Synthesis of catalytically active gold clusters on the surface of Fe₃O₄@SiO₂ nanoparticles

Abstract

This work proposes a novel method to obtain catalytically active gold clusters by using the water-soluble 5,10,15,20-Tetrakis(4-trimethyl-ammonio-phenyl)porphyrin under mild conditions instead of using strong reducing agents. Uniform gold clusters were obtained with a diameter comprised between 1 and 2 nm and long-term stability. Furthermore, the water-soluble porphyrin was immobilized on the SiO₂ shell of a core@shell Fe₃O₄@SiO₂ nanoparticles to directly synthesize gold clusters onto the surface of Fe₃O₄@SiO₂ nanoparticles featuring their magnetic recovery. Fe₃O₄@SiO₂@Au nanoparticles were found to be catalytically active for the reduction of 4-nitrophenol to 4-aminophenol using NaBH₄ and giving very high pseudo-first-order rate constant comprised between 0.7 and 2.7 min⁻¹. Our results demonstrate that Fe₃O₄@SiO₂@Au nanoparticles are stable catalysts and do not degrade during the catalytic process under the reaction conditions enabling their magnetic recuperation.

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Introduction

Gold has been used for centuries in jewelry due to its stability against oxidation and considered not suitable as catalyst. Haruta et al. reported the first major discovery in the field of catalysis by gold nanoparticles (Au NPs) showing that nanosized gold catalyzed the oxidation of carbon monoxide to carbon dioxide at room temperature by atmospheric air.¹ Nowadays, Au NPs have been recognized as an excellent catalyst and used in several processes to catalyze chemical reactions such as oxidation of alcohols² and aldehydes,³ epoxidation of propylene,⁴ selective reduction of chloronitrobenzenes,⁵ and carbon–carbon bond formation.⁶ The catalytic property of gold is affected by several factors such as nanoparticle size, morphology, and metal-oxide support interaction.^7–11 Unlike gold NPs whose optical properties are dominated by surface plasmon resonances, gold clusters possess molecule-like properties due to the quantum confinement effects resulting from “molecule-like” discrete orbital characteristics like HOMO–LUMO transition.^12,13 For long time the Schmid's method remained the best way to synthesize gold clusters,¹⁴ however, in 1994 Brust and Schiffrin published a novel method to obtain gold clusters. This method had a considerable impact on the synthesis of gold clusters due to their stability, controlled size, and narrow dispersity.¹⁵ Today, Au clusters can be obtained by mass-selected gas-phase synthesis that permits a perfect control over the number of atoms.¹⁶ However, this method produces a very small amount of clusters that limits their catalytic applications. The reactivity of gold clusters is strongly affected by their atomicity and location on a solid support that would enhance Au clusters recovery making crucial their design.^17–19 Current methods for the preparation of supported Au clusters require the immobilization of a precursor to a support and the removal of the ligands by post-synthesis treatment, trying to avoid cluster agglomeration during this step.^20–22 Therefore, it is desirable to develop a methodology that permits to obtain large quantities of stable clusters directly immobilized on a solid support for their use and subsequently recovery. The deposition and encapsulation of Au clusters onto different substrates has been reported by several groups and it is used to enhance their colloidal stability.^17,23 Haruta et al. reported the direct deposition of Au NPs on commercially available poly(methyl methacrylate) beads and their use for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) showing very interesting results.²⁴ Chang et al. synthesized a magnetically recoverable Au catalyst by adsorption–reduction of Au³⁺ ions on chitosan-coated iron oxide nanoparticles. The catalytic activity of the magnetically recoverable Au catalyst was illustrated performing the reduction of 4-NP to 4-AP.²⁵

Porphyrins are molecules rich in π–π electrons and there is an extensive knowledge on the formation of complexes between porphyrins and Au³⁺ ions with different properties and applications.^26–29 Sakamoto et al. proposed a novel strategy to obtain nano-platonic solids through the face coordination of porphyrin molecules on an inscribed Au nanoparticle with the adequate size.³⁰ Recently, porphyrin molecules assembled onto a clay surface have been used for the deposition of gold clusters.³¹ However, up to day there is no report of direct synthesis of Au nanoparticles/clusters using water-soluble porphyrins.

