Preparation and characterization of magnetic gold shells using different sizes of gold nanoseeds and their corresponding effects on catalysis

Abstract

The effect of gold nanoseeds with different sizes on the gold shell was investigated. Gold nanoparticles of two different sizes (∼3 nm and ∼15 nm) were prepared and attached to the surface of amine functionalized silica coated iron oxide nanoparticles. The gold nanoparticles assembled on the surface were used as seeds for further gold shell formation. It was observed that the amount of Au attached to iron oxide nanoparticles is higher for bigger gold nanoseeds as compared to smaller gold nanoseeds. Similarly, after the formation of gold shell, a higher amount of Au was found for larger gold nanoparticles. However, both transmission electron microscopy (TEM) and scanning electron microscopy (SEM) results show that a complete, uniform, and compact gold shell was formed in the case of using small gold nanoseeds, but for larger gold nanoseeds the shell formed was discontinuous and was not uniform, while most of the gold nanoparticles were found to be aggregated on the surface. The nanocomposites showed high efficiency in catalysis for the reduction of 4-nirophenol, among which Nanocomp-2, with a thin stable gold shell showed excellent catalytic activity and reusability. All of the nanocomposites have high magnetization values and can be easily separated from the reaction mixture using a magnet and can be reused.

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1. Introduction

Multifunctional nanocomposites are important and find many applications because of their unique properties arising from different components. Core shell nanocomposites have received great attention in the field of medicine, catalysis, optical devices, and biosensing. Iron oxide nanoparticles due to their magnetic, optical, and electrical properties, which are different from that of bulk material, have been widely investigated for various applications, such as magnetic resonance imaging (MRI),^1,2 drug delivery,^3,4 hyperthermia,^5,6 protein immobilization and separation,^7,8 catalysis,^9,10 and so on.

Various types of materials are used to stabilize nanoparticles including organic polymers, silica, metals, non metals, metal oxides etc.^11–15 However coating with noble metals such as gold nanoparticles, have been widely used as shell which also endow the core new functionalities in addition to stability and biocompatibility. Au nanoparticles have unique optical, electric, and catalytic properties.^16–19 Thus the combination of the two nanomaterials will result in nanostructures with combined magnetic and optical properties, and results in very important functional materials which have been used for wide beneficial applications in separation of proteins,^20,21 catalysis,^22,23 biosensing,^24,25 and photothermal therapy etc.²⁶ Due to the different nature of two surfaces it is difficult to coat metals, such as Au on the surface of iron oxide nanoparticles and may not be able to produce absorption in the near infra-red (NIR) region,²⁷ an important property of gold nanoshells which is used for biomedical applications. Various techniques have been utilized to fabricate magnetic iron oxide nanoparticles with gold. Some polymers such as poly (allylamine hydrochloride) (PAH), polypyrrole, and polyaniline etc., have been used for iron oxide gold core–shell nanocomposites.^28–30 However silica which is stable, biocompatible, and with good surface chemistry, has been widely used for gold nanoshells.^31,32 The surface of silica is negatively charged due to the silanol groups and the electrostatic repulsion limits the attachment of gold nanoparticles. Therefore some functional groups, usually with a positive surface charge, are attached to the silica surface which act as ligands for gold nanoparticles. Wu et al. reported the preparation of gold coated magnetite nanoparticles through easy and quick method of sonolysis using amino-modified magnetite nanoparticles as seeds and the reduction of HAuCl₄ on their surface, but the gold nanoparticles did not cover the whole surface of iron oxide nanoparticles.³³ Similarly, Wang et al. used polymer coated iron oxide nanoparticles for coating with gold shell.²⁸ The absorption maximum was shown to be shifted to NIR region, but the TEM results showed that iron oxide nanoparticles were not completely covered with gold shell, while some aggregation of gold nanoparticles were observed on the surface. Recently, in situ method to reduce Au³⁺ by Sn²⁺ and subsequently deposit highly dispersed gold nanoparticles directly onto the surface of Fe₃O₄@SiO₂ magnetic nanocomposites was also reported.³⁴

