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Magnetic–luminescent composite nanoparticles: Fabrication, characterization, magnetic and photophysical properties
Synthetic Metals 161 (2011) 1976–1981
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Magnetic–luminescent composite nanoparticles: Fabrication, characterization, magnetic and photophysical properties Liyan Wang a,b,∗ , Lei Huang c , Zhu Lin b , Xiaofeng Chen d , Yanhua Xu e a
Department of Stomotology, The Second People’s Hospital of FoShan, GuangDong, China Department of Orthodontics, School of Stomatology, The Fourth Military Medical University, XiAn, ShanXi, China Department of Stomotology, The First People’s Hospital of GuangZhou, GuangZhou, GuangDong, China d School of Materials Science and Engineering, South China University of Technology, GuangZhou, GuangDong, China e Department of Orthodontics, School of Stomatology, KunMing Medical College, KunMing, China b c
a r t i c l e
i n f o
Article history: Received 3 May 2011 Received in revised form 30 May 2011 Accepted 1 June 2011 Available online 31 July 2011 Keywords: Core–shell structure Luminescent Ru(II) complex Magnetic Fe2 O3 Composite material
a b s t r a c t In this paper, we construct core–shell structured nanoparticles, where magnetic Fe2 O3 nanoparticles are used as the inner core and SiO2 doped with phosphorescent Ru(II) complex is used as the outer shell. The obtained magnetic–luminescent composite nanoparticles are identified and characterized by XRD analysis, IR spectrum, field-emission scanning electron microscopy, transmission electron microscopy, and fluorescence image, which confirms the core–shell structure. Then the magnetic and photophysical properties of the composite nanoparticles are investigated in detail. Data suggest that the outer shell of SiO2 weakens the magnetic response of Fe2 O3 core by showing a smaller coercivity value compared with that of pure Fe2 O3 nanoparticles. The composite nanoparticles are red-emitting ones, and the emission signal is sensitive towards oxygen concentration variations. However, the decreased excited state lifetime and weak emission of the Ru(II) emissive center limit the sensitivity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent yeas, the design and construction of multifunctional nanostructured and nanocomposite materials have been considered to be a promising way to satisfy various demands because they can realize a combination of both enhanced functionality and multifunctional characters in contrast with their single-component counterparts [1,2]. Magnetic iron oxide nanoparticles, owing to the advantages of low toxicity and high saturation magnetization, have been widely employed in biomedical applications such as drug targeting, cell sorting and isolating, magnetic resonance imaging, and so on [3–5]. There is, however, a problem waiting to be conquered, which means that the magnetic nanoparticles tend to aggregate and correspondingly compromise their monodispersal. It is thus an important consideration to coat the bare magnetic nanoparticle cores with suitable shells. Amorphous silica seems to satisfy above requirements owing to its low toxicity and high compatibility with biological systems [6]. In addition, the silica surface can be readily functionalized with
∗ Corresponding author at: Department of Stomotology, The Second People’s Hospital of FoShan, GuangDong, China. Tel.: +86 0757 83635705; fax: +86 0757 83635705. E-mail address: [email protected] (L. Wang). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.041
amines, thiols, and carboxyl moieties, further favoring the conjunction of biomolecules. Several research groups have tried the combination of magnetic nanoparticles and fluorescent dyes into a single composite system, hoping to realize nanoparticles with optical activities and thus enlighten their applications in biomedical and biopharmaceutical fields. Among these efforts, pure organic dyes, quantum dots, and rare earth emitters have been explored, which are found to suffer from poor photostability for organic dyes, potential toxicity for quantum dots, and phase separation for rare earth emitters [7–9]. Therefore, it remains to be a challenge to achieve luminescent dyes owing good photostability, low toxicity, and high compatibility with supporting matrix. Phosphorescent Ru(II) complexes have been reported to exhibit strong emission signals and good photostability [10]. The highly emissive metal-to-ligand-charge-transfer (MLCT) state, suitable excited state lifetime, large stokes shift, and strong absorption in blue region of phosphorescent Ru(II) complexes make them promising to be utilized in optical materials. For practical applications in optical sensing devices, it is necessary to embed chemosensors into solid supporting matrix, allowing analyte transportation from surroundings. Indeed, the support may have quite stringent criteria for suitable performances [11]. Fortunately, most Ru(II) complexes are soluble in water and common organic solvents, which means that they can be introduced easily into supporting matrixes. Above mentioned characteristics make
L. Wang et al. / Synthetic Metals 161 (2011) 1976–1981
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Scheme 1. A fabrication route for the magnetic–luminescent composite nanoparticles of Ru@Fe2 O3 .
phosphorescent Ru(II) complexes promising candidates for luminescent dye in a composite system. Enlightened by above results, in this paper, we construct magnetic nanoparticles embedded with phosphorescent Ru(II) complex through covalent bond. The characterization, magnetic and photophysical properties of the obtained composite material are investigated in detail.
2. Experimental A fabrication route for the magnetic–luminescent composite nanoparticles (referred as Ru@Fe2 O3 ) is shown in Scheme 1. 3(triethoxysilyl)propyl isocyanate (TEPIC) and the 5% Pd/C catalyst were purchased from Aldrich (Milwaukee, WI, USA) and used without further purifications. Anhydrous RuCl3 (99.99%) was obtained from Acros Organics (Geel, Belgium). NH3 ·H2 O, tetraethoxysilane (TEOS), FeCl3 ·6H2 O, 2-propanol, hexane, chloroform, and ethanol were obtained from Beijing Chemical Company. The water used in this work was deionized. The starting Ru(II) complex of [Ru(bpy)2 (Phen-Si)]Cl2 was synthesized according to the literature procedure, where bpy = 2,2 -bipyridine, Phen-Si = 5-(di(1-(3(triethoxysilyl)propyl)urea-amino))-1,10-phenanthroline [12].
