Nanoparticles with Fe3O4−Nanoparticle Cores and Gold-Nanoparticle

Feb 9, 2011 - C , 2011, 115 (8), pp 3304–3312 ... Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challeng...
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Nanoparticles with Fe3O4-Nanoparticle Cores and Gold-Nanoparticle Coronae Prepared by Self-Assembly Approach Jia Tian, Fan Zheng, and Hanying Zhao* Key Laboratory of Functional Polymer Materials, Ministry of Education, Department of Chemistry, Nankai University, Tianjin 300071, China

bS Supporting Information ABSTRACT: Polystyrene (PS) brushes on Fe3O4 nanoparticles were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization, and after reduction reaction PS brushes with terminal thiol groups (HS-PS-Fe3O4NPs) were obtained. Citrate-stabilized gold nanoparticles (AuNPs) were prepared in aqueous solution and HS-PS-Fe3O4NPs were dispersed in toluene. Upon mixing of the aqueous solution with the toluene solution, a stable O/W emulsion was prepared. Hydrophilic gold nanoparticles interacted with hydrophobic Fe3O4 nanoparticles via Au-S interaction, and amphiphilic nanoparticle complexes were formed at the liquid-liquid interface. The stability of the emulsion was achieved by the formation of the amphiphilic nanoparticle complexes. Upon transfer of the emulsion into excess methanol, core-shell structures with Fe3O4nanoparticle cores and gold-nanoparticle coronae were fabricated. In this manuscript, the influences of the weight ratio of HS-PS-Fe3O4NPs to AuNP and volume ratio of toluene to water on the self-assembly structures were also investigated.

’ INTRODUCTION Self-assembly of nanoparticles has emerged as a powerful bottom-up method for fabricating ordered nanostructured materials with functional properties.1 The combination of inorganic components and organic polymeric materials provides a direct way to assemble inorganic nanoparticles into desired structures due to the diversity and different properties of the polymer materials.2 Over the past decade, block copolymers as templates for the synthesis of metallic or semiconducting nanomaterials have been reported.3 For example, simple mixing of CdSe nanoparticles with block copolymers in thin films resulted in the ordering of the nanoparticles driven by the phase-separation of the block copolymers.4 Hybrid polymers with nanoparticles are also able to self-assemble into advanced structures. In previous articles, we reported self-assembly of amphiphilic hybrid gold nanoparticles (AuNPs). In aqueous solution polystyrene (PS) with pendant hydrophilic AuNPs self-assembled into micelles with PS cores and AuNP coronae; however, in the presence of PS-coated Fe3O4 nanoparticles (Fe3O4NPs), the hybrid nanoparticles self-assembled into vesicles with AuNPs shells and PS/ PS-coated Fe3O4NPs walls.5 The hydrophilic AuNPs played a key role in the fabrication of the ordered structures. The surface properties of AuNPs can be controlled by selecting the species and proportions of the protected ligands. For example, amphiphilic hybrid AuNPs were prepared by grafting hydrophobic PS and hydrophilic poly[poly(ethylene glycol) methyl ether methacrylate] mixed polymer brushes to the nanoparticles.6 Similar to the amphiphlic block copolymers, amphiphilic hybrid AuNPs with charged nanoparticle head and r 2011 American Chemical Society

hydrophobic polymers tails assemble to different structures in aqueous solution. Researches on magnetic nanoparticles have received much attention due to the applications of the nanoparticles in biological sensor,7 target drug delivery,8 biomolecular separation,9 and gene therapy.10 In practical applications, surface treatment is important for magnetic nanoparticles. Many different polymerization strategies, such as atom transfer radical polymerization,11 nitroxide-mediated radical polymerization,12 ring-opening polymerization,13 and reversible addition-fragmentation chain transfer (RAFT) polymerization14 have been used in surfaceinitiated polymerization to introduce polymer brushes onto the surface of inorganic nanoparticles. Comparing with other polymerization methods, RAFT polymerization is one of the most versatile techniques due to the compatibility with a wide range of functionality in monomer types and polymerization conditions. Furthermore, the dithiocarbonate or trithiocarbonate groups in RAFT agents can be easily reduced to thiol groups,15 and interact with AuNPs. The coating of the magnetic nanoparticles with AuNPs improves chemical stability, biocompatibility, and established reactivity with thiolated compounds.16 Herein, we report preparation of nanoparticles with hydrophobic magnetic nanoparticle cores and hydrophilic AuNPs coronae based on self-assembly approach. PS brushes on Fe3O4 nanoparticles were prepared by RAFT polymerization, and after Received: November 29, 2010 Revised: January 17, 2011 Published: February 09, 2011 3304

