Magnetic and Electron Spin Relaxation Properties of - American

Mar 14, 2011 - and Maria Engström. †,‡. †. Center for Medical Image Science and Visualization (CMIV), Linköping University/US, SE-581 85 Linköping, ...
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Magnetic and Electron Spin Relaxation Properties of (GdxY1-x)2O3 (0 e x e 1) Nanoparticles Synthesized by the Combustion Method. Increased Electron Spin Relaxation Times with Increasing Yttrium Content  Hakan Gustafsson,*,†,‡ Maria Ahren,§ Fredrik S€oderlind,§ Jose M. Cordoba Gallego,§ Per-Olov K€all,§ Per Nordblad,^ Per-Olof Westlund,z Kajsa Uvdal,§ and Maria Engstr€om†,‡ †

Center for Medical Image Science and Visualization (CMIV), Link€oping University/US, SE-581 85 Link€oping, Sweden Department of Medical and Health Science (IMH), Division of Radiology, Link€oping University, SE-581 85 Link€oping, Sweden § Department of Physics, Chemistry and Biology (IFM), Link€oping University, SE-581 85 Link€oping, Sweden ^ Department of Engineering Sciences, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden  z Department of Chemistry, Umea University, SE-901 87 Umea, Sweden ‡

bS Supporting Information ABSTRACT:

The performance of a magnetic resonance imaging contrast agent (CA) depends on several factors, including the relaxation times of the unpaired electrons in the CA. The electron spin relaxation time may be a key factor for the performance of new CAs, such as nanosized Gd2O3 particles. The aim of this work is, therefore, to study changes in the magnetic susceptibility and the electron spin relaxation time of paramagnetic Gd2O3 nanoparticles diluted with increasing amounts of diamagnetic Y2O3. Nanoparticles of (GdxY1-x)2O3 (0 e x e 1) were prepared by the combustion method and thoroughly characterized (by X-ray diffraction, transmission electron microscopy, thermogravimetry coupled with mass spectroscopy, photoelectron spectroscopy, Fourier transform infrared spectroscopy, and magnetic susceptibility measurements). Changes in the electron spin relaxation time were estimated by observations of the signal line width in electron paramagnetic resonance spectroscopy, and it was found that the line width was dependent on the concentration of yttrium, indicating that diamagnetic Y2O3 may increase the electron spin relaxation time of Gd2O3 nanoparticles.

’ INTRODUCTION Image contrast in magnetic resonance imaging (MRI) is based on the intrinsic magnetic spin properties of the hydrogen nuclei present in different tissues in the body. Most important are the 1 H atoms of the water molecules, but even those of fatty tissues play a role. The creation of the MR image depends primarily on the hydrogen content (spin density) and the proton spin relaxation times T1 (spin-lattice relaxation, that is, the relaxation of the magnetization toward Boltzmann distribution) and T2 (spin-spin relaxation, that is, the decay of spin coherence in the transverse plane). Contrast agents are often used in MRI examinations of patients to provide additional image contrast r 2011 American Chemical Society

in certain tissues (e.g., tumors) by reducing the relaxation times of the hydrogen nuclei in the vicinity of the contrast agent. The most common contrast agents are gadolinium(III) chelates, which reduce both T1 and T2 (normally used to give additional image intensity in T1 weighted images) and superparamagnetic iron oxide nanoparticles (SPIOs), which reduce T2 much more than T1 (observed as a signal depletion in T2 weighted images as “negative” contrast).1 Received: November 30, 2010 Revised: February 15, 2011 Published: March 14, 2011 5469

