Eu3+ doped fiber-based germanotellurite (NZPGT) glasses with medium-low maximum phonon energy of 782 cm−1 have been fabricated and characterized. Judd-Ofelt intensity parameters Ω2 (6.25 × 10−20 cm2) and Ω4 (1.77 × 10−20 cm2) indicate a high asymmetrical and covalent environment around Eu3+ in the optical glasses. The spontaneous emission probability of the dominant transition 5D0→7F2 peaking at 612.5 nm and the corresponding maximum stimulated emission cross-section were derived to be 445.7 s−1 and 2.05 × 10−21 cm2, respectively, confirming the effectiveness of the red fluorescence emission. The quantum yield was derived to be 12% under 391 nm LED excitation, and approximately 88% photons have been demonstrated in wavelength range of 600−720 nm, indicating that Eu3+ doped NZPGT glasses under proper excitation conditions are promising optical materials for fiber-based irradiation light sources that are competent to activate diverse photodynamic therapy photosensitizers.
© 2013 Optical Society of America
Minimally invasive photodynamic therapy (PDT) has already been a clinically approved therapeutic modality that combines photosensitizers (PSs) and matching excitation light in the presence of ground state triplet oxygen (3O2) to produce a highly reactive oxygen species (ROSs) termed singlet oxygen (1O2) that kills malignant cells by apoptosis or necrosis [1–3]. Generally, such a therapeutic procedure comes down to a PS accumulating in the target tissue, and light irradiation conforming to the excitation band of the PS and being delivered to diseased target organs by a set of flexible fiber-optic device. One determinant of the efficacy of PDT irradiation light sources is the fluence that is codetermined by the translucency and the energy delivered to the tissue . In addition, the “therapeutic window” (600−1000 nm) where biological tissue scattering predominates over absorption is another important factor . Previous investigations on most tissue models have demonstrated that the efficient wavelength range for therapeutic activation of most PSs is typically from 600 to 800 nm (Q1 band) [6,7]. Consequently, high-intensity irradiation light sources in wavelength range of 600−800 nm are especially desirable in PDT modality.
At present, lasers and light emitting diodes (LEDs) are widely used as PDT light sources due to their special advantages such as favorable directivity and high fluence rates [8–11]. However, lasers with tremendous power density may mis-locate target diseased tissues, causing accidental damages to normal tissues. In terms of LEDs, low coupling efficiency with optical fiber catheters and narrow excited spectral bandwidth, to some extent, restrict their applications in PDT modality. Recently, investigations have been reported on the efficient fluorescence generated in rare-earth (RE) ions doped glass fibers and glass channel waveguides, which are deemed to be a new route for high-quality irradiation light sources for PDT modality [12,13]. As an active fluorescent center, Eu3+ emits intense fluorescence ranging from 570 to 720 nm wavelength region [14–17] that is within the scope of the maximum absorption regions of most PSs currently used in PDT modality or under clinical trials . Among oxide glasses, tellurite glasses have low maximum phonon energy, high refractive index, and good rare-earth ion solubility , but exhibit low glass stability and mechanical strength for fabricating high-quality optical fibers. With the introduction of GeO2, tellurite glasses are expected to possess better glass stability and mechanical strength for fiber fabrication.
In this work, Eu3+ doped fiber-based germanotellurite (NZPGT) glasses with medium-low maximum phonon energy have been fabricated and characterized. The thermal stability range was derived to be 101 °C, indicating that NZPGT glasses are potential optical materials for fiber fabrication. Judd-Ofelt intensity parameters Ω2 (6.25 × 10−20 cm2) and Ω4 (1.77 × 10−20 cm2) indicate a high asymmetrical and covalent environment around Eu3+ in the optical glasses. The spontaneous emission probability of dominant transition 5D0→7F2 peaking at 612.5 nm and the corresponding maximum stimulated emission cross-section were derived to be 445.7 s−1 and 2.05 × 10−21 cm2, respectively, confirming the effectiveness of the red fluorescence emission in the optical glasses. The quantum yield was derived to be 12% under 391 nm LED excitation, and approximately 88% photons have been demonstrated in wavelength range of 600−720 nm, indicating that Eu3+ doped NZPGT glasses under proper excitation conditions are promising optical materials for fiber-based irradiation light sources to activate diverse PDT photosensitizers.