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Scheme 1. (a) Scheme for the Preparation of PS Brushes on the Surface of Fe3O4 Nanoparticles (Fe3O4NPs) by Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization Method; after Reduction, Thiol Groups Were Introduced to the Polymer Chain Ends: (b) Scheme for the Preparation of Magnetic Nanopartcles with Fe3O4NPs Cores and AuNPs Coronae in Methanol

reduction reaction PS brushes with terminal thiol groups were obtained (part a of Scheme 1). In this article, the PS-grafted magnetic nanoparticles before and after the reduction reaction were assigned as RAFT-PS-Fe3O4NPs and HS-PS-Fe3O4NPs, respectively. Citrate-stabilized AuNPs were dispersed in aqueous solution, and HS-PS-Fe3O4NPs were dispersed in toluene. Upon mixing of the organic solution with the aqueous solution, AuNPs interacted with thiol terminal groups on PS brushes at oil-water interface, and amphiphilic nanoparticle complexes AuNPs-PS-Fe3O4NPs were formed. A stable O/W emulsion was obtained due to the formation of the amphiphilic nanoparticle complexes at the liquid-liquid interface. Upon addition of the emulsion to excess methanol, the amphiphilic nanoparticle complexes self-assembled into core-shell structures with hydrophobic Fe3O4NPs cores and hydrophilic AuNPs coronae (part b of Scheme 1).

’ EXPERIMENTAL SECTION Materials. Styrene (Tianjin Chemical Reagent Company, 98%) was washed with 5% NaOH aqueous solution, dried over anhydrous MgSO4, and distilled under reduced pressure. 2,2Azoisobutyronitrile (AIBN, Guo Yao Chemical Company, 98%) was purified by recrystallization from ethanol. HAuCl4 3 4H2O (Tianjin Chemical Reagent Company), sodium borohydride (NaBH4, Guo Yao Chemical Company, 96%), trisodium citrate dehydrate (Tianjin Chemical Reagent Company, 99%), N-(3hydrochloride (dimethylamino)propyl)-N0 -ethylcarbodiimide (EDC 3 HCl), N-hydroxysuccinimide (NHS) (Shanghai Medpep

Co., 99%), and 3-(triethoxysilyl)-propylamine (Aldrich, 98%) were used as received. FeCl3 3 6H2O (99%), FeCl2 3 4H2O (99.7%), 12 M HCl solution (36-38%), and solid NaOH (96%) were purchased from Tianjin agent company and used as received. RAFT chain transfer agent (S-1-dodecyl-S-(R,R0 -dimethyl-R00 -acetic acid) trithiocarbonate) was synthesized according to previous literature.17 Synthsis of Fe3O4NPs with Amino Groups on the Surface. In a round-bottomed flask, 1.0 g of Fe3O4NPs was dispersed in 30 mL of dry toluene after sonication. To above solution, 4.0 mL of 3-(triethoxysilyl)-propylamine was added dropwise. The solution was stirred at 95 °C for 14 h. After the reaction, the nanoparticles were collected at the bottom of the flask by a permanent magnet, and washed by ethanol until no blue precipitate was observed upon addition of Cu2þ to the supernatant, which indicated that all the unreacted 3-(triethoxysilyl)-propylamine was removed. Synthsis of Fe3O4NPs with RAFT Agent on the Surface. RAFT chain transfer agent (S-1-dodecyl-S-(R,R0 -dimethyl-R00 acetic acid) trithiocarbonate) was synthesized according to previous literature.17 RAFT agent (2.0 g, 5.5 mmol) and NHS (0.70 g, 8.2 mmol) were dissolved in 20 mL of dry N,Ndimethylformamide (DMF), and EDC 3 HCl (1.16 g, 8.24 mmol) was added into the above solution at 0 °C. The solution was stirred at room temperature for 24 h. After being concentrated at an elevated temperature, 40 mL of methanol was added to the solution and yellow solid product (RAFT-NHS) was obtained after filtration. NH2-coated Fe3O4NPs (0.30 g) were dispersed in 16 mL of DMF, and RAFT-NHS (0.13 g, 0.29 mmol) were 3305