The surface coverage degree and hence the shell thickness depends on various factors, like the type of link molecules on silica surface, solvent, pH, etc., and have been extensively investigated.³⁵ However, the effect of size of gold nanoseeds on shell uniformity and thickness is scarcely reported. It is very important to stabilize the catalysts so that they can be used in harsh conditions as well as can be recycled many times without the loss in activity. Different strategies have been utilized in order to achieve these goals. The core-satellite structure, in which the catalyst is protected with silica shell and then by selectively etching the silica shell with NaOH as etchant for producing pores has been reported.³⁶ Similarly, Lee et al. reported yolk–shell type structure also called as nanorattle for stabilization of the gold catalyst.³⁷ Recently, stable gold catalysts with magnetic properties and an outer mesoporous silica shell have been synthesized using CTAB as template for producing mesopores which is subsequently extracted with acetone refluxing.^23,38 The catalytic reaction takes place because of mesopores in silica shell allowing reactants to diffuse in and out.

However these reported approaches are laborious and require many steps. It is therefore, necessary to prepare core–shell structures for catalyst stability and good recyclability through simple steps. In this study we utilized the core–shell strategy to prepare stable magnetic gold shells using two different sizes of gold nanoseeds and then their effects on catalysis as well as reusability were investigated.

2. Experimental

2.1. Materials

Ferric chloride hexahydrate (FeCl₃·6H₂O), tetraethyl orthosilicate (TEOS), and trisodium citrate dihydrate were purchased from Tianjin Guangfu Technology Development Co., Ltd. (Tianjin, China). HAuCl₄·4H₂O was obtained from Fengyue Chemical Company (Tianjin, China). 3-Aminopropyl-triethoxysilane (APTES) and tetrakis (hydroxymethyl) phosphonium chloride (THPC) were purchased from Sigma-Aldrich. Hydroxylammonium chloride was provided by Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All other chemicals were of analytical grade and used as received. Deionized water was used throughout the experiment for solutions and dispersions preparation.

2.2. Methods

2.2.1. Synthesis of magnetic Fe₃O₄ nanoparticles. Iron oxide nanoparticles were prepared through solvothermal method. Briefly, FeCl₃·6H₂O (2.02 g) and sodium acetate (4.10 g) were quickly added into a mixed solution of ethylene glycol (50 mL) and stirred for about 30 min. The obtained yellow solution was transferred to a Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h. The autoclave was then naturally cooled to room temperature. The obtained black magnetite particles were washed three times with water and two times with ethanol, and dried in vacuum at 60 °C.

2.2.2. Synthesis of Fe₃O₄@SiO₂ and APTES-modified Fe₃O₄@SiO₂ nanocomposites. First Fe₃O₄@SiO₂ nanoparticles were prepared through sol–gel method in which Fe₃O₄ (0.10 g) nanoparticles were dispersed in ethanol and water mixture (100 mL, 4 : 1 v/v) through ultrasonication. A concentrated aqueous solution of ammonia (3.0 mL, 28 wt%) was added into the mixture. Then TEOS (80 μL) was added very slowly drop by drop. The reaction mixture was stirred for 10 h at room temperature, then separated and washed with water and ethanol, finally dried in vacuum. The surface modification with amine groups was carried out by refluxing a mixture of silica coated iron oxide nanoparticles (130 mg) with APTES (1.50 mL) in toluene for 10 h. The obtained APTES-modified Fe₃O₄@SiO₂ nanoparticles were collected with a magnet, washed with ethanol and water, then dried in vacuum.

