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Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites
G Model
CCLET-3651; No. of Pages 5 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites Hong-Xiang Huang a,*, Wen-Li He b, Xiang-Hai Sheng b, Wen-Jie Wu b a b
Department of Macromolecular Science and State Key Laboratory of Molecular Engineering of Polymers, Shanghai 200433, China Department of Chemistry, Fudan University, Shanghai 200433, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 January 2016 Received in revised form 12 March 2016 Accepted 17 March 2016 Available online xxx
Luminescent silica nanocomposites functionalized with a Eu-complex have been prepared and characterized. The europium complex is composed of 2,20 -bipyridyl (BPy) and 2-(4-bromomethyl)phenylpropionic acid (BMPPA), which contains highly active benzyl bromide substituents and can covalently bind with poly(4-vinylpyridine) (P4VP)-modified silica nanoparticles (nanoSiO2P4VP) to form nanoSiO2P4VPEuBPy composites. Microscopic images revealed that the nanoSiO2P4VPEuBPy composites easily formed aggregates, due to an inter-particle binding caused by the benzyl bromide among the composites. The as-prepared nanocomposites showed the typical emissions of Eu(III) ions at the wavelengths from 580 nm to 750 nm designated to the 5D0 ! 7Fn transitions. Time-resolved fluorescence decay measurements revealed that the emission lifetime was approximately 0.204 ms and 0.576 ms for the nanoSiO2EuBPy composites, a little shorter than that in the Eu(BMPPA)3BPy complex. ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Eu-complex Nanocomposites Silica nanoparticles Fluorescence
1. Introduction Lanthanide complexes have attracted much attention in the past several decades for the development of advanced luminescent materials with applications in the fields of optoelectronics, imaging, markers, OLEDs, and fluorescent lamps [1–5]. These complexes give off relatively intense visible emissions under UV radiation followed by an efficient intramolecular energy transfer from the excited ligands to the central lanthanide ions [6]. Several kinds of ligands have been used as the so-called antenna to absorb UV light, such as b-diketone derivatives [7], shift bases [8], organic acids [9], and even proteins [10]. Large amount of lanthanide complexes have been designed and synthesized with their emission properties characterized in past several decades [11,12]. Recently, with the development of molecular assembling and nanotechnology, effects have been made on the immobilization of the lanthanide complexes on the surface of nanoparticles to form luminescent nanocomposites, which can be obtained through a one-step process by using mixtures of lanthanide salts or complexes with tetraethyl orthosilicate catalyzed by a base
* Corresponding author. E-mail address: [email protected] (H.-X. Huang).
(e.g., ammonium hydroxide) [13,14], or through a multi-step process by chemical reactions with modified silica nanoparticles [15,16]. In the latter case, the silica nanoparticles were first covalently functionalized with small organic species like azida [17] or more especially with a trimethoxysilyl group grafted diketones [18]. Because the lanthanide ions or their complexes are reacted with pre-fabricated silica nanoparticles, the particle sizes, shapes as well as their distribution could be pre-controlled, which is benefit for their further applications as candidates for the device fabrication. Here, a ternary europium complex with 2-(4-bromomethyl)phenylpropionic acid (BMPPA) and 2,20 -bipyridyl (BPy) ligands was newly synthesized, which was then connected with poly (4-vinylpyridine) (P4VP) functionalized silica nanoparticles (nanoSiO2P4VP) via an addition reaction to produce luminescent nanoSiO2P4VPEuBPy composites. These nanocomposites were characterized by using UV–vis absorption and infrared spectra, thermogravimetric (TG), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM). Fluorescence emission spectra revealed typical Eu(III) ion emissions at the wavelengths from 580 nm to 750 nm designated to the 5D0 ! 7Fn transitions with the emission lifetime of approximately 0.204 and 0.576 ms for the nanoSiO2EuBPy composites, and that of approximately 1.47 ms for the Eu-complex in chloroform solutions.
