Article pubs.acs.org/JPCC
Up-Conversion Luminescence of NaYF4:Yb3+/Er3+ Nanoparticles Embedded into PVP Nanotubes with Controllable Diameters Peng Zou, Xia Hong,* Yadan Ding, Zhenyi Zhang, Xueying Chu, Talgar Shaymurat, Changlu Shao, and Yichun Liu* Centre for Advanced Optoelectronic Functional Materials Research, Key Laboratory for UV Light-Emitting Materials and Technology of the Ministry of Education, Northeast Normal University, Changchun 130024, People’s Republic of China ABSTRACT: NaYF4:Yb3+/Er3+/polyvinylpyrrolidone (PVP) upconversion (UC) luminescent nanotubes were synthesized by the electrospinning technique. The diameters of the nanotubes were controllable by changing the electrospinning voltages. The structure and the optical properties were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) spectra, and fluorescence microscope. The NaYF4:Yb3+/Er3+/PVP nanocomposites simultaneously possessed UC luminescent property, active surfaces and tubular structure. PVP molecules were found to have an important influence on the UC luminescence of NaYF4:Yb3+/Er3+/PVP nanotubes. Compared with those in pure NaYF4:Yb3+/Er3+ nanoparticles, the relative intensities of violet, blue, and green to red emissions were increased in NaYF4:Yb3+/Er3+/PVP nanotubes. It is owing to the existence of PVP molecules, which might produce the passivation on NaYF4:Yb3+/Er3+ nanoparticles and reduce the nonradiative relaxation of Er3+ ion. The luminescent nanotubes with desirable UC properties are expected to be applied in biomedicine.
1. INTRODUCTION Luminescent nanotubes, as one of the most interesting nanomaterials, have been widely applied in the areas of electronic and optical devices, sensing, catalysis, and biomedicine in particular.1−7 Compared with conventional fluorescent labels, the UC nanomaterials offer low autofluorescence background, sharp emission bandwidths, superior photostability, large penetration depth, high temporal resolution, and minimum photodamage to living organisms.8−10 They have become promising alternatives to organic fluorophores and quantum dots. Various host materials such as fluorides, oxides, chlorides, bromides, oxysulfides, and oxyhalides were selected to synthesize the lanthanide-doped nanomaterials with high UC efficiency and controllable emission profile.11 Among them, NaYF4 with low phonon energy has been demonstrated to be the most efficient host material.12,13 Further improvement of the luminescence is expected through surface passivation of the UC nanomaterials to minimize the surface quenching effects induced by solvents or surface defects. Chun-Hua Yan et al. reported that the UC luminescence and the intensity ratio of green to red emission (fg/r) for NaYF4:Yb3+/Er3+ nanoparticles were enhanced after coating with an undoped NaYF4 shell.14 Amorphous SiO2 shell was also used to passivate the surfaces of NaYF4:Yb3+/Er3+ nanoparticles and increase their UC efficiency.15 Polyvinylpyrrolidone (PVP) is a biocompatible polymer with good hydrotropy and strong ability of complexation.16−19 Our previous works proved that PVP molecules could passivate © 2012 American Chemical Society
the surface defects of ZnO nanomaterials effectively and enhance their luminescent emission.20−22 If lanthanide ions doped NaYF4 nanoparticles were modified with PVP to form UC luminescent nanotubes, ameliorative UC luminescent property and active surfaces for further biological functionalization can be imparted. The tubular structure can also be beneficial for biomedical applications. The hollow structure could encapsulate drugs and provide a prolonged supply for drug storage and slow release. These PVP modified UC luminescent nanotubes would open the door to a broader variety of applications in bioimaging, drug delivery, biodetection, and disease therapy. Herein, NaYF4:Yb3+/Er3+/PVP UC luminescent nanotubes were synthesized by the electrospinning technique. The UC luminescence property of the composite nanotubes was compared with that of the pure NaYF4:Yb3+/Er3+ nanoparticles. The influence of PVP molecules on the UC luminescence of the nanotubes was analyzed from the aspect of energy transfer.
