Unveiling the Effects of Rare-Earth Substitutions on the Structure

Article Cite This: ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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Unveiling the Effects of Rare-Earth Substitutions on the Structure, Mechanical, Optical, and Imaging Features of ZrO2 for Biomedical Applications Kalaivani Srigurunathan,† Rugmani Meenambal,†,‡ Anupam Guleria,§ Dinesh Kumar,§ José Maria da Fonte Ferreira,∥ and Sanjeevi Kannan*,† †

Centre for Nanoscience and Technology, Pondicherry University, Puducherry 605 014, India Department of Clinical Pharmacology and Toxicology, National Institute of Mental Health and Neuro Science, Bangalore 560029, India § Centre of Biomedical Research, SGPGIMS Campus, Raibareli Road, Lucknow 226014, India ∥ Department of Materials and Ceramics Engineering, University of Aveiro, CICECO, Aveiro 3810 193, Portugal

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S Supporting Information *

ABSTRACT: The impact of selective rare-earth (RE) additions in ZrO2-based ceramics on the resultant crystal structure, mechanical, morphological, optical, magnetic, and imaging contrast features for potential applications in biomedicine is explored. Six different RE, namely, Yb3+, Dy3+, Tb3+, Gd3+, Eu3+, and Nd3+ alongside their variations in the dopant concentrations were selected to accomplish a wide range of combinations. The experimental observations affirmed the roles of size and dopant concentration in determining the crystalline phase behavior of ZrO2. The significance of tetragonal ZrO2 (t-ZrO2) → monoclinic ZrO2 degradation is evident with 10 mol % of RE substitution, while RE contents in the range of 20 and 40 mol % ensured either t-ZrO2 or cubic ZrO2 (c-ZrO2) stability until 1500 °C. High RE content in the range of 80−100 mol % still confirmed the structural stability of c-ZrO2 for lower-sized Yb3+, Dy3+, and Tb3+, while the c-ZrO2 → RE2Zr2O7 phase transition becomes evident for higher-sized Gd3+, Eu3+, and Nd3+. A steady decline in the mechanical properties alongside a quenching effect experienced in the emission phenomena is apparent for high RE concentrations in ZrO2. On the one hand, the paramagnetic characteristics of Dy3+, Tb3+, Gd3+, and Nd3+ fetched excellent contrast features from magnetic resonance imaging analysis. On the other hand, Yb3+ and Dy3+ added systems exhibited good Xray absorption coefficient values determined from computed tomography analysis. KEYWORDS: rare earth, ZrO2, structure, optical, mechanical, imaging compensation mechanisms. Further, Zr4+ ensures a strong covalent interaction with O2− that favors sevenfold coordination to accomplish partial stabilization.1 Mg2+,2,3 Ca2+,4,5 Y3+,6,7 Ce3+/4+,8 and La3+ 9 are widely used as stabilizers to accomplish either t-ZrO2 or c-ZrO2 at room temperature. The incorporation of dopants at ZrO2 lattice ensures two types of stabilization, namely, partially stabilized zirconia (PSZ) or fully stabilized zirconia (FSZ), depending on the type and dopant concentration. PSZ comprise t-ZrO2, c-ZrO2, and m-ZrO2 mixtures, while FSZ is attained through an appropriate dose of stabilizers. For instance, 16 mol % each of CaO and MgO and 8 mol % of Y2O3 accomplish FSZ, while PSZ is attained with same oxides, however, with minor contents.10

1. INTRODUCTION Zirconia (ZrO2) is of paramount importance in the biomedical applications field owing to its excellent properties like biocompatibility, mechanical strength, toughness, and thermal stability. The three well-known polymorphs of ZrO2 include the monoclinic (m-ZrO2) that is stable from room temperature (RT) to 1170 °C, tetragonal (t-ZrO2) with stability in the range of 1170−2370 °C, and the cubic (c-ZrO2) with an exceptional stability that exists beyond 2370 °C. Generally, the martensitic phase transformation among these polymorphs induces changes in shape and volume, which are a major concern to retain unique ZrO2 structure at RT. In this context, a wide range of dopants ranging from monovalent to trivalent elements have been used to retain either t-ZrO2 or c-ZrO2 structure. The substitution of dissimilar elements at the ZrO2 lattice develops oxygen vacancies, and, as a consequence, structural stabilization is accomplished through charge © XXXX American Chemical Society

Received: December 12, 2018 Accepted: March 21, 2019

A

DOI: 10.1021/acsbiomaterials.8b01570 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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ACS Biomaterials Science & Engineering Table 1. Precursor Concentrations Used during the Synthesis molar concentration of the precursors with their respective sample codes ZrOCl2

Yb(NO3)3

Dy(NO3)3

Tb(NO3)3

Gd(NO3)3

Eu(NO3)3

Nd(NO3)3

% of RE3+with respect to Zr4+

0.5 0.5 0.5 0.5 0.5

0.05 (1ZY) 0.1 (2ZY) 0.2 (4ZY) 0.4 (8ZY) 0.5 (10ZY)

0.05 (1ZD) 0.1 (2ZD) 0.2 (4ZD) 0.4 (8ZD) 0.5 (10ZD)

0.05 (1ZT) 0.1 (2ZT) 0.2 (4ZT) 0.4 (8ZT) 0.5 (10ZT)

0.05 (1ZG) 0.1 (2ZG) 0.2 (4ZG) 0.4 (8ZG) 0.5 (10ZG)

0.05 (1ZE) 0.1 (2ZE) 0.2 (4ZE) 0.4 (8ZE) 0.5 (10ZE)

0.05 (1ZN) 0.1 (2ZN) 0.2 (4ZN) 0.4 (8ZN) 0.5 (10ZN)

10 20 40 80 100

In vivo studies deduce the nucleation phenomena of t-ZrO2 to persuade spontaneous t- → m-ZrO2 transformation that is complemented by twinning and microcracking processes thus resulting in the catastrophic failure. Among stabilizers, MgOstabilized ZrO2 (Mg-PSZ) is reported to deliver comparably better mechanical strength than Y2O3-stabilized ZrO2 (YSZ); nevertheless, the use of Mg-PSZ is restrained due to the drawback of residual porosity, higher sintering temperature, and the presence of impurities. ISO 13356 certify the use of YSZ in biomedical application owing to its good biocompatibility, better mechanical properties, and considerable aging resistance in comparison with other stabilizers.11−13 Despite the aging related failures under in vitro and in vivo, YSZ is already commercialized for implant applications. In continuation with the availability of commercial YSZ, zirconia toughened alumina (ZTA) also enjoys viable success due to the salient features.14 In case of YSZ, the accommodation of higher-sized Y3+ (1.019 Å) at the Zr4+ (0.84 Å) lattice sites of ZrO2 induces a considerable expansion of the resultant unit cell. In a similar manner, the incorporation of higher-sized RE is also expected to induce significant expansion of the ZrO2 unit cell. In the current scenario, investigations are mainly focused to either eliminate or delay the aging-related issues of ZrO2 through RE incorporation.15 The strategy to substitute 5 mol % of Sc3+ in YSZ ensured better structural stability in ZrO2 than the individual YSZ.16 Other than the doping strategies in YSZ, the elements like Gd3+, Dy3+, and Sc3+ in the range of 10−20 mol % were individually selected to ensure partial ZrO2 stabilization for applications in solid oxide fuel cells (SOFC) and thermal barrier coatings (TBC).17−19 Literatures also evince the luminescence and optical behavior of RE-doped ZrO2 for other applications such as display, sensor, and medical usage. Because of the low phonon energy (470 cm−1) and high host absorption coefficient, ZrO2 is considered an ideal material to host RE to achieve enhanced luminescence life.20 These features are considered a prime requirement for selective biomedical applications that involves in vivo imaging devoid of tissue damage, auto excitation, and easy photographic detection.21 In recent years, RE-doped materials gained significance in orthopedic and imaging applications. Single and cosubstitutions of Gd 3+, Dy3+, and Yb3+ in β-Ca3(PO 4)2 as a multifunctional bioprobe for magnetic resonance imaging (MRI), computed tomography (CT), and optical imaging application have been reported.22−25 Nevertheless, the poor mechanical compatibility restricts the use of β-Ca3(PO4)2 in hard-tissue replacements. An additional work on Dy3+-doped ZrO2−SiO2 binary oxide system presented good MRI contrast features.26 The investigation on RE-doped ZrO2 has been mainly focused on electrolyte for SOFC, TBC, and bioimaging nanoprobes. Despite the investigations available on the structural, mechanical, aging, optical, and contrast features

of RE-doped ZrO2, many of the studies limit their emphasis on individual or a couple of properties.27−30 In this context, the present work aims to develop RE-doped ZrO2 for biomedical applications, which intends to illustrate a detailed analysis of doping effects on the structure, mechanical, optical, and imaging contrast features. Ytterbium (Yb3+), dysprosium (Dy3+), terbium (Tb3+), gadolinium (Gd3+), neodymium (Nd3+), and europium (Eu3+) were selected for the proposed study. The rationale behind this selection is not only based on their role as a stabilizer and improved mechanical performance but also to impart inherent MRI (Dy3+, Gd3+, Tb3+, Nd3+), CT (Dy3+, Yb3+), and luminescence features that are expected to monitor the in vivo performance of the implant in a noninvasive manner.

2. MATERIALS AND METHODS 2.1. Synthesis of Powders. ZrO2-stabilized powders with a wide range of RE additions were synthesized through citrate-assisted sol− gel technique. The precursors used in this work include analytical grade zirconium oxychloride octahydrate (ZrOCl2·8H2O), rare-earth nitrates such as Yb(NO3)3, Dy(NO3)3, Tb(NO3)3, Gd(NO3)3, Eu(NO3)3, Nd(NO3)3, and citric acid (C6H8O7). The concentrations of RE precursors in ZrO2 were varied to obtain a wide range of combinations. Table 1 presents the sample codes along with their respective molar concentrations. In a brief description of the synthesis, an appropriate amount of RE(NO3)3 solution was slowly added to the ZrOCl2 stock solution under constant stirring conditions at 90 °C. This was followed by the addition of required amount of citric acid solution after 10 min. The resultant mixture was stirred until the formation of transparent viscous gel and subsequently dried at 120° and grounded to fine powders. 2.2. Powder Characterization. The characterization studies were performed after heat treatment of the synthesized powders at specific temperatures with a dwell time for 4 h. X-ray diffraction (XRD) studies using a high-resolution X-ray diffractometer (RIGAKU, ULTIMA IV) with Cu Kα radiation (λ = 1.5406 Å) were performed in the range of 5°−90° (2θ) with a scan rate of 0.02° s−1 produced at 40 kV and 30 mA. Standard International Centre for Diffraction Data (ICDD) cards were used for the phase analysis of XRD patterns. Quantitative structural analysis through Rietveld refinement of the powder XRD patterns were performed using GSAS-EXPUGI software package. The standard crystallographic information files (CIF) from American mineralogist crystal structure database were used to perform the structure refinement. CIF files for the refinement of m-ZrO2, t-ZrO2, c-ZrO2, c-Gd2Zr2O7, c-Eu2Zr2O7, and c-Nd2Zr2O7 were, respectively, obtained from Smith et.al, Howard et.al, Wyckoff et. al, Klee et. al, and Korneev et. al.31−35 The procedure for the structure refinement was accomplished in accordance with our previous reports.36 Vibrational modes of the heat-treated powders were obtained using the backscattering geometry of confocal Raman microscope (RENISHAW). An excitation wavelength of 785 nm by a semiconductor diode laser (0.5% of power) with the data acquiring time of 30 s was used to procure the Raman data. The absorption spectra of the powder samples were collected using UV−vis−NIR (NIR = near-infrared) spectrophotometer (PERKIN ELMER, LAMBDA, 650S). Luminescence spectrum was recorded using spectrofluorometer (FLUOROB


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ACS Biomaterials Science & Engineering

