Iron Oxide Coated Gold Nanorods: Synthesis, Characterization, and

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Langmuir 2008, 24, 6232-6237

Iron Oxide Coated Gold Nanorods: Synthesis, Characterization, and Magnetic Manipulation Anand Gole,* John W. Stone, William R. Gemmill, Hans-Conrad zur Loye, and Catherine J. Murphy* Department of Chemistry and Biochemistry, UniVersity of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208 ReceiVed December 19, 2007. ReVised Manuscript ReceiVed February 26, 2008 We report a simple process to generate iron oxide coated gold nanorods. Gold nanorods, synthesized by our three-step seed mediated protocol, were coated with a layer of polymer, poly(sodium 4-styrenesulfonate). The negatively charged polymer on the nanorod surface electrostatically attracted a mixture of aqueous iron(II) and iron(III) ions. Base-mediated coprecipitation of iron salts was used to form uniform coatings of iron oxide nanoparticles onto the surface of gold nanorods. The magnetic properties were studied using a superconducting quantum interference device (SQUID) magnetometer, which indicated superparamagnetic behavior of the composites. These iron oxide coated gold nanorods were studied for macroscopic magnetic manipulation and were found to be weakly magnetic. For comparison, premade iron oxide nanoparticles, attached to gold nanorods by electrostatic interactions, were also studied. Although control over uniform coating of the nanorods was difficult to achieve, magnetic manipulation was improved in the latter case. The products of both synthetic methods were monitored by UV-vis spectroscopy, zeta potential measurements, and transmission electron microscopy. X-ray photoelectron spectroscopy was used to determine the oxidation state of iron in the gold nanorod-iron oxide composites, which is consistent with Fe2O3 rather than Fe3O4. The simple method of iron oxide coating is general and applicable to different nanoparticles, and it enables magnetic field-assisted ordering of assemblies of nanoparticles for different applications.

Introduction Metallic nanoparticles show size- and shape-dependent optoelectronic properties, which make them useful for different applications.1 Synthetic protocols for producing metallic nanoparticles of different shapes and sizes have evolved over the last several years.1,2 The optoelectronic properties of metallic nanoparticles are exquisitely sensitive to the nature of their surface, which can form a basis for chemical sensing.3 Hence, research in the area of surface modification of nanoparticles has become increasingly important. Soft coatings such as bifunctional molecules,4a–c polymers,4d–f biological molecules,4g–i and hard * To whom correspondence should be addressed. E-mail: gole@ mail.chem.sc.edu (A.G.); [email protected] (C.J.M.). (1) (a) Daniel, M-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (b) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (c) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (d) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261. (e) Orendorff, C. J.; Sau, T. K.; Murphy, C. J. Small 2006, 2, 636. (2) (a) Murphy, C. J.; Sau, T. K.; Gole, A.; Orendorff, C. J. MRS Bull. 2005, 30, 349. (b) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Orendorff, C. J. Inorg. Chem. 2006, 45, 7544. (c) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (d) Perez-Juste, J.; Liz-Marzan, L. M.; Carnie, S.; Chan, D. Y. C.; Mulvaney, P. AdV. Funct. Mater. 2004, 14, 571. (e) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (f) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (g) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633. (3) (a) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (b) Obare, S. O.; Hollowell, R. E.; Murphy, C. J. Langmuir 2002, 18, 10407. (c) Kim, Y.; Johnson, R. C.; Hupp, J. T. Nano Lett. 2001, 1, 165. (d) Sastry, M.; Lala, N.; Patil, V.; Chavan, S. B.; Chittiboyina, A. G. Langmuir 1998, 14, 4138. (e) Aslan, K.; Zhang, J.; Lakowicz, J. R.; Geddes, C. D. J. Fluoresc. 2004, 14, 391. (4) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801. (b) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (c) Drechsler, U.; Erdogan, B.; Rotello, V. M. Chem.sEur. J. 2004, 10, 5570. (d) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846. (e) Kim, D.; Park, S.; Lee, J. H.; Jeong, Y. Y.; Jon, S. J. Am. Chem. Soc. 2007, 129, 7661. (f) Bhattacharjee, R. R.; Chakraborty, M.; Mandal, T. K. J. Phys. Chem. B 2006, 110, 6768. (g) Gole, A.; Dash, C.; Soman, C.; Sainkar, S. R.; Rao, M.; Sastry, M. Bioconjugate Chem. 2001, 12, 684. (h) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128. (i) Dujardin, E.; Mann, S. AdV. Mater. 2002, 14, 775.

