Synthesis, characterization, and oxygen sensing properties of Ru(II) complex covalently grafted to mesoporous MCM-41

ARTICLE IN PRESS Journal of Luminescence 130 (2010) 374–379

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Synthesis, characterization, and oxygen sensing properties of Ru(II) complex covalently grafted to mesoporous MCM-41 Xiudong Wu a, Luting Song a, Bin Li a,b,, Yanhong Liu b a b

Polyoxometalate Science Key Laboratory of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun 130024, PR China Key Laboratory of Excited State Processes, Changchun Institute of Optics Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, PR China

a r t i c l e in f o

a b s t r a c t

Article history: Received 3 September 2008 Received in revised form 18 September 2009 Accepted 24 September 2009 Available online 4 October 2009

Novel oxygen sensing materials consisting of [Ru(Bphen)2bpy]2 + (Bphen= 4,7-diphenyl-1,10-phenanthroline, bpy =2,20 -bipyridyl) portion covalently grafted to the backbones of the ordered functionalized mesoporous MCM-41 are synthesized by co-condensation of tetraethoxysilane (TEOS) and the functionalized Ru(II) complex [Ru(Bphen)2Bpy-Si]2 + using surfactant cetyltrimethylammoniumbromide (CTAB) as template. The Bpy-Si was used as not only one of the precursors of the sol–gel process but also the second ligand of Ru(Bphen)2Cl2?2H2O complex to prepare the functionalized mesoporous materials for oxygen sensors. Dye leaching shortcoming is overcome due to the Si–C bonds. The derivative mesoporous oxygen sensing materials are characterized by Fourier transform infrared (FT-IR), small angle X-ray diffraction (SAXRD), luminescence intensity quenching Stern–Volmer plots, and excited-state decay analysis. The mesoporous materials show higher sensitivity to the O2 concentration in N2 (I0/I100 =23.2) and shorter response time (1.2 s) in comparison with those based on sol–gel method. When the concentration of oxygen is 10%, the luminescence intensity of the oxygen sensor can be quenched by 89.9%, suggesting that it is highly sensing at low concentration of oxygen. & 2009 Elsevier B.V. All rights reserved.

Keywords: Oxygen sensor Mesoporous material Covalent grafting Ruthenium(II) complex

1. Introduction The determination of molecular oxygen (O2) concentrations, especially at low O2 concentration in water, has important ramifications in many areas ranging from environmental monitoring to biological, medical, analytical, and industrial chemistry. Optical oxygen sensors offer advantages over electrochemical devices, including faster response, lack of analyte consumption, no poison, and absence of electrical connections [1–10]. The ruthenium(II) diimine complexes are one of the most widely used oxygen-sensitive dyes for their appropriate properties, such as efficient quenching by molecular O2, relatively long luminescence lifetime determined by the metal-to-ligand charge-transfer (MLCT) excited state, fast response time, strong visible absorption in the blue–green region of the electromagnetic spectrum, large Stokes shift, and high photochemical stability [6,9,11]. The quenching process of RuII by O2 can be described as follows: RuII* +O2-RuII + O2*, where RuII denotes the complex and ‘‘*’’ is the excited state.

 Corresponding author at: Polyoxometalate Science Key Laboratory of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun 130024, PR China. Tel.: + 86 431 6176935; fax: + 86 431 6176935. E-mail address: [email protected] (B. Li).

0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.09.023

For practical applications in optical oxygen sensors, it is necessary to incorporate Ru(II) complexes in inert host matrix such as polymers [1,12–15], silica-based materials [6,7], etc. Ordered mesoporous silica materials with unique properties (high surface area, ordered pore structure of varying morphologies, and controllable pore size over wide ranges) have been investigated extensively as hosts since they were first introduced in 1992 [16– 18]. The mesoporous silica matrix is expected to be an effective support for preparation of oxygen sensors with excellent performances because its special structure favors the diffusion of analyst within the highly ordered and nearly parallel channel, which is necessary for higher sensitivity and faster response time. Furthermore, mesostructured silica materials offer the rigidity and photostability and, at the same time, have a well-defined hydrophilic/hydrophobic phase separation [5]. However, many studies were mainly focused on doping the mesoporous silica materials with the Ru(II) complexes in which only weak physical interactions exist between the Ru(II) complexes and the mesoporous materials. As a result there are significant drawbacks when these systems are used as oxygen sensors, including inhomogeneous distribution of both organic/inorganic complexes and leaching of dopants. In order to overcome the drawbacks mentioned above, an attractive approach covalently grafting the organic complexes and inorganic portions via the powerful covalent bands such as Si–CH2 has been found [19,20]. Recently, we have reported the synthesis of sol–gel-derived thin film based

