J. Phys. Chem. B 2005, 109, 21593-21601
21593
Monodispersed Core-Shell Fe3O4@Au Nanoparticles Lingyan Wang,† Jin Luo,† Quan Fan,† Masatsugu Suzuki,‡ Itsuko S. Suzuki,‡ Mark H. Engelhard,§ Yuehe Lin,§ Nam Kim,‡ Jian Q. Wang,‡ and Chuan-Jian Zhong*,† Department of Chemistry, State UniVersity of New York at Binghamton, Binghamton, New York 13902, Department of Physics, State UniVersity of New York at Binghamton, Binghamton, New York 13902, and EnVironmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: August 4, 2005; In Final Form: September 13, 2005
The ability to synthesize and assemble monodispersed core-shell nanoparticles is important for exploring the unique properties of nanoscale core, shell, or their combinations in technological applications. This paper describes findings of an investigation of the synthesis and assembly of core (Fe3O4)-shell (Au) nanoparticles with high monodispersity. Fe3O4 nanoparticles of selected sizes were used as seeding materials for the reduction of gold precursors to produce gold-coated Fe3O4 nanoparticles (Fe3O4@Au). Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, core-shell composition, surface reactivity, and magnetic properties have confirmed the formation of the core-shell nanostructure. The interfacial reactivity of a combination of ligand-exchanging and interparticle cross-linking was exploited for molecularly mediated thin film assembly of the core-shell nanoparticles. The SQUID data reveal a decrease in magnetization and blocking temperature and an increase in coercivity for Fe3O4@Au, reflecting the decreased coupling of the magnetic moments as a result of the increased interparticle spacing by both gold and capping shells. Implications of the findings to the design of interfacial reactivities via core-shell nanocomposites for magnetic, catalytic, and biological applications are also briefly discussed.
Introduction The ability to control the size and monodispersity in synthesizing and assembling metal-coated magnetic nanoparticles is important for exploring technological applications of the nanoscale core, shell, or their combinations. It is increasingly important for many applications involving magnetic nanoparticles, such as magnetic resonance imaging for medical diagnosis, high-density magnetic recording, controlled drug delivery, biological targeting or separation, and catalysis.1-13 While the synthesis of monolayer-capped iron oxide nanoparticles has been extensively studied,14-17 the synthesis of metal-coated iron oxide nanoparticles with controllable sizes and monodispersities is relatively limited. Recently, gold-coated magnetic core-shell nanoparticles are reported to enhance chemical stability by protecting the core from oxidation and corrosion, and to exhibit good biocompatibility and affinity via amine/thiol terminal groups. One example involves depositing gold onto iron oxide (Fe2O3 or partially oxidized Fe3O4) nanoparticles (9 nm) in an aqueous solution via hydroxylamine seeding to synthesize ∼60nm-sized particles.18 Using the reverse micelle method, goldcoated iron core-shell19-21 and gold (silver)-coated magnetite22 nanoparticles have also been synthesized. The assembly of gold nanoparticles on SiO2 cores has been shown to be a very effective approach to the formation of core-shell SiO2@Au nanoparticles.23 Most recently, a similar approach has been * Corresponding author. E-mail:
[email protected]. † Department of Chemistry, State University of New York at Binghamton. ‡ Department of Physics, State University of New York at Binghamton. § Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory.
