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Facile deposition of gold nanoparticles on core–shell Fe3O4@polydopamine as recyclable nanocatalyst
Accepted Manuscript Facile deposition of gold nanoparticles on core−shell Fe3O4@polydopamine as recyclable nanocatalyst Yan Zhao, Yaowen Yeh, Rui Liu, Jinmao You, Fengli Qu PII:
S1293-2558(15)00098-9
DOI:
10.1016/j.solidstatesciences.2015.04.010
Reference:
SSSCIE 5125
To appear in:
Solid State Sciences
Received Date: 9 September 2014 Revised Date:
10 March 2015
Accepted Date: 26 April 2015
Please cite this article as: Y. Zhao, Y. Yeh, R. Liu, J. You, F. Qu, Facile deposition of gold nanoparticles on core−shell Fe3O4@polydopamine as recyclable nanocatalyst, Solid State Sciences (2015), doi: 10.1016/j.solidstatesciences.2015.04.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
ACCEPTED MANUSCRIPT Facile deposition of gold nanoparticles on core−shell Fe3O4@polydopamine as recyclable nanocatalyst
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Yan Zhao a, Yaowen Yeh b, Rui Liu c, Jinmao You a, Fengli Qu a,c *
a College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, 273165, China
b Princeton Institute for the Science and Technology of Materials, Princeton
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University, Princeton, NJ, 08544 USA
c Department of Chemical and Biological Engineering, Princeton University,
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Princeton, New Jersey, 08544, USA
Correspondence: Email : [email protected] (F-L Qu)
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Tel/fax: Tel/Fax: (+1) 609-216-6252
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ACCEPTED MANUSCRIPT ABSTRACT: A simple and green method for the controllable synthesis of core−shell Fe3O4 polydopamine nanoparticles (Fe3O4@PDA NPs) with tunable shell thickness and their application as a recyclable nanocatalyst support is presented. Magnetite Fe3O4 NPs
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formed in a one-pot process by the hydrothermal approach with a diameter of ~240 nm were coated with a polydopamine shell layer with a tunable thickness of 15-45 nm. The facile deposition of Au NPs atop Fe3O4@PDA NPs was achieved by utilizing PDA as both the reducing agent and the coupling agent. The satellite nanocatalysts
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exhibited high catalytic performance for the reduction of p-nitrophenol. Furthermore, the recovery and reuse of the catalyst was demonstrated 8 times without detectible
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loss in activity. The synergistic combination of unique features of PDA and magnetic nanoparticles establishes these core−shell NPs as a versatile platform for potential applications.
KEYWORDS: core-shell nanoparticles, polydopamine, Fe3O4, catalyst 1. Introduction
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Gold nanoparticles (Au NPs) have received considerable attention because of their unique catalytic properties in a number of oxidation and reduction reactions [1-4]. Generally, Au NPs in solution exhibit strong tendency to aggregate because of
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the high surface energy, which leads to the decrease in catalytic activity [5]. Different solid matrices such as carbon nanotubes, silica, titania and other metal oxides [6-8] have been used as support of Au NPs to enhance the catalyst stability. However,
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time-consuming separation procedures such as filtration or centrifugation must be performed to isolate these catalysts from the reaction system [5]. Uniform core−shell nanoparticles with magnetically responsive cores and
functional shells are being widely investigated because of their potential applications in cell separation, enzyme immobilization, protein and nucleic acid purification, targeted drug delivery, environmental remediation separation, catalysis, and magnetic resonance imaging [9-11]. There have been extensive reports on the synthesis of magnetic Fe3O4 core−shell nanoparticles in which the shell layer was composed of either an inorganic (e.g., silica or gold) or an organic (e.g., polymer) material [12]. In 2
ACCEPTED MANUSCRIPT the case of polymer shell layers, polydopamine (PDA) has received significant attention as a candidate material because of its unique coating quality and functionality [13, 14]. Dopamine, the PDA monomer that contains both catechol and amine functional groups, self-polymerizes at alkaline pH values. During
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polymerization, PDA will spontaneously form a conformal and continuous coating layer atop nearly any material present in the reaction media including noble metals, metal oxides, semiconductors, ceramics, and synthetic polymers via the strong binding affinity of catechol functional groups [14]. The PDA layer thickness may be
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controlled by varying the dopamine concentration and reaction time [15].
Polydopamine-coated Fe3O4 nanoparticles (Fe3O4@PDA) have been exploited
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for use in numerous advanced applications, including the separation and enrichment of proteins in proteomic analysis [16], the detection of pollutants by matrix assisted laser desorption/ionization time-of-flight mass spectrometry [17], and magnetic solid-phase extraction adsorbent for the determination of trace polycyclic aromatic hydrocarbons in environmental samples [18]. PDA can serve as an adhesion layer to
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immobilize biological molecules, amine- and mercaptofunctionalized self-assembled monolayers, and metal films to the surface for secondary modification as biosensors [19, 20], support for biomineralization and freestanding films [21, 22].
