Trap Depth, and In Vivo

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Zn3Ga2Ge2O10:Cr3+ Uniform Microspheres: TemplateFree Synthesis, Tunable Bandgap/Trap-Depth, and In Vivo Rechargeable Near-Infrared Persistent Luminescence Qi Zhu, Junqing Xiahou, Yao Guo, Hailong Li, Chen Ding, Jing Wang, Xiaodong Li, Xudong Sun, and Ji-Guang Li ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00734 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Bio Materials

Zn3Ga2Ge2O10:Cr3+ Uniform Microspheres: Template-Free Synthesis, Tunable Bandgap/Trap-Depth, and In Vivo Rechargeable Near-Infrared Persistent Luminescence Qi Zhu1,2*, Junqing Xiahou1,2, Yao Guo1,2, Hailong Li3, Chen Ding3, Jing Wang4, Xiaodong Li1,2, Xudong Sun1,2 and Ji-Guang Li5*

1Key

Laboratory for Anisotropy and Texture of Materials (Ministry of Education),

Northeastern University, Shenyang, Liaoning 110819, China 2Institute

of Ceramics and Powder Metallurgy, School of Materials Science and

Engineering, Northeastern University, Shenyang, Liaoning 110819, China 3College

of Life and Health Sciences, Northeastern University, Shenyang, Liaoning

110015, China 4State

Key Laboratory of Optoelectronic Materials and Technologies, School of

Chemistry, Sun Yat-Sen University, Guangzhou, Guangdong 510275, China 5Research

Center for Functional Materials, National Institute for Materials Science,

Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan

*Corresponding author Dr. Qi Zhu Tel: +86-24-8368-1680 E-mail: [email protected]

Dr. Ji-Guang Li Tel: +81-29-860-4394 E-mail: [email protected]

Keywords: Near infrared persistent luminescence, monospheres, conduction band minimum, in vivo imaging 1

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Abstract Near-infrared (NIR) emitting persistent phosphors of Cr3+-doped zinc gallogermanate have emerged for in vivo bio-imaging with the advantages of no need for in situ excitation. However, it is challenging to synthesize well-dispersed and uniform spherical particles with high brightness, high resolution, and distinguished NIR long afterglow. In this work, Zn3Ga2Ge2O10:Cr3+ (ZGGC) monospheres were directly synthesized by a facile hydrothermal method with the assistance of citric anions (Cit3-), which emit a NIR emission at ~696 nm and exhibit excellent NIR persistent luminescence with rechargeability. Controlled experiments indicated that the shape evolution of ZGGC product is significantly affected by Cit3-, solution pH, and the duration and temperature of hydrothermal reaction. Furthermore, compositional influence on the crystal structure, bandgap, trap depth, and luminescence characteristics of ZnyGa2Ge2O10-δ:Cr3+ (y = 2.8, 3.0, 3.2) were investigated in details, which allows to construct an energy level diagram of the ZGGC host, Cr3+ ions, and electron traps. It was found that the bandgap and conduction-band minimum (CBM) are significantly affected by the Zn content, while the valence-band maximum (VBM) is not. The y = 3.0 sample exhibited the best persistent luminescence, owing to its deepest defects. The ZGGC-NH2 prepared through surface functionalization of ZGGC spheres showed distinguished NIR long afterglow, low toxicity, and great potential for in vitro cell imaging and in vivo bio-imaging in the absence of excitation. Moreover, the persistent-luminescence signal from the ZGGC-NH2 can be repeated in vivo through in situ recharge with external excitation of a red LED lamp, indicating that the ZGGC-NH2 is suitable for applications in long-term in vivo imaging.

2


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1. Introduction Long-persistent phosphors are “self-sustained” luminescent materials, whose emission can last for up to hours at room temperature after removal of the excitation source.1 Because the phosphors have great research interests, they have been widely applied in dark environment vision area, including emergency route display and identification symbol. However, the successful persistent phosphors, including Eu3+/Mg2+/Ti4+ co-doped Y2O2S and Eu2+/Tm3+/Ce3+ co-doped CaS (red emission), Eu2+/Dy3+ co-doped SrAl2O4 and Mn2+ doped MgAl2O4 (green emission), and Eu2+/Nd3+ co-doped CaAl2O4 and Eu2+,Dy3+ co-doped SrMgSi2O6:(blue emission),2-7 are luminescent in visible wavelength region, while the investigation on near-infrared (NIR, ~700-2500 nm) persistent phosphors is rather limited. Recently, the NIR persistent phosphors showed potential applications in bio-imaging field.8,9 Their advantage of long-lasting afterglow allows pre-excitation by UV or visible light, and permits detection and imaging in the absence of external illumination, thus the noise of background from in situ excitation can be reduced or even avoided. In addition, the NIR emission exhibits the great advantages of high sensitivity and deep penetration for in vivo imaging of whole body. Chen et al. first employed persistent phosphors and photo-sensitizers as the in vivo media for photodynamic therapy.9 Another in vivo agent of Ca0.2Zn0.9Mg0.9Si2O6:Eu2+,Mn2+,Dy3+ was used by Scherman et al. in in vivo bio-imaging.8 The same group then developed two new NIR-emitting phosphors with the compositions of CaMgSi2O6:Eu2+,Mn2+,Pr3+ and Ca2Si5N8:Eu2+,Tm3+.10,11 For NIR luminescence, Cr3+ ions are favorable luminescent centers in hosts owing to their spin forbidden 2E → 4A2 transitions at ~700 nm or spin-allowed 4T2 → 4A2 transitions at ~650-1600 nm. However, the host crystal-field environment determines the final emission.1,12 Recently, NIR-emitting persistent phosphors of Cr3+-doped gallate were synthesized by solid-state reaction in a reducing-free atmosphere.1 Two kinds of spinel-structured gallate persistent phosphors, including Cr3+-doped ZnGa2O4 and its variant form, have mainly been reported. The ceramic disc of ZnGa2O4:Cr3+ obtained from solid state reaction exhibits strong NIR emission and long-lasting afterglow, and much improvement was observed by partial Ge/Sn substitution of Ga 3



