J. Phys. Chem. C 2008, 112, 11211–11217
11211
Rare-Earth-Doped and Codoped Y2O3 Nanomaterials as Potential Bioimaging Probes Gautom Kumar Das and Timothy Thatt Yang Tan* School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, Singapore 637459, Singapore ReceiVed: March 10, 2008; ReVised Manuscript ReceiVed: May 5, 2008
The synthesis and characterization of various rare-earth (RE)-doped and codoped yttrium oxide (Y2O3) nanomaterials were undertaken in the current work. RE dopants such as terbium (Tb), europium (Eu), and erbium (Er) were successfully doped, and ytterbium (Yb)-Er was codoped into Y2O3 nanomaterials. The as-synthesized nanomaterials are highly dispersed in the organic phase. X-ray diffraction peaks can be assigned to cubic Y2O3, which confirms the crystallinity of the as-prepared Y2O3 samples. Room temperature photoluminescence (PL) spectra showed characteristic emission peaks of Tb-, Eu-, and Er-doped Y2O3 samples. Up-conversion spectra of green and red emissions were also shown for Er-Yb-codoped Y2O3 samples. It was found that the intensities of the green and red emissions could be altered by varying the dopant concentration. The nanocrystals were surface-functionalized with an amine (NH2) group via a reverse microemulsion method. The amine groups render the nanocrystals water-soluble and also afford them with the possibility of further functionalization by other biomolecules. in Vitro cytotoxicity studies showed that the synthesized nanocrystals have no appreciable toxicity on human hepatocellular carcinoma (Hep-G2) cells at concentrations of 0.007-0.063 mg/mL. Because of the Y2O3:RE nanomaterials’ well-dispersity in water, low toxicity, and good PL, they can potentially be used as fluorophores in bioimaging. 1. Introduction The endeavor to develop novel fluorescent nanomaterials in the fields of nanobiotechnology, photonics, and optoelectronics has been burgeoning from the past decade.1–3 There are several classes of nanomaterials currently being investigated or employed as fluorescent emitters. They include organic, metallorganic, and polymer-based dye, fluorescent proteins, and semiconductor nanocrystals. Generally, dye molecules photobleach and quench when exposed to harsh environments because of interactions with solvent molecules and reactive species such as oxygen or ions dissolved in solution.2 Colloidal II-VI semiconductor nanocrystals, known as quantum dots (QDs), are currently of great interest as light-emitting materials for biological labeling applications.4–9 The fundamental shortcoming involving the use of QDs, particularly in biological applications, is their toxicity. They are generally made of toxic metals such as Pb2+ or Cd2+, which are not suitable for bioapplications. In comparison, rare-earth (RE) nanophosphors offer an alternative for biological labeling and medical diagnostics because of their large Stokes shift, sharp emission spectra, long lifetime, multiphoton and up-conversion excitation, low toxicity, and reduced photobleaching over semiconductor nanocrystals like QDs and organic phosphor molecules.10–12 These materials have the ability to emit intensely at various wavelengths by an appropriate choice of color-center elements instead of variation in the particle size.13 In RE-doped nanophosphors, the RE ions 4f electronic levels are spatially localized to dimensions much smaller than the size of the nanoparticles and thus are not affected by the dimensions of the nanostructure. Yttrium oxide (Y2O3) has been widely investigated as the host for RE ions for optical application.14–17 It has a broad transparency range (0.2-8 µm) with a band gap of 5.6 eV, a * To whom correspondence should be addressed. E-mail: tytan@ ntu.edu.sg.