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Besides its biocompatibility and adhesiveness, PDA has other attractive material properties [23]. Polydopamine is known to reduce metal salts within a solution into metal nanoparticles via the catechol functional groups [24]. A range of metals
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including Au, Ag, Pt, and Cu have been successfully reduced and deposited atop PDA modified surfaces without the need for the addition of a reducing agent [25]. Aromatic amines are widely used in the synthesis of pharmaceuticals, dyes, and
agrochemicals [26]. Two general methods were used for the reduction of aromatic nitrocompounds in industry, which include stoichiometric reduction [27] and catalytic hydrogenation [28]. The catalytic hydrogenation is a convenient method for producing amines in high yield. Reduction of aromatic nitrocompounds using various nanoparticles prepared by different techniques has been investigated [29, 30] and suffers from limitations in reusability and recovery of the catalyst after the reaction. 3
ACCEPTED MANUSCRIPT Here, we present the controllable synthesis of core−shell Fe3O4 polydopamine nanoparticles (Fe3O4@PDA NPs) with tunable shell thickness to create a nanostructured metal catalyst support. Using PDA as a reducing agent, Fe3O4@PDA could be used as a platform to grow and support Au nanoparticles (Au/Fe3O4@PDA)
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for catalysis. The magnetic Au/Fe3O4@PDA satellite nanostructures were easily separated and recycled and exhibited high catalytic performance as illustrated by the reduction reaction of p-nitrophenol. 2. Experimental section
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2.1 Synthesis of Fe3O4
Magnetite Fe3O4 nanospheres were prepared by a previous report through a
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one-pot hydrothermal method [31]. Typically, 0.325 g of FeCl3 and 0.2 g of trisodium citrate dehydrate were dissolved in 20 mL of ethylene glycol under vigorous stirring for 1 h at room temperature till the solids dissolved. Then, 1.2 g of sodium acetate was added while stirring for 30 min. The mixture was sealed in a Teflon-lined stainless-steel autoclave for 12 h at 200 °C. After the reactor was cooled to room
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temperature, the obtained Fe3O4 nanoparticles was separated by the magnetic separation method and washed with ethanol and ultrapure water three times, respectively. The resultant nanoparticles were dispersed in water to 40 mg/mL after
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drying under vacuum for 12 h. 2.2 Synthesis of Fe3O4@PDA
To prepare core−shell Fe3O4@PDA nanoparticles, 1mL of as-prepared Fe3O4
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NPs suspensions were added in 20 mL of 2 mg/mL dopamine Tris solution (pH 8.5, 10 mM Tris−HCl buffer) under stirring and allowed to proceed for 8 h at room temperature. The product was separated and collected with a magnet and subsequently put through three wash cycles and dried under vacuum overnight. The thickness of PDA shell could be controlled by repeating PDA coating through the same procedure. 2.3 Synthesis of Au/Fe3O4@PDA A 2 mg portion of Fe3O4@PDA with tunable shell thickness were well dispersed in a 25 mL chloroauric acid (5μg/mL) aqueous solution. The mixture was stirred at
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Au/Fe3O4@PDA obtained by coating PDA for two times and three times were named as P2 and P3, respectively. 2.4 Catalytic study
Au/Fe3O4@PDA (0.1 mg) was added in 2 mL of p-nitrophenol (0.1 mM)
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containing freshly prepared aqueous solution of NaBH4 (2 mL, 0.1M). UV-Vis absorption spectra were recorded to monitor the change in the reaction mixture. After
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the reduction process was completed, the catalysts were magnetically separated from the mixture, washed several times and dried in a vacuum oven overnight for reuse in the next reaction cycle – this process was repeated 8 times. 3. Results and discussion
3.1 Characterizations of the synthesized nanoparticles
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The nanoscale hybrid system was exploited as illustrated in Scheme 1. Magnetic Fe3O4 NPs were formed in a one-pot process by the hydrothermal approach. The facile deposition of Au NPs atop Fe3O4@PDA NPs was achieved by utilizing PDA as
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both the reducing agent and the coupling agent. The excellent catalytical performance and good recyclability and easy separation ability of the satellite Au/Fe3O4@PDA nanocatalysts were evaluated by the reduction reaction of p-nitrophenol by NaBH4.
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Magnetic nanoparticles Fe3O4 were synthesized by a solvothermal reaction at
200 °C by reduction of FeCl3 with ethylene glycol in the presence of sodium acetate, an alkali source, and trisodium citrate, a biocompatible electrostatic stabilizer [31]. Figure 1A illustrates transmission electron microscopy (TEM) image of the magnetite nanoparticles. As revealed in Figure 1A, the NPs are nearly spherical in shape and uniform in size with a diameter of ∼240 nm. The Fe3O4 nanoparticles exhibit great stability and dispersion ability in water solution after storing for 30 days (Fig. S1) because of the presence of citrate groups. Figure 1B is the selected area electron diffraction pattern (SAED) on a separated Fe3O4 nanoparticle, which has the typical 5
ACCEPTED MANUSCRIPT diffraction pattern character of Fe3O4. PDA layer was formed onto the Fe3O4 NPs surfaces through autopolymerization of dopamine and Tris buffer was employed to adjust the solution pH according to previous reference [32]. In addition, the hydroxyl groups on the Fe3O4 surface may also bond with the catechol groups of dopamine
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through dehydration [33]. Figure 1C and 1D present of Fe3O4@PDA TEM images prepared via mixing Fe3O4 nanoparticles in a dopamine Tris solution. As illustrated, uniform Fe3O4@PDA NPs with a well-defined core−shell structure were successfully synthesized. That is, the Fe3O4 NPs, which represent the core, were successfully
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coated with a thin polydopamine shell layer of ∼15 nm in thickness after one time coating. TEM images of Fe3O4@PDA prepared by repeating PDA coating for two
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times (Fig. 1E and Fig. 1F) and three times (Fig. 1G and Fig. 1H) show that the thicknesses of the PDA shell are ~30 nm and ~45 nm, respectively. It is evidenced that the thickness of the PDA shell can be successfully and easily controlled by repeating PDA coating through the same procedure.
