Bright YAG:Ce Nanorod Phosphors Prepared via a Partial Wet

Apr 27, 2016 - Bright YAG:Ce Nanorod Phosphors Prepared via a Partial Wet Chemical ...... Tae-Joon Jeon , Sie-Wook Jeon , Yun Suk Huh , Young-Kyu Han...
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Bright YAG:Ce Nanorod Phosphors Prepared via a Partial Wet Chemical Route and Biolabeling Applications Daidong Guo,† Baojin Ma,† Lili Zhao,† Jichuan Qiu,† Wei Liu,† Yuanhua Sang,*,† Jerome Claverie,‡ and Hong Liu*,† †

State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China NanoQAM Research Center, Department of Chemistry, University of Quebec at Montreal, 2101 rue Jeanne-Mance, CP 8888, Montreal, Quebec H3C3P8, Canada

‡

ACS Appl. Mater. Interfaces 2016.8:11990-11997. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/08/19. For personal use only.

S Supporting Information *

ABSTRACT: Cerium-doped yttrium aluminum garnet (YAG:Ce) nanorods were prepared via a partial wet chemical route followed by a calcination process by using Al2O3 nanorods as both templates and the reactant. These novel well-crystallized YAG:Ce phosphors with a 200−300 nm diameter and a 2−3 μm length have a high specific surface area while being virtually devoid of surface defects. The YAG:Ce nanorod phosphors possess good luminescent properties compared with granular YAG:Ce phosphors. Photoluminescence quantum yields of YAG:Ce nanorod phosphors are higher than those of granular ones. The YAG:Ce nanorod phosphors exhibit two luminescent decay times due to their unique morphology. The YAG:Ce nanorods exhibited good cytocompatibility with bone marrow mesenchymal stem cells and can be used as biolabel nanoparticles in bioimaging. KEYWORDS: nanorod phosphors, partial wet chemical route, luminescent properties, biolabeling, YAG:Ce

1. INTRODUCTION

low surface defect concentration compared with granular nano/ microparticles.15,16 As is well-known, preparation methods of rare-earth doped phosphors are traditionally classified into two main categories: solid-state reaction and wet-chemical method.4,7−9,11,12,14,16 For solid-state reaction, the rare doped complex oxide phosphor ceramic powders or bulk ceramics can be prepared by calcining mixed oxides at high temperature and then grinding into microparticles to obtain more active emission sites. Generally, the crystallization degree of the phosphors synthesized by the solid-state reaction is high, which favors the luminescent properties. However, their luminescent properties are unsatisfactory due to the formation of large aggregated particles with limited surface area. Although these particles can be crushed by different methods, the size of the particles is still larger than phosphors synthesized by wet-chemical methods. In addition, the surface of crushed or milled phosphor particles is full of defects, which lower the fluorescent efficiency due to the surface quenching effect.17,18 In contrast, nanostructured phosphors obtained through wet-chemical methods are favored to obtain high light efficiency because of their small size and large surface area. However, the low crystallization degree of

Rare-earth doped phosphors have been extensively studied because of their numerous applications in many fields, such as solid-state lighting (cathode ray tubes, fluorescent lamps), displays (high resolution and projection TVs, plasma display panels), scintillation detectors (X-ray detectors), and biological diagnostics.1−7 Properties of phosphors, including optical, electrical, thermal, mechanical, and chemical properties are known to be dependent on their composition, morphology, and microstructure.2,8,9 For example, the luminescent properties of phosphors are influenced by the number of defects of the phosphor particles, which are responsible for fluorescence quenching effect.10−13 Furthermore, the luminescent property of phosphor nanoparticles varies with the total surface area of phosphor particle. Larger surface area means more luminescence sites are exposed, which contributes to the emission efficiency. While larger surface area also implies more defects present, which is adverse to the luminescence performance.14 Therefore, phosphors with fewer defects and larger surface areas theoretically lead to higher luminescence. However, the conflict between good crystallization for high efficiency and small particle size for more fluorescent sites is difficult to resolve. In recent years, one-dimensional (1D) phosphor nano/ micromaterials have attracted much attention because they have both a high surface area compared with bulk materials and a © 2016 American Chemical Society