2.2.3. Synthesis of Au nanoparticles. Two types of gold nanoparticles were prepared according to literature.^39,40 Citrate-stabilized Au nanoparticles (∼15 nm) were prepared as 100 mL aqueous solution of HAuCl₄ (5.0 × 10⁻⁴ M) was stirred under heating in a three-neck round-bottom flask. On boiling, trisodium citrate solution (5 mL, 1 wt%) was injected and its stirring was continued further for about 30 min. Small colloid Au nanoparticles (∼2 to 4 nm) were prepared by the reduction reaction of HAuCl₄ using THPC as reducing agent. In order to grow these small gold nanoparticles, the prepared gold nanoparticles were then kept in refrigerator for one to two weeks, and concentrated by rotary evaporator.

2.2.4. Preparation of Fe₃O₄@SiO₂@Au nanoseeds and Fe₃O₄@SiO₂@Au–shell. For preparation of Fe₃O₄@SiO₂@Au nanoseeds (Au 15 nm), 100 mL Au nanoparticles solution was mixed with 100 mL aqueous dispersion of APTES-modified Fe₃O₄@SiO₂ nanocomposites (0.04 g) through ultrasonication, and then stirred for 4 h at ambient temperature. For Fe₃O₄@SiO₂@Au nanoseeds (Au 3 nm), a dispersion of 10 mL (0.1 mg mL⁻¹), was mixed with colloidal gold nanoparticles (15 mL) and stirred for 3 h at ambient temperature. Fe₃O₄@SiO₂@Au nanoseeds were washed with deionized water and dried in vacuum.

For gold shell formation first potassium–gold (K–gold) solution was prepared by dissolving K₂CO₃ (0.025 g) in 100 mL of water. After stirring for 10 min, HAuCl₄ solution (1 mL, 1 wt%) was added to it while stirring for about 30 min and then aged overnight in a refrigerator at 4 °C. Gold nanoshells were prepared by stirring of 100 mL of K–gold solution with the dispersion of Fe₃O₄@SiO₂@Au nanoseeds (10 mL, 1 mg mL⁻¹) using hydroxylammonium chloride solution as a reducing agent forming violet to blue color formation which indicated the formation of gold shell. The formation of gold shell was also confirmed by its UV-vis absorption spectra. The obtained Fe₃O₄@SiO₂@Au nanoshells were washed with deionized water and dried in vacuum. Depending on the size of gold nanoseeds used, the four types of nanocomposites with gold nanoseeds and gold shells can be named as; Fe₃O₄@SiO₂@Au nanoseeds (Au 3 nm) = Nanocomp-1, Fe₃O₄@SiO₂@Au shell (Au 3 nm) = Nanocomp-2, Fe₃O₄@SiO₂@Au nanoseeds (Au 15 nm) = Nanocomp-3, and Fe₃O₄@SiO₂@Au shell (Au 15 nm) = Nanocomp-4.

2.2.5. Catalytic reduction of 4-nitrophenol (4-NP). In order to investigate the catalytic behavior of the prepared nanocomposites the reduction of 4-NP with NaBH₄ was carried out in the presence of nanocomposites. A very small amount of nanocomposites (3.75 × 10⁻³ mg mL⁻¹) was added into the equal mixture of 4-NP (0.2 mM) and freshly prepared NaBH₄ (80 mM) and the reduction was studied by recording the spectra at specific interval of time. The reaction was carried out at room temperature for all the nanocomposites.