The magnetic Fe3O4@PDA core−shell NPs were applied as a catalyst support for
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Au NPs by directly depositing Au NPs onto Fe3O4@PDA core−shell NPs. Au NPs were reduced and grown onto the surface of Fe3O4@PDA by facile means of stirring a mixture of Fe3O4@PDA and chloroauric acid at 90 °C for 30 min. In this process, the
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polydopamine shell layer served as both the reducing and the capping agent. The deposition of ∼15 nm diameter Au NPs on the surface of Fe3O4@PDA was visually confirmed by TEM (see Fig. 2A and Fig. 2B). More Au NPs were deposited when the
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PDA shell became thicker (see Fig. 2C, 2D and Fig. 2E, 2F), indicating that the thicker PDA shell facilitated the more Au NPs deposition. The size distribution of Au NPs was presented in Figure S2. The size of Au NPs showed strong dependency on the dosage of HAuCl4 [5] with a small size distribution range. At the same time, it was clearly shown that PDA maintained gold nanoparticles in a highly dispersed state without aggregation. The solution of Au/Fe3O4@PDA after magnetic separation was also measured using UV-Vis spectrometry, there was no detectable peak of Au NPs, indicating all Au nanoparticles were located on the nanoparticles. XRD patterns of the Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA were shown in 6
ACCEPTED MANUSCRIPT Figure 3. For Fe3O4, a high-intensity sharp peak at 2θ= 35.5°related to the (311) plane in electron diffraction pattern. Five additional weak peak at 30.1°, 43.1°, 54.4°, 57.0° and 62.6°corresponded to the (220), (440), (422), (511) and (440) planes, respectively. The result was matched well with the datebase of a face centered
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cubic lattice of Fe3O4 (JCPDS card No. 19-629). For Fe3O4@PDA, similar diffraction peaks as Fe3O4 indicated the amorphous coated PDA did not change the crystalline phase of Fe3O4. For Au/Fe3O4@PDA , new peaks at 2θof 38.1°, 43.2°and 65.0° assigned to the (111), (200) and (220) lattice planes of the Au NP electron diffraction.
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The electron diffraction pattern of Au NPs was provided in Figure S3.
FTIR spectrum was employed to examine the surface composition of the
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as-prepared Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA. As shown in Figure S4, the strong peak at 510 cm-1 was related to the vibration of the Fe-O function group, the strong peak at 1628 cm-1 and the wide peak at 3442 cm-1 are corresponding to the surface-adsorbed water and hydroxyl groups. For Fe3O4@PDA, the bands at 1604 cm-1 and 1508 cm-1 belonged to the C=C stretching vibrations of aromatic ring. For
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Au/Fe3O4@PDA, the weak peak at 1615cm-1 was presented due to the Au NPs. The dispersion ability and magnetic properties of the synthesized nanoparticles were evidenced in Figure S5.
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3.2 Catalysis efficiency of the synthesized Au/Fe3O4@PDA reduction
of
p-nitrophenol
to
p-aminophenol
is
known
to
be
thermodynamically favorable with a standard reduction potential of −0.76 V with a
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use of hydrogen source of NaBH4 that possesses the reduction potential of −1.33 V. P-nitrophenol could be reduced to p-aminophenol by the catalysis effect of gold NPs according to previous literatures [34, 35]. P-nitrophenol nevertheless does not spontaneously reduce due to the high activation energy of this reaction [36]. In the absence of Au/Fe3O4@PDA, the aqueous mixture of p-nitrophenol and NaBH4 shows a yellow color, which corresponds to p-nitrophenolate ion in alkaline conditions. The yellow color remains unaltered with time, which suggests that the reduction does not proceed without the presence of Au/Fe3O4@PDA catalyst. However, the addition of Au/Fe3O4@PDA with stirring causes fading and ultimate bleaching of the yellow 7
ACCEPTED MANUSCRIPT color of the reaction mixture in quick succession. The complete conversion of p-nitrophenol
could be visually appreciated by the color change of the solution from
yellow to clear (see Fig. 4A). Time-dependent UV-Vis absorption spectra of this reaction mixture showed the disappearance of the peak at 400 nm and a gradual
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development of a new peak at 300 nm corresponding to the formation of p-aminophenol. The gradual appearance of a peak at 300 nm indicated the corresponding production of p-aminophenol. The reduction of p-nitrophenol into p-aminophenol in the presence of P1 was completed in 20 min (Fig. 4B). Similarly,
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reduction in the presence of P2 and P3 were also investigated under similar conditions, and the reactions were completed in 12 min and 8 min (Fig. 4C and Fig. 4D). These
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results indicated that the reduction of p-nitrophenol by NaBH4 to p-aminophenol can be catalyzed efficiently by the synthesized Au/Fe3O4@PDA hybrid NPs. Au/Fe3O4@PDA
showed
remarkable
conversions
of
p-nitrophenol
to
p-aminophenol at high flow rates. As displayed in Figure 5A, the most rapid increase of the yield in P3 was observed, and the whole conversion was completed in 8 min for
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catalyst P3. Only 58% conversion was observed after 5 min for the catalyst P1, 79% and 87% conversion was observed after 5 min for the catalyst P2 and P3 respectively, the catalytic efficiency was better than other Au-deposited methods [37, 38].
