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Effect of gamma radiation on alkanethiolate-capped gold nanoparticles: Theoretical studies
Radiation Physics and Chemistry 120 (2016) 81–88
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effect of gamma radiation on alkanethiolate-capped gold nanoparticles: Theoretical studies M.E. Fernández-García, M. Pérez-Alvarez, D. Mendoza-Anaya, C. Gutiérrez-Wing n Instituto Nacional de Investigaciones Nucleares, Carr. México-Toluca S/N, La Marquesa, Ocoyoacac, Edo. De México C. P. 52750, Mexico
H I G H L I G H T S
Gamma radiation effects on alkanethiolate-capped Gold nanoparticles were simulated. Effects of gamma radiation depend on the Gold nanoparticle structure. Icosahedral nanoparticles were the most stable to gamma radiation. At the studied gamma irradiation doses, the metallic cores were stable up to 10 kGy. Major modifications can occur on the alkyl chains of the capping agent.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2015 Received in revised form 20 November 2015 Accepted 30 November 2015 Available online 2 December 2015
Theoretical studies of the effect of gamma irradiation on alkanethiolate-capped gold nanoparticles are presented. Icosahedral, decahedral and fcc nanoparticles protected with 1-dodecanethiolate (SC12) were obtained by molecular mechanics simulations, analyzing the effect of gamma irradiation through MonteCarlo. The studied doses were 1, 10 and 20 kGy. It was observed that slight structural modifications of the metallic core might occur and these are dependent on the shape of the nanoparticle. However, the most significant effect was observed on the organic passivating layer, where torsions, bending and scission of the alkyl chains were detected. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles Gamma radiation Monte Carlo Geometrical optimization
1. Introduction Since the great potential of nanoscale materials was exhibited, different scientific and technological fields have focused on the possibility of using their novel properties to fabricate new or improved materials on a number of applications. With no doubt, nanomaterials have occupied an important place in the development of systems or devices either for medicine, catalysis, electronics and aerospace. In some of these applications it is required that the nanomaterials withstand (Mariscal et al., 2010; Nara et al., 2004; Yourdshahyan and Rappe, 2002; Yuan et al., 2014; Askerka et al., 2012) harsh conditions as radiation fields, which have sufficient energy to induce changes in matter. For example, the new nanomaterials for the nuclear and thermonuclear power engineering (Andrievski, 2011), or nanodevices to be used in the space exploration (Dharani et al., 2013). On the other hand, radiation in combination with nanomaterials may be used to n
Corresponding author. E-mail address: [email protected] (C. Gutiérrez-Wing).
http://dx.doi.org/10.1016/j.radphyschem.2015.11.036 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
improve the function of the material in areas such as redox catalysis, environmental remediation, electronics and radiotherapy (Meisel, 2005). For example, for radiation therapy high-Z materials, such as Au nanoparticles, can be used to amplify the dose of delivered ionizing radiation (Cooper et al., 2014) or in electronics for the manufacturing of silicon solar cells in which Au nanoparticles are used to enhance their efficiency (Abdullah, 2013). Whatever the case, the information about the nanomaterials behavior under irradiation is very important especially when they will be exposed to an intense radiation field. For metallic nanoparticles specifically, there are many reports focused in the application of radiation to synthesize them, however, little information is available on the stability of nanoparticles when they are exposed to radiation of any kind, such as gamma rays. Furthermore, up to now, to our knowledge, there are no reports about the radiation effects on surface protectors over metallic nanoparticles. It should be mentioned that surface protector is very important in the dimensional stabilization of the nanoparticle, then, if the protector suffers any modification, the nanoparticle can destabilize and change its properties. In order to gain knowledge on the advantages or limitations of
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the application of nanoscale materials in a radiation field, in this paper we present a theoretical simulation analysis of the effects of gamma radiation on alkanethiolate-capped gold nanoparticles. The study was based on three of the most commonly found shapes of nanoparticles in the size range of 1–4 nm, icosahedron, decahedron and fcc. For this theoretical analysis, we simulated the behavior of gold nanoparticles exposed to ionizing radiation at a deposited energy per mass unit of 1, 10 and 20 kGy (Dose), considering that gamma radiation is widely used in medicine, industry and investigation. The theoretical analysis was performed using the PENELOPE (http://www.oecd-nea.org/lists/penelope. html) code of the Monte Carlo Method (Metropolis and Ulam, 1945).
ionizing radiation, energies such as those associated to 60Co, Compton scattering is the dominant interaction. In this process, a radiation beam transfers a fraction of its energy to an atomic electron, which is ejected. With its smaller energy, the radiation beam can undergo a photoelectronic effect by an atom: in this process, the atom emits an electron. These multiple interaction process can be simulated by the PENELOPE code of the MonteCarlo method. This simulation algorithm is based on a scattering model that combines numerical databases with analytical cross section models for the different interaction mechanism and is applicable to energies from a few hundred eV to 1 GeV PENELOPE-2008; Arqueros and Montesinos, 2003; Tajik et al., 2015).