2. EXPERIMENTAL SECTION 2.1. Materials. Synthesis was carried out using standard oxygen-free procedures and commercially available reagents. NaCl (99%), YCl3 (99.99%), YbCl3 (99.99%), ErCl3 (99.99%), ethylene glycol (EG, 99%), branched polyethylenimine (PEI, Received: November 29, 2011 Revised: February 8, 2012 Published: February 9, 2012 5787
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25 kDa), polyvinylpyrrolidone (PVP) powder (Mn ≈ 900 000), and tetraethyl orthosilicate (TEOS, 98%) were used as starting materials without further purification. 2.2. Synthesis of NaYF4:Yb3+/Er3+ Nanoparticles. NaYF4:18% Yb3+/2% Er3+ were synthesized by the hydrothermal method. Hydrophilicity polyethyleneimine (PEI) was used as the surfactant to control the growth of the crystals. In a typical procedure, 1.2 mmol of NaCl, 0.48 mmol of YCl3, 0.108 mmol of YbCl3, and 0.012 mmol of ErCl3 were mixed in a 9 mL EG transparent solvent to form a transparent solution. At the same time, 0.006 mmol PEI and 3.0 mmol of NH4F were added to a 6 mL EG solvent to form the other transparent solution. Afterward, the resulting mixtures were mixed together and agitated for 10 min, transferred into a 30 mL of Teflon-lined autoclave, and kept at 200 °C for 2 h. The product was collected by centrifugation, washed with ethanol several times, and finally redispersed in ethanol. 2.3. NaYF4:Yb3+/Er3+/PVP Nanotubes. The preparation of NaYF4:Yb3+/Er3+/PVP nanotubes were performed with electrospinning by three steps. First, 0.8 g of PVP powder was dissolved in 10 mL (6 mg/mL) of NaYF4:Yb3+/Er3+ nanoparticle−ethanol solution. Then, 0.8 mL of tetraethyl orthosilicate TEOS was slowly dropped into NaYF4:Yb3+/ Er3+/PVP ethanol solution to obtain the precursor. Afterward, the precursor was transferred into a plastic syringe for electrospinning. The positive voltages of 7 kV, 10 kV, and 13 kV were applied across a fixed collection distance of 16 cm between the tip of the needle and the grounded electrode. 2.4. Characterization. Transmission electron microscopy (TEM) ZEOL JEM-2100 (acceleration voltage of 200 kV) and scanning electron microscopy (SEM) FEI Quanta 250 were used to characterize the morphologies of the nanomaterials. Xray diffraction (XRD) measurements were carried out using a D/max 2500 XRD spectrometer (Rigaku) with Cu Kα line of 0.1541 nm (50 kV, 300 mA). Fourier transform infrared (FTIR) spectra were obtained on a Magna 560 FT-IR spectrometer with a resolution of 1 cm−1. The emission spectra were recorded at room temperature using a LS-55 PerkinElmer fluorescence spectrometer equipped with a 980 nm laser diode (YM0506PHB2, HaiTe). The luminescence images of nanotubes were taken by a fluorescence microscope (BX51, Olympus).
Figure 1. (a) TEM image of NaYF4:Yb3+/Er3+ nanoparticales. (b) HRTEM image of NaYF4:Yb3+/Er3+ nanoparticales. (c) SEM image of NaYF4:Yb3+/Er3+/PVP nanotubes. (d) Cross-section SEM image of NaYF4:Yb3+/Er3+/PVP nanotubes. (e−g) TEM images of NaYF4:Yb3+/Er3+/PVP nanotubes obtained under electrospinning voltages at 7 kV, 10 kV, and 13 kV, respectively.
