A Cr³⁺ luminescence based ratiometric optical laser power meter

Abstract

Chromium doped luminescent phosphors display numerous spectral features that can be useful for designing modern multi-functional materials. Here we study GdAl₃(BO₃)₄ nanoparticles doped with Cr³⁺ ions, which enable combining efficient light-to-heat conversion and efficient luminescent properties in homogenously doped materials avoiding complicated syntheses of hetero-structural or hybrid-designed materials. In particular, while the intentionally increased content of Cr³⁺ ions leads to increased self-heating in a relatively broad photoexcitation spectrum, the concurrently occurring bright luminescence intensity ratio between the ⁴T_2(g) level and the ²E_(g) level responds to local temperature variation. Such behavior inspired us to evaluate the possibility of ratiometrically measuring the delivered laser pump power. The presented strategy enables the development of luminescent sensors operating in a wide range of power densities. It has been proposed and explained how the performance of the sensor, including sensitivity and usable excitation density range of optical density meter, can be modified by the knowledgeable optimization of the Cr³⁺ dopant ion concentration. We believe that this work may be the beginning of a new research direction that will allow the development of highly sensitive remote optical density meters.

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Introduction

Nonradiative relaxation is typically considered as detrimental for luminescence and many efforts have been undertaken to prevent it.^1–5 In the case of materials doped with lanthanide ions, low phonon energy matrices and low dopant concentrations were proposed, mostly to eliminate multiphonon relaxation processes.^6,7 On the other hand, for transition metal ions, the preservation of a small shift between the parabolas of the excited and ground levels and a small electron-phonon coupling were found to reduce the probability of nonradiative processes.^8–10 Satisfying these requirements enables maintaining intense luminescence even at increased temperatures. Nevertheless, judicious exploitation of nonradiative processes and a wise combination of these features with other inherent properties of the luminescent nanoparticles allow developing luminescence-based multi-functional materials.^11–13 For example, strong susceptibility of nonradiative processes rates on parameters such as temperature, pressure and pH facilitates the development of luminescent sensors of those physical and chemical quantities. Simultaneously, purposeful enhancement of these processes may be additionally suitable for photothermal therapy of cancer.^14–16 The intentional management of probability of nonradiative processes can be achieved by adopting appropriate nanoparticle compositional architecture¹⁷ or by exploiting appropriate optical excitation wavelengths.¹⁸ The capability of accommodating efficient light to heat conversion with simultaneous susceptibility of ratiometric luminescence spectra to temperature within single phosphor enables designing multifunctional materials. Such phosphors may be applied to measure the excitation power.

