Synthesis and Application of Au–Fe3O4 Dumbbell-Like Nanoparticles

C H A P T E R

30 Synthesis and Application of Au Fe3O4 Dumbbell-Like Nanoparticles Xueping Zhang1,2 and Shaojun Dong1,2 1

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, P.R. China 2University of Chinese Academy of Sciences, Beijing, P.R. China

30.1 INTRODUCTION Hybrid nanoparticles consisting of noble metals and metal oxides nanoparticles often show improved physical/chemical properties over those of the individual component nanoparticles, or exhibit new properties that are not present in the individual component nanoparticles [1 5]. This could be attributed to the interfacial interactions that originate from electron transfer across the nanometer contact at the interface of these two nanoparticles [6 10]. For example, Au nanoparticles are usually chemically inert, but Au nanoparticles deposited on a metal oxide support exhibit superior catalytic activity for the CO oxidation reaction [11 13]. Besides, Au nanoparticles are known to have attractive optical properties with a well-defined plasmon resonance peak [14], while this peak can show a red shift by association with metal oxide nanoparticles [15 17]. On the other hand, magnetic Fe3O4 nanoparticles often show high magnetic response to an externally applied magnetic field and can be used for selective capture of targeting substrates, recyclable nanocatalysis, and magnetic-photonic purposes [18 23]. Thus, the combination of both Au and Fe3O4 into a single entity would lead to a hybrid nanostructure with advantageous and serendipitous properties from both individual Au and Fe3O4 nanoparticles. Au Fe3O4 dumbbell-like nanoparticles have recently undergone intensive investigation [3,24,25]. The dumbbell-like nanoparticles are heterogeneous nanostructures with two

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00033-4

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different functional nanoparticles in intimate contact with each other [26,27]. In contrast to other structures, Au Fe3O4 dumbbell-like nanoparticles have distinct advantages. First, in such a structure, each side of the nanoparticles is restricted to the nanometer scale, and a small variation in electron transfer across the interface between these two limited electron “nanoreservoirs” may lead to a drastic property change for each nanoparticle [3]. Second, Au Fe3O4 dumbbell-like nanoparticles contain both a magnetic (Fe3O4) and an optically active plasmonic (Au) unit and are suitable for simultaneous optical and magnetic detection [28]. Third, they offer two functional surfaces, and this can enhance the catalytic activity and facilitate the attachment of different chemical functionalities, making them especially attractive as multifunctional probes for target-specific imaging and delivery applications [29]. We therefore prefer to devote this chapter to summarizing the recent research progress in the synthesis of Au Fe3O4 dumbbell-like nanoparticles, and some significant factors that influence the preparation process, which would help shed light on the development of dumbbell-like nanoparticles with various components. Then, we illustrate the interesting optical and magnetic properties found in these hybrid nanoparticles, and highlight the potential applications of these nanohybrids in catalysis and biomedicine.

30.2 SYNTHESIS OF Au Fe3O4 DUMBBELL-LIKE NANOPARTICLES 30.2.1 General Strategy Sun’s group reported a general synthesis strategy for the Au Fe3O4 dumbbell-like nanoparticles [30]. As illustrated in Fig. 30.1, the Au Fe3O4 dumbbell-like nanoparticles were prepared by the decomposition of iron pentacarbonyl, Fe(CO)5, over the surface of the Au nanoparticles, followed by oxidation in air. The Au nanoparticles are either premade in the presence of oleylamine or synthesized in situ by injecting a HAuCl4 solution into the reaction mixture. Mixing Au nanoparticles with Fe(CO)5 in 1-octadecene in the presence of oleic acid and oleylamine and heating the mixture to reflux (B300 C) for 45 min followed by room-temperature air oxidation leads to the formation of Au Fe3O4 dumbbell-like nanoparticles. The size of the particles could be tuned from 2 to 8 nm for Au and 4 to 20 nm for Fe3O4. 30.2.1.1 Formation Mechanism This process is similar to the seed-mediated growth to form core/shell nanoparticles, while the difference is that the nucleation and growth is anisotropically centered on one

FIGURE 30.1 Schematic illustration of the growth of Au Fe3O4 dumbbell-like nanoparticles. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional Au Fe3O4 nanoparticles, Nano Lett. 5 (2005) 379 382. Copyright (2005) American Chemical Society.

