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Synthesis of surface amino-functionalized NaGdF4:Ce,Tb nanoparticles and their luminescence resonance energy transfer (LRET) with Au nanoparticles
Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 152–156
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of surface amino-functionalized NaGdF4 :Ce,Tb nanoparticles and their luminescence resonance energy transfer (LRET) with Au nanoparticles Congcong Mi a , Huanyu Gao a , Feng Li b , Shukun Xu a,∗ a b
Department of Chemistry, Northeastern University, Shenyang, 110819, PR China College of Science, Shenyang University, Shenyang, 110044, PR China
a r t i c l e
i n f o
Article history: Received 14 August 2011 Received in revised form 18 November 2011 Accepted 7 December 2011 Available online 16 December 2011 Keywords: NaGdF4 Solvothermal synthesis Au nanoparticles LRET
a b s t r a c t Luminescent/magnetic dual-functional NaGdF4 :Ce,Tb nanoparticles were first synthesized via a solvothermal method with rare earth stearate as the precursor, and the luminescence resonance energy transfer of NaGdF4 :Ce,Tb to Au nanoparticles was studied. The synthesized NaGdF4 :Ce,Tb were ellipsoidal nanoparticles, with pure hexagonal structure and an average size of 15 nm. The nanoparticles emit strong emission at 544 nm under the 246 nm excitation, and show good paramagnetic at room temperature. After surface amination by the typical Stöber method, the as-prepared NaGdF4 :Ce,Tb nanoparticles were served as the donor for the LRET with Au nanoparticles. Good linear relationships between the luminescence intensities of NaGdF4 :Ce,Tb (at 490 nm and 544 nm) and the concentration of Au nanoparticles were obtained, indicating a great potential application prospect of the LRET system in bioassays. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fluorescence resonance energy transfer (FRET) is a nonradiative process in which the electronic excitation energy of a donor chromophore is transferred to a nearby acceptor molecule via long-range dipole–dipole interactions [1]. To date, luminescent materials commonly used in FRET include organic dye [2], quantum dots [3,4] and rare earth doped down- or up-conversion nanoparticles [5,6], while, the FRET system with rare earth doped materials as the donor was commonly named as LRET [7]. Among them, the rare earth doped luminescent materials usually possess high quantum yield, long fluorescence lifetime and narrow emission peak. Especially, the rare earth doped upconversion nanoparticles are usually excited by near infrared light (NIR), the low excitation energy can effectively avoid auto-fluorescence from samples [8]. Among the rare earth doped luminescent materials, it is well known that Gd3+ ions doped nanoparticles always possess paramagnetism [9]. In recent years, nanomaterials that possess bifunctional properties have attracted more and more attention [10,11]. For example, nanomaterials exhibit optical and magnetic properties can realize the detection of samples by optical labeling or magnetic resonance image, and carry out separation and enrichment of samples at the same time [12]. Recently, researches on the synthesis and application of these bi-functional nanoparticles
∗ Corresponding author. Tel.: +86 2483681343. E-mail address: [email protected] (S. Xu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.022
have become the hotspot, there are some reports dedicated on it. The hydrophobic oleic acid-capped Tm/Yb/Er co-doped NaGdF4 nanoparticles were synthesized and applied as contrast agents for magnetic resonance imaging (MRI) in vivo [13]. The results indicated that these dual-functional nanoparticles exhibited excellent signal-to-noise ratio, low cytotoxicity, and can be served as a new platform for the detection of biological tissues. Based on the recent research, the major approaches for the preparation of rare earth doped nanoparticles in solution phase, include co-precipitation [14,15], thermal decomposition [16], sol–gel [17], polyol-mediated process [18], hydro/solvothermal [19,20], micro-emulsion [21], and son on. However, with the rapid development of nanotechnology, rare earth doped materials have demonstrated broad prospects in application of optics, optoelectronics, and bioassays, etc. [22,23]. Above all, the synthesis of high quality nanocrystals possessing small size, uniform shape, both high luminescent and magnetic intensity with a simple and rapid method is still of important significance. Recently, the analysis and detection based on the LRET system have been developed as a new kind of method with high sensitivity and selectivity. The method has been applied for the detection of nucleic acids [24], avidin [25], glucose [26] and immunoassay [27], etc. The lanthanides doped luminescent materials usually possess high quantum yield, long fluorescence lifetime and narrow emission peak [28,29], so that the signal to noise ratio can be greatly improved when lanthanides were used as the donor in LRET. Au nanoparticles itself cannot produce fluorescence, leading to avoid the interference from background. Moreover, the broad and
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2.3. Preparation of Au nanoparticles In this experiment, sodium citrate was used as the reductant for the preparation of gold nanoparticles. During the specific experimental procedure, 1 mL of 1% chlorine gold acid and 79 mL water were first mixed together and heated to 60 ◦ C, then 2 mL of 1% citric acid sodium solution was quickly added. After 30 min, Au colloid solution was obtained. 2.4. LRET between NaGdF4 :Ce,Tb and Au nanoparticles Scheme 1. The LRET diagram between NaGdF4 :Ce,Tb and Au nanoparticles.