Received: February 3, 2016 Accepted: April 27, 2016 Published: April 27, 2016 11990

DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997

Research Article

ACS Applied Materials & Interfaces wet-chemical synthesized nanoparticles normally results in particles with low luminescent properties due to the serious quenching effect caused by the surface and inside defect in the nanoparticles. Until now, the synthesis of nanosized phosphor particles with a high crystallization degree has been a challenge for the manufacturing of phosphors. In addition, the morphology of phosphor nanoparticles is also a very important factor to determine the phosphor properties. In recent years, we developed a partial wet-chemical route19,20 to synthesize complex oxide nanoparticles. By assembling a precursor layer of the second oxide component on the surface of the first oxide nanoparticles following a heat treatment step, a complex oxide compound mimicking the morphology of the first oxide nanoparticles can be obtained. In this route, the morphology and the crystallization degree of the particles is conveniently controlled to prepare 1D rod-like phosphors with good luminescent properties. The rod-like nanocrystals will have a higher crystallization degree compared with wet-chemical synthesized nanoparticles and have more fluorescent sites compared with solid-state reaction ceramic powders. Ceriumdoped yttrium aluminum garnet (YAG:Ce, (CexY1−x)3Al5O12) is a well-known phosphor with excellent luminescent properties.21−23 Although there are numerous literature reports regarding the synthesis of YAG:Ce granular nanoparticles with different synthesis routes,24−30 until now, no report regarding single crystalline YAG:Ce nanorods has been reported because of the highly symmetric crystallographic character of YAG. In this work, YAG:Ce nanorod phosphors with different doping concentrations were successfully prepared via a partial wet chemical method using as-synthesized Al2O3 nanorods as both a template and the reactant. The rod-like single crystalline YAG:Ce nanostructured phosphor has stronger luminescent property and higher photoluminescence quantum yield (QY) compared with granular YAG:Ce. Due to the unique morphology and structure, the YAG:Ce nanorod phosphors exhibit two luminescent decay times, which is different from granular YAG:Ce phosphors. In addition, YAG:Ce nanorod phosphors possess high cytocompatibility, as proven by the uptake of such phosphors by bone marrow stem cells, and can be used for biolabeling.

In the suspension liquid, the Y concentration was of 0.015 mol/L, and Ce was added with a predetermined ratio (2 at. % and 6 at. % Ce3+). After adding urea (urea:Y3+ = 20:1) and homogenizing at room temperature, the mixture was heated at 90 °C for 3 h under stirring to yield the YAG-precursor nanorods with Al2O3/Y-compound core− shell structure. The precursor was collected by filtration, washed with deionized water several times to remove byproducts, and then washed by anhydrous ethanol to inhibit hard agglomeration during the following drying process. After drying in the oven at 80 °C for 1 day, sieving, and then calcining at 1300 °C for 2 h, the 1D YAG:Ce nanorod phosphors were obtained in approximately 98% yield. (The evaluation process of yield is included in S1 of the Supporting Information). The crystal structure of the samples at different stages was determined using an X-ray diffraction (XRD, D8 Advance, Bruker Inc., Germany) apparatus equipped with a Cu Kα radiation source (λ = 0.15406 nm). The morphology of the samples was investigated using a field emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan) operated at an accelerating voltage of 10 kV and high resolution transmission electron microscope (HRTEM, JEM-2100F, JEOL, Japan) operated at 200 kV. The distribution of elements was obtained by energy-dispersive X-ray spectroscopy (EDS, X-MAX 80, Oxford Instruments, England) using a detector attached to the HRTEM. The luminescent properties, photoluminescence QY values and fluorescence lifetime of the samples was measured by using a fluorescence spectrophotometer (F-4500, Hitachi, Japan) and a steady/transient state spectrometer (FLS920, Edinburgh Instruments, Scotland) equipped with a 150-W xenon lamp at room temperature and the data were processed by the analytic system of the spectrometer system. The surface area of the samples was determined by the Brunauer−Emmett−Teller (BET) method using a physisorption analyzer (ASAP 2020, Micromeritics Instruments, Norcross, GA). To demonstrate the biolabeling capability, bone marrow mesenchymal stem cells (BMMSC) were incubated with YAG:Ce nanorod phosphors for 24 h using a particle concentration of 100 μg mL−1 in medium (Low-glucose Dulbecco‘s Modified Eagle Medium (LDMEM), supplemented with 10% fetal bovine serum and 1% penicillin and streptomycin). After washing 3 times with cold phosphate buffer solution (PBS), the cells were imaged with a fluorescence microscope (FV 300, Olympus, Japan). At the same time, the cytocompatibility and photoluminescence (PL) stability of the YAG:Ce nanorod phosphors was observed using the similar method with BMMSC incubated for 12, 24, and 48 h. The cell viability of BMMSC containing YAG:Ce nanorod phosphors was evaluated using the cell counting kit-8 (CCK-8) assay.