the diameter of the nanotubes (Figure 1e−g). The average outer diameters are decreased from 771 ± 84 nm, 572 ± 75 nm, to 423 ± 79 nm when the electrospinning voltages are 7 kV, 10 kV, and 13 kV, respectively. The main reason is that the increase of the voltages induces the increase of electrostatic stress on the jets. The higher electrostatic stress favors the formation of thinner fibers.26,27 The crystal structure and the composition analysis of the nanotubes were confirmed by XRD data (Figure 2). From the patterns of NaYF4:Yb3+/Er3+ nanoparticles, all of the peaks can be well-indexed in accord with cubic NaYF4 crystals. No other peak belonging to any impurities or other precursor compounds exists, suggesting good crystallinity of the product. For NaYF4:Yb3+/Er3+/PVP nanotubes, the peaks at 28.2°, 32.7°, 46.9°, and 55.7° from NaYF4:Yb3+/Er3+ nanoparticles can also be observed, which coincide with the (111), (200), (220), and (311) planes, respectively. FT-IR spectra further demonstrated the chemical components of NaYF4:Yb3+/Er3+/PVP nanotubes (Figure 3). For pure PVP nanofibers, the FT-IR spectrum presents the O−H group (3427 cm−1), methylene asymmetric and symmetric C−H stretching (2850 cm−1 and 2960 cm−1), CO stretching vibration (1660 cm−1), CH2 bending vibration (1445 cm−1), and C−N stretching vibration band (1280 cm−1). For NaYF4:Yb3+/Er3+/PVP nanotubes, besides the characteristic vibration bands of pure PVP, a new peak at 1092 cm−1 is observed, which assigns to the symmetric stretching of Si−O− Si group. It shows that, although TEOS evaporated rapidly, part of TEOS was residual. In addition, the vibration bands of
3. RESULTS AND DISCUSSION Electrospinning is a simple, convenient, and versatile technique for preparing 1D organic−inorganic nanocomposites and has been applied in many fields, such as membrane technology, tissue engineering, optical sensors, biosensors, superhydrophobic surfaces, and drug delivery.23−25 In this work, the electrospinning technique was chosen to prepare NaYF4:Yb3+/Er3+/PVP nanotubes through solvent evaporation induced phase separation. The morphologies of NaYF4:Yb3+/Er3+ nanoparticles and NaYF4:Yb3+/Er3+/PVP nanotubes were shown in Figure 1. NaYF4:Yb3+/Er3+ nanoparticles, as shown in Figure 1a, are spherical in shape with an average diameter of 34 ± 5 nm. Figure 1b shows the high resolution TEM (HRTEM) image of NaYF4:Yb3+/Er3+ nanoparticles. Lattice fringe with interplanar spacing of 0.31 nm corresponds to the (111) plane of cubic NaYF4 (JCPDS file number 77-2042). The general overview and the cross-section SEM images of NaYF4:Yb3+/Er3+/PVP nanotubes show that they are oriented randomly (Figure 1c,d). It was found that the electrospinning voltage greatly influenced 5788
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Figure 2. XRD patterns for (a) NaYF4:Yb3+/Er3+ nanoparticales and (b) NaYF4:Yb3+/Er3+/ PVP nanotubes.
Figure 4. (a) Normalized UC emission spectra of NaYF4:Yb3+/Er3+ nanoparticles (dash) and NaYF4:Yb3+/Er3+/PVP nanotubes (solid). (b) Optical microscopy photograph and (c) corresponding UC luminescence photograph of NaYF4:Yb3+/Er3+/PVP nanotubes under 980 nm excitation.