continuous absorption of Au nanoparticles can ensure the high efficiency of a LRET process. Therefore, Au nanoparticles were usually used as the receptor in LRET system. In this work, a novel method for the synthesis of luminescent/magnetic bi-functional NaGdF4 :Ce,Tb nanoparticles was developed via a two phase mechanism, with rare-earth stearate as the precursor, oleic acid, ethanol and water as the solvent. The nanoparticles possessed both optical and magnetic properties. After surface aminated modification through typical stöber method, the as-prepared nanoparticles were applied for the LRET with Au nanoparticles, and an effective LRET system was established through the covalent interaction of amino and carboxyl groups (Scheme 1). 2. Materials and methods 2.1. Materials Rare earth oxides (RE2 O3 , RE = Gd, Ce and Tb) of 99.99% purity, sodium fluoride (NaF), oleic acid (OA), stearic acid, tetraethyl orthosilicate (TEOS), 3-aminopropyltrimethoxysilane (APTES), isopropanol, chloroauric acid, sodium citrate and absolute alcohol were of analytical grade. All chemicals above were purchased from National Medicines Corporation Ltd. of China and used as received without further purification. 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) were purchased from Acros (USA). Triple-distilled water was used throughout the experiments. 2.2. Synthesis and modification of NaGdF4 :Ce,Tb nanoparticles NaGdF4 :Ce,Tb nanoparticles were prepared referencing to a solvothermal method previously reported by our group [30], based on the solid–liquid two-phase reaction mechanism. First, the rare earth stearate were synthesized according to the reported method, and used as the precursor. To synthesize of NaGdF4 :Ce,Tb nanoparticles, 4 mL of oleic acid, 18 mL of ethanol and 8 mL of water were mixed together. Then, 0.2100 g NaF and 1 mmol of rare earth stearate (C17 H35 COO)3 RE (RE = Gd0.75 Ce0.10 Tb0.15 ) was added. The mixture was stirred to form a homogeneous solution, then transferred to a autoclave, solvothermally treated at 120 ◦ C for 18 h. After finishing the reaction and cooling to room temperature, the product was purified by centrifugation and washing with ethanol for several times, dried at 60 ◦ C for 12 h. The modification of as prepared NaGdF4 :Ce,Tb nanoparticles was performed according to the typical stöber method [31]. First, 20 mg of NaGdF4 :Ce,Tb nanoparticles were dispersed in 70 mL of ethanol under sonication, then 20 mL H2 O and 2.5 mL NH3 ·H2 O were added. After stirred for about 10 min at 35 ◦ C, the aminated reaction was activated with the addition of TEOS and APTES, and kept at 35 ◦ C for 5 h. Finally, the product was precipitated and washed with ethanol by centrifuging, and dried at 60 ◦ C overnight.
The NaGdF4 :Ce,Tb nanoparticles were dispersed in water under sonication to form a 2 mg mL−1 colloidal solution. Then, a mixture of 200 L 200 g mL−1 EDC and different volumes (0–350 L) of 48 g mL−1 Au was added, the final solution was adjusted to 2 mL and oscillated at 37 ◦ C for 2 h. After reaction, the solution was detected on a LS-55 luminescence spectrometer under the 246 nm excitation. 2.5. Characterization The size and morphology of prepared NaGdF4 :Ce,Tb nanoparticles were observed on a H-7650 transmission electron microscope (TEM, HITACHI Co., Japan) operating at an accelerating voltage of 100 kV. X-ray powder diffraction (XRD) measurements were carried out by an X’ Pert Pro diffractometer (PANalytical Co., Holand) equipped with Cu K␣ radiation ( = 0.15406 nm). The optical property was examined on a LS-55 luminescence spectrometer (PerkinElmer Co., USA). The hysteresis loop was measured on a vibrating sample magnetometer (VSM 7407, LakeShore, USA). Fourier transform infrared (FT-IR) spectra of the modified nanoparticles were measured on a Spectrum One (B) spectrometer (PerkinElmer Co., USA). The absorption spectra of Au nanoparticles were recorded on a UV-2100 UV–vis spectrometer (Ruili Analytical Instrument Company, Beijing, China). 3. Results and discussion 3.1. Morphology and structure characterization of NaGdF4 :Ce,Tb nanoparticles In this experiment, a solid–liquid mechanism was applied for the preparation of NaGdF4 :Ce,Tb nanoparticles. Rare earth stearate was used as the precursor, a mixture of oleic acid, ethanol and water was the reaction medium. During the reaction, rare earth ions were slowly released from the precursor and reacted with F− and Na+ ions to form NaGdF4 :Ce,Tb nanoparticles. Meanwhile, during formation of the nanoparticles, oleic acid in the reaction system can be covered at the particle’s surface, avoid their aggregation and play an important role for the size and morphology controlling. Fig. 1a shows the typical TEM image of the NaGdF4 :Ce,Tb nanoparticles. From the image it can be seen that, the nanocrystals were almost uniform ellipsoidal particles with an average size of about 15 nm. The XRD patterns of NaGdF4 :Ce,Tb nanoparticles were represented in Fig. 1b. Compared with the standard pattern, it can be concluded that the nanoparticles possess pure hexagonal structure. All diffraction peaks of as prepared NaGdF4 :Ce,Tb nanoparticles are matched well with the peaks in standard pattern (Hexagonal JCPDS NO. 00-027-0699). 3.2. Luminescent properties of NaGdF4 :Ce,Tb nanoparticles Fig. 2 represents the emission spectrum of NaGdF4 :Ce,Tb nanoparticles monitored at 246 nm. The seven peaks located in 381, 413, 435, 490, 544, 577 and 622 nm were all the typical emission of Tb3+ ions, which are corresponding to the 5 D3 –7 FJ (J = 6, 5,
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Fig. 1. TEM image (a) and XRD pattern (b) of NaGdF4 :Ce,Tb nanoparticles.