in ZnGa2O4:Cr3+.13 Recently, it was reported that micrometric gallogermanates (Zn3Ga2Ge2O10:Cr3+) could be responsible for the strong emission at 650-1000 nm, extending beyond the typical 690-750 nm, and super-long afterglow of more than 360 h.1 Then, the same research group created the first up-conversion near-infrared persistent phosphor of Zn3Ga2GeO8:Cr3+ co-doped with the up-converting ion pair of Yb3+/Er3+.14 Upon excitation at 980 nm, the phosphor emits near-infrared persistent emission peaking at 700 nm a and with a afterglow of more than 24 h. However, these bulky materials cannot be used for in vivo bio-imaging directly. Soon afterwards, functionalized NIR-emitting Cr3+-doped ZnGa2O4 and Cr3+/Pr3+ or Cr3+/Eu3+ co-doped zinc gallogermanate persistent luminescent nanoparticles with super long afterglow were synthesized for in vivo bio-imaging.12,15-18 Novel porous nanoprobes of Zn1.1Ga1.8Ge0.1O4:Cr3+,Eu3+@SiO2 have been prepared using nano-sized spheres of mesoporous silica as templates for tumor imaging and drug load.18 The in vivo imaging could be carried out in small animals after subcutaneous injection in the absence of light excitation. Then, core-shell structured NIR luminescence multimodal probes of ZnGa2O4:Cr3+,Sn4+@MSNs (mesoporous silica nanoparticles) coated with Gd2O3 were prepared for MRI and luminescent imaging.19-21 It is widely accepted that the particle size, shape, and dimensionality of nanomaterials significantly affect the chemical and physical properties. Especially, scientists and researchers have paid increasing attention to well-dispersed and uniform luminescent nanomaterials with high brightness and high resolution, due to their wide applications in display and in vivo imaging fields. Because the resolution is closely related to the pixel size, smaller and spherical particles with uniform size are highly desired. In addition, uniform spheres can reduce light scattering, thus they are the charming particle shape in vivo imaging area. However, there are few literatures published on the synthesis of uniformly spherical gallogermanates, along with the influence of their composition on long-persistent luminescence. Accordingly, in this work, we hydrothermally synthesized Zn3Ga2Ge2O10:Cr3+ (ZGGC) monospheres in the presence of citric anions (Cit3-). The morphological regulation and the formation mechanism for the synthesized spheres have been studied. Furthermore, compositional influence on the 4


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crystal structure, bandgap, trap depth, and luminescence characteristics of ZnyGa2Ge2O10-δ:Cr3+ (y = 2.8, 3.0, 3.2) were investigated in details. The prepared NIR-emitting persistent phosphor spheres of ZGGC have a potential application in repeatable in vivo imaging.

2. Experimental 2.1 Synthesis The cation sources were Zn(NO3)3·6H2O, Ga2O3, GeO2, and Cr(NO3)3·9H2O purchased from China pharmaceutical group chemical reagent Co. LTD (99.99% pure, Shanghai, China), and the other reagents were purchased from Shenyang Chemical Reagent Factory (analytical grade, Shenyang, China). Gallium nitrate was prepared by dissolving the oxide in nitric acid (HNO3) via hydrothermal treatment. GeO2 was dissolved

in

dilute

ammonium

hydroxide

solution.

Typically,

Zn3(Ga1.995Cr0.005)Ge2O10 products were hydrothermally synthesized from the mixed cation solutions at a proper temperature in the presence of citric acid monohydrate (Cit3-), according to our previous work.22 Then the dried hydrothermal product was calcined at 800 °C in air for 2 hours. In order to study the influence of synthetic parameters on particle morphology, the molar ratio of Cit3- to total cations [R, R = Cit3-/M, M = total cations], reaction temperature and time, and pH value were widely varied, as shown in Table S1.