ARTICLE IN PRESS X. Wu et al. / Journal of Luminescence 130 (2010) 374–379

on a covalently grafted Ru(II) complex [21]. It demonstrates good linear Stern–Volmer plot and greatly improved anti-leaching. Unfortunately, its sensitivity, especially at low concentration, is not high enough and further improvement is required. Here we extend our previous work and prepare novel Ru(II) covalently grafted functionalized mesoporous MCM-41 hybrid materials. The oxygen sensing properties of the obtained hybrid materials are investigated.

2. Experimental 2.1. Chemical regents The anhydrous RuCl3 (99.99%), 5% Pd/C catalyst along with the 3-aminopropyltriethoxysilane (APS) and cetyltrimethylammoniumbromide (CTAB) were obtained from Aldrich (Milwaukee, WI, USA) and were used without further purification. The tetrathoxysilane (TEOS) and EtOH were purchased from Tianjin Chemicals Company. The concentrated NH3  H2O and thionyl chloride (SOCl2, A.R.) were obtained from Shanghai Chemical Company. SOCl2 was used after distillation in vacuo. The complex bis(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) chloride dihydrate, Ru(Bphen)2Cl2  2H2O was synthesized and purified as described in the literature [22]. 4,40 -Dimethyl-2,20 -bipyridyl and 2,20 -bipyridine-4,40 -dicarboxylic acid were prefabricated as the literature [22]. The water used in our present work is deionized. 2.2. Sample preparation 2.2.1. Synthesis of 2,20 -bipyridine functionalized Bpy-Si hydrolysable ligand. The ligand (denoted as Bpy-Si, as shown in Scheme 1) was prepared using 2,20 -bipyridine-4,40 -dicarboxylic acid and APS as the starting materials according to the literature [23,24]. The

375

detailed synthetic procedure can be briefed as follows: 2,20 bipyridine-4,40 -dicarboxylic acid (0.49 g; 2.0 mmol) was dissolved in excess distilled SOCl2 (8 ml) and refluxed for 4 h, the excess SOCl2 was eliminated by evaporation from the yellow solution, and the residual was reacted with APS under nitrogen for 4 h at room temperature in the presence of excess pyridine and using diethyl ether as a solvent (40 ml). Pyridinium hydrochloride was filtered and evaporation of the residual organic solvent gave the alkoxysilane-modified bipyridine ligand Bpy-Si. 2.2.2. Synthesis of the hydrolysable Ru (Bphen)2(Bpy-Si)Cl2 A mixture of Ru(Bphen)2Cl2 and Bpy-Si in anhydrous ethanol was refluxed for 8 h in nitrogen atmosphere to give a transparent deep red solution, indicating that the complexation reaction between Bpy-Si and Ru(Bphen)2Cl2 had finished. The molar ratio of Bpy-Si to Ru(Bphen)2Cl2 was 1.02:1. Finally the ethanol was rotary evaporated off and the residue was dried in vacuo without any further purification. 2.2.3. Synthesis of functionalized MCM-41 mesoporous materials covalently linked with the Ru(P) complex (Denoted as Ru-MCM-41) The synthetic procedure of functionalized mesoporous silica Ru-MCM-41 was similar to the previous publication with some minor modifications [25]. As a typical run, the synthetic procedure was as follows: concentrated NH3  H2O (18 ml) was mixed with deionized water (39 ml) and CTAB (1.65 g) at 35 1C. To this homogeneous solution, tetraethoxysilane (TEOS, 7.5 ml) and Ru(Bphen)2Bpy-Si were added under vigorous stirring. The molar ratio of the synthetic mixture was Ru(Bphen)2Bpy-Si/TEOS/CTAB/ NH3  H2O/H2O= 0.001:1.0:0.139:3.76:66.57. The mixture was stirred for 10 h at room temperature and then transferred into a Teflon bottle sealed in an autoclave, which was then heated at 100 1C for 48 h. Then the resulting product was filtered, washed with H2O, and dried for 12 h at 60 1C. The surfactant was removed by acid/solvent extraction using a solution of 1 M HCl in ethanol. This mixture was refluxed for 7 h, then filtered and washed with EtOH to remove the residual HCl. The product was dried at 60 1C for 12 h in vacuo and bisque Ru-MCM-41 powder was obtained for measurement. 2.3. Instrumentation and Measurements

Scheme 1. Synthesis procedure and the predicted structure of Ru-MCM-41.

