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Inorganic materials conspicuously change their properties by decreasing their size to the nanoscale as is the case with quantum dots that offer the possibility to tune the bandgap, and hence, the emission wavelength.1–4 For example, gold nanoparticles change the absorbance of the surface plasmon from ultraviolet wavelength in macroscopic gold to visible in the nanoparticles.5 In the case of magnetic materials, the size reduction leads to changes in their magnetic behavior that may result in superparamagnetism. Thus, iron oxide particles constituted by multiple magnetic domains can become superparamagnetic if their size is reduced in such a way that there is only one magnetic domain per nanoparticle.6 Superparamagnetic nanoparticles can be guided in a magnetic field, and when the field is switched off the magnetization disappears and the particles become magnetically inactive, while for ferromagnetic ones this is not the case. These nanoparticles are of interest in the preparation of ferrofluids since their small size allows an excellent dispersion in liquids.
One of the major advantages of nanoparticles is their high specific surface, while perhaps their major drawback is their lack of stability that leads to aggregation and precipitation. A common way to prevent aggregation is by functionalizing the nanoparticle surface with polymers which can stabilize the system with steric impediments.7–10 Moreover, the nanoparticle with its polymer shell constitutes a hybrid system that mixes the properties of both components. Thus, it is possible to prepare nanoparticles with simultaneous hydrophilic and lypophilic properties using amphyphilic copolymers with a hydrophilic and a lypophilic block. The most interesting of these copolymers possess surfactant properties and they can self assemble forming micelles and liquid-crystalline structures. The combination of these properties with those of the magnetic nanoparticle at the core could lead to magnetic surfactants that can be used to prepare emulsions reactive to external magnetic fields. These materials could find applications in medicine and pharmacology like carriers of proteins, DNA,11 peptides, hormones, antitumor agents,12–14 as image diagnostic tools,14 and biosensors.15–17
The aim of the present work was to synthesize magnetic nanoparticles with surfactant properties. For that, an amphyphilic diblock copolymer was covalently bound to the nanoparticles' surface with the purpose of providing them with miscibility in organic and aqueous solvents, as well as with steric impediments that prevent aggregation. The surface of γ-Fe2O3 nanoparticles of 10 nm diameter was modified with a specific diblock copolymer of polystyrene-SiCl2-poly(2-vinylpyridine) (abbreviated from here on as PS-b-P2VP) and a magnetic emulsion was prepared with the new surfactant.
The iron chloride precursors (FeCl3·6H2O and FeCl2·4H2O) were purchased from Panreac. The monomers (styrene and 2-vinylpyridine), initiator (sec-BuLi), polymerization solvent (THF), and the linking reagent (tetrachlorosilane or SiCl4) were purified according to literature and more details are given elsewhere.18 Hydrogen chloride and sodium hydroxide (Panreac), were purchased with maximum purity and used without further purification. A commercial magnet of NdFeB with a magnetic field of 0.4T was purchased from www.supermagnete.de.
Synthesis of the Diblock Copolymer
The block copolymer exhibiting middle active Cl groups of the PS-SiCl2-P2VP type was synthesized using anionic polymerization in combination with chlorosilane chemistry.19 All glassware manipulations were arranged according to the reactions necessary and the polymerization procedures and/or substitutions of chlorine atoms were performed through the break-seal approach. The living anionic polymerization of styrene is considered a trivial procedure, but the polymerization of a monomer such as 2-vinylpyridine containing functional groups is not straightforward due to the side reactions by the strong reactivity of the carbanion.20 The preparation of the materials can be divided in three steps. First, the PS(−)Li(+) chains are prepared in ∼1 h via anionic polymerization of styrene, using sec-BuLi as initiator, with THF at −78 °C as solvent, followed by the substitution of the Li(+) using large excess (1000:1) of tetrachlorosilane (SiCl4). After complete substitution of only one chlorine atom from the living ends, the excess of the SiCl4 is removed through the high vacuum line as described in the literature19 leaving only the intermediate product of the PS-SiCl3 type. The third step is the initiation of anionic polymerization of 2VP through sec-BuLi in THF at −78 °C in another polymerization apparatus. Before completion of the polymerization, two monomeric units of styrene are added in the system to avoid back-bitting reactions due to the nitrogen atom of the aromatic pyridine group being adjacent to the lithium cation. The incorporation of the styrene modified P2VP(−)Li(+) chains is performed by a titration reaction to selectively substitute only one of the three remaining chlorine atoms of the PS-SiCl3 intermediate product, leading to the final PS-SiCl2-P2VP diblock copolymer (or as mentioned above PS-b-P2VP) with a ratio of PS and P2VP chains of 1:1. In all cases THF was used as the solvent since it is considered a nonselective solvent for both segments (PS and P2VP, respectively).
