- Email: [email protected]
Fluorescence enhancement of europium complexes by core–shell Ag@SiO2 nanoparticles
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Fluorescence enhancement of europium complexes by core–shell Ag@SiO2 nanoparticles Hai-Xia Zhang, Xue-Mei Lin, Ai-Ling Wang, Yong-Liang Zhao, Hai-Bin Chu ⇑ College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot 010021, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Three kinds of Ag@SiO2 with shell
thickness of around 10, 15 and 25 nm were prepared. Six complexes of europium with halo-benzoic acid were synthesized. Ag@SiO2 nanoparticles can significantly enhance the fluorescence of the complexes. When the thickness of SiO2 shell is 25 nm, enhancement of fluorescence is the biggest. The order of enhancement for the complexes with different halogen atom is F < Cl < Br.
a r t i c l e
i n f o
Article history: Received 14 October 2014 Received in revised form 2 July 2015 Accepted 3 July 2015 Available online 6 July 2015 Keywords: Europium complex Halobenzoic acid Ag@SiO2 core–shell nanoparticles Metal-enhanced fluorescence
a b s t r a c t Three kinds of core–shell Ag@SiO2 nanoparticles with shell thickness of around 10, 15, and 25 nm, respectively, have been prepared by modified Stöber method and used for fluorescence enhancement. Six kinds of europium complexes with halobenzoic acid have been synthesized. Elemental analysis and lanthanide coordination titration show that the complexes have the compositions of Eu(p-XBA)3H2O and Eu(o-XBA)32H2O (X = F, Cl, Br). The fluorescence spectra investigation indicates that the introduction of Ag@SiO2 nanoparticles into the europium complexes’ solution can significantly enhance the fluorescence intensities of the complexes. The sequence of enhancement factors for halobenzoic acid complexes with different halogen atoms is F < Cl < Br, and the fluorescence enhancement factors increase as the excitation wavelength of complexes increase. When the thickness of the SiO2 shell is 25 nm, the fluorescence intensity of the europium complexes can reach a maximum enhancement factor of 5.1. The fluorescence enhancement mechanism may be the metal-enhanced fluorescence resulting from surface plasmon resonance of nanoparticles. And the nanoparticles near the complexes can effectively prevent complexes from the interaction with the solvent molecules, leading to a decrease of nonradiative energy transfer and the suppression of luminescence quench. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Lanthanide complexes have important applications in fluorescence materials, electroluminescence devices and fluorescence ⇑ Corresponding author. E-mail address: [email protected] (H.-B. Chu). http://dx.doi.org/10.1016/j.saa.2015.07.022 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
probes for biological systems [1,2]. However, there exist some restrictions for the practical application of the lanthanide complexes, essentially owing to their weak luminescence intensities, short lifetimes and low quantum efficiencies [3]. Substantial research efforts have shown that noble-metal (Au, Ag) nanoparticles can enhance the fluorescence intensities, radiative decay rates and fluorescence quantum yields of lanthanide complexes [4]. The
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
effect of metal-enhanced fluorescence is related to the distance between the fluorophore and the metal surface [5]. The direct contact between lanthanide complexes and metal nanoparticles could enhance the non-radiative energy transfer, which might lead to the quench of fluorescence, but far distance between them may result in no fluorescence enhancement. Therefore, an appropriate distance can optimize the metal-enhanced fluorescence of lanthanide complexes. Since the silica surface chemistry can covalently introduce versatile surface functionalities (e.g., COOH, CHO, NH2, or NCO) with readily available coupling agents [6,7], it is desirable to coat the metal nanoparticles with a shell of silica to permanently preserve their optical properties for effective functionalization [8– 10]. Zhang et al. have reported Ag@SiO2 nanoparticles can enhance the fluorescence intensity of Rose Bengal [11–13], but the effect of Ag@SiO2 nanoparticles on the fluorescence of lanthanide complexes are few reported [14,15]. Aromatic carboxylate ligands have been widely used in lanthanide complexes for their diverse coordination modes, strong bonding ability and efficient energy transfer to the lanthanide ions [16–18]. Especially, the halogen atoms (F, Cl, Br) in halogenated benzoic acids are electron withdrawing groups, which may have an important influence on the luminescence excitation and emission of center lanthanide ions. Therefore, six kinds of europium complexes with different halogenated benzoic acids (Eu(p-XBA)3H2O and Eu(o-XBA)32H2O, X = F, Cl, Br) are synthesized to investigate the effect of halogenation on the metal-enhanced fluorescence of lanthanide complexes. In order to study the effect of distance between lanthanide complexes and metal nanoparticles on the metal-enhanced fluorescence, we develop core–shell Ag@SiO2 nanoparticles with various shell thickness of around 10, 15 and 25 nm [19–22]. Results demonstrate that the introduction of Ag@SiO2 nanoparticles into the europium complexes’ solution can significantly enhance the fluorescence intensity of the complexes and the fluorescence enhancement is the strongest when the shell thickness of Ag@SiO2 nanoparticle is 25 nm. The enhancement factors for halo-benzoic acid complexes vary with different halogen atoms. And the fluorescence enhancement mechanisms are also discussed. 2. Experimental section 2.1. Chemical reagents Eu2O3 (99.99%), HCl, AgNO3, ammonia, anhydrous ethanol, p-fluorobenzoic acid (p-FBA), p-chlorobenzoic acid (p-ClBA), p-bromobenzoic acid (p-BrBA), o-fluorobenzoic acid (o-FBA), o-chlorobenzoic acid (o-ClBA), o-bromobenzoic acid (o-BrBA), N,N-dimethyl formamide (DMF), tetraethoxysilane (TEOS), sodium citrate and other agents are all of analytical grade and used as recieved. 2.2. Preparation of Ag@SiO2 nanoparticles 2 mL 0.20 mol L1 sodium citrate aqueous solution was added into 500 ml of boiling aqueous solution containing 2 mL 0.25 M AgNO3 under vigorous stirring [23–26]. After boiled for 15 min, the solution was cooled to room temperature [1]. The as-prepared silver colloid was centrifuged at 500 rpm for 1 h to remove large nanoparticles, and the remaining silver nanoparticles was diluted to 500 mL with water. The core–shell Ag@SiO2 nanoparticles were synthesized as follows. 10 mL as-obtained silver colloids were transferred respectively to three 500 mL round bottom flasks containing 150 mL ethanol and 1 mL 0.20 mol L1 sodium citrate. And the pH values of the solutions were adjusted to 9 via the addition of 30 wt.%
717
ammonia. Subsequently, 5 mL, 15 mL and 35 mL 10 mmol/L TEOS ethanol solutions were added into each flask within 4 h under vigorous stirring, respectively. And the resulting solutions were allowed to further react for 24 h at room temperature. Three kinds of Ag@SiO2 nanoparticles (No.1, No.2 and No.3) were collected by centrifugation at 8000 rpm for 30 min and washed with ethanol for three times.
