Controlling Physical Properties of Iron Nanoparticles during Assembly

Sep 9, 2013 - Fei Gao , Soraya Aminane , Shi Bai , and Andrew V. Teplyakov ... Timothy Miller , Laurent Pirolli , Fei Deng , Chaoying Ni , Andrew V. T...
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Controlling Physical Properties of Iron Nanoparticles during Assembly by “Click Chemistry” Yue Liu,† Neelam RamaRao,‡ Timothy Miller,† George Hadjipanayis,‡ and Andrew V. Teplyakov*,† †

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States

‡

S Supporting Information *

ABSTRACT: The use of chemical functionalization of nanoparticles for controlled assembly relies on selective surface modification that promotes highly specific chemical transformations. It is often also important to control physical properties of the materials produced by this approach, and in the case of iron nanoparticles, magnetic properties are often the target of the applications. Here, iron nanoparticles are assembled using “click chemistry”, with two batches of iron nanoparticles functionalized with 5-hexynoic acid and 5-azidopentanoic acid, respectively. The functionalization process and the final assembly are followed by a combination of spectroscopic techniques, scanning electron microscopy, and density functional theory calculations. Most importantly, the magnetic properties of the material produced can be retained, as demonstrated by vibrating sample magnetometer measurements, with substantial changes in the morphology of the material. The observed changes are attributed to partial oxidation of the produced functionalized surfaces, and the suggestions for further improvements are made. in a number of fields, including polymer and material science24−26 and drug delivery.27−29 In a recent work by White et al.20 it was demonstrated that iron oxide nanoparticles can be functionalized with 5-hexynoic acid and, following this modification step, can “click” with the azide molecules. Here, we use a surface chemical functionalization approach to actually induce assembly of iron nanoparticles based on two different surface functionalities to alter the morphology of the produced material and to examine how such an approach affects the resulting physical properties. Our previous experience with nanostructure assemblies obtained by surface chemical modification ranging from large organic molecules30 and biomolecules31,32 to nanoparticles33,34 on the solid surface is expanded here to nanoparticles assembly. In this work, two batches of amorphous iron nanoparticles were functionalized with 5-hexynoic acid and 5-azidopentanoic acid, respectively. This surface functionalization step was followed by “click chemistry” to assemble the iron nanoparticles with two different functional groups into a unified material. X-ray photoelectron spectroscopy (XPS) was used to confirm the nature of iron oxide on the surface of the nanoparticles studied. A combination of XPS, Fourier-transform infrared spectroscopy (FT-IR), and time-of-flight secondary iron mass spectroscopy (ToF-SIMS) was used to confirm surface functionalization of the nanoparticles and verify the click chemistry. FT-IR studies supported by density functional theory (DFT) calculations

1. INTRODUCTION In recent years, there has been a growing interest in magnetic materials due to their wide range of applications, ranging from data storage1 and biotechnology2,3 to catalysis.4,5 In order to obtain magnetic materials with targeted magnetic properties, the assembly of magnetic nanoparticles is often required. Previously, such an assembly has been achieved through noncovalent interactions.6,7 However, the very nature of these interactions implies that the binding is weak, and the assembly may break down under processing conditions, which often require heating or agitation. Thus, covalent attachment of nanoparticles into a target assembly is highly desired and has been the target of research for several years.8−10 However, chemical assembly of iron nanoparticles that would preserve the desired magnetic properties has not been demonstrated so far. Iron nanoparticles have a number of very attractive properties, and their magnetic properties are often the key to a variety of potential applications.11,12 Comparing to other magnetic materials, such as the ones that are based on cobalt or nickel, which can be toxic,13 iron nanoparticles have received more attention from both fundamental and applied points of view.14−16 Since in ambient conditions iron nanoparticles are usually covered with iron oxides,17−19 a number of chemical pathways can be designed to functionalize their surface. In particular, organic reactions with a phosphonic acid group20,21 or a carboxylic acid group20,22,23 have been demonstrated. A common type of “click chemistry”, which is a type of cycloaddition processes, is based on the reaction between azide and alkyne functional groups to form triazole rings. Because of fast rate and high selectivity, “click reactions” have been applied © XXXX American Chemical Society

