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A New Fluoride Luminescence Quencher Based on a Nanostructured Covalently Bonded Terbium Hybrid Material Qianming Wang,*,† Chaoliang Tan,† Hongyu Chen,† and Hitoshi Tamiaki‡ School of Chemistry and EnVironment, South China Normal UniVersity, Guangzhou 510006, People’s Republic of China, and Department of Bioscience and Biotechnology, Ritsumeikan UniVersity, Kusatsu, Shiga 525-8577, Japan ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: July 4, 2010
The preparation of a novel covalently bonded luminescent hybrid material and its spectrophotometric anion sensing property were reported. The fluorescent receptor (lanthanide complex) was embedded in mesoporous silica through a sol-gel approach, and the nanoscale rod material exhibited excellent photophysical properties (luminescence and thermal stability). It shows significant changes in its fluorescence upon hydrogen binding to fluoride ions in DMSO/H2O (1/1), and the luminescence varies from green to blue (3-4 s). Proton NMR titration studies also indicated that the organic ligand was participating in hydrogen-bonding interactions with fluoride guest anions. The detection limit for fluoride anions could reach 1 µM, and the reusability cycles were performed more than 10 times. Introduction Detecting fluoride is of a significant issue in the analysis of water environment and biochemical monitoring fields. Several decades ago, solid-state ion selective electrodes or devices have already been developed.1-3 Lately, there has been much attention on the fabrication of a group of anion-selective electrodes that were founded on the basis of chemical interaction processes.4 Among all of the current approaches, the development of luminescent material with fluoride selective optical sensing properties is potentially important and would provide possible alternatives to the ions electrodes. In the field of photoluminescence, the design of optical molecular devices, such as lanthanide complexes, is extensively studied due to their special luminescence properties with high color purity and long lifetimes as well as their potential functions in biomedical assays, time-resolved microscopy, and electron luminescent devices.5 Particularly, some lanthanide complexes were recently found to be effective in luminescent sensors for chemical species because they can interact with various guest anions to show guest-dependent lanthanide-centered luminescent changes. However, the quantum efficiency, photo/thermo stabilities, and solubility of the above molecular-based luminescent probes may restrict their uses for practical applications.6 These problems could be circumvented by adding lanthanide compounds in hybrid networks through a sol-gel method. In tracing the development of this work, certain pioneering studies concerning lanthanide optical materials have evolved from the common physical doping technique to the fabrication of covalently linked organosilica-based inorganic-organic hybrids.7-13 The latter approach focuses on organic functionalities that covalently attached to the host backbone, and the enhanced quantum yield or stabilities are successfully achieved.11 Because direct excitation of the lanthanide ion is very inefficient, we selected aromatic anthranilic acid that is suitable * To whom correspondence should be addressed. E-mail: qmwang@ scnu.edu.cn. Tel: 86-20-39310187. Fax: 86-20-39310187. † South China Normal University. ‡ Ritsumeikan University.
for energy transfer14 to terbium emissions as the antenna chromophore. The strategy is illustrated in Figure 1. The rational design is to generate the triethoxysilane terminal hydrolysis and covalently cross-link to the silica surface. As expected, this terbium-containing composite material exhibits much stronger green emissive optical properties compared with conventional complexes. More importantly, we characterize the hybrid material as a fluoride anion receptor. To the best of our knowledge, this is the first case of a covalently anchored terbium luminescent hybrid bearing anion recognition functions, and its photophysical properties have been discussed systematically. Experimental Section All the starting materials were obtained from commercial suppliers and used as received. FT-IR spectra were measured within the 4000-400 cm-1 region on an infrared spectrophotometer, Prestingge-21, with the KBr pellet technique. The 1H NMR spectra were recorded at 293 K on a Varian 400 (400 MHz) using TMS as an internal standard, and visible and fluorescence spectra, lifetime, and absolute quantum yields were measured on an Agilent 8453 spectrophotometer and Edinburgh FLS920 spectrometer, respectively. LC-MS was measured at an Agilent LC-MS equipment. Thermogravimetric analysis was carried out on a STA409PC system under air at a rate of 10 °C/min. Dynamic light scattering was measured at BI-200SM. SEM was measured using a Tescan 5136MM scanning electron microscope. Tetraethoxysilane (TEOS) was provided by Fluka Company. Bis(anthranilic acid) was synthesized according to the literature.15 The MCM-41 material was prepared based on ref 16. Synthesis of Ligand 1. Bis(anthranilic acid) (256 mg, 1 mmol) was dissolved in 20 mL of chloroform, and 3-(triethoxysilyl)propyl isocyanate (271 mg, 1.1 mmol) was added during the stirring. The mixture was refluxed for 6 h, and the termination of the reaction was monitored by thin layer chromatography. Hexane was used to precipitate the crude compound. Column chromatography was performed to purify the initial product. CH2Cl2/MeOH ) 10/1 as an elution solvent was adopted, and the purified product was attained as the main
10.1021/jp105035v 2010 American Chemical Society Published on Web 07/26/2010
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Figure 2. Infrared spectra of ligand 1 and the terbium-containing hybrid material.
