Enhanced near infrared persistent luminescence of Zn₂Ga_2.98Ge_0.75O₈:Cr_0.02³⁺ nanoparticles by partial substitution of Ge⁴⁺ by Sn⁴⁺

Abstract

Spinel-phase Zn₂Ga_2.98Ge_0.75–xSn_xO₈:Cr_0.02³⁺ (ZGGSO:Cr³⁺) nanoparticles with various Sn⁴⁺ concentrations were prepared by a hydrothermal method in combination with a post-annealing in vacuum at high temperature. For these nanoparticles, the observed near infrared (NIR) persistent luminescence peaked at ∼697 nm and originates from the ²E, ⁴T₂ (⁴F) → ⁴A₂ transitions of Cr³⁺ and the afterglow time exceeds 800 min. For both the interior and surface Cr³⁺ ions in the ZGGSO host, it can be found that the increased energy transfer from Cr³⁺ to the deep trap (anti-site defects, ) after the substitution of Ge⁴⁺ by Sn⁴⁺ plays a key role in enhancing the persistent luminescence of the ZGGSO:Cr³⁺ nanoparticles. Strikingly, this energy transfer process can be controlled through the variations in the crystal field strength and the trap depths. Our results suggest that not only Sn⁴⁺ substitution can improve in vivo bioimaging but also the existence of deep traps in ZGGSO:Cr³⁺ nanoparticles is helpful for retracing in vivo bioimaging at any time.

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Introduction

Nowadays, the materials and techniques related to in vivo bioimaging have attracted increasing attention because of their unique advantages, such as high sensitivity, contrast, resolution and in situ imaging during detection and therapy.^1–8 Among them, the strategy adopting near infrared (NIR) persistent luminescence nanoparticles (PLNPs) is one of the most promising techniques because it does not need external excitation and possesses high penetrability during bioimaging.^9–19 In particular, Cr³⁺ doped zinc gallate (ZGO:Cr³⁺) based PLNPs have been employed because its 3d³ electron configuration permits a narrow-band emission around 700 nm, which falls in the tissue optical window (650–1000 nm).^20–28

To understand how to enhance the persistent luminescence of ZGO:Cr³⁺, many efforts have been made by researchers. For example, Bessière et al. believed that, for Cr³⁺ doped ZnGa₂O₄ (ZGO:Cr³⁺) powders with spinel structure, the NIR persistent emissions at ∼695 nm originate from the distorted octahedral Cr³⁺ ions surrounded by anti-site pairs.^29–31 In particular, Pan et al. found that the NIR afterglow time of Cr³⁺ doped zinc gallogermanate (Zn₃Ga₂Ge₂O₁₀:0:5% Cr³⁺) ceramics can be reached to more than 360 hours through the substitution of Ga³⁺ by Ge⁴⁺ due to the increased number of anti-sites defects.^32,33 Our previous study found that, for Cr³⁺ doped zinc gallogermanate (Zn₂Ga_2.98−xGe_0.75O₈:Cr_0.020, Pr_x) nanoparticles, the substitution of Ga³⁺ by Pr³⁺ can simultaneously decrease the particle size and enhance the NIR persistent luminescence.³⁴ The related mechanism can be attributed to the generated anti-site defects and the suppressed energy transfer of the excited electrons from the ⁴T₂ level to the deep trap level related to anti-site pairs around the distorted octahedral Cr³⁺ ions.^31,34–37 However, for Cr³⁺ doped zinc gallogermanate nanoparticles, the effects of the variations in the crystal filed strength around Cr³⁺ ions and the depths of deep trap levels related to anti-sites on influencing the energy transfer from Cr³⁺ to the deep trap level still remain unclear.

Because cation substitution is an efficient approach to vary the crystal filed environments around luminescent centers, such as the bond length and crystal field strength, the substitution of Ge⁴⁺ by Sn⁴⁺ was employed in Cr³⁺ doped zinc gallogermanate nanoparticles.³⁸ Here, the compositions of the samples with various Sn⁴⁺ concentrations (x) can be represented as Zn₂Ga_2.98Ge_0.75−xSn_xO₈:Cr_0.020³⁺. Furthermore, it is also essential to understand how Sn⁴⁺ substitution and the deep trap affect the bioimaging applications. Thus, this study can give the comprehension of how to design new Cr³⁺ doped zinc gallogermanate nanoparticles for in vivo bioimaging.

