2D Assembling of Magnetic Iron Oxide Nanoparticles Promoted by

Apr 26, 2010 - Institut de Physique et de Chimie des Matériaux de Strasbourg, UMR 7504, CNRS-UdS-ECPM, 23 rue du. Loess, BP 43, F-67034 Strasbourg ...
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J. Phys. Chem. C 2010, 114, 9041–9048

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2D Assembling of Magnetic Iron Oxide Nanoparticles Promoted by SAMs Used as Well-Addressed Surfaces Benoıˆt P. Pichon,*,† Arnaud Demortie`re,† Matthias Pauly,† Karin Mougin,‡ Alain Derory,† and Sylvie Be´gin-Colin† Institut de Physique et de Chimie des Mate´riaux de Strasbourg, UMR 7504, CNRS-UdS-ECPM, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France, and Institut de Chimie des Surfaces et Interfaces, UPR 9069, 15 rue Jean Starcky, BP 2488, F-68057 Mulhouse Cedex, France ReceiVed: March 2, 2010; ReVised Manuscript ReceiVed: April 6, 2010

Well-addressed surfaces which consist of self-assembled monolayers (SAMs) of organic molecules have been used to control the assembling of magnetic iron oxide nanoparticles (NPs) coated with oleic acid molecules. The assembling occurs through interactions between the surface of NPs and specific functional terminal groups at the interface of the SAMs and the NP suspension. Polar head groups such as carboxylic acid and hydroxyl promote the assembling of NPs whereas nonpolar groups such as methylene avoid the assembling onto SAMs. The kinetics of the NP assembling is highly dependent on the polarity of terminal groups of the SAMs. Carboxylic acid groups have been shown to promote faster formation of a dense monolayer of NPs than hydroxyl groups. In addition, oleic acid used as a coating agent enables the high stability of the NP suspension and its concentration influences dramatically the kinetics of the assembling. The mechanism proceeds by a ligand exchange process between surfactant on the NP surface and terminal groups at the interface of the SAM with the suspension. I. Introduction In the field of nanotechnologies, magnetic inorganic nanoparticles rapidly appeared to be essential building blocks related to the fabrication of specific nanodevices dealing with potential applications like ultrahigh density magnetic storage media and nanoscaled magnetic sensors.1,2 The preparation of controlled assemblies of magnetic nanoparticles (NPs) onto substrates is now a hot topic in Materials Science and has been widely studied throughout different strategies.3-8 Indeed well-adapted deposition methods have to be selected to deposit low size distribution, uniform shape, and non-aggregated nanoparticles. Currently one promising way to easily prepare NPs assemblies deals with the control of the interactions between NPs and substrates by addressing their chemical functionalities.9 Well-addressed surfaces can be easily prepared by using selfassembled monolayers (SAMs) of organic molecules10 providing terminal head groups with specific functionalities. These functions have the ability to interact with the surface of nanoparticles and thus assist chemically the assembling of NPs at the interface of the SAMs.7,11-17 These groups act as driving agents to form a homogeneous monolayer of nanoparticles. Therefore SAMs represent very convenient, active, and tunable surfaces as the terminal group may be easily modified. Most studies deal with uncoated metallic NPs (mainly Au),18-23 Pt,24 or quantum dots (CdSe)24-26 which interact directly with thiol or amine groups whereas metal oxides (SiO2, TiO2, Al2O3)16,24,27-29 interact with carboxylic acid or hydroxyl head groups. SAMs and NPs can also be functionalized with organic head groups designed to achieve more specific interactions between two complementary * To whom correspondence should be addressed. E-mail: [email protected]. Fax: + 33 (0) 3 88 10 72 47. Tel: + 33 (0) 3 88 10 71 33. † Institut de Physique et de Chimie des Mate´riaux de Strasbourg. ‡ Institut de Chimie des Surfaces et Interfaces.