2.2.6. Characterization. The morphology of the nanoparticles and nanocomposites was determined by JEM-2100 transmission electron microscope (TEM) (Japan) operated at 200 kV and Hitachi S-4800 scanning electron microscope (SEM) (Japan) which was also used for energy-dispersive X-ray spectroscopy (EDX) analysis. X-ray diffraction (XRD) pattern of the product was performed on an X'Pert pro Philips X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) and scan range (2θ) was from 10° to 90° and magnetic properties of the samples were measured vibrating sample magnetometer (VSM, Lakeshore 730) at room temperature. FT-IR measurements were carried out on a Nicolet Nexus 670 FT-IR spectrometer (GMI Inc., MN, America) equipped with a Germanium attenuated total reflection (ATR) accessory, a DTGS KBr detector and a KBr beam splitter, at room temperature. All spectra were taken via attenuated total reflection method with resolution of 4 cm⁻¹ and 60 scans. Zeta potential and dynamic light scattering (DLS) were carried out using a Zetasizer Nano ZS/ZEN 8600 (Malvern instruments). Thermo Scientific ESCALAB 250Xi with a monochromatic Al Kα X-ray source operated at 200 W was used to record X-ray photoelectron spectra (XPS). A double-beam TU-1901 UV-visible spectrophotometer (Beijing Purkinje General Instrument Co., Ltd. China) was used to measure the absorption spectra of nanoparticles.

3. Results and discussion

3.1. Characterization of the prepared nanocomposites

The morphology of iron oxide nanoparticles prepared through solvothermal method is shown in Fig. 1. Both TEM and SEM results indicated that the particles are roughly spherical in shape with a wide range of size 200–400 nm. The same results were also obtained from DLS analysis as given in the inset of Fig. 1. The outer surface can be seen as rough, which reflects that the iron oxide nanoparticles are actually formed of many nanocrystals. The nanoparticles were then coated with a thin silica layer (∼30 nm) using TEOS as silica precursor. This silica layer provides a surface for attaching amine functional groups by reacting silica coated iron oxide nanoparticle with APTES. The presence of amine groups on the surface of iron oxide is very important which are responsible for attachment of gold nanoseeds on the surface of iron oxide nanoparticles.

Fig. 1 TEM (a) and SEM (b) images of iron oxide nanoparticle. Inset is the size distribution of iron oxide nanoparticles.

FTIR results of the magnetic iron oxide nanoparticles, silica coated nanoparticles, and that of amine functionalized silica coated iron oxide nanoparticles are given in Fig. S1 (ESI†), in which the important peaks have been given. The spectra of Fe₃O₄ nanoparticles showed peaks at 582 and 3421 cm⁻¹ which are due to the Fe–O, and hydroxyl groups respectively. The new absorption peaks on Fe₃O₄@SiO₂ nanoparticles were Si–O–Si (1076 cm⁻¹), Si–O (443 cm⁻¹), and the surface silanol groups Si–OH (955 cm⁻¹), these spectra indicated the presence of silica (Fig. S1b†). After reaction with APTES the bands arising from Si–O groups are at 1154 and 1080 cm⁻¹. C–H stretching vibration of the propyl appears at 2925 cm⁻¹. The broad band appearing at 3431 cm⁻¹ can be assigned to the N–H stretching vibration, and the other appearing at 1631 cm⁻¹ to the bending mode of NH₂ group (Fig. S1c†). However the results of the FTIR are not so distinct, therefore to prove the presence of amine groups, XPS analysis was done for Fe₃O₄@SiO₂–NH₂ nanoparticles. It can be seen from Fig. S2 (ESI†) that the peak assigned to N1s at 399.4 eV is distinct and clearly indicated. The peak for N1s is also shown in the inset of figure. This confirmed that amine groups are attached to silica coated iron oxide nanoparticles. Zeta potential is an important technique used to know about the surface charge of nanoparticles. Since amine groups were introduced on the surface of silica, it is necessary to measure the zeta potential of the amine functionalized nanoparticles. Silica is negatively charged due to –S–OH groups and therefore will not favor the attachment of negatively charged gold nanoparticles because the latter are mainly assembled through electrostatic attraction on alkylamines.²³ The results of zeta potential at different pH are shown in Fig. S3 (ESI†). It can be seen that the amine functionalized silica coated iron oxide nanoparticles have very high positive potential (+21 mV) even at basic pH of 8. This proved the successful attachment of NH₂ groups and showed that the resulting nanoparticles are richly covered with amine groups and are highly positively charged. Thus the nanoparticles provide a better surface for attaching gold nanoseeds.