http://dx.doi.org/10.1016/j.cclet.2016.03.031 1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031
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CCLET-3651; No. of Pages 5 H.-X. Huang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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2. Experimental 2.1. Materials Europium oxide (99.99%) was purchased from Yuelong Chemical Plant (Shanghai, China). (4-Chloromethylphenyl)trimethoxylsilane, P4VP, and Ludox AS40 (containing 40 wt% SiO2 with an average particle size of 12 nm) were from Sigma–Aldrich Co., Ltd. BMPPA and BPy were from J&K Scientific LTD. All chemicals were used as received without further purification. 2.2. Synthesis of Eu(BMPPA)3BPy complex and its silica nanocomposites Europium chloride was prepared by dissolving 0.813 g Eu2O3 (2.31 mmol) in the dilute HCl solution, which was heated on water bath until the solvent was evaporated. The white powders of EuCl3 were then dissolved in methanol, followed by addition of 0.72 g BPy (4.62 mmol) to form the Eu–BPy complex solution, to which a methanol solution containing 3.37 g BMPPA (13.86 mmol) and 0.78 g KOH (13.93 mmol) was added. The mixtures were refluxed overnight to produce white precipitates [19], which were finally filtrated, washed with plenty of methanol, recrystallized from chloroform-methanol solution, and dried under vacuum. Calc. For C40H38N2Br3O6Eu2H2O: C, 44.8; H, 3.90; N, 2.61%. Found: C, 44.0; H, 3.90; N, 2.31%. Selected IR (KBr, cm1), 3340, 3045, 3370, 2970, 2931, 2854, 1730, 1600, 1568, 1458, 1410, 1230, 849, 764, 604. The preparation processes of the present nanocomposites are shown in Fig. 1. Firstly, the P4VP functionalized silica hybrid of nanoSiO2P4VP was prepared based on the literature method with the use of silica gel in methanol as starting materials [18]. Then, the luminescent nanoSiO2P4VPEuBPy composites were prepared by stirring the mixtures of nanoSiO2P4VP hybrids and excess
Eu(BMPPA)3BPy complex in a DFM/CH3CN mixed solution at room temperature overnight. The suspension was isolated by centrifugation at 4000 rpm. The white powders of nanoSiO2P4VPEuBPy composites were washed thoroughly with plenty of DMF, methanol and finally dried in a vacuum overnight. 2.3. Characterization TG analysis was performed on an STDQ600 instrument (TA Ltd., USA) under a constant air flow atmosphere from room temperature to 800 8C at a rate of 10 8C/min. Elemental analysis was performed on a VARIO EL3 elemental analyzer. The europium content was analyzed by using Perkin Elmer Optima 8000 inductively coupled plasma optical emission spectrometer (ICP-OEP). Fourier transform infrared spectra (FITR) were measured by using a Nicolet NEXUS 470 spectrometer, operating at a resolution of 0.5 cm1 at 25 8C. XPS spectra were recorded using a VGESCALAB MKII multifunction spectrometer, with nonmonochromatized Mg-Ka X-rays as the excitation source. The system was carefully calibrated by Fermi-edge of nickel, Au 4f2/7 and Cu 2p2/3 binding energy. Pass energy of 70 eV and step size of 1 eV were chosen when taking spectra. In the analysis chamber pressures of 1–2 107 Pa were routinely maintained. The binding energies obtained in the XPS analysis were corrected by referencing the C1s peak to 284.60 eV. Field emission transmission electron microscopy (FETEM) images of the SiO2 nanoparticles before and after functionalization were recorded using a JEM-2100F field emission electron microscope. The nanoparticles were deposited on 230-mesh copper grid covered with Formvar. UV–vis spectra were measured with the use of Shimadzu UV2550 UV–vis spectrophotometer. Fluorescence spectra were recorded by using Shimadzu RF-5300PC spectrophotometer. The
Fig. 1. Schematic representation for the synthesis of Eu(BMPPA)3BPy complex and preparation of its functionalized silica nanocomposites.
Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031
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CCLET-3651; No. of Pages 5 H.-X. Huang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
It ¼ A þ B1 eðt=t1 Þ þ B2 eðt=t 2 Þ I0
(1)
50
40
Transmission
absolute quantum yield (QY) was obtained with the use of an integrating sphere measurement excited at 270 nm. Time-resolved single photon fluorescence measurements were performed using an Edinburgh Instruments FLS920 spectrometer with microsecond pulsed lamp as the light source. Lifetimes were estimated from the decay of the main transitions with the use of the Edinburgh Instruments software package. The excited wavelength was set at 270 nm. The fluorescence decay curves followed with first-order kinetics or two exponential decay kinetics, as displayed in the logarithmic plots of
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-1
where I0 and It are the fluorescence intensity at times zero and t, respectively, Bi is the pre-exponential factor, and t is the experimental lifetime of the excited state of the europium complex. 3. Results and discussion 3.1. Thermogravimetric, FTIR, and XPS analysis Fig. 2 shows the TG curves for the nanoSiO2P4VP and nanoSiO2P4VPEnBPy composites from room temperature to 800 8C. Both curves revealed a mass decrease below 120 8C (about 6% in weight), ascribed to the evaporation of physically adsorbed water. Then, the mass decrease was observed in the temperature range 250–600 8C, attributed to decomposition of the organic species covalently attached on the silica surfaces and the Eucomplex (for the nanoSiO2P4VPEnBPy composites). In this temperature range, the total mass decrease observed was about 19% in weight for the nanoSiO2P4VP composites, and 22% for the nanoSiO2P4VPEuBPyones, respectively. The difference of the mass decrease (3%) could be attributed to the Eu-complex attached. Finally, the residuals observed at above 600 8C corresponded to 72–75% of the total weight and were attributed to the formation of SiO2 powders following decomposition of organic species and loss of water [20,21]. Fig. 3 shows FTIR spectrum of the nanoSiO2P4VPEuBPy composites, which revealed following features. Firstly, the broad band centered at 3445 cm1 was attributed to adsorbed water molecules and the hydroxyl groups on the nanoparticle surfaces [21]. Secondly, several weak peaks were recorded at approximately 2940 and 2890 cm1 were assigned to the aliphatic –CH2 stretching vibration bands and confirmed that the P4VP substituent had anchored onto the silica core surface. Thirdly, a sharp peak recorded at approximately 1650 cm1 was attributed to the formation of the COO bond of Eu-complex. Fourthly, several small bands were recorded between 1530 and 1380 cm1,
Wavenuumber (cm ) Fig. 3. Infrared spectrum of the nanoSiO2P4VPEuBPy composites.
