Preparation of Fluorous Solvent-Dispersed Fe3O4 Nanocrystals: Role

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Preparation of Fluorous Solvent-Dispersed FeO Nanocrystals: Role of Oxygen in Ligand Exchange Youngsun Kim, Sangyoup Lee, and Sehoon Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00526 • Publication Date (Web): 27 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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Preparation of Fluorous Solvent-Dispersed Fe3O4 Nanocrystals: Role of Oxygen in Ligand Exchange Youngsun Kim,a Sangyoup Lee,b and Sehoon Kima* a) Center for Theragnosis, Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14gil, Seongbuk-gu, Seoul 02792, Republic of Korea b) Center for Bionics, Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea

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ABSTRACT

Perfluorinated ligand-passivated Fe3O4 nanocrystals were prepared through a biphasic ligand exchange method. It was found that dissolved oxygen in the reaction media predominantly determined the degree of ligand exchange and the resultant dispersion property of nanocrystals in a fluorous solvent. X-ray photoelectron spectroscopic analyses revealed that dissolved oxygen molecules bind to the surface iron species of nanocrystals in competition with the carboxylate moiety of ligands during the exchange reaction, lowering the degree of ligand exchange and colloidal stability significantly. Reducing the oxygen content of the fluorous phase by N2 bubbling was found to result in a highly stable dispersion of phase-transferred Fe3O4 nanocrystals with a single-particle size distribution maintained for a few months.

KEYWORDS Ligand exchange, Perfluorinated materials, Colloidal stability, Molecular oxygen, Iron oxide nanocrystals,


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INTRODUCTION Perfluorinated materials have widely been used as ultrasound imaging agents,1,2 coating matrices in organic-based film lamination,3,4 and working fluids of organic Rankine cycle.5–7 This material category, especially for those served as a liquid phase, has several attractive aspects: for instance, high chemical and thermal stability, low heat of vaporization, low refractive index, and low viscosity, compared to hydrocarbon counterparts in general. Being of specific interest, fluorous colloidal systems consisting of inorganic nanocrystals (NCs) and perfluorinated solvents have been studied. Quantum dots with compositions of CdSe/ZnS8,9 and InGaP/ZnS10 were integrated into water-dispersed perfluorocarbon emulsion droplets as multimodal bio-imaging contrast agents, and other fluorous colloids based on gold11 and iron oxide12 NCs were also reported. In developing nanocolloid systems, uniform dispersion and colloidal stability are key requirements to ensure the size-motivated material characteristics. A general and simple strategy for the preparation of fluorous NC colloids is to exchange ligands on the crystal surface with perfluorinated ones. In this approach, colloidal properties would be correlated to how effectively NCs are passivated by perfluorinated ligands. Ligand exchange is subjected to the interfacial nature between ligands and the crystal surface. In particular, the role of dissolved oxygen in the medium is to be considered when the ligand exchange reaction is conducted in perfluorinated solvents that generally have high oxygen solubility. For example, the dissolved oxygen content in perfluorodecalin is 10-20 times higher than that in water or organic solvents, being similar to the value in atmosphere.13 Therefore, the interaction between the crystal surface and surrounding molecular oxygen would be a more predominant governing factor in fluorous colloidal systems.


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Here, we investigate how dissolved oxygen molecules in the ligand exchange step affect the exchange degree and concomitant colloidal stability. A biphasic ligand exchange scheme was adopted in this study, which is composed of an organic phase (toluene) with oleic acid-capped Fe3O4 NCs and an immiscible fluorous phase (perfluorodecalin) with perfluorinated ligands. Fe3O4 was chosen for its well-established synthetic methods and superparamagnetic properties that will be utilized in further applications. Based on the reaction condition-dependent atomic ratios and temporal evolution of hydrodynamic size of colloids, we found that removal of oxygen from the reaction media can effectively enhance the degree of ligand exchange and thus colloidal stability of the resulting NC dispersion under the air-equilibrated storage condition. The oxygen species that bind to the surface iron ions were identified as a main factor for determining the degree of ligand exchange.

