Synthesis and Characterization of Photoluminescent Eu(III

Aug 12, 2009 - Encapsulating a Ternary Europium Complex in a Silica/Polymer Hybrid Matrix for High Performance Luminescence Application. Xiaoguang Hua...
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Synthesis and Characterization of Photoluminescent Eu(III) Coordination Halloysite Nanotube-Based Nanohybrids Chaoying Wan, Ming Li, Xin Bai, and Yong Zhang* State Key Laboratory of Metal Matrix Composites, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, China ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: July 21, 2009

Multicarboxyl polymer-functionalized halloysite nanotubes (HNTs) were synthesized by controlled surfaceinitiated atom transfer radical polymerization (SI-ATRP) of methyl methacrylate (MMA) and/or hydroxyethyl methacrylate (HEMA) to form HNT-(PMMA-b-PHEMA) or HNT-PHEMA hybrids, followed by esterification of the hydroxyl groups on the PHEMA blocks using excessive succinic anhydride in pyridine. The obtained products, HNT-(PMMA-b-PSEMA) or HNT-PSEMA with multicarboxyl groups on the external layers, were coordinated with triplet europium ions (Eu3+) in the presence of 1,10-phenanthroline (phen). For comparison purposes, PSEMA and PMMA-b-PSEMA copolymer were also synthesized via the same procedure and used as macromolecular ligands for coordination with Eu3+. The microstructure and fluorescence properties of the four kinds of Eu3+ coordination complexes were characterized via FTIR, FESEM, EDS, 1H NMR, TGA, and fluorescence spectroscopy. The conversion of hydroxyl groups to carboxyl groups during the esterification reactions was up to 100% for the synthesized polymers and modified HNT-based hybrids. The Eu3+ coordination HNT-based hybrids exhibited efficient narrow bandwidth emission of red light with high spectral purity when excited at 266 nm. The HNT-(PMMA-b-PSEMA)-Eu complex even gave 1.63 and 1.85 times higher emission intensities than those of HNT-PSEMA-Eu and (PMMA-b-PSEMA)-Eu complexes, respectively. The improved luminescence properties of both of the Eu3+ coordination HNT-based hybrids are attributable to the efficient intramolecular energy transfer from the hybrid ligands and phen to Eu3+ ions, and the rigid HNTs framework also plays a positive role in the enhancement of the emission intensity. Such photoluminescent Eu3+ coordination HNT-based hybrids are expected for spectroscopy probes, fluorescent plastics, and fluoroimmunoassay applications. Introduction Rare-earth complexes are important luminescent materials besides semiconductive quantum dots, organic dyes, and fluorescent proteins, and have received intensive attention in developing photoactive materials for biological applications.1-3 Compared with conventional luminescent materials, lanthanide complexes have high photoluminescence efficiency along with a narrow emission spectrum, large Stokes shifts, and long fluorescent lifetime, which make them useful alternatives to radioactive probes and organic dyes.4 In particular, lanthanides are nontoxic for humans,5 and the related coordination polymers are effective catalysts and antimicrobial agents.6 It is well-established that rare-earth complexes are characterized by highly efficient intraenergy conversion from the ligand singlet (S1) to the triplet (T1), and hence to the excited state of the central rare-earth metal ions.7 The metal ions exhibit sharp spectral bands corresponding to 5Dx-7Fx transitions. Since both singlet and triplet excitations are involved in the luminescence process, high rare-earth metal ion excitation efficiency is expected by optimizing energy transfer in solid state systems where the ions are doped into an appropriate molecular host matrix.3,4,8 Lanthanides doped inorganic or organic materials have been fabricated via sol-gel chemistry or coordination reactions.8-14 The direct inclusion or in situ formation of chelated lanthanides * To whom correspondence should be addressed. Phone: +86 21 5474 3257. Fax: +86 21 5474 1297. E-mail: [email protected].

inside sol-gel-derived matrices leads to the formation of hybrid materials with generally improved luminescent properties.9-11 For example, the condensation of trialkoxysiyl-functionalized chromophores in the presence of lanthanides leads to the formation of nanostructured hybrids with strong fluorescence emission even at low lanthanide ion concentration.11 Functionalized carbon nanotubes (CNTs) are found to be able to elongate fluorescence lifetime of the rare-earth complex, then the CNTs hosted lanthanide complexes have found important applications in drug carrier and biomarker fields.15-17 Clay minerals are attractive inorganic hosts for functional inorganic-organic hybrids because of their swelling properties and charged surfaces which could be easily modified. Clays have been successfully used as vectors for delivery of DNA to cells.18,19 Clay-modified electrodes have been developed as electrochemical sensors.20 Functionalized clays with luminescent properties also find their way in photoelectricity and fluorescence probe applications.21-23 Tetsuka et al.21 incorporated quantum dots into clay hosts and prepared flexible CdSe/ZnS-clay films with high photoluminescence efficiency that are promising materials for optoelectronic devices such as light emitting devices. Celedon et al.22 intercalated complexes of triplet europium ions (Eu3+) and terbium ions (Tb3+) with 2,2bipyridine and 1,10-phenanthroline (phen) into natural bentonite, and found the luminescence intensity of the intercalated products was comparable to that of the free complexes and the stability of the complex against hydrolysis was improved. Lezhnina et