Synthesis of Magnetic Nanoparticles
Magnetic nanoparticles with 10 nm diameter were synthesized using the Massart coprecipitation method.18 A mixture of 8.6496 g of FeCl3·6H2O and 3.1213 g of FeCl2·4H2O were dispersed in 25 mL MiliQ-grade water and sonicated for 1 h. This solution with acidic pH was added dropwise into 250 mL of a solution NaOH 1.5 M, under N2 atmosphere, and stirred for 30 min. The sudden change in the pH affects the iron ion solution leading to precipitation as small iron oxide crystals. Depending on the Fe+3 and Fe+2 ratios, different iron oxides could be obtained (magnetite, maghemite, and hematite). In addition with the aim of obtaining nanoparticles with different size, the synthesis temperature was varied from 25 to 80 °C. Magnetic nanoparticles were separated via magnetic decantation and washed with distilled water, a process which was repeated at least for three times.
Synthesis of Surfactant-Magnetic Nanoparticles
PS-SiCl2-P2VP (5 g) were dissolved in extra dry THF and poured in a flask containing 75 mg of magnetic nanoparticles. The mixture was left to react for 5 days under sonication. Once the reaction was complete, the nanoparticles were centrifuged at 21.000 rpm in THF for 3 h, and subsequently, the supernatant was decanted. This operation was repeated five times. The resulting product was lyophilized and analyzed.
Methods of Characterization
X-ray diffraction measurements were performed using a Philips X'Pert PW3050 diffractometer. The diffractograms were recorded in reflection mode using a 2θ interval between 1.5 and 35, recording at 0.01° steps per second with an estimated error in the reflection positions of ±0.2°. Silicon powder was used to calibrate the sample to detector distance. The temperature of the samples was measured with a precision of ±3 °C.
The size and morphology of the nanoparticles were measured with a JEOL JSM-2100 electron microscope working at 200 KV. Micrographs were recorded with a magnification between ×1500 up to ×6000. TEM grids were prepared drop cast technique.
Infrared measurements were recorded using a Nicolet IR200 spectrometer. Samples were analyzed using potassium bromide tablets enriched with 2% up to 5% nanoparticles.
The nanoparticles' hydrodynamic diameter was measured using a Malvern-NanoZS system equipped with a He–Ne laser working at 633 nm and a Peltier for temperature control. The contact angle measurements were carried out using a video camera connected to a goniometer.
The composition of the samples was determined using a Mettler TGA-DSC1. The measurements were performed in a temperature range between 25 °C and 700 °C under oxygen atmosphere to ensure that all organic compounds in the sample were totally decomposed.
Magnetization was measured using a SQUID class MPMS from Quantum Design working at 25 °C and scanning magnetic fields between −5 T and 5 T.
The specific surface of the magnetic nanoparticles was calculated from the Brunauer, Emmett, and Teller Isotherm method (BET) measured with a porosimeter Micrometrics ASAP 2020 version 3.00.
Characterization of the Magnetic Nanoparticles
The Massart coprecipitation carried out at different temperatures produced ferrofluids consisting of magnetic nanoparticles with different hydrodynamic diameters ranging from 5 nm to 10 nm. Figure 1 shows a TEM micrograph of magnetic nanoparticles synthesized at 80 °C.
The variation of temperature during the synthesis leads to variation on hydrodynamic diameters of the magnetic nanoparticles. Figure 2 represents the hydrodynamic diameter of the synthesized nanoparticles obtained by dynamic light scattering as a function of the synthesis temperature.