4) and 5 D4 –7 FJ (J = 6, 5, 4, 3) transitions of Tb3+ ions [32], respectively. Among them, intensity of peak at 544 nm is the strongest. In the digital photograph inserted in Fig. 2, the colloid solution of NaGdF4 :Ce,Tb nanoparticles exhibit bright green fluorescence under 246 nm excitation, this result is consistent with the emission spectrum, and fully proves that the synthesized NaGdF4 :Ce,Tb nanoparticles possess excellent optical properties. 3.3. Magnetic properties of NaGdF4 :Ce,Tb nanoparticles According to literature reported before, the materials doped with Gd3+ displayed paramagnetism and could be used as contrast enhancement agent in magnetic resonance imaging [33]. Here, the magnetic property of as prepared NaGdF4 :Ce,Tb nanoparticles was also detected through the measurement of magnetic hysteresis loop, and the result is shown in Fig. 3. As can be seen from the graph, the saturation magnetization intensity of the NaGdF4 :Ce,Tb nanoparticles is 133.42 memu g−1 , and the coercive force is nearly zero, which indicate that the NaGdF4 :Ce,Tb nanoparticles showed good super-paramagnetism at room temperature. Fig. 4 shows the digital photograph of NaGdF4 :Ce,Tb nanoparticles without and with magnetic field treatment under visible and ultraviolet light. It can be seen clearly that the synthesized NaGdF4 :Ce,Tb nanoparticles are well dispersed in water to form a homogeneous solution. While, when an external magnetic field was
Fig. 2. Emission spectra of NaGdF4 :Ce,Tb nanoparticles and the photograph of corresponding aqueous solutions under the UV lamp.
Fig. 3. Hysteresis loop of the NaGdF4 :Ce,Tb nanoparticles.
put aside, the NaGdF4 :Ce,Tb nanoparticles moved quickly to the magnet and presented bright green light under 246 nm excitation, as shown in Fig. 4b. The above phenomenon illustrated that the synthesized NaGdF4 :Ce,Tb nanoparticles possess super-paramagnetic, could be used for the separation and enrichment of sample during detection. Due to the existence of Gd3+ , NaGdF4 :Ce,Tb nanoparticles possess both optical and magnetic properties, could be
Fig. 4. Bright field (a) and dark field (b) photographs of the NaGdF4 :Ce,Tb colloid solution without (left) and with (right) an external magnet.
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Fig. 5. TEM image (a) and FT-IR spectra (b) of NaGdF4 :Ce,Tb nanoparticles before and after surface modification.
extended for the applications in targeted drug delivery and MRI imaging, etc. 3.4. Surface modification of NaGdF4 :Ce,Tb nanoparticles For the combination with Au to develop a LRET system, the NaGdF4 :Ce,Tb nanoparticles were first modified with aminosilane coupling agent by the typical stöber method. Fig. 5a is the TEM image of NaGdF4 :Ce,Tb nanoparticles after modification, there is a obviously silicon layer at outer surface of the nanoparticles. FT-IR spectra of NaGdF4 :Ce,Tb nanoparticles in Fig. 5b showed that after modification, the two peaks at 2922 and 2848 cm−1 , corresponding to the asymmetric and symmetric stretching vibrations of methylene group, which existed in the oleic acid molecules on the surface of nanoparticles, disappeared. Meanwhile, there is a strong peak located at 1076 cm−1 corresponding to the symmetrical stretching vibration of Si O bond. Both the TEM and FT-IR results show that the surface of the NaGdF4 :Ce,Tb nanoparticles has been successfully amino-functionalized. 3.5. LRET between NaGdF4 :Ce,Tb and Au nanoparticles Fig. 6 represents the emission spectrum of NaGdF4 :Ce,Tb nanoparticles and the UV–visible absorption of Au nanoparticles. As can be seen from the graph, the NaGdF4 :Ce,Tb nanoparticles exhibits four peaks from 450 nm to 650 nm under 246 nm excitation, and Au nanoparticles have a broad and continuous absorption
Fig. 6. Emission spectrum of NaGdF4 :Ce,Tb nanoparticles (a) and UV–vis absorption spectrum of Au nanoparticles (b).