attributed to the C5 5N, C–C, and C5 5C vibrations of pyridyl and benzyl rings. Finally, the vibration bands recorded at approximately 1110 cm1 and 795 cm1 were ascribed to the nanoparticle Si–O–Si asymmetric and Si–O symmetric stretching bands [22]. Elemental composition of the nanoSiO2P4VPEuBPy composites was analyzed by using XPS and ICP techniques. Fig. 4 shows the high resolution XPS spectra, which revealed seven peaks at the binding energy of 67.0, 102.4, 127.6/144.4, 197.6, 284.6, 399.2– 401.2, and 533.0 eV, which could be assigned to Br(3d), Si(2p), Eu(4d), Cl(2p), C(1s), N(1s), and O(1s), respectively. Si and O belonged to the nano-SiO2 core, while C, N Br, Cl and part of O to the modified organic layer and the ligands of BMPPA and BPy. Eu was from the coordinated central ions of the complex. Based on the ICP measurement, the accurate content of the Eu(III) ions was about 26.9 mg/g (nano-composites). Thus, the FTIR, XPS and ICP results confirmed formation of the nanoSiO2P4VPEuBPy composites. 3.2. Morphology characterization Microscopic images for the nanoSiO2BenCl and nanoSiO2P4VPEuBPy composites were observed by using TEM. As shown in Fig. 5, before functionalization with the Eu-complex, the nanoparticles were well separated with an average diameter of approximately 12 nm, which was in agreement with the data provided by the company. Image of the nanoSiO2BenP4VP composites was similar to that of the nanoSiO2BenCl (not shown). However, when the Eu(BMPPA)3BPy complex was coordinated on the particle surface, although the sizes and shapes did not changed significantly, the particles largely aggregated as shown in Fig. 5B. Possible reasons may be an inter-particle binding caused by the benzyl bromide substituents between two or more nanoparticles as the carton shown in Fig. 5C, in which it can be seen that the C(1s)
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Si(2pp)
Mass (%)
Br(3d) N(1s)
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Cl(2p)
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70
Eu(4d)
b
0
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o
Temperature ( C)
66 72 102 108 128 144 1922 200 280 290 396 402 528 5336
Binding energy (eV) Fig. 2. Thermogravimetric curves of (a) nanoSiO2P4VP and (b) nanoSiO2P4VPEuBPy composites.
Fig. 4. High resolution XPS spectra of the nanoSiO2P4VPEuBPy composites.
Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031
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Fig. 5. FETEM images of (A) nanoSiO2BenCl particles and (B) the nanoSiO2P4VPEuBPy composites. (C) Possible inter-particle binding mechanism in the aggregates of the nanoSiO2P4VPEuBPy composites.
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(A) Intensity (arb. unit)
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Fig. 6. (A) Steady-state fluorescence emission spectra for (a) Eu(BMPPA)3BPy complex and (b) nanoSiO2P4VPEuBPy composites. (B) Time-resolved fluorescence decay curves for (a) Eu(BMPPA)3BPy complex and (b) nanoSiO2P4VPEuBPy composites collected at 612 nm.
