Magnetically recyclable catalytic activity of spiky magneto-plasmonic nanoparticles

Abstract

Spiky magnetoplasmonic nanoparticles (NPs) with an Fe₃O₄ core and epitaxial Au branches have been successfully fabricated for the magnetically recyclable catalysis of the 4-nitrophenol reduction. The epitaxial Au branches in the spiky magnetoplasmonic NPs lead to enhanced catalytic activity. Because of high magnetization, the spiky NPs exhibit good separation ability and reusability, which can be repeatedly applied for the nearly complete reduction of 4-nitrophenol for at least four successive cycles. The unique properties make spiky magnetoplasmonic NPs an ideal platform to study various heterogeneous catalytic processes that can be potentially applied in a wide variety of fields in bio-separation, catalysis, and sensing devices.

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Introduction

Novel metal nanoparticles (NPs) exhibiting exceptional physical, chemical, and biological properties have been created that demonstrate high potential for biomedicine, catalysis, sensors, and energy conversion. In particular, Au NPs have been observed to play an important role in several catalytic processes, including low-temperature CO oxidation, reductive catalysis of chlorinated or nitrogenated hydrocarbons, and organic synthesis, because their large surface-area-to-volume ratio allows effective utilization of Au NPs.^1–5 However, the high surface energy of Au NPs often leads to aggregation and decreased catalytic activity. Moreover, the high cost and limited supply has severely restricted the applications of nanocatalysts. Therefore, the improvement of catalytic efficiency and reduction of the use amounts of catalyst are the top priorities for practical applications.^6–8

Multifunctional magnetoplasmonic nanomaterials have recently received much attention because of their unique optical, catalytic, and electrochemical properties, which make them suitable materials for potential applications in various fields.^9–14 Typically, magnetoplasmonic nanomaterials are composed of gold and magnetic nanomaterials. The magnetic nanomaterials as a core can be viable alternatives to conventional materials for catalyst supports.^7,8,15 Their insoluble and superparamagnetic natures enable trouble-free separation of the nanocatalysts from the reaction mixture using an external magnet, which eliminates the necessity of catalyst filtration. Moreover, the combination of Au nanomaterials with a magnetic core not only provides catalytic activity but can also be reclaimed via magnetic separation after use.⁷ The electron transfer across the interface between Au and Fe₃O₄ leads to a dramatic change in the physicochemical properties, thus offering an ideal platform to study the multifunctionality of nanomaterials.^16–20

In the previous reports, many researchers developed various novel nanocatalysts for catalytic reduction of 4-nitrophenol. Chen et al. report one-pot method to synthesize core–shell Au@resorcinol–formaldehyde (RF) nanospheres with multiple cores as catalyst to reduce 4-nitrophenol, which exhibit high catalytic activity and recyclability after five cycles.²¹ An et al. fabricated a nanocomposite catalyst containing both magnetite (Fe₃O₄) and palladium (Pd) nanoparticles with high catalytic activity and recyclability.²² Lin et al. fabricated the dumbbell- and flower-like Au–Fe₃O₄ heterostructures by thermal decomposition of iron–oleate complex using Au NPs as seed for magnetically recyclable catalysis. The morphologies of Fe₃O₄ in dumbbell- and flower-like Au–Fe₃O₄ heterostructures are different, leading to the change in catalytic property of the nanocatalysts.⁶

Previously, we reported a unique metallic core–shell nanocomplex, i.e., spiky gold-coated iron supraparticles (Fe₃O₄@Au SPs) with diameters of 105–185 nm and strong magnetization using the self-assembly method, which has received considerable attention because of the unique surface morphology and electromagnetic bifunctionality.^13,23 This structure exhibits unique physiochemical properties because of their well-defined shape and distinctive topological structure. However, the separation process is a bottleneck for the operation of this nanomaterial for catalysis. Even though the separation is technically quite easy using magnetic NPs and external magnetic force, the process requires over 12 h, and many additional processes are performed to ensure complete separation from reaction system.¹⁴ Therefore, rapid magnetically catalytic reduction has become increasingly important for magnetoplasmonic nanomaterials.