with the maximum peak at 527 nm. There is a well overlap between the two spectra, which is in accordance with the first basic condition for LRET. In this LRET system, the modified NaGdF4 :Ce,Tb nanoparticles served as the energy donor and Au nanoparticles was the receptor. As shown in Fig. 7(a), the luminescence intensity of the NaGdF4 :Ce,Tb nanoparticles is weakened gradually, along with the increase of Au nanoparticles (0–7.2 g mL−1 ), and finally
Fig. 7. Luminescence spectra of NaGdF4 :Ce,Tb with different concentration of Au nanoparticles (a) and linear relationships between the luminescent intensity of NaGdF4 :Ce,Tb nanoparticles and the concentrations of Au nanoparticles (b) at 490 nm and 544 nm.
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stabilized. This is because that in this process, the concentration of NaGdF4 :Ce,Tb nanoparticles was controlled and changeless, while the increasing of Au nanoparticles resulted in the enhancement of the luminescence quenching. However, when the concentration of Au nanoparticles was increased to 8.4 mg mL−1 , the energy transfer between NaGdF4 :Ce,Tb and Au nanoparticles got to stop because the amount of the donor (NaGdF4 :Ce,Tb) was depleted and, thus, could not interact with the increasing acceptor (Au) concentration, so the luminescence intensity of NaGdF4 :Ce,Tb nanoparticles no longer changed. There are two linear relationships between the luminescence intensity of NaGdF4 :Ce,Tb nanoparticles and concentration of Au nanoparticles in the range of 0–7.2 mg mL−1 (Fig. 7b) for the emission at 490 nm and 544 nm, respectively. For the emission of NaGdF4 :Ce,Tb nanoparticles at 490 nm, the linear equation is I = 6.07 × 102 − 40.19c, with the correlation coefficient R of 0.9952. Another linear equation at 544 nm is I = 7.26 × 102 − 25.93c with the correlation coefficient of 0.9929. These results demonstrate the successful energy transfer between amino-modified NaGdF4 :Ce,Tb and Au nanoparticles. 4. Conclusion Luminescent/magnetic bi-functional NaGdF4 :Ce,Tb nanoparticles were first synthesized via a solvothermal method with rare earth stearate as the precursor, based on solid–liquid mechanism. The synthesized nanoparticles are regular particles with an average size of about 15 nm. Due to the existence of Tb3+ and Gd3+ ions, the NaGdF4 :Ce,Tb nanoparticles possess excellent green luminescence and super-paramagnetism at the same time. After surface aminated modification, the NaGdF4 :Ce,Tb nanoparticles were combined with Au nanoparticles to form a luminescence resonance energy transfer system, with NaGdF4 :Ce,Tb as the donor and Au as the receptor, and good linear relationships between the luminescence intensities of NaGdF4 :Ce,Tb and the concentration of Au nanoparticles were obtained. It is expected that the prepared nanoparticles and LRET system established here can be developed for various bioassays with magnetic separation and concentration through the paramagnetism of NaGdF4 :Ce,Tb nanoparticles. Acknowledgments This work was funded by the National Natural Science Foundation Council, People’s Republic of China (grant no. 20875011) and supported by Northeastern University on PhD students. References [1] K.E. Sapsford, L. Berti, I.L. Medintz, Materials for fluorescence resonance energy transfer analysis: beyond traditional donor–acceptor combinations, Angew. Chem. Int. Ed. 45 (2006) 4562–4588. [2] F. Vetrone, R. Naccache, C.G. Morganb, J.A. Capobianco, Luminescence resonance energy transfer from an upconverting nanoparticle to a fluorescent phycobiliprotein, Nanoscale 2 (2010) 1185–1189. [3] Z.Q. Li, Y. Zhang, S. Jiang, Multicolor core/shell-structured upconversion fluorescent nanoparticles, Adv. Mater. 20 (2008) 4765–4769. [4] D.Z. Yang, S.K. Xu, Q.F. Chen, Y. Wang, One system with two fluorescence resonance energy transfer (FRET) assembles among quantum dots, gold nanoparticles and enzyme, Colloids Surf. A 329 (2008) 38–43. [5] J.Q. Gu, J. Shen, L.D. Sun, C.H. Yan, Resonance energy transfer in steady-state and time-decay fluoro-immunoassays for lanthanide nanoparticles based on biotin and avidin affinity, J. Phys. Chem. C 112 (2008) 6589–6593. [6] L.Y. Wang, Y.D. Li, Green upconversion nanocrystals for DNA detection, Chem. Commun. 255 (2006) 7–2559. [7] P.R. Selvin, J.E. Hearst, Luminescence energy transfer using a terbium chelate: improvements on fluorescence energy transfer, Proc. Natl. Acad. Sci. 91 (1994) 10024–10028. [8] F. Wang, X.G. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976–989.