2.1. Fabrication of Fe2 O3 nanoparticles Fe2 O3 nanoparticles were synthesized as follows. Firstly, aqueous solution of 2.0 × 10−2 M FeCl3 was aged at 100 ◦ C for 50 h, which was then cooled to temperature. The resulted residue was collected and washed with plenty of water and ethanol, and later dried in N2 atmosphere at 80 ◦ C for 5 h.
2.2. Fabrication of Ru@Fe2 O3 The magnetic–luminescent composite nanoparticles of Ru@Fe2 O3 were fabricated as follows: 72.7 mM Fe2 O3 nanoparticles were added into a mixed solution of 0.45 M of ammonia, 3.05 M of water and 10 mL of 2-propanol. The solution was then exposed to an ultrasonic bath for 120 min at 25 ◦ C. 2 mM TEOS and [Ru(bpy)2 (Phen-Si)]Cl2 (100 mg/g) were added into the solution. The mixture was exposed to an ultrasonic bath for another 120 min at 25 ◦ C. The resulted residue was collected and washed with plenty of water and ethanol.
2.3. Measurements and methods X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max-Ra X-ray diffractometer using a Cu target radiation source ˚ Energy-dispersive analysis of X-ray (EDAX) and ( = 1.5418 A). field-emission scanning electron microscopy (FE-SEM) image were measured on a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) images and the selected-area election diffraction (SAED) pattern were obtained with a JEM-2010 transmission election microscope made by Japanese JEOL Company. Fluorescence image was recorded by a Nikon TE2000-U fluorescence microscopy upon green light excitation. IR absorption spectra were measured in the range 400–4000 cm−1 using an FT-IR spectrophotometer (Model Bruker Vertex 70 FT-IR) with a resolution of ±4 cm−1 using the KBr pellet technique. For UV–vis measurements, powder samples were dispersed in absolute ethanol by ultrasonic to form a uniform suspension, which were performed on an UV-3101PC UV–vis–NIR scanning spectrophotometer (SHIMADZU) at room temperature. Magnetic characteristics were studied using a vibrating sample magnetometer (VSM) (Lake Shore Company) at room temperature. The photoluminescence (PL) spectra were recorded
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L. Wang et al. / Synthetic Metals 161 (2011) 1976–1981
Fig. 1. XRD patterns of pure Fe2 O3 and Ru@Fe2 O3 . Fig. 2. IR spectrum of Ru@Fe2 O3 , [Ru(bpy)2 (Phen-Si)]Cl2 , and magnetic Fe2 O3 nanoparticles.
3. Results and discussion at room temperature with a Hitachi F-4500 spectrophotometer equipped with a continuous 150-W Xe-arc lamp. For luminescence decay measurement, a 355-nm light generated from the Nd3+ : YAG laser combined with a third-harmonic-generator was used as the pump, with a repetition frequency of 10 Hz and pulse duration of 10 ns. A two-channel TEKTRONIX TDS-3052 oscilloscope was used to record fluorescence decay curves. The oxygen sensing properties were discussed on the basis of PL intensity signals using the same Hitachi F-4500 fluorescence spectrophotometer. For the Stern–Volmer plot measurement, oxygen and nitrogen were mixed at different concentrations via gas flow controllers and passed directly to the sealed gas chamber. We typically allowed 1 min between changes in the N2 /O2 concentration to ensure that a new equilibrium point was established. The time-scanning curve was obtained similarly.
3.1. Characterization and morphology of Ru@Fe2 O3 3.1.1. XRD and IR spectra Fig. 1 shows the XRD patterns of pure Fe2 O3 and Ru@Fe2 O3 . The labeled diffraction peaks correspond to the typical rhombohedral structure of Fe2 O3 , which confirms the existence of Fe2 O3 core. But the (0 1 8) peak centering at ∼58◦ is rather weak, which can be explained as follows. Considering the core–shell structure of Ru@Fe2 O3 , where the Fe2 O3 core is covered by the outer shell of SiO2 , the effective diffraction of Fe2 O3 core may be weakened and is thus too weak to be detected, leading to the absence of (0 1 8) peak. Fig. 2 shows the IR spectrum of Ru@Fe2 O3 , along with those of [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles for comparative purpose. As for [Ru(bpy)2 (Phen-Si)]Cl2 , the character peaks
Fig. 3. TEM (left), SEM (top right corner), and fluorescence (bottom right corner) images of Ru@Fe2 O3 .
L. Wang et al. / Synthetic Metals 161 (2011) 1976–1981
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Fig. 4. Magnetic characters of Ru@Fe2 O3 sample and Fe2 O3 nanoparticles (inset).