inorganic coatings (to form core-shell structures) such as AuSiO2,5a–d Au-Ag,5e,f and Au-Pt5g have been developed. One of the more interesting core-shell systems is that of a magnetic core-gold/silver shell combination.6 This system has the potential advantages of magnetic separation from precursor materials and the possibility of facile surface modification due to the well-known chemistry of gold and silver. Simple gold/ silver thiol chemistry can be used to tag bifunctional molecules or antibodies to such particles. These antibody-modified particles can then target different antigens and further be easily purified/ separated by application of a permanent magnet.6e,f This makes the system very attractive for magnetic, biocatalytic, and biological applications.6 The inverse system, a gold/silver core and a magnetic shell, is also an interesting system to study. Noble metal nanoparticle properties, such as plasmon band (transverse and longitudinal) positions, enhancements in surface enhanced Raman scattering, plasmon light scattering, and so forth, can be modulated depending on their size and shape.1,2 Hence, coating these differently shaped particles by different soft/hard materials has the advantages of retaining metal particle shape with its attendant optical properties, increasing the suite of surface chemistries available for further functionalization, (5) (a) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (b) Graf, C.; Dembski, S.; Hofmann, A.; Ruhl, E. Langmuir 2006, 22, 5604. (c) Shuhua, L.; Minyong, H. AdV. Funct. Mater. 2005, 15, 961. (d) Kobayashi, Y.; Correa-Duarte, M. A.; Liz-Marzan, L. M. Langmuir 2001, 17, 6375. (e) Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 8440. (f) Xie, F.; Baker, M. S.; Goldys, E. M. J. Phys. Chem. B 2006, 110, 23085. (g) Qian, L.; Sha, Y.; Yang, X. Thin Solid Films 2006, 515, 1349. (6) (a) Xu, Z.; Hou, Y.; Sun, S. J. Am. Chem. Soc. 2007, 129, 8698. (b) Wang, L.; Luo, J.; Fan, Q.; Suzuki, M.; Suzuki, I. S.; Engelhard, M. H.; Lin, Y.; Kim, N.; Wang, J. Q.; Zhong, C. J. J. Phys. Chem. B 2005, 109, 21593. (c) Yu, H.; Chen, M.; Rice, P. M.; Wang, S. X.; White, R. L.; Sun, S. Nano Lett. 2005, 5, 379. (d) Mikhaylova, M.; Kim, D. K.; Bobrysheva, N.; Osmolowsky, M.; Semenov, V.; Tsakalatos, T.; Muhammed, M. Langmuir 2004, 20, 2472. (e) Park, H.-Y.; Schadt, M. J.; Wang, L.; Lim, I.-I. S.; Njoki, P. N.; Kim, S. H.; Jang, M.-Y.; Luo, J.; Zhong, C.-J. Langmuir 2007, 23, 9050. (f) Liang, Y.; Gong, J.-L.; Huang, Y.; Zheng, Y.; Jiang, J.-H.; Shen, G.-L.; Yu, R.-Q. Talanta 2007, 72, 443.

10.1021/la703975y CCC: $40.75  2008 American Chemical Society Published on Web 05/17/2008