In this work, spinel-phase ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations were prepared by a hydrothermal method in combination with a post-annealing in vacuum at 800 °C. The effects of Sn⁴⁺ substitution on bond lengths, crystal filed strength, trap depth and the energy transfer from Cr³⁺ to the deep traps were studied in detail. For these nanoparticles, the observed near infrared (NIR) persistent luminescence peaked at ∼697 nm originates from the ²E, ⁴T₂ (⁴F) → ⁴A₂ transitions of Cr³⁺ and the afterglow time exceeds 800 min. For both the interior and surface Cr³⁺ ions in the ZGGSO host, it can be found that the increased energy transfer from Cr³⁺ to the deep trap (anti-site defects, ) after the substitution of Ge⁴⁺ by Sn⁴⁺ plays a key role in enhancing the persistent luminescence of the ZGGSO:Cr³⁺ nanoparticles. Meanwhile, this energy transfer process can be controlled through the variations in the crystal field strength and the trap depths. Strikingly, our results suggest that an 808 nm stimulation is an efficient way to retrace in vivo bioimaging in the case that the persistent luminescence of NIR ZGGSO:Cr³⁺ nanoparticles is undetectable.

Synthesis and characterization

Zn₂Ga_2.98Ge_0.75−xSn_xO₈:Cr_0.02³⁺ (ZGGSO:Cr³⁺) nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063, 0.125 and 0.250) were prepared by a hydrothermal method in combination with a post-annealing in vacuum at high temperature. Gallium(III) nitrate hydrate (Ga(NO₃)₃·xH₂O, 99.9%) (Sigma-Aldrich), zinc acetate (Zn(CH₃COO)₂, 99.99%) (Aladdin), chromium(III) nitrate nonahydrate (Cr(NO₃)₃·9H₂O, 99.95%) (Aladdin), tin(IV) chloride (SnCl₄, 99.99%) (Alfa Aesar) and germanium(IV) chloride (GeCl₄, 99.9999%) (Alfa Aesar) were used as starting materials. They were dissolved in deionized water according to the proposed metal mole ratio. Then the solution was transferred into a Teflon-line steel autoclave with a capacity of 40 ml and heated at 180 °C for 4 h in a drying oven. The generated precipitates were washed with deionized water three times and then dried at 80 °C for 6 h to obtain as-prepared ZGGSO:Cr³⁺ nanoparticles. The final nanoparticles were acquired by annealing the as-prepared nanoparticles at 800 °C in vacuum (pressure: 2.3 × 10⁻³ Pa) for 1.5 h.

The crystal structure of the samples was examined by a Rigaku D/MAX-2500 X-ray diffraction (XRD) spectrometer using Cu Kα radiation (0.15418 nm). The morphologies and compositions of the samples were characterized by means of a transmission electron microscope (TEM, JEOL-2100F). The Raman spectra were recorded by using a LabRAM HR Evolution Raman spectrometer (HORIBA) with a 488 nm excitation. The Rietveld refinements of XRD patterns were performed by GSAS program. The emission and excitation spectra, thermoluminescence (TL) and photostimulated luminescence (PSL) were recorded using a SHIMADZU RF-5301pc spectrophotometer equipped with a R928 photomultiplier tube. Especially, the emission spectrum at 80 K was obtained by using the above spectrophotometer in combination with INSTEC LN2-SYS Liquid Nitrogen Cooling Systems. The heating rate was 5 K min⁻¹ and an 808 nm laser was used as a stimulation source. The afterglow decay curves were recorded by using a Zolix Omin λ-300 spectrophotometer. The measurements related to the luminescence kinetics of the samples were taken by an equipment combined with a LeCroy digital oscilloscope (62MXs-B), a Zolix Omin λ-300 spectrophotometer, an INNOLAS laser (SpitLight 600) and an INNOLAS optical parametric oscillator. X-ray photoelectron energy spectra (XPS) were obtained by using a Thermo ESCALAB 250 electron energy spectrometer with Al Kα radiation (1486.6 eV). All tests were performed at room temperature (RT) except for the TL measurements and part of emission spectra.