10.1021/jp9051648 CCC: $40.75  2009 American Chemical Society Published on Web 08/12/2009

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SCHEME 1: Illustration of the Synthesis Process of HNT-Br

al.23 investigated the effects of iron impurities on the luminescence properties of rare-earth doped clay hybrids. As naturally abundant clay, halloysite nanotubes (HNTs, Al2Si2O5(OH)4 · 2H2O) have a tubular structure quite similar to that of MWNTs, but they are cheap, toxin-free, and biocompatible. The tubular structure of HNTs composes one silica sheet and one alumina sheet on each silicate layer, with a diameter of 50 nm, a lumen of 15 nm, and a length of 500-1000 nm.24 The hollow tubular structure and charged surface of HNTs make them especially suited for loading/release materials. So far, HNTs have been studied for potential biomedical applications including drug controlled release systems,25 nanoreactors for enzymes,26 and stem cell attachments and proliferation.27 In engineering fields, HNTs have been applied to immobilize corrosion inhibitors28 for active corrosion protection coatings, or compounded with polymers to improve their mechanical properties29 and flame retardancy.30 HNTs are able to be modified via noncovalent or covalent approaches. Covalent functionalization via atom transfer radical polymerization (ATRP) through “grafting from” or “grafting to” approaches is a promising method for tailoring the surface properties of nanoparticles.31,32 ATRP allows for the polymerization of most vinyl monomers in a controlled/“living” manner onto the nanoparticles surface.31 Polymer brushes have been successfully grafted onto the surface of montmorillonite and palygorskite clays via surface-initiated polymerization (SIP) as reviewed by Liu.32 Since the structure of ligand plays an important role in the luminescence performance of rare-earth ions, in this paper, two multicarboxyl polymer-functionalized HNT hybrids were synthesized for coordination with Eu3+ to elucidate the effect of the structures of macromolecular ligands on the luminescence performance of Eu3+. The designing idea originated from the following two aspects. First, the carboxylic acids or polyacids are most often used as ligands for luminescent rare-earth complexes because of their strong coordination ability and good thermal stability.4,5 Second, carboxylic polymers are important candidates in biomaterial synthesis.33,34 By combining the two aspects, in this paper, poly(hydroxyethyl methacrylate) (PHEMA) and poly(methyl methacrylate)block-poly(hydroxyethyl methacrylate) (PMMT-b-PHEMA) brushes were grafted from HNT surfaces via the SI-ATRP approach. The esterification of the two products produced multicarboxyl polymer-functionalized HNT hybrids, which were potential macromolecular ligands for Eu3+ coordination. The microstructure and fluorescence properties of the Eu3+ coordination HNT-based hybrids were investigated in detail. The corresponding polymeric Eu3+ complexes from PHEMA or PMMA-b-PHMEA were investigated as well to illustrate the effect of HNTs framework on the fluorescent properties of the complexes. The photoluminescent Eu3+ coordination HNT-based hybrids with enhanced fluorescence properties open the door to a new exciting approach to fluorescent nanohybrids. Experimental Section Materials. Halloysite nanotubes (HNTs) were obtained from NaturalNano, Inc., USA. Hydroxyethyl methacrylate (HEMA)