Received: June 18, 2013 Revised: August 19, 2013

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pentahydrate and 2.9 mg of sodium ascorbate were added to the mixture. The reaction time was 24 h, and the resulting product was washed three times with methanol. The nanoparticles were collected with a magnetic rod and dried with a stream of nitrogen gas. 2.6. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were collected using an ESCAlab 220i-XL electro spectrometer (VG Scientific, UK) with monochromatic Al Kα X-ray source. The instrument was operated with a power of 100 W with the nominal spot size of 400 μm2. The background pressure was ∼1.3 × 10−7 Pa. Survey spectra were collected from 0 to 1200 eV with the resolution of 1 eV and a pass energy of 100 eV. High-resolution spectra were collected by using pass energy of 20 eV with the resolution of 0.1 eV. The spectra were analyzed using Casa software. 2.7. X-ray Powder Diffraction (XRD). The structure and phase information on the synthetic Fe nanoparticles before and after “click chemistry” reaction was collected by X-ray powder diffraction (XRD, Rigaku Ultima IV instrument) measurements by using Cu Kα radiation. The spectra were collected with the steps of 0.05° and in the 2-theta (2Θ) range starting from 20°. 2.8. Fourier-Transform Infrared Spectroscopy (FT-IR). FT-IR spectra were collected on Nicolet Magna-IR 560 spectrometer with an MCT detector cooled with liquid nitrogen. Nanoparticles were mixed with ∼100 mg of KBr to prepare the FT-IR samples and pressed into pellets. All the spectra were collected in the transmission mode with 32 scans per spectrum at a resolution of 4 cm−1. 2.9. Density Functional Theory (DFT) Calculations. The Gaussian 09 suite of programs37 were used to perform the DFT calculations with B3LYP38,39 functional and 6-311+G(d,p) basis set. Geometry optimization, vibrational frequency calculations, and prediction of the core level XPS spectra for the N 1s region were performed. A simple Fe2O3 model cluster was used to investigate the reactions of functionalized carboxylic acids with the iron nanoparticles.40 The theoretically predicted XPS spectra of the N 1s region were calibrated based on our previous work with a previously determined correction factor of 8.76 eV for the 6-311+G(d,p) basis set to be compared to the experimental spectra.41,42 2.10. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). The ToF-SIMS spectra were recorded on an ION-TOF instrument (GmbH, Muenster, Germany) model “TOF-SIMS V”. A liquid metal ion beam with energy of 25 keV and Bi+ ions in a high-current bunched mode with angle of incidence at 45° and beam current at 1.5 pA at the target were used to collect the spectra. Both positive and negative spectra were taken, 50 scans for each set. The analyzer had a 2 keV extraction and 10 keV post acceleration energy. 2.11. Scanning Electron Microscopy (SEM). The morphology of the iron nanoparticle samples was investigated by SEM (Zeiss Auriga 60 FIB/SEM). The images were collected by secondary electrons (in-lens detector) with accelerating energy of 15 keV and a working distance of 5.0 mm. Selected images in the Supporting Information were collected on a different SEM (JEOL JSM-6335F). The images were collected with accelerating energy of 15 keV and a working distance of 8.0 mm. 2.12. Vibrating Sample Magnetometer (VSM) Measurements. Magnetic properties were measured by VSM in a vacuum chamber at room temperature. The maximum applied

were employed to monitor each step of surface functionalization of the iron nanoparticles. Scanning electron microscopy (SEM) was used to examine the structural changes in the material following “click chemistry” and also to confirm that only the reaction between differently functionalized nanoparticles causes this change. Finally, a vibrating sample magnetometer (VSM) was used to investigate the magnetic properties of the iron nanoparticles before and after reaction. Xray diffraction (XRD) studies were also used to verify that the iron nanoparticles were amorphous throughout chemical modification procedure. Based on this analysis, a general approach to producing this type of magnetic material and retaining magnetic properties will be developed, and suggestions on improving the retention of magnetic properties even further will be offered.

2. EXPERIMENTAL METHODS 2.1. Chemicals. The following chemicals were obtained from Sigma-Aldrich Co.: 5-hexynoic acid (97%), 5-azidopentanoic acid (≥97.0%), copper sulfate pentahydrate (99.995% trace metals basis), (+)-sodium L-ascorbate (crystalline, ≥98%), dimethyl sulfoxide (≥99.9%). Hexanes (Fisher Scientific, HPLC grade), chloroform (Fisher Scientific, ≥99.8%), ethanol (Fisher Scientific), iron chloride tetrahydrate (Sigma-Aldrich, >98%), sodium borohydride (Alfa Aesar, 95%), and benzyl azide (Alfa Aesar, 94%) were used as received. DI water (resistance 18 MΩ·cm) from a Milli-Q water system (Milipore) available at the University of Delaware was used to rinse the samples and/or as a solvent, as described below. Iron nanoparticles (∼25 nm) were synthesized as described in the next section. Iron nanoparticles (100 nm) were purchased from SS Nanomaterials. 2.2. Synthesis of Iron Nanoparticles. Iron nanoparticles (∼25 nm in diameter) were synthesized according to a previously published standard procedure.35,36 1 g of FeCl2· 4H2O was shaken to dissolve in 250 mL of ethanol until a yellow solution was formed. 0.588 g of NaBH4 was dissolved in 250 mL of DI water. After that, the two solutions were mixed together. After 30 s, 500 mL of ethanol was added to the mixture. The nanoparticles formed following this procedure were then washed three times with ethanol to remove water and the excess of reactants. Based on XRD, the produced nanoparticles are amorphous. 2.3. Attachment of 5-Hexynoic Acid to Iron Nanoparticles. The procedure for attaching 5-hexynoic acid to iron nanoparticles was based on the previously reported work.20 A mixture of 1:1 wt % ratio of 5-hexynoic acid and iron nanoparticles was dispersed in hexanes and sonicated for 30 min. The nanoparticles were collected by a magnetic rod and washed with ethanol 3 times. The nanoparticles were then dried by a stream of nitrogen gas. 2.4. Attachment of 5-Azidopentanoic Acid to Iron Nanoparticles. A mixture of 1:1 wt % ratio of 5azidopentanoic acid and iron nanoparticles was dispersed in chloroform and sonicated for 15 min.20 The nanoparticles were collected by a magnetic rod and washed with hexanes three times. The nanoparticles were then dried by a stream of nitrogen gas. 2.5. Assembly of Iron Nanoparticles through “Click Chemistry”. 10 mg of Fe nanoparticles functionalized with 5azidopentannoic acid and 10 mg of Fe nanoparticles functionalized with 5-hexynoic acid were dispersed in 5 mL of solvent (4:1 ratio of DMSO:H2O). 2.3 mg of copper sulfate B