1540 cm−1
of 1650 and are attributed to the absorption from –CONH– moiety [11,12]. The peaks of 470 and 575 cm−1 of Fe2 O3 nanoparticles are assigned to the character of Fe–O vibrations [13]. As for Ru@Fe2 O3 , the absorption peak of 1090 cm−1 which is assigned to the vibration of Si–O–Si confirms the successful formation of SiO2 framework. The typical characteristic peaks of –CONH– moiety and Fe–O vibrations, centering at 1540 cm−1 , 1650 cm−1 (assigned to the vibration of –CONH– moiety), and 1690 cm−1 (assigned to the vibration of C O bond), can all be found in Ru@Fe2 O3 IR spectrum, suggesting that the Ru(II) complex is successfully embedded into SiO2 backbone. In addition, above peaks show some red-shift tendency compared with corresponding ones of [Ru(bpy)2 (Phen-Si)]Cl2 , peaking at 1545 cm−1 , 1655 cm−1 , and 1712 cm−1 , respectively. After being grafted onto SiO2 backbone, the free vibrations of –CONH– and C O moieties are effectively suppressed due to the rigid microenvironment, leading to the red shift in Ru@Fe2 O3 sample [11,12]. 3.1.2. TEM, FE-SEM, and fluorescence images Fig. 3 shows the TEM image of Ru@Fe2 O3 , which confirms the core–shell structure of Ru@Fe2 O3 sample, and the diameter of the inner Fe2 O3 core is measured to be ∼60 nm. The FE-SEM figure of Ru@Fe2 O3 shown in Fig. 3, suggests that all particles render a mean diameter of ∼160 nm, showing uniform and monodisperse morphology, which suggests that the introduction of outer SiO2 shell can effectively block the aggregation of magnetic Fe2 O3 core. The thickness of the outer shell of SiO2 is then calculated to be ∼50 nm, which is believed to be adjustable by modulating the radio of TEOS:[Ru(bpy)2 (Phen-Si)]Cl2 [14]. In order to further confirm the successful introduction of Ru(II) complex, the fluorescence image of Ru@Fe2 O3 sample is also given as Fig. 3. Bright red emission is observed, and we attribute this red emission to the triplet metal-to-ligand-charge-transfer (3 MLCT) excited state, which further confirms that Ru(II) complex is successfully grafted onto the backbone of the SiO2 outer shell. 3.2. Magnetic property of Ru@Fe2 O3 Fig. 4 shows the magnetic character of Ru@Fe2 O3 sample, along with that of Fe2 O3 nanoparticles for comparative purpose. The field-dependent magnetization curves of both samples are found to be hysteretic. Ru@Fe2 O3 sample exhibits a strong magnetic response towards varying magnetic field, showing a large magnetic hysteresis loop as shown in Fig. 4. The coercivity value of Ru@Fe2 O3 sample is measured to be as high as ∼130 G, while that of Fe2 O3 nanoparticles is 155 G. In addition, the remnant magnetization to saturation magnetization ratio of Ru@Fe2 O3 sample is measured to
Fig. 5. UV–vis absorption spectra of Ru@Fe2 O3 , [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles.
be 0.35, compared with 0.26 of Fe2 O3 nanoparticles. Data comparison suggests that the outer shell of SiO2 some what compromises the magnetic response of Fe2 O3 core, however, the magnetic character still qualifies Ru@Fe2 O3 as a promising magnetic material. 3.3. Photophysical characteristics of Ru@Fe2 O3 3.3.1. UV–vis absorption Fig. 5 shows the UV–vis absorption spectrum of Ru@Fe2 O3 , along with those of [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles for comparative purpose. The absorption spectrum of [Ru(bpy)2 (PhenSi)]Cl2 is composed of two strong peaks of 223 nm and 264 nm in high energy region, along with a weak absorption ranging from 340 nm to 550 nm. The high energy band is assigned to the → * electronic transition of diamine ligand according to the literature report [11]. While, the low energy band is tentatively attributed to singlet metal-to-ligand-charge-transfer (1 MLCT) transition. As for the absorption spectrum of Fe2 O3 nanoparticles, a broad absorption band is observed, without giving any vibronic progressions, which means that a charge-transfer excited state is responsible for the absorption. Here, we assign the absorption to the charge transfer of d(Fe) → d*(Fe) and d(Fe) → d*(O). Above two typical absorption characters of [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles can both be found in the absorption spectrum of Ru@Fe2 O3 , confirming the successful formation of the composite material. Indeed, the absorption spectrum of Ru@Fe2 O3 can be considered as a simple adduct of two absorption spectra, which means that there is no obvious interaction between Ru(II) complex localizing in SiO2 outer shell and the inner core of Fe2 O3 . We attribute this phenomenon to the quarantine effect from SiO2 . 3.3.2. PL emission and excited state lifetime Fig. 6 shows the emission spectra of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state. It can be observed that [Ru(bpy)2 (PhenSi)]Cl2 exhibits a red emission centering at 587 nm, and this emission comes from the MLCT excited state as above mentioned. The emission of Ru@Fe2 O3 peaking at 592 nm shows a red shift tendency, compared with that of [Ru(bpy)2 (Phen-Si)]Cl2 . We attribute the causation of the red shift to the interaction between Ru(II) emissive center and the hydroxyl groups of SiO2 , which can stabilize the excited state and thus lead to the red shift of emission. The inset of Fig. 6 shows the PL intensity versus time characteristics of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state. Both emissions are found to follow a biexponential decay pattern. As for [Ru(bpy)2 (Phen-Si)]Cl2 , its decay curve is composed
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Fig. 6. Emission spectra of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state. Inset: PL intensity versus time characteristics of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state.
of 1 = 0.076 s (A1 = 1.863) and 2 = 0.528 s (A2 = 0.088), respectively. The decay curve of Ru@Fe2 O3 is composed of 1 = 0.053 s (A1 = 2.036) and 2 = 0.264 s (A2 = 0.358), respectively. From their comparison, it can be found that the excited state lifetime of the Ru(II) emissive center shows a decrease tendency when grafted onto the backbone of SiO2 in Ru@Fe2 O3 . We attribute this phenomenon to the quenching effect of Fe2 O3 core. In general, the observation of effective excitation band in → * state absorption region and the shorter decay lifetime from → * state than those from the lower MLCT state can suggest an efficient potential surface crossing (PSC) from the higher → * state to the lower MLCT state. As shown in Fig. 5, there is a strong absorption of [Ru(bpy)2 (Phen-Si)]Cl2 in → * state region, suggesting an efficient potential surface crossing from higher → * state to MLCT state of [Ru(bpy)2 (Phen-Si)]Cl2 . Therefore, the assignment of 1 (major contribution to decay) to the emission from the → * state should be believable. And this component assignment is consistent with the short-lived → * transition nature. 3.3.3. PL response towards oxygen Related references report that the emission signals of Ru(II) complexes are sensitive towards molecular oxygen due to the energy transfer process between excited state Ru(II) complexes and molecular oxygen [11,12], making Ru(II) complexes potential chemosensors for oxygen detection. Here, we also intend to
Fig. 8. Stern–Volmer plot of Ru@Fe2 O3 at various oxygen concentrations. Inset: PL intensity responses of Ru@Fe2 O3 sample when exposed to periodically varied 100% N2 and 100% O2 atmospheres.