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specific crystal plane around the seeding nanoparticles, and not uniformly distributed as in the synthesis of core/shell structures [3]. The epitaxial growth of Fe3O4 onto the Au nanoparticles surface is favored by the lattice mismatch between both materials since the ˚ ) is almost double that of the Au (4.08 A ˚ ). The conlattice parameter of the Fe3O4 (8.345 A trolled nucleation and growth of only one Fe3O4 on each Au seeding nanoparticle can be ascribed to the suitable electron transfer. As the Fe3O4 nanoparticles nucleate on Au nanoparticles, the polarized plane induces a change in the charge at the interface, and the free electrons from the Au nanoparticles will compensate for that charge change. However, the Au particle has only a very limited source of electrons, so this compensation makes all other facets of the Au nanoparticle electron deficient and unsuitable for multinucleation. Therefore, only dumbbell-like nanoparticles can be produced. 30.2.1.2 Characterization Fig. 30.2A shows the transmission electron microscopy (TEM) image of the Au Fe3O4 dumbbell-like nanoparticles with Fe3O4 at around 14 nm and Au at 8 nm. The Au particles appear black and Fe3O4 are light colored in the image because Au has a higher electron density and allows fewer electrons to transmit. Fig. 30.2B is the high angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image of the dumbbelllike nanoparticles. The brightness in the image reflects the intensity of scattered electrons from different substance, which is proportional to the atomic number (Z) [31]. In Fig. 30.2B, the brighter dots refer to the Au particles since they have higher Z compared to the Fe3O4 particles. Fig. 30.2C displays a typical high-resolution TEM (HRTEM) image of a dumbbell-like particle with Fe3O4 at 12 nm and Au at 8 nm. In the structure, a Fe3O4 (1 1 1) plane grows onto an Au (1 1 1) plane, giving the dumbbell-like structure.

30.2.2 Other Modified Strategies The Au Fe3O4 dumbbell-like nanoparticles were also synthesized by a modified Sun’s method, replacing highly toxic Fe(CO)5 with a safe Fe precursor (Fe oleate, Fe(OL)3). Doong et al. synthesized Au Fe3O4 dumbbell-like nanoparticles by simply refluxing a mixture which contains Fe(OL)3, Au colloid dispersion, oleic acid, oleylamine, and

FIGURE 30.2 (A) TEM image of the 8 14 nm Au Fe3O4 particles; (B) HAADF-STEM image of the 8 9 nm Au Fe3O4 particles; and (C) HRTEM image of one 8 12 nm Au Fe3O4 particle. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional Au Fe3O4 nanoparticles, Nano Lett. 5 (2005) 379 382. Copyright (2005) American Chemical Society.

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1-octadecene at 310 C for 30 min [32]. The size of the synthesized dumbbell-like nanoparticles ranges from 12 to 16 nm. Our group prepared Au Fe3O4 dumbbell-like nanoparticles by mixing pre-made Au nanoparticles with Fe(OL)3 in a solution of oleylamine and oleic acid in 1-octadecene and heating the mixture to reflux (320 C) for 40 min followed by room-temperature air oxidation [33]. Interestingly, the Fe3O4 domain presents a squarelike shape (Fig. 30.3), which is different from the spherical shape when Fe(CO)5 was used as Fe-precursor.

30.2.3 Influence Factors The size and morphology of the Au Fe3O4 dumbbell-like nanoparticles are closely related to the synthetic conditions, so in this section we discussed several important factors that influence the synthesis of Au Fe3O4 dumbbell-like nanoparticles. 30.2.3.1 The Molar Ratio of Au and Fe Precursors Grzybowski et al. discovered that different molar ratio of Au and Fe precursors led to a gradual change of morphology from dumbbell-like to flower-like [34]. When more Fe precursor was used relative to Au precursor, the formation of nanoflowers was preferred, and much larger flower petals were observed when the relative amount of Fe precursor increased. Fig. 30.4 shows the Au Fe3O4 nanoparticles prepared by refluxing different amounts of Fe(CO)5 in 1-octadecene at the presence of Au nanoparticles, oleic acid, and oleylamine for 50 min. When the molar ratio of Au and Fe precursors was 1:1 (Fig. 30.4A), the nanoparticles are dumbbell-like dimers comprising a smaller (5 8 nm) Fe3O4 domain attached to a larger (10 nm) Au part. The size of the Fe3O4 domain increases with the increase of the molar ratio of Au and Fe precursors and, at 1:2.5, is similar to that of the Au particle (Fig. 30.4B). At higher values of Au:Fe, multiple Fe3O4 domains form on each Au nanoparticle. For example, at 1:6 (Fig. 30.4C), about 60% of the composite particles have two and about 20% have three Fe3O4 domains around the Au core, while at 1:10 (Fig. 30.4D), about 80% of the particles have three or four Fe3O4 leaves per individual Au nanoparticle. 30.2.3.2 The Reaction Temperature The reaction temperature could influence the sizes of Au seeds. For example, Sun et al. observed that injecting HAuCl4 solution into the reaction mixture containing Fe(CO)5 at FIGURE 30.3 TEM images of Au Fe3O4 nanoparticles in water with different magnification. Source: Reproduced with permission from Y.M. Zhai, L.H. Jin, P. Wang, S. Dong, Dual-functional Au Fe3O4 dumbbell nanoparticles for sensitive and selective turn-on fluorescent detection of cyanide based on the inner filter effect, Chem. Commun. 47 (2011) 8268 8270. Copyright (2011) Royal Society of Chemistry.