2.2 Surface functionalization 100 mg of ZGGC powder was dispersed into 40 mL sodium hydroxide solution (5 mmol/L), followed by 1 h sonication treatment. After stirring at room temperature for 24 h, the large-sized particles were removed through centrifuging the resulting suspension at 1000 rpm for 10 min. Finally, the as-obtained ZGGC-OH particles were collected by centrifuging the supernatant at 10,000 rpm for 10 min, which were then washed three times by deionized water. A mixture containing 10 mg of ZGGC-OH particles and 4 mL of dimethylformamide (DMF) was prepared by 10-min sonication treatment, and then 5



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the suspension was vigorously stirred at room temperature for one day after adding 40 microliter of 3-aminopropyl-triethoxysilane (APTES). The resultant ZGGC-NH2 particles were obtained by high-speed centrifugation at ten thousand rpm for ten minutes. The unreacted APTES was removed by washing with DMF three times.

2.3 Characterization techniques X-ray diffractometry (XRD, Model SmartLab, Rigaku, Tokyo, Japan) was used for phase identification, and the data were measured through operating at 40 kV/40 mA using nickel filtered Cu Kα radiation and a scanning speed of 6.0° 2θ/min. X-ray photoelectron spectroscopy (XPS) data were measured using an X-ray photoelectron spectrometer (Model Axis Supra, Kratos Analytical Ltd., Manchester, UK) with monochromatized Al Kα X-ray radiation. The measurements were performed using an ultrahigh vacuum chamber with a base pressure below 3×10-9 Torr at room temperature. The binding energies were calibrated by using C 1s (284.8 eV) of carbon impurities as reference. Fourier transform infrared (FT-IR) spectra were obtained by the standard KBr method operated on a Nicolet iS5 spectrometer (Thermal Fisher Scientific, USA). Field emission scanning electron microscopy (FE-SEM ， Model JSM-7001F, JEOL, Tokyo) and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo) were employed to observe the morphologies of products. Fluorospectrophotometer (Model FP-8600, Jasco, Tokyo) was employed to analyze the photoluminescence of phosphors, and the persistent luminescence was analyzed with

a

Model

JY

FL3-21

spectrophotometer

(Horiba,

Kyoto,

Japan).

Thermoluminescence (TL) glow curves of the samples were recorded from 293 K (20 oC)

to 450 K (177 oC) with a heating rate of 1 K·s-1 on a spectrometer (Model

FJ-427A TL, Beijing Nuclear Instrument Factory), after exposure to a 254-nm UV light for 5 min at room temperature. Diffuse reflectance spectra of the samples were taken on a UV-Vis-NIR spectrophotometer (UV-3600 Plus, Shimadzu, Kyoto) at room temperature. Laser Scanning Confocal Microscope (LEICA TCS SP2, Germany) was employed to measure the cell images. The afterglow decay images and in vivo imaging were taken by an In-Vivo Imaging System (Kodak FX Pro, USA) in a dark 6


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environment.

2.4 Cytotoxicity assay CCK-8 (Cell Counting Kit-8) assay was employed to analyze the cytotoxicity of ZGGC-NH2. Briefly, cells (RAW264, A549-1 and HEK293T) were seeded into 96-well plates and cultured in a incubator (Thermo, USA) at 37 oC for 24 h with 5% CO2 in the environment. After removing the old medium, new medium (100 µL/well) with different concentrations of ZGGC-NH2 (10, 50, 100, 250, 500, 1000 µg/mL) was added into the culture plates. Six experimental groups and one blank control group were set up. Each group of 3 duplicate wells was cultured in the incubator for 24 h. Subsequently, 10 µL of CCK-8 (10% of the medium volume) was added to each well for another 0.5 h culture. Then, a microplate reader (Thermo, USA) was employed to measure the absorbance at 450 nm.

2.5 Cells imaging HEK293T cells were chosen for cell imaging, and they were cultured in DMEM containing 10% FBS and then seeded in a 35-mm culture dish for 2 h in a CO2 incubator. The cells were cultured with ZGGC-NH2 particles and DAPI dye, respectively, according to our previous work.23 After several-times of washing with PBS, the cells for further characterization were obtained.

2.6 In vivo imaging The ZGGC-NH2 solution (500 μL, 1 mg·mL-1) was injected into the belly of the mice, including a C57BL/6J adult mouse and a BALB/cA-nu adult mouse, through a subcutaneous method after 254-nm UV light irradiation for 10 minutes. Then, the NIR afterglow signals were monitored by a Kodak In-Vivo Imaging System FX Pro. The in vivo imaging was monitored in the absence of an illumination source in the whole process. Additionally, the ZGGC-NH2 solution (500 μL, 1 mg·mL-1) after 254-nm UV light irradiation for 10 minutes was injected into a mouse (BALB/cA-nu adult mouse) through the tail vein.19 Then, the NIR afterglow signals were monitored by a Kodak 7



In-Vivo Imaging System FX Pro. For in vivo recharge imaging, a red LED lamp (6 W) was used as the light source, and the imaging signal was detected just after in situ excitation for 120 s.19 In all of the imaging experiments, the picture was taken with a 45-s exposure time The university animal care and use committee approved the animal experiments and studies.

