pubs.acs.org/Langmuir © 2010 American Chemical Society
Micelles as “Fluorescence Protector” for an Europium Complex in Microcapsules Cui Wang,† Renjie Zhang,*,† and Helmuth M€ohwald‡ †
Key Laboratory of Colloid and Interface Chemistry of Ministry of Education, Shandong University, 250100 Jinan, China, and ‡Department of Interfaces, Max Planck Institute of Colloids and Interfaces Sciences, 14424 Potsdam, Germany Received April 12, 2010. Revised Manuscript Received May 6, 2010 Microcapsules with excellent fluorescence enhancement are assembled by using cetyltrimethylammonium bromide (CTAB) micelles to enrich Eu(DBM)3Phen (DBM and Phen are dibenzoylmethane and 1,10-phenanthroline, respectively) on CaCO3 particles by the LbL technique. Compared to microcapsules without micelles, the fluorescence intensity of microcapsules with micelles increases 9 times, larger than the 6 times increase of absorbance. This unexpected fluorescence enhancement is attributed to the “fluorescence protector” effect of CTAB micelles in microcapsules. Energy loss from nonradiative deactivation through energy transfer to high-energy O-H vibrations from the emissive 5D0 state of Eu(III) is greatly prohibited. The new strategy using micelles in this work not only enriches europium complexes during assembly in aqueous solution but also yields a fluorescence enhancement ratio larger than the enrichment ratio.
Introduction Excellent performance of water-insoluble molecules can be achieved by encapsulating them in the hydrophobic cores of micelles1-4 on two-dimensional (2D) planar solid substrates by the novel layer-by-layer (LbL) technique.5-8 Photoactive multilayers with high efficiency of emission were prepared due to the decreased aggregation of chromophore groups in micelles relative to that without micelles.9 Short photoresponse time of photochromic azobenzene was also achieved in films, where micelles separated aggregates.10 However, in the literature such micelles were only assembled on 2D planar solid substrates. It is a challenge to obtain micelles enriching functional molecules on threedimensional (3D) colloidal particles, which is a fundamentally new strategy to obtain advanced materials by the LbL self-assembly on the molecular level. Herein, on the basis of our previous work,11-13 especially considering the fluorescence emission process of europium complexes, we employ micelles to enrich and increase the fluorescence *Corresponding author. E-mail:
[email protected]. (1) Emoto, K.; Iijima, M.; Nagasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2000, 122, 2653. (2) Ma, N.; Zhang, H. Y.; Song, B.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 5065. (3) Cho, J. H.; Hong, J. K.; Char, K.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9935. (4) Ma, N.; Wang, Y. P.; Wang, B. Y.; Wang, Z. Q.; Zhang, X.; Wang, G.; Zhao, Y. Langmuir 2007, 23, 2874. (5) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (6) Decher, G.; Lehr, B.; Lowack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 677. (7) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; M€ohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (8) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395. (9) Zapotoczny, S.; Golonka, M.; Nowakowska, M. Macromol. Rapid Commun. 2005, 26, 1049. (10) Ma, N.; Wang, Y.; Wang, Z.; Zhang, X. Langmuir 2006, 22, 3906. (11) Zhang, R. J.; Cui, J. W.; Lu, D. M.; Hou, W. G. Chem. Commun. 2007, 1547. (12) Cui, J. W.; Zhang, R. J.; Lin, Z. G.; Li, L.; Jin, W. R. Dalton Trans. 2008, 895. (13) Zhang, R. J.; Lu, D. M.; Lin, Z. G.; Li, L.; Jin, W. R.; M€ohwald, H. J. Mater. Chem. 2009, 19, 1458.