Iron Oxide Coated Gold Nanorods

and, in the case of magnetic coatings, makes the composite nanomaterials amenable to magnetic separation. The coating of metallic nanoparticles with a thin shell of a magnetic material is synthetically challenging, and only a few reports demonstrate this possibility.7 Wiggins et al. used sequential synthesis to form gold-iron-gold “nano-onion” structures using a reverse microemulsion system.7a Teng et al. used a sequential synthesis method to form a Pt core and an Fe2O3 shell. They first synthesized platinum particles by reduction of platinum salts in organic media and then thermally decomposed iron pentacarbonyl on the Pt surface to yield core-shell particles.7b Tzitzios et al. overcame the use of toxic iron pentacarbonyl by using Pt(acac)2 and Fe(acac)3 and an inexpensive polyethylene glycol, PEG-200, as a reducing agent. They also lowered the synthesis temperature by doping the platinum core with Ag or Au.7c Most of these reports discuss spherical nanoparticles, and some involve high temperatures and toxic reagents.7 None of the reports so far demonstrate iron oxide coating of 1-D nanostructures. Earlier, we and others have modified 1-D nanostructures with different materials such as polymers,8 bifunctional molecules,9 biomolecules,10 silica,11 and other inorganic materials.12 Herein, we report the synthesis of iron oxide coated gold nanorods which we briefly mentioned in a recent review article.2b The as-prepared gold nanorods are coated with a single layer of a negatively charged polymer. This facilitates electrostatic binding of iron ions which are further treated to form bound iron oxide particles.2b,13a The composite is further characterized by a host of different techniques and subjected to magnetic manipulation. For comparison, premade iron oxide particles synthesized by literature methods13b were also coated on the gold nanorod surface and subjected to magnetic manipulation. This general methodology can be extended to other shapes, and such iron oxide coated particles could be of great use for forming ordered assemblies and for bioimaging and cell-labeling applications.

Experimental Details Materials. Gold chloride (HAuCl4 · 3H2O), trisodium citrate, sodium borohydride (NaBH4), ascorbic acid, poly(sodium 4-styrenesulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMAC; 35 wt % in water), and iron(II) chloride were purchased from Aldrich and used as received. Cetyltrimethylammonium bromide (CTAB; Sigma Ultra, 99%) and iron(III) chloride were purchased from Sigma and used without further purification. All salts were of highest quality available. All experiments were carried out in Nanopure water (resistivity ) 18.2 mΩ). (7) (a) Wiggins, J.; Carpenter, E. E.; O’Connor, C. J. J. Appl. Phys. 2000, 87, 5651. (b) Teng, X.; Black, D.; Watkins, N. J.; Gao, Y.; Yang, H. Nano Lett. 2003, 3, 261. (c) Tzitzios, V.; Niarchos, D.; Hadjipanayis, G.; Devlin, E.; Petridis, D. AdV. Mater. 2005, 17, 2188. (8) (a) Gole, A.; Murphy, C. J. Chem. Mater. 2005, 17, 1325. (b) Ding, H.; Yong, K.-T.; Roy, I.; Pudavar, H. E.; Law, W. C.; Bergey, E. J.; Prasad, P. N. J. Phys. Chem. B 2007, 111, 12552. (c) Vial, S.; Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M. Langmuir 2007, 23, 4606. (9) (a) Cai, L. T.; Skulason, H.; Kushmerick, J. G.; Pollack, S. K.; Naciri, J.; Shashidhar, R.; Allara, D. L.; Mallouk, T. E.; Mayer, T. S. J. Phys.Chem. B 2004, 108, 2827. (b) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (10) (a) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (b) Gole, A.; Murphy, C. J. Langmuir 2005, 21, 10756. (c) Liao, H.; Hafner, J. H. Chem. Mater. 2005, 17, 4636. (d) Huang, X.; El-Sayed, I. H.; Qian, W.; El-Sayed, M. A. Nano Lett. 2007, 7, 1591. (11) (a) Obare, S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2001, 1, 601. (b) Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan, L. M. Chem. Mater. 2006, 18, 2465. (c) Wang, C.; Ma, Z.; Wang, T.; Su, Z. AdV. Funct. Mater. 2006, 16, 1673. (12) (a) Ah, C. S.; Hong, S. D.; Jang, D. J. J. Phys. Chem. B 2001, 105, 7871. (b) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 108, 5882. (c) Huang, C. C.; Yang, Z.; Chang, H. T. Langmuir 2004, 20, 6089. (13) (a) Kang, Y. S.; Risbud, S.; Rabolt, J. F.; Stroeve, P. Chem. Mater. 1996, 8, 2209. (b) Sahoo, Y.; Goodarzi, A.; Swihart, M. T.; Ohulchanskyy, T. Y.; Kaur, N.; Furlani, E. P.; Prasad, P. N. J. Phys. Chem. B 2005, 109, 3879. (c) Lee, D.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 123.

