Alexis Duval
email: alexis.duval@univ-rennes.fr

Yan Suffren

Mourad Benabdesselam

Patrick Houizot

Tanguy Rouxel

Luminescence and mechanoluminescence of 1 Ba 4 Si 6 O 16 :Eu 2+ , RE phosphors 2 3

come

INTRODUCTION

The luminescence properties of oxides, oxynitrides and nitrides from the BaO-SiO2-Si3N4 system were reported in the past few years, such as BaSi2O5:Eu 2+ 1 2 (𝜆 𝑒𝑚 = 500-505 nm), Ba3Si5O13-δNδ:Eu 2+ 3 (broad emission bands from 400 to 620 nm), Ba5Si8O21:Eu 2+ 4 5 6 ( 𝜆 𝑒𝑚 = 473-480 nm), Ba4Si6O16:Eu 2+ 7 8 9 10 ( 𝜆 𝑒𝑚 = 496-515 nm), β-BaSiO3:Eu 2+ 6 11 (𝜆 𝑒𝑚 = 560-565 nm), Ba2SiO4:Eu 2+ 12 13 14 15 (𝜆 𝑒𝑚 = 503-510 nm), Ba3SiO5:Eu 2+ 6 16 17 18 19 ( 𝜆 𝑒𝑚 = 585-596 nm), BaSi2O2N2:Eu 2+ 20 21 22 ( 𝜆 𝑒𝑚 = 498-500 nm) or Ba2Si5N8:Eu 2+ 23 (𝜆 𝑒𝑚 = 585 nm). This chemical system offers a broad range of emission colors, ranging from blue to orange.

The luminescence properties of Ba4Si6O16:Eu 2+ were reported as early as the 60s, but were mostly studied during the last 15 years with various luminescent centers such as Eu 2+ , Pr 3+ and Ce 3+ 8 24 25 . In the crystal, the rare-earths (RE) substitute for Ba 2+ ions thanks to close ionic radii 26 . Its space group is P21/c 27 (a = 12.477 Å, b = 4.685 Å, c = 13.944 Å, β = 93.54°, Z = 2), which is similar to the non-standard P21/a formerly used 8 . Ba4Si6O16:Eu 2+ is a potential phosphor for applications in white light emitting diodes (wLEDs), but synthesis complications including mixed phases like Ba3Si5O13 and Ba5Si8O21 and high sintering temperatures limit its use 10 .

Ba4Si6O16 exhibits an intense long-lasting phosphorescence (LLP) as well as elastico-mechanoluminescence (EML) 28 when doped with Eu 2+ , in a similar way as BaSi2O2N2.

Recall that elastico-mechanoluminescence is the emission of light resulting from an elastic deformation. This phenomenon has recently aroused great interest, because of potential applications of mechanoluminescent materials for energy storage or as stress sensors 29 . The BaO-SiO2 binary diagram shows a glass-forming region with a BaO content from 20 to 50 mol. % 30 , along with congruent crystallization at given BaO contents. This allowed the formation of Ba4Si6O16:Eu 2+ , Ho 3+ -containing glass-ceramics exhibiting EML thanks to uniformly dispersed active crystals 28 . In our previous work (ref. 28 ), we have reported mechanoluminescence in a new glass-ceramic material mainly consisting of Ba4Si6O16:Eu 2+ , RE crystalline phase. In this preliminary work, we focused on the choice for the RE co-dopant and we have reported the first mechanical testing results, which revealed significant differences with the behavior of SrAl2O4:Eu 2+ , Dy 3+ . The present manuscript provides an in-depth investigation of the role of the co-dopant in the e -trapping energetics and in the physics of the luminescence mechanism. LLP behavior as well as EML properties are related to the characteristics of electron traps within the active crystalline phase. Trap depths and concentrations were determined from thermally stimulated luminescence experiments.

Experiments with different loading rates and loading modes allow to propose a mechanism of the EML phenomenon in the particular crystal.

