Ionic Liquid Mediated Synthesis and Surface Modification of

Article pubs.acs.org/Langmuir

Ionic Liquid Mediated Synthesis and Surface Modification of Multifunctional Mesoporous Eu:GdF3 Nanoparticles for Biomedical Applications Sonia Rodriguez-Liviano,† Nuria O. Nuñez,† Sara Rivera-Fernández,‡ Jesus M. de la Fuente,‡ and Manuel Ocaña*,† †

Instituto de Ciencia de Materiales de Sevilla (CSIC-US), Americo Vespucio 49, Isla de La Cartuja, 41092 Sevilla, Spain Instituto de Nanociencia de Aragon, Universidad de Zaragoza, Mariano Esquillor s/n, Zaragoza, 50018, Zaragoza, Spain

‡

S Supporting Information *

ABSTRACT: A procedure for the synthesis of multifunctional europium(III)-doped gadolinium(III) fluoride (Eu:GdF3) nanoparticles (∼85 nm) with quasispherical shape by precipitation at 120 °C from diethylene glycol solutions containing lanthanide chlorides and an ionic liquid (1-Butyl, 2methylimidazolium tetrafluoroborate) as fluoride source has been developed. These nanoparticles were polycrystalline and crystallized into a hexagonal structure, which is unusual for GdF3. They were also mesoporous (pore size = 3.5 Å), having a rather high BET surface area (75 m2 g−1). The luminescent and magnetic (relaxivity) properties of the Eu:GdF3 nanoparticles have been also evaluated in order to assess their potentiality as “in vitro” optical biolabels and contrast agent for magnetic resonance imaging. Finally, a procedure for their functionalization with aspartic-dextran polymers is also reported. The functionalized Eu:GdF3 nanoparticles presented negligible toxicity for Vero cells, which make them suitable for biotecnological applications.

1. INTRODUCTION Multifunctional nanoparticles are the subject of much attention, especially in the biotechnological field.1−6 Examples of these multifunctional nanomaterials are those suitable for both optical and magnetic resonance (MRI) imaging,5,7−17 which combine the high sensitivity of optical imaging for in vitro applications with the excellent spatial resolution and depth for in vivo and in vitro application associated with the MRI imaging or sensing.8,9,12,13,17 In such materials, the optical functionality may be afforded by quantum dots,7 fluorescent dyes,7,18 or rare earth based compounds,8−15 the latter being preferred due to their lower toxicity and higher stability.19 For MRI, the most frequently used contrast agent is superparamagnetic iron oxide,7,12,18,19 although different kinds of Gd(III)-containing compounds are gaining great interest,11,13,16,20 since these cations are the ideal paramagnetic relaxation agents due to their large magnetic moment and nanosecond time scale electronic relaxation time.22 Moreover, the Gd compounds can be easily doped with luminescent lanthanide (Ln) ions, resulting in materials with dual functionality consisting of a single phase. In some previous reports, these optical and MRI agents have been incorporated into mesoporous structures (usually, mesoporous silica) so that a third functionality is attained, since such porous composites may be used as carriers for drug delivery.3,4 The availability of such mesoporous nanoparticles © 2013 American Chemical Society

able to carry specific drugs and able to allow detection of their location (MRI) and how they interact with the tissue to be treated is highly desirable to monitor optimally disease treatment. Moreover, an additional property such as fluorescence emission would allow a proper following of the nanoparticles for in vitro research, helping to obtain a deep understanding of the action mechanism of this multifunctional system at the cellular level. Among the possible Gd-based compounds to be used to prepare multifunctional nanoparticles, fluorides are preferred for optical imaging purposes because they have lower vibrational energies than oxides, and consequently, the quenching of the exited state of the Ln cations is minimized, resulting in a higher quantum efficiency of the luminescence.23,24 Crystal structure is another important issue. For example, it has been reported that for Ln-doped NaREF4 (RE = rare earth cation) nanophosphors the hexagonal structure exhibits higher luminescence efficiency than the cubic phase.25 Uniform gadolinium fluoride (GdF3) nanoparticles doped with Ln cations with elongated26,27or rhombic28,29 morphologies have been recently synthesized at >200 °C and their Received: November 14, 2012 Revised: February 12, 2013 Published: February 13, 2013 3411

