Influence of a Silica Interlayer on the Structural and Magnetic

Article pubs.acs.org/Langmuir

Influence of a Silica Interlayer on the Structural and Magnetic Properties of Sol−Gel TiO2‑Coated Magnetic Nanoparticles Laura De Matteis,† Rodrigo Fernández-Pacheco,†,‡ Laura Custardoy,†,‡ María L. García-Martín,§ Jesús M. de la Fuente,†,∥ Clara Marquina,*,⊥,# and M. Ricardo Ibarra†,‡,⊥ †

Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, Mariano Esquillor s/n, 50018 Zaragoza, Spain Laboratorio de Microscopías Avanzadas (LMA) - Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, Mariano Esquillor s/n, 50018 Zaragoza, Spain § Centro Andaluz de Nanomedicina y Biotecnología (BIONAND), Severo Ochoa, 35, Parque Tecnológico de Andalucía, 29590 Málaga, Spain ∥ Fundación ARAID, María de Luna 11, 50018 Zaragoza, Spain ⊥ Departamento de Física de la Materia Condensada, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain # Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain ‡

S Supporting Information *

ABSTRACT: Superparamagnetic iron oxide nanoparticles coated with titanium dioxide have been synthesized, growing the titanium dioxide directly either on the magnetic nuclei or on magnetic nanoparticles previously coated with a semihydrophobic silica layer. Both coatings have been obtained by sol−gel. Since it is well-known that the existence of the intermediate silica layer influences the physicochemical properties of the material, a detailed characterization of both types of coatings has been carried out. The morphology, structure, and composition of the synthesized nanomatrices have been locally analyzed with subangstrom spatial resolution, by means of aberration corrected transmission electron microscopy (HRTEM and STEM-HAADF). Besides magnetization measurements, proton relaxivity experiments have been also performed on water suspensions of the as-synthesized nanoparticles to investigate the role of the silica interlayer in the relaxometric properties. The silica interlayer leads to nanoparticles with much higher water stability and to higher relaxivity of the suspensions.

1. INTRODUCTION Titanium dioxide is widely used due to its physical and chemical stability, low cost, nontoxicity, resistance to corrosion, and photocatalytic properties.1 TiO2-based materials, both commercially available and newly synthesized, that differ in morphology and properties, have been employed for wastewater treatment and water contamination control,2,3 prevention of material biodeterioration,4−6 microbial contamination control, and infections treatment.7,8 TiO2 nanoparticles have been used as well for biomedical applications, as for example for cancer treatment9 or as support of phosphonated gadolinium chelates for multimodal imaging-therapeutic nanoprobes.10 Among the different synthesis methods to produce TiO2 materials, sol−gel is one of the most frequently used for the development of new materials for a very broad range of applications.1 It is a cheap, low-temperature, and a mild condition synthetic approach that allows a fine control of the final product composition, structure, and shapes (fibers, films, monoliths, and particles). Sol−gel allows the formation of coatings of different nature (i.e., crystalline phase), morphology, and properties depending on the reaction conditions.11−16 © 2014 American Chemical Society

Titanium dioxide is also a suitable inorganic material for coating iron oxide magnetic nanoparticles (as magnetite and/or maghemite) to obtain magnetic field-responsive photocatalysts.17−20 It is well-known that inorganic coatings preserve the magnetic core from further oxidation.21,22 Therefore, in addition to its photoactivity, the TiO2 coating is intended to prevent the loss of the magnetic properties of the core, which would deprive the particle of its magnetic functionality. However, it has been reported that a TiO2 coating directly on the iron oxide core leads to undesired interphase interactions that drastically affect the physicochemical properties of the material, decreasing its photoactivity and worsening the magnetic properties. An intermediate SiO2 layer has been proposed as a strategy to overcome these drawbacks.17,19,23−26 Most of the works focus on the evaluation of the photocatalytic performances of the synthesized materials depending on the absence/presence of the intermediate silica coating. Received: January 31, 2014 Revised: April 1, 2014 Published: April 3, 2014 5238

dx.doi.org/10.1021/la500423e | Langmuir 2014, 30, 5238−5247

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Because of the high reactivity of the titanium precursors used to obtain sol−gel coatings, a heterogeneous TiO2 nucleation usually occurs.11,12,27 However, it has been reported that optimizing the sol−gel reaction conditions a continuous TiO2 layer can be obtained.16 Moreover, the characteristics of the substrate also influence the growing of the coating:28 having or not the silica intermediate layer implies working with an amorphous support (the silica covering the magnetic nuclei) or with a crystalline one (the magnetic nuclei themselves). Given the relevance of the modification on the nanoparticles properties depending on the existence of the intermediate silica layer, an in-depth characterization at subnanometric scale of both types of coatings is necessary. With this aim in mind the sol−gel method has been used to obtain magnetic iron oxide nanoparticles with a TiO2 outer coating, synthesizing the titanium dioxide directly on the magnetic cores (TiO2-coated nanoparticles) or on magnetic nuclei previously coated with a semihydrophobic silica layer also obtained by sol−gel (particles hereafter called TiO2/SiO2coated nanoparticles). Aberration corrected transmission electron microscopy (TEM) techniques are the most suitable techniques to locally analyze the morphology, structure, and composition of complex systems at subnanometer scale.21,29 In the present case HRTEM and HAADF-STEM have been used to study the morphology of the TiO2- and TiO2/SiO2-coated nanoparticles and to perform a detailed spatial analysis of their chemical composition. In addition, a comprehensive characterization of the physicochemical properties of the resulting materials has been carried out, including the characterization of their magnetic properties. Proton relaxivity measurements have been also performed, since recent works have shown that relaxometric studies provide insight not only on the magnetic properties of the core but also on the properties of the coating.30

