Understanding Mercapto Ligand Exchange on the Surface of FePt

Shang-Wei Chou , Chien-Liang Liu , Tzu-Ming Liu , Yu-Fang Shen , Lun-Chang Kuo , Cheng-Ham Wu , Tsung-Yuan Hsieh , Pei-Chun Wu , Ming-Rung Tsai , Che-...
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Langmuir 2006, 22, 7732-7737

Understanding Mercapto Ligand Exchange on the Surface of FePt Nanoparticles Hitesh G. Bagaria,†,‡ Earl T. Ada,§ Mohammed Shamsuzzoha,§ David E. Nikles,‡,| and Duane T. Johnson*,†,‡ Departments of Chemical and Biological Engineering and Chemistry, Central Analytical Facility, and Center for Materials for Information Technology, The UniVersity of Alabama, Tuscaloosa, Alabama 35487 ReceiVed January 15, 2006. In Final Form: June 22, 2006 Tailoring the surface of nanoparticles is essential for biological applications of magnetic nanoparticles. FePt nanoparticles are interesting candidates owing to their high magnetic moment. Established procedures to make FePt nanoparticles use oleic acid and oleylamine as the surfactants, which make them dispersed in nonpolar solvents such as hexane. As a model study to demonstrate the modification of the surface chemistry, stable aqueous dispersions of FePt nanoparticles were synthesized after ligand exchange with mercaptoalkanoic acids. This report focuses on understanding the surface chemistry of FePt upon ligand exchange with mercapto compounds by conducting X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies. It was found that the mercapto end displaces oleylamine on the Pt atoms and the carboxylic acid end displaces the oleic acid on the Fe atoms, thus exposing carboxylate and thiolate groups on the surface that provide the necessary electrostatic repulsion to form stable aqueous dispersions of FePt nanoparticles.

Introduction Magnetic nanoparticles have a number of biological applications. Some of them are in vivo applications, such as magnetic fluid hyperthermia and magnetothermal drug delivery, and some are in vitro applications, such as biosensors and magnetophoretic separations. These applications can be attributed to the size similarity between nanoparticles and many biological entities such as proteins, viruses, and cells in addition to their magnetic properties. A number of reviews discuss these applications of magnetic nanoparticles.1-3 The suitability of the magnetic nanoparticles for a certain biological application depends on two important aspects: the magnetic properties and the surface characteristics. The magnetic moment of the particle determines the sensitivity of the particle to an applied magnetic field. Due to their small size, almost all magnetic nanoparticles are superparamagnetic; that is, they respond only in the presence of a magnetic field. We are interested in studying FePt nanoparticles for biological applications since they have superior magnetic properties, and several procedures to make monodisperse FePt nanoparticles of different sizes (2-9 nm) are available.4-8 These procedures use * Corresponding author: phone 205-348-8402; fax 205-348-7558; e-mail [email protected]. † Department of Chemical and Biological Engineering. ‡ Center for Materials for Information Technology. § Central Analytical Facility. | Department of Chemistry. (1) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D 2003, 36, R167-R181. (2) Proceedings of the Fifth International Conference on Scientific and Clinical Applications of Magnetic Carriers; Hafeli, U., Zborowski, M., Eds.; J. Magn. Magn. Mater. 2005, 293, 1-736. (3) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995-4021. (4) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (5) Sun, S.; Anders, S.; Thomson, T.; Baglin, J. E. E.; Toney, M. F.; Hamann, H. F.; Murray, C. B.; Terris, B. D. J. Phys. Chem. B 2003, 107, 5419-5425. (6) Elkins, K. E.; Vedantam, T. S.; Liu, J. P.; Zeng, H.; Sun, S.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1647-1649. (7) Liu, C.; Wu, X.; Klemmer, T.; Shukla, N.; Yang, X.; Weller, D.; Roy, A. G.; Tanase, M.; Laughlin, D. J. Phys. Chem. B 2004, 108, 6121-6131. (8) Chen, M.; Liu, J. P.; Sun, S. J. Am. Chem. Soc. 2004, 126, 8394-8395.