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Because the concentration of sodium borohydride largely exceeds the concentration of p-nitrophenol, the reduction rate can be assumed to be independent of the borohydride concentration. Therefore, pseudo-first-order rate kinetics with regard to the
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p-nitrophenolate concentration could be used to evaluate the catalytic rate. Figure 5B shows the plot of ln(Ct/C0) versus time. The apparent rate constant k of P1 calculated from the ln(Ct/C0) vs. time plot was 0.17 min-1. The kinetic constant k of P2 and P3 were 0.34 min-1 and 0.40 min-1, revealing that the Au/Fe3O4@PDA has a good and high catalytic efficiency for the reduction of p-nitrophenol. Since catalysis being dependent on accessibility and size distribution of Au NPs [5]. The fast reaction kinetics of the p-nitrophenol reduction is believed to result from the satellite nanocatalyst characteristics, such as the highly dispersed gold nanoparticles in each satellite nanocatalyst and small size distribution of Au NPs. PDA is used as supports 8
ACCEPTED MANUSCRIPT for the Au NPs effect the diffusion of p-nitrophenol to the surface of Au NPs and thus enforce the catalytic properties. 3.3 Stability and reusability of the synthesized Au/Fe3O4@PDA Stability against coalescence is a very important issue for nanocrystal-based
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catalysts [39-43]. The stability of Au/Fe3O4@PDA was investigated by repeating the reduction reaction with the same catalyst 8 times (Fig. 6A). After each reaction, the catalyst was recycled by magnetic separation, followed by washing with distilled water and drying in vacuum overnight at room temperature. Although, many catalytic
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studies have been reported in the literature using nanoparticles as catalysts, there are only a few reports where the catalysts were recovered for further use in consecutive
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cycles [13, 44]. As shown in Figure 6A, the catalytic efficiency of the Au/Fe3O4@PDA remained nearly constant even after the eighth recycle. These results suggest that the magnetic removable Au/Fe3O4@PDA was not deactivated or poisoned during the catalytic or separation processes and could be recovered almost completely. The solution after magnetic separation was measured using UV-Vis
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spectrometry. There was no detectable peak of Au NPs after every recycle (8 recycles), which was coincident with the stability experiments. Au NPs were still clearly visualized on the surface of Fe3O4@PDA after 8
reaction cycles (Fig. 6B).
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Clearly, the presence of the PDA shell was efficient in working as a capping agent to stabilize the nanoparticles by preventing their aggregation. Reusability of the catalyst due to easy separation is another advantage in
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industrial applications. Although recyclability is usually regarded as an advantage of nanoparticle-based catalysts, their practical applications in liquid-phase reactions still suffer from both low efficiency of separation and reduced
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resulting from nanoparticle coagulation [45]. The Au/Fe3O4@PDA described here was designed to overcome these challenges. In this system, the separation of products and recycling of catalysts are easy and straightforward. The catalyst can be recovered efficiently from the reaction solution by using external magnetic fields for many cycles without significant losses. Concurrently, the PDA layer effectively stabilizes the catalyst nanoparticles and prevents the reduction in activity due to coagulation, 9
ACCEPTED MANUSCRIPT making the catalyst reusable after multiple cycles of reactions. The green synthesis, without the addition of any toxic reducing agents, stability, and recyclability of Au/Fe3O4@PDA makes it an attractive platform for nanocatalyst. 4. Conclusion
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In summary, a simple and low toxic method to prepare Fe3O4@PDA core−shell nanoparticles with tunable shell thickness as magnetic catalyst support was developed. The deposition of Au NPs atop Fe3O4@PDA can be accomplished through a simple and green method, and the resulting hybrid satellite nanocatalyst exhibited high
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catalytic performance and good reusability for the reduction of p-nitrophenol. The recyclable catalyst is straightforward to separate with fast catalytic ability and
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excellent stability, which is important for application under realistic technical conditions. The strategy presented in this work provides a facile and versatile approach towards designing Fe3O4-based hybrid nanoparticles for biological, energy, and environmental applications. Acknowledgments
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We are grateful for the support of the National Natural Science Foundation of China (21375076, 21275089), the Scientific Research Starting Foundation for Returned
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Overseas (Ministry of Education of China) and the Project of Shandong Province Science and Technology Program (2013GGX10207).
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Figure captions Scheme
1.
The
process
of
depositing
gold
nanoparticles
on
core-shell
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Fe3O4@polydopamine as satellite nanocatalysts and the catalytic application for p-nitophenol reduction.
Figure 1. TEM image of Fe3O4 nanoparticles (A) and SAED of Fe3O4 nanoparticles (B). TEM images of Fe3O4@PDA core−shell nanoparticles obtained for one
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coating layer (C, D), two coating layers (E, F) and three coating layers (G, H).
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Figure 2. TEM images of P1 (A, B), P2 (C, D) and P3 (E, F).
Figure 3: XRD pattern of the synthesized Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA. Figure 4. Reaction scheme and illustration of reaction color change (A), UV-Vis spectra of p-nitrophenol after the addition of catalyst of P1 (B), P2 (C) and P3 (D).
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Figure 5. Conversion of p-nitrophenol versus reaction time (A) and relationship of ln(Ct/C0) versus reaction time (B) for the catalyzed reduction of p-nitrophenol by P1, P2 and P3.
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Figure 6. Graphic illustration of the conversion of p-nitrophenol versus the catalyst cycles (A) and TEM image of Au/Fe3O4@PDA NPs after 8 reaction cycles
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Figure 6.
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19
ACCEPTED MANUSCRIPT Highlights
Core−shell Fe3O4@PDA nanoparticles with tunable-shell thickness was developed.
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The application of Fe3O4@PDA as recyclable nanocatalyst support was presented. Deposition of Au NPs atop Fe3O4@PDA NPs with a high density was achieved.
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The nanocatalysts exhibited high catalytic performance with good stability.
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Facile deposition of gold nanoparticles on core−shell Fe3O4@polydopamine as recyclable nanocatalyst
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Yan Zhao a, Yaowen Yeh b, Rui Liu c, Jinmao You a, Fengli Qu a,c *
University, Qufu, 273165, China
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a College of Chemistry and Chemical Engineering, Qufu Normal
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b Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ, 08544 USA
c Department of Chemical and Biological Engineering, Princeton
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University, Princeton, New Jersey, 08544, USA
Correspondence: Email : [email protected] (F-L Qu)
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Tel/fax: Tel/Fax: (+1) 609-216-6252
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Supplementary Figures Figure S1: TEM image of Fe3O4 NPs after storing for 30 days. Figure S2: The size distribution of Au NPs.
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Figure S3: The electron diffraction pattern of Au NPs. Figure S4: FTIR spectra of the synthesized Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA.