may be adsorbed on their surface without the protection of PVP molecules. These bonds can bridge the energy gap between 4I11/2 and 4I13/2 through one or several photons and make for red emissions.31 Moreover, an amount of nonradiative centers exist on the surface of nanoparticles. The adjacent doped ions could transfer the energy to nonradiative centers and enhance the nonradiation relaxation. If PVP molecules were modified on the surface of NaYF4:Yb3+/Er3+ nanoparticles, the passivation of PVP could effectively eliminate the surface trap-states and suppress the energy quenching in energy transfer processes. The nonradiative pathways and the red emissions of the nanoparticles could be reduced.32,33 In brief, the existence of PVP molecules could provide surface passivation on NaYF4:Yb3+/Er3+ nanoparticles and make the relative intensities of green, blue, and violet to red emissions increase. The luminescence images of NaYF4:Yb3+/Er3+/PVP nanotubes were taken by a fluorescence microscope (Figure 4b,c). Under 980 nm excitation, the green light of NaYF4:Yb3+/ Er3+/PVP nanotubes was observed. It indicates the formation of the NaYF4:Yb3+/Er3+/PVP nanocomposite structure once again and provides the possibility of using the UC nanotubes for bioimaging. To better understand the UC mechanism in NaYF4:Yb3+/ 3+ Er nanoparticles and NaYF4:Yb3+/Er3+/PVP nanotubes, the number of photons (n) involved in the UC process was investigated. The intensities of the UC emissions were recorded as a function of the 980 nm excitation intensity density in log− log plots (Figure 5). In NaYF4:Yb3+/Er3+ nanoparticles, the n values for red, green, and blue emissions are 1.24, 1.83, and 2.03, respectively (Figure 5a). It means that red and green emissions resemble a two-photon process, and blue emissions are a three-photon process. In NaYF4:Yb3+/Er3+/PVP nano-
Figure 3. (a) FT-IR spectra of (1) PVP fibers and (2) NaYF4:Yb3+/ Er3+/PVP nanotubes. (b) Enlarged view of selected area in panel a.
2804−3017 cm−1 and 1380−1493 cm−1 containing C−H, C− N, and amide stretching vibrations have a slight variation compared with pure PVP, which may result from the presence of NaYF4:Yb3+/Er3+ nanoparticles. The absorption peak of the CO group in pure PVP nanofibers located at 1660 cm−1 shifts to 1669 cm−1 in the composite nanotubes, as shown in Figure 3b. It indicates that the lanthanide ions of NaYF4:Yb3+/ Er3+ nanoparticles might interact with the oxygen atoms of the carbonyl group in PVP composite nanotubes. The similar results have been obtained in other inorganic nanoparticles/ PVP nanocomposites.20,28−30 The lanthanide ions were proved to accept a pair of electrons from carbonyl oxygen. The luminescent properties of NaYF4:Yb3+/Er3+ nanoparticles and NaYF4:Yb3+/Er3+/PVP nanotubes were studied. Figure 4a shows the normalized UC emission spectra of these nanomaterials. Under 980 nm near-infrared (NIR) laser excitation, both of the samples exhibit violet, blue, green, and red emissions. The emission peaks at 381 nm, 409 nm, 520 nm, 541 nm, and 653 nm can be assigned to 4G11/2, 2H9/2, 2H11/2, 4 S3/2, and 4F9/2 to 4I15/2 transitions of Er3+, respectively. Compared with those in NaYF4:Yb3+/Er3+ nanoparticles, the relative intensities of green, blue, and violet to red emissions are enhanced in NaYF4:Yb3+/Er3+/PVP nanotubes, which may arise from the existence of PVP molecules. It is known that the surface property of UC nanomaterials could influence the intensity ratios of emissions, e.g., fg/r. For NaYF4:Yb3+/Er3+ nanoparticles, some groups of high phonon energy (e.g., O−H) 5789
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Figure 6. UC mechanisms of NaYF4:Yb3+/Er3+ nanoparticles and NaYF4:Yb3+/Er3+/PVP nanotubes under 980 nm excitation at room temperature. The dashed-dotted, dashed, dotted, and full arrows represent photon excitation, energy transfer, multiphonon relaxation, and emission processes, respectively.
