Article pubs.acs.org/cm
Fe3O4@SiO2 Core/Shell Nanoparticles: The Silica Coating Regulations with a Single Core for Different Core Sizes and Shell Thicknesses H. L. Ding, Y. X. Zhang,* S. Wang, J. M. Xu, S. C. Xu, and G. H. Li* Key Laboratory of Material Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, People’s Republic of China ABSTRACT: In this work, we present the coating regulations of Fe3O4 nanoparticles (NPs) by the reverse microemulsion method to obtain the Fe3O4@SiO2 core/shell NPs. The regulation produces the core/shell NPs with a single core and with different shell thicknesses, and it especially can be applied to different sizes Fe3O4 NPs and avoid the formation of core-free silica particles. Our results reveal that the silica coating parameters suitable for Fe3O4 NPs with certain size are not definitely applicable to that with other sizes, and the match of the number of Fe3O4 NPs with aqueous domain is essential. We found that the small aqueous domain is suitable to coat ultrathin silica shell, while the large aqueous domain is indispensable for coating thicker shells. To avoid the formation of core-free silica particles, the thick silica shell can be achieved by increasing the content of either TEOS through the equivalently fractionated drops or ammonia with a decreased one-off TEOS. The ligand exchange between the intermediate processes of the silica coating is evidenced. Our results provide not only a strategy for synthesizing uniform Fe3O4@ SiO2 core/shell NPs with controlled shell thickness, but also a regulation that can be applied to preparation other core−shell NPs. KEYWORDS: Fe3O4 nanoparticles, core/shell, SiO2 coating regulations, single core, shell thickness
1. INTRODUCTION
For practical applications, it is required that every iron oxide NP should be coated with a homogeneous silica shell without core-free silica particles, regardless of the size of the NPs. As a heating source and magnetic guidance, iron oxide NPs play an important role in hyperthermia and targeted drug delivery, and the existence of core-free silica particles will lead to a loss in the effective dose of iron oxide NPs. Unequal core number and silica shell thickness will result in an irregular magnetic field, giving rise to uneven heating in hyperthermia and tissue distribution of the targeted drugs.1,8 During the past decade, the silica coating of single hydrophobic NPs via the reverse microemulsion method has been extensively investigated.21,26−32 However, most of the previous work was focused on silica coating of single-size NPs with less effort devoted to the coating of NPs of different sizes, particularly the influence factors in achieving one-to-one coating.33 The specific properties of core NPs, such as the saturation magnetization, luminescence intensity, the intensity of surface-enhanced Raman scattering signal, will decrease as the inert silica shells thickness increases, although silica shells can effectively prevent core NPs from corrosion of the surrounding media,4 and an ultrathin silica shell is helpful in avoiding this decrease. The size distribution of Fe3O4@SiO2
Superparamagnetic iron oxides nanoparticles (NPs) hold much promise for applications in biomedical fields, such as targeted drug delivery, magnetic resonance contrast, therapeutic agents, and bioseparation.1−12 Different methods have been explored to synthesize the superparamagnetic NPs. The iron oxides NPs prepared by one-pot method, such as coprecipitation, hydrothermal synthesis, thermal decomposition in aqueous media etc., are water-soluble and usually have a size polydispersion,13−15 while that synthesized through thermal decomposition in nonaqueous media have relatively high quality, such as the near monodispersion, but are hydrophobic and are not suitable for biological applications.16−20 To solve this problem, it is required to transfer the hydrophobic NPs to aqueous media by surface modification, such as polymer, silica, or carbon coating techniques.4,9,21−25 Among them, the silica coating is a very good surface modifier, because of its excellent biocompatibility and stability, nontoxicity, and easily furthered conjugation with various functional groups, thus enabling the coupling and labeling of biotargets with selectivity and specificity.2−6 The Stöber synthesis and reverse microemulsion methods are two of the most common approaches for silica coating, in which the former cannot be applied to the NPs that are insoluble in the polar media; however, the latter is an excellent alternative strategy for silica coating of oil-dispersed NPs.21,23,24 © 2012 American Chemical Society
