Fast Assembling of Magnetic Iron Oxide Nanoparticles by Microwave

Article pubs.acs.org/cm

Fast Assembling of Magnetic Iron Oxide Nanoparticles by Microwave-Assisted Copper(I) Catalyzed Alkyne−Azide Cycloaddition (CuAAC) Delphine Toulemon,† Benoît P. Pichon,*,† Cédric Leuvrey,† Spyridon Zafeiratos,‡ Vasiliki Papaefthimiou,‡ Xavier Cattoen̈ ,§,∥ and Sylvie Bégin-Colin† †

Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 (CNRS−UdS), 23, rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France ‡ Institut de Chimie et Procédés pour l’Énergie, l’Environnement et la Santé, UMR 7515 (CNRS−UdS), 25 rue Becquerel, 67087 Strasbourg Cedex 2, France § Institut Charles Gerhardt Montpellier, UMR 5253 (CNRS−UM2−ENSCM−UM1), 8 Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France S Supporting Information *

ABSTRACT: Two dimensional (2D) nanoparticles (NP) assemblies have become very attractive due to their original collective properties, which can be modulated as a function of the nanostructure. Beyond precise control on nanostructure and easy way to perform, fast assembling processes are highly desirable to develop efficient and popular strategies to prepare systems with tunable collective properties. In this article, we report on the highly efficient and fast 2D assembling of iron oxide nanoparticles on a self-assembled monolayer (SAM) of organic molecules by the microwave (MW)-assisted copper(I) catalyzed alkyne−azide cycloaddition (CuAAC) click reaction. Microwave irradiation favors a dramatic enhancement of the assembling reaction, which was completed with maximum density in NPs within one hour, much faster than the conventional CuAAC click reactions that require up to 48 h. Moreover, the MWassisted click reaction presents the great advantage to preserve specific reactions between alkyne and azide groups at SAM and NP surfaces, respectively, and also to avoid undesired reactions. To the best of our knowledge, this is the first time this approach is performed to nanoparticles assembled on surfaces. KEYWORDS: nanoparticle, assembly, click chemistry, CuAAC, microwave, magnetism, iron oxide

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INTRODUCTION

between terminal functional groups on both NPs and SAMs surfaces meets most of these objectives. A crucial point is that the assembling reaction has to proceed with high yield and high rates. Over the past years, the copper(I) catalyzed azide−alkyne cycloaddition (CuAAC) click reaction,18−21 which was developed initially for small-molecule organic synthesis, has proved to be a very simple and efficient method for surface modification of substrates22,23 and nanoparticles.24 Very recently, this approach has been reported by Kinge et al.25 and our group26 as a very useful tool to address the assembling of magnetic NPs on SAMs by the irreversible covalent formation of triazole linkages. Such a strategy strongly prevents the formation of unspecifc NP assemblies, which may be driven by dipolar interactions.26 Depending on the reaction time, we

Owing to their great ability to modulate their physical properties as a function of their nanostructure, 2D nanoparticle (NP) assemblies have become a highly attractive field of research for the development of new applications such as biosensors for molecule detection, high density magnetic storage, or magneto-resistive media.1−4 Indeed, collective properties are ruled by the fine control on the preparation of mono- and multilayer assemblies on surfaces.5−8 For instance, assemblies with well-defined interparticle distances enable to modulate the dipolar interactions between magnetic NPs and thus the overall magnetic properties.9−11 Patterned NPs assemblies on surfaces have been obtained in a very efficient way by functionalizing substrates using self-assembled monolayers (SAMs) of organic molecules.12−17 Because of the increasing interest for the assembly of NPs and their original collective properties, easy-to-process methods are now required to develop device miniaturization. Among current assembling techniques, deposition of NPs driven by specific interactions © 2013 American Chemical Society

