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Nd-doped NASICON-type nanophosphors for near-infrared excitation and emission Nd-doped NASICON-type nanophosphors for near-infrared excitation and emission
Watanabe, Mizuki; Itoh, Masahiro; Oka, Ryohei; Ida, Shintaro; Masui, Toshiyuki
2023-04-03 00:00:00
JOURNAL OF ASIAN CERAMIC SOCIETIES https://doi.org/10.1080/21870764.2023.2186842 Nd-doped NASICON-type nanophosphors for near-infrared excitation and emission a b c d b Mizuki Watanabe , Masahiro Itoh , Ryohei Oka , Shintaro Ida and Toshiyuki Masui a b Graduate School of Science and Technology, Niigata University, Niigata, Japan; Center for Research on Green Sustainable Chemistry, Department of Chemistry and Biotechnology, Faculty of Engineering, Tottori University, Tottori, Japan; Field of Advanced Ceramics, Department of Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Showa, Aichi, Japan; Institute of Industrial Nanomaterials, Kumamoto University, Kumamoto, Japan ABSTRACT ARTICLE HISTORY Received 25 November 2022 Neodymium-doped phosphates, (Nd Gd ) Zr (PO ) (0 ≤ x ≤ 1), were synthesized by co- 1-x x 0.33 2 4 3 Accepted 28 February 2023 precipitation. (Nd Gd ) Zr (PO ) was obtained as a single-phase and was confirmed to be 1-x x 0.33 2 4 3 a NASICON-type structure consisting of a three-dimensional network of PO tetrahedra sharing KEYWORDS corners with ZrO octahedra. The particle size of the (Nd Gd ) Zr (PO ) samples was in the 6 1-x x 0.33 2 4 3 Word; nanophosphor; NIR 3+ nanoscale, which is suitable for in vivo optical imaging. The (Nd Gd ) Zr (PO ) samples 1-x x 0.33 2 4 3 emission; Nd ; NASICON- 3+ showed characteristic luminescence corresponding to the f – f transitions of Nd . The highest type structure emission intensity at 1072 nm with excitation at 824 nm was observed for (Nd Gd ) Zr 0.75 0.25 0.33 2 (PO ) , which was 4.5 times higher than that of Nd Zr (PO ) . The near-infrared (NIR) emis- 4 3 0.33 2 4 3 sion intensity of this nanophosphor was significantly higher than that of indocyanine green, which is actually used as an in vivo optical probe reagent. 1. Introduction Ag S is a low toxicity QD, but has a QY of only 15% [24], and SWCNTs have low toxicity and good photostabil- Bioimaging using optical probes is an essential tech- ity, but low QY (about 10%) [18]. The up-conversion nology for biomedical research and clinical diagnosis. phosphorescent materials that convert NIR light into In particular, optical probes that emit near-infrared visible light exhibit critically low QY because of the (NIR) light with NIR excitation have attracted attention inevitable multiphoton excitation in the emission as a promising material to dramatically improve in vivo mechanism. Therefore, new NIR luminescent materials optical imaging. The NIR light in the wavelength range for optical probes with high QY are needed, since low of 700–1400 nm is called the “optical transmission QY generates extra infrared (IR) emission and causes window” and provides deep tissue penetration, undesirable heat load during in vivo optical imaging. reduced photodamaging effects, and low autofluores - Rare-earth doped ceramic phosphors that emit light cence and light scattering. Thus, safe and high signal- based on f-f transitions usually exhibit low long-term to-noise ratio images can be obtained [1–9]. cytotoxicity, low photobleaching, long luminescence life- Indocyanine green (ICG) is well known as the only time, and thermal and chemical stability. They also gen- practical reagent capable of excitation and emission erally have high QYs because the luminescence within the narrow wavelength region of the “optical mechanism is classified as a downshift process. transmission window” of tissues [10]. However, there 3+ Phosphors containing