The gold nanoseeds prepared were of two different sizes. The size of gold nanoseeds prepared through the reduction of HAuCl₄ using sodium citrate as reducing agent were observed to be about 15 nm from the results of TEM and SEM of Nanocomp-3. Similarly, the gold nanoseeds prepared by the reaction of HAuCl₄ with THPC were observed by TEM to be very small (2–4 nm). The gold nanoparticles prepared were reddish in color (15 nm) or brownish red (3 nm) with absorption maximum at about 520 nm (Fig. 2a and c) gold nanoparticles were attached to amino modified iron oxide nanoparticles simply by dispersing and stirring which resulted in richly covered surfaces. These gold nanoparticles attached to iron oxide nanoparticles act as seeds for gold shell formation. Gold ions were reduced on the surface of Fe₃O₄@SiO₂@Au nanoseeds using hydroxylamine as reducing agent which led to the formation of gold shells. This was proved by recording the UV-vis spectra of nanocomposites dispersed in deionized water. As shown in Fig. 2(a–d), the band of plasmon resonance was red-shifted from 520 nm to near infra-red (NIR), above 800 nm. This plasmon resonance property of gold nanoshells in NIR is very important and can be used for photothermal therapy because in this region the blood and tissues show minimum light absorption.⁴¹ Coating thickness is controlled by the ratio of gold solution amount and gold nanoseeds coated iron oxide nanoparticles, hence the plasmon resonance and the optical absorbance can be controlled which depends on the ratio of core and gold shell. Plasmon resonance also depends upon the morphology and surrounding medium.⁴² The colors of the nanocomposites also reflect the formation of gold shells as shown in the insets of Fig. 2(b and d), indicating a bluish color for both nanocomposites (Nanocomp-2 and Nanocomp-4).

Fig. 2 UV-vis spectra of (a) colloidal Au (3 nm) solution, (b) Nanocomp-2, (c) colloidal Au (15 nm) solution, and (d) Nanocomp-4.

Fig. 3 (a–d) shows the TEM of prepared nanocomposites with gold nanoseeds of 3 nm (Nanocomp-1 and Nanocomp-2). It can be seen that the silica surface of silica coated iron oxide nanoparticles is completely covered with very small Au nanoseeds (Nanocomp-1). When Au ions are reduced on the surface covered with these nanoseeds, a complete and uniform gold shell is formed as shown in Fig. 3c and d (Nanocomp-2). Similarly the corresponding SEM images of Nanocomp-1 and Nanocomp-2 are shown in Fig. S4 (ESI†), which also indicated that a compact and homogeneous gold shell was resulted using 3 nm gold nanoseeds. TEM images for nanocomposites prepared using 15 nm gold nanoseeds (Nanocomp-3, and Nanocomp-4) show that a large number of gold nanoseeds are present on the surface as can be seen in Fig. 4(a and b). When the gold solution was reduced on the surface a thick shell of gold was formed. However the gold shell is not uniform and is discontinuous. Large number of gold aggregates are found on the surface (see Fig. 4c and d). The corresponding SEM images of the nanocomposites are shown in Fig. S5 (ESI†), and further support this fact. A large number of gold nanoseeds can be observed on the surface, but the nanoseeds are far apart from one another. Also some nanoseeds are found to be aggregated on the surface. This results in a discontinuous gold shell with large voids on the surface and large aggregates of gold nanoparticles. This is because the interaction of small gold nanoparticles with one another is small and therefore densely and uniformly distributed on the surface covering it completely. While in the case of large size gold nanoparticles (∼15 nm) the interaction with one another is high and the electrostatic repulsion causes the particles to be far apart from one other. The surface covering with gold nanoseeds in this case is not uniform and large spaces are found among the nanoseeds. When the gold solution is reduced on such surface the shell formed is not uniform and thus results in larger aggregates of gold nanoparticles irregularly arranged on the surface.