The infrared absorption spectra were measured in the region of 400–4000 cm  1 by a Fourier transform infrared (FT-IR) spectrophotometer (Model Perkin-Elmer Model 580B) with a resolution of 74 cm  1 using the KBr pellet technique. Small angle X-ray diffraction (SAXRD) patterns were recorded with a Rigaku-Dmax 2500 diffractometer using Cu Ka1 (l = 0.154 05 nm) radiation at a 0.021 (2y) scanning step. The oxygen sensing properties of the obtained samples were investigated based on the luminescence intensity quenching instead of the excited-state lifetime because it is more complicated to measure excited-state lifetime compared with luminescence intensity. Luminescence intensities were characterized using a Hitachi-4500 fluorescence spectrophotometer equipped with a xenon lamp (150 W) operating in the 200–900 nm range. The excitation wavelength was 490 nm. The excitation spectra were obtained monitoring at the wavelength of peak emission (610 nm). For the Stern–Volmer plot measurement, oxygen and nitrogen were mixed at different concentrations via gas-flow controllers and flowed directly into the gas chamber sealed with a close fitting suba-seal rubber lid equipped with two (IN and OUT) tubes [19]. We typically allowed 1 min between changes in the N2/O2 concentrations to ensure that a new equilibrium point had been

ARTICLE IN PRESS 376

X. Wu et al. / Journal of Luminescence 130 (2010) 374–379

established. Equilibrium was evident when the luminescence intensity remained constant (7 2%). The sensor-response curves were obtained using the same instruments. All experiments were carried out at atmospheric pressure of 760 Torr. In the luminescence lifetime measurement of the mesoporous MCM-41 materials covalently bonded with Ru(P) complex, a 266 nm light generated from the fourth-harmonic-generator pumped by the pulsed Nd:YAG laser was used as excitation source. The Nd:YAG laser was with a line width of 1.0 cm  1, pulse duration of 10 ns and repetition frequency of 10 Hz. All measurements were performed at room temperature.

3. Results and discussion 3.1. FT-IR spectra results The complex [Ru(Bphen)2bpy]Cl2 has been covalently grafted to the backbones of the MCM-41 for oxygen sensors by using the double-role Bpy-Si compound, as a second ligand for Ru(Bphen)2Cl2 and a precursor of the mesoporous materials. The synthesis procedure is outlined in Scheme 1. The presence of [Ru(Bphen)2bpy]2 + covalently bonded to the network of the modified silicate is characterized by FTIR spectra. The FT-IR spectra of hydrolysable [Ru(Bphen)2(bpy-Si)]Cl2 complex (a), the as-synthesized mesoporous materials Ru-MCM-41 (b), and surfactant-extracted Ru-MCM-41 (c) are shown in Fig. 1. In Fig. 1a, the adsorption bands at 1627 cm  1 due to the adsorption of amide groups (CONH) indicate that APS has been successfully grafted on to 2,20 -bipyridine [23,24]. Further evidence for this is the presence of the stretching n(NH, 3393 cm  1) and bending d(NH, 1556 cm  1) vibration modes together with the stretching vibration of Si–O at 1105 cm  1 and the bending vibration of Si–O at 457 cm  1 (from APS) [23,24]. In Fig. 1b and c, the adsorption bands at 1086 cm  1 (nas, Si–O), 800 cm  1 (ns, Si–O), and 465 cm  1 (d, Si–O–Si) (n represents

Fig. 2. Small angle X-ray diffraction patterns of (a) the as-synthesized Ru-MCM-41, and (b) surfactant-extracted Ru-MCM-41.