trivalent neodymium ions (Nd ) are problems to be solved, such as low emission inten- show emissions around 880, 1060, and 1340 nm due to sity, low photostability, shallow penetration depth, and 3+ the f-f transition of Nd upon NIR excitation, which is low quantum yield (QY) [4,6]. In addition to organic suitable for in vivo optical imaging [25–30]. However, the fluorescent dyes, several materials have been investi- 3+ excitation and emission efficiencies of Nd -doped phos- gated that are excited by NIR light and emit NIR: semi- phors are usually insufficient, because the f-f transitions conductor quantum dots (QDs) [11–15], carbon-based 3+ of Nd are parity forbidden. Doping the host lattice with materials such as single-walled carbon nanotubes 3+ high concentrations of Nd solves this problem [31–33], (SWCNTs) [16–20], and up-conversion phosphors [21– 3+ but excess doping of Nd ions above an appropriate 24]. Unfortunately, however, there are concerns with concentration usually quenches the luminescence. This is these materials. The QDs often contain toxic elements due to the migration of the excitation energy among such as cadmium, arsenic, lead, and mercury, making activator ions [34] and is called “concentration quench- them unsuitable for in vivo use. A concern with less ing”. One means of solving the problem is to select a host toxic NIR emitting QDs is their low QY. For example, CONTACT Mizuki Watanabe mwatanabe@eng.niigata-u.ac.jp Graduate School of Science and Technology, Niigata University8050 Ikarashi 2-no- cho, Niigata, 950-2181, Japan © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 M. WATANABE ET AL. (Nd O , 99.9%) and phosphoric acid (H PO , 85.0%) 2 3 3 4 were purchased from Kishida Chemical Co. Ltd. (Japan). Zirconium chloride oxide octahydrate .8 H (ZrCl O O, 99.0%) and nitric acid (HNO ) were pur- 2 2 3 chased from FUJIFILM Wako Pure Chemical Co. (Japan). Indocyanine green (ICG) used as a luminescence stan- dard was purchased from Tokyo Chemical Industry Co., Ltd. (Japan). All reagents in this study were used as received without further purification. 2.2. Synthesis of (Nd Gd ) Zr (PO ) (0 ≤ x ≤ 1) 1-x x 1/3 2 4 3 nanoparticles (Nd Gd ) Zr (PO ) (0 ≤ x ≤ 1.0) nanoparticles were 1-x x 0.33 2 4 3 synthesized by the precipitation method [41]. Nd O 2 3 and Gd O weighed in stoichiometric ratios were dis- 2 3 solved in diluted nitric acid solution (20 cm ), and then a stoichiometric amount of ZrCl O solution (92.136 g −1 L ) was added. A stoichiometric amount of H PO 3 4 solution (8.5%) was then added subsequently to the Figure 1. Crystal structure of the NASICON-type Ln Zr mixed solution under constant stirring, heated at 50°C 0.33 2 (PO ) (Ln = Nd, Gd) with a space group of P-3c1. 4 3 for 1 h, and then evaporated to dryness in the air at 80°C for 12 h. The resulting powder was carefully ground and calcined in a muffle furnace (Isuzu, EPTR- material with a large distance between the activator ions 26K) at 700 − 900°C for 5 − 20 h. To test sample disper- in the lattice to suppress concentration quenching. sibility, the sample powders (0.01 g) were dispersed in The Na ion super ionic conductor (NASICON) struc- deionized water (10 mL) using ultrasound for 5 min. ture, which is thermally and chemically stable, nontoxic, The solutions were left to rest at room temperature and has long distances between the active agent ions, is for one hour to allow for sedimentation, after which one of the leading candidates for this study [35–37]. The the coarse particle residue was removed by decanta- NASICON-type structure in the space group of P-3c1 tion. The resulting solution is treated as sample illustrated by the VESTA program is shown in Figure 1 solution. [38]. This structure adopts a three-dimensional network