Here we study GdAl₃(BO₃)₄ nanoparticles doped with Cr³⁺ ions as a model material. The choice of the host material and luminescent active ions was motivated by their unique properties (Fig. 1). Luminescence of Cr³⁺ ions occurs, depending on the strength of the interacting crystal field, from ²E_(g) and/or ⁴T_2(g).^19–24 In the case of a strong crystal field, the emission spectrum is dominated by the spectrally narrow band associated with the ²E_(g) → ⁴A_2(g) electronic transition, whereas in the case of a weak crystal field, the emission spectrum of Cr³⁺ ions consists of the broad ⁴T_2(g) → ⁴A_2(g) band. Both energy levels are thermally coupled, i.e., for strong and intermediate crystal field host materials, an increase in temperature results in an increase in the population of the ⁴T_2(g) level with respect to ²E_(g). Therefore in the case of intermediate crystal field materials for which both emission bands are observed in the spectrum, an increase in the probability of nonradiative processes involved in the conversion of light to heat will contribute to an increase in the emission intensity of ⁴T_2(g) →⁴A_2(g) with respect to the ²E_(g) →⁴A_2(g). The increase in the temperature of the phosphor depends on the efficiency of the light to heat conversion processes and the excitation power density. Hence, the ratio of the emission intensity associated with the ⁴T_2(g) →⁴A_2(g) and ²E_(g) →⁴A_2(g) bands can be exploited as a luminescent optical excitation density indicator. To enhance the sensitivity of this optical power meter, a high concentration of Cr³⁺ ions was used to facilitate energy migration to the killer centers. The GdAl₃(BO₃)₄ belongs to the family of the RX₃(BO₃)₄,^25,26 which is very frequently used as a host material for Cr³⁺ based phosphors. The modification of the host material composition significantly affects the spectroscopic properties of the phosphor. Low temperature emission spectra of YAl₃(BO₃)₄:Cr³⁺ are dominated by the ²E → ⁴A₂ emission band whereas the ⁴T₂ → ⁴A₂ band cannot be observed.^27–29 Although the emission intensity of ⁴T₂ → ⁴A₂ increases at room temperature, the spectrum is still dominated by the ²E → ⁴A₂ band.³⁰ When Y³⁺ ions are replaced by Gd³⁺ the ⁴T₂ → ⁴A₂ emission intensity starts to dominate over the ²E → ⁴A₂ band. More spectacular changes in the shape of the emission band, however, are found when the Al³⁺ ions are substituted by Sc³⁺. This directly affects the shape of the octahedra surrounding the Cr³⁺ ions and thus only the ⁴T₂ → ⁴A₂ band can be observed for YSc₃(BO₃)₄:Cr³⁺ and GdSc₃(BO₃)₄:Cr³⁺.³⁰ Brik et al. revealed that when Cr³⁺ occupies the distorted octahedral site of Sc³⁺ ions in (Ce,Gd)Sc₃(BO₃)₄:Cr³⁺ the intense broad band of the ⁴T₂ → ⁴A₂ electronic transition with the maxima at 910 nm can be achieved.³¹ Replacement of Gd³⁺ by larger La³⁺ results in the change of the emission band maxima to 963 nm in LaSc₃(BO₃)₄.^32–34 Wu et al. proposed a strategy that when the Y³⁺ ions are introduced into the system to occupy partially La³⁺ and Sc³⁺ sites, the significant blue shift of the emission band to the 850 nm in Y_0.57La_0.72Sc_2.71(BO₃)₄:Cr can be observed.³⁵

Fig. 1 Schematic representation of the principle of operation of Cr³⁺ based ratiometric laser power meter: the luminescence spectra of Cr³⁺ ions in GdAl₃(BO₃)₄ consist of two emission bands associated with the radiative transitions from both ²E_(g) and ⁴T_2(g) states. Upon low excitation density (top), low heat is generated, hence the ²E_(g) → ⁴A_2(g) emission band dominates in the spectrum, whereas upon high excitation density (bottom), high heat generated in the system leads to the thermalization of the ⁴T_2(g) state and thus a relative increase of the ⁴T_2(g) → ⁴A_2(g) band intensity with respect to the ²E_(g) → ⁴A_2(g) is observed. Therefore, the LIR of these two bands can be exploited as a measure of laser beam power.

The GdAl₃(BO₃)₄ host material is characterized by two important advantages especially beneficial for the purpose of this study. First of all, it consists of an Al³⁺ octahedral site which could be occupied by the Cr³⁺ ions and even a high dopant concentration does not affect the crystal field strength. Second, it is characterized by high phonon energy which further increases the probability of light-to-heat conversion required for the developed application.^36–38 Besides, GdAl₃(BO₃)₄ is highly mechanically and thermally stable which enables developing a laser beam power meter operating even in the high excitation power density range without any degradation or decomposition of the material. The spectrally broad absorption bands of Cr³⁺ ions in the visible range associated with the ⁴A_2(g) → ⁴T_1(g) and ⁴A_2(g) → ⁴T_2(g) electronic transitions allow for extending the useful spectral range for which the luminescence response of the developed power meter will be recorded. Understanding the photophysics and materials sciences, let us propose a new method to measure the laser beam power as a simple proof-of-concept demonstration of multi-functional luminescent nanoparticles.

Experimental

Synthesis

Gadolinium(III) oxide (Gd₂O₃ REacton 99.999% purity, Alfa Aesar), aluminum(III) nitrate hydrate (Al(NO₃)₃·9H₂O Puratronic 99.999% purity, Alfa Aesar), chromium(III) nitrate hydrate (Cr(NO₃)₃·9H₂O 99.99% purity, Alfa Aesar), boric acid (H₃BO₃ 99.97% purity, Aldrich Chemistry), D-sorbitol (C₆H₁₄O₆ >98.0% purity POL-AURA), citric acid (C₆H₈O₇ 99% purity, Sigma-Aldrich), and n-hexane (Avantor) were used as the starting materials for synthesis without further purification.

