Limitations of Self-Regenerative Antioxidant Ability of Nanoceria

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C: Physical Processes in Nanomaterials and Nanostructures

Limitations of Self-Regenerative Antioxidant Ability of Nanoceria Imposed by Oxygen Diffusion Yuri Malyukin, Pavel Maksimchuk, Vladyslav Seminko, Elena Okrushko, and Nikolai Spivak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03982 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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The Journal of Physical Chemistry

Limitations of Self-Regenerative Antioxidant Ability of Nanoceria Imposed by Oxygen Diffusion

Yuri Malyukin†*, Pavel Maksimchuk†, Vladyslav Seminko†, Elena Okrushko†, Nikolai Spivak‡ †

STC “Institute for Single Crystals”, Institute for Scintillation Materials, National Academy of Sciences of Ukraine, 61072, 60 Nauky Ave., Kharkiv, Ukraine.

‡

Zabolotny Institute of Microbiology and Virology, National Academy of Sciences of Ukraine, 03680, 154 Akademika Zabolotnogo St., Kyiv, Ukraine.

Corresponding Author * Email: [email protected].

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ABSTRACT:

Nanoceria (CeO2-x) is a unique antioxidant material able to self-regeneration of its properties after interaction with oxidants. In our paper the dynamics of nanoceriaoxidant interaction accompanied by reversible Ce3+↔Ce4+ transitions of cerium ions was studied for nanoceria water colloidal solutions using conventional spectroscopic techniques. We have shown that interaction of nanoceria with hydrogen peroxide leads to Ce3+ → Ce4+ oxidation accompanied by quenching of Ce3+ luminescence of nanoceria, and recovery of initial Ce3+ luminescence intensity occurs with sufficient time delay (up to few days). The role of oxygen transport within ceria nanoparticles in regeneration of antioxidant properties of nanoceria after interaction with an oxidant is discussed. Involvement of oxygen diffusion into recovery of nanoceria antioxidant properties hampers the redox activity of ceria nanoparticles making it size- and temperature-dependent.


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1. INTRODUCTION

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), superoxide anions (O2-) and hydroxyl radicals (·OH) are indispensable for signal transmission and immune response in living cells.1,2 However, the concentration of ROS in the cell needs to be under strict regulation due to strong destructive effect of the most active types of ROS (such as ·OH) on the DNA and lipid membranes.3,4 Combined action of superoxide dismutase (SOD) transferring O2- to H2O2 and catalase or peroxiredoxin transferring H2O2 to H2O provides only partial removal of ROS excess. Different uncontrolled factors like cell irradiation, leakages from the respiratory electron transport chain or immune response of the cells to various external agents can lead to instantaneous growth of ROS content surpassing the redox ability of enzymes. Cellular imbalance between ROS activity and endogenous antioxidant defence can be covered by continuous supply of dietary antioxidants (vitamins), but the discussions on the dosages of these antioxidants required for normal functioning of the human organism are still continuing.5,6 Recently a new type of antioxidant materials able to reversible ROS neutralization based on cerium oxide nanocrystals (nanoceria) was proposed.7-9 Besides well-known catalytic and redox properties of cerium oxide,10,11 the ability of nanoceria to recover its antioxidant properties after oxidation or reduction by ROS was determined. These studies have shown a clear distinction between nanoceria and traditional biological antioxidants (like beta-carotene, tocopherols, ubiquinol, or ascorbic acid) which are either unable to reversible oxidationreduction reactions or require specific molecules like glutathione or thioredoxin to renew their antioxidant properties.12,13 In contrast to this, in 7 was shown that even single dose of nanoceria (10 nM) was able to provide long-time protection against radical insult for brain microglia cells, so, the capability of cerium oxide


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nanoparticles to regenerate their antioxidant properties after interaction with oxidizing agent was deduced. In the number of papers the ability of nanoceria to act both in SOD-like and catalase-like ways was shown.14,15 Reversibility of ceria redox properties and longterm stability makes it a reliable sensor for hydrogen peroxide detection

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.