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Figure 1. TEM images of (a) Fe3O4NPs, (b) AuNPs, and (c) RAFT-PS-Fe3O4NPs. TEM specimens of Fe3O4NPs and AuNPs were prepared from aqueous dispersions, and TEM specimen of RAFT-PS-Fe3O4NPs was prepared from THF dispersion. The scale bars in (a) and (b) represent 100 nm, and the scale bar in (c) represents 50 nm.

added into the dispersion at 0 °C. The reaction was conducted for 20 h under sonication. The Fe3O4NPs were collected at the bottom of the flask by a permanent magnet, washed by dichloromethane, and dried in vacuum. Synthesis of PS Polymer Brushes on the Surfaces of Fe3O4NPs. RAFT agent-coated Fe3O4NPs (150 mg) were dispersed in toluene (0.5 mL) under sonication, and AIBN (3.9 mg, 0.024 mmol) and styrene monomer (0.5 mL) were added into the solution. After three freeze-pump-thaw cycles, RAFT polymerization of styrene was conducted at 72 °C for 8 h. The PS-coated Fe3O4NPs were precipitated in methanol. To remove possible free PS, the hybrid nanoparticles were redispersed in 5 mL of chloroform; after centrifugation the hybrid nanoparticles at the bottom of the vial were collected and dried in vacuum. Synthesis of HS-PS-Fe3O4NPs. PS-coated Fe3O4NPs (0.4 g) were dispersed in 10 mL of THF, and NaBH4 (100 mg) was added under nitrogen atmosphere. The solution was stirred at room temperature for 24 h. The HS-PS-Fe3O4NPs were collected at the bottom of the flask by a permanent magnet, washed by dichloromethane, precipitated in methanol, and dried in vacuum.

Synthesis of Nanoparticles with Fe3O4NPs Cores and AuNPs Coronae. A typical procedure was described as follows.

HS-PS-Fe3O4NPs (0.9 mg) were dispersed in 0.15 mL of toluene; the solution was added to 3 mL of an aqueous dispersion of AuNPs under sonication and an emulsion was obtained. Nanoparticles with Fe3O4NPs cores and AuNPs coronae were prepared by adding the emulsion to 6-fold of methanol, and a violet homogeneous emulsion was obtained. Characterization. The apparent molecular weight and molecular weight distribution of the polymer were determined on a gel permeation chromatograph (GPC) equipped with a Hitachi L-2130 HPLC pump, Hitachi L-2350 column oven operated at 40 °C, three Varian PL columns with 5-600 K, 500-30 K, and 100-10 K molecular ranges, and a Hitachi L-2490 refractive index detector. THF was used as eluent at a flow rate of 1.0 mL/ min. Molecular weights were calibrated on PS standards. Before GPC measurement, 24 mg of PS-Fe3O4NPs were dipersed in 2 mL of THF, and 0.3 mL of concentrated HCl solution was added. After 20 min, the dispersion turned to yellow which indicated that the Fe3O4NPs were dissolved by HCl solution and free PS was obtained. Transmission electron microscopy (TEM) 3306