influenced by the environment and the surface-trap states, the nonradiative relaxations of 2H11/2/4S3/2 to 4F9/2 and 4I11/2 to 4 I13/2 are increased. They can quench the green emissions and populate the red emitting level. Besides, the nonradiative decay from 4I11/2 to 4I13/2 can also increase. The 4F9/2 level is populated via energy transfer from the excited Yb3+ ions, leading to more red emission by the two-photon process and a decrease in fg/r. For NaYF4:Yb3+/Er3+/PVP nanotubes, the nonradiative relaxation of 4I11/2 to 4I13/2 can be reduced through the passivation of PVP on NaYF4:Yb3+/Er3+ nanoparticles. The 4 F7/2 level of Er3+ ion is populated from the 4I11/2 level and produces the green and red UC emissions. However, part of the populated 4F9/2 level may be excited to the 4G11/2 level via the phonon-assisted energy transfer, which causes violet and blue emissions by the two-photon process.31
Figure 5. Excitation power density dependence of blue, green, and red UC emissions of (a) NaYF4:Yb3+/Er3+ nanoparticles and (b) NaYF4:Yb3+/Er3+/PVP nanotubes as a function of pump power at 980 nm.
tubes, the n values for red, green, and blue emissions are 1.28, 1.45, and 1.77, respectively (Figure 5b). All of the emissions are the two-photon process. Compared with those in NaYF4:Yb3+/ Er3+ nanoparticles, the n values for green and blue emissions decrease, and for red emissions, n increases in UC nanotubes. The changes of n values indicate that the UC mechanism in NaYF4:Yb3+/Er3+/PVP nanotubes is different from that of NaYF4:Yb3+/Er3+ nanoparticles. The UC procedures of NaYF4:Yb3+/Er3+ nanoparticles and NaYF4:Yb3+/Er3+/PVP nanotubes are schematically illustrated in Figure 6. For NaYF4:Yb3+/Er3+ nanoparticles, the UC process consists of excited state absorption and the energy transfer UC. Under 980 nm excitation, Yb3+ ion in ground state 2 F7/2 absorbs a photon and transits to excited state 2F5/2. Er3+ ion is then excited from ground state 4I15/2 to excited state 4I11/2 via energy transfer from the neighboring Yb3+ ion. The second 980 nm photon, or energy transfer from Yb3+ ion, can populate the 4F7/2 level of Er3+ ion. Er3+ ion can relax nonradiatively to the 2H11/2 and 4S3/2 levels and further relax to 4F9/2 level, which leads to green and red emissions by the two-photon process. Furthermore, some of 4F7/2 is transferred into 4G11/2 followed by nonradiative relaxations to 2H9/2 level, leading to blue emissions from 2H9/2 to 4I15/2 and violet emissions from 4G11/2 to 4 I15/2 by the three-photon process. Since the UC luminescence of NaYF4:Yb3+/Er3+ nanoparticles may be
4. CONCLUSIONS The NaYF4:Yb3+/Er3+/PVP nanotubes with varied diameters were synthesized successfully via electrospinning. Compared with the UC luminescence properties of pure NaYF4:Yb3+/Er3+ nanoparticles, the relative intensities of green, blue, and violet to red emissions in NaYF4:Yb3+/Er3+/PVP nanotubes were increased. It is owing to the existence of PVP molecules, which might produce the passivation on NaYF4:Yb3+/Er3+ nanoparticles and reduce the nonradiative relaxation of Er3+ ion. The luminescent nanotubes with desirable UC properties are expected to have potential applications in biomedicine, such as bioimaging, biodetection, drug delivery, disease diagnostics, and therapy.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (X.H.);
[email protected] (Y.L.). Notes
The authors declare no competing financial interest. 5790
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ACKNOWLEDGMENTS
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REFERENCES
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This work is supported by the National Natural Science Foundation of China (Grant Nos. 51072031 and 50725205), Scientific and Technological Developing Scheme of Jilin Province (Grant No. 201101011), the Fundamental Research Funds for the Central Universities (No. 10JCXK002), and the Funds for Young and Middle-Aged Leading Talents and Innovation Teams in Science and Technology (Grant No. 20121802).
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