Figure 1. XRD patterns of the compositions recorded after heat treatment at 1500 °C. The patterns of (a) Yb3+, (b) Dy3+, (c) Tb3+, (d) Gd3+, (e) Eu3+, and (f) Nd3+ substitutions in ZrO2. The values 10, 20, 40, 80, and 100 correspond to the RE concentrations in mol %. Std m-, t-, c-ZrO2, respectively, refer to XRD reflections of ICDD standard card Nos. 01-083-0944, 01-079-1765, 01-071-4810. MAX-4, HORIBA SCIENTIFIC) equipped with xenon lamp source. The magnetic behavior of the samples was assessed through Physical Properties Measurement System-Vibrating Sample Magnetometer (SYSTEM QUANTUM DESIGN) with the magnetic field ranging from −1 to 1 T. Microstructural features of the sintered specimen prior to polishing were analyzed using high-resolution scanning electron microscope (FEI-QUANTA, HRSEM, FEG-200) coupled with energy-dispersive spectroscopy (EDS) system. ImageJ software was used to generate the histograms of average grain size of the sintered specimens. 2.3. Mechanical Evaluation. Selective mechanical properties such as hardness and Young’s modulus were evaluated at room temperature through Nanoindentation machine (BRUKER). A brief explanation of the specimen preparation for mechanical testing is

explained as follows. Initially, the synthesized powders were treated at 700 °C to remove all the volatile impurities and subsequently ballmilled (Retsch) for 2 h and then pressed into the form of pellets (13 mm diameter and 1 mm thickness) using a semiautomatic hydraulic press (Kimaya Engineers) under an applied force of 10 kN for 30 s. The resultant specimens were sintered at 1500 °C for 4 h and subsequently mirror polished in a sequential manner with SiC sheets and diamond paste prior to indentation tests. A triangular pyramid (Berkovich) diamond tip indenter (50 nm radius of curvature) with an indent load of 5 mN was used. A typical depth-sensing (load/ unload−displacement curves) test that comprises a loading segment followed by a dwell time under a specific maximum load and finally an unloading segment was performed to determine the selective mechanical properties. All the specimens were analyzed through a C


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Table 2. Phase Fraction Data of All the RE-Substituted ZrO2 Systems Derived from the Structure Refinement of the Powder XRD Patterns Recorded at Selective Temperatures phase fraction 1400 °C mol % of RE at ZrO2 10%

20%

40%

80%

100%

RE 3+

Yb Dy3+ Tb3+ Gd3+ Eu3+ Nd3+ Yb3+ Dy3+ Tb3+ Gd3+ Eu3+ Nd3+ Yb3+ Dy3+ Tb3+ Gd3+ Eu3+ Nd3+ Yb3+ Dy3+ Tb3+ Gd3+ Eu3+ Nd3+ Yb3+ Dy3+ Tb3+ Gd3+ Eu3+ Nd3+

m-ZrO2

t-ZrO2

2.75

97.25 100 95.75 94.25 83.20 25.70 100 55.75

4.25 5.75 16.80 20.70

12.50

33.50

42.60

1500 °C

c-ZrO2

c-RE2Zr2O7

100 100 100 57.50

t-ZrO2

4.85 16.00 34.25

95.15 84.00 65.75 36.75 67.20 44.75

28.00 55.25

53.60 44.25 100 100 100 54.00 100 100 100 100 100 58.40 100 100 100 80.60 2.20

m-ZrO2

c-ZrO2

c-RE2Zr2O7

63.25 4.80 100 100 100 100 100

21.50

78.50

92.70

100 100 100 100 100 7.30 100 100 100

19.40 97.80 100

100 100 100 100 100 100

42.50 100 100

100 100 100

ms; and the spin echo time varied from 13 to 227 ms. The T1 and T2 relaxation times were then obtained from the nonlinear least-squares fits to the measured signal intensities at various TI and TE values, respectively. The calculation of r1 and r2 relaxivities values from the phantoms containing different sample concentration was done in accordance with the previous study.37 Matlab software, The Mathworks Inc., was used to ascertain the relaxation rate maps.

single indent mode with a maximum of 10 indents for each specimen at random locations. 2.4. In Vitro X-ray CT Imaging. Among the investigated compositions, Yb3+ and Dy3+ doped systems were selected for X-ray CT analysis. The samples for the CT analysis were prepared by dispersing appropriate amounts of powders in deionized water. The particle sizes of the powders that were selected for both the CT and MRI analysis are illustrated in Figure S1. CT phantom images alongside Hounsfield (HU) units were obtained using a multislice spiral CT system (GE HISPEED CT/e). The following conditions were followed to acquire phantom images: 120 kVp and 160 mA: field of view (FOV) = 54.07 mm × 146.00 mm, thickness = 0.9 mm, exposure time = 800 ms/rotation. The images were analyzed using the Kodak molecular imaging software embedded in the CT scanner. 2.5. In Vitro MR Imaging. T1 and T2 relaxivity of selective REdoped ZrO2 systems were determined using a 3 T MRI scanner (Siemens Skyra) equipped with the head coil. The samples were prepared by the appropriate dilution of 40 mol % RE-substituted ZrO2 with assorted concentrations of 0.2, 0.4, 0.6, 0.8, 1 mM in 10 mL of deionized water. T1 and T2 maps and the corresponding relaxation time measurements were performed using the inversion recovery and turbo spin echo pulse sequence, respectively. The following measurement parameters were used in the inversion recovery pulse sequence: FOV = 17 cm; matrix size = 256 × 320; slice thickness = 2 mm; spacing gap = 0; repetition time (TR) = 4000 ms; time to echo (TE) = 12 ms, and inversion time varied from 100 to 4000 ms, while the parameters used in turbo spin echo pulse sequence for T2 measurements were as follows: FOV = 17 cm; matrix size = 410 × 512; slice thickness = 2 mm; spacing gap = 0; TR = 6000

3. RESULTS 3.1. Phase Analysis. The influence of RE additions (Yb3+, Dy3+, Tb3+, Gd3+, Eu3+, and Nd3+) at the ZrO2 structure during elevated heat treatments was analyzed through XRD analysis. The diffraction patterns of all the compositions after treatments at 1500 and 1400 °C are presented, respectively, in Figures 1 and S2. The X-ray reflections indicate the crystallization of m-ZrO2, t-ZrO2, and c-ZrO2 polymorphs that were mainly dependent on the concomitant effects of dopant concentrations, type of element, and temperatures. The diffraction patterns were analyzed using the standard ICDD Card Nos. 01-083-0944, 01-079-1765, 01-071-4810, respectively, for m-ZrO2, t-ZrO2, and c-ZrO2. A mixture of diffraction peaks analogous to m-ZrO2, t-ZrO2, and c-ZrO2 were observed at the RE dopant concentrations of 10 mol %, while a minor increment in the RE content to 20 mol % yielded either t-ZrO2 or c-ZrO2 for Yb3+, Dy3+, Tb3+, Gd3+, and Eu3+ at 1400 °C. D


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Figure 2. Refined diffraction patterns of the selective RE-substituted ZrO2 at 1500 °C. (a) 100 mol % Yb3+, (b) 10 mol % Tb3+, (c) 100 mol % Gd3+, (d) 100 mol % Eu3+, and (e) 100 mol % Nd3+.

Further increment in RE concentrations to 40 mol % and above induced t- → c-ZrO2 transition at 1400 °C. The t/c- → m-ZrO2 phase degradation observed at 1400 °C for lower RE concentrations becomes more apparent at 1500 °C, which is justified by a sharp increment in the intensity of XRD reflections of m-ZrO2. On the contrary, the leftover compositions indicated the crystallization of single c-ZrO2 at both 1400 and 1500 °C. Further, a gradual shift of the X-ray reflections toward the lower 2θ angle with simultaneous increment in the RE additions is noticed at 1500 °C in all the investigated compositions. It is worthy to mention that XRD reflections typical of RE oxides, namely, Yb2O3, Dy2O3, Tb2O3, Gd2O3, Eu2O3, and Nd2O3 are not witnessed in any of the compositions. 3.2. Quantitative Analysis. In continuation with the qualitative information gained from the phase analysis, all the compositions were subjected to Rietveld refinement to gain quantitative information on the phase fractions and lattice

deviation in the unit cell due to the influence of various RE at the ZrO2 structure. The complete phase fraction data determined from the refinement are presented in Table 2. The refined diffraction patterns of selective compositions at 1500 and 1400 °C are presented, respectively, in Figures 2 and S3. 3.2.1. Role of RE on the Phase Composition. The phase fraction data (Table 2) confirm the crystallization of various ZrO2 polymorphs alongside RE zirconates (RE2Zr2O7) at both 1400 and 1500 °C. The crystallization of specific structures established good similarity with the respective space groups of m-ZrO2 with P21/c (14), t-ZrO2 with P42/nmc (137), c-ZrO2 with Fm3̅m (225), (c-Gd2Zr2O7, c-Eu2Zr2O7, and c-Nd2Zr2O7) with Fd3̅m (227).31−35 The invariable coexistence of m-ZrO2 and t-ZrO2 is apparent in all the systems with 10 mol % of dopant concentrations. It is also noticed that a sharp rise in the phase content of m-ZrO2 is apparent during the escalation of heat treatment from 1400 to 1500 °C. In case of 20 mol % RE, E


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Figure 3. Lattice behavior of the t-ZrO2, c-ZrO2, and c-RE2Zr2O7 crystal structures due to the substitution of various RE in the concentration range of 20−100 mol % at 1500 °C. (a) 20, (b) 40, (c) 80, and (d) 100 mol % of RE concentrations.

3.2.2. RE on the ZrO2 Crystal Structures. The lattice changes in ZrO2 structure due to the assorted concentrations of RE addition (20−100 mol %) are presented in Figures 3 and S4. The preceding investigation on the phase composition revealed invariable t- → m-ZrO2 transition in all the RE dopant concentrations with 10 mol %. This result affirms the limitation of RE content to stabilize either t-ZrO2 or c-ZrO2 structures at elevated heat treatments. The increment in dopant concentrations of various RE induced either the continuous lattice expansion of a particular unit cell or t- → c-ZrO2 phase transition. In case of higher-sized RE, the dopant concentrations also played a critical role to persuade the formation of cubic RE2Zr2O7. In case of both Yb3+ and Dy3+, a gradual increment in the dopant concentration from 20 to 100 mol % induced t- → c-ZrO2 phase transition, and subsequently a gradual expansion of the c-ZrO2 unit cell is obvious at both the investigated temperatures. This lattice expansion is also justified by the consistent upsurge in occupancy values of Yb3+ and Dy3+ at ZrO2 lattice. Figure 4 presents the occupancy of 40 mol % of individual RE at the lattice of c-ZrO2 unit cell. On the contrary, Tb3+ displayed a steady lattice expansion of the c-ZrO2 unit cell with a corresponding increment in the dopant content from 20 to 100 mol %. The comparatively higher-sized RE in the order of Gd3+ and Eu3+ signified the consistent expansion of c-ZrO2 unit cell despite the additional existence of their respective RE2Zr2O7 at 1400 °C in case of 80−100 mol % dopant concentration. Nevertheless, Gd2Zr2O7 and Eu2Zr2O7 were determined as single components for both 80 and 100 mol % dopant concentration at 1500 °C, and further Eu2Zr2O7 underwent comparatively a larger expansion