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FIGURE 30.4

TEM images of Au Fe3O4 nanoparticles prepared in 1-octadecene at the presence of oleic acid and oleylamine with Au:Fe initial molar ratios of (A) 1:1, (B) 1:2.5, (C) 1:6, and (D) 1:10. The dark portions of Au Fe3O4 nanoparticles correspond to Au while lighter ones to Fe3O4. All scale bars are 20 nm. Source: Reproduced with permission from Y. Wei, R. Klajn, A.O. Pinchuk, B.A. Grzybowski, Synthesis, shape control, and optical properties of hybrid Au/Fe3O4 “nanoflowers”, Small 4 (2008) 1635 1639.

120 C led to B2 nm Au particles, while injection at 160 C or 180 C gave 4 or 6 nm Au particles [30]. In a facile synthesis of monodisperse Au nanoparticles, they prepared Au nanoparticles with sizes tunable from 1 to 10 nm through a burst nucleation by carefully controlling the reaction temperature at which the reducing solution was injected into the precursor solution [35]. The nanoparticle size and the reaction temperature had a linear correlation, as listed in Table 30.1. This can be explained by the classic La Mer theory [36,37]: injection of the reducing agent into the precursor solution at a relatively high temperature results in the burst nucleation consuming most of the precursors, leading to fast nucleation and growth processes, and ultimately produces smaller Au nanoparticles. 30.2.3.3 The Refluxing Time Velasco et al. investigated the influence of the refluxing time in the reaction, and discovered that the morphology of Au Fe3O4 nanoparticles evolved from dumbbell-like to flower-like and finally a core shell structure with a prolonged refluxing time [38]. The synthesis of Au Fe3O4 nanoparticle is based on the method developed by Chen et al. for the preparation of FeAu nanoparticles [39], which was adapted from Sun et al. for the synthesis of monodisperse FePt nanoparticles [40]. But, in this case, the iron precursor, Fe (CO)5, was injected when the solution of oleylamine and oleic acid in dioctyl ether was heated to 200 C. With different refluxing time of 30, 90, and 180 min, the authors observed that the morphology of hybrid nanoparticles changed from dumbbell-like to flower-like and finally a core shell structure. As is explained above, after the addition of Fe(CO)5, Fe3O4 nanoparticles start to nucleate epitaxially onto one single Au surface, leading to the formation of Au Fe3O4 dumbbell-like nanoparticles. As the refluxing time increases, the

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TABLE 30.1 Average Size of Au Nanoparticles (Au NPs) Synthesized at Different Temperatures Reaction temperature ( C)

Avg. Au NP size (nm)

2

9.5

10

8.1

15

7.3

20

6.4

25

5.3

35

3.3

40

2.4

Reproduced with permission from S. Peng, Y. Lee, C. Wang, H. Yin, S. Dai, S. Sun, A facile synthesis of monodisperse Au nanoparticles and their catalysis of CO oxidation, Nano Res. 1 (2008) 229 234. Copyright (2008) Springer Nature.

number of Fe3O4 nanoparticles increases and they nucleate onto the gold free facets. After 90 min, flower-like nanoparticles are formed with 5 nm Fe3O4 nonuniform petals linked to a 7 nm Au core, whereas after 180 min, most of the Au nanoparticles seem to be completely surrounded by an Fe3O4 layer forming a core shell structure with a diameter of about 12 nm, where the Au core was estimated to be about 6 nm [38]. 30.2.3.4 The Solvent Polarity As we mentioned above, Sun et al. prepared Au Fe3Oh4 dumbbell-like nanoparticles with 1-octadecene as solvent in the presence of oleic acid and oleylamine [30]. However, when the solvent was changed from 1-octadecene to diphenyl ether, which has a higher polarity, they obtained flower-like Au Fe3O4 nanoparticles. Fig. 30.5 shows the TEM images of such flower-like nanoparticles with the faceted Au core being B8 nm and radial length of the Fe3O4 at B4 nm, indicating clearly the multinucleation of Fe3O4 on the faceted Au seeds. This observation is in accordance with the formation mechanism: if the Au nanoparticle has more electrons to compensate for the charge at the plane, the nucleation of Fe3O4 nanoparticles on Au seeds would occur at more facets of the Au nanoparticle, while these extra electrons could be offered by high polarity solvents in the synthesis.

30.3 OPTICAL AND MAGNETIC PROPERTIES The epitaxial linkage between Au and Fe3O4 in the Au Fe3O4 dumbbell-like nanoparticles has a significant effect on the optical and magnetic properties of individual nanoparticles.