EXPERIMENTAL SECTION Materials and Instrumentation. Iron(III) oxide (hydrated, FeO(OH)), oleic acid (OA), 1octadecene (ODE), triethylamine (TEA), 2H,2H,3H,3H-heptadecafluoroundecanoic acid (PFL), 1,1,1,2,2,3,4,5,5,5-decafluoro-3-methoxy-4-(trifluoromethyl)pentane (FS), and perfluorodecalin (PFD) were purchased from Sigma-Aldrich Inc., Alfa Aesar, or Tokyo Chemical Industry Co., Ltd. All the reagents were used as received. Transmission electron microscopy (TEM) was conducted with Tecnai F20 G2 from FEI, and powder x-ray diffraction (XRD) patterns were obtained using D8 Advance from BRUKER. Fourier transform infrared spectroscopy (FTIR) was conducted with Thermo Mattsonmodel Infinity Gold FT-IR. Temporal evolution of the average hydrodynamic size of NCs was recorded by the dynamic light scattering (DLS) method at 25 oC


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(Zen 3600 Nano-ZS, Malvern Instruments). Atomic ratio and species survey were conducted with x-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, ULVAC PHI) with monochromator Al Ka (1486.6 eV) anode (24.5 W, 15 kV) for x-ray source. All the XPS spectra were calibrated with adventitious C 1s peak at 284.6 eV. Synthesis of OA-capped Fe3O4. Organic-dispersed Fe3O4 NCs (11 nm in diameter) were synthesized through a literature method.14 Briefly, 0.089 g (1.00 mmol) of FeO(OH) and 1.13 g (4.00 mmol) of OA were mixed in 5 g of 1-octadecene, and the mixture was stirred for 60 min at 320 oC under nitrogen atmosphere. NCs were separated through extraction with 1:1 chloroform/methanol (v/v). After centrifugal washing with 1:10 chloroform/acetone (v/v), NCs were re-dispersed in hexane for storage. Biphasic Ligand Exchange. To a 5 mL anhydrous toluene solution of OA-capped Fe3O4 NCs (10 mg/mL) was added 3 mL PFD containing 200 mg of PFL and 0.2 ml of TEA. The toluene/PFD biphasic mixture was then stirred for 2 h at 60 oC. For the purpose of this study, 10 mL of PFD was pretreated by bubbling it with nitrogen gas or air flow for 60 min in an ice bath. After the phase transfer, the upper toluene layer was decanted and NCs in the PFD phase was collected by magnet with subsequent washings with toluene and PFD. Final NCs were obtained after being dried in vacuum, and dispersed in FS for the colloidal stability measurement.

RESULTS AND DISCUSSION As shown in Figure 1, OA-capped NCs (OA-NCs) were transferred from a pristine toluene phase to a PFD phase in the presence of excess perfluorinated ligands (PFL). Here, the reaction


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was conducted under nitrogen atmosphere. At the initial stage of reaction, it was observed when stirring is halted, that most of the NCs were swiftly moved to the interface between the two phases. According to a change in the optical density of toluene phase, around 85 % of the pristine NCs were extracted from that phase (see Figure S1 in the Supporting Information). In the course of time, those NCs were gradually transferred to the PFD phase, and the transfer rate was recorded as 93 % after 2 h. The successful phase transfer in the given condition implies that ligands of the toluene-dispersed OA-NCs were exchanged sufficiently with PFL in a statistical manner due to its high concentration in PFD. Note that we used PFL possessing a crystalcoordinating moiety of carboxylic acid, analogous to that of OA, to exclude the possibility of unwanted effects (e.g. changes in ligand exchange degree due to difference in binding affinity between pristine and exchanging ligands). We chose PFD as a reaction medium for its high capacity of oxygen and immiscible character with toluene. After removal of excess ligands and other reagents, separated NCs were found to be well dispersed in a perfluorinated ether solvent (FS) but not in organic solvents such as chloroform, hexane, and toluene.

Figure 1. Schematic diagram and photographs of phase transfer of Fe3O4 NCs by ligand exchange in the presence of excess perfluorinated ligand (PFL) molecules, under nitrogen atmosphere.


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The presence of PFL on the surface of ligand-exchanged NCs could be identified by FTIR spectroscopy (see Figure S2 in the Supporting Information) as well as indirectly by the observed phase transfer phenomenon (see photographs in Figure 1). Figure 2 shows TEM images and XRD patterns of pristine OA-NCs and ligand-exchanged NCs (pF- NCs). The average diameter of NCs was retained before (11.3 ± 1.1 nm) and after (11.8 ± 1.1 nm) ligand exchange. In addition, no distinct alteration in colloidal morphology and crystal structure was observed by ligand exchange under nitrogen atmosphere. The XRD patterns of both OA-NCs and pF-NCs well coincide with the magnetite crystal structure (PDF#74-0748).