2.3. Synthesis of europium complexes 3 mmol p-FBA (or p-ClBA, p-BrBA, o-FBA, o-ClBA and o-BrBA) was first dissolved in the mixed solvent of 10 mL ethanol and 2 mL distilled water with vigorously stirring. The pH value of the reaction mixture was adjusted to about 6 with ammonia. And 1 mmol EuCl3 was dropwise added to the solution. The pH value of the mixture was readjusted to 6, after which the mixture was heated to 60 °C and stayed at 60 °C for 3 h. Then the mixture was left still at room temperature overnight. After filtered, washed with ethanol for several times, and dried to constant weight, the white precipitates Eu(p-FBA)3H2O, Eu(p-ClBA)32H2O, Eu(p-BrBA)3H2O, Eu(o-FBA)3H2O, Eu(o-ClBA)3H2O and Eu(o-BrBA)32H2O were obtained, respectively.
2.4. Characterization Silver and Ag@SiO2 nanoparticles were characterized by JEM-2100 Transmission Electron Microscope (TEM). The composition of C, H was determined by Vario EL Cube elemental analyzer. The contents of europium were determined by EDTA titration method with xylenol orange as the indicator. The UV spectra were recorded on a Shimadzu UV-265 spectrophotometer. Infrared spectra were obtained on a Nicolet Nexus 670 FT-IR spectrometer. Fluorescence spectra were obtained on an Edinburgh FLS-920 fluorescence spectra-photometer. 2.00 mL 1.0 103 mol L1 of the complex solution was transferred into quartz cell, after which 30 lL Ag@SiO2 solution was added. The volume effect was so small that could be ignored. The solution was allowed to stand for 3 min to equilibrium, after which the fluorescence spectra were detected.
3. Results and discussion 3.1. Morphology analysis of core–shell structure Ag@SiO2 nanoparticles TEM images of silver nanoparticles are shown in Fig. 1A. Most of the silver nanoparticles are nearly spherical and their average diameter is 60–70 nm. Fig. 1B–D showed almost all the Ag nanoparticles were coated with SiO2 shell. And the three kinds of Ag@SiO2 core–shell nanoparticles (No.1, No.2 and No.3) possess a shell thickness of 10, 15 and 25 nm, respectively. UV–vis absorption spectra are an efficient measurement for tracking metallic colloid formation process [27]. The surface plasmon absorption in the visible region is very sensitive to particle size, shape and the properties of the surrounding medium [13]. According to the Mie’s theory, the maximum absorption peak of silver nanoparticles would move to longer wavelength with the increasement of the coating thickness [28,29]. Fig. 1E shows the UV–vis spectra of citrate-protected silver nanoparticles and Ag@SiO2 nanoparticles with different shell thickness. The surface plasmon bands of Ag@SiO2 nanoparticles red-shift about 8– 20 nm compared with the uncoated silver nanoparticles, which may result from the increase of the local refractive index around the particles [30,31].
718
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
Fig. 1. TEM images of silver nanoparticles (A) and core–shell Ag@SiO2 nanoparticles with different thickness. (B) No.1, (C) No.2, (D) No.3. The UV–vis absorption spectra of the nanoparticles (E).
3.2. Composition analysis of the complexes
3.3. UV absorption spectra of the complexes
As shown in Table 1, the results of elemental analyses and EDTA titrations indicate that the complexes have the formulas of Eu(p-FBA)3H2O, Eu(p-ClBA)32H2O, Eu(p-BrBA)3H2O, Eu(o-FBA)3 2H2O, Eu(o-ClBA)3H2O and Eu(o-BrBA)3H2O, respectively. The small values of molar conductivity (11.4–20.5 S cm2 mol1) of the complexes reveal that these complexes are all non-electrolytes. The complexes are easily soluble in DMF and DMSO, but slightly soluble in methanol, ethanol and water.
The UV absorption spectra of the ligands p-FBA, p-ClBA, p-BrBA, o-FBA, o-ClBA, o-BrBA and the corresponding Eu3+ complexes were determined in the range of 200–400 nm with DMF as solvent and reference, and the slit width was 2 nm. The absorption spectra are shown in Fig. S1. The maximum peaks of the six ligands locate at 265, 266, 267, 276, 278 and 279 nm, respectively. The spectra of the complexes are similar to those of the corresponding ligands, and the absorption peaks shift about 2–6 nm. The results indicate
719
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722 Table 1 Composition analysis (%) and molar conductivities (S cm2 mol1) of the complexes. Complexes
C
H
RE
km
Eu(p-FBA)3H2O Eu(p-ClBA)32H2O Eu(p-BrBA)3H2O Eu(o-FBA)32H2O Eu(o-ClBA)3H2O Eu(o-BrBA)3H2O
43.14(42.73) 38.24(38.35) 32.84(32.63) 41.26(41.46) 39.93(39.43) 32.32(32.63)
2.43(2.90) 2.82(2.91) 2.07(2.22) 3.53(3.15) 2.14(2.68) 2.01(2.22)
25.47(25.74) 23.45(23.10) 20.18(19.68) 25.45(24.98) 23.57(23.76) 19.65(19.68)
11.8 11.4 13.5 18.7 20.5 17.6
Remark: the values in brackets are theoretical values.
Table 2 Infrared spectra data of the ligands and complexes. Compounds
mO–H
mC@O(COOH)
masðCOO Þ
msðCOO Þ
dOH(COOH)
m(O–M)
p-FBA Eu(p-FBA)3H2O p-ClBA Eu(p-ClBA)32H2O p-BrBA Eu(p-BrBA)3H2O o-FBA Eu(o-FBA)32H2O o-ClBA Eu(o-ClBA)3H2O o-BrBA Eu(o-BrBA)3H2O
3458–2563 3431.82 3432–2555 3432.62 3557–2558 3431.80 3467–2666 3438.77 3457–2556 3432.38 3433–2547 3432.67
1682.05 – 1685.91 – 1679.67 – 1691.68 – 1689.61 – 1685.32 –
– 1540.37 – 1538.32 – 1536.99 – 1532.52 – 1544.35 – 1540.52
– 1408.96 – 1411.37 – 1409.15 – 1406.28 – 1407.26 – 1409.03
1129.79 – 1129.64 – 1128.43 – 1128.79 – 1265.59 – 1127.86 –
– 556.35 – 551.36 – 560.45 – 553.63 – 557.47 – 563.43
Fig. 2. Schematic diagram of synthetic route for Ag@SiO2 nanoparticles and Ag@SiO2@Eu-complexes.
Table 3 Fluorescence data of 5D0 ? 7F2 of europium complexes and Ag@SiO2@Eu complexes. Complexes
Excitation wavelength (nm)
Emission wavelength (nm)
Intensity (a.u.)
Enhancement factor
Eu(p-FBA)3H2O
Pure complex +No.1 +No.2 +No.3
270 272 274 275
616 616 616 616
8298 10,980 13,650 14,920
– 1.32 1.64 1.80
Eu(p-ClBA)32H2O
Pure complex +No.1 +No.2 +No.3
279 281 282 283
616 616 616 617
8121 19,300 26,170 28,650
– 2.38 3.22 3.53
Eu(p-BrBA)3H2O
Pure complex +No.1 +No.2 +No.3
394 394 394 394
615 616 616 616
1521 2885 3976 6387
– 1.90 2.61 4.20
Eu(o-FBA)32H2O
Pure complex +No.1 +No.2 +No.3
281 282 284 284
616 617 617 616
1080 1462 2364 3260
– 1.35 2.19 3.02
Eu(o-ClBA)3H2O
Pure complex +No.1 +No.2 +No.3
286 288 290 291
617 616 616 616
6966 13,770 17,680 24,220
– 1.98 2.54 3.48
Eu(o-BrBA)3H2O
Pure complex +No.1 +No.2 +No.3
394 394 394 394
617 617 616 617
1219 2660 5334 6222
– 2.18 4.38 5.10
720
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
Fig. 3. Fluorescent emission spectra of Eu complexes and Ag@SiO2@Eu-complexes with excitation spectra as insets. (A) Eu(p-FBA)3H2O, (B) Eu(p-ClBA)32H2O, (C) Eu(pBrBA)3H2O, (D) Eu(o-FBA)32H2O, (E) Eu(o-ClBA)3H2O and (F) Eu(o-BrBA)3H2O.