investigate the PL response of Ru@Fe2 O3 towards various molecular oxygen concentrations. Fig. 7 shows the emission signal response of Ru@Fe2 O3 upon changing oxygen concentration from 0% to 100%. With increasing oxygen concentrations, the emission intensity at 593 nm decreases obviously. The intensity ratio of I0 /I100 , where I0 is the luminescence intensity under 100% N2 atmosphere and I100 is that under 100% O2 atmosphere, is measured to be 1.48. This value is much lower than literature ones in which Ru(II) complexes are grafted onto silica matrix of MCM-41 and SBA-15 [11,12], and we are giving an explanation as follows. As mentioned above, the doptant concentration is low (100 mg/g), and the Fe2 O3 core seems to quench the excited state of the Ru(II) complex. The emission from the Ru(II) complex is thus pretty weak, leading to the limited value of I0 /I100 . What is more, the decreased excited state lifetime also decreases the chance for excited state Ru(II) center to be attacked by oxygen, which is also responsible for the unsatisfactory value of I0 /I100 . Generally, in a homogeneous medium with a single-exponential decay, the intensity form of Stern–Volmer equation with dynamic quenching is described as follows [11,12,15], where I is the luminescent intensity. The subscript 0 denotes a value in the absence of quencher, KSV is the Stern–Volmer constant, and [O2 ] is O2 concentration. I0 = 1 + KSV [O2 ] I
(1)
As above mentioned, the contribution of 1 to decay lifetime is much larger than that of 2 . Here we neglect the minor contribution of 2 , and the plot of I0 /I versus [O2 ] should be linear with identical slopes of KSV . The Stern–Volmer plot of Ru@Fe2 O3 at various oxygen concentrations shown in Fig. 8 fits well with Eq. (1) and exhibits a good linear relationship with increasing [O2 ], which confirms that the microenvironment around the Ru(II) emissive center is homogeneous, and the neglect of minor contribution from 2 is reasonable. The emission signal shows a quick response towards atmosphere variations, as shown by the inset of Fig. 8. Upon on periodic atmosphere variation from pure N2 to pure O2 then back to pure N2 , the maximum emission intensity shows a decrease tendency, which is called photobleaching, which means that the photostability of the emissive center needs to be further improved. 4. Conclusion Fig. 7. Emission signal response of Ru@Fe2 O3 upon changing oxygen concentration from 0% to 100%.
In conclusion, we construct core–shell structured nanoparticles of Ru@Fe2 O3 , where magnetic Fe2 O3 nanoparticles are used
L. Wang et al. / Synthetic Metals 161 (2011) 1976–1981
as the inner core and SiO2 doped with phosphorescent Ru(II) complex is used as the outer shell. The obtained magnetic–luminescent composite nanoparticles are identified and characterized by XRD analysis, IR spectrum, FE-SEM, TEM, and fluorescence image, which confirms the core–shell structure. Then the magnetic and photophysical properties of the composite nanoparticles are investigated in detail. Data suggest that the outer shell of SiO2 weakens the magnetic response of Fe2 O3 core by showing a smaller coercivity value compared with that of pure Fe2 O3 nanoparticles. The composite nanoparticles are red-emitting ones peaking at 592 nm, and the emission signal is sensitive towards oxygen concentration variations with a sensitivity of 1.48. However, the decreased excited state lifetime and weak emission of the Ru(II) emissive center limit the sensitivity. A slight photobleaching is obtained for Ru@Fe2 O3 , which means that the photostability should be further improved. Above conclusion may be useful for future design of magnetic–luminescent composite materials. Acknowledgment The authors gratefully thank the NNSFC (Grant No. 30700964) for financial support.