was purchased from Tokyo Chemical Industry Co. Ltd., Japan, and the inhibitor was removed by passing a column of alumina and distilling in vacuum. Methyl methacrylate (MMA) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. It was washed by 5% NaOH solution several times and dried over calcium hydride overnight, and then distilled in vacuum at 50 °C. Copper bromide (CuBr, 98%) was washed with acetic acid, followed by methanol to remove impurities. 2,2′-Bipyridine (bpy) was recrystallized several times from acetone and dried in vacuum before use. Anhydrous methanol (MeOH, 99.8%), anhydrous ethanol (99%), toluene (99%), anisole (99%), pyridine (99%), chloroform (99%), dimethylformamide (DMF, 99%), and diethyl ether (99%) were stored in the presence of 4 Å molecular sieves and used without further purification. Aluminum oxide (neutral, 200-300 mesh) and succinic anhydride (SA) were used as received. All these chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd., China. 2-Bromoisobutyryl bromide, tert-butyl R-bromoisobutyrate, triethylamine, and N,N-dimethylaminopyridine (DMAP) were purchased from Aldrich and used as received. γ-Aminopropyltrimethoxysilane (APTES, Dow Corning Co. Ltd., China) was used as received. Synthesis of HNT-Br Initiator. Vacuum-dried HNTs were treated with APTES in dry toluene at 80 °C for 5 h, then the resulting HNT-NH2 was washed and dried in vacuum at 60 °C overnight. The dried HNT-NH2 (1 g) was dispersed in chloroform (30 mL) and degassed through dry Argon. Triethylamine (0.54 g, 5.3 mmol) and DMAP (0.24 g, 2.1 mmol) were added into the degassed solution, and then 2-bromoisobutyryl bromide (0.49 g, 2.1 mmol) with chloroform (4 mL) was added dropwise into the solution via syringe at 0 °C. The synthesis progress was shown in Scheme 1. The mixture was stirred at 0 °C for 5 h and at room temperature for 48 h. The resulting HNT-Br was centrifuged and washed several times, and then dried overnight in vacuum at 50 °C. ATRP of PHEMA and PMMA-b-PHEMA Block Copolymer. A solution of HEMA (3 mL) in methanol was mixed with tert-butyl R-bromoisobutyrate (55.1 mg, 0.247 mmol), bpy (96.4 mg, 0.617 mmol), and CuBr (36.1 mg, 0.247 mmol) under dry argon. The reaction was conducted at 20 °C for 6 h. The resulting product was diluted with methanol and passed through a column of alumina to remove the catalysts. The obtained solution was precipitated into excessive diethyl ether and then filtered and vacuum-dried. The dissolving-precipitation-drying process was repeated several times and white PHEMA powders were obtained. MMA (0.5 g) was dissolved in 0.25 mL of DMF and degassed by bubbling through dry argon for 15 min, then the tert-butyl R-bromoisobutyrate (11.2 mg, 0.05 mmol), bpy (19.5 mg, 0.125 mmol), and CuBr (7.31 mg, 0.05 mmol) were added subsequently under dry argon. The reaction was conducted at 60 °C for 10 h. The resulting product was diluted with DMF and passed through a column of alumina to remove the catalysts. Then the obtained solution was precipitated into excessive

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SCHEME 2: Illustration of the Synthesis and Coordination Process of Eu3+ Coordination Functionalized HNT-Based Hybrid Complexes

methanol. The dissolving-precipitation-drying process was repeated several times before PMMA macroinitiator was obtained. The PMMA macroinitiator (0.3 g) was dissolved in DMF and mixed with HEMA (1 g, 7.69 mmol), bpy (19.5 mg, 0.125 mmol), and CuBr (7.31 mg, 0.05 mmol) under dry argon. The reaction was conducted at 60 °C for 12 h. The resulting product was diluted with DMF and passed through a column of alumina to remove the catalysts. Then the obtained solution was precipitated into excess diethyl ether. The dissolving-precipitation-drying process was repeated several times before PMMAb-PHEMA was obtained. SI-ATRP of HNT-PSEMA and HNT-(PMMA-b-PSEMA). HNT-PHEMA was first synthesized by ATRP of HEMA initiated with HNT-Br in the presence of the CuBr/bpy catalyst according to Scheme 2. Typically, HNT-Br (50 mg) was dispersed in 3 mL of methanol and degassed by bubbling through dry argon, then HEMA (250 mg, 1.92 mmol) and CuBr/ bpy (11.24/30 mg) were added subsequently under argon. The reaction was conducted at 20 °C for 48 h. Then the resulting mixture was diluted with methanol and centrifuged. The obtained solid was redispersed in methanol and precipitated into diethyl ether. This treatment was repeated several times to remove the absorbed catalyst and free polymer. The supernate of the final solution obtained by centrifugation was dropped into excess diethyl ether, and no precipitate was observed indicating that free PHEMA was removed after the complete extraction. The sample was dried in vacuum as HNT-PHEMA. The HNT-PHEMA hybrid with different HNTs/HEMA ratios (1:1 w/w) was synthesized under the same condition. HNT-PMMA was synthesized by ATRP of MMA (250 mg, 2.5 mmol) with HNT-Br (50 mg) in DMF in the presence of CuBr/bpy (14.6/39 mg) catalyst according to Scheme 2. The resulting HNT-PMMA was washed with DMF several times