dx.doi.org/10.1021/la4001076 | Langmuir 2013, 29, 3411−3418

Langmuir

Article

tubes were heated at 120 °C in a conventional oven for 15 h. The resulting dispersions were cooled down to room temperature, centrifuged to remove the supernatants and washed twice with ethanol and once with double distilled water. Finally, the precipitates were dispersed in Milli-Q water. For specific analyses, the powders were dried at room temperature. For functionalization, aspartic-dextran (20% oxidized) solutions (0.18 mmol dm−3) were first prepared following a procedure previously reported by Fuentes et al36 (details are provided in the Supporting Information), and their pH was adjusted to 8. Then, a weighted amount of Eu:GdF3 nanoparticles was added to such solutions so that their final concentration was 0.2 mg cm−3. The resulting dispersions were sonicated in cold water for 1 h. After this treatment, the so functionalized nanoparticles were washed several times with Milli-Q water by centrifugation and finally redispersed in Milli-Q water. 2.2. Characterization. Particle shape was examined by transmission (TEM, Philips 200CM) electron microscopy. Particle size distributions were obtained by counting several hundreds of particles from the TEM micrographs and from dynamic light scattering (DLS) measurements (Zetasizer NanoZS90, Malvern). The quantitative composition of the samples was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Horiba Jobin Yvon, Ultima 2). The crystalline structure of the prepared samples was assessed from X-ray diffraction patterns (XRD, Panalytical, X́ Pert Pro with an XCelerator detector) collected at intervals of 0.02° (2θ) and an accumulation time of 4000 s. Crystallite size was obtained from the XRD reflection appearing at 2θ ∼ 28.5°, using the Scherrer formula. The infrared spectra (FTIR) of the nanophosphors diluted in KBr pellets were recorded in a JASCO FT/IR-6200 Fourier transform spectrometer. Thermogravimetric analyses (TGA) were performed in air at a heating rate of 10 °C min−1, using a Q600 TA Instrument. BET surface area measurements were carried out by N 2 physisorption at 77 K using a Micromeritics ASAP 2010 apparatus. Cumulative pore volume and average pore diameter were subsequently determined from the desorption curve by the BJH method. Before measurements, the samples were heated under vacuum at 150 °C for 2 h. ζ-Potential measurements were carried out with a Malvern Zetasizer Nano-ZS90 apparatus. The excitation and emission spectra of the nanophosphors dispersed in water (0.5 mg cm−3) were recorded in a Horiba JobinYvon Fluorolog3 spectrofluorometer operating in the front face mode. Lifetime measurements were obtained under pulsed excitation at 266 nm by using the fourth harmonics of a Nd:YAG laser (Spectra Physics model DCR 2/2A 3378) with a pulse width of 10 ns and a repetition rate of 10 Hz. The fluorescence was analyzed through an ARC monochromator model SpectraPro 500-i and then detected synchronously with an EMI-9558QB photomultiplier and recorded by a Tektronix TDS420 digital oscilloscope. The photographs showing the luminescence of aqueous suspensions of the nanophosphors were taken under illumination with ultraviolet radiation (λ = 273 nm). 2.3. Magnetic Relaxivity Measurements and MRI Phantom Analysis. 1H NMR relaxation times T1 and T2 were measured at 1.47 T in a Relaxometer (BrukerMinispec spectrometer) at different concentrations of nanoparticles (0.1, 0.05, 0.025, 0.0125, 0.00625 mg cm−3) in water at 298 K. T1 and T2 values were determined by the inversion−recovery method and by the Carr−Purcell−Maiboom−Gill sequence, respectively. Images were postprocessed using dedicated IDL 6.2 (Exelis VIS Inc., Boulder, CA) software with homemade written scripts. Relaxivities (r1, r2) were obtained from the slopes of the curves 1/T1 or 2 vs the concentration of Gd(III) expressed in mM. Phantoms were obtained at 9.4 T in a BrukerBiospec MRI 2.4. Cell Culture. Vero cells (monkey kidney epithelial cells) were cultured in Dulbecco’s modified Eagle’s medium (DMEM), supplemented with 10% fetal bovine serum (FBS), 5% glutamine, and 5% penicillin/streptomycin. Cell cultures were incubated at 37 °C and equilibrated in 4% CO2 and air.

luminescent properties have been analyzed. It should be noted that, in all cases, these nanoparticles crystallized into the orthorhombic structure, which is the thermodynamically stable GdF3 polymorph. However, a metastable hexagonal GdF3 phase has been recently obtained under certain experimental conditions (using NaBF4 as fluoride source30 or doping with La3+ ions31), which has been shown to be a more efficient host for the preparation of Ln:GdF3 phosphors.31 Much fewer studies can be found on the evaluation of the magnetic relaxivities of GdF3-based nanoparticles, although the first report on this subject indicated that this system is a good candidate as contrast agent for MRI imaging.22 It must be also mentioned that, to the best of our knowledge, the potentiality of Ln-doped GdF3 nanoparticles for dual optical and MRI imaging purposes has received little attention and that the few available reports deal with orthorhombic GdF3.27,32,33 Moreover, no information can be found in the literature regarding the synthesis of mesoporous GdF3 nanoparticles, which could find applications for targeted drug delivery.16 Herein, we report for the first time a procedure for the synthesis of mesoporous europium(III)-doped GdF3 nanoparticles (∼85 nm) with quasispherical shape based on homogeneous precipitation reactions carried out at 120 °C in diethylene glycol medium using an ionic liquid (1-butyl-2methylimidazolium tetrafluoroborate) as fluoride source. It is also shown that our synthesis process drives to the stabilization of hexagonal GdF3, which, as above-mentioned, is more efficient than the typical orthorhombic phase. The suitability of the obtained nanophosphors for optical and MRI bioimaging is also investigated. For such a purpose, the luminescence properties of this material have been optimized by adjusting the Eu doping level, and the r1 and r2 relaxivities associated with the optimum nanoparticles have been evaluated. Finally, it is also well-known that the modification of the nanoparticles surface is important for most bioapplications for further adding functional ligands such as antibodies and drugs.34 However, little work has been reported dealing with the functionalization of GdF3-based nanophosphors. In fact, we have only found in the literature one work on the functionalization of undoped GdF3 nanoparticles with citric acid.22 In this paper, we also describe a procedure for the functionalization of the Eu-doped GdF3 nanoparticles with aspartic-dextran polymers, which provide anchors for adding other functional ligands of interest. The so functionalized nanophosphors were not toxic for cells, which, along with the above-described properties, make them potential candidates for biotechnological applications including optical and MRI imaging and as drug delivery vehicle.