Received: September 2, 2012 Revised: October 29, 2012 Published: October 31, 2012 4572
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2. EXPERIMENTAL SECTION
core/shell NPs with an ultrathin silica shell reported up to now is usually broad, which will greatly affect the uniformity of magnetic properties.29 On the other hand, the thick shells, after being etched to a hollow mesoporous structure (such as Fe3O4@hollow mSiO2), can not only minimize the decrease of above-mentioned specific properties, but also have a high loading amount when applied to biological molecules encapsulation.34−41 In general, the silica shell of different thicknesses has its own specific function, and the control of it is very important for different biological applications. It has been reported that the thickness of silica shells could be tuned by changing the amount of NPs and TEOS, but the tuning is very limited and free-core silica or multicore silica constantly appeared.21,27,29,42,43 Lee and co-workers found that the SiO2 shell thickness basically remains constant as the TEOS content increases up to a certain extent,27 but they ignored the confinement of aqueous domain size on the overall size of Fe3O4@SiO2 core/shell NPs. Zhang and co-workers reported that a suitable ratio of Igepal CO-520 to ammonia, which determines the number and size of aqueous domain, is crucial to a one-to-one matching of the number of core NPs with that of aqueous domains, and they found that excessive TEOS content would cause undesirable core-free SiO2 particles;29 however, they overlooked the heterogeneous or homogeneous nucleation of SiO2. Although the respective influence of Igepal CO-520, ammonia, number of core NPs, and TEOS on the coating quality of Fe3O4@SiO2 core/shell NPs has been extensively studied,21 a systematic investigation of the relationship of these factors is still lacking. The silica coating mechanism in the reverse microemulsion system has been discussed recently.31,32 Selvan and coworkers,21 as well as Darbandi and co-workers,26 proposed a ligand exchange mechanism to explain the coating process of the silica shell, but no direct evidence was given. By observing the fluorescence quenching efficiency of QDs core, Koole and co-workers thought that there is a ligand exchange, but the change of the coordination modes through ligand exchange on the surface of NPs is still not clear.24 Vogt and co-workers suggested the ligand exchange of oleic acid and hydrolyzed TEOS by FTIR spectra, but they ignored whether or not a ligand exchange existed between oleic acid and surfactant (such as Triton-X 100 or Igepal CO-520).44 More direct evidence is needed to clarify the ligand exchange of oleic acid and Igepal CO-520 in the intermediate process. Herein, we report a systematic study on the coating regulation of Fe3O4@SiO2 core/shell NPs with a single core and with different shell thicknesses for differently sized Fe3O4 NPs via the reverse microemulsion method. The roles and correlations of the factors, such as Igepal CO-520, ammonia, the number of Fe3O4 NPs, and TEOS, were analyzed and discussed. It was found that the match of the number of Fe3O4 NPs and amount of Igepal CO-520 is essential in realizing the silica coating Fe3O4 NPs of different sizes; the silica shell thickness can be adjusted by changing amount of Fe3O4 NPs and TEOS under the conditions that the corresponding ammonia content is modified simultaneously, and an equivalently fractionated drop of TEOS is very effective in avoiding the formation of core-free silica particles. The coating mechanism was discussed, together with the Fourier transform infrared (FTIR) analysis of the ligand exchange between oleic acid and Igepal CO-520 in the intermediate process.
2.1. Chemicals. 1-Octadecene (tech. 90%), polyoxyethylene (5) nonylphenylether (Igepal CO-520) were purchased from Aldrich. Oleic acid (tech. 90%) was obtained from Alfa Aesar. Aqueous ammonia (25%−28%), tetraethyl orthosilicate (TEOS, 98%), sodium hydroxide, iron(III) chloride hexahydrate (99.99%), ethanol, hexane, and cyclohexane are of analytical reagent grade. All the chemicals were used as-received without further purification. The water used was purified through a Millipore system. 