Eco-Friendly Luminescent Hybrid Materials Based on EuIII and LiI Co-Doped Chitosan

Biopolymer-based materials have been of particular interest as alternatives to synthetic polymers due to their low toxicity, biodegradability and biocompatibility. Among them, chitosan is one of the most studied ones and has recently been investigated for the application as solid state polymer electrolytes. Furthermore, it can serve as a host for luminescent species such as rare earth ions, opening up the possibility of combined electro-optical functionality, of particular interest for electroluminescent devices. In this study, we perform a fundamental, initial, investigation of chitosan based luminescent materials doped with Eu and Li triflate salts, from the structural, photophysical and conducting points of view. Because the host presents a broad emission band in the blue to green, while Eu emits in the red, fine-tuning of emission colour and/or generation of white light is proven possible, by proper combination of optimized composition and excitation scheme. Europium lifetimes (D0) are in the range 270-350 μs and quantum yields are up to 2%. Although Li does not interfere with the luminescent properties, it grants ion-conducting properties to the material suggesting that a combination of both properties could be useful in the development of electro-luminescent devices.


Introduction
The low toxicity, biodegradability and biocompatibility of chitosan makes it one of the most widely studied polymers from the application point of view.2][3] From the physicochemical standpoint, chitosan is a water soluble polymer that can be formed into films, hydrogels and scaffolds, under mild acidic conditions.Moreover, the polycationic character confers to chitosan a high affinity for the association and delivery of therapeutic hydrophilic macromolecules such as proteins, hormones and DNA, thus effectively protecting their bioactivity against enzymatic and hydrolytic degradation. 3,4As chitosan is water soluble, it is possible to mix it with different inorganic salts and produce polymer electrolytes (PEs) for electrochemical devices assembling. 5PEs are composed of a host macromolecule and a guest salt, and have been widely studied in the field of solid state electrochemistry, because of commercial interest in their application.Polymer electrolytes based on natural polymers have attracted a lot of attention due to their mechanical, optical and electrical properties. 6The mixed cation approach, which basically involves the use of two guest salts instead of a single one, is attractive and usually results in an improvement of the conductivity of PEs. 7 Besides the technological aspects, biopolymer based materials call attention for the possibility of developing environmentally friendly (multi)functional materials that can combine, for instance, high ionic conductivity and efficient luminescent properties. 8,9In this regard, trivalent lanthanide ions are the most studied luminescent species due to their versatile narrow band emissions in a large spectral range (from visible to near-infrared).Application opportunities are found in, but not restricted to, lighting devices, displays, amplifiers and lasers.The long lifetime of the lanthanide ions excited states allows the use of time-resolved spectroscopy to suppress background Vol. 26, No. 12, 2015   fluorescence, reaching very low detection limits, which is relevant for biological applications such as biomarkers and cell imaging. 10Complexes and salts can display high quantum efficiencies in the near-ultraviolet (Ce III ), visible (Tm III in blue; Tb III and Er III in green; Dy III in yellow; Sm III in orange; Eu III in red) and near-infrared (Nd III , Er III , Tm III and Yb III ).
Studies of the family of Eu III -doped d-Ut(300)-based di-urethanesils hybrids prepared by the sol-gel process indicate that the materials are promising full colour emitters with quantum yield ranging from 0.7 to 8.1%. 11Studies of other hybrids co-doped with Eu III , Tb III and Tm III have evidenced the chromaticity of the emission, and it was shown that their optical features were strongly influenced by environmental conditions, such as temperature variations, suggesting the application of these materials in sensoring. 12u III -doped DNA and agar membranes 9 have also been recently investigated, as well as Eu III -doped chitosan, which showed interesting spectroscopic properties.Recently, Tsvirko et al. 13 have studied absorption, photoluminescence and time resolved decay properties of chitosan films doped with Eu III β-diketonate complex.They have observed that these new materials present very efficient luminescence in the visible region with a quantum efficiency of about 47%. Rosen and Binnemans 14 proposed chitosan modified with ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) stationary phase for trivalent rare-earth ions separation using dilute nitric acid solution as eluent.Liu et al. 15 have studied bio-organic-inorganic hybrids containing silica, chitosan and Eu III or Tb III ions.They have observed that the Eu III ions have the same local coordination in the chitosan core of these core-shell (chitosan-silica) hybrids as in Eu III doped chitosan.Additionally, they have also found a small amount of Eu III in the silica shell.
Because the host polymer emits broadly in the blue to green spectral region and Eu III in the red, 15 the combination offers the possibility of tuning the emission colour and obtaining white light, through control of composition and proper excitation scheme.In this contribution, we extend the investigation to natural-polymer-based membranes, containing Eu III and Li I , which might offer the possibility of increased electrochemical functionality.These new samples can potentially exhibit both the emission properties due to the Eu III ions and conductivity due to Li I ions, thus they can be potential candidates for electrochemical devices such as displays.To that end, the chitosan-based membranes are characterized not only from their structural and morphological points of view, but also with respect to their ionic conductivity and photophysical properties.The CIE (Comission Internationale d'Éclairage) chromaticity coordinates are given for several samples along with the respective quantum yields and excited state lifetime values.To the best of our knowledge, this is the first time such an approach is used for chitosan based materials co-doped with Eu III and Li I ions.