3. Results and discussion 2. Experimental Minimum energy configuration models were obtained through geometrical optimization. The minimum energy configuration models were determined using a universal potential, with a maximum number of iterations of 5000 steps and a convergence criterion of 0.000 1 kcal/mol. Models of icosahedral, cuboctahedral and decahedral gold nanoparticles of 55 atoms respectively, were constructed and geometrically optimized to refine the geometry of their atomic structure. This process was performed through an iterative sequence, which adjusts the atomic coordinates to bring the system to a minimum energy structure. The algorithm SMART (Ermer, 1976) was used to perform these calculations, which is a combination of steps descent methods, conjugate gradient and NewtonRaphson. A Universal (Rappé et al., 1992) potential was used. Once these gold nanostructures models Au55ico, Au55dec Au55cub were obtained, they were capped by 20 molecules of SC12 each, and the system was again optimized to a minimum of energy. The energy of the systems was calculated through density functional theory (DFT), based on the Kohn and Sham theory, by applying a local density approximation (LDA) (Kohn and Sham, 1965; Barnard and Curtiss, 2006; Carr et al., 2012; Zhang et al., 2009). Because of the stochastic nature of radiation interaction, Monte-Carlo simulations techniques are very convenient to know the ionizing radiation effects in a specific material. Consequently, the theoretical simulation analysis of the irradiation process on gold nanoparticles-SC12 with radiation such as gamma rays, as those of 60Co, were performed using the Monte-Carlo Method. When radiation passes through matter, it interacts with atoms (electrons and nucleus) that conform this matter. Depending on the ionizing radiation energy, different interactions mechanisms can take place: coherent scattering (Rayleigh), incoherent scattering (Compton), photoelectric effect and pair production. For
On the first stage of this study the minimum energy models of the gold nanostructures capped by SC12, shown in Fig. 1, have been calculated. These are labeled as Au55ico, Au55dec Au55cub. The model of an icosahedron capped with SC12 was built, placing each SC12 over the exposed planes on the surface, at hollow sites on the face (111), with a distance S–Au of 2.4 Å (Majumder et al., 2002; Mazzarello et al., 2007). SC12 in a cuboctahedron structure, were placed in hollow sites over (100) and (111) facets with an average distance of S–Au of 2.4 Å. For the decahedron structure, the SC12 were placed on the hollow sites on the surface planes (111) and (100), with an S–Au distance 2.4 Å average. The capping molecules were attached on three-coordinated hollow sites for (111) planes and tetra-coordinated hollow sites for (100) planes, as it has been reported for similar systems (Ching Shih et al., 2007; Sheppard et al., 2011; Luedtke and Landman, 1996; Wilson and Johnston, 2002). The beam direction of the radiation was applied to interact with planes (111) and (100) for each system, having finally two different results for the cuboctahedral and decahedral models and one for the icosahedron. 3.1. Icosahedron The calculated energy of a 55 atoms-icosahedron with 20 molecules of SC12 is originally of 418970.70 eV. The model of this system is shown in Fig. 2a. Studies for this system were based on the interaction of the radiation beam with the (111) plane, as indicated by the arrow in Fig. 2b. It was observed that the energy of the system increases as the radiation dose rises. At 1, 10 and 20 kGy, the energy obtained for each system is 418964.46 eV, 418961.44 eV and 418945.86 eV respectively. The resulting structures obtained after an irradiation at 20 kGy are presented, showing the type of modifications that could be expected. Fig. 2b
Fig. 1. Models of 1-dodecanothiolate-capped gold nanoparticles. (a) icosahedron, (b) decahedron and (c) cuboctahedron.
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Fig. 2. Icosahedral structure capped with SC12 molecules. (a) Before and (b) after irradiation at 20 kGy. Insets in both figures show the icosahedron.
shows the models of the resulting structures at 20 kGy. The metallic core of the nanoparticle does not suffer modifications, suggesting that this energy increase can be associated to a modification of the SC12 as presented below. The S–Au distances are modified from 2.4 A to 2.8 Å after the irradiation. The distance between sulfurs (S–S) of adjacent SC12 groups in the model prior to irradiating, ranges from 3.3 to 4.8 and after irradiation these distances range from 2.05 to 6.2 Å. Insets in Figs. 2a and b show the models of the metallic cores, before and after irradiation. The icosahedron structure remained unchanged, with only minor distortions in the Au–Au bonding, which are within the reported known values. The C–C, C–H, C–S experimental reported bond distances from a SC12 molecule are 1.5, 1.09 and 1.819 Å, respectively (CRC, 2001), and the four s bonds of C form angle within the alkyl chain of 109°, as it is known for an sp3 hybridization. After irradiating an ico55, it is observed that 40% of the SC12 molecules remain without distortions along the chain axis (bending or bond length modifications) as shown in Fig. 3a; the bond distances of C–C, C–H and C–S are 1.539, 1.69 and 1.809 Å respectively, close to those experimental distances reported elsewhere (Saul, 1974). 20% are bent at the end of the chain, between C8 and C9, as presented in Fig. 3b. In these irradiated SC12 molecules, even when the angle between adjacent C atoms remains the same as that of an sp3 hybridization, the carbon chain is modified with a rotation of 92°
along the chain axis, mainly between C8 and C9, where bending takes place. However, the C–C, C–H and C–S interatomic distances remain almost the same with values of 1.54, 1.141 and 1.8 Å respectively. Another 20% exhibited bending in the S–C bond forming an angle of 114.206°, the distances of C–C, C–H and C–S are 1.544, 1.141 and 1.810 Å. Inset in Fig. 3c shows this torsion at the beginning of the alkyl chain between C1 and C4. The remaining 20% is bent almost in the middle of the alkyl chain between C6 and C8, as indicated on the inset in Fig. 3d. This bending takes place between C–C–C forming an angle of 113°, however the distances C–C, C–H and C–S are 1.546, 1.141 and 1.809 Å, which show no significant changes respect those reported experimentally in the literature. The C–S–Au angle reported experimentally and theoretically (Shelley et al., 2002) within the chain axis is 96.5° and 97.1°, respectively. In the model before irradiation the average angle is 96.45°, after irradiation these angles are changed within the range of 77–120°. 3.2. Cuboctahedron The original cuboctahedral system with 20 SC12 is presented in Fig. 4a. First, the beam direction of the radiation was applied over (111) plane of the nanostructures, as indicated by the arrow in Fig. 4b. The energy of the system before irradiation was 418957.302 eV and after irradiation with doses of 1, 10 and
Fig. 3. Behavior of the SC12 molecules on the surface of an icosahedral particle after the simulation of the irradiation process (dose of 20 kGy). This analysis indicates that SC12 molecules can be present in a number of modes (a) without changes along the alkyl chain; (b) with a torsion at end of the alkyl chain; (c) with a torsion between the sulfur atom and the alkyl chain; (d) with a torsion in the middle of the alkyl chain.