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30.3.1 Optical Properties The Au Fe3O4 dumbbell-like nanoparticles show a red shift in the plasma resonance of Au because of the intimate contact between the Au and Fe3O4 [30]. It is known that for Au particles with sizes ranging from 5 to 20 nm in diameter, freely mobile electrons are trapped in the small Au metal box and show a characteristic collective oscillation frequency of the plasma resonance, giving rise to the plasma resonance band at around 520 nm [14]. The exact absorption position varies with particle morphology and particle surface coating. Fig. 30.6 shows the UV vis spectra of the Au and Au Fe3O4 dumbbell-like nanoparticles dispersed in hexane. The peaks at 520 nm for Au nanoparticles are independent of the size and concentration of the particles but the width increases with the decreased nanoparticles size (Fig. 30.6A and B). However, once attached to Fe3O4, the Au particles show plasmon resonance absorption at 538 nm (Fig. 30.6C and D), an 18 nm red shift from that of the pure Au nanoparticles. As the dielectric environment for all the nanoparticles in this measurement is the same, the only difference comes from the size and dumbbell-like

FIGURE 30.5 (A) Low resolution TEM and (B) HRTEM images of flower-like Au Fe3O4 nanoparticles. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional Au Fe3O4 nanoparticles, Nano Lett. 5 (2005) 379 382. Copyright (2005) American Chemical Society.

FIGURE 30.6 UV vis spectra of the Au and Au Fe3O4 dumbbell-like nanoparticles dispersions in hexane: (A) 8 nm Au; (B) 4 nm Au; (C) 7 14 nm Au Fe3O4; and (D) 3 14 nm Au Fe3O4. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional Au Fe3O4 nanoparticles, Nano Lett. 5 (2005) 379 382. Copyright (2005) American Chemical Society.

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structure. Previous studies demonstrate that excess electrons on the Au particles can cause the plasmon resonance absorption shift to shorter wavelength; whereas electron deficiency will shift the absorption to longer wavelength [14]. The red shift of the plasmon resonance spectra for Au Fe3O4 dumbbell-like nanoparticles indicates that the Au nanoparticles in the Au Fe3O4 dumbbell-like nanoparticles are electron deficient, which is likely caused by the interface communication between Au and Fe3O4.

30.3.2 Magnetic Properties The interface communication between the nanoscale Au and Fe3O4 also leads to the change of magnetization behaviors of the Fe3O4 nanoparticles, especially for those smaller than 8 nm. Fig. 30.7 shows the hysteresis loops measured at room temperature for Au Fe3O4 dumbbell-like nanoparticles with Au being 3 nm and Fe3O4 14 nm (Fig. 30.7A) and 6 nm (Fig. 30.7B), respectively. Like pure Fe3O4 nanoparticles, the Au Fe3O4 dumbbell-like nanoparticles are superparamagnetic at room temperature. The 3 14 nm dumbbell-like nanoparticles show loops similar to the 14 nm Fe3O4 nanoparticles with saturation moment reaching 80 emu/g (Fig. 30.7A), a value that is close to the related Fe3O4 nanoparticles due to the negligible weight percentage of 3 nm Au in the nanocomposite. The 3 6 nm dumbbell-like nanoparticles, however, show a loop of slow increase in moment with the field up to 5 T (Fig. 30.7B). The slope is likely caused by surface spin canting of the small Fe3O4 nanoparticles [41], since the pure Fe3O4 nanoparticles of the same size do not show such a magnetization behavior. This spin canting is further aggravated upon the connection with Au nanoparticles [42].

FIGURE 30.7 Hysteresis loops of the Au Fe3O4 dumbbell-like nanoparticles measured at room temperature: (A) 3 14 nm Au Fe3O4 and (B) 3 6 nm Au Fe3O4 nanoparticles. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbelllike bifunctional Au Fe3O4 nanoparticles, Nano Lett. 5 (2005) 379 382. Copyright (2005) American Chemical Society.

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30.4 POTENTIAL APPLICATIONS The unique properties make Au Fe3O4 dumbbell-like nanoparticles show high potential for applications in the fields of catalysis, sensors, and biomedicine [24,43 48].

30.4.1 Catalysis Au Fe3O4 dumbbell-like nanoparticles have recently undergone intensive research as highly efficient catalysts for carbon monoxide oxidation [43,49,50], reduction of nitrophenols [32], and reduction of hydrogen peroxide [51]. The superparamagnetic nature of the Au Fe3O4 dumbbell-like nanoparticles enables trouble-free separation of the catalysts from the reaction mixture for recycling by using an external magnet [52]. Furthermore, the hybrid material matrix significantly stabilizes the Au nanoparticles from agglomeration and leaching, leading to a catalytic lifetime enhancement [53,54]. 30.4.1.1 Catalysis of Carbon Monoxide Oxidation Dai et al. reported the solution/suspension deposition of Au Fe3O4 dumbbell-like nanoparticles on amorphous carbon supports with further calcination at 300 C in 8% O2/He for 1 h to remove the residue of organic surfactant on the particle surface [43]. The treated nanoparticles exhibited significant catalytic activity for the CO oxidation reaction at room temperature. Fig. 30.8 summarizes the light-off curves for the CO oxidation reaction catalyzed by Au Fe3O4 dumbbell-like nanoparticles deposited on various supports. The reactivity shown on Au Fe3O4/C is especially interesting as it is well-known that Au nanoparticles loaded directly on the carbon supports are inactive for the CO oxidation reaction [55]. In contrast, the Au Fe3O4 nanoparticles supported on carbon are highly active for this reaction, showing 100% CO conversion at 50 C. This enhanced catalytic activity arises clearly from the local modification of the electronic structure of Au by Fe3O4 through the interface interaction. 100