In this work, we report a simple strategy for the synthesis and immobilization of Au clusters onto the surface of a Fe₃O₄@SiO₂ nanoparticles. Interestingly, we found that the water-soluble 5,10,15,20-Tetrakis(4-trimethyl-ammonio-phenyl)porphyrin used in this study is able to reduce HAuCl₄ in a controlled manner resulting in the production of Au clusters. The encapsulation of the water-soluble porphyrin inside the silica shell of Fe₃O₄@SiO₂ nanoparticles permitted the direct synthesis of Au clusters on the silica surface. The nanoparticles Fe₃O₄@SiO₂@Au were used to catalyze the reduction of 4-NP to 4-AP with NaBH₄ and the catalyst showed a high rate of conversion and the ability to maintain a 94% of catalytic activity for at least 6 cycles.

Materials and methods

Materials

5,10,15,20-Tetrakis(4-trimethyl-ammonio-phenyl)porphyrin tetra(p-toluenesulfonate) (TTMAPP) dye content 90%, 1-octadecene 90%, iron(III) citrate 99%, HAuCl₄·3H₂O ACS reagent ≥49.0% Au basis, Igepal® CO-520 average M_n 441, oleic acid technical grade 90%, hexane ReagentPlus® ≥99%, NaBH₄ ReagentPlus® ≥99%, 4-nitrophenol reagent grade 98%, tetraethyl orthosilicate 98% (TEOS), methanol ACS reagent ≥99.5%, and acetone ACS reagent ≥99.5% were purchased from Sigma-Aldrich and used as received without any further treatment. NH₃ 29% was purchased from Panreac. TEM copper grid mesh-200 coated with formvar/carbon was purchased from 2SPI.

Synthesis of hydrophobic Fe₃O₄ nanoparticles

In a 100 mL three-neck round bottom flask were added 10 mL of 1-octadecene, 1156 mg of oleic acid (4 mmol), and 245 mg of iron(III) citrate (1 mmol). The mixture was heated for one hour at 120 °C to remove any low boiling-point compounds of the solvent in an inert atmosphere of nitrogen. Then, the solution was heated for three hours at 305 °C to start the process of the thermal decomposition of the iron citrate. The color of the solution changed from brown to black indicating the formation of iron oxide nanoparticles. The reaction was cooled down at room temperature and the nanoparticles were precipitated by centrifugation at 8000 rpm with a solution of acetone/hexane 1 : 1 v/v, the supernatant was discarded and the solid redispersed. The process was repeated two times and the Fe₃O₄ nanoparticles were stored in hexane with a final concentration of 10 mg mL⁻¹.

Synthesis of core@shell Fe₃O₄@SiO₂ nanoparticles with encapsulated porphyrin

To a solution of 4.5 mL hexane were added 240 μL of Igepal® CO-520 (average M_n 441, 0.544 mmol) and 20 μL of hydrophobic Fe₃O₄ nanoparticles in hexane (10 mg mL⁻¹). A solution of TTMAPP was prepared by dissolving 40 mg of porphyrin in 1 mL of NH₃ 29%. The ammonia solution (40 μL) was added to the Fe₃O₄ nanoparticles solution and vigorously mixed to obtain a homogenous microemulsion. Finally, 30 μL of TEOS was added under stirring and the reaction was continued for 18 hours. The addition of methanol disrupted the microemulsion and the porphyrin-encapsulated Fe₃O₄@SiO₂ nanoparticles were centrifuged at 8000 rpm and the supernatant discarded. The core–shell nanoparticles were washed two additional times with ethanol, centrifuged, and finally dispersed in water at a concentration of 2 mg mL⁻¹.

Deposition of the Au clusters on the surface of the Fe₃O₄@SiO₂ nanoparticles

25 μL of HAuCl₄ 30 mmol L⁻¹ (0.75 mmol) were mixed with 5 mL of the porphyrin-encapsulated Fe₃O₄@SiO₂ nanoparticles and kept at room temperature for 12 hours without stirring. After this time, the Au cluster decorated Fe₃O₄@SiO₂ nanoparticles were centrifuged at 8000 rpm for 10 minutes, the supernatant was discarded to remove any unreacted HAuCl₄ and any unanchored Au clusters/nanoparticles and the process was repeated two times. The Fe₃O₄@SiO₂@Au solid was then redispersed in methanol at 2 mg mL⁻¹.