3. Results and discussion 3.1 Fabrication and characterization of ZGGC monospheres Zn3Ga2Ge2O10:Cr3+ (ZGGC) monospheres can be hydrothermally synthesized in the presence of citric anions (Cit3-). Typically, ZGGC spheres with diameters of 200 ± 20 nm were synthesized via a 18-h hydrothermal reaction at the temperature of 180 °C (Sample S3, Figure 1a). The X-ray diffraction (XRD) patterns allowed for the identification of the obtained spheres along with spinel-structured (cubic) ZGGC (ZnGa2O4, JCPDS No. 71-0843), as shown in Figure 1g. The cell constant a (a = b = c) calculated from the diffraction data is ~0.8335 nm, which is similar to that of cubic ZnGa2O4 (JCPDS No. 71-0843) of a = b = c = 0.8330 nm. In fact, Zn3Ga2Ge2O10 is a solid solution of ZnGa2O4 and Zn2GeO4, which has been mentioned in previously published literature.17,24 The average crystallite size is ~12 nm by profile-broadening analysis of the (311) Bragg reflection according to Scherrer equation. Through observation of the TEM morphology of ZGGC (Figure 1a), it can be concluded that the spheres were aggregates consisting of a number of nanocrystals. The (220), (400), and (440) planes were well resolved using selected-area electron diffraction (SAED) analysis (Figure 1b) and are indicative of the spherical particles having polycrystalline characteristics and high crystallinity. The spheres investigated in this study were direct solid solutions, because elemental mapping results found that all metal elements were evenly distributed among the particles, as shown in Figure 1c-f. The synthesis of nano/micromaterials with ideal morphologies is the pursued goals in inorganic chemistry/material fields, but the morphologies depend on not only the intrinsic structure of the target products but also the growth parameters in a solution reaction 8


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system. Therefore, morphology regulation of the ZGGC spheres was performed and described in the following section.

Figure 1. (a) TEM morphology, (b) SAED pattern, (c-f) elemental mapping, and (g) XRD pattern of ZGGC spheres (S3). (c-f) are the element distributions of Zn, Ga, Ge, Cr, respectively. Controlled experiments indicate Cit3- addition played a critical role in determining the crystal structures and morphologies of the final ZGGC. In the absence of Cit3-, the product was mainly composed of nanoparticles with diameters of < 100 nm (Figure S1b). However, the XRD results indicated that this sample is monoclinic Ga2O3 (JCPDS No. 43-1012, Figure S1a). These XRD patterns are different from those obtained from samples containing Cit3-, as shown in Figure 2d. In addition, Figure 2 9



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presents the field-emission scanning electron microscopy morphologies and XRD patterns of the particles synthesized with the R values ranging from 0.5:1 to 2:1 (Samples S2-S4).The resultant products were clearly identified along with the spinel-structured (cubic) ZGGC (ZnGa2O4, JCPDS No. 71-0843) in the presence of Cit3-. However, small particles composed of some nanospheres were obtained at R = 0.5:1, which then aggregated into larger irregular particles (Figure 2a). As the amount of Cit3- increased (R = 1:1), the product became monospheres with diameters of approximately 200 nm (Figure 2b). As observed in Figure 2c, the larger spheres (diameter of 500 ± 100 nm) partly aggregated due to the high Cit3- content (R = 2:1). In our system, Cit3- initially reacted with Zn2+, Ga3+, Ge4+, and Cr3+ to form Zn2+-Ga3+-Cit3--Ge4+-Cr3+complexes.25

Because

the

complexes

restricted

the

concentration of free cations, Cit3- is closely related to the crystal nucleation and growth of the final products. In addition, Cit3- can affect the growth rate of various crystal facets through absorbing to the crystal surface and act as a binder, thus monospheres were formed from aggregation of nanosized crystals.25 As the Cit3content increased (i.e., the R value got larger), the nucleation density decreased and, thus, the resultant colloidal particles increased in size. However, under a low pH of 5 in the absence of Cit3-, the products containing Zn and Ge would be amphoteric and unable to exist, leading to the solely formation of Ga2O3. The crystal structures and morphologies are also closely related to the duration, temperature, and pH of hydrothermal reaction. There was no precipitate formed at 180 °C and pH = 5 for 6 h (Sample S7). However, extending the reaction time from 6 to 12 h yielded monospheres with a diameter of ~180 nm (Figure 3a and Figure S2a, Sample S8). Larger spheres were observed after a longer reaction duration, with ~200 nm for 18 h (Figure 3b and Figure S2b, Sample S3) and ~350 nm for 24 h (Figure 3c and Figure S2c, Sample S9), indicating particle growth. Similarly, no precipitation was observed at the hydrothermal temperature of 150 °C (Sample S5). However, a higher reaction temperature of 200 °C yielded spheres with a diameter of ~100 nm (Figure 3d, Sample S6), which are smaller than the typical Sample S3 obtained at 180 °C. The above results indicate the original nucleation and growth occurred at temperatures 10


