Silica-Coated Ga(III)-Doped ZnO: Yb3+, Tm3+ Upconversion

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Silica-Coated Ga(III)-Doped ZnO: Yb3+, Tm3+ Upconversion Nanoparticles for High-Resolution in Vivo Bioimaging using NearInfrared to Near-Infrared Upconversion Emission Yuemei Li,† Rui Wang,*,† Wei Zheng,† and Yongmei Li*,‡ †

School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China NHC Key Laboratory of Hormones and Development (Tianjin Medical University), Tianjin Key Laboratory of Metabolic Diseases, Tianjin Medical University Metabolic Diseases Hospital & Tianjin Institute of Endocrinology, 300070 Tianjin, China

‡

Downloaded by BETHEL UNIV at 22:54:20:496 on May 24, 2019 from https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b01056.

S Supporting Information *

ABSTRACT: Photon avalanche has attracted much attention due to the unique nonlinear luminescence dynamics and highefficiency upconversion luminescence properties. In this paper, we present a simple but effective method to prepare Yb/Tm/ GZO@SiO2 (GZO = gallium-doped zinc oxide). Low-threshold photon-avalanche luminescence was obtained below the milliwatts (2.5 mW) in Yb/Tm/GZO@SiO2 core/shell nanoparticles. Compared with Yb/Tm/GZO nanoparitlces, nearinfrared (NIR) upconversion luminescence intensity of Yb/Tm/GZO@SiO2 was enhanced ∼12 times. Furthermore, Yb/Tm/ GZO@SiO2 acted as luminescence probe, which realized the red fluorescence imaging of myocardial tissue. With injection of 6 mg/kg Yb/Tm/GZO@SiO2, the red NIR fluorescence imaging of heart tissue became brighter. The experimental results of histological assessments of representative organs indicated that there was low toxicity of Yb/Tm/GZO@SiO2 in vivo. This study proved that Yb/Tm/GZO@SiO2 posed a promising potential for bioimaging applications.

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INTRODUCTION Upconverison nanoparitcles (UCNPs) have drawn a great deal of attention because of their unique upconversion luminescence property and a wide variety of biological applications, such as bioimaging, photodynamic therapy, biomarker, and biological probe.1−4 UCNPs have the ability to convert nearinfrared (NIR) excitation radiation into visible light, which is anti-Stokes luminescence.5 UCNPs provide the highest signalto-noise ratio, and low toxicity for in vitro imaging is reliable and effective, but it suffers from limited stability and lacked high resolution for bioimaging practical applications.6−8 The luminescence properties of UCNPs are closely related to those of host materials. For example, NaYF4 nanoparicles are the most efficient host materials due to their low phonon energy.9,10 Compared with fluorides, oxide nanocrystals have highest chemical stability and lowest environmental harm.11,12 Lanthanide-doped inorganic oxide nanoparticles can lead to the development of high stability and sensitivity in vivo bioimaging.13,14 However, the challenge remains to achieve a reliable and efficient upconversion luminescent (UCL) at © XXXX American Chemical Society

room temperature. So, the most important problem that needs enhancement is the UCL efficiency.15 For this, the photon avalanche (PA), due to its unique nonlinear excitation dynamics and high intensity of efficiency upconversion, has been reported as better process, which can improve the upconversion efficiency of UCNPs.16 Currently, the PA process is observed in a variety of lanthanide-doped materials such as Pr3+, Er3+, Ho3+, and Tm3+ ions.17−19 According to the reports, there are two necessary conditions to induce photon avalanche in the rare-earth-doped system as follows: resonance excited-state absorption and efficient cross relaxation energy transfer.20,21 Resonance excited-state absorption depends on the matching between the transition energy gap and the excitation photon energy. Cross relaxation energy transfer is determined by the distance between rare-earth ions.22−24 Therefore, the concentrations of rare earths have effect on cross relaxation energy transfer.23 Received: April 11, 2019

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DOI: 10.1021/acs.inorgchem.9b01056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic diagram of Yb/Tm/GZO@SiO2 synthesis; (b) SEM image of Yb/Tm/GZO upconversion nanoparitcles; (c) SEM image of Yb/Tm/GZO@SiO2 core/shell nanoparitlces; (d, e) TEM images of Yb/Tm/GZO@SiO2 core/shell nanoparitlces; (f) EDX mapping of a typical Yb/Tm/GZO@SiO2.

