Optically Active Magnetic Composites with Responsive Silica Shells

Apr 13, 2010 - Optically Active Magnetic Composites with Responsive Silica Shells. Ana B. Dávila-Ibáñez, M. Arturo López-Quintela, José Rivas and Vero...
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J. Phys. Chem. C 2010, 114, 7743–7750

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Optically Active Magnetic Composites with Responsive Silica Shells Ana B. Da´vila-Iba´n˜ez,† M. Arturo Lo´pez-Quintela,† Jose´ Rivas,† and Veronica Salgueirino*,‡ Departamento de Fı´sica Aplicada y Quı´mica Fı´sica, UniVersidade de Santiago de Compostela, 15782, Santiago de Compostela, Spain, and Departamento de Fı´sica Aplicada, UniVersidade de Vigo, 36310 Vigo, Spain ReceiVed: January 8, 2010; ReVised Manuscript ReceiVed: March 26, 2010

The paper describes the synthesis of relatively large magnetic plasmonic composite particles. Their magnetic characteristics (MS and HC) reach sufficiently increased values if compared to other hybrid composites, owing to a compromise between size, percentage of the soft magnetic material, and to a remarkable morphology consisting of inner (smaller) cobalt boride clusters assembled as larger spheres (magnetic interactions) and stabilized with an outer shell of silica. These magnetically stable silica-coated cobalt boride spheres were optically functionalized, acquiring a characteristic surface plasmon. Outstandingly, due to the silica formation procedure (acid- or acid- and base-catalyzed), silica shells between the optical and magnetic functionalities were proven to undergo chemical and morphological changes translated into an optical response, broadening their field of application. Introduction The complexity of magnetic core/shell structured nanocomposites that exhibit more than two different properties has undergone a huge increase, highly desirable for simultaneous and efficient technological applications.1 Although in certain cases, these composites can be attained by direct syntheses, generally, unique combinations of properties are accomplished by grouping nanoparticles of different sizes, compositions and functionalities, and indeed, particular systems can be attained offering exceptional shapes and symmetries.2 Thus, much effort has been devoted to the development and functionalization of hybrid assemblies comprising different colloids or decorated with secondary components.3,4 Composites with characteristics, such as optical and magnetic properties, are gaining increased attention, and the interest in having both characteristics altogether is entirely justified.1,5 On one hand, magnetic nanoparticles have attracted increasing interest among researchers of various fields due to their promising applications in magnetoelectronics, sensors, and biomedicine. The majority of current research is focused on the synthesis of magnetic nanoparticles with high magnetocrystalline anisotropies and coercitivities for future magnetic recording applications,6-9 although low coercivity superparamagnetic nanoparticles are also required in particular biotechnology areas.10 These characteristics make them readily identified by a magnet or a magnetic sensing device from the ocean of biomolecules so that, once coupled with a target agent, they can serve as nanovectors and interact specifically. A key advantage in magnetism lies in the ability to control motion at a distance without perturbing the biological system, as would occur, for example, with a large electric field. Consequently, magnetic adsorbents, carriers, and modifiers can be used for the immobilization, isolation, modification, detection, determination, and removal of a variety of biologically active compounds, xenobiotics, cellular components, and cells.11-24 * To whom correspondence should be addressed. E-mail: vsalgue@ uvigo.es. † Universidade de Santiago de Compostela. ‡ Universidade de Vigo.

Numerous attempts to synthesize monodisperse magnetic nanoparticles with controllable sizes have been reported. This control in the final size is based on different parameters in order to minimize the thermal fluctuations of the magnetization at room temperature.25 Therefore, nowadays, the focus of synthetic efforts appears to be shifting to creation of secondary structures of nanocrystals, either by self-assembly or through direct solution growth.26-29 Manipulation of the secondary structures is highly desired in order to tie together the size-dependent intrinsic properties of the individual constituents30 and the collective properties due to interactions between the subunits in the final composites,31 as both aspects play important roles in the displayed magnetism.32 The advantage of secondary structures is actually reflected by the magnetic force that can be applied to a magnetic nanostructure.10 It depends on the magnetic induction B and on its magnetic moment µ, and since µ ) MSV, where MS is the saturation magnetization of the material and V is the volume, a larger size (built up from smaller subunits) is generally preferable for a strong enough magnetic response. Thus, there is a compromise between the final size of the secondary nanostructures, the saturation magnetization given by the chosen material, and the magnetic interactions established between the subunits. A suitable solution can come from relative large composites built up from smaller subunits of a soft magnetic material, such as cobalt boride. On the other hand, the noble-metal nanostructures are beneficial not only because of their relative ease of biofunctionalization but also for plasmonic biosensing and other applications. These applications are generally based on the fact that, in a small metal particle, the dipole created by the electric field of the light wave sets up a surface polarization charge, effectively acting as a restoring force for the “free electrons”. This gives a wavelength absorption condensed into the widely known surface plasmon band (SPB).33-38 Additionally, if both functionalities (magnetic and optical) are together in the same composite, an electronic communication across the junction can drastically change the local electronic structure when linking optically active nanoparticles to magnetic structures, leading to a supplementary dimension of control in

