Synthesis and Characterization of Novel Lanthanide(III) Complexes

Jul 1, 2010 - Novel photoactive lanthanide hybrids covalently grafted on functionalized periodic mesoporous organosilicons (PMOs) by Schiff-base deriv...
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J. Phys. Chem. C 2010, 114, 12505–12510

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Synthesis and Characterization of Novel Lanthanide(III) Complexes-Functionalized Mesoporous Silica Nanoparticles as Fluorescent Nanomaterials Daojun Zhang, Xuemin Wang, Zhen-an Qiao, Duihai Tang, Yunling Liu, and Qisheng Huo* The State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: May 9, 2010; ReVised Manuscript ReceiVed: June 13, 2010

Lanthanide(III) (Eu and Tb)-imidazoledicarboxylic acid complexes were immobilized on colloidal mesoporous silica with diameter smaller than 100 nm by covalent bond grafting technique and uniform and monodisperse luminescent Eu-idc-Si and Tb-bidc-Si functionalized mesoporous silica hybrid nanomaterials (MSNs) were obtained. The lanthanide(III) complexes-functionalized MSNs were characterized by fluorescence spectra, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption-desorption, and powder X-ray diffraction. The hybrid nanomaterials Eu-idc-Si and Tb-bidc-Si functionalized MSNs show strong red and green photoluminescence upon irradiation with ultraviolet light, respectively. Both hybrid nanomaterials exhibit long lifetimes. The mesoporous silica nanoparticles are stable colloid and may have some advantages for potential applications in drug delivery or optical imaging. Introduction Since the discovery of ordered mesoporous silica materials, much attention has been focused on the organic moiety functionalized mesoporous silicas with different potential applications due to their large surface area, uniform pore size distributions and adjustable pore sizes over the range 1.5-30 nm.1-5 Bulk mesoporous materials with particle sizes in the micrometer range are less appropriate for drug delivery and sensor purposes because settlement of larger particles in solutions occurs rapidly. In comparison with the conventional mesoporous silica materials, mesoporous silica nanoparticles (MSNs) can escape from phagocytes in reticuloendothelial system (RES). The stable small-sized colloidal MSNs in a physiological environment are allowed a long blood circulation. Not surprisingly, MSNs have been functionalized for use in drug delivery, cell labeling, and controlled release of therapeutics.6-8 Recently, a lot of efforts have been devolved to the synthesis of mesoporous silica nanoparticles (MSNs).9-13 It is difficult to synthesize discrete and monodisperse mesoporous silica particles smaller than 100 nm in high yields. We recently reported a good way to synthesize monodisperse mesoporous silica nanoparticles in high yield, and the particle size can be controlled from 25 to 200 nm.14 Specifically, this kind of nanometer-sized mesoporous materials appears to be more effective as a host material for lanthanide complexes than bulk mesoporous silica in biomedical imaging, drug delivery, and gene therapy. Lanthanide complexes are a class of useful luminophores because they exhibit high-quantum efficiency, sharp and intense emission lines, long lifetimes and high color purity under ultraviolet excitation, through protecting metal ions from vibrational quenching, and increasing light absorption cross section by the well-known “antenna effect”.15-17 However, they have not so far been used extensively in practical applications as phosphor devices mainly due to their poor thermal stabilities and low-mechanical strength. Many lanthanide complexes have * To whom correspondence should be addressed. Tel: +86-43185168602. Fax: +86-431-85168624. E-mail: [email protected].

