Tunable Multicolor Upconversion Emissions and Paramagnetic

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Tunable Multicolor Upconversion Emissions and Paramagnetic Property of Monodispersed Bifunctional Lanthanide-Doped NaGdF4 Nanorods Guozhong Ren,†,‡ Songjun Zeng,*,†,§ and Jianhua Hao*,§ †

Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, 411105, People's Republic of China Science and Technology on Electro-Optical Information Security Control Laboratory, Sanhe, 065201, China § Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong ‡

ABSTRACT: In this paper, highly monodispsered ultrasmall hexagonal phase NaGdF4 nanorods were synthesized via a hydrothermal method using oleic acid as a stabilizing agent. The tunable multicolor upconversion (UC) emissions, including green, yellow, blue, and white emissions, can be readily achieved from lanthanide (Ln)-doped NaGdF4 nanorods under the excitation of a 980 nm diode laser. The calculated chromaticity coordinates (CIE-X = 0.346, CIE-Y = 0.357) are close to those of the standard white light (CIE-X = 0.33, CIE-Y = 0.33), and the white UC emissions can be tuned from blue-white to white by adjusting the doped contents of Ho3+ in the Yb3+/Tm3+/Ho3+ triply doped NaGdF4 nanorods. In addition, the ultrasmall NaGdF4 nanocrystals also exhibit paramagnetic properties at 293 K. The measured magnetizations of the NaGdF4:20%Yb3+/0.2% Er3+ and NaGdF4 nanocrystals were about 1.49 and 1.86 emu/g at 20 kOe, respectively, which were close to the reported values of other nanoparticles for bioseparation. Moreover, the NaGdF4 nanocrystals can be readily attracted by a small magnet, which shows it has potential application in cell isolating. It is expected that these multifunctional ultrasmall NaGdF4 nanorods including tunable UC colors and intrinsic paramagnetic properties may have potential applications in color displays, biolables, bioseparation, and magnetic resonance imaging.

1. INTRODUCTION In recent years, the study of the lanthanide (Ln)-doped UC nanomaterials in both fundamental and technological research has attracted considerable attention due to their wide and important applications in many fields, including color displays and solid-state lasers.1 4 More importantly, Ln-doped UC nanocrystals, which can convert a longer wavelength radiation (e.g., near-infrared light) to shorter wavelength fluorescence (e.g., visible light), are emerging as a new class of fluorescent nanoparticles for application in biological fluorescence imaging.5 7 Compared with the conventional bioimaging materials, such as semiconductor quantum dots8 11 and organic dyes,12,13 lanthanide-doped UC nanomaterials have several advantages, including weak autofluorescence,14 deep penetration of noninvasive NIR excitation, low radiation damage, and photobleaching.15,16 Monodispersible nanocrystals of lanthanide-doped rare-earth compounds, such as oxides, fluorides, and vanadates, have gained increased attention due to their unique luminescence properties.17 20 Among these UC nanomaterials, sodium rare earth (RE) fluoride was considered to be a promising and most efficient host lattice since it normally has lower phonon energy, which decreases the nonradiative relaxation probability and subsequently increases the luminescent quantum yields.21 In addition, the magnetic properties of these sodium RE fluoride nanoparticles is crucial to their uses in some biological applications, such as bioseparation r 2011 American Chemical Society