oleic acid and oleylamine ligands to stabilize FePt nanoparticles in nonpolar solvents such as hexane. The surface characteristics of the nanoparticles determine the biocompatibility and the specific binding ability of the particles.3,9 Hence, to make FePt suitable for biological applications, the oleyl stabilizers will need to be replaced by hydrophilic, biocompatible, and specific binding ligands. Ligand exchange can be used to achieve this goal. Several reports have studied ligand exchange on Cd alloy,10 Au,11 and FePt12-15 nanoparticles. Ligand exchange involves adding to the nanoparticle solution an excess of the ligand that is required to displace the original ligands on the nanoparticles’ surface. In the case of FePt, the ligand to be introduced on the surface of FePt should displace the oleic acid and oleylamine ligands. Inspired by the thiol ligand exchange on gold11 and platinum16 nanoparticles, our research group has conducted ligand exchange of alkanethiols15 and mercaptoalkanoic acids14,15 on FePt with oleyl ligands. Ligand exchange with mercaptoalkanoic acids made FePt nanoparticles dispersible in alkaline water. Our preliminary high-resolution X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies on ligand exchanged FePt gave us two intriguing results: (i) the presence of the bidendate (1550 cm-1) peak17,13 in the FTIR spectrum of the FePt nanoparticles after ligand exchange with dodecanthiol and (ii) the presence of sulfur in both the thiolate and the oxidized (9) Berry, C. C.; Curtis, A. S. G. J. Phys. D 2003, 36, R198-R206. (10) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (11) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (12) Xu, C.; Xu, K.; Gu, H.; Zhong, X.; Guo, Z.; Zheng, R.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 3392-3393. (13) Salgueirino-Maceira, V.; Liz-Marza´n, L. M.; Farle, M. Langmuir 2004, 20, 6946-6950. (14) Mabry, J. K.; Tackett, L. B.; Bagaria, H. G.; Sun, X. C.; Ada, E. T.; Shamsuzzoha, M.; Sun, K.; Wang, L. M.; Johnson, D. T.; Nikles, D. E. Mater. Res. Soc. Symp. Proc. 2005, 853E, I4.5.1-3. (15) Sun, X. C.; Thode, C. J.; Mabry, J. K.; Harrell, J. W.; Nikles, D. E.; Sun, K.; Wang, L. M. J. Appl. Phys. 2005, 97, 10Q901. (16) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876-9880. (17) Shukla, N.; Liu, C.; Jones, P. M.; Weller, D. J. Magn. Magn. Mater. 2003, 266, 178-184.

10.1021/la0601399 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/29/2006

Mercapto Ligand Exchange on FePt Nanoparticles Chart 1. Chemical Structures of (a) Oleic Acid, (b) Oleylamine, (c) Dodecanethiol (DDT), (d) 11-Mercaptoundecanoic Acid (MUA), and (e) 3-Mercaptopropionic Acid (MPA)

sulfur forms in the sulfur 2p XPS spectrum of the mercaptoalkanoic acid ligand-exchanged FePt. The first result indicates the presence of oleic acid on the FePt surface even after ligand exchange with dodecanethiol. The second result indicates the presence of sulfur in a form other than the thiolate on the mercaptoalkanoic acid ligand-exchanged FePt. The present study is directed toward understanding the mercapto ligand exchange on FePt nanoparticles. Materials and Methods Materials. The chemicals required for FePt synthesis are iron(II) chloride (Sigma-Aldrich), platinum(II) acetylacetonate (SigmaAldrich), 1,2-hexadecanediol (Sigma-Adlrich), 1.0 M solution of lithium triethyl borohydride (LiEt3BH) in tetrahydrofuran (THF) (Acros), diphenyl ether (Acros), oleic acid (Fisher), and oleylamine (Sigma-Aldrich). For the ligand exchange, dodecanethiol (DDT), 3-mercaptopropionic acid (MPA), 11-mercaptoundecanoic acid (MUA), dodecanedioic acid (DDA), cyclohexanone, and tetrahydrofuran (THF) were used as purchased from Sigma-Aldrich. Particle Preparation. The procedure for synthesizing FePt nanoparticles is described by Sun et al.5 Briefly, the procedure involved heating 0.5 mmol of iron and 0.5 mmol of platinum salts in 25 mL of diphenyl ether in an inert N2 atmosphere to 100 °C, followed by the injection of 0.5 mmol of each oleic acid and oleylamine. The solution was further heated to 200 °C and held at this temperature for 20 min. The reducing agent LiEt3BH in THF was then added dropwise over a period of 5 min. The N2 supply position was adjusted to sweep away the low-boiling THF vapors until the temperature reached the boiling point of diphenyl ether at ∼255 °C. The N2 supply was then restored to the original position and the solution was refluxed for 90 min. The solution was then cooled to room temperature under N2 atmosphere. The FePt nanoparticles were cleaned by cyclic precipitation and redispersion with ethanol and hexane, respectively. Finally the particles were stored in hexane. This hexane solution was used for ligand-exchange studies. FePt nanoparticles with oleic acid and oleylamine will henceforth be called the as-made FePt. Ligand-Exchange Procedures. The ligand exchange of the asmade FePt nanoparticles with DDT, MUA, and MPA has been reported earlier.14,15 For completeness, the procedures are described here. The chemical structures of all the ligands used in this study are shown in Chart 1. Dodecanethiol. Approximately 20 mg of FePt nanoparticles in 1 mL of hexane was added to 1 mL of DDT in a 10 mL centrifuge tube. The mixture was shaken well for 1 min, and ethanol was added to precipitate the particles. The particles were separated by centrifugation at 3500 rpm for 5 min and the supernatant was discarded. The precipitate was dispersed in ∼10 mL of hexane and centrifuged. The dark brown supernatant was discarded. This procedure of dispersion in hexane and centrifugation to clean the particles was repeated until a clear supernatant was observed. Finally