Herein, we developed a facile, efficient route for the synthesis of a novel spiky morphology of monodisperse Fe₃O₄@Au magnetoplasmonic NPs (as shown in Fig. 1). A magnetic core with a size of 100 nm was used for effective separation in terms of quantity and time. The epitaxial Au branches in the spiky magnetoplasmonic NPs lead to an enhanced catalytic property. Therefore, the spiky NPs exhibit excellent dual functions, which cannot only undergo rapid catalytic reduction of 4-nitrophenol in the presence of NaBH₄ but also be easily recycled using an external magnetic field.

Fig. 1 Illustration of the synthetic chemistry of spiky Fe₃O₄@Au nanocatalysts.

Experimental Section

Materials

Polyethyleneimine (PEI, branched, M.W., ∼10 000), iron(III) chloride hexahydrate (FeCl₃·6H₂O), hydrogen tetrachloroaurate(III) trihydrate (HAuCl₄·3H₂O, 99.9%), sodium borohydride (NaBH₄), sodium acetate anhydrous (NaAc), trisodium citrate (C₆H₅Na₃O₇·3H₂O), 4-nitrophenol, and hydroquinone were obtained from Sigma-Aldrich. Deionized water (>18.2 mΩ cm⁻¹) was used throughout the experimental procedure. All the chemicals were of analytical grade and were used as received.

Synthesis of PEI-coated Fe₃O₄ NPs

The Fe₃O₄@PEI NPs were prepared via the one-pot solvothermal method following the strategy developed by Wang et al.²⁴ First, 0.68 g FeCl₃·6H₂O was dissolved in 20 mL of ethylene glycol by ultrasonic treatment to form a transparent solution, and 1.8 g NaAc and 0.5 g PEI were added to the reaction solution. The mixture was stirred vigorously for 20 min at 60 °C and then transferred to an autoclave and reacted at 220 °C for 2 h. The resultant black products were rinsed with water and ethanol several times in the presence of an external magnet and dried in a vacuum.

Synthesis of spiky Fe₃O₄@Au magneto-plasmonic NPs

For the synthesis of spiky magnetoplasmonic NPs, 4 nm Au NPs were first prepared according to the method reported by Murphy et al.^25,26 Then, 2 mg of the well-dried Fe₃O₄@PEI NPs was sonicated in 10 mL of Au NP solution for 10 min and then mechanically stirred at room temperature for 2 h. The Fe₃O₄@PEI–Au NPs were then magnetically separated from the mixture and washed several times with water. Finally, the Fe₃O₄@PEI–Au NPs were dispersed in 2 mL of deionized water.

The spiky Au-shell coating was achieved via reduction of Au³⁺ on the surface of the Fe₃O₄@PEI–Au using the self-assembly method described in our previous publication.²³ Typically, 100 μL of aqueous HAuCl₄ (20 mM) was added into 2 mL of Fe₃O₄@PEI–Au solution with vigorous stirring. Subsequently, 1 mL of hydroquinone (30 mM) was added dropwise. The solution was then stirred at room temperature for 30 min. The final product was magnetically washed several times with water.

Catalytic reaction

The reduction of 4-nitrophenol by water-soluble spiky Fe₃O₄@Au magnetoplasmonic NPs in the presence of NaBH₄ was performed to examine the catalytic activity and recyclability of the spiky Fe₃O₄@Au nanocatalysts. Amounts of 0.5 mL of deionized water, 1 mL of 20 mM 4-nitrophenol, and 2 mL of 10 mM NaBH₄ solutions were added into a quartz cuvette followed by the addition of 0.25 mg of spiky Fe₃O₄@Au NPs to the mixture. The solution color changed rapidly from yellow to transparent as the reaction proceeded. UV-vis spectrometry was used to record the change in absorbance at a time interval of 1 min.