10.1021/jp100190j  2010 American Chemical Society Published on Web 04/13/2010

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the different properties displayed.5,39 These composites, because of their magnetic responsiveness, facile bioconjugation, and localized surface plasmon resonance displayed, open up new possibilities for in vivo and in vitro molecular and cell biological applications, including chemical composition analyses and magnetophoretic cell sorting. Herein, the strategy developed focuses on the synthesis of relatively large magnetic and plasmonic composite particles. Their saturation magnetization and coercivity have increased sufficiently to ∼8.5 emu/g and ∼30 Oe (compared to other hybrid nanostructures) owing to a remarkable morphology consisting of inner (smaller) cobalt boride clusters assembled into larger spheres stabilized with an outer shell of silica and to the increased ratio of magnetic material. There is a compromise between the final size of the secondary nanostructures, the saturation magnetization given by the cobalt boride soft magnetic material, and the magnetic interactions established between the subunits. These magnetically stable silica-coated cobalt boride spheres, optically functionalized, acquire a characteristic SPB that can be tuned in a relatively wide range of wavelengths, depending on the loading and nature (Au/Ag) of the metallic nanoparticles. Remarkably, depending on the silica formation procedure, acid- or acid- and base-catalyzed silica shells (linking both functionalities) were formed so that chemical, morphological, and optical responses of the composites can be tuned, broadening their spectra of applications. Experimental Section The synthesis of silica-coated cobalt boride nanospheres was performed as follows; 0.1 mL (0.4 M) of cobalt chloride hexahydrate (Fluka) in H2O was added to an aqueous solution (100 mL) of NaBH4 (4.4 mM) (Riedel de Haen) and citric acid monohydrate (Riedel de Haen) (2 × 10-6 M) under mechanical stirring. Immediately following the involved reactions,40 400 mL of an ethanolic solution containing 15 µL of TEOS (tetraethoxysilane) (Aldrich) was added. After 15 min, the solution was centrifuged and the precipitate redispersed in ethanol (40 mL).41 Once the Co2B spheres were coated with silica (precipitated in gently acid medium) (see Scheme 1, panel 1), a second shell was deposited, although, in this case, exploiting the TEOS hydrolysis in basic medium (see Scheme 1, panel 2). For that, 3.5 mL of NH4OH and 40 µL of TEOS (in 10 mL of EtOH) were added to 40 mL of EtOH containing the silica-coated cobalt boride spheres under moderate mechanical stirring and left to react during 6 h. Amino groups were attached to the surface of both types of cobalt boride spheres (coated with an outer shell of silica and precipitated in acid or in acid and basic media) by refluxing them in ethanol in the presence of (3-aminopropyl) trimethoxysilane (APS) (Aldrich). Different volumes (40-100 µL) of APS were added to 40 mL of ethanol solution containing the silicacoated cobalt boride spheres. After that, the solution was diluted up to 100 mL and refluxed for 60 min. The sample was centrifuged and dispersed in 40 mL of ethanol twice. Adsorption of APS to the outer surface of the silica shell was checked using photoluminescence spectroscopy. For that, a free dye, the fluorescamine (4-phenylspiro-(furan-2(3H)-1′phthalan)-3,3′-dione (Aldrich)), was added (5 mL, 1 mM) (a) to a solution of free APS (0.45 µM, 30 mL) in ethanol, (b) to a solution of silica-coated cobalt boride particles without APS attached (40 mL), and (c) to a solution of silica-coated cobalt boride nanoparticles that have undergone the process of reflux in ethanol in the presence of APS (40 mL).