Langmuir, Vol. 22, No. 18, 2006 7733 the particles were dispersed in 1 mL of hexane to give a green solution. This solution was used for further analysis. 11-Mercaptoundecanoic Acid. Cyclohexanone was chosen as the solvent for the ligand exchange as it dissolved both MUA and the FePt nanoparticles. A solution of MUA was prepared by mixing 2.5 g of MUA with 5 mL of cyclohexanone in a 10 mL centrifuge tube. To this ∼50 mg of FePt particles dispersed in 0.5 mL of hexane was added, and the mixture was shaken well. After 10-45 min the particles precipitated. The solution was centrifuged at 3500 rpm for 5 min. The supernatant was discarded, and approximately 10 mL of cyclohexanone was added to the precipitate to remove FePt with oleic acid and oleylamine ligands and any excess MUA. The solution was again centrifuged and the supernatant was discarded. The precipitate was mixed with ∼10 mL of ethanol to remove any remaining MUA. The solution was again centrifuged and the supernatant was discarded. As a final cleaning step, the precipitate was mixed with ∼10 mL of acetone. The solution was centrifuged and the supernatant was discarded. A small fraction of this cleaned precipitate was mixed with acetone and used for XPS and FTIR analysis. The other fraction was used to produce water-dispersible FePt nanoparticles by adding ∼2.25 mL of deionized water followed by 0.25 mL of 1 N NaOH solution. 3-Mercaptopropionic Acid. In a 10 mL centrifuge tube, 1 mL of 3-MPA was mixed with 1 mL of cyclohexanone and 0.5 mL of FePt dispersed in hexane (∼10 mg) and shaken well. After 10-30 min the particles started precipitating. It was interesting to note that when FePt was dispersed in hexane with 0.5% (volume basis) of each oleic acid and oleylamine, the particles did not precipitate even after 2 days. In that case the particles can be precipitated by the addition of ethanol. The solution was centrifuged at 3500 rpm for 5 min and the supernatant was discarded. The precipitate was cleaned with cyclohexanone, ethanol, and acetone just as described for MUA ligand exchange. A small fraction of the cleaned precipitate was mixed with acetone and used for XPS and FTIR analysis. Just as with MUA ligand-exchanged FePt, the cleaned particles can be made water-dispersible by addition of deionized water and NaOH. Characterization Techniques. The composition, structure, size, and magnetic properties of the prepared FePt particles were characterized by use of a Phillips XL 30 scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS), a Rigaku thin-film X-ray diffractometer (XRD), a Hitachi 8000 transmission electron microscope (TEM), and a Princeton Measurements Corporation alternating gradient magnetometer (AGM), respectively. For SEM-EDS, XRD, and AGM analysis, samples were prepared by drop-drying the FePt nanoparticles on Si substrates. FePt dispersion was dropped and dried on carbon-coated copper grids (Electron Microscopy Sciences) for TEM. The process of ligand exchange was studied by a combination of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). A Bio-Rad model FTS-40 FTIR having a spectral resolution of 4 cm-1 was used to collect the IR vibrational spectra. FePt nanoparticles with oleic acid, oleylamine, and DDT ligands were dissolved in hexane, while those with MUA and MPA ligands were dissolved in acetone. Each sample in the appropriate solution was drop-dried in air on a ZnSe window prior to FTIR analysis. XPS was performed in a Kratos Axis 165 multitechnique spectrometer with a chamber base pressure of 1 × 10-10 Torr. Samples were mounted on a standard holder by pressing the particles onto double-sided carbon tape. A monochromatic Al X-ray source and a 165 mm mean radius concentric hemispherical analyzer were used to collect the photoelectron spectra of each sample. Survey and high-resolution spectra were collected at fixed analyzer pass energies of 160 and 80 eV, respectively. The instrumental energy resolution was 0.5 eV. The adventitious carbon C 1s peak at 285.0 eV was used as the binding energy reference for all spectra.

