Magnetite–Polypeptide Hybrid Materials Decorated with Gold

Article pubs.acs.org/JPCC

Magnetite−Polypeptide Hybrid Materials Decorated with Gold Nanoparticles: Study of Their Catalytic Activity in 4‑Nitrophenol Reduction G. Marcelo,* A. Muñoz-Bonilla, and M. Fernández-García Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), C/Juan de la Cierva 3, 28006-Madrid, Spain ABSTRACT: The preparation and physical−chemical characterization of gold decorated hybrid materials based on individual magnetite nanoparticles coated with a poly(γ-benzyl-L-glutamate) shell is here presented. First, the synthesis of hybrid magnetite−polypeptide is achieved using a mimetic adhesive molecule, dopamine, which is responsible of both, anchoring the magnetite and initiating the ringopening polymerization of γ-benzyl-L-glutamate N-carboxyanhydride. Besides, and in a second step, new molecules of dopamine are introduced to provide functionality to the poly(γ-benzyl-L-glutamate) by simple aminolysis reaction, which allow the growth and the stabilization of gold nanoparticles inside the polypeptide shell. Finally, the potential of these nanoparticles as a magnetic catalyst is proved by the reduction reaction of nitrophenol to aminophenol. The 100% efficiency with the magnetic recovery after eight catalytic cycles is also demonstrated.

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INTRODUCTION Magnetic nanoparticles, especially magnetite and maghemite, are very attractive for a broad range of applications, including magnetic fluids,1 data storage,2 biomedical applications3 or catalysis.4 In the later area, superparamagnetic nanoparticles (size ≤20 nm) of magnetite have satisfactory specific surface area, but the weak magnetic response makes difficult to efficiently separate them from solution using moderate magnetic field gradients.5 In the last years, the development of synthetic hydrophobic routes (thermal decomposition process) of magnetite nanoparticles has allowed the design of particles with the desired size and shape.6 Nanoparticles (hereafter referred as NPs) coming from these routes are uniform and highly crystalline; therefore, with strong magnetic response in relation to those obtained from coprecipitation methods. The preparation of larger paramagnetic particles (100 nm ≥ size ≥ 20 nm) without a great loss of surface area is highly desirable for catalysts support materials. However, they are poorly dispersed in aqueous solution due to their hydrophobic surface.6 To find applications in aqueous medium, magnetite nanocrystals have been coated with inorganic shells (e.g., gold and silica) or hydrophilic polymers.3 Among these, the last one has received a great interest in the recent years because the polymer protects the magnetic core from the environment also renders stable and water dispersible nanoparticles and last but not least provides the desired functionality to further modifications. Polymers can coat a single magnetic nanoparticle by both physical adsorption and through in situ polymerizations, such as the surface initiated atom transfer radical polymerization (SI-ATRP),7,8 reversible addition−fragmentation chain transfer (RAFT) polymerization,9 or even by ring-opening polymerization (ROP).10−13 © 2012 American Chemical Society

Coating magnetite surface with a polypeptide shell can be an interesting alternative because of its unique secondary structure, significant functionality, and superior biocompatibility.14 Polypeptide chains can reversibly change their conformations,15 such as α-helix, β-sheet, and random coil under appropriate external stimuli such as pH,16 solvent,17 and temperature,18,19 concomitantly resulting in changes of their water solubility. They are readily accessible by ROP of amino acid N-carboxy anhydrides (NCA). Moreover, metallic nanoparticles, especially gold nanoparticles, have attracted intensive attention for their numerous applications in catalysis, sensing, imaging, diagnostics, therapy, and delivery.20−24 In particular, their large specific surface area made them highly catalytic active and then attractive tools for catalysis.25 However, the separation and recovery of the metal nanoparticles from the reaction mixture solution are difficult because of their size. A plausible approach to solve this matter is the use of magnetic particles as a platform for the metal nanoparticles. In this sense, the loading of metallic nanoparticles in the magnetite polymeric shell has been achieved by magnetite modification with a functional polymeric shell that contains functional groups to stabilize the nucleation and growth of metallic nanoparticles.26−29 This strategy has not been extensively used probably due to the tedious synthetic strategies necessary to obtain uniform magnetite nanoparticles coated with a polymeric shell. Furthermore, it has been recently demonstrated that catechol-containing molecules present the ability to adhere to almost any material of either organic or inorganic character to Received: September 14, 2012 Revised: October 25, 2012 Published: November 1, 2012 24717

