Facile synthesis of high dispersion γ-Fe₂O₃–Au nanoparticles within mesoporous silica spheres

Abstract

High dispersion γ-Fe₂O₃ combined with Au nanoparticles (denoted as γ-Fe₂O₃–Au NPs) coated by mesoporous silica spheres (denoted as m-SiO₂) were synthesized by a facile one-pot method process. The dispersion of γ-Fe₂O₃ seed is extremely important to the formation of γ-Fe₂O₃–Au@m-SiO₂ composite, by using a certain amount of neutral β-CD as stabilizing agent, high dispersion γ-Fe₂O₃ NPs with 16 nm can be prepared. Besides, different number of γ-Fe₂O₃–Au NPs within composite can be tuned with the addition amount of γ-Fe₂O₃ seed. TEM images showed the ordered morphology, good dispersion, and uniform particle size of γ-Fe₂O₃–Au NPs inside composite. Furthermore, XRD, M–H curves and N₂ sorption characterization confirmed γ-Fe₂O₃–Au@m-SiO₂ composite possessed good magnetic property, uniform mesopore distribution and high surface area.

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1. Introduction

Functionalized mesoporous materials are attracting increasing interest for adsorption, separation, catalysis, drug delivery, sensors, photonics, machines, and nanovalves.¹ Recently, much effort has been devoted to developing novel functionalized approaches for introducing various nanoparticles into mesoporous materials, which is an important goal of material scientists. The two major strategies involve introducing metal or metal oxide nanoparticles to mesoporous materials composite, one is incorporating or immobilizing nanoparticles by post nanoparticle synthesis,^2–4 and the other is synthesizing the nanoparticles within the framework of mesoporous composite.^5–7

Among these nanoparticles, magnetic nanoparticles such as Fe₃O₄ and γ-Fe₂O₃ have attracted extensive attention because of their unique magnetic properties.^8,9 Moreover, gold nanoparticles also have gained significant attention because of their well-known surface plasmon resonance properties¹⁰ and high catalytic activity.¹¹ Many studies have shown that magnetic nanoparticles are able to combine with Au NPs. For example, a dumbbell-like Au–Fe₃O₄ NPs was reported by Sun and colleagues, these nanoparticles retain the magnetic and optical properties of each single component and permit potential applications as optical reporter and magnetic handles for bioassays.^12,13 Chen et al. prepared a porous silica shell on to the Fe₃O₄–Au core nanoparticles. They verified the porous silica shell does not affect the optical and catalytic properties of Au NPs dramatically.¹⁴ Yin group reported a porous silica protected Fe₃O₄/SiO₂/Au composite, which were synthesized by a series of simple sol–gel and surface-protected etching processes, the Fe₃O₄ cores have an average diameter of 140 nm, and the composite showed very good uniform structure and the BET surface area and pore volume are 136.6 m² g⁻¹ and 0.25 cm³ g⁻¹, respectively.¹⁵

However, it is still a great challenge to coat iron oxide combined with Au NPs on mesoporous silica uniformly and quantitatively, especially, by using iron oxide as core, Au NPs should in situ combined with iron oxide in the formation of iron oxide–Au@m-SiO₂ composite.^16,17

In the present work, we devote to gain a uniform size of γ-Fe₂O₃–Au NPs inside mesoporous silica spheres with high dispersion and good magnetic property, through Au NPs in situ growth to avoid the aggregation of nanoparticles. By a facile one-pot method, we have synthesized successfully an ordered mesoporous pore channel and high surface area of mesoporous silica spheres coated on both magnetic iron oxide and Au NPs. By adjusting the addition amount of as-synthesized γ-Fe₂O₃ seed, the nanoparticles numbers and nanoparticles aggregation inside the final composite will be controlled. It was found that γ-Fe₂O₃–Au@m-SiO₂ composite showed good magnetic property, high nanoparticles dispersion, uniform mesopore size, high surface area and big pore volume.