2. EXPERIMENTAL SECTION 2.1. Nanoparticles Synthesis. The standard procedure for the synthesis of Eu:GdF3 nanoparticles was as follows. Proper amounts of gadolinium(III) chloride hexahydrate (GdCl3·6H2O, Aldrich, 99.9%) and europium(III) chloride hexahydrate (EuCl3·6H2O, Aldrich, 99%) were first dissolved in 6 cm3 of diethylene glycol (DEG, Sigma-Aldrich, 99%) under magnetic stirring while heating at low temperature (∼75 °C) to favor dissolution. The lanthanide ions (Gd + Eu) concentration was kept constant (0.02 mol dm−3) in all experiments, whereas the Eu/(Eu + Gd) ratio was varied from 5 to 15% in order to investigate the effect of this parameter on the morphological and luminescent properties of the precipitated nanoparticles. These solutions were cooled down to room temperature, after which they were admixed with 4 cm3 of 1-butyl-2-methylimidazolium tetrafluoroborate (BMIMBF4, C8H15BF4N2, Aldrich ≥97%). After homogenization, the final solutions (total volume = 10 cm3) placed in tightly closed test 3412


Langmuir

Article

2.5. Viability Testing. Cell viability and proliferation were analyzed by the MTT colorimetric assay.37 For the cytotoxicity assay 5000 cells were seeded in each well of 96-well plates and grown for 24 h. After 24 h, the medium was replaced with fresh medium containing the nanoparticles in varying concentrations. After cultivation again for 24 h, 20 μL of MTT dye solution (5 mg/mL in PBS) was added to each well. After 3 h of incubation at 37 °C and 5% CO2, the medium was removed, the cells were washed with fresh medium, and formazan crystals were dissolved in 100 μL of DMSO. The absorbance of each well was read on a microplate reader (Biotek ELX800) at 570 nm. The spectrophotometer was calibrated to zero absorbance using culture medium without cells. The relative cell viability (%) related to control wells containing cell culture medium without nanoparticles was calculated by Atest/Acontrol × 100. Each measurement was repeated at least five times to obtain the mean values and the standard deviation.

3. RESULTS AND DISCUSSION 3.1. Nanoparticles Synthesis. A well-known procedure for the synthesis of uniform colloidal particles is homogeneous precipitation, which can be achieved through a slow and controlled release of the precipitating anions or cations in the reaction medium.38 In this work, we have selected the former strategy using BMIMBF4 as fluoride reservoir, since the fluoroborate anion can be hydrolyzed at rather low temperatures,39 thus slowly liberating the fluoride anions required for the precipitation of GdF3 in solutions containing Gd3+ cations. In our case, the required water molecules are provided by the starting hydrated lanthanide salts.28,40 It should be noted that BMIMBF4 has been successfully used for the synthesis of several uniform lanthanide fluoride nanomaterials with different morphologies.40−42 Finally, the use of DEG as solvent is based on the polyols ability to act not only as solvent but also as complexing and/or capping agent, thus limiting particle growth.43,44 It was found that the aging at 120 °C for 15 h of 0.019 mol dm−3 GdCl3 and 0.001 mol dm−3 EuCl3 solutions [Eu/(Eu + Gd) mol ratio = 0.05] in DEG containing 40% by volume of BMIMBF4 resulted in rather homogeneous nanoparticles with quasispherical morphology and a mean diameter of 85 ± 13 nm, as obtained from the TEM micrographs (Figure 1a). It must be mentioned that their mean hydrodynamic diameter obtained from DLS measurements conducted on water suspensions (pH = 5.6) was higher (230 nm), indicating a certain degree of nanoparticle aggregation in such medium. The XRD pattern recorded for this sample (Figure 2) did not match any pattern reported for GdF3. However, it was very similar to that of hexagonal EuF3 (JCPDS No. 00-032-0373) appearing slightly shifted to higher 2θ values (lower interplanar spacing), which suggests a similar hexagonal symmetry for our GdF3 nanoparticles. It should be noted that although the thermodynamically stable GdF3 polymorph is that with orthorhombic structure (JCPDS No 00-012-0788), it has been previously reported that a kinetic stabilization of a hexagonal GdF3 phase takes place by using NaBF4 as fluoride source30 or by doping with La3+ cations.31 As we have shown, our synthesis protocol is also able to stabilize such a hexagonal GdF3 polymorph, although a precise set of experimental parameters is required, since the alteration of either the solvent nature, the amount of BMIMBF4 added, or the synthesis temperature induces important changes in the morphological and structural characteristics of the precipitated nanoparticles. Thus, heterogeneous and aggregated particles were obtained when DEG was replaced by EG (Figure S2a, Supporting Information) or the aging temperature was raised to 180 °C

Figure 1. (a and b) TEM images at different magnification of the nanoparticles prepared by heating at 120 °C for 15 h, DEG solutions containing 0.019 mol dm−3 of GdCl3, 0.001 mol dm−3 of EuCl3 [Eu/ (Eu + Gd) mol ratio = 0.05], and 40% by volume of BMIMBF4.