Results and Discussion The as-made nanoparticles had a composition of Fe47Pt53. XRD confirmed the disordered fcc structure of the nanoparticles (Figure 1a). Scherrer analysis on the (111) peak of the XRD gave a

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Figure 1. (a) XRD pattern and (b) TEM image of the as-made FePt nanoparticles.

crystallite size of ∼3 nm, which matched with the size from the TEM (Figure 1b), verifying the single-crystalline nature of the particles. The solubility of FePt nanoparticles before and after ligand exchange with MUA and MPA is shown in Figure 2 a-f. The as-made FePt nanoparticles disperse in hexane (Figure 2a) but not in water or alkaline water (Figure 2b) due to the presence of oleic acid and oleylamine on the particles. After ligand exchange with MUA (Figure 2c) and MPA (Figure 2d), the particles become soluble in water in the presence of NaOH. This suggests that the oleic acid and oleylamine ligands are replaced by MUA and MPA. The TEM images of the FePt with MUA and MPA ligands are shown in Figure 2 panels g and h, respectively. These images were obtained by drying the alkaline FePt dispersion on TEM grids. The gray features in the background are due to sodium ions. The images show that the particles are well dispersed after ligand exchange. The FTIR spectra of pure oleic acid and oleylamine surfactants shown in Figure 3 are labeled at their characteristic peaks.17 A FTIR spectrum of the as-made FePt particles, also shown in Figure 3, was collected after removal of the excess surfactants by washing the particles three times. One washing cycle involved precipitating the particles with ethanol and redispersing them in hexane (with no surfactants). The peaks at 1551 and 1711 cm-1 have been assigned to the bidendate (-COO-M) and monodendate (-CO-M) modes of oleic acid binding. The shoulder at 1591 cm-1 is due to the scissoring mode of the molecularly bonded oleylamine.17 Two reports in the literature have explored the use of FTIR to characterize the as-made FePt particles.13,17 Shukla et al.17 assign the bidendate peak at 1512 cm-1, and SalgueirinoMaceira et al.,13 at 1560 cm-1. Our assignment of the bidendate peak matches with the latter. The peaks around 3000 cm-1 are assigned to the -CH stretches.13,17 After ligand exchange with DDT, the FePt nanoparticles appear dark green, a cue indicating some change in the surface chemistry of the nanoparticles. The FTIR spectra of pure DDT, DDT ligand-

