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Luminescence properties of chitosan doped with europium complex
Journal of Luminescence 143 (2013) 128–131
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence properties of chitosan doped with europium complex M. Tsvirko a, E. Mandowska b, M. Biernacka b, S. Tkaczyk b, A. Mandowski b,n a b
Institute of Chemistry, Jan Dlugosz University, ul. Armii Krajowej 13/15, 42-200 Czestochowa, Poland Institute of Physics, Jan Dlugosz University, ul. Armii Krajowej 13/15, 42-200 Czestochowa, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 5 June 2012 Received in revised form 25 January 2013 Accepted 16 April 2013 Available online 23 April 2013
Chitosan is a natural, biodegradable and biocompatible polymer. It is transparent in UV–vis region. Nonetheless, appropriate doping may significantly change its optical properties. Chitosan film doped with Eu+3 β-diketonate complex was synthesized. Absorption, photoluminescence and time resolved decay properties of this material were investigated. It is shown, that the material exhibits excellent luminescence emission in VIS region with high quantum efficiency of approximately 47%. These exceptional properties allow for various applications of this material. In particular, it provides a good basis for the construction of biocompatible luminescent sensor. & 2013 Elsevier B.V. All rights reserved.
Keywords: Chitosan Eu+3 complex Luminescence Biosensor
1. Introduction In recent decades an increasing interest was observed on synthesis of lanthanides complexes possessing high quantum efficiencies in water environment. This trend of studies is developed by various applications of luminescent lanthanide probes for time-resolved immunoassays, time-resolved luminescent microscopy and optical sensors of pH, oxygen pressure, temperature and selected anions. Doping such complexes in biocompatible materials may be considered as alternative way to construct highly efficient luminescent systems for bio analytical applications [1–3]. Synthetic UV transparent polymers, especially poly(methyl methacrylate) (PMMA) doped by luminescence lanthanide complexes has been widely investigated [4]. On the other hand the complexes were introduced also to various hole transport polymers such as poly (N-vinylcarbazole) (PVK), poly[2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), and others to investigate mainly organic light emitting diodes (OLEDs) [5]. Luminescence quantum efficiency of Eu 5D0-7FJ (J ¼0, 1,…,4) luminescence is high even at room temperature (0.33 and 0.71 in toluene and PMMA films, respectively) [6]. This property is important for practical applications. The environmental requirements for safety and recyclability extorts to combine these functional materials with low cost, environmentally friendly biopolymers to synthesize hybrid materials. Natural, biodegradable and biocompatible chitosan produced from crab shells or shrimps is a promising candidate, as a matrix material, for various optical applications. This low cost produced polysaccharide has
n
Corresponding author. Tel.: +48 34 361 49 19x262; fax: +48 34 366 82 52. E-mail address: [email protected] (A. Mandowski).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.04.014
many interesting properties e.g. ability to form films, non-toxicity, excellent adsorption features etc. which makes it excellent to prepare functional hybrid materials [7]. Chitosan is used to prepare hydrogels, films, fibers or sponges in the biomedical domain, for which the biocompatilibity is essential. It is widely applied e.g. in medicine, pharmacy, cosmetics and agriculture because it exhibits antivirus and antiphage activities as well in bone tissue engineering etc. [8,9]. Chitosan nanoparticles, thin films and membranes seem to be particularly interesting for various applications. Chitosan nanoparticles doped with trivalent lanthanide ions could found medical application as fluorescent labels [10]. Composite materials based on PMMA thin films doped with europium complexes may be applied as UV radiation converters [6]. Pure chitosan film is transparent for UV–vis light [11] and may be used as a matrix for various chemical compounds. Therefore this polymer can be applied as a biocompatible sensor as well. This work presents luminescence properties of chitosan doped with Eu3+ fluorinated β-diketone (TTA) complex.
2. Experiment Chitosan is a natural polymer of acetylamino-α-glucose. A structural formula of chitosan is depicted in Fig. 1a. The investigated europium complex Eu(TTA)3DAPM, (TTA—thenoyltrifluoroacetone, DAPM—diantipyrylpropylmethane) is shown in Fig. 1b. The ligand DAPM saturates coordination number of Eu ion stabilizing structure of Eu(TTA)3DAPM. As a consequence, it improves luminescence properties of the investigated complex.
M. Tsvirko et al. / Journal of Luminescence 143 (2013) 128–131
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Fig. 1. Chemical structure of (a) chitosan and (b) Eu(TTA)3DAPM complex.