Figure 2. X-ray diffraction patterns recorded for the sample shown in Figure 1. The JCPDS file for hexagonal EuF3 is also included.

(Figure S2b, Supporting Information), which consisted of a mixture of crystalline phases (hexagonal and orthorhombic) (Figure S3, Supporting Information). The decrease of the amount of BMIMBF4 added from 40 to 10% (BMIMBF4/DEG ratio by volume) also resulted in a mixture of hexagonal and orthorhombic GdF3 (Figure S3, Supporting Information), although no important effects on particle morphology were 3413


Langmuir

Article

detected in this case (Figure S2c, Supporting Information). These findings suggest that the formation of hexagonal GdF3 nanoparticles requires a specific kinetic of precipitation, which can be achieved through the adjustment of temperature (120°) and BMIMBF4 concentration (BMIMBF4/DEG volumetric ratio = 40%) and by selecting the appropriate solvent (DEG), whose viscosity is one of the main factors affecting the nucleation and growth rate in the precipitation process.45−47 The nature of the fluoride source was also found to be a key factor in the preparation of our mesoporous nanoparticles, since the replacement of BMIMBF 4 by NaBF 4 in a concentration range from 0.1 to 0.3 mol dm−3 (maximum solubility in DEG) keeping constant the other synthesis parameters resulted in very heterogeneous samples, such as those illustrated in Figure S4 (Supporting Information). This suggests that the ionic liquid must play a role, not only as a fluoride source but also as a morphology-directing agent, as previously observed for the synthesis of YF3 nanorhombuses in ethylene glycol medium.40 The analysis of crystal size as obtained from the (111) XRD reflection (2θ ∼ 28.5) also gave important information on the structural features of the GdF3 nanoparticles. Thus, the obtained value was much lower (9 nm) than the mean particle diameter obtained from the TEM micrograph (85 nm), indicating their polycrystalline character, which can be visualized in the TEM micrograph taken at high magnification (Figure 1b). According to this figure, it also seems that the Eu:GdF3 nanoparticles are mesoporous. This suggestion was confirmed by the nitrogen physisorption measurements. Thus, a nitrogen adsorption/desorption isotherm type IV with a hysteresis loop was obtained (Figure S5, top, Supporting Information), which confirmed the mesoporous character of the sample. The average pore size calculated using the BJH method (Figure S5, bottom, Supporting Information) was 3.5 nm, whereas the pore volume and BET surface area were 0.25 cm3 g−1 and 75 m2 g−1, respectively. Finally, the increase of the Eu content from 5 to 10 and 15% [Eu/(Eu + Gd) mol ratio], with the other experimental conditions involved in the synthesis of the sample shown in Figure 1 kept constant, did not produce noticeable changes in the size, shape, and crystalline structure of the precipitated nanoparticles. The presence of Eu in all doped samples was confirmed by chemical analyses, which showed that the experimental Eu/(Eu + Gd) ratio was very similar to the nominal values (Table S1, Supporting Information). 3.2. Luminescent and Magnetic Properties. The excitation spectrum recorded for sample Eu0.05Gd0.95F3 (5% Eu content) by monitoring the Eu3+ emission at 590 nm (Figure 3, up) displays a strong band at 273 nm along with much weaker features in the 300−400 nm range, among which the most intense appeared at 393 nm. The latter are due to the direct excitation of the Eu3+ ground state to higher levels of the 4f-manifold, whereas the former can be ascribed to the electronic transition from the ground state level of Gd3+ (8S7/2) to the 6I11/2 excited levels,42 which indicates that an energy transfer from Gd3+ multiplets to Eu3+ electronic levels takes place in this sample. As observed in Figure 3 (bottom), the excitation of the Eu0.05Gd0.95F3 sample either at 272 or at 395 nm resulted in a similar emission spectrum, which presented several bands, due to the 5D0−7FJ (J = 1−4) electronic transitions characteristic of the Eu3+ cations,42 the most intense appearing at 590 nm, which is responsible for the strong orange-red luminescence (inset in Figure 3, bottom). In

Figure 3. Excitation spectrum (λem = 590 nm) of the Eu0.05Gd0.95F3 sample (top) and emission spectra recorded for the same sample using different excitation wavelengths (bottom). Inset: Photograph taken under UV illumination for the Eu0.05Gd0.95F3 nanophosphor in water suspension.

Figure 3 (bottom), it can be also observed that the intensity of the Eu3+ emissions was considerably higher when exciting through the Gd−Eu energy transfer band (λex = 272 nm) than by direct excitation of the Eu3+ electronic levels (λex = 395 nm), which is as expected from the excitation spectrum recorded for this sample. Therefore, the effects of Eu content on the luminescence of the Eu:GdF3 nanoparticles was studied for λex = 272 nm. Figure 4 shows that the increase of Eu concentration from 5% to higher values up to 15% gave rise to a decrease of the emissions intensity, indicating the presence of the well-

Figure 4. Emission spectra (λem = 272 nm) of the Eu:GdF3 samples with different Eu content. 3414