Received: April 22, 2013 Revised: June 20, 2013 Published: June 20, 2013 2849

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purchased from Strem Chemicals. Oleic acid, methanesulfonyl chloride, and 11-undec-1-ynol were purchased from Alfa Aesar. Triethylamine was obtained from Fluka and docosene from Aldrich. Synthesis of diethyl (12-bromododecyl)phosphonate. In a round-bottomed flask equipped with a short distillation apparatus, a mixture of 1,12-dibromododecane (65.6 g, 0.20 mol) and triethylphosphite (5.7 mL, 33 mmol) was heated at 160 °C for 4 h. The excess dibromododecane was then distilled off under vacuum, then the crude product was purified by column chromatography on silica gel (cyclohexane/ethyl acetate 3:1 to 1:1). Yield: 75%. 1H NMR (CDCl3, 400 MHz): 4.09 (m, 4H); 3.40 (t, J = 7.0 Hz, 2H); 1.84 (m, 2H); 1.78−1.66 (m, 2H); 1.64−1.52 (m, 2H); 1.44−1.23 (m, 22H). Synthesis of Diethyl (12-Azidododecyl)phosphonate. A mixture of diethyl (12-bromododecyl)phosphonate (1.83 g, 4.7 mmol) and sodium azide (1.54 g, 23.8 mmol) in ethanol (96%, 10 mL) was refluxed for 40 h. After evaporation of the solvent, water was added, and then, the mixture was extracted three times with dichloromethane. The combined organic fractions were washed with water then brine, and finally concentrated to afford diethyl (12azidododecyl)phosphonate (1.60 g, 97%). 1H NMR (CDCl3, 400 MHz): 4.09 (m, 4H); 3.25 (t, J = 7.0 Hz, 2H); 1.80−1.46 (m, 6H); 1.44−1.18 (m, 22H). Synthesis of (12-Azidododecyl)phosphonic Acid. Trimethylsilyl bromide (1.97 g, 12.9 mmol) was added to a solution of diethyl (12-azidododecyl)phosphonate (1.50 g, 4.7 mmol) in anhydrous dichloromethane (8 mL) under argon. The solution was stirred overnight at room temperature, and subsequently, the solvent was removed by rotary evaporation. Water (10 mL) was then added, and the mixture was stirred for two hours. The white solid was filtered off and air-dried. (12-Azidododecyl)phosphonic acid (1.17 g, 93%) was obtained as a white solid. 1H NMR (CDCl3, 400 MHz): 9.32 (br, 2H); 3.26 (t, J = 6.8 Hz, 2H); 1.80−1.67 (m, 2H); 1.65−1.52 (m, 4H); 1.40−1.22 (m, 16H). 13C NMR (CDCl3, 100 MHz): 51.4 (C−N3); 29.5−28.8; 26.7. 31P NMR (CDCl3, 400 MHz): 36.9. Synthesis of (11-Undec-1-ynyl)thiol. (11-Undec-1-ynyl)thiol was synthesized from (11-undec-1-ynyl)thioacetate, which has been synthesized following the synthesis pathway we have reported previously.26 Then, 300 mg of (11-undec-1-ynyl)thioacetate were dissolved in 20 mL of methanol. The solution was degassed thoroughly and backfilled with N2. One milliliter of concentrated HCl was added dropwise, and the entire mixture was refluxed under N2 atm for 5 h. The reaction was then stopped by adding 20 mL of ice cold water. The product was extracted twice with diethyl ether (20 mL), and the organic phase was washed twice with water (20 mL) and dried over MgSO4. Rotary evaporation yielded a yellow oil. 1H NMR (CDCl3, 400 MHz): 2.52 (q, J = 7.3 Hz, 2H); 2.18 (dt, J = 6.8 and 2.6 Hz, 2H); 1.94 (t, J = 2.6 Hz, 1H); 1.61 (m, 4H); 1.22−1.41 (m, 11H). 13 C NMR (CDCl3, 100 MHz): 85.6 (C terminal C−H); 67.9 (C alkyne); 39.1 (CH2−SH); 30.2−28.4 (6 CH2); 22.6 (CH2); 18.3 (CH2); 14.0 (CH2). Synthesis of Azide-Terminated Iron Oxide Nanoparticles (NP@N3). Iron stearate (Fe(stearate)2) (1.38 g, 2.2 × 10−3 mol) was dissolved in docosene (20 mL) in the presence of oleic acid (1.24 g, 3.3 × 10−3 mol). The mixture was kept at 110 °C for at least 4 h to avoid water residues and to completely dissolve the reactants. The temperature was then carefully raised to reflux with a heating rate of 5 °C·min−1 and kept under reflux without stirring for 120 min under air. After cooling to room temperature, the black suspension was washed 12 times with a mixture of hexane and acetone (v:v, 1:4) and centrifuged (14 000 rpm, 10 min). The obtained nanoparticles coated with oleic acid (NP@OA) were easily suspended in (THF) at a concentration of 1.67 mg/mL. Oleic acid was subsequently replaced by (12-azidododecyl)phosphonic acid (AP12N3) following direct exchange. A solution of AP12N3 (15 mg) in THF (10 mL) was added to 10 mL of the NP@OA suspension and stirred for 48 h at room temperature. Free molecules were removed by ultrafiltration (using a 30 kD membrane, Millipore) in 60 mL of THF. Preparation of Alkyne-Terminated Self-Assembled Monolayer (SAM-CC). Ion sputtered gold substrates were cleaned under O2/H2 plasma for 2 min and were soaked in a 10 mM ethanolic