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Scheme 1. Schematic Illustration of Fe3O4@PBLG NPs Preparation

form robust adhesion. 30 An example is given by 3,4dihydroxyphenylalanine (DOPA) that allows mussels to adhere at high binding strength and under wet conditions to a large variety of surfaces.30,31 Apart from their exceptional binding affinities to various substrates, catechol molecules are strongly studied for their unique reductive properties.32 The hydroxyphenols groups can suffer rapid oxidative self-conversion into their quinone form by releasing protons and electrons under mild reductive conditions. In this sense, several studies have previously demonstrated that various molecules containing phenolic compounds can be utilized as reducing agents for the synthesis and surface stabilization of gold nanoparticles.33,34 To the best of our knowledge, the approach of using the catechol groups incorporation into a magnetite polymeric shell that does not contain active functional groups to provide specific sites to stabilize the gold nanoparticles formation has not been explored. This strategy is a simple way to create specific architectures and consequently the purpose of the present work. In a previous article,10 we proposed a straightforward strategy to coat magnetite with a poly(benzyl-L-glutamate) shell by ROP initiated from a natural adhesive (dopamine) strongly attached to magnetite surface. In that article, the possibility of polypeptide shell modification was proved by incorporation

of a model compound, procaine, by aminolysis reaction. In the current work, we use the same strategy to modify the magnetite−polypeptide nanoparticles introducing new molecules of dopamine via aminolysis, which can act as support of gold NPs. The incorporation of active superficial groups in the polypeptide shell (dopamine molecules) affords to obtain a functional polypeptide shell where these catechol groups play an important role in the synthesis and stabilization of gold nanoparticles inside of the polypeptide shell. The resulting magnetic polymer−metal hybrids are used as magnetic catalyst model in the reduction process of 4-nitrophenol to 4aminophenol.

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EXPERIMENTAL SECTION Materials. Iron(III) acetylacetonate, oleic acid, dibenzyl ether, litium chloride (LiCl), sodium hydroxide (NaOH), chlorhydric acid (HCl), triethylamine (TEA), 4-dimethylaminopyridine (4-DMAP), dopamine hydrochloride, and sodium borohydride (NaBH4) were all purchased from Aldrich and used as received. γ-Benzyl-L-glutamate N-carboxyanhydride (BLG-NCA) was supplied by Isochem-SNPE (France). All organic solvents dimethylformamide (DMF), tetrahydrofuran (THF), methanol (MeOH), ethanol (EtOH), and 4-nitro24718

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Scheme 2. Schematic Illustration of Gold Functionalization of Polypeptide Shell to Achieve Fe3O4@PBLG@Au NPs

Synthesis of Magnetite Nanoparticles (Fe3O4 NPs). Fe3O4 NPs were prepared according to a previously described work.35 In a typical synthesis, iron(III) acetylacetonate (1.00 g) was added to a mixture of oleic acid (1.17 g) and benzyl ether (50.00 g). The mixture solution was degassed at room temperature for 1 h and then heated to 290 °C at the rate 14 °C/min under vigorous magnetic stirring. The reaction mixture was maintained at this temperature for 90 min. After cooling to room temperature, a mixture of toluene and hexane was added to the solution. The solution was then centrifuged to precipitate the Fe3O4 NPs. The separated precipitate was washed using chloroform at least four times. Functionalization of Magnetite with Dopamine. Biomimetic coating strategy drawn from mussel adhesive protein sequences mimicking mussel adhesion has been chosen to modify magnetite surface.36−41According to our previous work,10 250 mg of dopamine was added to 15 mg of oleic acidstabilized Fe3O4 NPs, dispersed in 3 mL of DMF. The dispersion was treated with a sonifier (45 min, settings: 20% amplitude, 3s on and 2s off). Then the particles were isolated with an external magnetic field and rinsed with EtOH at least three times. Particles after dopamine modification were named as Fe3O4@dopamine NPs. Polymerization of PBLG from Dopamine Adhered to Magnetite Surface (Fe3O4@PBLG) (Scheme 1). Prior to polymerization, the dopamine functionalized nanoparticles were neutralized with TEA in methanol. To do that, 200 μL of TEA were added to a 20 mg suspension of Fe3O4@ dopamine NPs in 2 mL of methanol. The mixture was left shaking overnight. Then several washes were performed with ethanol to remove TEA. The ring-opening polymerization of the BLG-NCA was initiated by the amine groups of the dopamine functionalized nanoparticles. A typical procedure was as follows: BLG-NCA (1.6 g) was placed into a round-bottom flask under argon atmosphere and dissolved in 20 mL of DMF previously dried. The Fe3O4@dopamine NPs (12.5 mg) were dispersed in 6 mL of dry DMF by ultrasonic treatment and vigorous magnetic stirring. The nanoparticles dispersion was then added to the BLG-NCA containing flask and the polymerization was initiated. The mixture was stirring at room temperature under argon atmosphere. After 3 days, the nanoparticles were isolated with an external magnetic field and, subsequently, washed several times with DMF and ethanol. The