Scheme 1. Schematic Representation of the Reaction Steps To Obtain (a) TiO2- and (b) TiO2/SiO2-Coated Magnetic Iron Oxide Nanoparticles

centrifugation the precipitate was recovered to be dried at the air. The powder (approximately 12 mg) was annealed at 550 °C for 1 h, allowing the complete transformation of amorphous titanium oxide into anatase.31 TiO2/SiO2-Coated Nanoparticles. A mixture of 15 mg of iron oxide nanoparticles suspended in 2.5 mL of water, 20 mL of absolute ethanol, and 375 μL of NH3 (30%) was prepared and left 20 min under sonication in order to obtain a good homogenization of the nanoparticles suspension. Then 0.220 mmol of TEOS was added, and the reaction was left under sonication for 30 min for the silica shell formation. Successively 1.1 mmol of TMES was added as coprecursor and hydrophobic agent to obtain an increase in the hydrophobicity of the coating surface. The mixture was sonicated other 30 min, and then it was left overnight under rotatory stirring. After that nanoparticle suspension was centrifugated at 24 000 rpm to remove the unreacted precursors, it was washed once with ethanol and then heated at 60 °C for 6 h under magnetic stirring. Finally, the sample was centrifugated again, washed once with ethanol, and, after another centrifugation, resuspended in 10 mL of fresh absolute ethanol. For the subsequent coating with titanium dioxide, a volume of 7.5 mL of the obtained silica-coated sample was put under sonication just before the addition of TBOT (0.145 mmol). Then 1.6 mL of a separate mixture EtOH:H2O in the molar ratio 5:1 was added immediately to the mixture NPs/precursor to start the sol−gel coating reaction; the final reaction mixture was left under sonication for 2 h, and after that it was left under gentle rotator stirring for 20 h. Then nanoparticles were centrifugated at 22 000 rpm for 10 min in order to remove the unreacted precursor, and they were washed once with absolute ethanol. After a further centrifugation the precipitate was recovered to be dried at the air. The powder (≈20 mg) was annealed at 550 °C for 1 h, allowing the complete transformation of titanium oxide into anatase.31 2.2.2. Nanoparticle Filtration. After annealing, nanoparticles were resuspended in Milli-Q water in a diluted concentration, and they were filtered with a low ash content paper filter with a 2−3 μm pores suitable for small particle suspensions. The filters were changed every 20 mL of filtered suspension. Finally, the recovered nanoparticles were centrifuged and resuspended in a final volume of 1 mL. 2.2.3. X-ray Diffraction (XRD). X-ray diffraction spectra were obtained by using a D-Max Rigaku instrument equipped with a

2. EXPERIMENTAL SECTION 2.1. Materials. Iron salts FeCl3·6H2O and FeCl2·4H2O were purchased from Sigma together with ethoxytrimethylsilane (TMES) and tetraethyl orthosilicate (TEOS). Titanium(IV) n-butoxide (TBOT) was purchased by Alfa Aesar. Absolute ethanol and ammonia 30% were purchased from Panreac. Milli-Q H2O has been used in all the experiments. 2.2. Methods. 2.2.1. Nanoparticle Synthesis. TiO2-coated and TiO2/SiO2-coated iron oxide magnetic nanoparticles were prepared following two slightly different sol−gel synthetic routes. The synthetic steps are reported in Scheme 1. In both cases the iron oxide core was synthesized by the coprecipitation method reported by De Matteis et al.21 Briefly, the magnetic core was prepared by coprecipitation method as follows: FeCl3·6H2O and FeCl2·4H2O in the ratio 3:1 were dissolved in water, and a proper amount of 30% NH3·H2O was then added quickly into the solution under a gently stirring. The color of the solution turned from orange to black immediately, indicating that the formation of the particles started. The suspension was left to rest for 20 min. Iron oxide nanoparticles were first separated magnetically to remove the reaction mixture and then were washed three times with water. Finally, the particles were resuspended either in water or in absolute ethanol, depending on the following synthetic step. TiO2-Coated Nanoparticles. 8.3 mg of iron oxide nanoparticles resuspended in 7.2 mL of absolute ethanol was put under sonication, and 0.145 mmol of TBOT was added. Almost immediately 4.1 mL of an EtOH: H2O mixture in the molar ratio 1:5.5 was added to start the sol−gel reaction. The nanoparticle suspension in the reaction mixture was kept under sonication for 2 h, and after that it was left under gentle rotator stirring for 20 h. Then nanoparticles were centrifuged at 22 000 rpm for 10 min in order to remove the unreacted precursor, and they were washed once with absolute ethanol. After a further 5239