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exchanged FePt, and the as-made FePt are shown in Figure 4. To ensure the collection of spectra from the adsorbed surfactants only, the DDT ligand-exchanged FePt particle were washed as described earlier. The ligand-exchanged particles have -CH stretches (peaks around 3000 cm-1) similar to that of DDT. The ratio of the peak intensity of the amine scissoring peak at 1590 cm-1 to the bidendate peak at 1560 cm-1 in the ligand-exchanged particles is reduced considerably compared to the as-made particles. These signatures indicate that DDT displaces oleylamine preferentially, leaving a significant amount of oleic acid undisplaced. The FTIR spectra of the MUA ligand, the cleaned MUA ligandexchanged FePt particles, and the as-made FePt particles are shown in Figure 5. The intensity ratio of the -CH stretches around 3000 cm-1 to the peaks around 1500 cm-1 are different in the ligand-exchanged particles compared to the as-made particles, indicating changes in the surface chemistry of the particles. The peak at 1560 cm-1 due to bidendate binding is present after the ligand exchange, but it is unclear if this peak is from oleic acid or MUA. The intensity of the amine scissoring mode peak at 1585 cm-1 is reduced, indicating partial removal of oleylamine. These signatures indicate that MUA is present at the surface of the particle. An additional peak at 1600 cm-1 is also seen that could not be attributed to any known stretches and warrants further investigation. The FTIR spectra of the MPA ligand-exchanged FePt nanoparticles, shown in Figure 6, revealed results very similar to those of MUA. The salient features include a small -CH stretch peak in accordance with the short chain of MPA, and the feature at 1600 cm-1 was more pronounced here compared to the case of MUA. The high-resolution Fe2p XPS spectra of the as-made and ligand-exchanged FePt nanoparticles are shown in Figure 7a. Two prominent features of the Fe2p spectra are the oxidized iron state at 711 eV18,19 and the elemental iron state at 707.5 eV.18,19 Oleic acid, MUA, and MPA are ligands that have carboxylate groups. The Fe2p spectra of FePt with these ligands show relatively more Fe in the oxidized state than in the elemental Fe state in comparison to FePt with DDT. This suggests the strong affinity of iron for carboxylate group. Even after ligand exchange with DDT, the Fe2p spectrum shows the presence of Fe in the oxidized state at 711 eV. This state may be attributed to the presence of an iron oxide layer and undisplaced oleic acid on the FePt after ligand exchange with DDT, which was also observed as a bidendate feature at 1551 cm-1 in the FTIR spectrum of DDT ligand-exchanged FePt (Figure 4b). These results suggest that iron has a greater affinity to bind to the carboxylate group over the mercapto group. An earlier study on ligand exchange

Figure 2. (a) FePt with oleic acid and oleylamine ligands (as-made FePt) in hexane, (b) as-made FePt in water, (c) FePt with MUA ligands in alkaline water, (d) FePt with MPA ligands in alkaline water, (e) FePt with MUA ligands in alkaline water and hexane, (f) FePt with MPA ligands in alkaline water and hexane, (g) TEM image of FePt with MUA ligands, and (h) TEM image of FePt with MPA ligands.

Mercapto Ligand Exchange on FePt Nanoparticles

Figure 3. FTIR spectra of (a) pure oleylamine, (b) pure oleic acid, and (c) as-made FePt nanoparticles

Figure 4. FTIR spectra of (a) pure dodecanthiol (DDT), (b) DDT ligand-exchanged FePt, and (c) as-made FePt nanoparticles.

Figure 5. FTIR spectra of (a) pure mercaptoundecanoic acid (MUA), (b) MUA ligand-exchanged FePt, and (c) as-made FePt nanoparticles.

of mercaptohexadecanoic acid (MHA) on iron oxide nanoparticles also reports the higher affinity of iron for the carboxylate group.20 Furthermore, a recent study on adsorption of alkanethiol on 316L stainless steel showed that octadecylmercaptan could not be adsorbed when a passivating iron and chromium oxide film was present on stainless steel.21 The high-resolution Pt4f XPS spectra of the as-made and the ligand-exchanged FePt nanoparticles are shown in Figure 7b. The binding energy of the Pt4f peak ranges from 71 eV for MPA (18) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy, reissue edition; Physical Electronics: Eden Prairie, MN, 1995. (19) Stahl, B.; Ellrich, J.; Theissmann, R.; Ghafari, M.; Bhattacharya, S.; Hahn, H.; Gajbhiye, N. S.; Kramer, D.; Viswanath, R. N.; Weissmu¨ller, J.; Gleiter, H. Phys. ReV. B, 2003, 67, 014422. (20) Liu, Q.; Xu, Z. Langmuir 1995, 11, 4617-4622. (21) Ruan, C. M.; Bayer, T.; Meth, S.; Sukenik, C. N. Thin Solid Films 2002, 419, 95-104.