and magnetic resonance imaging (MRI).22 However, studies on the sodium RE fluoride nanoparticles mainly focus on crystal phase and shape control, luminescent properties, and biocompatibility.23 25 There is little study of the magnetic properties. Recently, Hao’s group reported the intrinsic magnetic and luminescent properties of multifunctional GdF3:Eu3+ nanoparticles.22 Several groups had successfully used the lanthanide-doped Gd2O3 host as bifunctional nanoprobes for optical and magnetic resonance imaging, which implied that the lanthanide-doped NaGdF4 host might also be used as a single-phase bifunctionality for biomedical imaging, such as optical and MRI.26 28 In addition, compared with the lanthanide-doped Gd2O3 host, the lanthanide-doped NaGdF4 host may have a higher UC luminescence efficiency due to its lower phonon energy.21 Very recently, Li’s group successfully used NaGdF4-based UC nanocrystals for imaging of small animals29 and Gd3+-doped UC nanocrystals for multimodality imaging combining UC luminescence imaging, MRI, and positron emission tomography (PET),30,31 which further revealed that the lanthanide-doped NaGdF4 and Gd3+doped nanomaterials had potential application in multimodal bioimaging. However, there are few reports on the monodispersed Received: July 8, 2011 Revised: September 5, 2011 Published: September 06, 2011 20141

dx.doi.org/10.1021/jp2064529 | J. Phys. Chem. C 2011, 115, 20141–20147

The Journal of Physical Chemistry C

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multifunctional NaGdF4 nanorods with finely tuned UC emissions and paramagnetic properties. In this paper, multifunctional ultrasmall hexagonal phase NaGdF4 nanorods with intrinsic UC luminescence including tunable multicolor emissions and paramagnetic properties have been reported and studied in detail.

2. EXPERIMENTAL SECTION 2.1. Synthesis of High-Quality Ln3+-Doped NaGdF4 Nanorods. Monodispersed NaGdF4 nanorods doped with different

Ln3+ ions were synthesized by a hydrothermal method using oleic acid as a stabilizing agent.32,33 In a typical synthesis, 1.2 g of NaOH was dissolved in 2 mL of deionized water and then 8 mL of ethanol and 20 mL of oleic acid were added under vigorously stirring to form a transparent homogeneous solution. After stirring for 20 min, 1 mmol (total amounts) of Ln(NO3)3 (Ln = Gd, Yb, Er, Tm, and Ho) with designed compositions was added to the solution under vigorous stirring. An 8 mL portion of NaF (1.0 M) aqueous solution was then added, and the resulting mixture was stirred for another 30 min. Finally, the resulting solution was transferred into a 50 mL stainless Teflon-lined autoclave, sealed, and kept at 190 °C for 24 h. After reaction, the system was cooled to room temperature naturally and the products were deposited at the bottom of the vessel. The asprepared samples were separated by centrifugation and washed with ethanol and deionized water several times to remove oleic acid and other remnants and then dried in air at 60 °C for 12 h. Undoped NaGdF4 nanocrystals were synthesized by using the same method at 190 °C for 24 h. 2.2. Characterization. The crystal phase structures and phase compositions of the as-prepared NaGdF4 doped with 20% Yb3+ and 0.2% Er3+ and undoped NaGdF4 nanocrystals were examined by powder X-ray diffraction (XRD) using a D/max-γA System X-ray diffractometer at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15418 nm). The scan was performed in the 2θ range from 10° to 65° with a scanning rate of 0.04°/s and a step size of 0.04°. The morphology and microstructure were characterized by transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) images using a JEOL-2100 TEM equipped with an Oxford Instrument energy-dispersive X-ray spectroscopy (EDS) system operating at 200 kV. The samples for TEM assays were prepared as follows: as-prepared nanocrystals were dispersed in nonpolar cyclohexane solvent to form a homogeneous colloidal suspension, and then one drop of the suspension was placed on the TEM copper grid covered with carbon film. The UC emission spectra were recorded by a spectrophotometer (R500) under the excitation of an unfocused 980 nm laser diode (LD). The photography images were taken from the 1 wt % of the as-prepared NaGdF4 nanorods doped with Yb3+/Er3+, Yb3+/Ho3+, Yb3+/Tm3+, and Yb3+/Tm3+/ Ho3+ dispersed in cyclohexane under the excitation of a 980 nm LD with an excitation power density of 5 W/cm2. The magnetization as a function of the applied magnetic field ranging from 20 to 20 kOe was measured using a Lakeshore 7410 vibrating sample magnetometer at room temperature.