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Figure S5: (a) Fe3O4, (b) Fe3O4@PDA and (c) Au/Fe3O4@PDA dispersed
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in water (A), stored for 5 min (B), and after magnetic separation (C).
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Figure S1
Figure S2
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Figure S3
Figure S4
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Figure S5
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S1293-2558(15)00098-9
DOI:
10.1016/j.solidstatesciences.2015.04.010
Reference:
SSSCIE 5125
To appear in:
Solid State Sciences
Received Date: 9 September 2014 Revised Date:
10 March 2015
Accepted Date: 26 April 2015
Please cite this article as: Y. Zhao, Y. Yeh, R. Liu, J. You, F. Qu, Facile deposition of gold nanoparticles on core−shell Fe3O4@polydopamine as recyclable nanocatalyst, Solid State Sciences (2015), doi: 10.1016/j.solidstatesciences.2015.04.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
ACCEPTED MANUSCRIPT Facile deposition of gold nanoparticles on core−shell Fe3O4@polydopamine as recyclable nanocatalyst
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Yan Zhao a, Yaowen Yeh b, Rui Liu c, Jinmao You a, Fengli Qu a,c *
a College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, 273165, China
b Princeton Institute for the Science and Technology of Materials, Princeton
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University, Princeton, NJ, 08544 USA
c Department of Chemical and Biological Engineering, Princeton University,
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Princeton, New Jersey, 08544, USA
Correspondence: Email : [email protected] (F-L Qu)
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Tel/fax: Tel/Fax: (+1) 609-216-6252
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ACCEPTED MANUSCRIPT ABSTRACT: A simple and green method for the controllable synthesis of core−shell Fe3O4 polydopamine nanoparticles (Fe3O4@PDA NPs) with tunable shell thickness and their application as a recyclable nanocatalyst support is presented. Magnetite Fe3O4 NPs
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formed in a one-pot process by the hydrothermal approach with a diameter of ~240 nm were coated with a polydopamine shell layer with a tunable thickness of 15-45 nm. The facile deposition of Au NPs atop Fe3O4@PDA NPs was achieved by utilizing PDA as both the reducing agent and the coupling agent. The satellite nanocatalysts
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exhibited high catalytic performance for the reduction of p-nitrophenol. Furthermore, the recovery and reuse of the catalyst was demonstrated 8 times without detectible
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loss in activity. The synergistic combination of unique features of PDA and magnetic nanoparticles establishes these core−shell NPs as a versatile platform for potential applications.
KEYWORDS: core-shell nanoparticles, polydopamine, Fe3O4, catalyst 1. Introduction
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Gold nanoparticles (Au NPs) have received considerable attention because of their unique catalytic properties in a number of oxidation and reduction reactions [1-4]. Generally, Au NPs in solution exhibit strong tendency to aggregate because of
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the high surface energy, which leads to the decrease in catalytic activity [5]. Different solid matrices such as carbon nanotubes, silica, titania and other metal oxides [6-8] have been used as support of Au NPs to enhance the catalyst stability. However,
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time-consuming separation procedures such as filtration or centrifugation must be performed to isolate these catalysts from the reaction system [5]. Uniform core−shell nanoparticles with magnetically responsive cores and
functional shells are being widely investigated because of their potential applications in cell separation, enzyme immobilization, protein and nucleic acid purification, targeted drug delivery, environmental remediation separation, catalysis, and magnetic resonance imaging [9-11]. There have been extensive reports on the synthesis of magnetic Fe3O4 core−shell nanoparticles in which the shell layer was composed of either an inorganic (e.g., silica or gold) or an organic (e.g., polymer) material [12]. In 2
ACCEPTED MANUSCRIPT the case of polymer shell layers, polydopamine (PDA) has received significant attention as a candidate material because of its unique coating quality and functionality [13, 14]. Dopamine, the PDA monomer that contains both catechol and amine functional groups, self-polymerizes at alkaline pH values. During
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polymerization, PDA will spontaneously form a conformal and continuous coating layer atop nearly any material present in the reaction media including noble metals, metal oxides, semiconductors, ceramics, and synthetic polymers via the strong binding affinity of catechol functional groups [14]. The PDA layer thickness may be
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controlled by varying the dopamine concentration and reaction time [15].
Polydopamine-coated Fe3O4 nanoparticles (Fe3O4@PDA) have been exploited
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for use in numerous advanced applications, including the separation and enrichment of proteins in proteomic analysis [16], the detection of pollutants by matrix assisted laser desorption/ionization time-of-flight mass spectrometry [17], and magnetic solid-phase extraction adsorbent for the determination of trace polycyclic aromatic hydrocarbons in environmental samples [18]. PDA can serve as an adhesion layer to
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immobilize biological molecules, amine- and mercaptofunctionalized self-assembled monolayers, and metal films to the surface for secondary modification as biosensors [19, 20], support for biomineralization and freestanding films [21, 22].