Results and discussion

Fig. 1a shows XRD patterns of Zn₂Ga_2.98Ge_0.75−xSn_xO₈:Cr_0.02³⁺ (ZGGSO:Cr³⁺) powders with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063, 0.125 and 0.250). For the samples with Sn⁴⁺ substitution concentrations ranging from 0 to 0.125, the diffraction peaks correspond well to those of the standard ZnGa₂O₄ sample (JCPDS: 38-1240). This result suggests that single phase Zn₂Ga_2.98Ge_0.75−xSn_xO₈:Cr_0.02³⁺ with spinel structure can be obtained. It can be explained by the replacement of Ga³⁺ by Ge⁴⁺, Sn⁴⁺ and Cr³⁺ ions due to the fact that their ionic radii, Ge⁴⁺(0.54 Å, CN = 6), Cr³⁺(0.615 Å, CN = 6) and Sn⁴⁺(0.69 Å, CN = 6), are similar to that of Ga³⁺(0.62 Å, CN = 6).^39,40 Meanwhile, as the Sn⁴⁺ substitution concentration reaches to 0.250, additional XRD peaks related to impurity phases, such as Ga₂O₃, ZnO, GeO₂ and SnO₂ appear. It can be concluded that the maximum value of Sn⁴⁺ substitution concentration is lower than 0.250. In addition, for each sample with a certain Sn⁴⁺ concentration, the intensity ratio of (222) diffraction peak to (311) peak (I(222)/I(311)) was calculated and shown in Table 1. It can be found that the intensity ratios of the samples are in the range of 0.142–0.155, which is closer to that of ZnGa₂O₄ with normal spinel structure (0.13), but smaller than that of Zn₂GeO₄ with inversed spinel structure (0.20). This result indicates that the obtained samples maintain a normal spinel structure like ZnGa₂O₄.⁴⁰ In particular, a slight shift of the (311) peak to lower angles with an increase of Sn⁴⁺ substitution concentration can be observed, as shown in Fig. 1b, further proving that the crystal lattice spacing is increased due to the substitution of Ge⁴⁺ by Sn⁴⁺.

Fig. 1 (a) XRD patterns of ZGGSO:Cr³⁺ powders with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063, 0.125 and 0.250). (b) The (311) peak position as a function of Sn⁴⁺ substitution concentration.

Table 1 The intensity ratio of (222) diffraction peak to (311) peak (I(222)/I(311)) and the size distribution of the ZGGSO:Cr³⁺ powders as a function of Sn⁴⁺ substitution concentration

Sample	I(222)/I(311)	Mean (nm)	Standard deviation (nm)
x = 0	0.144	37.7	6.4
x = 0.015	0.147	38.2	6.0
x = 0.030	0.151	38.0	5.9
x = 0.063	0.142	36.9	5.6
x = 0.125	0.147	37.5	7.5
x = 0.250	0.148	37.4	6.8

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To confirm the influence of Sn⁴⁺ substitution on the particle sizes of the samples, TEM images and EDS of ZGGSO:Cr³⁺ samples with various Sn⁴⁺ substitution concentrations (x = 0, 0.015, 0.030, 0.063, 0.125 and 0.250) are shown in Fig. 2a and b. It can be found that the average particle sizes of these samples are in the range 37.4–38.2 nm, as listed in Table 1. For all the samples, EDS signals related to Zn, Ga, Ge and O atoms in the ZGGO host can be observed. The signals related to Sn atom can be detected when the substitution concentration is equal to or larger than 0.063. The aforementioned results suggest that Sn⁴⁺ substitution has little influence on their particle sizes.

Fig. 2 (a) TEM images and (b) EDS of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063, 0.125 and 0.250).

To further unravel the effect of Sn⁴⁺ substitution on the microstructure, the Rietveld refinements of XRD patterns for Zn₂Ga_2.98Ge_0.75−xSn_xO₈:Cr_0.02³⁺ nanoparticles with various Sn⁴⁺ substitution concentrations ranging from 0 to 0.125 are shown in Fig. 3. For each of the samples, the obtained goodness of fitting parameters, R_wp, R_p and χ², presented in Table 2, can ensure the reliability of the data fitting. Fig. 3b shows the calculated lattice constant, cell volume and Zn–O, Ga–O and Cr–O bond lengths as a function of Sn⁴⁺ substitution concentration. It can be found that these parameters increase with an increase of Sn⁴⁺ substitution concentrations because the ionic radius (0.62 Å) of Sn⁴⁺ is larger than that of Ge⁴⁺ (0.54 Å). In particular, the lattice constants and unit cell volumes show a slower increase as the Sn⁴⁺ substitution concentration is larger than 0.030. This suggests that it is difficult for Sn⁴⁺ ions substituting for interior Ge⁴⁺ ions and more Sn⁴⁺ ions appear on the surface of nanoparticles when x is larger than 0.030. This result is consistent with the XRD data, showing that the diffraction peaks related to SnO₂ appear when x is equal to 0.250.

Fig. 3 (a) The Rietveld refinements of XRD data for ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063, 0.125 and 0.250). (b) The calculated lattice constant, cell volume and Zn–O, Ga–O and Cr–O bond lengths as a function of Sn⁴⁺ substitution concentration.