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The Journal of Physical Chemistry C There is currently an increasing interest to use MRI for in vivo cell tracking and molecular imaging. MRI cell tracking is an important part of, for example, stem cell therapy2 where the migration of transplanted cells can be monitored in the body when labeled with a contrast agent. MRI cell tracking is commonly performed using SPIO nanoparticles due to their strong negative contrast, allowing reasonable contrast for a low number of transplanted cells. However, it is sometimes difficult to distinguish SPIO-labeled cells from other hypointense (dark) regions. The possibility to study molecular processes in vivo (molecular imaging) is an evolving technique to provide additional contrast to regions with high concentrations of a specific molecule of interest.3,4 Molecular processes in vivo can be studied if ligands with a specific affinity to the molecular target can be attached to a contrast agent. Recently, Spuentrup et al.5 reported their initial results on molecular imaging of thrombosis in humans using a fibrin-specific contrast agent. Gadolinium(III) chelates provide relatively low signal enhancement, 1,6 and consequently, a very large number of labeled cells or target molecules are required for an MRI study, limiting the number of clinical applications where MRI cell tracking and molecular imaging can be practically useful. It is, therefore, of interest to develop contrast agents capable of inducing a stronger positive contrast. A conceivable strategy to achieve this is to assemble a large number of paramagnetic atoms (e.g., Gd3þ) within small volumes, for example, nanoparticles,7 micelles, or emulsions.8 Another promising strategy for increased positive contrast is confining of, for example, gadolinium chelates inside nanoporous structures, such as silicon particles9 or apoferritin nanometric cages.10-12 Nanoparticles of Gd2O3 can carry high payloads of gadolinium to a molecular target, providing a strong local positive contrast, or to detect a small number of labeled cells. We have recently shown that Gd2O3 nanoparticles have a high relaxivity compared with the commonly used gadolinium(III) chelates,13-17 indicating that Gd2O3 particles are promising for future use in MRI cell tracking and molecular imaging. Paramagnetic relaxation enhancement,18 that is, the reduction of the relaxation times for hydrogen nuclei in the presence of an MRI contrast agent, is an effect of the interaction between the magnetic moment of the unpaired electrons in the contrast agent and that of the hydrogen nucleus of a water molecule in close vicinity to the agent. Relaxivity (denoted r1 and r2 for the spinlattice and spin-spin relaxivity, respectively) is the degree of paramagnetic relaxation enhancement for a given concentration of paramagnetic centers (e.g., Gd3þ ions) in units of mM-1 s-1. The relaxivity depends on several parameters, for example, the rotational correlation time of the complex, the water residence time in the first coordination sphere of the complex, the water exchange rate, and the electron spin relaxation times of the unpaired electrons.19 It is important to understand the contribution from each of these factors to the total relaxivity of a given system in order to be able to further optimize the system for maximum relaxivity in the magnetic field strength of interest.20,21 This can be achieved by theoretical modeling in combination with relaxivity measurements in nuclear magnetic dispersion (NMRD) experiments. In NMRD experiments, the relaxivity is measured as a function of the magnetic field and theoretically interpreted in combination with measurements of electron spin relaxation times using electron paramagnetic resonance (EPR).22,23 Key parameters for the gadolinium(III) chelate relaxivity at high field strengths (B g 1.5 T) are, typically, the rotation

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correlation time, hydration, and water exchange time, whereas the electron spin relaxation time is mostly important for the relaxation at lower magnetic field strengths.1 However, although there are theoretical models to describe the mechanisms behind the relaxivity of gadolinium chelates,18,19,24 it is more demanding to develop theoretical models for magnetically coupled systems, for example, Gd2O3 nanoparticles. It remains less certain whether the rotation correlation time, hydration, and water exchange time are key parameters for high relaxivity in coupled systems at field strengths relevant to MRI, or if the electron spin relaxation times have an impact on the relaxivity even at these fields. The aim of this work is, therefore, to investigate how the magnetic susceptibility and electron spin relaxation rate behavior of Gd2O3 nanoparticles changes, when the paramagnetic particles are diluted with increasing amounts of diamagnetic Y2O3. Gd2O3 and Y2O3 are isostructural cubic oxides and are believed to form a complete solid solution (GdxY1-x)2O3 covering the whole range 0 e x e 1. In a previous experimental and quantumchemical study performed by us, the surface interactions between organic molecules and RE2O3 (RE = Gd, Y) nanocrystals were studied and where diamagnetic Y2O3 was used as a proxy for paramagnetic Gd2O3.25 The (GdxY1-x)2O3 nanoparticles in the present study were synthesized by the combustion method and characterized by powder X-ray diffraction (XRD), transmission electron spectroscopy (TEM), thermogravimetry and mass spectroscopy (TG-MS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR).