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Figure 2. TEM images of dried droplets of O/W emulsion with AuNPs in aqueous phase and HS-PS-Fe3O4NPs in oil phase at low (a) and high magnification (b). The emulsion was casted on a carbon-coated copper grid and dried in air. The scale bars in (a) and (b) represent 500 and 50 nm, respectively.

observations were carried out on a Tecnai G2 20 S-TWIN electron microscope equipped with a Model 794 CCD camera. TEM specimens were prepared by dipping copper grids into solutions and dried in air. The thermal properties of the nanocomposites were measured by thermogravimetric analysis (TGA). The samples were heated to 800 °C at a heating rate of 10 K/min under nitrogen atmosphere on a Netzsch TG 209. Fourier transform infrared absorption spectra (FTIR) were obtained on a Bio-Rad FTS 6000 system using diffuse reflectance sampling accessories. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a Kratos Axis Ultra DLD spectrometer employing a monochromated Al KR X-ray source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multichannel plate and delay line detector. All XPS spectra were recorded using an aperture slot of 300  700 μm, survey spectra were recorded with pass energy of 160 eV, and high-resolution spectra were recorded with pass energy of 40 eV. The magnetic properties of the nanoparticles were studied by using a vibrating sample magnetometer (LDJ 9600-1, USA) at room temperature by cycling the field from -6 to 6 kOe. UV-vis absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer using a quartz cell of 1 cm path length.

’ RESULTS AND DISCUSSION Fe3O4NPs were synthesized by coprecipitation in aqueous solution without any surfactants. On the basis of the TEM result, the average diameter of Fe3O4NPs was determined to be about 12 nm (part a of Figure 1). The Fe3O4NPs agglomerated into clusters as the anisotropic dipolar attraction. The magnetic hysteresis loops show that the Fe3O4NPs is ferromagnetic, and the magnetization of the nanoparticles is about 60 emu/g, the coercivity is about 78 Oe. Part b of Figure 1 presents a TEM image of citrate-stabilized AuNPs. The average size of AuNPs is about 5 nm. Amino groups were incorporated onto the surface of Fe3O4NPs (H2N-Fe3O4NPs) by condensation reaction of 3(triethoxysilyl)-propylamine and Fe3O4NPs. RAFT agent-anchored Fe3O4NPs (RAFT-Fe3O4NPs) were prepared via a

reaction between H2N-Fe3O4NPs and RAFT-NHS in dry DMF in the presence of triethylamine. PS brushes on Fe3O4NPs were prepared by RAFT polymerization. The terminal trithiocarbonate groups on PS brushes were reduced to thiol groups by NaBH4. To determine the grafting density of the RAFT agent on Fe3O4NPs, the surface contents of nitrogen and sulfur were measured by elemental analysis and ion chromatography, respectively. The elemental analysis result showed that the nitrogen content of H2N-Fe3O4NPs was 0.95 wt %, corresponding to 0.68 mmol amino groups/g H2N-Fe3O4NPs. The ion chromatography result indicated that the sulfur content of RAFT-Fe3O4NPs was 1.22 wt %, which suggested that the grafting density of RAFT agent on Fe3O4NPs was about 0.8 molecules/nm2. XPS is a powerful tool in surface study of materials. XPS spectra can provide information on the type and number of different species of the elements in materials. The modification of Fe3O4NPs was also verified by XPS result. Wide XPS spectra of H2N-Fe3O4NPs and RAFT-Fe3O4NPs were shown in Figure S1 of the Supporting Information. The peaks of H2NFe3O4NPs corresponding to the binding energies of C1s at 284.4 eV, O1s at 531.3 eV, Fe2p at 711.2 eV and Si2s at 153.1 eV, were observed. N1s spectrum of NH2-Fe3O4NPs was shown in the insert of part a of Figure S1 of the Supporting Information, which indicated that the binding energy of N1s was at 400.0 eV. XPS spectrum of RAFT-Fe3O4NPs showed the presence of C1s at 284.5 eV, O1s at 529.3 eV, Fe2p at 709.1 eV, N1s at 399.1 eV, Si2s at 152.2 eV, and S2p at 163.3 eV (part b of Figure S1 of the Supporting Information). S2p spectrum of RAFT-Fe3O4NPs was also presented in the insert of part b of Figure S1 of the Supporting Information. The XPS results confirmed the grafting of RAFT agent to Fe3O4NPs. FTIR spectra of the original and modified Fe3O4NPs were shown in Figure S2 of the Supporting Information. On the FTIR spectrum of RAFT-Fe3O4NPs (spectrum b), the band at 1156 cm-1 was attributed to the —C(CH3)2— skeletal vibration of RAFT agent. However, the characteristic CdS band of RAFT agent at around 1068 cm-1 was overlapped by the absorption peak of Fe3O4NPs. After RAFT polymerization, PS brushes were 3307