EXPERIMENTAL

Ba4Si6O16:Eu 2+ , RE crystals were synthesized starting from dried BaCO3 (Sigma-Aldrich 99 %), Gd2O3, Lu2O3, Dy2O3, Sc2O3, Eu2O3 (Sigma-Aldrich 99.9 %), SiO2 (Sigma-Aldrich 99.5 %), Pr2O3, Sm2O3 (Fischer 99.9 %), Tb4O7 (Fischer 99.998 %), Tm2O3 (Alfa Aesar 99.9 %), Yb2O3, La2O3, CeO2, Nd2O3 (Rhône-Poulenc 99.99 %), Y2O3 (Acros 99.99 %), and Ho2O3, Er2O3 (Rhône-Poulenc 99.999 %). Three series of Ba4Si6O16:Eu 2+ , RE crystals were studied, with i) a fixed cationic composition: 35 Emission and excitation spectra of Ba4Si6O16:Eu 2+ , Ho 3+ were recorded from 77 to 300 K in quartz cuvettes with an optical cryostat (OptistatCF from Oxford Inst.) coupled to a liquid nitrogen bath under a N2 atmosphere, and from 300 to 380 K in a solid holder using a F-3004 Jobin-Yvon heating Peltier module.

Appropriate filters were used to remove the residual excitation laser light, the Rayleigh scattered light and associated harmonics from the spectra. All spectra were corrected for the instrumental response function.

Solid-state UV-visible absorption measurements were obtained with a PerkinElmer Lambda 650 spectrometer using a 60 mm integrated sphere. 4 Quantum yield measurements were recorded using a G8 GMP integrating sphere according to: 92 Φ = (𝐸 𝑐 -𝐸 𝑎 )/(𝐿 𝑎 -𝐿 𝑐 ), where 𝐸 𝑐 is the integrated emission spectrum of the sample, 𝐸 𝑎 is the 93 integrated "blank" emission spectrum, 𝐿 𝑎 is the "blank" absorption, and 𝐿 𝑐 is the sample 94 absorption at the excitation wavelength. Samples were introduced in specific capillaries for a 95 G8 integrating sphere, and placed directly inside the integrating sphere. 96 Rapid luminescence decays were measured directly with the fluorescence spectrometer 97 connected to an additional time-correlated single photon counting (TCSPC) module and a 98 360 nm pulsed delta diode. Lifetimes and quantum yields are the averages of three independent 99 measurements.

Luminance, expressed in mcd•m -2 , was measured on a 1.5 cm 2 sample pellet using a Konica Minolta LS-150 luminancemeter with an integration time of 3 s under UV irradiation (𝜆 𝑒𝑥𝑐 = 365 nm) in a lab-made black box. The working distance between the luminancemeter and the pellet surface was 45 cm. The irradiation was realized with a lab-made source constituted by 1 LED CUN66B1B from Roithner at 365 nm plugged in a Keysight Technologies E36104B benchtop power supply that provides the desired current (mA) and voltage (V). The incident beam angle is 30° with respect to the normal of the pellet surface and the distance LED-sample is 12 cm. In order to avoid saturation the current was fixed at 90 mA and the voltage at 3.32 V, with an effective power of 0.57 mW•cm -2 .

Thermally stimulated luminescence (TSL) experiments were performed from 293 to 573 K with a heating rate of 1 K•s -1 . The delay between the end of the UV illumination and reading was 40 s. During heating, the TSL signal was detected by means of a UV-visible photomultiplier tube.

LLP emission spectra were collected by an optical multichannel analyzer (OMA), consisting of a multimode optical fiber connected to an integrated spectrograph equipped with a CCD array , where 𝑃 is the load, 𝑑 the diameter (11.5 mm) and ℎ the height (6 mm) of the disk 31 . The LLP intensity and the EML intensity were quantified consistently using a Zyla 5.5 sCMOS Andor Technology high sensitivity camera, by irradiating each sample for 2 minutes using a 365 nm handheld UV light with an effective power of 1.22 mW•cm -2 prior to mechanical loading.

RESULTS AND DISCUSSION

Luminescence properties of Ba 4 Si 6 O 16 :Eu 2+ , RE

In a Ba4Si6O16 crystal, SiO4 tetrahedra are corner-sharing to form 2-periodic single chains 27 FIG. 1 (a). Three single chains are linked into triple chains, running along [010] with a 21 symmetry, therefore forming 6-membered rings 5 7 . Two non-equivalent VIII-coordinated Ba 2+ sites exist in Ba4Si6O16, where Ba(1) is located in between two triple chains while Ba(2) is located between two single chains.