Yb3+ (0.985 Å) that possessed the lowest size among the investigated REs displayed the only existence of t-ZrO2 at 1400 °C, and subsequently the complete transformation to c-ZrO2 is determined at 1500 °C. A relatively higher-sized Dy3+ (1.027 Å) perceived t-ZrO2 and c-ZrO2 mixtures at 1400 °C followed by the apparent crystallization of unique c-ZrO2 at 1500 °C. A further size increment in the order of Tb3+ (1.040 Å), Gd3+ (1.053 Å), and Eu3+ (1.066 Å) marked the sole existence of cZrO2 at both 1400 and 1500 °C in case of their 20 mol % additions. A blend of m-ZrO2, t-ZrO2, and c-ZrO2 mixtures at 1400 °C and further increment to 1500 °C displayed the mZrO2 and t-ZrO2 combination in case of 20 mol % Nd3+ (1.109 Å) that possessed the higher size among the investigated elements. 40 mol % RE additions witnessed the sole existence of cZrO2 at both 1400 and 1500 °C in case of all the elements excluding Nd3+, which signified the t-ZrO2 and c-ZrO2 mixtures at both the temperatures. Both the 80 and 100 mol % of Yb3+, Dy3+, and Tb3+ additions affirmed the presence of single-phase c-ZrO2 at both 1400 and 1500 °C. The presence of c-ZrO2 and Gd2Zr2O7 mixtures at 1400 °C and the unique Gd2Zr2O7 at 1500 °C is observed for both 80 and 100 mol % Gd3+ additions. The samples with 80 and 100 mol % of Eu3+ indicated the combination of c-ZrO2 and Eu2Zr2O7 and single Eu2Zr2O7 at 1400 °C, respectively. The invariable and even more noticeable presence of Eu2Zr2O7 for both dopant concentrations is also observed at 1500 °C. The uniform prevalence of Nd2Zr2O7 is obvious at both 80 and 100 mol % of Nd3+ at 1400 and 1500 °C. F


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bands at ∼110−190 and ∼200−520 cm−1, while a similar amount of Eu3+ displayed three distinct broad bands at ∼110− 165, ∼225−440, and ∼510−645 cm−1, which is mainly attributed to the merge of bands corresponding to c-ZrO2 at both the investigated temperatures. In continuation with the broad bands witnessed for 40 mol % Gd3+, a deliberate improvement in the peak intensity of these bands is noticed for both 80 and 100 mol % Gd3+ at 1400 °C. This particular progress in the peak intensity is accounted for the c-ZrO2 → cGd2Zr2O7 phase transition, which is in accordance with the quantitative analysis. Further, the spectra recorded at 1500 °C authenticate this phase transition, as the more distinct bands of c-Gd2Zr2O7 are noticed at 136, 324, 415, and 601 cm−1. In comparison with the Gd3+ system, a contrasting trend was perceived for 80 mol % Eu3+ that demonstrated specific bands at three different regions, namely, ∼280−345, ∼370−430, and ∼530−625 cm−1 respective of c-ZrO2 and c-Eu2Zr2O7 mixtures at 1400 °C. Further increment in the dopant concentration to 100 mol % Eu3+ validates the c-ZrO2 → c-Eu2Zr2O7 phase transition with more discrete bands determined at ∼260−355, ∼370−415, ∼500−560, and ∼560−622 cm−1. Nonetheless, the crystallization of discrete c-Eu2Zr2O7 is attained with a strong increment in the band intensity for 80 and 100 mol % of Eu3+ at 1500 °C. A considerable distortion in the Raman bands is noticed for the higher-sized Nd3+. 40 mol % Nd3+ exhibited a combination of t-ZrO2 and c-ZrO2 bands, while the additions on the higher side in the order of 80 and 100 mol % affirm the formation of unique c-Nd2Zr2O7 bands at 303, 408, 512, and 579 cm−1 at 1400 °C. A considerable improvement in the intensity of these particular bands is perceived at 1500 °C. 3.4. Optical Analysis. 3.4.1. Absorption. The absorption spectra in UV, visible, and NIR regions of all the REsubstituted ZrO2 compositions are presented in Figure S6a−f. The f−f electronic transitions corresponding to the ground state of RE3+ were assigned in accordance with the standard energy-level schemes. Yb3+ additions exhibited a characteristic absorption band (Figure S6a) in the NIR region at 910, 972, and 1026 nm that attributes to the stark splitting of 2 F7/2→2F5/2 transition.42 Dy3+ induced (Figure S6b) a gradual upsurge in the absorption intensity as a function of its enhanced additions. The characteristic Dy3+ absorption bands are perceived at 320, 351, 387, 447, 477, 748, 793, 887, 1083, 1268 nm, which are attributed to the respective transitions from 6H15/2 to 4L9/2, 4I9/2, 4I13/2, 4I15/2, 4F9/2, 6F1/2 + 6F3/2, 6 F5/2, 6F7/2, 6F9/2 + 6H7/2, and 6H9/2 + 6F11/2.43,44 Distinct Tb3+ bands (Figure S6c) are detected in the visible region at ∼225 and 270 nm characteristic of 7F6 →5D4 transitions.45,46 The typical Gd3+ (Figure S6d) absorption peaks are specified at 220−243 nm characteristic of 8S7/2 to 6IJ/2 transition, while a broad band in the region of ∼278−350 nm is evident for concentrations beyond 40 mol % of Gd3+ distinctive of the 8 S7/2 to 6P7/2 transition.47,48 The characteristic absorption bands of Eu3+ (Figure S6e) that transpire from the valence to conduction band are evident at ∼200−300 nm, whereas high Eu3+ content specified additional bands at ∼393 and ∼464 nm of the 7F0→5L6 and 7F0→5D2 transitions, respectively.49−51 Absorption peaks characteristic of Nd3+ (Figure S6f) are observed over UV, visible, and NIR regions at 356 (4I3/2 → 4 D3/2 + 4D5/2), 431 (4I3/2 → 2P1/2), 458 (4I3/2 → 4G11/2), 480 (4I3/2 → 2D3/2), 524 (4I3/2 → 4G9/2 + 4G7/2 + 2K13/2), 589 (4I3/2 → 4G5/2 + 2G7/2), 679 (4I3/2 → 4F9/2), 745 (4I3/2 → 4F7/2 + 4S3/2), 808 (4I3/2 → 2H9/2 + 4F5/2), and 880 nm (4I3/2 →

Figure 4. Occupancy values of 40 mol % of various RE at the lattice of t/c-ZrO2 unit cell.

of the unit cell than Gd2Zr2O7. The investigated highest-sized Nd3+ led to the formation of m- ZrO2, t- ZrO2, and c-ZrO2 mixtures at both 1400 and 1500 °C for 20 mol % dopant concentration. 40 mol % of Nd3+ assured the t-ZrO2 and cZrO2 mixtures at both 1400 and 1500 °C, while the 80 and 100 mol % of Nd3+ formed single Nd2Zr2O7. These results enable inferring about the lattice distortion persuaded by Nd3+ at the ZrO2 structure for 20 and 40 mol % dopant concentrations. Nevertheless, a considerable expansion of the Nd2Zr2O7 unit cell is witnessed in the order of 80 and 100 mol % of Nd3+. 3.3. Raman Analysis. Raman spectra were recorded to corroborate the results obtained from qualitative and quantitative X-ray analysis. Generally, the typical Raman active modes for m-ZrO2 are 18 (9Ag + 9Bg), 6 for t-ZrO2 (A1g + 2B1g + 3Eg), and single F2g mode is represented for c-ZrO2, Gd2Zr2O7, Eu2Zr2O7, and c-Nd2Zr2O7, and their characteristic peak positions are listed in Table 3.38−41 Raman spectra of the compositions recorded after heat treatments at 1500 and 1400 °C are illustrated, respectively, in Figures 5 and S5. Table 3. Characteristic Peak Positions of m/t/c- ZrO2, cGd2Zr2O7, c-Eu2Zr2O7, and c-Nd2Zr2O7 m-ZrO2 t-ZrO2 c-ZrO2 c-Gd2Zr2O7 c-Eu2Zr2O7 c-Nd2Zr2O7

97, 179,191, 225,303, 333, 347, 381, 475, 504, 537,560, 616, and 640 cm−1 147, 261, 318, 463, 611, and 641 cm−1 155, 278, 367, 579, and 617 cm−1 248, 331, 410, and 554 cm−1 303, 390, 541, and 585 cm−1 250, 303, 406, 516, and 579 cm−1

In accordance with the XRD results, 10 mol % of RE inclusions in ZrO2 system exhibited a blend of bands that correspond to m-ZrO2, t-ZrO2, and c-ZrO2, while the typical bands analogous to m/t- ZrO2 demonstrated a diminishing trend during the phase transition to c-ZrO2 in case of 20 mol % RE. The RE increments in the order of 40−100 mol % for Yb3+, Dy3+, and Tb3+ originated bands representative of c-ZrO2 for 40 mol %, and subsequently these bands perceived a gradual distortion and simultaneous broadening for 80 and 100 mol % additions at both 1400 and 1500 °C. In contrast to the above results, 40 mol % of Gd3+ unveiled two distinct broad G


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Figure 5. Raman spectra of individual RE additions in ZrO2 recorded after heat treatment at 1500 °C. Patterns of (a) Yb3+, (b) Dy3+, (c) Tb3+, (d) Gd3+, (e) Eu3+, and (f) Nd3+ substitutions in ZrO2. The labels 10, 20, 40, 80, and 100 refer to the RE concentrations in mol %. The annotations M, T, C, and * correspond to m-ZrO2, t-ZrO2, c-ZrO2, and c-RE2Zr2O7, respectively.

497 nm (4F9/2−6H15/2) and 583 nm (4F9/2→6H13/2); nevertheless, the band ∼452 nm corresponding to the host lattice emission is also observed,56,57 while the excitation of Tb3+ systems at 285 nm (Figure 6c) displayed peaks ∼488 and 544 nm respective of electronic transitions such as5D4→7F6 and 5 D4→7F5. This typical transition is obvious for 10 and 20 mol % of Tb3+ additions, while the apparent broadening and the quenching effect alongside the detection of weak band at ∼468 nm typical of 5D3−7FJ (J = 5, 4) transitions were observed for

F3/2).52,53 The upsurge in the absorption intensity with simultaneous increment in RE content is obvious in all the ZRE systems. 3.4.2. Emission. The excitation of Yb3+ additions at 980 nm (Figure 6a) originated bands at 1013, 1042, 1086, and 1112 nm characteristic of transitions from excited (2F5/2) to ground states (2F7/2). A gradual concentration quenching effect is also observed for incremental Yb3+ additions.54,55 The excitation of Dy3+ at 350 nm (Figure 6b) ensured typical emissions at 483− 4

H


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Figure 6. Emission spectra of various RE-substituted ZrO2 ceramics recorded at different excitations. Emission behavior of (a) Yb3+, (b) Dy3+, (c) Tb3+, (d) Gd3+, (e) Eu3+, and (f) Nd3+ substitutions at different excitations. The labels 10, 20, 40, 80, and 100 refer to the RE concentrations in mol %.

high Tb3+ content beyond 40 mol %.58,59 Excitation of Gd3+ systems at 285 nm (Figure 6d) exhibited strong band ∼490− 520 nm specific of 4f7→ 4f6 transition, and moreover the weak emission of Gd3+ is also determined at 445−455 and 460−475 nm.60,61 All the Eu3+ compositions excited at 393 nm (Figure 6e) ensured red emission at 590 nm (5D0→7F1), 606 nm (5D0→7F2), 633 and ∼645−665 nm (5D0→7F3), and 715 nm (5D0→7F4). A gradual concentration quenching is also witnessed for high Eu3+ contents for the dominant 715 nm peak.62−66 Excitation of Nd3+ added compositions at 795 nm (Figure 6f) unveiled bands at 1015, 1068, 1082, and 1113 nm characteristic of 4F3/2→4I11/2 transitions alongside the bands at 1137, 1178, 1194, and 1267 nm for 4F3/2→4I13/2 transitions. Ten and 20 mol % of Nd3+ exhibited more intense bands at 1068 and 1194 nm, while the Nd3+ additions at the higher side enunciate strong band at 1194 nm typical of Nd3+ transition in the NIR region.67,68 3.5. Morphological and Mechanical Features. The scanning electron micrographs, EDX spectra, and histograms

of grain size distribution are presented in Figures 7, and S7− S9. The RE concentrations at optimum level (20 and 40 mol %) displayed dense microstructures complimented by the small-sized grains, and on the contrary, an increment in the RE concentrations beyond 40 mol % enunciated higher-sized grains. Nevertheless, RE additions in ZrO2 system unveiled better densification that is justified by the absence of pores in microstructures. Load versus displacement graphs determined from the indentation are displayed in Figures 8 and S10. The uniform loading profile devoid of pop ups from the indentation graphs accomplish the attainment of better densification. The selective mechanical properties, namely, Young’s modulus and hardness (Table 4), determined from indentation ensured a gradual reduction with a simultaneous increment in RE additions, and this result corroborated with the morphological analysis. A gradual rise in the grain growth with incremental RE additions is perceived, and this is mirrored in mechanical data. A close observation of the mechanical data revealed a good uniformity amid 20 and 40 mol % RE substitutions in I


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Table 4. Young’s Modulus and Hardness Data of All the RESubstituted ZrO2 Specimens Determined from the Load versus Displacement Curves of Indentation Experiments ZrO2:Yb

Figure 7. Morphological features of the selective RE-substituted ZrO2 specimens sintered at 1500 °C. 40 mol % (a) Yb3+, (b) Dy3+, (c) Tb3+, (d) Gd3+, (e) Eu3+, and (f) Nd3+ substitutions in ZrO2.