Langmuir, Vol. 24, No. 12, 2008 6233 Instrumentation. Purified samples of nanorods before and after iron oxide coating were studied by UV-vis spectroscopy (Varian model Cary 500 Scan UV-vis spectrophotometer), zeta potential and light scattering measurements (Brookhaven Zeta PALS instrument). For transmission electron microscopy (TEM), carbon-coated copper grids were immersed in the nanorod solutions for 15 min, removed, drained, air-dried, and imaged with a Hitachi H-8000 TEM instrument operating at an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDS) measurements were performed on an ESEM FEI Quanta 200 instrument. The magnetic properties of the composite were studied using a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra instrument with a monochromatic Al KR (1486.6 eV) source employing a hemispherical energy analyzer detector with eight channeltron electron multipliers. Synthesis of Polymer-Coated Gold Nanorods. Gold nanorods were synthesized by our well-documented three-step seeding protocol.2a,b,e After purification by centrifugation, the nanorods, bearing a cationic bilayer of CTAB, were coated with a single layer of poly(sodium 4-styrenesulfonate) (PSS) as described in our earlier reports.8a,10b Briefly, to 1 mL of purified aqueous nanorod solution, 100 µL of 10 mM aqueous NaCl solution and 200 µL of PSS (10 mg/mL stock solution prepared in 10 mM aqueous NaCl solution) were added and the solution was stirred for 30 min. The nanorods were further purified (centrifugation at 7000 rpm for 6 min) and resuspended in 1 mL of DI water. Iron Oxide Coating of Gold Nanorods. In Situ Coating (Method A). First, an aqueous solution containing iron(II) and iron(III) salts was prepared. This was done by adding 5.2 g (0.032 mol) of FeCl3 and 2 g (0.016 mol) of FeCl2 to 25 mL of DI water containing 0.85 mL of 12.1 N HCl. To 1 mL of PSS-coated gold nanorods, 200 µL of this iron salt solution was added. The solution was vortexed for 1 min and allowed to sit undisturbed for 1 h. After 1 h, 2 mL of 0.1 M NaOH was added and the resulting solution was allowed to sit for an additional 1 h. The yellow-orange color formed upon addition of NaOH indicates the formation of iron oxide particles.13a The resulting solution was purified twice (centrifugation at 7000 rpm for 5 min each) to remove any unbound iron oxide particles. The iron oxide coated nanorods were resuspended in 1 mL of DI water and were stable for several weeks. Scheme 1 shows the general synthetic procedure. This method to produce iron oxide nanoparticles is similar to that reported in the literature by Kang et al.,13a with the only difference being that the reaction is carried out in situ on the surfaces of gold nanorods. 1. General Protocol Used to Prepare in Situ Iron Oxide Coated Gold Nanorods (Method A)

Ex Situ Coating (Method B). Preparation of Citrate-Capped Iron Oxide Nanoparticles: Citrate-capped Fe3O4 nanoparticles were prepared according to ref 15, although our results (see below) were more consistent with Fe2O3 formation. Briefly, a mixture of 0.86 g of FeCl2 and 1.40 g of FeCl3 was dissolved in 40 mL of degassed DI water and heated to 80 °C under nitrogen. While stirring vigorously, 5 mL of NH4OH (29.5%) was slowly added, and the resulting solution was heated for an additional 30 min. The supernatant