It can be observed that the increment of the synthesis temperature leads to an increase of the nanoparticles' hydrodynamic diameter as it was reported by other authors.21–23 This result agrees well with the nucleation and crystallization theory which predicts an increment of the crystal size when the synthesis temperature is increased. However, independent of the size, the magnetic nanoparticles present a crystalline structure which was studied by X-Ray diffraction. The diffraction patterns of magnetic nanoparticles with different size are presented in Figure 3. The d-spacing of the reflections was calculated using the Bragg equation:
where λ = 1.541 Å is the X-ray wavelength and θ is the scattering angle. All detected diffraction peaks (see Table 1) are attributed to cubic γ-Fe2O3 phase (maghemite) or cubic Fe3O4 (magnetite). The Rietveld analysis showed that the γ-Fe2O3 phase represents 93% of the crystalline contribution being the cubic Fe3O4 phase 7%. Furthermore, the calculated lattice parameter was found equal to 8.33 ± 0.04 Å which is far closer to the lattice parameter of maghemite (8.351 Å), than that of magnetite (8.396 Å). These results strongly indicate that the main crystalline phase of the synthesized nanoparticles can be identified as the cubic γ-Fe2O3 phase.
Table 1. Position, d-Spacing, and Miller Indices of the Reflections
One of the most important characteristics of magnetic nanoparticles is their magnetic moment which eventually determines their potential use. The magnetic properties were determined using the SQUID technique and the results are shown in Figure 4. One can observe the absence of hysteresis which at low field is the signature of ferromagnetic behavior, thus suggesting that each nanoparticle has only one magnetic domain, and hence, exhibits superparamagnetism. It is also worth to note that the nanoparticles' magnetic moment do not fully saturate at high field as is indicated by the absence of a clear plateau. The inset in Figure 4 shows the dependence of the maximum value of the magnetic moment as a function of the nanoparticle diameter. This is attributed to surface phenomena associated with the small size of the nanoparticles and their high specific surface. This result points to the conclusion that magnetic nanoparticles with 10 nm in diameter exhibit the highest magnetic moment, and for this reason, they were used as the inorganic core in the preparation of the hybrid material.
Characterization of the Surfactant-Magnetic Nanoparticles
To provide the nanoparticles with surfactant behavior, we have carried out a modification on their surface immobilizing PS-b-P2VP (Mn = 14,700 g/mol, PDI = 1.04). This copolymer has a dichlorosilane group (Cl-Si-Cl) between both blocks which is reactive towards the hydroxyl groups on the surface of the iron oxide (see Scheme 1) resulting to the covalent attachment of the copolymer on the nanoparticles.
The immobilization of the diblock copolymer on the nanoparticles surfaces was verified using FTIR spectroscopy and the results are shown in Figure 5.
The FTIR spectrum of pure γ-Fe2O3 nanoparticles exhibits three characteristic peaks at 3400 cm−1, 637 cm−1, and 572 cm−1 attributed to the hydroxyl groups on the nanoparticles surface, the Fe-O-Fe and Fe-OH bonds, respectively.24 It can be seen in Figure 5, that the FTIR spectrum of the pure copolymer is similar to that of the surfactant-magnetic nanoparticles with absorbance bands at 698, 752, 788, 1436, 1477, 1594, and 3030 cm−1 characteristics of poly(styrene)-copoly(2-vinylpyridine). Furthermore, we have tried to adsorb the same diblock-copolymer without the silane reactive group onto the surface of the maghemite and the FTIR results showed that cleaning yields magnetic nanoparticles without copolymer. These results confirm the functionalization of the nanoparticles with the diblock copolymer.
The TEM micrograph in Figure 6 shows the polymer disposition around the magnetic nanoparticles, where it is easily observed that a polymer corona of PS-b-P2VP surrounds the magnetic nanoparticles.
The polymer corona swells in appropriate solvents like THF increasing the hydrodynamic diameter of the hybrid nanoparticles. Figure 7 shows the hydrodynamic diameter of the magnetic nanoparticles measured by dynamic light scattering (DLS) before and after their surface was functionalized with the diblock copolymer.
As can be seen in Figure 7, the functionalized nanoparticles' hydrodynamic diameter increases from 10 nm to 34 nm, which is a reasonable increment considering that the polymer average molecular weight per number (Mn) is ∼14,700 g/mol. On the other hand, DLS measures the hydrodynamic diameter resulting from the size of the inorganic core and the polymer corona that swells in a good solvent increasing the nanoparticles diameter. By contrast, at the high vacuum condition of the TEM the magnetic nanoparticles are surrounded by a thin layer of the collapsed polymer and therefore their diameters are smaller than the 34 nm observed by DLS. Scheme 2 represents the arrangement of the copolymer around the nanoparticle and the estimated thickness of each layer.
