Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Influence of Silica Surface Coating on Operated Photodynamic Therapy Property of Yb3+-Tm3+: Ga(III)-Doped ZnO Upconversion Nanoparticles Yuemei Li,† Rui Wang,*,† Yanling Xu,† Wei Zheng,† and Yongmei Li*,‡ †
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001,China Key Laboratory of Hormones and Development (Ministry of Health), Tianjin Key Laboratory of Metabolic Diseases, Tianjin Metabolic Diseases Hospital & Tianjin Institute of Endocrinology, Tianjin Medical University, 300070 Tianjin, China
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ABSTRACT: Photodynamic therapy (PDT) is a noninvasive therapeutic technique. Upconversion nanoparticles (UCNPs) hold promise for photodynamic therapy (PDT). UCNPs with antistokes emission can improve the tissue penetration depth of PDT. However, the low upconversion efficiency poses a strong limit on further development of PDT. The core/shell structure Yb/Tm/GZO@SiO2 UCNPs are designed, which can form multistep cascade energy transfer from sillica shell to core Ga-doped Yb/Tm/ZnO. Compared with Yb/Tm/ZnO upconversion (UC) semiconductor nanoparticles (SNCs), the multistep cascade energy transfer process provides about seven times enhanced UCL emission. For the UCL-optimized core/ shell upconversion SNCs of Yb7/Tm0.5/G3ZO@mSiO2, the Yb3+ energy transfer efficiency is determined to be as high as ∼81% under 980 nm laser excitation. In addition, the Yb7/Tm0.5/G3ZO@mSiO2 core/shell UCNPs have an excellent PDT treatment for Hela cells under 980 nm laser excitation.
1. INTRODUCTION Lanthanide-doped upconversion nanoparticles (UCNPs) have been a focus in biological applications due to their unique upconversion luminescence properties, such as low cytotoxicity, long fluorescence lifetime, and high chemical stability.1−4 The photodynamic therapy (PDT) is of great significance in cancer therapy. The PDT technique is the use of photosensitizer to produce cytotoxic reactive oxygen, and the singlet oxygen (1O2) damages cancer cells. Compared with traditional therapy methods, the PDT technique has high therapeutic efficiency and low side influences. Upconversion nanoparticles (UCNPs) have the ability of converting longer-wavelength near-infrared (NIR) light to shorter wavelength visible luminescence. Therefore, UCNPs can be excited by NIR radiations, and NIR light is the biological window (700−1000 nm).5−9 UCNPs improve the penetration depth of PDT treatment. However, the low upconversion luminescence (UCL) efficiency is the main reason why it cannot be used widely in PDT theatment practical applications.10 A system with high efficiency UCL requires that real energy levels of rare earth ions are embedded in an appropriate host lattice.11−13 Fortunately, the great improvement of upconversion (UC) semiconductor nanoparticles (SNCs) provides a significant way to overcome this problem. Compared with insulators, SNCs have large exciton Bohr radius and outstanding quantum confinement effect.12,14 Upconversion SNCs can be tailored though bandgap engineering and size control, which improve © XXXX American Chemical Society
the energy transfer efficiency from the excited host to rare earth ions.15−17 Zinc oxide has been identified as a promising host candidate for upconversion emission due to its a widedirect-gap and long-term stability.18−21 In this work, we successfully synthesized Yb/Tm/ZnO upconversion SNCs via a hydrothermal method.13 In order to improve the UC efficiency, Ga3+ ions were effectively incorpotated into the lattice of ZnO SNCs. Ga3+-doped with charge compensation reduced the discrepancy of ionic radius and charge between Yb/Tm ions and the ZnO host cations.13 Doped Ga3+ made the charge density of O2− increase and improved the energy transfer efficiency via oxygen bridges.12,14 Meanwhile, silica shell coating not only improved the biocompatibility but also ehanced UCL efficiency.22,23 The silica shell coating increased the numbers of luminescence centers in SNCs. The activated luminescence center ions on the surfaces of SNCs contribute to the additional UC luminescence. Furthermore, Ga3+ ions and lanthanide ions have certain biological toxicity. SiO2 shell coating reduced the toxicity of Yb/Tm/GZO UCNPs. We measured the Hela cell viability with Yb/Tm/GZO@SiO2 under 980 nm laser; the core/shell Yb/Tm/GZO@SiO2 had an excellent photodynamic therapy for Hela cells. Received: April 27, 2018
