One-pot gradient solvothermal synthesis of the Ag/Au–Fe₃O₄ composite nanoparticles and their applications

Abstract

Herein, we report for the first time a simple and reliable one-pot methodology for the facile synthesis of the Ag/Au–Fe₃O₄ composite NPs. The as-prepared Ag/Au–Fe₃O₄ composite NPs have good crystallinity and saturation magnetization (38.021 emu g⁻¹), resulting in their magnetic recycling. They have good catalytic activity and stability during the reduction of 4-nitrophenol in the presence of NaBH₄. These Ag/Au–Fe₃O₄ composite NPs also have good antibacterial effect for Escherichia coli O1634. Excellent SERS was also shown for the detection of 2-nitroaniline using these Ag/Au–Fe₃O₄ composite NPs. The nanostructure makes the Ag/Au–Fe₃O₄ composite NPs stable and has a high-enhancement effect for Raman detection.

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Introduction

In recent years, due to their special properties and functionality, magnetic composite nanoparticles (NPs) played a more and more important role in the fields of high technology such as information storage,^1,2 color imaging,³ microwave absorption,⁴ medical diagnosis.⁵ Magnetic Fe₃O₄ NPs modified with a variety of functional modalities have been widely used in the fields such as molecular biology, cell separation and classification, targeted drug therapy. To date, a variety of synthesizing methods for magnetic composite Fe₃O₄ NPs have been developed such as chemical co-precipitation,⁶ inverse microemulsion,⁷ ultrasound irradiation,⁸ laser pyrolysis,⁹ thermal decomposition^10–12 and solvothermal method.¹³ The nanosized Fe₃O₄@graphene yolk–shell nanoparticles were prepared by a hydrothermal method, which can be used for controlled anticancer drug delivery with magnetic and pH-sensitive properties.¹⁴ The Ag/Au–Fe₃O₄ composite NPs inherit from the three components excellent stability, solvent compatibility, low toxicity, catalytic activity, antibacterial property and magnetic separability, all of which would greatly enhance their potential application.^15–24 For example, Au NPs have been found to play an important role in several catalytic processes, including low-temperature CO oxidation, reductive catalysis of chlorinated or nitrogenated hydrocarbons, and organic synthesis.^25–33 Similarly, composites containing Ag NPs have good antibacterial properties and excellent surface enhanced Raman scattering (SERS).^{21–24,34–36} Thus, considerable effort has been devoted to developing Ag/Au–Fe₃O₄ composite NPs, which enables full use of their functions.

Up to now, many methods have been developed for the synthesis of Ag/Au–Fe₃O₄ composite NPs.^34,37 Shen et al. prepared multifunctional Fe₃O₄@Ag/SiO₂/Au core–shell microspheres that displayed long-range plasmon transfer of Ag to Au, leading to an enhanced Raman scattering.³⁴ Wang et al. synthesized Fe₃O₄/Ag/Au composites for immunoassay based on surface plasmon resonance biosensor.³⁶ The Fe₃O₄/Ag/Au composites have the advantages of three components of Fe₃O₄, Ag and Au NPs. However, all of these methods include multiple steps, which are often accompanied by some disadvantages, such as low efficiency, tedious processing steps, and use of a hazardous Fe precursor. It has been an important challenge to synthesize Ag/Au–Fe₃O₄ composite NPs with uniform stability and high surface activity by a simple method.

In this study, we use a simple and reliable one-pot methodology for the facile synthesis of the Ag/Au–Fe₃O₄ composite NPs. The as-prepared Ag/Au–Fe₃O₄ composite NPs have good crystallinity and saturation magnetization (38.021 emu g⁻¹), resulting in good magnetic recycling. They show good catalytic activity during the reduction of 4-nitrophenol (4-NP) in the presence of NaBH₄. The as-prepared Ag/Au–Fe₃O₄ composite NPs have excellent SERS for 2-nitroaniline (2-NA) solution. These Ag/Au–Fe₃O₄ composite NPs have good antibacterial effect for Escherichia coli O1634. These excellent properties endow the as-prepared Ag/Au–Fe₃O₄ composite NPs with various potential applications, such as magnetic hyperthermia, surface plasmon resonance biosensor.