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field to measure the magnetization of the iron nanoparticles was 30 kOe. The data points were taken every 20 s.

observed, as well as the satellite features, which indicates that there were no substantial changes of the nanoparticle surface oxide layer following this step. Following surface functionalization of the iron nanoparticles with 5-hexynoic acid, the 707 eV feature and the satellite are still observed. The overall shape of the spectrum is somewhat different from the spectra in plots Figure 1a−c; however, all the key features are retained. It is possible that there is additional surface oxidation occurring during this surface modification step that manifests itself in changes of magnetic properties as will be further discussed in section 3.5. After “click chemistry” between two types of Fe nanoparticles with complementary functional groups, the peak at ∼707 eV representing metallic iron seems to be diminished, as indicated in Figure 1e. However, following all the surface modification steps and “click” attachment, it is also clear that the oxide layer of the nanoparticles assembled became more complex,48,49 and it is more difficult to identify the specific type of iron species on the surface of the resulting material. Figure 2 shows the XPS investigation of the N 1s spectral region following each surface modification step. Figure 2a

3. RESULTS AND DISCUSSION 3.1. XPS Study of Surface Modification of Iron Nanoparticles. Before the functionalization of iron nanoparticles, XPS was used to characterize their surface. On the basis of the previous studies, it is expected that Fe(0) should manifest itself in a peak near 707 eV.18,43 Figure 1 shows the

Figure 1. XPS spectra of Fe 2p region of: (a) the Fe2O3 nanopowder standard sample; (b) iron nanoparticles (25 nm); (c) Fe nanoparticles modified with 5-azidopentanoic acid; (d) Fe nanoparticles modified with 5-hexynoic acid; (e) result of the “click chemistry” reaction between Fe nanoparticles modified with 5-hexynoic acid and Fe nanoparticles modified with 5-azidopetanoic acid in DMSO/water solvent.

corresponding spectral region collected for the iron nanoparticles and for the standard Fe2O3 nanopowder sample. The small peak located at 706.9 eV was assigned to the metallic iron at the core of the nanoparticles. The features with binding energies at 724.7 and 711.3 eV were observed for iron 2p1/2 and iron 2p3/2, respectively,18 which confirmed the presence of an iron oxide layer on the surface of nanoparticles.18,43 The identity of the surface oxide can be confirmed by the comparison with the standard Fe2O3 spectrum in Figure 1a, with a characteristic satellite feature observed approximately 8 eV higher than the main 2p3/2 peak.44 It should be noted that satellite peaks are not observed for Fe3O4.44,45 Thus, the identity of iron oxide on the surface of iron nanoparticles in our studies can be confirmed as primarily Fe2O3. On the basis of a number of previous studies,20,46,47 a carboxylic group can readily react with iron oxides so that it can be used to functionalize the surface oxide layer of the iron nanoparticles. Plots c, d, and e in Figure 1 follow successive modification steps of iron nanoparticles. After functionalization with 5azidopentanoic acid, the peak around 707 eV is still clearly

Figure 2. XPS spectra of the N 1s spectral region with the vertical lines representing the theoretical predictions (and models schematically presented next to corresponding spectra): (a) Fe nanoparticles modified with 5-hexynoic acid; (b) Fe nanoparticles modified with 5-azidopentanoic acid; (c) Fe nanoparticles modified with 5azidopentanoic acid reacted with Fe nanoparticles modified with 5hexynoic acid in DMSO/water overnight; (d) Fe nanoparticles modified with 5-azidopentanoic acid reacted with 5-hexynoic acid in DMSO/water overnight.

shows the control experiment for the Fe nanoparticles modified with 5-hexynoic acid with no nitrogen signal observed following the modification, as expected. Figure 2b corresponds to the Fe nanoparticles modified with azide. We fitted the spectra obtained with three peaks, since it is expected that the three nitrogen atoms in the azide functional group would have distinctly different chemical environments. The peaks were located at 398.9, 400.5, and 404.2 eV, respectively, which C