In addition, thermogravimetric measurements were carried out to evaluate the amount of copolymer immobilized on the nanoparticles surface, and to quantitatively estimate the number of copolymer chains attached to the nanoparticles surface (see Fig. 8).
As can be seen in Figure 8, the weight loss of pure γ-Fe2O3 nanoparticles is around 10% at 700 °C. Similar weight loss has been previously observed by other authors25–27 and is attributed to dehydration of the hydroxyl groups on the nanoparticles surface. Thus, the 20% weight loss of the functionalized γ-Fe2O3 nanoparticles is explained by the dehydration of the hydroxyl groups together with an additional weight loss due to the copolymer decomposition.
The specific area of the surfactant-magnetic nanoparticles was measured with BET and the result was found to be 93 m2/g. Then, the number density of chains is obtained from the following equation:
where ρ(chains/nm2) is the immobilized polymer chains density, Δweight is the weight loss due to the organic material decomposition, MWpolymer is the molecular weight of the polymer equal to 14,000 g/mol, and SBET is the specific area of a gram of particles measured through BET. From the above expression we get a chain density of 0.046 chains/nm2. Assuming the magnetic nanoparticles are spheres of 10 nm diameter, we calculate an average of 15 chains immobilized per nanoparticle.
This copolymer was chosen due to its selective miscibility in different solvents for the two blocks. For example, polystyrene is miscible in toluene but immiscible in slightly acid water, whereas poly(2-vinylpyridine) is immiscible in toluene but miscible in water at pH = 4. In a mixture of toluene–water the blocks are positioned in such a way where most of the polystyrene blocks are oriented towards the organic phase whereas the poly(2-vinylpyridine) blocks are oriented towards the aqueous phase allowing the creation of an emulsion with the magnetic nanoparticles at the interface. For this reason, the amphiphilic nature of these nanoparticles was investigated by measuring the contact angle of a film prepared with the functionalized magnetic nanoparticles in the presence of toluene (good solvent for polystyrene), water at pH = 4 [good solvent for poly(2-vinylpyridine)], and water at pH = 7 (bad solvent for both polymers). Figure 9 shows the drop profile and a scheme, based in articles of Stamm and coworkers,28–30 of the possible arrangement of the polymer chains depending on the solvent used.
The contact angles obtained for the hybrid nanoparticles were 7.4° for toluene, 7.1° for water at pH = 4, and 76.6° for water at pH = 7. These values are very different from those measured for the iron nanoparticles without polymer: 32° for toluene, 15° for water at pH = 4, and 20° for water at pH = 7. As can be seen from Figure 9, in toluene the polystyrene chains expand whereas the poly(2-vinylpyridine) chains collapse. The situation reverses in the case of water at pH = 4. However, with water at pH = 7 both polymer chains collapsed decreasing the wetting. This result shows that the immobilized diblock copolymer provides amphiphilic character to the nanoparticles. This property was used to prepare a magnetic o/w emulsion of toluene in water at pH = 4. As is illustrated in Scheme 3, the magnetic nanoparticles will remain at the drop interface with the polystyrene chains extended towards the toluene and the poly(2-vinylpyridine) chains towards the aqueous phase, thus, stabilizing the emulsion.
The left part in Figure 10 shows an optical microscopy image of the toluene in water at pH = 4 emulsion stabilized with the surfactant nanoparticles. As is shown in the right part of Figure 10, under an external magnetic field the emulsion droplets become somewhat distorted and aligned in the direction of the field.
In the emulsion prepared with the functionalized magnetic nanoparticles the droplets present an intense dark interface indicating that most of the nanoparticles are situated at the toluene water interface. These results illustrate how the surfactant-magnetic nanoparticles can be used to stabilize emulsions and provide them with structure in the presence of a magnetic field.
We have synthesized magnetic nanoparticles with a size of ∼10 nm. These magnetic nanoparticles have been functionalized with an amphiphilic diblock copolymer of the PS-b-P2VP type (or PS-SiCl2-P2VP) contributing to their colloidal stability and giving them selected miscibility properties. The magnetic nanoparticles and the functionalized magnetic nanoparticles have been characterized, whereas their composition and properties were also investigated. The surfactant-magnetic nanoparticles have been used to prepare an emulsion in which the emulsion droplets become oriented under external magnetic field.
This work was supported by the Ministry of Science and Technology (MAT2009-14234) and the BSCH-UCM program for research groups (GR58/08). Partial support of COST Action D43 is also acknowledged.