high refractive index, better thermal conductivity, and low photon energy that make it an attractive choice as the host material.18 The bulk of Y2O3:RE research has been focused primarily on their Stokes luminescence and down- and upconversion fluorescence properties. In contrast, however, few works addressed their morphological characteristics. Very recently, Wang et al.15 showed the self-assembly of Y2O3:Eu nanocrystals to nanodisks, while Si et al.19 reported the formation of Y2O3 nanoplates. A flowerlike,20 wirelike21 structure of REdoped Y2O3 has also been reported in the literature. In our previous work, we reported the shape evolution of Y2O3:Tb nanomaterials from nanocrystals to nanorods using long-chain alkylamines as the stabilizing agent.22 In the present work, we aim to study the morphology and photoluminescence (PL) of Y2O3 nanomaterials when doped with other RE ions including europium (Eu) and erbium (Er) and codoped with ytterbium (Yb) and Er. We have also investigated the effect of dopant concentrations on the PL of Tb-, Eu-, and Er-doped and Yb-Er-codoped Y2O3 nanomaterials. In order to demonstrate the potential of the synthesized nanomaterials in bioimaging, we have further surface-functionalized the nanocrystals with amine (NH2) groups via a reverse microemulsion method so as to render them water-soluble and to afford them the ability for further bioconjugation. in Vitro cytotoxicity studies of the nanomateirals were also undertaken. 2. Experimental Section Chemicals. Terbium(III) chloride hexahydrate (99.9%), ytterbium(III) chloride hexahydrate (99.99%), europium(III) chloride hexahydrate (99.9%), erbium(III) chloride hexahydrate (99.9%), tetramethylammonium hydroxide (TMAH; 25 wt % in methanol), Igepal CO-520 [polyoxyethylene(5)nonyl phenyl ether], and oleylamine (technical grade, 70%) were purchased from Aldrich and used without further purification. Yttrium oxide (99.99%), HNO3 (analytical reagents, 70%), and oleic acid
10.1021/jp802076n CCC: $40.75 2008 American Chemical Society Published on Web 07/02/2008
11212 J. Phys. Chem. C, Vol. 112, No. 30, 2008
Figure 1. Typical TEM images of Y2O3:RE obtained at 280 °C: (a) after 10 min, indicating the formation of nanocrystals; (b) after 30 min, indicating the formation of a mixture of nanocrystals and nanorods; (c) after 2 h, indicating the formation of nanorods only.
(technical grade, 90%) were purchased from Alfa Aesar. NaOH (reagent grade, 97%, beads) and (3-aminopropyl)trimethoxysilane (APS; 97%) were purchased from Fluka. Ethanol, hexane, cyclohexane, and chloroform were of analytical reagent grade and were used as received. Synthesis of Y2O3:RE3+ Nanocrystals and Nanorods. A yttrium (RE) oleate complex was first prepared by reacting Y2O3 powder, oleic acid, sodium hydroxide, and rare-earth chlorides (i.e., TbCl3, YbCl3, EuCl3, and ErCl3). Typically, 10 mmol of Y2O3 was dissolved in 4 mL of HNO3 (70%), and x mmol of rare-earth chloride (varied from 0.05 to 2 mmol) was then added. To this precursor, 21 mL of oleic acid, 100 mL of hexane, 80 mL of ethanol, and 60 mL of water were added, and then the mixture was stirred for 1 h. Then, 26.5 g of NaOH was added and aged in a silicon oil bath at 70 °C for 4 h. After that, the upper organic layer containing the yttrium-(RE)-oleate complex was collected using a separatory funnel, washed with 30 mL of distilled water, and dried in an oven overnight at 70 °C to evaporate the water and hexane. Y2O3:RE3+ nanocrystals or
Das and Tan nanorods were then synthesized by the thermal decomposition of the yttrium-(RE)-oleate complex in the presence of longchain alkylamines. In a typical synthesis process for nanocrystals and nanorods, 0.5 mmol of the yttrium (RE) oleate complex was dissolved in 5 mL of oleylamine in a three-necked flask. The mixture solution was then heated to the crystallization temperature at 5 °C/min under a N2 environment. Nanocrystals and nanorods were grown in the solution by varying the reaction time at a fixed temperature. Reaction was halted by cooling the reaction solution to room temperature and precipitating with excess ethanol. The precipitate was collected by centrifugation and the supernatant decanted. This washing process was repeated several times, and the purified nanomaterials then dispersed in hexane. Silica Coating and Amine Functionalization. Silica coating of the nanocrystals was conducted according to the process described by Selvan et al.23 At first, micelles were prepared by dissolving 0.2 g of Igepal CO-520 in 4 mL of cyclohexane, followed by stirring vigorously for 1 h. Meanwhile, nanocrystals were being redispersed in chloroform at a concentration of 4 mg/mL. Next, 500 µL of nanocrystals in chloroform was added to the micelle solution, and the mixture was stirred for 15 min. Subsequently, 20 µL of APS was added, and the mixture was stirred for another 1 h. Then, 20 µL of TMAH in methanol was added. After 1 h more of stirring, 20 µL of deionized water was added, and