2.2. Synthesis. Fe3O4 NPs. Fe3O4 NPs were prepared according to the literature.20 In a typical synthetic procedure, 2 mmol FeCl3·6H2O was dissolved in 6 mL of H2O, then 8 mL of ethanol, 14 mL of hexane, and 1.9 mL of oleic acid was added to the above solution and stirred at room temperature for 30 min. Then, 0.24 g NaOH was added to the above solution and stirred in a closed vessel at 70 °C for 4 h. The resultant solution was separated from two different layers, using a separatory funnel. The above organic layer containing Fe(oleate)3 complex was collected, washed with deioned water three times, and then heated at 80 °C overnight in order to evaporate hexane. The sticky Fe(oleate)3 precursor was dispersed in 0.32 mL of oleic acid and 12.5 mL of 1-octadecene. The mixture solution was degassed with N2 for 30 min at room temperature, and then heated at 320 °C for 30 min under N2 flow. The solution was cooled to room temperature and precipitated by excess ethanol, and the precipitate was collected by centrifugation and the supernatant decanted. The isolated solid was redispersed in hexane and then precipitated with ethanol. The precipitation−redispersion process was repeated for several times to purify the as-prepared iron oxide NPs. By altering the oleic acid amount and reaction time, Fe3O4 NPs ranging in size from ∼3 nm to 19 nm can be obtained. Fe3O4@SiO2 NPs. A reverse microemulsion method was used to prepare Fe3O4@SiO2 core−shell NPs. Typically, 0.5 g of Igepal CO520 was dispersed in 11 mL of cyclohexane and sonicated for 10 min. Then, 0.5 mL of a 12.2-nm Fe3O4 solution (2.5 mg/mL in cyclohexane) (i.e., 1.25 mg Fe3O4) was added to the above solution with continuous stirring; subsequently, 0.2 mL of ammonium hydroxide (25%−28%) was added to the above mixture solution. Finally, 0.28 mL of TEOS was added via the equivalently fractionated drop method (adding 35 μL per 16 h). The resulting Fe3O4@SiO2 core−shell NPs were collected after centrifuging and washing, and then were redispersed in ethanol. By adjusting the size of Fe3O4 NPs and the amount of TEOS (include adding fractionated dose of TEOS per 16 h), as well as the ratio of ammonia and Igepal CO-520, singlecore Fe3O4@SiO2 NPs with shell thicknesses ranging from 2 nm to 20 nm can be achieved. 2.3. Characterization. X-ray diffraction (XRD) pattern was obtained on a Rigaku D/Max γA X-ray diffractometer with Cu Kα radiation (λ = 1.5405 Å). Transmission electron microscopy (TEM) experiments were performed on a JEOL 2010 system operated at an accelerating voltage of 200 kV. The Fourier transform infrared (FTIR) spectroscopy performed by a Bruker Vector-22 FTIR spectrometer operating in transmission mode was used to characterize the entire process of coating silica. The transmission spectra for NPs from different processes were received after making pellets with KBr powder. The magnetic properties were performed on a superconducting quantum interference device (SQUID). The field-dependent magnetization curves were recorded from 0 to ±4 T at 300 K. Temperature-dependent magnetization curves were measured under zero-field-cooled/field-cooled (ZFC−FC) mode from 5 K to 300 K in a static magnetic field of 100 Oe.
3. RESULTS AND DISCUSSION 3.1. Characterization of Fe3O4 and Fe3O4@SiO2 Core/ Shell NPs. Figure 1 shows typical TEM images of nearly monodispersed Fe3O4 NPs of different diameters, demonstrating that the oleate-capped Fe3O4 NPs prepared by the thermal decomposition method have a uniform size distribution and can be dispersed excellently in nonpolar solvent such as cyclo4573
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a large area, in which a spherical core/shell structure with just single-core Fe3O4 NPs and a 19.8-nm-thick SiO2 coating can be seen. The corresponding XRD pattern shows a broad wave packet at ca. 10°−30° and several diffraction peaks, as shown in the inset in Figure 2, in which the packet comes from amorphous SiO2 and the diffraction peaks at 35.2°, 42.8°, and 62.6° can be indexed to the (311), (400), and (411) planes of Fe3O4 with a cubic inverse spinel structure. 3.2. SiO2 Coating of Differently Sized Fe3O4 NPs. The uniform coating of silica on Fe3O4 NPs with a single core without core-free silica particles via a reverse microemulsion method is strongly dependent not only on the molar ratio of ammonia to Igepal CO-520, but also on the size and content of the loaded hydrophobic Fe3O4 NPs. The coating parameters suitable for certain-sized Fe3O4 NPs is not automatically adaptable to other size Fe3O4 NPs. We take the silica coating of 8.8- and 12.2-nm Fe3O4 NPs as an example to demonstrate this matter. The optimal silica coating regulations obtained in our previous report for 8.8-nm Fe3O4 NPs with a single core are listed in Table 1 (the corresponding sample is denoted as sample a128), and the corresponding morphology is shown in Figure 3a. When we applied these regulations to the silica Figure 1. TEM images of Fe3O4 NPs with sizes of (a) 3−5 nm, (b) 8.8 nm, (c) 10.1 nm, (d) 12.2 nm, (e) 13.5 nm, and (f) 19.1 nm. The insers show the corresponding silica coating. Scale bar = 20 nm.