Da´vila-Iba´n˜ez et al. SCHEME 1: Illustration of the Two Synthetic Procedures Followed in Order to Have Magnetic and Optically Active Nanocomposites with One or Two Types of Silica between the Two Characteristic Functionalities

Both types of silica-coated cobalt boride nanoparticles (labeled as 1 and 2) became optically active by attaching gold or silver nanoparticles on their surface through an electrostatic binding with the amino groups previously anchored. For that, gold and silver nanoparticles were synthesized as follows. Aqueous solutions of gold nanoparticles (15 nm average diameter) were prepared by reduction of chloroauric acid (Aldrich) with sodium citrate (Aldrich) according to the standard sodium citrate reduction method.42 Silver nanoparticles (12 nm average diameter) were also prepared by rapidly adding AgClO4 (Aldrich) to a vigorously stirred, ice-cold aqueous solution containing NaBH4 and sodium citrate.43 These metallic (gold and silver) nanoparticles were further stabilized with poly(vinylpyrrolidone) (PVP) (Sigma-Aldrich). For that, PVP-10 (mol wt ) 10 000) was dissolved in water (25 g/L) by ultrasonication and subsequently mixed with the aqueous solutions of metallic nanostructures. To guarantee complete adsorption and full coverage of the metallic surface by PVP molecules, the reaction mixture was stirred for at least 12 h at room temperature. The 15 nm gold and 12 nm silver nanoparticles were centrifuged and redispersed in ethanol.44 PVP-stabilized gold and silver nanoparticles were then added to silica-coated cobalt boride ethanol solutions and gently stirred (for about 30 min). To drive the metallic counterparts on the magnetic spheres’ surface, the amino groups previously anchored on the silica were brought into play, favoring their junction with the metallic surface, so that this metallic loading onto the magnetic composites directly depends on the initial quantities of APS used. Finally, metallic particle excess was removed by centrifugation and optical and magnetically active composites were dispersed in ethanol. Experiments including controlled fractions (1 and 10 vol %) of water added to the ethanolic solutions of the synthesized composites were carried out and stopped at different periods of time by centrifugation. Both types of composites (with acid- or with acid- and base-catalyzed silica) were further dispersed in ethanol. Additionally, the composites with the two types of silica were also centrifuged and transferred to aqueous and to

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phosphate buffered saline (PBS, containing 150 mM NaCl, ionic strength equivalent to 154 mM NaCl (Aldrich), at 37 °C) solutions. TEM measurements were performed on a JEOL JEM-1010 microscope operating at 100 kV. Samples for TEM were prepared by depositing them upon a carbon-coated copper grid. SEM measurements were performed on a JEOL JSM-6700F. Magnetic measurements were performed on a superconducting quantum interference device (SQUID magnetometry). Photoluminescence spectra were collected using a Cary Eclipse fluorescence spectrophotometer. UV-vis spectra were collected using a Shimadzu UV-3101PC UV-visible spectrometer over the range of 200-1100 nm. Results and Discussion Two types of magnetic and optically active nanocomposites were synthesized according to Scheme 1. The main difference between them comes from the fact that the silica with between the optical and magnetic functionalities was formed following acid-catalyzed (1) or subsequent acid- and base-catalyzed (2) procedures of hydrolysis and condensation of tetraethoxysilane (TEOS). Although the overall structures are fairly complex and combine multiple functionalities, the synthetic steps are rather simple and easy to carry out, as outlined in Scheme 1 and in the Experimental Section. For these two syntheses, cobalt ions (Co2+) in solution were reduced and allowed to combine with sodium borohydride (NaBH4), according to the following reaction:40