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DOI: 10.1021/acs.inorgchem.8b01169 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. (a) Schematic illustration of the Yb/Tm/GZO@SiO2 synthesis. (b) SEM image of Yb/Tm/GZO and (c) TEM image of Yb/Tm/GZO; inset is HTEM of Yb/Tm/GZO core samples. (d) SEM image of Yb/Tm/GZO@SiO2 and (e) TEM image of Yb/Tm/GZO@SiO2; inset is HTEM of Yb/Tm/GZO@SiO2 core/shell nanopartilces. (f) EDS of Yb/Tm/GZO@SiO2 core/shell nanopartilces and (g) EDS map of Yb/Tm/ GZO@SiO2 core/shell nanoparticles. B
DOI: 10.1021/acs.inorgchem.8b01169 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. (a) Upconversion emission spectra of Yb/Tm/GZO and Yb/Tm/GZO@SiO2 nanoparticles under 980 nm laser excitation. (b) Relative UC emission intensities of the Yb/Tm/ZnO, Yb/Tm/GZO, and Yb/Tm/GZO@SiO2 nanoparticles with 980 nm laser, respectively. (c) Luminescence decays of Tm3+ ions at 475 nm for the Yb/Tm/GZO and core/shell structured Yb/Tm/GZO@SiO2. (d) Luminescence decays of Yb3+ ions at 980 nm for Yb/Tm/ZnO, Yb/Tm/GZO nanoparticles, and (e) Yb/Tm/GZO@SiO2 core/shell nanoparticles.
utilized GZO:Yb3+ 7 mol %/Tm3+ 0.5 mol % as a core with high efficient energy transfer between Yb3+ and Tm3+ ions (Figure S2 and S3). Scanning electron microscopy (SEM) and TEM images of the core upconversion SNCs showed that the morphology of core particles was overlapping nanorod and hexagonal phase structure (Figure 1a−c). High-resolution TEM (HTEM) results as shown in Figure 1c demonstrate the lattice spacing of the core UCNPs is 0.52 nm, corresponding to (002) of ZnO with X-ray diffraction (XRD) patterns (Figure S4). To obtain high efficiency upconversion, the SiO2 shell was coated on the surface of the Yb/Tm/GZO UCNPs by the Stöber method. Figure 1d and Figure 1e show sizes of core/ shell UCNPs. The diameter of core UCNPs (Yb7/Tm0.5/ G3ZO) was about 208 nm, and the thickness of the shell structure (SiO2) was about 25 nm. As shown in Figure 1f−g, the EDS map results of Yb/Tm/GZO@SiO2 present Yb, Tm, Si, Zn, Ga, and O elements, which further proved the obtained core/shell structure of UCNPs. To examine the effect of the core/shell structure on the upconversion luminescence (UCL), upconversion emission properties of two structures, Yb/Tm/GZO and Yb/Tm/ GZO@SiO2, were compared. Under 980 nm excitation, the upconversion (UC) spectra of the Yb/Tm/GZO and Yb/Tm/
2. RESULTS AND DISCUSSION 2.1. Design of Efficient Oxide Core/Shell Upconversion Semiconductor Nanocrystals. The wet chemical approach method is one of the most effective ways to synthesize the upconversion semiconductor nanocrystals (SNCs). We incorporated Yb3+ and Tm3+ ions into the Ga3+-doped ZnO (GZO) SNCs through a hydrothermal method. The reaction process is expressed in eqs 1−4). Zn 2 + + H 2O → Zn(OH)24 −
(1)
Ga 3 + + Zn(OH)24 − → Zn1 − xGax(OH)−4
(2)
Zn1 − xGax(OH)−4 + Ln 3 +(Tm 3 +/Yb3 +) → Zn1 − xGax(OH)4 −Yb/Tm
Zn1 − xGax(OH)4 −Yb/Tm → Yb/Tm/GZO
(3) (4)
For this process, the first step is hydrolysis, with the production of Zn1−xGax(OH)−. During the hydrolysis process of the Zn1−xGax(OH)4−, Yb3+ and Tm3+ ions can be readily doped into the GZO host ionic ions via oxygen bridges, which successfully obtained Yb/Tm/GZO upconversion SNCs. We C
DOI: 10.1021/acs.inorgchem.8b01169 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. SEM images, energy migration cartons, and energy transfer mechanisms of (a) the Yb/Tm/ZnO, (b) Yb/Tm/GZO, and (c) Yb/Tm/ GZO@SiO2 nanoparticles. (d) Energy transfer mechanism in nanoparticles with Ga3+-doped and SiO2 shell.