the mixture was stirred for 30 min. At this stage, some globules were formed and settled at the bottom of the flask, leaving the upper solution transparent. The globules of Y2O3:RE nanocrystals were then collected, and the transparent organic phase was discarded. After this, the functionalized nanocrystals were washed with chloroform and ethanol for the complete removal of excess surfactant and other reactants. The silica-coated Y2O3:RE were then dispersed in deionized water. in Vitro Cytotoxicity Studies of Y2O3:Tb Nanocrystals in Water. Human hepatocellular carcinoma cells (Hep-G2) were seeded at a concentration of 2 × 104 cells/mL and cultured with a 125 µL/well medium in 96-well flat-bottom microassay plates for 24 h before the addition of the sample solutions. A desired amount of amine-functionalized Y2O3:Tb nanocrystals was diluted with Dulbecco’s Modified Eagle’s Medium without fetal bovine serum. A volume of 50 µL of the sample solution was then added into each well, and the cells were cultured at 37 °C for 48 h. To evaluate cell viability, 10 µL of alamar blue was added to the culture media and incubated for 4 h at 37 °C. The optical absorbance was read at 560 and 600 nm to quantify the amount of dye reduction using a GENios Pro multilabel counter (Tecan). The experiments were repeated three times to ensure reproducibility. Characterization. Transmission electron microscopy (TEM) images were obtained on a JEOL 3010 electron microscope operating on 200 kV. The X-ray diffraction (XRD) patterns of the as-synthesized nanomaterials were collected using a LabXShimadzu XRD6000 diffractometer with Cu KR used as the X-ray source (λ ) 1.5406 Å). PL spectra were collected on a Shimadzu RF-5301 PC spectrofluorophotometer using a 150 W xenon lamp as the excitation source. Up-conversion fluorescence spectra were recorded using a Fluoromax-4 Jobin Yvon spectrofluorometer equipped with a 150 W xenon lamp. Y2O3:RE nanomaterials were dispersed at a concentration of 0.25 mg/ mL in hexane in standard quartz cuvettes at room temperature for all of the PL spectra measurement. Fourier transform infrared (FTIR) spectra were recorded using a Digilab FTS3100 spectrometer with a resolution of 4 cm-1.
Rare-Earth-Doped and Co-Doped Y2O3 Nanomaterials
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11213
Figure 2. XRD pattern of as-prepared Y2O3:RE: (a) nanorods; (b) nanocrystals.
3. Results and Discussion In our previous study, it was found that the temperature, the reaction time, the amount of surfactant, and the type of alkylamine influenced the morphology and size of Y2O3:Tb nanocrystals.22 In the current study, however, we only investigate the effect of the reaction time to observe the trends of morphological evolution for other RE dopants in Y2O3. The reaction temperature was fixed at 280 °C. It was observed that at different aging times the morphology of the nanomaterials was changed from nanocrystals to nanorods for all of the REdoped and codoped Y2O3. The typical observation for all of the aforementioned RE-doped and codoped Y2O3 is shown in the TEM images in Figure 1. Nanocrystals were formed by heating of the reaction mixture for 10 min. As seen in Figure 1, they are highly monodispersed and around 3 nm in size. Further heating of the solution for 30 min generated a mixture of nanocrystals and nanorods (Figure 1b), while extending the heating time to 2 h resulted in the formation of nanorods of lengths in the range of 25-30 nm (Figure 1c). As explained, the formation of nanorods was attributed to the self-assembly of the nanocrystals along the longitudinal axis to nanorods due to dipole-dipole interactions.22 Owing to this phenomenon, the diameters of the nanorods were same as those of the nanocrystals. We have observed the same trend of morphological evolution for all RE-doped and codoped Y2O3 nanomaterials, which is expected because the small amount of dopants that are of similar ionic diameter to the Y3+ ion in the host material should not exert a significant influence on the nanoparticle morphology. The Y2O3:RE nanocrystals with high surface energy approached each other and minimized their surface energy via oriented attachment by fusing end-to-end longitudinally and resulted in a rodlike structure.24,25 The crystal structure of the as-synthesized nanocrystals and nanorods was analyzed by powder XRD. Figure 2 shows the powder XRD pattern of as-prepared RE-doped Y2O3:RE nanomaterials. The diffraction peaks obtained from XRD confirmed the crystallinity of as-prepared Y2O3 nanomaterials. The peaks are assigned to cubic Y2O3 (JCPDS file no. 89-5592). Broadening of the peaks suggests a small crystalline size. Intense peaks of nanorods at (440) in Figure 2a indicate their anisotropic growth due to the formation of nanorods at longer heating time. XRD analysis further suggests that Y2O3:RE crystal growth is independent of the type and concentration of the RE dopant within the scope of the current investigation.