groups like covalent bonding12,16 and hydrophobic30 or electrostatic23,17 interactions. Although SAMs represent a very versatile approach to chemically assist the assembling of NPs, it has been poorly reported on magnetic NPs. It mainly results from the difficulty to control low size dispersion and high stability of suspension. Therefore most studies require the functionalization of NPs to control the assembling of single and independent NPs onto substrates. Specific functionalities such as unsaturated terminated alkylene chains ensure their direct immobilization onto activated silica wafers via very precise experimental conditions such as thermal hydrosilylation at 110 °C11,13,14 or UV-initiated bonding.15 Biofunctional groups such as the avidin/biotine couple have also been used to specifically assemble functionalized NPs by multihydrogen bonding. Therefore, few studies have dealt with the investigation of the coverage density of magnetic NPs by modulating the chemical nature of the SAM. In that context, we report here a very direct, versatile, and fast one-pot method to control the assembling of magnetic NPs by using chemical well-addressed surfaces. Highly stable suspensions of non-aggregated magnetic iron oxide nanoparticles coated by oleic acid in organic solvents were used. Thus, iron oxide NPs are fully considered as independent building blocks and avoid aggregates which limit the control of magnetic properties. Well-addressed surfaces consist of SAMs of ω-mercaptoalkane molecules adsorbed on gold substrates which display terminal groups with specific functionalities to interact with NPs at the interface with the suspension (Scheme 1). Several terminal groups with different polarity and metal coordination abilities have been used. Indeed, the carboxylic acid group is a coordinating and chelating group whereas the hydroxyl group is only a coordinating group. On the other hand, the methylene group is neither a coordinating nor a chelating group and is used as a reference. The formation of NPs assemblies is investigated as a function of the different terminal

10.1021/jp101872u  2010 American Chemical Society Published on Web 04/26/2010

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SCHEME 1: Immobilization of Oleic Acid Coated Iron Oxide Nanoparticles onto SAMs Displaying Functional Groups at the Interface of the Solution

groups. The density of the NP monolayer depends on the chemical nature of the terminal groups of the SAMs and on the ratio between surfactant free molecules in solution and the one absorbed onto NPs. In addition, the reaction has been demonstrated to occur through a one-pot molecular exchange, i.e., no post functionalization of NPs is required to ensure their assembly onto the SAM. II. Experimental Section a. Synthesis of Oleic Acid Coated Iron Oxide Nanoparticles. The synthesis of oleic acid coated iron oxide nanoparticles was described previously by Hyeon et al.31 In this report, it was carried out following an adapted method detailed in ref 32. It consists of the preparation of an iron(III)/oleate complex (Fe(oleate)3), which is thermally decomposed in a high boiling solvent in the presence of oleic acid. Fe(oleate)3 was prepared from FeCl3 · H2O (10.8 g, 40 mmol, 97%, Aldrich), which was dissolved in 60 mL of H2O (Milli-Q) and 80 mL of ethanol. This solution was mixed with a solution of sodium oleate (36.5 g, 120 mmol, 82%, Riedel-de Hae¨n) dissolved in hexane (140 mL) and refluxed at 70 °C for 4 h. The organic phase containing the iron oleate complex was separated, washed three times with distilled water (30 mL) to extract salts, and dried with MgSO4, then finally hexane was evaporated. The resulting iron oleate complex was a reddish-brown viscous solution and was stored at 4 °C. A combination of Fe(oleate)3 (2 g, 2.2 × 10-3 mol), oleic acid (1.24 g, 3.3 × 10-3 mol), and octyl ether (20 mL) was stirred for 1 h to dissolve the reactants. The temperature was carefully raised to reflux with a heating rate of 5 deg/min without stirring for 120 min under air. After cooling to room temperature, the black suspension of nanocrystals was washed 3 times by addition of ethanol and centrifugation (8000 rpm, 10 min). The obtained nanocrystals could be easily suspended in various organic solvents to raise a highly stable suspension that can be stored for several months. The size monodispersity of NPs was improved by applying a size selection precipitation process.33 The nanoparticles were suspended in hexane at a concentration of 1 mg/mL and precipitated by adding the same volume of acetone followed by centrifugation. The precipitate was redispersed in tetrahydrofurane (THF) to prepare a highly stable suspension of coated nanoparticles with a specific concentration of 3.7 mg · mL-1. b. Preparation of Self-Assembled Monolayers. An ion sputtered gold substrate (50 nm) was cleaned under O2 plasma and then soaked in a 10 mmol ethanolic solution of 11mercaptoundecanoic acid (MUA, Aldrich), 11-mercaptoundecanol (MUD, Aldrich), or mercaptododecane (MMD, Aldrich) at room temperature for 24 h. The substrates were then washed with absolute ethanol and dried under a stream of N2. SAMs were obtained and respectively named SAM-COOH, SAM-OH, and SAM-CH3.