demonstrated for the formation of gold-coated Fe3O4 nanoparticles, which involved the attachment of 2-3-nm-sized gold nanoparticles via 3-aminopropyl triethylsilane onto 10-nm-sized Fe3O4 nanoparticles.24 The formation of thin films of magnetic nanoparticles has also attracted considerable interest. For example, using a polymer-mediated assembly method, thin films of polymer-FePt nanoparticles and arrays have been deposited on Si substrates.6,25 By using an evaporation method together with the applied magnetic field parallel to a substrate (e.g., HOPG), thin film assemblies of magnetic nanoparticles have been prepared and studied.26 While these prior studies have shown viabilities of synthesizing and assembling core-shell types of nanoparticles, the precise control of the continuous nature of the metal coating, the coating thickness, and the size monodispersity, and the thin film assembly of the nanoparticles remain to be some of the major challenges. In comparison with the large volume of studies on the synthesis of magnetic nanoparticles, the above challenges are related to the fact that relatively little is understood about the control of the composition and interfacial reactivity of the nanomaterials. In our efforts to develop iron oxide core-shell nanocomposites, we have recently demonstrated the viabilities of producing core-shell Fe oxide@Au nanoparticles via coating the presynthesized iron oxide nanoparticles with gold shells and forming molecularly mediated thin film assemblies via interfacial reactivities at the gold shell.27 One of the important aspects of our synthesis of the Fe3O4@Au nanoparticles is the formation of the gold shell at the iron oxide nanocrystal cores with high monodispersity and controllable surface capping properties
10.1021/jp0543429 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/27/2005
21594 J. Phys. Chem. B, Vol. 109, No. 46, 2005 SCHEME 1. Illustration of the Core-Shell Fe3O4@Au Nanoparticle with an Outmost Organic Shell Encapsulation (R ) -CO2H or -NH2).
(Scheme 1), which facilitates the subsequent control and manipulation of the interparticle interactions and reactivities. Such core-shell nanostructures could find applications that explore the electronic, magnetic, catalytic, sensing, and chemical or biological properties of the nanocomposite materials. To explore the magnetic properties, the formation of a gold shell with a controllable assembly allows better stability and tunability for the construction of ordered arrays.6 In a recent study of the unique catalytic properties of nanoscale gold, it is found that gold nanoparticles supported on iron oxide exhibit excellent catalytic properties for water-gas shift reactions.28,29 In the area exploring applications in biology and medicine, the use of magnetic nanoparticles and a magnetic filed in vivo or in vitro can either remotely position or selectively filter biological materials.30,31 In magnetic resonance imaging, the presence of the particles at a given site can alter the contrast of certain types of cells by several orders of magnitude. The possibility of using magnetic nanoparticles to improve the effectiveness of cell manipulation and DNA sequencing could also aid the development of pharmaceuticals, drug delivery systems, and magnetic separation technologies for rapid DNA sequencing. These applications should benefit a great deal from the ability to control the surface and interparticle spatial properties of the magnetic nanoparticles. Experimental Section Chemicals. Iron(III) acetylacetonate (Fe(acac)3, 99.9%), 1,2hexadecanediol (C14H29CH(OH)CH2(OH), 90%), oleylamine (OAM, C9H18dC9H17NH2, 70%), oleic acid (OA, C9H18dC8H15COOH, 99%), phenyl ether (C12H10O, 99%), 1,9-nonanedithiol (NDT, SHC9H18SH, 95%), mercaptoundecanoic acid (MUA, SHC11H22CO2H, 95%), and other solvents (hexane and ethanol) were purchased from Aldrich. Gold acetate (Au(OOCCH3)3, or Au(ac)3, 99.9%) was purchased from Alfa Aesar. All chemicals were used as received. Synthesis. The preparation of Fe3O4@Au involved an initial synthesis of Fe3O4 nanoparticles as seeds and a subsequent reduction of Au(OOCCH3)3 in the presence of the seeds. Fe3O4 nanoparticles seeds were synthesized using a modified protocol.16 Briefly, 0.71 g Fe(acac)3 (2 mmol) was mixed in 20 mL of phenyl ether with 2 mL of oleic acid (∼6 mmol) and 2 mL of oleylamine (∼4 mmol) under argon atmosphere with vigorous stirring. 1,2-Hexadecanediol (2.58 g, 10 mmol) was added into the solution. The solution was heated to 210 °C and refluxed for 2 h. After refluxing for 2 h and cooling to room temperature, the reaction solution was directly used without separation. In a typical synthesis, 10 mL of the phenyl ether reaction solution
