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Fluorescence-Magnetism Functional EuS Nanocrystals with Controllable Morphologies for Dual Bioimaging Yuanqing Sun, Dandan Wang, Tianxin Zhao, Yingnan Jiang, Yueqi Zhao, Chuanxi Wang, Hongchen Sun, Bai Yang, and Quan Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13831 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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ACS Applied Materials & Interfaces

Fluorescence-Magnetism Functional EuS Nanocrystals with Controllable Morphologies for Dual Bioimaging Yuanqing Sun,† Dandan Wang,§ Tianxin Zhao, † Yingnan Jiang, † Yueqi Zhao, † Chuanxi Wang,‡,* Hongchen Sun,§ Bai Yang, † and Quan Lin†,*

†

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun, 130012, P. R. China ‡

China-Australia Joint Research Centre for Functional Molecular Materials, School of Chemical

& Material Engineering, Jiangnan University, Wuxi, 214122, P. R. China §

School of Stomatology, Jilin University, Changchun, 130041, P.R. China

KEYWORDS EuS nanocrystals, controllable morphologies, fluorescence, magnetism, dual bioimaging

ACS Paragon Plus Environment

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ABSTRACT

Multiple functions incorporated in one single component material offer important applications in biosystems. Here we prepared a divalent state of rare earth EuS nanocrystals (NCs), which provides luminescent and magnetic properties, by using both 1-Dodecanethiol (DT) and oleylamine (OLA) as reducing agents. The resultant EuS NCs exhibit controllable shapes, uniform size and bright luminescence with a quantum yield as high as 3.5 %. OLA as a surface ligand plays an important role in tunable morphologies, such as nanowires, nanorods, nanospheres et al. Another attractive nature of the EuS NCs is their paramagnetism at room temperature. In order to expand the biological applications, the resultant EuS NCs were modified with amphiphilic block copolymer F127 and transferred from oil to water phase. The excellent biocompatibility of EuS NCs is demonstrated as well as preservation of their luminescence and paramagnetic properties. The EuS NCs offer multifunction and great advantages of bright luminescence, paramagnetic, controllable morphologies and good biocompatibility promising applications in the field of simultaneous magnetic resonance and fluorescence bioimaging.


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1. INTRODUCTION Magnetic resonance imaging (MRI) is one of the most popular techniques in modern clinical diagnosis, as it is able to offer deep tissue penetration, provide admirable three-dimensional soft tissue details, imaging by non-invasive detection. 1−3 Nevertheless, the spatial resolution of this technique is not ideal, which enables to resolve only objects larger than a few micrometers in size 4 and often magnetic hardware (cardiac pacemaker, heart stent, joint replacement etc.) is not suitable for MRI online observation during surgery. By contrast, fluorescence imaging (FI) possesses much higher sensitivity and potential application in real-time imaging compared to MRI.5, 6 However, the spatial resolution of fluorescence imaging in turbid media and the depth perception are very poor.7 Therefore the evolution of multimodal contrast agents for bioimaging is getting more and increasing influence in clinical diagnosis and surgical protocols. In recent years, multi-functional nanoparticles (NPs) combining fluorescence and magnetic resonance imaging have gained great attention.8−12 It is worth noting that the fabrication of ritualistic fluorescence-magnetic NPs has been mostly focused on integrating several materials with different functions into one system, otherwise it meets the problem that the efficacy of the individual functions might be impaired because their partial counteraction. Thus, it is necessary to develop a single nanomaterial with multi-functions without the above problems. Nanomaterials based on rare-earth elements have attracted much attention in the past few decades due to their unique optical, electric, and magnetic properties13−17 as well as many advanced applications in solar cells, biological labeling and imaging.18−21 Most of these researches focus on trivalent lanthanide nanomaterials including oxides, fluorides, phosphates, and vanadates.22−25 However, there have been many limitations for preparing divalent lanthanide nanomaterials. An important kind of divalent lanthanide nanomaterial are Eu2+ chalcogenide