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Fig. 4. Simulation of the effect of gamma radiation on a cuboctahedral structure with SC12 molecules, at a dose of 20 kGy. The radiation vector interacts over planes (111). Behavior of the passivated structure (a) before irradiation, (b) after irradiation. Insets in both figures show the metallic core without SC12 molecules.
20 KGy it was modified to 41.8958.502 eV, 418956.181 eV and 418931.604 eV respectively. At 1 and 10 KGy, the energy of the system remains almost the same; however, at 20 kGy a significant increase of the energy was observed. This increase can be attributed to the modifications induced in the metallic core and the alkyl chains. The resulting structures obtained after an irradiation at 20 kGy are presented, showing the type of modifications that could be expected. Fig. 4a and b shows the images of the initial and final model after irradiation process. Before the irradiation of the samples, the S–Au distance is 2.4 Å and after irradiation this distance is modified, ranging from 4.961 Å to 3.068 Å. The inset of Fig. 4a and b shows the gold nanostructure only, before and after irradiation. The inset in Fig. 4b shows the cuboctahedron deformation after an irradiation dose of 20 KGy. In the system cuboctahedron-SC12, 25% of SC12 did not exhibit changes along the alkyl chain, the distances between atoms within SC12 are very similar to those reported experimentally (Ermer, 1976). Fig. 5a shows the system cuboctahedron-SC12, where the SC12 remained unchanged – inset. 20% have a torsion at the beginning of the chain between S–C, with a C1–C2–C3 angle of 109.9° – inset in Fig. 5b. 10% have a torsion between C5–C7, forming in the chain axis an angle of 109° before and 111.7° after irradiating, shown in Fig. 5c, and the inset shows the modified
SC12. 15% of SC12 exhibit a torsion in the middle, between C5–C7, forming an angle of 119° in the chain axis (Fig. 5d). The remaining 30% has a slight torsion at the end of the SC12 chain, between C8 and C9 atoms, as shown in Fig. 5e, were the distances C–C, C–H and C–S are 1.546, 1.141 and 1.809 Å, remain close to those reported experimentally. The S–Au bond distances are average 3.2 Å, however, as a result of the irradiation, it was found that a rupture of S–Au bond having a final distance of S–Au of 4.4 Å, average. In Fig. 6a and b, the models of the cuboctahedral systems are presented. When the beam direction of the radiation is applied over a (100) plane as indicated by the arrow, the energy of the cuboctahedron-SC12 system is not greatly modified, obtaining an energy of 418958.129 eV, 418953.979 and 418943.944 eV at 1, 10 and 20 KGy respectively. Fig. 6 shows this cuboctahedronSC12 system before (a) and after (b) irradiation at a dose of 20 kGy. Inset in these figures presents only the metallic core at each stage. In this case, 70% of the SC12 do not show modifications (Fig. 7a). The other 30% exhibit a torsion at the beginning of the alkyl chain between C1 and C 2 and at the end between C10 and C11 (Fig. 7b). No structural changes were detected in the metallic core due to radiation.
Fig. 5. Behavior of SC12 molecules on a cubctahedral particle after the simulation of the irradiation process over (111) planes, at a dose of 20 kGy. This analysis indicates that SC12 molecules can be present (a) without changes along the alkyl chain, (b) with a torsion at the beginning of the alkyl chain, (c) and (d) with a torsion in the middle of the alkyl chain, and (e) with a torsion at the end of the alkyl chain.
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Fig. 6. Simulation of the effect of gamma radiation on a cuboctahedral structure with SC12 molecules, at a dose of 20 kGy. The radiation vector interacts over planes (100). Behavior of the passivated structure (a) before irradiation, (b) after irradiation. Insets in both figures show the metallic core without SC12 molecules.
3.3. Decahedron As mentioned before, simulation of the gamma radiation interaction with a decahedron-SC12 system were performed with a beam direction of the radiation applied over the (111) and (100) planes, similar to the study of the system cuboctahedron-SC12. It was observed that the behavior of the system was similar when radiation interacts with any of the two planes. After receiving gamma radiation on a (111) plane, with doses of 1, 10 and 20 kGy, the energy of the system was of 418955.72 eV, 418950.75 eV and 418913.21 eV respectively. For the same doses, when the radiation is incident over the (100) planes the energy of the system was 418955.74 eV, 418946.66 eV and 418927.40 eV respectively. Fig. 8 shows the simulated model before (Fig. 8 a) and after (Fig. 8 b) the irradiation process over (111) planes of a decahedronSC12 system at a dose of 20 kGy. Observe on the insets of these figures the corresponding uncovered nanoparticle models; similar to the previous case, it is observed that radiation induces deformation on the nanoparticle. The initial energy of the system is of 418955.74 eV while the final energy after receiving a dose of 20 kGy is 418913.21 eV. In this case, the energy increase is higher than when irradiation is directed to the (100) plane, this can be attributed to the fact that over the (111) plane there is a rupture of the C–C bonds at different sites of the chains. The effect of irradiation on the SC12 molecules attached to the (111) plane of the gold nanostructures was analyzed as presented in Fig. 10. 57% of these organic chains did not exhibited structural modifications along their structure (Fig. 9a) and their S–C, C–C and
C–H bond distances are 1.804, 1.535 and 1.1 Å, respectively. On the other hand, 33% of the overall chains were bent at the end of the structure, between C7 and C8, forming an angle in the chain axis of 117–119.08° (Fig. 9b). Finally, in 20% of the chains a rupture in different C–C bonds was observed, one between carbon atoms C11 and C12 and another between C9 and C10 atoms (Fig. 9c). The energy calculations of decahedral particles were also performed before and after irradiation over a (100) plane (Fig. 10a and b). The energy before irradiation is 418955.74 eV and after irradiation at a dose of 20 kGy is 418927.40 eV. It is observed that there is an increase in the overall energy of the system. This could be related to the gold nanostructure deformation and the S–Au bond breaking, thus the SC12 chain is segregated from the gold nanostructure. Measurements of the distance S–Au, before irradiating is 2.4 Å average and after irradiation distances range from 3.365 to 3.043 Å. These distances correspond to the alkyl chains that remain bonded to the nanoparticle. Notice that 70% of alkyl molecules remain without distortions along the chain. Insets in Figs. 10a and b show the uncovered gold nanostructure, before and after irradiation of the system. Note the decahedron deformation after the irradiation process, in Fig. 10b. 30% of alkyl chains are bent in the middle, between C7 and C8 (Fig. 11a), forming C–C–C angles of 118° within the chain axis (inset). 50% of the alkyl chains remain without changes along the carbon chain (Fig. 11b), and 20% of the alkyl chains exhibit a torsion at the end (Fig. 11c) as shown in the inset. The C–C, C–H and C–S distances are 1.546, 1.141 and 1.809 Å respectively, similar to those reported experimentally. The bond distances S–Au are 3.2 Å average; however, as a result of irradiation it was found that the S–
Fig. 7. Behavior of SC12 molecules on a cuboctahedral particle after the simulated irradiation process over the (100) plane, at a dose of 20 kGy. This analysis indicates that SC12 molecules can be (a) without changes along the alkyl chain, and (b) with a torsion at the beginning in the middle or at the end of the alkyl chain.