Langmuir

Article

known concentration quenching effect for Eu contents above 5%. Consequently, the Eu0.05Gd0.95F3 nanophosphor was used for further studies. In order to gain further information on the luminescence efficiency of this sample, luminescence lifetime measurements were carried out. The decay curve obtained at an emission wavelength of 618 nm (5D0→7F2 transition) could be fitted with a single exponential function (Figure S6, Supporting Information). By using eq 1, a lifetime (τ) value of 7.9 ms resulted, which was close to the highest values previously reported for Eu:GdF3 nanoparticles (from 4.5 to ∼10 ms),42,48,49 indicating, therefore, that the luminescence efficiency of our sample is also among the highest so far reported for this kind of material. ∞

⟨τ ⟩ =

∫0 tI(t ) dt ∞

∫0 I(t ) dt

(1)

The measurement of the longitudinal (T1) and transversal (T2) proton relaxation times as a function of the gadolinium ion concentration at 1.5 T gave for the Eu0.05Gd0.95F3 sample r1 and r2 values of 1.22 and 6.08 s−1 mM−1, respectively (Figure 5). Figure 6. T 2 parametric MRI phantom images (9.4 T) of Eu0.05Gd0.95F3 nanophosphor showing transversal relaxation times, respectively, of nanoparticle solutions: (a) control (water) and (b−f) serial dilutions (0.1, 0.05, 0.025, 0.0125, 0.00625 mg cm−3).

(4.98) obtained for our nanoparticles, they cannot be used as positive contrast agent (T1-weighted images become brighter as increasing the nanoparticles concentration), which requires r2/ r1 values close to unity.13,50,51 3.3. Functionalization and Cell Viability. Dextran and its derivatives (carboxy, amino, etc.) are branched polymers that have been widely used for the functionalization of nanoparticles with biomedical applications.51 In this work we selected aspartic-dextran (20% oxidized), which was synthesized as described in the Supporting Information. The comparison between the FTIR spectrum of the nanoparticles before and after functionalization with asparticdextran clearly demonstrated the success of the herein proposed procedure. Thus, before functionalization, the FTIR spectrum only showed bands at 80 for concentrations up to 0.5 mg/ mL.

Figure 7. FTIR spectra recorded for the Eu0.05Gd0.95F3 sample before and after functionalization with aspartic-dextran. The spectrum of the latter is also included for comparison.

Figure 9. Cytotoxicity profiles of functionalized nanoparticles with Vero cells, as determined by MTT assay. Percentage of viability of cells was expressed relative to control cells (n = 5). Results are represented as mean ± standard deviations.

4. CONCLUSIONS The heating at 120 °C for 15 h of 0.02 mol dm−3 lanthanide (Gd + Eu) chloride solutions in diethylene glycol in the presence of 1-butyl-2-methylimidazolium tetrafluoroborate, which acts as fluoride source, yields europium(III)-doped gadolinium(III) fluoride (Eu:GdF3) nanoparticles (∼85 nm) with quasispherical shape. These nanoparticles crystallized into a hexagonal structure, which is an unusual GdF3 phase, and were formed through an ordered aggregation of smaller subunits. As a consequence, they were mesoporous (pore size = 3.5 Å) and presented a rather high BET surface area (75 m2 g−1), which confers them potential applications for targeted drug delivery. In addition, the herein reported nanoparticles exhibited strong red luminescence and high transverse relaxivity (r2) values, which makes them suitable as in vitro optical biolabels and negative contrast agent for magnetic resonance imaging. Finally, this multifunctional material was nontoxic for cells and could be further functionalized with aspartic-dextran species, which provides anchors for the further addition of functional ligands such as antibodies and drugs.

Figure 8. TGA curves obtained for the Eu0.05Gd0.95F3 nanophosphor before and after functionalization with aspartic-dextran.

antibodies and drugs. It should be noted that the functionalization process had no appreciable effects on the morphological characteristics of the nanoparticles as observed by TEM (Figure S7, Supporting Information). However, it was observed that the mean hydrodynamic diameter obtained from DLS measurements decreased after functionalization from 230 nm (unfunctionalized system) to 180 nm (at pH = 5.6) indicating that the aspartic-dextran polymers adsorbed on the nanoparticles surface favor their dispersibility in water. Notice that such a mean diameter value is still higher than that obtained from TEM for the unfunctionalized particles (85 nm). This behavior might be due, at least in part, to the presence of the adsorbed polymer molecules, which must increase the nanoparticles hydrodynamic diameter. Biocompatibility studies of the functionalized Eu:GdF3 nanophosphors were undertaken by evaluating the cell viability of Vero cells by the MTT assay.37 This assay relies on the mitochondrial activity of cells fibroblasts and represents a parameter for their metabolic activity. The MTT (3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay is a simple nonradioactive colorimetric assay to measure cell cytotoxicity, proliferation, or viability. MTT is a yellow, water-

■

ASSOCIATED CONTENT

S Supporting Information *

Preparation and FTIR characterization of aspartic-dextran; FTIR spectra of dextran, aspartic acid, and aspartic-dextran polymer; nominal and experimental (ICP) composition for the Eu:GdF 3 nanophosphors; TEM images of the Eu:GdF 3 nanoparticles synthesized under different conditions; nitrogen adsorption/desorption isotherm (top) and pore size distribution (bottom) measured for the Eu0.05Gd0.95F3 sample; decay curve obtained for the 5 D0 → 7 F 2 transition in the 3416