Fig. 3 TEM images of (a and b) Nanocomp-1 (c and d) Nanocomp-2.

Fig. 4 TEM images of (a and b) Nanocomp-3 and (c and d) Nanocomp-4.

The nanocomposites with gold nanoseeds and gold nanoshells were also analyzed using energy-dispersive X-ray spectroscopy (EDX) technique (Fig. S6, ESI†). All the spectra show peaks of all the precursor elements in nanocomposites such as Fe, Si, O, and Au. For Nanocomp-1 in which 3 nm gold nanoseeds were used, only 3.6% of Au by weight was found. This small amount corresponds to small sizes of the gold nanoparticles. After gold shell formation this amount increased to 21.6%, indicating a thin gold shell on the surface. The amount of Au for Nanocomp-3, with 15 nm gold nanoparticles was 13% which is more than that of Nanocomp-1. This increased amount corresponds to the larger sizes of gold nanoseeds. Similarly, on gold shell formation this amount increased to 26% indicating thick shell and large aggregates of gold nanoparticles. The wide angle XRD patterns of iron oxide nanoparticles and iron oxide nanoparticles with gold shells are shown in Fig. 5. For Fe₃O₄ nanoparticles, the major peaks at about 30.3°, 35.5°, 43.2°, 53.6°, 57.2°, and 62.7° were observed which are assigned to the diffraction from (220), (311), (400), (422), (511), and (440) planes of the face-centered cubic (fcc) lattice of Fe₃O₄ (JCPDS card no. 79-0418), respectively. The values and sharp peaks indicated that Fe₃O₄ nanoparticles are pure and crystalline. The new peaks appeared in iron oxide nanoparticles with gold shells at 38.2°, 44.4°, 64.5°, and 77.7°, which correspond to the (111), (200), (220), and (311) lattice planes of the cubic-phase Au, respectively. The broad peak appearing in Fig. 5(b and c) indicates the silica. The room-temperature saturation magnetization (M_s) values were measured and are shown in Fig. 6. The saturation magnetization (M_s) of uncoated Fe₃O₄ nanoparticles is 71 emu per g, which decreased to 58 emu per g after coating with a layer of silica. The M_s value of Nanocomp-1 slightly decreased to 52 emu per g, and further decreased on gold shell formation to 43 emu per g. As compared to Nanocomp-1 and Nanocomp-2, the decrease in saturation magnetization for Nanocomp-3, and Nanocomp-4 is sharp with values of 46 and 39 emu per g respectively, which can be explained due to the larger sizes of gold nanoseeds and thick gold shell. The magnetic values of the nanocomposites are very important in bioseparation, targeted therapy, catalysis, and sensors etc.

Fig. 5 The wide angle XRD patterns of (a) Fe₃O₄, (b) Nanocomp-2 and (c) Nanocomp-4.

Fig. 6 Room temperature magnetic hysteresis curves of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b), Nanocomp-1 (c), Nanocomp-2 (d), Nanocomp-3 (e), and Nanocomp-4 (f).