Fig. 4. The enhancement times at different emission peaks of complexes (A) Eu(p-BrBA)3H2O and (B) Eu(o-BrBA)3H2O by adding Ag@SiO2 with shell thickness of around 10, 15 and 25 nm.
that the ligands coordinate with the europium ions in the complexes [32]. 3.4. Infrared spectra IR data of the ligands and the complexes are shown in Table 2. In comparison with the ligands, the complexes show obvious
absorption band shifts within the frequencies of 4000–400 cm1. Three characteristic absorption peaks of p-FBA are as follows. The bands occurring in the region 3458–2563 cm1 could be attributed to stretching vibration of hydroxyl groups. The band around 1129.79 cm1 originates from the rocking vibration of hydroxyl and the band around 1682.05 cm1 could be assigned to vibration peak of carbonyl groups. These three peaks all disappear in the
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
spectra of complexes, proving that the p-FBA ligands successfully bond to the europium ions. In the Eu(p-FBA)3H2O complex, the asymmetric stretching vibration frequency mas(COO) is at 1540 cm1 and the symmetric stretching vibration frequency ms(COO) is at 1409cm1. The results suggest that oxygen atoms of carboxyl groups in p-FBA coordinate with europium ions [33]. Other ligands exhibit similar changes in their characteristic absorption peaks after the formation of europium complexes.
3.5. Fluorescent properties The schematic diagram of synthetic route for Ag@SiO2 nanoparticles and Ag@SiO2@Eu-complexes was shown in Fig. 2. The complexes were dissolved in ethanol at a concentration of 5 103 mol L. And the Ag@SiO2 nanoparticles were dispersed uniformly in ethanol and the concentration of silver was about 4 105 mol L1. The Ag@SiO2@Eu-complexes were obtained by mixing the solutions of complexes (2.00 mL) and the solutions of Ag@SiO2 nanoparticles (30 lL) (see Supplemental material). Because the complexes are positively charged and the Ag@SiO2 nanoparticles are negatively charged, the complexes can easily get deposited or interact with nanoparticles by electrostatic interaction. The fluorescence intensities of Ag@SiO2@Eu-complexes were measured after standing for 3 min at room temperature. The excitation spectra were obtained by detecting the characteristic emission of europium ion at 614 nm and emission and excitation slits are all 5 nm. As shown in Table 3 and Fig. 3, the fluorescence intensities of the complexes are enhanced by adding Ag@SiO2 nanoparticles. The results indicate that europium complexes are situated on the surface of the Ag@SiO2 nanoparticles and Ag@SiO2@Eucomplexes may have formed. As shown in Table 3, the excitation wavelengths of the complexes are related with the ligands and the addition of Ag@SiO2 nanoparticles. The sequences for excitation wavelengths of both Eu(o-XBA)3 (X = F, Cl and Br) and Eu(p-XBA)3 (X = F, Cl and Br) are Br > Cl > F. The two Br-substituted complexes exhibit much longer excitation wavelength than F-substituted and Cl-substituted complexes. After the introduction of Ag@SiO2 nanoparticles, the excitation wavelengths red-shift by 1–5 nm for Eu(o-XBA)3 (X = F, Cl) and Eu(p-XBA)3 (X = F, Cl). However, the excitation wavelengths of Eu(o-BrBA)3H2O and Eu(p-BrBA)3H2O do not shift after the addition of Ag@SiO2 nanoparticles. As shown in Fig. 3, all the emission spectra show five typical emission bands at 580, 592, 617, 652 and 696 nm, which can be ascribed to the 5D0 ? 7F0, 5D0 ? 7F1, 5D0 ? 7F2, 5D0 ? 7F3 and 5 D0 ? 7F4 transitions of the Eu3+ ion, respectively. Among them the luminescence intensity of 5D0 ? 7F2 transition is the strongest. Fig. 4 shows the enhancement times at different emission peaks of Eu(o-BrBA)3H2O and Eu(p-BrBA)3H2O after the addition of Ag@SiO2 nanoparticles with shell thicknesses of around 10, 15 and 25 nm. The locations of emission peaks have no obvious effect on the enhancement times of the complexes, especially when the shell thickness of nanoparticles is 25 nm. Therefore, fluorescence intensities of 5D0-7F2 of the pure complexes and Ag@SiO2@ Eu-complexes and enhancement factors are listed in Table 3 for comparison. For example, the fluorescence enhancement sequence for Eu(o-BrBA)3H2O by Ag@SiO2 with different shell thickness is No.3 > No.2 > No.1, with an enhancement factor of 5.10, 4.36, and 2.36, respectively. It suggests that when the shell thickness is in the range of 10–25 nm, the fluorescence enhancement factor increases with the increasement of shell thickness [14]. And the distance from the complexes to the metal surface in No.3 nanoparticles (25 nm) is most suitable in this work [34]. Moreover, the enhanced order for different halogen-substituted complexes is
721
F < Cl < Br, which may relate with the sequence of their excitation wavelengths. The fluorescence enhancement of the complexes by adding Ag@SiO2 nanoparticles may mainly result from two aspects. First, the surface plasmon resonance absorption of the Ag@SiO2 nanoparticles can lead to the metal-enhanced fluorescence of the europium complexes. As the excitation wavelengths of the complexes seem to play more important roles than the emission wavelengths in the enhancement factors of the fluorescence of complexes, the excitation enhancement may be dominant in the metal-enhanced fluorescence. When the excitation wavelength is close to absorbance wavelength of nanoparticles, the light energy absorbed by nanoparticles can be transferred to complexes more effectively, which results in larger enhancement factors for Br-substituted complexes than other halogenated complexes. Second, the silver nanoparticles on the surface of europium complexes can effectively prevent complexes from the interaction with the solvent, leading to a decrease of nonradiative energy transfer and suppressed luminescent quenching [10,35–37]. Therefore, the fluorescence intensities of the complexes are enhanced. There might be other reasons leading to the enhanced fluorescence of the lanthanide complexes [38–42]. Further investigations on the mechanisms of the metal-enhanced fluorescence need to be conducted. 4. Conclusions Three kinds of Ag@SiO2 core–shell nanoparticles with shell thickness of around 10, 15, 25 nm and six europium complexes of halo-benzoic acid have been prepared. Introducing Ag@SiO2 nanoparticles into the solution of europium complexes can significantly enhance the fluorescence intensities of complexes. The enhancement factors depend strongly on the shell thickness of the nanoparticles. The sequence of enhancement factor of the nanoparticles with different shell thickness is No.3 (25 nm) > No.2 (15 nm) > No.1 (10 nm). The three kinds of nanoparticles can enhance the fluorescence intensities of Eu(o-BrBA)3H2O by 5.10, 4.36 and 2.36, respectively. The enhancement sequence for different halogen-substituted complexes is F < Cl < Br. The enhancement factor increases with the increase of excitation wavelength of the complex. These results show the fluorescence enhancement of the lanthanide complexes can be achieved at an appreciable level with Ag@SiO2 core–shell nanoparticles. And the complexes with enhanced fluorescence may find more applications in fluorescence materials and devices. Acknowledgements The research work is supported by the National Natural Science Foundation of China (21161013), Natural Science Foundation of Inner Mongolia (2011MS0202), the Opening Foundation for Significant Fundamental Research of Inner Mongolia (2010KF03), and the Program of High-level Talents of Inner Mongolia University (21300-5155104). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.07.022. References [1] W.N. Wu, N. Tang, L. Yan, J. Fluoresc. 18 (2008) 101–107. [2] H.A. Azab, S.A. El-Korashy, Z.M. Anwar, B.H.M. Hussein, G.M. Khairy, Spectrochim. Acta A 75 (2010) 21–27.
