Preparation and fluorescent recognition properties for fluoride of a nanostructured covalently bonded europium hybrid material

Preparation and fluorescent recognition properties for fluoride of a nanostructured covalently bonded europium hybrid material

JOURNAL OF RARE EARTHS, Vol. 33, No. 9, Sep. 2015, P. 905 Preparation and fluorescent recognition properties for fluoride of a nanostructured covalen...

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JOURNAL OF RARE EARTHS, Vol. 33, No. 9, Sep. 2015, P. 905

Preparation and fluorescent recognition properties for fluoride of a nanostructured covalently bonded europium hybrid material YU Xudong (余旭东), LI Jingyin (李景印), LI Yajuan (李亚娟)*, GENG Lijun (耿丽君), ZHEN Xiaoli (甄小丽), YU Tao (于 涛) (College of Science, Hebei University of Science and Technology, Shijiazhuang 050080, China) Received 11 June 2014; revised 7 July 2015

Abstract: A novel covalently bonded Eu3+-based silica hybrid material was designed and its spectrophotometric anion sensing property was studied. The fluorescent receptor (europium complex) was covalently grafted to the silica matrix via a sol-gel approach. FTIR, UV-vis spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescent spectra were characterized, and the results revealed that the hybrid material with nanosphere structure displayed excellent photophysical property. In addition, the selective anion sensing property of the hybrid material was studied by UV-vis and fluorescence spectra. The results showed that the hybrid material exhibited a smart response with fluoride anions. Keywords: lanthanide ion; hybrid material; anion recognition; sensors; rare earths

Fluoride is one of the most important anions in chemical and biological systems, therefore, the development of the new fluoride receptors including colorimetric and fluorescent chemosensors has attracted extensive attention in recent years[1–5]. Recently, the extensively used methods for achieving fluoride recognition have mainly used the following materials: (1) metal ion coordination complexes[6]; (2) hydrogen bonding moieties such as (thio)ureas, ammonium, imidazole, and imidazolium[7–13]; (3) ammonium or guanidinium moieties[14]. Among them, the study on chemosensors based on lanthanide complexes is still a challenging and high topic area because of their special luminescence properties with high color purity and long lifetime as well as their potential applications in the field of luminescent devices and biomedical assays[15,16]. However, the poor photo/thermo stabilities are often considered to be a great impediment in practical applications. The problem could be solved by incorporating the lanthanide complexes into hybrid networks through a sol-gel method[17,18]. Two methods were extensively used to obtain this goal: (a) physical doping technique; (b) fabrication of covalently linked organic-inorganic hybrids. In the past years, considerable efforts have been focused on the lanthanide organic-inorganic hybrid materials, especially the hybrid materials in which the lanthanide complexes are covalently grafted onto inorganic networks[19–21]. In these species, good stability and enhanced quantum yield have been achieved. However, the study of organic-inorganic hybrid materials covalently

bonded to lanthanide complexes has so far been mainly focused on the luminescence properties, the other functionality of the hybrids such as sensing properties for guest and magnetic properties have been rarely reported. Therefore, it would be highly attractive to investigate the host-guest interaction and sensing properties of lanthanide complex grafted hybrids. Herein, in this paper, according to the principle of hostguest chemistry, a new kind of luminescent Eu3+ based silica hybrid with urea group for colorimetric and luminescent sensing toward F– was fabricated and studied. The design of dual functional organic ligand is the key point for the construction of this hybrid material. Herein, we designed and synthesized a kind of azo-based ligand, which not only could coordinate to europium ions as well as sensitize the luminescence of europium ion through “antenna effect”, but also could be covalently grafted onto silica matrix through the modification with 3-(triethoxysilyl)-propyl-isocyanate (TEPIC). To the best of our knowledge, this was the first report of covalently bonded europium hybrid material with anion recognition.