Table 2 The related fitting parameters for the Rietveld refinements of the XRD data as a function of Sn⁴⁺ substitution concentration

Sample	x = 0	x = 0.015	x = 0.030	x = 0.063	x = 0.125
Rwp	10.14%	9.41%	10.68%	9.61%	10.93%
Rp	7.09%	7.01%	7.92%	6.95%	7.64%
a = b = c (nm)	8.339	8.347	8.34	8.351	8.361
α = β = γ (°)	90	90	90	90	90
Volume (nm^3)	579.941	581.547	581.940	582.557	584.477

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To illuminate the change in the microstructure induced by Sn⁴⁺ substitution, Fig. 4a and b show a cell of ZnGa₂O₄ with a spinel structure (AB₂X₄ formula) and Raman spectra of ZGGSO:Cr³⁺ nanoparticles with different Sn⁴⁺ substitution concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125), respectively. In Fig. 4a, it can be found that a cell of ZGO contains a number of ZnO₄ tetrahedrons and GaO₆ octahedrons, which are centered by Zn²⁺ (occupying the A sites) and Ga³⁺ (occupying the B sites), respectively. On the basis of the study of Gorkom et al., the allowed Raman modes, A_1g, E_g and T_2g, originating from the symmetric stretching vibrations of ZnO₄ groups can be found.⁴¹ Therefore, in Fig. 4b, for each of the samples, it is reasonable to detect the obvious Raman peaks at around 720, 630 and 610 cm⁻¹, which are attributed to the A_1g, E_g and T_2g modes related to the Zn–O tetrahedron vibrations in the ZGGSO host, respectively.^34,42 This result further suggests that the ZGGSO:Cr³⁺ nanoparticles with normal spinel structure were obtained. In particular, the Raman peaks related to the A_1g mode show an obvious shift to lower wavenumbers with an increase of Sn⁴⁺ substitution concentration. This phenomenon might be explained by the increased Zn–O bond length due to partial substitution of Ge⁴⁺ by Sn⁴⁺. For the Y₂O₃-stabilized ZrO₂ system, the similar result was reported by Hemberger et al., who believed that the Raman band shift of the Zr–O stretching vibrations depends on the environments around the participating ions, such as Zr⁴⁺ ions, and the Raman band shift to lower frequencies is due to the increased Zr–O bond length with increasing Y₂O₃.⁴³ Because Sn⁴⁺ substitution also leads to the increased Cr–O and Ga–O bond lengths, it can be deduced that Sn⁴⁺ substitution might induce not only the change in the environments around Zn²⁺ ions, but also those around Cr³⁺ and Ga³⁺ ions.

Fig. 4 (a) A cell of ZnGa₂O₄ with a spinel structure (AB₂X₄ formula). (b) Raman spectra of ZGGSO:Cr³⁺ nanoparticles with different Sn⁴⁺ substitution concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125).

To confirm the variation in the environments around Cr³⁺ ions induced by Sn⁴⁺ substitution, excitation and emission spectra of ZGGSO:Cr³⁺ with various Sn⁴⁺ substitution concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125) are shown in Fig. 5a and b, respectively. For each of all the samples, it can be observed that each excitation spectrum contains several of absorption peaks ranging from 220 to 660 nm when the monitored wavelength is 697 nm. The obvious excitation peaks at around 250, 440 and 592 nm can be attributed to the charge transfer from O²⁻ to Cr³⁺, the ⁴A₂ → ⁴T₁(⁴F) and ⁴A₂ → ⁴T₂ (⁴F) transitions of Cr³⁺ ions, respectively. From room temperature emission spectra upon 592 nm excitation, it can be observed that there exist two emission peaks at 697 and 712 nm, respectively. On the basis of the previous study, they might be related to the transitions from the ²E and ⁴T₂ (⁴F) states to the ⁴A₂ state, respectively.^26,27,44,45 To better discern the NIR emissions from the ²E and ⁴T₂ energy levels of Cr³⁺ ions doped in ZGGO nanoparticles, emission spectra at RT and 80 K are shown in the inset of Fig. 5b. For RT emission spectrum, it can be observed that there exists a narrow emission at ∼697 nm superimposed on a broad emission band at ∼712 nm. At 80 K, the contribution of the narrow emission to the emission spectrum becomes more dominating compared to the broad emission band. The above result is in agreement with the study reported by Chen et al., who believed that the narrow emission at ∼697 nm and the broad emission band at ∼712 nm originated from the E(²G) → ⁴A₂ and ⁴T₂ (⁴F) → ⁴A₂ transitions, respectively.⁴⁶ In particular, Chen et al. found that the appearance of a narrow emission band superimposed on the broad emission band suggested that the Cr³⁺ ions were located in an intermediate crystal-field environment and the crystal field strength is close to the crossing between the ⁴T₂ (⁴F) and ²E levels as shown in Fig. 5c.^46,47 To further prove the above conclusion, Fig. 5d shows normalized emission spectra recorded at RT of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations upon 592 nm excitation. From the enlarged part, it can be found that the intensity ratio of the 712 nm emission peak to the 697 nm emission peak increases with increasing Sn⁴⁺ substitution concentration. Our result is similar to that reported by Chen et al., who found that, for Cr³⁺ doped Bi₂(Ga/Al)₄O₉, the substitution of Al³⁺ for Ga³⁺ could lead to the decreased ratio of broadband emission to the narrow emission and the stronger crystal field strength around Cr³⁺ ions.⁴⁸ In our case, it can be deduced that Sn⁴⁺ substitution will give rise to the weakened crystal field strength around Cr³⁺ ions. Furthermore, on the basis of the study reported by Hermus et al., the relationship between the crystal field strength (D_q) around the luminescence center and the distance between the central atom (Cr³⁺) and the ligand (O²⁻) (R) can be expressed in the following equation:⁴⁹