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Fig. 8. Simulation of the effect of gamma radiation over (111) planes of a decahedral structure with SC12 molecules, at a dose of 20 kGy. Behavior of the passivated structure (a) before and (b) after irradiation, inset shows the cuboctahedral structure without SC12 before and after irradiation respectively.
Fig. 9. Behavior of SC12 molecules on a decahedral particle after the simulated irradiation process over a (111) plane, at a dose of 20 kGy. This analysis showed SC12 molecules (a) without change of the alkyl chain, (b) with a torsion at the end of the alkyl chain and (c) broken C–C bonds on the alkyl chain.
Fig. 10. Simulation of the effect of gamma radiation over (100) planes of a decahedral structure with SC12 molecules, at a dose of 20 kGy. Models (a) before and (b) after irradiation. Insets show the decahedral structures without SC12, at each stage.
Au bond breaks in 50% of the SC12 chains. Fig. 12 shows a graph of the energies of each model nanoparticle-SC12, before and after being irradiated at the different doses studied. It can be noted that at doses of 1 and 10 kGy the energy of the systems remains close to that of the original value, before irradiation. At these doses, the main modifications were observed on the conformational shape of the alkyl chains, including torsion, bending and restructuration of the alkanethiolate chain on the surface of the metallic core. Also, slight shifting in the position of the atoms within the nanoparticles was observed, without losing the final shape. However, when the irradiation dose increased up to
20 kGy, the energy of the system increased substantially, being more evident when irradiation interaction is through the (111) planes. This energy increase is associated to a higher degree of modification in the alkyl chains, where even scission of these chains or breaking of the S–Au bond were registered along with bending and torsion events. Also, a dose of 20 kGy is enough to cause damage of the metallic core, observing highly modified crystal structures, including detachment of some gold atoms in the cuboctahedral structure. However, the icosahedral structure remained with minimum structural modifications, being the changes in alkyl chains the main responsible for energy increase of this system.
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Fig. 11. Behavior of SC12 molecules on a decahedral particle after the simulated irradiation process, at a dose of 20 kGy. This analysis indicates that SC12 molecules can be (a) bent in the middle of the alkyl chain; (b) without changes along the alkyl chain and/or (c) with a torsion at the end of the alkyl chain.
Fig. 12. System energy vs gamma radiation dose of the different systems nanoparticle-SC12. Observe that up to 10 kGy, the system energy remains almost constant, similar to the original model.
In order to observe the effect of irradiation on the metallic core of the passivated system, the alkanethiolate molecules were eliminated, allowing to analyze the energy of only the nanostructure at the different irradiation doses (Fig. 13). Fig. 13 shows a graph of the calculated energy of each nanoparticle structure, icosahedral, cuboctahedral and decahedral, where it can be observed that the behavior follows a similar tendency, where the major effect takes place at 20 kGy. However, within the slight changes that can be noted, the icosahedral system in this case has a greater energy than the cuboctahedral structure irradiated over the (100) planes. From these results it is suggested that this increase in energy where only metallic cores are compared can be attributed to a higher disorder in the metallic core in the icosahedral nanostructure. Different possible events that can take place during the gamma radiation interaction with the studied systems are presented in this work. However, it is important to note that these phenomena will be influenced by the environment around the nanostructures, as suspending media such as a solvent or a solid support and capping agents. Also, another consideration known for radiation processes as the ones presented in this study is that events are probabilistic, so the percentages of each modification can vary.
Overall, it is observed that structural changes can take place not only due to the radiation dose, they also depend on the orientation of the particle respect to the beam direction of the radiation. Consequently, when radiation interacts over (111) planes the system energy increases in all studied systems. The radiation was directed over the different planes exposed by the nanoparticles. It was observed that the possible effects that might occur during this process include major modifications on the alkyl chains of the alkanethiolate molecules such as bending, torsion and scission, which eventually could lead to crosslinking. However, an important effect was observed on the nanoparticles, depending on the shape of these structures, being the icosahedral nanoparticles system the most stable at the irradiation conditions studied.
4. Conclusions This study reports different scenarios that can take place during the gamma irradiation of the system of 1-dodecanethiolate-capped gold nanoparticles with different shapes. Based on the studied conditions and radiation doses applied, it was observed that the metallic core in the analyzed systems is highly stable to gamma
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Fig. 13. System energy vs gamma radiation dose of the nanoparticles with different shapes at different orientations. The alkanethiolate molecules were removed after irradiation, to observe the effect on the metallic core.
irradiation up to 10 kGy. At 20 kGy, some modifications on the nanoparticles, as atomic displacements and detachment of atoms form the metallic core, start to be more evident. However, the organic capping agent can go through different modifications including conformational changes, even scission and detachment from the metallic nanoparticle. This behavior has to be considered in the process of designing an application.
Acknowledgments This research was financially supported by CONACyT through grant CB-169682 and by ININ project CA-216.