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nanocrystals (NCs), including EuX (X= O, S, Se, Te).26−29 These NCs are promising materials for optical isolators and opto-magnetic devices based on their unique photo- and magnetoproperties30,

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derived from their 4f-5d electronic transitions and spin configuration in the

divalent state of Europium, and it is only this state providing fluorescence and paramagnetism. EuS is a kind of magnetic semiconductor with a Curie temperature of 16.8 K, an energy gap of 1.6 eV, and a spin splitting with a gap of 0.36 eV.32 Latterly, EuS NCs have revealed high potential as opto-magnetic and luminescent material.33, 34 Although single-source precursors and liquid-phase methods have been introduced for synthesis of magnetic and luminescent EuS NCs,35,

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these time-consuming methods often require complicated devices and additional

ligands for Eu3+, which are relatively complex and unsuitable for large-scale production. The preparation of colloidal nanocrystals via a reduction method has been considered as a suitable method to resolve this problem.37−40 Our previous work reported a simple amine reduction means for synthesis of divalent EuSe NCs showing excellent luminescence and magnetic properties for great potential in the field of fluorescence-magnetic dual imaging.41 Most Eu2+ nanomaterials are synthesized in organic phase. These nanoparticles can only be dissolved in organic solvents for example hexane and chloroform, which greatly limits their application in biosystems. In order to improve their potential value in the field of biological imaging or more profound biological applications, Eu2+ NCs must be favorably dispersible and stable in the biological environment. It has become a lasting challenge to obtain superior and biocompatible EuS NCs with outstanding fluorescent and magnetic properties also with tunable morphologies and biostability. In this report, we describe a multi-functional EuS NC, which offers paramagnetic and fluorescent properties in one single material. In addition, the EuS NCs possess controlled shapes


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and uniform sizes. After modification with the amphiphilic block copolymer Pluronic F127 and transfer from organic phase to water phase, EuS NCs exhibit good water-dispersibility and biocompatibility. Comparing with Gd (III) MRI contrast agent only showing paramagnetic property, our EuS NCs integrate both fluorescence and magnetic properties. The resultant water soluble EuS NCs are further explored by T1-weighted MRI and labeling cells to demonstrate the fluorescence-magnetic dual imaging functions, indicating a high potential in biological applications. 2. EXPERIMENTAL SECTION 2.1. Materials. Europium oxide (Eu2O3,＞99 %) was purchased from Shanghai Yuelong Co., China. Oleylamine (OLA, technical grade, 70 %), oleic acid (OA, technical grade, 90 %), 1Octadecene (ODE, technical grade, 90 %) and Pluronic F127 were provided by Sigma-Aldrich. 1-Dodecanethiol (DT, 98 %) was obtained from Aladdin. Acetone, methanol, chloroform and hydrochloric solution were commercially available products from Beijing Chemical Works and used without further purification. Europium chloride hexahydrate (EuCl3·6H2O) was prepared by dissolving europium oxide in 36.5 wt % hydrochloric solution at 90 °C, and then the water was gradually evaporated completely. Deionized water was used throughout. 2.2. Synthesis of EuS NCs by One-step Method. The EuS NCs were synthesized by a one-step method. In a typical synthesis, EuCl3·6H2O (0.2 mmol), 1-Dodecanethiol (3.0 mL), oleylamine (3.4 mL), oleic acid (0.44 mL) and 1-Octadecene (5.0 mL) were mixed, vacuumized at room temperature and then heated up to 100 °C in a vacuum line for 2 h with stirring. Then the system was gradually heated up to 290 °C under nitrogen atmosphere, maintaining these conditions for various time spans. The resulting solution was naturally cooled to room temperature and then the product was precipitated by adding mixture of methanol and acetone to remove unreacted