2CoCl2 + 4NaBH4 + 9H2O f Co2B + 4NaCl + 12.5H2 + 3B(OH)2 The two reagents were rapidly mixed in the presence of citric acid, yielding nanoparticles of Co2B with tunable sizes, depending on the chosen [citrate ions]/[Co2+] ratio. For the experiments herein included, we have carried out the synthesis using the ratio [citrate ions]/[Co2+] ) 0.10. For the first silica coating step, the hydrolysis and condensation of TEOS were carried out under the gentle acidic conditions (acid-catalyzed) given by the citric acid present in solution. Figure 1a-c shows TEM images of typical spherical cobalt boride nanostructures likewise synthesized and further coated with silica in an aqueous/ethanolic solution. The resulting thin silica shell is required to minimize interactions and aggregation and to protect the magnetic cores from degradation. Silica-coated cobalt boride colloids appear to be fairly spherical and relatively monodisperse. The average diameter of the magnetic spheres was centered about 92 nm (fitting statistical analyses with standard deviations within 19%). Because of the acid-catalyzed silica deposition, an ≈5 ( 2 nm silica shell surrounds every magnetic core, offering a very homogeneous coating of the cobalt boride clusters assembled into spherical nanostructures (see the HRTEM image included in Figure 1b). Once this initial step is accomplished and in order to produce the second type of composites (Scheme 1, panel 2), a second coating with silica was carried out, although, in this case, following a basecatalyzed silica condensation (see the Experimental Section). Figure 1d includes a TEM image of a representative cobalt boride magnetic sphere onto which the two types of silica (acidand, afterward, base-catalyzed) were deposited, reaching an average shell thickness of ≈15 ( 5 nm. The acid-catalyzed silica shell thickness can almost, but not quite, tune with this synthetic procedure. Precipitated acid-

Figure 1. TEM images of the silica-coated cobalt boride spheres used, with an average size distribution centered around 92 nm (a). Successive acid- or acid- and base-catalyzed silica deposition yields silica shells ≈5 ( 2 (b, c) and ≈15 ( 5 nm (d) thick, respectively.

catalyzed silica from lower amounts of TEOS was not enough to cover the whole surface of the available colloids generated so that they end up dissolved in the initial H2O/EtOH solution. Precipitated acid-catalyzed silica from higher amounts of the precursor tends to form thicker silica shells around the cobalt boride, but the excess of silicon oxide favors the formation of fractal-like structures that aggregate the magnetic colloids. The base-catalyzed silica shell, on the other hand, can indeed be tuned and helps to smooth the surface of the colloids as its thickness is increasing (that depends linearly with the TEOS concentration). Because of the outer silica coating, independent of the acid or basic catalysis, the magnetic spheres’ surface is terminated by silanol groups that can react with various coupling agents to covalently attach specific ligands. Thus, both types of magnetic spheres accordingly synthesized were used as supports to drive optically active nanoparticles. With the term “optically active”, we refer to the phenomenon of surface plasmon band (SPB) already mentioned. Metallic nanoparticles can exhibit an SPB in different regions of the spectrum. In the case of gold and silver, this SPB is located in the visible region (around 520 and 400 nm, respectively).33 Taking that into account, metallic nanoparticles were driven to the surface of the magnetic spheres, and for that, a previous step consisting of the silica shell aminofunctionalization by attaching (3-aminopropyl)trimethoxysilane (APS) molecules was carried out. This process was characterized and proven by means of fluorescence spectra, taking advantange of the fluorescamine’s (4-phenylspiro-(furan-2(3H)-1′-phthalan)3,3′-dione) ability to conjugate with primary amino groups. This compound reacts directly with primary amines to form the same fluophors (390 nm excitation, 475 nm emission) as are generated in the ninhydrin-phenylacetaldehyde reaction. The resulting fluorescence is proportional to the amine concentration, and the fluorophors are stable over several hours.45 This covalent bonding between this fluorophore and the primary amino groups from the APS anchored permitted, therefore, the tuning of a fluorescent signal as a function of the number of amino groups

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Figure 2. PL spectra of the Co2B@SiO2 spheres functionalized with -NH2 groups from the APS molecules before and after conjugation with fluorescamine (black and red spectra). The green spectrum corresponds to APS-fluorescamine conjugates in EtOH solution, used as reference.