have been able to tune the average interparticle distance so as to produce either assemblies of quasi noninteracting NPs or high density assemblies of NPs, which display collective properties.26 Although this approach represents an important step toward the assembling of NPs on SAMs in a controlled fashion through specific interactions, it suffers from very slow kinetics. Indeed, reaction times are longer (up to 48 h) than the ones of reactions between alkyne and azide molecular derivatives in solution and require large amounts of catalysts (up to 10 times as much as the molar amount of azide and alkyne groups). The assembling kinetics of NPs on SAMs strongly depend on the probability that nanoparticles reach the corresponding functional groups at the SAM surface. Several parameters may slow down the kinetics of the reaction: (i) the Brownian motion that rules the mobility of nanoparticles in solution, (ii) the number of functional groups that are available at the SAM surface, and (iii) the intermolecular interactions between these groups that are favored by the tight packing of molecules and thus reduce their reactivity. Although dense monolayers of NPs can be obtained by specific interactions using the CuAAC reaction, the assembly of NPs has been reported to occur much faster by using other types of SAMs.12,15,27 Therefore, a general and versatile strategy that enables fast assembling of NPs on SAM surfaces still represents an unfilled goal. Recently, microwave (MW)-assisted synthesis has been developed for CuAAC reactions. This approach is highly appealing because of its major advantages such as decrease of reaction time from hours to minutes, improved reaction yields, absence of side products, and reproducibility. Originally reported for molecular synthesis,21,28,29 MW-assisted CuAAC reactions have very recently emerged for surface functionalization. Gold29 and iron oxide30 NPs and SAMs on silicon wafers31−33 have been functionalized by this method. All these studies have demonstrated the acceleration of the CuAAC reaction under exposure to MW irradiations from hours to minutes. This has been established for relatively low power (40−100 W), which resulted in mild to elevated temperatures (70−150 °C). In all cases, these reactions featured a fastmoving molecular derivative reacting with an immobile substrate or a particle in slow-motion. It is worth noting that these studies have been exclusively performed by using the Sharpless conditions in the presence of Cu(II) catalyst and a reducing agent (ascorbic acid) in aqueous and polar organic solvent mixtures. Nevertheless, the alternative approach in the presence of Cu(I) and triethylamine34 is much more suited for NPs coated with hydrophobic molecules since such NPs form unstable suspensions of aggregates in hydrophilic media. Herein, we report on the development of a new way to accelerate the assembling of slow-motion magnetic iron oxide NPs on immobile SAMs by using the MW-assisted CuAAC click reaction. The reaction proceeds under hydrophobic conditions,29,30 and results in dramatically faster kinetics compared to conventional heating, with an easy control of surface coverage. This approach could be applied to the assembly of a wide range of functionalized nanoparticles and nano-objects. To the best of our knowledge, the benefits of MW-assisted CuAAC click reaction has never been reported for NPs assembling addressed by SAMs.

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EXPERIMENTAL SECTION

Chemicals. Tetrahydrofurane (THF), methanol, and ethanol were purchased from Carlo Erba and used as received. Iron stearate was 2850