been incorporated into solid matrices, such as sol-gel-derived hybrid materials18-24 and mesoporous silica materials.25-28 Incorporation of lanthanide complexes into these matrices has not only improved the photo and thermal stabilities of the complexes but also avoided the self-quenching resulting from the concentration effect. Up to now, lanthanide complexes can be incorporated into mesoporous silica matrices, such as MCM41 or SBA-15, by either simply doping method or by covalent bond grafting technique.29-33 Until recently, Zhang reported the fabrication and characterization of magnetic mesoporous silica nanospheres covalently bonded with near-infrared luminescent lanthanide complexes.34 Reddy prepared an organic-inorganic mesoporous luminescent hybrid material by linking a ternary Eu3+ complex to the functionalized MCM-41 particles with 250-300 nm in diameter.35 However, to date there are rare reports on the synthesis and characterization of lanthanide(III) complexes-functionalized mesoporous silica nanoparticles, especially to get monodisperse and stable suspensions of colloidal lanthanide(III) complexes-functionalized mesoporous silica with diameter smaller than 100 nm. Furthermore, to explore novel lanthanide(III) complexes-functionalized hybrid luminescent nanomaterials, we need to select proper organic ligand to protect and sensitize rare earth ions. The ligands of 4,5-imidazoledicarboxylic acid (H3idc) and benzimidazole-5,6-dicarboxylic acid (H3bidc) both as multidentate N- or O-donor ligand has been exploited to build luminescent lanthanide metal-organic frameworks (MOFs).36-38 Yang used H3idc to create Eu-MOFs with intense red luminescence36,37 and Zheng used H3bidc to create Tb-MOFs with intense green luminescence,38 respectively. On the basis of these reasons, in this paper we use the colloidal mesoporous silica nanoparticles as host materials to immobilize imidazoledicarboxylic acid-lanthanide complexes through covalent bonds and investigate their fluorescent properties. Experimental Section Materials. Tetraethoxysilane (TEOS 98%), 3-isocyanatepropyltriethoxysilane (ICPTES), benzimidazole-5,6-dicarboxylic acid (H3bidc), and imidazole-4,5-dicarboxylic acid (H3idc) were purchased from Aldrich. Cetyltrimethylammonium chloride

10.1021/jp1042156  2010 American Chemical Society Published on Web 07/01/2010

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SCHEME 1: Synthesis Procedures of idc-Si and bidc-Si

(C16H33N(CH3)3Cl, CTAC, 25 wt % in water), diethanolamine (NH(CH2CH2OH)2, DEA), NH3 · H2O (28 wt % aqueous solution), Tb(NO3) · 6H2O, Eu(NO3) · 6H2O, and other chemicals were purchased from Beijing Chemical Works. All chemicals were used without further purification. Synthesis of Alkoxysilane Modified Imidazole-4,5-dicarboxylic Acid (idc-Si) and Benzimidazole-5,6-dicarboxylic Acid (bidc-Si) (Scheme 1).24 The alkoxysilane-modified imidazole-4,5-dicarboxylic acid (idc-Si) was prepared as follows: 0.625 g (4 mmol) of H3idc was first dissolved in 10 mL of pyridine with stirring and then 0.990 mL (4 mmol) of 3-isocyanatepropyltriethoxysilane was added dropwise to the solution. The mixture was heated at 70 °C for 7 h. The solvent was removed by rotoevaporation and the residue was dried at 60 °C under vacuum. Elemental analysis for idc-Si: C15H25N3O8Si. Calcd: C, 44.65; H, 6.25; N, 10.41%. Found: C, 42.31; H, 6.12; N, 10.89%. 1H NMR (DMSO, 300 MHz): 0.50 (2H, t), 1.14 (9H, t) 1.67 (2H, m), 2.97 (2H, t), 3.50 (6H, q), 7.50 (1H, t), 7.92 (1H, t), 8.65 (1H, s), 12.70 (2H, s). The alkoxysilanemodified benzimidazole-5,6-dicarboxylic acid (bidc-Si) was prepared by using the similar procedure. C19H27N3O8Si. Calcd: C, 50.32; H, 6.00; N, 9.26%. Found: C, 49.91; H, 6.12; N, 9.76%. 1H NMR (DMSO, 300 MHz): 0.50 (2H, t), 1.14 (9H, t) 1.40 (2H, m), 2.90 (2H, t), 3.70(6H, q), 4.25 (1H, t), 7.40 (1H, s), 7.70 (1H, d), 8.30 (1H, s), 8.50 (1H, s), 12.80 (2H, s). Preparation and Template Removal of Mesoporous Silica Nanoparticles. High-quality mesoporous silica nanoparticles were synthesized according to the procedure that we recently reported14 with a slight modification. The following is a typical preparation for ∼80 nm sized particles: 64 mL of water, 9.0 g of ethanol, 10.4 g of CTAC solution (25 wt %), and 0.2 g of DEA were mixed and stirred in a water bath at 60 °C for 30 min. Then 7.3 mL of TEOS was added dropwise into the mixture within 15 min with stirring. The solution turned white gradually and was stirred for 2 h. Finally, the mixture was cooled to room temperature. The products were centrifugalized, washed with an excess amount of water and ethanol, and then dried under vacuum. The surfactants were extracted from the nanoparticles with acidified ethanol. As-synthesized materials were refluxed in a mixture solution of 120 mL of ethanol and 30 mL of hydrochloric acid (36-38%) for 4 to 8 h. The extraction step was repeated three times. The product was dried at 60 °C under vacuum.