According to the molar host composition 14Na2O−10ZnO−7PbO−19GeO2−50TeO2 (NZPGT), Eu3+ doped NZPGT glasses were prepared from high-purity Na2CO3, ZnO, PbO, GeO2, and TeO2 powders. Additional 0.2wt% and 1.2wt% Eu2O3 were introduced into the NZPGT glass composition based on the host weight. Firstly, the raw materials were well grinded in an agate mortar and preheated in pure Pt crucibles at 270 °C for 3 h. Then the glass melts were wobbled every ten minutes when melted at 880 °C for 30 min, and finally quenched in a preheated aluminum mold. Subsequently, the glass samples were immediately annealed at 270 °C for 3 h to diminish the inhomogeneity in the Eu3+ ion distribution caused by the quenching process, and slowly cooled down to room temperature. For optical measurements, the annealed glass samples were sliced and polished into pieces with parallel sides.
The infrared transmittance spectrum of 0.2wt% Eu2O3 doped NZPGT glasses was obtained by a Spectrum One-B Fourier transform IR (FTIR) spectrometer. The differential thermal analysis (DTA) scan was carried out by a WCR-2D differential thermal analyzer at a rate of 15 °C/min from room temperature to 900 °C. The density of 1.2wt% Eu2O3 doped NZPGT glasses was obtained to be 5.117 g∙cm−3 by the Archimedes method. The refractive indices were derived to be 1.9300 at 632.8 nm and 1.8849 at 1536 nm using a Metricon 2010 prism coupler, respectively. The refractive indices at all the other wavelengths can be solved by the Cauchy’s equation  with A = 1.8757 and B = 21751 nm2 for the coming Judd-Ofelt (J-O) analysis. The absorption spectrum was detected by a Perkin-Elmer UV-VIS-NIR Lambda 19 double beam spectrophotometer with wavelength accuracy of ± 0.15 nm for UV/VIS region and ± 0.6 nm for NIR region. The excitation and emission spectra were measured using a Jobin Yvon Fluorolog-3 spectrophotometer with spectral accuracy of 0.5 nm equipped with an R928 photomultiplier (PMT) tube as detector and a CW Xe-lamp as pump source. The spectral power distributions were measured using a 30 cm diameter integrating sphere equipped with an Ocean Optics USB4000 CCD detector with optical resolution of ~1.5 nm FWHM connected by a 400 μm-core optical fiber. The schematic diagram of the experimental setup for quantum yield measurement is depicted in Fig. 1.The currents of commercially available 391 and 456 nm LEDs as pump sources were both fixed at 20 mA. A standard halogen lamp (EVERFINED062) was used for calibrating this measurement system and its spectral power distribution was obtained through fitting the factory data based on the blackbody radiation law. The LED pump sources were rounded by black tapes except the emitting surfaces were mounted in the integrating sphere. For quantitative characterization and analysis on absolute spectral properties of fluorescence, Eu3+ doped NZPGT glasses were put on the LED pump sources and covered their tops completely. All pictures were taken with a Sony SLT-α200 digital camera.
3. Results and discussion
3.1 Phonon energy and thermal properties of Eu3+ doped NZPGT glasses
The FTIR spectrum of 0.2wt% Eu2O3 doped NZPGT glasses is shown in the inset of Fig. 2.The maximum phonon energy of Eu3+ doped NZPGT glasses, to some extent, can be derived by the IR transmission side band and was estimated to be 782 cm−1 by the empirical formula , in which R is the wavenumber at 10% transmittance. The derived maximum phonon energy 782 cm−1 is ascribed to more distorted TeO4 due to the lengthening of one Te-O axial bond of the bipyramidal site [20,21]. The DTA curve of 0.2wt% Eu2O3 doped NZPGT glasses is presented in Fig. 2. The transition temperature Tg, the crystallization onset temperature Tx, and the crystallization temperature Tc were derived to be 295, 396, and 416 °C, respectively. Generally, thermal stability range (ΔT = Tx−Tg) of core and cladding glasses should be larger than 100 °C to obtain wide operating temperature range and avoid crystallization during fiber drawing [22–24]. Here, the ΔT of 0.2wt% Eu2O3 doped NZPGT glasses was derived to be 101 °C, indicating that Eu3+ doped NZPGT glasses are potential candidates for fiber materials.
In addition, another two critical thermal property parameters, i.e. the thermal stability parameter H and the Sadd-Poulain criterion S, are introduced to further evaluate the ability of NZPGT glasses against crystallization. The H parameter and the S criterion are defined as and, respectively. For 0.2wt% Eu2O3 doped NZPGT glasses, the H parameter and the S criterion were derived to be 0.34 and 6.85, respectively, which is similar to the glass system K2O−Nb2O5−GeO2−TeO2  and demonstrates that the introduction of GeO2 can extend the thermal stability range, and improve the chemical and thermal stability of the tellurite glasses for fiber drawing.