hexane. The size of Fe3O4 NPs can be tuned from ∼3 nm to 19 nm simply by changing the reaction time and oleic acid content. The insets in Figure 1 demonstrate that the Fe3O4@ SiO2 core−shell NPs structure with a single core without corefree silica particles for differently sized Fe3O4 NPs can be realized under optimal silica coating regulations in the reverse microemulison process, which will be described in detail below. Figure 2 shows a TEM image of 12.2-nm Fe3O4@SiO2 NPs on
Figure 3. TEM images of Fe3O4@ SiO2 NPs with Fe3O4 sizes of (a) 8.8 nm and (b−d) 12.2 nm. Panels (a)−(d) correspond to samples a1−a4 in Table 1.
coating of 12.2-nm Fe3O4 NPs (marked as sample a2 in Table 1), it was found that there is a large number of core-free silica particles, as shown in Figure 3b. To solve this problem, we increased the content of Fe3O4 NPs from 0.8 mg to 1.6 mg, and, intriguingly, the number of the core-free silica particles dramatically decreased, as shown in sample a3 (see Figure 3c). With further increases in the Fe3O4 NPs content to 2.0 mg, the core-free silica particles completely disappeared; see sample a4 (Figure 4d). In the reverse microemulsion system, the size and number of the aqueous domains are dependent on the ratio of ammonia to Igepal CO-520 (denoted as R).45−47 With a fixed R-value, the
Figure 2. TEM image showing 12.2-nm Fe3O4@SiO2 NPs with a 19.8nm-thick shell; the inset shows the corresponding XRD pattern.
Table 1. Parameters for Applying a SiO2 Coating to Differently Sized Fe3O4 NPs sample
Fe3O4 (mg)
size (nm)
Igepal CO-520 (g)
ammonia (μL)
TEOS (μL)
shell thickness (nm)
core-free silica?
a1 a2 a3 a4
0.8 0.8 1.6 2.0
8.8 12.2 12.2 12.2
1.36 1.36 1.36 1.36
100 100 100 100
75 75 75 75
8.5 7.6 7.2 5.7
no yes yes no
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75 μL to 150 μL, keeping other parameters constant, as listed in Table 2, the SiO2 shell thickness increases accordingly; unfortunately, the core-free silica particles appears, as shown in Figure 4b. To avoid the formation of core-free silica particles, an equivalently fractionated drop method was adopted, in which the 300-μL TEOS is divided into four individual droppings, and the following drop is carried out; after the former is finished with continuously stirring for 16 h, the corefree silica particles completely disappeared, as listed in Table 2 and Figure 4c, in which the SiO2 shell thickness increases to 13.3 nm. It is interesting to note that, in avoiding the formation of core-free silica particles, the fractionated drop of 300 μL of TEOS is much more effective than a one-off drop of 150 μL of TEOS, indicating that the equivalently fractionated drop method is very powerful in obtaining a thicker silica shell with single-core Fe3O4 NPs. To demonstrate the effectiveness of the fractionated drop, the TEOS content was increased to 600 μL and eight individual droppings was applied, and it was found the shell thickness of the Fe3O4@SiO2 NPs increased to 14.9 nm without core-free silica particles. The classical or modified La Mer theory has been frequently used to explain the growth of amorphous particles, and it is interpretable for the optimal reaction conditions.48−57 For example, Pemberton and co-workers utilized the modified La Mer model with an additional dimension of the particle number to explain the formation of sub-100-nm silica particles.53 Ma and co-workers also used the modified La Mer theory to account for the preparation of monodispersed silica particles with binary sizes.56 Although the La Mer theory cannot quantitatively describe the nucleation and growth of silica coating, it can qualitatively describe the optimal reaction condition of silica coating in the present study. Based on La Mer-like theory, the formation of NPs can be divided into three stages: monomer concentration increasing stage, nucleation stage, and growth stage. The silica coating processes can be well-described with the aid of this theory, as described in Figure 5. It is known that the energy barrier governing the heterogeneous nucleation is lower than that governing the homogeneous nucleation, and it is proportional to the supersaturating condition of the solution,58−60 (i.e., the concentration of hydrolyzed TEOS in our case, which is denoted as C). The hydrolyzed TEOS monomers are formed with the addition of TEOS, and this is the first stage, as shown in Figure 5. The concentration of the monomers generally increases as the TEOS amount increases, and when exceeds its solubility concentration (denoted as Cs), a heterogeneous nucleation occurs on the surface of the Fe3O4 NPs. When the monomer concentration surpasses the homogeneous nucleation threshold (denoted as Chomo), a spontaneously aggregation will take place (stage II in Figure 5). Further increases in the size of the nucleus are accompanied by consumption of the monomers, and as the monomer amount decreases below the homogeneous nucleation concentration, the heterogeneous nucleation will reappear (stage III in Figure 5, in which both
Figure 4. TEM images of 12.2-nm Fe3O4@ SiO2 NPs with different TEOS amounts: (a) 75, (b) 150, (c) 300 (4 times fractionated drop), and (d) 600 μL (8 times fractionated drop). Panels (a)−(d) correspond to samples b1−b4 in Table 2. Scale bar = 20 nm.