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organic agents after centrifugation. The above purification steps were repeated twice. Finally, the purified EuS NCs were redispersed in chloroform with a concentration of 2 mg/mL. 2.3. Preparation of F127-Coated EuS NCs. 100 mg of F127 was dissolved with 15 mL water. Then, 2 mL chloroform solution containing 8 mg EuS NCs was mixed with it at room temperature. The milky turbid liquid was formed under the stirring condition. The mixture turned to clarification after complete evaporation of the organic phase and sonication for 10 min. Then the water soluble F127-coated EuS NCs were obtained. The solution concentration of F127coated EuS NCs was 4 mg mL-1. The water soluble EuS NCs was directly used in the following investigations. 2.4. Cellular Imaging. HeLa cells (210 cells/mL) were cultured in a 96-well plate with 10 % fetal bovine serum, and 1 % penicillin/streptomycin at 37 °C in a 5 % CO2 incubator for several days. Before experiment, the cells were washed with Dulbecco’s phosphate buffer saline (DPBS pH=7.4) for two times and incubated with F127-coated EuS NCs (20 µg/mL) for 24 h (only 20 µg F127-coated EuS NCs of up to 150 µL of culture medium (105 cells) was added). Prior to fix the cells in the well plate for measurement with a confocal fluorescence microscope, the cells were washed with DPBS for 3 times to remove the redundant EuS NCs. 2.5. In Vivo MRI Study. Male BALB/c-nu mice were purchased from Nanjing Pengsheng Biological Technology Co. Ltd., and used under protocols approved by Jilin University Laboratory Animal Center. To develop the tumors model, the mice were planted with HeLa tumors. 150 µL Dulbecco's modified eagle medium (DMEM) containing 2.0×106 of HeLa cells was injected subcutaneously into the right back leg. After a week, the average size of the tumor reached to ~60 mm3, and then the mice were administrated by intratumoral injection with 80 µL of 50 µg/mL concentration of F127-coated EuS NCs. After carried out for 1 h, the mice were


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anaesthetized and imaged on a 3.0 T clinical MRI scanner equipped with a specially made holder for small animal imaging, using a T1-weighted sequence. 2.6. Characterization Methods. Transmission electron micrographs (TEM), high-resolution TEM (HRTEM) and energy dispersed X-ray (EDX) characterization were performed with a JEM-2100F field emission electron microscope operating at 200 kV. Photoluminescence (PL) experiments were performed with a Shimadzu RF-5301 PC spectrofluorimeter and UV-vis absorption was performed with Lambad 800 UV–Vis spectrophotometer. X-ray photoelectron spectroscopy (XPS) was investigated using a VG ESCALAB MKII spectrometer with a Mg KR excitation (1253.6 eV) and X-ray powder diffraction (XRD) investigation was carried out using a Siemens D5005 diffractometer at room temperature. The confocal microscopy images were taken by an Olympus Fluoview FV1000. The magnetic hysteresis loop was performed on the LakeShore 7410 at room temperature. T1-weighted images of the F127-coated EuS NCs in different concentrations determined by europium and in vivo MRI were scanned under a 3.0T clinical MRI scanner (Siemens Magnetom Trio Tim) at room temperature. 3. RESULTS AND DISCUSSION The reaction is sketched in Scheme 1. The multifunctional EuS NCs were prepared with a reaction between EuCl3·6H2O and 1-Dodecanethiol (DT) at 290 °C in the presence of Oleylamine (OLA) and oleic acid (OA). The as-prepared (divalent state) EuS NCs exhibit blue luminescence and paramagnetism, and the experimental parameters could be adjusted to control the EuS NCs morphologies of nanowires, nanorods, and nanospheres. After decoration with F127 EuS NCs were transferred to water, obtaining good dispersibility, biocompatibility and low biotoxicity. We expect and therefore sketch that EuS NCs possess great potential both in the fields of cell luminescence imaging and MRI.