The chitosan used for sample preparation was purchased from the Institute of Sea Fisheries in Gdynia (Poland). The deacetylation and viscosity of this chitosan were 73% and 256 mPa s, respectively. The 0.75 g of dry chitosan was dissolved in 85 mL (1%) acetic acid in distilled water. Solution of Eu(TTA)3DAPH was prepared by dissolving 30 mg of Eu(TTA)3DAPH in 10 mL of methanol. Then, the solutions of acetate chitosan and Eu(TTA)3DAPH in methanol were mixed in proper amounts to obtain 5.45% weight of europium complex relative to chitosan mass. Final solution was deposited at smooth poly(methyl methacrylate) (PMMA) or glass surfaces and dried for 24 h at 313 K. The obtained layers were homogeneous and transparent. Thickness of the layers was in the range 5– 100 mm, depending on the amount of deposited solution. Absorption spectra were measured using Schimadzu spectrophotometer UV-240PC at wavelength range 230–400 nm with resolution of 0.1 nm, in air, at room temperature. Photoluminescence (PL) measurements were carried out in vacuum at 80.5 K, after excitation by 350 nm light using 75 W Xenon arc lamp. More details of the measurement equipment are given in some earlier papers [12,13]. Wavelength resolution of PL measurements was 0.3 nm. Finally, numerical calibration of photoluminescence spectra was done taking into account sensitivity of the whole measurement system. Time-resolved measurements of luminescence excitation and emission spectra as well as decay kinetics were carried out using spectrofluorimeter FLUORAT-02-PANORAMA LUMEX. The light source was a high pressure flash Xe-lamp. It operates with 1 μs pulse and repetition frequency of 25 Hz. Emission of luminescence may be detected in spectrum range 210–730 nm with resolution of 15 nm. The measurements were carried out in ambient conditions. The delay and duration times were adjusted to investigated samples.
3. Results Absorption spectra of TTA,DAPM Eu(TTA)3 DAPM in methanol solution and Eu(TTA)3DAPM (5.45%) in chitosan film are shown in Fig. 2. Absorption peaks of TTA (337 nm) and DAPM ligands (276 nm) in methanol solution correspond to peaks obtained for Eu(TTA)3DAPM in methanol solution (336 nm and 283 nm) and europium-chitosan film (355 nm and 291 nm). Absorption peaks
Fig. 2. Absorption spectra of (a) diantipyrylpropylmethane (DAPM), thenoyltrifluoroacetone (TTA) and Eu(TTA3)DAPM in methanol solution and (b) europium complex Eu(TTA3)DAPM in methanol solution and in chitosan film (5.45% Eu(TTA3) DAPM, film thickness 24 μm).
of Eu complex in solution and chitosan film are slightly shifted to larger wavelengths as compared with absorption peaks of TTA and DAPM in methanol solution. Chitosan film in the region 220–400 nm is transparent. Photoluminescence (PL) spectrum of Eu(TTA)3DAPM (5.45%) doped chitosan film is shown in Fig. 3. Excitation at 350 nm produces five narrow emission peaks centered at 579 nm, 591 nm, 612 nm, 652 nm, and 708 nm which are related to 5 D0-7FJ transitions (J¼ 0, 1, 2, 3, 4) of Eu3+ ion. The peaks may reveal fine structure, because 7FJ (J ¼1, 2, 3, 4) levels are degenerate and split into 2 J+1 sublevels. Luminescence due to non-degenerate 5D0-7F0 transition appears as a single peak, which should be fitted by one gaussian (peak maximum 579.7 nm) in energy domain. The gaussian fit of this band is shown in the inset in Fig. 3. It indicates that in chitosan Eu(TTA)3DAPM film there is one dominant type of the complex.
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M. Tsvirko et al. / Journal of Luminescence 143 (2013) 128–131
Fig. 3. Photoluminescence spectrum of Eu(TTA)3DAPM (5.45%) in chitosan film measured at T ¼ 80.5 K, excitation by Xe 75 W lamp, λex ¼ 350 nm. The inset shows gaussian fit of 5D0-7F0 peak.