3. RESULTS AND DISCUSSION 3.1. Phase, Morphology, and Microstructure Characterization. The phase composition of the as-prepared samples

were detected by the powder XRD. Figure 1 shows the typical XRD patterns of as-prepared NaGdF4 nanorods codoped with

Figure 1. Typical XRD patterns of the NaGdF4:20%Yb3+/0.2% Er3+ and undoped NaGdF4 nanocrystals.

20%Yb3+/0.2%Er3+ and undoped NaGdF4 nanocrystals. As demonstrated in Figure 1, all of the strong and sharp diffraction peaks could be readily assigned as hexagonal phase NaGdF4 based on the standard XRD pattern (JCPDS 27-0667). No other impurity phase is observed, which indicates that the pure hexagonal phase NaGdF4 with a high crystalline nature can be readily prepared using this general solution-based method. Notably, compared with the undoped NaGdF4, the diffraction peaks of NaGdF4 nanorods codoped with 20%Yb3+/0.2%Er3+ shifted toward higher diffraction angles, which indicated the decrease of the unit cell volume owing to the larger ionic radius of Gd3+ (Gd3+: r = 1.193 Å)34 substituted by the relatively smaller Yb3+ (Yb3+: r = 1.125 Å).34 In addition, compared with the 20%Yb3+/0.2%Er3+doped NaGdF4 nanorods, the diffraction peaks of undoped NaGdF4 nanocrystals broadened, indicating that the average crystalline size decreases, which was further verified by the following TEM characterization. The size and morphology of NaGdF4 nanorods doped with 20%Yb3+/0.2%Er3+ and undoped NaGdF4 nanocrystals were characterized by TEM. As shown in Figure 2a, the doped samples exhibit high monodispersity and a rodlike morphology with a diameter of about 8 nm and a length of about 80 nm. Figure 2b shows the typical TEM image of the undoped NaGdF4 nanocrystals. It can be seen that the undoped NaGdF4 nanocrystals present an elongated particle morphology with an average size of about 16 nm, which is consistent with the above XRD results. Figure 2c shows the typical HRTEM image of the individual NaGdF4 nanorod. As demonstrated in Figure 2c, the nanorod presents high structural uniformity. The inset of Figure 2c shows the corresponding fast Fourier transformation (FFT) image taken from the white rectangular area of the HRTEM image. The regular diffraction spots recorded along the [010] zone axis unambiguously demonstrate that the nanorod has a singlecrystalline nature, which can be readily indexed as the hexagonal phase NaGdF4. The interplanar distances of HRTEM are determined to be 5.2 and 3.56 Å, corresponding to the (100) and (001) lattice planes of the hexagonal phase NaGdF4, respectively, which is consistent with the XRD analysis. As also revealed by the HRTEM image, the preferred growth direction of hexagonal phase NaGdF4 nanorods is along the [001] direction. Figure 2d shows the typical HRTEM image of the individual undoped NaGdF4 nanoparticle with a d-spacing of 5.25 Å, corresponding 20142

dx.doi.org/10.1021/jp2064529 |J. Phys. Chem. C 2011, 115, 20141–20147


Figure 2. TEM and HRTEM images of the as-prepared NaGdF4 nanocrystals: (a) NaGdF4:20%Yb3+/0.2% Er3+ nanorods, (b) undoped NaGdF4 nanocrystals, (c) HRTEM image of the individual NaGdF4:20%Yb3+/0.2% Er3+ nanorods, and (d) HRTEM image of the individual undoped NaGdF4 nanocrystal. (e) EDS of the NaGdF4:20%Yb3+/0.2% Er3+ nanorods (mainly composed of Na, Gd, Yb, and F elements). (f) EDS of the undoped NaGdF4 nanocrystals (mainly composed of Na, Gd, and F elements). Insets of (c) and (d) show the corresponding FFT of HRTEM images.

