Photoluminescent Porous Alginate Hybrid Materials Containing

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Biomacromolecules 2008, 9, 1945–1950

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Photoluminescent Porous Alginate Hybrid Materials Containing Lanthanide Ions Fengyi Liu,†,‡ Luis D. Carlos,*,† Rute A. S. Ferreira,† Joa˜o Rocha,§ Maria Concetta Gaudino,‡ Mike Robitzer,‡ and Franc¸oise Quignard*,‡ Department of Physics, CICECO, University of Aveiro, Portugal, Institut Charles Gerhardt- Montpellier, Mate´riaux Avance´s pour la Catalyse et la Sante´, UMR5253 CNRS-ENSCM-UM2-UM1, 8 rue de l’Ecole Normale, 34296 Montpellier, France, and Department of Chemistry, CICECO, University of Aveiro, Portugal Received February 25, 2008; Revised Manuscript Received April 2, 2008

The photoluminescence features of Eu3+-, Tb3+-, Tb3+/Eu3+-alginate aerogel (hydrogel and alcogel) and Eu3+alginate xerogel hybrids were investigated. The Eu3+-alginate aerogel and alcogel exhibit the highest 5D0 quantum efficiencies (9.9 and 8.2%, respectively), while the hydrogel and xerogel have lower values (5.2 and 5.6%, respectively). The Tb3+/Eu3+ hybrids are multiwavelength emitters in which the emission color can be tuned across the chromaticity diagram from the red toward the yellowish-green spectral regions, crossing the white area by selecting the excitation wavelength.

Introduction Organic-(bio)-inorganic hybrid materials are attracting much interest because they combine at the nanoscale the characteristics of organic (or bio) and inorganic components.1 In the future, hybrid materials will have to meet certain environmental requirements, such as being recyclable and safe. Hence, the use of abundant, low cost, and environmentally friendly biopolymers, such as polysaccharides, to synthesize hybrid materials is gaining much attention.2 Alginates, abundant natural polysaccharides produced by brown algae, are block copolymers of (1-4) linked β-Dmannuronic (M) and R-L-guluronic (G) residues (Scheme 1). Alginates are extensively used for the entrapment of biologically active materials for applications in the controlled release of drugs, cosmetics, biological catalysis, and enzyme transport in detergents.3 Alginates are used as immobilizing agents in various applications because they form heat-stable gels with divalent and trivalent cations. In addition, alginates are used for the preparation of several silica-based hybrid materials by polycondensation reactions of different siliceous precursors, such as sodium silicate,4 aminopropyl silicate,5,6 and tetra-alkoxysilane.7 These polymers are used as templating agents for the controlled formation of metallic and magnetic nanoparticules.8 Hybrid materials containing trivalent lanthanide ions (Ln3+) have been widely used as photoluminescent materials since they combine properties of sol-gel derived hosts (e.g., shaping, tunable refractive index and mechanical properties, etc.) and the well-known emission features of Ln3+ ions (e.g., sharp 4f transitions, long lifetimes, 10-3 s range, and high emission quantum yields, etc.).9–12 It is also well-known that Ln3+ ions may be used as biological fluorescent labels because of their high photochemical stability, high quantum yield, and low toxicity.13 The hydrophilic nature of alginates is beneficial for * To whom correspondence should be addressed. Tel.: 351 234 370 946 (L.C.); 33 467 163 460 (F.Q.). Fax: 351 234 378 197 (L.C.); 33 467 163 470 (F.Q.). E-mail: [email protected] (L.C.); [email protected] (F.Q.). † Department of Physics, University of Aveiro. ‡ Institut Charles Gerhardt- Montpellier. § Department of Chemistry, University of Aveiro.