The GdAl₃(BO₃)₄ borates doped with (1; 5; 10)% Cr³⁺ were synthesized according to the modified polymer precursor method, described in detail in previous work.^39,40 In general, this method consist of the complexation of metal cations by given carboxylic acid (e.g. citric acid) and further polymerization reaction which occurs between the used carboxylic acid and polyhydroxyl alcohol (e.g.D-sorbitol) resulting in the formation of a polymeric resin. In order to compensate for the loss of boric acid due to its evaporation during the synthesis and annealing process, an appropriate excess of both Al³⁺ and B³⁺ ions. The ratios were estimated on the basis of the percentage of the additional GdBO₃ phase received from X-ray powder diffraction patterns. First, gadolinium nitrate was obtained through the dissolution of gadolinium oxide in slightly diluted hot ultrapure nitric acid followed by a triple recrystallization process. Then, appropriate amounts of gadolinium, aluminum and chromium nitrates were dissolved in deionized water and an aqueous solution of citric acid was added in order to form citrate complexes of metal ions. Citric acid was added in the ratio of 3 : 1 with respect to the number of moles applied to the metal and boron ions and in the ratio of 3 : 2 with respect to D-sorbitol. Subsequently, an earlier prepared solution of D-sorbitol and boric acid was added and then the complete solution was stirred for 3 h at about 100 °C. Then, the solutions were heated for a few days at 90 °C to obtain crumbly resins. Afterward, the resins were pre-calcined under the following conditions: 400 °C/16 h and 700 °C/16 h with a heating rate of 5 °C min⁻¹ under an air atmosphere, with grinding in n-hexane between successive annealing processes. Such prepared powders were annealed at 1100 °C for 5 h.

Characterization

The X-ray powder diffraction (XRPD) patterns were measured with the use of a PANalitycal X’Pert diffractometer, equipped with an Anton Paar TCU 1000 N temperature control unit, using Ni-filtered Cu-K_α radiation (V = 40 kV, I = 30 mA). Transmission electron microscopy images were taken on a Philips CM-20 SuperTwin microscope using an accelerating voltage of 160 kV and a 0.25 nm spatial resolution. Excitation spectra and luminescence decay profiles were recorded using an FLS 1000 fluorescence spectrometer from Edinburgh Instruments equipped with a R928P side window multiplier tube from Hamamatsu as a detector as well as a 450 W halogen lamp and a μFlash lamp (40 Hz repetition, 20 ms time width of the excitation pulse) as the excitation sources. The temperature increase curves and thermovision images were collected using a T540 camera from FLIR.

The excitation power dependent emission spectra were collected using a home-built optical setup, illustrated schematically in Fig. S1 (ESI†). The excitation beam (445 nm) provided by the laser diode was collimated and then passed through the neutral filter with variable optical density (NDM4/M, Thorlabs), used to adjust the power density of the excitation beam. Next, this beam was split into two fractions – one part was transmitted by the beam splitter and was focused with an objective lens (Genetic Pro, Plan 20×, NA = 0.40, Delta Optical) on the sample enclosed in a quartz cuvette, while the other part of the light was reflected (R = 3%) by the beam splitter and directed to the power meter head (S121C, Thorlabs) connected with the computer to monitor the power of the beam. This reference power is proportional to the power of the light transmitted by the beam splitter, therefore the power of the light illuminating sample was possible to reconstruct. The light emitted by the sample was then collected by the lens and, and after passing through the longpass filter (FEL0600, Thorlabs), it was focused on the entrance slit (10 μm) of the spectrograph (Shamrock 500i, Andor) equipped with a CCD camera (Newton 920, BEX2-DD, Andor) to record the emission spectra. All of the recorded spectra were collected with an acquisition time of 50 ms.