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of β-diketone europium complexes (Eu(DBM)3Phen, where DBM and Phen are dibenzoylmethane and 1,10-phenanthroline, respectively) on CaCO3 particles by the LbL technique. An unexpected fluorescence enhancement is observed and a corresponding scheme is provided, which we name as the “fluorescence protector” effect of cetyltrimethylammonium bromide (CTAB) micelles, both increasing the content of Eu(DBM)3Phen in microcapsules and prohibiting nonradiative deactivation from the Eu(III) in micelles to H2O molecules in the surrounding. The microcapsules with the most intense fluorescence achieved so far and excellent monochromaticity are of great significance both in sciences like chemistry and in applications for laser materials,14,15 fluorescence immunoassays,16 and simultaneous multicolor bioassays.17
Experimental Section Materials and Methods. Poly(sodium 4-styrenesulfonate)
(PSS, Mw ≈ 70 kDa) and poly(allylamine hydrochloride) (PAH, Mw ≈ 55 kDa) were obtained from Sigma-Aldrich Inc. All commercial polyelectrolytes were used without further purification. Disodium ethylenediaminetetraacetic acid (EDTA), cetyltrimethylammonium bromide (CTAB), Ca(NO3)2, Na 2CO 3, NaCl, NaOH, and HCl were all of analytical reagents. Eu(DBM)3Phen was synthesized according to literature methods [J. Am. Chem. Soc. 1964, 86, 5125]. Pure water with a resistivity of 18.2 MΩ 3 cm was used in all experiments. Fabrication of CaCO3 Microparticles. 100 mL of Na2CO3 (0.33 M) solution was rapidly added to an equal volume of Ca(NO3)2 (0.33 M) solution at room temperature. The precipitate was thoroughly washed with pure water three times after intense agitation for 1 min with a magnetic stirrer. The products were filtered off through a membrane, rinsed with a drop of ethanol, and then air-dried. The procedure resulted in homogeneous, spherical CaCO3 microparticles of 4-6 μm in diameter. (14) Whan, R. E.; Crosby, G. A. J. Mol. Spectrosc. 1962, 8, 315. (15) Balzani, V. Tetrahedron 1992, 48, 10443. (16) Li, M.; Selvin, P. R. J. Am. Chem. Soc. 1995, 117, 8132. (17) Huhtinen, P.; Kivela, M.; Kuronen, O.; Hagren, V.; Takalo, H.; Tenhu, H.; Lovgren, T.; Harma, H. Anal. Chem. 2005, 77, 2643.
Published on Web 05/21/2010
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Fabrication of CTAB Micelles Enriching Eu(DBM)3Phen. CTAB micelles were prepared according to the literature
method.18 1 mL of 7.5 10-3 g/mL aqueous solution of CTAB was mixed with 1 mL of 1.35 10-4 g/mL aqueous suspension of Eu(DBM)3Phen under stirring. The mixture was left overnight under stirring. Microcapsule Preparation. Polyelectrolyte microcapsules doped with β-diketone europium complex were prepared by alternative incubation of CaCO3 microparticles (∼107 microparticles) in 2 mg/mL PAH containing 0.5 M NaCl, 2 mg/mL PSS solution containing 0.5 M NaCl, and micelles solubilizing Eu(DBM)3Phen. The pH of the polymer solutions was adjusted to 6.5 by addition of HCl or NaOH. Each adsorption cycle was completed with 10 min incubation in polyelectrolyte solutions or 1 h incubation in CTAB-Eu(DBM)3Phen micelles followed by washing procedures. The latter included four centrifugation steps (2000 rpm, 1 min), and three suspensions in 0.05 M NaCl solutions for 4 min before assembling the next layer were used to remove nonbound polymers. In the last washing step only pure water was used. Deposition of CTAB micelles enriching Eu(DBM)3Phen lasted 1 h. The CaCO3 cores of microcapsules were decomposed by 0.2 M EDTA solution at neutral pH for 30 min. The suspension was then centrifuged (3000 rpm, 1 h) and washed three times with pure water. Polarizing Optical Microscopy (POM). POM was performed using a Carl Zeiss Axioskop 40 microscope (Germany). Electrophoretic Mobility. Electrophoretic mobility of microcapsules was determined in water using a Powereach JS94H microelectrosphoretic instrument. Each value was averaged for 10 parallel