The fine structure of the transition 5D0-7F1 reveals three components in luminescence spectrum (Fig. 3). This observation indicates low symmetry group characterizing Eu+3 local environment. The relative higher intensity of electric-dipole hypersensitive 5D0-7F2 transition in comparison with the magnetic-dipole transition (5D0-7F1) is in agreement with this supposition [14,15]. To get more information on this issue additional measurements with higher signal-to-noise ratio should be performed (e.g. in low temperature). Photoluminescence spectra of Eu(TTA)3DAPM doped chitosan film and Eu(TTA)3DAPM in solution are similar. It indicates that surrounding of Eu+3 is the same in methanol solution and in chitosan film and the structure of the complex Eu(TTA)3DAPM in chitosan film remains unchanged. Fig. 4a, b and Fig. 5 show time resolved luminescence spectra of Eu(TTA3)DAPM in chitosan film. While recording the spectrum for time delay 20 μs and time duration 1000 μs (Fig. 4b), it was possible to observe only transition from 5D0 level of Eu3+ ion. By changing time delay to 3 μs and time duration to 0.5 μs (Fig. 4a) it was possible to measure 5D1-7FJ (J ¼1, 2) transitions of Eu3+ ion in shorter wavelength region. It indicates that lifetime of 5D1 state is of the order of 1 μs. Decay curves of Eu+3 5D0 emission were investigated by monitoring the emission at 617 nm under excitation by 360 nm light as shown in Fig. 5. The figure presents two fits: single and two exponential. It was found that the decay curve for Eu(TTA3)DAPM (1.5%) in chitosan film can be well fitted by bi-exponential function with corresponding lifetimes of (242 712) μs and (53577) μs. The deviation from a single exponential behavior can occur due to several reasons, such as, for instance, energy transfer, large distribution of similar Eu3+ local sites, etc. However, it may also indicate the existence of two chemical microenvironments around Eu+3 ion. It should be stressed that similar ratio of lifetimes were obtained for Eu(TTA)3(H2O) and Eu(TTA3)DAPM in toluene (220 μs and 490 μs, respectively) as well in PMMA (350 μs and 510 μs, respectively) [6]. This allows us to make the hypothesis that during incorporation of Eu(TTA3)DAPM complex in chitosan a small part of the complexes changes the neutral ligand DAPM to H2O. The short lifetime component may be associated with hydrated complex Eu(TTA)3(H2O)n (n ¼1, 2). Main participation (86%) in the
Fig. 4. Time resolved photoluminescence spectra of Eu(TTA3)DAPM (5.45%) in chitosan film at room temperature (λex ¼ 370 nm): (a) time delay 3 μs, time duration 0.5 μs and (b) time delay 20 μs, time duration 1000 μs.
decay curve relates to long lifetime component whereas the quantity of short-living component is only 14%. These remarks relate also to the analysis of transition 5D0-7F0 in photoluminescence spectra of doped chitosan film (Fig. 3). The small deviation of the gaussian profile as compared to experimental data, especially in short wavelength region, may correspond to the luminescence of hydrated complex as well. Important parameter characterizing the emission of light from excited states is the quantum yield φ. Photoluminescence of lanthanide complexes generally contain light absorbed by chromophore, which is transferred to photosensitize the lanthanide ion [16]. Overall luminescence quantum yield of the complex (φtot) under excitation of the chromophore is dependent on the efficiency of sensitization (ηs) and the quantum yield of the lanthanide luminescence (φLn) jtot ¼ ηs jLn
ð1Þ
where φtot can be measured but the factors ηs and φLn are not easy available. The knowledge of these two factors is important to
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131
Eu3+ and Tb3+ containing chitosan–silica hybrids the calculated quantum efficiencies and lifetimes at room temperature of different Eu3+ systems were 4–6% and 160–220 μs, respectively [21]. Eu(TTA)3DAMP complex in chitosan film have ca. 10 times higher quantum efficiency and ca. 2.5 higher lifetime then mentioned systems. It is a consequence of surrounding environment of Eu3+ ion. Lanthanide containing chitosan–silica hybrids have in first coordination shells 5–6 water molecules and/or chitosan/silica hydroxyl groups. Organic ligands (TTA and DAPM) reduce non radiative processes from 5D0 levels by excluding water molecules from Eu3+ coordination sphere. These ligands have good absorption (e.g. for TTA ε ¼ 1.5 104 M−1 cm−1) and excellent “antenna effect” which results in enhanced luminescence due to energy transfer from ligand to ion.
4. Conclusions
Fig. 5. Typical luminescence decay profile of Eu(TTA3)DAPM (1.5%) in chitosan film at room temperature, (λex ¼ 360 nm and λem ¼ 617 nm). The black points represent experimental data. The dotted line represents unsatisfactory single exponential plot. The solid line represents a two exponential fit to the equation I(t) ¼ A exp(–t/τ1) +B exp(–t/τ2). Best fit was achieved for the following parameters: A ¼ 0.28195, B¼0.76115, τ1 ¼ 242 μs and τ2 ¼ 535 μs with coefficient of determination, R2 ¼ 0.999924.