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rotating Cu anode and a graphite monocromator (therefore working with Kα1 = 1.5405 Å). The diffractometer worked at 40 kV and 80 mA. Data were recorded in the 2θ range between 10° and 80° by using a step of 0.03° and 1 s/step. 2.2.4. Compositional Analysis. The composition analysis of the synthesized samples was carried out by means of inductive coupled plasma−optic emission spectrometry (ICP-OES) in a IRIS ADVANTAGE Thermo Jarrel Ash equipment, after digestion of the samples in HCl/HNO3 mixture. The error of element determination was in the present cases lower than 3%. 2.2.5. Dynamic Light Scattering Analysis. The hydrodynamic size of particles in liquid suspension was measured in a Brookhaven 90Plus DLS instrument by means of the photocorrelation spectroscopy (PCS) technique. 2.2.6. Electrophoretic Mobility (Z-Potential Analysis). Point of zero charge (isoelectric point, IEP) of particle surface was determined by measuring the potential vs pH of a 0.05 mg/mL nanoparticle suspension in 5 mM KCl with a Plus particle size analyzer (Brookhaven Instruments Corporation). Particles were added to the KCl water solutions at fixed pH, and after 24 h a stable pH value for the obtained suspensions was measured and used in the following analysis. 2.2.7. Apparent Surface Coverage (ASC) Calculation. The ASC of the TiO2 coating on the TiO2/SiO2-coated nanoparticles was obtained by the equation32

direct measurement of the longitudinal (T1) and transversal (T2) relaxation times. These values were obtained by performing time domain 1H NMR measurements. The experiments were carried out in a Bruker Minispec MQ-60 NMR spectrometer working at 1.41 T magnetic field, 60 MHz operating frequency, and at 37 °C. For T1 measurements, an inversion recovery pulse sequence (IR) was used; T2 measurements were conducted by using a spin echo pulse sequence type Carr-Purcell-Marboom-Gill (CPMG). The relaxivity was studied as well deriving the relaxation times from magnetic resonance images (MRI) of different concentrations of the nanoparticles suspensions, measured at 9.4 T in a Bruker Biospec TM 94/20 USR, at room temperature. Images were postprocessed using dedicated IDL 6.2 (Exelis VIS Inc., Boulder, CA) software with homemade written scripts.

3. RESULTS AND DISCUSSION 3.1. Nanoparticle Synthesis. A new sol−gel protocol was developed to obtain magnetic nanoparticles coated with an external TiO2 surface grown on an intermediate semihydrophobic silica coating (henceforth TiO2/SiO2-coated nanoparticles). Their properties were compared with those of nanoparticles synthesized with TiO2 directly coating the magnetic core (henceforth TiO2-coated nanoparticles). The synthetic steps are reported schematically in Scheme 1. Superparamagnetic iron oxide nanoparticles (between 5 and 10 nm, as derived from TEM images), obtained through a previously described coprecipitation method,21 were used as magnetic cores in both synthetic patterns. In the case of TiO2/ SiO2-coated nanoparticles general methods to obtain silica materials with the proper degree of hydrophobicity33−35 were chosen as starting point to develop a new synthetic protocol for nanoparticle silica coating. This new method allowed achieving the optimal compatibility with ethanol required to produce a well-dispersed and stable nanoparticles suspension during the subsequent TiO2 sol−gel synthesis. The reaction was carried out in two steps as schematically shown in Scheme 1; the addition of TMES as coprecursor, in the optimized amount during the second step, permitted to add hydrophobic moieties (−CH3) on the thin silica coating produced by TEOS. Reaction times were carefully optimized for each step (optimized reaction times are reported in detail in the Experimental Section together with the reactant amounts). The conditions in the TiO2-coating reaction were slightly different depending on the presence of the intermediate silica surface, and they were independently adjusted to obtain the highest stability of the nanoparticles in the reaction medium (see Experimental Section). In both cases no acid was added as catalyst in the sol−gel reaction, differently from most of the works reported in the literature for TiO2 nanoparticles and coating protocols.17,36 It is well-known that the presence of acid in the reaction medium leads to a faster hydrolysis rate: this means that hydrolysis can be almost complete before massive condensation occurs. If condensation starts early, the incorporation of alkyl groups inside the network occurs, and therefore a disordered or defected structure is formed. For this reason sol−gel reaction is usually carried out under strong acidic conditions when a pure rutile phase is desired. On the contrary, in this work a close to neutral pH condition was used in the reaction, in order to achieve both an operative condition that allows the homogenization of the reactive suspension before a massive reaction starts and also to obtain anatase, the phase of interest because of its high photocatalytic activity.19,37 In fact, during the condensation process, the elongation of linear −Ti−O−Ti− chains lead to rutile while a lower