CO conversion (%)

80

60

40 a) b) c) d)

20

0 –100

–50

0 50 Temperature (ºC)

Au-Fe3O4 Au-Fe3O4/SiO2 Au-Fe3O4/TiO2 Au-Fe3O4/C 100

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FIGURE 30.8 CO oxidation conversion light-off curves of (A) Au Fe3O4: Au Fe3O4 nanoparticles calcined at 300 C for 1 h; (B) Au Fe3O4/SiO2: Au Fe3O4 deposited on SiO2 was calcined at 500 C for 1 h; (C) Au Fe3O4/TiO2: Au Fe3O4 deposited on TiO2 was calcined at 300 C for 1 h; (D) Au Fe3O4/C: Au Fe3O4 deposited on carbon was calcined at 300 C for 1 h. Source: Reproduced with permission from H. Yin, C. Wang, H. Zhu, S.H. Overbury, S. Sun, S. Dai, Colloidal deposition synthesis of supported gold nanocatalysts based on Au Fe3O4 dumbbell nanoparticles, Chem. Commun. (2008) 4357 4359. Copyright (2008) Royal Society of Chemistry.

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30.4.1.2 Catalysis of Nitrophenol Reduction Lin and Doong reported the synthesis of Au Fe3O4 dumbbell-like nanoparticles by thermal decomposition of Fe(OL)3 at Au seeds at 310 C for the catalytic reduction of p-nitrophenol and 2,4-dinitrophenol [32]. They discovered that the Au Fe3O4 dumbbelllike nanoparticles show higher catalytic efficiency than the pure Au nanoparticles, which was attributed to the synergetic effect that occurs at the interface of Au and Fe3O4. It is believed that the electronic structures of both the metal and metal oxide support are modified by electron transfer across the interface, giving rise to oxygen vacancies on the interfacial metal oxide support that become active sites for oxygen absorption and activation [24]. The as-prepared Au Fe3O4 dumbbell-like nanoparticles with magnetic properties can be easily recycled by an external magnet after the catalytic reduction. Fig. 30.9 shows the magnetically recyclable reduction of nitrophenols in the presence of Au Fe3O4

FIGURE 30.9

Catalytically recyclable reduction of (A) p-nitrophenol and (B) 2,4-nitrophenol by Au Fe3O4 dumbbell-like nanoparticles in the presence of NaBH4. Conversion efficiency of (C) p-nitrophenol in six successive cycles of reduction and (D) 2,4-nitrophenol in seven successive cycles of reduction by Au Fe3O4 and citratestabilized Au nanocatalysts. Source: Reproduced with permission from F.-H. Lin, R.-A. Doong, Bifunctional Au Fe3O4 heterostructures for magnetically recyclable catalysis of nitrophenol reduction, J. Phys. Chem. C 115 (2011) 6591 6598. Copyright (2011) American Chemical Society.

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dumbbell-like nanoparticles. The catalysts can be successfully recycled and reused for at least six successive cycles of reaction with stable conversion efficiency of around 100%. In contrast, the conversion efficiency of nitrophenols by citrate-stabilized Au nanoparticles drops dramatically after the second cycle. It is obvious that the presence of Fe3O4 nanoparticles makes the dumbbell-like heterostructures a promising bifunctional probe for magnetically recyclable catalytic reduction. 30.4.1.3 Catalysis of Hydrogen Peroxide Reduction Sun et al. proposed a unique protocol to investigate the synergetic effect in Au Fe3O4 dumbbell-like nanoparticles for catalyzing the reduction of H2O2 [51]. The strategy started with the synthesis of Au Fe3O4 dumbbell-like nanoparticles, and then single component Au and Fe3O4 nanoparticles were obtained from the Au Fe3O4 dumbbell-like nanoparticles by a controlled etching of the composite particles (Fig. 30.10A), which ensured that the individual Au and Fe3O4 nanoparticles have the same structural features as their corresponding domains in the Au Fe3O4 dumbbell-like nanoparticles. The catalytic reduction (A)

H 2SO 4

Au

KI/I2

Au-Fe3O4

Fe3O4

(B) (a)

I/mA (mg Fe3O4)

–1

0

–1

I/mA (mg Au)

(b)

40

–40 –80 –120 –160

10 0 –10 –20 –30 –40

–0.4 –0.2 0.0

0.2

0.4

0.6

V/V vs. Ag/AgCl

(d)

0.1

0.0

–0.1

–0.1

–0.2 –0.3

0.6

–0.4 –0.2 0.0 0.2 0.4 V/V vs. Ag/AgCl

0.6

0.1

0.0 I/mA

I/mA

(c)