optimize luminescence of lanthanide complex. If the radiative lifetime τR is known, φLn can be calculated using observed luminescence lifetime τobs τ φLn ¼ obs ð2Þ τR 5
3+
The radiative lifetime of D0 state of Eu may be calculated from emission spectrum using the following equation: 1 I tot ¼ AMD;0 n3 ð3Þ τR I MD The formula is based on the assumption that the energy of the D0-7Fj transition and its dipole strength are constant [17]. Here, n is the refractive index of the medium (for chitosan film n ¼ 1.5 [17]), AMD,0 is the spontaneous emission probability for the 5 D0-7F1 transition in vacuum (AMD,0 ¼14.65 s−1 11) and Itot/IMD is the ratio of the total integrated emission intensity 5D0-7FJ (J¼ 0– 6) of corrected Eu+3 emission spectrum to integrated emission intensity of 5D0-7F1 band [1]. In our measurements the spectrum consists of 5D0-7FJ (J ¼0–4) bands therefore, only the upper limit of the τR (3) could be calculated. From Fig. 3 the estimated ratio is Itot/IMD ¼17.9. Such approximation is typically used in transition probability and quantum yield analysis of Eu complexes, as the intensities of NIR band (J−5,6) are rather low [17]. Previously, it was shown that kinetic analysis reveals two components with lifetimes of (242 712) μs and (53577) μs. Main participation (86%) in the decay curve relates to long lifetime component whereas the quantity of short-living component is only 14%. In the case of two lifetime components it is not possible to exactly calculate the quantum yield. However, it may be roughly estimated by neglecting the short component. This way we estimate the upper limit of the quantum yield. The maximal (ηs ¼ 1) quantum yield φLn of Eu(TTA3)DAPM complex in chitosan film calculated from Eq. (3) is ca. 47% assuming that the whole luminescence goes through the long-living component. Only few papers reported luminescence of Ln3+ doped chitosan polymers, mainly in application for waveguiding, biology and cancer therapy [18–20]. In the investigated photoluminescence of 5
Preparation method of biocompatible Eu(TTA)3DAPM chitosan film was developed. It was shown that the structure of Eu complexes did not change during sample preparation. Investigated films have very good luminescence properties of Eu+3 after UV excitation with estimated quantum yield of approximately 47%. During several days of measurements the optical properties of the material did not change. It provides a good basis for further application of this material for the construction of luminescence sensors especially for biological, medical and agriculture applications.
Acknowledgments This work was partly supported by research Project no. N N313 442737 from the Polish Ministry of Science and Higher Education over the years 2009–2012. References [1] J.-C.G. Bunzli, Chem. Rev. 110 (2010) 2729. [2] K. Binnemans, Chem. Rev. 109 (2009) 4283. [3] C.P. Montgomery, B.S. Murray, E.J. New, R Pal, D. Parker, Acc. Chem. Res. 42 (2009) 925. [4] Hong-Guo Liu, Yong-Ill Lee, Seongtae Park, Kiwan Jang, Sang Su Kim, J. Lumin. 110 (2004) 11. [5] Junji Kido, Yoshi Okamoto, Chem. Rev. 102 (2002) 2357. [6] V.Y. Venchikov, M.P. Tsvirko, J. Appl. Spectros. 68 (2001) 1036. [7] N.V. Majeti, Ravi Kumar, React. Funct. Polym. 46 (2000) 1. [8] M. Rinaudo, Prog. Polym. Sci. 31 (2006) 603. [9] G.A. González-Aguilar, E. Valenzuela-Soto, J. Lizardi-Mendoza, F. Goycoolea, M. A. Martínez-Téllez, M.A. Villegas-Ochoa, I.N. Monroy-García, J.F. Ayala-Zavala, J. Sci. Food Agric. 89 (2009) 15. [10] Y. Tian, J.C. Yu, X.H. Qi, X.W. Wu, R.N. Hua, S.D. Fan, J Mater. Sci.: Mater. Electron. 20 (2009) 439. [11] A. Synytsya, P. Blafkova, A. Synytsya, J. Copıkova, J. Spevacek, M. Uher, Carbohydr. Polym. 72 (2008) 21. [12] E. Mandowska, A. Mandowski, J. Swiatek, W. Mazela, P. Czub, J. Pielichowski, Nonlinear Opt. Quantum Opt. 36 (2007) 325. [13] E. Mandowska, W. Mazela, P. Czub, A. Mandowski, J. Pielichowski, J. Swiatek, Macromol. Symp. 212 (2004) 269. [14] S. Cotton, Lanthanide and Actinide Chemistry, John Wiley & Sons, Ltd, Chichester, 2006(p. 69). [15] K. Binnemans, P. Lenaerts, K. Driesen, C. Gorller-Wlrand, J. Mater. Chem. 14 (2004) 191. [16] D. Parker, J.A.G. Williams, J. Chem. Soc. Dalton Trans. (1996) 3613. [17] M.H.V. Werts, R.T.F. Ronald Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002) 1542. [18] Hao Jiang, W. Su, S. Caracci, T.J. Bunning, T. Cooper, W.W. Adams, J. Appl. Polym. Sci. 61 (1996) 1163. [19] F. Wang, Y. Zhang, X. Fan, M. Wang, Nanotechnology 17 (2006) 1527. [20] J.Y. Paeng, M.J. Kim, J.H. Kang, B.C. Shin, H. Myoung, Int. J. Oral Maxill Surg. 34 (1) (2005) 154. [21] Fengyi Liu, D. Luis Carlos, Rute A.S. Ferreira, Joa~o Roch, J. Phys. Chem. B 114 (2010) 77.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence properties of chitosan doped with europium complex M. Tsvirko a, E. Mandowska b, M. Biernacka b, S. Tkaczyk b, A. Mandowski b,n a b
Institute of Chemistry, Jan Dlugosz University, ul. Armii Krajowej 13/15, 42-200 Czestochowa, Poland Institute of Physics, Jan Dlugosz University, ul. Armii Krajowej 13/15, 42-200 Czestochowa, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 5 June 2012 Received in revised form 25 January 2013 Accepted 16 April 2013 Available online 23 April 2013
Chitosan is a natural, biodegradable and biocompatible polymer. It is transparent in UV–vis region. Nonetheless, appropriate doping may significantly change its optical properties. Chitosan film doped with Eu+3 β-diketonate complex was synthesized. Absorption, photoluminescence and time resolved decay properties of this material were investigated. It is shown, that the material exhibits excellent luminescence emission in VIS region with high quantum efficiency of approximately 47%. These exceptional properties allow for various applications of this material. In particular, it provides a good basis for the construction of biocompatible luminescent sensor. & 2013 Elsevier B.V. All rights reserved.