2. EXPERIMENTAL PROCEDURES

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The crystal structure of samples at different synthesis stages were investigated by powder XRD, and the results are shown in Figure 1. All the diffraction peaks of the Al2O3-precursor prepared in the first stage (Figure 1 (a)) can be identified to be the diffraction peaks of NH4Al(OH)2CO3 (JCPDS card no. 42−0250), without any other peak, indicating the precursor is single phase of NH4Al(OH)2CO3. After calcining at 1200 °C for 2 h, the XRD peaks of the sample (Figure 1 (b)) correspond to rhombohedral α-Al2O3 (JCPDS card no. 10-0173), without another crystal phase of Al2O3, which indicates that the precursor is transformed into pure Al2O3 from NH4Al(OH)2CO3. Most peaks in the XRD pattern of the second stage precursor for YAG:6 at. % Ce3+ (Figure 1c) can be indexed to the diffraction peaks of α-Al2O3. However, two additional bulges at 2θ approximately 30 and 50° were detected, as shown in Figure 1c (inset is the magnification of Figure 1b,c); these peaks are ascribed to amorphous Ycompounds. In addition, with the same amount of alumina in both samples to be tested, the diffraction peak intensities of

Reactants Al(NO3)3·9H2O (Alfa Aesar, 99.99%), Y(NO3)3·6H2O (Alfa Aesar, 99.99%), Ce(NO3)3·6H2O (Sinopharm Chemical Reagent Co., Ltd., Specpure) and urea (Sinopharm Chemical Reagent Co., Ltd., Specpure) were used as purchased without further purification. A two-pot method was used for the preparation of 1D YAG:Ce nanorod phosphors. In the first pot, Al2O3 nanorods were prepared; these nanorods served as the template to prepare YAG:Ce in a second pot. Briefly, Al2O3 nanorods were synthesized through a hydrothermal method followed by a calcination process. Al(NO3)3·9H2O was dissolved in deionized water with vigorous stirring until the solution become transparent (Al concentration = 0.8 mol/L). Urea (urea:Al3+ = 10:1 in mol) powder was weighted and then added to the above solution under vigorous stirring to obtain a transparent solution mixture. The solution was transferred into a Teflon-lined stainless autoclave (25 mL volume) and then heated at 150 °C for 5 h. The white powder was collected by filtration, washed with deionized water and ethanol, and then dried at 80 °C for 24 h to obtain the Al2O3 precursor nanorods. The above precursor was calcined at 1100 °C for 2 h, and approximately 1 g of Al2O3 nanorods was obtained. The second pot is the preparation of YAG:Ce. Briefly, stoichiometric amounts of Ce(NO3)3·6H2O, Y(NO3)3·6H2O and Al2O3 nanorods were suspended in deionized water with sonication at 200 W/h for 5 h. 11991

DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997

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Figure 2. SEM images of the samples: (a) Al2O3 precursor; (b) Al2O3 samples obtained after calcining at 1200 °C; (c) precursor of YAG:6 at. % Ce3+; (d) YAG:Ce samples obtained after 1300 °C.

