that left the PEG shell significantly (similar to 20%) exceeded the respective value for electrons that left the uncoated GNP. Thus, rational design of GNP-based radiosensitizers should include critical assessment of physico-chemical properties of the material used for surface modification.