’ EXPERIMENTAL METHODS Materials. Gadolinium(III) nitrate hexahydrate (Gd(NO3)3 3 6H 2 O, 99.9%) and glycine (H 2 NCH2 COOH, 99%) were purchased from Aldrich. Yttrium(III) nitrate hexahydrate (Y(NO3)3 3 6H2O, 99.9%) was purchased from Acros Organics. Sample Preparation. Nanoparticles of Gd2O3, Y2O3, and (GdxY1-x)2O3 with x varying between 0 and 1 were synthesized using the combustion method, as described by Zhang et al.26 Solutions containing 80 mL of deionized water, glycine (100 mM), and various concentrations of the rare earth nitrates (x mM and (1-x) mM for Gd and Y, respectively) were thoroughly mixed and boiled until all water had evaporated. After further heating, the mixtures self-ignite, and very fine powders of nanoparticles are formed. Pellets were pressed using a manual pellet press (tablet diameter, 3 mm; tablet height, approximately 1-2 mm; and weight, approximately 15-30 mg). X-ray Diffraction. (XRD) measurements were performed on as-prepared samples with a Philips PW 1820 powder diffractometer using Cu KR radiation (λ = 1.5418 Å, 40 kV, 40 mA). Transmission Electron Microscopy. High-resolution transmission electron microscopy (TEM) studies were performed with a FEI Tecnai G2 electron microscope, operated at 200 kV. Samples were prepared by dispersing (GdxY1-x)2O3 powders in ethanol. The samples were placed in an ultrasonic bath for 10 min, and 1-2 drops of the dispersion were dried on an amorphous carbon-coated copper grid. Thermal Analysis. Thermogravimetric (TG) analysis was performed using a Netzsch STA 449C Jupiter apparatus. Approximately 10 mg of material was loaded into a sintered Al2O3 crucible, and the temperature was increased from room temperature to 1100 C at a heating rate of 10 C min-1. The temperature was held at 200 C during 30 min. The measurements were conducted under a helium flow (100 mL min-1). In situ gas 5470

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Figure 2. Variation of the unit cell length a vs x of (GdxY1-x)2O3, where a = 10.64 ( 0.01 þ (0.23 ( 0.02)x Å.

Figure 1. X-ray powder diffraction patterns of (GdxY1-x)2O3 with x = 0 (a), 0.01 (b), 0.1 (c), 0.25 (d), 0.5 (e), 0.75 (f), 0.9 (g), and 1 (h).

analysis was performed using a mass spectrometer (Netzsch: QMS 403C A€eolos) through a heated transfer capillary. FT-IR Spectroscopy. Infrared spectroscopy measurements in transmission mode were performed on a PerkinElmer FT-IR Spectrum 1000 spectrometer. The samples were mixed with dry KBr and pressed into pellets. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Microlab 310F instrument equipped with a hemispheric analyzer and an unmonochromatized Al KR photon (1486.6 eV) source. The pressure in the analysis chamber was approximately 4  10-8 mbar during the measurements. Si(100) substrates for XPS measurements were cleaned for 10 min in a 5:1:1 mixture of Milli-Q water, 25% hydrogen peroxide, and 30% ammonia at 80 C and rinsed in Milli-Q water. Approximately 1 mg of (GdxY1-x)2O3 powder was dispersed in 1 mL of ethanol and placed in an ultrasonic bath for 20-30 min. Drops of the dispersions were dried on the Si substrates and immediately inserted into the instrument. All peak positions were calibrated according to the position of the Gd 3d5/2 peak. EPR Measurements. X-band EPR spectroscopy was performed using a BRUKER Elexsys E580 EPR spectrometer equipped with the standard resonator ER 4102ST. Tablets were measured in a WILMAD EPR sample tube with a 5 mm inner diameter and a flat bottom (Q-5M-6M-0-200m-FB) resting on an in-cavity pedestal to ensure equal tablet position for all measurements. Preliminary experiments indicated that no change in EPR line widths or other spectral changes could be detected for increasing applied microwave powers, which indicated that there was no microwave power saturation. Measurements were thus performed using an applied microwave power of 4 mW, 650 mT sweep width, 0.5 mT modulation amplitude, 20 ms time constant, and four 168 s sweeps added together for each spectrum. All EPR measurements were performed in room temperature. A measurement was performed on the empty sample tube and subtracted from each measured tablet spectra to eliminate weak EPR signals present in the empty resonator and sample tube. Reported EPR spectra are mass-normalized. Relative EPR signal