Three series of Ba4Si6O16:Eu 2+ , RE crystals were designed as to study both the incidence of the various RE co-doping, the Eu:RE ratio and the Eu+RE quantity on the luminescence properties. An excitation wavelength of 365 nm was chosen for the following measurements to remain consistent with the EML experiments discussed later in the article. The emission spectra consist of a broad emission band from 400 to 700 nm characteristic of the 4f 6 5d 1 → 4f 7 transition of Eu 2+ , with a maximum ranging from 516 to 526 nm. While its position is RE-dependent, no conclusions could be obtained regarding its trend. All Ba4Si6O16:Eu 2+ , RE crystals exhibited a green emission, with CIE coordinates x = 0.302, y = 0.478 when RE = Ho 3+ FIG. 2 (b). In the literature, the 𝜆 𝑚𝑎𝑥 position of the broad emission band of Ba4Si6O16:Eu 2+ , RE crystals ranges from 494 up to 526 nm. Indeed, its position is slightly redshifted when the europium concentration is increased 7 . Here, the emission maximum of Ba4Si6O16:Eu 2+ , Ho 3+ crystals with a fixed Eu:Ho ratio but with increasing europium concentrations from 1 to 3 cat. % increases from 517 to 525 nm FIG. 2 (c). Typically, Eu 2+ concentrations from 0.08 to 1.5 cat. % lead to emission maxima ranging from 495 to 505 nm 9 10 32 33 , while higher concentrations up to 4 cat. % lead to a maximum emission at 526 nm 7 . Hence, the reported emission color of Ba4Si6O16:Eu 2+ , RE phosphors ranges from bluish-green to green, and can easily be tuned by choosing the right RE co-doping as well as Eu 2+ concentration. The higher the Eu 2+ concentration is and the smaller the distance between the europium ions becomes, which means that the probability of energy transfer from higher-energy 5d levels Eu 2+ ions to lower-energy 5d levels Eu 2+ ions increases, resulting in the end in an emission band shifting towards longer wavelength 9 .

Several absorption peaks were observed when RE = Pr 3+ , Nd 3+ , Ho 3+ , or Er 3+ , with positions at 538, 547, and 638 nm in the case of Ho 3+ corresponding to transitions from the ground state 5 I8 to 5 F4, 5 S2, and 5 F5 respectively 34 . The absorption spectrum of Ba4Si6O16:Eu 2+ , Ho 3+ revealed both the broad absorption band of Eu 2+ at short wavelength (< 400 nm), as well as various Ho 3+ intraconfigurational transitions FIG. 2 (d). The absorption and the quantum efficiency were estimated at 79 ± 10 % and 17 ± 10 % respectively with 𝜆 𝑒𝑥𝑐 = 365 nm. Li et al. 7 The luminescence decay was studied over a duration of 6000 ns for RE = Ho 3+ and Er 3+ . It can be fitted using equation (1):

𝐼(𝑡) = 𝐴 1 exp (- 𝑡 𝜏 1 ) + 𝐴 2 exp (- 𝑡 𝜏 2 ) (1)
Where 𝐼(𝑡) is the luminescence intensity at a given time 𝑡, 𝐴 However, the kinetics of the LLP decay depends significantly on the selected RE FIG. 3 (b).

While the steady-state luminescence (SSL) intensity is roughly similar whatever the RE, the LLP intensity changes significantly. For example, it is increased by two orders of magnitude as Ho 3+ substitutes Sm 3+ . The RE leading to the highest LLP intensity at times smaller than 250 s are, in descending order, Ho 3+ Nd 3+ Pr 3+ and Er 3+ . The luminescence of Ba4Si6O16:Eu 2+ , Ho 3+ is visible to the naked eye beyond a day after the excitation source is stopped. FIG. 3 (c FIG. 4. Luminance of Ba4Si6O16:Eu 2+ , RE crystals reported here or in the literature 10 32 33 , as well as that of well-known phosphors such as Y3Al2Ga3O12:Ce 3+ , Yb 3+ (YAGG) 35 , SrAl2O4:Eu 2+ , Dy 3+ 36 , BaSi2O2N2:Eu 2+ 37 . The SSL intensity of Ba4Si6O16:Eu 2+ , Ho 3+ is also indicated.