and centrifuged. The HNT-PMMA (25 mg) was mixed with HEMA (125 mg) and CuBr/bpy (7.3/19.5 mg) catalyst in DMF, then the SI-ATRP reaction was conducted at 60 °C for 20 h under argon. The resulting HNT-(PMMA-b-PHEMA) was washed with DMF and precipitated in diethyl ether several times. The esterification reactions of HNT-PHEMA, HNT-(PMMAb-PHEMA) hybrids, PHEMA, and PMMA-b-PHEMA copolymer were conducted by using 2 mol equiv of SA per HEMA. Taking the esterification reaction of HNT-PHEMA as an example, HNT-PHEMA (100 mg, 0.52 mmol of -OH) and SA (104 mg) were dispersed in dry pyridine and reacted at room temperature for 24 h. The resulting mixture was washed with pyridine and precipitated into excess diethyl ether several times. The final sample was obtained by centrifugation and dried under vacuum at 60 °C for 5 h. Such prepared HNT-PSEMA and HNT-(PMMA-b-PSEMA) were dissolved in CD3OD and DMSOd6, respectively, for 1H NMR analysis. The above synthesis process is shown in Scheme 2. Coordination Reactions. The molar ratio of [Eu3+:macromolecular ligand:phen] is 1:3:1. Typically, HNT-PSEMA (20 mg, 0.069 mmol of COOH) was dispersed in 5 mL of anhydrous ethanol, next 0.9 mL of phen solution was added dropwise and the pH value of the mixed solution was adjusted to 6-7 with NaOH aqueous solution. Then 0.9 mL of Eu(NO3)3 ethanol solution was dropped slowly into the mixed solution within 10 min. The reaction mixture was stirred for 5 h at room temperature, followed by aging overnight. The resulting product was washed with ethanol repeatedly, and then dried under vacuum at 50 °C. Measurements. 1H NMR spectra were measured in CDCl3 (PMMA), CD3OD (PHEMA and PSEMA), or DMSO-d6 (PMMA-b-PHEMA and PMMA-b-PSEMA) on a MERCURYplus 400 NMR spectrometer. Fourier transform infrared (FTIR) spectra were recorded on a PE Paragon 1000 spectrometer, using

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Figure 2. GPC traces of (a) PMMA macroinitiator and (b) PMMAb-PHEMA copolymer.

Figure 1. 1H NMR spectra recorded for (a) PMMA in CDCl3, (b) PHEMA in CD3OD, and (c) PMMA-b-PHEMA copolymer in DMSO-d6.

the KBr disk method. Luminescence spectra were recorded in the 500-700 nm range on a Perkin-Elmer LS50B luminescence spectrometer. Molecular weights and polydispersities of homopolymers were measured by gel permeation chromatography (GPC) at 70 °C, using PE Series 200 and an RI detector. The GPC eluent was HPLC grade DMF containing 0.01 M LiBr with the flow rate of 1 mL/min. Calibration was carried out by using polystyrene standards. Thermogravimetric analysis (TGA) was conducted on a PE TGA-7 instrument at a heating rate of 20 deg min-1 under nitrogen. The surface of samples was observed and detected by field emission scanning electron microscopy (FESEM, S-4100, JEOL), using an Oxford Energy Dispersive spectrometer (EDS). Results and Discussion Synthesis of PHEMA and PMMA-b-PHEMA Block Copolymer. Polymerization reactions of MMA and HEMA in bulk were performed at 60 and 20 °C, respectively. The resulting PMMA was used as a macroinitiator for copolymerization with HEMA. The obtained PMMA, PHEMA, and PMMA-b-PHEMA copolymer were characterized by 1H NMR and GPC (Figures 1 and 2). As shown in Figure 1, the chemical shifts of protons on PMMA are described as follows: 1H NMR (CDCl3) (ppm) δ 3.56 (-OCO-CH3), 1.77 (C-CH2-C), 0.75-0.92 (-CH3). For PHEMA: 1H NMR (CD3OD) (ppm) δ 4.83 (-OH), 3.89 (-CH2-OCO), 3.58 (-CH2-OH), 2.1-1.1 (-CH2-C), 1.3-0.7 ppm (-CH3). For the PMMA-b-PHEMA copolymer: 1H NMR (DMSO-d6) (ppm) δ 4.79 (-OH), 3.88 (COOCH2-CH2OH), 3.56 (-OCH2-CH2OH, COOCH3, overlapped), 1.77 (C-CH2-C), 0.75-0.92 (-CH3). The above results prove the