Langmuir

Article

nanoparticles as an optical imaging nanoprobe and T1 magnetic resonance imaging contrast agent. Adv. Mater. 2009, 21, 4467−4471. (12) Yu, X.; Shan, Y.; Li, G.; Chen, K. Synthesis and characterization of bifunctional magnetic-optical Fe3O4@SiO2@Y2O3: Yb3+,Er3+ nearinfrared-to-visible up-conversion nanoparticles. J. Mater. Chem. 2011, 21, 8104−8109. (13) Johnson, N. J. J.; Oakden, W.; Stanisz, G. J.; Prosser, R. S.; van Veggel, F. C. J. M. Size-tunable, ultrasmall NaGdF4 nanoparticles: Insights into their T-1 MRI contrast enhancement. Chem. Mater. 2011, 23, 3714−3722. (14) He, M.; Huang, P.; Zhang, C.; Hu, H.; Bao, C.; Gao, G.; He, R.; Cui, D. Dual phase-controlled synthesis of uniform lanthanide-doped NaGdF4 upconversion nanocrystals via an OA/ionic liquid two-phase system for in vivo dual-modality imaging. Adv. Funct. Mater. 2011, 21, 4470−4477. (15) Zhou, L.; Gu, Z.; Liu, X.; Yin, W.; Tian, G.; Yan, L.; Jin, S.; Ren, W.; Xing, G.; Li, W.; Chang, X.; Hu, Z.; Zhao, Y. Size-tunable synthesis of lanthanide-doped Gd2O3 nanoparticles and their applications for optical and magnetic resonance imaging. J. Mater. Chem. 2012, 22, 966−974. (16) Liu, Z.; Liu, X.; Yuan, Q.; Dong, K.; Jiang, L.; Li, Z.; Ren, J.; Qu, X. Hybrid mesoporous gadolinium oxide nanorods: A platform for multimodal imaging and enhanced insoluble anticancer drug delivery with low systemic toxicity. J. Mater. Chem. 2012, 22, 14982−14990. (17) Moros, M.; Pelaz, B.; Lopez-Larrubia, P.; Garcia-Martín, M. L.; Grazu, V.; de la Fuente, J. M. Engineering biofunctional magnetic nanoparticles for biotechnological applications. Nanoscale 2010, 2, 1746−1755. (18) Wang, F.; Chen, X.; Zhao, Z.; Tang, S.; Huang, X.; Lin, C.; Cai, C.; Zheng, N. Synthesis of magnetic, fluorescent and mesoporous core-shell-structured nanoparticles for imaging, targeting and photodynamic therapy. J. Mater. Chem. 2011, 21, 11244−11252. (19) Ma, Z. Y.; Dosev, D.; Nichkova, M.; Gee, S. J.; Hammock, B. D.; Kennedy, I. M. Synthesis and bio-functionalization of multifunctional magnetic Fe3O4@Y2O3:Eu nanocomposites. J. Mater. Chem. 2009, 19, 4695−4700. (20) Bridot, J. L; Faure, A.-C; Laurent, S.; Rivière, C.; Billotey, C.; Hiba, B.; Janier, M.; Josserand, V.; Coll, J.-J.; Elst, L. V.; Muller, R.; Roux, S.; Perriat, P.; Tillement, O. Hybrid gadolinium oxide nanoparticles: Multimodal contrast agents for in vivo imaging. J. Am. Chem. Soc. 2007, 129, 5076−5084. (21) Pinho, S. L. C.; Faneca, H.; Geraldes, C. F. G. C.; Rocha, J.; Carlos, L. D.; Delville, M. H. Silica nanoparticles for bimodal MRIoptical imaging by grafting Gd3+and Eu3+/Tb3+ complexes. Eur. J. Inorg. Chem. 2012, 2828−2837. (22) Evanics, F.; Diamente, P. R.; van Veggel, F. C. J. M.; Stanisz, G. J.; Prosser, R. S. Water-soluble GdF3 and GdF3/LaF3 nanoparticlesphysical characterization and NMR relaxation properties. Chem. Mater. 2006, 18, 2499−2505. (23) Stouwdam, J. W; van Veggel, F. C. J. M. Near-infrared emission of redispersible Er3+, Nd3+, and Ho3+ doped LaF3 nanoparticles. Nano Lett. 2002, 2, 733−737. (24) Yan, R.; Li, Y. Down/up conversion in Ln3+-doped YF3 nanocrystals. Adv. Funct. Mater. 2005, 15, 763−770. (25) He, M.; Huang, P.; Zhang, C.; Hu, H.; Chenchen, B.; Gao, G.; He, R.; Cui, D. Dual phase-controlled synthesis of uniform lanthanidedoped NaGdF4 upconversion nanocrystals via an OA/ionic liquid twophase system for in vivo dual-modality imaging. Adv. Funct. Mater. 2011, 21, 4470−4477. (26) Xu, Z.; Li, C.; Yang, D.; Wang, W.; Kang, X.; Shang, M.; Lin, J. Self-templated and self-assembled synthesis of nano/microstructures of Gd-based rare-earth compounds: Morphology control, magnetic and luminescence properties. Phys. Chem. Chem. Phys. 2010, 12, 11315−11324. (27) Tian, Y.; Yang, H-Y; Li, K.; Jin, X. J. Monodispersed ultrathin GdF3 nanowires: Oriented attachment, luminescence, and relaxivity for MRI contrast agents. Mater. Chem 2012, 22, 22510−22516.