present (given by the APS used; see the Experimental Section) in the surface of the magnetic nanostructures. Figure 2 shows typical room-temperature photoluminescence (PL) spectra of APS molecules that have reacted with fluorescamine, free in solution (green spectrum, used as reference) or linked to the surface of the silica-coated cobalt boride nanoparticles (red spectrum) using a 390 nm excitation wavelength. Without fluorescamine present in the solution, the silica-coated cobalt boride spheres, even functionalized with amino groups, do not show PL (black spectrum). Once this anchoring process was carried out, metallic gold and silver nanoparticles were driven to the surface of the magnetic spheres, as taking advantage of the affinity between the -NH2 groups from the APS molecules already anchored and the metallic surface of the nanoparticles. TEM and SEM images in Figure 3a-c reveal the composite nature with the different metallic nanoparticles attached to the surface of acidcatalyzed silica-coated cobalt boride nanoparticles. The process works exactly in the same way with the acid- and basedcatalyzed silica shells. Different loadings of gold and silver nanoparticles on the surface of the silica-coated Co2B spheres were carried out, depending on the homogeneous distribution and quantitative anchoring of -NH2 groups from the APS molecules. Au or Ag nanoparticles have large scattering cross sections and tunable optical properties46 retained once on the surface of the magnetic silica-coated cobalt boride. No optical quenching was found due to the sufficient silica shell thickness that avoids any electric interaction between the two magnetic and metallic components (even in the case of the thinnest silica shell). Figure 3d,e includes the UV-vis spectra of the metallic nanoparticles (Au and Ag) before (in aqueous solution) (solid lines) and after their deposition onto the silica-coated cobalt boride spheres (in ethanolic solution) (dashed lines), where both appear red shifted and broader. To validate the optical properties displayed by the composites, the following parameters causing different effects can be taken into account. First, gold nanoparticles interact with other gold nanoparticles deposited in the same magnetic spheres, as being closer enough in the same composite (in the case of silver, this effect is not so important as the number of metallic nanoparticles onto the composites is much lower, underlying the fact that metallic loading onto the magnetic composites can be tuned). Additionally, because the metallic nanoparticles were synthesized in aqueous solution and later transferred to ethanol when

Da´vila-Iba´n˜ez et al. surrounded by PVP molecules, some group themselves before being fixed to the surface of the magnetic colloids. These interactions between the metallic nanoparticles (grouped or not) also cause the SPB to red shift47,48 because, when the electromagnetic fields from two different nanoparticles interact, a complex, lower energy (higher wavelength) SPB is produced.49 At this point, it is important to underline the fact that the magnetic cobalt boride material does not interact with the metallic nanoparticles because of the sufficient thickness of the silica shell in between (∼5 nm in the thinnest shell). We have previously demonstrated possible interactions between magnetic and metallic nanoparticles, implying a magnetic influence onto the SPB only for distances smaller than ≈3 nm.39 Second, due to the bigger size of the magnetic spheres present in solution, there is an increased light-scattering effect, also contributing to the red shift of the SPB and to an increase in absorption over the whole range of the spectra obtained. Third, the metallic nanoparticles are generally synthesized in aqueous solution, displaying the characteristic SPB already mentioned. However, in order to fix them on the surface of the acidcatalyzed silica-coated cobalt boride spheres, they were transferred to ethanol. When the same nanoparticles are compared, but immersed in solvents with different refractive indexes, their SPB positions also become affected, contributing, in this case, as changing from water to ethanol, to the red shift of both gold and silver SPBs. Lastly, the broader line widths can be justified as reflecting a distribution of resonances due to the structural heterogeneity in terms of number of metallic nanoparticles attached to every silica-coated cobalt boride sphere. Because of that, gyromagnetic contrast as rotating the nanocomposites magnetically may become an option for bioimaging, synchronizing the magnetic moment with polarized excitation or emission.50 To fully characterize the magnetic samples, magnetization measurements were carried out at 5 and 300 K. The optical and magnetically active composites were precipitated from the ethanolic solution, and the dried sample was measured. The magnetic cores of the silica-coated Co2B nanoparticles are formed by smaller subunits, as can be appreciated in Figure 1b,c. When formed in the first step of the synthesis, these subunits of Co2B were led to assemble into the thermodynamically stable spherical shape and further coated with silica. With this multiple core morphology, the magnetic properties of nanoparticulate materials stem not only from the intrinsic properties of the material itself but also from the interactions established. Cobalt boride is a classical type of amorphous alloy (see the diffraction pattern obtained via TEM in the inset of Figure 1b) characterized by generally reduced magnetic moments that reflect the presence of the nonmagnetic atoms needed to stabilized the glassy state.25 However, because the spherical assembly sets up the mentioned interactions between the subunits (intraparticle interactions), higher-magnetization structures with a stronger magnetic response are provided. Figure 4a,b shows the magnetization plot as a function of magnetic field, reflecting magnetic nanostructures with an increased saturation magnetization even at room temperature (∼8.5 emu/ g) so that an easy external magnetic manipulation can be carried out. Composites built up using magnetic materials and silica have also been reported but generally with an increased ratio of the diamagnetic material so that the saturation magnetization can only reach values close to ≈1.5 emu/g.51,52 Noteworthy, the composites reach the saturation magnetization using external magnetic fields lower than 0.2 T (increased magnetic suscep-

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Figure 3. TEM and SEM images of the composites obtained by driving metallic nanoparticles, gold (a, b) and silver (c), using the aminofunctionalized silica shell to link both magnetic and metallic nanostructures. Scale bar: 100 nm. UV-vis spectra of the metallic gold (d) and silver (e) nanoparticles before (solid lines) and after (dotted lines) being driven onto the magnetic silica-coated cobalt boride spheres.