Keywords: Chitosan Eu+3 complex Luminescence Biosensor
1. Introduction In recent decades an increasing interest was observed on synthesis of lanthanides complexes possessing high quantum efficiencies in water environment. This trend of studies is developed by various applications of luminescent lanthanide probes for time-resolved immunoassays, time-resolved luminescent microscopy and optical sensors of pH, oxygen pressure, temperature and selected anions. Doping such complexes in biocompatible materials may be considered as alternative way to construct highly efficient luminescent systems for bio analytical applications [1–3]. Synthetic UV transparent polymers, especially poly(methyl methacrylate) (PMMA) doped by luminescence lanthanide complexes has been widely investigated [4]. On the other hand the complexes were introduced also to various hole transport polymers such as poly (N-vinylcarbazole) (PVK), poly[2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), and others to investigate mainly organic light emitting diodes (OLEDs) [5]. Luminescence quantum efficiency of Eu 5D0-7FJ (J ¼0, 1,…,4) luminescence is high even at room temperature (0.33 and 0.71 in toluene and PMMA films, respectively) [6]. This property is important for practical applications. The environmental requirements for safety and recyclability extorts to combine these functional materials with low cost, environmentally friendly biopolymers to synthesize hybrid materials. Natural, biodegradable and biocompatible chitosan produced from crab shells or shrimps is a promising candidate, as a matrix material, for various optical applications. This low cost produced polysaccharide has
n
Corresponding author. Tel.: +48 34 361 49 19x262; fax: +48 34 366 82 52. E-mail address: [email protected] (A. Mandowski).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.04.014
many interesting properties e.g. ability to form films, non-toxicity, excellent adsorption features etc. which makes it excellent to prepare functional hybrid materials [7]. Chitosan is used to prepare hydrogels, films, fibers or sponges in the biomedical domain, for which the biocompatilibity is essential. It is widely applied e.g. in medicine, pharmacy, cosmetics and agriculture because it exhibits antivirus and antiphage activities as well in bone tissue engineering etc. [8,9]. Chitosan nanoparticles, thin films and membranes seem to be particularly interesting for various applications. Chitosan nanoparticles doped with trivalent lanthanide ions could found medical application as fluorescent labels [10]. Composite materials based on PMMA thin films doped with europium complexes may be applied as UV radiation converters [6]. Pure chitosan film is transparent for UV–vis light [11] and may be used as a matrix for various chemical compounds. Therefore this polymer can be applied as a biocompatible sensor as well. This work presents luminescence properties of chitosan doped with Eu3+ fluorinated β-diketone (TTA) complex.
2. Experiment Chitosan is a natural polymer of acetylamino-α-glucose. A structural formula of chitosan is depicted in Fig. 1a. The investigated europium complex Eu(TTA)3DAPM, (TTA—thenoyltrifluoroacetone, DAPM—diantipyrylpropylmethane) is shown in Fig. 1b. The ligand DAPM saturates coordination number of Eu ion stabilizing structure of Eu(TTA)3DAPM. As a consequence, it improves luminescence properties of the investigated complex.
M. Tsvirko et al. / Journal of Luminescence 143 (2013) 128–131
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Fig. 1. Chemical structure of (a) chitosan and (b) Eu(TTA)3DAPM complex.