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Scheme 1. Schematic illustration of the preparation of biocompatible EuS nanocrystals and further application in dual bioimaging. The as-prepared EuS NCs emerged with amazingly different, but controllable morphologies by adjusting experimental parameters. Figure 1 shows transmission electron microscopy (TEM) images of as-prepared EuS NCs after various reaction times. Long nanowires (A1) with length of 200 nm were obtained after reaction at 290 °C for 5 min (Figure 1a). When the reaction mixture was maintained at 290 °C for 20 minutes, the morphology of EuS NCs (A2) changed considerably from long nanowires to short and highly monodisperse nanorods with length of 30 nm and width of 7 nm (Figure 1b). Extending the reaction time to 1 h, the EuS NCs nanorods (A3) became shorter and nonuniform, and near-spherical NCs developed (Figure 1c). Further increasing the reaction time to 3 h, most of the EuS NCs (A4) changed to spherical shape with diameter 14 nm (Figure 1d). The inset high-resolution TEM image in the Figure 1 exhibits lattice distances of EuS NCs of about 2.96 Å, attributed to the (200) spacings of cubic EuS.31 Further


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prolonging the reaction time to 6 h, the morphology of EuS NCs was not obviously changed (Figure S1).

Figure 1. TEM images of EuS NCs prepared with a precursor of EuCl3 0.2 mmol, DT 3 mL, OLA 3.4 mL and OA 0.44 mL at 290 °C, with different reaction time: a), 5 min (A1); b), 20 min (A2); c), 1 h (A3); d), 3 h (A4). The inset shows the high-resolution TEM image of EuS NCs. The optical properties of the as-prepared EuS NCs were firstly investigated. Figure 2a reveals the excitation (Ex) and emission (Em) spectra of sample A2. When the EuS NCs were irradiated by UV light of 375 nm, a strong blue emission was observed at a wavelength of 475 nm with a full width at half maximum (FWHM) of 32 nm. This emission is due to the transition from the ground state of 4f7 to the spin-orbit multiplets of 4f65d1 of Eu (II)42, 43. The inset in Figure 2a is a photograph of EuS NCs solution irradiated by 365 nm wavelength of UV light, bright blue emission of EuS NCs dispersed in chloroform was observed. Further to confirm absent of Eu (III) in the system, the emission line of EuS NCs was measured with excitation wavelength at 395 nm which is the optimal excitation wavelength of Eu (III)14, 44, as shown in the Figure S2. Clearly, there are only the emission peak of Eu (II) at 480 nm and no discrete luminescence lines of Eu (III) in the wavelength range from 580 nm to 650 nm confirming that the europium ions of the


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resultant EuS NCs are divalent state instead of trivalent state. This is because OLA and DT play a role of reducing agents in the formation of EuS NCs in the reaction system41. The quantum yield of the resultant EuS NCs was measured to be 3.5 % using quinine sulfate as the reference at room temperature. The four kinds of EuS NCs (A1, A2, A3 and A4) show the emission line at the same wavelength at 475 nm in the Figure S3. Figure S4 displays the UV-Vis absorption spectra of the resultant EuS NCs dispersed in chloroform. The absorption band of the four samples appears at 300 nm, which is attributed to the ligand-to-metal charge transfer (LMCT) S (II)-Eu (II).31 As well known, the electronic configuration of Eu (0) is 4f75s26s2. Eu (II) results from losing two 6s electrons of Eu (0). Consequently, due to seven single electrons in the 4f orbital of Eu (II), whose electronic structure is similar to that of trivalent gadolinium, nanomaterials containing Eu (II) are supposed to possess magnetic property. Figure 2b shows magnetization measurements of EuS NCs of nanorods A2 at room temperature. The absence of a hysteresis loop indicates that these EuS NCs exhibit paramagnetism, i.e. coercivity (Hc) of 0 Oe at 298 K. The paramagnetic character is also the same as that of EuS NCs samples A1, A3, and A4 (Figure S5). It is noticeable that this paramagnet is similar in magnitude as Gd3+ contrast agents,45 and this is to our knowledge the first report to prepare paramagnetic EuS NCs. The combination of high magnetism and fluorescence quantum yield indicates, that as-prepared EuS NCs can have important application in MRI.46