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Besides its biocompatibility and adhesiveness, PDA has other attractive material properties [23]. Polydopamine is known to reduce metal salts within a solution into metal nanoparticles via the catechol functional groups [24]. A range of metals
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including Au, Ag, Pt, and Cu have been successfully reduced and deposited atop PDA modified surfaces without the need for the addition of a reducing agent [25]. Aromatic amines are widely used in the synthesis of pharmaceuticals, dyes, and
agrochemicals [26]. Two general methods were used for the reduction of aromatic nitrocompounds in industry, which include stoichiometric reduction [27] and catalytic hydrogenation [28]. The catalytic hydrogenation is a convenient method for producing amines in high yield. Reduction of aromatic nitrocompounds using various nanoparticles prepared by different techniques has been investigated [29, 30] and suffers from limitations in reusability and recovery of the catalyst after the reaction. 3
ACCEPTED MANUSCRIPT Here, we present the controllable synthesis of core−shell Fe3O4 polydopamine nanoparticles (Fe3O4@PDA NPs) with tunable shell thickness to create a nanostructured metal catalyst support. Using PDA as a reducing agent, Fe3O4@PDA could be used as a platform to grow and support Au nanoparticles (Au/Fe3O4@PDA)
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for catalysis. The magnetic Au/Fe3O4@PDA satellite nanostructures were easily separated and recycled and exhibited high catalytic performance as illustrated by the reduction reaction of p-nitrophenol. 2. Experimental section
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2.1 Synthesis of Fe3O4
Magnetite Fe3O4 nanospheres were prepared by a previous report through a
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one-pot hydrothermal method [31]. Typically, 0.325 g of FeCl3 and 0.2 g of trisodium citrate dehydrate were dissolved in 20 mL of ethylene glycol under vigorous stirring for 1 h at room temperature till the solids dissolved. Then, 1.2 g of sodium acetate was added while stirring for 30 min. The mixture was sealed in a Teflon-lined stainless-steel autoclave for 12 h at 200 °C. After the reactor was cooled to room
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temperature, the obtained Fe3O4 nanoparticles was separated by the magnetic separation method and washed with ethanol and ultrapure water three times, respectively. The resultant nanoparticles were dispersed in water to 40 mg/mL after
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drying under vacuum for 12 h. 2.2 Synthesis of Fe3O4@PDA
To prepare core−shell Fe3O4@PDA nanoparticles, 1mL of as-prepared Fe3O4
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NPs suspensions were added in 20 mL of 2 mg/mL dopamine Tris solution (pH 8.5, 10 mM Tris−HCl buffer) under stirring and allowed to proceed for 8 h at room temperature. The product was separated and collected with a magnet and subsequently put through three wash cycles and dried under vacuum overnight. The thickness of PDA shell could be controlled by repeating PDA coating through the same procedure. 2.3 Synthesis of Au/Fe3O4@PDA A 2 mg portion of Fe3O4@PDA with tunable shell thickness were well dispersed in a 25 mL chloroauric acid (5μg/mL) aqueous solution. The mixture was stirred at
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Au/Fe3O4@PDA obtained by coating PDA for two times and three times were named as P2 and P3, respectively. 2.4 Catalytic study
Au/Fe3O4@PDA (0.1 mg) was added in 2 mL of p-nitrophenol (0.1 mM)
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containing freshly prepared aqueous solution of NaBH4 (2 mL, 0.1M). UV-Vis absorption spectra were recorded to monitor the change in the reaction mixture. After
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the reduction process was completed, the catalysts were magnetically separated from the mixture, washed several times and dried in a vacuum oven overnight for reuse in the next reaction cycle – this process was repeated 8 times. 3. Results and discussion
3.1 Characterizations of the synthesized nanoparticles
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The nanoscale hybrid system was exploited as illustrated in Scheme 1. Magnetic Fe3O4 NPs were formed in a one-pot process by the hydrothermal approach. The facile deposition of Au NPs atop Fe3O4@PDA NPs was achieved by utilizing PDA as
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both the reducing agent and the coupling agent. The excellent catalytical performance and good recyclability and easy separation ability of the satellite Au/Fe3O4@PDA nanocatalysts were evaluated by the reduction reaction of p-nitrophenol by NaBH4.
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Magnetic nanoparticles Fe3O4 were synthesized by a solvothermal reaction at
200 °C by reduction of FeCl3 with ethylene glycol in the presence of sodium acetate, an alkali source, and trisodium citrate, a biocompatible electrostatic stabilizer [31]. Figure 1A illustrates transmission electron microscopy (TEM) image of the magnetite nanoparticles. As revealed in Figure 1A, the NPs are nearly spherical in shape and uniform in size with a diameter of ∼240 nm. The Fe3O4 nanoparticles exhibit great stability and dispersion ability in water solution after storing for 30 days (Fig. S1) because of the presence of citrate groups. Figure 1B is the selected area electron diffraction pattern (SAED) on a separated Fe3O4 nanoparticle, which has the typical 5
ACCEPTED MANUSCRIPT diffraction pattern character of Fe3O4. PDA layer was formed onto the Fe3O4 NPs surfaces through autopolymerization of dopamine and Tris buffer was employed to adjust the solution pH according to previous reference [32]. In addition, the hydroxyl groups on the Fe3O4 surface may also bond with the catechol groups of dopamine
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through dehydration [33]. Figure 1C and 1D present of Fe3O4@PDA TEM images prepared via mixing Fe3O4 nanoparticles in a dopamine Tris solution. As illustrated, uniform Fe3O4@PDA NPs with a well-defined core−shell structure were successfully synthesized. That is, the Fe3O4 NPs, which represent the core, were successfully
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coated with a thin polydopamine shell layer of ∼15 nm in thickness after one time coating. TEM images of Fe3O4@PDA prepared by repeating PDA coating for two
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times (Fig. 1E and Fig. 1F) and three times (Fig. 1G and Fig. 1H) show that the thicknesses of the PDA shell are ~30 nm and ~45 nm, respectively. It is evidenced that the thickness of the PDA shell can be successfully and easily controlled by repeating PDA coating through the same procedure.