Results and discussion

The GdAl₃(BO₃)₄ crystallizes in a trigonal crystal system with a R₃₂ (no. 155) space group isostructural to CaMg₃(CO₃)₄.^36–38,41 It consists of octahedrally coordinated Al³⁺ sites and octahedrally coordinated Gd³⁺, (BO₃)³⁻ groups are arranged in sheets of planar triangles. Due to the similarities in the ionic charge and the ionic radii the Al³⁺ sites of C₂ point symmetry are preferentially occupied by the Cr³⁺ ions. Due to the relatively large Al³⁺–O²⁻ bond length (1.902 A) and the intermetallic Al³⁺–Al³⁺ distance even the high concentration of Cr³⁺ (up to 10% molar in respect to the Al³⁺ ions) does not affect the average Cr³⁺–O²⁻ distance and thus the crystal field strength. The XRD patterns measured for GdAl₃(BO₃)₄ powders indicate that independently of Cr³⁺ concentration, pure-phased GdAl₃(BO₃)₄ structures were obtained (Fig. 2b). The representative TEM images of the GdAl₃(BO₃)₄:Cr³⁺ powders reveal that the synthesized GdAl₃(BO₃)₄ consists of submicrometric well-crystallized particles of an average size of around 400 nm (Fig. 2c–e). Additionally, no visible influence of the dopant concentration on the morphology of synthesized powders was observed

Fig. 2 Visualization of the octahedral (AlO₆)⁹⁻ and octahedral (GdO₆)⁹⁻ sites in GdAl₃(BO₃)₄ (a) and the XRD patterns of GdAl₃(BO₃)₄:Cr³⁺ doped with different concentrations of Cr³⁺ ions (b) and representative TEM images of the GdAl₃(BO₃)₄:1%Cr³⁺ (c) and GdAl₃(BO₃)₄:10%Cr³⁺ (d and e).

The luminescence spectra of materials doped with Cr³⁺ ions result from the d–d electronic transitions. In the case of the transition metal ions of 3d³ electronic configuration observed for Cr³⁺ ions, depending on the crystal field strength of the host material the spectrum is dominated either by narrow band ²E_(g) → ⁴A_2g emission (strong crystal field) or broad emission band ⁴T_2(g) → ⁴A_2(g) for the weak crystal field. However, in order to develop a ratiometric optical excitation density sensor based on the emission intensity ratio from the ²E_(g) level to that from the ⁴T_2(g) level, one should use materials with an intermediate crystal field for which both bands are observed in the spectrum simultaneously (Fig. 3a). Therefore, GdAl₃(BO₃)₄ was used as a host material in this study. Representative emission spectra recorded for 200 mW cm⁻² of λ_exc = 445 nm optical excitation at room temperature shown in Fig. 3b reveal a strong dependence of the emission spectra on the Cr³⁺ ion concentration. As the Cr³⁺ content increases, the intensity of the broad ⁴T_2(g) → ⁴A_2(g) band increases relative to the ²E_(g) → ⁴A_2(g) one. Although such a change may suggest a modification in the crystal field strength with increasing dopant ion concentrations,⁴² a careful analysis of the excitation spectra clearly indicates that the positions of the ⁴A_2(g) →⁴T_2(g) and ⁴A_2(g) →⁴T_1(g) absorption bands of Cr³⁺ ions are independent of the dopant ion concentration. The determined strength of the crystal field (based on eqn (S1)–(S3), ESI†) confirms this observation and Dq/B ∼ 2.48 is obtained for all analyzed dopant concentrations (Fig. 3c). Thus, the observed change in the shape of the emission spectrum can be explained by more efficient thermalization of the ⁴T_2(g) level relative to the ²E_(g) level for high dopant concentrations. The enhanced efficiency of the light to heat conversion process in materials with high Cr³⁺ ion concentrations is related to two factors. The first of them is a more efficient absorption of the excitation radiation associated with an increase in the absorption cross-section observed for heavily doped particles. The second effect is related to a more efficient diffusion of energy across the excited levels of Cr³⁺ ions to killing centers. When such a center is reached the nonradiative recombination of the electron takes place. The probability of this process increases with the concentration of Cr³⁺ ions. The interaction of these two effects promotes efficient optical heating of the phosphor. To confirm this hypothesis, the temperature of the GdAl₃(BO₃)₄:Cr³⁺ powders upon a constant optical excitation density of 2 W cm⁻² was observed using a thermovision camera (Fig. 3d). The results obtained after 2 minutes of continuous optical excitation clearly indicate that while for GdAl₃(BO₃)₄:1%Cr³⁺ the recorded temperature was 58 °C, 143 °C was recorded for GdAl₃(BO₃)₄:10%Cr³⁺. The efficient depopulation of the ²E_(g) level by thermalization with increasing Cr³⁺ ion concentrations is also reflected in the kinetics of the luminescence. The determined probability of nonradiative processes (from eqn (S4), ESI†) clearly increases with increasing Cr³⁺ ion concentrations. These results confirm that the material efficiently converts light to heat and a clear indicator of this may be the shape of the GdAl₃(BO₃)₄:Cr³⁺ emission spectrum.