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Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of surface amino-functionalized NaGdF4 :Ce,Tb nanoparticles and their luminescence resonance energy transfer (LRET) with Au nanoparticles Congcong Mi a , Huanyu Gao a , Feng Li b , Shukun Xu a,∗ a b
Department of Chemistry, Northeastern University, Shenyang, 110819, PR China College of Science, Shenyang University, Shenyang, 110044, PR China
a r t i c l e
i n f o
Article history: Received 14 August 2011 Received in revised form 18 November 2011 Accepted 7 December 2011 Available online 16 December 2011 Keywords: NaGdF4 Solvothermal synthesis Au nanoparticles LRET
a b s t r a c t Luminescent/magnetic dual-functional NaGdF4 :Ce,Tb nanoparticles were first synthesized via a solvothermal method with rare earth stearate as the precursor, and the luminescence resonance energy transfer of NaGdF4 :Ce,Tb to Au nanoparticles was studied. The synthesized NaGdF4 :Ce,Tb were ellipsoidal nanoparticles, with pure hexagonal structure and an average size of 15 nm. The nanoparticles emit strong emission at 544 nm under the 246 nm excitation, and show good paramagnetic at room temperature. After surface amination by the typical Stöber method, the as-prepared NaGdF4 :Ce,Tb nanoparticles were served as the donor for the LRET with Au nanoparticles. Good linear relationships between the luminescence intensities of NaGdF4 :Ce,Tb (at 490 nm and 544 nm) and the concentration of Au nanoparticles were obtained, indicating a great potential application prospect of the LRET system in bioassays. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fluorescence resonance energy transfer (FRET) is a nonradiative process in which the electronic excitation energy of a donor chromophore is transferred to a nearby acceptor molecule via long-range dipole–dipole interactions [1]. To date, luminescent materials commonly used in FRET include organic dye [2], quantum dots [3,4] and rare earth doped down- or up-conversion nanoparticles [5,6], while, the FRET system with rare earth doped materials as the donor was commonly named as LRET [7]. Among them, the rare earth doped luminescent materials usually possess high quantum yield, long fluorescence lifetime and narrow emission peak. Especially, the rare earth doped upconversion nanoparticles are usually excited by near infrared light (NIR), the low excitation energy can effectively avoid auto-fluorescence from samples [8]. Among the rare earth doped luminescent materials, it is well known that Gd3+ ions doped nanoparticles always possess paramagnetism [9]. In recent years, nanomaterials that possess bifunctional properties have attracted more and more attention [10,11]. For example, nanomaterials exhibit optical and magnetic properties can realize the detection of samples by optical labeling or magnetic resonance image, and carry out separation and enrichment of samples at the same time [12]. Recently, researches on the synthesis and application of these bi-functional nanoparticles
∗ Corresponding author. Tel.: +86 2483681343. E-mail address: [email protected] (S. Xu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.022
have become the hotspot, there are some reports dedicated on it. The hydrophobic oleic acid-capped Tm/Yb/Er co-doped NaGdF4 nanoparticles were synthesized and applied as contrast agents for magnetic resonance imaging (MRI) in vivo [13]. The results indicated that these dual-functional nanoparticles exhibited excellent signal-to-noise ratio, low cytotoxicity, and can be served as a new platform for the detection of biological tissues. Based on the recent research, the major approaches for the preparation of rare earth doped nanoparticles in solution phase, include co-precipitation [14,15], thermal decomposition [16], sol–gel [17], polyol-mediated process [18], hydro/solvothermal [19,20], micro-emulsion [21], and son on. However, with the rapid development of nanotechnology, rare earth doped materials have demonstrated broad prospects in application of optics, optoelectronics, and bioassays, etc. [22,23]. Above all, the synthesis of high quality nanocrystals possessing small size, uniform shape, both high luminescent and magnetic intensity with a simple and rapid method is still of important significance. Recently, the analysis and detection based on the LRET system have been developed as a new kind of method with high sensitivity and selectivity. The method has been applied for the detection of nucleic acids [24], avidin [25], glucose [26] and immunoassay [27], etc. The lanthanides doped luminescent materials usually possess high quantum yield, long fluorescence lifetime and narrow emission peak [28,29], so that the signal to noise ratio can be greatly improved when lanthanides were used as the donor in LRET. Au nanoparticles itself cannot produce fluorescence, leading to avoid the interference from background. Moreover, the broad and
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2.3. Preparation of Au nanoparticles In this experiment, sodium citrate was used as the reductant for the preparation of gold nanoparticles. During the specific experimental procedure, 1 mL of 1% chlorine gold acid and 79 mL water were first mixed together and heated to 60 ◦ C, then 2 mL of 1% citric acid sodium solution was quickly added. After 30 min, Au colloid solution was obtained. 2.4. LRET between NaGdF4 :Ce,Tb and Au nanoparticles Scheme 1. The LRET diagram between NaGdF4 :Ce,Tb and Au nanoparticles.