%ASC = MWTiO2(IEPSiO2 − IEPTiO2 /SiO2) [MWSiO2(IEPTiO2 /SiO2 − IEPTiO2) − MWTiO2(IEPTiO2 /SiO2 − IEPSiO2)]

where MWTiO2 and MWSiO2 are the molecular weights of titanium dioxide and silica, respectively. The subscripts TiO2, SiO2, and TiO2/ SiO2 refer to the TiO2-, SiO2-, and TiO2/SiO2-coated nanoparticles, respectively. 2.2.8. Surface Area Analysis. The total surface area of the samples was determined by a low-temperature gas (argon) adsorption technique with an ASAP 2020 V3.00H instrument and by BET (Brunauer, Emmett, and Teller) analysis. 2.2.9. Transmission Electron Microscopy (TEM). A preliminary observation of the synthesized particles was carried out by bright field (BF) imaging in a FEI Tecnai T20 operated at 200 kV. The samples for all TEM techniques were prepared by resuspending the powder in water under sonication, putting a drop of the suspension directly on a TEM holey-carbon copper grid, and letting it dry in an air atmosphere before putting it under vacuum inside the microscope. High-resolution TEM (HRTEM) images were obtained in a Cs-image corrected Titan (FEI) operated at 300 kV. Correction of the spherical aberration allows a subangstrom spatial resolution. The crystalline structure for each particle was calculated from diffractograms obtained by applying a fast Fourier transform (FFT) to selected areas in the images. Scanning transmission electron microscopy and high angle annular dark field (STEM-HAADF) images and electron dispersive X-ray spectra (EDS) were obtained using an EDAX detector coupled to a Tecnai F30 (FEI) operated at 300 kV, equipped with a HAADF detector. A small probe (about 1 nm) is formed, and the sample is scanned, obtaining a whole range in energy spectrum for each point with a dispersion of 0.5 eV/ channel. The acquisition time for each spectrum was 5 s. That way, 2D maps of each element and RGB compositions of three components can be represented to understand the spatial distribution of Fe, Si, and Ti within the complex entities. 2.2.10. Magnetization Measurements. Magnetization measurements were performed in a high sensitivity magnetometer MPMS-XL (Quantum Design Inc.) with superconducting quantum interference detection (SQUID). The temperature dependence of the magnetization of powders and filtrated aqueous samples was measured from 5 to 320 K and to 250 K, respectively. Magnetization isotherms up to 5 T were recorded at 5 K and at the respective highest temperature. 2.2.11. Relaxivity Studies. The relaxivity of filtered aqueous suspensions was studied by two different methods. First of all, the longitudinal and transversal relaxivities were determined from the 5240


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symmetry bridging between shorter chains, favored by slightly higher pH values, favors the formation of anatase.14,38 After the reaction, the nanoparticles were annealed in a tubular oven to complete the crystallization of the TiO2 coating into anatase. The samples were analyzed by X-ray diffraction to prove the presence of both anatase and the magnetic iron oxide phase (magnetite/maghemite) (see Supporting Information). No rutile contribution was found in XRD spectra (with respect to the JCPDS file no. 21-1276), and no oxidation of the iron oxide core to hematite was observed. This result indicates that in both cases the coating completely covers the magnetic core and protects it from oxidation even well above room temperature, which can be of great potential interest for technological applications. 3.2. Morphological Characterization. The morphology of the samples with only TiO2 coating and of those with the intermediate silica shell is shown in the BF-TEM images in Figures 1a and 2a. For comparison, an image of the magnetic

Figure 2. (a) BF TEM and (b) HRTEM image of TiO2/SiO2-coated nanoparticles. (c) FFT of the yellow square area in the (b) image. (d) Maps obtained after the IFFTs of anatase (101) (green) and magnetite/maghemite (311) (red) reflections filtered separately.

characteristic spinel structure of magnetite and/or maghemite, confirms the presence of magnetic iron oxide nuclei. Filtering separately the diffraction spots of anatase and those corresponding to the magnetite/maghemite (Figures 1c and 2c) and applying the inverse fast Fourier transform (IFFT), it is possible to obtain (colored) maps of the two types of coated magnetic particles (Figures 1d and 2d). In both cases the magnetic nuclei (in red) and crystalline anatase (in green) are observed. In the case of the TiO2-coated nanoparticles it is not possible to identify a well-defined layer around the iron oxide nuclei, either in Figure 1a or in the HRTEM image. Instead, sets of crystallographic planes corresponding to magnetite/ maghemite and to anatase appear homogeneously distributed all over the image, suggesting magnetic iron oxide nuclei in an anatase matrix. In the case of TiO2/SiO2-coated nanoparticles, an amorphous intermediate silica coating envelops the magnetitc inclusions forming a matrix structure with TiO2 individual nanoparticles on its surface. The measurement of the hydrodynamic diameter of the particles in water suspension was not possible at this point of the work because the broad size dispersion of the sample made them not stable enough to give a reproducible measure. In order to narrow the size distribution and improve the sample stability, the annealed powders were resuspended in water and subsequently filtered (see Experimental Section) with the aim of removing both the largest matrices in the as-synthesized sample and the large powder aggregates possibly produced during the annealing step. After filtration a substantial increase in particle stability was observed, being much higher in the case of the TiO2/SiO2-coated particles. DLS measurements could be successfully carried out, allowing the determination of the hydrodynamic diameter distribution of both samples. DLS data of filtered nanoparticles with or without the intermediate silica layer are reported in the Supporting Information. Size distribution data of SiO2-coated nanoparticles before TiO2 coating are also reported for comparison. Water suspensions

Figure 1. (a) BF TEM and (b) HRTEM image of TiO2-coated nanoparticles. (c) FFT of the yellow square area in the (b) image. (d) Maps obtained after the IFFTs of anatase (101) (green) and magnetite/maghemite (311) (red) reflections filtered separately.