Figure 3. HRTEM images of (Gd0.01Y0.99)2O3 nanocrystals (a), and a (Gd0.5Y0.5)2O3 sample showing clear (222) lattice fringes with d ≈ 3 Å (b).

intensities were estimated by double integration of the massnormalized EPR spectra. No reference sample was used. The magnetic field strength was calibrated using a polycrystalline sample of 2,2-diphenyl picrylhydrazyl (DPPH) with g = 2.0037 ( 0.0002. Magnetic Susceptibility. Measurements of the dc-magnetic susceptibility were performed in a Quantum Design MPMS SQUID magnetometer. The magnetic moment of the samples was measured as function of temperature in a magnetic field of 1000 Oe.

’ RESULTS The aim of this study is to investigate changes in the magnetic susceptibility and the electron spin relaxation times in (GdxY1-x)2O3 nanoparticles as a function of the amount of diamagnetic yttrium within the oxide. Nanoparticles with increasing amounts of yttrium were prepared and thoroughly characterized by means of XRD, TEM, TG/DTA, FT-IR, and MPMS SQUID. XRD. Selected diffractograms of (GdxY1-x)2O3 for x values in the range of x = 0 to x = 1 are shown in Figure 1. The linear variation of the unit cell length (a) with the amount of Gd is presented in Figure 2. As the amount of Gd increases in the (GdxY1-x)2O3 solid solution, the unit cell length expands, and the 2θ of the diffraction peaks shifts to lower values. The relationship between a and x follows Vegard’s law, with a = 10.64 ( 0.01 þ (0.23 ( 0.02)x. TEM. Figure 3a shows a representative HRTEM image of aggregated, indicating the presence of more or less spherical 5471

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The Journal of Physical Chemistry C (Gd0.01Y0.99)2O3 nanoparticles. A single nanoparticle from the (Gd0.5Y0.5)2O3 sample is shown in Figure 3b. A histogram of the mean particle diameter as a function of the x value is shown in Figure 4. The histogram is based on the measurements of two orthogonal diameters for each clearly discernible nanoparticle. No significant size difference of the nanoparticles was observed for the four samples with x = 0.01, 0.10, 0.25, and 0.50, as based

Figure 4. Histogram showing the mean particle diameter for different (GdxY1-x)2O3 samples and the corresponding standard deviations. The mean particle diameter for all samples (5.9 nm) is also displayed.

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on at least 25 measured particles for each sample. The mean particle size for all samples was 5.9 ( 1.8 nm. The TEM studies showed that all samples consisted of agglomerated, highly crystalline (GdxY1-x)2O3 nanosized particles where (222), with a plane distance of about 3 Å, is the most commonly observed lattice plane (Figure 3). No significant size difference was observed between particles of different x values (Figure 4). TG-MS. Thermogravimetric analyses and supplementary gas evolution analyses of selected samples are shown in Figure 5a-d. A weight loss of about 20% is recorded for all samples, and the main gases evolved, as determined by mass spectroscopy, are H2O, NO, and CO2. Some of the H2O is lost already at the first heating ramp up to 200 C, and still more evolves when the temperature is further raised. Evolution of CO2 is observed at about 250 C, followed by NO at 350 C. FT-IR. IR spectra of three (Gd0.75Y0.25)2O3 samples (one assynthesized sample, one as-synthesized sample that was washed twice in H2O and then in MeOH, and one sample that was used in a TG run, i.e. heated to 1100 C) are shown in Figure 6, spectra a-c. While spectra a and b in Figure 6 are essentially identical, the spectrum of the heat-treated sample differs significantly from the other two in the ∼1500 and ∼550 cm-1 regions, as discussed below. XPS. The Gd 3d spectra of the (GdxY1-x)2O3 samples on Si(100) substrates are presented in Figure 7. The spectrum of the pure Gd2O3 sample (x = 1) was collected as a reference and is shown in Figure 7, spectrum e. The peak positions of the Gd 3d spin orbit doublets are 1188 eV (3d5/2) and 1220 eV (3d3/2).