722
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
[3] A.L. Wang, X.Y. Wei, H.X. Zhang, B. Yue, Y.R. Qu, J. Kang, Z.X. Wang, H.B. Chu, Y.L. Zhao, Dalton Trans. 43 (2014) 2620–2628. [4] K. Ray, R. Badugu, J.R. Lakowicz, J. Am. Chem. Soc. 128 (2006) 8998–8999. [5] K. Aslan, M.J.R. Previte, Y.X. Zhang, C.D. Geddes, J. Phys. Chem. 112 (2008) 18368–18375. [6] Y. Fu, J.R. Lakowicz, J. Phys. Chem. B 110 (2006) 22557–22562. [7] R.D. Badley, T.F. Warren, F.J. McEnroe, R.A. Assink, Langmuir 6 (1990) 792–801. [8] S. Liu, Z. Zhang, M. Han, Anal. Chem. 77 (2005) 2595–2600. [9] B. Yang, N. Lu, D. Qi, R. Ma, Q. Wu, J. Hao, X. Liu, Y. Mu, V. Reboud, N. Kehagias, C.M.S. Torres, F.Y.C. Boey, X. Chen, L. Chi, Small 6 (2010) 1038–1043. [10] X. Fang, H. Song, L. Xie, Q. Liu, H. Zhang, X. Bai, B. Dong, Y. Wang, W. Han, J. Chem. Phys. 131 (2009) 1–7. [11] R. Zhang, Z. Wang, C. Song, J. Yang, J. Li, A. Sadaf, Y. Cui, Chem. Phys. Chem. 12 (2011) 992–998. [12] D. Li, D. Li, Y. Li, J.S. Fossey, Y. Long, J. Mater. Chem. 20 (2010) 3688–3693. [13] R. Gunawidjaja, T. Myint, H. Eilers, Powder Tech. 210 (2011) 157–166. [14] W. Xu, X. Bai, S. Xu, Y. Zhu, L. Xia, H. Song, RSC Adv. 2 (2012) 2047–2054. [15] Y.R. Qu, X.M. Lin, A.L. Wang, Z.X. Wang, J. Kang, H.B. Chu, Y.L. Zhao, J. Lumin. 154 (2014) 402–409. [16] Y.G. Sun, X.F. Gu, F. Ding, P.F. Smet, E.J. Gao, D. Poelman, F. Verpoort, Cryst. Growth Des. 10 (2010) 1059–1067. [17] J.H. Luo, H.W. Xu, Y. Liu, Y.S. Zhao, L.L. Daemen, C. Brown, T.V. Timofeeva, S.Q. Ma, H.C. Zhou, J. Am. Chem. Soc. 130 (2008) 9626–9627. [18] H.J. Sun, X.T. Fu, H.B. Chu, Y. Du, X.M. Lin, X. Li, Y.L. Zhao, J. Photochem. Photobiol. A 219 (2011) 243–249. [19] S. Zhang, F. Ren, W. Wu, J. Zhou, L. Sun, X. Xiao, C. Jiang, J. Nanopart. Res. 14 (2012) 1105–1118. [20] Y. Fu, J. Zhang, J.R. Lakowicz, Langmuir 24 (2008) 3429–3433. [21] Y. Cho, S.S. Lee, J.H. Jung, Analyst 135 (2010) 1551–1555. [22] L. Guo, A. Guan, X. Lin, C. Zhang, G. Chen, Talanta 82 (2010) 1696–1700. [23] D. Cheng, Q. Xu, Chem. Commun. (2007) 248–250.
[24] O. Siiman, L.A. Bumm, R. Callaghan, C.G. Blatchford, M. Kerer, J. Phys. Chem. 87 (1983) 1014–1023. [25] L.M. Liz-Marzán, M. Giersig, P. Mulvaney, Langmuir 12 (1996) 4329–4335. [26] S. Underwood, P. Mulvaney, Langmuir 10 (1994) 3427–3430. [27] M.G. Ventura, A.J. Parola, A.P. Matos, J. Non-Cryst. Solids 357 (2011) 1342– 1349. [28] G. Schneider, G. Decher, N. Nerambourg, R. Praho, M.H.V. Werts, M. Blancharddesce, Nano Lett. 6 (2006) 530–536. [29] K. Ray, R. Badugu, J.R. Lakowicz, Chem. Mater. 19 (2007) 5902–5909. [30] W. Ni, Z. Yang, H. Chen, L. Li, J. Wang, J. Am. Chem. Soc. 130 (2008) 6692–6693. [31] Z. Yang, W. Ni, X. Kou, S. Zhang, Z. Sun, L. Sun, J. Wang, C. Yan, J. Phys. Chem. C 112 (2008) 18895–18903. [32] B. Yue, Y.N. Chen, H.B. Chu, Y.R. Qu, A.L. Wang, Y.L. Zhao, Inorg. Chim. Acta 414 (2014) 39–45. [33] Y. Zhao, H. Chu, F. Bai, D. Gao, H. Zhang, Y. Zhou, X. Wei, M. Shan, H. Li, Y. Zhao, J. Organomet. Chem. 716 (2012) 167–174. [34] M. Martini, P. Perriat, M. Montagna, R. Pansu, C. Julien, O. Tillement, S. Roux, J. Phys. Chem. C 113 (2009) 17669–17677. [35] O.L. Muskens, V. Giannini, J.A. Sánchez-Gil, J. Gómez Rivas, Nano Lett. 7 (2007) 2871–2875. [36] M.H. Chowdhury, K. Ray, K. Aslan, J.R. Lakowicz, C.D. Geddes, J. Phys. Chem. C 111 (2007) 18856–18863. [37] K. Sobhani, S.L. Fink, B.T. Cookson, N.J. Dovichi, Electrophoresis 28 (2007) 2308–2313. [38] J. Zhang, Y. Fu, J.R. Lakowicz, J. Phys. Chem. C 111 (2007) 1955–1961. [39] C.S. Bonnet, J. Massue, S.J. Quinn, T. Gunnlaugsson, Org. Biomol. Chem. 7 (2009) 3074–3078. [40] L. Guo, W. Li, X. Shi, X. Sun, J. Lumin. 129 (2009) 639–644. [41] M.L. Viger, L.S. Live, O.D. Therrien, D. Boudreau, Plasmonics 3 (2008) 33–40. [42] K. Okamoto, S. Vyawahare, A. Scherer, J. Opt. Soc. Am. B: Opt. Phys. 23 (2006) 1674–1678.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Fluorescence enhancement of europium complexes by core–shell Ag@SiO2 nanoparticles Hai-Xia Zhang, Xue-Mei Lin, Ai-Ling Wang, Yong-Liang Zhao, Hai-Bin Chu ⇑ College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot 010021, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Three kinds of Ag@SiO2 with shell
thickness of around 10, 15 and 25 nm were prepared. Six complexes of europium with halo-benzoic acid were synthesized. Ag@SiO2 nanoparticles can significantly enhance the fluorescence of the complexes. When the thickness of SiO2 shell is 25 nm, enhancement of fluorescence is the biggest. The order of enhancement for the complexes with different halogen atom is F < Cl < Br.