Therefore, the NIR UCL intensity was ∼12 times higher than that of the Yb/Tm/GZO UCNPs. The shell coating not only enhanced the avalanche mutation effect but also increased NIR UC luminescent efficiency.

However, when the distance between neighboring rare-earth ions (Ln3+) is larger than the critical distance, the quantum efficiency of cross relaxation is low. Rare-earth-doped materials have high luminous threshold and avalanche threshold, which makes it difficult for the photon avalanche to be induced.25,26 In our previous study, we proposed a Stöber wet chemical method to synthesize core−shell Yb/Tm/GZO@SiO 2 (UCNPs@SiO2; GZO = gallium-doped zinc oxide). It was reported that the silica shell-coated reduced the concentration quenching and enhanced the blue and red visible upconversion emission intensity of Yb/Tm/GZO.27−30 In this work, we found that the NIR upconversion emission intensity of Yb/ Tm/GZO@SiO2 increased in a highly nonlinear manner with increasing density powers, indicating occurrence of photon avalanche of Tm3+ with coating silica shell. The photon avalanche process was first investigated in core−shell structure Yb/Tm/GZO@SiO2. The energy mismatch of two crossrelaxation levels of Tm3+ ions was smaller and belongs to nearresonant energy transfer. The cross-relaxation process in Tmdoped nanocrystal mainly depends on the concentration of Tm3+ ions in energy-transfer process. The results indicated that the shell (SiO2) can increase concentration of Tm3+ ions participating in energy transfer, enhancing the cross-relaxation energy transfer. Most importantly, the avalanche threshold of Yb/Tm/GZO@SiO2 was remarkably declined compared with the Yb/Tm/GZO, which made photon avalanche in dopedlanthanide nanocrystals easier to induce. The photon avalanche levels of Tm3+ ions correspond to NIR upconversion emission.

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RESULTS AND DISCUSSION Yb/Tm/GZO and Yb/Tm/GZO@SiO2 nanoparticles were prepared by hydrothermal and Stöber methods, respectively (Figure 1a). Figure 1b was a scanning electron microscopy (SEM) image of Yb/Tm/GZO UCNPs that showed the UCNPs structure, presenting nanoscale flower. GZO nanopariticles codoped with Yb and Tm ions were synthesized with a size distribution in the range of 40∼80 nm, as shown in Figure 1b. SEM and transmission electron microscopy (TEM) images of the Yb/Tm/GZO@SiO2 nanoparticles displayed core/shell structure (Figure 1c,d). To further verify the size of shell, the shell thickness of Yb/Tm/GZO@SiO2 was tested in Figure 1e. Yb/Tm/GZO@SiO2 derived from 0.6 mL of tetraethyl orthosilicate (TEOS) possessed shell thickness of 12.5 nm. Energy dispersive X-ray spectroscopy (EDX) mapping of Yb/Tm/GZO@SiO 2 UCNPs (Figure 1f) performed the coexistence of elements Zn, Ga, Yb, Tm, and Si, especially that the size of Si map was larger than the maps of Zn, Ga, Yb, and Tm. These results confirmed the formation of core/shell structure and the coexistence of Si element on the surface of Yb/Tm/GZO core nanoparicles. In addition, X-ray diffraction (XRD) analysis of Yb/Tm/GZO and Yb/Tm/ B