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Figure 3. (a) TEM image of nanoparticles with Fe3O4NPs cores and AuNPs coronae prepared by adding O/W emulsion to 6-fold of methanol. The weight ratio of HS-PS-Fe3O4NPs to AuNP is 3:1. (b) A magnified TEM image of nanoparticles with Fe3O4NPs cores and AuNPs coronae. (c) A TEM image of nanoparticles prepared at a weight ratio of 1:1. (d) A TEM image of nanoparticles prepared at a weight ratio of 6:1.

prepared on the surface of Fe3O4NPs. On the spectrum of RAFT-PS-Fe3O4NPs, the characteristic bands of PS at around 698, 765, 1156, and 3025 cm-1 were observed. The H2N-Fe3O4NPs, RAFT-Fe3O4NPs, RAFT-PSFe3O4NPs, and HS-PS-Fe3O4NPs were analyzed by TGA and the results were shown in Figure S3 of the Supporting Information. The H2N-Fe3O4NPs was found to have 3 wt % weight loss in the range between 100 and 800 °C (curve a), which was mainly attributed to the loss of the coupling agent 3-(triethoxysilyl)-propylamine. The content of nitrogen on Fe3O4NPs determined by TGA was about 0.73 wt %, which agreed well with elemental analysis result. After grafting of RAFT agent, the weight loss of RAFT-Fe3O4NPs in the same temperature range was 11 wt % (curve b). The additional weight loss was attributed to the grafting of RAFT agent to the nanoparticles. After RAFT polymerization, the nanocomposite showed 62.3 wt % weight loss (curve d). It is worthy of note that after the reduction reaction, HS-PS-Fe3O4NPs was found to have 62 wt % weight loss (curve c). The small difference in the weight loss between RAFT-PS-Fe3O4NPs and HS-PS-Fe3O4NPs was

attributed to the reduction of the RAFT agent at polymer chain ends. Magnified TGA curves showing the difference between HS-PS-Fe3O4NPs and RAFT-PS-Fe3O4NPs were presented in the insert of Figure S3 of the Supporting Information. THF is a good solvent for PS brushes, so RAFT-PSFe3O4NPs were well dispersed in THF. A TEM image of RAFT-PS-Fe3O4NPs prepared from THF solution was presented in part c of Figure 1. After grafting of polymer chains to the surface, the Fe3O4NPs aggregated in the gray PS matrix composed of PS brushes. It was also noted that the aggregation density of RAFT-PS-Fe3O4NPs is not as high as the bare Fe3O4NPs due to the polymer brushes on the nanoparticles. The number average molecular weight and molecular weight distribution of PS brushes were 37 k and 1.56, respectively. Hydrophilic AuNPs were dispersed in aqueous solution and hydrophobic HS-PS-Fe3O4NPs were dispersed in toluene, and a stable emulsion was obtained by mixing two solutions via sonication. AuNPs interacted with thiol groups on HS-PSFe3O4NPs at oil-water interface via ligand exchange between thiol groups and organic salts,18 and nanoparticle complexes 3308

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Figure 4. TEM images of self-assembly structures prepared at different volume ratios of toluene to water: (a) 1:20, (b) 2:20, (c) 5:20. The weight ratio of HS-PS-Fe3O4NPs to AuNP was controlled at 3:1.