to the (100) lattice planes of hexagonal phase NaGdF4. The inset of Figure 2d shows the corresponding FFT image, which presents a 6-fold symmetry and can be also indexed as the hexagonal phase NaGdF4, recorded along the [001] zone axis. During the TEM assays, the corresponding elemental components of the NaGdF4 nanorods doped with 20%Yb3+/0.2%Er3+ and undoped NaGdF4 nanocrystals were detected by the energydispersive X-ray spectroscopy (EDS) analysis. The EDS result of

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NaGdF4:20%Yb3+/0.2%Er3+ (Figure 2e) reveals that the nanorods are mainly composed of Na, Gd, Yb, and F. Figure 2f shows the EDS result of the undoped NaGdF4 nanocrystals, which also indicates that the main composition is Na, Gd, and F. Note that the strong signals for C and Cu come from the TEM copper grid and the covered carbon film. On the basis of the above XRD and TEM analysis, it is concluded that the shape and size of the NaGdF4 nanocrystals can be readily controlled by doping Yb3+, which is consistent with the previous reports for structural control of UC nanocrystals via a doping method.35,36 With increasing the Yb3+ contents, the NaGdF4 nanocrystals have a higher tendency to form one-dimensional nanostructures. 3.2. Tunable Multicolor Upconversion Emissions of the Ln3+-Doped NaGdF4 Nanorods. It is well known that sodium rare earth fluorides have been considered as ideal host materials for UC luminescence. Various UC emissions, including red, green, and blue luminescence, can be achieved by doping different Ln ions. Under a 980 nm LD excitation, the bright eyevisible green, yellow, blue, and white light emissions via the frequency UC process can be achieved in the Yb3+/Er3+-, Yb3+/ Ho3+-, Yb3+/Tm3+-, and Yb3+/Tm3+/Ho3+-doped hexagonal phase NaGdF4 nanorods, respectively. Figure 3 shows the typical UC luminescence spectra of NaGdF4 nanorods doped with different Ln ions, including Yb3+/Er3+, Yb3+/Ho3+, Yb3+/Tm3+, and Yb3+/Tm3+/Ho3+. As can be observed, the intense green and weak red emissions centered at 520, 545, and 664 nm were observed in NaGdF4:20%Yb3+/0.2%Er3+ (Figure 3a). According to the simplified energy level diagram shown in Figure 4, the green emission bands of the Er3+ ions centered at 520 and 545 nm were attributed to the electronic transition of 2H11/2/4S3/2 f 4I15/2 of the Er3+ ions. The 664 nm red emission was ascribed to the 4 F9/2 f 4I15/2 transition. Figure 3b shows the UC luminescence spectra from NaGdF4:20%Yb3+/0.2%Ho3+. As demonstrated, there were two well-known emission bands centered around at 538 and 644 nm, which were associated with the 5S2/5F4 f 5I8 (538 nm) and 5F5 f 5I8 (644 nm) of the Ho3+ ions, respectively. The intense blue (450 and 477 nm) and weak red emissions (649 nm) were observed in NaGdF4:20%Yb3+/0.2%Tm3+ (Figure 3c), corresponding to the 1D2 f 3F4, 1G4 f 3H6, and 1 G4 f 3F4 transitions of Tm3+, respectively. On the basis of energy-matching conditions, the possible UC mechanisms from the NaGdF4:20%Yb3+/0.2%Tm3+ nanorods are illustrated in the simplified energy level diagrams (Figure 4). Under the excitation of a 980 nm LD, the Yb3+ ions are excited from the 2F7/2 level to the 2F5/2 level, then transfer their energies to the nearby Tm3+ ions. The 3H5, 3F2, and 1G4 levels of Tm3+ can be populated by three successive energy transfers (ET) from Yb3+ to Tm3+.37 Consequently, the blue emission at 477 nm is generated by the 1 G4 f 3H6 transition of the Tm3+ ions, and the red emission at 649 nm is due to the 1G4 f 3F4 transitions of Tm3+ ions. Because of the large energy mismatch about 3500 cm 1, the 1D2 level of Tm3+ cannot be populated by the fourth photon from Yb3+ via ET to the 1G4 energy level. Therefore, the cross relaxation (CR) process of 3F2 + 3H4 f 3H6 + 1D2 between Tm3+ ions may alternatively play an important role in populating the 1D2 level, which may result in the blue emission centered at 450 nm.38 As shown in Figure 3d, the white light emissions, including blue (450 and 477 nm), green (538 nm), and red (644 nm), were achieved from the 20%Yb3+/0.2%Tm3+/0.2%Ho3+-doped NaGdF4 nanorods (Figure 3d). As compared with Figure 3b,c, it is easy to assign the origins of the observed emission bands. The blue emissions centered at 450 and 477 nm were generated from the 20143