1 Experimental 1.1 Materials Organic solvents, i.e. DMSO, Et3N, CH2Cl2, etc., were analytical pure grade and obtained from Sinopharm Chemical Reagent Co., Ltd. DMSO was dried by CaH2

Foundation item: Project supported by the National Natural Science Foundation of China (21401040, 21301047) and the Natural Science Foundation of Hebei Province (B2014208160, B2014208091) * Corresponding author: LI Yajuan (E-mail: [email protected]; Tel.: +86-311-81668532) DOI: 10.1016/S1002-0721(14)60503-2

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and distilled under vacuum. 4-Amino antipyrine, NaNO2, 3-(triethoxysilyl)-propyl-isocyanate (TEPIC) and αnaphthyamine were obtained from Sinopharm Chemical Reagent Co., Ltd. Eu(NO3)3 was obtained by dissolving Eu2O3 in concentrated nitric acid. 1.2 Synthetic procedures 1.2.1 Synthesis of the Azo-based organic ligand 4-Amino antipyrine (120 mol, 2.5 g) and HCl (6 mol/L, 5.8 mL) were stirred in ice bath, NaNO2 solution (1 g, 5 mL H2O) was added dropwise into the above solution, the mixture was then stirred for 1 h and neutralized with Na2CO3. The solid was purified by chromatography (SiO2:CH2Cl2:CH3OH=20:1), purple solid was obtained (0.6 g), which was denoted as Azo. The 1HNMR data were placed in the supporting information. Anal. Calcd for C21H19N5O: C, 70.57; N, 19.59; H, 5.36; Found: C, 70.50; N, 19.63; H, 5.37. ESI for (C21H19N5O+H)+: 358.71. 1.2.2 Synthesis of the precursor Azo-Si Organic ligand Azo (1 mmol, 0.358 g) was first dissolved in 20 mL of dehydrate tetrahydrofuran (THF), and 3-(triethoxysilyl)-propyl-isocyanate (TEPIC, 1 mmol, 0.247 g) was added into the solution with stirring. The mixture was heated at 60 °C in a covered flask for approximately 12 h in a nitrogen atmosphere, then the mixture was concentrated and purified by chromatography (SiO2: CH2Cl2: CH3OH=20:1, 10:1), finally red solid was obtained. 1HNMR: 0.59-0.65 (m, 2H), 1.49–1.17 (t, J=2.5 Hz, 9H), 1.51–1.56 (m, 2H), 2.74 (s, 3H), 3.13– 3.16 (m, 2H), 3.39 (s, 3H), 3.74–3.79 (m, 6H), 7.57–7.60 (m, 3H), 7.69–7.71 (m, 1H), 7.71–7.44 (m, 2H), 7.74– 7.73 (d, J=7.5 Hz, 1H), 7.77–7.79 (d, J=8 Hz, 1H), 8.27– 8.28 (d, J=8 Hz, 1H), 8.33–8.34 (d, J=8.5 Hz, 1H), 8.70 (s, 1H), 8.91–8.92 (d, J=8.5 Hz, 1H), 9.49 (s, 1H). 1.2.3 Synthesis of the precursor europium hybrid material The Azo-derived hybrid material containing Eu3+ was prepared as follows: while being stirred, the precursor

Azo-Si (82 mg) was soaked in an appropriate amount of Eu(NO3)3 (20.18 mg) and 2,2′-bipyridine (bpy) (7.06 mg) DMF solution with the molar ratio of Eu3+:Azo-Si:bpy= 1:2:1. The mixture was stirred at room temperature for 2 h followed by the addition of H2O (19 mg) and TEOS (tetraethoxysilane, 56 mg). After stirring for 24 h, the mixture was dried at 60 °C for 6 d under vacuum. The obtained hybrid material was denoted as Eu(Azo-Si)2(bpy). The synthesis procedure and predicted structure of the hybird is presented in Scheme 1. 1.3 Characterization FTIR spectra were measured within the 4000–400 cm–1 region on a Nicolet 6700 spectrophotometer with the KBr pellet technique. 1HNMR spectra were recorded on a Mercuryplus instrument, at 500 and 125 Hz, respectively. Elemental analysis was carried out on a VARIOEL3 apparatus (ELEMENTAR). Absorption spectra were measured on a Lambda 35-UV/VIS spectrometer, Perkin-Elmer precisely. ESI-MS data were recorded on a Waters Quattro Micro API LC-MS-MS spectrometer (Waters, USA). The X-ray diffraction pattern was generated by using a Bruker AXS D8 instrument (Cu target; l=0.1542 nm) with a power of 40 kV and 50 mA. SEM images of the xerogels were obtained by using SSX-550 (Shimadzu) and FE-SEM S-4800 (Hitachi) instruments. TEM was performed on a JEOL JEM2011 apparatus operating at 200 kV. The fluorescence excitation and emission spectra were obtained on a Hitachi F-4500 spectrophotometer.