phenol (4-NP) were purchased from Scharlau. Hydrogen tetrachloroaurate (III) (HAuCl4) was obtained from Alfa Aesar. Measurements and Equipments. Scanning electron microscopy (SEM) measurements were performed using a field emission scanning electron microscope (FE-SEM) (Hitachi, SU 8000, Japan) at 5 kV in transmitted electron imaging mode. To prepare the SEM samples a dispersion of magnetite particles in hexane, in THF in case of Fe3O4@PBLG NPs or in EtOH in case of Fe3O4@PBLG-Au NPs was dropped onto a carbon-coated copper grid. The FTIR spectra of KBr pellets were recorded using a PerkinElmer Spectrum 2000 FTIR spectrometer incorporating a deuterated triglycine sulphide (DTGS) detector and an extended range KBr beamsplitter. X-ray diffraction (XRD) patterns were recorded in the reflection mode by using a Bruker D8 Advance diffractometer provided with a PSD Vantec detector (from Bruker, Madison, Wisconsin). CuKα radiation (λ = 0.1542 nm) was used, operating at 40 kV and 40 mA. The parallel beam optics was adjusted by a parabolic Göbel mirror with horizontal grazing incidence Soller slit of 0.12° and LiF monochromator. The equipment was calibrated with different standards. A step scanning mode was employed for the detector. The diffraction scans were collected within the range of 2θ = 4−80°, with a 2θ step of 0.024° and 0.5 s per step. The hydrodynamic diameter of each nanoparticle sample in solution (0.11 mg/mL in DMF or EtOH) was determined by dynamic light scattering measurements using a Malvern Zetasizer Nano ZS series equipment. Nanoparticle dispersions were sonicated for 10 min and left to equilibrate for 2 min before the measurements. Thermogravimetric analyses (TGA) were carried out on a TGA Q500−0885 equipment of TA Instrumental Analysis. Dynamic experiments were performed at a heating rate of 10 °C/min from room temperature up to 800 °C under a nitrogen atmosphere (50 cm3/min). X-ray photoelectron spectra (XPS) were recorded with a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and a MgKα (hν = 1253.6 eV) nonmonochromatic X-ray source. The samples were degassed in the pretreatment chamber at room temperature for 1 h prior to being into the instrument’s ultrahigh vacuum analysis chamber. The spectra were calibrated in relation to the C 1s binding energy (284.6 eV), which was applied as an internal standard. UV−vis spectra were recorded in a PerkinElmer Lambda 16 spectrophotometer. Particles were dispersed by using a Vibra-cell75186 ultrasonic processor. 24719

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Figure 1. FE-SEM images of Fe3O4 NPs. Histogram of the magnetite edge length distribution obtained from FE-SEM images.

resulting hybrid nanoparticles are named as Fe3O4@PBLG NPs. Decoration of Polypeptide Shell with Gold Nanoparticles (Fe3O4@PBLG@Au NPs) (Scheme 2). The loading of the polypeptide shell with gold NPs was performed after two separated steps. The first step was the aminolysis reaction between the PBLG segment and dopamine. Briefly, 4 mg of Fe3O4@PBLG nanoparticles were dispersed in 10 mL of DMF in an ultrasounds bath and then 220 mg of dopamine, 70 mg of 4-dimethylaminopyridine, and 120 μL of TEA were added. The mixture was stirred at 40 °C for 36 h. The resultant nanoparticles termed as Fe3O4@PBLG-dopamine NPs were isolated from reaction media with an external magnetic field and further were purified with EtOH at least twice. In the second step, 4 mg of Fe3O4@PBLG-dopamine NPs were dispersed in 10 mL of DMF. Then 0.75 mL of DMF solution containing 4 mM HAuCl4 was added and the mixture was stirred for 2 h. After that, 1.5 mL of NaBH4 0.05 M in DMF was added to the mixture. Immediate gold nanoparticles formation is observed as a consequence of a change in dispersion color and a notable increase in nanoparticles density. The samples were washed several times with EtOH. The resulting nanoparticles were identified as Fe3O4@PBLG@Au NPs. Catalytic Reactions with Fe3O4@PBLG@Au NPs as Catalyst. In a typical reaction, 0.03 mg of catalyst (Fe3O4@ PBLG@Au NPs) was added to 0.10 mL of aqueous 4-NP solution (5 mM), 1.0 mL of fresh NaBH4 (0.2 M) aqueous solution, and 2 mL ultrapure water in a quartz cuvette at room temperature, and the mixture was quickly subjected to UV−vis measurements. The initially obtained data can be designated as the starting point for the reaction, t = 0. Afterward, the solution was measured at different times to track the catalytic reaction.