Fig. 3 Simplified configurational coordination diagram of the Cr³⁺ energy levels in an intermediate crystal field strength (a), room temperature emission (λ_exc = 445 nm, solid line) and excitation (λ_em = 690 nm, dashed line) spectra of GdAl₃(BO₃)₄:Cr³⁺ (b); the influence of the Cr³⁺ concentration on the Dq/B (c); the temperature of the powder upon 2 W cm⁻² of λ_exc = 445 nm optical excitation determined using a thermovision camera (d) and probability of nonradiative processes (e).

To experimentally verify the hypothesis that an emission intensity ratio of Cr³⁺ ions can be exploited to develop a ratiometric optical excitation power density sensor, the emission spectra of GdAl₃(BO₃)₄:Cr³⁺ were recorded as a function of 445 nm optical excitation power density from 1.2 to 4300 W cm⁻². The representative spectra of the GdAl₃(BO₃)₄:1%Cr³⁺ clearly show that besides a significant increase in emission intensity, the shape of the emission spectrum also changes with photoexcitation density (Fig. 4a). With increasing laser beam power density the intensity of the ⁴T_2(g) → ⁴A_2(g) band significantly increases with respect to the ²E_(g) → ⁴A_2(g) band and for 4300 W cm⁻² the intensity of both bands is comparable. Analogous changes are also observed for the samples with the other concentrations (Fig. S2 and S3, ESI†). However, the dynamics of these changes for the power density regime below 1000 W cm⁻² increase with the increasing dopant ion concentration. In order to quantitatively describe the observed changes in the shape of the emission spectrum, the emission intensity ratio (LIR) is defined as follows:

As can be observed for samples with low Cr³⁺ ion concentrations, the LIR is a linear function of the optical excitation power density and increases monotonically from 2.23 to 2.94 in the 1.3 to 4300 W cm⁻² excitation density range (Fig. 4b). However, the growing number of dopant ions clearly affects the LIR. For 5%Cr³⁺ the LIR in the same range of excitation power density increases from 2.68 to 4.52, whereas for 10% Cr³⁺, it increased from 3.17 to 4.93. Significantly, for high dopant ion concentrations the most rapid increase in the LIR is observed below 2700 W cm⁻² and 1800 W cm⁻² for 5%Cr³⁺ and 10%Cr³⁺, respectively. Above these power density values, the LIR begins to saturate and a further increase of the excitation density does not lead to a meaningful change in the LIR. A qualitative description of these changes can be achieved by analyzing the absolute sensitivity of the luminescent power meter defined as follows:where ΔLIR represents the change in the LIR corresponding to a ΔP change in power density. In the case of 1%Cr³⁺, S_A takes an almost constant value over the entire range of the analyzed power density equal to S_A = 0.18 W⁻¹. On the other hand, for 10%Cr³⁺, S_A reaches very high values for low excitation power densities S_A = 2.68 W⁻¹ for 2 W cm⁻² and increasing the power density causes a monotonic decrease in S_A. It should be noted here that for power densities above 1700 W cm⁻², S_A values fall below those reported for 1%Cr³⁺. This result confirms that high dopant ion concentrations are preferable for the development of low excitation density luminescence based sensors. However, very interesting results were observed for the 5%Cr³⁺ sample. In this case, for 1.2 W cm⁻² even a higher value of S_A = 5.11 W⁻¹ was observed exceeding that observed for 10%Cr³⁺. However, even for the higher power density regime, the observed S_A is slightly higher than that for the 1%Cr³⁺ counterpart. Above 2.23 W cm⁻², the S_A, in this case, saturates at about 0.21 W⁻¹. Since the LIR is a dimensionless parameter, comparing the performance of different luminescence-based power meters is difficult to perform. For this purpose, a relative sensitivity parameter defined analogously to the parameter described for luminescence thermometry was introduced as follows:Similar to S_A, the lowest S_R values were recorded for 1%Cr³⁺ at about S_R = 0.07% W⁻¹. By increasing the concentration of Cr³⁺ ions, an increase in the maximum relative sensitivity was obtained for S_R = 0.05% W⁻¹ and S_R = 0.07% W⁻¹ for 5%Cr³⁺ and 10%Cr³⁺, respectively. Analogously to S_A, the highest S_R values are observed for high dopant concentrations. However, it should be emphasized that increasing the concentration simultaneously narrows the useful power density range. In order to prove that the observed changes in the emission spectra of GdAl₃(BO₃)₄:Cr³⁺ are not related to the sample decomposition, and the LIR values for 11 cycles of high and low power density were examined. The obtained results confirmed the high reproducibility of the obtained results (Fig. S4, ESI†). For the design of a luminescence-based power meter, the response time of the sensor to a change in the excitation power density is important. For this purpose, the kinetics of LIR changes for a constant excitation power density of 2.3W cm⁻² for GdAl₃(BO₃)₄:1%Cr³⁺ was investigated. It is clearly seen that after about 1s the LIR value assumes a constant value which indicates the sensitivity of the proposed meter to dynamic changes in the excitation power density. Changes in the shape of the emission spectrum under different excitation density conditions can be achieved not only for the λ_exc = 445 nm but for any wavelength matching the absorption bands of the Cr³⁺ ions (see for instance the emission spectra obtained for λ_exc = 532 nm – Fig. S5, ESI†).