Optical and microscopic characterization

The absorbance of the spiky Fe₃O₄@Au NPs was measured using UV-vis spectroscopy (SCINCO, S310, South Korea). The morphologies and sizes of the NPs were characterized using high-resolution transmittance electron microscopy (HRTEM, JEOL, JEM-3010, Japan). The surface potentials and particle size distribution of the NPs were monitored using a zeta-sizer (Malvern Instruments, Nano ZS, UK). Magnetic measurements were performed using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL-7, USA) with a maximum applied continuous field of 30 000 Oe at room temperature.

Results and discussion

The morphologies of the magnetoplasmonic NPs were characterized by TEM. We first synthesized PEI-coated Fe₃O₄ (Fe₃O₄@PEI) NPs using the solvothermal method. As illustrated in Fig. 2A and B, the as-synthesized Fe₃O₄@PEI NPs were monodisperse, spherical, and 100 nm in size. Moreover, we have measured the FT-IR spectra of pure PEI and Fe₃O₄@PEI NPs to prove PEI coated on the surface of Fe₃O₄ NPs. Fig. S1(a)† shows the FT-IR spectra of the pure PEI, the peaks around 1310 and 1652 cm⁻¹ were assignable to scissoring vibration of –NH₂, while the peaks at 2836 and 2954 cm⁻¹ could be attributed to the asymmetric and symmetric stretching vibrations of –CH₂– in PEI, respectively. Fig. S1(b)† shows the FT-IR spectra of the as-prepared Fe₃O₄@PEI, which observed the characteristic absorption peak of Fe₃O₄ at 598 cm⁻¹. Furthermore, the peaks around 1305 and 1646 cm⁻¹ were in agreement with the –NH₂-scissoring vibration in PEI. Upon the Au NPs adsorption on the Fe₃O₄ NPs, negative charged Au NPs combined with the positive charged PEI of the Fe₃O₄ NPs via electrostatic attraction, and ionic bonds were established. The zeta potentials of the Fe₃O₄@PEI NPs and Au NPs were +33 mV and −22.4 mV, respectively (Fig. S2†). Therefore, the Fe₃O₄@PEI NPs were further functionalized by the Au NPs through the electrostatic force. The PEI on the surface of the Fe₃O₄ NPs induced amino groups, which could combine Au NPs with the surface of the magnetic core. The TEM images in Fig. 2C and D demonstrate that the Fe₃O₄@PEI NPs were well surrounded by 4 nm Au NPs to form core-satellite nanostructures. Using Au NPs on the surface of Fe₃O₄ NPs as seeds, the epitaxial Au branches were grown on the surface of Fe₃O₄ NPs to form the spiky magnetoplasmonic NPs, as illustrated in Fig. 2E and F. A previous study demonstrated that the Au seeds can be prepared under the reduction of hydroquinone to increase the branching of the final nanostructures.²³ In this study, we used the core-satellite Fe₃O₄@Au NPs described above to seed the growth of spiky Fe₃O₄@Au NPs in aqueous hydroquinone solution, a condition that is known to promote anisotropic growth.

Fig. 2 TEM and HRTEM images of Fe₃O₄@PEI NPs (A and B), core-satellites Fe₃O₄@Au NPs (C and D), and spiky Fe3O4@Au NPs (E and F).

The STEM images of Fe₃O₄ and spiky Fe₃O₄@Au NPs revealed that the Au NPs were well dispersed on the Fe₃O₄@PEI surface, as shown in Fig. 3A and B. The brightness in the image reflects the intensity of the scattered electrons from different substances and is proportional to the atomic number (Z). In the micrographs, the Au on the surface of the Fe₃O₄ NPs, which have higher Z compared with the Fe₃O₄ NPs, are imaged as brighter dots. The line mapping of the selected NPs indicates the inclusion of an Fe core and epitaxial Au branches as a single Fe₃O₄@Au nanostructure (Fig. 3C). This result indicates that Fe, Au, and O are present in the same NPs. To investigate the surface modification, XPS measurement was also carried. Fig. S3† showed the XPS spectra of the spiky Fe₃O₄@Au NPs. The feature peak of Fe2p and O1s for iron oxide were identified at 710.96 eV and 530.2 eV. Moreover, the peak ascribed to Au4f, C1s and N1s indicated the existence of Au and PEI coated on the surface of Fe₃O₄ NPs.