2. Experimental section

2.1 Mesoporous composite materials preparation

Synthesis of chemicals. All reagents were of analytic grade, and deionised water was used throughout the experiments. Cetyltrimethylammonium bromide (CTAB, 98%), β-cyclodextrin (β-CD, >99%) were purchased from Shanghai Chemical Reagent Co. Sodium hydroxide (NaOH, beads), ferrous chloride tetrahydrate (FeCl₂·4H₂O), tetrachloroaurate (HAuCl₄·4H₂O), ammonium hydroxide (NH₃·H₂O, 28% by weight in water), tetraethylorthosilicate (TEOS, 98%), and ethanol were purchased from Guangzhou Chemical Reagent Co.

Preparation of γ-Fe₂O₃ nanoparticles. The γ-Fe₂O₃ NPs were prepared by the following procedures. First, 0.3 g ferrous chloride tetrahydrate (FeCl₂·4H₂O) dissolved in 40 mL deionized water, to obtain the well-dispersed γ-Fe₂O₃ NPs, 0.3 g β-cyclodextrin (β-CD) was added as structural dispersing agent and stirred strongly at room temperature. After stirring for 30 minutes, 5 mL aqueous ammonia solution was poured rapidly into the above solution. The solution was continued stirring for 1 h at room temperature, and then the resultant solution was transferred into a Teflon-lined stainless autoclave and aged at 140 °C for 4 h. The precipitate was centrifuged at 6000 rpm for 5 min, washed five times with deionized water and five times with ethanol. Finally, the collected magnetic nanoparticles were dispersed in ethanol for use as γ-Fe₂O₃ seed.

One-pot synthesis of γ-Fe₂O₃–Au@m-SiO₂ composite. The mesoporous composite containing both γ-Fe₂O₃ and Au NPs inside were prepared by a one-pot method. Firstly, 2 mL well-dispersed γ-Fe₂O₃ seed solution (5 mg mL⁻¹) was added into 200 mL deionized water and 80 mL ethanol and stirred for 15 min. Subsequently, 1.2 g cetyltrimethylammonium bromide (CTAB) was poured into the above solution and stirred at 70 °C for 15 min. Then 4 mL ethanol solution of tetrachloroaurate (HAuCl₄·4H₂O, 20 mg mL⁻¹) was injected and stirred for 5 minutes. Afterwards, 400 μL NaOH solution (2 M) was injected to produce the nucleation of the nanoparticles instantaneously. After 30 min stirring, 4 mL TEOS was added by dropwise additions to the aforementioned solution followed by the second addition of 800 μL NaOH solution (2 M) to the above suspension for triggering the gold nanoparticles in situ formation. With the addition of the basic solution, the resulting solution turned to a typical purple colour of thin coated Au@m-SiO₂ particles was observed, and the γ-Fe₂O₃–Au@m-SiO₂ composite formation process was finished in 2 h. Finally, the precipitate was filtered and washed with deionized water and ethanol for 5 times, and dried at 70 °C for 1 h in vacuum drying oven. To removal the CTAB template, the synthesized γ-Fe₂O₃–Au@m-SiO₂ composite were stirred and refluxed in ethanol for three times extraction. After extraction, the particles were consecutively filtered, washed twice with ethanol.

We denoted the synthesized γ-Fe₂O₃–Au@m-SiO₂ composite samples as FASC-number, such as FASC-2, FASC-5 and FASC-10. FASC means γ-Fe₂O₃–Au@m-SiO₂ composite; the number indicates the adding amount of γ-Fe₂O₃ seed, such as 2 mL, 5 mL and 10 mL of 5 mg mL⁻¹ solution, respectively.

2.2 Mesoporous composite materials characterization

X-Ray diffraction (XRD) patterns were obtained with an X'Pert Pro MPD X-ray diffractometer (PANalytical, Netherlands) using Cu Kα radiation. N₂ adsorption–desorption isotherms, and pore-size distributions were obtained using a Micromeritics ASAP 2020 analyzer at 77 K. Transmission electron microscopy (TEM) was carried out on a JEM-2010 microscope (JEOL, Japan) using an accelerating voltage of 200 kV. The M/H hysteresis loop was recorded with a Quantum Design MPMS XL-7 superconducting quantum interference device magnetometer (SQUID).