Synthesis of Eu-idc-Si and Tb-bidc-Si Complexes Functionalized MSNs (Scheme 2). Idc-Si (270 mg) was dissolved in anhydrous toluene (20 mL) and with anhydrous ethanol (5 mL). The mixture was stirred under nitrogen for 10 min. Then mesoporous silica nanoparticles (400 mg) were added as solid. After about 30 min, 0.5 mol of Eu(NO3)3 was added. The reaction mixture was refluxed for 12 h. The collected solid was washed copiously with ethanol and distilled water to rinse away any surplus idc-Si and then dried under vacuum. The same synthesis method was used to prepare Tb-bidc-Si functionalized MSNs. Characterization. The particle morphologies and dimensions of the samples were determined by scanning electron microscopy (SEM) using a JEOL JSM-6700F operating at an accelerating voltage of 5 kV and by transmission electron microscopy (TEM) using a JEOL JEM-3010 transmission electron microscope operating at 300 kV. Powder X-ray diffraction (XRD) data were collected on a SIEMENS D5005 diffractometer with Cu KR radiation at 40 kV and 30 mA. Fourier transform infrared spectra (FTIR) were recorded with a JASCOFT/IR-420 spectrophotometer within the wavenumber range 4000-400 cm-1 at a resolution of 4 cm-1 using the KBr pressed pellet technique. The thermogravimetric and differential thermal analysis (TG-DTA) curves were recorded with a PerkinElmer Pyris Diamond TGA/DTA thermal analyzer at a temperature-increase rate of 10 °C min-1 under nitrogen atmosphere. Elemental analysis was performed on a Perkin-Elmer 2400 Series II CHNS/O elemental analyzer. The adsorption-desorption isotherms of nitrogen were measured at 77 K using a Micrometeritics TriStar 3000 system. The pore size distributions were calculated from the adsorption branches of N2 adsorptiondesorption isotherms on the basis of the Barrett-Joyner-Halenda (BJH) model. The emission and excitation spectra of the samples were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with both continuous (450 W) and pulsed xenon lamps. Lifetime measurement was performed with the same spectrophotometer and detectors using a Pulsed Hydrogen lamp (nF900, Edinburgh) with a pulse width of about 1.6 ns. Results and Discussion In recent years, mesoporous silica nanoparticles have been drawing much attention. Bein et al. reported a high-yield synthesis procedure to prepare mesoporous silica nanoparticles

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SCHEME 2: Synthesis Procedures to Immomolization of Lanthanide(III) Complexes on Mesoporous Silica Nanoparticles (the Proposed Scheme Here Represents Illustrative Examples of Possible Coordination)

with 50 to 100 nm sizes by addition of triethanolamine (TEA) as a complexing agent.12,13 Our recent study found that the uniform, monodisperse, and stable mesoporous silica nanoparticles can be prepared in mild synthesis conditions (pH 6-10). The particle size can be controlled from 25 to 200 nm by varying synthesis parameters and adding suitable additive agents including alcohols, amines, or inorganic bases.14 As a progression research, we constructed the novel lanthanide(III) complexes-

functionalized mesoporous silica nanoparticles as photoluminescent nanomaterials. The SEM and TEM images of MSNs and functionalized MSNs are shown in Figure 1. The microscopic morphologies of surfactant-extracted MSNs (panels a,d) and Eu-idc-Si (panels b,e) and Tb-bidc-Si (panels c,f) complexes functionalized MSNs are clearly observed in SEM and TEM images, and it can be seen that the obtained mesoporous silica nanoparticles are

Figure 1. SEM (a-c) and TEM (d-f) images of lanthanide(III) complexes-functionalized mesoporous silica nanoparticles. (a,d) MSNs, (b,e) Eu-idc-Si-MSNs, and (c,f) Tb-bidc-Si-MSNs.

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Figure 2. XRD patterns of surfactant-extracted MSNs (a), Eu-idc-SiMSNs (b), and Tb-bidc-Si-MSNs (c).

Figure 4. N2 adsorption-desorption isotherms of surfactant-extracted MSNs (a), Eu-idc-Si-MSNs (b), Tb-bidc-Si-MSNs (c).

Figure 3. IR spectra of as-synthesized MSNs (a), surfactant-extracted MSNs (b), Eu-idc-Si- MSNs (c), and Tb-bidc-Si-MSNs (d).