measurements. Fluorescence Micrographs. Fluorescence micrographs were obtained with a mercury lamp and a 60 oil immersion objective and a DP70 CCD camera on an Olympus IX 81 microscope (Japan). Scanning Electron Microscopy (SEM). SEM measurements were carried out on a JEOL JSM-6700F (Japan) instrument at an operation voltage of 3 keV. A drop of the microcapsule suspension was applied to an aluminum foil, air-dried overnight, and then sputtered with gold. Transmission Electron Microscopy (TEM). TEM characterization was performed using an H-600 TEM (Japan). Microcapsule suspensions were dropped onto 230-mesh copper grids covered with a Formvar film and observed under an accelerating voltage of 100 kV with a vacuum of 10-6 Torr. Counting of Microcapsules. The number of microcapsules in a hemocytometer was counted under an optical microscope and averaged for three parallel measurements. 10 μL of microcapsule solution was dropped on the counting chamber of 0.1 μL in volume and then sealed with a cover glass. UV Spectra and the Working Curve of Absorbance. UV absorption spectra of Eu(DBM)3Phen in CHCl3 were recorded with quartz cells on a HP-8453 diode array spectrophotometer with a resolution of 2 nm and averaged for three parallel measurements. Eu(DBM)3Phen was extracted by 2 mL of CHCl3 from microcapsules with and without micelles after they were dried. The working curve of absorbance was obtained by plotting absorbance of Eu(DBM)3Phen solution in CHCl3 at the concentration of 1.0 10-6-6.0 10-5 g/mL.
Results and Discussion The LbL assembly of poly(allylamine hydrochloride) (PAH), poly(sodium 4-styrenesulfonate) (PSS), and CTAB micelles containing Eu(DBM)3Phen on CaCO3 microparticles is illustrated in Figure 1. The alternating change of ζ-potential data during the assembly (Figure 2) demonstrates that PAH, PSS, and CTAB (18) Liu, X. K.; Zhou, L.; Geng, W.; Sun, J. Q. Langmuir 2008, 24, 12986.
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Figure 1. Scheme of the LbL assembly of PAH, PSS, and CTABEu(DBM)3Phen on a CaCO3 microparticle.
Figure 2. ζ-potential values during the deposition of PAH, PSS, and CTAB-Eu(DBM)3Phen on CaCO3 microparticles. The layer numbers 1, 3, 5, and 7 correspond to PAH assembly, layer numbers 2, 4, 6, 8, 10, and 12 correspond to PSS adsorption, and layer numbers 9 and 11 correspond to CTAB-Eu(DBM)3Phen assembly.
Figure 3. Fluorescence emission spectra of pyrene in (A) CTAB solution under cmc, (B) microcapsules with CTAB micelles, and (C) CTAB micelles solution.
micelles containing Eu(DBM)3Phen are successfully deposited on CaCO3 particles. Pyrene, as a fluorescence probe, is used to characterize the presence of CTAB micelles in the microcapsules. The ratio of the intensity of the first peak (I1) to that of the third peak (I3) decreases, indicating a more hydrophobic environment.19 As shown in Figure 3, the ratio of I1 (374 nm)/I3 (383 nm) in the emission spectrum of pyrene in microcapsules is 0.94, lower than the I1/I3 of pyrene both in CTAB aqueous solutions above the critical micellar concentration (cmc) (0.96) and below the cmc (1.10). So CTAB micelles with hydrophobic cores formed by aliphatic chains are confirmed in microcapsules. Solid and hollow microcapsules doped with CTAB micelles containing Eu(DBM)3Phen emit brilliant fluorescence (Figure 4A,B), which can be observed by the naked eye under (19) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
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Figure 6. Plot of absorbance at 346 nm against concentration of Eu(DBM)3Phen in chloroform.
Figure 4. (A) Fluorescence micrograph of CaCO3|(PAH/PSS)4/ [CTAB-Eu(DBM)3Phen/PSS]2 microcapsules, (B) fluorescence micrograph of (PAH/PSS)4/[CTAB-Eu(DBM)3Phen/PSS]2 hollow microcapsules, (C) SEM micrograph of hollow luminescent microcapsules (PAH/PSS)4/[CTAB-Eu(DBM)3Phen/PSS]2, and (D) TEM micrograph of hollow luminescent microcapsules (PAH/ PSS)4/[CTAB-Eu(DBM)3Phen/PSS]2.