Transmission electron microscopy (TEM)

Au clusters and Fe₃O₄@SiO₂ nanoparticles decorated with Au clusters were analyzed using high-resolution TEM JEM-2100 (JEOL Ltd. Tokyo, Japan) working at 200 kV with an ADF Gatan detector (Gatan Inc, Pleasanton, USA). TEM copper grids with mesh 200 covered with formvar/carbon and lacey formvar/carbon were purchased from 2SPI. TEM grid was prepared by deposition of a 10 μL drop of the different solutions and blotted after 90 seconds using filter paper. X-ray energy dispersive spectroscopy (EDS) analysis was performed using an Oxford INCA (Scanservice Corporation, Tustin, USA) device connected with the TEM microscope.

X-ray photoelectron spectroscopy (XPS)

The chemical composition of the sample with Au clusters was examined by XPS surface measurements. In a typical experiment Au clusters were obtained as follow. 10 mL of a water solution of TTMAPP porphyrin (0.1 mmol L⁻¹, 1 mmol) were placed in a 25 mL glass vial and mixed with 25 μL of HAuCl₄ (30 mmol L⁻¹, 1.5 mmol) and the color of the solution rapidly changed from red-wine to dark green. After 20 minutes from the beginning of the reaction, the color of the solution gradually changed to dark brown-orange typical of Au clusters. The solution was held without stirring for 3 hours at room temperature and finally centrifuged at 8000 rpm for 10 minutes in order to separate Au nanoparticles formed during the synthesis. The high-resolution XPS spectra C1s, O1s, Au4f, and survey spectra were recorded using a Thermo Scientific K-Alpha instrument (Thermo Fisher Scientific Inc, East Grinstead, UK). Monochromatic X-ray source Al Kα (1486.6 eV) was used for all samples and experiments. The X-ray monochromatic spot was 400 μm in diameter. Residual vacuum in the analysis chamber was maintained at around 6 × 10⁻¹⁰ mbar. The binding energies (BEs) positions were referenced to the C1s on unsputtered surfaces. Charge referencing was done by setting the binding energy of C1s hydrocarbon peak at 285.0 eV. High-resolution XPS spectra of Au4f were also acquired using the snapshot mode. When a spectrum is acquired in snapshot mode, the acquisition is faster allowing more acquisition with the same scan time and the spectra is high quality when the detector has a large number of channels, as in our case. We have also employed an electron flood gun to minimize surface charging. Avantage software (5.932v) was used to determine the atomic concentrations from the XPS peak areas using the Shirley background subtraction technique and the Scofield sensitivity factors.^32,33 Some fragments of the samples were fixed to the sample holder with a metal clamp to ensure the electrical contact between holder and sample.

Ultraviolet-visible (UV-vis) spectroscopy

UV-vis spectra of the free-base porphyrin before and after the reaction with HAuCl₄ were taken using a Variant Cary-Bio300 at room temperature (Agilent, Santa Clara, Ca, USA). UV-vis was also used to monitor the catalytic reduction of 4-NP to 4-AP. The experiments were performed in a crystal quartz cuvette (path length 1 cm) mixing 200 μL of 4-NP (2 mmol L⁻¹), 1.4 mL of NaBH₄ (100 mmol L⁻¹), different volumes of the dispersed Fe₃O₄@SiO₂@Au (2 mg mL⁻¹), and deionized water to reach a fixed volume of 3 mL.