a r t i c l e
i n f o
Article history: Received 14 October 2014 Received in revised form 2 July 2015 Accepted 3 July 2015 Available online 6 July 2015 Keywords: Europium complex Halobenzoic acid Ag@SiO2 core–shell nanoparticles Metal-enhanced fluorescence
a b s t r a c t Three kinds of core–shell Ag@SiO2 nanoparticles with shell thickness of around 10, 15, and 25 nm, respectively, have been prepared by modified Stöber method and used for fluorescence enhancement. Six kinds of europium complexes with halobenzoic acid have been synthesized. Elemental analysis and lanthanide coordination titration show that the complexes have the compositions of Eu(p-XBA)3H2O and Eu(o-XBA)32H2O (X = F, Cl, Br). The fluorescence spectra investigation indicates that the introduction of Ag@SiO2 nanoparticles into the europium complexes’ solution can significantly enhance the fluorescence intensities of the complexes. The sequence of enhancement factors for halobenzoic acid complexes with different halogen atoms is F < Cl < Br, and the fluorescence enhancement factors increase as the excitation wavelength of complexes increase. When the thickness of the SiO2 shell is 25 nm, the fluorescence intensity of the europium complexes can reach a maximum enhancement factor of 5.1. The fluorescence enhancement mechanism may be the metal-enhanced fluorescence resulting from surface plasmon resonance of nanoparticles. And the nanoparticles near the complexes can effectively prevent complexes from the interaction with the solvent molecules, leading to a decrease of nonradiative energy transfer and the suppression of luminescence quench. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Lanthanide complexes have important applications in fluorescence materials, electroluminescence devices and fluorescence ⇑ Corresponding author. E-mail address: [email protected] (H.-B. Chu). http://dx.doi.org/10.1016/j.saa.2015.07.022 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
probes for biological systems [1,2]. However, there exist some restrictions for the practical application of the lanthanide complexes, essentially owing to their weak luminescence intensities, short lifetimes and low quantum efficiencies [3]. Substantial research efforts have shown that noble-metal (Au, Ag) nanoparticles can enhance the fluorescence intensities, radiative decay rates and fluorescence quantum yields of lanthanide complexes [4]. The
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
effect of metal-enhanced fluorescence is related to the distance between the fluorophore and the metal surface [5]. The direct contact between lanthanide complexes and metal nanoparticles could enhance the non-radiative energy transfer, which might lead to the quench of fluorescence, but far distance between them may result in no fluorescence enhancement. Therefore, an appropriate distance can optimize the metal-enhanced fluorescence of lanthanide complexes. Since the silica surface chemistry can covalently introduce versatile surface functionalities (e.g., COOH, CHO, NH2, or NCO) with readily available coupling agents [6,7], it is desirable to coat the metal nanoparticles with a shell of silica to permanently preserve their optical properties for effective functionalization [8– 10]. Zhang et al. have reported Ag@SiO2 nanoparticles can enhance the fluorescence intensity of Rose Bengal [11–13], but the effect of Ag@SiO2 nanoparticles on the fluorescence of lanthanide complexes are few reported [14,15]. Aromatic carboxylate ligands have been widely used in lanthanide complexes for their diverse coordination modes, strong bonding ability and efficient energy transfer to the lanthanide ions [16–18]. Especially, the halogen atoms (F, Cl, Br) in halogenated benzoic acids are electron withdrawing groups, which may have an important influence on the luminescence excitation and emission of center lanthanide ions. Therefore, six kinds of europium complexes with different halogenated benzoic acids (Eu(p-XBA)3H2O and Eu(o-XBA)32H2O, X = F, Cl, Br) are synthesized to investigate the effect of halogenation on the metal-enhanced fluorescence of lanthanide complexes. In order to study the effect of distance between lanthanide complexes and metal nanoparticles on the metal-enhanced fluorescence, we develop core–shell Ag@SiO2 nanoparticles with various shell thickness of around 10, 15 and 25 nm [19–22]. Results demonstrate that the introduction of Ag@SiO2 nanoparticles into the europium complexes’ solution can significantly enhance the fluorescence intensity of the complexes and the fluorescence enhancement is the strongest when the shell thickness of Ag@SiO2 nanoparticle is 25 nm. The enhancement factors for halo-benzoic acid complexes vary with different halogen atoms. And the fluorescence enhancement mechanisms are also discussed. 2. Experimental section 2.1. Chemical reagents Eu2O3 (99.99%), HCl, AgNO3, ammonia, anhydrous ethanol, p-fluorobenzoic acid (p-FBA), p-chlorobenzoic acid (p-ClBA), p-bromobenzoic acid (p-BrBA), o-fluorobenzoic acid (o-FBA), o-chlorobenzoic acid (o-ClBA), o-bromobenzoic acid (o-BrBA), N,N-dimethyl formamide (DMF), tetraethoxysilane (TEOS), sodium citrate and other agents are all of analytical grade and used as recieved. 2.2. Preparation of Ag@SiO2 nanoparticles 2 mL 0.20 mol L1 sodium citrate aqueous solution was added into 500 ml of boiling aqueous solution containing 2 mL 0.25 M AgNO3 under vigorous stirring [23–26]. After boiled for 15 min, the solution was cooled to room temperature [1]. The as-prepared silver colloid was centrifuged at 500 rpm for 1 h to remove large nanoparticles, and the remaining silver nanoparticles was diluted to 500 mL with water. The core–shell Ag@SiO2 nanoparticles were synthesized as follows. 10 mL as-obtained silver colloids were transferred respectively to three 500 mL round bottom flasks containing 150 mL ethanol and 1 mL 0.20 mol L1 sodium citrate. And the pH values of the solutions were adjusted to 9 via the addition of 30 wt.%
717
ammonia. Subsequently, 5 mL, 15 mL and 35 mL 10 mmol/L TEOS ethanol solutions were added into each flask within 4 h under vigorous stirring, respectively. And the resulting solutions were allowed to further react for 24 h at room temperature. Three kinds of Ag@SiO2 nanoparticles (No.1, No.2 and No.3) were collected by centrifugation at 8000 rpm for 30 min and washed with ethanol for three times.
2.3. Synthesis of europium complexes 3 mmol p-FBA (or p-ClBA, p-BrBA, o-FBA, o-ClBA and o-BrBA) was first dissolved in the mixed solvent of 10 mL ethanol and 2 mL distilled water with vigorously stirring. The pH value of the reaction mixture was adjusted to about 6 with ammonia. And 1 mmol EuCl3 was dropwise added to the solution. The pH value of the mixture was readjusted to 6, after which the mixture was heated to 60 °C and stayed at 60 °C for 3 h. Then the mixture was left still at room temperature overnight. After filtered, washed with ethanol for several times, and dried to constant weight, the white precipitates Eu(p-FBA)3H2O, Eu(p-ClBA)32H2O, Eu(p-BrBA)3H2O, Eu(o-FBA)3H2O, Eu(o-ClBA)3H2O and Eu(o-BrBA)32H2O were obtained, respectively.