Pichon et al. c. Exposure of SAMs to a Suspension of Iron Oxide Coated Nanoparticles. SAMs were immersed directly in the suspension of oleate coated nanoparticles in THF with different concentrations at room temperature for different times from 10 to 180 min. The modulations of concentration and immersion times enable us to study the kinetics of the immobilization of NPs. The substrates were then placed in an ultrasonic bath in THF to remove any physisorbed nanoparticles and finally dried under a stream of nitrogen. d. Characterization Techniques for Nanoparticles and SAMs. The X-ray diffraction (XRD) pattern was recorded at room temperature with a D500 Siemens diffractometer equipped with a quartz monochromator (Co KR1 0.17890 nm). The average crystallite size ∆ was estimated by using the DebyeScherrer equation: ∆(2θ) ) 0.9λ/(L cos(θ0)), where λ is wavelength of Co KR1 and L is the full width at half-maximum. Nanoparticles were analyzed with a TOPCON model 002B transmission electron microscope (TEM), operating at 200 kV, with a point-to-point resolution of 0.18 nm. The polydispersity in size was calculated from the size measurements of more than 300 nanoparticles. Dynamic light scattering (DLS) measurements were performed on the suspension of nanoparticles in tetrahydrofurane, using a nanosize MALVERN (nano ZS) apparatus. Thermogravimetric measurements were performed on dried powder samples by using a SETARAM TGA 92 from 20 to 600 at 5 deg/min under air. The solid supports were studied before and after being exposed to the suspension of nanoparticles with a JEOL 6700 scanning electronic microscope equipped with a field emission gun (SEM-FEG) operating at an accelerating voltage of 3 KV. Atomic force microscopy (AFM) was also performed with a Digital Instrument 3100 microscope coupled to a Nanoscope IIIa recorder. Measurements were done in the tapping mode onto substrates before and after exposure to the suspension of nanoparticles. ω-Mercaptoalkane molecules adsorbed on gold substrates which result in SAMs were characterized by polarization modulation infrared reflection-absorption spectroscopy (PMIRRAS) performed on gold substrates after being immersed in thiol solution, using a IF66S Bruker spectrometer with a liquidnitrogen-cooled mercury cadmium telluride (MCT) detector. Typical data was derived from 2000 scans at a resolution of 1 cm-1, 85° beam angle of incidence and 74 kHz modulation frequency. X-ray photoelectron spectroscopy (XPS) was performed with a ThermoVGScientific photoelectron spectrometer equipped with a twin anode, providing unchromatized Al KR radiation (1486.6 eV). The spectrometer, which was equipped with a multichannel detector, operated in the constant resolution mode with pass energy of 20 eV. The total resolution of the system was estimated to be 0.55 eV. The energy scale is referred to the Au 4f7/2 line at a binding energy of 84 eV. The spectra were fitted by using a linear background and the Gaussian function. Magnetic measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design model MPMS-XL). Magnetizations versus applied magnetic field curves were recorded at 290 and 5 K. Zero field cooling (ZFC) and field cooling (FC) curves were recorded between 5 and 300 K under exposure of an applied magnetic field of 50 G. ZFC curves were first recorded from 5 to 300 K after the temperature had been decreased without applying any magnetic field. The FC curves were recorded between 5 and 300 K after the temperature was returned to 5 K under a magnetic field of 50 G. Aliphatic coated nanoparticles

