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New optical material europium EDTA complex in polyvinyl pyrrolidone films with fluorescence enhanced by silver plasmons
Optical Materials 34 (2011) 351–354
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
New optical material europium EDTA complex in polyvinyl pyrrolidone films with fluorescence enhanced by silver plasmons Renata Reisfeld a,⇑, Tsiala Saraidarov a, Gerard Panzer b, Viktoria Levchenko a, Michael Gaft c a
The Hebrew University of Jerusalem, Chemistry Institute, E. Safra Campus, 91904 Jerusalem, Israel LPCML UMR 5620 CNRS, University of Lyon, 69622 Villeurbanne, France c LDS – Laser Distance Spectrometry Ltd., 11 Granit St., P.O. Box 3257, 49514 Petah Tikva, Israel b
a r t i c l e
i n f o
Article history: Received 2 March 2011 Received in revised form 6 April 2011 Accepted 8 April 2011 Available online 12 May 2011 Keywords: Fluorescence intensification Steady state and dynamic fluorescence EuEDTA complex Polyvinyl pyrrolidone Silver nanoparticles
a b s t r a c t In our search for efficient Luminescent Solar Concentrators (LSC) we have prepared polyvinyl pyrrolidone (PVP) films incorporated by ethylenediamine tetraacetic acid (EDTA) complex of europium and co-doped with silver nanoparticles (NPs). Steady state fluorescence was studied under weak and strong excitation. Dynamical study was performed by second harmonic of Nd laser. Under weak excitation the fluorescence of europium co-doped with silver plasmons increased by a factor of three and excited by continuous laser by a factor of 50. The lifetimes of films doped by the complex were 755 ls and co-doped with silver nanoparticles 946 ls. This is the first finding that the photon density accumulates the number of plasmons interacting with electronic states of europium increasing its transition probability resulting in the strong intensification of fluorescence. In dynamical measurements of lifetimes a single pulse does not provide enough energy to create such number of plasmons. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Photoactive lanthanide complexes in combination with metal nanoparticles (NPs) are of both scientific and technological interest because of their large Stokes shifts, narrow emission bandwidths, and long emission lifetimes. Due to these characteristics, they are suitable candidates for applications as light-emitting diode (LED) [1,2], laser materials [3,4] biosensors and fluoroimmunoassay [5– 10]. It is well established that silver nanoparticles (NPs) can interact with species incorporated in a dielectric medium and influence their optical properties. It is only recently understood that the strong interaction of light with noble metal NPs and the dielectric medium in which they are imbedded is of both fundamental and practical significance [11]. Luminescence properties of EuIII complex/polyvinyl pyrrolidone composite nano-fibers were prepared and characterized by Zhang et al. [12]. The composite nano-fibers demonstrated a significant improvement of luminescent efficiency compared with the complex Eu(TTA)3phen without polyvinyl pyrrolidone. The enhancement and quenching of emission of EuEDTA complex in presence of Au NPs and Au–ZnO core-shell nanoparticles were demonstrated by Halder and Patra [13]. The same effects of enhancing and quenching of Eu complex by Silver NPs were obtained in solution phase by Nabika and Deki [14]. Lakowicz et al. [15] reported the effects of silver islands on the fluorescent ⇑ Corresponding author. E-mail address: [email protected] (R. Reisfeld). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.007
intensity and decay times of lanthanide chelates in PVA films. Hayakawa et al. [16] described the influence of silver islands on the enhancement of fluorescence from Eu3+ ion doped silica gels. Malta and Couto dos Santos [17] demonstrated the fluorescence yield of rare-earth ions in glasses in presence of Ag particles and they proposed the energy transfer between Eu3+ and Ag particles. It should be noted that majority of cases the interaction of EuEDTA complex with silver NPs results in quenching effects which is contrary to our results where the enhancement is observed. The main motivation for this work is to obtain composite material based on polyvinyl pyrrolidone (PVP) and in situ synthesis of EuEDTA complex with and without silver NPs. As it will be shown an appreciate increase of fluorescence of EuEDTA–PVP composite with presence of silver NPs with comparison to the fluorescence of the same complex without silver plasmons. This phenomenon observed is the result of interaction of the complex with silver plasmons. It is known that metal NPs can result in strong scattering of incident light and greatly enhanced local fields, and can also lead to enhanced Raman scattering.