The chitosan used for sample preparation was purchased from the Institute of Sea Fisheries in Gdynia (Poland). The deacetylation and viscosity of this chitosan were 73% and 256 mPa s, respectively. The 0.75 g of dry chitosan was dissolved in 85 mL (1%) acetic acid in distilled water. Solution of Eu(TTA)3DAPH was prepared by dissolving 30 mg of Eu(TTA)3DAPH in 10 mL of methanol. Then, the solutions of acetate chitosan and Eu(TTA)3DAPH in methanol were mixed in proper amounts to obtain 5.45% weight of europium complex relative to chitosan mass. Final solution was deposited at smooth poly(methyl methacrylate) (PMMA) or glass surfaces and dried for 24 h at 313 K. The obtained layers were homogeneous and transparent. Thickness of the layers was in the range 5– 100 mm, depending on the amount of deposited solution. Absorption spectra were measured using Schimadzu spectrophotometer UV-240PC at wavelength range 230–400 nm with resolution of 0.1 nm, in air, at room temperature. Photoluminescence (PL) measurements were carried out in vacuum at 80.5 K, after excitation by 350 nm light using 75 W Xenon arc lamp. More details of the measurement equipment are given in some earlier papers [12,13]. Wavelength resolution of PL measurements was 0.3 nm. Finally, numerical calibration of photoluminescence spectra was done taking into account sensitivity of the whole measurement system. Time-resolved measurements of luminescence excitation and emission spectra as well as decay kinetics were carried out using spectrofluorimeter FLUORAT-02-PANORAMA LUMEX. The light source was a high pressure flash Xe-lamp. It operates with 1 μs pulse and repetition frequency of 25 Hz. Emission of luminescence may be detected in spectrum range 210–730 nm with resolution of 15 nm. The measurements were carried out in ambient conditions. The delay and duration times were adjusted to investigated samples.
3. Results Absorption spectra of TTA,DAPM Eu(TTA)3 DAPM in methanol solution and Eu(TTA)3DAPM (5.45%) in chitosan film are shown in Fig. 2. Absorption peaks of TTA (337 nm) and DAPM ligands (276 nm) in methanol solution correspond to peaks obtained for Eu(TTA)3DAPM in methanol solution (336 nm and 283 nm) and europium-chitosan film (355 nm and 291 nm). Absorption peaks
Fig. 2. Absorption spectra of (a) diantipyrylpropylmethane (DAPM), thenoyltrifluoroacetone (TTA) and Eu(TTA3)DAPM in methanol solution and (b) europium complex Eu(TTA3)DAPM in methanol solution and in chitosan film (5.45% Eu(TTA3) DAPM, film thickness 24 μm).
of Eu complex in solution and chitosan film are slightly shifted to larger wavelengths as compared with absorption peaks of TTA and DAPM in methanol solution. Chitosan film in the region 220–400 nm is transparent. Photoluminescence (PL) spectrum of Eu(TTA)3DAPM (5.45%) doped chitosan film is shown in Fig. 3. Excitation at 350 nm produces five narrow emission peaks centered at 579 nm, 591 nm, 612 nm, 652 nm, and 708 nm which are related to 5 D0-7FJ transitions (J¼ 0, 1, 2, 3, 4) of Eu3+ ion. The peaks may reveal fine structure, because 7FJ (J ¼1, 2, 3, 4) levels are degenerate and split into 2 J+1 sublevels. Luminescence due to non-degenerate 5D0-7F0 transition appears as a single peak, which should be fitted by one gaussian (peak maximum 579.7 nm) in energy domain. The gaussian fit of this band is shown in the inset in Fig. 3. It indicates that in chitosan Eu(TTA)3DAPM film there is one dominant type of the complex.
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M. Tsvirko et al. / Journal of Luminescence 143 (2013) 128–131
Fig. 3. Photoluminescence spectrum of Eu(TTA)3DAPM (5.45%) in chitosan film measured at T ¼ 80.5 K, excitation by Xe 75 W lamp, λex ¼ 350 nm. The inset shows gaussian fit of 5D0-7F0 peak.