where Z and e are the charges of the anion and electron, respectively, and r is the radius of the 3d wave functions. It can be found that the increased R value, namely, the increased Cr–O bond length, induced by Sn⁴⁺ substitution, gives rise to the weakened crystal filed strength, D_q. Thus, for ZGGSO:Cr³⁺ nanoparticles, it is possible that Sn⁴⁺ substitution will lead to the weakened strength of crystal field around Cr³⁺ ions and the enhanced transition from the ⁴T₂ state to the ⁴A₂ state (emission located at 712 nm).

Fig. 5 (a) Excitation (λ_em = 697 nm) and (b) emission (λ_em = 592 nm) spectra at RT of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ substitution concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125). The inset gives the emission spectra of ZGGSO:Cr³⁺ without Sn⁴⁺ at RT and 80 K, respectively. (c) Tanabe–Sugano diagram for Cr³⁺ ion in an octahedral site. The position of the intermediate crystal field strength (D_q/B = ∼2.3) is marked by the red solid line. (d) The normalized emission spectra recorded at RT. The inset gives the enlarged region.

To understand how Sn⁴⁺ substitution influences the persistent luminescence of nanoparticles, afterglow decay curves of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ substitution concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125) monitored at 697 nm are shown in Fig. 6. For each of the samples, it can be found that the afterglow time exceeds 800 min (>13 h). This result is consistent with the fact that the obvious phosphorescence at 812 min after the stoppage of 254 nm irradiation can be detected as shown in the inset of Fig. 6. In particular, the afterglow intensities show an initial rise and a subsequent decrease with increasing Sn⁴⁺ substitution concentration from 0 to 0.125. The strongest afterglow intensity of the ZGGSO:Cr³⁺ nanoparticles with x = 0.015 can be obtained and its intensity recorded at 60 min after stopping 254 nm irradiation is ∼1.7 times as strong as that of the ZGGO:Cr³⁺ nanoparticles (x = 0). This result suggests that Sn⁴⁺ substitution can enhance the afterglow intensity of ZGGO:Cr³⁺ nanoparticles. According to our previous study on Pr³⁺ doped ZGGO:Cr³⁺ nanoparticles,^50,51 the enhanced persistent luminescence can be explained by the increased number of the distorted Cr³⁺ ions and more generated anti-site defects around them due to Sn⁴⁺ substitution.^38,52

Fig. 6 The afterglow decay curves of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ substitution concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125) upon 254 nm excitation. The inset shows their phosphorescence spectra.

To unravel the effect of Sn⁴⁺ substitution on the depth of the traps, TL spectra of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125) are shown in Fig. 7. For each of all TL spectra, it can be observed that there exists a single broad TL peak at around 350 K. In particular, the TL peak position shifts to lower temperatures with increasing Sn⁴⁺ concentration. Generally, the trap depths can be obtained through the following equation:

E_t = T_m/500 (2)

where E_t is the calculated trap depth and T_m is the temperature corresponding to the TL peak position. It can be found that the E_t value decreases from 0.72 to 0.68 eV with increasing Sn⁴⁺ substitution amounts. It can be concluded that the trap depths are altered through Sn⁴⁺ substitution.⁵³ Based on the study reported by Bessière et al., the traps with a depth of ∼0.7 eV can be attributed to the contribution of anti-site defects around distorted Cr³⁺ ions.³¹ According to our previous study on Pr³⁺ doped ZGGO:Cr³⁺ nanoparticles, the decrease of trap depth with increasing Sn⁴⁺ concentration indicates that anti-site defects are closer to distorted Cr³⁺ ions due to the increased numbers of the generated anti-site defects and distorted Cr³⁺ ions after Sn⁴⁺ substitution.³⁴

Fig. 7 TL spectra of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125).