Figure 5. TG curves of Eu-idc-Si-MSNs and Tb-bidc-Si-MSNs.

uniform in size with a diameter around 80 nm. The load of lanthanide complexes does not nearly change the morphology and monodisperse characteristic of MSNs. Figure 2 shows the XRD patterns of the MSNs (panel a) and functionalized MSNs (panels b,c) samples. The results show that the disordered mesostructures of MSNs are maintained after the extraction of surfactant and the functionalization of lanthanide complexes. In contrast to the multiple sharp peaks of highly ordered MCM41 bulk materials, our samples only give one broad Bragg peak at low angles (2θ ) 1.9 to 2.0). XRD peaks become broadened or disappear with decreasing of particle size or disordering of mesostructure. The IR spectra of samples before and after extraction showed that the template was successfully removed for the CH stretching vibrations between 2830 and 2970 cm-1, and the C-H bending vibrations at 1470 cm-1 almost completely disappeared after extraction with ethanol (Figure 3a,b). Figure 3c,d shows the FTIR spectra of Eu-idc-Si and Tb-bidcSi functionalized MSNs, respectively. The peaks at 1696 and 1555 cm-1 in Figure 3c and 1665 and 1560 cm-1 in Figure 3d can be assigned to the CONH groups of idc-Si and bidc-Si. The νs(COO-) vibration at 1385 cm-1 in the FTIR spectra for the functionalized MSNs gives proof of the coordination of the carboxylic group to the metallic ion with the oxygen atoms.22,32 In order to investigate the pore structure of MSNs and Euidc-Si and Tb-bidc-Si functionalized MSNs materials, the measurement of the nitrogen adsorption-desorption isotherms was carried out. For the nitrogen adsorption-desorption at 77 K, the extraction and functionalized MSN samples give the typical type IV isotherms with a hysteresis (Figure 4). The specific area is calculated by the Brunauer-Emmett-Teller (BET) method and the pore size is calculated by BJH model. The BET surface areas are 913, 416, and 283 m2g-1, respec-

Figure 6. Photographs of the lanthanide(III) complexes-functionalized mesoporous silica nanoparticles under UV light irradiation (Tb-bidcSi-MSNs, green color, Eu-idc-Si-MSNs, red color).

tively, and pore volumes are 1.02, 0.46, and 0.46 cm3g-1, respectively. As expected, the surface area and pore volume of the lanthanide(III) complexes-functionalized mesoporous silica nanoparticles decrease considerably compared to MSNs samples, which is consistent with the presence of anchored organic ligands in the pore channels of MSNs. Thermogravimetric curves (Figure 5) showed two distinct weight loss stages. The weight loss below 100 °C was related to adsorbed water and weight loss above 200 °C was assigned to decomposition of organic species and water from the condensation of -SiOH groups. The weight losses for Eu-idc-Si and Tb-bidc-Si functionalized MSNs were 28.7 and 29.0 mass %, respectively, which agree with the adsorption capacity of the loaded organic ligands of these MSNs materials.

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Figure 7. Excitation and emission spectra for the Tb-bidc-Si-MSNs with λex ) 309 nm (a) and Eu-idc-Si-MSNs with λex ) 285 nm (b).

The hybrid nanomaterials Eu-idc-Si and Tb-bidc-Si functionalized MSNs shows a strong red and green photoluminescence upon irradiation with ultraviolet light, respectively (Figure 6). The excitation and emission spectra of Tb-bidc-Si and Euidc-Si functionalized MSNs materials are shown in Figure 7. The maximum absorption of Tb-bidc-Si and Eu-idc-Si is located around 309 and 285 nm, respectively. Upon excitation at 309 nm, the obtained Tb-bidc-Si functional MSNs materials give characteristic Tb3+ emission. The emissions at 491, 545, 584, and 621 nm of functionalized MSNs nanocomposites can be assigned to the transitions from the 5D4 level to the 7FJ (J ) 6, 5, 4, 3) levels, of which the 5D4f7F5 emission is the most prominent one. No emission from the ligand could be observed, which indicates that the surrounding aromatic bidc ligands absorb and transfer energy efficiently to central Tb3+ ion. In addition, the halfwidth of the strongest band is less than 10 nm, indicating that the functionalized MSNs exhibit high fluorescence intensity and color purity. Upon excitation at 285 nm, the Eu-idc-Si functional MSNs materials give characteristic Eu3+ emission, the five prominent emission peaks at 580, 598, 616, 650, and 698 nm in the emission spectra can be attributed to the 5D0f7FJ (J ) 4, 3, 2, 1, 0) transition with red emission for J ) 2 as the dominant feature. There is a very weak broadband emission (in the blue and green spectral regions) from the triplet state of the idc, which indicates that there is a nearly efficient energy transfer from the triplet states of the organic ligand to the central Eu3+ ion. The luminescence decay curves of Eu-idc-Si and Tb-bidc-Si functionalized MSNs are shown in Figure 8a,b. The curves obtained from time-resolved luminescence experiments could not fit to a single-exponential function, whereas a biexponential function yields a good fit