However, any detailed mechanism of nanoceria redox action is absent up to now due to large number of possible intermediate steps of ROS-nanoceria interaction and absence of consensus between different authors. Currently, two main concepts of nanoceria activity were proposed. First one is based on the simple Ce3+/Ce4+ switching on the surface of nanoceria,14,15 while the second one implies formation/annihilation of surface oxygen vacancies.18 In our recent papers we have applied new approach to the study of nanoceria redox activity. In shown that besides defect

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and charge-transfer

21

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we have

luminescence ceria

nanoparticles demonstrates 5d→4f luminescence of Ce3+ ions which intensity depends on the size of nanoparticle and atmosphere of pre-treatment. In

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was

shown that addition of hydrogen peroxide to water colloidal solutions of nanoceria leads to quenching of Ce3+ luminescence. This effect was used for understanding the time dynamics of nanoceria-oxidant interaction. Depending on H2O2 concentration different patterns of Ce3+ oxidation were revealed, and unexpected Ce3+↔Ce4+ oscillations accompanied by corresponding oscillations of Ce3+ luminescence intensity were observed.23 On the base of these investigations of nanoceria action, in

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we proposed that not only surface oxygen vacancies, but

also all subsystem of oxygen vacancies in nanoparticle can be involved in nanoceria redox activity. However, conformation of this statement needed detailed study of nanoceria redox activity on the different stages of ROS-nanoceria interaction. An example of Ce3+↔Ce4+ reversible transition monitoring using the methods of optical spectroscopy can be found in

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. In

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after H2O2 addition to


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nanoceria water colloidal solution sufficient redshift of the transmission edge was observed, which was accompanied by the eye-visible change of the colloidal solution color. This shift was ascribed to Ce3+→Ce4+ oxidation by means of Ce3++·OH→Ce4++OH- reaction taking place on the surface of the nanoparticle. The reverse transition (Ce4+ →Ce3+) lasted for months, so even after 30 days of storage in the dark place the regeneration of the initial spectra of the colloidal solution still was not complete.24 This observation of the authors

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can be explained using our

ideas about the decisive role of oxygen diffusion both on the stages of oxidation of Ce3+ ions within nanoceria volume (Ce3+→Ce4+) and their subsequent recovery (Ce4+ →Ce3+). In order to understand the role of oxygen diffusion in Ce4+ →Ce3+ transition processes in nanoceria, water colloidal solutions of cerium oxide nanocrystals with different sizes (2 nm and 10 nm) were obtained. Characterization of obtained materials (TEM images and XRD patterns) is shown in Supplement. As in our previous studies, intensity of Ce3+ luminescence was used as a measure of Ce3+/Ce4+ ratio in the nanoparticle. 2.EXPERIMENTAL SECTION 2.1. Materials and methods Aqueous solutions of ceria nanoparticles were obtained using the following reagents: CeCl3 (99 %, Acros organics), hexamethylenetetramine (99.2%), sodium citrate (99.8%, Merck). CeCl3 solution (100 ml, 2 mM) was mixed with 100 ml of hexamethylenetetramine solution (4 mM) and stirred by means of magnetic stirrer for 3 h at room temperature. After that 1.8 ml NH4OH and 0.6 ml of H2O2 were added into the solution. Then, the solution was put in round-bottom flask and refluxed for 5 h. As a result, transparent colorless solutions were obtained. The solution was evaporated in a rotary evaporator flask under vacuum at the bath temperature of 70оC to 30 ml. A solution of 2 M NaCl was added to the obtained ACS Paragon Plus Environment