2. Experimental procedure and measurements 2.1. EuEDTA complex in PVP The EuEDTA complex in PVP matrix was formed as follows: appropriate stock solutions of desired metal ion EuIII chloride and ligands (ethylenediamine tetraacetic acid–EDTA disodium salt
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R. Reisfeld et al. / Optical Materials 34 (2011) 351–354
dihydrate) in 1:1 ratio were mixed in solution of sodium hydroxide/water/ethanol. Then 5 ml of this solution were incorporated in 5 ml of 20% PVP solution in ethanol followed by stirring for 3 h at 70 °C. The obtained solution was divided in two parts: (1) To one half of the solution was added 0.5 ml water and stirred. (2) To another half of the solution was added 0.5 ml of citrate capped silver NPs solution obtained separately. Concentration of Eu+3 ions in PVP solution was 0.013 mM/mL, concentration of Ag+1 ion in water was 0.0029 mM/mL and in the PVP solution of EuEDTA was 0.0003 mM/mL. Films were formed on microscope slides using drop-casting process, dried at 28 °C for 24 h and heated at 120 °C for 1 h. 2.2. Silver NPs Silver NPs were obtained using procedure described in ref. [18]. One sample of preparation is: silver nitrate and sodium citrate (1:3) were dissolved in distillated water and heated via stirring to achieve boiling. During boiling 0.1 M of ammonium hydroxide was added. After the solution become opaque the heating was stopped. To the cold solution hydroxyl ethyl-cellulose (HEC) dissolved in water (0.5%) was added as stabilizer.
Fig. 2. Fluorescence spectra of Eu:EDTA in PVPD film (blue) and the same film with presence of Ag NPs (red lines). The excitation was measured at the maximum of fluorescence – 615 nm and the fluorescence was measured at the maximum of excitation – 393 nm. Intensity enhancement = 497/178 = 279%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. Measurements Absorption spectra were measured using a Milton Roy Spectronic M-3000 diode array spectrophotometer. Steady state fluorescence was measured using a JASCO FP770 spectrofluorimeter equipped with a xenon lamp and a Renishaw spectrometer (grating of 1800 lines mm 1) recording the emission induced by a 532 nm excitation laser diode reduced to avoid saturation to 1.5 mW on the samples after focalization by a Olympus 50 objective. In this last configuration the laser beam was focused perpendicularly to the film surface avoiding the glass slide contribution and the emission collected in the same direction to the ICCD. Decay times were measured using the 532 nm second harmonic of pulsed Nd:YAG laser (160 mJ/pulse) and a set of spectra recorded at a geometry of 90° using the kinetic mode of an Andor ICCD with a step of 100 ls after the laser pulse and a duration of 5 ms each. SEM images of films cross section were obtained by high resolution scanning electron microscopy (HRSEM) (Sirion FEI Company) uses Shottky type Field Emission Source and allows wide range of accelerating voltages from 200 V to 30 kV. It is able to achieve resolutions of 1.5 nm at 10 kV and 2.5 nm at 1 kV.
Fig. 3. Steady state luminescence of Eu:EDTA (blue; 10) and Eu:EDTA Ag NPs (red) under 532 nm continuous excitation. Enhancement: 5000%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussions EDTA or ethylenediaminetetraacetic acid is a polyprotic acid for complexing metal ions. It is containing four carboxylic acid groups
Fig. 1. Chemical structure of ethylenediamine tetraacetic acid (EDTA) – classic form (left) and at high pH alkaline conditions (right).
Fig. 4. Absorption spectrum of citrate capped Ag NPs obtained in water and dissolved in ethanol, corresponding to a size of 35–38 nm.
R. Reisfeld et al. / Optical Materials 34 (2011) 351–354
Fig. 5. Determination of the size of silver NPs versus the absorption maximum (from [18]).