with a molar ratio of 3/1 in ethanol under basic conditions. The crude complex was washed with ethanol and water two times consecutively. EA found: C, 50.85; H, 5.88; N, 7.39%. Anal. Calcd for C72H100N9O23Si3Tb: C, 50.79; H, 5.92; N, 7.40. Results and Discussion
Figure 1. Scheme of the preparation and structure of the terbiumcontaining hybrid material. Right cuvette: terbium hybrid material, 10-4 M dispersed in DMSO/H2O ) 1/1. Left cuvette: in [Bu4N]F ) 10-6 M.
band (427 mg, yield 85%). IR (KBr): 3319, 1682 (CdO), 1657, 1583, 754, 721, 526 cm-1. 1H NMR (DMSO-d6): δ ) 12.16, 10.08 (each 1H, s, NH), 8.80 (1H, d, J ) 1.2 Hz, Hd), 8.61 (1H, d, J ) 1.2 Hz, Ha), 8.21 (1H, d, J ) 1.6 Hz, Hd′), 7.82 (1H, d, J ) 1.6 Hz, Ha′), 7.47 (1H, t, J ) 1.6 Hz, Hb′), 7.28 (1H, t, J ) 1.6 Hz, Hc), 7.05 (1H, t, J ) 1.6 Hz, Hc′), 3.82, 3.21 (6H+2H, m, OCH2 × 3, NCH2), 1.67 (2H, m), 1.17 (9H, t, J ) 5.6 Hz, OCCH3 × 3), 0.67 (2H, t, J ) 2.8 Hz, SiCH2). MS (LC-MS) found m/z 526 [M + Na]+. Calcd for C24H33N3O7Si, 503. Synthesis of Sol-Gel Hybrid Material. Ligand 1 (100 mg) and Tb(NO3)3 · 6H2O (29 mg) were dissolved in ethanol, to which TEOS was dropped into the above solution (molar ratio of ligand 1/TEOS ) 1/1). MCM-41 (0.5 g) was then added, and the whole mixture was titrated by 2-3 drops of aq. NH3 · H2O to accelerate the hydrolysis. After 5 h, the thermal curing was maintained at 80 °C overnight until the sample solidified. The powder material was washed with ethanol and water three times and dried in vacuum. IR (KBr): 3421, 1644 (CdO), 1657, 1385, 1198, 1072, 860, 536 cm-1. For comparison purposes, the terbium complex (Tb(1)3 · 2H2O) was prepared based on ligand 1 and Tb(NO3)3 · 6H2O
FT-IR measurements were performed on silylated ligand 1 and the sol-gel derived material (Figure 2). The broad band located at 3421 cm-1 was attributed to silanols, the stretching υ(NH) vibration, and remaining alcohol molecules. Three adjacent peaks at around 2931 cm-1 belong to stretching vibrations of -CH2- in grafted 3-(triethoxysilyl)propyl isocyanate. The sharp bands located at 1682 and 1657 cm-1 could be assigned to carbonyl stretching vibrations of -COOH and -CONH moieties. The bending vibration mode of NH was clearly evidenced by the peak at 1583 cm -1. In contrast to ligand 1, the IR spectra of amorphous luminescent xerogel show that the CdO vibration of -COOH shifted to a lower frequency (υa, 1644 cm-1; ∆υ ) 38 cm-1) due to the complexation of lanthanide ions. In addition, the new absorption band emerged at 1385 cm-1, which corresponded to the symmetric stretching vibration of COO-, again proving that coordinated bonds formed between carboxylic groups and the lanthanide center.7 The Si-C vibration at 1198 cm-1 exhibited that this bond is relatively stable during hydrolysis and polycondensation processes. The broad band at 1072 cm-1 that was ascribed to be Si-OEt showed that the condensation reaction was incomplete, but it will not affect the chelating capability of ligand 1 in xerogels. Proton NMR was applied to precisely study the interaction between the model silylated ligand 1 and the anion by monitoring the chemical shift of the proton resonances (Figure 3). Addition of 3 equiv. of fluoride resulted remarkable changes of NMR signals: First, all the amide and urea resonances completely disappeared due to the strong hydrogen binding to fluoride. Moreover, the aromatic protons Ha, Hd, and Hd′ showed upfield shifts, whereas Ha′, Hb, and Hc gave downfield shifts. It seems that the phenyl protons (Ha, Hd, and Hd′) will strengthen their shielding effects due to the closer distances to the hydrogen-bond donor units, such as NH and COOH. The exceptional case is Ha′, and it is estimated that the adjacent urea bond will decrease its electron density. Therefore, the deshield-
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Figure 3. NMR spectra of ligand 1 and 3 equiv of [Bu4N]F in DMSO-d6. Figure 5. Emission spectra of the sol-gel derived material and terbium complex, dipped in [Bu4N]F-containing DMSO/H2O (1/1) and in the presence of other anions. (F concentration )1 µM, pH ) 6.0). Excitation ) 289 nm.