Results and discussions

The porphyrin structure is made of conjugated double bonds and they posses a typical UV-vis absorption spectrum displaying an extreme intense band in the range of 380–500 nm (Soret band) with a molar extinction coefficient of 10⁵ mol L⁻¹ cm⁻¹, and the Q bands at longer wavelengths in the range of 500–750 nm with a lower extinction coefficient of 10⁴ mol L⁻¹ cm⁻¹.³⁴ Porphyrins are well known for their ability to form stable metalloporphyrin complexes and, depending on the ionic radius of the metal ion, the porphyrin complex can be in-plane (r_ion < 90 pm) or out-of-plane.³⁴ The formation of metalloporphyrin complexes can be monitored using spectrophotometric techniques, such as UV-vis, following the displacement of the Soret band of the porphyrin. The formation of an in-plane porphyrin complex generates a blue shift of the Soret band respect to the free-base porphyrin in its UV-vis absorption spectra. The bathochromic shift is primarily due to the distortion of the macrocycle and not to the electronic properties of the metal.³⁴ Au(III) has ionic radius of 85 pm and it can fit properly into the cavity of the porphyrin ring forming very stable in-plane complexes.^27,34,35 For this reason, we monitored the evolution of the Soret and Q bands during the reaction between the TTMAPP porphyrin and HAuCl₄. Fig. 1a shows the Soret band at 413 nm of the free-base TTMAPP porphyrin (red line) at neutral pH. After the addition of the first aliquot (5 μL) of HAuCl₄ the intensity of the Soret band decreased conspicuously (black line) probably due to the redox reaction between the electron donor porphyrin and the acceptor HAuCl₄. The addition of the second aliquot (5 μL) had the effect to slightly decrease the intensity of the Soret band (green line), however, it is also visible the presence of a shoulder on the left part of the band probably due to the formation and simultaneous presence of the Au(III)–TTMAPP complex. The third and forth addition of HAuCl₄ (5 + 5 μL) provoked the complete blue-shift of the Soret band at 404 nm probably due to total transformation of the remaining free-base TTMAPP into Au(III)–TTMAPP complex, and the last addition of HAuCl₄ (5 μL) decreased the intensity of the Soret band at 404 nm. The inset shows a magnification of the UV-vis spectra of the Q bands that confirms the contemporary presence of free-base TTMAPP and Au(III)–TTMAPP complex. Fig. 1b shows the reaction performed at acidic pH and the course of the reaction was different. The addition of 60 μL of HCl 10 mM caused the total red-shift of the Soret band from 413 to 432 nm (black line) due to the protonation of the pyrrole rings that induces a conformational change of the planar structure of the porphyrin ring.³⁴ The first addition of 5 μL of HAuCl₄ caused a small decreased of the Soret and Q bands, while further additions of HAuCl₄ (20 μL) strongly decreased the intensity of the Soret and Q bands without the formation of Au(III)–TTMAPP complex. Fig. 1c and d show the TEM pictures of the gold clusters obtained with a diameter size of 1.01 ± 0.2 nm (Fig. S1†) and long-term stability using the two different conditions.

Fig. 1 UV-vis spectra of TTMAPP porphyrin before and after the addition of different aliquots of HAuCl₄ at neutral pH (a), and acidic pH (b). The inset shows the Q bands of the UV-vis spectra. TEM pictures showing the obtained Au clusters at neutral pH (c), and acidic pH (d). Scale bar is 20 nm.

The products obtained using the different acidic pH conditions and TTMAPP were further characterized using X-ray photoelectron spectroscopy (XPS) to determine the possible presence of different Au oxidation states (Au³⁺, Au⁺, and/or Au⁰). First, the solutions containing Au clusters were centrifuged to separate gold nanoparticles formed during the synthesis.

High-resolution XPS spectra of Au4f were acquired using the snapshot mode and used for the subsequent chemical state assignment of Au species. The XPS study of the sample prepared using the reaction between TTMAPP and HAuCl₄ at neutral pH shows that Au was present in different oxidation states (Fig. 2a and b). The high-resolution spectrum of the Au4f core level was deconvoluted into three pairs of peaks due to Au4f7/2 and Au4f5/2 spin–orbit coupling. The first pair of peaks Au4f7/2A and Au4f5/2A at Binding Energies (BEs) of 83.6 and 87.6 eV is related to elemental gold (Au⁰), the other two pairs are related to the two gold oxide states Au⁺ (Au4f7/2B and Au4f5/2B at BEs of 85.3 and 89.3 eV) and Au³⁺ (Au4f7/2C and Au4f5/2C at BEs of 88.3 and 92.3 eV). On the basis of relative peak areas, their respective atomic percentages were estimated as 15% for Au⁰, 20% for Au⁺, and 65% for Au³⁺. For the reaction at acidic pH, the high-resolution XPS spectrum of the Au4f core level was deconvoluted obtaining two pairs of peaks due to Au4f7/2 and Au4f5/2 spin–orbit coupling (Fig. 2c and d). The first pair with BE at 83.9 and 88.0 eV are related to Au⁰ and the other pair is related to the stable gold oxide state, Au⁺ (BEs of 85.4 and 89.2 eV).^36,37 On the basis of relative peak areas, their respective atomic percentages were estimated as 47% for Au⁰ and 53% for Au⁺ indicating higher conversion efficiency during the reaction between TTMAPP and HAuCl₄ at acidic pH without the production of Au(III)–TTMAPP complex. To facilitate the purification of the Au clusters and provide a solid support that would allow their stability and recyclability, we synthesized Au clusters on the surface of Fe₃O₄@SiO₂ nanoparticles using a synthetic approach shown in Scheme 1.