Experimental

Chemicals

Ferric chloride hexahydrate (FeCl₃·6H₂O), sodium acetate (NaAc), trisodium citrate, sodium hydroxide (NaOH), 1,2-propylene glycol (PG), HAuCl₄, silver nitrate (AgNO₃), polyvinyl pyrrolidone (PVP), NaBH₄, 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) were of analytical grade and purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. All chemicals were used as received without any further purification.

Synthesis of the Ag/Au–Fe₃O₄ composite NPs by one-pot methodology

The Ag/Au–Fe₃O₄ composite NPs were prepared by a gradient solvothermal method. Firstly, 0.5 g FeCl₃·6H₂O were completely dissolved in 5 mL PG as recorded solution A. Then, 0.9 g NaAc and 0.15 g trisodium citrate were dissolved in 10 mL PG as recorded solution B. Secondly, solution A and solution B were mixed together under magnetic stirring, and their pH was adjusted to proper value. Third, 1.5 mL of the above solution and 13.5 mL PG were mixed together under magnetic stirring. Finally, 0.2 mL 0.5% PVP aqueous solution, 0.4 mL 2.0% HAuCl₄ solution and 0.4 mL 1.0% AgNO₃ solution were added into the abovementioned solution. The solution was sealed in a Teflon lined stainless steel autoclave. The reactor was maintained 150 °C for 1 h and then the temperature rose up to 200 °C and was maintained for 6 h. The heating rate from 150 °C to 200 °C was 1.67 °C min⁻¹. After cooled to room temperature, the black products were separated magnetically. The obtained sediments were washed with ethanol and deionized water three times to eliminate organic and inorganic impurities and then dried at ambient temperature.

Application of the Ag/Au–Fe₃O₄ composite NPs for catalytic reduction of 4-NP

The reduction of 4-NP by the Ag/Au–Fe₃O₄ composite NPs in the presence of NaBH₄ was carried out to examine the catalytic activity and recyclability of the Ag/Au–Fe₃O₄ composite NPs. Firstly, 4-NP and the Ag/Au–Fe₃O₄ composite NPs were dispersed in deionised water to form aqueous suspension and their concentrations were 18 mmol L⁻¹ and 1 mg mL⁻¹, respectively. Secondly, 34.0 mg NaBH₄ were added into 6 mL deionised water and stirred uniformly at room temperature. Thirdly, 1 mL 4-NP solution and 1 mL the Ag/Au–Fe₃O₄ composite NPs were added into the abovementioned solution. The color of the mixture gradually changed from yellow to colorless. During the reaction, a quantitative portion of the mixture was taken at regular intervals and analysed by UV/Vis spectroscopy. After the reaction, the catalysts were magnetically recycled. As described above, catalysts were reused for the reduction of 4-NP time after time.

Application of the Ag/Au–Fe₃O₄ composite NPs for SERS of 2-NA

In order to examine SERS of the Ag/Au–Fe₃O₄ composite NPs, 2-NA solution was used for assay. Firstly, the Ag/Au–Fe₃O₄ composite NPs were dispersed in deionised water to form an aqueous suspension and its concentration was 1 mg mL⁻¹. Every time 200 μL Ag/Au–Fe₃O₄ composite NPs suspension was placed on a glass slide, then dried at ambient temperature and set aside. Secondly, 2-NA was dispersed in deionised water to form aqueous solutions and their concentrations were 5 × 10⁻⁴, 5 × 10⁻⁵, 5 × 10⁻⁶ and 5 × 10⁻⁷ mol L⁻¹. Thirdly, 25 μL different concentration solutions were placed on the glass slide and then SERS was tested. Finally, different carriers were also tested such as Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs and glass slides.

Application of the Ag/Au–Fe₃O₄ composite NPs for antibacterial effect on Escherichia coli O1634

In order to test antibacterial effect of the Ag/Au–Fe₃O₄ composite NPs, culture of Escherichia coli O1634 was carried out. Firstly, 5.7 g fungi medium and 7.0 g nutrient agar were dispersed in 200 mL deionised water to form aqueous solutions. Secondly, the fungi medium solution was put into a high temperature sterilizer for sterilizing it for 30 min and then cooled in a super clean bench. Thirdly, Escherichia coli O1634 was inoculated in solutions, shaken well and then evenly poured into the fungus Petri dishes. They were punched by hole puncher after waiting for fungi medium solution condensation. Subsequently, 20 μL, 1, 1.25, 2.5, 5 mg mL⁻¹ Ag/Au–Fe₃O₄ composite NPs solutions were poured in the holes. After waiting for 30 min, the fungus Petri dishes were turned over and cultivated for 24 h in a 4 °C oven. Finally, antibacterial effect of the Ag/Au–Fe₃O₄ composite NPs for Escherichia coli O1634 was measured.