cores coated with the semihydrophobic silica layer before the TiO2 coating is shown in the Supporting Information. A change in the morphology of the particles can be appreciated depending on the presence of the silica layer. The presence of an intermediate amorphous silica coating enveloping magnetic nuclei and of crystalline TiO2 on the SiO2 coating is confirmed by HRTEM (see Figure 2b). The fast Fourier transform (FFT) of the HRTEM images yields structure information on each particle, since it provides the reciprocal space representation of the real space image. Each diffraction spot corresponds to a family of planes in Bragg’s condition. Thus, the distance of these diffraction spots to the central transmitted beam and the angles between them give us the crystalline structure for each particle. The presence of spots at 3.5 Å, distance corresponding to the (101) planes of the anatase structure confirms the presence of TiO2 crystallized in that phase. Likewise, the presence of spots at 2.5 Å, a distance that corresponds to the (311) planes of the 5241


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of TiO 2 /SiO 2 matrices resulted composed of several populations with hydrodynamic diameters ranging from 180 and 350 nm; each population represents between 15% and 30% of the sample. In the case of the water suspensions of the TiO2 matrices a large population of 300 nm diameter prevailed. Taking into account the observed difference in the stability of the two types of suspensions, it is possible that, despite the smaller size of the TiO2-coated nanoparticles (as derived from TEM images), the absence of the silica layer would probably decrease their water stability. Therefore, the probability of aggregation in water suspension would be higher than that of the TiO2/SiO2-coated nanoparticles, leading to the higher amount of large particles detected by DLS. 3.3. Compositional Characterization. Further information about the relation between the morphology and composition of the two kinds of samples was derived after a compositional analysis realized by ICP before and after filtration. The ratios between Fe and Ti and Fe and Si in the case of silica-coated nanoparticles were calculated, and results are reported in Table 1. Such ratios are strongly related to the

Scheme 2. Schematic Representation of the Two Different Nanomatrices Obtained and the Effect of Filtration Process: (a) TiO2-Coated and (b) TiO2/SiO2-Coated Nanoparticles

Table 1. Elemental Composition of TiO2- and TiO2/SiO2Coated Magnetic Nanoparticles before and after Filtration, Obtained from ICP Analysis

addressed to the special nature of the TiO2 coating. As reported in the literature,27,39 and as suggested by the particle morphology seen in BF-TEM images in Figure 1, titanium oxide coating commonly grows in the shape of small nanograins all around the particle surface. This discontinuous nucleation and growth of the TiO2 on the magnetic nuclei is characteristic of the sol−gel coating method.16 It can lead to an increase of the effective surface, which can be an advantage for catalysis applications. In the case of only TiO2-coated particles titanium oxide crystalline grains coated the iron oxide nanoparticles and juxtaposed, leading to a particle with magnetic nuclei entrapped in a TiO2 matrix. A similar disposition was observed in the case of TiO2-coated cobalt ferrite nanoparticles by Li and coworkers.24 In the case of nanoparticles with the silica layer, such TiO2 nanograins grew on this silica coating. However, the high water stability of the particles with an intermediate silica layer suggests that the TiO2 coating grows in a different way on silica than on the magnetic nuclei. It is possible that the TiO2 nuclei did not cover completely the silica surface, leading to particles with an external surface composed of both TiO2 and SiO2. Taking into account the difference between SiO2 and TiO2 surface charge, Z-potential measurements (reported in Figure 3) have been used to investigate the presence of silica on the external surface. The derived IEP of TiO2-coated nanoparticles is consistent with the measure reported in the literature for nanoparticles of

sample TiO2 coated Fe (μM) Ti (μM) Si (μM) Fe/Ti Ti/Si Fe/Si

filtered TiO2 coated

86 44

15 8

1.95

1.88

TiO2/SiO2 coated

filtered TiO2/SiO2 coated

73 18 7 4.06 2.57 10.43

43 28 23 1.53 1.22 1.87

nanoparticle volume/surface ratio. If we consider the volume of the particles being that of the magnetic nuclei, the volume/ surface ratio changes depending on the number of magnetic nuclei coated together to form a particle. In the case of only TiO2-coated nanoparticles we can consider that the Fe/Ti ratio does not change after filtration, indicating that all the nanomatrices are identical, and that the filtration process improved the stability of the nanoparticles water suspension, probably eliminating the biggest aggregates formed by the interaction of more than one of those identical entities. The described effect of filtration is schematically reported in Scheme 2. On the contrary, in the case of nanomatrices with the silica intermediate layer, both Fe/Ti and Fe/Si decreased after filtration due to an increase of the total surface per volume, which means a decrease in size and an increase of the Ti and Si content. These results indicate that filtration selected the small nanomatrices (those with the smallest number of magnetic nuclei) by eliminating the ones with more nuclei (the biggest ones), yielding to the decrease of the Fe content. In the case of TiO2-coated particles the Fe/Ti ratio remains constant, suggesting an improvement of the sample quality in terms of homogeneity and stability, by eliminating the largest aggregates formed by physicochemical interactions between similar TiO2-coated particles, as represented in Scheme 2. As already mentioned, a highly stable sample was obtained in the case of nanoparticles possessing the intermediate silica layer. The higher stability of these nanomatrices can be

Figure 3. Z-potential analysis (▲,SiO2-coated; ■, TiO2/SiO2-coated; □, TiO2-coated). 5242