of AO (A = Ti, Zr, and Hf) octahedra sharing corners with 2.3. Characterization PO tetrahedra, and M ions (M = alkaline or alkaline earth metal), which can be substituted by rare earth ions, are The sample composition was analyzed by X-ray fluores - present in the interstitials [39,40]. The interstitial sites are cence spectrometry (XRF; Rigaku, ZSX Primus). The crys- separated from each other by a distance of more than 5 Å tal structure was identified by X-ray powder diffraction [41]. Although several studies have been reported on Nd (XRD; Rigaku Ultima IV) using Cu-Kα radiation (40 kV and -containing oxides with NASICON-type structures such 40 mA). The data were collected by step scanning in the as Nd Zr (PO ) [35,40,42], to our knowledge their 0.33 2 4 3 2θ range from 20° to 80° with a step size of 0.02° and luminescence properties have not been investigated −1 a scan rate of 6° min . Rietveld refinement of the and their potential as in vivo optical imaging probes has resulting XRD patterns obtained in the 2θ range from not been explored. In this study, nano-sized 10° to 120° by an X-ray diffractometer (Bruker D2 (Nd Gd ) Zr (PO ) (0 ≤ x ≤ 1.0) phosphors were 1-x x 0.33 2 4 3 PHASER) with monochromatic CuKα radiation (10 mA synthesized by a precipitation method and their fluores - and 30 kV) was performed by the RIETAN-FP software cence properties were investigated. To suppress the con- package [43]. The lattice parameters and lattice volumes 3+ centration quenching of Nd Zr (PO ) , Gd , which is 0.33 2 4 3 were calculated from the XRD peak angles refined with chemically and thermally stable and whose ionic radius is α-Al O as a standard material using the CellCalc 2 3 3+ close to that of Nd , was selected as a buffer ion. Ver. 2.20 software. Transmission electron microscope images were taken at an acceleration voltage of 300 kV (TEM; Hitachi H-9000NAR). The average particle size 2. Experimental was estimated by measuring the maximum diameter of 200 particles in one direction on the TEM images. The 2.1. Materials NIR photoluminescence (PL) excitation and emission Gadolinium oxide (Gd O , 99.9%) was purchased from 2 3 spectra of the sample powders were measured at Shin-Etsu Chemical Co. Ltd. (Japan). Neodymium oxide room temperature using a fluorescence spectrometer JOURNAL OF ASIAN CERAMIC SOCIETIES 3 (Horiba, Fluorolog-3), where the emission spectra were sample synthesized at 700°C was obtained in the amor- obtained for excitation at 824 nm and the excitation phous phase. On the other hand, the samples synthe- spectra were recorded for emission at 1072 nm. sized at 800°C and above yielded a trigonal NASICON- type structure in a space group of P-3c1 as the main phase, although Zr O(PO ) and ZnO are detected as 2 4 2 2 3. Results and discussion impurities. This result indicates that the precursor obtained as an amorphous phase crystallized upon heat- Figure 2 shows the XRD patterns of the Nd Zr (PO ) 0.33 2 4 3 ing at 750°C and above. Since a single phase was samples after heating at 700–900°C in air for 20 h. The Figure 2. XRD patterns of the Nd Zr (PO ) samples heated at 700–900°C in air for 20 h. 0.33 2 4 3 Figure 3. XRD patterns of the Nd Zr (PO ) samples heated at 800°C for 5–20 h in air. 0.33 2 4 3 4 M. WATANABE ET AL. obtained at 750°C but the crystallinity was low, the opti- than 10 h. From this result, the optimum reaction time to obtain a single phase while suppressing parti- mum reaction temperature was 800°C, although a very cle growth was determined to be 5 h. small amount of ZrO was observed. The composition of each sample was confirmed to In addition, the samples were synthesized at differ - be stoichiometric by XRF analysis. Figure 4 shows the ent reaction times to determine the optimum reaction XRD