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were easily dispersed in eicosane to limit the formation of any assemblies during the measurements and to decrease the magnetic dipolar interaction between the nanoparticles. The substrate containing the monolayer of nanoparticles was placed in a parallel direction to that of the applied magnetic field. ZFC and FC curves were normalized on the basis of the magnetization value corresponding to the maximum of the ZFC curves of each sample. III. Results Oleic acid coated iron oxide nanoparticles were synthesized by thermal decomposition of an iron(III)/oleate complex (Fe(oleate)3) in a high boiling solvent in the presence of oleic acid molecules.31,32 Transmission electron microscopy (TEM) images showed highly uniform nanoparticles with a narrow size distribution centered at 11.8 ( 1.4 nm (see the Supporting Information). The electron diffraction pattern and the X-ray diffraction (XRD) pattern have confirmed the formation of an iron oxide spinel structure (see the SI).34 The calculated parameter a ) 8.374 Å is intermediate to that of magnetite (8.396 Å) and maghemite (8.346 Å) according to JCPDS cards numbers 19-629 and 39-1346, respectively. This observation is significant of nanoparticles which mainly consist of magnetite partially oxidized at their surface.34 The average crystallite size of about 11.5 nm estimated by using the Scherrer formula is in good agreement with the mean size deduced from TEM measurements. Because the nonaggregation of nanoparticles in suspensions is a determinant parameter to obtain homogeneous and dense films, dynamic light scattering (DLS) measurements were conducted. A monomodale distribution of the particle size is observed and centered on 13.8 nm, which is a value in good agreement with the hydrodynamic diameter of the NPs, taking into account the oleate coating (two times a layer of about 2 nm) and the first coordination sphere of solvent molecules. In addition, the initial weight ratio between NPs and oleic acid was calculated from TGA measurements which revealed the samples to consist of 15% by weight oleic acid (see SI). To explore specific interactions between NPs in THF suspension and substrates covered with SAMs, the terminal function of SAMs was either polar carboxylic acid or hydroxide groups or a nonpolar methyl group. Moreover, the carboxylic acid group is a coordinating and chelating group whereas the hydroxyl group is only a coordinating group. On the other hand, the methylene group is neither a coordinating nor a chelating group and is used as a reference. SAMs were prepared in a very common way in ethanol by chemisorption of thiol groups onto a gold surface. The spontaneous assembly of ω-mercaptoalkyl molecules was driven by intermolecular interactions.10 11Mercaptoundecanoic acid (MUA), 11-mercaptoundecanol (MUD), and 12-mercaptododecane (MDD) lead to the corresponding SAMs named respectively SAM-COOH, SAM-OH, and SAMCH3. SAMs were characterized by polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and show the presence of organic molecules containing alkyl chains (see the SI).35 Bands corresponding to CH2 stretching vibration modes at 2920-2921 cm-1 (νas) and 2850-2852 cm-1 (νs) and CH2 scissoring modes at 1461-1457 cm-1 (δ) are displayed by all spectra. Bonds of CH3 stretching at 2963 cm-1 (νas) and 2875 cm-1 (νs) are only observed for SAM-CH3 and correspond to the terminal methylene groups. This confirms the adsorption of MDD molecules onto gold substrates. These bands are not observed in other spectra related to SAM-OH and SAM-COOH. A band related to carboxylic acid groups is observed at 1596

Figure 1. SEM pictures of SAM-COOH, SAM-OH, SAM-CH3, and gold substrate after exposure to a suspension of oleate coated nanoparticles at room temperature for 10 min.