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References [1] H. Kim, M. Achermann, L.P. Balet, J.A. Hollingsworth, V.I. Klimov, J. Am. Chem. Soc. 127 (2005) 544–546. [2] Z.Y. Liu, G.S. Yi, H.T. Zhang, J. Ding, Y.M. Zhang, J.M. Xue, Chem. Commun. 6 (2008) 694–696. [3] A.S. Lubbe, C. Bergemann, H. Riess, Cancer Res. 56 (1996) 4686–4793. [4] A. Dyal, K. Loos, M. Noto, S.W. Chang, C. Spagnoli, K. Shafi, A. Ulman, M. Cowman, R.A. Gross, J. Am. Chem. Soc. 125 (2003) 1684–1685. [5] B.T. Dayane, L.R.V. Lucas, L.D. Evandro, I. Rosangela, K.K. Pedro, S.B. Maurício, M.R. Liane, Langmuir 23 (2007) 8194–8199. [6] C. Barbe, J. Bartlett, L. Kong, K. Finnie, H.Q. Lin, M. Larkin, S.B.A. Calleja, G. Calleja, Adv. Mater. 16 (2004) 1959–1966. [7] Y. Lu, Y.D. Yin, B.T. Mayers, Y.N. Xia, Nano Lett. 2 (2002) 183–186. [8] D.K. Yi, T. Selvan, S.S. Lee, G.C. Papaefthymiou, D. Kundaliya, J.Y. Ying, J. Am. Chem. Soc. 127 (2005) 4990–4991. [9] S.Y. Yu, H.J. Zhang, J.B. Yu, C. Wang, L.N. Sun, W.D. Shi, Langmuir 23 (2007) 7836–7840. [10] L.M. Rossi, L.F. Shi, F.H. Quina, Z. Rosenzweig, Langmuir 21 (2005) 42774280. [11] B. Lei, B. Li, H. Zhang, L. Zhang, W. Li, J. Phys. Chem. C 111 (2007) 11291– 11301. [12] B.F. Lei, B. Li, H.R. Zhang, S.Z. Lu, Z.H. Zheng, W.L. Li, Y. Wang, Adv. Funct. Mater. 16 (2006) 1883. [13] J.L. Zhang, R.S. Srivastava, R.D.K. Misra, Langmuir 23 (2007) 6342–6351. [14] L. Levy, Y. Sahoo, K.S. Kim, E.J. Bergey, P.N. Prasad, Chem. Mater. 14 (2002) 3715–3721. [15] L. Shi, B. Li, S. Yue, D. Fan, Sens. Actuators B: Chem. 137 (2009) 386–392.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Magnetic–luminescent composite nanoparticles: Fabrication, characterization, magnetic and photophysical properties Liyan Wang a,b,∗ , Lei Huang c , Zhu Lin b , Xiaofeng Chen d , Yanhua Xu e a
Department of Stomotology, The Second People’s Hospital of FoShan, GuangDong, China Department of Orthodontics, School of Stomatology, The Fourth Military Medical University, XiAn, ShanXi, China Department of Stomotology, The First People’s Hospital of GuangZhou, GuangZhou, GuangDong, China d School of Materials Science and Engineering, South China University of Technology, GuangZhou, GuangDong, China e Department of Orthodontics, School of Stomatology, KunMing Medical College, KunMing, China b c
a r t i c l e
i n f o
Article history: Received 3 May 2011 Received in revised form 30 May 2011 Accepted 1 June 2011 Available online 31 July 2011 Keywords: Core–shell structure Luminescent Ru(II) complex Magnetic Fe2 O3 Composite material
a b s t r a c t In this paper, we construct core–shell structured nanoparticles, where magnetic Fe2 O3 nanoparticles are used as the inner core and SiO2 doped with phosphorescent Ru(II) complex is used as the outer shell. The obtained magnetic–luminescent composite nanoparticles are identified and characterized by XRD analysis, IR spectrum, field-emission scanning electron microscopy, transmission electron microscopy, and fluorescence image, which confirms the core–shell structure. Then the magnetic and photophysical properties of the composite nanoparticles are investigated in detail. Data suggest that the outer shell of SiO2 weakens the magnetic response of Fe2 O3 core by showing a smaller coercivity value compared with that of pure Fe2 O3 nanoparticles. The composite nanoparticles are red-emitting ones, and the emission signal is sensitive towards oxygen concentration variations. However, the decreased excited state lifetime and weak emission of the Ru(II) emissive center limit the sensitivity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent yeas, the design and construction of multifunctional nanostructured and nanocomposite materials have been considered to be a promising way to satisfy various demands because they can realize a combination of both enhanced functionality and multifunctional characters in contrast with their single-component counterparts [1,2]. Magnetic iron oxide nanoparticles, owing to the advantages of low toxicity and high saturation magnetization, have been widely employed in biomedical applications such as drug targeting, cell sorting and isolating, magnetic resonance imaging, and so on [3–5]. There is, however, a problem waiting to be conquered, which means that the magnetic nanoparticles tend to aggregate and correspondingly compromise their monodispersal. It is thus an important consideration to coat the bare magnetic nanoparticle cores with suitable shells. Amorphous silica seems to satisfy above requirements owing to its low toxicity and high compatibility with biological systems [6]. In addition, the silica surface can be readily functionalized with
∗ Corresponding author at: Department of Stomotology, The Second People’s Hospital of FoShan, GuangDong, China. Tel.: +86 0757 83635705; fax: +86 0757 83635705. E-mail address: [email protected] (L. Wang). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.06.041
amines, thiols, and carboxyl moieties, further favoring the conjunction of biomolecules. Several research groups have tried the combination of magnetic nanoparticles and fluorescent dyes into a single composite system, hoping to realize nanoparticles with optical activities and thus enlighten their applications in biomedical and biopharmaceutical fields. Among these efforts, pure organic dyes, quantum dots, and rare earth emitters have been explored, which are found to suffer from poor photostability for organic dyes, potential toxicity for quantum dots, and phase separation for rare earth emitters [7–9]. Therefore, it remains to be a challenge to achieve luminescent dyes owing good photostability, low toxicity, and high compatibility with supporting matrix. Phosphorescent Ru(II) complexes have been reported to exhibit strong emission signals and good photostability [10]. The highly emissive metal-to-ligand-charge-transfer (MLCT) state, suitable excited state lifetime, large stokes shift, and strong absorption in blue region of phosphorescent Ru(II) complexes make them promising to be utilized in optical materials. For practical applications in optical sensing devices, it is necessary to embed chemosensors into solid supporting matrix, allowing analyte transportation from surroundings. Indeed, the support may have quite stringent criteria for suitable performances [11]. Fortunately, most Ru(II) complexes are soluble in water and common organic solvents, which means that they can be introduced easily into supporting matrixes. Above mentioned characteristics make
L. Wang et al. / Synthetic Metals 161 (2011) 1976–1981
1977
Scheme 1. A fabrication route for the magnetic–luminescent composite nanoparticles of Ru@Fe2 O3 .
phosphorescent Ru(II) complexes promising candidates for luminescent dye in a composite system. Enlightened by above results, in this paper, we construct magnetic nanoparticles embedded with phosphorescent Ru(II) complex through covalent bond. The characterization, magnetic and photophysical properties of the obtained composite material are investigated in detail.