upconversion SNCs, the decay lifetime performed positive correlation with the UC energy transfer efficiency, which can be quantified by eq 5.
GZO@SiO2 nanocrystals performed two UC emission peaks at around 475 nm, 652 nm, and 800 nm, consistent with the 1G4 → 3H6, 1G4 → 3F4, and 3F2,3 → 3H6 transitions of Tm3+, respectively (Figure 2a). Obviously, as illustrated in Figure 2a and b, the Yb/Tm/GZO showed significant luminescence enhancement compared with the Yb/Tm/ZnO upconversion SNCs. The Yb/Tm/GZO@SiO2 enhanced UCL intensity by seven times compared with the Yb/Tm/GZO core nanoparticles. According to the relative UC luminescence decay lifetime curve, by comparison of the obtained Yb/Tm/ZnO, Yb/Tm/GZO, and Yb/Tm/GZO@SiO2 upconversion SNCs (Figure 2c), we can conclude that the luminescence decays of Tm3+ ions were at 475 nm for the core/shell structured Yb/ Tm/GZO@SiO2 (415 us), which were significantly prolonged compared with Yb/Tm/ZnO (96 us) and Yb/Tm/GZO (286 us) upconversion SNCs.13 It indicated that the nonradiative relaxations were obviously suppressed in Yb/Tm/GZO@SiO2 core/shell structures. In addition, in this system of
ET = 1 −
τDA τD
(5)
where ET is the efficiency of energy transfer and the τDA and τD stand for the effective lifetimes of the energy acceptor and energy donor, respectively. It can be seen that efficient energy transfer was produced from Yb3+ ions (donor) to Tm3+ ions (acceptor). Following eq 5 and measuring the decay lifetimes of samples (Figure 2d), the energy transfer efficiency of the particles gradually increases from ∼16% to 54% with Ga3+doped. Particularly, the energy transfer efficiency of Yb3+ ions in Yb/Tm/GZO@SiO2 UCNPs was determined to be as high as ∼81% (Figure 2e). 2.2. Energy-Cascaded of Upconversion SNCs. To realize high efficiency energy transfer upconversion in core/ D
DOI: 10.1021/acs.inorgchem.8b01169 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry shell structure SNCs, we analyzed the UC energy transfer mechanism in Yb/Tm/ZnO, Yb/Tm/GZO, and Yb/Tm/ GZO@SiO2 nanoparticles. As shown in Figure 3a, the UC mechanism of Yb/Tm/ZnO nanoparticles shows that the blue UC emission peak at 475 nm originated from three photon energy transfer (ET) processes, corresponding to the 3H6 → 3 H5 (ET1), 3F4 → 3F2,3 (ET2), and 3H4 → 1G4 (ET3) transitions of Tm3+ ions, respectively. Meanwhile, Yb/Tm/ ZnO nanoparticles show the red UC luminescence from 652 nm is a cooperative sensitization UC mechanism, which is consistent with 1G4 → 3F4 transitions of Tm3+ ions (Figure S5). However, because the surface dopant ions of Yb3+ and Tm3+ cannot diffuse into the ZnO host lattice, surface-related upconversion luminescence quenching enhancement lead to low energy transfer (ET). In stark contrast, the Yb/Tm/GZO upconversion SNCs exhibit the more optimum UCL emission compared with the Yb/Tm/ZnO UCNPs. We assume that this obvious difference in the energy transfer efficiency of the UC emission can be attributed to Ga3+ ions doping. This means that when the Ga3+ substitute for Zn2+, Yb/Tm ions can be easily embedded in the host lattice due to similar ionic radius and the same charge. The obvious enhancement in the intensity of the UC emission is further revealed by the fact that Ga3+-doped provides an intermediate state energy level for host to Tm3+ energy transfer (Figure 3b). For the Yb/Tm/ GZO@SiO2, the energy-cascaded upconversion was