Figure 3. Room temperature PL emission and excitation spectra of (a) Y2O3:Tb, (b) Y2O3:Eu, and (c) Y2O3:Er nanomaterials.
The room temperature PL spectra of different RE-doped Y2O3 nanomaterials dispersed in hexane are shown in Figure 3. Emission peaks at different wavelengths correspond to the transition of electrons in respective RE3+ ions. Figure 3a shows PL excitation and emission spectra of Y2O3:Tb nanomaterials. The emission spectra of Y2O3:Tb nanomaterials under 235 nm excitation display four emission bands at 490, 545, 585, and 620 nm, which can be assigned to 4f f 4f transitions within Tb3+ ions. The most intense peak at 545 nm corresponds to the 5D f 7F transition, while the peaks at 490, 585, and 620 nm 4 5 correspond to 5D4 f 7F6, 5D4 f 7F4, and 5D4 f 7F3 transitions, respectively.26 PL excitation and emission spectra of Y2O3:Eu nanomaterials are shown in Figure 3b. Three emission peaks have been found at 590, 612, and 625 nm under 255 nm excitation. The most intense peak at 612 nm corresponds to the
11214 J. Phys. Chem. C, Vol. 112, No. 30, 2008
Figure 4. Effect of the dopant concentration on PL for (a) Tb-doped, (b) Eu-doped, and (c) Er-doped Y2O3 nanomaterials. 5D 0
f 7F2 transition within Eu3+ ions, while the other peaks at 590 and 625 nm correspond to 5D0 f 7F1 and 5D0 f 7F3 transitions, respectively.15,21 Figure 3c shows excitation and emission spectra of Y2O3:Er. Two emission peaks at 525 and 553 nm were found at 245 nm excitation. The intense peak at 525 nm corresponds to the 2H11/2 f 4I15/2 transition, while the other at 553 nm corresponds to the 4S3/2 f 4I15/2 transition.27 To investigate the optimum dopant amount, Tb, Eu, and Er dopant concentrations (mol %) were varied. As shown in Figure 4, no shift in the spectral position was observed by varying Tb,
Das and Tan Eu, and Er ion concentrations, but the PL intensity changed considerably. The inset of Figure 4a shows the PL intensity as a function of the Tb concentration at 545 nm for the 5D4 f 7F5 transition, while insets of parts b and c of Figure 4 show the PL intensity as a function of the Eu concentration at 612 nm for the 5D0 f 7F2 transition and the Er concentration at 525 nm for the 2H11/2 f 4I15/2 transition, respectively. By variation of Tb, Eu, and Er concentrations in the range of 0.5-20 mol %, the highest PL intensity was observed at 5 mol % for all RE dopant concentrations. The trend of the dopant concentration dependence luminescence is consistent with the trends reported in the literature. Park et al.28 reported 8% Tb dopant to be the highest PL intensity, while Muenchausen et al.17 reported optimum Tb at 1.8% in the Y2O3 host. For the Eu dopant, Zeng et al.20 reported that the highest PL was obtained at 4% Eu dopant in Y2O3, while Pang et al.29 reported that at 6% in the Y2O3 host. Yet in another report, Yi et al.27 found that 4% Erdoped Y2O3 give the highest luminescence. Thus, the luminescence property is dependent not only on the dopant concentration but also on some other factors such as the synthesis process, host material, size of the particle, and temperature.30,31 This is because the luminescence property is related to the exact energy level positions, cross sections, dielectric constant, and typical phonon energy of the host material. The general trend is an initial increase of the PL intensity up to a critical concentration, at which the PL intensity is the maximum. Higher PL intensity is attributed to the larger number of luminescent centers in the respective emitting level. With the increment of doped RE3+ ions, more excited ions are transited to the respective emitting level (for example, 5D4 for Tb, 5D0 for Eu, and 2H11/2 for Er). Therefore, increasing the concentration of RE ions will increase the intensity of PL up to a critical concentration, of which the luminescence is maximum. However, PL decreases beyond the critical concentration. At too high RE3+ concentrations, interaction among RE3+ ions causes an output-limiting effect. Studies on Tb-17,32,33 and Eu-31,34,35 doped systems suggested that concentration quenching beyond an optimal dopant concentration is due to energy transfer (ET) between adjacent luminescence centers via cross-relaxation. For the Er-doped system, Lei et al.36 explained concentration quenching in terms of a cooperative up-conversion process. At too high Er3+ ion concentration when two Er ions come into very close proximity, an excited Er3+ ion deexcites the neighboring excited Er3+ ion by ET, and itself returns to the