TSL experiments provide key elements to better understand the LLP intensity and decay, such as the trap depths and distributions. Trap depth 𝐸 and concentration 𝑛 0 can be calculated from the TSL intensity 𝐼(𝑇) according to 38 :

𝐼(𝑇) = [( (𝑙-1)𝑠 𝛽 ) × ∫ exp (- 𝐸 𝑘 𝐵 𝑇 ) 𝑑𝑇 + 1 𝑇 𝑇 0 ] - 𝑙 𝑙-1 𝑠𝑛 0 exp (- 𝐸 𝑘 𝐵 𝑇 ) (2)
Where 𝑘 𝐵 is the Boltzmann constant, 𝑇 the temperature, 𝑙 the kinetic order, 𝑠 the frequency factor, and 𝛽 the heating rate (1 K•s -1 ). 𝐸 , 𝑙 and 𝑠 were deduced according to previously described methods 39 40 . TSL curves are plotted in FIG. 5 (a). TSL measurements of Ba4Si6O16:Eu 2+ , Ho 3+ carried out with various delay times.

In Ba4Si6O16:Eu 2+ , RE crystals, three recurring TSL peaks were observed FIG. 5 (a) and FIG. 5 (b). The first one lies in the 303-363 K region, while the second and third ones are systematically located at temperatures of 398 K and 523 K respectively. The only exceptions are i) RE = Dy 3+ , for which the width of the first peak suggests two combined peaks ii) Er 3+ , which also exhibits two peaks in the 303-363 K region, and iii) Tm 3+ that exhibits a third peak at 473 K. Such a recurrence in the peak position hints that the nature of those traps is similar regardless of the RE co-doping. Further experiments would be required to elucidate the physical and structural origins of these electron traps.

In addition, TSL experiments performed with different delay times with RE = Ho 3+ evidenced a gradual shift from the first TSL peak towards higher temperatures FIG. 5 (c), which implies that at least for this co-dopant there is actually a distribution of trap depths rather than a discrete energy level. This is consistent with preliminary DFT calculations performed on the Ba4Si6O16 crystal, which also show the presence of a distribution of energy levels arising from oxygen vacancies 28 . This distribution of energies would extend from at least 𝐸 𝐶 3 = 0.694 to 10 distribution of trap depths. For example, Dy 3+ co-doping does not induce a shift of the first TSL peak position with increasing delay times 10 .

𝐸 𝐶 1 = 0.
FIG. 6. LLP decay (< 60000 s) of Ba4Si6O16:Eu 2+ , Ho 3+ fitted with various distribution shapes.

The inset shows the LLP decay at short times (< 500 s).

In order to get more insight into the LLP behavior of Ba4Si6O16:Eu 2+ , RE crystals at the scale of a few minutes, only traps with a depth of 1 eV at most need to be considered, which correspond to the first peak in the 303-363 K region. Thus, the second and third peaks, which correspond to deeper traps are excluded from this study. The high sensitivity camera used for LLP experiments does not allow to quantify the luminescence intensity immediately after the excitation source is stopped. Therefore, the luminescence intensity at an arbitrarily chosen time of 250 s was used as a reflection of the LLP lifetime. As can be seen in FIG. 7, there is an

Eu(1) and Eu(2) emission bands

As europium ions are incorporated in Ba4Si6O16 crystals, Eu 2+ will occupy the two non-equivalent Ba 2+ sites. It was proposed that, because both Ba(1) and Ba(2) sites are coordinated by eight oxygen atoms 27 , the two europium ions environments are relatively similar, in agreement with the fact that two broad emission bands with close emission maximum are observed 7 10 32 . Indeed, while the 4f electrons of divalent europium are quite insensitive to their surroundings, this is not the case of the 5d electrons 32 . Consistently, the broad emission band could be deconvoluted into two bands at 𝜆 𝑚𝑎𝑥 = 500 and 550 nm, corresponding to the two non-equivalent europium sites, namely Eu(1) and Eu(2) 32 . This is supported by i) a shift of the broad emission band maximum towards longer wavelength when the delay time increases, which suggests the existence of two different lifetimes for each site 32 , and ii) a significantly different excitation spectra when 𝜆 𝑒𝑚 = 500 or 550 nm.