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Figure 2. TEM images of OA-NCs (A) and pF-NCs (B) with corresponding size histograms (insets), and XRD patterns thereof (C).

We then investigated the effect of dissolved oxygen in the perfluorinated reaction medium on the dispersion property of the resulting pF-NCs. Ligand exchange was conducted under two different (oxygen-deficient and oxygen-rich) conditions with PFD that was pretreated with N2 or air purging (N2-pF-NCs and air-pF-NCs, respectively). It turned out that both cases undergo


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phase transfer of NCs into PFD. For air-pF-NCs, a transfer rate was calculated as ~ 92 % after 2 h reaction (see Figure S1 in the Supporting Information). After completion of ligand exchange, both samples were dispersed in FS and air-equilibrated to reach the dissolved oxygen-rich storage condition. Figure 3a describes temporal changes in the average hydrodynamic size of two types of pF-NCs under the air-equilibrated storage condition. For measurement, the storage stock of N2-pF-NCs (10 mg/mL) was diluted by 100 times in FS. The particle concentration of air-pFNCs was adjusted to have an optical density similar to the diluted N2-pF-NCs. In the case of asprepared N2-pF-NCs, the average hydrodynamic size was reliably measured as 15.0 ± 2.2 nm, comparable to that of OA-NCs in toluene (16.2 ± 2.3 nm). In contrast, as-prepared air-pF-NCs showed a wide deviation in size along serial measurements, ranging from 20 to 80 nm. Some regions of aggregated crystals were observed in TEM images of as-prepared air-pF-NCs (Figure S3), indicative of relatively poor dispersion stability of air-pF-NCs in FS. As shown in Figure 3a, N2-pF-NCs maintained the initial size distribution stably in the oxygen-rich storage condition for a few months (Figure S4), whereas air-pF-NCs appeared to gradually agglomerate with time; the average size of air-pF-NCs was kept increasing larger than 100 nm after 7 d of storage with apparent precipitation (Figure 3b). All these results suggest that dissolved oxygen content at the moment of ligand exchange reaction (not during storage) exerts a significant influence on the dispersion stability of the phase-transferred pF-NCs.


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Figure 3. (A) Temporal evolution of average hydrodynamic diameters of pF-NCs in FS under the air-equilibrated condition. Each value was obtained by averaging the results of sequential 50 measurements. (B) Photographs of pF-NCs dispersions on 7th day of storage under the airequilibrated condition.

In order to analyze factors that affect the dispersion property, surface elemental concentrations for two types of pF-NCs were examined by XPS as summarized in Table 1 (for whole XPS spectra and raw data of atomic concentrations, see Figure S5 and Table S1 in the Supporting Information). Two types of pF-NCs show a large difference in F/Fe and C/Fe ratios. Those ratios of N2-pF-NCs are around six-fold higher than those of air-pF-NCs. Considering the chemical structure of PFL (C11H5F17O2), higher values for C/Fe than F/Fe would be ascribed to the presence of OA remnant, which is further confirmed by the presence of remnant C-H stretch vibrations characteristic of the methyl terminus of OA (see Figure S2 in the Supporting Information). Taking the carbon content from OA (C18H34O2) into account, it is calculated that surface-adsorbed ligands are composed of approximately 70 % PFL and 30 % OA in number for both pF-NCs. Assuming that iron contents of the crystallites remain the same during ligand


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exchange for both cases, the ratios of F/Fe and C/Fe would directly reflect the amounts of surface-adsorbed ligands (PFL/OA). Therefore, the N2/air values for F/Fe and C/Fe suggest that air-pF-NCs have ~6 times less surface-bound ligands than N2-pF-NCs, implying less efficient or hindered ligand exchange under the oxygen-rich condition. Regarding oxygen contents, contributions would come from crystal lattice, ligands and possibly surface-bound non-carbon oxygen species (e.g., O2). As in the cases of fluorine and carbon contents, the amount of organic oxygens belonging to the carboxyl group of ligands (PFL/OA) would be similarly (~6 times) less in air-pF-NCs. However, the overall oxygen content (O/Fe) of air-pF-NCs was shown to be only 1.5 times less than that of N2-pF-NCs, implying the abundance of non-ligand oxygen on the surface of air-pF-NCs. Lattice or adsorbed non-carbon oxygen species could be probable candidates.