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above 150 °C and reaction durations longer than 6 h. As the hydrothermal temperature increased, the original nucleation density arising from the faster hydrolysis of cations (Zn2+, Ga3+, Ge4+, and Cr3+) increased and, thus, smaller resultant spheres were observed. Although the samples were all spheres, the particle size decreased from ~200 to ~180 nm as the pH increased from 5 to 7. Further increasing the pH to 9 resulted in the appearance of nanoplates, which were identified along with rhombohedral Zn2GeO4 (JCPDS No. 11-0687). Under acidic conditions (pH = 5), the original particles were positively charged, which resulted in more adsorption of Cit3- ions on the particle surface and, thus, yielded larger particles. However, less positive charge led to less adsorption of Cit3- ions on the particle surface at the pH of 7, contributing to the formation of smaller spheres. When the pH reached 9, Cit3- played an invalid role in complexation of cations, and thus the mixture of Zn2GeO4 (rhombohedral) and ZnGa2O4 (cubic) was yielded. After calcining the spheres (Sample S3) at 800 °C for 2 h, XRD patterns in Figure 4e indicated that the resultant products are spinel-structured (cubic) ZGGC (ZnGa2O4, JCPDS No. 71-0843) and TEM images (Figure 4a, d) showed that the morphology of the original particles were maintained. However, smaller particles with a mean diameter of ~180 nm were observed, because water loss and densification from grain growth took place during the calcination. Since higher temperature induced crystallite growth and better crystallinity, sharper diffraction peaks in the XRD patterns, clearer symmetry spots in the SAED patterns, and better resolved lattice fringes in the high-resolution transmission electron microscopy images were observed for Sample S3 calcined at 800 ºC (Figure 4b, c). Figure 4f and Figure S3 present the X-ray photoelectron spectroscopy (XPS) survey spectra and detailed XPS spectra of Sample S3 calcined at 800 ºC. No other elements were detected in addition to the original components and contaminating carbon. The C1s peak at 284.8 eV from carbon contamination was used as a reference. The detailed XPS spectra of Ga 2p, Ge 3d, and Cr 2p core levels were obtained and are presented in Figure S3. In Figure S3b, the peak at 1118.1 eV is assigned to Ga 2p3/2 core-level electrons while the peak at 1144.9 eV is assigned to Ga 2p1/2 ones. Meanwhile, the peaks at 577.1 and 586.6 eV in 11



Figure S3d are assigned to Cr 2p3/2 and Cr 2p1/2 core-level electrons, respectively. The high-resolution spectra of the Ge 3d core levels, shown in Figure S3c, are deconvoluted into the two peaks at ~32.4 and 34.3 eV, indicating two coordination environments are available for Ge in the sample, which is mainly due to the inverted spinel structure.13

Figure 2. (a-c)

FE-SEM morphology and (d) XRD patterns of ZGGC products, with

various molar ratios R of Cit3-:M : (a) 0.5:1 (S2, i in (d)), (b) 1:1 (S3, ii in (d)), (c) 2:1 (S4, iii in (d)). The scale bar is 850 nm.

12


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Figure 3. FE-SEM morphology of the hydrothermal products, with (a-c) synthesized at 180 oC and pH = 5 for 12 h (S8), 18 h (S3) and 24 h (S9), (d) synthesized at 200 oC and pH = 5 for 18 h (S6), (e, f) synthesized at 180 oC for 18 h with pH = 7 (S10) and pH = 9 (S11). (g) are XRD patterns of the products from (b), (e), and (f). The scale bar in (a-f) is 850 nm.

13



O 1s

Ga 2p3/2

Zn 2p3/2

Cr 2p1/2 Cr 2p3/2

400000

Ga 2p1/2

500000

Zn 2p1/2

(f) 600000

300000 200000

C 1s

100000

Ge 3d

Intensity/(cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 1200

1000

800

600

400

200

Binding Energy/(eV)

Figure 4. (a, d) TEM, (b) SAED pattern, (c) HR-TEM images, (e) XRD pattern, and (f) XPS survey spectra of ZGGC spheres calcined at 800 oC (S3).

3.2 Persistent-luminescence properties of ZGGC monospheres Figure 5a displays the excitation and emission spectra of the ZGGC spheres (Sample S3, calcined at 800 ºC) at room temperature. There are three main excitation bands in the excitation spectrum monitored at 696 nm, which are charge-transfer band of O2--Ga3+ peaking at 244 nm, 4A2 → 4T1 (te2) and 4A2 → 4T1 (t2e) transitions peaking at 410 nm, and 4A2 → 4T2 transition peaking at 563 nm.12,19,20 Because the 563-nm band ranges from 500 to 630 nm, the ZGGC spheres can be excited by a red LED light (Figure S4), which has been reported by other research groups.12,18,20 The electrons of Cr3+ ions can be excited from ground state to energy levels of deep traps by visible-light excitation. Although the excitation energy is below the ionization threshold, the traps can be filled from the energy-matched energy levels of Cr3+ through the tunneling process.1,12,18,20 Therefore, long afterglow can be produced through the reverse tunneling recombination process.1,12,18,20 Because the excitation band at 244 nm exhibited the strongest intensity, the most effective excitation wavelength was the charge-transfer band. Upon UV excitation at 244 nm, the powder sample exhibited a dominating near-infrared (NIR) emission band at 696 nm (2E→4A2 transition of Cr3+), because Cr3+ ions occupy distorted sites.12 The NIR 14


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persistent-luminescence decay curves of the ZGGC spheres were shown in Figure 5b, which were monitored at 696 nm after 5-min illumination with 254-nm UV light at room temperature. It was found that the ZGGC sample could continuously emit intense NIR afterglow longer than 120 minutes, which is in good agreement with the time-dependent persistent luminescence spectra in Figure 5c. The NIR afterglow decay images in Figure 6a presented the direct evidence that the afterglow can last longer than 240 minutes. To evaluate the rechargeability of ZGGC powders, they were second in situ excited after 4 h by a red light (LED lamp, 6W) for 2 minutes. Repeated NIR afterglow signal was detected after excitation and found to last for longer than 3 h (Figure 6b). The above results indicate red LED light can repeatedly recharge ZGGC powders.