continuous absorption of Au nanoparticles can ensure the high efficiency of a LRET process. Therefore, Au nanoparticles were usually used as the receptor in LRET system. In this work, a novel method for the synthesis of luminescent/magnetic bi-functional NaGdF4 :Ce,Tb nanoparticles was developed via a two phase mechanism, with rare-earth stearate as the precursor, oleic acid, ethanol and water as the solvent. The nanoparticles possessed both optical and magnetic properties. After surface aminated modification through typical stöber method, the as-prepared nanoparticles were applied for the LRET with Au nanoparticles, and an effective LRET system was established through the covalent interaction of amino and carboxyl groups (Scheme 1). 2. Materials and methods 2.1. Materials Rare earth oxides (RE2 O3 , RE = Gd, Ce and Tb) of 99.99% purity, sodium fluoride (NaF), oleic acid (OA), stearic acid, tetraethyl orthosilicate (TEOS), 3-aminopropyltrimethoxysilane (APTES), isopropanol, chloroauric acid, sodium citrate and absolute alcohol were of analytical grade. All chemicals above were purchased from National Medicines Corporation Ltd. of China and used as received without further purification. 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) were purchased from Acros (USA). Triple-distilled water was used throughout the experiments. 2.2. Synthesis and modification of NaGdF4 :Ce,Tb nanoparticles NaGdF4 :Ce,Tb nanoparticles were prepared referencing to a solvothermal method previously reported by our group [30], based on the solid–liquid two-phase reaction mechanism. First, the rare earth stearate were synthesized according to the reported method, and used as the precursor. To synthesize of NaGdF4 :Ce,Tb nanoparticles, 4 mL of oleic acid, 18 mL of ethanol and 8 mL of water were mixed together. Then, 0.2100 g NaF and 1 mmol of rare earth stearate (C17 H35 COO)3 RE (RE = Gd0.75 Ce0.10 Tb0.15 ) was added. The mixture was stirred to form a homogeneous solution, then transferred to a autoclave, solvothermally treated at 120 ◦ C for 18 h. After finishing the reaction and cooling to room temperature, the product was purified by centrifugation and washing with ethanol for several times, dried at 60 ◦ C for 12 h. The modification of as prepared NaGdF4 :Ce,Tb nanoparticles was performed according to the typical stöber method [31]. First, 20 mg of NaGdF4 :Ce,Tb nanoparticles were dispersed in 70 mL of ethanol under sonication, then 20 mL H2 O and 2.5 mL NH3 ·H2 O were added. After stirred for about 10 min at 35 ◦ C, the aminated reaction was activated with the addition of TEOS and APTES, and kept at 35 ◦ C for 5 h. Finally, the product was precipitated and washed with ethanol by centrifuging, and dried at 60 ◦ C overnight.
The NaGdF4 :Ce,Tb nanoparticles were dispersed in water under sonication to form a 2 mg mL−1 colloidal solution. Then, a mixture of 200 L 200 g mL−1 EDC and different volumes (0–350 L) of 48 g mL−1 Au was added, the final solution was adjusted to 2 mL and oscillated at 37 ◦ C for 2 h. After reaction, the solution was detected on a LS-55 luminescence spectrometer under the 246 nm excitation. 2.5. Characterization The size and morphology of prepared NaGdF4 :Ce,Tb nanoparticles were observed on a H-7650 transmission electron microscope (TEM, HITACHI Co., Japan) operating at an accelerating voltage of 100 kV. X-ray powder diffraction (XRD) measurements were carried out by an X’ Pert Pro diffractometer (PANalytical Co., Holand) equipped with Cu K␣ radiation ( = 0.15406 nm). The optical property was examined on a LS-55 luminescence spectrometer (PerkinElmer Co., USA). The hysteresis loop was measured on a vibrating sample magnetometer (VSM 7407, LakeShore, USA). Fourier transform infrared (FT-IR) spectra of the modified nanoparticles were measured on a Spectrum One (B) spectrometer (PerkinElmer Co., USA). The absorption spectra of Au nanoparticles were recorded on a UV-2100 UV–vis spectrometer (Ruili Analytical Instrument Company, Beijing, China). 3. Results and discussion 3.1. Morphology and structure characterization of NaGdF4 :Ce,Tb nanoparticles In this experiment, a solid–liquid mechanism was applied for the preparation of NaGdF4 :Ce,Tb nanoparticles. Rare earth stearate was used as the precursor, a mixture of oleic acid, ethanol and water was the reaction medium. During the reaction, rare earth ions were slowly released from the precursor and reacted with F− and Na+ ions to form NaGdF4 :Ce,Tb nanoparticles. Meanwhile, during formation of the nanoparticles, oleic acid in the reaction system can be covered at the particle’s surface, avoid their aggregation and play an important role for the size and morphology controlling. Fig. 1a shows the typical TEM image of the NaGdF4 :Ce,Tb nanoparticles. From the image it can be seen that, the nanocrystals were almost uniform ellipsoidal particles with an average size of about 15 nm. The XRD patterns of NaGdF4 :Ce,Tb nanoparticles were represented in Fig. 1b. Compared with the standard pattern, it can be concluded that the nanoparticles possess pure hexagonal structure. All diffraction peaks of as prepared NaGdF4 :Ce,Tb nanoparticles are matched well with the peaks in standard pattern (Hexagonal JCPDS NO. 00-027-0699). 3.2. Luminescent properties of NaGdF4 :Ce,Tb nanoparticles Fig. 2 represents the emission spectrum of NaGdF4 :Ce,Tb nanoparticles monitored at 246 nm. The seven peaks located in 381, 413, 435, 490, 544, 577 and 622 nm were all the typical emission of Tb3+ ions, which are corresponding to the 5 D3 –7 FJ (J = 6, 5,
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Fig. 1. TEM image (a) and XRD pattern (b) of NaGdF4 :Ce,Tb nanoparticles.