cm-1 (νCdO) and is related to the absorption of MUA to form SAM-COOH. The broad band at 3708-3050 cm-1 (νOH) in SAM-COOH corresponds to adsorbed water molecules. These spectral data show that SAMs of organic molecules were formed at the gold surfaces after immersion in solutions of the corresponding ω-mercaptoalkyl molecules. In a first step, SAMs were separately introduced into a suspension of iron oxide NPs in THF with a specific concentration of 3.7 mg · mL-1 for only 10 min. Then the substrates were extensively rinsed with THF and studied by scanning electron microscopy (SEM) (Figure 1). A large and dense array of nanoparticles was observed on the SAM-COOH (Figure 1a), which corresponds to a density of 4090 ( 126 NP/µm2. Assuming an ideal hexagonal superlattice of NPs with a size of 15.8 nm (11.8 nm sized NPs shelled by a layer of oleic acid molecules with an approximate thickness of 2 nm), the theoretical value is 4590 NP/µm2. In contrast, the density (2873 ( 24 NP/µm2) is 30% lower on SAM-OH than SAM-COOH, which demonstrates weaker interactions between the SAM-OH and the NPs (Figure 1b). Almost no particles (145 ( 22 NP/µm2) are present on the SAM-CH3 after immersion in the suspension and only gold grains from the substrate can be observed in a cloudy fashion because of the SAM-CH3 (Figure 1c). A blank experiment was also performed by using a gold substrate uncovered by SAM that was immersed in a suspension of NPs. The surface remains uncovered by NPs after rinsing showing the need to chemically address the surface (Figure 1d). These results points out the importance of the functional terminal groups which direct the assembling of NPs through specific interactions. The 2D assembling of NPs requires coordinating head groups at the interface of the SAM with the suspension. Moreover chelating groups favor stronger interactions at the NPs surface and result in a higher density after only 10 min of immersion. To obtain a better insight in the mechanism and the kinetics of the assembling we studied the NPs density on the SAMs as a function of time by plunging the SAMs in the NP suspension for different times. Whereas no significant modulation of the NP density was observed when SAM-COOH and SAM-CH3 were immersed in the suspension of NPs at longer times (90 min), a dramatic increase in the NP density was observed on SAM-OH. The assembling of NPs at different times from 10

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Figure 2. SEM pictures of SAMs-OH after immersion in a suspension of nanoparticles coated with oleic acid: (a) 10, (b) 20, (c) 30, (d) 60, (e) 90, and (f) 180 min.

Figure 3. UV/vis spectra of SAMs-OH after immersion in a suspension of nanoparticles coated with oleic acid after different times (a) and in suspensions with different concentrations in NPs (c) and their corresponding reflectance (closed circles) values at 300 nm plotted vs nanoparticles densities (open circles) as a function of time (b) and concentration in NPs (d).

to 180 min was studied by SEM (Figure 2). Pictures related to immersion times from 10 to 60 min look very similar. The density in NPs was calculated very precisely by using the image processing Visilog 5.2 software from NOESIS (Figure 3b). The density appears to increase continuously and linearly from 10 min (2873 ( 56 NPs per µm2) to 180 min (4263 ( 132 NP per µm2). The latter corresponds to the highest and maximum density and is similar to the density of NPs on SAM-COOH after only 10 min (4090 ( 126 NP/µm2). Although SEM pictures recorded in several regions of the samples have shown the assembling to depend on the immersion time, we performed a complementary kinetic study by UV/vis spectrophotometry on the same samples (Figure 3a) which provides information at the scale of the whole samples. The

linear decrease in reflectance values at 300 nm with time from 13.3% to 10.2% is in good agreement with the increase of the density in NPs deduced from SEM pictures (Figure 3b). In contrast, the reflectance values measured on SAM-COOH after the same immersion times do not show any differences and all values are 10.3((0.25)%. This observation confirms that the highest density in NPs is obtained on SAM-COOH within very short times. Therefore the kinetics of the NPs assembling on SAMs is clearly dependent on the polarity of the terminal groups, more precisely on its chelating properties. The density in NPs may also be highly depending on their concentration in suspensions when the kinetics of the assembling is slow. Indeed the immersion of SAMs-OH for 10 min in suspensions with different concentrations in NPs from 7.4 to

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Figure 4. SEM pictures of SAMs-OH after immersion for 10 min in suspensions of nanoparticles coated with oleic acid with different concentrations: (a) 7.4, (b) 3.7, (c) 1.84, (d) 0.74, (e) 0.37, and (f) 0.037 mg · mL-1.