Figure 1. XRD patterns of (a) Al2O3-precursor NH4Al(OH)2CO3; (b) Al2O3 samples obtained after calcining at 1200 °C, α-Al2O3; (c) precursor of YAG:6 at. % Ce3+: α-Al2O3 and Y-compound; (d) samples obtained after 1300 °C: YAG:Ce. Inset is a magnification of spectra b and c. Stars (★) indicate Al2O3, and dots (●) indicate CeO2.

nanorods, and the diameter of the YAG:Ce nanorods is approximately 250−400 nm (Figure 2d), which is smaller than the precursor nanorods. The morphology and the microstructure of the as-prepared and YAG:6 at. % Ce3+ samples were further characterized using TEM and HRTEM. The YAG:Ce precursor particles (Figure 3a) are rod-like nanostructures of 3−5 μm in length and 300− 500 nm in diameter, which is consistent with the results from the SEM observation. The YAG:Ce precursor nanorod exhibits a clear interface between the core and the shell (Y-compound thickness approximately 20−25 nm) due to the different electron penetrability corresponding to different compositions. In addition, it is well-known that under electron exposure, the

Al2O3 in the YAG:Ce-precursor sample are lower than those of the as-synthesized Al2O3, which is caused by the blocking effect of the Y-compound precursor on the Al2O3 nanoparticles. After the YAG:6 at. % Ce3+ precursor sample is calcined at 1300 °C (Figure 1d), almost all of diffraction peaks of the sample can be identified as Y3Al5O12 (JCPDS card no. 72-1315). Because of the difficulty in high Ce3+ doping concentration in YAG:6 at. % Ce3+ phosphor, some very week diffractions peaks can be detected, which can be ascribed to α-Al2O3 phase as shown in Figure 1b and CeO2 phase. The residual Al2O3 and CeO2 are of very small amount. In addition, the diffraction peaks of YAG:2 at. % Ce3+ nanorod phosphors can be identified as pure Y3Al5O12. No impurity or obvious shifting of the peaks can be detected in their XRD patterns, which implies that the YAG:Ce-precursor has fully transformed into YAG:Ce. (The phase of all four kinds of phosphors are shown in Figure S1, Supporting Information). The morphologies of the samples were characterized by scanning electron microscopy, as shown in Figure 2. The Al2O3-precursor (Figure 2a) mainly consists of straight rod-like particles with a length of approximately 3−5 μm and a diameter of approximately 200−500 nm. After calcination, the crystallized Al2O3 (Figure 2b) is of similar length as the precursor nanorods; however, the nanorods after calcination are curved and have a less smooth surface, and the diameter of the nanorods decreases to 100−250 nm. The variation of the morphology is caused by the decomposition reaction of NH4Al(OH)2CO3 and the recrystallization process of the Al2O3 precursor during calcination. After precipitation in Y3+ ions, Ce3+ ions and urea solution, the YAG:Ce precursor nanorods (Figure 2c) have a rough surface with a diameter ranging from 300 to 500 nm, which is much larger than that of Al2O3 nanorods. As discussed in our previous work,19,20 the reaction should be a layer of Ycompounds assembling on the surface of the Al2O3 nanorods to form a core−shell structure. After calcination, the final YAG:Ce particles appear to maintain a rod-like nanostructure with a smooth surface. The length of the YAG:Ce nanorods is approximately 3−5 μm, which is same as the YAG:Ce precursor