Characterization

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on a JEOL JEM 2010F electron microscope operating at 200 kV. Energy-dispersive X-ray (EDX) spectroscopy measurements were obtained using a Hitachi S-4800 scanning electron field emission microscope. X-ray-powder diffraction patterns were accumulated on a Japan Rigaku D/max γA X-ray diffractometer by using graphite monochromatised Cu-Kα radiation (λ = 1.5418 Å). UV/Vis spectra were obtained by using a JASCO V-570 spectrophotometer at room temperature. The magnetic hysteresis loops of the samples were recorded on a Model PPMS-9 Physics Property Measurement System. The SERS spectra for the 532 nm probes were recorded using a Renishaw inVia spectrometer equipped with a microscope and a 50× objective (numerical aperture of 0.75).

Results and discussion

The X-ray diffraction (XRD) patterns of the synthesized Fe₃O₄, Au–Fe₃O₄, Ag–Fe₃O₄ and Ag/Au–Fe₃O₄ composite NPs are shown in Fig. 1. For all NPs, three diffraction peaks at 35.6°, 57.3° and 62.7° were indexed to the (311), (511) and (440) planes of the Fe₃O₄ cubic inverse spinel phase. As for Au–Fe₃O₄ composite NPs, four extra diffraction peaks at 38.2°, 44.2°, 64.4° and 77.6° were indexed to the (111), (200), (220) and (311) planes of the Au cubic phase. As for Ag–Fe₃O₄ composite NPs, four extra diffraction peaks at 38.1°, 44.1°, 64.3° and 77.5° were indexed to the (111), (200), (220) and (311) planes of the Ag cubic phase, whereas for Ag/Au–Fe₃O₄ composite NPs, four extra diffraction peaks at 38.3°, 44.3°, 64.6° and 77.7° were indexed to the (111), (200), (220) and (311) planes of the Ag and Au cubic phase. As shown in Fig. 1, Ag/Au–Fe₃O₄ composite NPs show good crystallinity. Therefore, Ag/Au–Fe₃O₄ composite NPs were successfully prepared, which was confirmed by the XRD pattern.

Fig. 1 XRD patterns of the Fe₃O₄ NPs, Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs and Ag/Au–Fe₃O₄ composite NPs.

In order to analyse the morphology of the synthesized Ag/Au–Fe₃O₄ composite NPs, the observations by transmission electron microscopy (TEM) were carried out. Fig. 2 shows the TEM images of the prepared Ag/Au–Fe₃O₄ composite NPs, together with Fe₃O₄ NPs, Au–Fe₃O₄ composite NPs and Ag–Fe₃O₄ composite NPs for comparison. The as-prepared Ag/Au–Fe₃O₄ composite NPs display a narrow size distribution. Fig. 3 shows the high resolution transmission electron microscopy (HR-TEM) images of the prepared Fe₃O₄ NPs, Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs and Ag/Au–Fe₃O₄ composite NPs. As shown in Fig. 3d, the interplanar spacings are 0.204, 0.235, 0.253 and 0.296 nm, which are consistent with the (200) plane of the Ag cubic phase, the (111) plane of the Au cubic phase, the (311) plane and (220) plane of the Fe₃O₄ cubic inverse spinel phase, respectively. The existence of both Au and Ag in the Ag/Au–Fe₃O₄ composite NPs is confirmed by the HR-TEM images. For Fe₃O₄ NPs, the interplanar spacings are 0.209 and 0.296 nm, which are consistent with the (400) plane and (311) plane of the Fe₃O₄ cubic inverse spinel phase, respectively. Similarly, both the Au cubic phase and the Fe₃O₄ cubic inverse spinel phase are found in the Au–Fe₃O₄ composite NPs. In the HR-TEM image of Ag–Fe₃O₄ composite NPs, the Ag cubic phase and the Fe₃O₄ cubic inverse spinel phase are observed. Through the abovementioned analysis of experimental data, the preparation of the Ag/Au–Fe₃O₄ composite NPs is achieved by the one-pot gradient solvothermal process.