AuNPs-PS-Fe3O4NPs were formed at the interface. Because AuNPs were hydrophilic and HS-PS-Fe3O4NPs were hydrophobic, amphiphilic AuNPs-PS-Fe3O4NPs could stabilize toluene droplets in aqueous solution. The role of amphiphilic nanoparticle complex is very similar to molecular surfactants used in emulsions. Part a of Figure 2 showed a TEM image of the emulsion stabilized by the amphiphilic nanoparticle complexes dried on a carbon-coated copper grid. No individual AuNPs were observed outside of the droplet structures, which confirmed the formation of the complexes at oil-water interface. A magnified TEM image (part b of Figure 2) showed that Fe3O4NPs were located inside the structures, and AuNPs formed a layer around the structures. This result proved that AuNPs interacted with HS-PS-Fe3O4NPs at oil-water interface (Scheme 1). Nanoparticles with Fe3O4NPs cores and AuNPs coronae were prepared by adding the emulsion to 6-fold of methanol. A homogeneous solution was obtained as water and toluene were both miscible with methanol. As methanol is a precipitant for PS, PS chains collapse on the surface of Fe3O4NPs, and the hydrophilic AuNPs are on the surface to stabilize the nanoparticles. Part a of Figure 3 showed a TEM image of the nanoparticles in methanol. The weight ratio of HS-PS-Fe3O4NPs to AuNPs

was 3:1. The average size of the nanoparticles was about 80 nm. Part b of Figure 3 presented a magnified TEM image of nanoparticles. On the TEM image, it could be found that there were a couple of Fe3O4NPs in the core and a number of AuNPs in the corona. Part of surface of a specific AuNP on the corona was embedded inside PS phase due to the Au-S interaction. To investigate the influence of the weight ratio of HS-PSFe3O4NPs to AuNPs on the fabrication of the core-shell structured nanoparticles, the nanoparticles were prepared at different weight ratios. Parts c and d of Figure 3 showed TEM images of the nanoparticles prepared at two different weight radios. When the weight ratios were kept at 1:1 and 6:1, the size of the nanoparticles ranged from 13 to 55 nm and 100 to 270 nm, which indicated that the size of the nanoparticles increased with the weight ratio of HS-PS-Fe3O4NPs to AuNPs. With the increase in the weight ratio, there are less AuNPs in the nanoparticle complexes and the stabilizing ability of the nanoparticle complexes becomes weak, so the size of the selfassembled structures in methanol becomes big. The influence of the volume ratio of toluene to water on the self-assembled structures was also investigated. Figure 4 showed TEM images of nanoparticles prepared at different volume ratios of toluene to 3309

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Figure 5. (a) TEM image of self-assembly structures after being redispersed in THF; (b) a schematic illustration of amphiphilic nanoparticle complexes AuNPs-PS-Fe3O4NPs.

water. With the increase in the volume ratio, the size of selfassembled structures increased as well. When the volume ratio reached 5:20, the nanoparticles aggregated together and precipitates were observed. To further prove the formation of amphiphilic nanoparticle complexes at the liquid-liquid interface, the nanoparticles in methanol were collected at the bottom of a flask by the effort of a permanent magnet and then redispersed in THF. A TEM image of the nanoparticles after being redispersed in THF was shown in part a of Figure 5. Although THF is a good solvent for PS, nanoparticles with Fe3O4NPs in the cores and AuNPs in the coronae were still observed in the image. It was also noted that comparing with the self-assembly structures prepared in methanol fewer Fe3O4NPs and AuNPs were involved in the structures prepared in THF, which indicated that a particle in methanol might self-assemble into smaller particles when it was redispersed in THF solution. The observation of core-shell structures proved the formation of AuNPs-PS-Fe3O4NPs nanoparticle complexes due to the Au-S interaction. A magnified TEM image of AuNPs-PS-Fe3O4NPs in THF was shown in the insert of part a of Figure 5. In THF, PS chains stretch into the solution from the surface of Fe3O4NPs, and connect with AuNPs at the chain ends. The structure of AuNPs-PS-Fe3O4NPs nanoparticle complex was illustrated in part b of Figure 5. On the basis of the TEM result, we calculated the distance between Fe3O4NPs and AuNPs and found that the distances were in the range between 12 and 25 nm. We also estimated root-mean-square end-to-end distance of a free polymer having the same molecular weight in their unperturbed state.19 The calculation result showed that the end-to-end distance was about 12.7 nm. Because THF is a good solvent for PS, the end-to-end distance of a PS brush chain in THF is longer than a free polymer chain in unperturbed state. The aggregation of AuNPs leads to a red shift of the plasmon absorption because of the electronic coupling interactions between neighboring particles.20 In aqueous solution, the plasmon absorption band of citrate-stabilized AuNPs appeared with a maximum absorbance at 516 nm (curve a in Figure 6). However, in self-assembly structures in methanol, the maximum absorbance of AuNPs red-shifted to 536 nm (curve b in Figure 6)