6234 Langmuir, Vol. 24, No. 12, 2008 was removed, and fresh DI water was added followed by addition of 2 mL of an aqueous sodium citrate solution (0.5 g/mL). The resulting solution was heated to 95 °C for 90 min and allowed to cool to room temperature. Twenty microliter aliquots of the sample were added to microcentrifuge tubes, diluted to 1.5 mL, purified two times (centrifugation at 14 000 rpm for 15 min each), and resuspended in ∼1.5 mL of DI water. This solution was used in subsequent rod coating experiments. Iron Oxide Coating of Gold Nanorods Using Premade Iron Oxide Particles: Gold nanorods were prepared as described previously.2a,b,e To 1 mL of these purified CTAB-capped nanorods, 200 µL of the above-prepared iron oxide solution containing purified citrate-capped iron oxide particles was added. The solution was vortexed briefly (∼1-2 s) and allowed to sit undisturbed for ∼30 min. To this solution, 200 µL of poly(diallyldimethylammonium chloride) (PDADMAC; 35 wt % in water, 10 mg/mL) was added and vortexed briefly, and the final solution was allowed to sit undisturbed for an additional 45 min. (From our trial experiments, we found that PDADMAC was required to stabilize the composite system.) The rods, now coated with Fe2O3 particles, were purified from unbound Fe2O3 by centrifuging once at 7000 rpm for 5 min and resuspending in 1 mL of DI water. A neodymium iron boride (Nd2Fe14B) magnet was used to remove unbound Fe2O3 particles present in the rod solution which were not removed via centrifugation. Magnetic Manipulation. A neodymium iron boride (Nd2Fe14B) (∼1 T) magnet was used for the magnetic manipulation of the iron oxide coated gold nanorods synthesized by both A and B methods (above). For method A, the following procedure was used to prepare a polymer nanocomposite: A 1% by weight solution of poly(vinyl alcohol) (PVA) was prepared by dissolving 0.5 g of PVA (average MW 124 000-186 000) in 50 mL of deionized water. To a chamber slide was added 2 mL of this solution followed by the addition of 3 mL of iron oxide coated nanorods (prepared by method A). Upon addition of the nanorods, the solution became noticeably colored. The chamber slide was placed in a drying oven at ∼40 °C with the Nd2Fe14B magnet sitting beneath the PVA/nanorods solution for 24 h. As a control, a second chamber slide was prepared as described above and allowed to dry in the absence of a magnetic field. In addition, vials containing the iron oxide coated gold nanorods prepared by both methods (method A and B) were placed near a Nd2Fe14B magnet for 2-48 h and studied for any possible movement.

Results and Discussion Scheme 1 shows details of the procedure used for the formation of iron oxide coated gold nanorods using method A. Incubating iron salts (Fe2+ and Fe3+) with polymer-coated gold nanorods leads to binding of these cations to the negatively charged polymer. Upon addition of NaOH, coprecipitation of these salts leads to the formation of iron oxide particles in situ on the nanorod surface. This method is adapted from the work of Kang et al.13a who demonstrate the synthesis of iron oxide particles by a coprecipitation method. The main difference is that we have used this methodology to synthesize iron oxide particles in situ on the surface of gold nanorods. For comparison, gold nanorods were also coated with premade iron oxide particles (method B) as described in the Experimental Details section. The as-prepared iron oxide particles, by literature methods, are negatively charged and are supposed to be Fe3O4.13b,c By simply mixing these particles with CTAB-coated as-prepared gold nanorods, electrostatic interactions drive the assembly of these particles. A layer of a cationic polymer, PDADMAC, further stabilizes the system against aggregation. Figure 1 shows the UV-vis spectra of PSS-coated gold nanorods before (curve 1) and after coating with iron oxide particles (method A, curve 2) and as-prepared gold nanorods coated with premade iron oxide particles (method B, curve 3). The transverse surface plasmon band due to gold nanorods can be clearly seen in curve 1 (Figure 1). After coating with iron oxide particles (curve 2), there is a change in the slope

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Figure 1. UV-vis spectra of PSS-coated gold nanorods before (curve 1) and after (curve 2) modification with iron oxide particles by method A. (Curve 3) As-prepared gold nanorods coated with premade iron oxide nanoparticles (method B).

