# Spherical Gd3TaO7 Particles with Phase-Stabilized Structure for Multimodal Bioimaging Application

Fernanda Hediger Borges, York E Serge-Correales, Robert Mauricot, Ana Beatriz Acosta, Franck Desmoulin, Carine Pestourie, Henrique Luis Piva, Antônio Claudio Tedesco, Marc Verelst, Rogéria Rocha Gonçalves

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Fernanda Hediger Borges, York E Serge-Correales, Robert Mauricot, Ana Beatriz Acosta, Franck Desmoulin, et al. Spherical Gd3TaO7 Particles with Phase-Stabilized Structure for Multimodal Bioimaging Application. ACS Applied Nano Materials, 2025, 8 (38), pp.18649-18661. 10.1021/acsman.5c03858. hal-05524832

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# Spherical Gd<sub>3</sub>TaO<sub>7</sub> Particles with Phase-Stabilized Structure for Multimodal Bioimaging Application

Fernanda Hediger Borges, York E. Serge-Correales, Robert Mauricot, Ana Beatriz Acosta, Franck Desmoulin, Carine Pestourie, Henrique Luis Piva, Antônio Claudio Tedesco, Marc Verelst, and Rogéria Rocha Gonçalves*

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Cite This: ACS Appl. Nano Mater. 2025, 8, 18649–18661

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Supporting Information

**ABSTRACT:** We report the polyol-mediated synthesis of spherical Gd₃TaO₇ particles with controlled morphology, phase purity, and luminescence, engineered as trimodal nanoprobes for bioimaging. The method yields monodisperse particles (~170 nm) that retain their spherical morphology even after high-temperature annealing at 800 and 1000 °C, overcoming the morphological instability and aggregation typically observed in tantalate-based systems. Structural characterization confirms the stabilization of the disordered cubic fluorite-related phase, with Eu³⁺ ions acting as structural and luminescent probes. The doped particles exhibit broad-band red emission and long-lived excited states, suitable for optical detection. Notably, luminescence is significantly enhanced via postsynthetic coordination with 2-thenoyltrifluoroacetone (TTA), enabling clear intracellular visualization through confocal microscopy. In parallel, undoped Gd₃TaO₇ particles show excellent performance as T₂-weighted MRI contrast agents (r₂ = 71.5 mM⁻¹ s⁻¹ at 7 T) and demonstrate strong X-ray attenuation for CT imaging, attributed to the presence of Gd and Ta high-Z elements. Confocal imaging confirms cellular uptake, while cytotoxicity assays in NIH-3T3 mouse fibroblasts demonstrate over 95% viability up to 1000 µg/mL, attesting to their biocompatibility. This work establishes Gd₃TaO₇-based nanoprobes as robust, compositionally engineered candidates for integrated MRI, CT, and optical imaging, with enhanced luminescence achievable via TTA sensitization for advanced bioimaging applications.

**KEYWORDS:** polyol-mediated synthesis, morphological control, multimodal imaging, magnetic resonance, computed tomography, optical imaging, luminescence

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## 1. INTRODUCTION

Over the last few decades, significant attention has been paid to oxides with fluorite (MO₂ or M₄O₈)-related structured materials. The variety of substitutions and occupations by different cations in place of M metal results in a large variety of structures and compositions.¹⁻⁴ The versatility of fluorite-derived structures (pyrochlores, weberites, and disordered cubic defect fluorite), with general formulas A₂B₂O₇ and A₃BO₇ (A = rare earth (RE), B = transition metal), results in catalytic, magnetic, dielectric, and piezoelectric properties, among others.⁵⁻⁸ They have a wide range of applications, such as solid oxide fuel cells (SOFCs),⁹ solid electrolytes,¹⁰ photocatalysts,⁵ high-temperature conductivity,¹¹ and one of the hot topics of the research of these materials, as thermal barrier coating (TBCs).¹²,¹³ The different possibilities of cation positions in these structures enable the modulation of some thermal properties and stability of the materials by incorporating other metals into their lattice.¹⁴,¹⁵

RE tantalates and niobates have been explored as potential optical materials. These materials are chemically and thermally

stable, with a relatively low phonon energy (~800 cm⁻¹), and high solubility of RE³⁺ ions,¹⁶ capable of incorporating a high RE³⁺ concentration, which makes them suitable hosts for photonic applications. Yttrium tantalates and niobates have shown a high quantum yield from both upconversion and downshifting processes, when compared with other oxides in the literature.¹⁷⁻¹⁹ Gd₃TaO₇ has gained attention as an optical material for white light-emitting diodes (WLEDs),²⁰,²¹ as well as its thermometric sensing properties based on its luminescence features, affording a maximum relative temperature sensitivity of approximately 0.92% K⁻¹ for Eu³⁺-doped Gd₃TaO₇.

Received: August 19, 2025

Revised: August 29, 2025

Accepted: September 2, 2025

Published: September 12, 2025

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RE₃TaO₇ has an anion-deficient fluorite-related structure. Three types of structures were proposed in the first publications for these compounds: ordered orthorhombic weberite, disordered pyrochlore, and disordered fluorite.²² Despite the growing research on these oxides, the attribution of their crystalline structures remains controversial.²³,²⁴ The problem lies in attributions based only on the ionic radius of the RE. If only this factor is considered as the determining condition, for larger, medium, and smaller RE ions, the material adopts the orthorhombic weberite Cmcm (for La-Nd), C222₁ or Ccmm (Nd-Ho, Y), and Fm$\overline{3}$m (Ho-Yb), respectively.⁷,²⁵ Despite the similar structural motifs shared by these three structures, they differ in the polyhedral coordination of both Ta and RE cations and the orientation of the TaO₆ polyhedra tilt axis.²⁶ In the weberite structures, long-range order can be seen, which is lost for the disordered defect fluorite structure.²⁶ However, factors other than the RE ionic size must be considered when attributing the crystalline phase. Some studies have reported defective fluorite structures for niobates and tantalates with medium-sized RE ions.⁵,¹¹,²⁷ In addition, the size of the cation and the synthesis conditions (such as temperature, pressure, and synthetic route) can also play an important role in the stabilization of the crystal structure.²⁸,²⁹ Another point to consider is the use of limited laboratory X-ray diffraction experiments to assess the crystalline phase of the formed compound,²⁶ and Raman spectroscopy, which can offer decisive insights into the definition of the crystalline phase, has rarely been employed.

As previously mentioned, the synthetic route is also critical for structural formation. Among the different methodologies, the most common for synthesizing RE tantalates and niobates is solid-state reactions.⁷,²³ Gadolinium tantalates and niobates synthesized by this route afforded orthorhombic Gd₃MO₇, as predicted by the structure based only on the medium-sized Gd³⁺ ionic radii. However, a few authors have reported the formation of the cubic Gd₃MO₇ fluorite structure with space group Fm$\overline{3}$m, such as Chesnaud and Ray,¹¹,²⁷ by preparing these materials via citrate, combustion, and sol-gel routes. Previous studies of our group also revealed that cubic Y₃TaO₇ was formed by the sol-gel method even when the predicted phase was assumed to be orthorhombic based on the ionic radii,¹⁸,²⁴ showing that the stabilization of the crystalline phases strongly depends on the processing conditions.