Figure 4. Temperature dependence magnetization vs field curves of the magnetic and optically active composites.

tibility (χ)), rendering their use suitable in the presence of small commercial magnets.

The magnetic interactions between the subunits forming the cores can also be established between the silica-coated cobalt boride spheres (interparticle interactions). Intraparticle interactions influence the magnetic properties much more than the interparticle ones because the subunits are touching each other through an interface that favors the direct exchange coupling. The interparticle interactions, on the contrary, depend on the distance between the composites that is determined, in this case, by the silica shell thickness, that is, ∼10 nm for the thinner silica shell, and the metallic nanoparticles deposited on their surface. The determination of these inter- and intraparticle interactions effects is complex because several causes can interplay, including the disordered arrangement of particles (or subunits forming the particles) with volume distribution and easy directions in random positions. In addition, the thermal fluctuations influencing the magnetic moments do not simplify the problem.53 On the other hand, this fact may become very advantageous when working with these magnetic nanostructures because it permits combining the ability to harness the sizedependent properties of the nanostructures themselves with the possibility to tune the collective behavior due to interactions in between.27,31,32 For the present case, these magnetic colloids offer remarkable coercivities that reach 135 and 30 Oe if cooling the sample to 5 K or measured at 300 K (Figure 4b). The reason for this behavior is also related to the distance between the magnetic cores. Because of the silica shell and metallic nanoparticles that keep the Co2B cores far enough during the magnetic characterization (the sample was measured as powder), the magnetic dipolar coupling that leads to cooperative magnetization reversals of adjacent particles was not completely prevented but highly hindered because of two main reasons; even in the case of composites with the thinnest silica shells, there are at least ∼40 nm in between the magnetic cores, and at 300 K, the

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Figure 5. TEM images (a, b) and UV-vis spectra of composite structures with acid-catalyzed silica, affected by different H2O/EtOH ratios in different periods of time (c) and of the composite structures with the acid- and base-catalyzed silica, once transferred to pure water and PBS solutions (d).

magnetic moments of the composites are fluctuating as a function of the thermal energy. These two characteristics justify the small coercivity field at room temperature, avoiding the aggregation of the composites and permitting their manipulation for the subsequent experiments. The composites with the silica shell linking the magnetic and metallic nanoparticles were formed exploiting two different hydrolysis and condensation reactions,54 depending on the acidcatalyzed or the successive acid- and base-catalyzed procedures (see Scheme 1). To check envisioned activities in fields, such as sensing, or for in vitro applications (in vivo applications are extremely hindered due to the high percent of cobalt in the magnetic core), the magnetic and optically active nanostructures were subjected to the following tests in which controlled fractions (1 and 10 vol %) of water were added to the ethanolic composite solutions, in restricted periods of time. In the presence of water in the solution, the composites with acid-catalyzed silica were subjected to characteristic morphological changes. Figure 5a shows a representative TEM image of the composites where an increase in the typical thickness and roughness of the silica shell can be appreciated. This image (corresponding to 1% of water in the solution and after approximately 3 min) clearly demonstrates a partial gelation process the silica shell has undergone, proving the morphological changes mentioned. Because of this partial gelation, a relatively loose structure is formed, offering a thicker, although less dense, shell of sol-gel-