4) and 5 D4 –7 FJ (J = 6, 5, 4, 3) transitions of Tb3+ ions [32], respectively. Among them, intensity of peak at 544 nm is the strongest. In the digital photograph inserted in Fig. 2, the colloid solution of NaGdF4 :Ce,Tb nanoparticles exhibit bright green fluorescence under 246 nm excitation, this result is consistent with the emission spectrum, and fully proves that the synthesized NaGdF4 :Ce,Tb nanoparticles possess excellent optical properties. 3.3. Magnetic properties of NaGdF4 :Ce,Tb nanoparticles According to literature reported before, the materials doped with Gd3+ displayed paramagnetism and could be used as contrast enhancement agent in magnetic resonance imaging [33]. Here, the magnetic property of as prepared NaGdF4 :Ce,Tb nanoparticles was also detected through the measurement of magnetic hysteresis loop, and the result is shown in Fig. 3. As can be seen from the graph, the saturation magnetization intensity of the NaGdF4 :Ce,Tb nanoparticles is 133.42 memu g−1 , and the coercive force is nearly zero, which indicate that the NaGdF4 :Ce,Tb nanoparticles showed good super-paramagnetism at room temperature. Fig. 4 shows the digital photograph of NaGdF4 :Ce,Tb nanoparticles without and with magnetic field treatment under visible and ultraviolet light. It can be seen clearly that the synthesized NaGdF4 :Ce,Tb nanoparticles are well dispersed in water to form a homogeneous solution. While, when an external magnetic field was
Fig. 2. Emission spectra of NaGdF4 :Ce,Tb nanoparticles and the photograph of corresponding aqueous solutions under the UV lamp.
Fig. 3. Hysteresis loop of the NaGdF4 :Ce,Tb nanoparticles.
put aside, the NaGdF4 :Ce,Tb nanoparticles moved quickly to the magnet and presented bright green light under 246 nm excitation, as shown in Fig. 4b. The above phenomenon illustrated that the synthesized NaGdF4 :Ce,Tb nanoparticles possess super-paramagnetic, could be used for the separation and enrichment of sample during detection. Due to the existence of Gd3+ , NaGdF4 :Ce,Tb nanoparticles possess both optical and magnetic properties, could be
Fig. 4. Bright field (a) and dark field (b) photographs of the NaGdF4 :Ce,Tb colloid solution without (left) and with (right) an external magnet.
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Fig. 5. TEM image (a) and FT-IR spectra (b) of NaGdF4 :Ce,Tb nanoparticles before and after surface modification.
extended for the applications in targeted drug delivery and MRI imaging, etc. 3.4. Surface modification of NaGdF4 :Ce,Tb nanoparticles For the combination with Au to develop a LRET system, the NaGdF4 :Ce,Tb nanoparticles were first modified with aminosilane coupling agent by the typical stöber method. Fig. 5a is the TEM image of NaGdF4 :Ce,Tb nanoparticles after modification, there is a obviously silicon layer at outer surface of the nanoparticles. FT-IR spectra of NaGdF4 :Ce,Tb nanoparticles in Fig. 5b showed that after modification, the two peaks at 2922 and 2848 cm−1 , corresponding to the asymmetric and symmetric stretching vibrations of methylene group, which existed in the oleic acid molecules on the surface of nanoparticles, disappeared. Meanwhile, there is a strong peak located at 1076 cm−1 corresponding to the symmetrical stretching vibration of Si O bond. Both the TEM and FT-IR results show that the surface of the NaGdF4 :Ce,Tb nanoparticles has been successfully amino-functionalized. 3.5. LRET between NaGdF4 :Ce,Tb and Au nanoparticles Fig. 6 represents the emission spectrum of NaGdF4 :Ce,Tb nanoparticles and the UV–visible absorption of Au nanoparticles. As can be seen from the graph, the NaGdF4 :Ce,Tb nanoparticles exhibits four peaks from 450 nm to 650 nm under 246 nm excitation, and Au nanoparticles have a broad and continuous absorption
Fig. 6. Emission spectrum of NaGdF4 :Ce,Tb nanoparticles (a) and UV–vis absorption spectrum of Au nanoparticles (b).