Another important aspect of the synthetic route is the morphology and size of particles. For some oxides, such as Nb₂O₅ (and Ta₂O₅), achieving uniform nanoscale particles is a significant challenge.³⁰ The fast reactivity of Nb and Ta alkoxides and the requirement of a postsynthesis annealing step at high temperatures to obtain the crystalline phase are among the reasons for the difficulty in obtaining nanoparticles from these materials. The formation of RE tantalates and niobate nanoparticles with controlled shapes is a complex process. Although the number of publications on these materials has increased, most of these studies have not reported on the morphology and size control of the particles. To the best of our knowledge, Du et al.³¹ were the only researchers to report niobates and tantalates up to 7.5 nm obtained by hydrothermal synthesis, although with irregular shapes. In this sense, the polyol method, also considered a high-temperature sol-gel process,³⁰ has been proven effective in designing RE₃TaO₇ with good control over size and shape.

In this study, we propose this novel synthesis of a cubic Gd₃TaO₇ (undoped and doped with Eu³⁺ ions) with structural,

morphological, and size control using a polyol-mediated method. To the best of our knowledge, there have been no reports on the synthesis of gadolinium tantalates using this methodology. In the polyol-mediated method, which is a very versatile method for the synthesis of binary and ternary oxides,³⁰ the polyol can act simultaneously as a ligand and stabilizer, coordinating the metal precursor and the particle surface, avoiding agglomeration, and ensuring controlled particle growth.³² In addition, polyol also acts as a chelating agent, forming a new tantalum precursor (tantalum glycolate), which is more humidity resistant and less reactive than tantalum ethoxide.³³

Controlling the size and morphology of these RE₃TaO₇ materials paves the way for their use in several biomedical applications. In the current context of seeking imaging resolution improvement in bioimaging tools and diagnosis, multimodal imaging (the combination of two or more imaging modalities) has become an attractive solution to benefit from the advantages of different techniques.³⁴,³⁵ Regarding the goal and challenge of obtaining good contrasting capacities in different techniques in a single probe,³⁶ Gd³⁺-based materials have been widely explored because of Gd³⁺ seven unpaired electron (4f⁷) configuration and high atomic number (Z = 64), affording great paramagnetic and X-ray absorption properties.³⁶,³⁷ Among these materials, Gd₃TaO₇ offers promising characteristics, by combining its high atomic number and high density,³⁸ as a multimodal imaging contrast agent (CA).

In this study, we explored the possibility of controlling the size, morphology, and structure of gadolinium tantalate particles, which is one of the challenges of these materials. To the best of our knowledge, this is the first size/morphology-controlled synthesis of well-dispersed Gd₃TaO₇ particles with suitable scale for cellular uptake and safe performance in biological applications. Herein, we show the stabilization of the cubic Gd₃TaO₇ single crystalline structure, using Raman spectroscopy to shed light on phase characterization. We also investigated the luminescent properties of the Eu³⁺-doped sample, where the Eu³⁺ ions acted as structural probes, providing more information about the intrinsic structural disorder of the host lattice, as well as its absolute quantum yield. Moreover, the synthesized red luminophore could be advantageous for use in bioimaging as a probe *in vitro* and *in vivo*.

The application of this compound as a multimodal imaging probe was evaluated using different imaging techniques, where its relaxivity values and the possibility of IRM applications, as well as in CT. Cytotoxicity tests revealed the viability of using these particles in biological applications. In this sense, we present herein a very interesting system with morphological, size, and structural control showing that these particles have great potential as multimodal imaging probes for *in vivo* tumor tracking in future technological applications.

## 2. EXPERIMENTAL PROCEDURES

### 2.1. Gd₃TaO₇ Particles Synthesis.

**2.1.1. Preparation of Gadolinium and Tantalum Precursors.** Gadolinium acetate was prepared by dissolving ~5 g of Gd₂O₃ (Rhône-Poulenc, 99.99%) in an excess of acetic acid solution (500 mL of deionized water and 12 mL of acetic acid glacial), using a flask equipped with a reflux condenser. The suspension was heated to 90 °C in an oil bath with vigorous stirring until the oxide was completely dissolved, and the solution was then vacuum-filtered twice to remove impurities. Next, the solution was heated again to evaporate water. Finally, the resulting precipitate was air-dried overnight at 70 °C in an oven.
<figure><img src="image_11.png" /><figcaption>**Figure 1.** Synthetic route for preparation of gadolinium tantalates scheme.</figcaption></figure>

A tantalum precursor, which is more stable than tantalum ethoxide was used in this synthesis, which was prepared as described elsewhere,³⁹ based on the method reported by Li et al.⁴⁰ A 46 mM stock solution of tantalum glycolates was prepared by adding tantalum ethoxide (Sigma-Aldrich, 99.98%) to ethylene glycol (EG) (Sigma-Aldrich, 99.0%). The solution was bubbled with argon for approximately 30 min to purge oxygen and water in the reaction mixture. The resulting mixture was vigorous stirred overnight. Because tantalum ethoxide is very sensitive and reactive to humidity, this step is important for the formation of a tantalum precursor more resistant to hydrolysis. The stock solution could be kept for a few days under stirring without the formation of precipitates resulting from tantalum hydrolysis.

### 2.1.2. Undoped and Eu³⁺-Doped Gd₃TaO₇ Particles.
In a typical synthesis of undoped gadolinium tantalates (Gd₃TaO₇), 12.5 mL of the stock solution of tantalum glycolate was added to a mixture containing 37.5 mL of EG and 1.734 mmol of Gd acetate, under magnetic stirring in a three-necked round-bottom flask. Next, the temperature of the mixture was increased at a rate of 5 °C/min to 190 °C and maintained for 2 h under reflux. The resulting white slurry was cooled to room temperature and 50 mL of ethanol was subsequently added. The solid was separated by centrifugation, washed 3 times with ethanol and dried at 70 °C. Finally, the powder was annealed at 800 and 1000 °C for 2 h under ambient atmosphere, using a heating rate of 5 °C/min, followed by natural cooling to room temperature. Eu³⁺-doped Gd₃TaO₇ sample (Gd₃TaO₇:Eu³⁺) was synthesized following the same methodology, where Eu³⁺ acetate (Aldrich, 99.9%) was added after the Gd³⁺ precursor. The sample was doped with 1 mol % Eu³⁺ (0.01734 mmol) with respect to Gd³⁺ content (Gd₂.₉₇Eu₀.₀₃TaO₇). The as-prepared samples were labeled as GTO and GTOE for the undoped and Eu³⁺-doped samples, respectively, and as GTO_800, GTO_1000, GTOE_800, and GTOE_1000 after annealing at 800 and 1000 °C. **Figure 1** presents a schematic representation of the described synthesis.