derived silicon oxide networks. The average thickness increases because the branches diffuse out of the composites, making the appearance of a more flexible and rough structure more pronounced. These morphological changes were simultaneously reflected on the optical signal displayed. The optical properties of these composite structures reveal both dielectric changes near the gold nanoparticles’ surface and electromagnetic coupling between them, in short periods of time after adding the 1% (vol) of water to the ethanolic solution of composites. Figure 5c monitors by UV-vis spectroscopy the transition the characteristic extinction (absorption + scattering) spectrum has undergone, becoming the SPB slightly red shifted as a function of time and as the total diameter of the composites increases. This was also reflected by the noticeable increasing in scattering of light (the long tails in the red and green spectra) due to the turbidity given by the branches formed. If the water to ethanol ratio increases (10 vol %, blue spectrum) the silica gets partially dissolved. The partial dissolution of the silica shell is dictated by the water concentration and etching time, able in the latest terms to reach the complete dissolution of the magnetic core (Figure 5b). This leads to free gold nanoparticles and gold-silica aggregates in solution. As a result, the system ends up displaying a spectrum with two well-defined bands, located at 525 and 570 nm, respectively. These two characteristic bands can be associated with the already mentioned free gold nanoparticles and to uncontrollably ag-

Optically Active Magnetic Composites gregated and electromagnetically coupled nanoparticles shown in Figure 5b. Accordingly, three distinct steps in this process can be established: a first etching progression consistent with the partial gelation of silica, accompanied by an increase in the scattering; a second step in which there is a progressive red shift of the SPB because of the fractal-like structure formed as the silica is dissolved (at this point, there is already some magnetic cores completely dissolved); and the third step, characterized by the final split of the SPB, is given by the presence of the mentioned free gold nanoparticles and additional gold-silica uncontrolled aggregates with no presence of magnetic material. These three steps are carried out in different periods of time, depending on the H2O/EtOH ratio. The whole process undergone by the acid-catalyzed silica shell is observed after 3 min as no further optical changes are observable upon prolonged exposure of the composites to the ethanol solution or upon increased proportions of water. The key factor in this experiment is the acid catalysis used for the silica shell deposition, which held alkoxide groups protonated. The electron density of the silicon atoms is, therefore, withdrawn, rendering it more electrophilic and thus more susceptible to attack by water (even with small volume percentages). Taking into account the chemistry of silica,55 it can be said that sol-gel-derived silicon oxide networks, under acid-catalyzed conditions, yield primarily linear or randomly branched polymers, which entangle and form additional branches, favoring gelation and partial dissolution and, therefore, the formation of the silica-based structures observed in the TEM images of Figure 5a,b. The same experiments were also carried out with composites built with silica deposited onto the magnetic cores using successive acid- and base-catalyzed silica condensation steps, but neither gelation nor dissolution of silica took place when the same amounts of water were added. The composites were, after that, transferred to aqueous and phosphate buffered saline (PBS) (the latter was carried out at 37 °C) solutions, keeping the same morphology and optical properties (the transfer of the composites from water to PBS solution just induces a small deviation of less than ∼5 nm in the SPB due to the variation of the local dielectric environment once transferred, as shown in Figure 5d). A similar procedure consisting of the successive acid- and base-catalyzed silica condensation but onto carbon nanotubes has been reported, accomplishing, however, the subsequent dissolution of the inner acid-catalyzed silica.56 In our case, the dissolution of the inner (acid-catalyzed) silica shell was prevented because of the absence of amino groups from APS (inside and as a structural component of the acid-catalyzed silica matrix) and PAH (poly(allylamine hydrochloride)). Both compounds were used in the carbon nanotubes case; the acidcatalyzed silica condensation took place from APS and TEOS, and the positively charged polyelectrolyte PAH helped to yield stable dispersions of carbon nanotubes. In our case, the amino groups from the APS molecules are attached only on the basecatalyzed silica outer surface and once this silica is already formed. There are no amino groups inside both types of silica, and those in the outside are linked to the surface of the metallic nanoparticles so that they cannot favor the dissolution of the silica shells. These simple tests reported demonstrate that the magnetic and optically active composites (with acid-catalyzed silica) can be conveniently used to directly detect the H2O content, for example, in organic solvents, although progressive experiments that would allow us to specify the type of organic solvent and to quantify the optical response, the convenient periods of time, and the detection range will be required. On