The fine structure of the transition 5D0-7F1 reveals three components in luminescence spectrum (Fig. 3). This observation indicates low symmetry group characterizing Eu+3 local environment. The relative higher intensity of electric-dipole hypersensitive 5D0-7F2 transition in comparison with the magnetic-dipole transition (5D0-7F1) is in agreement with this supposition [14,15]. To get more information on this issue additional measurements with higher signal-to-noise ratio should be performed (e.g. in low temperature). Photoluminescence spectra of Eu(TTA)3DAPM doped chitosan film and Eu(TTA)3DAPM in solution are similar. It indicates that surrounding of Eu+3 is the same in methanol solution and in chitosan film and the structure of the complex Eu(TTA)3DAPM in chitosan film remains unchanged. Fig. 4a, b and Fig. 5 show time resolved luminescence spectra of Eu(TTA3)DAPM in chitosan film. While recording the spectrum for time delay 20 μs and time duration 1000 μs (Fig. 4b), it was possible to observe only transition from 5D0 level of Eu3+ ion. By changing time delay to 3 μs and time duration to 0.5 μs (Fig. 4a) it was possible to measure 5D1-7FJ (J ¼1, 2) transitions of Eu3+ ion in shorter wavelength region. It indicates that lifetime of 5D1 state is of the order of 1 μs. Decay curves of Eu+3 5D0 emission were investigated by monitoring the emission at 617 nm under excitation by 360 nm light as shown in Fig. 5. The figure presents two fits: single and two exponential. It was found that the decay curve for Eu(TTA3)DAPM (1.5%) in chitosan film can be well fitted by bi-exponential function with corresponding lifetimes of (242 712) μs and (53577) μs. The deviation from a single exponential behavior can occur due to several reasons, such as, for instance, energy transfer, large distribution of similar Eu3+ local sites, etc. However, it may also indicate the existence of two chemical microenvironments around Eu+3 ion. It should be stressed that similar ratio of lifetimes were obtained for Eu(TTA)3(H2O) and Eu(TTA3)DAPM in toluene (220 μs and 490 μs, respectively) as well in PMMA (350 μs and 510 μs, respectively) [6]. This allows us to make the hypothesis that during incorporation of Eu(TTA3)DAPM complex in chitosan a small part of the complexes changes the neutral ligand DAPM to H2O. The short lifetime component may be associated with hydrated complex Eu(TTA)3(H2O)n (n ¼1, 2). Main participation (86%) in the
Fig. 4. Time resolved photoluminescence spectra of Eu(TTA3)DAPM (5.45%) in chitosan film at room temperature (λex ¼ 370 nm): (a) time delay 3 μs, time duration 0.5 μs and (b) time delay 20 μs, time duration 1000 μs.
decay curve relates to long lifetime component whereas the quantity of short-living component is only 14%. These remarks relate also to the analysis of transition 5D0-7F0 in photoluminescence spectra of doped chitosan film (Fig. 3). The small deviation of the gaussian profile as compared to experimental data, especially in short wavelength region, may correspond to the luminescence of hydrated complex as well. Important parameter characterizing the emission of light from excited states is the quantum yield φ. Photoluminescence of lanthanide complexes generally contain light absorbed by chromophore, which is transferred to photosensitize the lanthanide ion [16]. Overall luminescence quantum yield of the complex (φtot) under excitation of the chromophore is dependent on the efficiency of sensitization (ηs) and the quantum yield of the lanthanide luminescence (φLn) jtot ¼ ηs jLn
ð1Þ
where φtot can be measured but the factors ηs and φLn are not easy available. The knowledge of these two factors is important to
M. Tsvirko et al. / Journal of Luminescence 143 (2013) 128–131
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Eu3+ and Tb3+ containing chitosan–silica hybrids the calculated quantum efficiencies and lifetimes at room temperature of different Eu3+ systems were 4–6% and 160–220 μs, respectively [21]. Eu(TTA)3DAMP complex in chitosan film have ca. 10 times higher quantum efficiency and ca. 2.5 higher lifetime then mentioned systems. It is a consequence of surrounding environment of Eu3+ ion. Lanthanide containing chitosan–silica hybrids have in first coordination shells 5–6 water molecules and/or chitosan/silica hydroxyl groups. Organic ligands (TTA and DAPM) reduce non radiative processes from 5D0 levels by excluding water molecules from Eu3+ coordination sphere. These ligands have good absorption (e.g. for TTA ε ¼ 1.5 104 M−1 cm−1) and excellent “antenna effect” which results in enhanced luminescence due to energy transfer from ligand to ion.
4. Conclusions
Fig. 5. Typical luminescence decay profile of Eu(TTA3)DAPM (1.5%) in chitosan film at room temperature, (λex ¼ 360 nm and λem ¼ 617 nm). The black points represent experimental data. The dotted line represents unsatisfactory single exponential plot. The solid line represents a two exponential fit to the equation I(t) ¼ A exp(–t/τ1) +B exp(–t/τ2). Best fit was achieved for the following parameters: A ¼ 0.28195, B¼0.76115, τ1 ¼ 242 μs and τ2 ¼ 535 μs with coefficient of determination, R2 ¼ 0.999924.