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Figure 2. a) Typical excitation (Ex) and emission (Em) spectra of EuS NCs A2. b) Magnetization curve of EuS nanorods A2. Inset: photograph of the EuS NCs irradiated by 365 nm wavelength of UV light. The DT in the preparation not only serve as the sulfur source, which decomposed at 230 °C and released the S2-, but also act as a weak reducing agent involved the formation of europium (II) compounds. To verify this result, a series of experiments was designed. First, when the reaction with the absence of OLA and DT was carried out for 3 h at 290 °C in absence of OLA and DT, the characteristic luminescence line of Eu (III) at 590 nm and 613 nm was observed, which means that there was no reduction (Figure S6). Then, when DT was introduced into the reaction mixture at 290 °C for 3 h, a highly intensive luminescence peak at 475 nm was observed with


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excitation at 375 nm with the FWHM about 33 nm, whose bright blue emission was attributed to Eu (II) chalcogenide NCs (Figure 3). Besides, the characteristic absorption band of Eu (II) emerged at 300 nm (Figure S7). This result manifests that DT played a role of the reducing agent in the preparation of EuS NCs. However, as Figure S8 shows, no uniform nanocrystals were formed without using OLA in the reaction. Finally, only when the OLA, DT and OA coexisted in the system, the morphology-controlled divalent EuS NCs with blue fluorescence (Figure 3) could be obtained. Consequently, it is easy to understand that OLA acted as not only a reductant but also an efficient capping ligand in the regulation and control of the morphologies of the nanocrystals.47, 48

Figure 3. PL spectra of EuS NCs prepared from fixing the concentration of EuCl3 0.018 M, DT 1.037 M and OA 0.105 M, and changing the content of OLA: a) without OLA, b) with OLA 0.623 M. Chemical compositions and status of EuS NCs were determined by X–ray photoelectron spectroscopy (XPS), and the results prove that the nanocrystals contain the elements of Eu, S, C, and O (Figure S9). The presence of O suggests that the surface of EuS NCs had been oxidized after exposure to air for days. Moreover, the XPS analysis shown in Figure S10a confirms that


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Eu (II) and Eu (III) are coexisting. Probably the Eu (II) ions on the surface could be partially oxidized to Eu (III) ions upon exposure to air.31 The zoom-in spectrum of S 2p is displayed in Figure S10b and the S results from the decomposition of DT at 230 °C. X-ray diffraction (XRD) analysis verifies the formation of EuS NCs (Figure S11). The diffraction peaks are indexed and juxtaposed with the JCPDS file for bulk EuS (JCPDS, pdf file 26-1419). The diffraction peaks of 25.90°, 29.95° and 42.88° in 2θ are assigned to the (111, lattice spacing 3.44 Å), (200, lattice spacing 2.98 Å) and (220, lattice spacing 2.11 Å) planes of the typical face-centered cubic EuS crystal structure. The observed spacings of EuS NCs in HRTEM (Figure. 1) are consistent with the (200) planes of the XRD spectrum. It also indicates that the EuS NCs have been successfully synthesized. The energy dispersive X-ray (EDX, Figure S12) spectrum quantifies that the composition is constituted by Eu and S, in which composition stoichiometry (atomic fraction) is 47 % (Eu): 53 % (S), corresponding to bulk EuS. For applications in medical imaging EuS NCs with fluorescent and paramagnetic properties need to be transferred from oil phase to water phase without losing both functions. Then through self-assembly strategy, the amphiphilic polymer Pluronic F127 as a surfactant becomes an appropriate candidate. The procedure of F127 coating of EuS NCs is illustrated in Scheme 2. The chloroform solution of EuS NCs is blended with the F127 aqueous solution. The system changes into an oil-in-water micro emulsion via ultrasonication and continuous stirring.49 By the hydrophobic interaction between PPO blocks and ligands of the EuS NCs surface, F127 is able to be modified on the EuS NCs due to the oil beads serving as templates in the micro emulsion.50 After evaporation of organic phase, PPO blocks are expected to graft on EuS NCs, and PEO blocks to stretch towards water phase. In this way, EuS NCs are stably dispersed in water.