Fig. 3 STEM images of Fe₃O₄@PEI NPs (A) and spiky Fe₃O₄@Au NPs (B); line map curve of spiky Fe₃O₄@Au NPs in dark field.

The spiky Fe₃O₄@Au NPs can be separated using a magnet, as shown in inset (a) of Fig. 4. These results indicate that the spiky Fe₃O₄@Au NPs were separable using high-gradient magnetic filtration. After magnetic separation, the Fe₃O₄@Au NPs were still well dispersed in aqueous solutions (data not shown). The magnetic property of the spiky Fe₃O₄@Au NPs was measured using the SQUID technique. Fig. 4 shows the spiky Fe₃O₄@Au NPs hysteresis loops measured at room temperature. The saturation magnetization (Ms) of the spiky Fe₃O₄@Au NPs was 75 emu g⁻¹ at room temperature.

Fig. 4 Magnetization moments estimated for spiky Fe₃O₄@Au NPs at room temperature. Inset (a) shows the spiky Fe₃O₄@Au NPs entirely separated by a magnet.

The catalytic reduction of 4-nitrophenol in the presence of NaBH₄ was selected as a model reaction to investigate the activity of the core-satellite Fe₃O₄@Au NPs and spiky Fe₃O₄@Au NPs. The reaction is particularly easy to follow because only one product, 4-aminophenol, forms, and the extent of reaction can be determined by measuring the change in the UV-vis absorbance at 400 nm. In general, the catalytic activity is also related to the amount of gold on Fe₃O₄ NPs. Therefore, we measured the Au and Fe content of core-satellite Fe₃O₄@Au NPs and spiky Fe₃O₄@Au NPs by Inductively Coupled Plasma (ICP), as shown in Table S1.† We observed that there is 2.58 ppm Au in the core-satellite Fe₃O₄@Au NPs, while the loading amount of gold of spiky Fe₃O₄@Au NPs is 6.98 ppm. To avoid the impact of Au content for the catalytic activity of core-satellite Fe₃O₄@Au NPs and spiky Fe₃O₄@Au NPs, we adjusted the concentration of spiky Fe₃O₄@Au NPs to make the same loading amount of gold with core-satellite Fe₃O₄@Au NPs for the catalytic reaction. Fig. 5 presents typical UV-vis spectra and reveals the concentration change of the 4-nitrophenol compounds in the presence of Fe₃O₄@Au NPs and NaBH₄. The original absorption peak of 4-nitrophenol is centered at 317 nm and shifts to 400 nm after the addition of freshly prepared NaBH₄ solution, indicating the formation of 4-nitrophenolate ions. The peak starts to decrease when the reduction proceeds in the presence of Fe₃O₄@Au nanocatalysts. Furthermore, the absorption peak at 400 nm decreases with the concomitant increase in the peak intensity at 300 nm within 8 min after the addition of spiky Fe₃O₄@Au nanocatalysts (Fig. 5B), whereas the catalytic reaction can be finished after 16 min with the addition of core-satellite Fe₃O₄@Au NPs (Fig. 5A). The new absorption peak at 300 nm is the characteristic peak of 4-aminophenol, indicating the reduction of 4-nitrophenol to form 4-aminophenol.

Fig. 5 UV-vis spectral changes in 4-nitrophenol catalyzed by (A) core-satellite Fe₃O₄@Au NPs and (B) spiky Fe₃O₄@Au NPs. The linear relationship of ln (C/C₀) as a function of time for 4-nitrophenol catalyzed by (C) core-satellite Fe₃O₄@Au NPs and (D) spiky Fe₃O₄@Au NPs.