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Figure 6. FTIR spectra of (a) pure mercaptopropionic acid (MPA), (b) MPA ligand-exchanged FePt, and (c) as-made FePt nanoparticles.

ligand-exchanged FePt to 71.7 eV for the as-made FePt. The observed shift is systematic with respect to the length of the ligand on FePt. However, a recent study on the binding of MPA and MHA on Pt films shows that the mercapto group binds to the Pt surface and that this bonding does not change the binding energy of Pt.22 Other studies also show that the bonding of alkanethiols on Pt films does not change the binding energy of Pt.23,24 Further experimentation needs to be done to understand the observed Pt4f peak shift. The S2p XPS spectra of the DDT, MUA, and MPA ligandexchanged FePt nanoparticles are shown in Figure 7c. The DDT ligand-exchanged FePt shows the S2p peak at a binding energy of 162.2 eV along with a broad shoulder, confirming the presence of DDT on the particle. On the basis of the Fe2p and Pt4f XPS results, we believe that this feature is due to thiol adsorbed on Pt. Studies on adsorption of alkanethiols on Pt films also report the S2p peak for adsorbed thiol at 162.5 eV.22,24,25 The S2p spectra of MUA and MPA ligand-exchanged FePt nanoparticles also show the presence of adsorbed thiol at 162.5 eV. In addition, they show a feature around 163.5 eV that is similar to the S2p spectra of free MUA and MPA ligands (Figure S2 in Supporting Information). It is unlikely that this feature is due to the presence of unbound MUA or MPA since the samples have been washed thoroughly. We believe that this feature is from MUA or MPA that is bound to the particle surface through its carboxylate end, thus exposing the free thiol end to the solution. Despite the similarity between S2p spectra of MUA and MPA ligandexchanged FePt in the 162-164 eV region, a glaring difference between them is the absence of the oxidized sulfur around 167 eV for MUA ligand-exchanged FePt. It is well-known that selfassembled monolayers (SAM) of short-chain alkanethiols on Au26 and Ag27 films are easily oxidized compared to that of long-chain alkanethiols due to the greater disorder in short-chain SAMs. Hutt et al. studied the UV oxidation of alkanethiols on Au26 and Ag27 films under ambient conditions for up to 200 min with SAMs of alkanethiols ranging from propanethiol to octadecanethiol. Their kinetic studies showed that the rate constant for oxidation of propanethiol on Au films was almost an order of magnitude higher than for the oxidation of dodecanethiol. A recent study shows that alkanethiols of different chain lengths (22) Brito, R.; Tremont, R.; Feliciano, O.; Cabrera, C. R. J. Electroanal. Chem. 2003, 540, 53-59. (23) Li, Z.; Chang, S. C.; Williams, S. R. Langmuir 2003, 19, 6744-6749. (24) Laiho, T.; Lukkari, J.; Meretoja, M.; Laajalehto, K.; Kankare, J.; Leiro, J. A. Surf. Sci. 2005, 584, 83-89. (25) Petrovykh, D. Y.; Kimura-Suda, H.; Opdahl, A.; Richter, L. J.; Tarlov, M. J.; Whitman, L. J. Langmuir 2006, 22, 2578-2587. (26) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657-6662. (27) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174-184.

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Figure 7. (a) Fe2p, (b) Pt4f, (c) S2p, (d) C1s, and (e) O1s XPS spectra of the as-made particles and DDT, MUA, and MPA ligand-exchanged particles.

on Pt films25 get oxidized even when exposed to air for long periods (1.5 h to 55 days). In our study, MUA and MPA ligandexchanged FePt samples were exposed to air for ∼2 days before the XPS data were acquired. Therefore, the short-chain MPA adsorbed on the FePt surface through its thiol end may have oxidized, giving rise to the S2p feature at 167 eV. In contrast, the MUA thiol being a long-chain alkanethiol is better protected from oxidation; hence the S2p feature at 167 eV is absent here. The carbon C1s XPS spectra are shown in Figure 7d. The adventitious carbon peak corresponding to the hydrocarbon tail of the ligands on the FePt nanoparticles is observed at a binding energy of 285 eV.18 In addition, a higher binding energy peak at 288 eV also appears in all the four spectra that corresponds to the carboxylate carbon on the particle.19 The carboxylate carbon feature is rather faint in the as-made FePt and the DDT ligandexchanged FePt, probably due to the long hydrocarbon tail of oleic acid. The relative ratio of the carboxylate feature to the hydrocarbon tail is in the order MPA > MUA > as-made (oleic acid), which is in accordance with the ligand’s chain length. The oxygen O1s XPS spectra of the as-made and ligandexchanged FePt nanoparticles are shown in Figure 7e. Two distinct features at 530 and 531.5 eV are seen on the as-made and DDT ligand-exchanged FePt. The binding energy of iron oxides is known to be at 529-530 eV,18 suggesting the presence of iron oxide. On the basis of the XPS of free MUA and MPA ligands,