The magnetic Fe3O4@PDA core−shell NPs were applied as a catalyst support for
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Au NPs by directly depositing Au NPs onto Fe3O4@PDA core−shell NPs. Au NPs were reduced and grown onto the surface of Fe3O4@PDA by facile means of stirring a mixture of Fe3O4@PDA and chloroauric acid at 90 °C for 30 min. In this process, the
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polydopamine shell layer served as both the reducing and the capping agent. The deposition of ∼15 nm diameter Au NPs on the surface of Fe3O4@PDA was visually confirmed by TEM (see Fig. 2A and Fig. 2B). More Au NPs were deposited when the
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PDA shell became thicker (see Fig. 2C, 2D and Fig. 2E, 2F), indicating that the thicker PDA shell facilitated the more Au NPs deposition. The size distribution of Au NPs was presented in Figure S2. The size of Au NPs showed strong dependency on the dosage of HAuCl4 [5] with a small size distribution range. At the same time, it was clearly shown that PDA maintained gold nanoparticles in a highly dispersed state without aggregation. The solution of Au/Fe3O4@PDA after magnetic separation was also measured using UV-Vis spectrometry, there was no detectable peak of Au NPs, indicating all Au nanoparticles were located on the nanoparticles. XRD patterns of the Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA were shown in 6
ACCEPTED MANUSCRIPT Figure 3. For Fe3O4, a high-intensity sharp peak at 2θ= 35.5°related to the (311) plane in electron diffraction pattern. Five additional weak peak at 30.1°, 43.1°, 54.4°, 57.0° and 62.6°corresponded to the (220), (440), (422), (511) and (440) planes, respectively. The result was matched well with the datebase of a face centered
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cubic lattice of Fe3O4 (JCPDS card No. 19-629). For Fe3O4@PDA, similar diffraction peaks as Fe3O4 indicated the amorphous coated PDA did not change the crystalline phase of Fe3O4. For Au/Fe3O4@PDA , new peaks at 2θof 38.1°, 43.2°and 65.0° assigned to the (111), (200) and (220) lattice planes of the Au NP electron diffraction.
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The electron diffraction pattern of Au NPs was provided in Figure S3.
FTIR spectrum was employed to examine the surface composition of the
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as-prepared Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA. As shown in Figure S4, the strong peak at 510 cm-1 was related to the vibration of the Fe-O function group, the strong peak at 1628 cm-1 and the wide peak at 3442 cm-1 are corresponding to the surface-adsorbed water and hydroxyl groups. For Fe3O4@PDA, the bands at 1604 cm-1 and 1508 cm-1 belonged to the C=C stretching vibrations of aromatic ring. For
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Au/Fe3O4@PDA, the weak peak at 1615cm-1 was presented due to the Au NPs. The dispersion ability and magnetic properties of the synthesized nanoparticles were evidenced in Figure S5.
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3.2 Catalysis efficiency of the synthesized Au/Fe3O4@PDA reduction
of
p-nitrophenol
to
p-aminophenol
is
known
to
be
thermodynamically favorable with a standard reduction potential of −0.76 V with a
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use of hydrogen source of NaBH4 that possesses the reduction potential of −1.33 V. P-nitrophenol could be reduced to p-aminophenol by the catalysis effect of gold NPs according to previous literatures [34, 35]. P-nitrophenol nevertheless does not spontaneously reduce due to the high activation energy of this reaction [36]. In the absence of Au/Fe3O4@PDA, the aqueous mixture of p-nitrophenol and NaBH4 shows a yellow color, which corresponds to p-nitrophenolate ion in alkaline conditions. The yellow color remains unaltered with time, which suggests that the reduction does not proceed without the presence of Au/Fe3O4@PDA catalyst. However, the addition of Au/Fe3O4@PDA with stirring causes fading and ultimate bleaching of the yellow 7
ACCEPTED MANUSCRIPT color of the reaction mixture in quick succession. The complete conversion of p-nitrophenol
could be visually appreciated by the color change of the solution from
yellow to clear (see Fig. 4A). Time-dependent UV-Vis absorption spectra of this reaction mixture showed the disappearance of the peak at 400 nm and a gradual
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development of a new peak at 300 nm corresponding to the formation of p-aminophenol. The gradual appearance of a peak at 300 nm indicated the corresponding production of p-aminophenol. The reduction of p-nitrophenol into p-aminophenol in the presence of P1 was completed in 20 min (Fig. 4B). Similarly,
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reduction in the presence of P2 and P3 were also investigated under similar conditions, and the reactions were completed in 12 min and 8 min (Fig. 4C and Fig. 4D). These
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showed
remarkable
conversions
of
p-nitrophenol
to
p-aminophenol at high flow rates. As displayed in Figure 5A, the most rapid increase of the yield in P3 was observed, and the whole conversion was completed in 8 min for
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catalyst P3. Only 58% conversion was observed after 5 min for the catalyst P1, 79% and 87% conversion was observed after 5 min for the catalyst P2 and P3 respectively, the catalytic efficiency was better than other Au-deposited methods [37, 38].
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Because the concentration of sodium borohydride largely exceeds the concentration of p-nitrophenol, the reduction rate can be assumed to be independent of the borohydride concentration. Therefore, pseudo-first-order rate kinetics with regard to the
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p-nitrophenolate concentration could be used to evaluate the catalytic rate. Figure 5B shows the plot of ln(Ct/C0) versus time. The apparent rate constant k of P1 calculated from the ln(Ct/C0) vs. time plot was 0.17 min-1. The kinetic constant k of P2 and P3 were 0.34 min-1 and 0.40 min-1, revealing that the Au/Fe3O4@PDA has a good and high catalytic efficiency for the reduction of p-nitrophenol. Since catalysis being dependent on accessibility and size distribution of Au NPs [5]. The fast reaction kinetics of the p-nitrophenol reduction is believed to result from the satellite nanocatalyst characteristics, such as the highly dispersed gold nanoparticles in each satellite nanocatalyst and small size distribution of Au NPs. PDA is used as supports 8
ACCEPTED MANUSCRIPT for the Au NPs effect the diffusion of p-nitrophenol to the surface of Au NPs and thus enforce the catalytic properties. 3.3 Stability and reusability of the synthesized Au/Fe3O4@PDA Stability against coalescence is a very important issue for nanocrystal-based
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catalysts [39-43]. The stability of Au/Fe3O4@PDA was investigated by repeating the reduction reaction with the same catalyst 8 times (Fig. 6A). After each reaction, the catalyst was recycled by magnetic separation, followed by washing with distilled water and drying in vacuum overnight at room temperature. Although, many catalytic
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studies have been reported in the literature using nanoparticles as catalysts, there are only a few reports where the catalysts were recovered for further use in consecutive
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cycles [13, 44]. As shown in Figure 6A, the catalytic efficiency of the Au/Fe3O4@PDA remained nearly constant even after the eighth recycle. These results suggest that the magnetic removable Au/Fe3O4@PDA was not deactivated or poisoned during the catalytic or separation processes and could be recovered almost completely. The solution after magnetic separation was measured using UV-Vis
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spectrometry. There was no detectable peak of Au NPs after every recycle (8 recycles), which was coincident with the stability experiments. Au NPs were still clearly visualized on the surface of Fe3O4@PDA after 8
reaction cycles (Fig. 6B).