patterns of the (Nd Gd ) Zr (PO ) (0 ≤ x ≤ 1.0) time to obtain a single-phase sample. Figure 3 shows 1-x x 0.33 2 4 3 samples. All samples were obtained in a single phase the XRD patterns of the Nd Zr (PO ) samples 0.33 2 4 3 with a trigonal NASICON-type structure in a space obtained by heating the precursor at 800°C for 5 to group of P-3c1 (Inorganic Crystal Structure Database: 20 hours in air. A single phase with NASICON-type ICSD No. 245204). Figure 5 shows the composition structure was obtained by heating at 800°C for less Figure 4. XRD patterns of the (Nd Gd ) Zr (PO ) (0 ≤ x≤1.0) samples heated at 800°C for 5 h in air. 1-x x 0.33 2 4 3 Figure 5. Composition dependence on lattice volume of the (Nd Gd ) Zr (PO ) (0 ≤ x≤1.0) samples. 1-x x 0.33 2 4 3 JOURNAL OF ASIAN CERAMIC SOCIETIES 5 dependence on lattice volume of the (Nd Gd ) Zr (PO ) (0 ≤ x < 1.0) samples excited at 824 nm. Three 1-x x 0.33 2 4 3 3+ (PO ) samples, estimated from the peak angles of characteristic peaks due to the f-f transition of Nd 4 3 X-ray diffraction. The lattice volume decreased mono- were observed at 910 nm, 1072 nm, and 1309 nm in tonically with increasing Gd content, indicating that the near-infrared region for all samples, which were 3+ 4 4 4 4 4 smaller Gd (ionic radius: 0.0938 nm for 6 coordina- attributed to the F → I , F → I , and F 3/2 9/2 3/2 11/2 3/ 3+ 4 tion) [44] was substituted with larger Nd (ionic → I transitions, respectively. Because the emis- 2 13/2 radius: 0.083 nm for 6 coordination) [44] to form solid sion spectra between 800 and 850 nm should over- solutions. As shown in Table 1, the a parameter lap with the excitation spectra peaked at 824 nm, an 4 4 increased whereas the c parameter decreased with emission peak related to the F → I transition 5/2 9/2 4 4 increasing Gd content. Similar behavior has been was not found. No emission due to the F → I 5/2 9/2 observed in other solid solutions with NASICON-type transition was seen even under excitation with multi- structures, which can be explained by structural ple wavelengths, most likely because the lumines- changes [45]: the NASICON-type M Zr (PO ) con- cence intensity was too low. Therefore, the 0.33 2 4 3 4 4 tains columns between two adjacent ZrO octahedral emission due to the F → I transition was left 6 5/2 9/2 4 4 faces along the c axis, and the decrease in the ionic out of the following discussion. The F → I tran- 3/2 11/2 radius of the M cation causes the c parameter sition showed the strongest emission line at 1072 decreases [35–37]. On the other hand, the nm, which was more than five times higher than the a parameter increases with decreasing ionic radius of other emission lines in Figure 6. This characteristic the M cation, due to the correlated rotation of the PO can be attributed to the large crystal field splitting of 3+ 4 tetrahedron connecting the columns parallel to the the Nd F metastable manifold, as observed in 3/2 3+ 3+ ZrO octahedron. The introduction of Gd , which is other Nd -doped ceramic phosphors with low sym- 3+ 3+ smaller than Nd , into the (Nd Gd ) Zr (PO ) (0 ≤ metry around Nd [46]. 1-x x 0.33 2 4 3 x ≤ 1.0) system resulted in an increase in the The intensity of each transition depends on the a parameter and a decrease in the c parameter. branching ratio (β), defined as the relative emission Figure 6 shows the photoluminescence emission probability for each transition. The branching ratio spectra at room temperature of the (Nd Gd ) Zr can be estimated from the relative contribution to 1-x x 0.33 2 Table 1. Lattice parameters of the (Nd Gd ) Zr 1-x x 0.33 2 (PO ) (x = 0, 0.50, and 1) samples. 