y ) A1 exp(-t/τf) + A2 exp(-t/τs) + y0 where A1 and A2 are the pre-exponential factors obtained from the curve fitting, and τf and τs stand for the lifetimes for the fast term and slow term, respectively. The values of τf and τs are 372 and 893 µs for Eu-idc-Si and 864 and 1450 µs for Tbbidc-Si. The luminescence decay curves indicate that the chemical environments of metal center ions in the materials are not uniform.39-41 The average lifetime (τav) can be calculated using the following equation

τav ) (A1τf2 + A2τs2)/(A1τf + A2τs) The average lifetime (τav) is 0.51 ms of Eu3+ ion for Eu-idcSi and 1.26 ms of Tb3+ ion for Tb-bidc-Si, respectively. A

Figure 8. Luminescence decay curve of the functionalized nanomaterials Eu-idc-Si-MSNs (a) and Tb-bidc-Si-MSNs (b).

similar biexponential decay was observed in periodic mesoporous organosilica containing a similar Tb3+ and Eu3+ chelate and is attributable to site-to-site heterogeneity in the solid state. It appears that the lifetime in hybrid nanomaterials is longer than that in other hybrid materials.41 Conclusions We introduced 4,5-imidazoledicarboxylic acid (H3idc) and benzimidazole-5,6- dicarboxylic acid (H3bidc)-lanthanide complexes to mesoporous silica particles with the diameters smaller than 100 nm and successfully constructed two novel uniform and monodisperse luminescent hybrid nanomaterials. The hybrid nanomaterials exhibit long fluorescence lifetimes and show a strong red and green photoluminescence upon irradiation with ultraviolet light, respectively. The colloidal stability of mesoporous silica nanoparticles in water and ethanol may provide some advantages for the potential applications of MSN-based luminescent nanomaterials in drug delivery or optical imaging.

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Acknowledgment. We greatly acknowledge financial support from the National Nature Science Foundation of China (Grant Nos. 20788101 and 20671041). References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem.Commun. 1993, 680. (4) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (5) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279. (6) Angelos, S.; Liong, M.; Choi, E.; Zink, J. I. Chem. Eng. J. 2008, 137, 4. (7) Slowing, I. I.; Vivero-Escoto, J. L.; Wu, C.-W.; Lin, V. S.-Y. AdV. Drug DeliVery ReV. 2008, 60, 1278. (8) Cheng, S.-H.; Lee, C.-H.; Yang, C.-S.; Tseng, F.-G.; Mou, C.-Y.; Lo, L.-W. J. Mater. Chem. 2009, 19, 1252. (9) Cai, Q.; Luo, Z. S.; Pang, W. Q.; Fan, Y. W.; Chen, X. H.; Cui, F. Z. Chem. Mater. 2001, 13, 258. (10) Rathousky, J.; Zukalova, M.; Kooyman, P. J.; Zukal, A. Colloids Surf., A 2004, 241, 81. (11) Nooney, R. I.; Thirunavukkarasu, D.; Chen, Y. M.; Josephs, R.; Ostafin, A. E. Chem. Mater. 2002, 14, 4721. (12) Moller, K.; Kobler, J.; Bein, T. AdV. Funct. Mater. 2007, 17, 605. (13) Kobler, J.; Moller, K.; Bein, T. ACS Nano 2008, 2, 791. (14) Qiao, Z. A.; Zhang, L.; Guo, M. Y.; Liu, Y. L.; Huo, Q. S. Chem. Mater. 2009, 21, 3823. (15) Tan, M. Q.; Ye, Z. Q.; Wang, G. L.; Yuan, J. L. Chem. Mater. 2004, 16, 2494. (16) G. Bu¨nzli, J.-C.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048. (17) Escribano, P.; Julia´n-Lo´pez, B.; Planelles-Arago´, J.; Cordoncillo, E.; Viana, B.; Sanchez, C. J. Mater. Chem. 2008, 18, 23. (18) Franville, A.-C.; Zambon, D.; Troin, R. M. Y. Chem. Mater. 2000, 12, 428. (19) Lu, L. H.; Liu, F. Y.; Sun, G. Y.; Zhang, H. J.; Xi, S. Q.; Wang, H. S. J. Mater. Chem. 2004, 14, 2760.

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