2.4. Characterization Silver and Ag@SiO2 nanoparticles were characterized by JEM-2100 Transmission Electron Microscope (TEM). The composition of C, H was determined by Vario EL Cube elemental analyzer. The contents of europium were determined by EDTA titration method with xylenol orange as the indicator. The UV spectra were recorded on a Shimadzu UV-265 spectrophotometer. Infrared spectra were obtained on a Nicolet Nexus 670 FT-IR spectrometer. Fluorescence spectra were obtained on an Edinburgh FLS-920 fluorescence spectra-photometer. 2.00 mL 1.0 103 mol L1 of the complex solution was transferred into quartz cell, after which 30 lL Ag@SiO2 solution was added. The volume effect was so small that could be ignored. The solution was allowed to stand for 3 min to equilibrium, after which the fluorescence spectra were detected.
3. Results and discussion 3.1. Morphology analysis of core–shell structure Ag@SiO2 nanoparticles TEM images of silver nanoparticles are shown in Fig. 1A. Most of the silver nanoparticles are nearly spherical and their average diameter is 60–70 nm. Fig. 1B–D showed almost all the Ag nanoparticles were coated with SiO2 shell. And the three kinds of Ag@SiO2 core–shell nanoparticles (No.1, No.2 and No.3) possess a shell thickness of 10, 15 and 25 nm, respectively. UV–vis absorption spectra are an efficient measurement for tracking metallic colloid formation process [27]. The surface plasmon absorption in the visible region is very sensitive to particle size, shape and the properties of the surrounding medium [13]. According to the Mie’s theory, the maximum absorption peak of silver nanoparticles would move to longer wavelength with the increasement of the coating thickness [28,29]. Fig. 1E shows the UV–vis spectra of citrate-protected silver nanoparticles and Ag@SiO2 nanoparticles with different shell thickness. The surface plasmon bands of Ag@SiO2 nanoparticles red-shift about 8– 20 nm compared with the uncoated silver nanoparticles, which may result from the increase of the local refractive index around the particles [30,31].
718
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
Fig. 1. TEM images of silver nanoparticles (A) and core–shell Ag@SiO2 nanoparticles with different thickness. (B) No.1, (C) No.2, (D) No.3. The UV–vis absorption spectra of the nanoparticles (E).
3.2. Composition analysis of the complexes
3.3. UV absorption spectra of the complexes
As shown in Table 1, the results of elemental analyses and EDTA titrations indicate that the complexes have the formulas of Eu(p-FBA)3H2O, Eu(p-ClBA)32H2O, Eu(p-BrBA)3H2O, Eu(o-FBA)3 2H2O, Eu(o-ClBA)3H2O and Eu(o-BrBA)3H2O, respectively. The small values of molar conductivity (11.4–20.5 S cm2 mol1) of the complexes reveal that these complexes are all non-electrolytes. The complexes are easily soluble in DMF and DMSO, but slightly soluble in methanol, ethanol and water.
The UV absorption spectra of the ligands p-FBA, p-ClBA, p-BrBA, o-FBA, o-ClBA, o-BrBA and the corresponding Eu3+ complexes were determined in the range of 200–400 nm with DMF as solvent and reference, and the slit width was 2 nm. The absorption spectra are shown in Fig. S1. The maximum peaks of the six ligands locate at 265, 266, 267, 276, 278 and 279 nm, respectively. The spectra of the complexes are similar to those of the corresponding ligands, and the absorption peaks shift about 2–6 nm. The results indicate
719
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722 Table 1 Composition analysis (%) and molar conductivities (S cm2 mol1) of the complexes. Complexes
C
H
RE
km
Eu(p-FBA)3H2O Eu(p-ClBA)32H2O Eu(p-BrBA)3H2O Eu(o-FBA)32H2O Eu(o-ClBA)3H2O Eu(o-BrBA)3H2O
43.14(42.73) 38.24(38.35) 32.84(32.63) 41.26(41.46) 39.93(39.43) 32.32(32.63)
2.43(2.90) 2.82(2.91) 2.07(2.22) 3.53(3.15) 2.14(2.68) 2.01(2.22)
25.47(25.74) 23.45(23.10) 20.18(19.68) 25.45(24.98) 23.57(23.76) 19.65(19.68)
11.8 11.4 13.5 18.7 20.5 17.6
Remark: the values in brackets are theoretical values.
Table 2 Infrared spectra data of the ligands and complexes. Compounds
mO–H
mC@O(COOH)
masðCOO Þ
msðCOO Þ
dOH(COOH)
m(O–M)
p-FBA Eu(p-FBA)3H2O p-ClBA Eu(p-ClBA)32H2O p-BrBA Eu(p-BrBA)3H2O o-FBA Eu(o-FBA)32H2O o-ClBA Eu(o-ClBA)3H2O o-BrBA Eu(o-BrBA)3H2O
3458–2563 3431.82 3432–2555 3432.62 3557–2558 3431.80 3467–2666 3438.77 3457–2556 3432.38 3433–2547 3432.67
1682.05 – 1685.91 – 1679.67 – 1691.68 – 1689.61 – 1685.32 –
– 1540.37 – 1538.32 – 1536.99 – 1532.52 – 1544.35 – 1540.52
– 1408.96 – 1411.37 – 1409.15 – 1406.28 – 1407.26 – 1409.03
1129.79 – 1129.64 – 1128.43 – 1128.79 – 1265.59 – 1127.86 –
– 556.35 – 551.36 – 560.45 – 553.63 – 557.47 – 563.43
Fig. 2. Schematic diagram of synthetic route for Ag@SiO2 nanoparticles and Ag@SiO2@Eu-complexes.
Table 3 Fluorescence data of 5D0 ? 7F2 of europium complexes and Ag@SiO2@Eu complexes. Complexes
Excitation wavelength (nm)
Emission wavelength (nm)
Intensity (a.u.)
Enhancement factor
Eu(p-FBA)3H2O
Pure complex +No.1 +No.2 +No.3
270 272 274 275
616 616 616 616
8298 10,980 13,650 14,920
– 1.32 1.64 1.80
Eu(p-ClBA)32H2O
Pure complex +No.1 +No.2 +No.3
279 281 282 283
616 616 616 617
8121 19,300 26,170 28,650
– 2.38 3.22 3.53
Eu(p-BrBA)3H2O
Pure complex +No.1 +No.2 +No.3
394 394 394 394
615 616 616 616
1521 2885 3976 6387
– 1.90 2.61 4.20
Eu(o-FBA)32H2O
Pure complex +No.1 +No.2 +No.3
281 282 284 284
616 617 617 616
1080 1462 2364 3260
– 1.35 2.19 3.02
Eu(o-ClBA)3H2O
Pure complex +No.1 +No.2 +No.3
286 288 290 291
617 616 616 616
6966 13,770 17,680 24,220
– 1.98 2.54 3.48
Eu(o-BrBA)3H2O
Pure complex +No.1 +No.2 +No.3
394 394 394 394
617 617 616 617
1219 2660 5334 6222
– 2.18 4.38 5.10
720
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
Fig. 3. Fluorescent emission spectra of Eu complexes and Ag@SiO2@Eu-complexes with excitation spectra as insets. (A) Eu(p-FBA)3H2O, (B) Eu(p-ClBA)32H2O, (C) Eu(pBrBA)3H2O, (D) Eu(o-FBA)32H2O, (E) Eu(o-ClBA)3H2O and (F) Eu(o-BrBA)3H2O.