2. Experimental A fabrication route for the magnetic–luminescent composite nanoparticles (referred as Ru@Fe2 O3 ) is shown in Scheme 1. 3(triethoxysilyl)propyl isocyanate (TEPIC) and the 5% Pd/C catalyst were purchased from Aldrich (Milwaukee, WI, USA) and used without further purifications. Anhydrous RuCl3 (99.99%) was obtained from Acros Organics (Geel, Belgium). NH3 ·H2 O, tetraethoxysilane (TEOS), FeCl3 ·6H2 O, 2-propanol, hexane, chloroform, and ethanol were obtained from Beijing Chemical Company. The water used in this work was deionized. The starting Ru(II) complex of [Ru(bpy)2 (Phen-Si)]Cl2 was synthesized according to the literature procedure, where bpy = 2,2 -bipyridine, Phen-Si = 5-(di(1-(3(triethoxysilyl)propyl)urea-amino))-1,10-phenanthroline [12].
2.1. Fabrication of Fe2 O3 nanoparticles Fe2 O3 nanoparticles were synthesized as follows. Firstly, aqueous solution of 2.0 × 10−2 M FeCl3 was aged at 100 ◦ C for 50 h, which was then cooled to temperature. The resulted residue was collected and washed with plenty of water and ethanol, and later dried in N2 atmosphere at 80 ◦ C for 5 h.
2.2. Fabrication of Ru@Fe2 O3 The magnetic–luminescent composite nanoparticles of Ru@Fe2 O3 were fabricated as follows: 72.7 mM Fe2 O3 nanoparticles were added into a mixed solution of 0.45 M of ammonia, 3.05 M of water and 10 mL of 2-propanol. The solution was then exposed to an ultrasonic bath for 120 min at 25 ◦ C. 2 mM TEOS and [Ru(bpy)2 (Phen-Si)]Cl2 (100 mg/g) were added into the solution. The mixture was exposed to an ultrasonic bath for another 120 min at 25 ◦ C. The resulted residue was collected and washed with plenty of water and ethanol.
2.3. Measurements and methods X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max-Ra X-ray diffractometer using a Cu target radiation source ˚ Energy-dispersive analysis of X-ray (EDAX) and ( = 1.5418 A). field-emission scanning electron microscopy (FE-SEM) image were measured on a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) images and the selected-area election diffraction (SAED) pattern were obtained with a JEM-2010 transmission election microscope made by Japanese JEOL Company. Fluorescence image was recorded by a Nikon TE2000-U fluorescence microscopy upon green light excitation. IR absorption spectra were measured in the range 400–4000 cm−1 using an FT-IR spectrophotometer (Model Bruker Vertex 70 FT-IR) with a resolution of ±4 cm−1 using the KBr pellet technique. For UV–vis measurements, powder samples were dispersed in absolute ethanol by ultrasonic to form a uniform suspension, which were performed on an UV-3101PC UV–vis–NIR scanning spectrophotometer (SHIMADZU) at room temperature. Magnetic characteristics were studied using a vibrating sample magnetometer (VSM) (Lake Shore Company) at room temperature. The photoluminescence (PL) spectra were recorded
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Fig. 1. XRD patterns of pure Fe2 O3 and Ru@Fe2 O3 . Fig. 2. IR spectrum of Ru@Fe2 O3 , [Ru(bpy)2 (Phen-Si)]Cl2 , and magnetic Fe2 O3 nanoparticles.
3. Results and discussion at room temperature with a Hitachi F-4500 spectrophotometer equipped with a continuous 150-W Xe-arc lamp. For luminescence decay measurement, a 355-nm light generated from the Nd3+ : YAG laser combined with a third-harmonic-generator was used as the pump, with a repetition frequency of 10 Hz and pulse duration of 10 ns. A two-channel TEKTRONIX TDS-3052 oscilloscope was used to record fluorescence decay curves. The oxygen sensing properties were discussed on the basis of PL intensity signals using the same Hitachi F-4500 fluorescence spectrophotometer. For the Stern–Volmer plot measurement, oxygen and nitrogen were mixed at different concentrations via gas flow controllers and passed directly to the sealed gas chamber. We typically allowed 1 min between changes in the N2 /O2 concentration to ensure that a new equilibrium point was established. The time-scanning curve was obtained similarly.
3.1. Characterization and morphology of Ru@Fe2 O3 3.1.1. XRD and IR spectra Fig. 1 shows the XRD patterns of pure Fe2 O3 and Ru@Fe2 O3 . The labeled diffraction peaks correspond to the typical rhombohedral structure of Fe2 O3 , which confirms the existence of Fe2 O3 core. But the (0 1 8) peak centering at ∼58◦ is rather weak, which can be explained as follows. Considering the core–shell structure of Ru@Fe2 O3 , where the Fe2 O3 core is covered by the outer shell of SiO2 , the effective diffraction of Fe2 O3 core may be weakened and is thus too weak to be detected, leading to the absence of (0 1 8) peak. Fig. 2 shows the IR spectrum of Ru@Fe2 O3 , along with those of [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles for comparative purpose. As for [Ru(bpy)2 (Phen-Si)]Cl2 , the character peaks
Fig. 3. TEM (left), SEM (top right corner), and fluorescence (bottom right corner) images of Ru@Fe2 O3 .