built, which exploited a core−shell system consisting of a core upconverting SNC and silica shell coating on the core SNC surface (Figure 3c). The Yb3+/Tm3+ in the core particles surface can take part in the energy transfer process, and owing to a core/shell being obtained, the surface-related UCL quenching in the core is suppressed by the shell. Specifically, the steps of energy transfer for Yb/Tm/GZO@SiO2 nanocrystals can be classified as follows (Figure 3d). First, under the 980 nm excitation, part of the energy absorbed by Yb3+ ions (intermediate sensitizers) is on the surface of Yb/Tm/GZO nanoparticles. Second, intermediate sensitizers in the shell cross the core lattice to Yb3+ ions (sensitizers) in the core. Third, the charge compensation is caused by Ga3+ doping of the inner core, which produced defect-mediated energy transfer from the GZO host to Yb/Tm ions (host sensitized). Last but not least, the energy from sensitizers of Yb3+ ions transfers to Tm3+ of the core and obtained high upconversion emission via a classical ETU mechanism. Therefore, compared with Yb3+ and Tm3+ ions doped with the ZnO SCNs, this multistep cascade energy strategy of Yb/Tm/GZO@SiO2 enhanced upconversion luminescence efficiency due to harvested energy. 2.3. Operated Photodynamic Therapy Applications of Yb/Tm/GZO@SiO2 Nanocrystals. Silica shell coating improved passive tumor target efficacy, prolonged blood circulation time, and increased the biocompatibility. In order to research the application of Yb/Tm/GZO@SiO2 UCNPs, in vitro photodynamic therapy (PDT) treatment of Hela cells with Yb/Tm/GZO@SiO2 nanoparitlce was carried out with 980 nm laser excitation. After incubated with different concentrations of Yb/Tm/GZO@SiO2 nanoparticles (100 μg·mL−1 ∼ 800 μg·mL−1) for 24 h, 48 h, and 72 h, respectively, Hela cells were irradiated at a 980 nm laser with 0.8 W/cm2 for 20 min (the cell radiated at 1 min intervals), and the cell viability was determined with a CCK8 method. As shown in Figure 4, Hela cell viability can be quantified using the following eq 6.
Figure 4. (a) The Hela cell viability of Yb/Tm/GZO@SiO2 with different times and concentrations without 980 nm irradiation (0.8 W/cm2). (b) The Hela cell viability of Yb/Tm/GZO@SiO2 with different times and concentrations with 980 nm irradiation (0.8 W/ cm2).
cell viability (%) =
A s − Ab × 100% Ac − Ab
(6)
where As stands for the absorbance of test Hela cells with Yb/ Tm/GZO@SiO2 nanocomposites; Ab is the absorbance of control Hela cells without Yb/Tm/GZO@SiO2 nanocomposites, and Ac is the absorbance of blank samples containing culture medium (without Hela cells and Yb/Tm/GZO@ SiO2). For unirradiated Hela cells, Yb/Tm/GZO@SiO2 nanoparticles had no effect on the cell viability (Figure 4a), despite the existence of a high concentration of 800 μg/mL and a long time of 72 h. As shown in Figure 4b, under 980 nm laser irradiation, Hela cell viability decreased from 71.04% ± 0.06 to 34.69% ± 0.02 (P < 0.01) at nanoparticle concentration of 400 μg/mL with the increased incubation times. For 800 μg/mL concentration of nanoparticle, the viability of Hela cells gradually decreased from 37.88% ± 1.76 to 19.63% ± 0.53 with the increased incubation times. The cell viability realized the Yb/Tm/GZO@SiO2 significantly inhibited the viability of Hela cells. It is indicated that Yb/Tm/ GZO@SiO2 has an excellent application in photodynamic therapy.