Wang et al. of Fe3O4 nanoparticles (∼0.33 mmol Fe3O4), 0.83 g (2.2 mmol) of Au(OOCCH3)3, 3.1 g (12 mmol) of 1,2-hexadecanediol, 0.5 mL (∼1.5 mmol) of oleic acid, 3 mL (∼6 mmol) of oleylamine were added into 30 mL of phenyl ether. In this case, the mole ratio of the Au precursor to the iron oxide nanoparticles was approximately 7:1. Under argon atmosphere and vigorous stirring, the reaction solution was heated to 180-190 °C at 10 °C/min and was kept at this temperature for 1.5 h. After cooling to room temperature, ethanol was added into the solution. A dark-purple material was precipitated and separated by centrifuging. The precipitated product was washed with ethanol, and redispersed in hexane in the presence of ∼75 mM each of oleic acid and oleylamine. The nanoparticle solution appeared dark purple. The product nanoparticles dispersed in hexane, which can be further separated by centrifugation to obtain the desired core-shell Fe3O4@Au nanoparticles. A Hermle Labortechnik GMbH Z 233 M2 centrifuge was used. Assembly. Details of the thin film assembly procedures were reported previously.27,32,33 Briefly, NDT or MUA molecules were mixed with nanoparticles in a hexane solution, 10 mM NDT + 0.2 µM Fe3O4@Au nanoparticles for the NDT assembly, and 0.06 mM MUA + 0.1 µM Fe3O4@Au for the MUA assembly. The concentration of Fe3O4@Au nanoparticles was estimated by using several parameters, including the initial feeding of Fe, the density of iron oxide, the average diameter of the particle, and the yield of the resulting core-shell nanoparticles. The nanoparticles were assembled as a thin film on different substrates, including cleaned glass slides and gold film coated glass slides. The glass slides were used for spectrophotometric measurement. The substrates were immersed vertically in the solution to ensure the film was free of powder deposition. The thin film was thoroughly rinsed with hexane and dried under nitrogen or air before characterization. The thickness (or coverage) was controlled by assembling time and monitored by surface plasmon resonance absorbance and mass loading.32,33 It is important to point out that the assembly scheme via the thiol-thiol exchange works only for the Fe oxide@Au core-shell nanoparticles due to the Au-SR binding affinity. In contrast, it does not work for iron oxide nanoparticles because of the lack of binding affinity. Characterization. Transmission electron microscopy (TEM). TEM was performed on Hitachi H-7000 electron microscope (100 kV). The nanoparticle samples dispersed in hexane solution were cast onto a carbon-coated copper grid sample holder, followed by evaporation at room temperature. UltraViolet-Visible Spectroscopy (UV-Vis). Ultravioletvisible spectra were acquired with a HP 8453 spectrophotometer. The spectra were collected over the range of 200-1100 nm. Direct Current Plasma-Atomic Emission Spectroscopy (DCPAES) The composition was analyzed using the direct current plasma-atomic emission spectroscopy, which was performed by using an ARL Fisons SS-7 direct current plasma-atomic emission spectrometer. Measurements were made on emission peaks at 267.59 and 259.94 nm, for Au and Fe, respectively. The nanoparticle samples were dissolved in concentrated aqua regia, and then diluted to concentrations in the range of 1-50 ppm for analysis. Calibration curves were made from dissolved standards, with concentrations from 0 to 50 ppm in the same acid matrix as the unknowns. Detection limits based on three standard deviations of the background intensity were 0.008 and 0.005 ppm for Au and Fe, respectively. Standards and unknowns were analyzed 10 times each for 3 s counts. Instrument reproducibility, for concentrations greater than 100 times the detection limit, results in (2% error.