DOI: 10.1021/acs.inorgchem.9b01056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. NIR upconversion emission spectra (λex = 980 nm) of (a) Yb/Tm/GZO nanoparticles and (b) Yb/Tm/GZO@SiO2 core/shell nanoparticles with different pump powers from 1.5 to 7.5 mW. (c) NIR (830 nm) upconversion emission intensities of Yb/Tm/GZO and Yb/Tm/ GZO@SiO2 with different pump powers from 1.5 to 7.5 mW. (d) Double logarithmic fitting of 830 nm NIR upconversion intensity with different pump power from 1.5 to 7.5 mW.

the excitation power was 2.5 mW, NIR emission intensity at 830 nm band of Yb/Tm/GZO@SiO2 increased in rapidly nonlinear manner, indicating photon avalanche threshold of Yb/Tm/GZO@SiO2 was 2.5 mW. These results proved that the photon avalanche process did not occur at less than 2.5 mW. When the excitation power increased to more than 2.5 mW, the cross-relaxation energy transfer between Tm3+ increased, and the avalanche mechanism established, which led to the nonlinear growth of fluorescence intensity. It was reported that efficient cross relaxation is the condition of photon avalanche, and the generation of efficient cross relaxation depends on the distance between rare-earth ions.31,32 The coating of SiO2 enabled Tm3+ on the Yb/Tm/ GZO surface to participate in the upconversion luminescence process and increased the concentration of Tm3+ ions. Increasing the concentration of Tm3+ ions can reduce the spacing of rare-earth ions and increase the probability of cross relaxation, which effectively enhanced the photon avalanche effect. Moreover, the decay lifetimes of NIR UCL (830 nm) with different power density were studied in Figure S3. According to equations S2 and S3, the decay lifetimes of 830 nm emission were calculated with increasing excitation powers (Table 1). With increasing the power density from 1.0 to 8.5 mW, the decay lifetimes of NIR UCL (830 nm) increased from 226 to 788 μs. When the excitation power increased to 2.5 mW, the slow decay lifetime (τs) dramatically increased to 1901 μs, which indicated that the PA process was produced. As a result, the silica shell coating decreased the low threshold of photon

GZO@SiO2 were shown in Figure S1. Silica coating has no changed hexagonal phase of Yb/Tm/GZO. There was no wide diffraction peak of silica, because the crystallinity and content of core nanoparticles were much higher than those of silica shell. To investigate how different pump powers influence the NIR upconversion emission properties of the nanoparticles, the photoluminescence spectroscopy for Yb/Tm/GZO and Yb/Tm/GZO@SiO2 was shown in Figure 2. Yb3+ excited at 980 nm transferred energy to Tm3+, which resulted in the characteristic Tm3+ NIR emission with peaks at 700 and 830 nm. According to equation S1, the numbers of n can be determined from the slopes of the fitting lines (Figure S2). The slopes were 2.0 ± 0.1 and 2.1 ± 0.2, corresponding to UCL emssions at 700 and 830 nm, respectively. The upconversion mechanism of 700 and 830 nm emissions were two photos process. The intensity of 700 and 830 nm NIR emissions for Yb/Tm/GZO and Yb/Tm/GZO@SiO2 nanoparticles gradually increased with increase of the excitation power from 1.5 to 7.5 mW (in Figure 2a,b). Its value was considered to depend on the excitation power density; NIR emission intensities of Yb/Tm/GZO@SiO2 core/shell UCNPs increased in a highly nonlinear (in Figure 2c). It means that the Tm3+ ion doping Yb/Tm/GZO@SiO2 core−shell system has a remarkable nonlinear absorption process for excited photons. The PA process occurred in Yb/Tm/GZO@SiO2. As performed in the Figure 2d, photoluminescence process of Yb/Tm/GZO@SiO2 exhibited a significant nonlinear and a minimum excitation threshold, which was due to the nonresonant absorption of 980 nm excited photons by the ground state of Tm3+ ions. When C