## 2.2. Characterization.
2.2.1. Powder Analysis.
2.2.1.1. Transmission Electron Microscopy (TEM). Microscopy images were obtained using a CM20 microscope operating at 200 kV. ImageJ software was used to calculate the particle size distribution of ~300 particles using the TEM images.

2.2.1.2. X-ray Diffraction (XRD). The samples were characterized by X-ray using a Bruker D8 Advance diffractometer in a Bragg-Brentano configuration equipped with a linear detector and a Kα Cu source, with λ = 1.5406 Å in the range of 10° to 90°.

2.2.1.3. Vibrational Spectroscopy. Raman spectra of the GTO_800 and GTO_1000 samples were performed at room temperature using a LabRAM HR Evolution (Horiba) micro-Raman in the 100–1500 cm⁻¹ range, with a 0.3 cm⁻¹ resolution, using a laser

He-Ne ($\lambda$ = 633 nm) as the excitation source. For the GTOE_1000 sample, the Raman spectrum was also acquired in the 50–1200 cm⁻¹ range with an acquisition time of 70 s per scan, over two cycles, using a Horiba Scientific Xplora Plus Raman platform, mounted in an Olympus BX43F microscope ($\lambda$ = 785 nm).

2.2.1.4. Visible Photoluminescence. The measurements were acquired using a spectrofluorometer Fluorolog 3 Horiba Scientific (model FL3-22), with double monochromators on excitation and emission and photomultipliers Hamamatsu R928 for detection in the visible range. For the emission/excitation spectra the excitation source was a 450 W Xe lamp, and a 450 W Xe pulsed lamp was used for lifetime measurements.

2.2.1.5. Quantum Yield Measurements. The absolute emission quantum yields of GTOE_800 and GTOE_1000 were measured at room temperature using Quantaurus-QY Plus UV-NIR absolute PL (QY C13534-11, Hamamatsu). The equipment uses a 150 W xenon lamp as the light source, a monochromator for wavelength selection, and an integrating sphere as the sample chamber.

2.2.2. Multimodal Imaging Applications (Proof of Concept).
2.2.2.1. MRI Measurements. First, an aqueous suspension of ~10 mg/g NPs stock solution was prepared. Gelatin phantoms (2 mL flat-bottom plastic vials, 6 mm in diameter) were then produced by diluting this suspension into 10 wt % gelatin with different concentrations, ranging from 0.10 to 1.01 mg/g for the GTO_1000. Gd in the GTO_1000 sample was quantified by EDS using two different instruments (Supporting Information (SI)). This approach was necessary because the sample was completely insoluble in any acid, making other possible analyses unfeasible, such as inductively coupled plasma-optical emission spectroscopy (ICP-OES). MR measurements were performed on a Bruker Pharmascan 70/16 system (Bruker Biospin, Ettlingen, Germany) equipped with a 7 T magnet and a 16 cm horizontal bore. A transmit-receive volume coil with an inner diameter of 70 mm was used for simultaneous MRI acquisition of nine vials. MRI relaxometry was used to estimate the $T_2$. $T_2$ was determined using a multispin echo sequence with 30 echo times (TE) and a long repetition time (TR > 5 × $T_1$). The acquisition parameters were as follows: 30 images were acquired with TE ranging from 6.5 to 188 ms in 6.5 ms steps. The field of view was set to 65 × 32 mm with a matrix size of 65 × 32 data points. A slice thickness of 6 mm was used, with an averaging factor of 8 and a repetition time (TR) of 7000 ms. An exponential fit was applied to the signal intensities of the images for $T_2$ quantification using Paravision 6 software (Bruker).

2.2.2.2. X-ray Tomography Scan. Similarly, 10 wt % gelatin phantoms were prepared by diluting a ~90 mg/g NPs stock solution with concentrations ranging from ~5 to 40 mg/g for GTO_1000 and GTOE_10000, respectively. Cone-beam CT scans were performed
using the CT component of a nanoScan PET/CT (MEDISO Ltd., Hungary) *in vivo* scanner. Acquisition parameters were a semicircular trajectory with 720 projections/rotation, 300 ms exposure time, 70 kV tube voltage, 240 µA tube current and 1.2 optical magnification. The data were reconstructed using a 125 µm isotropic voxel-matrix.

**2.2.3. Cells Assays.** 2.2.3.1. *Cell Culture.* NIH-3T3 fibroblasts were obtained from the American Type Culture Collection (ATCC-CRL 1658). Cells were cultured in Dulbecco's Modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin-streptomycin in a humidified incubator at 37 °C under a 5% CO₂ atmosphere.

**2.2.3.2. Cell Viability.** Rezasurin assay was used to assess cell viability after incubation with particles for 3 h. Briefly, Rezasurin solution in PBS was added to each well and incubated for 4 h. The fluorescence emission (λ<sub>ex</sub> = 560 nm, λ<sub>em</sub> = 590 nm) of each well was measured using a Cytation 5-Cell Imaging Multi-Mode Reader (Biotek Instruments, Winooski, VT, USA), and the results were plotted as a percentage of cell survival relative to the control.

**2.2.3.3. Cell Imaging.** To analyze the intracellular interaction between cells and particles, confocal microscopy was performed. Briefly, cells (20,000 cells) were seeded onto glass coverslips and then incubated with 1000 µg/mL colloidal suspension for 3 h. After treatment, cells were washed with PBS, fixed, and permeabilized. The slides where then stained with 2-thenoyltrifluoroacetone (TTA) for an increase in fluorescence signal of Eu³⁺, Phalloidin-Alexa 488 for evaluate the F-Actin filaments, and mounted with glass slides using ProLong Gold Mounting Medium with DAPI (Life Technologies) as counterstaining, slides were observed under a Leica TCS-SP8 confocal laser scanning microscope (Leica Microsystems, Wetzlar, Germany), signals were detected at the excitation and emission wavelengths as follows: Eu³⁺ particles (405 nm, 600–670 nm), phalloidin–to stain actin filaments of the cytoskeleton (488 nm, 500–520 nm), and DAPI–to stain cell nuclei (405 nm, 440–460 nm), respectively.