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solution of 11-(undec-1-ynyl)thiol at room temperature for 24 h. The SAM was then rinsed with copious amounts of pure ethanol and used directly after drying under air. Nanoparticles Assembling. The assembling of nanoparticles by microwave-assisted CuAAC click reaction was performed using an Anton Paar microwave device (Monowave 300) with a 10 mL vial by immersing SAM-CC (5 × 5 mm2) in 5 mL of a solution of NP@N3 in THF (0.67 mg/mL). Then, 0.5 mL (3.70 mmol) of triethylamine and 6.5 mg (6.7.10−3 mmol) of CuBr(PPh3)3 were added. The MW CuAAC reaction was performed under a maximum temperature of 100 °C, which was controlled by an infrared sensor, while the reaction vial was cooled by compressed air flow. A maximum power of 50 W at a frequency of 2.45 GHz was applied to the reaction medium. The reaction time was varied from 2 min to 1 h. The power, temperature, and pressure were recorded during the assembling reaction. Characterization Techniques. Transmission electron microscopy (TEM), high resolution TEM (HRTEM), and electron diffraction (ED) were performed with a TOPCON model 002B TEM, operating at 200 kV, with a point-to-point resolution of 0.18 nm. The size distribution was calculated from the size measurement of more than 100 nanoparticles by using the Image J software. Granulometry measurements were performed on a nanosize MALVERN (nano ZS) apparatus for each NP suspension. Fourier transform infrared (FTIR) spectroscopy was performed using Digilab Excalibur 3000 spectrophotometer (CsI beamsplitter) in the energy range 4000−400 cm−1. Scanning electron microscopy (SEM) was performed using a JEOL 6700 microscope equipped with a field emission gun (SEM-FEG) operating at an accelerating voltage of 3 kV. Atomic Force Microscopy (AFM) was performed in the tapping mode using a Digital Instrument 3100 microscope coupled to a Nanoscope IIIa recorder. Collected data were analyzed with Nanotec WSXM software. 35 Polarization modulation infrared reflection−absorption spectroscopy (PMIRRAS) was performed on gold substrates after being immersed in thiol solution, using a IF66S Bruker spectrometer with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. The Xray photoelectron spectroscopy (XPS) measurements were carried out in an ultrahigh vacuum (UHV) setup equipped with a VSW ClassWA hemispherical electron analyzer (150 mm radius) with a multichanneltron detector. A monochromated AlKα X-ray source (1486.6 eV; anode operating at 240 W) was used as incident radiation. The base pressure in the measurement chamber was ∼1 × 10−9 mbar. XP spectra were recorded in the fixed transmission mode using pass energy of 22 eV resulting in an overall energy resolution better than 0.4 eV. Prior to individual elemental scans, a survey scan was taken for all the samples to detect all of the elements present. The CASA XPS program with a Gaussian−Lorentzian mix function and Shirley background subtraction was employed to deconvolute the XP spectra. Magnetic curves were recorded at 300 and 5 K by applying a magnetic field in the plane of the substrate by using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS SQUID-VSM dc magnetometer).

Figure 1. Schematic representation of the MW-assisted CuAAC click reaction between azido-terminated NPs and alkyne-terminated SAMs, which result in the formation of irreversible triazole linkages.

thioacetate derivative.26 The assembling of thiol molecules as a SAM was confirmed by the thickness measured by ellipsometry (1.1 ± 0.1 nm) and also by PM-IRRAS spectroscopy and XPS. In contrast to the very few studies that report only on surface functionalization by microwave-assisted CuAAC reaction from Cu(II) catalyst associated to a reducing agent (ascorbic acid),31−33 the assembling of NPs was performed by dipping the SAM-CC in a mixture of the NP@N3 suspension in THF, triethylamine, and the CuBr(PPh3)3 catalyst, which enable the reaction to proceed in nonaqueous, aprotic solvents.34 The CuAAC reaction was carried out in a sealed tube under microwave irradiations with a maximum power of 50 W with the aim not to exceed 100 °C. More details related to the syntheses and assembling processes are available in the experimental section of the Supporting Information. The assembling of NP@N3 on SAM-CC was characterized by scanning electron microscopy (SEM) (Figure 2a).

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RESULTS AND DISCUSSION The assembling process operating by click chemistry was conducted between azide-terminated NPs (NP@N3) and alkyne-terminated SAM (SAM-CC) (Figure 1). Iron oxide NPs coated with oleic acid were prepared by the thermal decomposition of iron stearate in docosene (bp 365 °C) and were further functionalized by (12-azidododecyl)phosphonic acid (AP12) following a ligand exchange procedure.9,26 NPs exhibiting a narrow size distribution centered at 19.8 ± 1.6 nm were obtained and formed a highly stable suspension in tetrahydrofurane (THF) according to TEM micrographs and granulometry measurement, respectively. Prior to the assembling reaction, the alkyne terminated SAM was prepared by the adsorption of (11-mercaptoun)dec-1-yne in ethanol on a gold substrate after plasma cleaning. This molecule was synthesized by performing the acidic deprotection of the corresponding

Figure 2. (a) SEM image of NPs assembled on SAM after 1 h of MWassisted CuAAC click reaction. (b) Density in NPs assembled on SAM as a function of the reaction time. (c) Height AFM images and (d) cross-section profile corresponding to the line in panel c.