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oxidized nitrogen-containing functional groups following the reaction, according to our previous work.41,42,57 3.2. FT-IR Investigation of the Iron Nanoparticles Modification and Assembly. The left panel of Figure 3

matched our theoretical prediction (399.4, 400.5, and 403.3 eV) very well; this is perfectly consistent with previously published studies of surfaces modified with azide functional groups.50,51 The peak at 404.2 eV corresponds to the middle electron-deficient nitrogen in the azide functional group. The peaks at 400.5 and 398.9 eV correspond to the outside nitrogen atoms. The 400.5 eV peak is assigned to the azide nitrogen attached to the carbon chain, while the 398.9 eV peak is assigned to the electron-rich terminal nitrogen. The areas of the fitted peaks correspond approximately to the expected 1:1:1 ratio. After the “click chemistry” reaction between iron nanoparticles modified with azide and iron nanoparticles modified with alkyne functional groups, a broad peak at 397.2 eV was recorded (Figure 2c). If the “click” reaction was conducted between the iron nanoparticles modified with 5-azidopentanoic acid and free 5-hexynoic acid, a similar broad feature was observed but shifted to higher binding energy of 398.0 eV, as indicated in Figure 2d. On the basis of our computational investigation of the triazole species, the binding energies for the three nitrogen atoms of the triazole ring were predicted to be at 401.1, 400.1, and 399.0 eV, respectively. Although the exact positions of the XPS features are difficult to calibrate, it is important to realize that the spread of the peaks corresponds to the observed width of the experimental feature, as suggested by the comparison in Figure 2c, where the dashed lines represent the relative positions of the peaks corresponding to three binding energies predicted computationally. These results compare very well with previously reported studies. For example, the “click chemistry” reaction between functionalized single-walled carbon nanotubes with alkyne termination and zinc porphyrins52 also resulted in similar core level shifts. Azide-modified graphite surface reaction with ethynylferrocene was reported to present a broad feature around 399 eV.53 The reaction of azideterminated poly(PEGMA) brushes with 10-undecynoic acid showed slightly higher binding energies (1:2 ratio of 401.5 and 400.2 eV peaks).54 Our experimentally observed XPS feature at 397.2 eV may be affected by the changes in surface morphology and charge redistribution upon assembly.55 One of the better comparisons is probably given by the previously published study of the triazole ring formation during iron oxide stabilization reactions for production of tumor-targeting superparamagnetic iron oxide nanoparticles, where the observed peak was reported at 398.3 eV.56 The exact areas of the observed features are difficult to compare because of different samples used for the measurements and also possibly because of different screening effects in these different materials. One possible point of semiquantitative comparison is between the iron nanoparticles modified with 5azidopentanoic acid and then exposed to 5-hexynoic acid for click reaction and the Fe nanoparticles modified with 5azidopentanoic acid reacted with Fe nanoparticles modified with 5-hexynoic acid. Assuming that the main reaction of the formation of a triazole ring is the same in both cases, the intensity comparison of the N 1s signal in those two samples suggests that nearly 75% of nitrogen atoms reacted by exposing iron nanoparticles modified with azide to 5-hexynoic acid also reacted with Fe nanoparticles modified with 5-hexynoic acid. It should again be emphasized that this is only a semiquantitative comparison. Even more importantly, the absence of XPS features above 400 eV after “click chemistry” helps to rule out the presence of

Figure 3. (a) DFT-predicted infrared spectrum of 5-hexynoic acid on Fe2O3 cluster. (b) DFT-predicted infrared spectrum of 5-hexynoic acid. (c) FT-IR spectra of 5-hexynoic acid-modified iron nanoparticles. (d) DFT-predicted infrared spectrum of 5-azidopentanoic acid on Fe2O3 cluster. (e) DFT-predicted infrared spectrum of 5-azidopentanoic acid. (f) FT-IR spectra of 5-azidopentanoic acid-modified iron nanoparticles.

shows FT-IR spectra of 5-hexynoic acid-modified iron nanoparticles, DFT-predicted spectra of 5-hexynoic acid, and DFTpredicted spectra of 5-hexynoic acid-modified iron nanoparticles. A small peak was observed in the experimental data at 2112 cm−1 representing the stretching of a carbon−carbon triple bond. The position of this feature matches both the literature value20 and our theoretically predicted spectral features. Another peak located at 3288 cm−1 was assigned to the C−H stretching of the ≡C−H functional group.58 Peaks located at 1414 and 1551 cm−1 were assigned to carboxylate stretching, which also matches the DFT calculation and literature values.47 The right panel of Figure 3 compares the FT-IR spectra of 5azidopentanoic acid reacted with Fe nanoparticles and DFTpredicted spectra of a free 5-azidopentanoic acid and 5azidopentanoic acid on a cluster representing iron oxide, respectively. A sharp peak located at 2098 cm−1 corresponds to the asymmetric stretching of the azide moiety.20,59 Most of the other absorptions correspond to the carboxylate species.47,60,61 Complete comparison of the vibrational features can be found in Tables S1−S3 of the Supporting Information that summarizes the key experimental and computational observations. Most importantly, the functionalized iron nanoparticles clearly do not exhibit the absorption features corresponding to the respective carboxylic acids but do exhibit the peaks confirming surface functionalization and retention of the desired functionality on the outside. Thus, on the basis of these FT-IR results, we can conclude that we successfully modified iron nanoparticles with functionalized carboxylic acids to introduce azide or alkyne groups amenable to further modification. Figures 4c,d summarize the key FT-IR measurements describing “click chemistry” reaction between iron nanoD

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nanoparticles modified with 5-azidopentanoic acid and then followed by “click chemistry”, ToF-SIMS was used. Both positive (Figure S2) and negative spectra were taken; however, only the negative spectra are shown in Figure 5. The most useful spectral regions correspond to 26 and 42 amu.