## 3. RESULTS AND DISCUSSION

**3.1. Morphological Characterization.** Undoped (GTO) and Eu³⁺-doped Gd₃TaO₇ (GTOE) particles were prepared using ethylene glycol as a solvent, resulting in spherical particles, as confirmed by transmission electron microscopy (TEM). Controlled hydrolysis of the precursors leads to a consistent nucleation and growth process, ensuring the spherical morphology of the particles, which in turn helps control their sizes and prevent agglomeration.⁴¹

Figure 2d shows the TEM micrography of the as-prepared GTOE sample, confirming the formation of spherical particles with an average diameter of 235 nm (standard deviation of 67 nm). The particles exhibited consistent morphology and shape, although with a moderate degree of polydispersity, as confirmed by their particle size distribution (Figure 2a). The mean size decreased to 173 and 170 nm for the samples annealed at 800 and 1000 °C for 2 h, with standard deviations of approximately ± 55 and ± 53 nm, respectively (Figure 2b,c). Even at high annealing temperatures, the particles retained their spherical shapes. Similarly, the diameters of the as-prepared undoped sample and its corresponding annealed sample (GTO_1000) were approximately 204 and 164 nm, respectively (standard deviations of ±56 and ±50 nm, respectively), as shown in Figures S1 and S2 (Supporting Information, SI). Dynamic light scattering (DLS) measurements confirmed the absence of agglomerate formation. Additionally, these measurements indicated that the annealed samples exhibited good colloidal stability, with average particle sizes around 200 nm, corresponding to well-dispersed particles remaining in suspension (Figures S3 and S4, SI). Table S1

<figure><img src="image_12.png" /><figcaption>**Figure 2.** (a, b, c) Size distribution histogram and (d, e, f) TEM micrograph (average size ± standard deviation) of (a, d) as-prepared GTOE, (b, e) GTOE_800 and (c, f) GTOE_1000, respectively.</figcaption></figure>

summarizes the size values obtained by TEM and DLS for GTO and GTOE samples.

**3.2. Structural and Vibrational Properties.** Figure 3 shows the X-ray diffractograms of the undoped and Eu³⁺-doped samples, annealed at 800 and 1000 °C. The diffractograms of the as-prepared samples (not shown here) were amorphous, so that the annealing step was necessary to obtain the crystalline phase. The annealed samples were crystallized in a pure single crystalline phase and the diffraction patterns were attributed to the cubic defect fluorite-related structure Gd₃TaO₇, space group Fm$\overline{3}$m (ICSD #109055). For other RE tantalates possible structural attributions, such as pyrochlore and orthorhombic, minor peaks (relative intensities less than 5%) can be detected.⁸ No additional peaks related to fluorite-related superstructures were observed in these diffraction patterns apart from the main peaks, confirming the formation of a cubic phase. In this structure, Gd³⁺ and Ta⁵⁺ randomly share the same symmetry site (4a Wyckoff position) at a 3:1 ratio;⁶,⁴³ one oxygen vacancy is created per unit cell distributed at a concentration of 1/8 on the anion site.¹,⁶ Subramani et al. described the surroundings of Gd³⁺/Ta⁵⁺ as 7 oxygen +1 vacancy (occupying the 8c position) coordination.⁴⁴ All of these factors contribute to the intrinsic disorder in defect fluorite.⁴⁵

The attribution of this phase is not easy to determine and the divergence in the literature dates back several decades, since the 1960s, for both RE tantalates and niobates, where they are assigned as weberite-, pyrochlore-, or cubic defect fluorite-type structures.⁸ This divergence in the literature relies on attributions based on the RE ionic radii. It is well established in literature that, in general, only the size of the RE ion influences which structure it is going to crystallize. For Gd³⁺, which is classified as a medium-sized RE³⁺, the expected assigned lattice based on the literature was assumed to have an orthorhombic structure.³,⁷,⁴⁶ However, from the diffractograms obtained in this work, we do not see any additional diffraction peaks that can confirm the formation of a superlattice structure, which can be indexed to the orthorhombic phase. Other studies have proven that, besides the RE ionic size, processing conditions, such as different synthetic routes and dopant concentrations, can also be determinants of the
<figure><img src="image_13.png" /><figcaption>**Figure 3.** (a) X-ray diffractograms of the GTO and GTOE samples annealed at 800 and 1000 °C. (b) Unit cell of cubic Gd<sub>3</sub>TaO<sub>7</sub> was created using VESTA software.<sup>42</sup></figcaption></figure>

resulting structural crystallization.<sup>24,44,46</sup> Gadolinium tantalates do have a weberite (C222<sub>1</sub>) ↔ cubic defect fluorite (Fm$\overline{3}$m) phase transition demonstrated, but this is confirmed for temperatures above 1800 K.<sup>47,48</sup> Using this synthetic methodology, it was possible to stabilize this cubic structure at lower temperatures. Different papers criticize this problem in literature, since the structure is directly related to physicochemical properties, as well as, in this work, the luminescent properties. The same issue was observed for the Y<sub>3</sub>TaO<sub>7</sub> compounds and is discussed in detail in ref 24. In addition to the diffractograms, our attribution of cubic Gd<sub>3</sub>TaO<sub>7</sub> was also supported by the Raman spectra (Figure 4).

<figure><img src="image_14.png" /><figcaption>**Figure 4.** Raman spectra of GTO_800, GTO_1000, GTOE_800, and GTOE_1000.</figcaption></figure>

**Figure 4** illustrates the Raman spectra of the GTO and GTOE samples annealed at 800 and 1000 °C. First, the crystalline groups of these samples cannot be attributed to orthorhombic Gd<sub>3</sub>TaO<sub>7</sub>, Ccmm, or the C222<sub>1</sub> space group. The first space group possesses 27 Raman-active modes (8A<sub>8</sub> + 8B<sub>1g</sub> + 5B<sub>2g</sub> + 6B<sub>3g</sub>), whereas the latter has 63 Raman-active modes (14A + 19B<sub>1</sub> + 17B<sub>2</sub> + 16B<sub>3</sub>).<sup>29</sup> The Raman spectra herein do not resemble those reported by Yang et al.,<sup>20</sup> who obtained C2<sub>2</sub>2<sub>1</sub> orthorhombic Gd<sub>3</sub>TaO<sub>7</sub>. Instead, these spectra are very similar to those observed for other cubic fluorite-related tantalates (RE = Y, Gd, Sm, Yb, and Lu).<sup>5,13,24,46,49,50</sup> Raman spectrum was expected to be broader due to the structural disorder of this crystalline phase and confirm the formation of the cubic phase.

In contrast, for the Fm$\overline{3}$m space group, only a single active Raman mode (F<sub>2g</sub>) was expected to be observed. However,

two broad bands are observed in these spectra. Although no additional peaks attributed to the pyrochlore structure were detected within the sensitivity of the XRD equipment, these Raman spectrum profiles revealed the existence of lower-symmetry structural nanodomains in the cubic Fm$\overline{3}$m lattice.<sup>29</sup> This can be explained by the short-range order generated in the structure, which is typical of oxygen-deficient compounds, as confirmed by Kovyazina et al.<sup>51</sup> In A<sub>2</sub>B<sub>2</sub>O<sub>7</sub>/A<sub>3</sub>BO<sub>7</sub> compounds, the tendency of cations A (larger cation) and B (smaller cation) to be in a different coordination than that in the disordered fluorite promotes local ordering of the vacancies,<sup>44,52</sup> but is not transmitted to the whole structure (long-range disorder). The oxygen vacancies are, then, ordered as in a cubic pyrochlore structure (space group Fd$\overline{3}$m), which has six Raman-active modes (A<sub>1g</sub> + E<sub>g</sub> + 4F<sub>2g</sub>). Because of this short-range ordering, more than one band is observed in the spectra. However, long-range intrinsic disordering of the cubic fluorite structure is reflected in the spectra as broad bands. The band centered at approximately 770 cm<sup>-1</sup> corresponds to the F<sub>2g</sub> mode, which is assigned to the Ta—O stretching mode.<sup>49</sup>