size and number of the aqueous domains will be constant. Basically, if the number of Fe3O4 NPs equals the number of the aqueous domains, a one-to-one silica coating could be realized. For the coating of 8.8-nm Fe3O4 NPs, the addition of 0.8 mg of Fe3O4 NPs just fulfills the one-to-one silica coating regulations. However, for 12.2-nm Fe3O4 NPs with equal mass, the number of NPs is much less than that of the 8.8-nm Fe3O4 NPs and does not match the number of aqueous domains. As the mass of 12.2-nm Fe3O4 NPs increases to 2.0 mg, the number of NPs will match the number of aqueous domains, thus reaching the basic requirement of one-to-one coating silica without core-free silica. 3.3. Different SiO2 Shell Thicknesses. The thickness of the silica shell increases as the amount of TEOS increases, while the core-free silica particles inevitably appear when the TEOS content is increased to a certain extent.27,29,42 In the following section, the influence of the parameters, such as the content of Fe3O4 NPs, Igepal CO-520, ammonia, and TEOS, on the shell thickness and formation of core-free silica particles will be analyzed. The 12.2-nm Fe3O4 NP is selected to better describe the story, and optimal regulations for the realization of Fe3O4@ SiO2 NPs with a single core without core-free silica particles and with different shell thicknesses are determined. To further demonstrate the importance of the match between the number of Fe3O4 NPs and aqueous domains in the realization of one-to-one coating silica without core-free silica particles, the amount of Fe3O4 NPs and Igepal CO-520 was decreased to 1.25 mg and 0.5 g (listed in Table 2 as sample b1), respectively, with other parameters the same as sample a4, it was found that the Fe3O4@SiO2 core/shell NPs with a single core and a 7.0-nm silica coating without core-free silica can be fulfilled (as shown in Figure 4a), because, in this case, the number of Fe3O4 NPs matches that of the number of aqueous domains, because of the decrease in the amount of both Fe3O4 NPs and Igepal CO-520. As the TEOS content increases from
Table 2. Parameters for Fabricating 12.2-nm Fe3O4@SiO2 NPs with Different TEOS Amounts sample
Fe3O4 (mg)
size (nm)
Igepal CO-520 (g)
ammonia (μL)
TEOS (μL)
shell thickness (nm)
core-free silica?
b1 b2 b3 b4
1.3 1.3 1.3 1.3
12.2 12.2 12.2 12.2
0.5 0.5 0.5 0.5
100 100 100 100
75 150 300 (4 times) 600 (8 times)
7.0 8.7 13.3 14.9
no yes no no
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Figure 6. TEM images of 12.2-nm Fe3O4@ SiO2 NPs with different ammonia amounts: (a) 50 μL (sample c1), (b) 100 μL (sample c2), and (c) 200 μL (sample c3). In panel (d), the TEOS content was decreased to 35 μL (sample c4). Scale bar = 20 nm. Figure 5. (a) La Mer-like diagram: hydrolyzed TEOS (monomers) concentration against time on homogeneous nucleation and heterogeneous nucleation, (b) the existence of Fe3O4@SiO2 core/ shell NPs and SiO2 NPs in the reaction production when C > Chomo at some moment, (c) only the existence of Fe3O4@SiO2 core/shell NPs in the reaction production when C < Chomo at any moment.