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Clearly, the presence of the PDA shell was efficient in working as a capping agent to stabilize the nanoparticles by preventing their aggregation. Reusability of the catalyst due to easy separation is another advantage in
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industrial applications. Although recyclability is usually regarded as an advantage of nanoparticle-based catalysts, their practical applications in liquid-phase reactions still suffer from both low efficiency of separation and reduced
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resulting from nanoparticle coagulation [45]. The Au/Fe3O4@PDA described here was designed to overcome these challenges. In this system, the separation of products and recycling of catalysts are easy and straightforward. The catalyst can be recovered efficiently from the reaction solution by using external magnetic fields for many cycles without significant losses. Concurrently, the PDA layer effectively stabilizes the catalyst nanoparticles and prevents the reduction in activity due to coagulation, 9
ACCEPTED MANUSCRIPT making the catalyst reusable after multiple cycles of reactions. The green synthesis, without the addition of any toxic reducing agents, stability, and recyclability of Au/Fe3O4@PDA makes it an attractive platform for nanocatalyst. 4. Conclusion
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In summary, a simple and low toxic method to prepare Fe3O4@PDA core−shell nanoparticles with tunable shell thickness as magnetic catalyst support was developed. The deposition of Au NPs atop Fe3O4@PDA can be accomplished through a simple and green method, and the resulting hybrid satellite nanocatalyst exhibited high
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catalytic performance and good reusability for the reduction of p-nitrophenol. The recyclable catalyst is straightforward to separate with fast catalytic ability and
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excellent stability, which is important for application under realistic technical conditions. The strategy presented in this work provides a facile and versatile approach towards designing Fe3O4-based hybrid nanoparticles for biological, energy, and environmental applications. Acknowledgments
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We are grateful for the support of the National Natural Science Foundation of China (21375076, 21275089), the Scientific Research Starting Foundation for Returned
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Overseas (Ministry of Education of China) and the Project of Shandong Province Science and Technology Program (2013GGX10207).
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Figure captions Scheme
1.
The
process
of
depositing
gold
nanoparticles
on
core-shell
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Fe3O4@polydopamine as satellite nanocatalysts and the catalytic application for p-nitophenol reduction.
Figure 1. TEM image of Fe3O4 nanoparticles (A) and SAED of Fe3O4 nanoparticles (B). TEM images of Fe3O4@PDA core−shell nanoparticles obtained for one
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coating layer (C, D), two coating layers (E, F) and three coating layers (G, H).
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Figure 2. TEM images of P1 (A, B), P2 (C, D) and P3 (E, F).
Figure 3: XRD pattern of the synthesized Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA. Figure 4. Reaction scheme and illustration of reaction color change (A), UV-Vis spectra of p-nitrophenol after the addition of catalyst of P1 (B), P2 (C) and P3 (D).
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Figure 5. Conversion of p-nitrophenol versus reaction time (A) and relationship of ln(Ct/C0) versus reaction time (B) for the catalyzed reduction of p-nitrophenol by P1, P2 and P3.
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Figure 6. Graphic illustration of the conversion of p-nitrophenol versus the catalyst cycles (A) and TEM image of Au/Fe3O4@PDA NPs after 8 reaction cycles
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(B).
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Scheme 1.
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Figure 1.
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Figure 2.
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Figure 3
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Figure 4.
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Figure 5.
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Figure 6.
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Core−shell Fe3O4@PDA nanoparticles with tunable-shell thickness was developed.
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The application of Fe3O4@PDA as recyclable nanocatalyst support was presented. Deposition of Au NPs atop Fe3O4@PDA NPs with a high density was achieved.
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The nanocatalysts exhibited high catalytic performance with good stability.
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Facile deposition of gold nanoparticles on core−shell Fe3O4@polydopamine as recyclable nanocatalyst
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Yan Zhao a, Yaowen Yeh b, Rui Liu c, Jinmao You a, Fengli Qu a,c *
University, Qufu, 273165, China
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a College of Chemistry and Chemical Engineering, Qufu Normal
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b Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, NJ, 08544 USA
c Department of Chemical and Biological Engineering, Princeton
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University, Princeton, New Jersey, 08544, USA
Correspondence: Email : [email protected] (F-L Qu)
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Tel/fax: Tel/Fax: (+1) 609-216-6252
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Supplementary Figures Figure S1: TEM image of Fe3O4 NPs after storing for 30 days. Figure S2: The size distribution of Au NPs.
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Figure S3: The electron diffraction pattern of Au NPs. Figure S4: FTIR spectra of the synthesized Fe3O4, Fe3O4@PDA and Au/Fe3O4@PDA.
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Figure S5: (a) Fe3O4, (b) Fe3O4@PDA and (c) Au/Fe3O4@PDA dispersed
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in water (A), stored for 5 min (B), and after magnetic separation (C).
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