Eu0.05Gd0.95F3 sample, after excitation at 266 nm; TEM image of the Eu:GdF3 nanoparticles after functionalization with aspartic-dextran. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 34-954489533. Fax: 34-954460665. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by Junta de Andaluciá (grant FQM6090), the Spanish CICYT (grant MAT2011-23593), Fondo Social Europeo, and ERC-SartingGrant NANOPUZZLE. S.R.L. wants to thank the Spanish Ministerio de Ciencia y Tecnologiá for a FPI fellowship. J.M.F. wants to thank ARAID for financial support. The authors thank D. Alcántara and M. L. Garciá Martiń for MRI images, V. Grazu for fruitful discussion, and I. Echaniz for technical support.

■

REFERENCES

(1) Yoon, T. J.; Kim, J. S.; Kim, B. G.; Yu, K. N.; Cho, M.-H.; Lee, J.K. Multifunctional nanoparticles possessing a “magnetic motor effect” for drug or gene delivery. Angew. Chem., Int. Ed. 2005, 44, 1068−1071. (2) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C.-H.; Park, J.-G.; Kim, J.; Hyeon, T. Magnetic fluorescent delivery vehicle using uniform mesoporous silica spheres embedded with monodisperse magnetic and semiconductor nanocrystals. J. Am. Chem. Soc. 2006, 128, 688−689. (3) Liong, M.; Lu, J.; Kovochich, M.; Xia, T.; Ruehm, S. G.; Nel, A. E.; Tamanoi, F.; Zink, J. Multifunctional inorganic nanoparticles for imaging, targeting, and drug delivery. ACS Nano 2008, 2, 889−896. (4) Zhang, F.; Braun, G. B.; Pallaoro, A.; Zhang, Y.; Shi, Y.; Cui, D.; Moskovits, M.; Zhao, D.; Stucky, G. D. Mesoporous multifunctional upconversion luminescent and magnetic “nanorattle” materials for targeted chemotherapy. Nano Lett. 2012, 12, 61−67. (5) Tian, G.; Gu, Z.; Liu, X.; Zhou, L.; Yin, W.; Yan, L.; Jin, S.; Ren, W.; Xing, G.; Li, S.; Zhao, Y. Facile Fabrication of rare-earth-doped Gd2O3 hollow spheres with upconversion luminescence, magnetic resonance, and drug delivery properties. J. Phys. Chem. C 2011, 115, 23790−23796. (6) Luwang, M. N.; Chandra, S.; Bahadur, D.; Srivastava, S. K. Dendrimer facilitated synthesis of multifunctional lanthanide based hybrid nanomaterials for biological applications. J. Mater. Chem. 2012, 22, 3395−3403. (7) Mulder, W. J.M.; Griffioen, A. W.; Strijkers, G. J.; Cormode, D. P.; Nicolay, K.; Fayad, Z. A. Magnetic and fluorescent nanoparticles for multimodality imaging. Nanomedicine 2007, 2, 307−324. (8) Kumar, R.; Nyk, M.; Ohulchanskyy, T. Y.; Flask, C. A.; Prasad, P. N. Combined optical and MR bioimaging using rare earth ion doped NaYF4 nanocrystals. Adv. Funct. Mater. 2009, 19, 853−859. (9) Das, G. K; Heng, B. C.; Ng, S.-C.; White, T.; Loo, J. S. C.; D’Silva, L.; Padmanabhan, P.; Bhakoo, K. K.; Selvan, S. T.; Tan, T. T. Y. Gadolinium oxide ultranarrow nanorods as multimodal contrast agents for optical and magnetic resonance imaging. Langmuir 2010, 26, 8959−8965. (10) Ryu, J.; Park, H.-Y.; Kim, K.; Kim, H.; Yoo, J. H; Kang, M.; Im, K.; Grailhe, R.; Song, R. Facile synthesis of ultrasmall and hexagonal NaGdF4: Yb3+, Er3+ nanoparticles with magnetic and upconversion imaging properties. J. Phys. Chem. C 2010, 114, 21077−21082. (11) Park, Y. I.; Kim, J. H.; Lee, K. T.; Jeon, K.-S.; Na, H. B.; Yu, J. H.; Kim, H. M.; Lee, N.; Choi, S. H.; Baik, S.-I; Kim, H.; Park, S. P.; Park, B.-J.; Kim, Y. W.; Lee, S. H.; Yoon, S.-Y.; Song, I. C; Moon, W. K.; Suh, Y. D.; Hyeon, T. Nonblinking and nonbleaching upconverting 3417