Figure 4. Fluorescence microscopy graph of the terbium-containing sol-gel derived material.
ing effect plays a primary role. Anyway, the above data firmly prove that there existed a favorable interaction between amide/ urea and the fluoride anion that also affects the benzene ring chromophore, as will be discussed in the spectroscopy studies. Aromatic carboxylic acid and its derivatives are proved to be effective in chelating terbium ions. The energy-transfer mechanism follows the so-called antenna effect through ultraviolet absorption and a triplet-state energy migration process, which has been extensively studied. Fluorescence microscopy images demonstrated that the hybrid material exhibited green luminescence and were uniformly distributed (Figure 4). The excitation spectrum was dominated by a broad band covering from 250 to 400 nm that corresponded to the absorption of the organic ligand (Figure S1, Supporting Information). When exciting the resulting hybrid materials at 289 nm (maximum), we can find four characteristic intraconfiguration f-f transitions of terbium ions from 5D4 excited states to the different J levels of the ground term 7FJ (J ) 6, 5, 4, 3), in the emission spectra (Figure 5). The 5D4 f 7F5 emission at 545 nm was the most dominant transition, and strong green luminescence was, therefore, observed. Another blue band at around 410 nm was assigned to be the organic ligand and hybrid silica host.17 The latter silica matrix may exist radiative recombination between NH2+ and N- in ureasils. In contrast to the common terbium complex, the emission of the organic ligand in the ureasil-containing hybrid was dramatically reduced, showing that these siliceous hybrid matrixes shielded Tb3+ ions from the environment more effectively than the conventional complex and energy-transfer efficiency might increase. Luminescence decays of both samples fit a single-exponential principle, suggesting that all Tb3+ ions have the same coordination mode (figures not shown). The radiative lifetime of Tb3+ in the sol-gel
Figure 6. Intensity ratio of the 545 and 420 nm in DMSO/H2O (1/1) upon titration with [Bu4N]F in the presence of mixed anions (20 equiv of H2PO4-, HSO4-, and AcO-). Excitation ) 289 nm; pH ) 6.0.
material (490 µs) is slightly shorter than that of its complex (577 µs), indicating that a partial quenching effect existed due to remaining ethanol or silanol groups.18 The efficiency of the energy transfer could be detected by measuring the absolute quantum yields of both the hybrid and the complex (MgO as reflecting standard). The Φ value for the latter (5.6%) is lower than that of the covalent hybrid (8.7%), suggesting that a more effective energy migration would be achieved by the hydrolyzed ligand. The ability of recognizing several anions was also studied using emission spectroscopy in DMSO/H2O (1/1, terbium material concentration ) 10-4 M). When the F- concentration reached 10-6 M, the luminescence of terbium ions decreased substantially compared with the original intensity, whereas the emission due to the organic component and silica backbone increased at least 4-fold than that of intact hybrids after 3-4 s. In this way, the presence of blue luminescence was clearly seen
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Figure 7. TGA curves of the hybrid material and terbium complex.