The first-order kinetics was used to evaluate the rate constants for the 4-nitrophenol reduction by core-satellite Fe₃O₄@Au NPs and spiky Fe₃O₄@Au NPs. The concentration of 4-nitrophenol at time t is denoted as C, and the initial concentration of 4-nitrophenol at t = 0 is denoted as C₀. C/C₀ is measured from the relative intensity of the absorbance (A/A₀). The rate constant k for 4-nitrophenol reduction is 0.0116 min⁻¹ using core-satellite Fe₃O₄@Au NPs (Fig. 5C), whereas the rate constant k for 4-nitrophenol reduction catalyzed by spiky Fe₃O₄@Au NPs is 0.058 min⁻¹ (Fig. 5D). These results clearly indicate that the catalytic efficiency as well as the rate constants for nitrophenol reduction by spiky Fe₃O₄@Au NPs is higher than those of core-satellite Fe₃O₄@Au NPs. Moreover, it also demonstrated that the epitaxial Au branch nanostructures on the Fe₃O₄ NPs enhanced catalytic activity.

Considering that the amount of catalysts was the same, this difference of catalytic activity is due to the following factors: the spiky magnetoplasmonic NPs have epitaxial Au branches, implying their higher surface area compared with core-satellite Fe₃O₄@Au NPs. When spiky Fe₃O₄@Au NPs are used for catalytic reduction, BH⁴⁻ and 4-nitrophenol are first diffused from the aqueous solution to the surface of the nanoparticles, and then, the exposed Au on the spiky NPs serves as a catalyst to transfer electrons from BH⁴⁻ to 4-nitrophenol, leading to the production of 4-aminophenol.⁶ Nevertheless, the absorption peak at 400 nm did not significantly change when the Fe₃O₄ NPs were added (Fig. S4 in the ESI†). This result also revealed the catalytic activity of spiky Fe₃O₄@Au NPs from another viewpoint.

The as-prepared spiky Fe₃O₄@Au NPs exhibited both catalytic and magnetic properties after the catalytic reduction. Fig. 6 shows the magnetically recyclable reduction of 4-nitrophenol in the presence of spiky Fe₃O₄@Au nanocatalysts. The catalysts can be successfully recycled and reused for at least four successive cycles of reaction with a stable conversion efficiency of approximately 100%. It is apparent that the presence of the Fe₃O₄ core makes the spiky magnetoplasmonic NPs a promising bifunctional probe for magnetically recyclable catalytic reduction. When the reduction is complete, the spiky nanocatalysts can be easily and rapidly collected from the solution within 15 s using an external magnetic field and then redispersed into the reaction solution for the next cycle of catalysis. We also measured the TEM image of spiky Fe₃O₄@Au NPs after four successive cycles of reaction, as show in Fig. S5.† We observed that the gold branch still immobilized on the surface of Fe₃O₄ NPs.

Fig. 6 Catalytically recyclable reduction of 4-nitrophenol by spiky Fe₃O₄@Au NPs in the presence of NaBH₄.

Conclusion

In this study, we developed a facile, efficient route for the synthesis of spiky Fe₃O₄@Au magnetoplasmonic nanocatalysts. These nanoparticles exhibit dramatic bifunctionality for reusability and catalytic reduction in successive cycles of reduction. The catalytic performance of spiky Fe₃O₄@Au NPs is excellent for 4-nitrophenol reduction in the presence of NaBH₄. This research opens an avenue to the preparation of highly efficient nanocatalysts for practical applications.

Acknowledgements

This study was supported by grants from the Natural Science Foundation of China (Grant No. 21177132 and 51272255); and the National Key Basic Research Program of China (Grant No. 2013CB934303); and also supported by a grant of the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (HI13C0862); and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013004637; 2014R1A1A2007222).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08425c

‡ These authors contributed equally to this work.

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