increase in ammonia content, the TEOS content must appropriately decrease to avoid the homogeneous nucleation of monomers. The above results demonstrate that the increase in the content of both ammonia and TEOS can increase the shell thickness of Fe3O4@SiO2 NPs, but the good match between these two components is essential to avoid the formation of core-free silica particles. The regulations listed in Table 3 for sample c4 can obtain Fe3O4@SiO2 NPs with a SiO2 shell thickness of 6.3 nm. Using the regulations of sample a4 but increasing the TEOS content to 140 μL, and when four fractionated drops was applied, the SiO2 shell thickness can increase to 13.4 nm, as shown in Figure 7a (similar to the manipulation of sample b3), which provides further evidence that an increase in ammonia content promotes the growth of the silica shell. With the aid of the fractionated drops, the shell thickness can increase further to 19.8 nm when the TEOS content is increased to 280 μL (divided into eight droppings), as shown in Figure 7b. Based on these results, it is speculated that the SiO2 shell thickness can be further increased by continuing increasing either the TEOS content with more fractionated drops, or the ammonia content with decreased one-off TEOS content. Note that a low ammonia content is suitable to obtain a thin silica shell, as listed in Table 3 for samples c1−c3 and Figure 6. Using the regulations of sample c1 but decreasing the TEOS content to 20 μL, the shell thickness can be even reduced to ∼2.0 nm (see Figure 7c), fulfilling the realization of a ultrathin silica shell. The low ammonia content makes the aqueous domain possess bound water,45 resulting in an effective restriction for the hydrolysis of TEOS and, thus, the formation of thin SiO2 shell. Based on the above results and analysis, one can deduce that, for low ammonia content, a small aqueous domain will effectively confine the growth of silica particles, because of
coating and monomers nuclei growth occur). Therefore, once the monomer concentration is higher than that of homogeneous nucleation (C > Chomo), regardless of the time, both Fe3O4@SiO2 core/shell NPs and SiO2 particles will coexist in the final products (see Figure 7b). To ensure coated Fe3O4 NPs with just a single core, without core-free SiO2 particles, the monomer concentration must fulfill the condition Cs < C < Chomo throughout the reaction process (see Figure 6c). The fractionated drop method can make the above-mentioned condition always meet, in which fresh TEOS is added after the previous TEOS is basically consumed. The ammonia content also affects the shell thickness of Fe3O4@SiO2 NPs. When the ammonia content was increased from 50 μL to 200 μL, keeping other parameters constant, the SiO2 shell thickness increases only slightly, from 5 nm to 8.3 nm, as listed in Table 3 and Figures 6a−c, but a small amount of core-free silica particles appeared, as shown in sample c3 (see Figure 6c). In this case, when the TEOS content was decreased to 35 μL (sample c4 in Table 3), the core-free silica particles disappeared, as shown in Figure 6d. This is due to the fact that the aqueous domain will increase in size as the content of ammonia increases,45 and the enlarged aqueous domain has more free aqueous space, resulting in the formation of more monomers and, thus, an increase in SiO2 shell thickness. On the other hand, the enlarged aqueous domain will accelerate the hydrolysis of TEOS, and the supplying rate of the monomers will exceed its consumption rate: in this case, the condition Cs < C < Chomo will be broken within a very short time, resulting in the formation of core-free silica particles. Therefore, with an
Table 3. Parameters for Fabricating 12.2-nm Fe3O4@SiO2 NPs with Different Ammonia and TEOS Amounts sample
Fe3O4 (mg)
size (nm)
Igepal CO-520 (g)
ammonia (μL)
TEOS (μL)
shell thickness (nm)
core-free silica?
c1 c2 c3 c4
1.3 1.3 1.3 1.3
12.2 12.2 12.2 12.2
0.5 0.5 0.5 0.5
50 100 200 200
75 75 75 35
5.0 7.0 8.3 6.3
no no yes no
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Figure 7. TEM images of 12.2-nm Fe3O4@ SiO2 NPs with a TEOS content of (a) 140 μL (four times fractionated drop) and (b) 280 μL (eight times fractionated drop), and (c) 20 μL. Scale bar = 20 nm.