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Fig. 4. Magnetic characters of Ru@Fe2 O3 sample and Fe2 O3 nanoparticles (inset).
1540 cm−1
of 1650 and are attributed to the absorption from –CONH– moiety [11,12]. The peaks of 470 and 575 cm−1 of Fe2 O3 nanoparticles are assigned to the character of Fe–O vibrations [13]. As for Ru@Fe2 O3 , the absorption peak of 1090 cm−1 which is assigned to the vibration of Si–O–Si confirms the successful formation of SiO2 framework. The typical characteristic peaks of –CONH– moiety and Fe–O vibrations, centering at 1540 cm−1 , 1650 cm−1 (assigned to the vibration of –CONH– moiety), and 1690 cm−1 (assigned to the vibration of C O bond), can all be found in Ru@Fe2 O3 IR spectrum, suggesting that the Ru(II) complex is successfully embedded into SiO2 backbone. In addition, above peaks show some red-shift tendency compared with corresponding ones of [Ru(bpy)2 (Phen-Si)]Cl2 , peaking at 1545 cm−1 , 1655 cm−1 , and 1712 cm−1 , respectively. After being grafted onto SiO2 backbone, the free vibrations of –CONH– and C O moieties are effectively suppressed due to the rigid microenvironment, leading to the red shift in Ru@Fe2 O3 sample [11,12]. 3.1.2. TEM, FE-SEM, and fluorescence images Fig. 3 shows the TEM image of Ru@Fe2 O3 , which confirms the core–shell structure of Ru@Fe2 O3 sample, and the diameter of the inner Fe2 O3 core is measured to be ∼60 nm. The FE-SEM figure of Ru@Fe2 O3 shown in Fig. 3, suggests that all particles render a mean diameter of ∼160 nm, showing uniform and monodisperse morphology, which suggests that the introduction of outer SiO2 shell can effectively block the aggregation of magnetic Fe2 O3 core. The thickness of the outer shell of SiO2 is then calculated to be ∼50 nm, which is believed to be adjustable by modulating the radio of TEOS:[Ru(bpy)2 (Phen-Si)]Cl2 [14]. In order to further confirm the successful introduction of Ru(II) complex, the fluorescence image of Ru@Fe2 O3 sample is also given as Fig. 3. Bright red emission is observed, and we attribute this red emission to the triplet metal-to-ligand-charge-transfer (3 MLCT) excited state, which further confirms that Ru(II) complex is successfully grafted onto the backbone of the SiO2 outer shell. 3.2. Magnetic property of Ru@Fe2 O3 Fig. 4 shows the magnetic character of Ru@Fe2 O3 sample, along with that of Fe2 O3 nanoparticles for comparative purpose. The field-dependent magnetization curves of both samples are found to be hysteretic. Ru@Fe2 O3 sample exhibits a strong magnetic response towards varying magnetic field, showing a large magnetic hysteresis loop as shown in Fig. 4. The coercivity value of Ru@Fe2 O3 sample is measured to be as high as ∼130 G, while that of Fe2 O3 nanoparticles is 155 G. In addition, the remnant magnetization to saturation magnetization ratio of Ru@Fe2 O3 sample is measured to
Fig. 5. UV–vis absorption spectra of Ru@Fe2 O3 , [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles.
be 0.35, compared with 0.26 of Fe2 O3 nanoparticles. Data comparison suggests that the outer shell of SiO2 some what compromises the magnetic response of Fe2 O3 core, however, the magnetic character still qualifies Ru@Fe2 O3 as a promising magnetic material. 3.3. Photophysical characteristics of Ru@Fe2 O3 3.3.1. UV–vis absorption Fig. 5 shows the UV–vis absorption spectrum of Ru@Fe2 O3 , along with those of [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles for comparative purpose. The absorption spectrum of [Ru(bpy)2 (PhenSi)]Cl2 is composed of two strong peaks of 223 nm and 264 nm in high energy region, along with a weak absorption ranging from 340 nm to 550 nm. The high energy band is assigned to the → * electronic transition of diamine ligand according to the literature report [11]. While, the low energy band is tentatively attributed to singlet metal-to-ligand-charge-transfer (1 MLCT) transition. As for the absorption spectrum of Fe2 O3 nanoparticles, a broad absorption band is observed, without giving any vibronic progressions, which means that a charge-transfer excited state is responsible for the absorption. Here, we assign the absorption to the charge transfer of d(Fe) → d*(Fe) and d(Fe) → d*(O). Above two typical absorption characters of [Ru(bpy)2 (Phen-Si)]Cl2 and Fe2 O3 nanoparticles can both be found in the absorption spectrum of Ru@Fe2 O3 , confirming the successful formation of the composite material. Indeed, the absorption spectrum of Ru@Fe2 O3 can be considered as a simple adduct of two absorption spectra, which means that there is no obvious interaction between Ru(II) complex localizing in SiO2 outer shell and the inner core of Fe2 O3 . We attribute this phenomenon to the quarantine effect from SiO2 . 3.3.2. PL emission and excited state lifetime Fig. 6 shows the emission spectra of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state. It can be observed that [Ru(bpy)2 (PhenSi)]Cl2 exhibits a red emission centering at 587 nm, and this emission comes from the MLCT excited state as above mentioned. The emission of Ru@Fe2 O3 peaking at 592 nm shows a red shift tendency, compared with that of [Ru(bpy)2 (Phen-Si)]Cl2 . We attribute the causation of the red shift to the interaction between Ru(II) emissive center and the hydroxyl groups of SiO2 , which can stabilize the excited state and thus lead to the red shift of emission. The inset of Fig. 6 shows the PL intensity versus time characteristics of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state. Both emissions are found to follow a biexponential decay pattern. As for [Ru(bpy)2 (Phen-Si)]Cl2 , its decay curve is composed
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Fig. 6. Emission spectra of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state. Inset: PL intensity versus time characteristics of [Ru(bpy)2 (Phen-Si)]Cl2 and Ru@Fe2 O3 in solid state.