Figure 5. (a) PLE/PL spectra, (b) NIR persistent luminescence decay curve (monitored at 696 nm after 5-min illumination with 254-nm UV light), and (c) time-dependent persistent luminescence spectra of ZGGC powders (S3). 15



Figure 6. (a) NIR afterglow decay images of ZGGC powders (S3) obtained after stoppage of UV irradiation at 0-4 h. (b) is recharging NIR afterglow decay images obtained after stoppage of secondary excitation with a red light from LED lamp. Because zinc content is closely related to persistent-luminescence, a series of spinel-structured ZGGC spheres of ZnyGa2Ge2O10-δ:Cr3+ (y=2.8, 3.0, and 3.2) were generated in this work. Figure S5 shows the photoluminescence excitation (PLE)/photoluminescence (PL) spectra obtained for ZGGC spheres with different Zn contents. Obviously, the positions and shapes of emission peaks were independent on the Zn content. In the PLE spectra, however, it is interesting to find that the 410-nm transition (4A2 → 4T1) shifts to the longer wavelength side while the 563-nm transition (4A2 → 4T2) keeps at the same position by varying the Zn content from 2.8 to 3.2. In addition, stronger transition of 4A2 → 4T2 (t2e) (monitoring the 563-nm peak) and weaker transition of 4A2 → 4T1 (monitoring the 410-nm peak) were observed with the y value increasing from 2.8 to 3.2, thus leading to I563/I410 increase from 0.86 to 1.54. Actually, the overlap of 4A2 → 4T1 (te2) (at shorter wavelength side) transition and 4A2 → 4T1 (t2e) (at longer wavelength side) transition of Cr3+ yields the broad band peaking at 410 nm. The weakened 4A2 → 4T1 (te2) transition at a higher Zn content (y value), which is closely related to the conduction band minimum (CBM), contributes to the red-shift of the 410-nm peak. Figure 7d shows afterglow intensity and duration of ZnyGa2Ge2O10-δ:Cr3+ (y = 2.8, 3.0, and 3.2). Increasing Zn2+ content from 2.8 to 3.0 led to enhanced afterglow intensity, but weakened one was observed by further increasing Zn2+ content from 3.0 to 3.2. Estimations of bandgap energies for the 16


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samples were made from the absorption spectra, which were obtained via UV-vis absorption spectroscopy.26 The relationship between the absorption coefficient (α) and incident photon energy (hν) can be written as α = Bd(hν-Eg)1/2, where Bd is the absorption constant and Eg is the bandgap energy.26 Plots of (Ahυ)2 versus hυ based on the spectral data are presented in Figure 7a. Extrapolating the linear parts of the curves for the three samples yielded bandgaps of 4.95 (y = 2.8), 4.80 (y = 3.0), and 4.43 eV (y = 3.2). The valence-band XPS spectra of the samples were measured as shown in Figure 7b, with the similar edges of the maximum energy being at about 2.73, 2.89, and 2.82 eV for the y=2.8, 3.0, and 3.2 samples, respectively.27 Based on the values of bandgap and valence-band maximum (VBM), the calculated-conduction band minimum (CBM) is about -2.22, -1.91, and -1.61 eV for the y=2.8, 3.0, and 3.2 samples, respectively. Combined with the above results, a schematic of the energy level diagram was constructed, as shown in Figure 7e, which suggests that: (1) a narrower bandgap was resulted from a higher Zn content, (2) a higher Zn content led to an downshift in the conduction band minimum (CBM), and (3) the valence-band maximum (VBM) was not significantly affected by the Zn content. Based on the above results, a possible mechanism for the red-shift of the 410-nm peak in Figure S5 was proposed. At y = 2.8, the CBM is above the excited state of 4T1 (te2), while the 4T 1

(te2) state is near the CBM or just within conduction band at the y value of 3.0.

Further increasing the y to 3.2 causes 4T1 (te2) to completely submerge into the conduction band. The downshift of CBM and thus the swallowed 4T1 (te2) state contribute to the red-shift of the 410-nm peak (Figure S4). Figure 7c shows thermoluminescence (TL) glow curves of the Cr3+ emissions in the three samples. Only one main peak ranging from 293 (20 °C) to 450 K (177 °C) was observed. As the Zn concentration increased, peaks with maxima at 353 K for y = 2.8, 359 K for y = 3.0, and 349 K for y = 3.2 were observed in the Figure 7c. The following equation was used to estimate the electron-trap depths observed from the TL curves:24 E = Tm/500

(1)

where Tm is the temperature of peak maximum in TL curves. Therefore, the electron-trap depths calculated from the TL peaks were roughly being 0.71, 0.72, and 17



0.70 eV for y=2.8, 3.0, and 3.2, respectively. Because deep traps are difficult to be emptied at room temperature, a good afterglow would be observed for the samples. Therefore, the most intense persistent luminescence was obtained at y=3.0, owing to its deepest defects. (a)

(b)

y=2.8

y=2.8

2.73eV

y=3.0

2.89eV

Intensity/(a.u.)