stretching, d in-plane bending, s symmetric, and as asymmetric vibrations) substantiate the formation of the silica framework [23]. The n(Si–C) vibration located at 1227 cm  1 is still observed in the IR spectrum of the final materials, which indicates that there is not a great extent of Si–CH2 bond cleavage occurring [23]. Compared with Fig. 1b, the surfactant-extracted Ru-MCM41 (pattern c of Fig. 1) exhibits very weak n(C–H) vibrations in the region of 2700–3000 cm  1, confirming the removal of most of the surfactant CTAB. 3.2. SAXRD analysis X-ray diffraction patterns of the as-synthesized and surfactantextracted Ru-MCM-41 are presented in Fig. 2a and b, respectively. The two patterns clearly show a high-intensity (1 0 0) reflection at a low angle region ranging of 1–2.4 (2y) and three higher-angle reflections located at the higher-angle range that can be indexed for the (11 0), (2 0 0), (2 1 0) reflections. The presence of these four diffraction peaks indicates that typical highly ordered MCM-41 materials were obtained [16]. Acid extraction of the CTAB surfactant template did not lead the loss of the hexagonal ordered structure. After the surfactant was extracted, the diffraction intensities are found to increase due to the further cross-linking of silicates [26]. The value of a0 for the final MCM-41 pffiffiffi materials obtained from the SAXRD pattern (a0 ¼ 2d100 = 3) is 4.95 nm. 3.3. Optical and oxygen sensing properties analysis The room temperature emission spectra recorded for a typical covalently grafted Ru-MCM-41 oxygen sensor under different oxygen concentrations are presented in Fig. 3. Their fluorescence feature is the typical triplet MLCT process, which is attributed to the emission from the triplet MLCT excited state (3MLCT) to the ground state transition [27]. From Fig. 3, it can be observed that the emission spectra show a broad band ranging from 550 to 700 nm centering at 610 nm. The Ru(II) complex grafted to mesoporous materials could be quenched effectively by oxygen molecules. The energy transfer mechanism responsible for the quenching is as follows [28]: 

Fig. 1. Representative FT-IR spectra of (a) the hydrolysable functionalized Ru(II) complex [Ru(Bphen)2(bpy-Si)]Cl2, (b) the as-synthesized mesoporous materials Ru-MCM-41, and (c) surfactant-extracted Ru-MCM-41.

½RuðBphenÞ2 bpy2 þ þ O2 -½RuðBphenÞ2 bpy

2þ

 þ I O2

where * stands for an excited state. The extent of quenching is determined by the concentration of O2. The shape and position

ARTICLE IN PRESS X. Wu et al. / Journal of Luminescence 130 (2010) 374–379

Fig. 3. Room temperature emission spectra for Ru-MCM-41 oxygen sensor under different oxygen concentrations with an interval of 10%.

located at 610 nm is constant under different oxygen concentrations. But the relative intensity decreases markedly as increasing the oxygen concentration. The relative luminescent intensities of the Ru(Bphen)2(Bpy-Si)Cl2 covalently grafted MCM41 oxygen sensor decrease by 95.7% upon changing from pure nitrogen to pure oxygen. When the concentration of oxygen is 10%, the luminescence intensity of the oxygen sensor can be quenched by 89.9% (I0/I10 = 9.9, I0 and I10 denote the fluorescence intensities under 100% nitrogen and 10% oxygen condition, respectively), suggesting that it has a potential for the measurement of oxygen at low concentration. Optical sensors based on the luminescence quenching are examined by the Stern–Volmer relationship. In homogeneous media with a single exponential decay, the Stern–Volmer equation with dynamic quenching is as follows [29]: I0 =I ¼ t0 =t ¼ 1 þKSV pO2 ¼ 1þ kq t0 pO2

ð1Þ

which relates the ratio of the steady-state intensities or lifetime in the absence of quencher (I0 and t0 ) to the intensity or lifetime in the presence of quencher (I and t) through the dynamic Stern– Volmer quenching constant, KSV, the bimolecular quenching rate constant that describes the collisional encounter kinetics between the luminophore and quencher, kq, and the quencher concentration [Q]. For this ideal case, a plot of I0/I or t0/t versus [Q] (called the Stern–Volmer plot) will be linear with a slope equal to KSV and an intercept of unity. The lifetime decay of luminophore in homogeneous media can be described by a single exponential equation [9,11]: IðtÞ ¼ a expðt=tÞ