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Effect of gamma radiation on alkanethiolate-capped gold nanoparticles: Theoretical studies M.E. Fernández-García, M. Pérez-Alvarez, D. Mendoza-Anaya, C. Gutiérrez-Wing n Instituto Nacional de Investigaciones Nucleares, Carr. México-Toluca S/N, La Marquesa, Ocoyoacac, Edo. De México C. P. 52750, Mexico
H I G H L I G H T S
Gamma radiation effects on alkanethiolate-capped Gold nanoparticles were simulated. Effects of gamma radiation depend on the Gold nanoparticle structure. Icosahedral nanoparticles were the most stable to gamma radiation. At the studied gamma irradiation doses, the metallic cores were stable up to 10 kGy. Major modifications can occur on the alkyl chains of the capping agent.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2015 Received in revised form 20 November 2015 Accepted 30 November 2015 Available online 2 December 2015
Theoretical studies of the effect of gamma irradiation on alkanethiolate-capped gold nanoparticles are presented. Icosahedral, decahedral and fcc nanoparticles protected with 1-dodecanethiolate (SC12) were obtained by molecular mechanics simulations, analyzing the effect of gamma irradiation through MonteCarlo. The studied doses were 1, 10 and 20 kGy. It was observed that slight structural modifications of the metallic core might occur and these are dependent on the shape of the nanoparticle. However, the most significant effect was observed on the organic passivating layer, where torsions, bending and scission of the alkyl chains were detected. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles Gamma radiation Monte Carlo Geometrical optimization
1. Introduction Since the great potential of nanoscale materials was exhibited, different scientific and technological fields have focused on the possibility of using their novel properties to fabricate new or improved materials on a number of applications. With no doubt, nanomaterials have occupied an important place in the development of systems or devices either for medicine, catalysis, electronics and aerospace. In some of these applications it is required that the nanomaterials withstand (Mariscal et al., 2010; Nara et al., 2004; Yourdshahyan and Rappe, 2002; Yuan et al., 2014; Askerka et al., 2012) harsh conditions as radiation fields, which have sufficient energy to induce changes in matter. For example, the new nanomaterials for the nuclear and thermonuclear power engineering (Andrievski, 2011), or nanodevices to be used in the space exploration (Dharani et al., 2013). On the other hand, radiation in combination with nanomaterials may be used to n
Corresponding author. E-mail address: [email protected] (C. Gutiérrez-Wing).
http://dx.doi.org/10.1016/j.radphyschem.2015.11.036 0969-806X/& 2015 Elsevier Ltd. All rights reserved.
improve the function of the material in areas such as redox catalysis, environmental remediation, electronics and radiotherapy (Meisel, 2005). For example, for radiation therapy high-Z materials, such as Au nanoparticles, can be used to amplify the dose of delivered ionizing radiation (Cooper et al., 2014) or in electronics for the manufacturing of silicon solar cells in which Au nanoparticles are used to enhance their efficiency (Abdullah, 2013). Whatever the case, the information about the nanomaterials behavior under irradiation is very important especially when they will be exposed to an intense radiation field. For metallic nanoparticles specifically, there are many reports focused in the application of radiation to synthesize them, however, little information is available on the stability of nanoparticles when they are exposed to radiation of any kind, such as gamma rays. Furthermore, up to now, to our knowledge, there are no reports about the radiation effects on surface protectors over metallic nanoparticles. It should be mentioned that surface protector is very important in the dimensional stabilization of the nanoparticle, then, if the protector suffers any modification, the nanoparticle can destabilize and change its properties. In order to gain knowledge on the advantages or limitations of
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the application of nanoscale materials in a radiation field, in this paper we present a theoretical simulation analysis of the effects of gamma radiation on alkanethiolate-capped gold nanoparticles. The study was based on three of the most commonly found shapes of nanoparticles in the size range of 1–4 nm, icosahedron, decahedron and fcc. For this theoretical analysis, we simulated the behavior of gold nanoparticles exposed to ionizing radiation at a deposited energy per mass unit of 1, 10 and 20 kGy (Dose), considering that gamma radiation is widely used in medicine, industry and investigation. The theoretical analysis was performed using the PENELOPE (http://www.oecd-nea.org/lists/penelope. html) code of the Monte Carlo Method (Metropolis and Ulam, 1945).
ionizing radiation, energies such as those associated to 60Co, Compton scattering is the dominant interaction. In this process, a radiation beam transfers a fraction of its energy to an atomic electron, which is ejected. With its smaller energy, the radiation beam can undergo a photoelectronic effect by an atom: in this process, the atom emits an electron. These multiple interaction process can be simulated by the PENELOPE code of the MonteCarlo method. This simulation algorithm is based on a scattering model that combines numerical databases with analytical cross section models for the different interaction mechanism and is applicable to energies from a few hundred eV to 1 GeV PENELOPE-2008; Arqueros and Montesinos, 2003; Tajik et al., 2015).