3. Results and analysis

As shown in Fig. 1a, the as-prepared nanoparticles exhibited six peaks at 2θ = 30.24°, 35.63°, 43.28°, 53.73°, 57.24° and 63.93°, which were assigned to a typical γ-Fe₂O₃ phase diffractions from the (220), (311), (400), (422), (511) and (440) planes. The corresponding crystallite size of the γ-Fe₂O₃ NPs was 16 nm calculated by the Scherrer equation. Magnetic properties of the γ-Fe₂O₃ NPs were studied using their M–H curves. M–H curves of γ-Fe₂O₃ NPs at room temperature are shown in Fig. 1b. As expected, the magnetization curve shows no remanence or coercivity at room temperature, suggesting the superparamagnetic character essential for the magnetic separation and recycling as the particles are not subject to strong magnetic interactions in dispersion.¹⁶ The saturation magnetization (M_s) value of γ-Fe₂O₃ NPs is observed to be 36.05 emu g⁻¹ which is small compared to that of theoretical value of bulk γ-Fe₂O₃ (M_s = 74 emu g⁻¹)^18,19 since M_s generally decreases with a decrease in magnetic particle size.²⁰ From Fig. 1c, the γ-Fe₂O₃ NPs in ethanol solution showed very high dispersion, and no deposit showed up after 24 h static observation, which attributed to the addition of β-CD. In the synthesis process of nanoparticles, we found that the addition of β-CD played a crucial role in the dispersibility of the γ-Fe₂O₃ NPs, it was very easy to precipitate without β-CD addition, however, by adding a certain amount of β-CD as stabilizing agent, the resultant γ-Fe₂O₃ NPs showed very good dispersion, and the nanoparticles were not easy to precipitate in the bottom. Furthermore, to check the magnetic property of γ-Fe₂O₃ NPs, we put a magnet next to the vial, the high dispersion of γ-Fe₂O₃ NPs could be rapidly and completely adsorbed onto one side of the vial less than 5 min as shown in Fig. 1d. It should be pointed out that dispersibility, particles size and magnetic property are very important to prepare γ-Fe₂O₃–Au@m-SiO₂ composite with high-performance, after the above characterization, we have confirmed our prepared γ-Fe₂O₃ NPs could be a magnetic seed source for the subsequent synthesis of γ-Fe₂O₃–Au@m-SiO₂ composite.

Fig. 1 XRD analysis and magnetic property of γ-Fe₂O₃ nanoparticles. (a) XRD pattern; (b) M–H curves; (c) static observation; (d) magnetic response.

To obtain high functional performance composite, we investigated the effect of γ-Fe₂O₃–Au formation with the adding amount of γ-Fe₂O₃ seed. Fig. 2 showed TEM images of γ-Fe₂O₃–Au@m-SiO₂ composite (FASC) with different adding amount of magnetic γ-Fe₂O₃ seed. The number of particles within the composite increased gradually with the addition amount of γ-Fe₂O₃ seed. There were a few nanoparticles inside of FASC-2 as shown in Fig. 2a–c, which was attributed to a small amount γ-Fe₂O₃ seed addition (2 mL, 5 mg mL⁻¹). As shown in the low magnification (Fig. 2a), FASC particles have uniform size about 400–500 nm, besides which shows an average of 2–3 γ-Fe₂O₃–Au NPs inside of FASC-2 in Fig. 2b. When 5 mL of γ-Fe₂O₃ seed solution was added, it showed very high dispersion and high-loaded nanoparticles inside of FASC-5 as shown in Fig. 2d–f. The image of Fig. 2d clearly showed there were dozens of monodisperse γ-Fe₂O₃–Au NPs distributed in FASC-5. When 10 mL of γ-Fe₂O₃ seed solution was added, we found that a divergent core shape or bulk γ-Fe₂O₃–Au within FASC-10 particles as shown in Fig. 2g–i. We speculate this cluster morphology may be owing to a large number of γ-Fe₂O₃ seed addition, leading to bulky γ-Fe₂O₃ particles could not combine with Au NPs into monodisperse γ-Fe₂O₃–Au NPs. As we known, the porosity is present everywhere within the particles but only visible at the edge due to the mass-thickness contrast. In the high magnification images, such as Fig. 2c, f and i, we can see an obvious worm-like mesopore structure in the edges of particles.