Eu-complex is possibly connected with two or three nanoparticles, reversely, one nanoparticle may connect with many Eu-complex via the modified P4VP polymeric chain. 3.3. UV–vis absorbance, fluorescence and time-resolved fluorescence decays Absorption spectrum of the complex (curve not shown) in the methanol solutions showed broad bands at 245 nm and 290–310 nm, which could be attributed to the electron transition of the ligands of BMPPA and BPy. Both absorption bands appeared in the absorption spectrum of nanoSiO2P4VPEuBPy composites though its relative intensity became rather weak because of only one layer of Eu-complex covered on the surface of the nanoparticles. Based on their absorption and excitation spectra, the excited wavelength was set at 270 nm for the fluorescence emission and time-resolved fluorescence decay measurements discussed below. Fig. 6A shows the fluorescence emission spectra for the Eu(BMPPA)3BPy and nanoSiO2P4VPEuBPy composites in the chloroform solutions. It can be seen that, for the Eu(BMPPA)3BPy complex, five emission peaks were recorded at approximately 585, 595, 615, 655 and 702 nm, which were designated to be 5 D0 ! 7F0, 5D0 ! 7F1, 5D0 ! 7F2, 5D0 ! 7F3, and 5D0 ! 7F4 transitions [23]. Similar to those reported in the literatures [23,24], strong emissions appeared at around 600 nm. The absolute QY was about 0.02 for the Eu(BMPPA)3BPy complex in the chloroform solution. After the Eu-complex was anchored on the surface of the silica nanoparticles, the main emission still appeared at about 600 nm, designated to the transitions of 5D0 ! 7F1 and 5 D0 ! 7F2. The relative intensity at about 700 nm was slightly
increased as compared with that in the Eu(BMPPA)3BPy complex. Unfortunately, we failed to obtain the absolute QY of the nanocomposites because of their relatively lower emission intensity. Fig. 6B shows time-resolved fluorescence decay curves of the Eu(BMPPA)3BPy complex and nanoSiO2P4VPEuBPy composites in the chloroform solutions, excited at 270 nm and collected at 612 nm, respectively. A faster deactivation process was recorded for the nanoSiO2P4VPEuBPy composites than that for the Eu(BMPPA)3BPy complex. These decay curves were fitted with the first-order kinetics or two exponential decays (Eq. (1)). The emission lifetime was approximately 1.47 ms for the Eu-complex in solution based on the first-order kinetics. On the other hand, it was 0.204 ms and 0.576 ms for the nanoSiO2P4VPEuBPy composites. This difference may be attributed to the reason that the Eucomplex functionalized on the surface of the nanoparticles may be have different compositions or exist in different states as revealed by the TEM images.
4. Conclusions We have demonstrated preparation of europium complex functionalized luminescent silica nanoparticles through an addition reaction of the highly active benzyl bromide substituents in the Eu-complex with poly(4-vinylpyridine) modified nanoSiO2 particles. The excited energy of the ligands can be transferred to the Eu3+ ions either in the Eu-complex or in the functionalized silica nanoparticles resulting in typical lanthanide ion emissions. Considering about the unique structural features of the Eucomplex covered by the benzyl bromide substituents, we suggest that this Eu-complex and its functionalized nanoparticles can be
Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031
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used as building blocks to construct various supramolecular or nanoscale luminescent materials. Acknowledgment This work was supported by National Natural Science Foundation of China (No. 21373058). References [1] X.H. Wang, H.J. Chang, J. Xie, et al., Recent developments in lanthanide-based luminescent probes, Coord. Chem. Rev. 273–274 (2014) 201–212. [2] C.S. Stan, M. Popa, D. Sutiman, P. Horlescu, Photoluminescent red, green and blue monoliths of new Eu(III), Tb(III) and Y(III) complexes embedded in silica matrix, Electron. Mater. Lett. 10 (2014) 827–835. [3] S.J. Ryu, A. Kim, M.D. Kim, et al., Photoluminescenteuropium(III) complex intercalated in natural and synthetic clay minerals for enhanced latent fingerprint detection, Appl. Clay Sci. 101 (2014) 52–59. [4] J.P. Martins, P. Martı´n-Ramos, C. Coya, et al., Lanthanide tetrakis-b-diketonate dimers for solution-processed OLEDs, Mater. Chem. Phys. 147 (2014) 1157–1164. [5] Y. Hasegawa, Photofunctional lanthanoid complexes, coordination polymers, and nanocrystals for future photonic applications, Bull. Chem. Soc. Jpn. 87 (2014) 1029–1057. [6] W.X. Li, X.D. Xin, S.Y. Feng, et al., Fluorescence enhancement of europium(III) perchlorate by 1,10-phenanthroline on the 1-(naphthalen-2-yl)-2-(phenylsulthio)ethanone complex and luminescence mechanism, Luminescence 29 (2014) 810–817. [7] D.J. Wang, Y. Pi, H. Liu, et al., Synthesis and spectroscopic behavior of highly luminescent trinuclear europium complexes with tris-b-diketone ligand, J. Alloys Compd. 613 (2014) 13–17. [8] W.J. Wu, H.X. Huang, M. Chen, D.J. Qian, Synthesis and luminescent properties of a silylated-terpyridine derivative and its metalated complexes in solutions and selfassembled monolayers, Chin. Chem. Lett. 26 (2015) 343–347. [9] X.B. Sun, X.Z. Jin, W. Pan, J.P. Wang, Syntheses of new rare earth complexes with carboxymethylated polysaccharides and evaluation of their in vitro antifungal activities, Carbohyd. Polym. 113 (2014) 194–199. [10] X.B. Gao, J. Yu, N. Li, H.Y. Yin, J.H. Yang, The preparation and fluorescence properties of europium nanoparticles, Chin. Chem. Lett. 18 (2007) 1289–1292.