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Scheme 2. The process of transferring hydrophobic EuS NCs into water solution using pluronic F127. Comparison of the FT-IR spectrum of the EuS NCs with that of F127-coated EuS NCs proves that EuS NCs are coated successfully with F127 (Figure S13). Next we show that phase transfer does not affect the properties too much. The photo luminescence of EuS NCs (A2) in chloroform solution and F127-coated EuS NCs (B2) in aqueous solution, presented in Figure 4 reveals the same emission peaks at around 480 nm with the same FWHM before and after phase transfer of the EuS NCs. When the EuS NCs are transferred from oil phase into water phase, the ligand of EuS NCs changes from hydrophobic OLA to hydrophilic PEO blocks. In comparison to EuS NCs synthesized in oil phase, approximate 40 % of luminescence intensity is maintained. The reduced of EuS NCs luminescence upon transfer into water phase is accounted to water adsorbed on the EuS NCs surface, and this quenches the luminescence. Under visible light, F127-coated EuS NCs are evenly dispersed in water to form a transparent and stable solution. Under ultraviolet light, blue fluorescence is preserved for EuS NCs B2 after transfer to water phase (inset of Figure 4).


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Figure 4. Emission spectra of EuS NCs (A2) and F127-coated EuS NCs (B2). The inset image is a photograph of the F127-coated EuS NCs (B2) under visible light (a) and 365 UV radiation (b). Figure S14 shows TEM images of hydrophobic EuS NCs of nanorods (A2) synthesized in oil phase at 290 °C for 20 min and of EuS NCs after being modified with F127 (B2). The images indicate that EuS NCs A2 and B2 present remarkable dispersibility without aggregation. The size and morphology of B2 are well maintained compared to the EuS NCs synthesized in oil phase. These results also indicate that F127 is an effective surfactant to obtain a well dispersed aqueous solution of nanocrystals through self-assembly. Magnetization measurements confirm that the magnetism of EuS NCs after decoration with F127 and phase transfer is well preserved (Figure 5a). Modified EuS NCs (B2) are paramagnetic at 298 K. Such dual-functional EuS NCs are appropriate for a new type of fluorescence-magnetic imaging detection system. With respect to MRI imaging a series of EuS NCs with different molar concentrations were evaluated. The relaxation rate (r1) was computed via the slope of the concentration-dependent relaxation rate 1/T1 charts. As seen in Figure 5b, a linear relationship is observed, when 1/T1 was plotted against Eu2+ molar concentrations. The calculated r1 value is 19.87mM−1 s−1. Thus we firstly report the preparation of water-soluble EuS NCs and obtain a


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reasonably high relaxation rate (r1). The potential of modified EuS NCs as contrast agents was evaluated at room temperature. Figure 5c reveals, that F127-coated EuS NCs in aqueous solutions can be observed with their T1-weighted MR signal intensity of aqueous solutions, which is continuously increased with the increase of EuS NC concentration, resulting in brighter images. It demonstrates that the F127-coated EuS NCs efficiently enhance the longitudinal relaxation of protons. We also investigated the potential of F127-coated EuS NCs as a MRI contrast agent in vivo. The T1-weighted MRI was conducted on a HeLa tumor-bearing mouse using a 3.0T human MRI scanner 1h after intratumoral injection of EuS NCs. As shown in Figure 5d, the T1-weighted MRI images of tumor with and without injecting F127-coated EuS treatment show dramatic contrast difference. The tumor shows strong T1 signals after intratumoral injection of EuS NCs and exhibit obviously MR imaging property. In contrast, the signals intensity of tumor without injecting EuS NCs are the same level as the normal tissue. This result indicates as-prepared water soluble EuS NCs exhibit the potential for contrast agents in MRI.46, 51