optimize luminescence of lanthanide complex. If the radiative lifetime τR is known, φLn can be calculated using observed luminescence lifetime τobs τ φLn ¼ obs ð2Þ τR 5
3+
The radiative lifetime of D0 state of Eu may be calculated from emission spectrum using the following equation: 1 I tot ¼ AMD;0 n3 ð3Þ τR I MD The formula is based on the assumption that the energy of the D0-7Fj transition and its dipole strength are constant [17]. Here, n is the refractive index of the medium (for chitosan film n ¼ 1.5 [17]), AMD,0 is the spontaneous emission probability for the 5 D0-7F1 transition in vacuum (AMD,0 ¼14.65 s−1 11) and Itot/IMD is the ratio of the total integrated emission intensity 5D0-7FJ (J¼ 0– 6) of corrected Eu+3 emission spectrum to integrated emission intensity of 5D0-7F1 band [1]. In our measurements the spectrum consists of 5D0-7FJ (J ¼0–4) bands therefore, only the upper limit of the τR (3) could be calculated. From Fig. 3 the estimated ratio is Itot/IMD ¼17.9. Such approximation is typically used in transition probability and quantum yield analysis of Eu complexes, as the intensities of NIR band (J−5,6) are rather low [17]. Previously, it was shown that kinetic analysis reveals two components with lifetimes of (242 712) μs and (53577) μs. Main participation (86%) in the decay curve relates to long lifetime component whereas the quantity of short-living component is only 14%. In the case of two lifetime components it is not possible to exactly calculate the quantum yield. However, it may be roughly estimated by neglecting the short component. This way we estimate the upper limit of the quantum yield. The maximal (ηs ¼ 1) quantum yield φLn of Eu(TTA3)DAPM complex in chitosan film calculated from Eq. (3) is ca. 47% assuming that the whole luminescence goes through the long-living component. Only few papers reported luminescence of Ln3+ doped chitosan polymers, mainly in application for waveguiding, biology and cancer therapy [18–20]. In the investigated photoluminescence of 5
Preparation method of biocompatible Eu(TTA)3DAPM chitosan film was developed. It was shown that the structure of Eu complexes did not change during sample preparation. Investigated films have very good luminescence properties of Eu+3 after UV excitation with estimated quantum yield of approximately 47%. During several days of measurements the optical properties of the material did not change. It provides a good basis for further application of this material for the construction of luminescence sensors especially for biological, medical and agriculture applications.
Acknowledgments This work was partly supported by research Project no. N N313 442737 from the Polish Ministry of Science and Higher Education over the years 2009–2012. References [1] J.-C.G. Bunzli, Chem. Rev. 110 (2010) 2729. [2] K. Binnemans, Chem. Rev. 109 (2009) 4283. [3] C.P. Montgomery, B.S. Murray, E.J. New, R Pal, D. Parker, Acc. Chem. Res. 42 (2009) 925. [4] Hong-Guo Liu, Yong-Ill Lee, Seongtae Park, Kiwan Jang, Sang Su Kim, J. Lumin. 110 (2004) 11. [5] Junji Kido, Yoshi Okamoto, Chem. Rev. 102 (2002) 2357. [6] V.Y. Venchikov, M.P. Tsvirko, J. Appl. Spectros. 68 (2001) 1036. [7] N.V. Majeti, Ravi Kumar, React. Funct. Polym. 46 (2000) 1. [8] M. Rinaudo, Prog. Polym. Sci. 31 (2006) 603. [9] G.A. González-Aguilar, E. Valenzuela-Soto, J. Lizardi-Mendoza, F. Goycoolea, M. A. Martínez-Téllez, M.A. Villegas-Ochoa, I.N. Monroy-García, J.F. Ayala-Zavala, J. Sci. Food Agric. 89 (2009) 15. [10] Y. Tian, J.C. Yu, X.H. Qi, X.W. Wu, R.N. Hua, S.D. Fan, J Mater. Sci.: Mater. Electron. 20 (2009) 439. [11] A. Synytsya, P. Blafkova, A. Synytsya, J. Copıkova, J. Spevacek, M. Uher, Carbohydr. Polym. 72 (2008) 21. [12] E. Mandowska, A. Mandowski, J. Swiatek, W. Mazela, P. Czub, J. Pielichowski, Nonlinear Opt. Quantum Opt. 36 (2007) 325. [13] E. Mandowska, W. Mazela, P. Czub, A. Mandowski, J. Pielichowski, J. Swiatek, Macromol. Symp. 212 (2004) 269. [14] S. Cotton, Lanthanide and Actinide Chemistry, John Wiley & Sons, Ltd, Chichester, 2006(p. 69). [15] K. Binnemans, P. Lenaerts, K. Driesen, C. Gorller-Wlrand, J. Mater. Chem. 14 (2004) 191. [16] D. Parker, J.A.G. Williams, J. Chem. Soc. Dalton Trans. (1996) 3613. [17] M.H.V. Werts, R.T.F. Ronald Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002) 1542. [18] Hao Jiang, W. Su, S. Caracci, T.J. Bunning, T. Cooper, W.W. Adams, J. Appl. Polym. Sci. 61 (1996) 1163. [19] F. Wang, Y. Zhang, X. Fan, M. Wang, Nanotechnology 17 (2006) 1527. [20] J.Y. Paeng, M.J. Kim, J.H. Kang, B.C. Shin, H. Myoung, Int. J. Oral Maxill Surg. 34 (1) (2005) 154. [21] Fengyi Liu, D. Luis Carlos, Rute A.S. Ferreira, Joa~o Roch, J. Phys. Chem. B 114 (2010) 77.