353
Fig. 8. Experimental and fitted decay curve of Eu3+ in Eu:EDTA at 514 nm emission (5D0 ? 7F2) allowing the determination of a decay time of 755 ls.
pium ion. Fig. 1 shows chemical structures of ethylenediamine tetraacetic acid (EDTA) – classic form and at high pH alkaline conditions. Luminescence spectra for the two types of PVP films doped by EuEDTA complex with and without presence of silver NPs are presented on Fig. 2. The excitation spectra for both samples were measured at 393 nm around the band associated with the intense transition 5D0 ? 7F2 of Eu3+ ions. The comparison of emission intensities in both cases show enhancement of fluorescence in the case of presence of silver NPs by 279%. Fig. 3 shows the highly resolved europium emission spectra obtained using continuous 532 nm radiation (1.5 mW) for PVP film
Fig. 6. HR SEM image of Eu:EDTA in PVPD film with Ag NPs revealing a size of 35 nm.
Fig. 7. Experimental and fitted decay curve of Eu3+ in Eu:EDTA with Ag NPS at 514 nm emission (5D0 ? 7F2) allowing the determination of a decay time of 946 ls.
and two amine groups with lone pair electrons. We are using the ability of EDTA to chelate or complex metal ions in 1:1 ratio of metal – to EDTA complex. In our case we obtained the fully deprotonated form of EuEDTA complex formatted at alkaline conditions when all the acidic protons are removed and bound to the euro-
Fig. 9. SEM images of cross-sections show thicknesses of (a) EuEDTA–PVP (24.4 lm) and (b) EuEDTA–Ag–PVP film (29.3 lm).
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R. Reisfeld et al. / Optical Materials 34 (2011) 351–354
Lifetime measurements were performed for both samples (Figs. 7 and 8) show 755 ls for the sample without silver NPs and 946 ls for the sample with presence of silver NPs. The SEM images (Fig. 9a and b) of cross-sections show respectively, 24.4 lm thickness of EuEDTA–PVP film and 29.3 lm thickness of EuEDTA–Ag–PVP film. The crystallographic structure of EuEDTA was reported by Mondry and Janicki [7] who demonstrated that EDTA-ligand is coordinated to EuIII ion via two nitrogen atoms and four oxygen atoms and the remaining three bonds providing 9-fold coordination of Eu ion are supplied by three water molecules as shown on Fig. 10a. We suggest the schematic representation of the intermolecular interactions between EuEDTA complex and the neighboring chain segments of PVP and believe that in our case the 9-fold coordination is supplied by PVP chain segment’s AC@O as shown in Fig. 10b. The replacement of water molecule by PVP chain segments in EuIII–EDTA complex can decrease non-radiative relaxations in europium. 4. Conclusion The new composite material based on EuEDTA complex synthesized in situ in PVPD show dramatic enhancement of fluorescence in presence of silver NPs. It has been shown how the fluorescence can be changed by interaction of electronic levels of the complex with the radiation field of Ag NPs. As the result of this interaction under weak excitation the fluorescence of europium co-doped with silver NPs increased by a factor of three and excited by continuous laser by a factor of 50. We are showing for the first time that the laser intensity controlling the photon density on the sample has a dramatic effect of the increase of the fluorescence. It accumulates the number of plasmons interacting with electronic states of Eu resulted in the increase of transition probabilities followed by strong intensification of fluorescence. In dynamical measurements of lifetimes a single pulse does not provide enough energy to create such number of plasmons. References
Fig. 10. (a) Structure of EDTA-ligand coordinating to EuIII ion via two nitrogen atoms and four oxygen atoms showing the remaining three bonds providing 9-fold coordination of Eu ion supplied by three water molecules; (b) schematic representation of the intermolecular interactions between EuEDTA complex and the neighboring chain segments of PVP.
doped EuEDTA complex and the same film co-doped by silver NPs. For better representation the emission spectrum without silver NPs was multiplied by 10 on the figure. The results reveal that under weak excitation the fluorescence of europium co-doped with silver plasmons increased by a factor of three and excited by continuous laser by a factor of at least 50. Fig. 4 displays the absorption spectrum of silver NPs obtained in water and dissolved in ethanol. Approximately size of silver NPs 35–38 nm shown on the Fig. 4 is obtained from the theoretical graph Fig. 5 [18] based on absorption maximum, and from the HR SEM image presented on Fig. 6.