Figure 3. TEM images of (a) the YAG:Ce precursor, with an inset of the HRTEM image of the Y-compound precursor shell, and (c) the YAG:6 at. % Ce3+ samples, with the HRTEM image inset. EDS element mapping results of (b) the Al2O3/Y-compound precursor and (d) the YAG:6 at. % Ce3+ samples for Al + Y, Al, and Y as well as the TEM image in a dark field. 11992

DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997

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Figure 4. Schematic illustration of the YAG:Ce formation process.

precursor. In the next stage, the as-synthesized Al2O3 nanorods serve both as a template and as the reactant to synthesize YAG:Ce nanorods. In the second stage, when the mixture suspension system of Al2O3, Y(NO3)3 and urea is heated, the negative ion group of OH−, CO32− and HCO3−, which form from the slow decomposition of urea when the temperature is below 83 °C, modified the surface of the template particle (Al2O3 nanorod core). When the temperature of the suspension is over 83 °C, the urea molecules in the suspension are rapidly decomposed, and the yttrium containing complex precursor nuclei abruptly form upon reaction between Y3+ ions and the precipitant anions (HCO3−, OH−, etc.), leading to a burst nucleation process.31 Because of the electrostatic attraction between the opposite surface charges of the Al2O3 nanorods and the yttrium containing precursor nuclei,19,20 these yttrium-precursor nuclei self-assembled onto the surface of Al2O3 nanorods to form a shell due to the electrostatic attraction. During the reaction process, the Y-precursor shell becomes thicker, leading to a core−shell structure, in which the core is Al2O3 and the shell is formed of yttrium-precursor particles. The YAG:Ce precursor nanorod grows during the oil-bath process until the Y3+ ions are completely consumed. During calcination, the Y-precursor layer decomposes and an in situ nanoscale solid state reaction occurs between the Y-based shell and the Al2O3 core. Afterward, the final product YAG:Ce is obtained. To compare the effect of the morphology of the YAG:Ce nanorods on the luminescent properties, granular YAG:Ce nanoparticles were synthesized via the same partial-wet chemical method by using granular Al2O3 as the template and the reactant. The product granular YAG:Ce nanoparticles are subspheroidal, with a size of approximately 200 nm and with a highly regular surface. The detailed characterization of the granular YAG:Ce nanoparticles is presented in the Supporting Information (Figure S3 of the Supporting Information).

Y-compound (Y(OH)CO3 or some other carbonate) will decompose and recrystallize. After high electron exposure, the polycrystalline character of the Y-compound, whose lattice structure is omnidirectional, is shown clearly in the precursor shell (Figure 3a inset). The core−shell structure is also validated by energy-dispersive X-ray spectroscopy (EDS; Figure 3b). Although the Al element is only present in the core, the Y element appears distributed all over the particle, as expected for a core−shell structure. The YAG:Ce nanorod particles (Figure 3c,d) are a homogeneous single crystal structure without a residual trace of the core−shell structure and are thinner than the precursor nanorods. From HRTEM images of YAG:Ce nanorods, we found that the nanorod is well crystallized, and the crystalline planes extend throughout the structure, with an interplanar spacing of 0.269 nm, which corresponds to the (420) plane of the YAG crystal. These results indicate that YAG:Ce nanorods obtained after calcination are of a high degree of crystallinity garnet phase, in good agreement with the XRD results (Figure 1d). According to the above results, a mechanism for the formation of YAG:Ce is proposed (Figure 4). The first stage of synthesis of YAG:Ce nanorods is preparation of the Al2O3 nanorods. During the hydrothermal process, when the temperature reaches 83 °C,31 urea starts to hydrolyze to generate a large number of precipitant groups (HCO3−, OH−, etc.). These groups can react with Al3+ to nucleate primary particles, which are colloidally unstable. These particles aggregate and assemble into the rod-like Al2O3-precursor with amorphous NH4Al(OH)2CO3. During the hydrothermal reaction process, the Al2O3-precursor grows both in length and diameter and forms a rod-like structure based on its crystallization character. (SEM images of the Al2O3-precursor at different hydrothermal reaction stages are given in Figure S2 of the Supporting Information.) After calcination, the Al2O3precursor is transformed into Al2O3 particles, which inherit the rod-like morphology of the NH4Al(OH)2CO3 nanorod 11993

DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997

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ACS Applied Materials & Interfaces 3.2. Luminescent Properties. The photoluminescent (PL) properties of YAG nanorods and granular YAG nanoparticles doped with 2 and 6 at. % Ce3+ were investigated. Form the PL spectrum in Figure 5, all four samples share a

doping concentration increasing from 2 to 6 at. % and the morphology changing from granule to nanorod, the luminescence intensity of the samples increases. The excitation PL spectrum monitored at 520 nm consists of an intense broad absorption band from 400 to 500 nm and a weakly relatively narrow absorption band from 320 to 360 nm, which is due to the electron transitions from a 4f ground state of Ce3+ to the different crystal field splitting of the excited 5d state.32 The PL intensity is a maximum at the excitation wavelength of 463 nm, with the strongest PL spectrum belonging to the YAG:6 at. % Ce3+ nanorod sample. The broad emission in the range of 480− 535 nm, with a luminescent maximum at approximately 520 nm, is ascribed to the electron transitions from the lowest crystal-splitting component of the 5d (2A1g) level to the ground state of Ce3+ (2F5/2, 2F7/2).25,30 When comparing the luminescence intensity of all of the samples, it is clear that YAG:Ce nanorod samples show stronger emission intensity than granular ones. From the results determined by the BET method, the surface area of a material increases when the size decreases. (The results of surface area of the samples are given in Supporting Information Table S1.) Why do the smaller surface area phosphors (YAG:Ce nanorod samples) possess stronger PL intensity than the lager surface area ones (granular YAG:Ce samples)? On one hand, a larger surface area means a higher number of surface defects, such as the organic or −OH groups, which are harmful to luminescence. The improvement of luminescence intensity originates from the smaller surface area of YAG:Ce nanorod samples, which possess relatively small number of surface

Figure 5. PL emission spectra at λex = 463 nm and PL excitation spectra at λem = 520 nm of YAG nanorods and granular YAG phosphors doped with 2 and 6 at. % Ce3+.

similar spectral pattern in the excitation and emission spectrum, albeit with distinct differences in the intensity. With the Ce3+

Figure 6. (Blue) Decay curves with (red) fitting and (green) residuals monitored at 530 nm and excited at 445 nm at room temperature for the samples. 11994

DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997

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is 66.85 and 70.09 ns, respectively, which is longer than that of granular YAG:Ce phosphors of the same doping levels (61.16 and 63.38 ns). (The process of calculating the average fluorescence lifetime is given in the Supporting Information, and the results are given in Table S2.) This once again proves that the crystallization of YAG:Ce nanorod phosphors is better than that of the granular YAG:Ce phosphors. 3.3. Biolabeling Application. The YAG:6 at. % Ce3+ nanorod phosphors dispersed in medium with the concentration of 100 μg mL−1 were incubated with BMMSC for 24 h and showed good cytocompatibility and good PL stability. (The results of cytocompatibility test and PL stability test of the YAG:Ce nanorod phosphors are given in the Supporting Information in Figures S4 and S5, respectively.) In addition, luminescent biolabeling results of YAG:6 at. % Ce3+ nanorod phosphors are given in Figure 7. The fluorescence microscopy