4 3 Sample a (nm) c (nm) Nd Zr (PO ) 0.8749 2.3173 0.33 2 4 3 (Nd Gd ) Zr (PO ) 0.8755 2.2921 0.50 0.50 0.33 2 4 3 Gd Zr (PO ) 0.8767 2.2788 0.33 2 4 3 Figure 6. Photoluminescence emission spectra at room temperature of the (Nd Gd ) Zr (PO ) (0 ≤ x < 1.0) samples under 1-x x 0.33 2 4 3 excitation at 824 nm. 6 M. WATANABE ET AL. 3+ the total integrated luminescence of the emission field around Nd increased with increasing Gd con- spectrum in Figure 6 as follows: tent [51,52]. Since the development of detectors for in vivo ∫ IðλÞdλ 850 optical imaging consisting of AsGaIn arrays, nano- β9 ¼ phosphors working in the so-called “second biolo- ∫ IðλÞdλ gical window” (1000–1400 nm) have attracted much attention. They can improve both image ∫ IðλÞdλ β11 ¼ resolution and transmission depth. In such detec- ∫ IðλÞdλ tors, the relative detection sensitivities at 910, 1072, and 1309 nm are close to 0.4, 0.8, and 0.8 (arbitrary units), respectively [53]. The emission due ∫ IðλÞdλ β13 ¼ 4 4 2 to the F → I transition at 910 nm was 3/2 9/2 ∫ IðλÞdλ observed in the region of low relative detection where I(λ) is the integrated emission intensity at sensitivity. The low emission probability of the 4 4 a certain wavelength. In this case, the background F → I transition at 1309 nm was not com- 3/2 13/2 intensity is removed. Note that the emission band pensated by the high relative detection sensitivity 4 4 due to the F → I transition is observed outside 3/2 15/2 in the corresponding spectral region. Moreover, 4 4 the detection range of our spectrometer, and the the emission intensity of the F → I transition 3/2 13/2 branching ratio corresponding to this transition is less is significantly reduced in the solvent due to than 1% [47–50]. Therefore, the contribution of this strong emission quenching by the hydroxyl group transition can be neglected in the following calcula- 4 4 [54]. Therefore, monitoring the F → I emis- 3/2 11/2 tions and discussion without affecting the results. sion intensity at 1072 nm will be suitable for in vivo Figure 7 shows change in the branching ratios for the optical imaging in the “second biological window” 4 4 4 4 4 4 F → I , F → I and F → I transitions 3/2 9/2 3/2 11/2 3/2 13/2 region using an AsGaIn array detector. The result with increasing Gd content. The branching ratios at 4 4 that the branching ratio of the F - I transi- 3/2 11/2 910 nm and at 1309 nm increased from 12.8 to 18.8% tion for the (Nd Gd ) Zr (PO ) (0 ≤ x < 1.0) 1-x x 0.33 2 4 3 and from 10.7 to 16.3%, while that at 1072 mm samples increased with decreasing Gd content decreased from 76.5 to 64.8%. It is suggested that the indicates that the samples with lower Gd content branching ratio can be adjusted according to the appli- are more suitable for in vivo optical imaging of the cation by changing the Gd content. The change in “second biological window” region. branching ratio implies a change in the spontaneous 4 4 4 4 Figure 8 shows the Gd concentration depen- emission probability of the F → I , I and I 3/2 9/2 11/2 13/2 4 4 dence of the photoluminescence emission peak transitions. The continuously enhanced F → I 3/2 9/2 4 4 intensity at 1072 nm for the (Nd Gd ) Zr and F → I transitions suggest that the crystal 1-x x 0.33 2 3/2 13/2 4 4 4 4 4 4 Figure 7. Gd content dependence on the branching ratios for the F → I , F → I , and F → I transitions in the 3/2 9/2 3/2 11/2 3/2 13/2 (Nd Gd ) Zr (PO ) (0 ≤ x < 1.0) samples under excitation at 824 nm. 1-x x 0.33 2 4 3 JOURNAL OF ASIAN CERAMIC SOCIETIES 7 Figure 8. Gd concentration dependence of the photoluminescence emission peak intensity at 1072 nm for the (Nd Gd ) Zr 1-x x 0.33 2 (PO ) (0 < x ≤ 1.0) samples under excitation at 824 nm. 4 3 (PO ) (0 < x ≤ 1.0) samples. Among the samples where X is the critical concentration, N is the num- 4 3 c synthesized in this study, the (Nd Gd ) Zr ber of cation sites in the unit cell, and V is the volume 