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[11] H. Xu, Q. Sun, Z.F. An, Y. Wei, X.G. Liu, Electroluminescence from europium(III) complexes, Coord. Chem. Rev. 293–294 (2015) 228–249. [12] M.L. Cable, J.P. Kirby, H.B. Gray, A. Ponce, Enhancement of anion binding in lanthanide optical sensors, Acc. Chem. Res. 46 (2013) 2576–2584. [13] Q.P. Li, B. Yan, Luminescent nanoparticles prepared by encapsulating lanthanide chelates to silica sphere, Colloid Polym. Sci. 292 (2014) 1385–1393. [14] Q. Zhang, Y. Sheng, K.Y. Zheng, et al., Novel organic–inorganic amorphous photoactive hybrid films with rare earth (Eu3+, Tb3+) covalently embedded into silicon–oxygen network via sol–gel process, Mater. Res. Bull. 70 (2015) 379–384. [15] Y.F. Shao, B. Yan, Multi-component hybrids of surfactant functionalized europium tetrakis(b-diketonate) in MCM-41(m) and polymer modified ZnO for luminescence integration, Microporous Mesoporous Mater. 193 (2014) 85–92. [16] J.X. Zhang, N. Prabhakar, T. Na¨reoja, J.M. Resenholm, Semiconducting polymer encapsulated mesoporous silica particles with conjugated europium complexes: toward enhanced luminescence under aqueous conditions, ACS Appl. Mater. Interfaces 6 (2014) 19064–19074. [17] S.N.A. Jenie, S. Pace, B. Sciacca, et al., Lanthanide luminescence enhancements in porous silicon resonant microcavities, ACS Appl. Mater. Interfaces 6 (2014) 12012–12021. [18] A.P. Duarte, M. Gressier, M.J. Menu, et al., Structural and luminescence properties of silica-based hybrids containing new silylated-diketonato europium(III) complex, J. Phys. Chem. C 116 (2012) 505–515. [19] Q. Zhang, Y. Sheng, K.Y. Zheng, et al., Novel luminescent lanthanide complexes assembling alumina/titania/silica hybrids through 2-phenylmalonic acid linkage, J. Non-Cryst. Solids 413 (2015) 34–38. [20] M.J. Zhou, D.L. Han, X.L. Liu, et al., Characterization and catalytic activity of a novel Fe nano-catalyst as efficient heterogeneous catalyst for selective oxidation of ethylbenzene, cyclohexene, and benzylalcohol, J. Mol. Catal. A-Chem. 172–173 (2015) 174–184. [21] Y. Yuan, N. Chen, R. Liu, S.W. Zhang, X.Y. Liu, A novel acrylic prepolymer/ methacrylate modified nano-SiO2 composite used for negative photoresist, Mater. Res. Bull. 50 (2014) 392–398. [22] R.K. Sharma, S. Sharma, Silica nanosphere-supported palladium(II) furfural complex as a highly efficient and recyclable catalyst for oxidative amination of aldehydes, Dalton Trans. 43 (2014) 1292–1304. [23] H.G. Liu, Y.I. Lee, S. Park, K. Jang, S.S. Kim, Photoluminescent behaviors of several kinds of europium ternary complexes doped in PMMA, J. Lumin. 110 (2004) 11–16. [24] F. Wang, J.H. Sun, J.P. Wang, S.Y. Bai, X. Wu, Eu3+-modification of luminescent hybrid bimodal mesoporous silicas with various anions (NO3, CH3COO, and Cl), Mater. Chem. Phys. 145 (2014) 471–475.
Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031
CCLET-3651; No. of Pages 5 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet
Original article
Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites Hong-Xiang Huang a,*, Wen-Li He b, Xiang-Hai Sheng b, Wen-Jie Wu b a b
Department of Macromolecular Science and State Key Laboratory of Molecular Engineering of Polymers, Shanghai 200433, China Department of Chemistry, Fudan University, Shanghai 200433, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 January 2016 Received in revised form 12 March 2016 Accepted 17 March 2016 Available online xxx
Luminescent silica nanocomposites functionalized with a Eu-complex have been prepared and characterized. The europium complex is composed of 2,20 -bipyridyl (BPy) and 2-(4-bromomethyl)phenylpropionic acid (BMPPA), which contains highly active benzyl bromide substituents and can covalently bind with poly(4-vinylpyridine) (P4VP)-modified silica nanoparticles (nanoSiO2P4VP) to form nanoSiO2P4VPEuBPy composites. Microscopic images revealed that the nanoSiO2P4VPEuBPy composites easily formed aggregates, due to an inter-particle binding caused by the benzyl bromide among the composites. The as-prepared nanocomposites showed the typical emissions of Eu(III) ions at the wavelengths from 580 nm to 750 nm designated to the 5D0 ! 7Fn transitions. Time-resolved fluorescence decay measurements revealed that the emission lifetime was approximately 0.204 ms and 0.576 ms for the nanoSiO2EuBPy composites, a little shorter than that in the Eu(BMPPA)3BPy complex. ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Eu-complex Nanocomposites Silica nanoparticles Fluorescence
1. Introduction Lanthanide complexes have attracted much attention in the past several decades for the development of advanced luminescent materials with applications in the fields of optoelectronics, imaging, markers, OLEDs, and fluorescent lamps [1–5]. These complexes give off relatively intense visible emissions under UV radiation followed by an efficient intramolecular energy transfer from the excited ligands to the central lanthanide ions [6]. Several kinds of ligands have been used as the so-called antenna to absorb UV light, such as b-diketone derivatives [7], shift bases [8], organic acids [9], and even proteins [10]. Large amount of lanthanide complexes have been designed and synthesized with their emission properties characterized in past several decades [11,12]. Recently, with the development of molecular assembling and nanotechnology, effects have been made on the immobilization of the lanthanide complexes on the surface of nanoparticles to form luminescent nanocomposites, which can be obtained through a one-step process by using mixtures of lanthanide salts or complexes with tetraethyl orthosilicate catalyzed by a base
* Corresponding author. E-mail address: [email protected] (H.-X. Huang).