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pure sol−gel TiO2.39,40 The Z-potential was measured on silicacoated magnetic nanoparticles before TiO2 coating, and the results are shown in Figure 3 for comparison. Our results are as well in good agreement with those reported in the literature.41,42 After TiO2 coating the IEP value is lower than that of nanoparticles without silica intermediate layer. This result together with the steeper decrease of the potential toward negative values when increasing the pH of the medium indicates the more acidic character of the surface of the SiO2/ TiO2-coated nanoparticles. Both observations point to the presence of silica in the external surface. On the basis of the derived IEP value of TiO2/SiO2-coated nanoparticles, the percentage of TiO2 on the nanoparticle surface can be obtained by calculating the percentage of apparent surface coverage (%ASC)32 (see Experimental Section). From this estimation resulted that TiO2 constitutes the 82% of the outer coating, confirming that TiO2 does not completely cover the silica interlayer. Specific surface area measurements were performed by BET analysis to further investigate the coexistence of both SiO2 and TiO2 on the external surface of the nanoparticles synthesized with the intermediate silica layer. The measurements were carried out before and after the TiO2 coating. Data are reported in Table 2, together with those corresponding to the TiO2-

Figure 4. (a) STEM-HAADF image of a TiO2/SiO2-coated nanoparticle. The white line corresponds to the EDS line-scan performed. Scale bar: 50 nm. (b) EDS spectrum taken at the point marked in the image; Fe K is extracted as an example. (c) Composition profiles corresponding to the Si, Ti, and Fe K-absorption edges.

point and represent a 2D map for each element. RGB superimposed maps of the three chemical elements can be observed in Figure 5b. The fact that the red component appears always associated with the blue one leading to pink spots means that Fe appears always located in those places where Si is present. Therefore, the SiO2 forms a continuous coating covering the magnetic iron oxide nuclei. On the contrary, the fact that the green component appears only in specific sites evidences that TiO2 is not uniformly coating the SiO2 surface. 3.4. Magnetic Characterization. To assess whether the presence of the intermediate silica layer influences the magnetic properties of the synthesized samples, magnetization measurements were performed as a function of temperature and applied magnetic field, in the as-synthesized powders as well as in their water suspensions obtained after filtration. For the calculation in emu/g, we have assumed as a first approximation that the magnetic nuclei are stoichiometric Fe3O4; the magnetite mass has been obtained from the iron content derived from the ICP measurements. The ZFC−FC temperature dependence of the magnetization of the TiO2-coated and the TiO2/SiO2-coated powders can be seen in Figure 6a. In both cases the ZFC and FC curves overlap from the highest measured temperature down to the so-called temperature of irreversibility, Tirr, which is ≈110 K. In an ideal superparamagnetic system Tirr coincides with the blocking temperature (TB), above which the system behaves as superparamagnetic. In the present case the ZFC− FC curves evidence the existence of a distribution of blocking temperatures, related to the size polydispersity of the magnetic nuclei in the nanomatrix. In such cases, the temperature at which a maximum appears in the ZFC curve (Tmax) is commonly accepted in the literature as an average blocking temperature.43−46 In our particular case Tmax is very close to Tirr, indicating a narrow size distribution of the magnetic nuclei in both types of nanomatrices, and therefore considering Tmax as the blocking temperature is a good approximation. In both cases TB ≈ Tmax = 100 K, a value which coincides with the blocking temperature of the magnetic nuclei synthesized by coprecipitation.21 For Stoner−Wohlfart particles, their volume is related to the TB obtained from magnetization measurements

Table 2. Specific Area and Pore Diameter of SiO2-, TiO2/ SiO2-, and TiO2-Coated Magnetic Nanoparticles 2

area (m /g) pore diameter (Å)

SiO2 coated

TiO2/SiO2 coated

TiO2 coated

111.1 9.93

77.2 7.57

69.9 7.46

coated nanoparticles. The specific area of these nanoparticles is similar to that of annealed sol−gel titanium oxide materials reported in the literature.15,19 The specific area of nanomatrices with the intermediate silica layer decreases after coating with TiO2 , but it is higher than that of the TiO2 -coated nanoparticles, indicating the presence of SiO2 on the external surface. The value is consistent with the ASC% value and confirms our assumption about the composition of the external surface. Advanced electron microscopy techniques are among the most appropriate to study the chemical composition of nanoparticle surfaces at nanometer scale. In our case a STEM-HAADF study has been carried out on the nanoparticles with the intermediate silica coating to obtain the distribution of silicon and titanium. The results are displayed in Figure 4. EDS line scans and a point spectrum (Figure 4b) have been obtained in the STEM-HAADF mode, and a chemical profile of the elements has been obtained by extracting the signal corresponding to the Fe K, Si K, and Ti K edges at each spectrum (see Figure 4c). Iron was detected in several regions of nanometric size inside the matrix while silica appears distributed along the whole profile. The TiO2 profile is as well noncontinuous, leaving the intermediate silica coating exposed in some places of the outer shell. To get a deeper insight into the direct visualization and spatial localization of the material constituents, a 2D-compositional mapping has been obtained by EDS analysis. It is reported in Figure 5. The sample (Figure 5a) is scanned with a small probe, and due to the electron−matter interaction X-rays are emitted. The energy of these X-rays is characteristic of each element; that way we can obtain a whole spectrum for each 5243