Representative micrographs recorded on different areas show the high efficiency of the microwave-assisted CuAAC click reaction. NPs cover the whole surface of the SAM with a density of 1470 ± 14 NP/μm2 after 1 h of reaction, which, when taking into account the adsorption process, is rather close to the maximum theoretical value (1820 NP/μm2) calculated for compact assemblies. These results were confirmed by AFM images and the corresponding cross-sectional profile, which 2851

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shows that NPs were assembled in a well-defined and dense monolayer (Figure 2c,d). In addition, extensive washings with THF and exposure to ultrasounds do not affect the structure of the NP assembly, which shows the irreversible formation of highly stable covalent triazole linkages between NPs and the SAM. Beyond the fact that NPs were assembled with a very high density, a very important point is that the microwave-assisted assembling reaction is dramatically faster than using conventional heating conditions for which the maximum density was obtained at 60 °C after 48 h.26 Because of the geometric considerations, unreacted alkyne and azide groups are expected to remain at the surface of SAM and NPs, respectively, after the assembling reaction. Therefore, surface analysis methods such as PM-IRRAS or XPS cannot reveal with certainty the occurrence of the assembling reaction by CuAAC. With the aim to gain better insight on this point, we performed additional counter experiments that indirectly prove the occurrence of the cycloaddition reaction. First, the reaction was performed identically as the main experiment without adding the Cu catalyst and resulted in the nondeposition of NPs on SAM by SEM (see Supporting Information). Although the microwave activation is highly efficient, it cannot overpass the role of the Cu catalyst. Second, the reaction was performed in the presence of methyl-terminated SAM, which also prohibited the assembling of NPs. These two experiments demonstrate the requirement of both Cu catalyst and alkyne groups at SAM surface to assemble NP@N3 by the CuAAC click reaction. More details on the kinetics of the microwave-assisted CuAAC reaction have been obtained by performing the experiment for different durations. Reaction times shorter than one hour resulted in lower NP densities (see Supporting Information). Plotting the NP density as a function of time shows the nonlinear increase of the kinetics (Figure 2b). Indeed, the NP assembling does not happen within the first few minutes. A minimum incubation time is necessary for iron oxide NPs and gold surface to accumulate energy, which induces local increase in temperature. Between 2 and 5 min, the assembling reaction starts and the density increases faster to reach 50% of the maximum value after 10 min. These results are correlated to the increase in temperature, which reaches its maximum (100 °C) only after 5 min of irradiation (see Supporting Information). Although 100 °C is necessary to obtain the maximum density in NPs after 1 h of reaction, the click reaction can also occur at lower temperature. For longer times, the kinetics decrease, which is related to the lower amount of empty space at the SAM surface that NPs can occupy. More generally, NPs that are suspended in solution are submitted to the Brownian motion, which results on their rather low mobility and statistic adsorption. Whatever the efficiency of the microwave-assisted CuAAC reaction, the probability of azide terminated NPs to react with free alkyne groups at SAM surface decreases as long as the NP density increases on SAM. In addition, the time needed (1 h) to reach a maximum density is rather long in comparison to studies that deal with the reaction of molecules with functional groups at the SAM surface by using the same reaction, which can be accomplished within a couple of minutes.28 This finding supports the argument of the low mobility of NPs in solution in comparison to molecules that are far smaller. Such an increase of reaction time has been also noticed for increasing steric hindrance of molecules.31