formation of PMMA-b-PHEMA copolymer via ATRP from the PMMA-Br. The ratio of polymerization degree of PMMA and PHEMA block in the PMMA-b-PHEMA copolymer is 1:5 as calculated according to the integral area of the peaks at 3.56 and 3.88 ppm in Figure 1c. The GPC results are shown as follows. PMMA: Mn ) 33 660, Mw/Mn ) 1.18. PHEMA: Mn ) 58 960, Mw/Mn ) 1.25. PMMA-b-PHEMA copolymer: Mn ) 140 900, Mw/Mn ) 1.59. The unimodal bell shape GPC curves demonstrates the purity of the synthesized copolymer. The shift of the retention time to a lower value indicates an increase of the molecular weight from PMMA to PMMA-b-PHEMA. Since the PHEMA and PMMA-b-PHEMA have multihydroxyl groups on the molecular chains, they can be facilely esterfied with SA to give multicarboxyl polymers which are good candidates for coordination with metal ions or act as templates for nanoparticle growth.35,36 The 1H NMR spectra of the esterification products of PHEMA and PMMA-b-PHEMA were compared and shown in Figure 3. As shown in Figure 3a, the attachment of succinic acid groups to PHEMA leads to the appearance of two new peaks at 4.2 and 4.4 ppm due to the shifts of the positions of the pendent ethylene protons. A new peak at 2.7 ppm is assigned to the methylene protons of the succinic groups. The disappearance of the ethylene peaks of the hydroxylethyl group at 4.1 and 3.6 ppm and the emergence of the doubles of 4.2 and 4.4 ppm for the ethylene group of PSEMA suggest the complete cinnamation of PHEMA. In Figure 3b, the esterification reaction of PMMAb-PHEMA with SA leads to the appearance of two new peaks at 4.2 and 4.4 ppm and the disappearance of the hydroxyl group at 4.79 ppm. Meanwhile, the integral area of the peak at 3.55 ppm decreases a lot, which indicates the environment of ethylene groups of PMMA-b-PHEMA changes after reaction with SA. The hydroxyl groups of PHEMA and PMMA-b-PHMEA are modified into carboxyl groups according to the above 1H NMR analysis. SI-ATRP of HNT-PSEMA and HNT-PMMA-b-PSEMA. The SI-ATRP reactions on HNTs surface were performed under the same procedures as those of the bulk polymer synthesis. The grafting quantities of initiators and polymers on HNTs were investigated by TGA (Figure 4). As shown in Figure 4a, in the temperature range of 100-600 °C, the total weight loss of raw HNTs, HNT-NH2, and HNT-Br is 13.4%, 16.5%, and 19.8%, respectively, indicating the organosilane APTES and the initiator were subsequently grafted onto the HNTs surface. The

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Figure 4. TGA results of (a) HNT grafted with PHEMA and (b) HNT grafted with PMMA-b-PHEMA hybrids.

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Figure 3. H NMR spectra for (a) PHEMA and PSEMA in CD3OD and (b) PMMA-b-PHEMA and PMMA-b-PSEMA in DMSO-d6.

grafting content of amino groups in the HNT-NH2 is 0.53 mmol/g of HNT-NH2 or 0.63 mmol/g of neat HNTs. The grafting content of the initiator in the HNT-Br is 0.23 mmol/g of HNT-Br or 0.29 mmol/g of neat HNTs. Therefore, about 46% of HNT-NH2 converted into HNT-Br after the amidation reaction. During the SI-ATRP process, the HNT-Br/HEMA feed ratio was changed from 1:1 to 1:5 in order to synthesize different HNT-PHEMA hybrids with different concentrations of hydroxyl groups. The total weight loss of the HNT-PHEMA increased from 28.6% to 73.9% in the temperature range of 100-600 °C. As calculated, the grafting content of HEMA (or -OH) is 0.84 mmol/g of HNT-PHEMA or 1.2 mmol/g of neat HNTs for the HNT-PHEMA (1:1), while it is 5.2 mmol/g of HNT-PHEMA or 19.9 mmol/g of neat HNTs for the HNT-PHEMA (1:5). This means the grafting amount of PHEMA on HNTs surface could be controlled by tailoring the feed ratio of HNT-Br and the monomer. After the esterification reactions, the total weight loss of HNT-PHEMA (1:5) was further increased from 73.9% to 83%, which is just explained by the grafting of SA chains to the PHEMA chains through the esterification reactions. The reacted SA content is 3.5 mmol/g of HNT-PSEMA or 20.5 mmol/g of neat HNTs. Therefore, the reaction conversion of