Fig. 2 TEM images of (a) Fe₃O₄ NPs, (b) Au–Fe₃O₄ composite NPs, (c) Ag–Fe₃O₄ composite NPs and (d) Ag/Au–Fe₃O₄ composite NPs.

Fig. 3 HR-TEM images of (a) Fe₃O₄ NPs, (b) Au–Fe₃O₄ composite NPs, (c) Ag–Fe₃O₄ composite NPs and (d) Ag/Au–Fe₃O₄ composite NPs.

Typical energy dispersive X-ray spectra (EDS) of the Au–Fe₃O₄ composite NPs, the Ag–Fe₃O₄ composite NPs and Ag/Au–Fe₃O₄ composite NPs are shown in Fig. 4. In Fig. 4a, it indicates the existence of Au, Fe and O in the Au–Fe₃O₄ composite NPs. The amount of Au NPs in the Au–Fe₃O₄ composite NPs is tested by EDS, which is 16.90%. Fig. 4b shows the existence of Ag, Fe and O in the Ag–Fe₃O₄ composite NPs. Similarly, the amount of Ag NPs in the Ag–Fe₃O₄ composite NPs is 25.37%. As shown in Fig. 4c, the amount of Au and Ag NPs in the Ag/Au–Fe₃O₄ composite NPs is 7.20% and 23.25%, respectively. Therefore, the existence of Au and Ag in the Ag/Au–Fe₃O₄ composite NPs was also confirmed by the EDS.

Fig. 4 EDX spectrum of (a) Au–Fe₃O₄ composite NPs, (b) Ag–Fe₃O₄ composite NPs and (c) Ag/Au–Fe₃O₄ composite NPs.

As shown in Fig. 5, the magnetic properties of the synthesized Fe₃O₄ NPs, Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs and Ag/Au–Fe₃O₄ composite NPs were measured by vibrating sample magnetometer (VSM) at room temperature. The magnetization of the synthesized Fe₃O₄ NPs, Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs and Ag/Au–Fe₃O₄ composite NPs, which were recorded at 2 T are 58.558, 37.571, 49.150 and 38.021 emu g⁻¹, respectively. Due to the good saturation magnetization, the Au–Fe₃O₄ composite NPs can be quickly magnetically recycled.

Fig. 5 Magnetic hysteresis curve at room temperature of the Fe₃O₄ NPs, Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs and Ag/Au–Fe₃O₄ composite NPs.

The catalytic performance of the Ag/Au–Fe₃O₄ composite NPs was investigated using the reduction of 4-NP to 4-aminophenol (4-AP) in the presence of NaBH₄ as a model reaction. Fig. 6 showed the typical UV/Visible absorption spectra of 4-NP catalyzed by the Fe₃O₄ NPs, Ag–Fe₃O₄ composite NPs and Ag/Au–Fe₃O₄ composite NPs. After adding the Ag/Au–Fe₃O₄ composite NPs, the peak intensity at 400 nm decreased with concomitant increase in peaks of 4-AP at 230 and 300 nm, as shown in Fig. 6c. The disappearance of the peak at 400 nm indicated the accomplishment of the reaction within 10 min, and the color of the mixture gradually changed from yellow to colorless. Fe₃O₄ NPs didn't have any catalytic performance within 2 h, as shown in Fig. 6a. The same amount of 4-AP was catalyzed by the Ag–Fe₃O₄ composite NPs in Fig. 6b. The reduction of 4-NP catalyzed by the Ag–Fe₃O₄ composite NPs needed 30 min. The Ag/Au–Fe₃O₄ composite NPs have more excellent catalytic performance than Ag–Fe₃O₄ composite NPs. Compared with the previously reported methods,^38,39 Ag/Au–Fe₃O₄ composite NPs show better catalytic performance. Because the reusability of the catalyst is an important issue for practical applications, the reusability of the Ag/Au–Fe₃O₄ composite NPs was further investigated. The Ag/Au–Fe₃O₄ composite NPs were recycled by a magnet. As shown in the Fig. 6d, it was noted that the Ag/Au–Fe₃O₄ composite NPs showed a complete conversion under each cycle. Our Ag/Au–Fe₃O₄ composite NPs can be reused for six cycles, which suggests that the Ag/Au–Fe₃O₄ composite NPs are not deactivated during the catalytic cycle processes. Through the abovementioned analysis of experimental data, the synthesized Ag/Au–Fe₃O₄ composite NPs have excellent catalytic activity for the reduction of 4-NP.