Fig. 2 (a) and (c) XPS survey scan spectra used to identify the elements. The black boxes highlight the region where the high-resolution XPS was performed for (b) the reaction at neutral pH and (d) at acidic pH.

Scheme 1 (a) Schematic representation of the synthetic approach used to obtain the recoverable Au clusters immobilized on Fe₃O₄@SiO₂ nanoparticles. (b) Illustration of the catalytic reduction of 4-NP to 4-AP using NaBH₄ catalyzed by Fe₃O₄@SiO₂@Au nanoparticles and their recovery.

The first step required the synthesis of iron oxide nanoparticles obtained by thermal decomposition using as precursor iron(III) citrate.³⁸ TEM image showed highly monodisperse iron oxide nanoparticles, which facilitates the next step of coating with a homogenous layer of SiO₂, and with an average diameter of 10 ± 1 nm (Fig. 3a). SAED pattern analysis of the sample revealed the crystalline structure of the nanoparticles (Fig. 3a inset) and based on the measured lattice spacing based on the rings of the SAED pattern the result matched with the known lattice spacing of bulk Fe₃O₄ (ICDD card no. 19.0629) along with their respective hkl indexes (Table 1).

Fig. 3 (a) TEM micrograph of the synthesized monodisperse iron oxide nanoparticles. In the inset, SAED shows the diffraction rings of the iron oxide nanoparticles. The lattice spacing measured using SAED matched with Fe₃O₄. (b) High-resolution TEM micrograph of the Fe₃O₄@SiO₂ core@shell nanoparticles. (c) and (d) TEM and HR-TEM micrographs showing the decoration of the Fe₃O₄@SiO₂ nanoparticles surface with Au clusters. (e) and (f) Colloidal dispersion of the Fe₃O₄@SiO₂@Au nanoparticles in water and their responsiveness to an external magnetic field.

Table 1 Measured lattice spacing, d (Å), based on SAED

	1	2	3	4	5
d (Å)	2.98	2.51	2.07	1.61	1.48
Fe3O4	2.97	2.53	2.1	1.62	1.48
hkl	220	311	400	511	440

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The encapsulation of iron oxide nanoparticles within the silica was obtained through TEOS hydrolysis in a reverse microemulsion. The presence of negative charges on the surfaces of the silica shell provides colloidal stability to the Fe₃O₄@SiO₂ nanoparticles.^39,40 TTMAPP porphyrin was dissolved in NH₃ 29%, that is the aqueous phase of the reverse microemulsion, to make possible the encapsulation within the silica matrix, and to reduce the gold salt in the following step. The high aqueous solubility of the TTMAPP makes possible an efficient encapsulation of porphyrin molecules within the silica matrix.⁴¹ TEM image shows the core@shell structure of Fe₃O₄@SiO₂ nanoparticles obtained in very high yield (Fig. S2†). The Fe₃O₄@SiO₂ nanoparticles were monodisperse and the average diameter calculated using the TEM micrograph was 43 ± 4 nm. Finally, the Fe₃O₄@SiO₂ nanoparticles were mixed with an aqueous solution of HAuCl₄. TEM and HR-TEM showed the presence of small dots on the surface of the silica shell (Fig. 3c and d). However, the presence of Au clusters on the surface of the SiO₂ nanoparticle does not limit the possibility that some of them growth inside the nano-channels existing on the SiO₂ shell. The solution presents a light orange color indicating the presence of encapsulated TTMAPP and iron oxide nanoparticles that are responsive to an external magnetic field (Fig. 3e and f). HR-TEM image showed that the size of the Au clusters obtained was slightly bigger than the Au clusters synthesized in water ranging from 1.1 to 3.7 nm with a mean diameter of 1.45 ± 0.6 nm (Fig. S3†). EDS analysis was used to obtain the elemental composition of the system Fe₃O₄@SiO₂@Au with an amount of Fe of 8.45%, Si of 89.29%, and Au of 2.26% confirming that the small dots on the surface of the silica shell were gold (Fig. S3†).