the HNT-PHEMA to HNT-PSEMA could be calculated as the carboxyl group content (20.5 mmol/g) divided by the hydroxyl group content (19.9 mmol/g), that is, about 100% conversion is achieved though minute deviation could arise from the TGA testing. Meanwhile, the appearance of HNT-PHEMA hybrid changed from white powder to gray elastomeric particles, which reflects that the surface properties of the HNT-PHEMA were modified due to the conversion of hydroxyl groups to carboxyl groups. The TGA results of HNT-PMMA, HNT-(PMMA-b-PHEMA), and HNT-(PMMA-b-PSEMA) are shown in Figure 4b. The total weight loss of HNT-PMMA and HNT-(PMMA-bPHEMA) is 39.1% and 58.9%, respectively, in the temperature range of 100-600 °C. The relatively low grafting contents of HEMA to HNT-PMMA might be due to the low initiator concentration and steric hindrance of HNT-PMMA initiators. The total weight loss of HNT-(PMMA-b-PSEMA) is 66.7%. As calculated, the grafting amount of HEMA (or -OH) is 6.1 mmol/g of net HNTs, and the grafting amount of SA (or -COOH) is 5.7 mmol/g of neat HNTs; therefore, the esterification conversion is about 93.4%. Characterization of the Synthesized Eu3+ Coordination Polymer and HNT-Based Hybrid Complexes. Figure 5 shows the FTIR spectra of the Eu3+ coordination polymer complexes and HNT-based hybrid complexes; the FTIR spectrum of phen is also included for comparison. In Figure 5a, the four samples exhibit a broad and strong absorption band at around 3449 cm-1 ascribed to Vs(-OH) and an absorption band at 1734 cm-1

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Figure 5. FTIR spectra of (a) Eu3+ coordination polymer complexes and (b) Eu3+ coordination HNT-based hybrid complexes.

ascribed to Vs(CdO). This confirms that carboxyl groups (COO-) exist in the four polymers and form hydrogen bondings.37 Comparing the FTIR spectrum of PSEMA with PSEMA-Eu, two new sharp peaks at 1571 and 1446 cm-1 appear, which should arise from the coordination reaction. Analyzing the structure of the coordination complex (Eu3+: PSEMA:phen 1:3:1 mol ratio), a coordination mechanism is proposed as follows. First, the COO- releases hydrogen proton and forms an OCO conjugated structure with homogenized bonding energy. Meanwhile, the lone electron pair of the oxygen atom in COO- transfers to the empty electron orbit of the Eu3+ ion, thus further weakening the covalent bonding of CO. As a result, the two new peaks at 1571 and 1446 cm-1 are different from the vibrations of CdO and CsO, and they are ascribed to the asymmetric and symmetric vibrations of carboxyl groups,

Vas(OCO) and Vs(OCO). This suggests that the oxygen atom of COO- coordinates with the Eu3+ ion.38 The difference between the peak positions of Vas(OCO) and Vs(OCO) is 125 (less than 200), indicating the COO- and Eu3+ form a bidentate complex in the PSEMA-Eu complex.39 Similar results are found in the spectra of (PMMA-b-PSEMA) and (PMMA-b-PSEMA)-Eu complexes. Besides, comparing the spectrum of phen and (PMMA-b-PSEMA)-Eu complex, the benzene ring of phen shifts from 1416 cm-1 to 1384 cm-1 obviously, as shown in the spectrum of the (PMMA-b-PSEMA)-Eu complex in Figure 5a, which suggests that phen participates in the coordination reaction. The intensities of the characteristic absorption bands of Vas(OCO) and Vs(OCO) in (PMMA-b-PSEMA)-Eu are relatively weak as compared to those in PSEMA-Eu complex

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Figure 6. Emission spectra of Eu3+ coordination polymer complexes (all the emission spectra were recorded with an excitation wavelength of 266 nm); the inserted photograph is the luminescence of HNT-(PMMA-b-PSEMA)-Eu hybrid complex under UV light.