ð2Þ

where I(t) is the luminescence intensity at time t and a is the preexponential factor. However, if luminophores are immobilized in two or more sites within the silicate matrix simultaneously in which one site is more heavily quenched than the others, the Eq. (1) above is generally not obeyed and the Stern–Volmer plots become nonlinear. Several quenching mechanisms have been proposed. The simplest being the two-site familiar Demas model, which in its basic approach, assumes that the oxygen concentration is uniform but the lifetime and quenching constants are different [7]. The intensity Stern–Volmer equation becomes I0 =I ¼ 1=½f01 =ð1þ KSV1 pO2 Þ þf02 =ð1 þ KSV 2 pO2 Þ

ð3Þ

where f0i is the steady-state fraction of light emitted from the i site and KSVi is its Stern–Volmer constant. If one fraction of the luminophore is nonquenchable (KSV2 =0), above Eq. (3) collapses

377

Fig. 4. Typical Stern–Volmer plot of the final Ru-MCM-41. The scatter points a: the experimental data; b: two-site Demas model fitting result; c: Lehrer model fitting result

to another similar Lehrer express as follows. The previously reports revealed that the Lehrer model can also be used to give excellent fit to the downward Stern–Volmer plots [7]. I0 =I ¼ ½f01 =ð1 þ KSV 1 pO2 Þ þ f02 

ð4Þ

In these above-mentioned Lehrer and Demas two-site models, there are two excited-state lifetime components for the luminescence species, and the excited lifetime decay analysis may be described by IðtÞ ¼ a1 expðt=t1 Þ þ a2 expðt=t2 Þ

ð5Þ

where I(t) represents the fluorescence intensity at time t, the subscripts 1 and 2 denote the assigned lifetime components, and ai denotes the pre-exponential factors. Fig. 4 describes the Stern–Volmer plot of the final covalently grafted oxygen sensor (Ru-MCM-41). The best fitting parameters are summarized in Table 1. It can be found that their sensitivity defined by I0/I100 (I0/I100 =23.2) is higher than that of our previous reported oxygen sensor using sol–gel method (I0/I100 =4.3), though the linearity is not as good as the sol–gel film [21]. The result can be attributed to the nearly parallel porous structure of the MCM-41 support with large surface areas and narrow pore size distribution. The luminophore confined within the MCM-41 highly ordered parallel-aligned porous channels have a higher locally effective quenching by O2, resulting in increased sensitivity. Although the plot is nonlinear resulting from site heterogeneity, both the Demas twosite model and the Lehrer model [11,19] can be used to fit the intensity quenching curve of the final sample well. The fitting parameters in Table 1 represent the best fits to the curve in Fig. 4. From the fitting results listed in Table 1, it can be observed the twosite Demas model fitting result is much better than the Lehrer model fitting results. It means that the Ru(II) complex molecules are distributed between at least two sites within the mesoporous silicate, which can be quenched by oxygen molecules at different degrees. 3.4. Quenching response curve and stability Fig. 5 demonstrates the typical dynamic response of the covalently grafted MCM-41 mesoporous materials on exposure to pure N2 and pure O2 atmospheres which was varied periodically. In general, 95% response time, i.e., tk (95%, N2-O2), is defined as the time required for the luminescent intensity to decrease by 95% on

ARTICLE IN PRESS 378

X. Wu et al. / Journal of Luminescence 130 (2010) 374–379

Table 1 Intensity-based Stern–Volmer oxygen quenching parameters of the functionalized mesoporous MCM-41 oxygen sensing materials from the fitting employing different models, i.e., Demas two-site model and Lehrer model. Samples

Ru-MCM-41 a b

Demas two-site modela

Lehrer modelb

KSV1 (O2%  1)

KSV2 (O2%  1)

f01 (c)

r2

KSV1 (O2%  1)

f01

r2

0.0095 7 0.0008

3.10137 0.4920

0.07857 0.0038

0.9991

0.9655 7 0.1240

0.9638 7 0.0020

0.9736

Terms are from Eq. (3). f01 + f02 = 1. Terms are from Eq. (4). f01 is the fraction of luminophore that is quenchable. KSV1 is the Stern–Volmer quenching constant of luminophore that is quenchable. KSV2 = 0.