Thermoluminescence experiments performed in this work from 77 to 380 K support another interpretation. When the emission spectra are measured at temperatures as low as 77 K, one can observe a second emission band appearing at 𝜆 𝑚𝑎𝑥 = 439 nm, in addition to the green emission band at 𝜆 𝑚𝑎𝑥 = 521 nm FIG. 8 (a). The intensity of this blue emission band gradually decreases up to 200 K where it is no longer visible, while the thermal quenching of the green emission band starts at 160 K but is not yet completed at 380 K. Furthermore, excitation spectra collected at 77 K with 𝜆 𝑒𝑚 = 500, 521, or 550 nm are identical FIG. 8 (b), but all-three were notably different from the one obtained with 𝜆 𝑒𝑚 = 439 nm. Indeed, the latter is shifted towards shorter wavelengths and its behavior in the 330-400 nm region is clearly unalike. This is in contradiction with what was previously reported 32 . This suggests that i) the blue and green emission bands are of different nature, which is clearly outlined when 𝜆 𝑒𝑥𝑐 = 312 nm, and ii) the green emission band is not composed of two subsequent emission bands. The interpretation proposed in this work is supported by emission spectra measured at various times after the excitation source is stopped, where no shift of the emission maximum was observed FIG. 8 (b), which was already noted in another study 45 . The previously observed shift is likely resulting from the presence of β-BaSiO3 as a secondary phase, which is revealed by the presence of a peak on the XRD pattern at 26° FIG. 1. The persistent luminescence properties of this crystal are known 11 and an emission maximum at 𝜆 𝑚𝑎𝑥 = 560 nm was identified, which explains the shift of the emission maximum towards longer wavelengths with time.

It is thus concluded that the two non-equivalent Eu 2+ sites are sufficiently different to give birth to two emission bands at low temperatures. Indeed, the Ba(1) polyhedron is more distorted than the Ba(2) one 27 , so that an 82 nm difference between the two emission bands maximum is observed. It was proposed that the higher distortion degree of the Ba(1) polyhedra would result in a weaker crystal field of Ba(1) than that of Ba(2) 10 , thus, the broad emission band of Eu (1) would be in a shorter wavelength region than that of Eu(2) 46 , i. e. the blue emission band would correspond to Eu(1) and the green one to Eu(2). However, it is still not clear as to why the thermal quenching of Eu(1) occurs at much lower temperatures than that of Eu(2).

The thermoluminescence behavior of Ba4Si6O16:Eu 2+ , Ho 3+ is very similar to that of monoclinic SrAl2O4:Eu 2+ , Dy 3+ . In the latter, a blue emission band also appears at low temperatures (𝜆 𝑚𝑎𝑥 = 445 nm) which arises from the presence of two non-equivalent Sr 2+ sites in the crystal structure 47 . In addition, the asymmetric green emission band of Ba4Si6O16:Eu 2+ , Ho 3+ is shifted to larger wavelengths just like the green emission band ( 𝜆 𝑚𝑎𝑥 = 520 nm) of strontium aluminate. In this latter case, this was discussed by Botterman et al. 47 in terms of the centroid shift and crystal field splitting. This suggests that the charge carriers involved in the EML of Ba4Si6O16:Eu 2+ , RE crystals slowly recombine with the luminescent center with time, and are thus also participating in the LLP at the scale of several minutes/hours.

Mechanoluminescence properties of

There seems to be a strong connection between the EML intensity and the LLP intensity FIG. 9 (c). Various mechanisms were proposed to explain the mechanically-induced luminescence 29 , among which those involving piezoelectricity are the most popular. It is supposed that, as a stress field is applied, the trap depths are changed thanks to piezoelectricity, ultimately altering the charge carriers recombination kinetics. Although Ba4Si6O16 is centrosymmetric (space group P21/c) and should therefore not exhibit piezoelectricity, centrosymmetry might be broken at the local scale by the presence of defects, so that piezoelectricity can subsequently be observed 29 . In Ba2Si2O2N2:Eu 2+ , the stress is assumed to induce a decrease of a trap depth leading to an increased luminescence intensity 20 . In the SrAl2O4:Eu 2+ , Dy 3+ compound, a mechanism with three discrete traps allowed to explain an increase in the luminescence intensity both during the loading and the unloading stages 49 . This correlation is therefore consistent with this latter mechanism: a larger trap concentration with depths ranging from 0.65 to 0.85 eV is assumed i) to result in a stronger LLP intensity, but also ii) in a stronger EML intensity, as more charge carriers will be able to recombine with the luminescent center when the trap depth decreases. where EML is more intense and therefore better resolved FIG. 9 (d). It is noteworthy that the EML behavior of the Ba4Si6O16:Eu 2+ , Ho 3+ /epoxy composite is identical to that of a mechanoluminescent glass-ceramic with the same cationic composition 28 . The mechanical loading and its holding lead to an increase in the luminescence intensity, while the unloading leads to a decrease of the luminescence intensity. Changes in irradiation time lead to an increase in both the luminescence and the mechanoluminescence intensity.