2 Results and discussion The successful preparation of the precursor Azo-Si was characterized by FT-IR spectrum. The FT-IR spectrum of the precursor Azo-Si is presented in Fig. 1. The spectrum is dominated by ν(C–Si, 1193 cm–1) and

Scheme 1 Synthesis procedure and predicted structure of the hybrid material Eu(bpy)(Azo-Si)2

YU Xudong et al., Preparation and fluorescent recognition properties for fluoride of a nanostructured covalently …

Fig. 1 FT-IR spectra of the precursor Azo-Si

ν(Si–O, 1078 cm–1) absorption peaks, characteristic of trialkoxylsilyl functions, and the band centered at 3434 cm–1 corresponds to the stretching vibration of grafted –NH– groups. Moreover, the bending vibration (δNH, 1537 cm–1) further proves the formation of amide groups. A series of strong bands at around 2987, 2942, 2928 cm–1 are originated from the vibrations of methylene –(–CH2)3– in the TEPIC. The band located at 1637 cm–1 is attributed to the absorption of the cross-linking reagent TEPIC, further proving that that TEPIC was successfully grafted onto the ligand Azo. UV-vis spectra, XRD, SEM as well as TEM experiments were performed to characterize the hybrids. Seen from Fig. 2, the peak of the precursor Azo-Si at 427 nm was narrowed and had little red shift from 427 to 433 nm after the coordination and sol-gel process, indicating the formation of Eu3+ complex. Also, the shoulder peak at 463 nm of the Eu3+-based hybrid Eu(bpy)(Azo-Si)2 was assigned to the metal-to-ligand charge transfer (MLCT) of Eu(bipy)3+ moiety coupled with the Azo-centered charge transfer[8]. As shown in Fig. 3, TEM and SEM images revealed that the hybrid formed sphere structure with average diameter of 200 nm. The only one broaden peak in XRD data also confirms that the hybrid was composed of good silica networks (Fig. 4). The ability of recognizing anions was also studied in the DMSO solution. After sonication for 5 min, the hy-

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brid could be suspended in DMSO for a long time without precipitation. The sensing properties of the hybrid solution toward anions were performed at 298 K in DMSO (5 mg/150 mL, the concentration of Azo-building block was less than 1×10–4 mol/L). With the titration of fluoride anions, two step binding events could be observed from Fig. 5(a). Firstly, with the addition of F– from 5×10–5 to 5×10–4 mol/L, the absorption peak at 431 nm increased due to the strong hydrogen bonding of urea group with F–; in the second step, with the addition of F– from 5×10–4 to 1×10–1 mol/L, the absorption peak at 431 nm decreased and broadened, accompanied by the obvious color changes from yellow to orange, which indicated that deprotonation process happened. The good isosbestic point at 448 nm also certified the two-step binding events between the hybrid material and F–. From Fig. 6, when plotting the fluorescent intensity against the fluoride concentration in the range of 5×10–5 to 4×10–4 mol/L, the calibration region follows the simple linear equation y=0.05808x+1.02084 by the least-squares fitting method and the correlation coefficient R=0.98146, which showed that the two variables was best fit with linear correlation. By the fluorescence titration experiments, it could be found that the fluoride was a good luminescence quencher for the hybrid. When exciting the result hybrid

Fig. 2 UV-vis spectra of the precursor Azo-Si and the Eu3+ based hybrid Eu(Azo-Si)2(bpy)

Fig. 3 TEM (a) and SEM (b) images of the hybrid material Eu(Azo-Si)2(bby)