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7749 the other hand, if acid- and base-catalyzed silica is used, the magnetic and optically active composites can be used in biorelated in vitro applications, taking advantage of their strong magnetic signal and SPB displayed. Conclusion The synthesis of relatively large magnetic plasmonic composite particles is reported. Their magnetic characteristics (MS and HC) reach sufficiently increased values if compared to other hybrid composites, owing to a compromise between size, percentage of the soft magnetic material, and to a remarkable morphology consisting of inner (smaller) cobalt boride clusters assembled as larger spheres (magnetic interactions) and stabilized with an outer shell of silica. These magnetically stable silica-coated cobalt boride spheres, optically functionalized, acquire a characteristic SPB. Outstandingly, due to the silica formation procedure (acid- or acid- and base-catalyzed), silica shells between the optical and magnetic functionalities were proven to undergo chemical and morphological changes translated into an optical response, broadening their field of application. Acknowledgment. The authors acknowledge the financial support of this work under Project MAT2008-06126 by the Spanish Ministerio de Ciencia e Innovacio´n, under project INCITE08PXIB209007PR by the regional government (Xunta de Galicia, Spain), and by the L’Ore´al-UNESCO Woman in Science Program. V.S. acknowledges the Ramo´n y Cajal Program fellowship (Ministerio de Ciencia e Innovacio´n). J.R. also acknowledges the partial financial support from the regional government (07TMT003206PR, Xunta de Galicia, Spain), NanoBioMed (CONSOLIDER-INGENIO 2010), and the Large Collaborative Project FP7 - 214685-2 MAGISTER - MAGnetIc Scaffolds for in vivo Tissue EngineeRing. References and Notes (1) Salgueirin˜o-Maceira, V.; Correa-Duarte, M. A. AdV. Mater. 2007, 19, 4131–4144. (2) Manoharan, V. N.; Elsesser, M. T.; Pine, D. J. Science 2003, 301, 483–487. (3) Ohnuma, A.; Cho, E. C.; Camargo, P. H. C.; Au, L.; Ohtani, B.; Xia, Y. J. Am. Chem. Soc. 2009, 131, 1352–1353. (4) Yu, H. K.; Mao, Z.; Wang, D. J. Am. Chem. Soc. 2009, 131, 6366– 6367. (5) Xu, C.; Xie, J.; Ho, D.; Wang, C.; Kohler, N.; Walsh, E. G.; Morgan, J. R.; Chin, Y. E.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 173– 176. (6) Wen, M.; Zhu, Y.-z.; Wu, Q.-s.; Zhang, F.; Zhang, T. J. Phys. Chem. C 2009, 113, 19883–19890. (7) Vasilakaki, M.; Trohidou, K. N. Phys. ReV. B 2009, 79, 144402. (8) Zhang, Y.; Sun, L.; Fu, Y.; Huang, Z. C.; Bai, X. J.; Zhai, Y.; Du, J.; Zhai, H. R. J. Phys. Chem. C 2009, 113, 8152–8157. (9) Cabot, A.; Alivisatos, A. P.; Puntes, V. F.; Balcells, L.; Iglesias, O.; Labarta, A. Phys. ReV. B 2009, 79, 094419. (10) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, R167–R181. (11) Landmark, K. J.; DiMaggio, S.; Ward, J.; Kelly, C.; Vogt, S.; Hong, S.; Kotlyar, A.; Myc, A.; Thomas, T. P.; Penner-Hahn, J. E.; Baker, J. R., Jr.; Banaszak Holl, M. M.; Orr, B. G. ACS Nano 2008, 2, 773–783. (12) Xie, J.; Xu, C.; Kohler, N.; Hou, Y.; Sun, S. AdV. Mater. 2007, 19, 3163–3166. (13) Lee, J.-H.; Lee, K.; Moon, S. H.; Lee, Y.; Park, T.; Cheon, J. Angew. Chem., Int. Ed. 2009, 48, 4174–4179. (14) Fu, A.; Hu, W.; Xu, L.; Wilson, R. J.; Yu, H.; Osterfeld, S. J.; Gambhir, S. S.; Wang, S. X. Angew. Chem., Int. Ed. 2009, 48, 1620–1624. (15) Xie, J.; Chen, K.; Lee, H.-Y.; Xu, C.; Hsu, A. R.; Peng, S.; Chen, X.; Sun, S. J. Am. Chem. Soc. 2008, 130, 7542–7543. (16) Wang, Z.; Lu, Y. J. Mater. Chem. 2009, 19, 1788–1798. (17) Fung, A. O.; Kapadia, V.; Pierstoff, E.; Ho, D.; Chen, Y. J. Phys. Chem. C 2008, 112, 15085–15088. (18) De las Cuevas, G.; Faraudo, J.; Camacho, J. J. Phys. Chem. C 2008, 112, 945–950.

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