0.75 0.25 0.33 2 (PO ) sample exhibited the highest emission, of the unit cell. For the (Nd Gd ) Zr (PO ) sys- 4 3 0.75 0.25 0.33 2 4 3 which was found to be 4.5 times higher than that tem, X = 0.75, N = 2, and V = 1521.8 Å , which are of the Nd Zr (PO ) sample without Gd. In rare- obtained in the Rietveld refinement (Table S1), result- 0.33 2 4 3 earth phosphors, concentration quenching gener- ing in a calculated R of 12.5 Å. Two general concen- ally occurs at luminescent ion concentrations tration quenching mechanisms have been proposed: below 5 mol %. However, higher critical concentra- nonradiative energy transfer between ions (exchange tions were obtained in the (Nd Gd ) Zr (PO ) interaction) and electric multipolar interaction. In the 1-x x 0.33 2 4 3 3+ system. This is because Nd , which acts as the exchange interaction model, the R between the sen- activating ion, is located at the M cation sites sitizer and the activator has to be shorter than 5 Å [55] more than 5 Å away. The Rietveld analysis of the and the emission and excitation spectra have to over- XRD pattern of the (Nd Gd ) Zr (PO ) sam- lap for radiation reabsorption. However, the critical 0.75 0.25 0.33 2 4 3 3+ ple revealed that the distance between the nearest distance between the Nd ions is longer than that 3+ neighbor Nd ions was 8.74894(0)Å; the Rietveld required for the exchange interaction and thus is not refinement profile of the sample is shown in Figure involved in the energy transfer. Therefore, the multi- 3+ S1, and the crystallographic data and structural polar interactions between Nd ions are considered to refinement parameters are summarized in Tables be involved in the energy transfer mechanism of the S1 and S2, respectively. It was experimentally con- (Nd Gd ) Zr (PO ) (0 ≤ x < 1.0) system. 1-x x 0.33 2 4 3 3+ firmed that the Nd ions are separated from each The (Nd Gd ) Zr (PO ) sample exhibited the 0.75 0.25 0.33 2 4 3 other by more than 5Å. Such long-range separa- highest emission intensity and high branching ratio 4 4 tion suppresses concentration quenching [36,37]. (72.2%) from F to I among the 3/2 11/2 The concentration quenching mechanism of phos- (Nd Gd ) Zr (PO ) (0 ≤ x < 1.0) powders. From 1-x x 0.33 2 4 3 phors was investigated by calculating the critical these results, it can be concluded that the sample 3+ distance (R ) between Nd ions. The critical dis- with x = 0.25 is most suitable for in vivo optical tance for energy transfer can be approximately imaging. estimated using the following relation proposed Figure 9 shows TEM photographs of the by Blasse [49]: (Nd Gd ) Zr (PO ) sample. Although agglom- 0.75 0.25 0.33 2 4 3 erated particles were observed, the average primary � �1 particle size was less than 100 nm. Since the particle 3V R ¼ 2 size suitable for in vivo optical imaging is 10–200 nm, 4πX N c 8 M. WATANABE ET AL. Figure 9. TEM images of (a) an aggregate and (b) a dispersed particle of the (Nd Gd ) Zr (PO ) sample particles. 0.75 0.25 0.33 2 4 3 Figure 10. Photographs of the sample solution (a) under daylight, (b) displaying the Tyndall effect under dark, (c) under NIR light (980 nm), and (d) ionized water under NIR light (980 nm). the (Nd Gd ) Zr (PO ) samples is expected to nm) as shown in Figure 10(c); the photograph of 0.75 0.25 0.33 2 4 3 be a candidate as an in vivo optical imaging material. deionized water without the sample is shown in Figure 10 shows photographs of the Figure 10(d), as a comparison. (Nd Gd ) Zr (PO ) sample dispersed in deio- The emission intensity of the (Nd Gd ) Zr 0.75 0.25 0.33 2 4 3 0.75 0.25 0.33 2 nized water under daylight and NIR light (980 nm). (PO ) sample was compared with that of indocya- 4 3 The solution is transparent (see Figure 10(a)). The nine green (ICG), a green