Monodispersed Core-Shell Fe3O4@Au Nanoparticles
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SCHEME 2. Illustration of the Chemistry and Processes Involved in the Synthesis of the Fe3O4 and Fe3O4@Au Nanoparticles
X-ray Powder Diffraction (XRD). The products were identified by powder X-ray diffraction. Powder diffraction patterns were recorded on a Scintag XDS 2000 θ-θ powder diffractometer equipped with a Ge(Li) solid-state detector (Cu KR radiation). The data were collected from 2θ ) 5-90° at a scan rate of 0.02° per step and 5 s per point. X-ray Photoelectron Spectroscopy (XPS). The XPS measurements were made by using a Physical Electronics Quantum 2000 scanning ESCA microprobe. This system uses a focused monochromatic Al KR X-ray (1486.7 eV) source for excitation and a spherical section analyzer. The instrument has a 16element multichannel detection system. The X-ray beam used was a 98 W beam with a 107 mm diameter that is rastered over a 1.4 mm by 0.2 mm rectangle on the sample. The X-ray beam is incident normal to the sample, and the X-ray detector is at 45° away from the normal. The collected data were referenced to an energy scale with binding energies for Cu (2p3/2) at 932.67 ( 0.05 eV and Au (4f) at 84.0 ( 0.05 eV. The percentages of individual elements detected were determined from the relative composition analysis of the peak areas of the bands. It is based on the relative peak areas and their corresponding sensitivity factors to provide relative compositions. Superconducting Quantum Interference DeVice (SQUID) Magnetometry. Magnetic measurements were performed using a SQUID magnetometer (Quantum Design MPMS XL-5). The temperature dependence of the magnetic susceptibility (χ ) M/H, where M is the magnetization and H is the applied magnetic field) was measured as a function of temperature in a magnetic field. Results and Discussion The discussion of the results is divided into four sections. First, the results for the separation of Fe3O4 and Fe3O4@Au nanoparticles are discussed. Second, the results from the characterization of the Fe3O4@Au nanoparticles are described. Third, the results from molecularly mediated thin film assemblies of the Fe3O4@Au nanoparticles are discussed. And last, we discuss the magnetic properties of the Fe3O4@Au nanoparticles. 1. Separation of Fe3O4 and Fe3O4@Au Nanoparticles. Scheme 2 illustrates the chemistry and processes involved in the synthesis of the Fe3O4 and Fe3O4@Au nanoparticles. It starts
with the Fe3O4 seeds formation according to well-established protocol.16 With the desired seeds, two new protocols were used in this work. First, gold is deposited onto Fe3O4 via reduction of Au(ac)3 in the presence of the seeds and capping agent by a reducing agent at elevated temperature (180-190 °C). This process involves thermally activated desorption of the capping layer, deposition of Au on the exposed Fe3O4 surface, and subsequent re-encapsulation of the Au surface by the capping agent (See Scheme 2). Second, the resulting Fe3O4@Au nanoparticles were separated by centrifugation, which is a process that we found to be very effective in separating Fe3O4 and Fe3O4@Au nanoparticles and different sizes as well. The nanoparticle solution was first centrifuged at 7000 rpm for 20 min, after which some precipitate was observed at the bottom of the tube and the top solution still showed a dark-purple color. The top dark-purple solution was transferred into a new centrifuge tube and recentrifuged at 14 000 rpm for 20 min, after which more dark precipitate was found at the bottom of the tube. The top solution displayed a yellow-brown color, which appears to be a very diluted solution of Fe3O4. This process can be repeated many times and the speed can be varied as one desires. Figure 1 shows a representative set of TEM images for the Fe3O4@Au nanoparticles collected from the above separation processes, including those collected from the precipitate after centrifugation at 7000 rpm (A), the precipitate by recentrifuging the top solution at 14 000 rpm (B), and the final top solution portion after both the 7000 and 14 000 rpm centrifugations (C). There are clear differences in sizes. The particles collected after the first 7000 rpm were clearly those with larger average core size (12.1 ( 1.4 nm), whereas the particles collected from the precipitate at 14 000 rpm by recentrifuging the top solution that resulted from that at 7000 rpm were those with a smaller average core size (6.6 ( 0.4 nm) (B). On the basis of the change of the absorbance for the surface plasmon resonance band, a rough estimate of the ratio of A vs B in terms of quantities yielded 12:88. In contrast, the particles collected from the top solution portion (C) were predominantly uncoated Fe3O4 nanoparticles (5.3 ( 0.5 nm), which is supported by the darkness contrast of the particles. The composition of the nanoparticles thus obtained was further analyzed by using the DCP-AES technique. Table 1
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Figure 1. TEM micrographs for Fe3O4@Au collected from the precipitate after centrifugation at 7000 rpm (A), the precipitate by recentrifuging the top solution at 14 000 rpm (B), and the final top solution portion after both the 7000 and 14 000 rpm centrifugations (C).