3.2 Spectral properties of Eu3+ doped NZPGT glasses
As labeled in Fig. 3(a), the absorption spectrum of 1.2wt% Eu2O3 doped NZPGT glasses presents five absorption bands that are attributed to the absorption transitions from the ground state 7F0 to the excited states 5L6, 5D3, 5D2, 5D1, and 7F6, respectively. The 7F0→5D1, 7F0→5D2, and 7F0→5D3 absorption bands are weak due to spin-forbidden rule. In contrast, the 7F0→5L6 and the 7F0→7F6 absorption transitions are stronger owing to spin-allowed rule. Doublet peaks at 534 nm (5D1) and 2082 nm (7F6), and shoulders on the broader peak at 394 nm (5L6) manifest the two thermally populated ground states 7F0,1. Because the closely spaced 7F0,1 ground levels have energy level differences on the order of kT, where k is Boltzmann’s constant and T is the absolute temperature, and they are easily thermal populated at room temperature unlike the other lanthanides that only have a single populated ground state . For Eu3+, the population CJ (J = 0, 1, 2, 3, 4) of any level 7FJ (J = 0, 1, 2, 3, 4) is given by27], and E0 is energy of the ground state. Given that T = 298 K, the fractional populations were derived to be 0.6615 (7F0), 0.3166 (7F1), 0.0213 (7F2), 5.24 × 10−4 (7F3), and 5.52 × 10−6 (7F4), respectively.
The excitation spectrum for the red fluorescence emission of Eu3+ doped NZPGT glasses is shown in Fig. 3(b), which covers 350−600 nm spectral range and indicates that the red fluorescence at 612.5 nm originating from the emission transition 5D0→7F1 can be efficiently achieved under the excitation of commercially available UV/violet/blue/green laser diodes (LDs) and LEDs, together with Ar+ laser.
As labeled in Fig. 3(c), three main emission peaks at 591.5, 612.5, and 701.0 nm originate from Eu3+ characteristic excited level 5D0 to the lower lying levels 7F1,2,4 transitions, respectively, under 466.5 nm excitation. Other five minor emission peaks at 510.0, 536.0, 553.0, 578.5, and 652.5 nm are ascribed to 5D2→7F3, 5D2→7F4, 5D1→7F2, 5D0→7F0, and 5D0→7F3 transitions, respectively. The commonly observed 5D0→7F0,1,2,3,4 transitions were clearly detected, meanwhile, the infrequent transitions from the upper lying states 5D2 and 5D1 were also observed due to the medium-low maximum phonon energy (782 cm−1) of Eu3+ doped NZPGT glasses, though the intensities of the emission peaks are weak. The contributions of electron-phonon anharmonicities relate to temperature and cause effects such as emission peak shift, the quantum efficiency on the polarization of the pumping beam, and the non-polar third rank polar tensor like optical second harmonic generation [28,29]. For the Eu3+ doped germanotellurite glasses, there exist contributions of electron-phonon anharmonicities due to the special glass network structure and the low maximum phonon energy of the optical glasses containing heavy metal oxide.
The hypersensitive transition 5D0→7F2 of Eu3+ (an electric-dipole transition governed by the selection rules ) is extremely sensitive to ligand symmetry and bond covalence , and tends to be much more intense in non-symmetric sites and high bond covalence. Meanwhile, the 5D0→7F1 transition is independent on ligand symmetry and bond covalence due to magnetic dipole-allowed rule, which makes the integrated fluorescence intensity ratio of 5D0→7F2/5D0→7F1, i.e. the asymmetry ratio R, a good criterion to estimate the site asymmetry and the chemical bond covalency of Eu3+ in NZPGT glasses [31–33]. Typicallly, higher value of R indicates lower ligand symmetry and higher bond covalency. Here, the R (3.58) of 1.2wt% Eu2O3 doped NZPGT glasses is higher than those of Eu3+ doped ZnO−TlO0.5−TeO2 (3.40) , CaO−La2O3−B2O3 (3.10) , and PbF2−WO3−TeO2 glasses (2.78) , which indicates that Eu3+ ions in NZPGT glasses occupy relatively lower ligand symmetry sites and higher bond covalence, and are beneficial for red fluorescence emission in the optical glasses.