Figure 5. UV absorption spectra of Eu(DBM)3Phen in microcapsules of (A) CaCO3|(PAH/PSS)4[CTAB-Eu(DBM)3Phen/PSS]2 and (B) CaCO3|(PAH/PSS)4[Eu(DBM)3Phen/PSS]2.
ultraviolet (UV) irradiation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs of hollow capsules show narrow size distribution and excellent integrity of microcapsules (Figure 4C,D). The dry hollow microcapsules appear as “flat balloons” after evaporation of water. Microcapsules with micelles and without micelles were prepared to reveal the micelle effects on the fluorescence emission of Eu(DBM)3Phen in microcapsules. The content of Eu(DBM)3 Phen in micelles was studied by measuring the absorbance of Eu(DBM)3 Phen extracted from microcapsules by chloroform. The absorption spectra of Eu(DBM)3Phen show two peaks at 270 and 346 nm (Figure 5), respectively; both can be assigned to the π-π* electronic transitions of the ligands.20 By calculating the absorbance at 346 nm on the basis of a working curve (Figure 6), each microcapsule with and without CTAB micelles carries respectively 0.628 and (20) Accorsi, G.; Armaroli, N.; Parisini, A.; Meneghetti, M.; Marega, R.; Prato, M.; Bonifazi, D. Adv. Funct. Mater. 2007, 17, 2975.
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Figure 7. Fluorescence emission spectra of Eu(DBM)3Phen in microcapsules of (A) CaCO3|(PAH/PSS)4[CTAB-Eu(DBM)3Phen/PSS]2 and (B) CaCO3|(PAH/PSS)4[Eu(DBM)3Phen/PSS]2.
0.109 pg of Eu(DBM)3Phen. The enrichment ratio by CTAB micelles is 6 for Eu(DBM)3Phen in microcapsules. The fluorescence emission spectra of microcapsules with and without micelles are shown in Figure 7. Two characteristic emission peaks at 591 and 612 nm correspond to the electric dipole transitions of 5D0 f 7F1 and 5D0 f 7F2 of the Eu(III) ion, respectively. The narrow peak at 612 nm shows the extraordinarily good monochromaticity of the Eu(III) emission. The fluorescence intensity of Eu(DBM)3Phen in micelle-containing microcapsules is about 9 times that of microcapsules without micelles. Similar to the reason for increased absorbance of microcapsules with micelles, the increased fluorescence intensity can be due to the enrichment effect by CTAB micelles. However, the different increases of fluorescence intensity (9) and absorbance (6) is worthy of detailed discussion. The enrichment accounts for a factor of 6, since fluorescence intensity generally increases linearly with the concentration of europium complexes.21 An and co-workers attribute the fluorescence enhancement of (1-cyano-trans-1,2-bis(40 -methylbiphenyl)ethylene, CN-MBE) nanoparticles to the synergetic effect of intramolecular planarization and J-type aggregate formation (restricted excimer formation).22 Chromophores (DBM and Phen) in europium complexes cannot form such a type of aggregate, since their binding to the Eu(III) ions does not allow packing in such a manner. So the Eu(DBM)3Phen fluorescence enhancement should be described differently. The additional increase in fluorescence intensity should be due to the “fluorescence protector” effect of CTAB micelles in microcapsules. For free Eu(DBM)3Phen molecules in water there exists energy loss through the nonradiative deactivation from the emissive 5D0 state of the Eu(III) to the high-energy O-H vibrations.23 (21) Zhang, R. J.; Liu, H. G.; Zhang, C. R.; Yang, K. Z.; Zhu, G. Y.; Zhang, H. W. Thin Solid Films 1997, 302, 223. (22) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (23) Sendor, D.; Kynast, U. Adv. Mater. 2002, 14, 1570.