we believe the feature at 531.5 eV is due to the carboxylate oxygen (Figure S2 in Supporting Information). The O1s spectrum of MUA ligand-exchanged FePt shows a broad feature at 532 eV and a small hump at 535.8 eV. The surface of the MUA ligand-exchanged FePt is expected to be covered with carboxylate, giving a strong feature at 532 eV. The iron oxide feature at 530 eV is probably buried due to the strong signal from carboxylate. The high binding energy hump at 535.8 eV is most likely due to adsorbed water molecules as reported earlier.28,29 In the case of MPA ligand-exchanged FePt, the O1s spectrum shows a broad feature at 531.5 eV and a shoulder at about 535.5 eV. We expect the 531.5 eV feature to have components of carboxylate, sulfate, and iron oxide, giving rise to the broadest feature among all the spectra shown in Figure 7e. The high binding energy shoulder at ∼535.5 eV is also probably due to adsorbed water molecules.28,29 To further understand the surface chemistry of MUA and MPA on FePt, ligand exchange on as-made FePt nanoparticles with dodecanedioic acid (DDA) was conducted. After cleaning of the particles to remove excess DDA, the particles failed to disperse in water with NaOH. Thus carboxylate functionality alone was (28) Gardner, S. D.; Singamsetty, C. S. K.; Booth, G. L. Carbon 1995, 33, 587-595. (29) Knipe, S. W.; Mycroft, J. R.; Pram, A. R.; Nesbitt, H. W.; Bancroft, G. M. Geochim. Cosmochim. Acta 1995, 59, 1079-1090.

Mercapto Ligand Exchange on FePt Nanoparticles

not sufficient to make water-dispersible FePt particles. This observation along with the above XPS results suggest that MUA and MPA ligand-exchanged FePt nanoparticles form clear aqueous dispersions in alkaline water due to the combined electrostatic repulsion due to carboxylate and thiolate. It is hypothesized that the carboxylate end of MUA binds to the iron atoms and the mercapto group binds to the platinum atoms, revealing the thiolate and carboxylate group to the solution, respectively, for electrostatic stabilization in an alkaline solution. This leads us to believe that in general, the bimetallic composition of the alloyed FePt particles necessitates separate chemistry for both the metals involved, unlike the single-metal nanoparticles. A recent article on the need to have both dopamineand thiol-terminated PEG ligands to make better dispersed FePt favors our hypothesis.30 This may very well be true for other alloyed bimetallic nanoparticles systems and needs to be verified.

Conclusions Ligand exchange on FePt nanoparticles was studied by FTIR and XPS. The alkanethiols preferentially displace the oleylamine on Pt atoms over the oleic acid on Fe atoms. Studies on (30) Hong, R.; Fischer, N. O.; Emrick, T.; Rotello, V. M. Chem. Mater. 2005, 17, 4617-4621.

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mercaptoalkanoic acid ligand-exchanged FePt suggest the binding of the carboxylate end to the iron atoms and the mercapto end to the platinum atoms. It was verified that the carboxylate functionality alone did not suffice to make stable water dispersions. Both mercapto and carboxylate functionalities were needed to make stable water dispersions of FePt nanoparticles. In conclusion, ligand exchange or any other surface chemistry process on bimetallic alloyed FePt nanoparticles requires separate considerations for both the atoms. This may be true for other alloyed bimetallic nanoparticles surfaces. Acknowledgment. We acknowledge partial support from the Center for Materials for Information Technology and its NSF Materials Research Science and Engineering Center award DMR-0213985, Department of Chemical & Biological Engineering, and the Graduate Council Creative Activity and Research Fellowship at the University of Alabama. We acknowledge the Department of Chemistry and Dr. Tonya Klein for making their FTIR instruments available for our studies. Supporting Information Available: XPS spectra of MUA and MPA ligands. This material is available free of charge via the Internet at http://pubs.acs.org. LA0601399