–0.4 –0.2 0.0 0.2 0.4 V/V vs. Ag/AgCl

–0.2 –0.3

–0.4 –0.2 0.0 0.2 0.4 V/V vs. Ag/AgCl

0.6

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FIGURE 30.10 (A) Selected etching of Au Fe3O4 dumbbelllike nanoparticles for the preparation of the Au nanoparticles and dented Fe3O4 nanoparticles. (B) I V curves of (a) Au Fe3O4 (black) and Au (gray) nanoparticles, (b) Au Fe3O4 (black) and Fe3O4 (gray) nanoparticles normalized by Au and Fe3O4 weight, respectively, (c) Au Fe3O4 catalyst (black) and carbon support (gray), and (d) Au Fe3O4 catalyst with (black) and without (gray) the addition of 4 mM of H2O2. Recorded in N2-saturated 0.1 M PBS with 4 mM of H2O2; scan rate: 50 mV/s. Source: Reproduced with permission from Y. Lee, M.A. Garcia, N.A.F. Huls, S. Sun, Synthetic tuning of the catalytic properties of Au Fe3O4 nanoparticles, Angew. Chem. Int. Ed. 49 (2010) 1271 1274.

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of H2O2 demonstrated that the Au Fe3O4 dumbbell-like nanoparticles had a higher catalytic activity than that of either Au or Fe3O4 nanoparticles (Fig. 30.10B). The enhanced catalytic activity of the Au Fe3O4 dumbbell-like nanoparticles for H2O2 reduction reaction resulted from the electronic interaction between Au and Fe3O4 domains in the composite nanoparticles.

30.4.2 Sensors Wei et al. reported the application of Au Fe3O4 dumbbell-like nanoparticles in the detection of the cancer biomarker, prostate specific antigen (PSA) [56]. As shown in Fig. 30.11, a sandwich-type structure was formed by immobilizing the primary anti-PSA antibody (Ab1) onto the graphene sheet (GS) surface, the PSA in the sample captured, and the Ab2 Au Fe3O4 label. The Au Fe3O4 dumbbell-like nanoparticles showed a synergetic effect in catalyzing H2O2 reduction, which was more active than either Au or Fe3O4 nanoparticles alone. The electrochemical signal emanating from Au Fe3O4 nanoparticles was in accordance with the concentration of PSA, which could be quantified for the detection of PSA. The immunosensor showed a wide linear range (0.01 10 ng/mL), low detection limit (5 pg/mL), good reproducibility, and stability. Our group demonstrate for the first time that the bifunctional Au Fe3O4 dumbbell-like nanoparticles can be used for sensitive and selective turn-on fluorescent detection of cyanide with Rhodamine B (RB) as the fluorescent probe based on the inner filter effect (IFE), and a “magnetic concentration-washing process” is proposed to effectively reduce the interference of dye pollution [33]. As shown in Fig. 30.12, Au Fe3O4 dumbbell-like nanoparticles can be mixed into a larger volume of sample solution and magnetically concentrated to the original volume after the reaction with cyanide. By doing so, a higher fluorescence recovery signal can be obtained. More importantly, if dye pollution exists in the sample, this fluorescent detection method can also be used. Most real sample solution can be removed during the magnetic concentration process. Then, a certain amount of buffer prepared with pure water was added to dilute the rest of the real sample, followed by another magnetic concentration (this procedure was named a “magnetic concentrationwashing process”). Repeating such procedures, very little interference from the real sample would be left. Cyanide can dissolve Au domains of Au Fe3O4 nanoparticles in the

FIGURE 30.11 Schematic representation of the immunosensor. Ab1, primary anti-PSA antibody; Ab2, secondary anti-PSA antibody; GC, glassy carbon electrode; GS, graphene sheet; PSA, prostate specific antigen. Source: Reproduced with permission from Q. Wei, Z. Xiang, J. He, G. Wang, H. Li, Z. Qian, et al., Dumbbell-like Au Fe3O4 nanoparticles as label for the preparation of electrochemical immunosensors, Biosens. Bioelectron. 26 (2010) 627 631. Copyright 2010 Elsevier.

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FIGURE 30.12

Schematic depiction of the turn-on fluorescent detection of cyanide based on the dual-functional Au Fe3O4 dumbbell-like nanoparticles. Source: Reproduced with permission from Y.M. Zhai, L.H. Jin, P. Wang, S. Dong, Dualfunctional Au Fe3O4 dumbbell nanoparticles for sensitive and selective turn-on fluorescent detection of cyanide based on the inner filter effect, Chem. Commun. 47 (2011) 8268 8270. Copyright (2011) Royal Society of Chemistry.