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Eu(DBM)3Phen has a higher solubility in organic solvent than in water due to organic ligands bound to the Eu(III). The Eu(DBM)3Phen molecules are enriched in the hydrophobic core of CTAB micelles. The cores repel water, so the energy loss from the Eu(III) is greatly decreased. This “fluorescence protector” effect of micelles enables the fluorescence enhancement ratio 9/6 = 1.5 times larger than the enrichment ratio, taking the microcapsules without micelles as reference. Microcapsules with the so far most intense fluorescence emission of europium complexes are therefore assembled. Above cmc CTAB easily forms spherical micelle.18 Different from lipids that can form vesicles, CTAB does not form vesicles of a double layer of CTAB molecules. CTAB as charged surfactant has a large resistance toward bending, which would become too compressed as a result of bending for the charges in the inner layer of CTAB molecules.24,25 Vesicles containing CTAB can only form when mixed with another kind of surfactant that compensates the surface charges of CTAB.26 Under POM the absence of birefringence of CTAB aggregates enriching Eu(DBM)3Phen rules out experimentally the formation of vesicles. As a result, above cmc CTAB forms micelle of a single layer of CTAB molecules, whose positively charged groups pointing outward. The “fluorescence protector” effect is attributed to the CTAB micelles encapsulating rare earth complexes in the hydrophobic core of micelles. Different from polystyrene microparticles with smooth surfaces, mesoporous CaCO3 microparticles with rough surfaces and many pores result in failure to observe the morphology of micelles in microcapsules either by scanning force microscopy in dry state or by freeze-fracture electron microscopy in hydration state. A scheme of the novel fluorescence enhancement in microcapsules with CTAB micelles is illustrated in Figure 8. First, the DBM ligand absorbs UV photons and relaxes to a triplet through an intersystem crossing, and then energy is transferred to the 5D0 level of Eu(III) by resonance coupling. Energy transfer from the 5 D0 has generally four pathways: the radiative and nonradiative energy transfer to the ground states of Eu(III), the back-donation to the triplet of DBM, and energy transfer to other molecules in the surrounding. Among them, the first three are nearly constant for the same compound, Eu(DBM)3Phen. The micelles separate Eu(DBM)3Phen from water (Figure 8A), while Eu(DBM)3Phen (24) Moya, S.; Richter, W.; Leporatti, S.; Baumler, H. B.; Donath, E. Biomacromolecules 2003, 4, 808. (25) Krishna, G.; Shutava, T.; Lvov, Y. Chem. Commun. 2005, 2796. (26) Zana, R.; Levy, H.; Danino, D.; Talmon, Y.; Kwetkat, K. Langmuir 1997, 13, 402.
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Figure 8. “Fluorescence protector” effect in (A) microcapsules with CTAB micelles superior to (B) microcapsules without micelles. Energy transfer from the Eu(III) to water molecules is prohibited in (A) due to the hydrophobic core of CTAB micelles.
in microcapsules without micelles suffers excitation energy losses to water (Figure 8B). So the last pathway decreases significantly in microcapsules with micelles.
Conclusions We successfully assembled cetyltrimethylammonium bromide (CTAB) micelles containing Eu(DBM)3Phen on 3D CaCO3 particles by the LbL technique. Micelles not only enable an increase of the content of Eu(DBM)3Phen in the microcapsules but also protect against losses through nonradiative deactivation from the Eu(III) in micelles to H2O molecules in the surrounding. This leads to a further enhanced fluorescence emission. The “fluorescence protector” effect provides a fluorescence enhancement ratio larger than the enrichment ratio. This work therefore describes an optimized strategy to obtain the highest fluorescence emission for a fixed amount of fluorescent chemicals. Acknowledgment. This work was financially supported by the National Nature Science Foundation of China (No. 20103005, 20533050), Award Foundation for Excellent Young Scientists in Shandong Province, China, Fok Ying-Tong Education Foundation for Youth Scientists, Ministry of Education, China (No. 81012), and the Max-Plank Society, Germany.
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