To further understand how Sn⁴⁺ substitution affects the surface distributions of Zn²⁺ and Ga³⁺ ions, XPS spectra of Zn 2p_3/2 and Ga 3d_5/2 core levels for ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125) are shown in Fig. 8a and b. These XPS peaks were calibrated by using the C 1s peak at 284.6 eV. For each of the samples with various Sn⁴⁺ concentrations, the binding energies of the Zn 2p_3/2 and Ga 3d_5/2 core levels can be obtained by fitting with a single Gaussian function, as listed in Table 3. On the basis of the afterglow spectral analysis and our previous XPS study on Pr³⁺ doped ZGGO:Cr³⁺ nanoparticles, it can be inferred that there merely exist Zn²⁺ ions in the tetrahedral sites and Ga³⁺ ions in the octahedral sites on the surface of the ZGGSO:Cr³⁺ nanoparticles, suggesting that Sn⁴⁺ substitution mainly leads to the generation of anti-site defects in the interior of the ZGGSO:Cr³⁺ nanoparticles.³⁴ Strikingly, the peak positions related to the Zn 2p_3/2 and Ga 3d_5/2 core levels exhibit an initial decrease and a following increase with increasing Sn⁴⁺ concentration. The decreased binding energies of the Zn 2p_3/2 and Ga 3d_5/2 core levels with a rise of Sn⁴⁺ concentration might be related to the increased Zn–O and Ga–O bond lengths due to Sn⁴⁺ substitution. For carbon nitride systems, the similar behavior were also reported by Titantah et al., who found that the N 1s binding energy decreases with increasing C–N bond length.⁵⁴ Meanwhile, the increased Zn 2p_3/2 and Ga 3d_5/2 binding energies can be attributed to the deformations of ZnO₄ tetrahedrons and GaO₆ octahedrons after Sn⁴⁺ substitution based on our previous study on Pr³⁺ doped ZGGO:Cr³⁺ nanoparticles.³⁴

Fig. 8 XPS spectra of (a) Zn 2p_3/2 and (b) Ga 3d_5/2 core levels for ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125).

Table 3 The binding energies of the Zn 2p_3/2 and Ga 3d_5/2 core levels and the related lifetimes of the ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ substitution concentrations^a

x	Zn 2p3/2	Ga 3d5/2	Decay (^4T2 → ^4A2)^d		Decay (^2E → ^4A2)^e
	BE (eV)^b	BE (eV)^c	τs (μs)	τi (μs)	τs (ms)	τi (ms)
a The samples are ZGGSO:Cr^3+ with various Sn^4+ concentrations (x = 0, 0.015, 0.030, 0.063, 0.125).b BE refers to the binding energies of the Zn 2p3/2 core level.c BE refers to the binding energies of the Ga 3d5/2 core level.d Emission decay of the ^4T2 → ^4A2 transition.e Emission decay of the ^2E → ^4A2 transition. τs and τi represent the lifetimes related to the surface and interior Cr^3+ ions, respectively.
0	1023.1	20.3	78	668	2.23	6.63
0.015	1022.3	19.8	17	249	2.24	6.88
0.030	1020.9	19.4	19	218	2.62	7.43
0.063	1022.0	19.7	32	257	2.12	6.75
0.125	1022.7	20.3	34	398	2.06	6.71

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To further illuminate the effect of Sn⁴⁺ substitution on the surroundings of Cr³⁺ ions, the decay curves of the 697 nm emission of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125) under 575 nm excitation are shown in Fig. 9. In analogy with our previous study on Pr³⁺ doped ZGGO:Cr³⁺ nanoparticles, for each of the samples with various Sn⁴⁺ concentrations, the fitted lifetimes, as listed in Table 3, consist of four components.³⁴ Herein, for ZGGSO:Cr³⁺ nanoparticles, all of the decay curves need to be fitted with a sum of four exponentials. The two longer lifetimes (unit: ms) are related to the ²E(²G) → ⁴A₂ transitions of Cr³⁺ due to the forbidden spin-selection rule. The two shorter lifetimes (unit: μs) are related to the allowed ⁴T₂ (⁴F) → ⁴A₂ transitions of Cr³⁺. Furthermore, Cr³⁺ ions doped in nanoparticles can be sorted into two different surroundings: surface and interior. The lifetime of the surface Cr³⁺ exhibits a shorter decay time because of Cr³⁺ occupying low-symmetry sites.⁴⁰

Fig. 9 The decay curves of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations (x = 0, 0.015, 0.030, 0.063 and 0.125) monitored at 697 nm under 575 nm excitation.