and broadening of the surface plasmon band of the gold nanorods. This is likely due to swamping of the plasmon band by contributions from the brown iron oxide particles (curve 2, Figure 1). In the case of gold nanorods coated with iron oxide by method B, there is a small red-shift of the plasmon band (curve 3), indicating surface modification of the gold nanorods. Zeta potential measurements are convenient to follow the surface modification of gold nanorods. The as-prepared nanorods are positively charged due to the bilayer of CTAB and have a zeta potential value of +28 ( 2 mV.8a,10b Upon modification with the negatively charged polymer (PSS), the zeta potential goes to -40 ( 2 mV, indicating surface modification by the polymer. These gold nanorods upon further modification by iron oxide particles (method A) become positively charged (+50 ( 3 mV). For method B, the as-prepared iron oxide particles are negatively charged (∼ -35 mV). These particles were electrostatically adsorbed to as-prepared CTAB-coated gold nanorods and further coated with the polymer PDADMAC. The zeta potential after this coating becomes +51 ( 2 mV. These variations in zeta potential at different steps support successful surface modification of the gold nanorods. TEM measurements were performed to determine the morphology of the iron oxide coated gold nanorods (method A, Figure 2; method B, Figure 3). For method A (Figure 2), the presence of a diffuse layer of a material uniformly coating the gold nanorods can be clearly seen. A closer look (Figure 2C and D) reveals a granular structure for the coating. We estimate the size of these granules is ∼8 nm. As mentioned earlier, the polymer PSS forms a primary coating layer on the gold nanorods. A single layer of polymer alone is not electron-dense enough to visualize by TEM (Supporting Information, Figure S1). Hence, the uniform coating is likely iron oxide. EDS studies on dropcast films of these samples on Si(111) substrates revealed the presence of iron (Supporting Information, Figure S2). In the case of method B (Figure 3), a coating of iron oxide particles on the gold nanorod surface can be clearly seen. Although we do not have good control over the surface coverage of the iron oxide particles by this method, magnetic manipulation was significantly easier compared to method A (see below). X-ray diffraction (XRD) experiments to distinguish between Fe2O3, Fe3O4, FeO, or other iron oxides were not informative in our case. Previous reports utilize X-ray diffraction as supplementary evidence to deduce the oxidation state of iron in

Iron Oxide Coated Gold Nanorods

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Figure 4. XPS core level spectra of Fe (2p) in iron oxide coated gold nanorods as prepared by method A (curve 1) and by method B (curve 2). The Fe (2p3/2) and Fe (2p1/2) peaks are labeled.

Figure 2. TEM images of gold nanorods coated with iron oxide particles by the in situ method (method A). Scale bars are all 20 nm.

Figure 3. Representative TEM images of gold nanorods coated with premade iron oxide particles (method B). Scale bars are all 100 nm.

iron oxides.14 However, as pointed out by others,6b,c maghamite (γ-Fe2O3) has the same cubic inverse spinel structure and nearly the same lattice parameter (a ) 0.8350 nm) as magnetite (Fe3O4; a ) 0.8396 nm), and hence, XRD is inadequate to distinguish between the two. In our case, we observed XRD peaks due to gold but did not observe peaks due to iron oxides in our iron oxide-gold nanorod composite system synthesized by method A (Supporting Information Figure S3). This is possibly due to two reasons. First, the small size of the iron oxide particles (14) (a) Sato, T.; Mousavand, T.; Ohara, S.; Umetsu, M.; Adschiri, T. Mater. Lett. 2007, 61, 4769. (b) Heuser, J. A.; Spendel, W. U.; Pisarenko, A. N.; Yu, C.; Pechan, M. J.; Pacey, G. E. J. Mater. Sci. 2007, 42, 9057. (c) Cao, H.; Zhu, M.; Li, Y. J. Magn. Magn. Mater. 2006, 305, 321.