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solution until the resulting solution became turbid. Then the solid phase was precipitated by centrifugation. The precipitate was separated and solution of sodium chloride was added again. The procedure of precipitate cleaning was repeated three times. After the last stage of centrifugation, solution of sodium citrate with molar ratio CeO2/NaCt of 1:1 was added to the precipitate. Size of nanoceria

obtained

from

the

mixture

of

cerium

(III)

chloride

and

hexamethylenetetramine (HMTA) taken in mole ratio 1:10 was ~10 nm. At further increase of HMTA excess size of obtained nanoparticles decreases to ~2 nm. СеО2х

nanoparticles were stabilized by sodium citrate with molar ratio 1:1. The

solutions were additionally dialyzed for 24 h against deionized water to remove the excess of ions and organics species. 2.2. Experimental techniques The photoluminescence spectra of nanoceria were excited by a continuouswave GKL-4UM He-Cd laser (λ=325 nm) and registered using the SDL-1 grating monochromator with the Hamamatsu R9110 PMT in the photon counting mode. The time evolution of Ce3+ luminescence intensity was registered at 430 nm. Influence of nanoceria before and after HP addition on the processes of ·OH generation at X-ray irradiation of water solutions was determined using standard coumarin test. Intensity of coumarin fluorescence (at λexc=325 nm) for irradiated nanoceria water solutions was compared with coumarin fluorescence intensity for the control sample (irradiated water solutions without nanoceria). Concentration of coumarin in all samples was equal to 10-4 mol. X-ray tube (U=30 kV, I=20 mA) was positioned at d=30 cm from the cuvette with irradiated solution. All samples were irradiated during 30 minutes. Concentrations of nanoceria in aqueous solutions were the similar in all experiments and equal to 1 g/l. Concentration of hydrogen peroxide in all experiments was equal to 5 mM. ACS Paragon Plus Environment

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3.RESULTS AND DISCUSSION The solid phase of colloidal solutions of cerium oxide (CeO2-x) nanocrystals obtained by the method described above was characterized using the methods of X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to TEM data the average sizes of nanoparticles were about 2 nm and 10 nm with narrow size dispersion (Figure 1) XRD patterns of obtained CeO2-x nanocrystals are shown in the Figure 2. The structure of the samples corresponds to JCPDS card No.34-0394, so the nanocrystals are characterized by FCC fluorite-type lattice and formation of any additional phases can be excluded.

Figure 1.ТЕМ of tested nanoceria specimens as obtained.

Figure 2. XRD of tested nanoceria specimens: 2 nm and 10 nm nanoceria as obtained.

Luminescence spectra of both 2 nm and 10 nm nanoceria (see Figure 3 and Figure 4) consisted of the single wide band determined by 5d→ 4f transitions of Ce3+ ions (Ce3+ band). Addition of hydrogen peroxide (HP) to nanoceria colloidal solution led to quenching of Ce3+ band due to oxidation of the part of Ce3+ ions to Ce4+ ones. As was shown previously,

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Ce3+/Ce4+ ratio, and so, Ce3+

luminescence intensity depended directly on the content of oxygen vacancies in ACS Paragon Plus Environment

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CeO2-x structure. Due to different sizes of nanoparticles, the concentration of oxygen vacancies for 2 nm and 10 nm nanoceria was different as well. Direct estimation of the absolute values of concentrations of oxygen vacancies (or concentrations of Ce3+ ions) for nanocrystals in water colloidal solutions is not possible, so to estimate (at least, roughly) these values we have used literature data. 26,27

In

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content of Ce3+ ions in ceria nanoparticles with different sizes was

determined using XPS methods. According to these data, for 3 nm nanoceria concentration of Ce3+ ions is equal to 44% (CeO1.78), for 6 nm nanoceria – 29 % (CeO1.855), and for 30 nm nanoceria – 17% (CeO1.915). Using these data and taking into account that according to

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the results for small nanoparticles can be slightly

overestimated, one can roughly estimate the oxygen stoichiometry for 2 nm and 10 nm nanoceria as ~CeO1.8 and ~CeO1.88, respectively.