Fig. 2 TEM images of FASC samples. (a–c) FASC-2; (d–f) FASC-5; (g–i) FASC-10.

To further verify both γ-Fe₂O₃ and Au NPs inside of FASC, we collected the corresponding electron dispersive spectroscopy (EDS) pattern for FASC-5 when we took the inset picture as shown in Fig. 3a. It confirmed the existence of Au, Fe and O in composite, demonstrating that adding γ-Fe₂O₃ seed can combine with Au NPs into monodisperse γ-Fe₂O₃–Au NPs as shown in Fig. 3.

Fig. 3 EDS spectrum of FASC-5 and corresponding TEM inset image.

The possible formation mechanism is schematically shown in Scheme 1. To prepare high dispersion γ-Fe₂O₃ NPs, we chose a neutral β-CD as structural dispersing agent, which is composed of seven glucose units, and it also possesses hydrophobic cavity and hydrophilic exterior. Firstly, a certain amount of β-CD dissolved together with FeCl₂·4H₂O in water, monodisperse γ-Fe₂O₃ NPs were formed by adding aqueous ammonia solution in air. It is worth noting that the pH of solution was adjusted by aqueous ammonia, β-CD would like to adsorb and assemble around γ-Fe₂O₃ NPs.²¹ Hence, β-CD adsorbed on the surface of γ-Fe₂O₃ NPs can isolate γ-Fe₂O₃ NPs through abundant hydrophilic groups of β-CD. Secondly, a certain amount of high dispersion γ-Fe₂O₃ seed source dispersed into CTAB solution, soon afterwards, CTAB micelle coated γ-Fe₂O₃ NPs were obtained with micelle self-assembly, CTAB-stabilized γ-Fe₂O₃ NPs act as seeds for the formation of spherical mesoporous silica particles.²² Thirdly, tetrachloroaurate was injected into the above solution, AuCl₄⁻ ions will disperse into the hydrophilic section of β-CD and CTAB micelle, the nucleation of the Au NPs were produced by first reduction of NaOH, afterwards, the framework of m-SiO₂ was formed by TEOS hydrolysis. Meanwhile, Au NPs could be mostly in situ reduced on the surface of γ-Fe₂O₃ NPs by second reduction of NaOH. Lastly, γ-Fe₂O₃–Au@m-SiO₂ composite were gained after template removal by extraction.

Scheme 1 The possible formation process of γ-Fe₂O₃–Au embedded in mesoporous silica spheres.

The high angle range XRD patterns of FASC-5 samples were shown in Fig. 4. It was observed that the as-synthesized FASC-5 sample exhibited four peaks at 2θ = 38.18°, 44.39°, 64.58° and 77.55°, which can be assigned to diffraction from the (111), (200), (220), and (311) planes of Au (space group: Fm3̄m/225) in Fig. 4a. Unfortunately, there were no diffraction peaks relating to γ-Fe₂O₃ phase. We conjecture that there are two reasons, one may be due to the addition amount of γ-Fe₂O₃ seed is less and the particle size is relatively small, the other one may be due to the coating effect of mesoporous silica, so the γ-Fe₂O₃ phase diffraction might be weaken or covered by the strong Au diffraction peaks. Besides the diffraction peaks of Au, a broadened peak centred at 22° could be ascribed to the (100) reflection of amorphous silica. Owing to the special structure of FASC, we gained the mesopore structure by using an extraction instead of calcination method, which could be avoided the agglomeration of Au and γ-Fe₂O₃ NPs during the high-temperature calcinations. It should be noted that the XRD pattern of FASC-5 after extraction always showed good Au diffraction peaks, which indicated that Au and γ-Fe₂O₃ NPs were still existed inside FASC after template removal.