(Ah



4.95eV

y=3.0 4.80eV

y=3.2 y=3.2

2.82eV

4.43eV

2

3

4

5

6

25

20

heV

(c)

15

10

5

0

Binding Energy/(eV)

(d)

349 353 359

y=2.8 y=3.0 y=3.2

Background y=2.8 y=3.0 y=3.2

Intensity/(a.u.)

1000000

Intensity/(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

100000 10000 1000 100

300

325

350

375

400

425

10

450

0

2000

Temperature/(K)

4000

t/(s)

18


6000

8000

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Figure 7. (a) Determination of bandgap energies, (b) Valence-band XPS spectra, (c) TL

glow

curves,

(d)

NIR

persistent

luminescence

decay

curves

of

ZnyGa2Ge2O10-δ:Cr3+ (y = 2.8, 3.0, 3.2), and (e) schematic illustration of energy levels according to the data from (a-c). To get better water-solubility and biocompatibility, the ZGGC spheres were treated using a two-step processing (Figure 8).18 Through treatment with sodium hydroxide, the surfaces of ZGGC spheres adsorbed hydroxyl groups (Figure S6), and the final formation of ZGGC-NH2 was achieved by linking the two groups of amino and hydroxyl (Figure S6). After functionalization, a luminescence solution was easily obtained by dispersing the ZGGC-NH2 probes in water. Figure 8 showed the luminescence spectra of the ZGGC-NH2 aqueous dispersion, which exhibited PLE and PL curves similar to the ZGGC powder. However, the quenching effect of water resulted in the weakened intensity of luminescence, which has been observed by other research groups.18,19 The above results indicate the ZGGC-NH2 solution has a potential application in in vivo imaging.

19



(b)

350

2

E→4A2

O2-→Ga3+

Intensity/(a.u.)

300 250

ex =238 nm

200

em=696 nm

150 100 50 0 -50 200

300

400

500

600

700

800

Wavelength/(nm)

Figure 8. (a) Schematic illustration showing the surface modification of ZGGC particles and (b) PL excitation and emission spectra of aqueous solution at room temperature. The digital photo in (a) is the appearance of ZGGC aqueous solution under the exposure to a UV light of 254-nm wavelength.

3.3 Cytotoxicity assay and cell imaging RAW264

A549-1

HEK293T

100

Cell viability/(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

80

60

40

20

0 0

10

50

100

250

500

1000

Concentration/(mg/L)

Figure 9. Cytotoxicity of ZGGC-NH2 performed by the CCK-8 (Cell Counting Kit-8) assay with RAW264, A549-1, and HEK293T cells. 20


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Assessment of the toxicity of ZGGC-NH2 is critical for future clinical applications, mainly due to that the amount of Cr3+ ions exceeding a concentration threshold is toxic to humans. Therefore, Cell Counting Kit-8 assay was performed on RAW264, A549-1, and HEK293T cells to assess the cytotoxicity of ZGGC-NH2 (Figure 9). Three different cells cultured with the medium containing 10-1000 µg/mL of ZGGC-NH2 for 24 h suggested no obvious toxicity in the test. The cell viability retained approximately above 88.3, 86.9, and 87.8% for RAW264, A549-1, and HEK293T cells, with the ZGGC-NH2 concentration up to 1000 µg/mL, further confirming the low toxicity of ZGGC-NH2 to the cells. Before in vivo imaging, HEK293T cells were chosen for in vitro imaging. As shown in Figure 10a, the outline of HEK293T cells cultured with ZGGC-NH2 can be precisely detected after 1-hour stoppage of excitation, owing to the intense afterglow signal from ZGGC-NH2 in the cells, although signal loss took place over time. The same cells stained with 0.5 mL DAPI dye were employed for comparison (Figure 10b, simultaneous excitation), but the signals are obviously weaker than that in Figure 10a. The stronger imaging afterglow signals suggest that ZGGC-NH2 has excellent potential for imaging cells in vitro in the absence of an excitation source.

Figure 10. (a) LSCM image of HEK293T cells cultured with ZGGC-NH2 after 1-hour stoppage of excitation and (b) appearance of the same cells stained with DAPI dye for reference (simultaneous excitation).

21



3.4 In vivo luminescence imaging

Figure 11. In vivo NIR afterglow images of a black mouse (C57BL/6J adult mouse) after subcutaneous injection with ZGGC-NH2 solution (500 μL, 1 mg·mL-1, 10-min exposure to a 254-nm UV light before injection).

Figure 12. (a) In vivo NIR afterglow images of a nude mouse (BALB/cA-nu adult mouse) after subcutaneous injection with ZGGC-NH2 solution (500 μL, 1 mg·mL-1, 10-min exposure to a 254-nm UV light before injection) and (b) In vivo recharging NIR afterglow decay images after 2-min secondary excitation with a red LED lamp (6 W) .