The catalytic reduction of 4-NP to the corresponding derivative 4-AP by NaBH₄ was chosen as a model reaction to investigate the efficacy of Fe₃O₄@SiO₂@Au.^42–44 The reduction of 4-NP to 4-AP using aqueous NaBH₄ is thermodynamically favorable (E₀ for 4-NP/4-AP = −0.76 V and H₃BO₃/BH₄⁻ = −1.33 V versus NHE). However, the large potential difference between donor and acceptor decreases the feasibility of this reaction. Gold nanoparticles can catalyze this reaction by facilitating the electron transfer from the donor BH₄⁻ to the acceptor 4-NP and overcome the kinetic barrier of the reaction. The 4-NP shows an absorbance peak at 317 nm, which red shifts to 400 nm when is mixed with NaBH₄ due to the formation of 4-nitrophenolate ions in the alkaline medium caused by NaBH₄.⁴⁵ Hence, the reaction progress can be followed by monitoring the decrease of the absorption at 400 nm of the 4-nitrophenolate ions by UV-vis spectroscopy. Since the concentration of NaBH₄ (1.4 × 10⁻⁴ mol) greatly exceeds that of 4-NP (4.0 × 10⁻⁷ mol), the reduction rate can be assumed to be independent of NaBH₄ concentration. Therefore, the apparent rate constant K_app can be evaluated by studying the pseudo-first-order kinetics respect to the 4-NP concentration, and determined from the slope of the linear correlation of ln(A/A₀) versus time (t), and is proportional to the total surface area S available.^42,45 The reduction of 4-NP was conducted with different amounts of Fe₃O₄@SiO₂@Au nanoparticles and the results are shown in Fig. 4a. Control experiments were performed to discard any catalytic effect of the TTMAPP porphyrin and/or Fe₃O₄@SiO₂ nanoparticles and the results are shown in Fig. S4.† Liang et al. used iron oxide nanoparticles as catalyst for the reduction of 4-NP, however, the concentration of Fe₃O₄ nanoparticles necessary to observe a catalytic effect was much larger than gold as nanoparticles.⁴⁶ From the control experiments, our system show no catalytic effect over the reduction of 4-NP during the observed time of 10 minutes. The linear relationship of ln(A/A₀) versus time (t) indicates that the reduction of 4-NP by Fe₃O₄@SiO₂@Au catalyst follows a pseudo-first-order kinetics. The calculated rate constants for 4-NP reduction were dependent on the amount of Fe₃O₄@SiO₂@Au used to catalyze the reaction, as well as the induction time as reported in Table 2 and Fig. 4d that shows a dependence between the amount of catalyst and the calculated K_app.

Fig. 4 (a) Time traces of the absorption of the 4-nitrophenolate ions measured at 400 nm during their reduction. The reaction was conducted using 200 μL of 4-NP (2 mmol L⁻¹), 1.4 mL of NaBH₄ (100 mmol L⁻¹), different volumes of the dispersed Fe₃O₄@SiO₂@Au (2 mg mL⁻¹): △0.66, ●1.00, ■1.65, and ▽3.30 μg, and deionized water to reach a fixed volume of 3 mL. The reaction starts after an induction time, t₀, which was dependent on the concentration of catalyst used. The linear fitting gave the slope where K_app can be determined; (b) conversion efficiency of 4-NP in six successive cycles of reduction catalyzed by NaBH₄ and the same Fe₃O₄@SiO₂@Au nanoparticles. (c) Schematic illustration of the magnetic recovery of the Fe₃O₄@SiO₂@Au nanoparticles after the reduction of the 4-NP by NaBH₄. (d) Different values of the obtained K_app represented as function of the catalyst amount.

Table 2 K_app values obtained using different amounts of catalyst and relative induction time

Catalyst amount	Kapp (min^−1)	Induction time (s)
0.66 μg	0.7	145
1.00 μg	1.1	108
1.65 μg	1.7	73
3.30 μg	2.7	54