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Figure 3. Upconversion luminescence spectra of the Ln-doped NaGdF4 nanorods: (a) NaGdF4:20%Yb3+/0.2% Er3+, (b) NaGdF4:20%Yb3+/0.2% Ho3+, (c) NaGdF4:20%Yb3+/0.2% Tm3+, and (d) NaGdF4:20%Yb3+/0.2% Ho3+/0.2% Tm3+. The photographs show the corresponding 1 wt % cyclohexane colloidal solutions of (e) NaGdF4:20%Yb3+/0.2% Er3+, (f) NaGdF4:20%Yb3+/0.2% Ho3+, (g) NaGdF4:20%Yb3+/0.2% Tm3+, and (h) NaGdF4:20% Yb3+/0.2% Ho3+/0.2% Tm3+. The samples were excited under a 980 nm LD with a power density of 5 W/cm2.

electronic transitions of 1D2 f 3F4, and 1G4 f 3H6 of Tm3+, respectively. The green UC emission region centered at 538 nm originated from the electronic transition of Ho3+ ions. It is noted that the weak red emission band at 649 nm in Figure 3c arising from the transition of 1G4 f 3F4 of Tm3+ ions is very close to the red emission band from Ho3+ ions (Figure 3b). It can be expected that the red emission centered at 644 nm in Figure 3d is ascribed to the electronic transition of Ho3+ ions due to the similar spectra shape between panels b and d in Figure 3. Figure 3e,f exhibits the photography images of the 1 wt % cyclohexane colloidal solutions of the as-prepared NaGdF4 nanorods doped with Yb3+/Er3+, Yb3+/Ho3+, Yb3+/Tm3+, and Yb3+/Tm3+/ Ho3+, respectively. From photographs, the bright eye-visible green, yellow, blue, and white light emissions can be observed under the excitation of a 980 nm LD with an excitation power density of 5 W/cm2. To further reveal the UC mechanism of

white light emission, the excitation power-dependent UC emissions of blue, green, and red were investigated. For nearly any UC mechanism, the output UC luminescent intensity (IUC) will be proportional to the infrared excitation (IIR) power39 IUC ∞IIR n where n is the absorbed photon numbers per visible photon emitted, and its value can be obtained from the slope of the fitted line of the plot of log IUC versus log IIRn. Figure 5 shows the double logarithmic plots of the emission intensity as a function of excitation power of the blue, green, and red emissions in the 20% Yb3+/0.2%Tm3+/0.2%Ho3+-doped NaGdF4 nanorods. As shown in Figure 5, the slopes of the linear fits of log(IUC) versus log(IIR) for the blue, green, and red emissions at 477, 538, and 644 nm in the 20%Yb3+/0.2%Tm3+/0.2%Ho3+-doped NaGdF4 sample are 20144



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Figure 4. Simplified energy level diagrams of Yb3+, Er3+, Ho3+, and Tm3+ and proposed mechanism of upconversion emissions.

Figure 5. Log log plots of the UC luminescence intensity versus excitation power in 20%Yb3+/0.2% Ho3+/0.2% Tm3+-doped NaGdF4 nanorods under the excitation of a 980 nm LD.