Scheme 1. Molecular Structure of R-L-Guluronic (G) and β-D-Mannuronic (M) Acid

the in vivo application of lanthanide-based hybrid materials. These promising in vivo applications include magnetic resonance imaging (MRI),14 biological labeling,15 fluoroimmunoassays, and nucleic acid hybridization assays.16 While supercritical drying stabilizes the gels porous network, affording desirable properties, such as high surface area and high porosity, evaporative drying yields a very dense and crystalline material. Both types of materials may be of interest for optical applications. The porous structures allow the construction of composites with many guest types, for example, organic molecules, inorganic ions, semiconductors, or polymers. These guest/host materials are promising candidates for applications such as sensors, solar cells, pigments, and lasers.17,18 While the very dense crystalline materials can be used as a chemically stable matrices for the hybrid materials which exhibit a total absence of scatting losses.19 Although it is known that alginates form gels with lanthanide ions,20 to the best of our knowledge, no studies are available on the photoluminescence properties of such lanthanide-alginate hybrid materials. Here, we wish to report photoluminescence studies of Ln3+-alginate materials (Ln ) Eu, Tb, and Eu/Tb), whereas for Eu3+-alginates the four states, hydrogel, alcogel, xerogel, and aerogel, were investigated; for the remaining samples, only the aerogel state is addressed.

Experimental Section Materials and Methods. Hydrogels. A 1% (w/w) solution of sodium alginate (Sigma-Aldrich from Macrocystis pyrifera, 3600 cP viscosity of a 2% solution, mannuronic/guluronic ratio of 1.82 by spectroscopic evaluation) in deionized water was added dropwise at room temperature to a LnCl3 (Aldrich) solution (0.01 M; Ln ) Eu, Tb) under stirring

10.1021/bm8002122 CCC: $40.75  2008 American Chemical Society Published on Web 05/22/2008

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using a syringe with a 0.8 mm diameter needle. Gel microspheres were thus formed, cured in the Ln3+ solution for 18 h, then separated from the cationic solution and washed with distilled water. For mixed lanthanide alginate gels Tb0.90/Eu0.10-alg and Tb0.40/Eu0.60-alg, the molar ratio of Tb/Eu in the lanthanide salt solution was 0.95:0.05 and 0.4:0.6, respectively. Alcogels. The hydrogel microspheres were successively immersed in a series of ethanol-water baths of increasing alcohol concentration (10, 30, 50, 70, 90, and 100%) during 30 min each. Aerogels. The alcogel microspheres were dried under supercritical CO2 conditions (74 bar, 31.5 °C) in a Polaron 3100 apparatus. The samples will be henceforth identified as Ln3+-alg, where Ln ) Eu, Tb, and Tb/Eu. Characterization of Materials. Scanning Electron Micrographs (SEM). The SEM of the aerogel microspheres were obtained using a Hitachi S-4500 apparatus after platinum metallization. Nitrogen Adsorption/Desorption Isotherms. The nitrogen adsorption/ desorption isotherms were recorded using a Micromeritics ASAP 2010 apparatus at 77 K after outgassing the sample at 323 K under vacuum until a stable 3 × 10-3 Torr pressure was obtained without pumping. Surface areas were evaluated by the BET method assuming that a monolayer of N2 molecules covers 0.162 nm2/molecule. ThermograVimetric Analysis. Thermogravimetric analysis was performed with a Netzsch TG 209 C apparatus under air (20 mL min-1, 25-850 °C, 5 °C min-1) on a 10 mg sample. Photoluminescence Spectroscopy. The photoluminescence spectra in the ultraviolet/visible (UV/vis) spectral ranges were recorded between 14 K and room temperature with a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Jobin Yvon-Spex) coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. The lifetime measurements were performed between 14 K and room temperature, with the setup described for the luminescence spectra using a pulsed Xe-Hg lamp (6 µs pulse at half-width and 20-30 µs tail).

Liu et al.

Figure 1. Eu3+-alg hydrogel (A), alcogel (B), aerogel (C), and xerogel (D) before UV radiation (upper row) and after UV (395 nm) radiation (lower row).

Figure 2. Cross-section micrograph of Eu3+-alg aerogel (upper row) and xerogel (lower row). Table 1. Metal Content and Surface Area of Ln3+-alg Aerogels