0.037 mg · mL-1 leads to decreasing densities from 3155 ( 103 to 2444 ( 105 NP/µm2 shown by SEM pictures (Figure 4). These observations are confirmed by UV/vis spectroscopy showing the increase in reflectance at 300 nm from 7.5% to 14.0-13.6% (Figure 3c,d). Therefore high concentrations in NPs make them to interact easily on the surface of the SAM-OH because of their low mobility in the suspension and the high stability of assemblies. At a lower concentration, NPs move faster in the solution but the probability to interact with the terminal groups of the SAM is lower. The density in NPs on the SAM becomes lower and assemblies are less stable, which lead to an equilibrium between NPs on the SAM and NPs in solution that favors the latter. Moreover, the reflectance becomes stable around 14.0-13.6% at concentrations lower than 0.74 mg · mL-1. It shows that such a phenomenon has not been considered below a precise concentration in NPs. We conducted another series of experiments which consist of the increase in the oleic acid/nanoparticles ratio by adding a precise amount of surfactant to the initial suspension of NPs. The initial ratio between NPs and oleic acid was calculated from TG analysis on coated nanoparticles after the washing steps followed by the evaporation of THF. Therefore, the initial suspension of NPs used with SAMs contains 15% in weight of oleic acid (see the SI). A second suspension was prepared by adding a precise amount of oleic acid molecules to reach 52% in weight of the solid. According to the size of the NPs and the mean molecular area occupied by a single oleic acid molecule (about 0.42 nm2/molecule)36-38 and considering a monolayer of surfactants, about 65% and 225% of the NP surface would be covered by oleic acid molecules when using the initial and the second suspensions, respectively. However, oleic acid molecules are characterized by an equilibrium between grafted molecules at the surface of nanoparticles and free molecules in the solvent. The nanoparticles covered by oleic acid molecules in suspension consist of a dynamic system. Therefore, the amount of oleic acid molecules in the initial suspension is not sufficient to cover the whole surface of NPs but leads to a high density in NPs on the SAM-COOH within 10 min of immersion. However, in the second suspension, the high amount of surfactant molecules, which is much more to cover the whole NPs surface, leads to a lower density of 1623 ( 22 NP/µm2 for the same time. This value is almost two times lower than the density of NPs assembled by using the initial suspension with the lowest amount of surfactant. It was confirmed by the

dramatic increase of the reflectance at 300 nm to 14.5%. These results show that the amount of oleic acid in solution clearly influences the assembling of NPs. Indeed the high amount of surfactant increases the coverage of NPs and limits the driving force, which consists of the high density of carboxylic acid terminal groups at the SAM surface when introduced in the NP suspension. The structure of the NPs assemblies immersed for 10 min in the initial suspension (15 wt % in oleic acid molecules) was investigated more precisely by tapping mode atomic force microscopy (AFM). The analyses were performed on several regions to characterize the SAMs before and after deposition of NPs (Figure 5). After immersion in NP suspension, the SAMCOOH (Figure 5a) and SAM-OH (Figure 5b) display higher roughnesses (2.4 ( 0.1 and 1.9 ( 0.1 nm, respectively) than the corresponding SAMs before immersion (0.8 ( 0.1 nm), which shows the presence of assembled NPs. Topographic height images confirmed the homogeneous and high coverage density in NPs on the SAM-COOH in contrast to the partial coverage on SAM-OH. On SAM-COOH, some unoccupied areas are observed as the occurrence of the darkest regions but do not exceed 5%. Such free areas enable us to confirm the formation of a monolayer of nanoparticles as revealed by the surface cross sections which show average height differences of about 11.5 nm, consistent with the particle size. Individual nanoparticles are not clearly observed and appear as irregularly shaped blocks because of their coating with oleic acid and of the usual tip effects upon scanning. SAMs of ω-mercaptoalkylene molecules (Figure 5c) show a relatively high and similar roughness to that of polycrystalline grain gold substrates (0.8 ( 0.1 nm, see the SI) and cannot be related to the presence of SAMs. Additional information was obtained by performing X-ray photoelectron spectroscopy (XPS) after the SAM-COOH was introduced into the suspension of oleic acid coated NPs (see the SI). The C(1s) signal exhibits a peak centered at a binding energy of 285 eV, which is characteristic of the alkane chains of the SAM and oleic acid molecules on the NP surface.39 Moreover, the immobilization of NPs onto the SAM-COOH was evidenced by the presence of Fe(2p) peaks at 710.7 and 724.0 eV. The magnetic properties of the 2D assembly of nanoparticles immobilized onto the SAM-COOH were studied and compared to the ones dispersed randomly in eicosane with a SQUID