Langmuir

Article

(28) Wang, H.; Nann, T. Monodisperse upconversion GdF3:Yb, Er rhombi by microwave-assisted synthesis. Nanoscale Res. Lett. 2011, 6, article no. 267 (29) Paik, T.; Ko, D-K; Gordon, T. R.; Doan-Nguyen, V.; Murray, C. B. Studies of liquid crystalline self-assembly of GdF3 nanoplates by inplane, out-of-plane SAXS. ACS Nano 2011, 5, 8322−8330. (30) Zhang, X.; Hayakawa, T.; Nogami, M.; Ishikawa, Y. Variation in Eu3+ luminescence properties of GdF3:Eu3+ nanophosphors depending on matrix GdF3 polytype. J. Alloys Compd. 2011, 509, 2076−2080. (31) Dong, C.; Raudsepp, M.; van Veggel, F. C. J. M. Kinetically determined crystal structures of undoped and La3+-doped LnF3. J. Phys. Chem. C 2009, 113, 472−478. (32) Ju, Q.; Liu, Y.; Tu, D.; Zhu, H.; Li, R.; Chen, X. Lanthanidedoped multicolor GdF3 nanocrystals for time-resolved photoluminescent biodetection. Chem.Eur. J. 2011, 17, 8549−8554. (33) Passuello, T.; Pedroni, M.; Piccinelli, F.; Polizzi, S.; Marzola, P.; Tambalo, S.; Conti, G.; Benati, D.; Vetrone, F.; Bettinelli, M.; Speghini, A. PEG-capped, lanthanide-doped GdF3 nanoparticles: Luminescent and T-2 contrast agents for optical and MRI multimodal imaging. Nanoscale 2012, 4, 7682−7689. (34) Fang, C.; Zhang, M. Multifunctional magnetic nanoparticles for medical imaging applications. J. Mater. Chem. 2009, 19, 6258−6266. (35) Chatterjee, D. K; Gnanasammandhan, M. K; Zhang, Y. Small upconverting fluorescent nanoparticles for biomedical applications. Small 2010, 6, 2781−2795. (36) Fuentes, M.; Mateo, C.; García, L.; Tercero, J. C.; Guisán, J. M.; Fernández-Lafuente, R. Directed covalent immobilization of aminated DNA probes on aminated plates. Biomacromolecules 2004, 5, 883−888. (37) Mosmann, T. J. J. Immunol. Methods 1993, 95, 55. (38) Matijević, E. Preparation and properties of uniform size colloids. Chem. Mater. 1993, 5, 412−426. (39) Jacob, D. S; Bitton, L.; Grinblat, J.; Felner, I.; Koltypin, Y.; Gedanken, A. Are ionic liquids really a boon for the synthesis of inorganic materials? A general method for the fabrication of nanosized metal fluorides. Chem. Mater. 2006, 18, 3162−3168. (40) Nuñez, N. O.; Ocaña, M. An ionic liquid based synthesis method for uniform luminescent lanthanide fluoride nanoparticles. Nanotechnology 2007, article no. 18455606. (41) Nuñez, N. O.; Quintanilla, M.; Cantelar, E.; Cussó, F.; Ocaña, M. Uniform YF3:Yb,Er up-conversion nanophosphors of various morphologies synthesized in polyol media through an ionic liquid. J. Nanoparticle Res. 2010, 12, 2553−2565. (42) Lorbeer, C.; Cybinska, J.; Mudring, A.-V. Facile preparation of quantum cutting GdF3:Eu3+ nanoparticles from ionic liquids. Chem. Commun. 2010, 46, 571−573. (43) Feldmann, C. Polyol-mediated synthesis of nanoscale functional materials. Adv. Funct. Mater. 2003, 13, 101−107. (44) Poul, L.; Ammar, S.; Jouini, N.; Fievet, F. Synthesis of inorganic compounds (metal, oxide and hydroxide) in polyol medium: A versatile route related to the sol-gel process. J. Sol-Gel Sci. Technol. 2003, 26, 261−265. (45) Dirksen, J. A.; Ring, T. A. Fundamentals of crystallization Kinetic effects on particle-size distributions and morphology. Chem. Eng. Sci. 1991, 46, 2389−2427. (46) Privman, V.; Goia, D. V.; Park, J.; Matijević, E. Mechanism of formation of monodispersed colloids by aggregation of nanosize precursors. J. Colloid Interface Sci. 1999, 213, 36−45. (47) Libert, S.; Gorshkov, V.; Privman, V.; Goia, D. V.; Matijević, E. Formation of monodispersed cadmium sulfide particles by aggregation of nanosize precursors. Adv. Colloid Interface Sci. 2003, 100, 169−183. (48) Wong, H.-T.; Chan, H. L.; Hao, J. H. Magnetic and luminescent properties of multifunctional GdF3:Eu3+ nanoparticles. Appl. Phys. Lett. 2009, 95, article no. 022512 (49) Tian, Y.; Tian, J.; Li, X.; Yu, B.; Shi, T. Facile synthesis of ultrasmall GdF3 nanowires via an oriented attachment growth and their luminescence properties. Chem. Commun. 2011, 47, 2847−2849. (50) Caravan, P.; Farrar, C. T.; Frullano, L.; Uppal, R. Influence of molecular parameters and increasing magnetic field strength on

relaxivity of gadolinium- and manganese-based T(1) contrast agents. Contrast Media Mol. Imaging 2009, 4, 89−100. (51) Hifumi, H.; Yamaoka, S.; Tanimoto, A.; Citterio, D.; Suzuki, K. Gadolinium-based hybrid nanoparticles as a positive MR contrast agent. J. Am. Chem. Soc. 2006, 128, 15090−15091. (52) Hastie, J. W; Hauge, R. H; Margrave, J. L. Geometries and entropies of metal trifluorides from infrared-spectraScF3, YF3, LaF3, CeF3, NdF3, EuF3, AND GdF3. J. Less Common Met. 1975, 39, 309− 334.

3418


Ionic Liquid Mediated Synthesis and Surface Modification of

Recommend Documents