3.2. Catalytic activity of the nanocomposites

The catalytic activity of the prepared nanocomposites was studied using the reduction of 4-NP by NaBH₄ as model reaction. UV-vis absorption spectra were measured at specific interval of time to monitor the changes in the reaction mixtures. The solution of 4-NP shows a peak at 317 nm, which is red shifted to 400 nm after the addition of freshly prepared NaBH₄ as shown in Fig. 7. This shift in peak is due to the formation of phenolate ions which are produced by the alkaline NaBH₄. It was observed that there was no change in the reaction mixture in the absence of any nanocomposite or in the presence of silica coated iron oxide nanoparticles even for very long time and hence no reduction of 4-NP occurred. However, after the addition of very small amount of any nanocomposite the reaction goes to completion very quickly within few minutes. Fig. 8 shows UV-vis spectra of the reaction mixture in the presence of all nanocomposites. It can be seen that the peak at 400 nm decreases as the yellow color of the mixtures fades and a new peak at 300 nm appears which gradually increases with time. The new peak corresponds to the production of 4-aminophenol (4-AP). The reaction is completed in 16 min when Nanocomp-1 is added to the reaction mixture (Fig. 8a). While it is completed very quickly in the presence of Nanocomp-2, Nanocomp-3, and Nanocomp-4 with 6, 12, and 10 min respectively. The spectra show an isosbestic point between two absorption bands indicating that just one product is produced during the reaction without any by-product. Since the concentration of NaBH₄ is in excess as compared to 4-NP, its concentration remains constant during the reaction. Therefore the reaction follows pseudo first-order kinetics with respect to the concentration of 4-NP. This was proved by the linear relations of ln(C_t/C₀) versus time for all the nanocomposites as shown in Fig. 9. Where C_t and C₀ are the concentrations of 4-NP at time t and 0, respectively which can be obtained from their corresponding absorbance values. The rate constant (k) was found to be 4.06 × 10⁻³ s⁻¹, 1.07 × 10⁻² s⁻¹, 5.97 × 10⁻³ s⁻¹, and 6.6 × 10⁻³ s⁻¹ for Nanocomp-1, Nanocomp-2, Nanocomp-3, and Nanocomp-4, respectively (See Table 1). The small rate constant for Nanocomp-1 is due to the small amount of Au on the surface of silica coated iron oxide nanoparticles. It can be seen that the rate constants are almost same for Nanocomp-3 and Nanocomp-4, although the amount of Au is higher for latter and with a thick gold shell. This is due to the incomplete shell and with large aggregates of Au on the surface of Nanocomp-4, which is responsible for the reduction in catalytic activity. However a large difference between the rate constants of Nanocomp-1 and other nanocomposites was found which is due to the uniform and thin gold shell of the Nanocomp-1 providing good surface area for reactants, thus indicates the highest rate constant and is found to be an excellent catalyst. When the mount of the nanocomposites was doubled, i.e. (7.5 × 10⁻³ mg mL⁻¹), in the reaction mixture the time of reaction was observed to be 10, 4, 6, and 5 min with rate constants of 7.21 × 10⁻³ s⁻¹, 1.94 × 10⁻² s⁻¹, 1.08 × 10⁻² s⁻¹, and 1.22 × 10⁻² s⁻¹ for Nanocomp-1, Nanocomp-2, Nanocomp-3, and Nanocomp-4, respectively, indicating that the rate constant increases with increase in catalyst amount. In this case Nanocomp-2 again showed highest rate constant, indicating its excellent catalytic activity. However, the turnover frequency (TOF) showed different trend in which the Nanocomp-1 showed higher TOF as compared to other nanocomposites. This is due to the smaller size of gold nanoparticles because TOF increases with decrease in size of gold particles.³⁷ In case of Nanocomp-2 and Nanocomp-4 with gold shells we can see that TOF is lower for Nanocomp-4, which is even lower than that of Nanocomp-3 without gold shell. This is due to the large size aggregations of gold on the surface of Nanocomp-4 and uniform gold shell on the surface of Nanocomp-2 (Table 1).

Fig. 7 UV-vis absorption spectra of 4-nitrophenol (4-NP) taken (a) before and (b) after the addition of NaBH₄.

Fig. 8 UV-vis absorption spectra of the reduction of 4-NP by NaBH₄ in the presence of (a) Nanocomp-1, (b) Nanocomp-2, (c) Nanocomp-3, and (d) Nanocomp-4.

Fig. 9 Plot of ln(C_t/C₀) versus time for each nanocomposite.