Fig. 4 The XRD patterns of FASC-5. (a) As-synthesized; (b) template removal by extraction.

The pore-size distribution (by BJH method) for different FASC samples under N₂ sorption characterization is shown in Fig. 5. It is noteworthy that the mesopore size decreases slightly with the addition of γ-Fe₂O₃ seed, however, all of them exhibit uniform mesopore distribution about 3 nm, and no macropore shows up between 10 and 250 nm.

Fig. 5 Pore-size distribution for FASC samples. (a) FASC-2; (b) FASC-5; (c) FASC-10.

The textural properties such as surface area, pore size, and pore volume of FASC samples are listed in Table 1. It is clear that all FASC samples possess high surface areas and large pore volumes. The BET surface areas are high up to 539–762 m² g⁻¹, and the pore volumes are 0.65–0.78 cm³ g⁻¹. However, we found that the value of surface area, pore size, and pore volume of samples slightly decreased with the addition of γ-Fe₂O₃ seed, we speculated that gradually increasing nanoparticles introducing into the composite might result in some of pore channels to be occupied or blocked by γ-Fe₂O₃–Au NPs, this can be observed from former TEM images.

Table 1 Textural properties of different FASC samples

Sample	SBET [m^2 g^−1]	BJH pore size [nm]	Pore volume [cm^3 g^−1]
FASC-2	762	3.11	0.78
FASC-5	620	2.99	0.71
FASC-10	539	2.88	0.65

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To determine the magnetic properties of FASC, magnetic measurements at room temperature were carried out on FASC samples. The magnetic hysteresis curves are given in Fig. 6, which shows typical ferromagnetic behaviours of the sample. The saturation magnetization (M_s) values of FASC samples were much lower than as-synthesized γ-Fe₂O₃ seed (M_s = 36.05 emu g⁻¹). The reduced magnetization of γ-Fe₂O₃–Au NPs coated by m-SiO₂ could result from the small particle surface effect,²³ which refers to the disordered alignment of surface atomic spins induced by reduced coordination and broken exchange between surface spins.²⁴ When only 2 mL of γ-Fe₂O₃ seed solution was added, the FASC-2 shows poorly magnetic property. Upon the addition amount, the M_s value of FASC-5 is closed to 1.5 emu g⁻¹, and the M_s value of FASC-10 is closed to 2.5 emu g⁻¹. Since it seems that the magnetic property of FASC could be improved by increasing γ-Fe₂O₃ seed addition.

Fig. 6 Room-temperature magnetic hysteresis loops of FASC samples. (a) FASC-2; (b) FASC-5; (c) FASC-10.

4. Conclusion

In summary, using a certain amount of β-CD as stabilizing agent, high dispersion γ-Fe₂O₃ NPs can be prepared by hydro-thermal synthesis, and then by using as-synthesized γ-Fe₂O₃ NPs as seed solution, a mesoporous composite (FASC) containing both γ-Fe₂O₃ and Au NPs inside were successfully prepared by one-pot method. The morphology, dispersibility, particle size of γ-Fe₂O₃–Au NPs were observed from TEM images. Furthermore, based on the above discussion of XRD, M–H curves and N₂ sorption characterization, et al., we have confirmed the mesopore structure and magnetic property of FASC. This γ-Fe₂O₃–Au@m-SiO₂ composite not only exhibits higher surface area and pore volume, but also it possibly possesses two special property, one is the magnetic γ-Fe₂O₃ can provide a magnetic targeting, the other is Au NPs with plasmonic properties can provide a plasmonic heating effect. Hence, the special property of FASC could make it a potential candidate for intelligent molecular machine under both magnetic targeting and plasmonic heating drug release applications. Besides, based on an easy magnetic separation and high catalytic activity of Au NPs, FASC may also be a good catalytic oxidation catalyst. More detailed work in these areas will be reported by our group in the near future.

Acknowledgements

This work was supported by the National Scientific Foundation of China (project no. 51302091, 51102099, 21303210), and the Fundamental Research Funds for the Central Universities, SCUT.

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