Figure 3. Luminescence spectra of 2 nm nanoceria at different times after hydrogen peroxide (HP) addition: (a) before and immediately after HP addition, (b) recovery of initial nanoceria luminescence. In the insets – time evolution of


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Ce3+ luminescence (430 nm) after HP addition at different time scales. λexc=325 nm, t = 22 ºC. In

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we have shown that the change in the luminescence spectra of

nanoceria during HP addition differed substantially from the case when only the luminescence of near-surface Ce3+ ions was quenched (for instance, by addition of DIOC3S dyes, for details see Figure S2 in Supplement). In this way, HP addition led not only to oxidation of near-surface Ce3+ ions to Ce4+ ones, but to the change of oxygen stoichiometry of nanoparticle as whole. Ce3+ luminescence intensity of 2 nm nanoceria 10 min after HP addition was about 0.22 of the initial one (Figure 3a, inset), for 10 nm nanoceria - about 0.35 of the initial one (Figure 4a, inset). Decrease in Ce3+ luminescence corresponds to the same decrease in the content of oxygen vacancies. So, taking our rough estimates for 2 nm and 10 nm nanoceria, after oxidation the oxygen stoichiometry of both nanoparticles should be almost similar (~CeO1.96).

Figure 4. Luminescence spectra of 10 nm nanoceria at different times after hydrogen peroxide (HP) addition: (a) before and immediately after HP addition, (b) recovery of initial nanoceria luminescence. In the insets – time evolution of ACS Paragon Plus Environment

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Ce3+ luminescence (430 nm) after HP addition at different time scales. λexc=325 nm, t = 22 ºC.

The decrease of Ce3+ luminescence accompanied by change of oxygen stoichiometry of nanoparticle as whole could be explained by involvement of oxygen diffusion in Ce3+→Ce4+ oxidation processes. In

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we have proposed the

model for this process including oxidation of surface Ce3+ ions, subsequent water splitting on Ce4+-V-Ce4+ sites and diffusion of resulting oxygen ions via single-file diffusion mechanism. Filling of equilibrium oxygen vacancies by oxygen turns the system out of thermodynamic equilibrium, so according to this model after relatively fast Ce3+→Ce4+ oxidation stage the reverse process should be observed – namely, the exit of oxygen out of nanoparticle accompanied by transfer of remaining electrons to Ce4+ ions and so, by Ce4+→Ce3+ reduction. However, these predictions required experimental confirmation. Processes of restoration of equilibrium content of Ce3+ ions (and, so, of oxygen vacancies) in nanoceria were studied using the same method as described above – i.e., by measuring the intensity of Ce3+ luminescence band with different time delays after oxidation. The resulting series of luminescence spectra for 2 nm and 10 nm nanoceria at 22 ̊C are shown in the Figure 3b and Figure 4b, respectively. The most striking feature of the obtained results is an enormous difference between the oxidation (Ce3+→Ce4+) and reduction (Ce4+→Ce3+) rates. While Ce3+→Ce4+ oxidation took few minutes, the reverse process (Ce4+→Ce3+ reduction) occurred within few days. The time dependencies of Ce3+ luminescence intensity shown in the insets to the Figure 3b and Figure 4b demonstrate that even after 120 hours this process in not complete. Increase of the size of nanoparticle slows down not only the oxidation (Figure 3a and 4a), but also the reduction process (Figure 3b and 4b). After 120 hours the intensity of Ce3+ luminescence for ACS Paragon Plus Environment

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2 nm nanoceria was about 95% of the initial one (before oxidant addition), while for 10 nm nanoceria – only 80% of the initial intensity. Increase of the temperature led to sufficient decrease of Ce4+→Ce3+ reduction time for both 2 nm and 10 nm nanoceria. The recovery of Ce3+ luminescence for 2 nm and 10 nm nanoceria after oxidant addition at 22 ̊C and 37 ̊C are shown in Figure 5a and Figure 5b. In contrast to the results obtained at 22 ̊C, at 37 ̊C within only 24 hours intensity of Ce3+ luminescence band for 2 nm nanoceria reached the value observed before oxidation, while for 10 nm nanoceria after 24 hours intensity of Ce3+ luminescence was about 95 % of the initial one. So, the process of recovery of initial Ce3+/ Ce4+ ratio in nanoceria is strongly temperature-dependent.