Table 1 Rates of reaction and turnover frequencies (TOF) for the reduction of p-nitrophenol by nanocomposites

Type of nanocomposites	Rate constant (s^−1)	TOF (s^−1)
Nanocomp-1	4.06 × 10^−3	1.14 × 10^−1
Nanocomp-2	1.07 × 10^−2	6.70 × 10^−2
Nanocomp-3	5.97 × 10^−3	4.87 × 10^−2
Nanocomp-4	6.60 × 10^−3	2.97 × 10^−3

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3.3. Stability and reusability of the nanocomposites

Stability and reusability of the catalysts are of utmost importance for its various applications. Due to the magnetic properties the nanocomposite catalysts can be easily separated from the reaction mixture by an external magnet and can be reused for many times. For recyclability, in order to make the separation easy and shorten the reaction time, 5 × 10⁻² mg mL⁻¹ amount of nanocomposites was used in the reaction mixture. Nanocomposites were separated with a magnet, washed with deionized water twice and then reused. Freshly prepared sodium borohydride was used for each cycle. It was found that all the nanocomposites showed good reusability with high conversion efficiency for 4-NP even after using for many cycles as shown in Fig. 10. Nanocomp-2 showed highest conversion (90%) of 4-NP even when reused for 15 cycles which arise from complete and uniform shell. However, Nanocomp-1 and Nanocomp-3 showed conversion efficiency of 78% and 74% respectively, while in case of Nanocomp-4 conversion efficiency of 80% was observed after 15 cycles. From the graph and these values of conversion efficiency we can see that there is a clear difference between Nanocomp-2 and other nanocomposites. The small decrease in the conversion efficiency in case of Nanocomp-2 after using for many cycles might be due to the loss of nanocomposites because of successive washing with water after each cycle. While for other nanocomposites the decrease in conversion is not only due to the loss of nanocomposites but also the detachment of Au nanoparticles from the surface. So the formation of uniform and stable shell of Nanocomp-2 not only resulted in excellent catalytic activity but also good reusability.

Fig. 10 Reusability of the nanocomposites as catalyst for the reduction of 4-NP with NaBH₄.

After reusing for ten cycles the TEM study for all the nanocomposites was carried out. The results showed that the nanoseeds from Nanocomp-1 and Nanocomp-3 detached, which is due to many cycles, and also due to the corrosion of silica layer as a result of basic reaction mixture (Fig. S7, ESI†). These structures look light in color which is due to the above reasons, but still there can be seen a large number of gold nanoparticles on the surface even after using for many cycles, which is responsible for their catalytic activity. Similarly, is case of Nanocomp-4 which has incomplete gold shell and thus results in the detachment of gold nanoseeds indicating its instability, hence decrease in its activity. In contrary, the TEM results of Nanocomp-2 showed that even after using for ten cycles the nanocomposite still has a continuous gold shell indicating its stability which is confirmed from the dark color of outer gold shell. The detachment of nanoseeds is prevented and thus the catalyst can be reused for many cycles without reduction in its catalytic activity.

4. Conclusions

In this study first iron oxide nanoparticles were prepared through solvothermal method and then amine functionalized after silica coating. The gold nanoparticles of two different sizes were synthesized and readily deposited on the amine functionalized nanoparticles. The gold nanoparticles on the surface act as seeds for gold shell formation. Gold nanoseeds of small sizes were uniformly distributed, and densely covered the surface which led to the formation of smooth and uniform gold shell after reduction of gold ions on the surface. As compared to small gold nanoseeds, the large nanoseeds did not result in smooth and uniform gold shell, and large aggregates of gold nanoparticles assembled on the surface. In spite of silica and gold shells, nanocomposites have high saturation magnetization values. All nanocomposites show high catalytic behavior in the reduction reaction of 4-NP. Of all the nanocomposites, Nanocomp-2 with complete gold shell showed excellent catalytic activity, reusability as well as high stability. This study will be helpful in many applications such as catalysis, targeted photothermal therapy, bioseparation, and so on.

Acknowledgements

This work was kindly supported by the National Natural Science Foundation of China (no. 21075056).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra44434a

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