Figure 6. UV-vis spectra of citrate-stabilized AuNPs in water (curve a), self-assembly structures with Fe3O4NPs cores and AuNPs coronae in methanol prepared at 3:1 weight ratio of HS-PS-Fe3O4NPs to AuNPs (curve b).

Figure 7. Magnetic hysteresis loops of (a) Fe3O4NPs, (b) RAFT-PSFe3O4NPs, and (c) AuNPs-PS-Fe3O4NPs.

because of the aggragation of AuNPs in the self-assembly structures. 3310

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: 86-022-2349-8703. E-mail: [email protected].

’ ACKNOWLEDGMENT This project was supported by National Natural Science Foundation of China (NSFC) under Contract No. 20774046 and 20874050. ’ REFERENCES

Figure 8. Photographs showing the methanol dispersion of magnetic nanoparticles with Fe3O4NPs cores and AuNPs coronae (vial a) and concentration of magnetic nanoparticles on the left side of the vial by a magnet (vial b).

Magnetic measurements conducted at 298 K showed that the magnetic moment of Fe3O4NPs was higher than those of RAFTPS-Fe3O4NPs and Au-PS-Fe3O4NPs but there was no significant change in the coercivity. As shown in Figure 7, the magnetization of the original Fe 3 O 4 NPs, RAFT-PSFe3O4NPs, and AuNPs-PS-Fe3O4NPs were about 60, 18, and 4.5 emu/g, respectively. The decreases of RAFT-PSFe3O4NPs and AuNPs-PS-Fe3O4NPs in magnetization were attributed to the grafting of PS to Fe3O4NPs and formation of nanoparticle complexes. The coercivity of all the three materials determined by the central loop shown in the inset of Figure 7 was 78 Oe. The effect of a permanent magnet on the dispersion of magnetic nanoparticles was shown in Figure 8. The nanoparticles were well dispersed in methanol due to the stabilization of AuNPs in the coronae (vial a in Figure 8). However, all of the nanoparticles were concentrated on the left side of the vial when the magnet was placed at the corresponding place (vial b in Figure 8). This result well demonstrated that the magnetic nanoparticles were embedded in the cores of the nanoparticles. When the magnet was removed, the nanoparticles were redispersed in methanol after ultrasonication meaning that the magnet only attracted the nanoparticles to the corresponding position and did not cause any structure change. This property allows tracking or separation of nanoparticles in a magnetic gradient.

’ CONCLUSIONS In summary, nanoparticles with Fe3O4NPs cores and AuNPs coronae were prepared based on self-assembly of nanoparticle complexes formed by citrate-protected AuNPs and HS-PSFe3O4NPs at oil-water interface. These core-shell structures have potential applications in catalysis, biomolecular separation, and gene therapy. This approach is a versatile method, and many different functional self-assembly structures can be prepared based on this approach. ’ ASSOCIATED CONTENT

bS

Supporting Information. XPS spectra, FTIR spectra, and TG curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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