(e.g., ammonium hydroxide) [13,14], or through a multi-step process by chemical reactions with modified silica nanoparticles [15,16]. In the latter case, the silica nanoparticles were first covalently functionalized with small organic species like azida [17] or more especially with a trimethoxysilyl group grafted diketones [18]. Because the lanthanide ions or their complexes are reacted with pre-fabricated silica nanoparticles, the particle sizes, shapes as well as their distribution could be pre-controlled, which is benefit for their further applications as candidates for the device fabrication. Here, a ternary europium complex with 2-(4-bromomethyl)phenylpropionic acid (BMPPA) and 2,20 -bipyridyl (BPy) ligands was newly synthesized, which was then connected with poly (4-vinylpyridine) (P4VP) functionalized silica nanoparticles (nanoSiO2P4VP) via an addition reaction to produce luminescent nanoSiO2P4VPEuBPy composites. These nanocomposites were characterized by using UV–vis absorption and infrared spectra, thermogravimetric (TG), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM). Fluorescence emission spectra revealed typical Eu(III) ion emissions at the wavelengths from 580 nm to 750 nm designated to the 5D0 ! 7Fn transitions with the emission lifetime of approximately 0.204 and 0.576 ms for the nanoSiO2EuBPy composites, and that of approximately 1.47 ms for the Eu-complex in chloroform solutions.
http://dx.doi.org/10.1016/j.cclet.2016.03.031 1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031
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CCLET-3651; No. of Pages 5 H.-X. Huang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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2. Experimental 2.1. Materials Europium oxide (99.99%) was purchased from Yuelong Chemical Plant (Shanghai, China). (4-Chloromethylphenyl)trimethoxylsilane, P4VP, and Ludox AS40 (containing 40 wt% SiO2 with an average particle size of 12 nm) were from Sigma–Aldrich Co., Ltd. BMPPA and BPy were from J&K Scientific LTD. All chemicals were used as received without further purification. 2.2. Synthesis of Eu(BMPPA)3BPy complex and its silica nanocomposites Europium chloride was prepared by dissolving 0.813 g Eu2O3 (2.31 mmol) in the dilute HCl solution, which was heated on water bath until the solvent was evaporated. The white powders of EuCl3 were then dissolved in methanol, followed by addition of 0.72 g BPy (4.62 mmol) to form the Eu–BPy complex solution, to which a methanol solution containing 3.37 g BMPPA (13.86 mmol) and 0.78 g KOH (13.93 mmol) was added. The mixtures were refluxed overnight to produce white precipitates [19], which were finally filtrated, washed with plenty of methanol, recrystallized from chloroform-methanol solution, and dried under vacuum. Calc. For C40H38N2Br3O6Eu2H2O: C, 44.8; H, 3.90; N, 2.61%. Found: C, 44.0; H, 3.90; N, 2.31%. Selected IR (KBr, cm1), 3340, 3045, 3370, 2970, 2931, 2854, 1730, 1600, 1568, 1458, 1410, 1230, 849, 764, 604. The preparation processes of the present nanocomposites are shown in Fig. 1. Firstly, the P4VP functionalized silica hybrid of nanoSiO2P4VP was prepared based on the literature method with the use of silica gel in methanol as starting materials [18]. Then, the luminescent nanoSiO2P4VPEuBPy composites were prepared by stirring the mixtures of nanoSiO2P4VP hybrids and excess
Eu(BMPPA)3BPy complex in a DFM/CH3CN mixed solution at room temperature overnight. The suspension was isolated by centrifugation at 4000 rpm. The white powders of nanoSiO2P4VPEuBPy composites were washed thoroughly with plenty of DMF, methanol and finally dried in a vacuum overnight. 2.3. Characterization TG analysis was performed on an STDQ600 instrument (TA Ltd., USA) under a constant air flow atmosphere from room temperature to 800 8C at a rate of 10 8C/min. Elemental analysis was performed on a VARIO EL3 elemental analyzer. The europium content was analyzed by using Perkin Elmer Optima 8000 inductively coupled plasma optical emission spectrometer (ICP-OEP). Fourier transform infrared spectra (FITR) were measured by using a Nicolet NEXUS 470 spectrometer, operating at a resolution of 0.5 cm1 at 25 8C. XPS spectra were recorded using a VGESCALAB MKII multifunction spectrometer, with nonmonochromatized Mg-Ka X-rays as the excitation source. The system was carefully calibrated by Fermi-edge of nickel, Au 4f2/7 and Cu 2p2/3 binding energy. Pass energy of 70 eV and step size of 1 eV were chosen when taking spectra. In the analysis chamber pressures of 1–2 107 Pa were routinely maintained. The binding energies obtained in the XPS analysis were corrected by referencing the C1s peak to 284.60 eV. Field emission transmission electron microscopy (FETEM) images of the SiO2 nanoparticles before and after functionalization were recorded using a JEM-2100F field emission electron microscope. The nanoparticles were deposited on 230-mesh copper grid covered with Formvar. UV–vis spectra were measured with the use of Shimadzu UV2550 UV–vis spectrophotometer. Fluorescence spectra were recorded by using Shimadzu RF-5300PC spectrophotometer. The
Fig. 1. Schematic representation for the synthesis of Eu(BMPPA)3BPy complex and preparation of its functionalized silica nanocomposites.
Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031
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It ¼ A þ B1 eðt=t1 Þ þ B2 eðt=t 2 Þ I0
(1)
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absolute quantum yield (QY) was obtained with the use of an integrating sphere measurement excited at 270 nm. Time-resolved single photon fluorescence measurements were performed using an Edinburgh Instruments FLS920 spectrometer with microsecond pulsed lamp as the light source. Lifetimes were estimated from the decay of the main transitions with the use of the Edinburgh Instruments software package. The excited wavelength was set at 270 nm. The fluorescence decay curves followed with first-order kinetics or two exponential decay kinetics, as displayed in the logarithmic plots of
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where I0 and It are the fluorescence intensity at times zero and t, respectively, Bi is the pre-exponential factor, and t is the experimental lifetime of the excited state of the europium complex. 3. Results and discussion 3.1. Thermogravimetric, FTIR, and XPS analysis Fig. 2 shows the TG curves for the nanoSiO2P4VP and nanoSiO2P4VPEnBPy composites from room temperature to 800 8C. Both curves revealed a mass decrease below 120 8C (about 6% in weight), ascribed to the evaporation of physically adsorbed water. Then, the mass decrease was observed in the temperature range 250–600 8C, attributed to decomposition of the organic species covalently attached on the silica surfaces and the Eucomplex (for the nanoSiO2P4VPEnBPy composites). In this temperature range, the total mass decrease observed was about 19% in weight for the nanoSiO2P4VP composites, and 22% for the nanoSiO2P4VPEuBPyones, respectively. The difference of the mass decrease (3%) could be attributed to the Eu-complex attached. Finally, the residuals observed at above 600 8C corresponded to 72–75% of the total weight and were attributed to the formation of SiO2 powders following decomposition of organic species and loss of water [20,21]. Fig. 3 shows FTIR spectrum of the nanoSiO2P4VPEuBPy composites, which revealed following features. Firstly, the broad band centered at 3445 cm1 was attributed to adsorbed water molecules and the hydroxyl groups on the nanoparticle surfaces [21]. Secondly, several weak peaks were recorded at approximately 2940 and 2890 cm1 were assigned to the aliphatic –CH2 stretching vibration bands and confirmed that the P4VP substituent had anchored onto the silica core surface. Thirdly, a sharp peak recorded at approximately 1650 cm1 was attributed to the formation of the COO bond of Eu-complex. Fourthly, several small bands were recorded between 1530 and 1380 cm1,
Wavenuumber (cm ) Fig. 3. Infrared spectrum of the nanoSiO2P4VPEuBPy composites.
attributed to the C5 5N, C–C, and C5 5C vibrations of pyridyl and benzyl rings. Finally, the vibration bands recorded at approximately 1110 cm1 and 795 cm1 were ascribed to the nanoparticle Si–O–Si asymmetric and Si–O symmetric stretching bands [22]. Elemental composition of the nanoSiO2P4VPEuBPy composites was analyzed by using XPS and ICP techniques. Fig. 4 shows the high resolution XPS spectra, which revealed seven peaks at the binding energy of 67.0, 102.4, 127.6/144.4, 197.6, 284.6, 399.2– 401.2, and 533.0 eV, which could be assigned to Br(3d), Si(2p), Eu(4d), Cl(2p), C(1s), N(1s), and O(1s), respectively. Si and O belonged to the nano-SiO2 core, while C, N Br, Cl and part of O to the modified organic layer and the ligands of BMPPA and BPy. Eu was from the coordinated central ions of the complex. Based on the ICP measurement, the accurate content of the Eu(III) ions was about 26.9 mg/g (nano-composites). Thus, the FTIR, XPS and ICP results confirmed formation of the nanoSiO2P4VPEuBPy composites. 3.2. Morphology characterization Microscopic images for the nanoSiO2BenCl and nanoSiO2P4VPEuBPy composites were observed by using TEM. As shown in Fig. 5, before functionalization with the Eu-complex, the nanoparticles were well separated with an average diameter of approximately 12 nm, which was in agreement with the data provided by the company. Image of the nanoSiO2BenP4VP composites was similar to that of the nanoSiO2BenCl (not shown). However, when the Eu(BMPPA)3BPy complex was coordinated on the particle surface, although the sizes and shapes did not changed significantly, the particles largely aggregated as shown in Fig. 5B. Possible reasons may be an inter-particle binding caused by the benzyl bromide substituents between two or more nanoparticles as the carton shown in Fig. 5C, in which it can be seen that the C(1s)
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Binding energy (eV) Fig. 2. Thermogravimetric curves of (a) nanoSiO2P4VP and (b) nanoSiO2P4VPEuBPy composites.