the limited space of the droplets and the depressed hydrolysis rate of TEOS. In this case, the thickness of the silica shell is also restricted. On the other hand, for high ammonia content, the large aqueous domain will be favorable to the growth of silica shells and silica particles, because of the free space of the droplets and the quick hydrolysis rate of TEOS. In order to increase the silica shell thickness and simultaneously avoid the formation of core-free silica particles, an effective strategy is to either add to the TEOS content with more fractionated drops, or increase the ammonia content with the decreased one-off TEOS content, keeping the concentration of hydrolyzed TEOS in the range of Cs < C < Chomo. 3.4. Strategies of Silica Coating with a Single Core without Core-Free Silica Particles. To obtain single-core Fe3O4@SiO2 NPs with an adjustable shell thickness without core-free silica particles, matching the number of Fe3O4 NPs with the number of aqueous domains is essential. Under a given ratio of ammonia to Igepal CO-520, which determines the size and number of aqueous domains, to ensure a one-to-one silica coating on Fe3O4 NPs, the content of Fe3O4 NPs must increase as the Fe3O4 NPs size increases. To adjust the SiO2 shell thickness, the aqueous domain size must be regulated by changing the ammonia concentration. A small aqueous domain is beneficial for obtaining an ultrathin silica shell, while a large aqueous domain is necessary for thicker shell coatings. Nevertheless, the large aqueous domain has more free water and will promote the hydrolysis of TEOS, leading to the nucleation and growth of hydrolyzed TEOS, and, thus, the formation of core-free silica particles. In this case, to avoid the formation of core-free silica particles, the content of TEOS must be reduced. A thicker silica shell can be achieved by increasing the TEOS content with equivalently fractionated drops. Figure 8 shows the TEM image of 12.2-nm Fe3O4@SiO2 NPs with different shell thicknesses, demonstrating the effectiveness of the strategy adopted in this study. The magnetic properties of the 12.2-nm Fe3O4 NPs with oleate capping and silica shell were investigated. From Figure 9a, one can see that the oleate-capped and silica-coated Fe3O4 NPs are all superparamagnetic at 300 K, because no hysteresis loop can be observed, and the saturation magnetization of the oleate-capped Fe3O4 NPs is ∼27.7 emu g−1 and is smaller than that of the commercial magnetite NPs, which is considered to be due to the existence of excessive physically absorbed oleic acid. However, the saturation magnetization of the silica-coated Fe3O4 NPs decreases as the silica shell thickness increases, and it has values of ∼30.0, 5.2, and 1.9 emu g−1 with shell thicknesses of 2.0, 12.1, and 18.5 nm, respectively. The blocking temperature of the silica-coated Fe3O4 NPs also decreases as the silica shell thickness increases and is ∼146, 135, and 133 K with shell thicknesses of 2.0, 12.1, and 18.5 nm, respectively; all these temperatures are less than 155 K, which is the temperature for the oleate-capped Fe3O4 NPs, as shown in
Figure 8. TEM image of 12.2-nm Fe3O4@SiO2 NPs with shell thicknesses of (a) 2.0 nm, (b) 6.3 nm, (c) 14.1 nm, and (d) 19.8 nm. Scale bar = 20 nm.
Figure 9. (a) Field-dependent magnetization and (b) ZFC-FC curves of oleate-capped Fe3O4 NPs and silica coated Fe3O4 NPs with different silica shell thicknesses.
Figures 9b. The decrease can be ascribed to the decrease in the dipolar interaction, which is due to the increased interparticle distance.30 The above results demonstrate that the magnetic property of Fe3O4@SiO2 NPs can be preserved, at the utmost, with an ultrathin silica shell thickness. 3.5. Coating Mechanism. Figure 10 shows the FTIR spectra of the silica coating on Fe3O4 NPs at different steps. From this figure, one can see that there are four vibration peaks for the oleate-capped Fe3O4 NPs (see curve (1) in Figure 10, in which the bands at 2924 and 2852 cm−1 can be ascribed to the asymmetric and symmetric stretch vibration modes of −CH2, respectively, and the band at 1712 cm−1 can be assigned to the stretch modes of CO). These three bands result from free oleic acid or physically absorbed oleic acid on the surface of the Fe3O4 NPs. The peak at 1402 cm−1 is attributed to the stretch vibration of −COO− and results from the bond of the carboxylate anions with Fe ions of Fe3O4, indicating the existence of the chemically absorbed oleic acid on the Fe3O4 surface.61−64 After the addition of Igepal CO-520 to the oleate4577
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coordination modes on the Fe3O4 NPs surface.62,69 These results provide clear evidence that there are effective ligand exchanges in the process of the silica coating on the Fe3O4 NPs surface in the reverse microemulsion system. Based on the above analysis, the silica coating mechanism is schematically illustrated in Scheme 1. First, once Igepal CO520 is mixed in cyclohexane solution, it spontaneously aggregates and forms the micelles in the cyclohexane solution, because of its hydrophilic groups. Second, as the Fe3O4 NPs are added to the solution, the ligand exchange between oleic acid and fractional Igepal CO-520 occurs on the surface of the Fe3O4 NPs with part of the Igepal CO-520 still in the micelle form. The Igepal CO-520 micelles will then be filled with ammonia upon its addition, and the micelle size is enlarged and forms a reverse microemulison system. Subsequently, the added TEOS will hydrolyze at the oil/water interface and perform the ligand exchange with Igepal CO-520 chemically absorbed on the Fe3O4 NPs surface and then transfer the NPs to the water phase. Finally, the hydrolyzed TEOS on the Fe3O4 NPs surface undergoes a condensation process and forms silica shells.