of 1 = 0.076 s (A1 = 1.863) and 2 = 0.528 s (A2 = 0.088), respectively. The decay curve of Ru@Fe2 O3 is composed of 1 = 0.053 s (A1 = 2.036) and 2 = 0.264 s (A2 = 0.358), respectively. From their comparison, it can be found that the excited state lifetime of the Ru(II) emissive center shows a decrease tendency when grafted onto the backbone of SiO2 in Ru@Fe2 O3 . We attribute this phenomenon to the quenching effect of Fe2 O3 core. In general, the observation of effective excitation band in → * state absorption region and the shorter decay lifetime from → * state than those from the lower MLCT state can suggest an efficient potential surface crossing (PSC) from the higher → * state to the lower MLCT state. As shown in Fig. 5, there is a strong absorption of [Ru(bpy)2 (Phen-Si)]Cl2 in → * state region, suggesting an efficient potential surface crossing from higher → * state to MLCT state of [Ru(bpy)2 (Phen-Si)]Cl2 . Therefore, the assignment of 1 (major contribution to decay) to the emission from the → * state should be believable. And this component assignment is consistent with the short-lived → * transition nature. 3.3.3. PL response towards oxygen Related references report that the emission signals of Ru(II) complexes are sensitive towards molecular oxygen due to the energy transfer process between excited state Ru(II) complexes and molecular oxygen [11,12], making Ru(II) complexes potential chemosensors for oxygen detection. Here, we also intend to
Fig. 8. Stern–Volmer plot of Ru@Fe2 O3 at various oxygen concentrations. Inset: PL intensity responses of Ru@Fe2 O3 sample when exposed to periodically varied 100% N2 and 100% O2 atmospheres.
investigate the PL response of Ru@Fe2 O3 towards various molecular oxygen concentrations. Fig. 7 shows the emission signal response of Ru@Fe2 O3 upon changing oxygen concentration from 0% to 100%. With increasing oxygen concentrations, the emission intensity at 593 nm decreases obviously. The intensity ratio of I0 /I100 , where I0 is the luminescence intensity under 100% N2 atmosphere and I100 is that under 100% O2 atmosphere, is measured to be 1.48. This value is much lower than literature ones in which Ru(II) complexes are grafted onto silica matrix of MCM-41 and SBA-15 [11,12], and we are giving an explanation as follows. As mentioned above, the doptant concentration is low (100 mg/g), and the Fe2 O3 core seems to quench the excited state of the Ru(II) complex. The emission from the Ru(II) complex is thus pretty weak, leading to the limited value of I0 /I100 . What is more, the decreased excited state lifetime also decreases the chance for excited state Ru(II) center to be attacked by oxygen, which is also responsible for the unsatisfactory value of I0 /I100 . Generally, in a homogeneous medium with a single-exponential decay, the intensity form of Stern–Volmer equation with dynamic quenching is described as follows [11,12,15], where I is the luminescent intensity. The subscript 0 denotes a value in the absence of quencher, KSV is the Stern–Volmer constant, and [O2 ] is O2 concentration. I0 = 1 + KSV [O2 ] I
(1)
As above mentioned, the contribution of 1 to decay lifetime is much larger than that of 2 . Here we neglect the minor contribution of 2 , and the plot of I0 /I versus [O2 ] should be linear with identical slopes of KSV . The Stern–Volmer plot of Ru@Fe2 O3 at various oxygen concentrations shown in Fig. 8 fits well with Eq. (1) and exhibits a good linear relationship with increasing [O2 ], which confirms that the microenvironment around the Ru(II) emissive center is homogeneous, and the neglect of minor contribution from 2 is reasonable. The emission signal shows a quick response towards atmosphere variations, as shown by the inset of Fig. 8. Upon on periodic atmosphere variation from pure N2 to pure O2 then back to pure N2 , the maximum emission intensity shows a decrease tendency, which is called photobleaching, which means that the photostability of the emissive center needs to be further improved. 4. Conclusion Fig. 7. Emission signal response of Ru@Fe2 O3 upon changing oxygen concentration from 0% to 100%.
In conclusion, we construct core–shell structured nanoparticles of Ru@Fe2 O3 , where magnetic Fe2 O3 nanoparticles are used
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as the inner core and SiO2 doped with phosphorescent Ru(II) complex is used as the outer shell. The obtained magnetic–luminescent composite nanoparticles are identified and characterized by XRD analysis, IR spectrum, FE-SEM, TEM, and fluorescence image, which confirms the core–shell structure. Then the magnetic and photophysical properties of the composite nanoparticles are investigated in detail. Data suggest that the outer shell of SiO2 weakens the magnetic response of Fe2 O3 core by showing a smaller coercivity value compared with that of pure Fe2 O3 nanoparticles. The composite nanoparticles are red-emitting ones peaking at 592 nm, and the emission signal is sensitive towards oxygen concentration variations with a sensitivity of 1.48. However, the decreased excited state lifetime and weak emission of the Ru(II) emissive center limit the sensitivity. A slight photobleaching is obtained for Ru@Fe2 O3 , which means that the photostability should be further improved. Above conclusion may be useful for future design of magnetic–luminescent composite materials. Acknowledgment The authors gratefully thank the NNSFC (Grant No. 30700964) for financial support.
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