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Figure 5. AFM images of SAM-COOH (a) and SAM-OH (b) after immersion in a suspension of oleic coated iron oxide nanoparticles and SAMCOOH (c) before immersion. Topographic height (left) image and surface cross-section (center) of panel observed at the horizontal line in the left figure and 3D image (right).

magnetometer. Magnetization curves show no hysteresis loop at 290 K and are consistent with the super paramagnetic behavior of NPs above the blocking temperature (TB). On the other hand, at 5 K the hysteresis indicates a coercitive field of about 200 Oe and is characteristic of ferrimagnetism. Both curves were normalized according to their saturation magnetization. The assembling of NPs in two dimensions does not affect their magnetic behavior, which is similar to that of dispersed NPs. In addition, ZFC/FC curves are characteristic of superparamagnetic iron oxide nanoparticles with a blocking temperature (TB) of 130 K (Figure 6). The immobilization of nanocrystals onto the SAM-COOH does not induce strong modifications of the dipolar interactions compared to those of the diluted sample suggesting that such an assembling method leads to 2D films with magnetically non-interacting NP.32,40,41 However, the 2D assembly exhibits a broader ZFC curve, i.e., a larger distribution of TB, than that related to the random nanoparticles diluted in eicosane. This can be explained by some closer packing of the nanoparticles in some areas of the monolayer with local magnetic interactions. Such small domains are consistent with SEM and AFM analyses. This behavior has already been observed in similar close-packed two-dimensional arrays.40

IV. Discussion The chemical nature of SAMs, which determines their surface activity, and the coating of NPs, which is related to the amount of surfactant in the solution, are key parameters to control the assembly of NPs onto SAMs. The functional terminal groups at the interface of SAMs with the suspension of NPs control the assembling and the surface density of NPs. Interestingly, the very low amount of NPs on SAM-CH3 in contrast to SAMCOOH and SAM-OH shows that non-polar and non-coordinative groups are not suited to assemble iron oxide NPs. As NPs are coated with oleic acid groups, which display alkylene chains at the surface, interactions with the alkylene chains of the SAMCH3 would have been expected to favor the assembling of NPs in a dense film. In contrast, few nanoparticles were observed even if the substrate was not placed into an ultrasonic bath after being introduced into the suspension of nanoparticles. This may be due to closely packed and ordered monolayers of alkylene chains which are non-permeable and thus prevent any van der Waals interactions with organic species as previously demonstrated with coated gold NPs.30 The few NPs observed on SAMCH3 may result from some defects in the SAM. In contrast, the assembling of NPs happens on SAMs decorated with polar and coordinative groups such as carboxylic

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Figure 6. Magnetization curves versus applied magnetic field at 290 K (circles) and 5 K (squares) for the 2D assembly of nanoparticles immobilized onto the SAM-COOH. (b) An enlargement of panel a. (c) Zero field cooled and field cooled curves of the 2D assembly of NPs (gray squares) and oleic acid coated nanoparticles dispersed in eicosane (black circles).