Fig. 5. Relative luminescence intensity of 610 nm emission as a function of time for a typical covalently grafted [Ru(Bphen)2(bpy-Si)]Cl2 mesoporous oxygen sensing materials on periodically cycling from 100% nitrogen to 100% oxygen atmosphere.

changing from 100% N2 to 100% O2. On the other hand, 95% recovery time, i.e., tk (95%, O2-N2), means the time required for the luminescence intensity to reach the 95% of its initial value recorded under 100% N2 on changing from 100% O2 to 100% N2 [5]. From Fig. 5, it can be observed that upon changing to pure O2, the emission intensity drops very quickly, while on changing to pure N2, the emission intensity increases and recovers to its initial value. This cycle is repeated in an alternating atmosphere of nitrogen and oxygen. As illustrated in Fig. 5, stable and reproducible signals are obtained with the mesoporous-optic oxygen sensor. The response time of the covalently grafted mesoporous oxygen sensor is only 1.2 s upon switching from pure nitrogen to pure oxygen, which is shorter than that of our previous oxygen sensor based on sol–gel method (7 s) [21]. And the recovery time is about 29 s on changing from 100% O2 to 100% N2. The response times are very short and the fast response obviously comes from the Si–CH2 covalent bond and the highly ordered and nearly parallel channel of the MCM-41 mesoporous materials. In addition, the recovery time is not short enough compared with the response time. This phenomenon is similar to the results obtained for Pt–OEP in a polymer medium [30] and this difference can be explained by the mathematical expressions developed elsewhere by Mills et al. to describe the diffusion-controlled dynamic response and recovery behavior of a hyperbolic-type sensor to changing analyte concentration [31].

3.5. Excited-state lifetimes of the functionalized mesostructured MCM-41 oxygen sensing materials Fig. 6 shows the typical excited-state intensity decay profile for the covalently grafted Ru(Bphen)2(Bpy-Si)2 + mesoporous

Fig. 6. Typical excited-state intensity decay profile for the covalently grafted Ru(Bphen)2(Bpy-Si)2 + mesoporous materials measured in the presence of pure N2 (a), and in the ambient atmosphere (b). The scatter line is the best fit to the experimental data in both (a) and (b). Residuals plots for double exponential fit are also shown.

ARTICLE IN PRESS X. Wu et al. / Journal of Luminescence 130 (2010) 374–379

materials measured in the presence of pure N2 (a) and in the ambient atmosphere (b). It is observed that the oxygen sensor exhibits double-exponential excited-state decay process. The luminescence intensity decay profiles were well described by a double-exponential decay and the decay time t1 and t2 were found to give well fit. It means that the Ru(II) molecules are distributed simultaneously between two or more sites within mesoporous silicate, which is responsible for nonlinear relationship of the Stern–Volmer plot (as shown in Fig. 4).

3.6. Complex-leaching analysis It is important to determine the concentrations of molecular oxygen (O2) in liquid phase. In order to investigate whether the luminophore leaches into the liquid phase, complex-leaching experiments were performed for the covalently grafted incorporated sample by soaking them in water, DMF and ethanol at 60 1C in a sealed cuvette under magnetic stirring and then dried at 100 1C in vacuo, as described previously [20]. This investigation reveals that the covalently grafted MCM-41 materials possess improved chemical durability in comparison with physically entrapped MCM-41. The integrated fluorescence intensities of the unquenched MLCT emission of the covalently grafted sample after stirring in each solvent for 5 days are nearly constant and the leaching effect is small enough to be neglected. This result reveals that the Si–CH2 covalent bonds between the Ru(II) complex and the mesoporous silica matrix can effectively prevent the dopant leaching. This result is similar to that of the previous report [20].

4. Conclusion Optical mesoporous MCM-41 materials containing covalently bonded Ru(II) diimine complex in a silicate network were prepared. The high sensitivity can be explained by the fact that the mesoporousity of the MCM-41 matrixes favors the oxygen diffused to the complex resulting in quick quenching. Dye leaching shortcoming is overcome since these luminescence molecules were covalently grafted to the Si–O network using the Si–C bonds. Most importantly, it is sensitive for oxygen at low concentration. These properties make them candidates for monitoring the dissolved oxygen in liquid phase, especially at low concentration.