In

3 varying

 3 Ba -60 Si -3 Eu -2 RE for each RE, ii) Eu 2+ :RE ratio for RE = Pr 3+ or Ho 3+ , and iii) varying Eu 2+ +RE quantity for RE = Ho 3+ .Stochiometric amounts were weighted and grounded during 15 minutes in ethanol in an agate mortar, and then fired 6 h at 1300 °C in an alumina crucible under a N2:H2 = 94:6 reducing atmosphere. Each specimen was grounded into a fine powder.The X-ray diffraction (XRD) patterns were collected using a PANalytical X'pert Pro diffractometer using Cu-Kα radiation (1.5418 Å).Solid-state excitation and emission spectra were recorded using a Horiba Jobin-Yvon Fluorolog III fluorescence spectrometer equipped with a Xe lamp operating at 450 W, a UV-vis-nearIR photomultiplier (Hamamatsu R928, sensitivity 190 -860 nm), or a Horiba Jobin-Yvon FluoroMax-4 Plus fluorescence spectrometer equipped with a Xe lamp operating at 150 W and a UV-vis photomultiplier (Hamamatsu R928, sensitivity 190 -860 nm). The luminescence spectra were realized on powder samples placed inside the solid-sample holder fixed in 30° rotation and blocked by a quartz blade.

  200-1100 nm. An appropriate grating covering the 200-1100 nm range was used. LLP and EML experiments were performed on powder/epoxy composites, where each Ba4Si6O16:Eu 2+ , RE crystal was mixed with an epoxy resin (obtained by mixing EPOFIX resin Struers and EPOFIX hardener Struers with a 90:10 concentration by weight) with a final powder content of 20 wt. %. Mechanical tests were carried out on disks using a Shimadzu-AGS-X testing machine with a 10 kN load cell and a SiC pusher. The stress 𝜎 induced at the center of a disk made in the composite material (epoxy + ceramic powder) during the diametral compression test is expressed as 𝜎 = 2𝑃 𝜋𝑑ℎ

2 XRDFIG. 1 .

 21 FIG. 1. (a) Crystal structure of Ba4Si6O16. Green, yellow, and blue polyhedra correspond to Ba(1), Ba(2), and Si-based polyhedra respectively. The local coordination of the Ba(1) and

FIG. 2 .

 2 FIG. 2. (a) Excitation spectrum of Ba4Si6O16:Eu 2+ , Ho 3+ , and emission spectra of Ba4Si6O16:Eu 2+ , RE crystals (b) CIE chromaticity coordinates of Ba4Si6O16:Eu 2+ , Ho 3+ (c) Dependence of the emission wavelength on the europium concentration in Ba4Si6O16:Eu 2+ , Ho 3+

  reported a similar absorption (80 %) but a considerably higher quantum efficiency (46 %) in Ba3.8Eu0.2Si6O15.85N0.1.

FIG. 3 .

 3 photographs at various times of the LLP decay in the case of Ba4Si6O16:Eu 2+ , Ho 3+ .

FIG. 5 .

 5 FIG. 5. (a) TSL measurements of Ba4Si6O16:Eu 2+ , RE crystals with a heating rate of 1 K•s -1 . (b) Zoom-in of the TSL measurements highlighting the recurring 398 and 523 K TSL peaks (c)

FIG. 7 .

 7 FIG. 7. Trap depth and concentration of the various Ba4Si6O16:Eu 2+ , RE crystals determined from equation (2).

FIG. 8 .

 8 FIG. 8. (a) Excitation spectra measured at 77 K and emission spectra measured at various temperatures (20 K steps) of Ba4Si6O16:Eu 2+ , Ho 3+ . The inset shows the emission intensity at various temperatures. (b) Excitation and emission spectra of Ba4Si6O16:Eu 2+ , Ho 3+ at 77 K. The inset shows the LLP emission spectra at ambient temperature various times after the excitation source is stopped.