[1] M.D. McGehee, T. Bergstedt, C. Zhang, A.P. Saab, M.B. O’Regan, G.C. Bazan, V.I. Srdanov, A.J. Heeger, Adv. Mater. 11 (1999) 1349. [2] J. Cybinska, J. Legendziewicz, V. Trush, R. Reisfeld, T. Saraidarov, J. Alloys Compd. 451 (2008) 94. [3] C. Piguet, J.C. Bunzli, G. Bernardinelli, G. Hopfgartner, A.F. Williams, J. Am. Chem. Soc. 115 (1993) 8197. [4] Renata Reisfeld, Tsiala Saraidarov, Opt. Mater. 28 (2006) 64. [5] S. Comby, G. Frédéric, J.-C.G. Bunzli, T.S. Saraidarov, R. Reisfeld, Chem. Phys. Lett. 432 (2006) 128. [6] V.M. Mukkala, M. Helenius, I. Hemmila, J. Kankare, H. Takalo, Helv. Chim. Acta 76 (1993) 1361. [7] A. Mondry, R. Janicki, Dalton Trans. (2006) 4702. [8] R. Reisfeld, M. Pietraszkiewicz, T.S. Saraidarov, V. Levchenko, J. Rare Earth 27 (2009) 4. [9] C. Molina, K. Dahmouche, Y. Messaddeq, S.J.L. Ribeiro, M.A.P. Silva, V. de Zea Bermudez, L.D. Carlos, J. Lumin. 104 (2003) 93. [10] R. Reisfeld, Struct. Bonding 106 (2003) 209. [11] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824. [12] X. Zhang, S.H. Wen, S.H. Hu, Q. Chen, H. Fong, J. Phys. Chem. C 114 (2010) 3898. [13] K.K. Haldar, A. Patra, Appl. Phys. Lett. 95 (2009) 063103. [14] H. Nabika, S.H. Deki, J. Phys. Chem. B 107 (35) (2003) 9161. [15] J.R. Lakowicz, B.P. Maliwal, J. Malicka, Z. Gryczynski, I. Gryczynski, J. Fluoresc. 12 (2002) 431. [16] T. Hayakawa, S.T. Selvan, M. Nogami, Appl. Phys. Lett. 74 (1999) 1513. [17] O.L. Malta, M.A. Couto dos Santos, Chem. Phys. Lett. 174 (1990) 13. [18] T.S. Saraidarov, V. Levchenko, R. Reisfeld, Phys. Status Solidi C 7 (11–12) (2010) 2648.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
New optical material europium EDTA complex in polyvinyl pyrrolidone films with fluorescence enhanced by silver plasmons Renata Reisfeld a,⇑, Tsiala Saraidarov a, Gerard Panzer b, Viktoria Levchenko a, Michael Gaft c a
The Hebrew University of Jerusalem, Chemistry Institute, E. Safra Campus, 91904 Jerusalem, Israel LPCML UMR 5620 CNRS, University of Lyon, 69622 Villeurbanne, France c LDS – Laser Distance Spectrometry Ltd., 11 Granit St., P.O. Box 3257, 49514 Petah Tikva, Israel b
a r t i c l e
i n f o
Article history: Received 2 March 2011 Received in revised form 6 April 2011 Accepted 8 April 2011 Available online 12 May 2011 Keywords: Fluorescence intensification Steady state and dynamic fluorescence EuEDTA complex Polyvinyl pyrrolidone Silver nanoparticles
a b s t r a c t In our search for efficient Luminescent Solar Concentrators (LSC) we have prepared polyvinyl pyrrolidone (PVP) films incorporated by ethylenediamine tetraacetic acid (EDTA) complex of europium and co-doped with silver nanoparticles (NPs). Steady state fluorescence was studied under weak and strong excitation. Dynamical study was performed by second harmonic of Nd laser. Under weak excitation the fluorescence of europium co-doped with silver plasmons increased by a factor of three and excited by continuous laser by a factor of 50. The lifetimes of films doped by the complex were 755 ls and co-doped with silver nanoparticles 946 ls. This is the first finding that the photon density accumulates the number of plasmons interacting with electronic states of europium increasing its transition probability resulting in the strong intensification of fluorescence. In dynamical measurements of lifetimes a single pulse does not provide enough energy to create such number of plasmons. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Photoactive lanthanide complexes in combination with metal nanoparticles (NPs) are of both scientific and technological interest because of their large Stokes shifts, narrow emission bandwidths, and long emission lifetimes. Due to these characteristics, they are suitable candidates for applications as light-emitting diode (LED) [1,2], laser materials [3,4] biosensors and fluoroimmunoassay [5– 10]. It is well established that silver nanoparticles (NPs) can interact with species incorporated in a dielectric medium and influence their optical properties. It is only recently understood that the strong interaction of light with noble metal NPs and the dielectric medium in which they are imbedded is of both fundamental and practical significance [11]. Luminescence properties of EuIII complex/polyvinyl pyrrolidone composite nano-fibers were prepared and characterized by Zhang et al. [12]. The composite nano-fibers demonstrated a significant improvement of luminescent efficiency compared with the complex Eu(TTA)3phen without polyvinyl pyrrolidone. The enhancement and quenching of emission of EuEDTA complex in presence of Au NPs and Au–ZnO core-shell nanoparticles were demonstrated by Halder and Patra [13]. The same effects of enhancing and quenching of Eu complex by Silver NPs were obtained in solution phase by Nabika and Deki [14]. Lakowicz et al. [15] reported the effects of silver islands on the fluorescent ⇑ Corresponding author. E-mail address: [email protected] (R. Reisfeld). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.04.007
intensity and decay times of lanthanide chelates in PVA films. Hayakawa et al. [16] described the influence of silver islands on the enhancement of fluorescence from Eu3+ ion doped silica gels. Malta and Couto dos Santos [17] demonstrated the fluorescence yield of rare-earth ions in glasses in presence of Ag particles and they proposed the energy transfer between Eu3+ and Ag particles. It should be noted that majority of cases the interaction of EuEDTA complex with silver NPs results in quenching effects which is contrary to our results where the enhancement is observed. The main motivation for this work is to obtain composite material based on polyvinyl pyrrolidone (PVP) and in situ synthesis of EuEDTA complex with and without silver NPs. As it will be shown an appreciate increase of fluorescence of EuEDTA–PVP composite with presence of silver NPs with comparison to the fluorescence of the same complex without silver plasmons. This phenomenon observed is the result of interaction of the complex with silver plasmons. It is known that metal NPs can result in strong scattering of incident light and greatly enhanced local fields, and can also lead to enhanced Raman scattering.
2. Experimental procedure and measurements 2.1. EuEDTA complex in PVP The EuEDTA complex in PVP matrix was formed as follows: appropriate stock solutions of desired metal ion EuIII chloride and ligands (ethylenediamine tetraacetic acid–EDTA disodium salt
352
R. Reisfeld et al. / Optical Materials 34 (2011) 351–354
dihydrate) in 1:1 ratio were mixed in solution of sodium hydroxide/water/ethanol. Then 5 ml of this solution were incorporated in 5 ml of 20% PVP solution in ethanol followed by stirring for 3 h at 70 °C. The obtained solution was divided in two parts: (1) To one half of the solution was added 0.5 ml water and stirred. (2) To another half of the solution was added 0.5 ml of citrate capped silver NPs solution obtained separately. Concentration of Eu+3 ions in PVP solution was 0.013 mM/mL, concentration of Ag+1 ion in water was 0.0029 mM/mL and in the PVP solution of EuEDTA was 0.0003 mM/mL. Films were formed on microscope slides using drop-casting process, dried at 28 °C for 24 h and heated at 120 °C for 1 h. 2.2. Silver NPs Silver NPs were obtained using procedure described in ref. [18]. One sample of preparation is: silver nitrate and sodium citrate (1:3) were dissolved in distillated water and heated via stirring to achieve boiling. During boiling 0.1 M of ammonium hydroxide was added. After the solution become opaque the heating was stopped. To the cold solution hydroxyl ethyl-cellulose (HEC) dissolved in water (0.5%) was added as stabilizer.
