Revealing the Mechanism of the Low-Energy Electron Yield Enhancement from Sensitizing Nanoparticles

Abstract

We provide a physical explanation for enhancement of the low-energy electron production by sensitizing nanoparticles due to irradiation by fast ions. It is demonstrated that a significant increase in the number of emitted electrons arises from the collective electron excitations in the nanoparticle. We predict a new mechanism of the yield enhancement due to the plasmon excitations and quantitatively estimate its contribution to the electron production. Revealing the nanoscale mechanism of the electron yield enhancement, we provide an efficient tool for evaluating the yield of emitted electron from various sensitizers. It is shown that the number of low-energy electrons generated by the gold and platinum nanoparticles of a given size exceeds that produced by the equivalent volume of water and by other metallic (e.g., gadolinium) nanoparticles by an order of magnitude. This observation emphasizes the sensitization effect of the noble metal nanoparticles and endorses their application in novel technologies of cancer therapy with ionizing radiation.PACS numbers: 36.40. Gk, Radiotherapy is currently one of the frequently used methods to treat cancer, which is a major health concern of nowadays. However, it has the side effects induced by the radiation in surrounding healthy tissues. One of the promising modern treatment techniques is ion-beam cancer therapy [1][2][3]. It allows one to deliver a higher dose to the target region, as compared to conventional photon therapy, and also to minimize the exposure of healthy tissue to radiation [1]. Approaches that enhance radiosensitivity within tumors relative to normal tissues have the potential to become advantageous radiotherapies. A search for such approaches is within the scope of several ongoing multidisciplinary projects [4,5].Metal nanoparticles (NPs) were proposed recently to act as sensitizers in cancer treatments with ionizing radiation [6][7][8]. Injection of such nanoagents into a tumor can increase relative biological effectiveness of ionizing radiation. It is defined as a ratio of the dose delivered by photons to that by a different radiation modality, leading to the same biological effects such as the probability of an irradiated cell death. During the past years, application of gold NPs acting as dose enhancers, in combination with photons, revealed an increase of cancer cell killing [9][10][11], while an advantage of using NPs in ion-beam cancer therapy is still to be thoroughly substantiated.It is currently acknowledged [3,[12][13][14]] that a substantial portion of biodamage by incident ions is related to the secondary electrons and free radicals produced due to ionization of the medium by the projectiles. Refs. [15][16][17] have explored the possibility of the low-energy electrons (LEE), having the kinetic energy from a few eV to several tens of eV, to be important agents of biodamage.In this Letter, we reveal the physical mechanism of enhancement of the LEE production by sensitizing (noble metal, in particular) NPs. We demonstrate that a significant inc...

Improving proton therapy by metal-containing nanoparticles: nanoscale insights

“…The de-excitation of collective modes is even higher for atoms rich in 5d electrons (ie, platinum). 14 The interaction of these emitted electrons with water molecules in the medium leads to the production of free radicals (HO • as precursor and by-products) concentrated in clusters around the NPs. We, thus, attribute the induction of nanosize molecular damages to the interaction of radical clusters with the plasmid DNA.…”

“…Subsequent simulation studies have led to first insights into this amplification effect. [11][12][13][14] Furthermore, nanostructures were found to not only amplify radiation effects, but also improve in situ tumor diagnostics. In particular, gadolinium-based NPs (GdBNs) are magnetic resonance imaging active agents for which amplification of cell killing was observed using high-energy photons [15][16][17] and carbon ions as incident radiation.…”

Nano-scale processes behind ion-beam cancer therapy

“…On the scale of electromagnetic interactions, the effect of the presence of biomolecules in the medium on ionization cross sections, stopping power, dose deposition in a cell was explored [6,7]. Another advance on this scale was the analysis of secondary electron production by nanoparticles [8,9]. The radial dose deposition as well as transport of secondary particles on the scales pertinent to this transport have also been investigated [10][11][12].…”

Modeling of nanoparticle coatings for medical applications

“…The sensitizing effect of metal NPs is related to an increased emission of secondary electrons compared to a similar volume of water [8]. These electrons in turn activate hydrolysis of the surrounding water medium resulting in an increased overall radical yield.…”