DOI: 10.1021/acs.inorgchem.9b01056 Inorg. Chem. XXXX, XXX, XXX−XXX

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showed that myocardial fibers were arranged orderly without rupture and no infiltration of inflammatory cells. Kidney tissue sections appeared as normal structures; the glomerulus and renal tubular structure could be distinguished easily, and no hydropic degeneration was observed in the kidney samples. Necrosis had no appearance in any of the groups. These results proved that Yb/Tm/GZO@SiO2 nanopartilces have a little systemic toxicity and pose a great potential for future biomedical applications. Because of the effect of PA of Yb/Tm/GZO@SiO2 core/ shell nanoparitcles, NIR emission at 830 nm intensity of Yb/ Tm/GZO@SiO2 was stronger than that of Yb/Tm/GZO core nanoparticles. To further investigate the in vivo imaging of Yb/ Tm/GZO and Yb/Tm/GZO@SiO2 nanoparticles, confocal imaging of myocardial tissue was studied in Figure 5. Heart was dissected and fresh-frozen in O.C.T. embedding medium (Sakura Finetechnical Co.) Ten-micrometer sections were cut from frozen blocks at −20 °C. Confocal imaging was done using OLYMPUS FV500 laser scanning confocal microscope. Figure 5 showed the multiphoton confocal imaging of myocardial tissue treated with the 6 mg/kg Yb/Tm/GZO and Yb/Tm/GZO@SiO2 following excitation at 980 nm using a femtosecond pulsed laser, respectively. The 830 nm emission (red color) of Yb/Tm/GZO under 980 nm excitation was detected, but the intensity of red NIR light was weak (Figure 5a,d). However, imaging of myocardial tissue at 830 nm band was remarkably enhanced bright red luminescent treated with 6 mg/kg Yb/Tm/GZO@SiO2 for 24 h (Figure 5b,e). Furthermore, with injection of 6 mg/kg Yb/Tm/GZO@SiO2 for 7 d (Figure 5c,f), bright red luminescence of myocardial tissue image can still be achieved, demonstrationg that the Yb/ Tm/GZO@SiO2 can effectively improve myocardial tissue imaging. These results suggested that Yb/Tm/GZO@SiO2 nanoparticles have an excellent ability in bioimaging applications, and silica shell coating improved the biocompatibility of Yb/Tm/GZO upconversion nanoparticles.

Table 1. Fluorescence Lifetime of 830 nm Emission in Yb/ Tm/GZO@SiO2 Upconversion Nanoparticles with Different Excitation Powers excitation power (mW)

slow decay lifetime (τs, μs)

fast decay lifetime (τf, μs)

decay lifetime (τ, μs)

1.0 1.5 2.5 4.5 6.5 8.5

226 360 1901 602 656 788

226 360 248 602 656 788

226 360 508 602 656 788

avalanche luminescence. The high NIR (830 nm) decay lifetime can be achieved after PA process. Figure 3 showed the energy-level structure and PA process of Yb/Tm/GZO@SiO2 UCNPs at 980 nm excitation. There was resonance between excited photon (Ep = hv) and excited state of Tm3+ under 980 nm lase, producing a strong resonance absorption of Tm3+ excited state (3F4 + hv → 1G4). The 3F4 state of Tm3+ excited at 980 nm transferred energy to 1G4. The cross relaxation occurred between 1G4 state of Tm3+ and 3H6 ground state of adjacent Tm3+; nonradiation relaxed to 3F2,3 level of Tm3+ (1G4 + 3H6 → 3F2,3 + 3F4). Because of the small gap between 3F2,3 and 3H4 levels of Tm3+, most of the photons at 3F2,3 state relaxed to 3H4 energy by fast nonradiation thermal relaxation. Because of the symmetrical energy-level structure of Tm ions, there was abundant cross relaxation. Tm3+ at the 3H4 level and ground state of adjacent Tm3+ at the 3H6 level produced cross relaxation; nonradiation relaxed to 3F4 level of Tm3+ (3H4 + 3H6 → 3F4 + 3F4). Two cross-relaxation effects led to increase in the number of ions at excited state (3F4 and 3 H4). Photon avalanche occurred under the effect of resonance excited-state absorption and cross relaxation. To further study the in vivo toxicity of Yb/Tm/GZO@SiO2 nanoparticles, histological assessments of heart and kidney tissue were examined in Kunming mouse injected with different doses of Yb/Tm/GZO@SiO2 through the tail vein (Figure 4). The histological results of heart and kidney 24 h and 7 d after injection of 1, 6, and 10 mg/kg Yb/Tm/GZO@ SiO2 nanoparitlces were shown in Figure 4. Macroscopic observations of tissue sections apparently suggested normal regular anatomic structures and hardly different from those of the control group. Cardiac muscle tissue in the heart samples