Figure 4. (a) Predicted IR spectra of “click chemistry” reaction between 5-hexynoic acid-modified Fe2O3 and 5-azidopentanoic acidmodified Fe2O3. (b) Predicted IR spectra of “click chemistry” reaction between 5-hexynoic acid and 5-azidopentanoic acid. (c) FT-IR spectra of “click chemistry” reaction between iron nanoparticles modified with 5-azidopentanoic acid and free 5-hexynoic acid. (d) FT-IR spectra of “click chemistry” reaction between iron nanoparticles modified with 5azidopentanoic acid and iron nanoparticles modified with 5-hexynoic acid.

Figure 5. ToF-SIMS results of CN− and N3− regions for (a) aluminum tape as a blank experiment, (b) iron nanoparticles modified with 5hexynoic acid, (c) iron nanoparticles modified with 5-azidopentanoic acid, (d) iron nanoparticles modified with 5-azidopentanoic acid reacted with iron nanoparticles modified with 5-hexynoic acid in DMSO/water solvent overnight, and (e) iron nanoparticles modified with 5-azidopentanoic acid reacted with 5-hexynoic acid in DMSO/ water solvent overnight. Only negative spectra are presented.

particles modified with 5-azidopentanoic acid and molecular 5hexynoic acid and between iron nanoparticles modified with 5azidopentanoic acid and iron nanoparticles modified with 5hexynoic acid, respectively. These spectra are completely consistent with previously published experimental infrared spectroscopy studies of 1,2,3-triazole.62 The azide peak initially located at 2098 cm−1 disappeared following click reaction in both cases. For “click chemistry” between azide-modified iron nanoparticles and alkyne functionality, a new peak located at 1551 cm−1 appeared, which indicated the CN stretching of the triazole ring.20,62 For “click chemistry” between nanoparticles modified with 5-azidopentanoic acid and nanoparticles modified with 5-hexynoic acid, there was a shoulder observed at 1551 cm−1. Although overlapping with carboxylate stretching, this is consistent with the previously observed vibrations of the triazole species, including the absorption at wavenumbers above 1600 cm−1.63,64 In our case, 1618 cm−1 feature was recorded for iron nanoparticles modified with 5-azidopentanoic acid and further “clicked” with 5-hexynoic acid and 1632 cm−1 peak for “click” reaction between complimentarily modified iron nanoparticles. These results were supported by the DFT calculations presented in Figure 4a,b. The stability of the structures prepared by “click chemistry” was confirmed by leaving them overnight in a preheated oven (∼100 °C). Following this procedure, the infrared spectra confirmed that all the absorption features remained unchanged except for the O−H stretching and bending modes corresponding to molecular water. 3.3. ToF-SIMS Studies of the “Click” Reaction between Functionalized Iron Nanoparticles. To further investigate the behavior of the N3 and CN structural elements for the

In the positive ToF-SIMS spectra, it is difficult to distinguish between C2H2O+ and N3 species due to their similar m/z value. On the other hand, for the negative spectra, C2H2O− was substantially less noticeable in the N3 region of the blank sample (Figure 5a). The spectra were calibrated by Fe+ concentration. A blank sample for iron nanoparticles modified with 5-hexynoic acid was also collected. There was essentially no signal observed in the 26 amu region and a very small peak in the 42 amu region due to organic carbon adsorption on the surface of iron nanoparticles. For iron nanoparticles modified with 5-azidopentanoic acid, a sharp CN− peak located at 26.006 and a sharp N3− peak located at 41.011 were clearly observed. After “click chemistry” reaction between azide/alkyne-modified nanoparticles, the N3− peak remained observable although with lower intensity. For the “click chemistry” of 5-azidopentanoic acid-modified nanoparticles reacted with molecular 5-hexynoic acid, the intensity of the peak remained at ∼11%, and for “click chemistry” between iron nanoparticles modified with 5azidopentanoic acid and iron nanoparticles modified with 5hexynoic acid, the peak remained at ∼25%. In order to understand these results, FT-IR studies presented in the previous section have to be recalled. These FT-IR results showed that following “click chemistry” reaction there was almost no peaks observed at ∼2100 cm−1 for the azide functionality. Thus, the remaining N3− species in the ToFSIMS spectra for this case must correspond to the N3 fragment E