Figure 8. Nitrogen adsorption-desorption curves of the terbiumcontaining hybrid material.
upon the ultraviolet light irradiation. More concentrated fluoride ions will trigger the complete quench of terbium emission peak. The mechanism for this sharp change could be explained as the hydrogen-bonding interaction between fluoride and urea and amide moieties of the organic chromophore. The strong fluoride binding changed the coordination conformation and affected the efficiency of energy transfer. Figure 6 gives the typical variations in emission intensities as a function of the concentration of [Bu4N]F. Interestingly, when plotting the fluorescence intensity against the anion concentration in the range of 0.05-0.8 µM, we observed that the calibration region follows the simple linear equation y ) -5.4962x + 5.1471 by the least-squares fitting method and the correlation coefficient R2 ) 0.9983, which shows the two variables, is best fit with linear correlation.
Figure 9. SEM of the terbium-containing hybrid material.
Wang et al. Gunnlaugsson et al described that hydrogen-bonding recognition is proved by addition of hydrogen-bonding solvents, such as alcohols.19 Therefore, ethanol was chosen to disperse the bulk material for 10 min, and a striking green luminescence was retrieved. This particular recognition process could be repeated for more than 10 cycles. Analogous experiments were conducted for other related tetrabutyl ammonium salts (Cl-, Br-, I-, HSO4-, AcO-, and H2PO4-) in the presence of a concentration of 10-3 M; the terbium emissions showed much lower affinities toward these guest anions (Figure S2, Supporting Information). The results again reveal that the urea and amide units have a dramatic effect on the affinity to fluoride anions. A hybrid composite gives rise to a paramount advantage of increasing the thermal stability so as to open opportunities for application in photonic devices. The thermal properties of both the hybrid and the pure Tb(1)3 complex have been recorded by means of TGA (Figure 7). Obviously, the TG curve of the hybrid is much more endurable under thermal treatment than the conventional complex in the range of 0-700 °C. Particularly, during the thermal decomposition temperature of the organic ligand (400-650 °C), the weight loss of the terbium complex is still much faster than that of its counterpart. This shows that the effective cross-linking of Si-O covalent bonds will restrict the mobility of embedded organic compounds and the decomposition temperature is enhanced. Classical nitrogen adsorption/desorption isotherms of the terbium covalent siloxane complex impregnated MCM host hybrid material is given in Figure 8. The particular surface area estimated from the BET formula is 520 m2/g, and the total pore volume is 0.21 cm3/g. Compared with pure mesoporous matrixes (specific surface area ) 1488 m2/g), the introduction of the Tb complex induced an obvious decrease of the above data, which means that relatively concentrated Tb-complex species would firmly bind to the mesoporous wall even after washing several times. The morphology of the synthesized hybrid material was investigated by way of scanning electron microscopy (Figure 9). The resulting solids have regular rodlike columns with the particle size of around 1 µm in length and 175 nm in width by average. Generally speaking, MCM-41-type scaffolds will show a packed mesoporous structure in the linked material. Here, in this case, it is estimated that the self-assembly of the ligand and the terbium complex with an infinite chainlike crystalline phase14,20,21 would extend the microstructure to a well-defined nanometer scale rod. In addition, this functionalized silica hybrid was constructed only through a common sol-gel approach without any template effect. A further assessment of dynamic light scattering (DLS) exhibited the aggregate solutions of the
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J. Phys. Chem. C, Vol. 114, No. 32, 2010 13883 Supporting Information Available: Excitation spectrum, emission spectra, and DLS trace of terbium-containing sol-gel derived material. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 10. Graph of the fluorescence peak intensity of terbium with alternated dipping into 1 µM DMSO/H2O (1/1) of fluoride anions.
same sample (Figure S3, Supporting Information). Results prove that the as-derived particles have average diameters in the range of 100-800 nm. Last, but not the least, the reusability of the luminescent hybrid material for anion sensing was studied. The characteristic green emission peak intensity (5D4 f 7F5) is shown in Figure 10. Repeated experiments were performed by rinsing the powders with ethanol and water three times for each case. Apparently, the hybrid exhibits an excellent reusability because slight changes were found after a test of 10 repeated cycles. Accordingly, this fast luminescence response opens a new possibility for rapid fluoride anion recognition in an aqueous environment. Conclusions This unique design of a covalently bonded organic-inorganic luminescent hybrid material exhibits a smart response through the hydrogen-bonding effect with fluoride anions. The photoluminescence color quickly changes from a striking green to a blue emission. This easy-to-make hybrid is able to donate urea units for anion binding. Here is the first example, to our knowledge, based on a lanthanide-containing hybrid material. These preliminary results will pave the way for the development of luminescent probes in biological studies that include unavoidable background noise signals. Acknowledgment. Q.W. appreciates the Start Funding of South China Normal University, No. G21117.
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