broadens the XRD peaks.15 Second, the closeness of the strong gold (111) diffraction peak at a 2θ value of ∼38° almost completely swamps the weaker iron oxide (311) diffraction peak occurring at a 2θ value of 36°. Hence, we turned to XPS analysis of our samples of iron oxide coated gold nanorods prepared by both methods A and B. Figure 4 shows the Fe core level spectrum for gold nanorods coated with iron oxide (Figure 4: curve 1, method A; curve 2, method B), and the characteristic doublet of Fe (2p3/2) and Fe (2p1/2) is present in both cases (Figure 4). The peak positions for Fe (2p3/2) and Fe (2p1/2) are 710.8 and 724.5 eV (method A) and 710.7 and 724.3 (method B), respectively, with C (1s) at 285 eV as an internal reference. Teng et al.7b have performed an XPS study of commercially available Fe3O4 and γ-Fe2O3 particles. They find that the Fe (2p3/2) peak occurs at 711.9 eV for Fe3O4 and 710.6 eV for γ-Fe2O3, whereas the Fe (2p1/2) peak occurs at 725.8 eV for Fe3O4 and 724.3 eV for γ-Fe2O3. Furthermore, the XPS data of Ni et al.16 for R-FeO(OH) nanorods puts the Fe (2p3/2) and Fe (2p1/2) peaks at 711.7 and 725.5 eV, respectively. Using these data for comparison, we assign our Fe (2p3/2) and Fe (2p1/2) peaks to γ-Fe2O3 particles coating the gold nanorods for both methods A and B. This is slightly surprising given that the literature procedure we used for the synthesis13a–c is reported to yield Fe3O4, although the possibility of Fe2O3 was not rigorously excluded.13b The magnetic properties of the composite nanoparticles prepared by method A were investigated using a SQUID magnetometer (analogous data for the as-prepared iron oxide nanoparticles alone is available in ref 13b). Figure 5 shows the field dependence of the nanoparticles at different temperatures. The saturation magnetization per gram of the sample measured at the lowest temperatures is relatively small (∼0.02 emu/g) compared to that of pure Fe2O3 nanoparticles of ∼10 nm diameter (∼65 emu/g),17 suggesting that only a small amount of magnetic oxide is present on the gold surface. Such a reduction in saturation magnetization has been seen before.6b As the temperature is increased, both the saturation magnetization and the magnetic hysteresis decrease, as expected for superparamagnetic iron oxides. A layer of iron oxide on the gold nanorod surface should make them macroscopically magnetic. To test this hypothesis, we attempted to manipulate the nanorods by placing a solution of (15) Klug, H. P.; Alexander, L. E. X-ray diffraction procedures for polycrystalline and amorphous materials; John Wiley & Sons: New York, 1974. (16) Ni, Y.; Ge, X.; Liu, H.; Zhang, Z.; Ye, Q.; Wang, F. Mater. Lett. 2001, 49, 185. (17) Taboada, E.; Rodriguez, E.; Roig, A.; Oro, J.; Roch, A.; Muller, R. N. Langmuir 2007, 23, 4583.

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Gole et al.

Figure 5. (A) Magnetic field sweep data for gold nanorods coated with iron oxide (method A) collected at several temperatures. (B,C) Plots showing the magnetic hysteresis for gold nanorods coated with iron oxide (method A) at 2 K (Plot B) and 300 K (plot C).