ZrO2 system, while enhanced RE additions demonstrated a considerable decline in the resultant data. 3.6. Aging Studies. The long-term instability in vivo is considered a major drawback of ZrO2 implants. The phase stability of the selective RE-doped ZrO2 specimens was tested under hydrothermal conditions. For this purpose, the speci-

1ZY

2ZY

4ZY

8ZY

10ZY

Young’s modulus (GPa) hardness (GPa) ZrO2:Dy

158.24 ± (8.17)

137.22 ± (1.60)

129.51 ± (2.25)

120.49 ± (2.08)

119.58 ± (8.10)

11.37 ± (0.61) 1ZD

10.90 ± (0.35) 2ZD

9.66 ± (0.47) 4ZD

9.05 ± (0.52) 8ZD

7.50 ± (0.43) 10ZD

Young’s modulus (GPa) hardness (GPa) ZrO2:Tb

156.65 ± (8.17)

153.43 ± (2.90)

155.19 ± (11.63)

132.14 ± (7.70)

127.64 ± (6.74)

11.07 ± (1.04) 1ZT

10.58 ± (0.91) 2ZT

10.11 ± (0.37) 4ZT

8.90 ± (1.02) 8ZT

7.29 ± (0.66) 10ZT

Young’s modulus (GPa) hardness (GPa) ZrO2:Gd

154.24 ± (5.17)

132.51 ± (1.25)

125.58 ± (5.10)

117.22 ± (1.60)

112.39 ± (1.08)

11.27 ± (0.41) 1ZG

10.67 ± (0.25) 2ZG

9.86 ± (0.27) 4ZG

8.01 ± (0.12) 8ZG

7.87 ± (0.43) 10ZG

Young’s modulus (GPa) hardness (GPa) ZrO2:Eu

161.81 ± (1.36)

154.60 ± (5.31)

128.70 ± (2.80)

115.75 ± (2.68)

112.79 ± (4.26)

10.84 ± (0.71) 1ZE

9.85 ± (0.59) 2ZE

9.61 ± (0.62) 4ZE

6.06 ± (0.14) 8ZE

5.42 ± (0.47) 10ZE

Young’s modulus (GPa) hardness (GPa) ZrO2:Nd

158.63 ± (5.82)

153.32 ± (6.60)

129.31 ± (5.72)

110.52 ± (1.41)

105.37 ± (2.38)

10.45 ± (0.98) 1ZN

9.47 ± (0.38) 2ZN

9.14 ± (0.42) 4ZN

6.47 ± (0.53) 8ZN

5.67 ± (0.53) 10ZN

Young’s modulus (GPa) hardness (GPa)

90.84 ± (5.10)

89.69 ± (4.84)

89.96 ± (5.39)

89.06 ± (1.60)

121.10 ± (4.33)

4.77 ± (0.63)

4.48 ± (0.41)

4.34 ± (0.52)

4.02 ± (0.11)

6.48 ± (0.21)

Figure 8. Indendation profile of selective RE-substituted ZrO2 specimens obtained at RT. 40 mol % (a) Yb3+, (b) Dy3+, (c) Tb3+, (d) Gd3+, (e) Eu3+, and (f) Nd3+ substitutions in ZrO2. J


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Figure 9. XRD patterns of selective RE-substituted ZrO2 specimens recorded after autoclave test. (a) 20 and (b) 40 mol % of different RE substitutions in ZrO2 specimens.

Figure 10. (a) CT values (HU) as a function of 40 mol % of Yb3+- and Dy3+-substituted ZrO2 samples. In vitro CT images at different concentrations of 40 mol % of (b) Yb3+ and (c) Dy3+.

mens prepared under a hydraulic press of size 0.6 mm × 0.3 mm sintered at 1500 °C were used. The resultant specimens were subjected to hydrothermal tests in autoclave with the conditions of 134 °C at 2 bar pressure and were periodically analyzed at regular intervals. The previous investigations state that 1 h of autoclave test is equivalent to four years stay in vivo, which is susceptible to induce gradual phase transformation at the surface.69−71 Devile et al. reported the 15% of t- → m-ZrO2 degradation in ZTA systems after 115 h of aging tests. The

improved aging resistance displayed by ZTA due to the substitution of higher-sized trivalent RE have been reported.72,73 Further, the ability of Gd3+ and Gd3+/Dy3+ cosubstitutions in ZTA to resist hydrothermal degradation until 64 and 128 h is also demonstrated.15,74 Here, the autoclave tests were performed for a 4−128 h period to induce artificial phase transformation. The XRD pattern of selective compositions recorded after autoclave tests (Figure 9) affirm the resistance to phase degradation barring Nd3+ system, which K


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Figure 11. RT magnetization curves of selective RE-substituted ZrO2 compositions recorded after heat treatment at 1500 °C. (a) 10, (b) 40, and (c) 100 mol % of dopant concentrations.

indicated t/c → m-ZrO2 transformation after autoclave tests. Nonetheless, 20 mol % of Nd3+ substitution still ensured good similarity in the phase behavior before and after autoclave tests. 3.7. Hounsfield Unit Measurement and X-ray CT Imaging in Vitro. Given the chemical and physical nature of high atomic-sized lanthanides, they are considered more effective in absorbing energetic photons of a polyenergetic spectrum than commercially available iodine.75,76 In particular, Yb3+- and Dy3+-doped ZrO2 systems were investigated for their potential application in X-ray CT imaging. In vitro X-ray CT phantom images (Figure 10) were acquired using 40 mol % of Yb3+- and Dy3+-doped ZrO2 systems of various concentrations in the range of 0.62, 1.25, 2.5, 5, and 10 mg mL−1. As the mass concentration of Yb 3+ and Dy 3+ gets increased, the simultaneous increment in the corresponding X-ray CT signal is noticed. This inference exhibited good corroboration with the other reported CT contrast agents at X-ray energies of 120 kV.77−79 The CT values of Yb3+ (362 HU) and Dy3+ (124 HU) doped systems determined at the concentration of 10 mg mL−1 ascribed to the K-edge of RE within the higher-energy region of X-ray spectrum. At 10 mg/mL mass concentration, the CT value for Dy3+-doped ZrO2 exhibited higher X-ray attenuation in comparison with Gd3+/Dy3+ dual-doped βCa3(PO4)2,80 NaGdF4,81 and DyNPs-Gd-Ir82 that revealed 161, 138, and 158 HU, respectively, while cit-NaLuF4:Yb, Tm has indicated excellent X-ray CT imaging ability with 160 HU at 10 mg/mL.83 The results also ensure the superior contrast efficacy displayed by Yb3+-doped ZrO2 systems on comparison with the reported systems such as Er3+-doped Yb2O384 and Yb3+-doped β-Ca3(PO4)2 systems.77

3.8. Magnetic Characteristics. The selective compositions were subjected to magnetic field measurements in the range from −10 000 to 10 000 Oe at RT using vibrating sample magnetometer (VSM). The resultant magnetization curves (Figures 11 and S11) ensure the paramagnetic response for all the compositions with the effective magnetic moment ranging from 0.02 to 2.76 emu g−1. In contrast with all other systems, the Nd3+ system exhibited relatively low magnetization values. These attained values are comparably less than their respective atomic magnetic moments and also for the complex that possess REs.85−87 Table 5 data unveil an upsurge in magnetization values (M) with incremental RE additions. The magnetization demonstrated a linear upsurge with the applied field, and the saturation of magnetization values were absent even at the maximum field. This indicates that an Table 5. Room-Temperature Magnetization Values for RESubstituted ZrO2 Compositions Recorded after Heat Treatment at 1500 °C ZRE systems ZrO2:Yb ZrO2:Dy ZrO2:Tb ZrO2:Gd ZrO2:Nd

L

magnetization (M) emu/g 1ZY 0.07 1ZD 0.47 1ZT 0.29 1ZG 0.32 1ZN 0.02

2ZY 0.16 2ZD 0.84 2ZT 0.50 2ZG 0.43 2ZN 0.05

4ZY 0.28 4ZD 1.59 4ZT 0.81 4ZG 0.86 4ZN 0.14

8ZY 0.29 8ZD 1.66 8ZT 1.02 8ZG 1.30 8ZN 0.14

10ZY 0.40 10ZD 2.76 10ZT 1.17 10ZG 1.32 10ZN 0.16


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Figure 12. (a) 1/T1 vs concentration for Gd3+-substituted ZrO2. (b) 1/T2 vs concentration for various RE-substituted ZrO2 (Dy3+, Tb3+, Gd3+, Nd3+). (c) T1 and (d) T2 weighted MR images, respectively.

4. DISCUSSION

energetically isolated spin state is never achieved due to paramagnetism. 3.9. Relaxivity Measurement and MR Imaging in Vitro. The paramagnetic behavior exhibited by the systems as inferred from the RT magnetization (Figure 11) illustrated an increasing magnetization trend with incremental dopant concentrations. To obtain consistent MRI contrastability results, a stabilizing concentration of 40 mol % RE-doped ZrO2 with magnetization values in the range of 0.28−1.59 emu g−1 was considered for analysis. T1 and T2 contrastability of the samples was tested based on the nature of dopant. The phantom images of various systems are shown in (Figure 12). A gradual raise in the image contrast for T1 maps and a steady depletion in the contrast intensity for T2 maps with incremental dopant concentrations were determined, and the corresponding relaxivity measurements correlate with the phantom images. The relaxivity values determined from the slope of 1/T1 and 1/T2 for different RE are as follows: 18.14 mM−1 s−1 for r1 of Gd3+ and 9.62, 5.15, 14.38, 3.24 mM−1 s−1 for r2 respective of Dy3+, Tb3+, Gd3+, and Nd3+. The attained values established good concurrence with the literature data.88−93

The stabilization of either t-ZrO2 or c-ZrO2 polymorphs at RT assisted by the substitution of elements (Ca2+, Mg2+, and Y3+) with dissimilar valences relative to Zr4+ are more common. The generation of oxygen vacancies to balance the net charge created by the accommodation of these elements at the ZrO2 lattice has been considered the prime reason for this metastable stabilization. Moreover, the dopant concentrations persuade ZrO2 stabilization in either tetragonal or cubic forms. However, in vivo case studies conducted on these ZrO2 implants still demonstrated the occurrence of t/c → m-ZrO2 transformation under suboptimal conditions. Other than the normally used stabilizers, rare earths of Gd3+, Sc3+, and Dy3+ have also been used; however, this was with the main motivation going toward other applications.17−19 In this context, the present study aimed to investigate the structural, mechanical, optical, and imaging contrast features of ZrO2 due to rare-earth substitutions. The synthesized materials are intended for various biomedical applications. Six different rare earths, namely, Dy3+, Gd3+, Yb3+, Eu3+, Tb3+, and Nd3+, were M