Fig. 2. Fluorescence spectra of Eu:EDTA in PVPD film (blue) and the same film with presence of Ag NPs (red lines). The excitation was measured at the maximum of fluorescence – 615 nm and the fluorescence was measured at the maximum of excitation – 393 nm. Intensity enhancement = 497/178 = 279%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.3. Measurements Absorption spectra were measured using a Milton Roy Spectronic M-3000 diode array spectrophotometer. Steady state fluorescence was measured using a JASCO FP770 spectrofluorimeter equipped with a xenon lamp and a Renishaw spectrometer (grating of 1800 lines mm 1) recording the emission induced by a 532 nm excitation laser diode reduced to avoid saturation to 1.5 mW on the samples after focalization by a Olympus 50 objective. In this last configuration the laser beam was focused perpendicularly to the film surface avoiding the glass slide contribution and the emission collected in the same direction to the ICCD. Decay times were measured using the 532 nm second harmonic of pulsed Nd:YAG laser (160 mJ/pulse) and a set of spectra recorded at a geometry of 90° using the kinetic mode of an Andor ICCD with a step of 100 ls after the laser pulse and a duration of 5 ms each. SEM images of films cross section were obtained by high resolution scanning electron microscopy (HRSEM) (Sirion FEI Company) uses Shottky type Field Emission Source and allows wide range of accelerating voltages from 200 V to 30 kV. It is able to achieve resolutions of 1.5 nm at 10 kV and 2.5 nm at 1 kV.
Fig. 3. Steady state luminescence of Eu:EDTA (blue; 10) and Eu:EDTA Ag NPs (red) under 532 nm continuous excitation. Enhancement: 5000%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussions EDTA or ethylenediaminetetraacetic acid is a polyprotic acid for complexing metal ions. It is containing four carboxylic acid groups
Fig. 1. Chemical structure of ethylenediamine tetraacetic acid (EDTA) – classic form (left) and at high pH alkaline conditions (right).
Fig. 4. Absorption spectrum of citrate capped Ag NPs obtained in water and dissolved in ethanol, corresponding to a size of 35–38 nm.
R. Reisfeld et al. / Optical Materials 34 (2011) 351–354
Fig. 5. Determination of the size of silver NPs versus the absorption maximum (from [18]).
353
Fig. 8. Experimental and fitted decay curve of Eu3+ in Eu:EDTA at 514 nm emission (5D0 ? 7F2) allowing the determination of a decay time of 755 ls.
pium ion. Fig. 1 shows chemical structures of ethylenediamine tetraacetic acid (EDTA) – classic form and at high pH alkaline conditions. Luminescence spectra for the two types of PVP films doped by EuEDTA complex with and without presence of silver NPs are presented on Fig. 2. The excitation spectra for both samples were measured at 393 nm around the band associated with the intense transition 5D0 ? 7F2 of Eu3+ ions. The comparison of emission intensities in both cases show enhancement of fluorescence in the case of presence of silver NPs by 279%. Fig. 3 shows the highly resolved europium emission spectra obtained using continuous 532 nm radiation (1.5 mW) for PVP film
Fig. 6. HR SEM image of Eu:EDTA in PVPD film with Ag NPs revealing a size of 35 nm.
Fig. 7. Experimental and fitted decay curve of Eu3+ in Eu:EDTA with Ag NPS at 514 nm emission (5D0 ? 7F2) allowing the determination of a decay time of 946 ls.
and two amine groups with lone pair electrons. We are using the ability of EDTA to chelate or complex metal ions in 1:1 ratio of metal – to EDTA complex. In our case we obtained the fully deprotonated form of EuEDTA complex formatted at alkaline conditions when all the acidic protons are removed and bound to the euro-
Fig. 9. SEM images of cross-sections show thicknesses of (a) EuEDTA–PVP (24.4 lm) and (b) EuEDTA–Ag–PVP film (29.3 lm).