Figure 2. Magnetization versus magnetic field at 300 K: M−H curves for Fe3O4 NPs (black) and Fe3O4@PBLG NPs (red).

field (M-H) curves indicating the ferrimagnetic nature of the Fe3O4 cubes. The synthesized Fe3O4 NPs have high saturation magnetization (Ms) and low coercivity values, 68.5 emu/g and 74.5 mT, respectively. Surface Functionalization of Fe3O4 with Dopamine (Fe3O4@dopamine NPs). The oleic exchange on magnetite surface by dopamine was confirmed by the information obtained by three different techniques. FTIR spectroscopy affords to identify dopamine bands. The characteristic bands of oleic acid remarkably disappear after the dopamine exchange, whereas a new absorption band centered at 1635 cm−1 appeared. This absorption band can be assigned to the C−C vibration of catechol rings. The new band at 1260 cm−1 can be assigned to the C−O single bond vibration of phenolic moieties. Positive ξ-potential of magnetite nanoparticles (35 mV) upon dopamine functionalization and XPS that permits to identify C, N, O and Fe, confirm the success of the ligand exchange, (see Figure 3). Magnetite Coated with a PBLG Shell (Fe3O4@PBLG NPs). As we described previously,10 Fe3O4@PBLG NPs were synthesized via ring-opening polymerization of γ-benzyl-L-

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RESULTS AND DISCUSSION Synthesis of Magnetite Nanoparticles (Fe3O4 NPs). Figure 1 shows FE-SEM images of the prepared magnetite particles by thermal decomposition of Fe(III) acetylacetonate in the presence of oleic acid. It reveals that the synthesized nanoparticles are well-defined cube shaped structures with a mean uniform edge length of 27 ± 7 nm. Figure 2 displays the magnetic behavior of the synthesized Fe3O4 NPs along with Fe3O4@PBLG NPs measured at 300 K. Both nanoparticles show hysteresis loops in the magnetization against magnetic 24720

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Figure 3. (A) FTIR spectra for Fe3O4 NPs (black) and Fe3O4@dopamine NPs (red), (B) XPS spectrum of the Fe3O4@dopamine NPs.

glutamate-N-carboxyanhydride (BLG-NCA)42 from the surface of Fe3O4@dopamine NPs using the terminated amine groups of the dopamine as ROP initiator. Figure 4 shows the SEM

Figure 5. FTIR spectra for Fe3O4@PBLG NPs (black) and Fe3O4@ PBLG-dopamine NPs (red).

Figure 4. FE-SEM image of Fe3O4@PBLG NPs.

image of Fe3O4@PBLG NPs. A polymeric shell with an average thickness of ca. 15 nm (taking into account the shell thickness in different areas of the picture) coating the Fe3O4 NPs can be observed. The amount of PBLG on magnetite surface was measured quantitatively by TGA. The weight percentage of PBLG was around 7.5%. Figure 5 depicts the FTIR spectrum of Fe3O4@ PBLG NPs, which shows a strong peak at 583 cm−1 associated to the Fe−O stretching vibration. Besides, two characteristic bands (697 and 750 cm−1) are attributed to the phenyl groups. The peaks in the region from 1500 to 1800 cm−1 reveal that PBLG is successfully coated onto the magnetite surface and, furthermore, the PBLG chains adopt a α-helix conformation.43 The diffractogram for Fe3O4@PBLG NPs is shown in Figure 6, where all of the diffraction peaks are consistent with the database in JCPDS file,44 and can be indexed according to the inverse spinel structure of magnetite. The signals belonging to the corresponding polypeptide shells are not observed. The coating of magnetite nanoparticles with a protected PBLG shell leads to a slightly decrease in saturation magnetization value, from 68.5 to 63.0 emu/g, due to the lower density of the magnetic component in the Fe3O4@PBLG sample (Figure 2). However, considering only the magnetic component in the sample (∼92.5%), the Ms value is practically

Figure 6. XRD patterns of the Fe3O4@PBLG NPs (black) and Fe3O4@PBLG@Au NPs (pink).

the same, which indicates that the magnetite preserves its characteristics. The magnetization property enables the Fe3O4@PBLG NPs to be easily separated from THF or DMF solutions under an external magnetic field in less than 30 s, which makes it suitable 24721

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Figure 7. FE-SEM images of Fe3O4@PBLG@Au NPs.