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CONCLUSIONS

In conclusion, we have synthesized Yb/Tm/GZO@SiO2 core/ shell nanoparticles. With the SiO2 shell, Yb/Tm/GZO@SiO2 was more prone to produce the photon avalanche process compared with Yb/Tm/GZO. A low-power threshold of 2.5 mW was obtained in Yb/Tm/GZO@SiO2. Because of the

Figure 3. Proposed PA mechanism of Yb/Tm/GZO@SiO2 under 980 nm excitation. D

DOI: 10.1021/acs.inorgchem.9b01056 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Histopathology of the kidney and heart after intravenous injection of 1, 6, and 10 mg/kg Yb/Tm/GZO@SiO2 at 24 h and 7 d, respectively. The scale bar is 100 mm.

Figure 5. Confocal imaging (top) and bright-field (bottom) images of myocardial tissue. (a, d) Following 24 h of incubation with 6 mg/kg of Yb/ Tm/GZO contents. (b, e) Following 24 h of incubation with 6 mg/kg of Yb/Tm/GZO@SiO2. (c, f) Following 7 d of incubation with 6 mg/kg of Yb/Tm/GZO@SiO2. The excitation at 980 nm was provided from a pulsed laser, and the red NIR emission was captured (120 × 10 oil lens, scale bar = 50 μm).

photon avalanche, the NIR UCL intensity of Yb/Tm/GZO@ SiO2 was stronger by ∼12 times than that of Yb/Tm/GZO. In

addition, we have measured the histopathology of kidney and heart, and no obvious histopathological lesions or abnormalE

DOI: 10.1021/acs.inorgchem.9b01056 Inorg. Chem. XXXX, XXX, XXX−XXX

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was dissected and fresh-frozen in O.C.T. embedding medium (Sakura Finetechnical Co.). Ten-micrometer sections were cut from frozen blocks at −20 °C. Confocal microscopy was used to observe the imaging of cardiac myocytes. Confocal imaging was done using OLYMPUS FV500 laser scanning confocal microscope (120 × 10 oil lens), receiving red NIR UC emission at 980 nm using a femtosecond pulsed laser.

ities were observed, indicating the Yb/Tm/GZO@SiO2 has no obvious toxicity. Particularly, we found that the red NIR (830 nm) fluorescence imaging of myocardial tissue enhanced brightness with the injection of Yb/Tm/GZO@SiO2. In summary, these nontoxic nanoparitcles can be considered as ideal candidates for diagnostic probes.

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EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