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of the triazole ring. At the same time, following the evolution of the CN− species after “click chemistry” reaction with either 5hexynoic acid or 5-hexynoic acid-modified nanoparticles, the peak shifted to higher m/z at 26.009 due to the triazole ring formation, which also gives rise to a very closely located peak corresponding to C2H2−. In other words, C2H2− species consisting of 12C and 1H isotopes should be located at 26.016 amu. A very small peak at this position is observed on the background spectra likely from carbonaceous impurities. As expected, the CN− fragment corresponding to the triazole ring overlaps the much smaller feature corresponding to the C2H2− species from the same structure, which shifts the overall position of the peak (CN− and C2H2− species) to slightly higher m/z. Thus, the CN− species that should be observed at 26.0036 amu and is indeed observed at this position for azideterminated nanoparticles is shifted to 26.009 amu following “click chemistry”. For the 26 amu region, the peak intensity following “click chemistry” of the iron nanoparticles modified with 5-azidopentanoic acid reacted with the iron nanoparticles modified with 5-hexynoic acid remained at ∼60% of that before reaction, which is consistent with the N3− region with peak intensity remaining at ∼25% because the peak recorded in the 26 amu region was a combination of CN and C2H2 species. For the “click chemistry” between iron nanoparticles with 5azidopentanoic acid and molecular 5-hexynoic acid, the peaks in the 26 region remained at ∼20% of starting point matching the changes in N3 region. Thus, based on XPS and FT-IR studies, the “click” reaction between iron nanoparticles modified with 5-azidopentanoic acid reacted with 5-hexynoic acid was more efficient comparing to the “click chemistry” between functionalized nanoparticles. This would be expected simply because of the steric reasons. However, the resulting peak intensity seemed to be smaller in ToF-SIMS of the “click chemistry” for the 5-azidopentanoic acid-modified nanoparticles reacted with pure 5-hexynoic acid. One of the obvious explanations for this discrepancy would be a change of particle morphology and the microscopy studies in the next section confirm this assumption. However, overall, together with XPS and FT-IR results, ToF-SIMS spectra prove the successful “click chemistry” reaction between the functionalized iron nanoparticles. 3.4. SEM Investigation of the Surface Modification and “Click Chemistry” for Iron Nanoparticles. SEM images were collected after each step of nanoparticle surface modification to follow the morphology changes in the produced material. On the basis of the results presented in Figure 6a−f, it can be concluded that regardless of surface chemical functionalization or absence thereof, the morphology of the iron nanoparticles produced remains essentially the same and shows the size of nanoparticles to be approximately 25 nm. In other words, performing chemical modification of iron nanoparticles with 5-azidopentanoic acid or with 5-hexynoic acid does not cause noticeable morphology change. The other question is whether there are any changes following “click chemistry”. Figure 7 compares the same images as in Figure 6 (as a reference) recorded for unmodified iron nanoparticles (Figure 7a,b) with the studies of chemical modification. Figure 7c,d examines the morphology of iron nanoparticles modified with 5-azidopentanoic acid and reacted with molecular 5-hexynoic acid. There is no obvious change in morphology, but there also remains a possibility that following this modification scheme, the carboxylic group at the end of the newly obtained functional layer could potentially react with

Figure 6. SEM images of iron nanoparticles after each step of modification: (a) 25 nm iron nanoparticles, magnification 50K; (b) 25 nm iron nanoparticles, magnification 200K; (c) 25 nm iron nanoparticles modified with 5-azidopentanoic acid, magnification 50K; (d) 25 nm iron nanoparticles modified with 5-azidopentanoic acid, magnification 200K; (e) 25 nm iron nanoparticles modified with 5-hexynoic acid, magnification 50K; (f) 25 nm iron nanoparticles modified with 5-hexynoic acid, magnification 200K.

open sites on a surface of iron nanoparticles. To eliminate this possibility, we performed a “click” reaction between iron nanoparticles modified with 5-hexynoic acid and benzyl azide presented in Figure 7e,f. In this set of studies any possibility of interaction of the outermost layer of a modified iron nanoparticle with another nanoparticle is eliminated, since the outermost layer is terminated with an unreactive benzyl group. Finally, following “click reaction” between iron nanoparticles with two different functionalities, the average size of the clustered nanoparticle features increased to approximately 100 nm and the morphology changed substantially, as indicated by the SEM images in Figure 7g,h. Thus, only in the case of complementary functionalized iron nanoparticles do we observe the morphology change, while the “click chemistry” by itself does not affect the morphology substantially. This set of experiments confirms the role of “click reaction” in controlling the morphology change and eliminates the possibility that this change is caused by surface redox processes or dissolution/solvent effects during “click” modification. In addition to the SEM measurements, we have performed selected TEM studies that are fully consistent with the SEM analysis and confirm that following “click chemistry” reaction, the formation of nanoparticle agglomerates is observed (see Figure S4). However, largely because of this aggregation,65,66 no additional information about individual nanoparticles could be obtained. We have also attempted to attach 100 nm Fe nanoparticles and 25 nm Fe nanoparticles in order to observe the specific changes in morphology (i.e., large nanoparticles surrounded by smaller nanoparticles). However, due to the wide size distribution of the 100 nm nanoparticles, it was impossible to clearly distinguish the morphology differences for the 100 nm nanoparticles and 25 nm nanoparticles. A series of control SEM images presented in the Supporting Information (iron nanoparticles kept in DMSO/water overnight; iron nanoF

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Figure 8. Magnetization measurements for (a) the best result of Fe nanoparticles modified with 5-azidopentanoic acid reacted with Fe nanoparticles modified with 5-hexynoic acid in DMSO/water solvent overnight; (b) iron nanoparticles modified with 5-hexynoic acid; (c) Fe nanoparticles modified with 5-azidopentanoic acid; and (d) unmodified iron nanoparticles.