Fig. 4. High resolution XPS spectra of the nanoSiO2P4VPEuBPy composites.
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Fig. 5. FETEM images of (A) nanoSiO2BenCl particles and (B) the nanoSiO2P4VPEuBPy composites. (C) Possible inter-particle binding mechanism in the aggregates of the nanoSiO2P4VPEuBPy composites.
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Fig. 6. (A) Steady-state fluorescence emission spectra for (a) Eu(BMPPA)3BPy complex and (b) nanoSiO2P4VPEuBPy composites. (B) Time-resolved fluorescence decay curves for (a) Eu(BMPPA)3BPy complex and (b) nanoSiO2P4VPEuBPy composites collected at 612 nm.
Eu-complex is possibly connected with two or three nanoparticles, reversely, one nanoparticle may connect with many Eu-complex via the modified P4VP polymeric chain. 3.3. UV–vis absorbance, fluorescence and time-resolved fluorescence decays Absorption spectrum of the complex (curve not shown) in the methanol solutions showed broad bands at 245 nm and 290–310 nm, which could be attributed to the electron transition of the ligands of BMPPA and BPy. Both absorption bands appeared in the absorption spectrum of nanoSiO2P4VPEuBPy composites though its relative intensity became rather weak because of only one layer of Eu-complex covered on the surface of the nanoparticles. Based on their absorption and excitation spectra, the excited wavelength was set at 270 nm for the fluorescence emission and time-resolved fluorescence decay measurements discussed below. Fig. 6A shows the fluorescence emission spectra for the Eu(BMPPA)3BPy and nanoSiO2P4VPEuBPy composites in the chloroform solutions. It can be seen that, for the Eu(BMPPA)3BPy complex, five emission peaks were recorded at approximately 585, 595, 615, 655 and 702 nm, which were designated to be 5 D0 ! 7F0, 5D0 ! 7F1, 5D0 ! 7F2, 5D0 ! 7F3, and 5D0 ! 7F4 transitions [23]. Similar to those reported in the literatures [23,24], strong emissions appeared at around 600 nm. The absolute QY was about 0.02 for the Eu(BMPPA)3BPy complex in the chloroform solution. After the Eu-complex was anchored on the surface of the silica nanoparticles, the main emission still appeared at about 600 nm, designated to the transitions of 5D0 ! 7F1 and 5 D0 ! 7F2. The relative intensity at about 700 nm was slightly
increased as compared with that in the Eu(BMPPA)3BPy complex. Unfortunately, we failed to obtain the absolute QY of the nanocomposites because of their relatively lower emission intensity. Fig. 6B shows time-resolved fluorescence decay curves of the Eu(BMPPA)3BPy complex and nanoSiO2P4VPEuBPy composites in the chloroform solutions, excited at 270 nm and collected at 612 nm, respectively. A faster deactivation process was recorded for the nanoSiO2P4VPEuBPy composites than that for the Eu(BMPPA)3BPy complex. These decay curves were fitted with the first-order kinetics or two exponential decays (Eq. (1)). The emission lifetime was approximately 1.47 ms for the Eu-complex in solution based on the first-order kinetics. On the other hand, it was 0.204 ms and 0.576 ms for the nanoSiO2P4VPEuBPy composites. This difference may be attributed to the reason that the Eucomplex functionalized on the surface of the nanoparticles may be have different compositions or exist in different states as revealed by the TEM images.
4. Conclusions We have demonstrated preparation of europium complex functionalized luminescent silica nanoparticles through an addition reaction of the highly active benzyl bromide substituents in the Eu-complex with poly(4-vinylpyridine) modified nanoSiO2 particles. The excited energy of the ligands can be transferred to the Eu3+ ions either in the Eu-complex or in the functionalized silica nanoparticles resulting in typical lanthanide ion emissions. Considering about the unique structural features of the Eucomplex covered by the benzyl bromide substituents, we suggest that this Eu-complex and its functionalized nanoparticles can be
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Please cite this article in press as: H.-X. Huang, et al., Fabrication and luminescent properties of Eu(BMPPA)3BPy complex and its functionalized silica nanocomposites, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.03.031