defects. On the other hand, the host material with bad crystallization would also present a number of defects in the phosphor body. These defects could affect the migration of photos and act as quenching sites, which could provide nonradiative recombination routes for electrons and holes in hosts, thus leading to fluorescence quenching and a decrease of the PL intensity. The improvement of luminescence intensity also originates from the good crystallization of the YAG:Ce nanorod samples, as indicated by the XRD and TEM results, which possess a small number of defects. Therefore, the YAG:Ce nanorod samples show a strong emission intensity. Moreover, the photoluminescence QY of the YAG:6 at. % Ce3+ nanorod phosphors is 30.28%, which is higher than that of granular YAG:6 at. % Ce3+ phosphors (26.36%). Similarly, the photoluminescence QY of the YAG:2 at. % Ce3+ nanorod phosphors and granular YAG:2 at. % Ce3+ phosphors are 40.12% and 32.77%, respectively. The results confirm that the YAG:Ce nanorod phosphors possess good luminescent properties compared with granular YAG:Ce phosphors. It is noted that although the luminescence intensities of the phosphors with more Ce3+ are higher, the photoluminescence QYs of them are lower. It may be caused by concentration quenching effect due to the high Ce3+ concentration, as well the difficulty in homogeneous doping of Ce with high concentration. The 5 nm red shift in the emission peak as the dopant concentration increases from 2 and 6 at. % is attributed to an increased selfabsorption and an increase in the YAG lattice parameter when Y3+ (1.019 Å) is substituted by Ce3+ (1.283 Å).23 The fluorescence lifetime of YAG:Ce phosphors with different morphology and different doping concentration were investigated by fluorescence spectrometers, and the corresponding decay curves monitored at 530 nm and excited at 445 nm at room temperature are shown in Figure 6. Obviously, the curves of the granular YAG:Ce phosphors are fitted into the single exponential function for fluorescence lifetime, I = A1exp(−t/τ1). The lifetimes are determined to be 61.16 and 63.38 ns for granular YAG:2 at. % Ce3+ and granular YAG:6 at. % Ce3+, respectively. However, the decay of YAG:Ce nanorod phosphors requires at least two exponential terms, with a short decay time τ1 and a long decay time τ2, and the decay can be well fitted into a function as I = A1exp(−t/τ1) + A2exp(−t/τ2). For the YAG:2 at. % Ce3+ nanorod phosphors, the short decay time τ1 is 28.39 ns, and the long decay time τ2 is 72.79 ns. For YAG:6 at. % Ce3+ nanorod phosphors, the decay times τ1 and τ2 are 31.06 and 75.48 ns, respectively. On one hand, the long lifetime τ2 is assigned to conventional emission, which reflect luminescence of Ce3+ ions in the bulk YAG sample.33 Due to the good crystallization of the interzonal region, the YAG:Ce nanorod samples present a long lifetime with a certain proportion. On the other hand, the short lifetime τ1 reflects the luminescence of Ce3+ on the surface, which possesses abundant defects.34 Because the proportion of surface atoms is large in both ends, resulting in more defects being introduced into these parts, the YAG:Ce nanorod samples also have a short lifetime feature. Therefore, the rod-like morphology leads to a significant differentiation between τ1 and τ2. In addition, the distribution of defects on the granular YAG:Ce is uniform, leaded to a single fluorescence lifetime. Thus, YAG:Ce nanorod phosphors possess two fluorescence lifetimes; this unique property will lead to special applications of these YAG:Ce nanorod phosphors. The average fluorescence lifetime τ35 of YAG:2 at. % Ce3+ nanorod phosphors and YAG:6 at. % Ce3+ nanorod phosphors

Figure 7. Fluorescence microscope images of BMMSC (a, green) after 24 h of exposure to YAG:6 at. % Ce3+ nanorod phosphors excited at 460−490 nm; (b, red) the actin filaments of the cells stained with phalloidin and excited at 510−550 nm and (c, blue) the nucleus with Hoechst 33258 excited at 330−385 nm. (d) The bright-field image of BMMSC in the same zone. (e) The image generated from stacking images a, b, and c. (f) Stacking image generated from images d and e. Scale bar for all images in this figure are 100 μm.