Fig. 4. The enhancement times at different emission peaks of complexes (A) Eu(p-BrBA)3H2O and (B) Eu(o-BrBA)3H2O by adding Ag@SiO2 with shell thickness of around 10, 15 and 25 nm.
that the ligands coordinate with the europium ions in the complexes [32]. 3.4. Infrared spectra IR data of the ligands and the complexes are shown in Table 2. In comparison with the ligands, the complexes show obvious
absorption band shifts within the frequencies of 4000–400 cm1. Three characteristic absorption peaks of p-FBA are as follows. The bands occurring in the region 3458–2563 cm1 could be attributed to stretching vibration of hydroxyl groups. The band around 1129.79 cm1 originates from the rocking vibration of hydroxyl and the band around 1682.05 cm1 could be assigned to vibration peak of carbonyl groups. These three peaks all disappear in the
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
spectra of complexes, proving that the p-FBA ligands successfully bond to the europium ions. In the Eu(p-FBA)3H2O complex, the asymmetric stretching vibration frequency mas(COO) is at 1540 cm1 and the symmetric stretching vibration frequency ms(COO) is at 1409cm1. The results suggest that oxygen atoms of carboxyl groups in p-FBA coordinate with europium ions [33]. Other ligands exhibit similar changes in their characteristic absorption peaks after the formation of europium complexes.
3.5. Fluorescent properties The schematic diagram of synthetic route for Ag@SiO2 nanoparticles and Ag@SiO2@Eu-complexes was shown in Fig. 2. The complexes were dissolved in ethanol at a concentration of 5 103 mol L. And the Ag@SiO2 nanoparticles were dispersed uniformly in ethanol and the concentration of silver was about 4 105 mol L1. The Ag@SiO2@Eu-complexes were obtained by mixing the solutions of complexes (2.00 mL) and the solutions of Ag@SiO2 nanoparticles (30 lL) (see Supplemental material). Because the complexes are positively charged and the Ag@SiO2 nanoparticles are negatively charged, the complexes can easily get deposited or interact with nanoparticles by electrostatic interaction. The fluorescence intensities of Ag@SiO2@Eu-complexes were measured after standing for 3 min at room temperature. The excitation spectra were obtained by detecting the characteristic emission of europium ion at 614 nm and emission and excitation slits are all 5 nm. As shown in Table 3 and Fig. 3, the fluorescence intensities of the complexes are enhanced by adding Ag@SiO2 nanoparticles. The results indicate that europium complexes are situated on the surface of the Ag@SiO2 nanoparticles and Ag@SiO2@Eucomplexes may have formed. As shown in Table 3, the excitation wavelengths of the complexes are related with the ligands and the addition of Ag@SiO2 nanoparticles. The sequences for excitation wavelengths of both Eu(o-XBA)3 (X = F, Cl and Br) and Eu(p-XBA)3 (X = F, Cl and Br) are Br > Cl > F. The two Br-substituted complexes exhibit much longer excitation wavelength than F-substituted and Cl-substituted complexes. After the introduction of Ag@SiO2 nanoparticles, the excitation wavelengths red-shift by 1–5 nm for Eu(o-XBA)3 (X = F, Cl) and Eu(p-XBA)3 (X = F, Cl). However, the excitation wavelengths of Eu(o-BrBA)3H2O and Eu(p-BrBA)3H2O do not shift after the addition of Ag@SiO2 nanoparticles. As shown in Fig. 3, all the emission spectra show five typical emission bands at 580, 592, 617, 652 and 696 nm, which can be ascribed to the 5D0 ? 7F0, 5D0 ? 7F1, 5D0 ? 7F2, 5D0 ? 7F3 and 5 D0 ? 7F4 transitions of the Eu3+ ion, respectively. Among them the luminescence intensity of 5D0 ? 7F2 transition is the strongest. Fig. 4 shows the enhancement times at different emission peaks of Eu(o-BrBA)3H2O and Eu(p-BrBA)3H2O after the addition of Ag@SiO2 nanoparticles with shell thicknesses of around 10, 15 and 25 nm. The locations of emission peaks have no obvious effect on the enhancement times of the complexes, especially when the shell thickness of nanoparticles is 25 nm. Therefore, fluorescence intensities of 5D0-7F2 of the pure complexes and Ag@SiO2@ Eu-complexes and enhancement factors are listed in Table 3 for comparison. For example, the fluorescence enhancement sequence for Eu(o-BrBA)3H2O by Ag@SiO2 with different shell thickness is No.3 > No.2 > No.1, with an enhancement factor of 5.10, 4.36, and 2.36, respectively. It suggests that when the shell thickness is in the range of 10–25 nm, the fluorescence enhancement factor increases with the increasement of shell thickness [14]. And the distance from the complexes to the metal surface in No.3 nanoparticles (25 nm) is most suitable in this work [34]. Moreover, the enhanced order for different halogen-substituted complexes is
721
F < Cl < Br, which may relate with the sequence of their excitation wavelengths. The fluorescence enhancement of the complexes by adding Ag@SiO2 nanoparticles may mainly result from two aspects. First, the surface plasmon resonance absorption of the Ag@SiO2 nanoparticles can lead to the metal-enhanced fluorescence of the europium complexes. As the excitation wavelengths of the complexes seem to play more important roles than the emission wavelengths in the enhancement factors of the fluorescence of complexes, the excitation enhancement may be dominant in the metal-enhanced fluorescence. When the excitation wavelength is close to absorbance wavelength of nanoparticles, the light energy absorbed by nanoparticles can be transferred to complexes more effectively, which results in larger enhancement factors for Br-substituted complexes than other halogenated complexes. Second, the silver nanoparticles on the surface of europium complexes can effectively prevent complexes from the interaction with the solvent, leading to a decrease of nonradiative energy transfer and suppressed luminescent quenching [10,35–37]. Therefore, the fluorescence intensities of the complexes are enhanced. There might be other reasons leading to the enhanced fluorescence of the lanthanide complexes [38–42]. Further investigations on the mechanisms of the metal-enhanced fluorescence need to be conducted. 4. Conclusions Three kinds of Ag@SiO2 core–shell nanoparticles with shell thickness of around 10, 15, 25 nm and six europium complexes of halo-benzoic acid have been prepared. Introducing Ag@SiO2 nanoparticles into the solution of europium complexes can significantly enhance the fluorescence intensities of complexes. The enhancement factors depend strongly on the shell thickness of the nanoparticles. The sequence of enhancement factor of the nanoparticles with different shell thickness is No.3 (25 nm) > No.2 (15 nm) > No.1 (10 nm). The three kinds of nanoparticles can enhance the fluorescence intensities of Eu(o-BrBA)3H2O by 5.10, 4.36 and 2.36, respectively. The enhancement sequence for different halogen-substituted complexes is F < Cl < Br. The enhancement factor increases with the increase of excitation wavelength of the complex. These results show the fluorescence enhancement of the lanthanide complexes can be achieved at an appreciable level with Ag@SiO2 core–shell nanoparticles. And the complexes with enhanced fluorescence may find more applications in fluorescence materials and devices. Acknowledgements The research work is supported by the National Natural Science Foundation of China (21161013), Natural Science Foundation of Inner Mongolia (2011MS0202), the Opening Foundation for Significant Fundamental Research of Inner Mongolia (2010KF03), and the Program of High-level Talents of Inner Mongolia University (21300-5155104). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.07.022. References [1] W.N. Wu, N. Tang, L. Yan, J. Fluoresc. 18 (2008) 101–107. [2] H.A. Azab, S.A. El-Korashy, Z.M. Anwar, B.H.M. Hussein, G.M. Khairy, Spectrochim. Acta A 75 (2010) 21–27.