Because the energy level position of the ⁴T₂ state is sensitive to the variation in the surroundings of Cr³⁺ ions (linearly depends on the variation in the crystal field strength), we focus on the effect of Sn⁴⁺ substitution on the lifetimes related to the ⁴T₂ → ⁴A₂ emissions of the surface and interior Cr³⁺. For both the surface and interior Cr³⁺ ions, it can be found that the lifetimes of Sn⁴⁺ doped nanoparticles are shorter than those of the undoped nanoparticles. Usually, the reciprocal of a lifetime (τ) can be expressed using a sum of radiative (W_R), nonradiative (W_NR) and energy transfer (W_ET) rates as follows:^34,55

1/τ = W_R + W_NR + W_ET (3)

It is known that the W_R value depends on the variation in the symmetry of the occupied sites and the W_NR value can be tuned by the variations of the particle size and phonon energy of the host after cation substitution. Based on the TEM observations and Raman, emission and absorption spectral analyses, it can be found that Sn⁴⁺ substitution leads to little variation in the W_R and W_NR values. Thus, in our case, the obtained lifetimes can be mainly ascribed to the contribution of the variation in the energy transfer rate after Sn⁴⁺ substitution. Since the emission intensity decreases with increasing Sn⁴⁺ concentration from Fig. 5b, it can be inferred that Sn⁴⁺ substitution gives rise to the increased energy transfer rate and the decreased lifetimes.

In addition, for both the surface and interior Cr³⁺ ions in the ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations, the corresponding lifetimes show an initial decrease and a following rise with increasing Sn⁴⁺ concentration. These phenomena might be explained by the variations in the ⁴T₂ level position of Cr³⁺ and the related trap depths after Sn⁴⁺ substitution. A schematic configuration coordinate diagram related to the energy transfer from Cr³⁺ to the deep traps under the ⁴A₂–⁴T₂ excitation is given in Fig. 10 based on our previous studies.^34,40 Here, the deep traps refer to the anti-site pair , which is close to the distorted Cr³⁺ ions, and the trap depths can be tuned by varying the distance between the anti-site pair and the distorted Cr³⁺ ion.⁴⁰ This distance will decrease with increasing the numbers of anti-site defects and distorted Cr³⁺ ions.³⁴ Because the numbers of the anti-site defects show an initial rise and a following decrease with increasing Sn⁴⁺ concentration, the distances between the anti-site pair and the distorted Cr³⁺ ions will initially decrease and then increase with increasing Sn⁴⁺ concentration. Thus, the variation in the deep trap depth will show a non-monotonic changing behaviour with increasing Sn⁴⁺ concentration. Meanwhile, Sn⁴⁺ substitution leads to the monotonously increased crystal field strength and the lowered bottom of the ⁴T₂ parabola. It can be deduced that Sn⁴⁺ substitution will cause the non-monotonic variations in the energy difference between the ⁴T₂ level and the deep traps related to the anti-site defects. Thus, the energy transfer from Cr³⁺ to the deep trap and the related lifetimes can be tuned through Sn⁴⁺ substitution. In particular, the sample (x = 0.015) with the shortest lifetimes corresponds to that with the strongest afterglow intensity, further indicating that the increased energy transfer from Cr³⁺ to the deep trap after Sn⁴⁺ substitution plays a key role in enhancing the persistent luminescence of the ZGGSO:Cr³⁺ nanoparticles. To prove the existence of the tunnelling mechanism, the afterglow decay curves of ZGGSO:Cr³⁺ nanoparticles without and containing Sn⁴⁺ (x = 0, 0.015) under 575 nm excitation are shown in Fig. 10b. For both of the samples, the strong persistent luminescence can be detected under visible light excitation. Strikingly, the afterglow intensity of the ZGGSO:Cr³⁺ containing Sn⁴⁺ (x = 0.015) is higher than that without Sn⁴⁺. This result further suggests that Sn⁴⁺ substitution can enhance the persistent luminescence through controlling the tunnelling process. The detailed explanation is given as follows. For the excited electrons in the ⁴T₂ level upon 575 nm excitation, the tunnelling process will happen in the case that the bottom of the ⁴T₂ parabola is far from the bottom of the conduction band of the host. Fortunately, for Cr³⁺ doped ZGO or ZGGO, it can be found that the ⁴T₂ level of Cr³⁺ is ∼1.6–1.8 eV below the bottom of conduction band of the host based on the reported studies.^33,34,56 Thus, the observed persistent luminescence of ZGGSO:Cr³⁺ nanoparticles under 575 nm irradiation may originate from the tunnelling process between the ⁴T₂ and the deep traps.

Fig. 10 (a) A schematic configuration coordinate diagram related to the energy transfer from Cr³⁺ ion to the deep traps under the ⁴A₂–⁴T₂ excitation. (b) The afterglow decay curves of ZGGSO:Cr³⁺ nanoparticles without and containing Sn⁴⁺ (x = 0, 0.015) upon 575 nm excitation. The inset shows their phosphorescence spectra.