Sample preparation
The electrolytes were prepared according to the following procedure: 0.20 g of chitosan (Sigma-Aldrich; medium molecular weight) was dispersed in 10 mL of 1% acetic acid solution (Sigma-Aldrich) under magnetic stirring over night to complete dissolution.Then, 0.25 g of glycerol (Himedia; 99.5%) and different quantities of europium (Sigma-Aldrich; 98%) and lithium triflate (Sigma-Aldrich; 99.995%) salts were added and the solutions were kept under stirring for a few minutes.Samples have been represented by the notation ChiEu n Li m , where n is the lanthanide salt mass and m is the lithium salt mass in the electrolytes (Table 1).The viscous solutions were poured on Petri plates and cooled at room temperature to form the electrolyte films.The films were subjected to a final drying procedure during which the temperature was raised from 25 to 60 ºC over a period of two days to form transparent membranes (Figure 1).During this period the oven was periodically evacuated and purged with dry argon.The thickness was determined with a micrometer (Mitutoyo), and varied between 0.070 and 0.175 ± 0.001 mm.After drying, the samples were conditioned in a glovebox under argon atmosphere.

Characterization techniques
The X-ray diffraction measurements were performed at room temperature in a PANalytical X'Pert Pro diffractometer equipped with an X'Celerator detector.The film samples were exposed to monochromated CuK α radiation with λ = 1.541Å over a scattering angle (2θ) range from 3 to 60 º.In these measurements, samples were placed on a Si wafer, in order to minimize diffuse scattering from the substrate.
The surface morphology of the films, prepared at different ratios, was evaluated using scanning electron microscopy (SEM) micrographs, obtained in a LEO 440 instrument.All film samples were pre-coated with a conductive layer of sputtered gold.The micrographs were taken at 10 kV at different magnifications.
Total ionic conductivities of samples were determined using a constant volume support equipped with gold blocking electrodes and located within a Buchi TO 50 oven.The sample temperature was evaluated by means of a type K thermocouple placed close to the electrolyte film and impedance measurements were carried out at frequencies between 65 kHz and 500 mHz using an Autolab PGSTAT-12 (Eco Chemie), over the temperature range 20 to 90 o C. Measurements of conductivity were effected during heating cycles.The reproducibility of recorded conductivities was confirmed by comparing the results obtained for a sample subjected to two heating-cooling-heating cycles.The excellent reproducibility of the results obtained using this procedure demonstrated the correct operation of the support and the mechanical stability of the samples.
Room temperature emission and excitation spectra of the polymer films, as well as excited state lifetime decays were measured in a Horiba Jobin Yvon spectrofluorimeter model Fluorolog FL3-221 using continuous wave or pulsed (flash) xenon lamps.Quantum yield measurements of Eu-doped and Eu/Li-co-doped samples were carried out in the integrating sphere model Quanta Phi F3029, from Horiba Jobin Yvon, properly connected to the spectrofluorimeter visible detector by optical fibers.By carefully testing various samples, with different sizes, but from the same batch, quantum yields showed a maximum absolute deviation of 10%, so that the presented values are an average.