because of the lower concentration of COO- in PMMA-bPSEMA than that in the PSEMA bulk. In Figure 5b, the spectrum of HNT-Br exhibits two Al-OH stretching vibrations at 3698 and 3626 cm-1, respectively. The bending vibration of Al-OH is observed at 912 cm-1 and the bands at 1035 and 790 cm-1 are attributed to Si-O-Si stretching vibrations.40 The absorption band at 1651 cm-1 belonged to Vs(NHCdO) formed in the amidation reaction between HNT-NH2 and 2-bromoisobutyryl bromide. Comparing the spectrum of HNT-PSEMA-Eu with that of HNT-PSEMA, two new peaks at 1571 and 1446 cm-1 are ascribed to Vas(OCO) and Vs(OCO), respectively. This is quite similar to the phenomena observed in Figure 5a. The appearance of the two absorption bands indicates the coordination reaction between COO- and Eu3+ occurred. It should be mentioned that the stretching vibration of -CH3 is at 1443 cm-1, so the band around 1443-1448 cm-1 should be the overlapping of both Vs(OCO) and Vs(-CH3), which is also observed in Figure 5a. For the HNT-based hybrids, the spectrum of HNT-(PMMAb-PSEMA) exhibits Vs(NHCdO) at 1651 cm-1 and Vs(-CH3) at 1443 cm-1 as well. After coordination with Eu3+ and phen, a new absorption band at 1571 cm-1 ascribed to Vas(OCO) appears, and the other Vs(OCO) at 1446 cm-1 is possibly overlapped with Vs(-CH3). Furthermore, the absorption bands at 750-650 cm-1 attributed to Vf(O-H) shift left and change their shapes, which may be due to the chelating of the COOwith Eu3+. From the above analysis, the coordination of Eu3+ with multicarboxyl polymers or the functionalized HNT-based hybrids is successfully achieved. Photoluminescence Properties. The fluorescence spectra of four kinds of Eu3+ coordination complexes are shown in Figure 6. Typical red luminescence lines of the Eu3+ ions at 594 and 618 nm are observed at the excitation wavelength of 266 nm for the four materials, which are attributed to 5D0 f 7F1 and 5 D0 f 7F2 transitions, respectively. Basically the 5D0 f 7F1 transition is due to the magnetic dipole transition and insensitive to their local structure environment, while the red band related to 5D0 f 7F2 is due to electric-dipolar transitions and sensitive to the coordination environment of the Eu3+ ions.4,7 The stronger emission peak at 618 nm than that at 594 nm means the 5D0 f 7F2 line dominates above the 5D0 f 7F1 one and the coordination environment of the central Eu3+ ions is in low

symmetry.41 When the multicarboxyl groups in the PSEMA or PMMA-b-PSEMA contact Eu3+, the lone electron pair in the oxygen atom of COO- of PSEMA or PMMA-b-PSEMA transfers to the empty electron orbit of the external layer of Eu3+ ion and forms a conjugate chelate ring. Such a structure has enlarged conjugated delocalization and easily absorbs UV energy. That is, the PSEMA or PMMA-b-PSEMA absorbs UV light and acts as an “antenna” to transfer energy from the polymeric chelates (S1 or T1) to the excited 4f levels of the Eu3+ ions. As a second ligand, phen is added in order to enhance the luminescent properties of Eu3+. Phen has a big conjugate π-bond and rigid plane structure and is easy to excite into the triplet state. The triplet state of phen (22 940 cm-1) is higher than the 5 D0 (17 250 cm-1) and 5D1 (19 020 cm-1) states of Eu3+ ions,42 thus it can efficiently transfer the UV light absorbed by itself and by the carboxyl groups of the macromolecular ligands to the central Eu3+ ions via resonance vibrations. On the other hand, the inclusion of phen has the chance to decrease the crosslinking extent of Eu3+ ions on the synthesized (co)polymers, thus avoiding the fluorescence quenching caused by the aggregation of Eu3+ ions. Intensification of the 5D0 f 7F2 emission band is observed not only for the Eu3+ coordination polymer complexes, but also especially for Eu3+ coordination HNT-based hybrid complexes. As shown in Figure 7a, the HNTs are hollow silicate tubes with nanosized diameter. Ferrum ions are not detected from the HNTs surface by EDS, which is beneficial for them to be templates for rare earth coordination.23 After SI-ATRP and coordination reactions, the surfaces of HNTs are covered by thick polymer layers, and the Eu3+ ions are detected on the surface of the complexes as shown in Figure 7b. The luminescence intensity of coordination complexes is related to the intensity of the ligand absorption, the efficiency of the ligand-to-metal energy transfer, and the efficiency of the metal luminescence.43 In the Eu3+ coordination HNT-based hybrid complexes, the energy transfer would be a three-dimensional process. The nanotubular HNTs provide a rigid framework for the covalent-bonded Eu3+ coordination polymer complexes, and inhibit aggregation of the polymer complexes at the same time (see the FESEM image inserted in Figure 7b), which accordingly dilute the Eu3+ ions in the complexes, and prevent the fluorescence concentration

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Figure 7. Morphology and EDS results of (a) raw HNTs and (b) HNT-(PMMA-b-PSEMA)-Eu hybrid complex.