3. Results and discussion 2. Experimental Minimum energy configuration models were obtained through geometrical optimization. The minimum energy configuration models were determined using a universal potential, with a maximum number of iterations of 5000 steps and a convergence criterion of 0.000 1 kcal/mol. Models of icosahedral, cuboctahedral and decahedral gold nanoparticles of 55 atoms respectively, were constructed and geometrically optimized to refine the geometry of their atomic structure. This process was performed through an iterative sequence, which adjusts the atomic coordinates to bring the system to a minimum energy structure. The algorithm SMART (Ermer, 1976) was used to perform these calculations, which is a combination of steps descent methods, conjugate gradient and NewtonRaphson. A Universal (Rappé et al., 1992) potential was used. Once these gold nanostructures models Au55ico, Au55dec Au55cub were obtained, they were capped by 20 molecules of SC12 each, and the system was again optimized to a minimum of energy. The energy of the systems was calculated through density functional theory (DFT), based on the Kohn and Sham theory, by applying a local density approximation (LDA) (Kohn and Sham, 1965; Barnard and Curtiss, 2006; Carr et al., 2012; Zhang et al., 2009). Because of the stochastic nature of radiation interaction, Monte-Carlo simulations techniques are very convenient to know the ionizing radiation effects in a specific material. Consequently, the theoretical simulation analysis of the irradiation process on gold nanoparticles-SC12 with radiation such as gamma rays, as those of 60Co, were performed using the Monte-Carlo Method. When radiation passes through matter, it interacts with atoms (electrons and nucleus) that conform this matter. Depending on the ionizing radiation energy, different interactions mechanisms can take place: coherent scattering (Rayleigh), incoherent scattering (Compton), photoelectric effect and pair production. For
On the first stage of this study the minimum energy models of the gold nanostructures capped by SC12, shown in Fig. 1, have been calculated. These are labeled as Au55ico, Au55dec Au55cub. The model of an icosahedron capped with SC12 was built, placing each SC12 over the exposed planes on the surface, at hollow sites on the face (111), with a distance S–Au of 2.4 Å (Majumder et al., 2002; Mazzarello et al., 2007). SC12 in a cuboctahedron structure, were placed in hollow sites over (100) and (111) facets with an average distance of S–Au of 2.4 Å. For the decahedron structure, the SC12 were placed on the hollow sites on the surface planes (111) and (100), with an S–Au distance 2.4 Å average. The capping molecules were attached on three-coordinated hollow sites for (111) planes and tetra-coordinated hollow sites for (100) planes, as it has been reported for similar systems (Ching Shih et al., 2007; Sheppard et al., 2011; Luedtke and Landman, 1996; Wilson and Johnston, 2002). The beam direction of the radiation was applied to interact with planes (111) and (100) for each system, having finally two different results for the cuboctahedral and decahedral models and one for the icosahedron. 3.1. Icosahedron The calculated energy of a 55 atoms-icosahedron with 20 molecules of SC12 is originally of 418970.70 eV. The model of this system is shown in Fig. 2a. Studies for this system were based on the interaction of the radiation beam with the (111) plane, as indicated by the arrow in Fig. 2b. It was observed that the energy of the system increases as the radiation dose rises. At 1, 10 and 20 kGy, the energy obtained for each system is 418964.46 eV, 418961.44 eV and 418945.86 eV respectively. The resulting structures obtained after an irradiation at 20 kGy are presented, showing the type of modifications that could be expected. Fig. 2b
Fig. 1. Models of 1-dodecanothiolate-capped gold nanoparticles. (a) icosahedron, (b) decahedron and (c) cuboctahedron.
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Fig. 2. Icosahedral structure capped with SC12 molecules. (a) Before and (b) after irradiation at 20 kGy. Insets in both figures show the icosahedron.
shows the models of the resulting structures at 20 kGy. The metallic core of the nanoparticle does not suffer modifications, suggesting that this energy increase can be associated to a modification of the SC12 as presented below. The S–Au distances are modified from 2.4 A to 2.8 Å after the irradiation. The distance between sulfurs (S–S) of adjacent SC12 groups in the model prior to irradiating, ranges from 3.3 to 4.8 and after irradiation these distances range from 2.05 to 6.2 Å. Insets in Figs. 2a and b show the models of the metallic cores, before and after irradiation. The icosahedron structure remained unchanged, with only minor distortions in the Au–Au bonding, which are within the reported known values. The C–C, C–H, C–S experimental reported bond distances from a SC12 molecule are 1.5, 1.09 and 1.819 Å, respectively (CRC, 2001), and the four s bonds of C form angle within the alkyl chain of 109°, as it is known for an sp3 hybridization. After irradiating an ico55, it is observed that 40% of the SC12 molecules remain without distortions along the chain axis (bending or bond length modifications) as shown in Fig. 3a; the bond distances of C–C, C–H and C–S are 1.539, 1.69 and 1.809 Å respectively, close to those experimental distances reported elsewhere (Saul, 1974). 20% are bent at the end of the chain, between C8 and C9, as presented in Fig. 3b. In these irradiated SC12 molecules, even when the angle between adjacent C atoms remains the same as that of an sp3 hybridization, the carbon chain is modified with a rotation of 92°
along the chain axis, mainly between C8 and C9, where bending takes place. However, the C–C, C–H and C–S interatomic distances remain almost the same with values of 1.54, 1.141 and 1.8 Å respectively. Another 20% exhibited bending in the S–C bond forming an angle of 114.206°, the distances of C–C, C–H and C–S are 1.544, 1.141 and 1.810 Å. Inset in Fig. 3c shows this torsion at the beginning of the alkyl chain between C1 and C4. The remaining 20% is bent almost in the middle of the alkyl chain between C6 and C8, as indicated on the inset in Fig. 3d. This bending takes place between C–C–C forming an angle of 113°, however the distances C–C, C–H and C–S are 1.546, 1.141 and 1.809 Å, which show no significant changes respect those reported experimentally in the literature. The C–S–Au angle reported experimentally and theoretically (Shelley et al., 2002) within the chain axis is 96.5° and 97.1°, respectively. In the model before irradiation the average angle is 96.45°, after irradiation these angles are changed within the range of 77–120°. 3.2. Cuboctahedron The original cuboctahedral system with 20 SC12 is presented in Fig. 4a. First, the beam direction of the radiation was applied over (111) plane of the nanostructures, as indicated by the arrow in Fig. 4b. The energy of the system before irradiation was 418957.302 eV and after irradiation with doses of 1, 10 and
Fig. 3. Behavior of the SC12 molecules on the surface of an icosahedral particle after the simulation of the irradiation process (dose of 20 kGy). This analysis indicates that SC12 molecules can be present in a number of modes (a) without changes along the alkyl chain; (b) with a torsion at end of the alkyl chain; (c) with a torsion between the sulfur atom and the alkyl chain; (d) with a torsion in the middle of the alkyl chain.