Figure 8. (A) Intensity distribution for Fe3O4@PBLG@Au NPs in EtOH (red). (B) Number distributions for Fe3O4@PBLG NPs in DMF (black) and Fe3O4@PBLG@Au NPs in EtOH (red).

substitution of the benzyl groups by more voluminous dopamine molecules in high extent can lead to a destabilization of the α-helices. Gold seeds were then generated inside the polypeptide shell by interaction of gold ions with catechol groups followed by fast in situ reduction using sodium borohydride. Studies performed in absence of external reductor (NaBH4) do not lead to gold NPs formation in the polypeptide shell. Besides, to estimate the importance of catechol groups in gold formation and stabilization the reduction reaction was carried out on the Fe3O4@PBLG NPs. Formation of gold NPs inside of polypeptide shell or over the polypeptide surface is not observed because the Fe3O4@PBLG NPs do not present functional groups to promote the nucleation or stabilization of gold NPs in the polypeptide shell, which confirms the necessity of catechol groups. In conclusion, catechol groups might act as sites where AuCl4− ions are reduced to gold nucleuses that immediately grow with the addition of NaBH4. Figure 7 displays the FE-SEM images of Fe3O4@PBLG@Au NPs. It can be noted that gold NPs with a uniform size of ca. 6 ± 1 nm are decorating the polypeptide coating. The small aggregation observed in the micrograph may be due to the FESEM sample preparation. Therefore, this system meets the basic requirements in the catalytic activity of gold.25 Considering that the final amine group is involved in an intramolecular termination step at the end of ROP,42 there are

to act as magnetic support of catalytic gold nanoparticles and facilitates the magnetic recovery of the catalyst. Fe3O4@PBLG@Au NPs. Redox reactions are commonly used to form gold NPs in solution with reducing agents such as sodium borohydride or sodium citrate.45−47 Catechol-containing molecules can be used as reductant for the generation of metal NPs.48,49 In addition, the formation of polymer-coated metal nanoparticles (gold or silver NPs) through reduction of metal cations with 3,4-dihydroxyphenylalanine (DOPA)containing poly(ethylene glycol) (PEG) has been recently described.50 By using the possibility of polypeptide modification by aminolysis reaction once it is anchored onto the magnetite surface,10 dopamine molecules were incorporated in the polypeptide shell structure. The change on the polypeptide structure upon modification with dopamine by aminolysis reaction was studied by FTIR. In Figure 5 is also depicted the Fe3O4@PBLG-dopamine FTIR spectrum. The characteristic absorption peaks of phenyl groups at 749 and 697 cm−1 completely disappeared after reaction with dopamine. The new band at around 1260 cm−1 could be assigned to the C−O single bond vibration of phenolic moieties.51 Moreover, the secondary structure of the polypeptide changes after dopamine incorporation. The position of amide I band is shifted at 1630 cm −1 indicating that the β-sheet conformation is the predominant.52,53 This may be explained considering that the 24722