Fig. 4 Emission spectra of GdAl₃(BO₃)₄:1%Cr³⁺ recorded as a function of excitation density of λ_exc = 445 nm (a); the dependence of luminescence intensity ratio (LIR) of 2E → 4A2 to 4T2 → 4A2 bands on excitation density dependence (b).

Conclusions

In summary, this manuscript presents a strategy for the development of a remote optical excitation power density sensor. The described approach exploits the luminescence of Cr³⁺ ions in GdAl₃(BO₃)₄ – a host material and an intermediate crystal field material. In such a case both emission bands ⁴T_2(g) → ⁴A_2(g) and ²E_(g) → ⁴A_2(g) are observed in the luminescence spectrum and the thermal coupling occurring between the emitting levels results in the increase of the emission intensity from the ⁴T_2(g) level with respect to the ²E_(g) level at elevated temperatures. Therefore, an increase in the power density resulting in the heating of the phosphor in terms of light to heat conversion results in a change in the luminescence intensity ratio of these bands. After calibration, such LIR variations become a measure of laser beam optical power density. As expected, increasing the concentration of Cr³⁺ ions results in a higher absorption coefficient, higher probability of nonradiative processes and thus increasing the optically induced temperature of the phosphor. Hence, for low dopant concentrations (1%Cr³⁺) a linear dependence of LIR on power density with relatively low absolute and relative sensitivity was observed. On the other hand, increasing the dopant ion concentration resulted in higher maximum sensitivities and narrowing the useful range of excitation power density. Similarly to Ln³⁺ doped materials, Cr³⁺ guarantees high photostability and sensitive luminescence-based quantification of temperature. Nevertheless, the most important advantages of Cr³⁺ ions over Ln³⁺ ions is their much higher absorption cross-section which is not compromised by the increasing temperature and more efficient self-heating. Moreover the larger spectral bandwidth of the absorption bands of Cr³⁺ enables a much wider spectral range of operation of such designed luminescence sensors. It should be noted here that the excitation density range of the Cr³⁺ based power meter depends on the energy separation between the ²E and the ⁴T₂ states. Therefore, it can be optimized according to the requirements of specific applications by the intentional optimization of the crystal field strength affecting the Cr³⁺ ions through host material composition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Center Poland (NCN) under project No. 2019/35/N/ST5/00553. K. E.-P. acknowledges the START Fellowship from the Foundation for Polish Science. The authors acknowledge MSc. Zuzanna Korczak for help in carrying out the experiments.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc02348b

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