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Fig. 4. Simulation of the effect of gamma radiation on a cuboctahedral structure with SC12 molecules, at a dose of 20 kGy. The radiation vector interacts over planes (111). Behavior of the passivated structure (a) before irradiation, (b) after irradiation. Insets in both figures show the metallic core without SC12 molecules.
20 KGy it was modified to 41.8958.502 eV, 418956.181 eV and 418931.604 eV respectively. At 1 and 10 KGy, the energy of the system remains almost the same; however, at 20 kGy a significant increase of the energy was observed. This increase can be attributed to the modifications induced in the metallic core and the alkyl chains. The resulting structures obtained after an irradiation at 20 kGy are presented, showing the type of modifications that could be expected. Fig. 4a and b shows the images of the initial and final model after irradiation process. Before the irradiation of the samples, the S–Au distance is 2.4 Å and after irradiation this distance is modified, ranging from 4.961 Å to 3.068 Å. The inset of Fig. 4a and b shows the gold nanostructure only, before and after irradiation. The inset in Fig. 4b shows the cuboctahedron deformation after an irradiation dose of 20 KGy. In the system cuboctahedron-SC12, 25% of SC12 did not exhibit changes along the alkyl chain, the distances between atoms within SC12 are very similar to those reported experimentally (Ermer, 1976). Fig. 5a shows the system cuboctahedron-SC12, where the SC12 remained unchanged – inset. 20% have a torsion at the beginning of the chain between S–C, with a C1–C2–C3 angle of 109.9° – inset in Fig. 5b. 10% have a torsion between C5–C7, forming in the chain axis an angle of 109° before and 111.7° after irradiating, shown in Fig. 5c, and the inset shows the modified
SC12. 15% of SC12 exhibit a torsion in the middle, between C5–C7, forming an angle of 119° in the chain axis (Fig. 5d). The remaining 30% has a slight torsion at the end of the SC12 chain, between C8 and C9 atoms, as shown in Fig. 5e, were the distances C–C, C–H and C–S are 1.546, 1.141 and 1.809 Å, remain close to those reported experimentally. The S–Au bond distances are average 3.2 Å, however, as a result of the irradiation, it was found that a rupture of S–Au bond having a final distance of S–Au of 4.4 Å, average. In Fig. 6a and b, the models of the cuboctahedral systems are presented. When the beam direction of the radiation is applied over a (100) plane as indicated by the arrow, the energy of the cuboctahedron-SC12 system is not greatly modified, obtaining an energy of 418958.129 eV, 418953.979 and 418943.944 eV at 1, 10 and 20 KGy respectively. Fig. 6 shows this cuboctahedronSC12 system before (a) and after (b) irradiation at a dose of 20 kGy. Inset in these figures presents only the metallic core at each stage. In this case, 70% of the SC12 do not show modifications (Fig. 7a). The other 30% exhibit a torsion at the beginning of the alkyl chain between C1 and C 2 and at the end between C10 and C11 (Fig. 7b). No structural changes were detected in the metallic core due to radiation.
Fig. 5. Behavior of SC12 molecules on a cubctahedral particle after the simulation of the irradiation process over (111) planes, at a dose of 20 kGy. This analysis indicates that SC12 molecules can be present (a) without changes along the alkyl chain, (b) with a torsion at the beginning of the alkyl chain, (c) and (d) with a torsion in the middle of the alkyl chain, and (e) with a torsion at the end of the alkyl chain.
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Fig. 6. Simulation of the effect of gamma radiation on a cuboctahedral structure with SC12 molecules, at a dose of 20 kGy. The radiation vector interacts over planes (100). Behavior of the passivated structure (a) before irradiation, (b) after irradiation. Insets in both figures show the metallic core without SC12 molecules.
3.3. Decahedron As mentioned before, simulation of the gamma radiation interaction with a decahedron-SC12 system were performed with a beam direction of the radiation applied over the (111) and (100) planes, similar to the study of the system cuboctahedron-SC12. It was observed that the behavior of the system was similar when radiation interacts with any of the two planes. After receiving gamma radiation on a (111) plane, with doses of 1, 10 and 20 kGy, the energy of the system was of 418955.72 eV, 418950.75 eV and 418913.21 eV respectively. For the same doses, when the radiation is incident over the (100) planes the energy of the system was 418955.74 eV, 418946.66 eV and 418927.40 eV respectively. Fig. 8 shows the simulated model before (Fig. 8 a) and after (Fig. 8 b) the irradiation process over (111) planes of a decahedronSC12 system at a dose of 20 kGy. Observe on the insets of these figures the corresponding uncovered nanoparticle models; similar to the previous case, it is observed that radiation induces deformation on the nanoparticle. The initial energy of the system is of 418955.74 eV while the final energy after receiving a dose of 20 kGy is 418913.21 eV. In this case, the energy increase is higher than when irradiation is directed to the (100) plane, this can be attributed to the fact that over the (111) plane there is a rupture of the C–C bonds at different sites of the chains. The effect of irradiation on the SC12 molecules attached to the (111) plane of the gold nanostructures was analyzed as presented in Fig. 10. 57% of these organic chains did not exhibited structural modifications along their structure (Fig. 9a) and their S–C, C–C and
C–H bond distances are 1.804, 1.535 and 1.1 Å, respectively. On the other hand, 33% of the overall chains were bent at the end of the structure, between C7 and C8, forming an angle in the chain axis of 117–119.08° (Fig. 9b). Finally, in 20% of the chains a rupture in different C–C bonds was observed, one between carbon atoms C11 and C12 and another between C9 and C10 atoms (Fig. 9c). The energy calculations of decahedral particles were also performed before and after irradiation over a (100) plane (Fig. 10a and b). The energy before irradiation is 418955.74 eV and after irradiation at a dose of 20 kGy is 418927.40 eV. It is observed that there is an increase in the overall energy of the system. This could be related to the gold nanostructure deformation and the S–Au bond breaking, thus the SC12 chain is segregated from the gold nanostructure. Measurements of the distance S–Au, before irradiating is 2.4 Å average and after irradiation distances range from 3.365 to 3.043 Å. These distances correspond to the alkyl chains that remain bonded to the nanoparticle. Notice that 70% of alkyl molecules remain without distortions along the chain. Insets in Figs. 10a and b show the uncovered gold nanostructure, before and after irradiation of the system. Note the decahedron deformation after the irradiation process, in Fig. 10b. 30% of alkyl chains are bent in the middle, between C7 and C8 (Fig. 11a), forming C–C–C angles of 118° within the chain axis (inset). 50% of the alkyl chains remain without changes along the carbon chain (Fig. 11b), and 20% of the alkyl chains exhibit a torsion at the end (Fig. 11c) as shown in the inset. The C–C, C–H and C–S distances are 1.546, 1.141 and 1.809 Å respectively, similar to those reported experimentally. The bond distances S–Au are 3.2 Å average; however, as a result of irradiation it was found that the S–
Fig. 7. Behavior of SC12 molecules on a cuboctahedral particle after the simulated irradiation process over the (100) plane, at a dose of 20 kGy. This analysis indicates that SC12 molecules can be (a) without changes along the alkyl chain, and (b) with a torsion at the beginning in the middle or at the end of the alkyl chain.