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The rate constants for the reduction of 4-NP catalyzed by Fe₃O₄@SiO₂@Au are higher than any previously reported values obtained by using Au nanoparticles or stabilized Au clusters.^{24,42,47–49} Kuroda et al. have used PMMA as a solid support for the deposition of gold nanoparticles and they reported a 4-NP reduction rate constant of 0.43 min⁻¹ using 5.7 mg L⁻¹ of catalyst.²⁴ Gao et al. used a lower amount of catalyst 0.25 mg L⁻¹ and the reduction rate constant was 0.44 min⁻¹.⁴⁷ Mahamallik et al. have used silica as solid support for the gold nanoparticles and they reported a 4-NP reduction rate of 0.25 min⁻¹ using 50 mg L⁻¹.⁴⁸ Comparing the dose of catalyst used in our work, we used a minimum of 0.22 mg L⁻¹ and a maximum of 1.1 mg L⁻¹ of catalyst obtaining a 4-NP reduction rate constant ranging between 0.7 and 2.7 min⁻¹. A different system such as the one proposed by Tzounis et al. showed that in the presence of SiO₂@Ag nanoparticles they obtained a similar reduction rate constant, however, they used a much higher amount of catalyst.⁵⁰ Comparing different nanoparticles having the same diameter, as the one reported by Xu et al., our system showed a superior performance. They used iridium oxide nanoparticles with a mean diameter of 1.7 nm to reduce 4-NP to 4-AP and the required time to achieve the total conversion was 25 minutes.⁵¹ Using 3.30 μg of catalyst the time required to achieve the total conversion of 4-NP to 4-AP was less than 1 minute. Our result seems to indicate that the Fe₃O₄@SiO₂@Au nanoparticles are superior catalysts, enhancing the catalytic efficiency and minimizing the required amount of catalyst. It is noteworthy to point out that the induction time, t₀, decreased when a higher amount of catalyst was used. The induction time has been reported by many groups during the catalytic reduction of 4-NP,⁴² however, its origin is not fully understood. Recently, Zhou et al. suggested that t₀ is related to a dynamic restructuring of the surface of the nanoparticles.⁵² In fact, when a catalyst is introduced in the reaction media and it is not the active species, but this is formed during the reaction, an induction period should be observed.⁵³ However, in heterogenous catalysis it is necessary to wait a certain time required for 4-NP to be adsorbed onto the surface of the catalyst and reach the equilibrium before the reaction begin. If this does not occur different induction times are required to reach the equilibrium using different amounts of Au catalyst. In our experiments a catalyst amount higher than 3.30 μg resulted in a t₀ impossible to measure (data not shown). The reduction (and absence) of the induction time might indicate that the equilibrium was reached faster as a higher amount of catalyst was used, thus decreasing t₀. The as-prepared Fe₃O₄@SiO₂@Au nanoparticles showed excellent catalytic activity and were collected by an external magnet after the catalytic reduction for successive cycles. The use of Fe₃O₄@SiO₂ as supporting material for the Au clusters permits the recovery, separation, and reuse of the catalyst. Fig. 4b shows the efficiency of conversion of the Fe₃O₄@SiO₂@Au catalyst after six consecutive cycles and the conversion efficiency slightly decreased with the number of cycles. This result might be assessed to the loss of small amounts of Fe₃O₄@SiO₂@Au nanoparticles during the magnetic separation and not to a significant change of the nanoparticles morphology and structure enabling their recyclability.

Conclusions

In summary, this is the first report of synthesis of water-stable Au clusters mediated by the redox reaction between the water-soluble TTMAPP porphyrins and HAuCl₄ as shown by XPS, UV-vis and TEM. The synthesized Au clusters have a homogenous size distribution with a diameter of 1.01 ± 0.2 nm and very long-term stability. Furthermore, the Au clusters were directly synthesized onto a core@shell nanoparticle made of Fe₃O₄@SiO₂ to produce a magnetically recoverable Au clusters. The iron oxide core provided magnetic responsiveness to the system, whereas the silica shell provided high colloidal stability to the system. The size of the Au clusters onto the Fe₃O₄@SiO₂ nanoparticles was comprised between 1.1 and 3.7 nm with a mean diameter of 1.45 ± 0.6 nm. The Fe₃O₄@SiO₂@Au nanoparticles were used as catalyst to study the reduction of 4-NP to 4-AP. The nanoparticles were able to catalyze the reaction giving a pseudo-first-order rate constant values comprised between 0.7 and 2.7 min⁻¹. In addition the Fe₃O₄@SiO₂@Au nanoparticles demonstrate to be a stable catalyst during the catalytic process enabling their recyclability due to the magnetic responsiveness of the Fe₃O₄@SiO₂ nanoparticles.

Competing financial interests

The authors declare no competing financial interests.

Acknowledgements

The authors acknowledge the Spanish Ministry MINECO for financial support (MAT2014-55065R). M. A. L. Q. acknowledges the Spanish Ministry MINECO (MAT2012-36754-C02-01) and the Xunta de Galicia (GRC2013-044, FEDER Funds). P. A. C. acknowledges the Spanish Ministry of Education for the FPU fellowship (AP2010-1163).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20055a

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