379

Acknowledgements The authors gratefully thank the financial supports of One Hundred Talents Project from Chinese Academy of Sciences and the National Natural Science Foundations of China (Grant no. 50872130). References [1] L. Huynh, Z. Wang, V. Stoeva, A. Lough, I. Manners, M.A. Winnik, Chem. Mater. 17 (2005) 4765. [2] M.E. Kose, A. Omar, C.a. Virgin, B.F. Carroll, K.S. Schanze, Langmuir 21 (2005) 9110. [3] B.H. Han, I. Manners, M.A. Winnik, Chem. Mater. 17 (2005) 3160. [4] M.E. Kose, B.F. Carroll, K.S. Schanze, Langmuir 21 (2005) 9121. [5] H. Zhang, Y. Sun, K. Ye, P. Zhang, Y. Wang, J. Mater. Chem. 15 (2005) 3181. [6] R.M. Bukowski, R. Ciriminna, M. pagliaro, F.V. Bright, Anal. Chem. 77 (2005) 2670. [7] Y. Tang, E.C. Tehan, Z. Tao, F.V. Bright, Anal. Chem. 75 (2003) 2407. [8] M.C. DeRosa, P.J. Mosher, G.P.A. Yap, K.S. Focsaneanu, R.J. Crutchley, C.E.B. Evans, Inorg. Chem. 42 (2003) 4864. [9] M.T. Murtagh, M.R. Shahriari, M. Krihak, Chem. Mater. 10 (1998) 3862. [10] C. McDonagh, B.D. MacCraith, A.K. McEvoy, Anal. Chem. 70 (1998) 45. [11] E.R. Carraway, J.N. Demas, B.A. DeGraff, J.R. Bacon, Anal. Chem. 63 (1991) 337. [12] Z. Wang, A.R. McWilliams, C.E.B. Evans, X. Lu, S. Chung, M.A. Winnik, I. Manners, Adv. Funct. Mater. 12 (2002) 415. [13] A. Mills, C. Tommons, R.T. Bailey, M.C. Tedford, P. Crilly, J. Analyst (Cambridge UK) 131 (2006) 495. [14] W. Xu, R.C. McDonough, B. Langsdorf, J.N. Demas, B.A. DeGraff, Anal. Chem. 66 (1994) 4133. [15] R.N. Gillanders, M.C. Tedford, P.J. Crilly, R.T. Bailey, J. Photochem. Photobiol. A 163 (2004) 193. [16] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, E.W. Sheppard, S.B. McCullen, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [17] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [18] M.E. Davis, Nature 417 (2002) 813. [19] B. Lei, B. Li, H. Zhang, S. Lu, Z. Zheng, W. Li, Y. Wang, Adv. Funct. Mater. 16 (2006) 1883. [20] H.R. Li, J. Lin, H.J. Zhang, L.S. Fu, Q.G. Meng, S.B. Wang, Chem. Mater. 14 (2002) 3651. [21] H. Zhang, B. Li, B. Lei, W. Li, S. Lu, Sensors Actuators B: Chem. 123 (2007) 508. [22] G. Sprintschnik, H.W. Sprintschnik, P.P. Kirsch, D.G. Whitten, J. Am. Chem. Soc. 99 (1977) 4947. [23] H.R. Li, J. Lin, H.J. Zhang, H.C. Li, L.S. Fu, Q.G. Meng, Chem. Commun. 13 (2001) 1212. [24] H. Li, J. Yu, F. Liu, H. Zhang, L. Fu, Q. Meng, C. Peng, J. Lin, New J. Chem. 28 (2004) 1137. [25] L.N. Sun, H.J. Zhang, C.Y. Peng, J.B. Yu, Q.G. Meng, L.S. Fu, X.M. Guo, J. Phys. Chem. B 110 (2006) 7249. [26] J. Hukkama˙ki, S. Suvanto, M. Suvanto, T.T. Pakkanen, Langmuir 20 (2004) 10288. [27] J.J. Ding, B. Li, H.R. Zhang, B.F. Lei, W.L. Li, Mater. Lett. 61 (2007) 3374. [28] J.N. Demas, E.W. Harris, R.P. McBride, J. Am. Chem. Soc. 99 (1977) 3547. [29] V.O. Stern, M. Volmer, Physik. Zeitschr. 20 (1919) 183. [30] A. Mills, A. Lepre, Anal. Chem. 69 (1997) 4653. [31] A. Mills, Q. Chang, Analyst (Cambridge UK) 117 (1992) 1461.