3. CONCLUSION In summary, by optimizing the doping concentrations of Ga3+ ions in the host sensitizing core, we present a system of energycascaded UCNPs using SiO2 coated Yb/Tm/GZO core/shell nanoparticles, which provides a way to obtain high-efficiency upconversion SNCs under 980 nm excitation. Yb/Tm/GZO@ mSiO2 UCNPs can reduce surface-related UC luminescence quenching and improve the energy transfer efficiency. Yb/Tm/ GZO@mSiO2 UCNPs form the multistep energy transfer, and E
DOI: 10.1021/acs.inorgchem.8b01169 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry ORCID
this enhances upconvertion ET efficiency as high as 81%. In addition, Hela cells viability decreased to 19.63% ± 0.53 with increased concentration of fabricated Yb/Tm/GZO@mSiO2 under 980 nm laser irradiation, while the Hela cells viability had not change too much without 980 nm laser irradiation. It is indicated that core/shell Yb/Tm/GZO@SiO2 upconversion SNCs are an excellent candidate for photodynamic therapy Hela cells.
Rui Wang: 0000-0002-1964-4776 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 11374080).
4. EXPERIMENTAL SECTION 4.1. Materials. All chemicals were purchased and used without further purification. GZO (3 mol % Ga): 7 mol % Yb, 0.5 mol % Tm were synthesized via a hydrothermal method.13 Zn(NO3)2·6H2O (99.99%), Ga(NO3)3·xH2O (99.99%) and 10 mL of NaOH (2 mol/ L) were mixed via magnetic stirring for 30 min. We dropped Yb(NO3)3·5H2O (99.99%) and Tm(NO3)3·5H2O (99.99%) in the mixture with magnetic stirring for 60 min. The turbid liquid was obtained. We removed the mixture liquid into a 50 mL reaction kettle at 150 °C for 24 h. To remove impurity, the samples were cleaned by ethanol and water for six times with centrifugation and finally dried in a 60 °C baking box. The Yb/Tm/GZO UCNPs were obtained. 4.2. Synthesis of Yb/Tm/GZO@SiO2 Upconversion Nanoparticles. The Yb/Tm/GZO@SiO2 UCNPs were prepared by the Stöber method. Yb/Tm/GZO nanoparticles were prepared by ultrasound in isopropyl alcohol. First, the well dispersed Yb/Tm/ GZO core samples were well stirred at 32 °C for 20 min, and 10 mL of deionized water was added. Second, 5 mL of NH3·H2O (25%) was added to the core samples with strong stirring for 10 min. Finally, 6 μL of TEOS was dropped into the mixture samples at 32 °C for 90 min. We cleaned the Yb/Tm/GZO@SiO2 in isopropyl alcohol and water ultrasonically and then dried it for 24 h at 80 °C. 4.3. Material Characterization. Scanning electron microscopy (TEM) was performed on the SU8000 SEM using a 15 kV focus voltage. Transmission electron microscopy (TEM) was performed on a JEOL 2010F field-emission TEM using a 200 kV focus voltage. The upconversion emission spectra of the nanoparticles have been recorded using 980 nm excitation (Figure S1). The decay lifetime of sample was performed on a computer with a 980 nm laser, MD03024 Mixed Domain Dscilloscope. 4.4. Cell Culture. The Hela cells were cultured in Dulbecco’s Modified Eagle Medium (Hyclone, Thermo Fisher, Beijing, China) supplemented with 10% fetal bovine serum, and all cells were maintained in a humidified incubator (Thermo Forma, USA) containing 5% CO2 at 37 °C. The Hela cells were cultured in RPMI 1640 (Hyclone, Thermo Fisher, Beijing, China) medium containing 10% fetal bovine serum and 1% penicillin−streptomycin. Cells were maintained in a humidified incubator (Thermo Forma, USA) containing 5% CO2 at 37 °C, and all the experiments were performed in a clean atmosphere.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01169.
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Optical path of upconversion luminescence test, emission spectra, XRD patterns, intensity versus input pump power, UC luminescence mechanism, Raman spectra, SEM images, process of SiO2 coating Yb/Tm/ GZO, and time-dependent bleaching of DPBF (PDF)
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DOI: 10.1021/acs.inorgchem.8b01169 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b01169 Inorg. Chem. XXXX, XXX, XXX−XXX