ground state by nonradiative relaxation. Hwang et al.37 investigated cooperative up-conversion for Er-doped phosphate glasses and reported a similar deexcitation mechanism. Thus, at too high Er dopant concentrations, neighboring excited Er3+ ions depopulate the 4I11/2 level, which, in turn, lowers the number of excited ions and, hence, the PL intensity. While PL has been investigated for single RE-doped systems, it has also been studied for a codoped system of which two different RE ions were doped in the same host material. Codoping can increase the efficiency of PL and in some cases can induce up-conversion PL between the donor and acceptor ions.38 In our investigation, Yb3+ was codoped with Er3+ and their room temperature PL was studied. Yb3+ was chosen as the codopant because it can induce ET to a Er3+ ion because the 2F5/2 level of Yb3+ and the 4I11/2 level of Er3+ are nearly resonant. In addition, Yb3+ has a much longer excited-state lifetime.18 We have studied the up-conversion fluorescent spectra of the Yb-Er-codoped Y2O3 nanomaterials. Figure 5a shows the up-conversion emission spectra of codoped samples (2 mol % Er-1 mol %Yb) excited at 980 nm. Three emission bands
Rare-Earth-Doped and Co-Doped Y2O3 Nanomaterials
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11215
Figure 6. Schematic of the different up-conversion mechanism of Yb-Er-codoped Y2O3 excited at 980 nm.
the Er3+ ions in the 4I11/2 level can be further excited to the level by excited-state absorption (ESA) process. The ions then undergo multiphoton relaxation to 2H11/2, 4S3/2 and 4F9/2 luminescent levels. Considering the first case where red emission was enhanced with increasing Yb concentrations. The red emission enhancement is certainly due to a larger population of Er3+ ions at the red emitting level 4F9/2. One of the most likely reasons is the higher efficiency of the cross-relaxation (ET1: 4F7/2 + 4I11/2 f 4F 4 4 40 9/2 + F9/2), which can directly populate the F9/2 level. Another possible route is the populated 4I13/2 level excited to the 4F9/2 level by ET from excited Yb3+ ions (ET2: 4I13/2 + 2F 4 2 39 However, unlike the red emission 5/2 f F9/2 + F7/2). enhancement, the green emission of the codoped sample was decreased with increased Yb concentration. Guo et al.39 proposed that the ET process of ET3 (4F7/2 + 2F7/2 f 4I11/2 + 2F5/2) depopulates the excited 4F7/2 level at higher Yb concentrations. This results in a smaller population of the 2H11/2 and 4S3/2 green emitting levels and causes a decrease in the green emission intensity. In a second case when Er3+ concentrations are 5-fold higher than Yb3+ concentrations, it is more likely that the upconversion is occurring via Er3+ ions. At first, Er3+ ions are excited to the 4I11/2 level via GSA. One ion subsequently transfers it energy to its neighboring Er3+ ion and returns to the 4I15/2 ground state, while the second ion further excited to the 4F7/2 level by an ET process (ET4: 4I11/2 + 4I11/2 f 4F7/2 + 4I 3+ ions are excited to the 4F 15/2). Hence, more Er 7/2 level and relax nonradiatively to the 2H11/2, 4S3/2, and 4F9/2 luminescent levels. In addition, Lin et al.41 proposed an ET mechanism (ET5: 4I 4 4 4 4 13/2 + I11/2 f F9/2 + I15/2), which may populate the F9/2 level at high Er3+ concentration. The combined effect of ET4 and ET5 results in red emission enhancement, while the green emission intensity remains relatively unchanged in the observed up-conversion spectra. To render these nanomaterials water-soluble as potential bioimaging probes and afford them the potential for further functionalization with biomolecules, silanization was carried out using a reverse microemulsion technique. A thin layer of silica coating was anticipated to produce the nanocrystals with amine (NH2) groups at the surface due to silanization. These amine groups can afford easy biomolecule functionalization. The existence of a silica framework and amine (NH2) groups on the surface was confirmed by FTIR spectroscopy. The FTIR spectrum in Figure 7a(i) was taken for as-synthesized Y2O3: Yb nanocrystals capped with oleic acid. The peaks at 3018, 2925, and 2850 cm-1 are due to dCH stretching and asymmetric and symmetric C-H stretching, respectively.42 The peaks at 1549 and 1454 cm-1 are attributed to CdC stretching and C-O-H in-plane bending bands. The peak at 570 cm-1 is for 4I 7/2
Figure 5. (a) Up-conversion spectra of Yb-Er (1:2 mol %) codoped Y2O3 at 980 nm excitation. (b) Concentration dependence up-conversion spectra at 980 nm excitation.