Table 1. Surface elemental ratios surveyed from wide-scan XPS for as-prepared N2-pF-NCs and air-pF-NCs Atomic ratioa F/Fe

C/Fe

O/Fe

N2-pF-NCs

20.3

22.2

3.73

air-pF-NCs

3.07

3.41

2.53

N2/airb

6.61

6.51

1.47

a

Surveyed spectral regions are F 1s, Fe 2p, C 1s and O 1s.

b

N2/air corresponds to the values obtained by dividing each atomic ratio for N2-pF-NCs with

that for air-pF-NCs.


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The origin of oxygen content disparity was examined by narrow XPS scan of the O 1s spectral regions. Here, OA-NCs were used as a standard for matching peak binding energy (BE) of components with literature values. As shown in Figure 4a-c, spectra of the O 1s regions are well fitted with four peaks centered at 530.0, 531.1, 532.1 and 533.2 eV that are sequentially labeled as component I~IV. Component I is assigned as lattice oxygen anions (OL2-) from Fe3O4 NCs. According to literatures, BE for component II is attributable to chemisorbed oxygen species (Oadsδ-) on the crystal surface or hydroxyl species (OH-) from lattice or surface.15,16 The possibility of OH- is likely excluded because lattice OH- is a profound component in iron(III) oxide-hydroxide (FeOOH) crystals, and because PFD has extremely low solubility of water for surface adsorption. Thus, in our system, it is more reasonable to identify surface Oadsδ- as a main contribution to component II. BEs for component III and IV lay in the range that corresponds to organic oxygen species like C-O and/or C=O or adsorbed H2O.16,17 Adsorbed H2O species is again likely excluded because of poor solubility of water in PFD. We assigned component III as monodentate carboxylate (C-O/C=O) and component IV as bidentate carboxylate (O-C=O), both of which stem from surface-adsorbed ligand molecules.17 Relatively low fraction of carboxylate moieties measured in pristine OA-NCs is ascribable to sparse ligand passivation by loss of OA during the harsh centrifugal washing step that was adopted for the sake of facile ligand exchange, and/or by steric hindrance of OA due to its tilted bulky structure. In the case of N2-pF-NCs and air-pF-NCs, one of the most distinct features is the fraction of component II (Figure 4d). A high level of component II in air-pF-NCs indicates that non-carbon oxygen species (Oadsδ-) outnumber organic ligand oxygens and are responsible for the high surface concentration of non-ligand oxygens for air-pF-NCs. In the case of N2-pF-NCs, however, it was shown that main oxygen


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species on the surface correspond to organic oxygens belonging to the carboxyl group of PFL and OA (component III and IV). Here, it is speculated that in the ligand exchange step, O2 dissolved in PFD, responsible for Oadsδ-, and carboxylates of PFL are in competition for coordination to the given available iron sites on the crystal surface. In this context, it is unambiguous that N2 bubbling of PFD can promote the ligand exchange with PFL by reducing the concentration of competing O2 molecules, leading to efficient PFL passivation and stable dispersion of NCs in a fluorous solvent. In contrast, aerated PFD with a higher oxygen content would lower the degree of ligand exchange by disturbing the ligand exchange reaction. As shown in Figure 3, it turned out that the resulting low ligand coverage of air-pF-NCs seems to be enough for initial phase transfer of NCs to PFD but insufficient to retain the dispersion stability in FS.


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Figure 4. (A-C) Narrow-scan XPS spectra of O 1s regions for the surface of OA-NCs, N2-pFNCs and air-pF-NCs. (D) Summary on BEs and relative fractions of component peaks for each sample.