2.54, 1.77 and 1.72, respectively. The results indicate that the green and red UC emissions only need two photons, whereas the three-photon process is needed for the blue UC emission, which is consistent with the previous report in Yb3+/Ho3+/Tm3+ doped NaYF4 nanocrystals.40 To further tune the white light emission, the UC properties of a series of NaGdF4 nanorods doped with different molar ratios of 20%Yb3+/0.2%Tm3+/x% Ho3+ (x = 0.1, 0.2, 0.6, and 1.0) were studied, as shown in Figure 6a. As the content of Ho3+ increases, the intensity of the green and red emissions increase, which further reveals that the green and red emissions originated from the Ho3+. Figure 6b shows the CIE 1931 chromaticity diagram, and the calculated chromaticity coordinates. The calculated chromaticity coordinates of the 20%Yb3+/0.2%Tm3+/x%Ho3+ (x = 0.1, 0.2, 0.6, and 1.0) doped NaGdF4 nanorods are (0.176, 0.151), (0.281, 0.334), (0.346, 0.357), and (0.398, 0.460), respectively. The chromaticity coordinate of the 20%Yb3+/0.2%Tm3+/0.6%Ho3+-doped NaGdF4 nanorods falls in the white region of the CIE 1931 chromaticity diagram, which is very close to that of standard white light (x = 0.33, y = 0.33), suggesting the potential application as a white light source. More importantly, with

Figure 6. (a) UC luminescence spectra of the 20%Yb3+/0.2%Tm3+/x% Ho3+ (x = 0.1, 0.2, 0.6, and 1.0) doped NaGdF4 nanorods under the excitation of a 980 nm LD. (b) The CIE 1931 chromaticity diagram and calculated chromaticity coordinates of UC luminescence corresponding to (a).

increasing the Ho3+ contents, the UC luminescent colors can be tuned from blue to blue-white to white and finally to yellow, which shows the tunable white light (from blue-white to white) outputs can be achieved by simply adjusting the doped Ho3+ content. Hopefully, these highly monodispersed Ln-doped NaGdF4 ultrasmall nanorods with finely tuned UC emission colors (especially for tunable white emissions) may have potential applications in color displays, white light source, multiplexed labels, and biomedical fluorescence imaging. 3.3. Paramagnetic Properties of the Ln3+-Doped and Undoped NaGdF4 Nanorods. Apart from the UC multicolor emissions, the magnetic properties of the Ln3+-doped NaGdF4 and undoped NaGdF4 nanorods were also investigated. The room-temperature magnetization (M) of NaGdF4 doped with 20% Yb3+ and 0.2% Er3+ and undoped NaGdF4 nanorods as a function of applied field (H) ( 20 to 20 kOe) is shown in Figure 7. As demonstrated, the as-prepared NaGdF4:20%Yb3+/ 0.2% Er3+ and undoped NaGdF4 nanorods both exhibit paramagnetic properties. The paramagnetic properties of the Gd3+ ions in NaGdF4 nanocrystals come from seven unpaired inner 4f electrons, which are closely bound to the nucleus and effectively shielded by the outer closed-shell electrons 5s25p6 from the 20145



Figure 7. Magnetization as a function of applied field for the as-prepared NaGdF4:20%Yb3+/0.2% Er3+ and undoped NaGdF4 nanocrystals.