organic dye that is in Tyndall effect was detected in the solution, as practical use as a bioimaging reagent. Figure 11 shown in Figure 10(b), showing that the sample shows the photoluminescence emission spectra of particles are disseminated as colloidal particles in the (Nd Gd ) Zr (PO ) sample and ICG at 0.75 0.25 0.33 2 4 3 deionized water. Despite the TEM image in room temperature. No photoluminescence emission Figure 9 showing particle agglomeration, this find - peak of ICG was observed under the same measure- ing suggested that the sample particles may be ment conditions. The emission spectrum of ICG was disseminated steadily in deionized water without multiplied by a factor of 200 for better comparison. sedimentation. The colloidal suspension exhibits The emission intensity of ICG at 910 nm was at least NIR luminescence under NIR light excitation (980 200 times lower than that of the (Nd Gd ) Zr 0.75 0.25 0.33 2 JOURNAL OF ASIAN CERAMIC SOCIETIES 9 Figure 11. Photoluminescence emission spectra at room temperature of the (Nd Gd ) Zr (PO ) sample under excitation at 0.75 0.25 0.33 2 4 3 824 nm and ICG under excitation at 780 nm. The emission spectrum of ICG was multiplied by a factor of 200 for better comparison. (PO ) sample. Furthermore, the emission intensity Acknowledgments 4 3 of ICG at 1072 nm, which will be suitable for in vivo This study was supported by the “Advanced Characterization optical imaging in the “second biological window” Nanotechnology Platform, Nanotechnology Platform region using an AsGaIn array detector, is too low. Program of the Ministry of Education, Culture, Sports, Therefore, it is noteworthy that the emission inten- Science and Technology (MEXT), Japan” at the Research Center for Ultra-High Voltage Electron Microscopy sity of (Nd Gd ) Zr (PO ) was significantly 0.75 0.25 0.33 2 4 3 (Nanotechnology Open Facilities) of Osaka University. The higher than that of ICG. authors thank Prof. Dr. Hidehiro Yasuda and Dr. Takao Sakata for their assistance with TEM observation. The authors thank Mr. Masato Iwaki for their assistance with Rietveld analysis. This study was financially supported by The 4. Conclusions Murata Science Foundation [No. H2992] and the Electric (Nd Gd ) Zr (PO ) (0 ≤ x ≤ 1.0) nanophosphors Technology Research Foundation of Chugoku. 1-x x 0.33 2 4 3 were successfully synthesized by a precipitation method. The nanoparticles had a particle size of Disclosure statement less than 100 nm and ranged from 10 to 200 nm, making them suitable for in vivo optical imaging No potential conflict of interest was reported by the authors. materials. The (Nd Gd ) Zr (PO ) (0 ≤ x < 1.0) 1-x x 0.33 2 4 3 nanophosphors exhibited NIR emission at 910 nm, 1072 nm, and 1309 nm due to the f – f transitions of Funding 3+ Nd under NIR excitation at 824 nm. The highest The work was supported by the Murata Science Foundation emission intensity was obtained in the sample con- 3+ [H2992] taining 75 mol% Nd . This indicates that the 3+ NASICON-type structure, in which the Nd ions are separated by more than 5 Å, can be highly 3+ ORCID doped with Nd while suppressing concentration quenching. Furthermore, the luminescence intensity Mizuki Watanabe http://orcid.org/0000-0003-4967-3719 of the (Nd Gd ) Zr (PO ) nanophosphor was 0.75 0.25 0.33 2 4 3 found to be significantly higher than that of ICG. References These results indicate that (Nd Gd ) Zr (PO ) 0.75 0.25 0.33 2 4 3 nanophosphor is a promising optical probe for [1] Weissleder R. A clearer vision for in vivo imaging. Nat in vivo optical imaging. Biotechnol. 2001;19(4):316–317. 10 M. WATANABE ET AL. [2] Luypaert J, Massart DL, Heyden YV. Near-infrared spec- [19] Kodach VM, Kalkman J, Faber DJ, et al. 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