J-O intensity parameters Ωt (t = 2, 4, 6) are important indicators to predict some radiative properties such as oscillator strengths, luminescence branching ratios, energy-transfer probabilities, and excited-state radiative lifetime , which are usually derived from absorption spectrum. Nevertheless, they were calculated from the emission spectrum due to the special energy level structure of Eu3+ in present work. The 5D0→7F2,4,6 transitions originating from Eu3+ are electronic-dipole allowed, and the spontaneous emission probability Aed from initial manifold to final manifold is given using the following expression38]. The 5D0→7F1 transition of Eu3+ is magnetic-dipole allowed, and the spontaneous emission probability of magnetic-dipole transition Amd can be derived by using the following expression31] in this work. Due to selection rules and the special energy level structure of Eu3+, the J-O intensity parameters Ωt can be solved from the ratios of the integrated fluorescence intensity of 5D0→7F2,4,6 transitions to the integrated fluorescence intensity of 5D0→7F1 transition as follows:
The intensity parameter Ω2 has been identified to be associated with the asymmetry and the covalence between RE ions and ligand anions, and Ω4,6 parameters are related to the bulk property and rigidity of the samples, respectively. In NZPGT glasses, the J-O intensity parameters Ω2 and Ω4 were solved to be 6.25 × 10−20 and 1.77 × 10−20 cm2, respectively, while the J-O intensity parameter Ω6 was not obtained due to the absence of 5D0→7F6 transition. For comparison, the J-O parameters Ωt of Eu3+ in various optical glasses are listed in Table 1. The Ω2 parameter (6.25 × 10−20 cm2) in NZPGT glasses is smaller than the value of the glass system ZnO−B2O3 (9.80 × 10−20 cm2) , and is comparable to the values of the glass systems MgO−PbO−B2O3−SiO2 (6.01 × 10−20 cm2)  and K2O−SrO−Al2O3−P2O5 (6.34 × 10−20 cm2) , showing a strong asymmetrical and covalent environment around Eu3+ in the optical glasses.
Using Ωt values, spontaneous transition probabilities Aij, branching ratios βij, and calculated radiative lifetime τrad were calculated and listed in Table 2. The β of 5D0→7F2 transition is dominant and is as high as 70.9%, indicating that the intense red fluorescence at 16327 cm−1, i.e. 612.5 nm, can be efficiently achieved in Eu3+ doped NZPGT glasses as irradiation light sources to activate diverse PDT photosensitizers.
Stimulated emission cross-section σem is a significant parameter to evaluate the energy extraction efficiency from RE doped optical materials, which can be solved using the Füchtbauer-Ladenburg formulaFig. 3(d), and the maximum stimulated emission cross-sections σem-max of those transitions are listed in Table 2. Here, the σem-max of dominant emission transition 5D0→7F2 peaking at 612.5 nm was derived to be 2.05 × 10−21 cm2, which is larger than those of Eu3+ doped LiF−Li2CO3−H3BO3 (0.16 × 10−21 cm2) , ZnF2−PbO−TeO2 (0.23 × 10−21 cm2) , and K2O−SrO−Al2O3−P2O5 glasses (1.28 × 10−21 cm2) , predicting that the red fluorescence can be efficiently extracted in Eu3+ doped NZPGT glasses as irradiation light sources to deliver sufficient energy intensity to activate diverse PDT photosensitizers.
3.3 Quantitative characterization and analysis on absolute spectral properties of fluorescence
Under different pump sources, 0.2wt% and 1.2wt% Eu2O3 doped NZPGT glasses vary in fluorescence colors and intensities as depicted in Fig. 4.
As shown in Figs. 4(a) and 4(b), 1.2wt% Eu2O3 doped NZPGT glasses exhibit stronger fluorescence than 0.2wt% Eu2O3 doped NZPGT glasses under 365 nm UV lamp radiation due to a higher Eu3+ concentration. However, 1.2wt% Eu2O3 doped NZPGT glasses exhibit different colors under 391 and 456 nm LED excitations as depicted in Figs. 4(c) and 4(d). In fact, such a phenomenon results from polychromatic light, i.e. violet light mixing red light gets orange light and blue light mixing red light gets magenta light.
In order to essentially reveal the absolute spectral properties of fluorescence in 1.2wt% Eu2O3 doped NZPGT glasses, spectral power distributions, Pon and Pside, were recorded using the integrating sphere method with commercially available 391 and 456 nm LED as pump sources when the glass sample was located on (Pon, curve 1 in Figs. 5(a) and 5(b)) and aside (Pside, curve 2 in Figs. 5(a) and 5(b)) the LED pump sources. During the integrating sphere measurement for quantum yield, there exists light scattering from the glass sample. Nevertheless, the integrating sphere can reflect the scattering light repeatedly until it is evenly distributed. Therefore, the scattering of light as well as other parasitic problems is negligible here.