**3.3. Luminescent Properties (Eu<sup>3+</sup>-Doped Samples).** Investigation of Eu<sup>3+</sup> ion luminescence is an insightful tool for attributing of the crystalline phase and exploring the local structure of this disordered Gd<sub>3</sub>TaO<sub>7</sub> host. Figure 5a exhibits the excitation spectra of GTOE samples annealed at 800 and 1000 °C, monitoring through Eu<sup>3+</sup> <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> transition. These spectra show several narrow peaks attributed to f–f transitions and a broad band monitored at smaller wavelengths than 300 nm. The broad band can be assigned to charge transfer (CT) between O<sup>2−</sup> and Ta<sup>5+</sup> of the host and between O<sup>2−</sup> filled 2p shell and Eu<sup>3+</sup> partially filled 4f shell.<sup>19,53,54</sup> For Eu<sup>3+</sup>-doped cubic Y<sub>3</sub>TaO<sub>7</sub>, the charge transfer band (CTB) is centered at approximately 270 nm, owing to the host absorption band.<sup>19,54</sup> On the other hand, Yang et al.<sup>20</sup> showed that, for the undoped orthorhombic Gd<sub>3</sub>TaO<sub>7</sub>, the O<sup>2−</sup> → Ta<sup>5+</sup> CT appears in the excitation spectrum as a broad band ranging between 250–365 nm, while the introduction of Eu<sup>3+</sup> gives rise to O<sup>2−</sup> → Eu<sup>3+</sup> CT, with a blue shift of the band to the region 230–330 nm. The narrow bands located around 274 nm were ascribed to Gd<sup>3+</sup> <sup>8</sup>S<sub>7/2</sub> → <sup>6</sup>I<sub>9</sub>, and at 307, 313, and 321 nm to Gd<sup>3+</sup> <sup>8</sup>S<sub>7/2</sub> → <sup>6</sup>P<sub>1</sub> transitions, respectively, while the narrow bands centered at 362, 393, 413, 464, and 531 nm were attributed to Eu<sup>3+</sup> <sup>7</sup>F<sub>0</sub> → <sup>5</sup>D<sub>4</sub>, <sup>7</sup>F<sub>0</sub> → <sup>5</sup>L<sub>6</sub>, <sup>7</sup>F<sub>0</sub> → <sup>5</sup>D<sub>3</sub>, <sup>7</sup>F<sub>0</sub> → <sup>5</sup>D<sub>2</sub>, and <sup>7</sup>F<sub>0</sub> → <sup>5</sup>D<sub>1</sub> transitions, respectively.

The emission spectra of GTOE_800 and GTOE_1000 at different excitation wavelengths are presented in Figure 5b.
<figure><img src="image_15.png" /><figcaption>**Figure 5.** (a) Excitation with fixed emission at 612 nm and (b) emission spectra, (c) luminescence decay curves for the Eu³⁺ ⁵D₀ excited state, and (d) CIE 1931 chromaticity diagram of the GTOE_800 and GTOE_1000 samples under different excitation wavelengths.</figcaption></figure>

<table><caption>**Table 1.** Asymmetric Ratio (⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ Intensity Ratio), ⁵D₀ Excited State Lifetimes (τ<sub>EXP</sub>) (±0.01 ms) and Intrinsic Quantum Yield (Q<sub>Eu</sub><sup>EXP</sup>), Judd–Ofelt Parameters, and Absolute Quantum Yield Values (%) of GTOE_800 and GTOE_1000 under Excitation at Different Wavelengths (265 nm-CTB, 31³ nm-Gd³⁺: ⁸S<sub>7/2</sub> → ⁶I<sub>7/2</sub>, and 39³ nm- Eu³⁺: ⁷F₀ → ⁵L<sub>6</sub>)</caption><thead><tr><th>annealing temperature (°C)</th><th>λ (nm)</th><th>5D<sub>0</sub> → 7F<sub>2</sub><br>5D<sub>0</sub> → 7F<sub>1</sub></th><th>Ω<sub>2</sub> (10<sup>-20</sup> cm<sup>2</sup>)</th><th>Ω<sub>4</sub> (10<sup>-20</sup> cm<sup>2</sup>)</th><th>A<sub>T</sub> (s<sup>-1</sup>)</th><th>τ<sub>RAD</sub> (ms)</th><th>τ<sub>EXP</sub> (ms)</th><th>Q<sub>Eu</sub><sup>EXP</sup> (%)</th><th>absolute quantum yield (%)</th></tr></thead><tbody><tr><td rowspan="3">800</td><td>265</td><td>4.2</td><td>6.8</td><td>3.3</td><td>541</td><td>1.85</td><td>1.81</td><td>98</td><td>8.4</td></tr><tr><td>313</td><td>4.1</td><td>6.6</td><td>3.2</td><td>528</td><td>1.89</td><td>1.71</td><td>90</td><td>4.6</td></tr><tr><td>393</td><td>4.1</td><td>6.7</td><td>3.6</td><td>537</td><td>1.86</td><td>1.55</td><td>83</td><td>11.1</td></tr><tr><td rowspan="3">1000</td><td>265</td><td>3.9</td><td>6.3</td><td>3.3</td><td>514</td><td>1.95</td><td>1.85</td><td>95</td><td>10.9</td></tr><tr><td>313</td><td>3.8</td><td>6.1</td><td>3.2</td><td>502</td><td>1.99</td><td>1.82</td><td>91</td><td>4.6</td></tr><tr><td>393</td><td>3.9</td><td>6.2</td><td>3.5</td><td>514</td><td>1.94</td><td>1.85</td><td>85</td><td>6.9</td></tr></tbody></table>

Here, broad emission bands correspond to ⁵D₀ → ⁷F<sub>1</sub> (J = 0, 1, 2, 3, and 4) Eu³⁺ transitions. The emission spectra obtained for this sample are similar to those of other Eu³⁺-doped RE niobates and tantalates that crystallize as cubic defective-fluorites, such as Y₃NbO₇<sup>55,56</sup> and Lu₃TaO₇.<sup>31,57</sup>