Materials. All chemical materials, ytterbium nitrate pentahydrate (Yb(NO3)3·5H2O, 99.99%), thulium nitrate pentahydrate (Tm(NO3)3·5H2O, 99.99%), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99.9%), gallium nitrate (Ga(NO3)3·xH2O, 99.99%), isopropyl alcohol, TEOS, citric acid sodium, ammonia (NH3·H2O, 25%), and sodium hydroxide (NaOH) were used directly as received without further purification. Synthesis of Yb/Tm/GZO. A hydrothermal method was as follows: Zn(NO3)2·6H2O (0.5 mol) and Ga(NO3)3·xH2O (10 mol %) were dissolved in deionized (DI, 10 mL) water and magnetically stirred for 30 min, and citric acid sodium was added, forming a clarified liquid, which was magnetically stirred for 20 min. NaOH (2 mol/L, 10 mL) was added into the above clarified liquid, forming a suspension solution that was magnetically stirred for 30 min. Tm(NO3)3·5H2O (with 0.5 mol %, 5 mL) and Yb(NO3)3·5H2O (7 mol %, 5 mL) were added into this suspension solution, which was magnetically stirred for 60 min. Subsequently, the solution was transferred into a 50 mL hydrothermal reactor. The solution was heated to 150 °C for 24 h. The resultant solution was cooled and cleaned by centrifuge in 3500 r/min for 15 min with copious amounts of ethanol and DI water. Yb/Tm/GZO was obtained after 24 h of drying in oven at 60 °C. Synthesis of Yb/Tm/GZO@SiO2. Isopropyl alcohol (40 mL) was added to Yb/Tm/GZO (0.1713 g) by ultrasonication for 30 min, forming a suspension solution. DI water (10 mL) was added into the mixture and was magnetically stirred for 10 min. NH3·H2O (5 mL, 25%) was dropped into suspension solution at 32 °C and magnetically stirred for 20 min. TEOS was dropped slowly into the aforementioned solution at 32 °C and magnetically stirred for 90 min. The amount of TEOS was 0.6 mL. The resultant solution was cooled to room temperature. The solution was washed six times with copious amounts of ethanol and DI water by centrifuge in 3500 r/min. The products were collected after drying in oven for 24 h at 80 °C. Characterization. Size and morphologies of UCNPs were determined at 15 kV using a focus voltage SU8000 SEM and at 200 kV using a focus voltage JEOL 2010F TEM. The upconversion emission spectra of the nanoparticles were measured using 980 nm excitation. The decay lifetime of sample was performed on a computer with 980 nm laser, MD03024 Mixed Domain Oscilloscope. In Vivo Toxicity and Imaging Studies. All animal experiments and procedures were approved by the Animal Ethical and Experimental Committee of the Tianjin Medical University Metabolic Diseases Hospital. Five weeks old male Kunming mice (weighed 23∼26 g) were purchased from the animal center of Military Medical Sciences Academy of the PLA (Permission No. SCXK-(A) 20120004). All experimental mice were maintained on a 12 h light/12 h dark cycle at 22 °C and 55 ± 5% relative humidity in a standard laboratory room at Tianjin Medical University. The mice were provided rodent chow and water ad libitum. After a week of acclimation, mice were randomly allocated into four groups (eight animals in each group): three sample groups (treated with final dose 1, 6, and 10 mg/kg of Yb/Tm/GZO@SiO2 body weight) and a vehicle group (treated with equivalent volume of phosphate-buffered solution (PBS)). Yb/Tm/GZO@SiO2 nanoparticles and PBS were injected via the tail vein, respectively. Eight mice from each group were weighed and sacrificed by CO2 asphyxiation after 24 h and 7 d. Heart and kidney tissue were excised and fixed in 10% PBS buffered formalin, and then the tissues were embedded in paraffin. Sections (4 μm thick) that had been deparaffinized and rehydrated were stained with hematoxylin-eosin (H&E) for histological assessment. Histological changes were visualized with a microscope (Olympus). Heart

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01056.

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X-ray diffraction spectra of Yb/Tm/GZO and Yb/Ym/ GZO@SiO2 nanoparticles, The correspongding log(UC intensity)−log(excitation power) curve of samples, schematic energy level diagram of Tm3+ and Yb3+ ions, as well as the proposed upconversion mechanism for generating 700 and 830 nm emissions under 980 nm excitation. Decay lifetime of 830 nm emission in Yb/ Tm/GZO@SiO2 core−shell upconversion nanoparticles with different excitation powers from 1.0 to 8.5 mW (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.W.) *E-mail: [email protected]. (Y.-m.L.) ORCID

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 Science & Technology Development Fund of Tianjin Education Commission for Higher Education (No. 2018KJ069) and Startup Funding of Scientific Research, Tianjin Medical University Metabolic Diseases Hospital and Tianjin Institute of Endocrinology (No. 2017DX07)

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DOI: 10.1021/acs.inorgchem.9b01056 Inorg. Chem. XXXX, XXX, XXX−XXX