Structural and morphological characterization
Figure 2 depicts the X-ray diffraction (XRD) patterns of film samples of ChiEu n Li m , i.e., non-doped chitosan and its mixture with either Eu(CF 3 SO 3 ) 3 or both Eu(CF 3 SO 3 ) 3 and LiCF 3 SO 3 .7][18][19][20] For samples of ChiEu 0.1 and ChiEu 0.1 Li 0.05 the same broad band is observed at 21º, similarly to DNA-Eu III samples. 21However, as observed in other studies, the doping of polymer matrices with inorganic salts can induce long range disorder in the diffraction patterns. 22This observation strongly suggests that an increase of the amorphous content was promoted by the presence of salts.As a consequence of the increase of the amorphous content some regions of matrix become more crystaline.Although it seems contradictory, it might be explained as the increased disorder created by the salt among the neighboring chitosan chains forces chain segments, which are not located within the The balance error was of ±0.00001g.amorphous domain, to gather in well-defined regions and organize in a more regular manner.Similar behavior was also previously observed for polymer electrolytes based on europium salt and polyethylene oxide (PEO). 23The polymer electrolytes ChiEu 0.1 Li 0.04 and ChiEu 0.1 Li 0.1 exhibit Bragg peaks at 21º, similar to ChiEu 0.1 Li 0.0 , ChiEu 0.1 Li 0.05 and non-doped chitosan matrix, and a very sharp lonely peak at 10º.Chitosan powder presents two peaks at 2θ = 10.9 and 20.1 o . 24,25The peak at about 10-11º of the ChiEu 0.1 Li 0.04 and ChiEu 0.1 Li 0.1 is much sharper than the peak of chitosan powder and resemble more the one found in free LiCF 3 SO 3 or a polymer-salt crystalline complex.Nunes et al. 26 also detected a peak at about 10-11º for the system d-U(2000) n LiCF 3 SO 3 , although this system presents a series of other peaks in salt rich domains.The position of these peaks coincides exactly with the location of the diffraction peaks of the crystalline complex with stoichiometry 3:1 that is formed between PEO and LiCF 3 SO 3 . 27The peak at 10 o , in the present study, can be attributed to the sample or to some impurity.No peak was detected at approximately 29º, which, in the case of the system U(2000) n Eu(CF 3 SO 3 ) 3 , was ascribed to the diffraction of Eu-rich domains. 28The obtained results prove that the addition of the guest europium triflate salt to chitosan matrix inhibits the formation of crystalline phases of free Eu(CF 3 SO 3 ) 3 or chitosan/salt complexes.[31] Scanning electron micrographs illustrating the morphology of chitosan and Eu(CF 3 SO 3 ) 3 and LiCF 3 SO 3 are shown in Figure 3.The good homogeneity without any phase separation and very good surface uniformity at the micrometer scale of samples containing 0.1 g of Eu III -triflate (without Li I ) can be observed in the SEM micrographs in Figure 3a.Similar results were obtained for agar-based films. 32Other SEM images reproduced in Figure 3 (b, c  and d) show that the samples containing mixture of both salts exhibit an irregular texture, and are very similar to the samples of solid polymer electrolyte based on polyethylene glycol (PEG) with high LiClO 4 concentration. 22The micrograph of ChiEu 0.1 Li 0.04 (Figure 3b) shows a porous morphology evolving from large to lower pore size.It was verified that this is not a surface effect and it is explained because the degree of porosity is due to the low evaporation temperature of the solvent during the sample preparation.At the same time, the polymer chains show a reduced mobility, which hinder their occupation of the free space left by the evaporated solvent. 33The images recorded for samples ChiEu 0.1 Li 0.05 and ChiEu 0.1 Li 0.1 reveal clearer lines that can be either related to undissolved or recrystallized salt, which confirms XRD data.