In a previous work,<sup>24</sup> we pointed out that there is a lack of studies correlating the structural and luminescent properties, especially in cases that diverge from the attribution of ionic radii theory. To the best of our knowledge, there are no publications on pure cubic defective-fluorite Eu³⁺-doped Gd₃TaO₇ with correctly assigned luminescence spectra. In this cubic fluorite-related structure (Fm$\overline{3}$m), Gd³⁺ is in a 4a Wyckoff position, in point group symmetry O<sub>h</sub>.<sup>7,29</sup> However,

the intrinsic disorder present in this structure, where the oxygen vacancies are randomly distributed, creating slightly different symmetry sites, leads to a lowering of the site symmetry as well as inhomogeneous broadening of the emission bands, resembling the behavior observed in glass materials.<sup>56</sup> The observation of ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₂, both electric dipole (ED) transitions, especially a higher relative intensity of the ⁵D₀ → ⁷F₂ transition over the ⁵D₀ → ⁷F₁ transition, indicates that Eu³⁺ ions are in a lower symmetry site and without an inversion center. When Eu³⁺ ions were incorporated into orthorhombic Gd₃TaO₇, the emission spectrum was completely different, with more defined Stark components, as shown by Yang et al.<sup>20</sup> This author observed
<figure><img src="image_16.png" /><figcaption>**Figure 6.** (a) $T_2$ relaxivity as a function of Gd concentration of aqueous gelatin suspensions of GTO\_1000 sample and (b) $T_2$-weighted MRI of these particles at 7 T increasing the $Gd^{3+}$ concentration (ranging from 0 up to 3.64 mM) (NPs concentration ranging from 0 to 1.01 mg/g).</figcaption></figure>

two splittings for $^5D_0 \rightarrow ^7F_0$, which indicates that $Eu^{3+}$ is in two different symmetry sites in this host ($Gd^{3+}$ can occupy a 7- and 8-fold coordination in this structure). In addition, $^5D_0 \rightarrow ^7F_4$ exhibited a high relative intensity. In this sense, $Eu^{3+}$ luminescence spectra can also be a conclusive tool that shows the formed crystalline phase, whether orthorhombic or cubic $Gd_3TaO_7$.

By changing the excitation wavelength (265 nm-CTB, 313 nm-directly in a $Gd^{3+}$ transition, $^8S_{7/2} \rightarrow ^6P_J$, and 392.5 nm-$Eu^{3+}$ f-f transition $^7F_0 \rightarrow ^5L_6$), the samples exhibited the same broad spectral profile. Even though the disorder in the lattice leads to a large bandwidth, $Eu^{3+}$ occupies the same average symmetry sites, as demonstrated by the insignificant variation in the asymmetric ratio values with changing wavelengths, as presented in Table 1. This same average occupation is also evidenced by the single exponential behavior of the $^5D_0$ excited-state emission decay curves of the GTOE\_800 and GTOE\_1000 samples upon excitation at the same wavelengths (Figure 5c). The curves were fitted using a first-order exponential and Table 1 displays the calculated lifetime values.

The annealing temperature did not lead to a significant increase in lifetime at any excitation wavelength. When the samples are excited at directly at $Eu^{3+}$ f-f transition (392.5 or 464 nm), the obtained value was ~1.50 ms for both annealing temperatures. For excitation under CTB, the lifetime values were higher (1.8-1.9 ms). It is known that the CTB intermediate state in near-resonance with the $^5D_0$ emitting level that mediates energy transfer causes an increase when compared to the lifetime measured by direct f-f transition excitation.<sup>19,58</sup> A smaller increase in the lifetime values when the sample is excited via $Gd^{3+}$ ions, which means another path via $Gd^{3+}$ energy transfer to $Eu^{3+}$, can be highlighted. For the samples annealed at 1000 °C, on the other hand, the experimental values do not show significant difference, suggesting improved homogeneity and better phase formation.

For bulk samples, Francis et al.<sup>46</sup> obtained lifetime values of 0.99 ms, 0.81 ms, 0.79 ms for $Eu^{3+}$-doped $Y_3TaO_7$, $Lu_3TaO_7$, $Gd_3TaO_7$ prepared by solid state route, Du et al.<sup>31</sup> obtained 0.93 ms for $Eu^{3+}$-doped $Y_3TaO_7$ prepared by hydrothermal synthesis, and Borges et al.<sup>19,54</sup> found values ranging from 1.5 to 2.2 ms for $Eu^{3+}$-doped $Y_3TaO_7$, prepared by sol-gel method GTOE particles exhibit lifetimes longer than those reported by other authors for different rare-earth tantalates. This difference was expected, considering that the present samples are not in the bulk form. In nanoparticulate systems, variations in the refractive index relative to the bulk materials can lead to longer lifetime values. In addition, differences in the local structure and symmetry of $Eu^{3+}$ sites may also contribute to this behavior, as previously discussed.

Finally, Figure 5d shows a similar orangish-red color emission in the Commission Internationale d'Éclairage (CIE) 1931 chromaticity diagram for these two samples with different excitation wavelengths. The $(x,y)$ color coordinates were calculated from the emission spectra shown in Figure 5b.

The $^5D_0$ excited state intrinsic quantum yield ($Q_{Eu}^{Eu}$) and the experimental Judd-Ofelt parameters were calculated for GTOE\_800 and GTOE\_1000 samples, in these three different excitation wavelengths (265, 313, and 392 nm). The refractive index used for these calculations was 1.8, based on previous work with a similar compound, $Y_3TaO_7$.<sup>54</sup> The $Q_{Eu}^{Eu}$ do not show significant variation between the two different annealing temperatures. In both of cases, we obtained high $Q_{Eu}^{Eu}$ values (~85-95%) for these samples.

The absolute emission quantum yield (QY), the ratio between the number of emitted photons and the number of absorbed photons, was measured for the GTOE\_800 and GTOE\_1000 samples, also indicated in Table 1. When excited at the CTB, values around 10% were obtained for both samples. For bulk $Eu^{3+}$-doped $Y_3TaO_7$ also excited at the CTB, the obtained values were comparable to the samples presented
in this work (QY = 11.0% for Y₃TaO₇:20 mol % Eu³⁺).¹⁹ When excited via Gd³⁺, at 313 nm, the QY values decreased by half.

**3.4. Multimodal Imaging Applications (Proof of Concept).** The main goal of this work is to use these particles for multimodal imaging applications, so that we carried out initial tests on MR and CT measurements on gelatin suspensions of undoped Gd₃TaO₇, mimicking the biological medium, as a proof of the concept (the same tests performed on GTOE particles are available in the SI). The cell viability and the confocal microscopy were equally tested on the Eu³⁺-doped sample (to verify the internalization of the particles in the cell by confocal microscopy based on their luminescence) to show the feasibility of these particles in biological applications.

**3.4.1. MR Imaging.** In vitro MRI analyses were performed in 10% gelatin suspensions, at a magnetic field of 7 T, as shown in Figure 6. Frequently, Gd³⁺-based materials (such as NaGdF₄, Gd-complexes, GdF₃, Gd₂O₃)⁵⁹⁻⁶³) are commonly used as T₁-weighted contrast due to Gd³⁺ high number of unpaired electrons. However, since the particle size directly influences on the magnetic property,⁶¹ for particles sizes over 100 nm it is normal to obtain a T₁ signal that is negligible⁶⁴ and therefore not studied here.

The T₂-weighted imaging, on the other hand, has shown promising results. In Figure 6b, it can be seen that these particles can be used as negative CA in MRI (darker images with the increasing of the Gd³⁺ concentration). Gd³⁺ concentration ranged from 0 up to 3.64 mM for GTO_1000 sample (based on the measure in equipment #1) was determined by EDS analysis (Figures S5–S8, SI). From the slope obtained by the variation of 1/T₂ as a function of Gd³⁺ concentration (Figure 6a), the transverse relaxivity (r₂) has presented a value of 75 mM⁻¹ s⁻¹ for the GTO_1000 sample. On the other hand, the quantification measured by equipment #2 resulted in a r₂ value of 68 mM⁻¹ s⁻¹. The average of both measurements yielded a r₂ value of 71.5 mM⁻¹ s⁻¹, with an observable uncertainty in the results. However, due to the complete insolubility of the sample in any acid, other quantification analysis was not feasible.