Figure 5. Recovery of Ce3+ luminescence for 2 nm (a) and 10 nm (b) nanoceria at 22 ºC and 37 ºC after HP addition. In the insets the formation of oxygen bubbles on the cuvette walls for both samples is shown.

As was mentioned above, the studies of self-regenerative activity of nanoceria in biological systems have resulted in two opposite views on the redox processes for ceria nanoparticles – first one associated with reversible Ce3+/Ce4+ ACS Paragon Plus Environment

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switching on the surface of nanoceria, and second one involving formation and annihilation of surface oxygen vacancies. It is worth noting that experimental results shown above cannot be explained within the frameworks of each of these concepts. Very slow temperature and size-dependent recovery of initial Ce3+/Ce4+ ratio (or, in other words, recovery of equilibrium concentration of oxygen vacancies) after HP addition observed in our experiments can be explained only by oxygen diffusion from the bulk to the surface and then out of ceria nanoparticle. This process must be accompanied by oxygen generation which was observed experimentally for nanoceria colloidal solutions sealed in quartz cuvettes immediately after HP addition. The series of photographs in the insets to Figure 5a and 5b shows the formation of oxygen bubbles on the cuvette walls during Ce4+→Ce3+ reduction process for colloidal solution of 2 nm and 10 nm nanoceria, respectively (the same photos in higher resolution are shown in Supplement). Accumulation of oxygen during annealing in oxidative (air) atmosphere and its release during annealing in neutral (Ar) atmosphere led to the same changes of luminescence spectra as oxidation-reduction cycles observed at HP addition to water colloidal solutions of nanoceria (see Figure S4 in Supplement). So, involvement of oxygen transport in both these processes is reliable. Temperature dependence of oxygen diffusion in nanoceria should obey the standard law: D=D0exp(-Ea/kT), where Ea is the diffusion activation energy. The diffusion parameters D0 and Ea depend strongly on the oxygen stoichiometry of CeO2-x lattice, according to

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for CeO1.8: D0 = 6.2·10-6 cm2/s, Ea = 0.16 eV, for

CeO1.92: D0 = 1.5·10-5 cm2/s, Ea = 0.52 eV, for CeO2: D0 = 1.9·10-4 cm2/s, Ea = 1.1 eV. The best approximation of our data for 2 nm nanoceria can be obtained at D0 = 1·10-4 cm2/s, Ea = 0.93 eV: at 300 K (22 ºC) D=1·10-4·exp(-0.93/0.0258)=2.2·10-20 cm2/s, t=x2/2D=125 hours; at 315 K (37 ºC) D=1·10-4·exp(-0.93/0.0271)=1.23·10-19 cm2/s, t=x2/2D=22.5 hours, where the mean square displacement x is equal to the radius of nanocrystal. The values of both diffusion parameters D0 and Ea lie ACS Paragon Plus Environment

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between the ones found in 28 for CeO1.92 and CeO2 structures in agreement with our rough estimate of the oxygen stoichiometry of nanocrystal after HP addition (CeO1.96). Taking the same values of diffusion coefficients for 10 nm nanocrystal and assuming that oxygen diffusion occurs within the whole nanoparticle (x=5 nm), one should obtain at 22 ºC t=x2/2D=3100 hours and at 37 ºC t=x2/2D=560 hours; more realistic results can be obtained assuming that diffusion takes place only within the layer of 1.5-2 nm from the surface of nanoparticle.

Figure 6. Concentration of ·OH radicals in X-ray irradiated water solutions with and without nanoceria determined by coumarin test. Control sample - water solutions without nanoceria. Results for 2 nm (a) and 10 nm (b) nanoceria before, 30 min after and 24 hours after HP addition are shown.