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concentrations of Yb3+, Dy3+, and Tb3+; however, they are with a distorted and broadened spectrum with respect to the incremental dopant concentrations. The gradual phase transformation from c-ZrO2 → RE2Zr2O7 is also apparent as the 80 mol % concentrations of Gd3+ and Eu3+ that enunciated the mixture of c-ZrO2 and RE2Zr2O7 bands at 1400 °C prior to the complete transformation to RE2Zr2O7 at 1500 °C. On the contrary, Nd3+ exhibited complete transformation to RE2Zr2O7 at both the investigated temperatures. Moreover, a gradual shift of the bands typical of RE2Zr2O7 is obvious as a function of enhanced dopant size in the order of Gd3+, Eu3+, and Nd3+. Generally, the stable lanthanides possess 3+ oxidation state and comprise the electronic configuration of 4fn5d16s2 with half-filled f orbitals.95,96 The absorption spectra recorded for six different RE-doped ZrO2 exhibited good similarities among each other; however, their corresponding emission spectra disclosed a wide characteristic that covered UV, visible, and NIR regions, thus attributing to the intraconfigurational f−f transition. The absorption and emission bands depend on two types of transitions: parity-allowed magnetic dipole transition (MD) and parity-forbidden electric dipole transitions (ED). An apparent upsurge in the absorption intensity of ∼200−250 nm with a simultaneous increment in the RE content is attributed to the charge transfer from O (2p) valence band to Zr4+ (4d) conduction band. This charge transfer ascends due to the oxygen vacancies generated by the RE substitution at ZrO2 lattice.49 On the contrary, absorption spectra of 80 and 100 mol % of Yb3+-doped composition specified the broadening of band ∼280−500 nm, which is ascribed to the cooperative absorption created by the excitation of neighboring Yb−Yb ions. This cooperative absorption is termed as a virtual state in which the oxygen bridges with Yb3+ neighbors to establish Yb−O−Yb linkages, and, as a consequence, overlapping of Yb-4f and O-2p occurs to enhance the interaction of active ions.97 The NIR emission of Yb3+ occurs due to 2F5/2→2F7/2 transition that comprises two energy states alongside seven split stark levels. Nonetheless, an increment in the dopant concentration leads to the resilient quenching of intense peak at ∼1042 nm. It has also been reported that enhanced Yb3+ content escalates the probability of Yb−Yb dimer to emit in the visible region, which results in the overall reduction of Yb3+ in NIR emission.98 The typical Dy3+ emission is centered ∼483−497 nm in the blue region that displays magnetic characteristic, while the other less and hypersensitive peak at 583 nm in the yellow region exhibits electrical features. Emission in the yellow region strongly depends on the crystalline field of the host, while blue emission is independent of the host matrix. However, the quenching effect attributed by the reduction in the emission intensity for high Dy 3+ concentrations (80 and 100 mol %) is ascribed to the cross relaxation phenomenon that occurs during the reduction in the distance between the average Dy3+ ion.99 Tb3+ substitutions emit less- and high-intense peaks, respectively, in blue and green regions during excitation in the UV range. The emission intensities of 544 and 488 nm peaks reach a maximum for 20 mol % Tb3+, while its addition beyond this limit originates a quenching effect. The cross relaxation between two ions due to multipolar interactions has been considered the prime reason for the quenching effect in most of the RE,100 while the case of Tb3+ depicts a contradictory reason for this quenching effect that is contributed by the stronger resonant energy transfer between

selected mainly on the basis of diverse physical and chemical properties displayed by these elements. Structural analysis emphasizes the concomitant roles of dopant concentration and the type of RE in determining the stability of the resultant ZrO2 polymorph. The invariable presence of traces of m-ZrO2, t-ZrO2, and c-ZrO2 are apparent in all the 10 mol % RE concentrations, while a sharp increment to 20 mol % yielded single c-ZrO2 phase in all the compositions except for Nd3+ in which m-ZrO2, t-ZrO2, and c-ZrO2 mixtures still retained at both 1400 and 1500 °C. A similar result is witnessed in the case of the dopant concentration with 40 mol %; nevertheless, t-ZrO2 and cZrO2 mixtures devoid of m-ZrO2 were obtained in Nd3+ compositions. Further increments in dopant concentrations to 80 and 100 mol % displayed contrasting results that were dependent on the size of the investigated RE. Yb3+ (0.985 Å), Dy3+ (1.027 Å), and Tb3+ (1.040 Å) specified c-ZrO2, whereas Gd3+ (1.053 Å), Eu3+ (1.066 Å), and Nd3+ (1.109 Å) enunciated the formation of their respective zirconates (RE2Zr2O7) at 1500 °C. It is also obvious from the results that Tb3+ remains the critical size limit to retain c-ZrO2, while the size of RE exceeding this limit instigates c-ZrO2 → RE2Zr2O7 phase transition as has been witnessed from Gd3+ and continued in the order of Eu3+ and Nd3+. The annealing temperature also plays an important role in which Gd3+ and Eu3+ perceived c-ZrO2 and RE2Zr2O7 mixtures at 1400 °C. The ratio between the two phases was still dependent on the size effect with relatively lower-sized Gd3+ comprising c-ZrO2 as a major fraction, whereas the higher-sized Eu3+ favored Eu2Zr2O7 as a dominant component. A close comparison of the existing results with the commonly investigated dopant, namely, Y3+ (1.019 Å) in ZrO2 revealed the fact that t-ZrO2 is retained until 8 mol %, whereas c-ZrO2 remains as single phase in the range of 8−100 mol % of Y2O3.94 These inferences reveal that the effects of dopant size and concentration and of annealing temperatures play key roles in the phase transformations in the order of t-ZrO2 → c-ZrO2 → RE2Zr2O7. These phase transformations are also mainly based on the stresses induced by the continuous lattice expansion of ZrO2 due to RE substitutions rather than the periodic rearrangement of atoms to articulate a new structure. In case of t- → c-ZrO2 transition, the substituted element occupies positions along a= b-axis, and this expansion continues until a stage where a- = b- = c-axis becomes unique to attain a cubic fluorite structure with Fm3̅ m space setting. Once again, the continuous accumulation of dopant in the resulting cubic lattice leads to an endless expansion whereby the structural defect is induced and, as a consequence, yields a pyrochlorite structure with Fd3̅m space setting, which is otherwise termed as distorted fluorite structure. Raman spectra coupled with the quantitative X-ray analysis confirm the phase degradation to m-ZrO2 at low RE3+ contents. The stabilization of c-ZrO2 is evident for RE dopant concentrations within 20 and 40 mol %, excluding Nd3+ substitution. In this last case, the degradation to m-ZrO2 was witnessed in the X-ray analysis. A gradual expansion of the cZrO2 lattice is also visible from the Raman analysis, which perceived a steady shift of the maximum intensity band (615 cm−1) toward the lower-frequency region as a function of enhanced size in the substituted RE. This Raman shift alongside considerable peak broadening is obvious at both 20 and 40 mol % of RE concentrations. The bands pertinent to c-ZrO2 are still obvious at both 80 and 100 mol % N


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persist in c-ZrO2 due to enhanced dopant concentrations that induce the formation of voluminous grains alongside simultaneous reduction in the grain boundary area thus retarding degradation. The current findings demonstrate the influence of both the dopant size and its content in playing a major role to restrict the aging behavior through the segregation of dopant at the boundaries of zirconia grain.72,102,103 Literature reports also emphasize the degradation of c-ZrO2 during prolonged aging in hydrothermal conditions.104 The aging studies that were performed according to ISO 13356 at 134 °C/2 bar for 128 h thus ensure the ability of the implants for their prolonged stay in vivo.105 X-ray CT imaging is one of the most powerful diagnostic techniques that generates anatomic information in biological tissues based on the differential X-ray absorption of high atomic number contrast agents administered within the body system. Despite the availability of iodinated contrast agents, the poor K-edge value of iodine (33 keV) with its low contrast efficacy is not optimal for X-ray attenuation.78 Moreover, iodine-based contrast agents are not an apt selection for patients with iodine hypersensitivity. Alternatively, metal-based nanoparticles of Au, Bi, Pt, and Ta are also reported as prospective X-ray CT contrast agents.76,79 In principle, these nanoparticles are expected to exhibit high photoelectron effects; however, they failed to show pronounced X-ray attenuation in clinical settings. It is noteworthy that the photoelectric effect also emphasizes the enhanced mass attenuation coefficients and the similar K-edge values of these metal nanoparticles and iodine deviate from the higherintensity region of X-ray spectrum, thus contributing to limited X-ray attenuation.106 In this pursuit, the Dy3+ and Yb3+ systems with respective K-edge values of 53.78 and 61.33 keV in the high-intensity region of X-ray spectrum with more X-ray absorption demonstrates potential interest. The magnetic characteristics were evaluated for all the systems except Eu3+-substituted system, which is mainly due to the dominant fluorescent imaging features demonstrated by Eu3+ rather than their applications in MRI.107,108 Magnetic studies performed at RT unveil unique paramagnetic response, and this paramagnetic behavior of RE demonstrates the potential ability to examine the MRI contrast features. Gd3+ substitutions revealed both T1 and T2 contrast features with T1 demonstrating positive contrast in a progressive manner as a function of concentration, while T2 exhibited negative contrast features with the concentration-dependent gradual decline in the contrast effect. On the contrary, Dy3+, Tb3+, and Nd3+ displayed T2 contrast features. The r1 and r2 relaxivity values of Gd3+ are, respectively, determined as 18.14 and 14.38 mM−1 s −1 , while r 2 values of Dy 3+ , Tb 3+ , and Nd 3+ are correspondingly determined as 9.62, 5.15, and 3.24 mM−1 s−1. These results ensure the comparable r1 and r2 values at high fields, making them appropriate candidates for positive and negative contrast agents.88−93 Despite the efficient contrast imaging effects, the toxicity profiles and biodistribution in vivo is a matter of further investigation.

ions with nonradiative interaction to persuade decline in emission intensity.58 Gd3+ expresses weak blue emission ∼440−480 nm and an intense green emission ∼490−520 nm, which is ascribed to the radiative and nonradiative transitions of Gd3+.61 In continuation with the other systems, Gd3+ substitutions also demonstrate quenching effects beyond 20 mol %. Nevertheless, a strong surge in the green emission beyond 40 mol % of Gd3+ substitution exhibits good connectivity with the results from physiochemical characterization that specified c-ZrO2 → c-Gd2Zr2O7 phase transition. Notice that the emission peak distinctive of c-Gd2Zr2O7 at 495 nm is hard to be determined due to the broad emission at 490−520 nm.60 Eu3+ substitutions in ZrO2 display orange and red emissions alongside two intense peaks at 590 and 606 nm for low concentrations. On the one hand, the prominent 606 nm peak is due to the forced electron dipole transition of Eu3+ (5D0→7F2) that is hypersensitive to the crystal environment. On the other hand, the 590 nm peak typical of 5D0→7F1 magnetic dipole transition is insensitive to the crystal environment.62 As usual, the enhanced dopant concentration leads to the quenching of both 590 and 606 nm peaks and consequently gives rise to a more intense red emission centered at 715 nm. In general, orange emission is favored by the occupancy of Eu3+ at a host crystallographic site with inversion symmetry (5D0→7F1), while the red emission is favored when the Eu3+ is located at the noninversion center of host site.63 The NIR emission of Nd3+ is obvious from lessintense peak at 1068 nm typical of 4F3/2→4I11/2 transition and higher-intensity peak ∼1198 nm ascribed to the potential 4 F3/2→4I13/2 transition. The apparent quenching is witnessed for 1068 nm peak at higher concentrations; however, such effect is negligible in 1198 nm peak for Nd3+ substitutions. This inverse behavior is attributed to the formation of cNd2Zr2O7 in higher concentrations as witnessed in Gd3+ systems. Morphological analysis revealed dense and pore-free microstructures in all the investigated systems. A gradual reduction in the hardness and Young’s modulus values is apparent with respect to the increment in dopant concentrations. Despite the size difference between the investigated RE, the trend in mechanical data with respect to their concentrations has been similar. This enables inferring that the size of RE plays a negligible role in the resultant mechanical data. However, it was also seen that enhanced dopant concentrations led to the phase transitions. This might correlate with the reduced mechanical data in the specimens with high dopant concentrations. Nonetheless, all the specimens unveil mechanical data better than the natural bone.101 The control of grain size and the metastable state of ZrO2 are considered the most important criteria to yield better mechanical features. The gradual reduction in mechanical data is also ascribed to the role of RE concentration in enhancing the grain size of the ZrO2 specimens, thereby leading to a gradual drop in the hardness and Young’s modulus. Thus, the indentation results specify the importance of dopant concentrations to accomplish a controlled grain size and fetch better mechanical properties. Here the results show that RE dopant concentration until 40 mol % reasonably yields better properties. Accelerated aging tests confirm the absence of m-ZrO2 in all the systems except higher-sized Nd3+-doped system. In general, t- → m-ZrO2 phase degradation is attributed to the dopant content, grain size, and oxygen vacancies. However, this condition does not