Figures 5a-c show Fe 2p3/2 spectral regions representing coordination sites for oxygen species. Fe3O4 crystal is a high-spin multivalent (Fe3+ at octahedral and tetrahedral sites; Fe2+ at octahedral sites) iron oxide compound known to have a broad envelope of Fe 2p3/2 spectrum.18 This spectral region of such materials is featured with narrow multiplet components. We conducted peak separation by fitting with a boundary condition of full-width at half-maxima


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(FWHM) between 0.8 and 1.2 eV for all components (for fitting details and whole spectra of Fe 2p regions, see Table S2 and Figure S6 in the Supporting Information). All spectra for three types of samples could be fitted with seven to eight components. Two lower-BE peaks (component I) and the rest higher-BE peaks (component II) were referred to as lower-oxidationstate (Fe2+) and higher-oxidation-state (Fe3+) species, respectively. Even though deconvolution of the broad Fe 2p3/2 spectral shape is somewhat complicated due to the presence of satellite peaks or surface contributions as well as overlaps between multiplet peaks, our curve fitting results seem to be reasonable with the integrated Fe3+/Fe2+ ratio of 2.1 for pristine OA-NCs (Figure 5d). Note that an ideal stoichiometry for Fe3+/Fe2+ in the magnetite structure is 2. However, all the experimental values of Fe3+/Fe2+ were estimated over 2, and the relative fractions of higheroxidation-state (Fe3+) species increased in the order of OA-NCs, N2-pF-NCs and air-pF-NCs. Such a tendency is in accordance with the increasing trend of surface-adsorbed oxygens over overall non-ligand oxygens (Oadsδ-/(Oadsδ- + OL2-)) observed in the O 1s spectral regions (Figure 4d). All these results suggest that adsorbed Oadsδ- species oxidize surface iron ions to the higheroxidation-state. In a schematic view, O2 molecules dissolved in PFD physically adsorbs to iron ions of the crystal surface in competition with carboxylates of ligands. Once adsorbed, they undergo charge exchange with crystals19 or take partial charges from tunneling electrons,15 resulting in oxidation of iron ions and stable iron-to-oxygen bond formation. Such charge transfer reactions would explain why component II in the O 1s spectra has a BE close to the lattice OL2- (chemical shift of around +1 eV).15 Due to the formation of iron-to-oxygen bonds and a resultant quasi-lattice oxygen layer on the crystal surface, carboxylates of PFL would be excluded from the coordination reaction when the oxygen concentration is high in the reaction media. It was observed that once ligands densely passivate the NCs, the presence of O2 does not


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significantly affect the already-formed ligand/crystal interface of N2-pF-NC, with the resulting colloidal dispersion in FS kept stable even after a few months of storage under the airequilibrated ambient condition.

Figure 5. (A-C) Narrow-scan XPS spectra of Fe 2p3/2 regions for the surface of OA-NCs, N2-pFNCs and air-pF-NCs. (D) Summary on BEs and relative fractions of component peaks for each sample. Percent concentrations correspond to partial summations of integrated components I and II.


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CONCLUSIONS We prepared perfluorinated ligand-passivated Fe3O4 NCs by organic-to-fluorous phase transfer of NCs through the biphasic ligand exchange reaction. When the concentration of oxygen gas dissolved in the fluorous reaction solvent was reduced by N2 bubbling, the resulting phasetransferred NCs exhibited highly stable colloidal dispersion in a fluorous solvent, retaining the average hydrodynamic size distribution for several months. In contrast, NCs obtained under the air-purged oxygen-rich condition were shown to readily aggregate and precipitate with time. The XPS analyses revealed that the adsorbed O2 molecules occupy the surface sites for ligand coordination and form stable iron-to-oxygen bonds by partial charge transfer, to limit the degree of ligand exchange under the aerated oxygen-rich reaction condition. These results conclude that the oxygen content of fluorous reaction media is a key factor that governs the degree of ligand exchange and concomitant colloidal stability of fluorous solvent-dispersed Fe3O4 NCs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], tel: +82-2-958-5924, fax: +82-2-958-5909 (S. Kim) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by grants from the Korea Institute of Energy Technology Evaluation and Planning (No. 20142020103790) and the Intramural Research Program of KIST. Supporting Information Temporal changes in the phase transfer ratio during ligand exchange, FTIR spectra, TEM and XRD data of air-pF-NCs, size distribution profiles by DLS, full-range XPS spectra and corresponding elemental contents, and fitting details on XPS Fe 2p3/2 regions. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1)

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Langmuir

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Table of Contents Graphic


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Preparation of Fluorous Solvent-Dispersed Fe3O4 Nanocrystals: Role

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