Results and Discussion The preparation of Ln3+-alg hybrids includes several steps illustrated in Figure 1A-C for the Eu3+-alg. The first step is the formation of a hydrogel by reaction of a sodium alginate solution with an Eu3+ solution. Dropping the sodium alginate solution into the Eu3+ coagulation bath forms mechanically stable hydrogel spheres, which may be extracted from the synthesis bath (Figure 1A). Alcogel spheres are obtained after exchange of water by ethanol through successive immersions in a series of ethanol-water baths of increasing ethanol concentration. This exchange induces a 20% volume shrinkage (Figure 1B).Two different procedures may be followed to dry the microspheres. One consists of the evaporation of the solvent at room temperature. This procedure leads to a xerogel. The ethanol evaporation of alcogel Eu3+-alg at room temperature yields xerogel spheres with a dramatic volume shrinkage (Figure 1D). The second consists in the extraction of the solvent above the critical point. This procedure leads to an aerogel. CO2 is chosen because of its low critical point of 31.5 °C at 70 bar. This technique is commonly processed with inorganic solids to achieve very high specific surface area.21 This procedure releases the porous texture quite intact by avoiding the pore collapse phenomenon and was successfully applied to polysaccharides gels.22 The alcogel spheres are placed in the supercritical drier at room temperature and ethanol is exchanged by liquid CO2. Temperature is increased to reach

sample

% wt Ln3+

S (BET) m2 · g-1

% wt H2O

Eu3+-alg Tb3+-alg Tb3+0.90/Eu3+0.10-alg Tb3+0.40/Eu3+0.60-alg

17.4 18.8 14.2 (Tb), 1.6 (Eu) 6.5 (Tb), 9.6 (Eu)

410 ( 5 383 ( 5 451 ( 5 484 ( 5

24.6 21 20 19

the critical point of CO2. The supercritical CO2 drying allows the formation of the aerogel from the alcogel (Figure 1C) with no shrinkage. Under UV radiation, all the Eu3+ hybrid materials are room temperature emitters (Figure 1). As shown by SEM, the Eu3+alg aerogel is an open network of fibrils 10-15 nm in diameter, while the xerogel is a very dense material (Figure 2). Nitrogen adsorption-desorption isotherms provided information on the textural properties of the Ln3+-alg aerogels.23,24 These hybrids exhibited high surface areas, between 380 and 480 m2 · g-1, comparable to those of transition metal- and alkaline earth-alginate aerogels.25,26 The Ln3+ content was determined by elemental analysis and/or thermogravimetric analysis considering that the residual mass corresponds to Ln2O3 oxide. The surface areas and compositions of the materials are reported in Table 1. All the samples exhibit a nominal 3:1 carboxylate groups to lanthanide coordination. Photoluminescence Studies. Figure 3A shows the emission of the Eu3+-alg alcogel under UV excitation. The spectrum

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Figure 3. (A) Room temperature emission spectrum of the Eu3+-alg alcogel excited at 330 nm. (B) Expansion of the 5D0 f 7F0-4 transitions region for the Eu3+-alg hydrogel (a), alcogel (b), xerogel (c), and aerogel (d) excited at 395 nm at room temperature. The red line (e) depicts the emission of the aerogel acquired at 14 K and excited at 395 nm.

consists of a series of lines ascribed to the intra4f6 5D0 f 7F0-4 transitions and of a low-intense broadband (380-550 nm), similar to that observed for the parent alginate biopolymer (not shown). The excitation spectrum of the Eu3+-alg alcogel monitored within the 5D0 f 7F2 transition (Figure S1 in Supporting Information) is formed of a series of intra4f6 lines ascribed to transitions between the 7F0,1 levels and the 5L6, 5 D1-4, 5G2-4, and 5H3,6 excited states superimposed on a lowintensity broadband which may be ascribed to the alginate biopolymer. These photoluminescence features resemble those observed for the Eu3+-alg hydrogel, xerogel, and aerogel. Figure 3B focus on the Eu3+ emission excited at the wavelength that maximizes the emission intensity (5L6 excitation, 395 nm). Both the local-field splitting of the 7F1 level in three Stark components and the presence of a single line for the nondegenerated 5D0 f 7 F0 transition (Figure 3B) indicate that all Eu3+ ions occupy the same low symmetry local environment without an inversion center, in accord with the higher intensity of the induced electricdipole 5D0 f 7F2 transition. Changing the excitation wavelength does not produce significant alterations in the energy, full-widthat-half-maximum (fwhm), and relative intensity of the 5D0 f 7 F0-4 transitions, indicating a similar Eu3+ local environment in all the materials. To compare the Eu3+ local environment in all the alginates, the fwhm of the nondegenerated 5D0 f 7F0 line was estimated by a Gaussian deconvolution, revealing large values for the fwhm (40-50 cm-1) that point out a broad distribution of similar Eu3+ local sites. This distribution may arise from either a different number of OH groups of the polysaccharide framework coordinated to the Eu3+ ions or to slightly different bond angles and bond lengths of the Eu3+ firstcoordination shells. Apart from a better resolved profile of the intra4f6 lines, in particular in the 5D0 f 7F2 transition, the 14 K and room temperature spectra are similar (exemplified for the aerogel in Figure 3B). The 5D0 emission decay curves of all the alginates were monitored within the 5D0 f 7F2 emission lines under direct intra4f6 excitation (5L6, 395 nm). The curves display a single exponential behavior, supporting the presence of a single type of Eu3+ local environment, yielding the lifetime values (τ) listed in Table 2. The Eu3+-alg aerogel and alcogel display identical values, ∼0.25 ms, whereas for the Eu3+-alg hydrogel and xerogel lower values were estimated. The 5D0 decay curve of the Eu3+-alg aerogel acquired at 14 K is also well fitted by a single exponential function yielding a lifetime value of 0.318 ( 0.006 ms, which is ∼27% higher than that measured at 300 K. Such an increase indicates the presence of thermally activated nonradiative channels (such as OH oscilators, for instance).