TABLE 1: Results from DCP Analysis of Metal Compositions in the Nanoparticles (Core Size: 5.2 nm) sample as-synthesized Fe3O4@Au powder separated Fe3O4@Au at 7000 rpm separated Fe3O4@Au at 14 000 rpm
atomic weight ratio Au:Fe
shell thickness d (nm)
44:56
0.4
71:29
1.0
66:34
0.8
shows a set of DCP data comparing the compositions of the separated precipitate nanoparticles with the product before separation. There are two important findings. First, there is a clear reduction of Fe in the separated precipitate nanoparticles in comparison with the original product, which is consistent with the TEM data that most of the uncoated Fe3O4 particles were left in the solution after 14 000 rpm. Second, the Au content for the particles collected after the first 7000 rpm were slightly larger (by ∼7%) than that for the particles collected from the precipitate at 14 000 rpm by recentrifuging the top solution that resulted from that at 7000 rpm, which may suggest the presence of either Fe3O4@Au with a thicker Au shell or a small fraction of Au particles in the particles separated from the first centrifuging at 7000 rpm. The large-sized particles in the Figure 1A could also be due to aggregation of two or three Fe3O4@Au particles. We note that the surface plasmon resonance band for the separated nanoparticles showed little change except for the absorbance decrease due to concentration change. Clearly, the composition data from the DCP analysis are quite consistent with the data from the TEM characterization. The results demonstrate that the separation of Fe3O4 and Fe3O4@Au nanoparticles is very effective. 2. Characterization of Fe3O4@Au Nanoparticles. 2.1. TEM. Figure 2 shows a representative set of TEM micrographs comparing Fe3O4 (A) and Fe3O4@Au (B). The average size of the Fe3O4 seeds obtained from 2 h reflux was 5.2 nm. There are two major findings from the morphological comparison of A and B in both figures. First, the particles after coating with Au appear much darker than the starting nanoparticles. Second, for the particles after coating with Au, the average particle size changed from 5.2 ( 0.4 nm (A) to 6.6 ( 0.4 nm (B). The average thickness of the Au coating is ∼0.7 nm. As evidenced by the interparticle spacing and its uniformity, the nanoparticles are individually isolated by the capping monolayer (oleylamine and oleic acid). Clearly, the TEM data serve as an important piece of evidence for the formation of Fe3O4@Au core-shell nanoparticles. Similar results were observed for particles prepared by slightly different seeding procedures in terms of refluxing time (Figure 3). For example, a 1.5 h reflux led to a smaller average size for the Fe3O4 seeds obtained (4.5 nm). For the particles after coating with Au, the average particle sizes changed from 4.5 ( 0.5 nm
to 6.3 ( 0.5 nm (Figure 3). The average thickness of the Au coating is ∼0.9 nm. 2.2. UV-Vis. Measurements of the surface plasmon (SP) resonance band of the nanoparticles provided an indirect piece of evidence supporting the formation of Fe3O4@Au core-shell morphology. Figure 4 shows a typical set of UV-vis spectra comparing Fe3O4 (curve a) and Fe3O4@Au (curves c and d) nanoparticles dispersed in hexane solution. In contrast to the largely silent feature in the visible region for Fe3O4 particles, Fe3O4@Au particles show a clear SP band at 535 and 550 nm for the different core sizes and Au thickness, characteristic of the optical property of gold nanostructures. This band showed a red-shift in comparison with pure Au nanoparticles capped