722
H.-X. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 716–722
[3] A.L. Wang, X.Y. Wei, H.X. Zhang, B. Yue, Y.R. Qu, J. Kang, Z.X. Wang, H.B. Chu, Y.L. Zhao, Dalton Trans. 43 (2014) 2620–2628. [4] K. Ray, R. Badugu, J.R. Lakowicz, J. Am. Chem. Soc. 128 (2006) 8998–8999. [5] K. Aslan, M.J.R. Previte, Y.X. Zhang, C.D. Geddes, J. Phys. Chem. 112 (2008) 18368–18375. [6] Y. Fu, J.R. Lakowicz, J. Phys. Chem. B 110 (2006) 22557–22562. [7] R.D. Badley, T.F. Warren, F.J. McEnroe, R.A. Assink, Langmuir 6 (1990) 792–801. [8] S. Liu, Z. Zhang, M. Han, Anal. Chem. 77 (2005) 2595–2600. [9] B. Yang, N. Lu, D. Qi, R. Ma, Q. Wu, J. Hao, X. Liu, Y. Mu, V. Reboud, N. Kehagias, C.M.S. Torres, F.Y.C. Boey, X. Chen, L. Chi, Small 6 (2010) 1038–1043. [10] X. Fang, H. Song, L. Xie, Q. Liu, H. Zhang, X. Bai, B. Dong, Y. Wang, W. Han, J. Chem. Phys. 131 (2009) 1–7. [11] R. Zhang, Z. Wang, C. Song, J. Yang, J. Li, A. Sadaf, Y. Cui, Chem. Phys. Chem. 12 (2011) 992–998. [12] D. Li, D. Li, Y. Li, J.S. Fossey, Y. Long, J. Mater. Chem. 20 (2010) 3688–3693. [13] R. Gunawidjaja, T. Myint, H. Eilers, Powder Tech. 210 (2011) 157–166. [14] W. Xu, X. Bai, S. Xu, Y. Zhu, L. Xia, H. Song, RSC Adv. 2 (2012) 2047–2054. [15] Y.R. Qu, X.M. Lin, A.L. Wang, Z.X. Wang, J. Kang, H.B. Chu, Y.L. Zhao, J. Lumin. 154 (2014) 402–409. [16] Y.G. Sun, X.F. Gu, F. Ding, P.F. Smet, E.J. Gao, D. Poelman, F. Verpoort, Cryst. Growth Des. 10 (2010) 1059–1067. [17] J.H. Luo, H.W. Xu, Y. Liu, Y.S. Zhao, L.L. Daemen, C. Brown, T.V. Timofeeva, S.Q. Ma, H.C. Zhou, J. Am. Chem. Soc. 130 (2008) 9626–9627. [18] H.J. Sun, X.T. Fu, H.B. Chu, Y. Du, X.M. Lin, X. Li, Y.L. Zhao, J. Photochem. Photobiol. A 219 (2011) 243–249. [19] S. Zhang, F. Ren, W. Wu, J. Zhou, L. Sun, X. Xiao, C. Jiang, J. Nanopart. Res. 14 (2012) 1105–1118. [20] Y. Fu, J. Zhang, J.R. Lakowicz, Langmuir 24 (2008) 3429–3433. [21] Y. Cho, S.S. Lee, J.H. Jung, Analyst 135 (2010) 1551–1555. [22] L. Guo, A. Guan, X. Lin, C. Zhang, G. Chen, Talanta 82 (2010) 1696–1700. [23] D. Cheng, Q. Xu, Chem. Commun. (2007) 248–250.
[24] O. Siiman, L.A. Bumm, R. Callaghan, C.G. Blatchford, M. Kerer, J. Phys. Chem. 87 (1983) 1014–1023. [25] L.M. Liz-Marzán, M. Giersig, P. Mulvaney, Langmuir 12 (1996) 4329–4335. [26] S. Underwood, P. Mulvaney, Langmuir 10 (1994) 3427–3430. [27] M.G. Ventura, A.J. Parola, A.P. Matos, J. Non-Cryst. Solids 357 (2011) 1342– 1349. [28] G. Schneider, G. Decher, N. Nerambourg, R. Praho, M.H.V. Werts, M. Blancharddesce, Nano Lett. 6 (2006) 530–536. [29] K. Ray, R. Badugu, J.R. Lakowicz, Chem. Mater. 19 (2007) 5902–5909. [30] W. Ni, Z. Yang, H. Chen, L. Li, J. Wang, J. Am. Chem. Soc. 130 (2008) 6692–6693. [31] Z. Yang, W. Ni, X. Kou, S. Zhang, Z. Sun, L. Sun, J. Wang, C. Yan, J. Phys. Chem. C 112 (2008) 18895–18903. [32] B. Yue, Y.N. Chen, H.B. Chu, Y.R. Qu, A.L. Wang, Y.L. Zhao, Inorg. Chim. Acta 414 (2014) 39–45. [33] Y. Zhao, H. Chu, F. Bai, D. Gao, H. Zhang, Y. Zhou, X. Wei, M. Shan, H. Li, Y. Zhao, J. Organomet. Chem. 716 (2012) 167–174. [34] M. Martini, P. Perriat, M. Montagna, R. Pansu, C. Julien, O. Tillement, S. Roux, J. Phys. Chem. C 113 (2009) 17669–17677. [35] O.L. Muskens, V. Giannini, J.A. Sánchez-Gil, J. Gómez Rivas, Nano Lett. 7 (2007) 2871–2875. [36] M.H. Chowdhury, K. Ray, K. Aslan, J.R. Lakowicz, C.D. Geddes, J. Phys. Chem. C 111 (2007) 18856–18863. [37] K. Sobhani, S.L. Fink, B.T. Cookson, N.J. Dovichi, Electrophoresis 28 (2007) 2308–2313. [38] J. Zhang, Y. Fu, J.R. Lakowicz, J. Phys. Chem. C 111 (2007) 1955–1961. [39] C.S. Bonnet, J. Massue, S.J. Quinn, T. Gunnlaugsson, Org. Biomol. Chem. 7 (2009) 3074–3078. [40] L. Guo, W. Li, X. Shi, X. Sun, J. Lumin. 129 (2009) 639–644. [41] M.L. Viger, L.S. Live, O.D. Therrien, D. Boudreau, Plasmonics 3 (2008) 33–40. [42] K. Okamoto, S. Vyawahare, A. Scherer, J. Opt. Soc. Am. B: Opt. Phys. 23 (2006) 1674–1678.