Figure 8. Photographs of 1 wt % cyclohexane colloidal solutions of the NaGdF4:20%Yb3+/0.2% Er3+ nanorods: (a) before magnetization (shows transparent colloidal solutions), (b) after magnetization by a magnet (the samples are aggregated when a magnet is present, marked by the red arrow), and (c, d) the corresponding UC luminescence of the samples before and after magnetization under the excitation of a 980 nm LD with a power density of 5 W/cm2, respectively.

crystal field.22 The magnetic moments associated with the Gd3+ ions are all localized and noninteracting, giving rise to paramagnetism.22 The magnetic mass susceptibilities of the as-prepared NaGdF4:20%Yb3+/0.2% Er3+ and undoped NaGdF4 nanocrystals are found to be 7.46 10 5 and 9.29 10 5 emu/gOe, respectively. The magnetization of the NaGdF4:20%Yb3+/0.2% Er3+ and undoped NaGdF4 nanorods is around 1.49 and 1.85 emu/g at 20 kOe, respectively, which is close to the value reported for nanoparticles used for common bioseparation.41 43 It is also noted that the magnetization and magnetic mass susceptibility of the NaGdF4 nanorods can be modified by doping Yb3+ ions.

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Figure 8 shows the photographs of 1 wt % cyclohexane colloidal solutions of NaGdF4:20%Yb3+/0.2% Er3+ nanorods. As shown in Figure 8a, the colloidal solution is transparent before magnetization. The samples are attracted to a one side of vial by a small magnet marked by a red arrow, as shown in Figure 8b. As a comparison, the corresponding optical photographs of UC emissions under the excitation of a 980 nm LD are presented in Figure 8c,d. As shown in Figure 8c, the green UC fluorescence of the sample without magnetization was observed from the center of the vial, which demonstrated that the nanocrystals were well dispersed to form a homogeneous colloidal solution. After magnetization, the UC fluorescence was observed from the corner of the vial marked by the green arrow shown in Figure 8d, which further revealed that the nanocrystals were aggregated together under the presence of a magnet. These results showed that the asprepared nanocrystals could be separated under the applied magnetic field. As it is well known that a synergistic combination of fluorescent probes and MRI contrast agents in a single material would help combine the advantages of each, while avoiding the disadvantages of the other. However, most of the multifunctional nanoparticles synthesized so far consist of MRI contrast agent (Fe3O4) and down-conversion phosphors, such as organic dyes and quantum dots, which are known to display some intrinsic limitations.44 Moreover, compared with the recently reported fluorescent-magnetic nanoprobes,45,46 this may provide a simple strategy for simultaneous cell imaging, cell isolating, and intracellular spatial control by only using a single-phase material (NaGdF4:Ln3+).

4. CONCLUSIONS Pure hexagonal phase NaGdF4 nanorods with high monodispersity were synthesized by a rational hydrothermal method using oleic acid as a stabilizing agent. The XRD and TEM analyses revealed that the shape and size of NaGdF4 nanocrystals can be readily tuned by doping Yb3+, which requires further extensive research and will be reported elsewhere. Under the excitation of the 980 nm LD, the bright eye-visible green, yellow, blue, and white light emissions in the hexagonal phase NaGdF4 host via the frequency UC process can be readily modified by doping the Yb3+/Er3+, Yb3+/Ho3+, Yb3+/Tm3+, and Yb3+/ Tm3+/Ho3+, respectively. The chromaticity coordinate of the 20%Yb3+/0.2%Tm3+/0.6%Ho3+-doped NaGdF4 nanorods is (x = 0.346, y = 0.357), which is very close to that of standard white light (x = 0.33, y = 0.33), suggesting the potential application as a white light source. More importantly, the UC luminescent colors can be tuned by simply adjusting the doped Ho3+ content from blue to white and finally to yellow in Yb3+/Tm3+/Ho3+ triply doped NaGdF4 nanorods. Besides the tunable UC emission, the NaGdF4 nanorods also exhibit paramagnetic properties at room temperature. More interestingly, the magnetization and magnetic mass susceptibility can also be readily modified by doping Ln ions, which promise further fundamental research. It is expected that these monodispersed multifunctional ultrafine NaGdF4 nanorods including tunable UC emission colors and paramagnetic properties may have potential applications in color displays, biolables, bioseparation, MRI, and intracellular spatial control. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.Z.). E-mail: apjhhao@polyu. edu.hk (J.H.). 20146



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Tunable Multicolor Upconversion Emissions and Paramagnetic

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