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Figure 5. EDS compositional mapping: STEM-HAADF image of (a) TiO2/SiO2-coated and (c) TiO2-coated particles and RGB mappings (b, d) of the respective selected areas. In (b): red = Fe; green = Ti; blue = Si. In (d): red = Fe; green and blue = Ti. Scale bar: 50 nm.

by means of the expression KV = 25kBTB, where K is the nanoparticle anisotropy constant and kB is the Boltzmann constant.47 As reported in ref 21, the upper limit for the diameter of the magnetic nuclei synthesized by coprecipitation derived from this expression and blocking at 100 K is 17 nm, assuming for K the value of bulk magnetite at room temperature.48 Also as an approximation, considering the same K for the magnetic nuclei in both the TiO2 and the TiO2/ SiO2 nanomatrices, the fact that the TB of the magnetic nuclei once embedded in the respective nanomatrices is as well 100 K would indicate that the magnetic nanoparticles keep their volume once coated by TiO2 or TiO2/SiO2. However, as it will be discussed in the following paragraph, the assumption of an identical magnetocrystalline anisotropy might not be realistic. The slope of both ZFC−FC magnetization curves in the superparamagnetic regime deviated from the Curie−Weiss behavior, pointing to the existence of dipolar interactions whose strength seems to be slightly higher in the case of the TiO2-coated magnetic nuclei.46 The magnetization isotherms at 300 K (see Figure 6b) confirmed the superparamagnetic behavior at room temperature, regardless of the presence of the intermediate silica coating. The small coercivity (less than 10 Oe) seen in the low field region of the isotherm (see Figure 6b inset) is due to the remanent field of the superconducting magnet. The magnetization values (65 emu/g for TiO2/SiO2coated nanoparticles and 40 emu/g TiO2-coated nanoparticles, at 5 T) were comparable (or even higher) to those reported in the literature.20,26,49 The comparison of these two values indicates that coating the magnetic nuclei with SiO2 prior to the TiO2 growth leads to nuclei with higher content of highly crystallized magnetic phase. The different approach to saturation observed at high fields in the isotherms of Figure 6b might suggest an extra contribution to the magnetocrystalline anisotropy. This extra contribution would appear as surface anisotropy, which would be originated by a thin shell of disordered noncompensated magnetic moments on the surface of the crystalline magnetic nuclei.50 At room temperature this surface anisotropy would be higher in the case of the TiO2/SiO2-coated nanoparticles, suggesting that this thin surface might be of different magnetic nature depending on the presence of the intermediate silica

coating: this could lead to the formation of phases with a different temperature dependence of the respective anisotropies, to glassy behavior, etc.51 Moreover, the existence of different magnetic phases on the surface of the magnetic nuclei depending on the coating might be also due to the formation of solid solutions between iron and titanium oxides, frequently reported in the literature.52 All together they could also account for the different slope of the temperature dependence of the FC magnetization curves at low temperatures seen in Figure 6a. However, surface and finite-size effects45,53 (leading to the existence of a dead magnetic layer,54 canting of surface spins,55 cation site disorder of magnetic ions at the magnetic nuclei surface,56,57 among other effects) cannot be excluded. Taking into account that in the present case this broad phenomenology seems to appear to be due mainly to an interface of nanometric thickness, the identification of these magnetic phases and the complete understanding of the observed magnetic behavior would require the use of techniques that probe the magnetization at local level and at nanometer scale, such as Mössbauer spectroscopy and/or X-ray magnetic circular dichroism (XMCD). However, such an in-depth magnetic characterization is far beyond the scope of this work. The relaxometric properties of water suspensions of the synthesized particles have been studied to get a better insight into the different coatings, analyzing whether the presence of the SiO2 intermediate layer influences the relaxivity. The relaxivity of the nanoparticles coated with an intermediate SiO2 layer has been measured at 37 °C and 60 MHz, before and after coating with TiO2. These results have been compared with the relaxivity of the nanoparticles coated with only TiO2. As corresponds to particles with a superparamagnetic iron oxide core, the longitudinal relaxivity is very small compared to the transversal one. The transversal relaxivity of the suspensions obtained with the SiO2-coated nanoparticles is 243 s−1 mM−1. After coating with TiO2 the transversal relaxivity decreases up to 125 s−1 mM−1. It has been reported that the relaxivity of superparamagnetic silica coated nanoparticles depends on the coating thickness. According to Pinho et al.30 for thicknesses above a critical limit, the SiO2 shell is permeable. The silica layer resulting from the synthesis is thick enough to be water permeable in its outer part. However, the presence of crystalline 5244


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that of the TiO2/SiO2 layer, the effective distance of closest approach of the water protons to the superparamagnetic core is higher, which leads to a slower relaxivity. As expected from the relaxivity values (in the range of those of Feridex IV and Endorem,58,59) the suspensions provide a good contrast in MR images (obtained at room temperature and applying a magnetic field of 9.4 T), according to Figure 7.

Figure 7. T2-weigthed MR images of water suspensions containing different concentrations of Fe in (A) SiO2/TiO2-coated nanoparticles and (B) TiO2-coated nanoparticles; water and Gd-doped water were used as control. Color scale: variation of T2 (ms). Images were taken applying 9.4 T at room temperature.