Table 2. 5D0 Lifetime (τ, ms), Radiative (kr, ms-1), Nonradiative (knr, ms-1) Transition Probabilities, Quantum Efficiency (η, %), and Number of Coordinated Water Molecules (and/or Polysaccharide Hydroxyl Groups; nw) in Eu3+-alg Hybrids

τ kr knr η nw

hydrogel

alcogel

aerogel

xerogel

0.204 ( 0.003 0.256 4.646 5.2 4.8 ( 0.1

0.249 ( 0.003 0.331 3.685 8.2 3.8 ( 0.1

0.251 ( 0.001 0.395 3.589 9.9 3.7 ( 0.1

0.210 ( 0.001 0.266 4.496 5.6 4.7 ( 0.1

To interpret the variation of the room temperature 5D0 lifetime values, the 5D0 radiative (kr) and nonradiative (knr) transition probabilities and the quantum efficiency (η) were estimated based on the emission spectra and on the 5D0 lifetimes27 (Table 2). The higher 5D0 lifetime value found for the Eu3+-alg alcogel and aerogel materials is related to the higher and smaller values of the radiative and nonradiative transition probabilities, respectively. Consequently, the Eu3+-alg aerogel and the alcogel exhibit the highest 5D0 quantum efficiencies (9.9 and 8.2%, respectively), while the Eu3+-alg hydrogel and xerogel have lower η values (5.2 and 5.6%, respectively). The larger values found for the two former alginates are due to a decrease in the nonradiative transition probability and an increase in the radiative transition probability with respect to that found in the Eu3+-alg hydrogel and xerogel. The difference in the nonradiative transition probability between the alginate hybrids may be rationalized on the basis of the diverse number of OH oscillators present in the Eu3+first coordination shell. The number of water molecules belonging to the Eu3+ first coordination sphere (nw) may be estimated using the empirical formula nw ) 1.11 × [τ-1 - kr - 0.31].28 The contribution to the nw value of OH oscillators in the first coordination spheres is half the contribution of one water molecule. As listed in Table 2, four and five water molecules were estimated for the Eu3+-alg alcogel, aerogel, hydrogel, and xerogel, respectively. In the present system, however, besides OH oscillators from water molecules, it is likely that hydroxyl groups of the polysaccharide framework also coordinate the metal ion and, therefore, nw in Table 2 refers to both types of OH oscillators. Eu3+ First Coordination Shell. The coordination of divalent cations with alginates has been much studied and fairly detailed models of Ca2+-alginate interaction are available in the literature.29–31 The role of the mannuronic and guluronic blocks was investigated and lead to the so-called egg-box like conformation proposition. The box possibly consists of six

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Figure 4. Room temperature (black line) and 14 K (red line) emission spectra of the Tb3+-alg aerogel excited at 305 and 320 nm, respectively.