Figure 5. Measured TG curves, and gas analysis evolution profiles for the decomposition of four (GdxY1-x)2O3 samples: x = 1 (a), x = 0.25 (b), x = 0.5 (c), and x = 0.75 (d). 5472

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Figure 6. FT-IR spectra of (Gd0.75Y0.25)2O3 as-synthesized (a), washed in MeOH and H2O (b), and heat-treated at 1100 C (c). The strong sharp peak at 1384 cm-1 is an artifact.

Figure 8. XPS spectra showing the Y 3p spin orbit doublet for (GdxY1-x)2O3 samples with x = 0.01 (a), 0.10 (b), 0.25 (c), 0.50 (d), and 1.0 (e).

Figure 7. XPS spectra showing the Gd 3d spin orbit doublet for (GdxY1-x)2O3 samples with x = 0.01 (a), 0.10 (b), 0.25 (c), 0.50 (d), and 1.0 (e).

Figure 9. EPR spectra for (GdxY1-x)2O3: x = 1.0, 0.90, 0.75, 0.50, 0.25, 0.1, 0.01, and 0.

The peak positions of the spin orbit doublet of Y 3p are 300.4 and 312.5 eV and are displayed in Figure 8. EPR. X-band EPR spectra for all samples (0 e x e 1) are shown in Figure 9. All spectra are centered on g ≈ 2. The pure Gd2O3 sample produced a broad, non-Lorentzian EPR spectrum, whereas those for the (GdxY1-x)2O3 particles with x = 0.5, 0.25, and 0.1 yielded more Lorentzian-shaped EPR peaks with slightly reduced EPR line widths. The reduced line width for x < 0.1 also makes it possible to observe three different sites with different geff (geff ≈ 6, geff ≈ 3, and geff ≈ 2). The mass-normalized EPR signal intensity (measured as double integrated spectra) showed a nonlinear dependence on the gadolinium concentration in the samples. Repeated measurements on tablets from different syntheses suggested a maximum for x = 0.8 ( 0.1. This result must be interpreted with care because the measurement error resulting from double integration of broad EPR lines may be large. The EPR line widths of the central line were found to be

dependent on the amount of Gd in the sample with reduced line widths for decreasing Gd content. Magnetic Measurements. The reciprocal magnetic susceptibility χ-1 as a function of temperature T (H = 1000 Oe) for a pure Gd2O3 nanocrystalline sample is shown in Figure 10. The samples showed only a paramagnetic (linear) response with increasing magnetic field at all temperatures. The measured μeff and θ values for (GdxY1-x)2O3 nanocrystals for different x values are reported in Table 1. It can be noted that, although the measured magnetic moments (μeff) of the samples, in general, are close to the expected value of 7.94 μB for Gd3þ 4f7 (the difference is usually 0 obeyed the CurieWeiss law χ¼

C T-θ

where χ is the magnetic susceptibility, T is the absolute temperature, C is the Curie constant, and θ is the Weiss constant. For pure, nanocrystalline Gd2O3, the expected weak antiferromagnetic behavior is observed with θ ≈ -2.9 K (Figure 10). This magnitude of θ is considerably smaller than that for the bulk compound, where values in the range of -17 ( 2 K have been reported.27,40 For still smaller Gd2O3 nanocrystals, θ tends toward zero, as recently observed by Fortin et al.14 As seen in Table 1, for low x values, θ similarly approaches 0 K, the value expected for free Gd3þ ions.41 The mean value of the magnetic moments (μeff) listed in Table 1 is 7.73 ( 0.15 μB and is close to the theoretical value for the 8S7/2 ground state of Gd3þ (L = 0), where μeff = 7.94 μB As earlier found by Schinkel and Van Amstel42 for bulk Gd2O3 and (Gd0.25Y0.75)2O3, μeff is not affected by dilution of the gadolinium content in the nanoparticles. As mentioned above, an interesting observation in Table 1 is that θ shows a nonlinear dependence on x, exhibiting a maximum magnitude (-4.19 ( 0.03 K) for x ≈ 0.9, rather than for x = 1. In the cubic RE2O3 (RE = Gd, Y) structure, RE1 is present in the positions 8b (site symmetry C3i) and RE2 in 24d (C2) in the Ia3 (No. 206) space group. Whereas the Gd1-O1 distances in Gd2O3 are all 2.300 Å, the Gd2-O1 distances vary from 2.284 to 2.408 Å. The coordination around the Gd1 atom can be described as a distorted octahedron and that of Gd2 as a distorted trigonal prism. (Structural details of Gd2O3 are given in Figure S2 and Table S1 in the Supporting Information.) The arrangement 5475