Fe nanoparticles is most likely caused by the presence of the nonmagnetic organic molecular layer as well as by oxidation of the topmost surface layer of the nanoparticles during the surface modification process. It has been previously suggested that smaller iron nanoparticles (less than 60 nm in diameter) have propensity to further oxidation compared to larger particles.67 However, after “click chemistry” reaction in DMSO/water, the magnetization decreased substantially. Highly dependent on the specific experimental conditions (including room temperature and humidity in the laboratory), our best magnetic value for the material produced following click chemistry was ∼50 emu/g, which corresponds to less than half of the initial magnetization. Overall, these results suggest that despite the success of the “click chemistry” in selective linking of iron nanoparticles and the desired change in the morphology of the produced material, the magnetic properties of the resulting mixture are diminished compared to the starting iron nanoparticles, which is very likely caused by oxidation of iron nanoparticles in the course of chemical modification. It must be emphasized that the iron nanoparticles used in these studies were amorphous. XRD experiments before and after “click chemistry” suggest that the material remains amorphous after assembly (Figure S3). This means that the produced surface oxide is also very likely amorphous, which would result in very low magnetization values.68−71 That is why the retention of the magnetic properties, albeit not complete, is of particular importance in this study. Thus, the remedies for retaining magnetic properties while assembling magnetic materials by chemical functionalization are clear: (1) The functionalization and assembly should be extremely efficient and fast to avoid surface oxidation. (2) The solvents used should not be a substantial source of oxygen for nanoparticle oxidation. (3) Ideally, purely thermal or photochemical methods that do not require catalysts (and in the case of “click” reaction water-based solvents to dissolve such catalysts) should be used. Further studies focused on the use of solvents that do not promote oxidation of iron nanoparticles and faster “click chemistry” reactions that would minimize the exposure of iron nanoparticles to the solvent in the first place are necessary for realistic practical applications.

Figure 7. SEM studies of the effect of “click chemistry” on the morphology of the nanomaterial produced: (a) 25 nm iron nanoparticles, magnification 50K (reference, same as in Figure 6a); (b) 25 nm iron nanoparticles, magnification 200K (reference, same as in Figure 6b); (c) 25 nm iron nanoparticles modified with 5azidopentanoic acid and reacted with 5-hexynoic acid, magnification 50K; (d) 25 nm iron nanoparticles modified with 5-azidopentanoic acid and reacted with 5-hexynoic acid, magnification 200K; (e) 25 nm iron nanoparticles modified with 5-hexynoic acid reacted with benzyl azide, magnification 50K; (f) 25 nm iron nanoparticles modified with 5-hexynoic acid reacted with benzyl azide, magnification 200K; (g) “click chemistry” between 25 nm iron nanoparticles modified with 5hexynoic acid and 25 nm iron nanoparticles modified with 5azidopentanoic acid, magnification 50K; (h) “click chemistry” between 25 nm iron nanoparticles modified with 5-hexynoic acid and 25 nm iron nanoparticles modified with 5-azidopentanoic acid, magnification 200K.

aprticles kept in DMSO/water with all the catalyst overnight; iron nanoparticles modified with azide kept in DMSO/water overnight; iron nanoparticles modified with alkyne in DMSO/ water overnight shown in Figure S2) were taken, showing that there were no apparent morphology changes without the “click chemistry” reaction. On the basis of all of these studies, we conclude that the increase in apparent particle size was only observed following “click” reaction between complementarily functionalized nanoparticles. 3.5. VSM Measurements for Functionalized Iron Nanoparticles and Opportunities To Retain Magnetic Properties Following Chemical Functionalization. To investigate the changes in magnetic properties after each step of modification, a VSM study was conducted, as summarized in Figure 8. Fe nanoparticles showed a magnetization of 106 emu/ g before modification. Following modification with 5azidopentanoic acid, the nanoparticles retained most of their magnetization at around 100 emu/g. Fe nanoparticles modified with 5-hexynoic acid retained approximately 80% of the original magnetization. The slight decrease of the magnetization of the G