with oleylamine (curve b) that were synthesized by the same method. While similar shifts have been reported for gold-coated iron oxide nanoparticles prepared in aqueous solution,18 our observation revealed that the core-shell nanoparticles with a thicker Au shell (curve c) displayed a smaller red-shift than those with thinner Au shells (curve d). This finding suggests that the surface plasmon resonance property of the Au shell is dependent on the thickness. As will be further supported by DCP analysis data described in a later subsection, it is unlikely to have a significant amount of pure gold nanoparticles in these core-shell nanoparticle samples. 2.3. XRD. In Figure 5, the XRD spectra for Fe3O4, Au, and Fe3O4@Au nanoparticles are compared. The data (curve c) showed diffraction peaks at 2θ ) 38.2°, 44.4°, 64.6°, 77.5°, and 81.7°, which can be indexed to (111), (200), (220), (311), and (222) planes of gold in a cubic phase (space group: Fm3m), respectively.34 Clearly, the XRD peaks for Fe3O4@Au nanoparticles (curve c) are similar to that for Au nanoparticles capped with oleylamine (curve b), but different from that for Fe3O4 nanoparticles (curve a). The absence of any diffraction peaks for magnetite is most likely due to the heavy atom effect from gold35 as a result of the formation of Au-coated Fe3O4 nanoparticles. The fact that the diffraction peaks from Fe3O4 were not observed is consistent with observations by others,22 thus providing strong evidence for complete coverage of the oxide core by Au supporting our TEM data, which supports the formation of Fe3O4@Au core-shell nanoparticles. 2.4. XPS. The relative surface composition of Fe3O4@Au was further analyzed by using XPS. A representative set of results is shown in Figure 6. Table 2 summarizes the binding energies (Eb) and elemental surface compositions (atom %) for Fe3O4@Au nanoparticles of two different core-shell sizes. Using the binding energy for C (1s) (285 eV) as an internal reference, we found that the band shape and binding energy between the two different core size nanoparticles are basically identical ((0.2 eV), indicating that they have the same oxide composition. On the basis of the relative elemental compositions (Table 2), we obtained the Fe:Au ratio of 37:63 for Fe3O4@Au nanopar-ticles derived from core size 5.2 nm and 32:68 for Fe3O4@Au
Monodispersed Core-Shell Fe3O4@Au Nanoparticles
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Figure 2. TEM micrographs (top panel) and size distributions (bottom panel) for Fe3O4 nanoparticles before (A) and after (B) coating with Au shell. (A) 5.2 ( 0.4 nm, (B) 6.6 ( 0.4 nm.
Figure 3. TEM micrograph and size distribution (6.3 ( 0.5 nm) for Fe3O4@Au nanoparticles derived from Fe3O4 cores of 4.5 ( 0.5 nm diameter.
Figure 4. UV-Vis spectra comparing Fe3O4 (a) and Fe3O4@Au (c, d corresponding to the samples in Figures 3 and 2B, respectively). A spectrum for Au nanoparticles capped with oleylamine is included for comparison (b).
nanoparticles derived from core size 4.5 nm. These values are indeed very consistent with the Fe:Au ratio determined from DCP analysis, which gave 34:66 for Fe3O4@Au derived from core size 5.2 nm and 29:71 for those derived from core size 4.5 nm. Assuming that the detected N and C come from the organic shells, the oleylamine (OAM) to oleic acid (OA) ratios are 37: 63 for Fe3O4@Au derived from core size 5.2 nm and 42:58 for that derived from core size 4.5 nm. For Fe3O4@Au nanopar-
Figure 5. XRD for Fe3O4 (a), Au (b), and Fe3O4@Au (c) nanoparticles.