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The Journal of Physical Chemistry C of the Gd atoms resembles that of an fcc lattice with the Gd1 atoms in the corner of the cube and the Gd2 atoms approximately in the face-centered positions, as depicted in Figure 11. In the structure, there are only Gd1-O1-Gd2 and Gd2-O1Gd2 bonds, with the intergadolinium distances varying between 3.6 and 4.1 Å. The Gd1-Gd1 distance is 5.4 Å, with no connecting oxygen between the metals. As discussed above, Gd3þ is suggested to preferentially occupy the 24d sites at lower x values, and this deviation from an even distribution of the paramagnetic atoms over the crystallographic sites might offer an explanation for the “anomalies” (i.e., nonlinearity with x) of the θ magnitudes, and the EPR signal intensities.

’ CONCLUSIONS Nanoparticles of cubic (GdxY1-x)2O3 (0 e x e 1) were prepared by the combustion method and characterized with respect to particle size, crystallinity, chemical composition, EPR, and magnetic properties. The size of the nanoparticles was in the range of 5-7 nm for all samples. XRD data showed that the cubic unit cell length basically followed Vegard’s law, with a = 10.64 ( 0.01 þ (0.23 ( 0.02)x Å. It can be noted that the unit cell length of the end points (x = 0 and 1) are slightly larger than that observed for bulk Y2O3 and Gd2O3, respectively, possibly as a result of crystal strain. TG-MS measurements showed that the combustion reaction is incomplete and that, besides adsorbed water, about 15 ( 2 wt % of nitrogen- and carbon-containing species remain in the as-synthesized samples. Given that the chemical composition of the remaining species is close to that of the starting material (as indicated by the TG-MS data), it can be concluded that about 6% of the rare earth atoms are not incorporated into the (GdxY1-x)2O3 lattice. It is likely that, for high x values, these “extraneous” Gd3þ ions will represent a purely paramagnetic contribution to the magnetic moment of the samples. The chemical composition and relative Gd/Y ratio of the nanoparticles was corroborated by XPS. The magnetic behavior of the samples as measured in the temperature range of 5-50 K (H = 1000 Oe) followed the Curie-Weiss law perfectly, with μeff = 7.73 ( 0.15 μB for all samples and with θ approaching 0 K for the lowest x values, as expected. An interesting observation is that θ showed its maximum magnitude (-4.19 ( 0.03 K) for x < 1 (actually x ≈ 0.9). A similar observation of nonlinearity with x was made for the EPR signal intensity, which exhibited a maximum for x ≈ 0.8. A possible explanation for the observed “anomalies” is that the distribution of diamagnetic Y and paramagnetic Gd may not be completely statistical over the two crystallographic sites (8b and 24d) in the cubic RE2O3 structure with decreasing x value. The EPR line width of the central line in the EPR spectra was found to be dependent on the amount of yttrium in the sample, with decreasing line widths for increasing amounts of diamagnetic Y2O3, indicating that the electron spin relaxation time of Gd2O3 nanoparticles could be changed by dilution with diamagnetic Y2O3. Further experiments are planned to study if diluted Gd2O3 nanoparticles have an increased relaxivity in water solutions at clinical relevant magnetic field strengths (B0 g 1.5 T). ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1: measured TG curves and gas analysis evolution profiles for the decomposition of Gd(NO3)3 3 6H2O (a), and glycine (H2NCH2COOH) (b).

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The coordination around the Gd1 and Gd2 atoms in cubic Gd2O3 is given in Figure S2. Selected angles and atomic distances of Gd2O3 (PDF 43-1014) are given in Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: þ46 10 103 1475. E-mail: hakan.l.gustafsson@ liu.se.

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dx.doi.org/10.1021/jp111368t |J. Phys. Chem. C 2011, 115, 5469–5477