acid and hydroxide groups despite the nonpolar coating of NPs by alkylene chains of oleic acid molecules. It suggests that polar and coordinative head groups of the SAMs succeeded in interacting directly with the polar surface of iron oxide NPs. Indeed, we demonstrate the density in NPs on the SAM to depend on the relative coating of NPs by modulating the amount of oleic acid molecules in the suspension. A low amount of surfactant favors a higher density in NPs on the SAMs than a high amount. This observation demonstrates that the immobilization of NPs on the SAMs is controlled by the equilibrium between free surfactant in the solution and surfactant which interact at the NPs surface. High amounts of surfactant displace the equilibrium to higher coverage of the NPs surface, which becomes less available to interact with polar terminal groups of the SAMs. Therefore repulsive interactions occur between aliphatic chains of oleic acid and the polar surface of SAMs and limit the immobilization of NPs on SAMs. In addition, the immersion of SAMs in the suspension of oleic acid coated NPs also modifies the equilibrium. The high density in polar and coordinating terminal groups at the interface of the SAMs and the suspension represents a fantastic driving force in comparison to carboxylic acid groups of diluted surfactant molecules. Therefore, the thermodynamic equilibrium is shifted to favor the interaction between SAMs and the NPs surface. This leads to the immobilization of NPs on SAMs instead being covered with oleic acid molecules in the suspension. The assembling of NPs on SAMs happens by the replacement of oleic acid molecules from the NPs surface by the SAMs through a ligand exchange process. Moreover, the equilibrium between free and grafted surfactants and the kinetics of the NPs assembling are also influenced by the coordinative and chelating abilities of the terminal head groups of the SAM. The SAMCOOH displays a higher density in NPs for the same immersion time as the SAM-OH, which is consistent with higher kinetics of the NPs assembling on SAM-COOH. Carboxylic acid groups which are chelating groups in contrast to hydroxyl groups lead to stronger interactions with the NPs surface and favor the faster immobilization of NPs on SAM-COOH than on SAM-OH. Moreover, longer immersion times of SAM-OH lead to higher densities in NPs which show that the kinetic is slower than that with SAM-COOH. Finally, these observations show that the ligand exchange process is favored by low amounts of surfactants and chelating terminal groups on the SAM. V. Conclusion In conclusion, a very direct, efficient, and versatile method has been presented to prepare 2D assemblies of magnetic

nanoparticles. The immobilization of iron oxide nanoparticles in a dense and very stable monolayer was easily controlled by the chemical nature of a SAM, which acts as a well-addressed surface. SAMs decorated with carboxylic acid terminal groups have the highest ability to interact with NPs because of their coordinating and chelating properties in contrast to hydroxide ones which only have coordinating properties. Moreover, the kinetic of the assembling of NPs on SAMs depends on the thermodynamic equilibrium between free surfactant molecules in the solution and those on the surface of NPs. It is clearly dependent on the experimental parameters such as the initial concentration of surfactant molecules and NPs, time, and functional terminal groups of the SAM.42 The fine-tuning of SAMs which consist of well-addressed surfaces offers unlimited possibilities to control the preparation of nanoparticles assemblies. Acknowledgment. The authors thank the Agence Nationale de la Recherche (ANR) and the Direction Ge´ne´rale des Arme´es (DGA) for financial support as well as P. Bernhardt from LMSPC in Strasbourg for XPS analyses. Supporting Information Available: Experimental procedure and characterizations of nanoparticles (XRD pattern, DLS, SAED), XPS, PM-IRRAS, TG analyses, and SEM picture. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Shi, W. L.; Sahoo, Y.; Zeng, H.; Ding, Y.; Swihart, M. T.; Prasad, P. N. AdV. Mater. 2006, 18, 1889. (2) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (3) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358–3371. (4) Desvaux, C.; Amiens, C.; Fejes, P.; Renaud, P.; Respaud, M.; Lecante, P.; Snoeck, E.; Chaudret, B. Nat. Mater. 2005, 4, 750. (5) Bre´chignac, C.; Houdy, P.; Lahman, M. Nanomaterials and Nanochemistry; Springer: Berlin, Germany, 2008. (6) Mammeri, F.; Bras, Y. L.; Daou, T. J.; Gallani, J.-L.; Colis, S.; Pourroy, G.; Donnio, B.; Guillon, D.; Be´gin-Colin, S. J. Phys. Chem. B 2008, 113, 734. (7) Kinge, S.; Crego-Calama, M.; Reinhoudt, D. N. ChemPhysChem 2008, 9, 20–42. (8) Demortie`re, A.; Launois, P.; Goubet, N.; Albouy, P. A.; Petit, C. J. Phys. Chem. B 2008, 112, 14583. (9) Hata, K.; Fujita, M.; Yoshida, S.; Yasuda, S.; Makimura, T.; Murakami, K.; Shigekawa, H.; Mizutani, W.; Tokumoto, H. Appl. Phys. Lett. 2001, 79, 692–694. (10) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990–1995. (11) Altavilla, C.; Ciliberto, E.; Aiello, A.; Sangregorio, C.; Gatteschi, D. Chem. Mater. 2007, 19, 5980.

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