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Fig. 8. Simulation of the effect of gamma radiation over (111) planes of a decahedral structure with SC12 molecules, at a dose of 20 kGy. Behavior of the passivated structure (a) before and (b) after irradiation, inset shows the cuboctahedral structure without SC12 before and after irradiation respectively.
Fig. 9. Behavior of SC12 molecules on a decahedral particle after the simulated irradiation process over a (111) plane, at a dose of 20 kGy. This analysis showed SC12 molecules (a) without change of the alkyl chain, (b) with a torsion at the end of the alkyl chain and (c) broken C–C bonds on the alkyl chain.
Fig. 10. Simulation of the effect of gamma radiation over (100) planes of a decahedral structure with SC12 molecules, at a dose of 20 kGy. Models (a) before and (b) after irradiation. Insets show the decahedral structures without SC12, at each stage.
Au bond breaks in 50% of the SC12 chains. Fig. 12 shows a graph of the energies of each model nanoparticle-SC12, before and after being irradiated at the different doses studied. It can be noted that at doses of 1 and 10 kGy the energy of the systems remains close to that of the original value, before irradiation. At these doses, the main modifications were observed on the conformational shape of the alkyl chains, including torsion, bending and restructuration of the alkanethiolate chain on the surface of the metallic core. Also, slight shifting in the position of the atoms within the nanoparticles was observed, without losing the final shape. However, when the irradiation dose increased up to
20 kGy, the energy of the system increased substantially, being more evident when irradiation interaction is through the (111) planes. This energy increase is associated to a higher degree of modification in the alkyl chains, where even scission of these chains or breaking of the S–Au bond were registered along with bending and torsion events. Also, a dose of 20 kGy is enough to cause damage of the metallic core, observing highly modified crystal structures, including detachment of some gold atoms in the cuboctahedral structure. However, the icosahedral structure remained with minimum structural modifications, being the changes in alkyl chains the main responsible for energy increase of this system.
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Fig. 11. Behavior of SC12 molecules on a decahedral particle after the simulated irradiation process, at a dose of 20 kGy. This analysis indicates that SC12 molecules can be (a) bent in the middle of the alkyl chain; (b) without changes along the alkyl chain and/or (c) with a torsion at the end of the alkyl chain.
Fig. 12. System energy vs gamma radiation dose of the different systems nanoparticle-SC12. Observe that up to 10 kGy, the system energy remains almost constant, similar to the original model.
In order to observe the effect of irradiation on the metallic core of the passivated system, the alkanethiolate molecules were eliminated, allowing to analyze the energy of only the nanostructure at the different irradiation doses (Fig. 13). Fig. 13 shows a graph of the calculated energy of each nanoparticle structure, icosahedral, cuboctahedral and decahedral, where it can be observed that the behavior follows a similar tendency, where the major effect takes place at 20 kGy. However, within the slight changes that can be noted, the icosahedral system in this case has a greater energy than the cuboctahedral structure irradiated over the (100) planes. From these results it is suggested that this increase in energy where only metallic cores are compared can be attributed to a higher disorder in the metallic core in the icosahedral nanostructure. Different possible events that can take place during the gamma radiation interaction with the studied systems are presented in this work. However, it is important to note that these phenomena will be influenced by the environment around the nanostructures, as suspending media such as a solvent or a solid support and capping agents. Also, another consideration known for radiation processes as the ones presented in this study is that events are probabilistic, so the percentages of each modification can vary.
Overall, it is observed that structural changes can take place not only due to the radiation dose, they also depend on the orientation of the particle respect to the beam direction of the radiation. Consequently, when radiation interacts over (111) planes the system energy increases in all studied systems. The radiation was directed over the different planes exposed by the nanoparticles. It was observed that the possible effects that might occur during this process include major modifications on the alkyl chains of the alkanethiolate molecules such as bending, torsion and scission, which eventually could lead to crosslinking. However, an important effect was observed on the nanoparticles, depending on the shape of these structures, being the icosahedral nanoparticles system the most stable at the irradiation conditions studied.
4. Conclusions This study reports different scenarios that can take place during the gamma irradiation of the system of 1-dodecanethiolate-capped gold nanoparticles with different shapes. Based on the studied conditions and radiation doses applied, it was observed that the metallic core in the analyzed systems is highly stable to gamma
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Fig. 13. System energy vs gamma radiation dose of the nanoparticles with different shapes at different orientations. The alkanethiolate molecules were removed after irradiation, to observe the effect on the metallic core.
irradiation up to 10 kGy. At 20 kGy, some modifications on the nanoparticles, as atomic displacements and detachment of atoms form the metallic core, start to be more evident. However, the organic capping agent can go through different modifications including conformational changes, even scission and detachment from the metallic nanoparticle. This behavior has to be considered in the process of designing an application.
Acknowledgments This research was financially supported by CONACyT through grant CB-169682 and by ININ project CA-216.
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