were observed: two in the green emission region centered at 525 and 535 nm correspond to the 2H11/2 f 4I15/2 and 4S3/2 f 4I 15/2 transitions, respectively, while a third peak appearing in the red region at 652 nm corresponds to the 4F9/2 f 4I15/2 transition.27 The concentration dependence of up-conversion spectra of Yb-Er-codoped Y2O3 was investigated and is shown in Figure 5b. Using a fixed Er concentration (1 mol %) and upon variation of the Yb concentration (1 and 5 mol %), it was found that the green emission intensity decreased while the red emission intensity increased. When the Yb concentration was fixed at 1 mol % and the Er concentration was varied (1 and 5 mol %), the red emission intensity increased while the green emission intensity was almost unchanged. The observed phenomenon is consistent with that reported by Guo et al.39 and Vetrone et al.40 in Yb-Er-codoped systems. The up-conversion mechanism in Yb-Er-codoped systems is well studied and occurs via two successive energy transfers (ETs) from the Yb ion to the Er ion.27,39,40 Figure 6 shows the schematic of the up-conversion mechanism. At 980 nm excitation, the Er3+ ions can be excited to the 4I11/2 excited state via two processes: ground-state absorption (GSA; 4I15/2 f 4I11/2) and ET from excited Yb3+ ions (2F5/2 + 4I15/2 f 2F7/2 + 4I11/2). The latter process is particularly dominant in codoped samples with higher Yb concentrations because Yb3+ ions have larger absorption cross sections than Er3+ ions around 980 nm.40 Upon absorption of 980 nm photons,
11216 J. Phys. Chem. C, Vol. 112, No. 30, 2008
Das and Tan
Figure 8. Cell viability test for Y2O3:Tb.
4. Conclusion
Figure 7. (a) FTIR spectra of (i) as-synthesized Y2O3:Tb and (ii) surface-functionalized Y2O3:Tb. (b) Luminescence of amine-functionalized Y2O3:Tb in deionized water at 365 nm illumination.
Y-O vibration bands.43 The spectrum of amine-functionalized nanocrystals is shown in Figure 7a(ii). The peak at 685 cm-1 is identified as the N-H bending band. The relatively strong peak at 1109 cm-1 is due to the overlay of the C-N stretching vibration, normally observed at 1000-1200 cm-1, and IR absorption of the Si-O-Si bond in the range of 1000-1130 cm-1.44,45 The peaks at 1570 and 1663 cm-1 are due to NH2 symmetric bending and N-H bending, respectively. The peak at 3635 cm-1 is assigned to N-H stretching for a terminal amine (NH2) group.44 Amine functionalization renders hydrophilic groups on the nanocrystal surface and enhances the dispersity of these nanomaterials in water. The trust of our proposition is to use these nanomaterials as bioimaging probes, and hence it is necessary to show that the amine-functionalized Y2O3:RE nanomaterials are well-dispersed in water and are luminescent. The photograph shown in Figure 7b indicates that the Y2O3:Tb nanomaterials are well-dispersed in deionized water and luminescent at 365 nm illumination. To the best of the authors’ knowledge, there is no open literature documenting the toxicity of RE-doped inorganic crystals. Some literature has indeed reported that RE ions are not highly toxic.46 However, it is necessary to conduct cytotoxicity studies on the synthesized nanocrystals because they are proposed to be used as probes for bioimaging. Preliminary cytotoxicity of the amine-functionalized nanocrystals was undertaken in human hepatocellular carcinoma (Hep-G2) cells. Cell viability data (Figure 8) reveal no appreciable toxicity when the cells were subjected to Y2O3:Tb nanocrystals at concentrations in the range of 0.007-0.063 mg/mL. More work is currently underway to further investigate the cytotoxicity of different RE-doped oxides at different conditions.