these particles near a permanent magnet. Two different studies were carried out. In the first study, the iron oxide coated gold nanorods (method A) were dispersed in a polymer matrix (PVA) and subjected to a magnetic field while curing the polymer (24 h, 40 °C). This was done by placing two ∼1 T magnets below the chamber slides containing the nanorods (Figure 6A, top panel photograph). The dark color of the nanorod solution can be used to visualize the nature of pattern formation due to the magnet field (Figure 6A). It can be clearly seen that the gold nanorods form a square pattern which takes the shape of the square Nd2Fe14B magnets (Figure 6A, central panel photograph), with concentration at the junction between the two magnets. A control experiment carried out under similar conditions in the absence of the magnets failed to show such a pattern (Figure 6A, bottom panel photograph). The accumulation of particles at the edges of the chamber plate is due to the effect of drying of the polymer containing the particles. Another set of experiments was carried out wherein a glass vial containing iron oxide-gold nanorod solution (method A) was placed near the Nd2Fe14B magnet for 12-48 h and studied for any possible movement. As shown in Figure 6B (both images), the iron oxide coated nanorods seem to saturate/collect near the magnet. We found that this movement was quite slow (∼48 h). For the gold nanorod-iron oxide composite prepared by method B, a similar experiment was carried out (Figure 6C). Before placement of the magnet, a uniform brown color, indicating dispersed nanorods (Figure 6 C, panel photograph on the left), is evident. Upon application of a magnetic field for a period of 16-24 h, visually the nanorods could be moved to one side of the vial (Figure 6 C, panel photograph on the right). This also supports the successful coating of the gold nanorods by iron oxide particles. The very slow magnet-induced movement of the gold nanorods coated by iron oxide is likely the result of the large relative mass of gold that the thin magnetic shell must pull. We estimated of the weight of the iron oxide present in the gold nanorod-iron oxide composite prepared by method A assuming a diameter of iron oxide coating of ∼8 nm (TEM of Figure 2), gold nanorod dimensions of ∼500 nm length and 25 nm thickness,2a,b,e,g and the bulk densities of Fe2O3 (5.2 g/cm3) and Au (19.3 g/cm3). We estimate ∼780 iron oxide nanoparticles per nanorod. Accounting for densities for Fe2O3 and Au, we estimate that the total mass of iron oxide per gold nanorod is ∼1.1 × 10-15 g and the mass of each gold nanorod is ∼4.8 × 10-15 g, meaning that ∼18% of the mass of one coated gold nanorod is due to the iron oxide

Figure 6. Magnetic manipulation of iron oxide coated gold nanorods. (A) Top photograph shows the initial position of the magnets. Central photograph shows the final position of the nanorods in the polymer, observed as dark lines following the contour of the magnet. Bottom photograph shows the results of the control experiment of the distribution of iron oxide coated gold nanorods (method A) dried overnight in the PVA matrix in the absence of a magnet, illustrating the effects of drying alone (accumulation of particles at the edges) and illustrating the lack of particles in the middle, where the two magnets had met in the central photograph. (B) Vial containing iron oxide coated gold nanorods (method A) before (top photograph) and after (lower photograph) application of a magnet for 48 h. The accumulation of particles (dark coloration) at the bottom right of the glass vial is shown by a red arrow. (C) Magnetic manipulation of iron oxide coated gold nanorods (method B) before (left panel) and after (right panel) application of a magnet for 16 h.

coating. Hence, such a low content of iron oxide is insufficient to move the large mass of the gold nanorods and lead to the longer time scales (hours) required for magnetic manipulation of the gold nanorods. Attempts to increase the thickness of the iron oxide coating and hence improve the magnetization leads to aggregation of the nanorods. More work is needed to achieve

Iron Oxide Coated Gold Nanorods

more useful magnetic properties and is currently underway in our laboratory.

Conclusion Simple mixing of iron salts with polymer-coated gold nanorods and further addition of a strong base leads to the formation of a coating of iron oxide on the surfaces of gold nanorods. XPS analysis suggests that the iron oxide present in the system is Fe2O3. SQUID studies indicate that the composite particles are superparamagnetic. These iron oxide coated gold nanorods were found to be mildly responsive to an externally applied magnetic field. The long time required for such magnetic manipulation is likely due to the thinness of the magnetic coating and the relatively large mass of the core gold nanorods. Premade iron oxide particles, when bound to gold nanorods by electrostatic interactions, were more responsive to a magnetic field, but the control over the uniformity and reproducibility of the coating was compromised.

Langmuir, Vol. 24, No. 12, 2008 6237

There is a great potential for in situ coating of iron oxide by the simple procedure mentioned herein. This general methodology would be an attractive way to form uniform coatings of magnetic material on different surfaces, hence proving beneficial as a means to develop magnetically responsive systems. Acknowledgment. The authors thank R. Haasch (University of Illinois at Urbana-Champaign) and S. Ma (Department of Chemical Engineering, University of South Carolina) for XPS measurements. We thank the National Science Foundation and the W. M. Keck Foundation for funding. Supporting Information Available: TEM image of PSSfunctionalized gold nanorods, EDS analysis of iron oxide coated gold nanorods prepared by method A, and powder XRD of iron oxide coated gold nanorods prepared by method A. This material is available free of charge via the Internet at http://pubs.acs.org. LA703975Y