The involvement of oxygen diffusion in regeneration of nanoceria redox properties after oxidation should provide a strong limitation for its biological antioxidant activity. The high number of papers dedicated to the self-regenerative antioxidant activity of nanoceria, however, omitted the question of time delay required for nanoceria to renew its antioxidant action after interaction with an oxidant molecule. In order to confirm the influence of relatively slow diffusion stage on the recovery of antioxidant ability of nanoceria we have used well-known ACS Paragon Plus Environment

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coumarin test 29 developed to estimate the content of ·OH hydroxyl radicals which are known as the most harmful type of ROS formed in biological systems. Water colloidal solutions of 2 nm and 10 nm nanoceria were irradiated by X-ray tube during 30 min at 37 ̊C. Splitting of water molecules under X-ray irradiation leads to formation of hydroxyl radicals: H2O→H·+·OH. In the Figure 6a and 6b the content of hydroxyl radicals after water irradiation determined by intensity of hydroxycoumarin fluorescence was taken as 100%. In the presence of nanoceria coumarin oxidation by ·OH radicals is strongly suppressed - 45 % of control for 2 nm nanoceria (Figure 6a) and 21 % for 10 nm nanoceria (Figure 6b). Such scavenging of ·OH radicals by nanoceria is well-known and its dependence on the Ce3+/Ce4+ ratio ions in nanoceria was shown elsewhere.30 In agreement with our predictions, the addition of hydrogen peroxide 30 minutes before irradiation of colloidal solution of nanoceria leads to sufficient hampering of nanoceria antioxidant action. For 2 nm nanoceria addition of H2O2 leads to stronger suppression of nanoceria antioxidant activity than for 10 nm nanoceria. As can be seen from luminescence spectra (Figure 3a and Figure 4a), for 2 nm nanoceria H2O2 addition leads to stronger decrease of Ce3+/Ce4+ ratio as compared to 10 nm nanoceria, and, so, to stronger suppression of antioxidant activity which correlates with presence and availability of Ce3+ sites on the nanoceria surface. These results are also consistent with the ones obtained from nanoceria luminescence spectra at 37 ̊C (Figure 5) showing that 30 minutes after oxidation only a small part of Ce4+ are reduced back to Ce3+ ones. Full recovery of the antioxidant properties of nanoceria required about 24 hours i.e. the same time at which the initial Ce3+/Ce4+ ratio corresponding to equilibrium concentration of oxygen vacancies was recovered.


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4. CONCLUSIONS In conclusion, the spectroscopic studies of Ce3+/Ce4+ ratio on the different stages of nanoceria-oxidant interaction have shown the key role of oxygen diffusion in the redox processes for ceria nanoparticle. Analysis of nanoceria antioxidant activity during ·OH scavenging has shown that oxygen diffusion is a sufficient limiting factor in the processes of self-regeneration of antioxidant properties of ceria nanoparticles after oxidation. Both size reduction and temperature increase lead to increase of the rate of oxygen diffusion and, so, facilitate regeneration of antioxidant properties of nanoceria.


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ASSOCIATED CONTENT Supporting Information. Interpretation of luminescent spectra (PDF). AUTHOR INFORMATION Corresponding Author *(Y.V.M.) Phone: +38 057 341-01-49 E-mail: [email protected].

Notes The authors declare no competing financial interest.

Acknowledgement

We are grateful to Dr. Vladimir Klochkov and Mrs. Olga Sedyh (Institute for Scintillation Materials, Kharkiv, Ukraine) for their help in obtaining ceria nanoparticles of different sizes.

Supporting Information Available: synthesis methods, experimental techniques, characterization of the samples, analysis of luminescence spectra at size and annealing atmosphere variation. This material is available free of charge via the Internet at http://pubs.acs.org.


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Limitations of Self-Regenerative Antioxidant Ability of Nanoceria

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