In comparison to other Gd³⁺-based NPs described in literature, GTO sample presented similar or higher r₂ values. Some of us also previously obtained r₂ values of 114 mM⁻¹ s⁻¹ at 7 T for 59 nm Gd₂O₂S nanoparticles,⁶⁴ and, more recently, ultrasmall Gd₂O₂S NPs were fabricated, ranging between 3–11 nm, that exhibited a r₂ value of 57 mM⁻¹ s⁻¹ at 7 T.⁶⁵

The use of such a high magnetic field as 7 T is particularly interesting in the case of T₂ CAs. Stronger magnetic field improves the signal-to-noise ratio and, consequently, increases the spatial resolution and sensibility of the measurement.⁶⁶ Moreover, while for T₁ CAs, an increase on the field strength means a decrease on the r₁ values, for T₂ CAs, it results in a proportional increase on r₂ values.⁶⁷ Thus, the results obtained herein are enough to consider that these particles can be monitored in future *in vivo* measurement as a T₂ negative CA in high-magnetic field MR.

**3.4.2. CT Imaging.** In order to overcome the limitation of CT, having low soft tissue contrast in this high spatial resolution imaging technique, heavy elements based CAs are used since they are expected to allow a greater X-ray attenuation. In fact, over the years, CAs based on Ta have been extensively explored due to its high atomic number (Z = 73) and high density (16.4 g/cm³).⁶⁸ On this premise, the selection of the materials herein studied, composed by both Ta

and Gd (Z = 64), are expected to be a good CA for CT measurement. Because the contrast in this type of measurement is dependent on the material density, Gd₃TaO₇, whose density is approximately 8.07 g/cm³,⁶⁹ was also tested as a CA for this imaging modality. CT measurements were performed at 70 kVp in gelatin phantom samples containing increasing concentrations of GTO_1000 (Figure 7a). To compare the

<figure><img src="image_17.png" /><figcaption>**Figure 7.** (a) CT imaging intensity with increasing GTO_1000 concentration as indicated (from 0 to 40 mg/mL). (b) Intensity values in terms of HU as a function of NPs concentration.</figcaption></figure>

contrast generated by the particles, a sample containing only 10% gelatin was used as control. Air was used as the reference (−1000 Hounsfield units (HU)). The contrast in the images increases with increasing concentration (Figure 7b). In these experiments, the concentrations required to obtain a linear response were high (from 0 to 40 mg/mL), and at lower concentrations (0.5–20 mg/mL), it was possible to observe a contrast that was already detected at concentrations of ~2 mg/mL (Figure S10). These results show that these materials have suitable properties as a CA for CT imaging and are capable of offering a proper contrast in a certain measurement. Note that the lower sensitivity of CT (~2 mg/mL) compared to MRI (~0.2 mg/mL) is well-known and is not surprising here. CT remains an interesting imaging technique because of its good resolution and ease of use.

**3.4.3. Cytotoxicity Assays and In Vitro Imaging.** Finally, when developing a multimodal imaging probe, it is essential to evaluate the degree of toxicity and biocompatibility of particles. To the best of our knowledge, this is the first report on the viability of Gd₃TaO₇ cells. Here, we tested the cell viability of uncoated GTOE_1000 particles (further surface modifications can be investigated in the future). Figure 8 shows the cytotoxicity assays in NIH-3T3 fibroblast cell lines. The concentration of the GTOE_1000 up to 2.5 mg/mL was tested for an incubation period of 3 h.

A resazurin assay was conducted to evaluate the cytotoxicity of the tested compounds on the cells. This is a well-established protocol for evaluating the metabolic activity of cells. Live cells reduce the blue resazurin dye to pink resorufin dye. The fluorescence of the dye was proportional to the cell viability after internalization of the particles and compared with the control. No cytotoxic effects were observed within the concentration range of 25–1000 µg/mL, with no statistical difference in cell viability across these concentrations. This
<figure><img src="image_18.png" /><figcaption>**Figure 8.** Viability of NIH-3T3 cells incubated with different concentrations of GTOE_1000 sample suspensions.</figcaption></figure>

indicates that the compound is safe for use in these cells within this concentration range. However, a decrease in cell viability

was observed at higher concentrations (75% and 68% viability at 1500 and 1750 µg/mL, respectively), suggesting a potential cytotoxicity at these elevated levels. Overall, the particles could be safely applied at concentrations of up to 1000 µg/mL. These results indicate the high biocompatibility and safe use of this material compared to other luminescent NPs described in the literature.

Additionally, we followed the intracellular uptake of GTOE_1000 particles using confocal fluorescence microscopy. Because the Eu³⁺ doping concentration was only 1 mol %, we were not able to observe their emission under excitation at 405 nm in the cellular medium. Hence, to confirm that the GTOE_1000 particles were incorporated into the cells and not only interacting on their outside, 2-thenoyltrifluoroacetone (TTA) (500 µL, 50 µg mL⁻¹) was used as a pretreatment to the fixed cell slides to improve the luminescence of the particles. A highly luminescent Eu³⁺-TTA complex is formed between the β-diketone compound and Eu³⁺ ions on the surface of the GTOE particles.⁷⁰ This complex was easily identified using confocal microscopy (Figure 9). The assay confirmed the presence of cell-internalized Eu³⁺ species inside the nucleus and perinuclear region, which may be attributed to the concentration of the complex in these areas, as clearly evidenced by the image overlay shown in Figure 9d.

## 4. CONCLUSIONS AND PERSPECTIVES

This study successfully demonstrated the preparation of well-controlled particle sizes and morphologies of undoped and Eu³⁺-doped Gd₃TaO₇ using a polyol-mediated process. The average particle size was ~200 nm for the as-prepared samples