presence of oxygen, forming a stable Au(CN)22 complex. With the erosion of the Au domains, there will be more light available to excite the RB and also more emission light be detected, because the Au domains can effectively reduce the emission of RB. As a result, the signal of fluorescence was enhanced gradually by increasing the concentration of cyanide. The proposed method obtained a good linear relationship between the fluorescence enhancement ratio and the cyanide concentration in the range of 4.0 3 1027 M to 1.2 3 1024 M (R2 5 0.994), and can be applied for real samples analysis. Huang et al. developed an effective sensitive interface to detect As(III) by using Au Fe3O4 dumbbell-like nanoparticles [57]. Fig. 30.13 illustrates the detection mechanism of As(III) on Au Fe3O4 dumbbell-like nanoparticles. According to their previous report [58], the synergistic effect of the excellent catalytic properties of Au nanoparticles and the good adsorption ability of 400 nm Fe3O4 nanoparticles can significantly improve the detection of As(III), and the adsorbed As(III) on 400 nm Fe3O4 nanoparticles will be directly reduced and oxidized on the Au surface (left side of Fig. 30.13). The adsorption capacity toward As(III) increases as the size of Fe3O4 nanoparticles decreases (B10 nm). Besides, active Fe(II) exposed on the surface of the Au Fe3O4 nanoparticles also has a role. The concentration of As(III) near the electrode surface gradually increases as a result of the adsorption of B10 nm Fe3O4 nanoparticles. The surface-activated Fe(II) can denote an electron to form Fe(III) in the reduction of As(III) to As(0) (right side of Fig. 30.13). The generated Fe(III) will then get an electron from the electrode or oxidation of As(0) to As(III) during the process of square wave anodic stripping voltammetry (SWASV), which completes the Fe(II)/Fe(III) cycle (right side of Fig. 30.13). The Fe(II) works as an electrocatalyst to mediate electron transfer between electrode and As(III). This mediation of the Fe(II)/Fe (III) cycle as well as the catalyst of Au nanoaprticles will efficiently enhance the electrochemical sensitivity toward As(III). As a result, the modified Au Fe3O4 dumbbell-like nanoparticles modified screen-printed carbon electrode (SPCE) serves as an efficient sensing interface for As(III) detection with an excellent sensitivity of 9.43 μA/ppb and a low detection limit of 0.0215 ppb.

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FIGURE 30.13 Ultrahighly sensitive electroanalysis of As(III) based on the adsorption of B10 nm Fe3O4 nanoparticles and the catalyst of B7 nm Au nanoparticles as well as the redox mediation by surface-active Fe(II) at Au Fe3O4 SPCE. Source: Reproduced with permission from S.S. Li, W.Y. Zhou, M. Jiang, Z. Guo, J.H. Liu, L. Zhang, et al., Surface Fe(II)/Fe(III) cycle promoted ultra-highly sensitive electrochemical sensing of Arsenic(III) with dumbbell-like Au/Fe3O4 nanoparticles, Anal. Chem. 90 (2018) 4569 4577. Copyright (2018) American Chemical Society.

30.4.3 Biomedicine Magnetically and optically active Au Fe3O4 dumbbell-like nanoparticles containing two different chemical surfaces are particularly suitable for selected nanoparticle functionalization with both targeting agents and drug molecules, which facilitates their application as multifunctional probes for target specific imaging [47,48] and delivery [59]. 30.4.3.1 Cell Imaging Sun et al. reported that the Au Fe3O4 dumbbell-like nanoparticles were suitable probe for A431 (human epithelial carcinoma cell line) cell imaging [60]. By functionalizing the surface of Fe3O4 and Au domains in Au Fe3O4 dumbbell-like nanoparticles with the epidermal growth factor receptor antibody (EGFRA) and HS-PEG-NH2, respectively, the nanohybrids could remain stable against aggregation in phosphate-buffered saline (PBS) or PBS containing 10% fetal bovine serum (FBS) at 37 C for 12 h. They then incubated the functionalized dumbbell-like nanoparticles with A431 cells in Dulbecco’s Modified Eagle’s Medium (DMEM) containing 10% FBS for 1 h. In their study, A431 cells labeled with 8 20 nm Au Fe3O4 hybrid nanoparticles were visualized with a scanning confocal microscope. The wavelength used for this image was 594 nm, which is close to the strong reflectance of the nanoparticles. Furthermore, A431 cells labeled with these hybrid nanoparticles can be also manipulated by using an external magnetic field, which is readily tracked under the optical microscope. To demonstrate the specific targeting, the authors incubated A431 cells and the 8 20 nm Au Fe3O4 hybrid nanoparticles without EGFRA. In this case, the reflection signal was much weaker, and the signal-to-noise ratio was much higher, thus indicating that the EGFRA-labeled nanoparticles had higher specificity in their attachment to A431 cells [60]. II. APPLICATIONS