image displayed in Figure 7a is obtained under excitation wavelength of 460−490 nm and is detected at the emission wavelengths greater than 510 nm. The Ce3+ luminescence can be observed under this excitation range, with the particle distribution as fasciculation. To indicate the location of the nanorod phosphors in BMMSC, the actin filaments of the cells were stained with phalloidin (Figure 7b) and the nucleus was stained with Hoechst 33258 (Figure 7c). From the stacking image (Figure 7e), it appears that YAG:Ce nanorod phosphors have been internalized by the cells because a green fluorescence is observed in the cytoplasm, with the fluorescence being more intense around the nucleus. Figure 7d shows the fluorescence images superimposed on bright-field images of the same zone. After stacking the bright-field images and Figure 7e, the cell configuration of BMMSC clearly appeared with staining and labeling (Figure 7f). Note that YAG:Ce phosphors and Hoechst 33258 can be excited at the same range of approximately 540 nm and their emission wavelengths are quite different. Under the same excitation wavelength, the nucleus and YAG:Ce phosphors can be observed in the same 11995

DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997

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thank Dr. Sujie Chang at State Key Laboratory of Crystal Materials in Shandong University for her help in the experiment proceeding.

scene with different colors (Figure 7c, blue for Hoechst 33258 and green for YAG:Ce phosphors). This mode could be applied to simplify the biolabeling process. In addition, due to the broad excitation and emission wavelength ranges of YAG:Ce, the Ce3+ luminescence can be detected while the actin filaments are observed (Figure 7 (b)). According to the above results, the YAG:Ce nanorod phosphors can label BMMSC and they show a relatively good biocompatibility.

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4. CONCLUSIONS One-dimensional YAG:Ce nanorod phosphors were prepared via a two-step procedure involving a hydrothermal method and a partial wet chemical route. The well-crystallized phosphors with a 200−300 nm diameter and a 2−3 μm length possess high specific surface area and also few surface defects due to their high crystallinity. Thus, the PL of the YAG:Ce nanorod phosphors is higher than their granular analogs, which contain a higher proportion of surface defects. Because of the unique morphology, YAG:Ce nanorod phosphors possess two luminescent decay times, a short lifetime τ1 and a long lifetime τ2. The photoluminescence QY of YAG:6 at. % Ce3+ and YAG:2 at. % Ce3+ nanorod phosphors are 30.28% and 40.12%, respectively, which are higher than those of granular ones. These phosphors were found to be able to label bone marrow mesenchymal stem cells efficiently and exhibit a relatively good biocompatibility. The good luminescent properties and biocompatibility of these YAG:Ce nanorod phosphors indicate that they are promising candidates for a more systematic biolabeling usage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01460. The evaluation process of yield; phase of YAG:Ce phosphors; morphologies and XRD patterns of Al2O3 precursor at different time from hydrothermal process; morphologies and XRD patterns of granular YAG:Ce samples; surface area of the YAG:Ce phosphors determined by BET method; average fluorescence lifetimes of YAG:Ce nanorod phosphors; proliferation of BMMSC in the solution with different YAG:6 at. %Ce nanorod phosphors concentration after 12, 24, 48 h, PL stability vs time (0, 12, 24, 48 h) of YAG:Ce nanorods when the material was dispersed in aqueous solution and in PBS buffer and mixed with biological cells. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for funding from the National Natural Science Foundation of China (Grant No. 51372142, 51402172), the Innovation Research Group (IRG: 51321091), the Science and Technology Development Plan Project of Shandong Province (2014GGX102004), and the Fundamental Research Funds of Shandong University (2014QY003, 2014JC019, 2015JC036). The authors would 11996

DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997

Research Article

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DOI: 10.1021/acsami.6b01460 ACS Appl. Mater. Interfaces 2016, 8, 11990−11997