Figure 10. FTIR spectra in the silica coating process of (1) the oleatecapped Fe3O4 NPs, (2) after the addition of Igepal CO-520, and (3) after the addition of TEOS.
capped Fe3O4 NPs, the band at 1402 cm−1 disappears, and the new peaks at 1117 cm−1 and 1064 cm−1, which do not match those of free Igepal CO-520, appear (the typical characteristic bands of Igepal CO-520 are observed at 1250 and 1120 cm−1, and are attributed to the stretch vibration of aromatic ether C− O−C and aliphatic ether C−O−C, respectively. Otherwise, the bands are observed at 1512 and 1456 cm−1, and are assigned to the stretch vibration of the aromatic ring CC),65 as shown in curve (2) in Figure 10. The band at 1064 cm−1 can be derived from the stretch vibration of C−O combined with Fe ions, and that at 1117 cm−1 is regarded as a shift to the stretch vibration peak of aliphatic ether C−O−C, indicating the occurrence of the ligand exchange between chemically absorbed oleic acid and Igepal CO-520 and the formation of the chemical absorption of Igepal CO-520 on the Fe3O4 surface. The peak at 1377 cm−1 belongs to the symmetric bending vibration of methyl C−H bonds, being stable in the peak position, and the band is not related to the ligand exchange. With the addition of TEOS, the bands at 1161, 1094, 955, 798, and 467 cm−1, which are attributed to the vibration modes of SiO2, appear66−68 and the above-mentioned new peaks at 1117 and 1064 cm−1 disappear, as shown in curve (3) of Figure 10; this indicates the occurrence of the ligand exchange between chemically absorbed Igepal CO-520 and hydrolyzed TEOS on the Fe3O4 NPs surface. The characteristic band near 600 cm−1, which arises from the stretch vibration of Fe−O bonds, has a shift with the change of surface ligands, further revealing the variation of
4. CONCLUSIONS In conclusion, we have presented the regulations of the controlled synthesis of Fe3O4@SiO2 core/shell nanoparticles (NPs) via a reverse microemulison method. The core/shell NPs have only a single Fe3O4 core without core-free SiO2 particles. The regulations can be applied to differently sized Fe3O4 NPs with different SiO2 shell thicknesses. Matching the number of Fe3O4 NPs with the number of aqueous domains is indispensable for fulfilling this objective. Under a given ratio of ammonia to Igepal CO-520, which determines the size and number of aqueous domains, as the size of the Fe3O4 NPs increases, the NPs mass must increase accordingly to match the number of NPs with the number of aqueous domains. It was found that a small aqueous domain is suitable to coat an ultrathin silica shell, while a large aqueous domain is necessary for thicker shell coatings. Although increasing the content of both ammonia and TEOS can increase the shell thickness of Fe 3 O 4 @SiO 2 NPs, a good match between these two components is essential to avoid the formation of core-free silica particles. We found that the effective strategy in increasing the SiO2 shell thickness is by increasing either the TEOS content with more fractionated drops, or the ammonia content
Scheme 1. Illustratation of the Coating Mechanism of SiO2 on the Surface of Fe3O4 NPs
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with the decreased one-off TEOS content. Fourier transform infrared (FTIR) analysis has provided evidence of the ligand exchange between oleic acid and Igepal CO-520, as well as between Igepal CO-520 and hydrolyzed TEOS during silica coating. Our results not only have provided regulations for silica coating on Fe3O4 NPs with a single core without core-free silica suitable for differently sized Fe3O4 NPs and shell thicknesses, but also a strategy that can be applied to the silica coating of other NPs, such as magnets, fluorescence, and metals. The realization of uniform core−shell NPs will be beneficial to biological and environmental applications.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +0086-(0)551-559-1437. Fax: +0086-(0)551-559-1434. E-mail:
[email protected] (G.H.L.),
[email protected] (Y.X.Z.). Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from National Basic Research Program of China (Grant No. 2013CB934304) and National Natural Science Foundation of China (Grant No. 51272255).
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NOTE ADDED AFTER ASAP PUBLICATION There was an error in the Acknowledgment in the version published ASAP November 29, 2012; the corrected version published ASAP December 3, 2012.
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