Although the assembling of NPs on SAM clearly demonstrates that organic functionalities are preserved upon the MW-assisted CuAAC reaction, we investigated the structure of SAM and NPs before and after exposure to MW irradiation. The reaction was performed without Cu catalyst to maintain both SAM and NPs under the same conditions. All spectra related to the following measurements can be seen in the Supporting Information. The composition of SAM was analyzed by XPS. Both C1s spectra exhibit a very similar main peak at 284.6 eV with a slight asymmetry toward to the high binding energy side. The main peak is related to sp3 carbon atoms in alkylene chains,36 while the peak asymmetry is caused by shake up satellite peaks and/or contribution of sp1 (alkyne) species36 and carbon connected to oxygen species. The S 2p spectra have been fitted by two S 2p3/2,1/2 doublets. For each S 2p doublet, a 1.3 eV spin orbit splitting and a 2:1 intensity ratio between the 3/2 and 1/2 components was used. The first doublet at a binding energy of 161.9 ± 0.1 eV (S 2p3/2) is related to thiol groups bound to the gold surface (S−Au).37 The second doublet at a higher BE of 163.6 ± 0.1 eV is attributed to some free thiol groups (unbound species).37 Moreover, the S/Au and S/C intensity ratios were calculated 0.09 ± 0.01 and 0.1 ± 0.01 and are practically unaffected by the MW process, indicating that microwave irradiation does not modify the composition of SAM. The SAM structure was also investigated by PM IRRAS. While νCC and νCC−H vibration modes33,36 corresponding to alkyne terminal groups were not observed as reported earlier,38,39 the νCH2 vibration modes are centered to 2918 and 2854 cm−1, which demonstrates that alkylene chains are in the all-trans conformation and remain tightly packed.26 However, granulometry and FTIR spectroscopy have been performed on NPs before and after exposure to MW irradiation. Similar spectra proved the stability of azide terminated NPs in the reaction medium. Finally, magnetic measurements have been performed on the assembly obtained after 1 h of MW-assisted CuAAC reaction. Magnetization was recorded against a magnetic field in the −7 to +7 T range (Figure 3). Curves recorded at 300 and 5 K display hysteresis loops with coercive fields (HC) of 50 and 400 Oe, respectively. These curves correspond to the ferrimagnetic behavior of spinel iron oxide NPs with sizes of 20 nm, which are featured by a rather high magnetic anisotropy in comparison to smaller iron oxide NPs that are super-

Figure 3. Magnetization recorded at 300 and 5 K against an applied field for NPs assembled after 1 h of MW assisted CuAAC click reaction. The inset depicts the full curves. 2852

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paramagnetic.40 As a consequence, SEM shows NPs to assemble in 1D chains after 10 min of reaction (see Supporting Information). This behavior may be correlated to strong and collective dipolar interactions, which may participate in the assembling reaction. More generally, these results show that the magnetic properties of these NPs are similar to others reported in the literature40 and do not seem to be affected by the microwave irradiations, even when the reaction is prolonged for one hour.

CONCLUSIONS To summarize, the major finding of this study is the unseen and very fast assembling of NPs on SAMs by performing the microwave-assisted CuAAC click reaction. Beyond the fact of formation of dense assemblies of NPs with a well-defined monolayer structure, this process promotes the enhancement of the kinetics of the reaction, reducing the reaction time from days to minutes. While the elevation of temperature of the reaction media is rather low in comparison to conventional experimental conditions, the mechanism of the reaction is expected to proceed through a local increase of the temperature at the interface between iron oxide NPs and gold substrate. This point is currently under investigation. Fortunately, microwave irradiation does not affect the functionalities and structures of both SAMs and NPs. Notably, the MW-assisted CuAAC click reaction was demonstrated to be carried out for the first time in the presence of a Cu(I) catalyst and alkynethiolate SAMs on a gold surface, which enables this assembling method to proceed for hydrophobic NPs as stable suspensions. Other advantages of this assembling method are the absence of side-products and that no purification process is required. Its potential adaptability to a wide range of nanoparticles and surfaces also renders it very promising to be used as a general assembling technique. ASSOCIATED CONTENT

S Supporting Information *

Molecule synthesis. Structural analysis of nanoparticles before and after exposure to microwave irradiation. SEM micrographs, FTIR, PM-IRRAS, and XPS spectra. Experimental data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(B.P.P.) E-mail: [email protected]. Tel: 0033 (0)3 88 10 71 33. Fax: 0033 (0)3 88 10 72 47. Present Address ∥

Institut Néel, UPR2940 CNRS/UJF, 25 rue des Martyrs, 38042 Grenoble, France. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. Laurent Douce and Dr. Julien Fouchet for providing the access to the microwave reactor and fruitful discussions. Funding was provided by Direction générale de l’armement (DGA) and région Alsace. 2853

dx.doi.org/10.1021/cm401326p | Chem. Mater. 2013, 25, 2849−2854

Chemistry of Materials

Article

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dx.doi.org/10.1021/cm401326p | Chem. Mater. 2013, 25, 2849−2854