SCHEME 3: Illustration of the Synthesis and Coordination Process of HNT-(PMMA-b-PSEMA)-Eu Hybrid Complexes

quenching. On the other hand, the existence of the HNTs framework enhances the structural rigidity of the complexes, which is helpful in reducing the energy dissipation from the nonradiative transition in the complexes. As a result, the HNTs framework covalent bonding with polymer complexes acts as an “antenna” and efficiently sensitizes the characteristic fluorescence of Eu3+ ions. As shown in Figure 6, the emission intensity of HNTPSEMA-Eu complex is 1.60 times higher than that of PSEMAEu complex, and it is 1.85 times higher for HNT-(PMMA-bPSEMA)-Eu complex than that for (PMMA-b-PSEMA)-Eu complex. The existence of PMMA blocks in the (PMMA-bPSEMA)-Eu and HNT-(PMMA-b-PSEMA)-Eu complexes dilutes the local concentration of Eu3+ ions in the complex as well, which explains the higher emission intensity values of the block copolymer and the copolymer modified HNT-based hybrid complexes than those of the corresponding PSEMA-Eu and HNT-PSEMA-Eu complexes. The HNT-(PMMA-b-PSEMA)Eu complex exhibiting strong red fluorescence under UV light is shown in the inserted photograph in Figure 6. The synthesis and coordination process are illustrated in Scheme 3. The HNTs are grafted with PMMA and PHEMA subsequently to give mutihydroxyl outlayers. Then the multihydroxyl groups on the external layers are modified into multicarboxyl groups through the esterification reaction with SA. The resulting multicarboxyl polymer-functionalized HNTs are used as templates or macromolecular ligands to chelate Eu3+ with phen. Since the mole ratio of [Eu3+:carboxyl groups:phen] is 1:3:1, the Eu3+ ions exist in asymmetric position in the resulting complexes. Three carboxyl groups participating in the coordination reactions may come from a single or more branched chains of the grafted polymer brushes. Conclusion Multicarboxyl PSEMA and PMMA-b-PSEMA grafted HNT hybrids were synthesized by employing the SI-ATRP method

and used as macromolecular ligands to coordinate with Eu3+ and phen. Typical red fluorescence of Eu3+ (5D0 f 7F1 and 5D0 f 7F2 transitions) of the complexes was observed at 594 and 618 nm, respectively, with the excitation wavelength of 266 nm. The carboxylic polymers absorbed UV light and transferred energy to the central Eu3+ ions like “antennas”, and this effect could be enhanced by phen. The emission intensities of the HNT-based hybrid complexes were even higher than those of the Eu3+ coordination polymer complexes, which indicates the HNTs framework plays a positive role in enhancing energy transfer efficiency to Eu3+ ions. The approach proposed in this paper provides an alternative way to synthesize luminescent rare-earth hybrid complexes, which takes advantage of the surface-initiated controlled/“living” radical technique to provide versatile functional surface of the nanoparticles. In this paper, multicarboxyl polymer-functionalized HNTs hybrids are obtained with different polymer brush thickness surrounded by various concentrations of carboxyl groups, which provide diversified macromolecular ligands for lanthanide coordination. The photoluminescent HNT-based hybrids are expected to be applied in fluorescent probes, biomarkers, sensors, and fluorescent plastics. Acknowledgment. Financial support from the National Natural Science Foundation of China (project nos. 50803035 and 50773036) and Shanghai Leading Academic Discipline Project (project no. B202) is gratefully acknowledged. References and Notes (1) Li, M.; Selvin, P. R. J. Am. Chem. Soc. 1995, 117, 8132–8138. (2) Yuan, J.; Wang, G.; Majima, K.; Matsumoto, K. Anal. Chem. 2001, 73, 1869–1876. (3) Qiu, G. M.; Xu, Y. Y.; Zhu, B. K.; Qiu, G. L. Biomacromolecules 2005, 6, 1041–1047. (4) Sinha, S. P. Complexes of The Rare Earth; Pergamon: London, UK, 1966. (5) Evans, C. H. Biochemistry of Lanthanides; 8th ed.; Plenum Press: New York, 1992. (6) Kapadia, M. A.; Patel, M. M.; Joshi, J. D. Inorg. Chim. Acta 2009, 362, 3292–3298. (7) Blasse, G.; Grabmeier, B. C. Luminescent Materials; SpringerVerlag: Berlin, Germany, 1994. (8) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J. P. AdV. Mater. 2003, 15, 1969–1994. (9) Kłonkowski, A. M.; Grobelna, B.; But, S.; Lis, S. J. Non-Cryst. Solids 2006, 352, 2213–2219. (10) Stone, B. T.; Costa, V. C.; Bray, K. L. AIChE J. 1997, 43 (11A), 2785–2792. (11) Franville, A. C.; Zambon, D.; Mahiou, R.; Troin, Y. Chem. Mater. 2000, 12, 428–435.

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