5. CONCLUSION The impacts of added rare-earth concentrations on the ZrO2 structure were explored. Depending on the type and size of RE, substantial structural variations can be induced in the resultant crystal structure of ZrO2. RE additions within the range of 20− 40 mol % are required to retain the unique crystal structure of O


Article

ACS Biomaterials Science & Engineering Notes

ZrO2. RE additions below and above this optimum range led to either t- → m-ZrO2 phase degradation or the voluminous expansion of the c-ZrO2 unit cell to yield a defective cubic fluorite structure of RE2Zr2O7. Both quantitative X-ray analysis and Raman spectra demonstrated good concurrence in the structural variations in ZrO2 due to assorted RE additions. The absorption and emission spectra unveiled the characteristic nature of RE rather than the bulk ZrO2 . Moreover, concentration quenching is a common phenomenon experienced in all the investigated RE. The results also deduced the detrimental effect witnessed in the mechanical performance due to excess RE inclusions in the ZrO2 structure. The paramagnetic response of the ZrO2 material influenced by RE are apparent with Dy3+, Tb3+, Gd3+, and Nd3+ presenting T1 or T2 contrast features that are likely to gain attention in MRI applications. In addition, the presence of Yb3+ and Dy3+ in ZrO2 confer additional features of high X-ray attenuation. Among the investigated compositions, ZrO2:Gd3+ and ZrO2:Dy3+ combinations display better mechanical and paramagnetic features alongside exceptional T1 and T2 contrast features for MRI, while the latter still express good X-ray attenuation for CT imaging. The overall outcome from the current investigation widens the possibility of RE-doped ZrO2 systems to be explored in biomedical applications.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial assistance received from Council of Scientific and Industrial Research, CSIR, India [Reference No. 01(2952)/ 18/EMR-II dated 01.05.2018], and Department of Science and Technology-Science and Engineering Research Board, DSTSERB, India [Reference No. EMR/2015/002200 dated 20.01.2016], are acknowledged. The facilities availed from Central Instrumentation Facility of Pondicherry Univ. are also acknowledged. K.S. acknowledges CSIR, India (Reference No. 09/559(0123)/18-EMR-I), for Senior Research Fellowship. The support from the Project CICECO−Aveiro Institute of Materials (Reference No. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement, is acknowledged.

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(1) Chevalier, J.; Gremillard, L. 1.6 Zirconia as a Biomaterial. In Comprehensive Biomaterials II; Elsevier, 2017; Vol. 8, pp 122−144. DOI: 10.1016/B978-0-12-803581-8.10245-0. (2) Jiang, L.; Guo, S.; Qiao, M.; Zhang, M.; Ding, W. Study on the Structure and Mechanical Properties of Magnesia Partially Stabilized Zirconia during Cyclic Heating and Cooling. Mater. Lett. 2017, 194, 26−29. (3) Govila, R. K. Strength Characterization of MgO-Partially Stabilized Zirconia. J. Mater. Sci. 1991, 26 (6), 1545−1555. (4) Saha, K. Innovative Chemical Method for Preparation of CalciaStabilized Zirconia Powders. Br. Ceram. Trans. 1995, 94 (3), 123− 127. (5) Wang, G.; Liu, X.; Ding, C. Phase Composition and In-Vitro Bioactivity of Plasma Sprayed Calcia Stabilized Zirconia Coatings. Surf. Coat. Technol. 2008, 202 (24), 5824−5831. (6) Papanagiotou, H. P.; Morgano, S. M.; Giordano, R. A.; Pober, R. In Vitro Evaluation of Low-Temperature Aging Effects and Finishing Procedures on the Flexural Strength and Structural Stability of Y-TZP Dental Ceramics. J. Prosthet. Dent. 2006, 96 (3), 154−164. (7) Paul, A.; Vaidhyanathan, B.; Binner, J. G. P. Hydrothermal Aging Behavior of Nanocrystalline Y-TZP Ceramics. J. Am. Ceram. Soc. 2011, 94 (7), 2146−2152. (8) Pandey, A. K.; Biswas, K. Influence of Sintering Parameters on Tribological Properties of Ceria Stabilized Zirconia Bio-Ceramics. Ceram. Int. 2011, 37 (1), 257−264. (9) Thangadurai, P.; Bose, A. C.; Ramasamy, S. Phase Stabilization and Structural Studies of Nanocrystalline La2O3-ZrO2. J. Mater. Sci. 2005, 40 (15), 3963−3968. (10) Vagkopoulou, T.; Koutayas, S. O.; Koidis, P.; Strub, J. R. Zirconia in Dentistry: Part 1. Discovering the Nature of an Upcoming Bioceramic. Eur. J. Esthet. Dent. 2009, 4 (2), 130−151. (11) Piconi, C.; Maccauro, G. Zirconia as a Ceramic Biomaterial. Biomaterials 1999, 20 (1), 1−25. (12) Kelly, J. R.; Denry, I. Stabilized Zirconia as a Structural Ceramic: An Overview. Dent. Mater. 2008, 24 (3), 289−298. (13) Lee, W. E.; Rainforth, M. Ceramic Microstructures: Property Control by Processing; Springer Science & Business Media, 1994. . (14) Aragón-Duarte, M. C.; Nevarez-Rascón, A.; Esparza-Ponce, H. E.; Nevarez-Rascón, M. M.; Talamantes, R. P.; Ornelas, C.; MendezNonell, J.; González-Hernández, J.; Yacamán, M. J.; Hurtado-Macías, A. Nanomechanical Properties of Zirconia- Yttria and Alumina Zirconia- Yttria Biomedical Ceramics, Subjected to Low Temperature Aging. Ceram. Int. 2017, 43 (5), 3931−3939. (15) Ponnilavan, V.; Poojar, P.; Geethanath, S.; Kannan, S. Gadolinium Doping in Zirconia-Toughened Alumina Systems and Their Structural, Mechanical, and Aging Behavior Repercussions. Inorg. Chem. 2017, 56 (18), 10998−11007.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsbiomaterials.8b01570. Particle size calculated from dynamic light scattering of 40 mol % RE-substituted ZrO2 ball milled specimens. XRD patterns of the investigated compositions recorded after heat treatment at 1400 °C. Refined diffraction patterns of selective RE-substituted ZrO2 at 1400 °C. Variations in lattice parameters of the t-ZrO2, c-ZrO2, and c-RE2Zr2O7 crystal structures due to various RE substitution in the concentration range of 20−100 mol % at 1400 °C. Raman spectra of individual REsubstituted ZrO2 recorded after heat treatment at 1400 °C. Absorption spectra of various RE-substituted ZrO2 compositions recorded at different excitations. Morphological features of 20 mol % RE-substituted ZrO2 specimens sintered at 1500 °C. EDX spectra of 40 mol % RE-substituted ZrO2 specimens sintered at 1500 °C. Histogram grain size distribution of 40 mol % REsubstituted ZrO2 specimens sintered at 1500 °C. Indentation profiles of 20 mol % RE-substituted ZrO2 specimens obtained at room temperature. Roomtemperature magnetization curves of selective REsubstituted ZrO2 compositions recorded after heat treatment at 1500 °C. Refinement agreement parameters of all the concentration of ZRE system at 1400 and 1500 °C (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 0091-413-2654973. ORCID

Dinesh Kumar: 0000-0001-8079-6739 José Maria da Fonte Ferreira: 0000-0002-7520-2809 Sanjeevi Kannan: 0000-0003-2285-4907 P


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(35) Korneev, V. R.; Glushkova, V. B.; Keler, E. K. Heats of Formation of Rare Earth Zirconates. Inorg. Mater. 1971, 7, 781−782. (36) Vasanthavel, S.; Awasthi, S.; Dhayalan, A.; Derby, B.; Kannan, S. Structural, Mechanical, Imaging and in Vitro Evaluation of the Combined Effect of Gd3+and Dy3+in the ZrO2-SiO2 Binary System. Inorg. Chem. 2018, 57 (8), 4602−4612. (37) Thangavel, K.; Saritaş, E. Ü . Aqueous Paramagnetic Solutions for MRI Phantoms at 3T: A Detailed Study on Relaxivities. Turkish J. Electr. Eng. Comput. Sci. 2017, 25 (3), 2108−2121. (38) Gazzoli, D.; Mattei, G.; Valigi, M. Raman and X-Ray Investigations of the Incorporation of Ca2+ and Cd2+ in the ZrO2 Structure. J. Raman Spectrosc. 2007, 38 (7), 824−831. (39) Fernandez Lopez, E.; Sanchez Escribano, V.; Panizza, M.; Carnasciali, M. M.; Busca, G. Vibrational and Electronic Spectroscopic Properties of Zirconia Powders. J. Mater. Chem. 2001, 11 (7), 1891−1897. (40) Shu, X.; Fan, L.; Lu, X.; Xie, Y.; Ding, Y. Structure and Performance Evolution of the System (Gd1‑XNdx)2(Zr1‑YCey2O7 (0⩽x, Y⩽1.0). J. Eur. Ceram. Soc. 2015, 35 (11), 3095−3102. (41) Li, H.; Li, N.; Li, Y.; Tao, Q.; Zhao, Y.; Zhu, H.; Ma, Y.; Zhu, P.; Wang, X. Pressure-Induced Disordering and Phase Transformations in Eu2Zr2O7pyrochlore. High Pressure Res. 2017, 37 (2), 256−266. (42) De La Rosa, E.; Solis, D.; Díaz-Torres, L. A.; Salas, P.; AngelesChavez, C.; Meza, O. Blue-Green Upconversion Emission in ZrO2: Yb3+ Nanocrystals. J. Appl. Phys. 2008, 104 (10), 103508. (43) Amjad, R. J.; Sahar, M. R.; Ghoshal, S. K.; Dousti, M. R.; Arifin, R. Synthesis and Characterization of Dy3+doped Zinc-Lead-Phosphate Glass. Opt. Mater. (Amsterdam, Neth.) 2013, 35 (5), 1103−1108. (44) Azizan, S. A.; Hashim, S.; Razak, N. A.; Mhareb, M. H. A.; Alajerami, Y. S. M.; Tamchek, N. Physical and Optical Properties of Dy3+: Li2O-K2O-B2O3glasses. J. Mol. Struct. 2014, 1076, 20−25. (45) Wakefield, G.; Keron, H. A.; Dobson, P. J.; Hutchison, J. L. Structural and Optical Properties of Terbium Oxide Nanoparticles. J. Phys. Chem. Solids 1999, 60 (4), 503−508. (46) Zmojda, J.; Kochanowicz, M.; Miluski, P.; Dorosz, D. SideDetecting Optical Fiber Doped with Tb3+ for Ultraviolet Sensor Application. Fibers 2014, 2 (4), 150−157. (47) Dhananjaya, N.; Nagabhushana, H.; Nagabhushana, B. M.; Rudraswamy, B.; Sharma, S. C.; Sunitha, D. V.; Shivakumara, C.; Chakradhar, R. P. S. Effect of Different Fuels on Structural, Thermo and Photoluminescent Properties of Gd2O3nanoparticles. Spectrochim. Acta, Part A 2012, 96, 532−540. (48) Rudraswamy, B.; Dhananjaya, N. Photoluminescence Properties of Gadolinium Oxide Nanophosphor. IOP Conf. Ser.: Mater. Sci. Eng. 2012, 40 (1), 012034. (49) Villabona-Leal, E. G.; Diaz-Torres, L. A.; Desirena, H.; Rodríguez-López, J. L.; Pérez, E.; Meza, O. Luminescence and Energy Transfer Properties of Eu3+and Gd3+in ZrO2. J. Lumin. 2014, 146 (3), 398−403. (50) Shwetha, M.; Eraiah, B. Influence of Europium (Eu 3+) Ions on the Optical Properties of Lithium Zinc Phosphate Glasses. IOP Conf. Ser.: Mater. Sci. Eng. 2018, 310, No. 012033. (51) Rasool, S. N.; Rama Moorthy, L.; Jayasankar, C. K. Optical and Luminescence Properties of Dy3+ ions in Phosphate Based Glasses. Solid State Sci. 2013, 22 (3), 82−90. (52) Mohan, S.; Thind, K. S.; Sharma, G. Effect of Nd3 + Concentration on the Physical and Absorption Properties of SodiumLead-Borate Glasses. Braz. J. Phys. 2007, 37 (4), 1306−1313. (53) Sardar, D. K.; Dee, D. M.; Nash, K. L.; Yow, R. M.; Gruber, J. B. Optical Absorption Intensity Analysis and Emission Cross Sections for the Intermanifold and the Inter-Stark Transitions of Nd3+(4f 3) in Polycrystalline Ceramic Y2O3. J. Appl. Phys. 2006, 100 (12), 123106. (54) Subhan, M. A.; Nakata, H. NIR and CT Luminescence Spectra of [Yb(TFN)(S-BINAPO)] and [Yb(HFA)(S-BINAPO)] Complexes. Spectrochim. Acta, Part A 2014, 130, 37−40. (55) Tamrakar, R. K.; Tiwari, N.; Dubey, V.; Upadhyay, K. Infrared Spectroscopy and Luminescence Spectra of Yb3+ Doped ZrO2 Nanophosphor. J. Radiat. Res. Appl. Sci. 2015, 8 (3), 399−403.