FIG. 9 .

 9 FIG. 9. (a) EML intensity of Ba4Si6O16:Eu 2+ , RE/epoxy composites (b) Peak EML intensity of Ba4Si6O16:Eu 2+ , RE/epoxy composites (c) Peak EML intensity plotted as a function of the luminescence intensity at 250 s (d) EML intensity of Ba4Si6O16:Eu 2+ , Ho 3+ /epoxy composite with stress rates of 1.8 and 8.8 MPa•s -1 .

4 FIG. 10 .

 410 FIG. 10.Energy level scheme of Ba4Si6O16:Eu 2+ , Ho 3+ under mechanical stress. For the sake of clarity, the trap distribution is shown as a continuous succession of trap depths.

  1 and 𝐴 2 are constants, and 𝜏 1 and 𝜏 2 are time constants describing the luminescence decay. Two regimes are observed in the beginning for a duration of about 200 s : a rapid one at t < 200 ns followed by a slower one, corresponding respectively to 𝜏 1 = 0.107 ± 0.002 µs and 𝜏 2 = 1.140 ± 0.004 µs FIG.3 (a), suggesting that in both cases the luminescent center is Eu 2+ while the RE act as sensitizers.

Yang et al.

10

also identified two lifetimes in Ba4Si6O16:Eu 2+ , RE crystals when RE = Eu 2+ , Dy 3+ , or Ho 3+ , however these lifetimes are significantly larger than those reported here (𝜏 1 = 4 µs and 𝜏 2 = 30.3 µs). In addition, Chen et al.

8

observed a decrease of the effective decay time with an increase of the europium content in Ba3.86-ySi6O16:0.07Ce 3+ ,0.07Li + , yEu 2+ , which suggests that a variation of the Eu 2+ :RE ratio and/or Eu 2+ +RE quantity induces a visible change of the specimens lifetime. This was attributed to energy transfers between RE 3+ co-dopants and Eu 2+ .

  924 eV, where 𝐶 3 is the top and 𝐶 1 the bottom of the trap distribution. It is slightly shallower than that reported by Yang et al. (which extended from 0.602 to 1.156 eV)10 , where its deepness was possibly overestimated because of the close second TSL peak at 398 K. As a

	matter of fact, short (< 500 s) and long time (up to 60000 s) luminescence decay measured at
	the peak emission wavelength would suggest that this distribution is close to a gaussian form
	FIG. 6 41 42 43 44 . It is noteworthy that RE co-doping in Ba4Si6O16 does not always lead to a

Ba 4 Si 6 O 16 :Eu 2+ , RE The

  EML response of Ba4Si6O16:Eu 2+ , RE/epoxy composites, obtained by subtracting the LLP intensity as recorded at rest (i. e. at σ = 0 MPa) to the mechanically-loaded specimen, was characterized by means of the diametral compression tests FIG.9 (a). Each test was performed with a 0.37 MPa•s -1 stress rate. It is recalled that the EML intensity is proportional to both the stress and the stress rate48 . In a similar way as with the previous LLP measurements, the RE co-doping has a pronounced effect on the intensities of the EML signal. The largest intensities are obtained with RE = Ho 3+ , Pr 3+ , Er 3+ , or Nd 3+ FIG. 9 (b). The mechanical stress was applied 90 s after the excitation source was stopped, as longer delays resulted in smaller EML intensity.

  particular, TSL experiments provide insight into the EML mechanism. Based on the existence of an electron trap distribution, we can propose an energy level scheme for Ba4Si6O16:Eu 2+ , Ho 3+ FIG.10. Although the identified charge carriers (holes and/or electrons) involved in the afterglow of Ba4Si6O16:Eu 2+ , RE phosphors are still unknown32 , it is assumed that electrons are the key players. Traps corresponding to the 398 K and 523 K TSL peaks are too deep to contribute to the EML effect. The Eu 2+ ions electronic configuration changes from the ground state 4f7 to the 4f 6 5d 1 excited state under UV light. Then, the electron can escape to the conduction band and eventually reaches energy levels located in the band gap, leaving behind Eu 3+ ions. The main effect of the mechanical loading is to change the depth of the trap distribution, i. e. the 𝐶 3 and/or the 𝐶 1 trap depth decreases (FIG.10illustrates the scenario where the depth of the whole trap distribution decreases). The recombination probability is therefore higher and an increase of the luminescence intensity follows. However, the mechanical loading empties the stocked charge carriers at a much higher rate than it would have at rest. As a matter of fact, a smaller luminescence intensity is observed after the stress is relieved. This model provides an explanation to the observed EML during both mechanical loading and unloading, in consistency i) with the EML mechanisms involving changes in trap depths through piezoelectricity, and ii) the decrease in the EML intensity with long delays prior to mechanical loading in Ba4Si6O16:Eu 2+ , RE crystals.