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(5) Lu, A. H.; Salabas, E. L.; Schuth, F. Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application. Angew. Chem., Int. Ed. 2007, 46, 1222−1244. (6) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Self-Assembly of Nanoparticles into Structured Spherical and Network Aggregates. Nature 2000, 404, 746−748. (7) Mori, H.; Muller, A. H. E.; Klee, J. E. Intelligent Colloidal Hybrids Via Reversible pH-Induced Complexation of Polyelectrolyte and Silica Nanoparticles. J. Am. Chem. Soc. 2003, 125, 3712−3713. (8) Janczewski, D.; Tomczak, N.; Liu, S.; Han, M.-Y.; Vancso, G. J. Covalent Assembly of Functional Inorganic Nanoparticles by “Click” Chemistry in Water. Chem. Commun. (Cambridge, U. K.) 2010, 46, 3253−3255. (9) Peng, J.; Yu, P.; Zeng, S.; Liu, X.; Chen, J.; Xu, W. Application of Click Chemistry in the Fabrication of Cactus-Like Hierarchical Particulates for Sticky Superhydrophobic Surfaces. J. Phys. Chem. C 2010, 114, 5926−5931. (10) Chan, E. W. L.; Yu, L. P. Chemoselective Immobilization of Gold Nanoparticles onto Self-Assembled Monolayers. Langmuir 2002, 18, 311−313. (11) Burke, N. A. D.; Stover, H. D. H.; Dawson, F. P. Magnetic Nanocomposites: Preparation and Characterization of PolymerCoated Iron Nanoparticles. Chem. Mater. 2002, 14, 4752−4761. (12) Carpenter, E. E. Iron Nanoparticles as Potential Magnetic Carriers. J. Magn. Magn. Mater. 2001, 225, 17−20. (13) Tartaj, P.; Morales, M. D.; Veintemillas-Verdaguer, S.; Gonzalez-Carreno, T.; Serna, C. J. The Preparation of Magnetic Nanoparticles for Applications in Biomedicine. J. Phys. D: Appl. Phys. 2003, 36, R182−R197. (14) Vidal-Vidal, J.; Rivas, J.; Lopez-Quintela, M. A. Synthesis of Monodisperse Maghemite Nanoparticles by the Microemulsion Method. Colloids Surf., A 2006, 288, 44−51. (15) Williams, D. N.; Ehrman, S. H.; Pulliam Holoman, T. R. Evaluation of the Microbial Growth Response to Inorganic Nanoparticles. Nat. Biotechnol. 2006, 4, 3−3. (16) Amiens, C.; Chaudret, B. Organometallic Synthesis of Nanoparticles. Mod. Phys. Lett. B 2007, 21, 1133−1141. (17) Li, X. Q.; Zhang, W. X. Iron Nanoparticles: The Core-Shell Structure and Unique Properties for Ni(II) Sequestration. Langmuir 2006, 22, 4638−4642. (18) Sun, Y. P.; Li, X. Q.; Cao, J. S.; Zhang, W. X.; Wang, H. P. Characterization of Zero-Valent Iron Nanoparticles. Adv. Colloid Interface Sci. 2006, 120, 47−56. (19) Bhargava, G.; Gouzman, I.; Chun, C. M.; Ramanarayanan, T. A.; Bernasek, S. L. Characterization of the “Native” Surface Thin Film on Pure Polycrystalline Iron: A High Resolution XPS and TEM Study. Appl. Surf. Sci. 2007, 253, 4322−4329. (20) White, M. A.; Johnson, J. A.; Koberstein, J. T.; Turro, N. J. Toward the Syntheses of Universal Ligands for Metal Oxide Surfaces: Controlling Surface Functionality through Click Chemistry. J. Am. Chem. Soc. 2006, 128, 11356−11357. (21) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.; Rafailovich, M.; Sokolov, J.; Gedanken, A. Self-Assembled Monolayers of Alkanesulfonic and Phosphonic Acids on Amorphous Iron Oxide Nanoparticles. Langmuir 1999, 15, 7111−7115. (22) Lattuada, M.; Hatton, T. A. Functionalization of Monodisperse Magnetic Nanoparticles. Langmuir 2007, 23, 2158−2168. (23) Zhang, L.; He, R.; Gu, H. C. Oleic Acid Coating on the Monodisperse Magnetite Nanoparticles. Appl. Surf. Sci. 2006, 253, 2611−2617. (24) Fournier, D.; Hoogenboom, R.; Schubert, U. S. Clicking Polymers: A Straightforward Approach to Novel Macromolecular Architectures. Chem. Soc. Rev. 2007, 36, 1369−1380. (25) Binder, W. H.; Sachsenhofer, R. “Click” Chemistry in Polymer and Materials Science. Macromol. Rapid Commun. 2007, 28, 15−54. (26) Acikgoz, S.; Aktas, G.; Inci, M. N.; Altin, H.; Sanyal, A. FRET Between BODIPY Azide Dye Clusters within PEG-Based Hydrogel: A

4. CONCLUSIONS In order to test the feasibility of using “click” reaction for assembly of iron nanoparticles, while retaining the magnetic properties of the material produced, two batches of iron nanoparticles were successfully modified with 5-hexynoic acid and 5-azidopentanoic acid, respectively. The success of this modification procedure as well as the completion of the “click” process following this preparation was demonstrated by combining FT-IR, XPS, and ToF-SIMS with DFT analysis. SEM images showed a substantial morphology change following “click chemistry”. The magnetometry study confirmed that the magnetic properties, although diminished, were promising for further applications of this approach to materials assembly. Most importantly, this study demonstrates that the “click” reaction in DMSO/water solvent led to the retention of magnetic properties. The decrease in magnetization was most likely caused by the surface oxidation of functionalized iron nanoparticles. In order to make the surface functionalization/ “click” assembly approach to work for retaining physical properties of materials that are relatively easily oxidizable, further work has to focus on finding a proper solvent to exclude water and oxygen and to determine a faster “click chemistry” reactions to decrease the exposure time to water and oxygen.

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ASSOCIATED CONTENT

* Supporting Information S

Additional ToF-SIMS and SEM studies, XRD investigation, complete assignment of the vibrational features observed in the experimental studies supported with DFT calculations. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel (302) 831-1969; Fax (302) 831-6335; e-mail andrewt@ udel.edu (A.V.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by ARPA-E project and by the National Science Foundation (CHE 1057374). Y.L. thanks Mr. Zachary Voras (Department of Chemistry and Biochemistry, University of Delaware) for his help with the XPS and ToFSIMS measurements. We gratefully acknowledge Mr. Niantzu Suen and Professor Svilen Bobev for their help with collection and discussion of the XRD data and Professor Chaoying Ni, Mr. Frank Kriss, and Dr. Fei Deng for their help with electron microscopy studies.

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REFERENCES

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