Nanocrystals and nanorods of various doped and codoped Y2O3:RE (RE ) Tb, Eu, Er, Yb) have been synthesized in the presence of long-chain alkylamine. It was found that the type of dopant has no effect on the morphology of the nanomaterials formed. Tb-, Eu-, and Er-doped and Er-Yb-codoped Y2O3 nanomaterials exhibit PL at room temperature. Under 980 nm excitation, green and red up-conversion emissions of Er-Ybcodoped Y2O3 have been observed. The dopant concentration plays an important role in the relative green and red upconversion intensities. In the first case, when the Yb concentration was increased 5-fold, the red emission enhancement was attributed to the enhanced population of the 4F9/2 level via ET1 (4F7/2 + 4I11/2 f 4F9/2 + 4F9/2) and ET2 (4I13/2 + 2F5/2 f 4F9/2 + 2F7/2), while green emission diminishment was attributed to ET3 (4F7/2 + 2F7/2 f 4I11/2 + 2F5/2), which depopulates the excited 4F7/2 level at higher Yb concentrations. In the second case, when the Er3+ concentration was 5 times higher than the Yb3+ concentration, the combined effect of ET4 (4I11/2 + 4I11/2 f 4F7/2 + 4I15/2) and ET5 (4I13/2 + 4I11/2 f 4F9/2 + 4I15/2) resulted in red emission enhancement while the green emission intensity remained relatively unchanged. Surface functionalization with amine groups via a silanization process renders the nanomaterials water-soluble and with the ability to afford further functionalization by other biomolecules. The Y2O3:Tb nanomaterials show no appreciable cytotoxicity in Hep-G2 cells at concentrations of 0.007-0.063 mg/mL. The RE-doped and codoped oxide synthesis method reported herein could be applied broadly to other oxide materials. This provides a generic synthesis method for tailoring functional properties in rare-earthdoped oxide nanomaterials for potential applications such as bioimaging. Up-conversion fluorophores are envisioned to have great potential in deep-tissue imaging. Acknowledgment. G.K.D. gratefully acknowledges a research scholarship from Nanyang Technological University. References and Notes (1) Burns, A.; Ow, H.; Weisner, U. Chem. Soc. ReV. 2006, 35, 1028. (2) Wang, F.; Tan, W. B.; Zhang, Y.; Fan, X.; Wang, M. Nanotechnology 2006, 17, R1. (3) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800.
Rare-Earth-Doped and Co-Doped Y2O3 Nanomaterials (4) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (5) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (6) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson, G. P.; Sundar, V. C.; Mikulec, F. V.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 12142. (7) Mamedova, N. N.; Kotov, N. A.; Rogach, A. L.; Studer, J. Nano Lett. 2001, 1, 281. (8) Selvan, S. T.; Tan, T. T.; Ying, J. Y. AdV. Mater. 2005, 17, 1620. (9) Tan, T. T.; Selvan, S. T.; Zhao, L.; Gao, S.; Ying, J. Y. Chem. Mater. 2007, 19, 3112. (10) Nichkova, M.; Dosev, D.; Gee, S. J.; Hammock, B. D.; Kennedy, I. M. Anal. Chem. 2005, 77, 6864. (11) Goldys, E. M.; Tomsia, K. D.; Jinjun, S.; Dosev, D.; Kennedy, I. M.; Yatsunenko, S.; Godlewski, M. J. Am. Chem. Soc. 2006, 128, 14498. (12) Gordon, W. O.; Carter, J. A.; Tissue, B. M. J. Lumin. 2004, 108, 339. (13) Yi, G.; Lu, H.; Zhao, S.; Ge, Y.; Yang, W.; Chen, D.; Guo, L.-H. Nano Lett. 2004, 4, 2191. (14) Vetrone, F.; Boyer, J. C.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. J. Phys. Chem. B 2003, 107, 1107. (15) Wang, H.; Uehara, M.; Nakamura, H.; Miyazaki, M.; Maeda, H. AdV. Mater. 2005, 17, 2506. (16) Camenzind, A.; Strobel, R.; Pratsinis, S. E. Chem. Phys. Lett. 2005, 415, 193. (17) Muenchausen, R. E.; Jacobsohn, L. G.; Bennett, B. L.; McKigney, E. A.; Smith, J. F.; Valdez, J. A.; Cooke, D. W. J. Lumin. 2007, 126, 838. (18) Dikovska, A. O. G.; Atanasov, P. A.; Jime´nez de Castro, M.; Perea, A.; Gonzalo, J.; Afonso, C. N.; García Lo´pez, J. Thin Solid Films 2006, 500, 336. (19) Si, R.; Zhang, Y.-W.; Zhou, H.-P.; Sun, L.-D.; Yan, C.-H. Chem. Mater. 2007, 19, 18. (20) Zeng, S.; Tang, K.; Li, T.; Liang, Z. J. Colloid Interface Sci. 2007, 316, 921. (21) Yin, S.; Shinozaki, M.; Sato, T. J. Lumin. 2007, 126, 427. (22) Zhang, Y.; Guo, J.; White, T.; Tan, T. T. Y.; Xu, R. J. Phys. Chem. C 2007, 111, 7893. (23) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2007, 46, 2448. (24) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751.