ticles, the detected Fe:O ratio was 3.0:4.2 for both samples, which are quite close to the Fe:O ratio of 3:4 expected for Fe3O4. 3. Characterization of Thin Film Assembly of Fe3O4@Au Nanoparticles. On the basis of our early work for the assembly of gold nanoparticles via interparticle covalent or hydrogen bonding linkages32,33 using 1,9-nonanedithiol (NDT) and mercaptoundecanoic acid (MUA), we examined the thiol-mediated binding chemistry for the assembly of Fe3O4@Au nanoparticles. As illustrated in Scheme 3, the basic chemistry involved an initial thiolate-oleylamine exchange reaction followed by cross-
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Figure 6. XPS spectra in the regions of Au (4f) (A) and Fe (2p) (B) for Fe3O4@Au nanoparticles derived from core sizes of 4.5 nm (dashed lines) and 5.2 nm (solid lines).
TABLE 2: Comparisons of Band Positions and Composition Ratios Based on XPS Analysis XPS data Eb/eV
Eb/eV
Fe3O4@Au
Au 4f7/2
Au 4f5/2
Au (atom %)
Fe 2p3/2
Fe 2p1/2
Fe (atom %)
Fe:Au
5.2 nm core 4.5 nm core
84.2 84.2
87.8 87.8
12.6 14.4
711 711
724.8 724.6
7.4 6.7
37:63 32:68
relative composition (atom %)
a
Fe3O4@Au
C
O
Fe
Au
N
Fe:O
Fe:Aua
5.2 nm core 4.5 nm core
63.7 63.7
15.0 13.7
7.4 6.7
12.6 14.4
1.3 1.5
3.0:4.2 3.0:4.2
34:66 29:71
Determined from DCP analysis.
TABLE 3: Formation of Thin Films on Glass Substrate for Different Nanoparticles and Linkers nanoparticle Fe3O4 Fe3O4@Au (Fe3O4 core size: 5.2 nm) Fe3O4@Au (Fe3O4 core size: 4.5 nm) Au[32,33]
linker
film assembly
λ (SP band) (nm)
NDT MUA NDT MUA NDT MUA NDT MUA
No No Yes Yes Yes Yes Yes Yes
610 600 600 590 590 580
linking via an alkyl chain for the case of using dithiol as a linker or hydrogen bonding for the case using carboxylic acid functionalized thiol as a linker. This assembly chemistry should work for the Au-coated iron oxide nanoparticles, but not for the iron oxide nanoparticles, because the thiolate binding is highly specific to Au. The binding chemistry and the thin film formation have been examined in several different ways. First, the thin film formation was examined by monitoring the optical property. Similar to thin film assemblies of gold nanoparticles,32 the color of the film formed ranged from brown to blue depending on the particle size, linker molecule, and film thickness. Table 3 summarizes the several sets of the experimental results. In comparison with the SP band for Fe3O4@Au nanoparticles in solution, the SP band is shifted to a longer wavelength for NDT- or MUA-mediated thin film assemblies of Fe3O4@Au nanoparticles. This trend is similar to those observed for the NDT- or MUA-mediated thin film assemblies of Au nanoparticles of similar sizes.32,33 The shift of the SP band is characteristic of the changes in interparticle spacing and dielectric properties as a result of the thin film assembly. In contrast, thin films were not detected for the assembly of Fe3O4 nanoparticles under the same condition. The results are consistent with the Au-thiolate binding chemistry responsible for
the formation of Fe3O4@Au thin films. We note that the nanoparticle solution became clear after 10 days for the case of NDT-mediated assembly, suggesting an assembly efficiency of ∼100%. In the case of MUA-mediated assembly, the solution showed a color change from dark red to light red after 5 days, suggesting assembly efficiency of