Ionic conductivity of electrolytes
The ionic conductivities of various electrolyte compositions over the temperature range from 20 to 90 ºC, and as a function of salt content, are illustrated in Figure 4.These obtained results show a continuous variation in conductivity that is almost linear with respect to 1/T over a range of salt compositions.This plot demonstrates that all the doped samples exhibit a typical behavior for polymer electrolytes where hopping mechanism of ionic charge species is predominant.This behavior is similar to that exhibited by the same matrices doped with different salts [34][35][36] and contrasts with that of semi-crystalline materials based on commercial polyethylene oxide (PEO) host matrices. 37he ionic conductivity (σ i ) values were calculated for each heating cycle using the relation of σ i = d/R b A, where  R b is the bulk resistance, d is the thickness and A is the area of the sample.As shown in Figure 4 (fittings) the ionic conductivity temperature dependence follows the Arrhenius equation of σ i = σ 0 exp(−E a /RT), where σ 0 is the pre-exponential factor, E a is the apparent activation energy for ion transport, R is the gas constant (8.314J mol -1 K -1 ) and T is the temperature.Table 2 and Figure 4 show the ionic conductivities (σ i ), and activation energy values (E a ), obtained from the above equations.
Over the whole range of studied temperatures, the sample with the highest ionic conductivity is the ChiEu 0.05 Li 0.2 with values of 5.38 × 10 -6 and 8.77 × 10 -5 S cm -1 , at 30 and 80 ºC, respectively.These values are higher than those obtained for analogous polymer electrolytes incorporating europium salt 30 and very similar to those doped with lithium salts. 38Besides temperature, ionic conductivity also depends strongly on the guest salt, both in type and concentration.Usually, increasing guest salt concentration results in an ionic conductivity increase, which is not at all observed in the present study.A high amount of Eu(CF 3 SO 3 ) 3 can play opposite effects.Although the number of charge carriers increases with the increase of salt amount, a high concentration of salt leads to the decrease of free volume, as well as available coordination sites.Additionally, the influence of ion pairing should also be taken into account.The fitted E a values of all the SPEs are depicted in Table 2, where one can observe that most of E a values decrease with the increase of ionic conductivity.This behavior was normally expected, since the lower the energy barrier, the easier the ionic movement.This is probably due to the above given reason.

Photophysical properties
Figure 5 presents the photoluminescence emission spectra of the representative sample ChiEu 0.3 measured with excitation at different wavelengths.Besides the typical bands of Eu III corresponding to transitions from the excited state 5 D 0 to lower lying 7 F J states, there is superimposed broad band whose energy in the blue-green spectral region is highly dependent on the chosen excitation wavelength.Similar broad bands have been observed for other bio-polymer hybrid electrolytes and their nature is attributed to the electron-hole recombination in the polymer host. 9,39At 360 and 394 nm there is concomitant excitation of the Eu III ion and the host, however, at 325 nm (30769 cm -1 ) only the host is excited (Eu III does not have an energy level corresponding to this wavelength).Therefore, the fact that Eu III emission is observed gives strong evidence of energy transfer from the host to the ion.This assumption is corroborated by examining the excitation spectrum in Figure 6, which was recorded by monitoring the europium emission at 616 nm.In this spectrum, not only the broad polymer band appears, suggesting once again the host → ion energy transfer, but one can also see why excitation at 394 nm is preferable in Eu III -doped systems to achieve higher emission intensities as seen in Figure 5. Furthermore, in the spectra recorded with excitation at 325 and 360 nm, a small structure around 390 nm is observed and it corresponds to self-absorption by the ion.This peak has also been observed in Eu III -based di-ureasil samples. 40he fact that the 5 D 0 → 7 F 0 transition band displays only one, well-defined peak, gives strong evidence that the Eu III ion occupies a sole site of symmetry in the SPE samples.Moreover, the considerable intensity (as judged by naked eye) of the hypersensitive transition 5 D 0 → 7 F 2 with respect to the purely magnetic dipole transition 5 D 0 → 7 F 1 indicates that such site is of low symmetry, with no inversion center.The fact that the intense polymer emission accompanies that of Eu III , even when excitation is done at 394 nm, suggests the combination of these bands to generate tunable visible (or white) light.
In Figure 7, a comparison of emission intensities and lineshapes is made for samples co-doped with Li I .It comes out as no surprise that no changes of relative intensities and lineshapes of Eu III bands, as a function of lithium concentration is observed.Still, a new functionality (PEs) is added and could potentially be combined with Eu III -emission in a multifunctional material.
In Figure 8, representative luminescence decay curves measured for the excited state 5 D 0 by monitoring the emission at 616 nm ( 5 D 0 → 7 F 2 ) in different ChiEu n Li m samples are presented.The sample ChiEu 0.3 was the only one to display nearly single-exponential decay.However, for the sake of comparison, averaged lifetime values were determined as the area below the curves corresponding to all samples, and also by assuming the value when intensities drop by a factor 1/e, yielding very similar results.The lifetime values for the samples containing lithium are slightly higher than for the sample, which does not contain this species.The values are in the range of those determined for Eu III -doped agar (194 µs), DNA (528 µs), 9 and for poly(εcaprolactone)/siloxane biohybrids (225 and 262 µs). 39In the latter case, the higher value also corresponds to a sample where the Li I concentration is higher indicating that it can be positively interfering by decreasing the amount of hydroxyl ions in the first coordination sphere of the RE ion.
By taking advantage of the concomitant emissions of polymer host and ion, and of the relative intensity variation depending on Eu III concentration and excitation wavelength, the colour tuning 41 can be achieved as indicated in Figure 9.     3 for all the samples, together with the quantum yield values ϕ, measured in an integrating sphere.The maximum ϕ value around 2.7% is to be expected for Eu III inserted in an environment where hydroxyl groups can act as efficient luminescence quenchers.Still, the values are comparable to other polymer doped hosts 42 and one, possible, way to increase efficiency would be to employ highly coordinated Eu III complexes for which quantum yields can be as high as 85%. 43