Figure 5. Room temperature emission spectra of the Tb3+/Eu3+-alg with [Tb/Eu] ) 0.90:0.10 excited at (1) 368 nm, and (2) 395 nm.

oxygen ligands from OH groups (5) plus a carboxylate oxygen of the subsequent residue, supplied by each polyguluronate chain. In the case of Co2+-alginate aerogel hybrids, we demonstrated that H2O coordinates the metal as well as adsorbs onto the polysaccharide.32 This observation was also reported in the case of freeze-drying solids.8,33 Thermogravimetric analysis of various M-alginate hybrids revealed that the xerogels retain 8-10% weight water and the aerogels 18-24% weight water.34 In the case of Europium, the aerogel retained 24 wt % H2O (Table 1). Whatever the states of the lanthanide hybrids, hydrogel, alcogel, xerogel, and aerogel, the presence of water molecules in the first Eu3+ coordination shell is therefore plausible. Indeed, the quantum efficiency of Eu3+-based hybrid materials reported in the literature (most of them modified by β-diketonate complexes) showed a wide range of values from 7.7-72.1%,35–40 demonstrating that η is strongly dependent on the Eu3+ local coordination, especially the first coordination sphere. In our case, the relative low quantum efficiency of the polysaccharide hybrids is due to the high nonradiative transition probability caused by the presence of water molecules and OH oscillators of the polysaccharide framework in the Eu3+ first coordination shell. Consequently, even if the trivalent cations-alginate complexation is scarcely reported,20,41 one can admit the coordination of three carboxylic groups per cation (confirmed by elemental analysis), and the implication of OH from the polysaccharide network in the coordination sphere of the lanthanide cation. Tb3+-alg, Tb3+/Eu3+-alg Hybrids. The photoluminescence features of the Tb3+- and Tb3+/Eu3+-alg hybrids were also investigated. Figure 4 compares the emission spectra of the Tb3+-alg aerogel in the temperature range 14-300 K. The spectra display a series of straight lines ascribed to the intra4f8 5 D4 f 7F6-0 transitions and the broadband already observed in the emission spectra of the Eu3+-alg hybrids (Figure 3A). The temperature decrease from 300 to 14 K induces essentially a decrease in the broadband relative intensity. The excitation spectrum monitored within the 5D4 f 7F5 transition (Figure S2 in Supporting Information) is formed of a series of intra4f8 transitions between the 7F6 level and the 5H7-4, 5D0-3, 5G6-2, and5L10-7 excited states. The 5D4 lifetime value was estimated in the temperature range 14-300 K by monitoring the emission

decay curves within the 5D4 f 7F5 transition under direct intra4f8 excitation (5D4, 488 nm). The emission decay curves display a single-exponential profile yielding lifetime values of 0.838 ( 0.007 ms (300 K) and 0.863 ( 0.013 ms (14 K). The 5 D4 lifetime decrease with increasing temperature reveals the presence of thermally activated nonradiative mechanism, similar to that observed for the Eu3+-alg. Figure 5 shows the emission spectra of a selected codoped Tb3+/Eu3+-alg under different excitation wavelengths. All the spectra display a large broadband similar to that already detected for Eu3+-alg and Tb3+-alg (Figures 3A and 4) and a series of intra4f lines, which are ascribed to the 5D4 f 7F6-0 transitions, under intra4f8 excitation (5L10, 368 nm), and to the 5D0 f 7F0-4 lines, for intra4f6 excitation (5L6, 395 nm). In this latter case, the Tb3+ 5D4 f 7F6,5 lines (marked with arrows in Figure 5) are also discerned, indicating that Tb3+-to-Eu3+ energy transfer occurs in these codoped alginates. However, due to the low intensity of the 5D4 f 7F6,5 transitions this energy transfer process seems to be inefficient, suggesting that the Eu3+ and Tb3+ ions are well shielded from interaction with each other. Excitation spectra were recorded at selected monitoring wavelengths, 488 nm (Tb3+, 5D4 f 7F6 transition) and 715 nm (Eu3+, 5 D0 f 7F4 transition) to avoid spectral overlap between the intra4f6 and intra4f8 levels, (not shown). The excitation spectra monitored within the Tb3+ emission lines resemble that observed for the Tb3+-alg (in Supporting Information), whereas in the spectra monitored within the Eu3+ emission lines only the 7F0 f 5L6 transition could be clearly discerned. The absence of Tb3+ intra4f8 lines in these latter excitation spectra confirms the above suggestion of an inefficient Tb3+-to-Eu3+ energy transfer process (Figure 5). The emission features of the Tb3+/ Eu3+-alg with [Tb/Eu] ) 0.40:0.60 resemble those of the [Tb/ Eu] ) 0.90:0.10. Room temperature lifetime values of the excited states, 5D0 (Eu3+) and 5D4 (Tb3+) for the Tb3+/Eu3+ codoped alginate with [Tb/Eu] ) 0.90:0.10 were monitored within the more intense line of the 5D0 f 7F4 (614 nm) and 5D4 f 7F6 (488 nm) transition under 395 and 318 nm excitation wavelength, respectively. The emission decay curves have a typical singleexponential behavior, yielding 5D0 and 5D4 lifetime values of 0.254 ( 0.004 ms and 0.800 ( 0.007 ms, respectively. The 5 D0 lifetime values is the same as that found in the Eu3+-alg hybrid, whereas the 5D4 lifetime sligthly decreases relatively