<figure><img src="image_19.png" /><figcaption>**Figure 9.** Intracellular distribution of GTOE_1000 particles at 1 mg/mL in NIH-3T3 cells was observed by confocal microscopy under excitation at 405 nm (emission detected at 600–670 nm). Images representing the emission of (a) DAPI, (b) phalloidin (Alexa-488), (c) particles, and (d) overlay of the images (a + b + c).</figcaption></figure>
and ~170 nm after annealing at 1000 °C. Even after annealing at high temperatures, the particles remained spherical and isolated, as verified using MET and DLS. The formation of a pure cubic Gd<sub>3</sub>TaO<sub>7</sub> crystalline phase was confirmed by XRD and Raman spectroscopy. Eu<sup>3+</sup> luminescence spectra exhibited inhomogeneous broadening of the emission bands. The intrinsic disorder of this defective fluorite Gd<sub>3</sub>TaO<sub>7</sub> structure, where the oxygen vacancies are randomly distributed, promotes a similar but slightly different average symmetry site occupied by Eu<sup>3+</sup> ions. This leads to a reduction in site symmetry and inhomogeneous broadening of the emission bands. A single exponential order function was used to fit the ⁵D<sub>0</sub> excited state emission decay curves, showing the same average site of Eu<sup>3+</sup> ions in this host. The absolute quantum yield of Eu<sup>3+</sup>-doped Gd<sub>3</sub>TaO<sub>7</sub> was measured, reaching values up to 11%. In addition, preliminary MRI and CT tests were conducted on gelatin suspensions of undoped Gd<sub>3</sub>TaO<sub>7</sub> to demonstrate their potential as contrast agents for multimodal imaging. Gd<sub>3</sub>TaO<sub>7</sub> exhibited a good transverse relaxivity response, making it suitable as a T<sub>2</sub> negative contrast agent for MR imaging. For CT, sensitive contrast of the particles was observed for concentrations above 2 mg/mL. Moreover, these uncoated particles showed promising results: they were not cytotoxic up to a concentration of 1000 µg/mL. These results show that materials based on gadolinium tantalates, with controlled size and morphology, offer properties suitable for their potential use as probes for in vivo tracking for trimodal imaging in the future.

## ASSOCIATED CONTENT

**Supporting Information**

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsanm.5c03858.

TEM images of undoped GTO sample and their size distribution histograms; DLS measurements on GTO and GTOE samples as-prepared and annealed at 800 and 1000 °C; SEM and EDS results for Gd<sup>3+</sup> quantification; MRI tests on GTOE_1000 sample; CT tests on GTO_1000 with lower NPs concentration (0–20 mg/mL) and on GTOE_1000 with high concentration (0–40 mg/mL) (PDF)

## AUTHOR INFORMATION

**Corresponding Author**

Rogéria Rocha Gonçalves – Department of Chemistry, Center of Nanotechnology and Tissue Engineering-Mater Lumen Laboratory, Faculty of Philosophy, Science and Letters of Ribeirão Preto, University of São Paulo, 14040-901 Ribeirão Preto, SP, Brazil; ● orcid.org/0000-0001-5540-7690; Email: rrgoncalves@ffclrp.usp.br

**Authors**

Fernanda Hediger Borges – Department of Chemistry, Center of Nanotechnology and Tissue Engineering-Mater Lumen Laboratory, Faculty of Philosophy, Science and Letters of Ribeirão Preto, University of São Paulo, 14040-901 Ribeirão Preto, SP, Brazil; Centre d'Élaboration des Matériaux et d'Études Structurales, CNRS, Université de Toulouse-UPS, 31055 Toulouse, France

York E. Serge-Correales – Department of Chemistry, Center of Nanotechnology and Tissue Engineering-Mater Lumen Laboratory, Faculty of Philosophy, Science and Letters of

Ribeirão Preto, University of São Paulo, 14040-901 Ribeirão Preto, SP, Brazil

Robert Mauricot – Centre d'Élaboration des Matériaux et d'Études Structurales, CNRS, Université de Toulouse-UPS, 31055 Toulouse, France

Ana Beatriz Acosta – Department of Chemistry, Center of Nanotechnology and Tissue Engineering-Mater Lumen Laboratory, Faculty of Philosophy, Science and Letters of Ribeirão Preto, University of São Paulo, 14040-901 Ribeirão Preto, SP, Brazil

Franck Desmoulin – ToNIC, Toulouse NeuroImaging Center, Université de Toulouse, Inserm, UPS, 31024 Toulouse, France; CREFRE, Centre Régional d'Exploration Fonctionnelle et de Ressources Expérimentales, Université de Toulouse, Inserm, UPS, ENVT, 31076 Toulouse, France

Carine Pestourie – CREFRE, Centre Régional d'Exploration Fonctionnelle et de Ressources Expérimentales, Université de Toulouse, Inserm, UPS, ENVT, 31076 Toulouse, France

Henrique Luis Piva – Department of Chemistry, Center of Nanotechnology and Tissue Engineering, Photobiology, and Photomedicine Research Group, FFCLRP, University of São Paulo, 14040-901 Ribeirão Preto, São Paulo, Brazil

Antônio Claudio Tedesco – Department of Chemistry, Center of Nanotechnology and Tissue Engineering, Photobiology, and Photomedicine Research Group, FFCLRP, University of São Paulo, 14040-901 Ribeirão Preto, São Paulo, Brazil; ● orcid.org/0000-0003-4198-9321

Marc Verelst – Centre d'Élaboration des Matériaux et d'Études Structurales, CNRS, Université de Toulouse-UPS, 31055 Toulouse, France

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsanm.5c03858

**Author Contributions**

F.H.B.: Methodology, investigation, conceptualization, data curation, formal analysis, visualization, writing—original draft, writing—review and editing. Y.E.S.—C.: data curation, investigation, formal analysis, writing—review and editing. R.M.: formal analysis, validation, writing—review and editing. A.B.A.: Data curation. F.D.: data curation, formal analysis, writing—review and editing. C.P.: Data curation, formal analysis, writing—review and editing. H.L.P.: data curation, investigation, formal analysis, writing—review and editing. A.C.T. validation, resources, funding acquisition, writing—review and editing. M.V.: conceptualization, validation, supervision, resources, funding acquisition, writing—review and editing. R.R.G.: conceptualization, methodology, validation, supervision, resources, funding acquisition, writing—review and editing.

**Funding**

The Article Processing Charge for the publication of this research was funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil (ROR identifier: 00x0ma614).

**Notes**

This work was supported by Brazilian funding agencies FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) [grant number 2020/00277–6, 2023/13509–0, 2020/05319–9, 2021/08111–2, and 2023/05351–8], CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) [#303110/2019–8], and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) [CAPES-
Cofecub scholarship process number 88887.662011/2022-00 and CAPES-Cofecub Ph-C 965/2 program]. We thank the National Council for Scientific and Technological Development (CNPq) for grant #404416/2021-7, #304687/2021-9 and CNPq-SCTIE-Decit-DGITIS-CGCIS #441673/2020-Innovative Platforms in Advanced Therapies from Brazilian Ministry of Health-MoH" and "Brazilian National Programme of Genomics and Precision Health-Genomas Brasil". We also thank the Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES), São Paulo Research Foundation (FAPESP) thematic project #2021/08111-2 #2013/50 181-1, FINEP project 01.10.0758.01, PRPI-USP PIPAE Project Agreement #21.1.10424.1.9, and USP-COFECUB Project Agreement #22.1.538.59.4 for their financial support. H.L.P. thanks FAPESP for the doctorate and RIA scholarships (#2020/04009-6, 2021/11803-3).

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

The authors would like to thank the Department of Chemistry (FFCLRP), University of São Paulo, the Centre d'Élaboration de Matériaux et d'Études Strutturales (CEMES-CNRS), the São Paulo State University (UNESP) of Araraquara, and Inserm for the opportunity to use their facilities.

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