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Figure 5. a) Magnetic characterization of the F127 coated EuS NCs. b) Relaxation rates 1/T1 of EuS coated with F127 for a series of europium concentrations in water at room temperature. c) T1-weighted MR images of F127-coated EuS NCs aqueous solutions (concentrations: 0, 0.5, 1.3, 2.1, and 4.4 mM). d) T1-weighted MR images of HeLa tumor-bearing mice without (left) and with intratumoral injection (right) of F127-coated EuS NCs. An MTT (3-(4, 5-dimethyl-2-thiazolyl)-2, 5-diphenyltetrazolium bromide) assay and apoptosis assay were used to evaluate the cytotoxicity of EuS NCs and the viability of cells. Figure 6 shows that the viability of HeLa cells remains above 80 % after incubation at a concentration below 60 µg/mL for 24 h. This result indicates that the F127-coated EuS NCs own satisfying biocompatibility and low cytotoxicity.

Figure 6. Viability of HeLa cells after incubation with different concentrations of F127-coated EuS NCs for 24 h in the cell medium as determined by a MTT assay. In order to further establish the feasibility of EuS NCs application in biological fluorescence imaging, we tested the EuS NCs internalization in HeLa cells. The EuS NCs incubated in the


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cells were investigated by confocal laser fluorescence microscopy. Figure 7 reveals bright (a), dark (b) and overlapped background photographs (c) of HeLa cells respectively, which were incubated with EuS NCs for 24 h. The bright-field image of HeLa cells (Figure 7a) shows that the cells incubated with EuS NCs maintain normal appearance, which indicates that as-prepared EuS NCs own good biocompatibility. The fluorescence image excited at 405 nm (Figure 7b) shows blue fluorescence within the cells, indicating the uptake by HeLa. The fluorescence signals are mostly distributed in the cytoplasm (Figure 7c). This indicates that the F127-coated EuS NCs can be used as fluorescent probe to realize biological fluorescence imaging. In order to develop more favorable application for fluorescent detection, we will try to adjust the emission peak of rare-earth material to long wavelength in future work. The as-prepared EuS NCs offer multifunction and great advantages of bright luminescence, paramagnetic, controllable morphologies and good biocompatibility for potential applications in the field of simultaneous magnetic resonance and fluorescence bioimaging.

Figure 7. (a) The bright, (b) dark and (c) overlapped background photographs of HeLa cells after incubation with EuS NCs for 24 h, acquired by laser scanning confocal microscopy. 4．．CONCLUSIONS In this work, we report the preparation of water soluble EuS NCs with intense luminescence and paramagnetic property. The divalent state of rare earth nanocrystals of EuS NCs are prepared by using DT as the sulfur source and DT and OLA as the co-reducing agent. In this state Eu (II) is


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fluorescent and paramagnetic and the resultant EuS NCs exhibit multifunction in one single nanomaterial

with

pronounced

intense

blue

luminescence,

paramagnetic

properties.

Morphologies of nanowires, nanorods, and nanospheres and dispersibility are controllable via experimental parameters. After modification with F127 and transfer from oil to water phase, the F127-coated EuS NCs were demonstrated to exhibit excellent biocompatibility and low biotoxicity. This offers applications in both fluorescence bioimaging and in vivo magnetic resonance imaging due to their combined favorable luminescent and magnetic contrast.

ASSOCIATED CONTENT Supporting Information UV-Vis absorption spectra and magnetization curves of EuS NCs prepared with various reaction time spans. Additional TEM images and PL spectrum of EuS NCs. XPS spectrum, X-ray diffractogram, EDX spectrum and FT-IR spectra of EuS NCs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Q.L.: Tel: +86 431-8516-8483; Fax: +86 431-8519-3423; E-mail: [email protected] *C.W.: Tel: +86 13701511677; E-mail: [email protected]


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Funding Sources The National Nature Science Foundation of China (Grants 51373061 and 21304090). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Grants 51373061 and 21304090). We would like to thank Prof. Helmuth Moehwald from the Max Planck Institute of Colloids and Interfaces, Germany for useful discussions and helpful suggestions.

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