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coordination shell. The Tb3+/Eu3+ materials are multiwavelength emitters for which the emission color can be tuned across the chromaticity diagram by selecting the excitation wavelength. Acknowledgment. We thank financial support from NoE “Functionalised Advanced Materials Engineering of Hybrids and Ceramics” (FAME), Fundac¸a˜o para a Cieˆncia e Tecnologia, Portugal (BPD/26097/2005), FEDER, and PTDC. Supporting Information Available. Excitation spectra of Eu3+-alg alcogel and Tb3+-alg aerogel. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes

Figure 6. CIE (1931) (x,y) chromaticity diagram showing the emission color coordinates of the Eu3+-, Tb3+-, and Tb3+/Eu3+-alg hybrids under different excitation wavelengths.

to that of the Tb3+-alg pointing out that for Tb3+ a nonradiative channel (not present in the pure Tb3+-alg) is open, which is in sound agreement with the mentioned Tb3+-to-Eu3+ energy transfer process. The emission features of Eu3+-, Tb3+-, and codoped Tb3+/ Eu3+-alg with [Tb/Eu] ) 0.90:0.10 were further quantified through the calculation of the emission (x,y) color coordinates following the standard procedure described by Commission Internationale d′E´clairage (CIE) for a 2° standard observer. Figure 6 displays a (x,y) chromaticity diagram showing the emission color coordinates under the wavelength that maximizes the lanthanide ion emission, namely, 395 and 320 nm for the Eu3+-alg and Tb3+-alg, respectively, and 368 (Tb3+, intra4f8 lines) and 395 nm (Eu3+, intra4f6) for the codoped Tb3+/Eu3+alg. The color coordinates of the Eu3+-alg (0.66, 0.34) are very close to the inorganic red standard phosphor Y2O3/Eu (0.65, 0.35), whereas the emission color of the Tb3+-alg (0.30, 0.46) lie within the yellowish-green spectral region due to the broadband contribution for the emission spectra (Figure 4). The emission color of the codoped Tb3+/Eu3+-alg resembles white to the naked eye as it lies within the white spectral region with (x,y) values of (0.26, 0.34) and (0.25, 0.26) for excitation wavelengths of 368 and 395 nm, respectively.

Conclusions The photoluminescence features of the Eu3+-, Tb3+-, and Tb3+/Eu3+-alg hybrids were investigated, whereas for Eu3+alg, the hydrogel, alcogel, aerogel, and xerogel materials were investigated and, for the Tb3+- and Tb3+/Eu3+-alg, only the aerogel state is addressed. Under UV radiation, all hybrids are room temperature emitters in the visible spectral range. The Eu3+-alginate aerogel and alcogel exhibit the highest 5D0 quantum efficiencies (9.9 and 8.2%, respectively), while the hydrogel and xerogel have lower quantum efficiency values (5.2 and 5.6%, respectively). The difference in the nonradiative transition probability may be rationalized on the basis of the different number of OH oscillators present in the Eu3+-first

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