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Fig. 4 XRD pattern of the europium hybrid Eu(bpy)(Azo-Si)2

solution at 275 nm, we can find four characteristic intraconfiguration f-f transitions of europium ions from 5D0 excited states to the different J levels of the ground term 7 FJ (J=1, 2, 3, 4), in the emission spectra (Fig. 5(b)). Among these transitions, the 5D0→7F2 transition at about 618 nm shows the most prominent emission. Therefore, strong red luminescence was observed. Another blue band at 400 nm was assigned to be the organic ligand

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and hybrid silica host[22]. In the first step recognition process, with the addition of F– range from 5×10–5 to 5×10–4 mol/L, the luminescence of the Eu3+ hybrid was quenched gradually, while there was no change at 400 nm (Fig. 5(b) and (c)), which revealed that the fluorescence of Eu(bpy)3+ was more sensitive to the recognition events between hybrid material and F– due to the electron transfer from the Azo-urea group to Eu(bpy)3+ moiety. From Fig. 6(b), when plotting the fluorescent intensity against the fluoride concentration in the range of 5×10–5 to 4×10–4 mol/L, the calibration region follows the simple linear equation y=510.7366+–1.27508×10–6x by the least-squares fitting method and the correlation coefficient R=0.9948, which also showed the two variables was best fit with linear correlation. Then the peak at 400 nm, which was assigned to the emission of ligand, was quenched after the further addition of F– range from 5×10–4 to 1×10–1 mol/L. From the above results, it can be deduced that two processes might happen in the binding event of Eu(bpy)(Azo-Si)2 with fluoride ion. Firstly, Eu(bpy)(Azo-Si)2 was bonded with fluoride ion through hydrogen bonding interaction via NH of urea unit. Sec ondly, the deprotonation process induced by fluoride

Fig. 5 UV-vis titrations of the hybrid Eu(Azo)2(bpy) with fluoride (a), fluorescent titration of Eu(Azo)2(bpy) with fluoride in the first step with low concentration of F– (b) and fluorescent titration of Eu(Azo)2(bpy) with fluoride in the second step with high concentration of F– (λem=275 nm) (c)

Fig. 6 (a) Absorbance changes of hybrid material (5 mg/150 mL) with F– at 400 nm range from 5×10–5 to 5×10–4 mol/L in the first step binding process and (b) fluorescence intensity changes of hybrid material (5 mg/150 mL) with F– at 620 nm range from 5×10–5 to 5×10–4 mol/L

YU Xudong et al., Preparation and fluorescent recognition properties for fluoride of a nanostructured covalently …

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Fig. 7 UV-vis titrations of Eu(bpy)(Azo)2 with anions such as AcO– (from 1×10–4 to 3×10–3 mol/L), Cl– (5×10–3 mol/L), Br– (5×10–3 mol/L) , I– (5×10–3 mol/L)

took place, resulting in the fluorescence quenching of Eu(bpy)(Azo-Si)2 at 400 nm. The two processes were also in accordance with the Uv-vis and FL spectra. Other anions such as AcO–, Cl–, Br– and I– were also added to test the selectivity of the hybrid solution. From Fig. 7, with the addition of AcO– ranging from 1×10–4 to 3×10–3 mol/L, the peak at 431 nm changed irregularly, indicating the formation of unstable hybrid-AcO– complex. With the addition of Cl–, Br–, I–, there were no obvious spectra or color changes. These results indicated that the hybrid material had high selectivity toward Famong the test anions such as AcO–, Cl–, Br–, I–.

3 Conclusions In summary, a new kind of covalently bonded europium hybrid material was designed and synthesized through sol-gel method. The hybrid material possessed nanosphere structure and exhibited a smart response with fluoride anions through the hydrogen bonding effect, which resulted in the obvious color and fluorescent changes. The hybrid networks were able to donate urea units for fluoride anion binding selectively. These results will pave the way for the development of luminescent probes in biological environment. Acknowledgements: Authors would like to thank the Foundation of Hebei University of Science and Technology (2014PT75) for financial support.

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