(16) Jiang, S.; Huang, X.; He, Z.; Buyers, A. Phase Transformation and Lattice Parameter Changes of Non-Trivalent Rare Earth-Doped YSZ as a Function of Temperature. J. Mater. Eng. Perform. 2018, 27 (5), 2263−2270. (17) Pastor, M.; Maiti, S.; Pandey, A.; Biswas, K.; Manna, I. Effect of Dysprosia Doping on Structural and Electrical Property of Stabilized Zirconia for Intermediate- Temperature SOFCs. Mater. Chem. Phys. 2011, 125 (1−2), 202−209. (18) Rahaman, M. N.; Gross, J. R.; Dutton, R. E.; Wang, H. Phase Stability, Sintering, and Thermal Conductivity of Plasma-Sprayed ZrO2-Gd2O3compositions for Potential Thermal Barrier Coating Applications. Acta Mater. 2006, 54 (6), 1615−1621. (19) Zhou, J.; Zhang, H.; Xu, H.; Xue, Q.; Huang, X.; Feng, Z.; Long, Z. Preparation and Electrical Characterization of Ultra-Fine Powder Scandia-Stabilized Zirconia. J. Rare Earths 2016, 34 (2), 181− 186. (20) Pihlgren, L.; Laihinen, T.; Rodrigues, L. C. V; Carlson, S.; Eskola, K. O.; Kotlov, A.; Lastusaari, M.; Soukka, T.; Brito, H. F.; Hö l sä , J. On the Mechanism of Persistent Up-Conversion Luminescence in the ZrO2:Yb3+,Er3+nanomaterials. Opt. Mater. (Amsterdam, Neth.) 2014, 36 (10), 1698−1704. (21) Liu, Y.; Zhou, S.; Tu, D.; Chen, Z.; Huang, M.; Zhu, H.; Ma, E.; Chen, X. Amine-Functionalized Lanthanide-Doped Zirconia Nanoparticles: Optical Spectroscopy, Time-Resolved Fluorescence Resonance Energy Transfer Biodetection, and Targeted Imaging. J. Am. Chem. Soc. 2012, 134 (36), 15083−15090. (22) Meenambal, R.; Poojar, P.; Geethanath, S.; Kannan, S. Substitutional Limit of Gadolinium in β-Tricalcium Phosphate and Its Magnetic Resonance Imaging Characteristics. J. Biomed. Mater. Res., Part B 2017, 105 (8), 2545−2552. (23) Meenambal, R.; Poojar, P.; Geethanath, S.; Sanjeevi, K. Structural Insights in Dy3+-Doped β-Tricalcium Phosphate and Its Multimodal Imaging Characteristics. J. Am. Ceram. Soc. 2017, 100 (5), 1831−1841. (24) Meenambal, R.; Nandha Kumar, P.; Poojar, P.; Geethanath, S.; Kannan, S. Simultaneous Substitutions of Gd3+ and Dy3+ in βCa3(PO4)2as a Potential Multifunctional Bio-Probe. Mater. Des. 2017, 120, 336−344. (25) Meenambal, R.; Kannan, S. Cosubstitution of Lanthanides (Gd 3+ /Dy 3+ /Yb 3+) in β-Ca3 (PO4)2 for Upconversion Luminescence, CT/MRI Multimodal Imaging. ACS Biomater. Sci. Eng. 2018, 4 (1), 47−56. (26) Vasanthavel, S.; Derby, B.; Kannan, S. Tetragonal to Cubic Transformation of SiO2-Stabilized ZrO2 Polymorph through Dysprosium Substitutions. Inorg. Chem. 2017, 56 (3), 1273−1281. (27) Yoshimura, M. Phase Stability of Zirconia. Am. Ceram. Soc. Bull. 1988, 67 (12), 1950−1955. (28) Li, P.; Chen, I. -W; Penner-Hahn, J. E. Effect of Dopants on Zirconia StabilizationAn X-ray Absorption Study: II, Tetravalent Dopants. J. Am. Ceram. Soc. 1994, 77 (5), 1281−1288. (29) Wang, C.; Zinkevich, M.; Aldinger, F. Phase Diagrams and Thermodynamics of Rare-Earth-Doped Zirconia Ceramics. Pure Appl. Chem. 2007, 79 (10), 1731−1753. (30) Chevalier, J.; Gremillard, L.; Virkar, A. V.; Clarke, D. R. The Tetragonal-Monoclinic Transformation in Zirconia: Lessons Learned and Future Trends. J. Am. Ceram. Soc. 2009, 92 (9), 1901−1920. (31) Smith, D. K.; Newkirk, W. The Crystal Structure of Baddeleyite (Monoclinic ZrO2) and Its Relation to the Polymorphism of ZrO2. Acta Crystallogr. 1965, 18 (6), 983−991. (32) Howard, C. J.; Hill, R. J.; Reichert, B. E. Structures of ZrO2 Polymorphs at Room Temperature by High-resolution Neutron Powder Diffraction. Acta Crystallogr., Sect. B: Struct. Sci. 1988, 44 (2), 116−120. (33) Robertson, J. M. Crystal Structures by R. W. G. Wyckoff. Acta Crystallogr. 1954, 7 (12), 867−867. (34) Klee, W. E.; Weitz, G. Infrared Spectra of Ordered and Disordered Pyrochlore-Type Compounds in the Series RE2Ti2O7, RE2Zr2O7and RE2Hf2O7. J. Inorg. Nucl. Chem. 1969, 31 (8), 2367− 2372. Q


Article

ACS Biomaterials Science & Engineering (56) Marzouk, M. A.; Ouis, M. A.; Hamdy, Y. M. Spectroscopic Studies and Luminescence Spectra of Dy2O3 Doped Lead Phosphate Glasses. Silicon 2012, 4 (3), 221−227. (57) Baéz-Rodríguez, A.; Alvarez-Fragoso, O.; García-Hipólito, M.; Guzmán-Mendoza, J.; Falcony, C. Luminescent Properties of ZrO2:Dy3+and ZrO2:Dy3++Li+films Synthesized by an Ultrasonic Spray Pyrolysis Technique. Ceram. Int. 2015, 41 (5), 7197−7206. (58) López-Romero, S.; García-Hipólito, M.; Aguilar-Castillo, A. Bright Green Luminescence from Zirconium Oxide Stabilized with Tb3+ Ions Synthesized by Solution Combustion Technique. World J. Condens. Matter Phys. 2013, 03 (04), 173−179. (59) Kumar, R.; Shanker, R.; Kotnala, R. K.; Chawla, S. Luminomagnetic K2Gd1‑XZr(PO4)3:Tbx3+phosphor with Intense Green Fluorescence and Paramagnetism. Phys. Status Solidi Appl. Mater. Sci. 2013, 210 (9), 1933−1937. (60) Singh, V. K.; Tripathi, S.; Mishra, M. K.; Tiwari, R.; Dubey, V.; Tiwari, N. Optical Studies of Erbium and Ytterbium Doped Gd2Zr2O7 Phosphor for Display and Optical Communication Applications. J. Disp. Technol. 2016, 12 (10), 1224−1228. (61) Tamrakar, R. K.; Bisen, D. P.; Brahme, N. Characterization and Luminescence Properties of Gd2O3phosphor. Res. Chem. Intermed. 2014, 40 (5), 1771−1779. (62) Tamrakar, R. K.; Bisen, D. P.; Upadhyay, K. Photoluminescence Behavior of ZrO2: Eu3+ with Variable Concentration of Eu3+ Doped Phosphor. J. Radiat. Res. Appl. Sci. 2015, 8 (1), 11−16. (63) Behrh, G. K.; Gautier, R.; Latouche, C.; Jobic, S.; Serier-Brault, H. Synthesis and Photoluminescence Properties of Ca2Ga2SiO7:Eu3+red Phosphors with an Intense5D0→7F4transition. Inorg. Chem. 2016, 55 (18), 9144−9146. (64) Sousa, F.; Lima, G.; Á vila, L.; et al. Incorporation of Europium III Complex into Nanoparticles and Films Obtained by the Sol-Gel Methodology. Mater. Res. 2010, 13 (1), 71−75. (65) Prakashbabu, D.; Ramalingam, H. B.; Hari Krishna, R.; Nagabhushana, B. M.; Chandramohan, R.; Shivakumara, C.; Thirumalai, J.; Thomas, T. Charge Compensation Assisted Enhancement of Photoluminescence in Combustion Derived Li+co-Doped Cubic ZrO2:Eu3+nanophosphors. Phys. Chem. Chem. Phys. 2016, 18 (42), 29447−29457. (66) Jeyaraman, J.; Shukla, A.; Sivakumar, S. Targeted Stealth Polymer Capsules Encapsulating Ln3+-Doped LaVO4 Nanoparticles for Bioimaging Applications. ACS Biomater. Sci. Eng. 2016, 2 (8), 1330−1340. (67) Duan, W.; Zhang, Y.; Wang, Z.; Jiang, J.; Liang, C.; Wei, W. Synthesis and Near-Infrared Fluorescence of K5NdLi2F10nanocrystals and Their Dispersion with High Doping Concentration and Long Lifetime. Nanoscale 2014, 6 (11), 5634−5638. (68) Pivin, J. C.; Podhorodecki, A.; Kudrawiec, R.; Misiewicz, J. Study of Neodymium Photoluminescence and Energy Transfer in Silicon-Based Gels. Opt. Mater. (Amsterdam, Neth.) 2005, 27 (9), 1467−1470. (69) Lee, T. H.; Lee, S. H.; Her, S. B.; Chang, W. G.; Lim, B. S. Effects of Surface Treatments on the Susceptibilities of Low Temperature Degradation by Autoclaving in Zirconia. J. Biomed. Mater. Res., Part B 2012, 100B (5), 1334−1343. (70) Deville, S.; Chevalier, J.; Gremillard, L. Influence of Surface Finish and Residual Stresses on the Ageing Sensitivity of Biomedical Grade Zirconia. Biomaterials 2006, 27 (10), 2186−2192. (71) Chevalier, J.; Gremillard, L.; Deville, S. Low-Temperature Degradation of Zirconia and Implications for Biomedical Implants. Annu. Rev. Mater. Res. 2007, 37 (1), 1−32. (72) Zhang, F.; Batuk, M.; Hadermann, J.; Manfredi, G.; Mariën, A.; Vanmeensel, K.; Inokoshi, M.; Van Meerbeek, B.; Naert, I.; Vleugels, J. Effect of Cation Dopant Radius on the Hydrothermal Stability of Tetragonal Zirconia: Grain Boundary Segregation and Oxygen Vacancy Annihilation. Acta Mater. 2016, 106 (2016), 48−58. (73) Zhang, F.; Chevalier, J.; Olagnon, C.; Van Meerbeek, B.; Vleugels, J. Slow Crack Growth and Hydrothermal Aging Stability of an Alumina-Toughened Zirconia Composite Made from La2O3Doped 2Y-TZP. J. Eur. Ceram. Soc. 2017, 37 (4), 1865−1871.