The reduction in the transversal relaxation time of the water protons (T2) is much higher in the case of the TiO2/SiO2coated nanoparticles than in the case of the TiO2-coated ones. For particle concentrations corresponding to Fe contents equal to or higher than 0.50 mM, the reduction of T2 is similar to that induced by a 5 mM water solution of a Gd-based contrast agent. The nonprogressive variation of the contrast with the concentration of nanoparticles seen in column B (i.e., corresponding to the TiO2-coated nanoparticles suspensions) was attributed to the sample precipitation during the time of the measurements, as a consequence of the increasing agglomeration appearing when the nanoparticle concentration increased. Therefore, the T2 values derived from the contrast observed in column B, and thus the transversal relaxivity, were not reliable. Particle agglomeration was not observed in the case of the TiO2/SiO2-coated nanoparticles, at least for particle concentrations corresponding to Fe contents lower than 1 mM. The higher stability of these suspensions supports the presence of silica in the outer shell of the particles, as a higher stability would be favored by the above-mentioned permeability of the silica coating.

Figure 6. (a) ZFC−FC temperature dependence of the magnetization of the TiO2/SiO2- and TiO2-coated nanoparticles. Measurements have been performed applying 50 Oe. (b) Magnetization isotherms at 300 K of the TiO2/SiO2- and TiO2-coated nanoparticles. Inset: low field region.

TiO2 grains on the silica surface reduces the permeability of the silica coating decreasing the proton diffusion by hindering the contact between the water molecules and the silica surface and therefore yielding a lower r2 (and r1). On the other hand, the relaxivity of the suspensions obtained with the TiO2/SiO2coated nanoparticles resulted to be higher than that of the suspensions obtained with the TiO2-coated nanoparticles (r2 = 41 s−1 mM−1). In both cases the longitudinal relaxation was almost negligible. The lower transversal relaxivity values of the TiO2-coated nanoparticles could be in part due to their lower magnetization at room temperature but are also influenced by the absence of the intermediate SiO2. The porosity of the TiO2coated nanoparticles surface is lower than that of the TiO2/ SiO2-coated nanoparticles (see Table 2). Therefore, although the thickness of the TiO2 coating was equal to or smaller than

4. CONCLUSIONS In this work two kinds of magnetic nanoparticles coated with an external TiO2 surface (with or without an intermediate silica coating) have been synthesized, and an in-depth characterization of both types of coatings has been carried out, studying the growth and spatial localization of the titanium dioxide either on the silica or directly on the magnetic core. TiO2 coating in absence of the silica interlayer leads to nanoparticles similar in size and composition, while the presence of the silica leads to particles less homogeneous in terms of the number of 5245


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Apoyo a la Investigación-SAI (Universidad de Zaragoza) and B. Gaspar for their support.

magnetic nuclei trapped in the SiO2. In this case, even if a larger particle size is obtained, the particles present much higher water stability. An in-depth electron microscopy characterization has been carried out by BF-TEM, HRTEM, and EDS analysis (elemental analysis and mapping) on the basis of STEMHAADF images. From this characterization the spatial distribution of TiO2 around the magnetic nuclei has been resolved, showing a different pattern depending on the nature of the support (amorphous silica or crystalline iron oxide). In both cases, the majority of the observed nanoparticles present a matrix-like structure with the coating wrapping more than one magnetic nucleus. Regarding their magnetic behavior, both types of nanoparticles are superparamagnetic at room temperature. The presence of the silica coating introduces no substantial differences in either the intensity of the dipolar interactions between the magnetic cores or the blocking temperature. On the contrary, it influences the magnetization values and the approach to saturation observed in magnetization isotherms at room temperature. The characterization of the different magnetic phases eventually present deserves a more complex analysis which oversteps the aim of this work. The differences observed in the relaxivity confirm the spatial distribution of TiO2 and SiO2 on the nanoparticles surface observed by the electron microscopy analysis. The presence of the permeable intermediate silica layer results to play an important role in the enhancement of the transversal relaxivity: besides increasing the particle stability, it reduces the effective closest approach distance of the water protons to the superparamagnetic core. The transversal relaxivity values of the suspensions obtained with the TiO2/SiO2- and TiO2-coated nanoparticles are similar to those of commercial magnetic resonance imaging contrast media. Therefore, both types of particles could be taken into consideration to be optimized as a potential MRI contrast agents.

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ASSOCIATED CONTENT

S Supporting Information *

XRD spectra of annealed powders of TiO2/SiO2- and TiO2coated nanoparticles; BF-TEM image of the magnetic cores coated with the semihydrophobic silica layer before the TiO2 coating; hydrodynamic diameter distribution of the SiO2coated, TiO2/SiO2-coated, and TiO2-coated nanoparticles, measured after filtering the liquid suspensions; inverse of the longitudinal and transversal relaxation times vs Fe concentration of magnetic nanoparticles with and without the SiO2 interlayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Ph +34 976 76 1213; Fax +34 976 76 1229; e-mail clara@ unizar.es (C.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by MAT2011-26851-C02-01, European Regional and Social Development Funds, Aragón Autonomous Government (DGA) through Research Groups and ARAID funds, and ERC-Starting Grant 239931-NANOPUZZLE. Authors also acknowledge the Servicio General de 5246


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