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nanoparticles did not reveal any absorption bands in the visible region of the spectrum, while hybrid Fe3O4@PBLG@Au NPs showed a broad SPR band and with a maximum in the yellow region at 580 nm. Catalytic Activity. Au-catalyzed reduction of 4-nitrophenol (4-NP) in the presence of NaBH4 is selected as a model system to demonstrate the use of Fe3O4@PBLG@Au NPs as recoverable catalyst−supports.55,56 The substrates and products of this reaction are easily detected by spectroscopic methods, and there is no appreciable byproduct formation. The reduction reaction does not proceed even in a large excess of NaBH4 without Au catalyst as could be evidenced by a non varying absorption spectrum with the main peak located at 400 nm, a wavelength characteristic of 4-nitrophenolate ions. However, when a trace amount of catalyst is introduced into the solution, the absorption at 400 nm decreases and absorption at 295 nm increases gradually indicating the reduction of 4-NP and formation of 4-aminophenol (4-AP), respectively. Part A of Figure 10 shows the UV−vis spectra as a function of reaction time for the reduction process. Visually, the bright-yellow solution gradually becomes colorless over 50 min indicating complete conversion of 4-NP to 4-AP. To reveal magnetic recovery of the Fe3O4@PBLG@Au NPs, the catalyst was quickly separated from solution using an external magnetic field, rinsed several times with deionized water, and dispersed into deionized water for the next cycle of catalysis. As shown in part B of Figure 10, the catalyst can be successfully recycled and reused in eight successive reactions, all with conversions of 100% within 50 min for the first five cycles. After the fifth cycle, the time required to reach complete reduction is longer increasing with the number of cycles. Moreover, the conversion starts decreasing after 8 cycles, that is, the reduction reaction does not reach full conversion after 24 h. This fact could be probably due to gradual loss of the catalyst by washing during the repeated magnetic separation. Although other factors such as gold NPs agglomeration can be taken into account.57−59

not free amino groups on hybrid material surface to link gold NPs and consequently the gold NPS are formed mostly inside of polypeptide shell. The hydrophilicity and the dispersion stability of magnetic nanoparticles change considerably upon the incorporation of gold nanoparticles in the polypeptide shell. Actually, Fe3O4@ PBLG@Au NPs are dispersible in water or EtOH solutions. Figure 8 displays both the number and intensity distributions for a dispersion of Fe3O4@PBLG@Au NPs in EtOH. The number distribution shows two populations. The first peak corresponds to the size of isolated Fe3O4@PBLG@Au NPs with the maximum centered at ca. 130 nm. The second belongs to large size aggregates with the maximum centered at ca. 420 nm. It is much less intense than the first peak indicating that most of Fe3O4@PBLG@Au NPs are dispersed in EtOH solution as isolated particles. In contrast, Fe3O4@PBLG NPs that were dispersible in DMF solutions, with a hydrodynamic diameter of ca. 400 nm, are no dispersible in EtOH solution. The XRD data of Fe3O4@PBLG@Au NPs, Figure 4, shows diffraction peaks at 2θ = 38.2°, 44.4°, 64.6°, which can be indexed to (111), (200), and (220) planes of gold in a cubic phase.54 To further confirm the formation of the gold nanoparticles inside the polypeptide shell of Fe3O4@PBLG NPs, surface plasmon resonance (SPR) of gold in the Fe3O4@ PBLG@Au NPs was spectrophotometrically recorded as can be observed in Figure 9. The spectrum of Fe3O4@PBLG

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CONCLUSIONS We have described a new successful approach to prepare uniform core−shell structures based on magnetite−polypeptide, designed with a highly hydrophobic and crystalline monodisperse magnetite core coated with a uniform and controlled shell of PBLG, which served as support of metallic gold nanoparticles. The polypeptide shell of PBLG allows the

Figure 9. UV−vis absorption spectra of Fe3O4@PBLG NPs (black) in DMF and Fe3O4@PBLG@Au NPs (blue) in EtOH.

Figure 10. (A) UV−vis spectra showing the gradual reduction of 4-NP with Fe3O4@PBLG@Au catalyst. (B) Conversion of 4-NP in nine successive cycles of reduction and magnetic separation of catalyst (●), time required for total conversion in each catalytic cycle (▲). 24723

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modification to incorporate dopamine molecules in its structure, which act as sites for the gold nucleation and growth and, at the same time, as adhesive to the metallic gold surface. Therefore, dopamine plays two essential roles in the hybrid material formation. First, dopamine acts as a biomimetic adhesive anchoring to the magnetite surface and as initiator that affords the ROP coating of magnetite surface with poly(γbenzyl-L-glutamate). In second place and after its incorporation into polypeptide shell, it acts as reductor participating in the formation and growth of gold NPs. In addition to that, in this stage the dopamine also acts as linker of gold NPs. The resulting Fe3O4@PBLG@Au NPs possess high magnetization value, water stability, and elevated performance in the catalytic reduction of 4-NP (conversion of 95% in 25 min). As a final point, we can conclude that the catalyst can be truly reusable because it keeps the 100% of its activity during eight catalytic cycles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank to the MINECO (Spain) for financial support (Project MAT2010-17016). G. Marcelo and A. Muñoz-Bonilla gratefully acknowledge CSIC and MINECO for her JAE-doc and Juan de la Cierva postdoctoral contracts. Authors thank Mr. D. Gómez for FE-SEM images.

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