(74) Ponnilavan, V.; Meenambal, R.; Kannan, S. Crystallization and Polymorphic Phase Transitions in Zirconia-Toughened Alumina Systems Induced by Dy3+/Gd3+ Cosubstitutions. Cryst. Growth Des. 2018, 18 (8), 4449−4459. (75) Pietsch, H.; Jost, G.; Frenzel, T.; Raschke, M.; Walter, J.; Schirmer, H.; Hutter, J.; Sieber, M. A. Efficacy and Safety of Lanthanoids as X-Ray Contrast Agents. Eur. J. Radiol. 2011, 80 (2), 349−356. (76) Lusic.; et al. X-Ray Computed Tomography Contrast Agents. Chem. Rev. 2013, 113 (3), 1641−1666. (77) Meenambal, R.; Kannan, S. Design and Structural Investigations of Yb3+substituted β-Ca3(PO4)2contrast Agents for Bimodal NIR Luminescence and X-Ray CT Imaging. Mater. Sci. Eng., C 2018, 91, 817−823. (78) De La Vega, J. C.; Häfeli, U. O. Utilization of Nanoparticles as X-Ray Contrast Agents for Diagnostic Imaging Applications. Contrast Media Mol. Imaging 2015, 10 (2), 81−95. (79) Hainfeld, J. F.; Slatkin, D. N.; Focella, T. M.; Smilowitz, H. M. Gold Nanoparticles: A New X-Ray Contrast Agent. Br. J. Radiol. 2006, 79 (939), 248−253. (80) Meenambal, R.; Nandha Kumar, P.; Poojar, P.; Geethanath, S.; Kannan, S. Simultaneous Substitutions of Gd3+and Dy3+in βCa3(PO4)2as a Potential Multifunctional Bio-Probe. Mater. Des. 2017, 120, 336−344. (81) He, M.; Huang, P.; Zhang, C.; Hu, H.; Bao, C.; Gao, G.; He, R.; Cui, D. Dual Phase-Controlled Synthesis of Uniform LanthanideDoped NaGdF4 Upconversion Nanocrystals via an OA/Ionic Liquid Two-Phase System for in Vivo Dual-Modality Imaging. Adv. Funct. Mater. 2011, 21 (23), 4470−4477. (82) Wang, S.; Shan, G.; Lu, Z.; Liao, Y.; Zhou, J. Gadolinium Complex and Phosphorescent Probe-Modified NaDyF4 Nanorods for T1- and T2-Weighted MRI/CT/Phosphorescence Multimodality Imaging. Biomaterials 2014, 35 (1), 368−377. (83) Sun, Y.; Peng, J.; Feng, W.; Li, F. Upconversion Nanophosphors Naluf4: Yb,Tm for Lymphatic Imaging in Vivo by RealTime Upconversion Luminescence Imaging under Ambient Light and High-Resolution x-Ray CT. Theranostics 2013, 3 (5), 346−353. (84) Liu, Z.; Li, Z.; Liu, J.; Gu, S.; Yuan, Q.; Ren, J.; Qu, X. LongCirculating Er 3+-Doped Yb 2O3 up-Conversion Nanoparticle as an in Vivo X-Ray CT Imaging Contrast Agent. Biomaterials 2012, 33 (28), 6748−6757. (85) Watson, A. D. The Use of Gadolinium and Dysprosium Chelate Complexes as Contrast Agents for Magnetic Resonance Imaging. J. Alloys Compd. 1994, 207−208 (C), 14−19. (86) Schumacher, D. P.; Wallace, W. E. Magnetic Characteristics of Some Lanthanide Nitrides. Inorg. Chem. 1966, 5 (9), 1563−1567. (87) Song, Y. R.; Yang, F.; Yao, M. Y.; Zhu, F.; Miao, L.; Xu, J. P.; Wang, M. X.; Li, H.; Yao, X.; Ji, F.; et al. Large Magnetic Moment of Gadolinium Substituted Topological Insulator: Bi 1.98Gd 0.02Se 3. Appl. Phys. Lett. 2012, 100 (24), 2010−2013. (88) Xu, W.; Lu, Y. A Smart Magnetic Resonance Imaging Contrast Agent Responsive to Adenosine Based on a DNA AptamerConjugated Gadolinium Complex. Chem. Commun. 2011, 47 (17), 4998−5000. (89) Bridot, J. L.; Faure, A. C.; Laurent, S.; Rivière, C.; Billotey, C.; Hiba, B.; Janier, M.; Josserand, V.; Coll, J. L.; et al. Hybrid Gadolinium Oxide Nanoparticles: Multimodal Contrast Agents for in Vivo Imaging. J. Am. Chem. Soc. 2007, 129 (16), 5076−5084. (90) Kattel, K.; Park, J. Y.; Xu, W.; Kim, H. G.; Lee, E. J.; Bony, B. A.; Heo, W. C.; Jin, S.; Baeck, J. S.; Chang, Y.; et al. Paramagnetic Dysprosium Oxide Nanoparticles and Dysprosium Hydroxide Nanorods as T2MRI Contrast Agents. Biomaterials 2012, 33 (11), 3254− 3261. (91) Park, J. Y.; Baek, M. J.; Choi, E. S.; Woo, S.; Kim, J. H.; Kim, T. J.; Jung, J. C.; Chae, K. S.; Chang, Y.; Lee, G. H. Paramagnetic Ultrasmall Gadolinium Oxide Nanoparticles as Advanced T 1 MRI Contrast Agent: Account for Large Longitudinal Relaxivity, Optimal Particle Diameter, and In Vivo T 1 MR Images. ACS Nano 2009, 3 (11), 3663−3669. R


Article

ACS Biomaterials Science & Engineering (92) Kumar, R.; Nyk, M.; Ohulchanskyy, T. Y.; Flask, C. A.; Prasad, P. N. Combined Optical and MR Bloimaging Using Rare Earth Ion Doped NaYF4nanocrystals. Adv. Funct. Mater. 2009, 19 (6), 853− 859. (93) Mishra, S. K.; Kannan, S. Doxorubicin-Conjugated Bimetallic Silver-Gadolinium Nanoalloy for Multimodal MRI-CT-Optical Imaging and PH-Responsive Drug Release. ACS Biomater. Sci. Eng. 2017, 3 (12), 3607−3619. (94) Vasanthavel, S.; Kannan, S. Structural Investigations on the Tetragonal to Cubic Phase Transformations in Zirconia Induced by Progressive Yttrium Additions. J. Phys. Chem. Solids 2018, 112, 100− 105. (95) Huang, C.-H. Rare_Earth_Coordination_Chemistry__Fundamentals_and_Applications; John Wiley & Sons, 2010. (96) Vuojola, J.; Soukka, T. Luminescent Lanthanide Reporters: New Concepts for Use in Bioanalytical Applications. Methods Appl. Fluoresc. 2014, 2 (1), 012001. (97) De la Rosa, E.; Salas, P.; Díaz-Torres, L. A.; Martínez, A.; Angeles, C. Strong Visible Cooperative Up-Conversion Emission in ZrO2:Yb3+ Nanocrystals. J. Nanosci. Nanotechnol. 2005, 5 (9), 1480− 1486. (98) Meza, O.; Diaz-Torres, L. A.; Salas, P.; de la Rosa, E.; AngelesChavez, C.; Solis, D. Cooperative Pair Driven Quenching of Yb(3+) Emission in Nanocrystalline ZrO(2):Yb(3+). J. Nano Res. 2009, 5, 121− 134. (99) Gu, F.; Wang, S. F.; Lü, M. K.; Zhou, G. J.; Liu, S. W.; Xu, D.; Yuan, D. R. Effect of Dy3+doping and Calcination on the Luminescence of ZrO2nanoparticles. Chem. Phys. Lett. 2003, 380 (1−2), 185−189. (100) Lu, F. C.; Bai, L. J.; Lu, Y.; Dang, W.; Yang, Z. P.; Lin, P. Photoluminescence Mechanism and Thermal Stability of Tb3+-Doped Y4Si2O7N2green-Emitting Phosphors. J. Am. Ceram. Soc. 2015, 98 (3), 867−872. (101) Zysset, P. K.; Edward Guo, X.; Edward Hoffler, C.; Moore, K. E.; Goldstein, S. A. Elastic Modulus and Hardness of Cortical and Trabecular Bone Lamellae Measured by Nanoindentation in the Human Femur. J. Biomech. 1999, 32 (10), 1005−1012. (102) Chevalier, J.; Cales, B.; Drouin, J. M. Low-Temperature Aging of Y-TZP Ceramics. J. Am. Ceram. Soc. 1999, 82 (8), 2150−2154. (103) Deville, S.; Chevalier, J.; Dauvergne, C.; Fantozzi, G.; Bartolome, J. F.; Moya, J. S.; Torrecillas, R. Microstructural Investigation of the Aging Behavior of (3Y-TZP)-Al2O3 Composites. J. Am. Ceram. Soc. 2005, 88 (5), 1273−1280. (104) Guo, X.; He, J. Hydrothermal Degradation of Cubic Zirconia. Acta Mater. 2003, 51 (17), 5123−5130. (105) Ramesh, S.; Sara Lee, K. Y.; Tan, C. Y. A Review on the Hydrothermal Ageing Behaviour of Y-TZP Ceramics. Ceram. Int. 2018, 44 (17), 20620−20634. (106) Wang, G.; Yu, H.; De Man, B. An Outlook on X-Ray CT Research and Development. Med. Phys. 2008, 35 (3), 1051−1064. (107) Wang, J.; Chen, Y.; Liu, H.; Ye, J.; Ge, W.; Dong, X.; Wang, X.; Li, Q.; Jiang, H.; Jiang, X. Rapid and Accurate Tumor-Target BioImaging through Specific in Vivo Biosynthesis of a Fluorescent Europium Complex. Biomater. Sci. 2016, 4 (4), 652−660. (108) Seveus, L.; Väisälä, M.; Syrjänen, S.; Sandberg, M.; Kuusisto, A.; Harju, R.; Salo, J.; Hemmilä, I.; Kojola, H.; Soini, E. Time-resolved Fluorescence Imaging of Europium Chelate Label in Immunohistochemistry and in Situ Hybridization. Cytometry 1992, 13 (4), 329− 338.

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