22


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Figure 13. (a) In vivo NIR afterglow images of a nude mouse (BALB/c adult mouse) after intravenous injection of ZGGC-NH2 and (b) Ex vivo NIR luminescence pictures of internal organs from the same mouse after 24 hours To estimate the potential of ZGGC-NH2 for use in bio-imaging as a luminescent probe, a live mouse was chosen for imaging study. After 10-min excitation with 254-nm UV light, 500 μL of the prepared ZGGC-NH2 aqueous solution ( 1 mg/mL) was injected into an adult C57BL/6J mouse by subcutaneous. NIR afterglow signals could be detected throughout the mouse in the absence of excitation light source (Figure 11), although signal loss took place over time. Very weak signals were detected after 30 min, mainly due to the melanin and black wool. Next, 500 μL of the pre-excited ZGGC-NH2 aqueous solution with the concentration of 1 mg/mL was subcutaneously injected into a BALB/Ca nude mouse and the afterglow luminescence signals were clearly stronger than those in the black mouse. The afterglow luminescence signals lasted for at least 60 min (Figure 12a). The above results indicate that ZGGC-NH2 is suitable for in vivo imaging, because it can emit a bright NIR light in the bio-imaging window without producing backdrop noise from in situ excitation. To assess the in vivo rechargeability of ZGGC-NH2, a second 2-min in situ excitation by a red light from LED lamp (6 W) was performed on the whole body of the same mouse after 1 hour.

Repeated NIR afterglow signal could be detected after

the red-light excitation, and the signal could last for longer than 10 min (Figure 12b). 23



Page 24 of 30

The above result indicates ZGGC-NH2 can be efficiently recharged by red LED light. Furthermore, a live mouse (BALB/Ca-nu mouse) was intravenously injected by ZGGC-NH2, which was then used for bio imaging. It could be easily to detect the NIR afterglow signals throughout the body of the nude mouse in the absence of excitation source. In order to study the relatively long-term bio-distribution of ZGGC-NH2 in live mouse, ex vivo NIR luminescence imaging was performed on the harvested organs from the same mouse. Through carful observation, no obvious luminescence signal could be detected in the heart, kidney, and stomach (Figure 13b). However, the organs of lungs, liver, and spleen exhibited intense signals, which are in good agreement with in vivo imaging on the whole body (Figure 13). The enrichment of ZGGC-NH2 in the lungs, liver, and spleen indicates ZGGC-NH2 is in a similar manner to other materials, such as noble metal particles,28 luminescent porous silicon nanoparticles,18,29

and

polyethylene

glycol-grafted

nanoparticles

for

NIR

emission,16,17,30 because reticulo endothelial system organs would take up the larger nanoparticles of hydrodynamic size exceeding one hundred nanometer.31-33

4. Conclusion In summary, spinel-structured (cubic) monospheres of Zn3Ga2Ge2O10:Cr3+ (ZGGC) were hydrothermally synthesized in the presence of citric anions (Cit3-), which emitted a dominated NIR emission at 696 nm (2E→4A2 transition of Cr3+ ions in distorted sites) and exhibited excellent NIR persistent luminescence properties with rechargeableability. The morphology of ZGGC products is significantly affected by synthetic parameters, including the dosage of Cit3-, pH value, reaction time and temperature. In addition, the influence of insufficient and excessive Zn2+ ions on the crystal structure, bandgap, trap depth, and persistent luminescence characteristics of ZGGC

has

been

presented.

The

ZGGC-NH2

prepared

through

surface

functionalization of ZGGC spheres showed distinguished long afterglow with efficient rechargeableability, low toxicity, and can be used as an NIR persistent-luminescence probe for applications in in vitro cell imaging and long-term in vivo bio imaging. 24


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ASSOCIATED CONTENT Supporting Information. The hydrothermal conditions for ZGGC synthesis; XRD pattern and FE-SEM image of sample S1; XRD patterns of the hydrothermal products; XPS detail spectra of ZGGC spheres calcined at 800 oC; PLE spectra of ZGGC powders (S3) and PLE/PL spectra of ZnyGa2Ge2O10-δ:Cr3+ (y=2.8, 3.0, 3.2); FT-IR spectra of ZGGC, ZGGC-OH, and ZGGC-NH2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Q. Z.). *E-mail: [email protected] (J.-G. L.). ORCID Qi Zhu: 0000-0001-5513-6309 Ji-Guang Li: 0000-0002-5625-7361

Author Contributions Q. Z. and J.-G. L. conceived the project; J. Q. X. H., Y. G. and H. L. L. carried out the experiments and data analysis; Q. Z. and J. Q. X. H. drafted the manuscript. All the authors were involved in the results discussion, and have read and approved the final manuscript. Notes The authors declare no competing financial interest. Acknowledgements This work was supported in part by the Fundamental Research Funds for the Central Universities (Grants N172002001 and N160204008) and the National Natural Science

25



Page 26 of 30

Foundation of China (Grant 51672039). Q. Zhu acknowledges the financial support from the China Scholarship Council (Grant 201806085026).

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Graphical Abstract

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Trap Depth, and In Vivo

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