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11217 (25) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (26) Leonard, J. P.; Gunnlaugsson, T. J. Fluoresc. 2005, 15, 585. (27) Yi, G.; Sun, B.; Yang, F.; Chen, D.; Zhou, Y.; Cheng, J. Chem. Mater. 2002, 14, 2910. (28) Park, J.-H.; Back, N. G.; Kwak, M.-G.; Jun, B.-E.; Choi, B.-C.; Moon, B.-K.; Jeong, J.-H.; Yi, S.-S.; Kim, J.-B. Mater. Sci. Eng. C 2007, 27, 998. (29) Pang, Q.; Shi, J.; Liu, Y.; Xing, S.; Gong, M.; Xu, N. Mater. Sci. Eng. B 2003, 103, 57. (30) Pires, A. M.; Heer, S.; Gudel, H. U.; Serra, O. A. J. Fluoresc. 2006, 16, 461. (31) Sharma, P. K.; Jilavi, M. H.; Nass, R.; Schmidt, H. J. Lumin. 1999, 82, 187. (32) Liu, Z.; Sun, B.; Xu, S.; Lian, J.; Li, X.; Xiu, Z.; Li, Q.; Huo, D.; Li, J.-G. J. Phys. Chem. C 2008, 112, 2353. (33) Wu, C.; Wang, Y. Mater. Lett. 2007, 61, 2416. (34) Liu, X.; Wang, X. Opt. Mater. 2007, 30, 626. (35) Park, C.-S.; Kwak, M.-G.; Choi, S.-S.; Song, Y.-S.; Hong, S.-J.; Han, J.-I.; Lee, D. Y. J. Lumin. 2006, 118, 199. (36) Lei, H.; Yang, Q.; Ou, H.; Chen, B.; Yu, J.; Wang, Q.; Xie, D.; Wu, J.; Xu, D.; Xu, G. Proc. SPIE 1999, 3622, 74. (37) Hwang, B.-C.; Jiang, S.; Luo, T.; Watson, J.; Sorbello, G.; Peyghambarian, N. J. Opt. Soc. Am. B 2000, 17, 833. (38) Salley, G. M.; Valiente, R.; Guedel, H. U. J. Lumin. 2001, 94-95, 305. (39) Guo, H.; Dong, N.; Yin, M.; Zhang, W.; Lou, L.; Xia, S. J. Phys. Chem. B 2004, 108, 19205. (40) Vetrone, F.; Boyer, J.-C.; Capobianco, J. A. J. Appl. Phys. 2004, 96, 661. (41) Lin, H.; Meredith, G.; Jiang, S.; Peng, X.; Luo, T.; Peyghambarian, N.; Yue-Bun, P. E. J. Appl. Phys. 2003, 93, 186. (42) Stuart, B. H. Infrared Spectroscopy: Fundamentals and Applications; John Wiley & Sons, Ltd.: New York, 2004; pp 70-85. (43) Nayak, A.; Sahoo, R.; Debnatha, R. J. Mater. Res. 2007, 22, 35. (44) Chong, A. S. M.; Zhao, X. S. J. Phys. Chem. B 2003, 107, 12650. (45) White, L. D.; Tripp, C. P. J. Colloid Interface Sci. 2000, 232, 400. (46) Hirano, S.; Suzuki, K. T. EnViron. Health Perspect. 1996, 104, 85.
JP802076N