Fig. 6 UV/Vis absorption spectra of 4-NP catalyzed by (a) Fe₃O₄ NPs, (b) Ag–Fe₃O₄ composite NPs and (c) Ag/Au–Fe₃O₄ composite NPs; (d) catalytically recyclable reduction of 4-NP by the Ag/Au–Fe₃O₄ composite NPs in the presence of NaBH₄.

In order to illustrate multifunctional properties of the synthesized Ag/Au–Fe₃O₄ composite NPs, SERS spectra of different amounts of 2-NA on the composite NPs were carried out as shown in Fig. 7a. With the increase of 2-NA solution concentration, SERS effect of the synthesized Ag/Au–Fe₃O₄ composite NPs was more significant. The linear range of solution concentration was 5 × 10⁻⁴ to 5 × 10⁻⁷ mol L⁻¹. Similarly, SERS spectra of 2-NA on the Ag/Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs, Au–Fe₃O₄ composite NPs and glass were carried out, as shown in Fig. 7b. The Ag/Au–Fe₃O₄ composite NPs have more SERS effect than both Au–Fe₃O₄ composite NPs and Ag–Fe₃O₄ composite NPs. Therefore, the synthesized Ag/Au–Fe₃O₄ composite NPs have excellent SERS effect for 2-NA solution.

Fig. 7 (a) SERS spectra of different amounts (5 × 10⁻⁴ to 5 × 10⁻⁷ mol L⁻¹) of 2-NA on the Ag/Au–Fe₃O₄ composite NPs; (b) SERS spectra of 2-NA on the Ag/Au–Fe₃O₄ composite NPs, Ag–Fe₃O₄ composite NPs, Au–Fe₃O₄ composite NPs and glass.

In order to further illustrate multifunctional properties of the synthesized Ag/Au–Fe₃O₄ composite NPs, antibacterial effect by the different concentrations composite NPs for Escherichia coli O1634 were carried out, as shown in Fig. 8. The Ag/Au–Fe₃O₄ composite NPs had clear bacteriostatic ring for Escherichia coli O1634. The average size of antibacterial effect of the composite NPs for Escherichia coli O1634 is as shown in Table 1. Although the concentrations of Ag/Au–Fe₃O₄ composite NPs were 1.0, 1.25, 2.5 and 5 mg L⁻¹, the average sizes of antibacterial effect of the composite NPs for Escherichia coli O1634 were 6.9, 8.6, 9.1 and 9.5 mm. Therefore, the synthesized Ag/Au–Fe₃O₄ composite NPs have excellent antibacterial effect for Escherichia coli O1634.

Fig. 8 Antibacterial effect on the (a) 5.0 mg L⁻¹, (b) 2.5 mg L⁻¹, (c) 1.25 mg L⁻¹, (d) 1.0 mg L⁻¹ Ag/Au–Fe₃O₄ composite NPs for Escherichia coli O1634.

Table 1 The average size of antibacterial effect of the Ag/Au–Fe₃O₄ composite NPs for Escherichia coli O1634

Number	The concentrations of Ag/Au–Fe3O4 composite NPs (mg mL^−1)	The average size (mm)
a	5.0	9.5
b	2.5	9.1
c	1.25	8.6
d	1.0	6.9

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Conclusions

In summary, we have successfully synthesized the Ag/Au–Fe₃O₄ composite NPs by one-pot method, which is based on a gradient solvothermal route. The as-prepared Ag/Au–Fe₃O₄ composite NPs have good crystallinity and saturation magnetization (38.021 emu g⁻¹), resulting in their magnetic recycling. They have good catalytic activity during the reduction of 4-NP in the presence of NaBH₄. The as-prepared Ag/Au–Fe₃O₄ composite NPs have excellent SERS for 2-NA solution. These Ag/Au–Fe₃O₄ composite NPs have good antibacterial effect for Escherichia coli O1634. This work might successfully synthesize similar composite structure by one-pot method on a large-scale and broaden practical applications of this kind of composite nanostructured materials.

Acknowledgements

The supports of the National Natural Science Foundation of China (project no. 61171049, 51273122, 61178035 and 81171454) and the Research Fund for the Distinguished Young Scholars of Sichuan University (2011SCU04B21) are acknowledged with gratitude.

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