To confirm the existence of the aforementioned deep traps in the ZGGSO nanoparticles, PSL spectra of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations monitored at 697 nm (stimulation wavelength: 808 nm) after the stoppage of 254 nm irradiation for 5 min are shown in Fig. 11. Here, in order to realize high-quality in vivo bioimaging, we employed the 808 nm laser in the PSL measurement. For each of the samples, it can be found that a strong PSL signal superimposes on the afterglow curve when the 808 nm laser is on (the electrons stored in the deep traps are released). In particular, among ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations, the sample with x = 0.015 shows the strongest NIR luminescence, further proving that Sn⁴⁺ substitution can enhance the NIR persistent luminescence via tuning the energy transfer from Cr³⁺ to the deep traps. Meanwhile, at 30 min after stopping 254 nm irradiation, the PSL intensity is ∼12 times as strong as the afterglow intensity without 808 nm stimulation. This result suggests that the strong NIR luminescence can be acquired using an 808 nm stimulation strategy in the case that the weak NIR afterglow intensity cannot be detected.

Fig. 11 PSL spectra of ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations monitored at 697 nm (stimulation wavelength: 808 nm) after the stoppage of 254 nm irradiation.

To demonstrate the effects of Sn⁴⁺ substitution and 808 nm stimulation on the bioimaging using NIR persistent luminescence nanoparticles, images of two pellets, pressed by the ZGGSO:Cr³⁺ nanoparticles without Sn⁴⁺ (x = 0) and containing Sn⁴⁺ (x = 0.015) upon 254 nm irradiation and 808 nm stimulation are shown in Fig. 12. The clear images of the two pellets were recorded using a digital camera under white light illumination, as shown in Fig. 12a. As the two pellets were taken at 1 min after stopping 254 nm irradiation, two red images can be observed, as shown in Fig. 12b. Strikingly, the image of the pellet containing Sn⁴⁺ is brighter than that without Sn⁴⁺. This result is consistent with the afterglow spectral analysis. While, taken at 15 min or later after stopping 254 nm irradiation, the two red images become less clear, as shown in Fig. 12c and d. To retrace bioimaging in the case that the afterglow intensity is undetectable, the images of the two pellets taken at 30 and 60 min after stopping 254 nm irradiation stimulated by an 808 nm laser are shown in Fig. 12e and f. It can be found that the two clear images, originating from the NIR PSL of ZGGSO:Cr³⁺ nanoparticles, reappear and the image of the pellet containing Sn⁴⁺ is still brighter than that without Sn⁴⁺. This result suggests that 808 nm stimulation is an efficient way to retrace the bioimaging using NIR persistent luminescence nanoparticles. In addition, the power density of 808 nm excitation is ∼7.8 mW cm⁻², which is much smaller than the critical value of photodamaged skin of human beings (330 mW cm⁻²). This further can ensure the accuracy and security of in vivo bioimaging using ZGGSO:Cr³⁺ NIR PLNPs at any time.^57,58 Our results suggest that ZGGSO:Cr³⁺ nanoparticles have potential applications in in vivo bioimaging.

Fig. 12 (a) The photos of two pellets without Sn⁴⁺ and containing Sn⁴⁺ under white light illumination. (b–d) The NIR images of the two pellets without Sn⁴⁺ and containing Sn⁴⁺ were taken at 1, 5 and 15 min after stopping 254 irradiation. (e and f) The NIR images of the two pellets without Sn⁴⁺ and containing Sn⁴⁺, originating from the NIR PSL stimulated by an 808 nm laser, taken at 30 and 60 min after stopping 254 nm irradiation.

Conclusions

Spinel-phase ZGGSO:Cr³⁺ nanoparticles with various Sn⁴⁺ concentrations were prepared by a hydrothermal method in combination with a post-annealing in vacuum at 800 °C. It can be found that Sn⁴⁺ substitution has little influence on their particle sizes and leads to the increased Zn–O, Ga–O and Cr–O bond lengths. For these nanoparticles, the observed near infrared (NIR) persistent luminescence peaked at ∼697 nm originates from the ²E, ⁴T₂ (⁴F) → ⁴A₂ transitions of Cr³⁺ and the afterglow time exceeds 800 min. For both the interior and surface Cr³⁺ ions in the ZGGSO host, it can be found that the increased energy transfer from Cr³⁺ to the deep trap (anti-site defects, ) after the substitution of Ge⁴⁺ by Sn⁴⁺ plays a key role in enhancing the persistent luminescence of the ZGGSO:Cr³⁺ nanoparticles. It can be found that this energy transfer process can be controlled through the variations in the crystal field strength and the trap depths. Strikingly, our results suggest that not only Sn⁴⁺ substitution can improve in vivo bioimaging but also the existence of deep traps in ZGGSO:Cr³⁺ nanoparticles is helpful for retracing in vivo bioimaging at any time.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 11674050, 11374047 & 11604044).

Notes and references

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