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30.4.3.2 Drug Delivery Sun et al. developed a drug nanocarrier by using Au Fe3O4 dumbbell-like nanoparticles with herceptin (Her2 antibody) attached on Fe3O4 nanoparticles and the anticancer drug cisplatin attached on the Au nanoparticles [61]. Such a structure takes the advantages of multitasking during therapy and also two kinds of functional ligands can work independently without disturbing each other. The specificity of the platin Au Fe3O4 Herceptin composite nanoparticles was examined through their preferred targeting of Sk-Br3 cells that are Her2-positive breast cancer cells with Her2-negative breast cancer cells (MCF-7) [62] as a control. Fig. 30.14 shows the reflection images of Sk-Br3 cells (Fig. 30.14A) and MCF-7 cells (Fig. 30.14B). The brighter image shown in Fig. 30.14A demonstrates that more composite nanoparticles are conjugated to the Sk-Br3 cells, indicating that under the same incubation concentration, Herceptin helps the preferred targeting of Sk-Br3 cells as

FIGURE 30.14 Reflection images of (A) Sk-Br3 and (B) MCF-7 cells after incubation with the same concentration of platin Au Fe3O4 Herceptin NPs. (C) Cisplatin and platin release curves at 37 C (pH 5 7). (D) pHdependent Pt release from platin Au Fe3O4 Herceptin at 37 C. Source: Reproduced with permission from C.J. Xu, B.D. Wang, S. Sun, Dumbbell-like Au Fe3O4 nanoparticles for target-specific platin delivery, J. Am. Chem. Soc. 131 (2009) 4216 4217. Copyright (2009) American Chemical Society.

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opposed to MCF-7 cells. The drug release profile was analyzed by using the dialysis bag in PBS, 37 C at pH 5 7. As shown in Fig. 30.14C, 80% of the free cisplatin diffused through the dialysis bag in 1 h, while for the nanoparticle conjugate, this release was reduced to only B25% in the same incubation time. The Pt release was pH dependent (Fig. 30.14D). It can be seen that the release of platin from the platin Au Fe3O4 Herceptin nanoparticles increases when the pH becomes lower. The comparison of the cell viability after incubation with the composite nanoparticles, control composite nanoparticles (without antibody), and cisplatin showed that the cell viability of the composite nanoparticles was lowest for its therapeutic effect. 30.4.3.3 Gene Transfection Gene therapy has the great potential for treating many diseases such as cancer. However, at present gene therapies in the clinic are still limited due to the low efficiencies of nonviral gene vectors and the receptor-dependent host tropism of adenoviral or low titers of retroviral vectors [63]. Recently, “Magnetofection” has been used to overcome these deficiencies by associating gene vectors with magnetic nanoparticles and application of a magnetic field [64]. For example, Xu et al. reported the usage of Au Fe3O4 dumbbell-like nanoparticles in gene transfection as well as micro-optical coherence tomography (μOCT) imaging [65]. As illustrated in Fig. 30.15, negatively charged plasmids can be conjugated with Au Fe3O4 nanoparticles premodified with cationic polymer, PEI. Given the presence of a magnetic component (Fe3O4), under a magnetic field, these nanocomposites provided a higher efficiency in transfecting adherent mammalian cells. Moreover, Au Fe3O4 nanoparticles (especially the Fe3O4 core) enabled the visualization of this transfection process through micro-optical coherence tomography (μOCT) technology. Although the Au didn’t show any specific contribution to the μOCT imaging and magnetofection here, considering the multirole applications of Au nanoparticles already demonstrated in multimodal imaging [66], the properties reported here FIGURE 30.15 Schematic illustration of Au Fe3O4 dumbbell-like nanoparticles for gene delivery and μOCT tracking. Source: Reproduced with permission from W. Shi, X. Liu, C. Wei, Z.J. Xu, S.S.W. Sim, L. Liu, et al., Micro-optical coherence tomography tracking of magnetic gene transfection via Au Fe3O4 dumbbell nanoparticles, Nanoscale 7 (2015) 17249-17253. Copyright (2015) Royal Society of Chemistry.

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are expected to trigger more cross-disciplinary research on Au Fe3O4 nanoparticles. Furthermore, by increasing the size, Au nanoparticles might dominate and greatly enhance the μOCT signal [67].

30.5 CONCLUSIONS Au Fe3O4 dumbbell-like nanoparticles offer an interesting platform to investigate the physical/chemical properties of materials based not only on each particle dimension and morphology, but also on the communication between the two different components. The interfacial interaction that exists between two different nanoparticles can induce significant change in the physical and chemical properties of both nanoparticles in the structure. As a result, the Au Fe3O4 dumbbell-like nanoparticles have shown interesting catalytic, optical, and magnetic properties. The distinct surface chemistry presented in the Au Fe3O4 dumbbell-like nanoparticles also facilitates the selected nanoparticles functionalization with either a targeting agent and/or a therapeutic agent, making these nanohybrids especially important for target-specific medical diagnostic and therapeutic applications. The research of Au Fe3O4 dumbbell-like nanoparticles could be categorized as follows: (i) developing general strategies to prepare the nanohybrids with controlled size and morphology of each component; (ii) understanding the synergetic effect and interface boundary sites, which play an important role in the modulation of physical and chemical properties; and (iii) exploring potential applications of these nanohybrids.

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