354
R. Reisfeld et al. / Optical Materials 34 (2011) 351–354
Lifetime measurements were performed for both samples (Figs. 7 and 8) show 755 ls for the sample without silver NPs and 946 ls for the sample with presence of silver NPs. The SEM images (Fig. 9a and b) of cross-sections show respectively, 24.4 lm thickness of EuEDTA–PVP film and 29.3 lm thickness of EuEDTA–Ag–PVP film. The crystallographic structure of EuEDTA was reported by Mondry and Janicki [7] who demonstrated that EDTA-ligand is coordinated to EuIII ion via two nitrogen atoms and four oxygen atoms and the remaining three bonds providing 9-fold coordination of Eu ion are supplied by three water molecules as shown on Fig. 10a. We suggest the schematic representation of the intermolecular interactions between EuEDTA complex and the neighboring chain segments of PVP and believe that in our case the 9-fold coordination is supplied by PVP chain segment’s AC@O as shown in Fig. 10b. The replacement of water molecule by PVP chain segments in EuIII–EDTA complex can decrease non-radiative relaxations in europium. 4. Conclusion The new composite material based on EuEDTA complex synthesized in situ in PVPD show dramatic enhancement of fluorescence in presence of silver NPs. It has been shown how the fluorescence can be changed by interaction of electronic levels of the complex with the radiation field of Ag NPs. As the result of this interaction under weak excitation the fluorescence of europium co-doped with silver NPs increased by a factor of three and excited by continuous laser by a factor of 50. We are showing for the first time that the laser intensity controlling the photon density on the sample has a dramatic effect of the increase of the fluorescence. It accumulates the number of plasmons interacting with electronic states of Eu resulted in the increase of transition probabilities followed by strong intensification of fluorescence. In dynamical measurements of lifetimes a single pulse does not provide enough energy to create such number of plasmons. References
Fig. 10. (a) Structure of EDTA-ligand coordinating to EuIII ion via two nitrogen atoms and four oxygen atoms showing the remaining three bonds providing 9-fold coordination of Eu ion supplied by three water molecules; (b) schematic representation of the intermolecular interactions between EuEDTA complex and the neighboring chain segments of PVP.
doped EuEDTA complex and the same film co-doped by silver NPs. For better representation the emission spectrum without silver NPs was multiplied by 10 on the figure. The results reveal that under weak excitation the fluorescence of europium co-doped with silver plasmons increased by a factor of three and excited by continuous laser by a factor of at least 50. Fig. 4 displays the absorption spectrum of silver NPs obtained in water and dissolved in ethanol. Approximately size of silver NPs 35–38 nm shown on the Fig. 4 is obtained from the theoretical graph Fig. 5 [18] based on absorption maximum, and from the HR SEM image presented on Fig. 6.
[1] M.D. McGehee, T. Bergstedt, C. Zhang, A.P. Saab, M.B. O’Regan, G.C. Bazan, V.I. Srdanov, A.J. Heeger, Adv. Mater. 11 (1999) 1349. [2] J. Cybinska, J. Legendziewicz, V. Trush, R. Reisfeld, T. Saraidarov, J. Alloys Compd. 451 (2008) 94. [3] C. Piguet, J.C. Bunzli, G. Bernardinelli, G. Hopfgartner, A.F. Williams, J. Am. Chem. Soc. 115 (1993) 8197. [4] Renata Reisfeld, Tsiala Saraidarov, Opt. Mater. 28 (2006) 64. [5] S. Comby, G. Frédéric, J.-C.G. Bunzli, T.S. Saraidarov, R. Reisfeld, Chem. Phys. Lett. 432 (2006) 128. [6] V.M. Mukkala, M. Helenius, I. Hemmila, J. Kankare, H. Takalo, Helv. Chim. Acta 76 (1993) 1361. [7] A. Mondry, R. Janicki, Dalton Trans. (2006) 4702. [8] R. Reisfeld, M. Pietraszkiewicz, T.S. Saraidarov, V. Levchenko, J. Rare Earth 27 (2009) 4. [9] C. Molina, K. Dahmouche, Y. Messaddeq, S.J.L. Ribeiro, M.A.P. Silva, V. de Zea Bermudez, L.D. Carlos, J. Lumin. 104 (2003) 93. [10] R. Reisfeld, Struct. Bonding 106 (2003) 209. [11] W.L. Barnes, A. Dereux, T.W. Ebbesen, Nature 424 (2003) 824. [12] X. Zhang, S.H. Wen, S.H. Hu, Q. Chen, H. Fong, J. Phys. Chem. C 114 (2010) 3898. [13] K.K. Haldar, A. Patra, Appl. Phys. Lett. 95 (2009) 063103. [14] H. Nabika, S.H. Deki, J. Phys. Chem. B 107 (35) (2003) 9161. [15] J.R. Lakowicz, B.P. Maliwal, J. Malicka, Z. Gryczynski, I. Gryczynski, J. Fluoresc. 12 (2002) 431. [16] T. Hayakawa, S.T. Selvan, M. Nogami, Appl. Phys. Lett. 74 (1999) 1513. [17] O.L. Malta, M.A. Couto dos Santos, Chem. Phys. Lett. 174 (1990) 13. [18] T.S. Saraidarov, V. Levchenko, R. Reisfeld, Phys. Status Solidi C 7 (11–12) (2010) 2648.