with the maximum peak at 527 nm. There is a well overlap between the two spectra, which is in accordance with the first basic condition for LRET. In this LRET system, the modified NaGdF4 :Ce,Tb nanoparticles served as the energy donor and Au nanoparticles was the receptor. As shown in Fig. 7(a), the luminescence intensity of the NaGdF4 :Ce,Tb nanoparticles is weakened gradually, along with the increase of Au nanoparticles (0–7.2 g mL−1 ), and finally
Fig. 7. Luminescence spectra of NaGdF4 :Ce,Tb with different concentration of Au nanoparticles (a) and linear relationships between the luminescent intensity of NaGdF4 :Ce,Tb nanoparticles and the concentrations of Au nanoparticles (b) at 490 nm and 544 nm.
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stabilized. This is because that in this process, the concentration of NaGdF4 :Ce,Tb nanoparticles was controlled and changeless, while the increasing of Au nanoparticles resulted in the enhancement of the luminescence quenching. However, when the concentration of Au nanoparticles was increased to 8.4 mg mL−1 , the energy transfer between NaGdF4 :Ce,Tb and Au nanoparticles got to stop because the amount of the donor (NaGdF4 :Ce,Tb) was depleted and, thus, could not interact with the increasing acceptor (Au) concentration, so the luminescence intensity of NaGdF4 :Ce,Tb nanoparticles no longer changed. There are two linear relationships between the luminescence intensity of NaGdF4 :Ce,Tb nanoparticles and concentration of Au nanoparticles in the range of 0–7.2 mg mL−1 (Fig. 7b) for the emission at 490 nm and 544 nm, respectively. For the emission of NaGdF4 :Ce,Tb nanoparticles at 490 nm, the linear equation is I = 6.07 × 102 − 40.19c, with the correlation coefficient R of 0.9952. Another linear equation at 544 nm is I = 7.26 × 102 − 25.93c with the correlation coefficient of 0.9929. These results demonstrate the successful energy transfer between amino-modified NaGdF4 :Ce,Tb and Au nanoparticles. 4. Conclusion Luminescent/magnetic bi-functional NaGdF4 :Ce,Tb nanoparticles were first synthesized via a solvothermal method with rare earth stearate as the precursor, based on solid–liquid mechanism. The synthesized nanoparticles are regular particles with an average size of about 15 nm. Due to the existence of Tb3+ and Gd3+ ions, the NaGdF4 :Ce,Tb nanoparticles possess excellent green luminescence and super-paramagnetism at the same time. After surface aminated modification, the NaGdF4 :Ce,Tb nanoparticles were combined with Au nanoparticles to form a luminescence resonance energy transfer system, with NaGdF4 :Ce,Tb as the donor and Au as the receptor, and good linear relationships between the luminescence intensities of NaGdF4 :Ce,Tb and the concentration of Au nanoparticles were obtained. It is expected that the prepared nanoparticles and LRET system established here can be developed for various bioassays with magnetic separation and concentration through the paramagnetism of NaGdF4 :Ce,Tb nanoparticles. Acknowledgments This work was funded by the National Natural Science Foundation Council, People’s Republic of China (grant no. 20875011) and supported by Northeastern University on PhD students. References [1] K.E. Sapsford, L. Berti, I.L. Medintz, Materials for fluorescence resonance energy transfer analysis: beyond traditional donor–acceptor combinations, Angew. Chem. Int. Ed. 45 (2006) 4562–4588. [2] F. Vetrone, R. Naccache, C.G. Morganb, J.A. Capobianco, Luminescence resonance energy transfer from an upconverting nanoparticle to a fluorescent phycobiliprotein, Nanoscale 2 (2010) 1185–1189. [3] Z.Q. Li, Y. Zhang, S. Jiang, Multicolor core/shell-structured upconversion fluorescent nanoparticles, Adv. Mater. 20 (2008) 4765–4769. [4] D.Z. Yang, S.K. Xu, Q.F. Chen, Y. Wang, One system with two fluorescence resonance energy transfer (FRET) assembles among quantum dots, gold nanoparticles and enzyme, Colloids Surf. A 329 (2008) 38–43. [5] J.Q. Gu, J. Shen, L.D. Sun, C.H. Yan, Resonance energy transfer in steady-state and time-decay fluoro-immunoassays for lanthanide nanoparticles based on biotin and avidin affinity, J. Phys. Chem. C 112 (2008) 6589–6593. [6] L.Y. Wang, Y.D. Li, Green upconversion nanocrystals for DNA detection, Chem. Commun. 255 (2006) 7–2559. [7] P.R. Selvin, J.E. Hearst, Luminescence energy transfer using a terbium chelate: improvements on fluorescence energy transfer, Proc. Natl. Acad. Sci. 91 (1994) 10024–10028. [8] F. Wang, X.G. Liu, Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976–989.
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