The total radiant flux, ΦE, of the luminescence can be calculated byFigs. 5(a) and 5(b), the radiant fluxes of 1.2wt% Eu2O3 doped NZPGT glasses under 391 and 456 nm LED excitations were solved to be 1402 and 9329 μW in visible light wavelength range of 380−780 nm, respectively, when the glass sample was located on the tops of the LEDs. In fluorescence emission wavelength range of 570−720 nm, the radiant fluxes were solved to be 55 and 108 μW, respectively.
Based on absolute spectral power distributions, photon distributions N(v) can be derived byEq. (8) with spectral power distributions Pon and Pside, respectively. The net absorption and emission photon distribution curves of 1.2wt% Eu2O3 doped NZPGT glasses were derived by subtracting the Nside component from the Non component as depicted in Figs. 5(c) and 5(d).
Quantum yield (QY) is being used as a selection criterion of luminesce materials for potential applications in solid-state lighting and is defined as the ratio of the number of emitted photons to the number of absorbed photons. Namely,Figs. 5(c) and 5(d).
As shown in the inset of Fig. 6, the QYs of 1.2wt% Eu2O3 doped NZPGT glasses under 391 and 456 nm LED excitations were calculated to be 12% and 4%, respectively, indicating that proper pump sources are crucial to obtain high external quantum yield in the optical glasses.
The absorption cross-section σabs(v) is calculated from the absorption spectrum by using the following formulaFig. 6, which reveals that Eu3+ are more beneficial in absorbing photons over NZPGT glass host under 391 nm LED excitation than that of under 456 nm LED excitation.
The broadband fluorescence ranging from 570 to 720 nm of 1.2wt% Eu2O3 doped NZPGT glasses under 391 and 456 nm LED excitations were divided into five equal parts with a step length of 30 nm, and the photon numbers of each wavelength interval were obtained by integrating the net emission photon distributions.
As a result, photon number percentages of different wavelength regions based on total emission photon numbers in wavelength range of 570−720 nm were labeled and shown in Figs. 7(a) and 7(b). Approximately 88% photons of the total fluorescence emitted from the glass sample ranging from 600 to 720 nm are qualified to match the maximum absorption regions of the Q1 bands of diverse clinical photosensitizers, such as Photofrin® (~630 nm), Metvix® (~630 nm), Foscan® (~652 nm), Laserphyfrin® (~654 nm), Photochlor® (~665 nm), Photosens® (~675 nm), and Visudyne® (~689 nm) [7,47,48]. More than 55% and 20% photons have been demonstrated in wavelength range of 600−630 nm and 690−720 nm, respectively, indicating that Eu3+ doped NZPGT glasses as irradiation light sources are especially efficient for Photofrin®, Metvix®, and Visudyne®. Therefore, high quantum yield, intense broadband fluorescence, and foreseeable amplified spontaneous emission (ASE) fluorescence generated in Eu3+ doped NZPGT glass fibers under proper excitation conditions can deliver sufficient energy intensity to activate diverse PDT photosensitizers, producing a highly reactive oxygen species (1O2) and then resulting in cancer cell death via apoptosis or necrosis in PDT modality.
Eu3+ doped fiber-based germanotellurite (NZPGT) glasses with medium-low maximum phonon energy of 782 cm−1 have been fabricated and characterized. The derived Judd-Ofelt parameters Ω2 (6.25 × 10−20 cm2) and Ω4 (1.77 × 10−20 cm2) indicate a high asymmetrical and covalent environment around Eu3+ in the optical glasses. The spontaneous emission probability of the dominant transition 5D0→7F2 peaking at 612.5 nm and the corresponding maximum stimulated emission cross-section were derived to be 445.7 s−1 and 2.05 × 10−21 cm2, respectively, confirming the effectiveness of the red fluorescence emission. The quantum yield was derived to be 12% under 391 nm LED excitation, and approximately 88% photons have been demonstrated in wavelength range of 600−720 nm, indicating that Eu3+ doped NZPGT glasses under proper excitation conditions hold great promise for fiber-based irradiation light sources that are qualified to activate diverse photodynamic photosensitizers.
This work was supported by the National Natural Science Foundation of China (61275075) and the Science and Technology Foundation of Liaoning Province, China (201202011).
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