Conclusions
In this work we developed a novel luminescent and ion conducting biohybrid electrolyte doped with LiCF 3 SO 3 and Eu(CF 3 SO 3 ) 3 by means of the solvent casting method.The Gaussian-shaped broad X-ray diffraction band confirms the predominantly amorphous nature of the studied SPEs.Ionic conductivity values in the range 5.38 × 10 -6 (30 ºC) to 8.77 × 10 -5 (80 ºC) S cm -1 were measured, which are higher than those of analogous polymer electrolytes singly doped with Eu III salts, and very similar to those doped with lithium salts.
Photophysical investigations indicate that by combining the emissions of the host matrix and the dopant Eu III ion it is possible to tune the final emission colour and ultimately generate white light.Except for a small increase in europium 5 D 0 lifetime values (270 to ca. 350 µs), the presence of Li I does not seem to interfere with the emissive properties, but offers the perspective of further developing and exploring these materials for multifunctional applications.The main benefit of this particular host-ions combination lies in the eco-friendly properties of the biodegradable and non-toxic host.

Figure 1 .
Figure 1.Photograph of a representative ChiEu n Li m film.Petri plate diameter was 5.2 cm.

Figure 4 .
Figure 4. Conductivity plot of the ChiEu n Li m samples.

Figure 6 .
Figure 6.Excitation spectrum of the sample ChiEu 0.3 measured by monitoring the emission at 616 nm.The characteristic transitions of Eu III are indicated by the respective bands.

Figure 5 .Figure 7 .
Figure 5. Emission spectra of the sample ChiEu 0.3 with excitation at different wavelengths.The characteristic transitions of Eu III are indicated nearby the respective bands.

In
Figure 9a, the CIE chromaticity diagram presents the resultant colours for various ChiEu m Li n samples, where the ChiEu 0.3 is closely placed at the crossing of x = 0.3 and y = 0.3 colour coordinates of the CIE 1931 diagram, upon excitation at 325 nm (Figure 9b).This sample is the closest one to the white in the diagram.The x-and y-chromaticity coordinates are presented in Table

Figure 9 .
Figure 9. CIE chromaticity diagrams showing the (x,y) colour coordinates of emissions of ChiEu m Li n samples.In (a), the dependence on Eu 3+ and Li + relative concentrations, λ exc = 394 nm; (b) dependence on excitation wavelength for ChiEu 0.3 sample.

Table 1 .
Relevant details of the synthetic procedure of the ChiEu n Li m samples

Table 2 .
Ionic conductivity and activation energy of the ChiEu n Li m samples