ACKNOWLEDGEMENTS

We acknowledge financial support from Région Bretagne and from the European Research Council (ERC Adv. Grant "DAMREG").

DATA AVAILABILITY

The data that support the findings of this study are available within the article and from the corresponding author upon reasonable request.

AUTHOR DECLARATIONS

Conflict of Interest


est destinée au dépôt

Encapsulation of bioactive phenolic extract from brewers' spent grain for use in functional foods Investigating bioequivalence in a cellular setup

CONCLUSIONS

 Transporter and metabolic enzymes were stimulated by both extracts. However the crude extract was a stronger modulator than the encapsulated extract, suggesting a small masking effect of the matrix on the absorption and metabolic pathways  Encapsulation seems however to protect the cells from IL8 expression (not shown) compared to the crude extract

Ongoing analyses (phenol absorption)

 Polyphenol analyses in the basal compartment of the cell system are still ongoing. It is therefore too early to draw conclusions on the equivalent bioavailability of both extracts or the impact of genetic modulations on the actual uptake and metabolic parameters The brewers' spent grain (BSG) is the main solid by-product from beer industry, representing up to 30% of the starting malted grain. The composition of BSG is variable according to the barley variety and quality, malting and mashing conditions but regardless of these factors it con-tains appreciable amounts of valuable molecules. One way to valorise BSG is by recovering these highvalue components such as phenolic compounds that can be reused as functional ingredients in food industry or other related sectors.

The extract is targeting the bakery industry as an antioxidant additive. To reduce the risks of degradation or oxidation during the baking process, as well as keep organoleptic properties, it was encapsulated. It is however necessary to prove that the bioactives are being delivered to the body in order to provide their health benefits.

The objective of the study was thus to validate in vitro whether the bioactive phenolic compounds are absorbed in a similar manner through the intestinal barrier.

Flore

  Depeint, Sonia Ancuţa Socaci, Anca Corina Fărcaş, Larbi Rhazi, Dan Cristian Vodnar To cite this version: Flore Depeint, Sonia Ancuţa Socaci, Anca Corina Fărcaş, Larbi Rhazi, Dan Cristian Vodnar. Encapsulation of bioactive phenolic extract from brewers' spent grain for use in functional foods Investigating bioequivalence in a cellular setup. Bioavailability 2018, Sep 2018, Norwich, Norfolk, United Kingdom. hal-04336221 Encapsulation of bioactive phenolic extract from brewers' spent grain for use in functional foods Investigating bioequivalence in a cellular setup Flore Dépeint a , Sonia Ancuţa Socaci b,c , Anca Corina Fărcaş b,c , Larbi Rhazi a , Dan Cristian Vodnar b,c a EA 7519 Transformations & Agro-ressources, Institut Polytechnique UniLaSalle, 19 rue Pierre Waguet, 60000 Beauvais, France. b Faculty of Food Science and Technology, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, 3-5 Calea Mănăştur, Cluj-Napoca, 400372, Romania. c Institute of Life Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, 3-5 Calea Mănăştur, Cluj-Napoca, 400372, Romania. CONTEXT Measuring bioavailability RESULTS (modulation of genetic expression)  Transporters were not modulated by either product  LPH and CBG expressions were increased by the crude but not encapsulated extract  UDPGT was stimulated by both extracts while SULT expression was only increased by the crude extract  COMT and CYP3A4 were not modulated by either product For the crude extract only (not shown):  Low dose only modulated UDPGT, COMT and GLUT2 at 2h while high dose modulated most genes up to 6h  There was time dependent modulation with highest expression at 2h, then reducing over time

OBJECTIVE

Mechanisms of intestinal absorption and metabolism

BSG extraction and encapsulation