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Aqueous Superparamagnetic Magnetite Dispersions with Ultra-High Initial Magnetic Susceptibilities Yunping Fei, Muhammad Iqbal, Seong D Kong, Zheng Xue, Charles P Mcfadden, Jesse L Guillet, Linda H Doerrer, Esen E Alp, Wenli Bi, Yi Lu, Chola Bhargava Dandamudi, Prashant Ranganath, Kevin J Javier, Mohsen Ahmadian, Christopher J Ellison, and Keith P. Johnston Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03702 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Aqueous Superparamagnetic Magnetite Dispersions with Ultra-High Initial Magnetic Susceptibilities Yunping Feiǁ†, Muhammad Iqbalǁ†‡, Seong D. Kong†, Zheng Xue†, Charles P. McFadden† Jesse L. Guillet⊥, Linda H. Doerrer⊥. Esen E. Alp#, Wenli Bi#, Yi Lu†, Chola B. Dandamudi†, Prashant. J. Ranganath†, Kevin J. Javier†, Mohsen Ahmadian$, Christopher J. Ellison†, and Keith P. Johnston†* †

McKetta Department of Chemical Engineering, University of Texas at Austin, Austin,

TX 78712 Department of Chemistry, Boston University, Boston, MA 02215

⊥ #

Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439

$

Advanced Energy Consortium & Bureau of Economic Geology, University of Texas at

Austin, Austin, TX 78758

*

ǁ

Address correspondence to: [email protected]

The authors contributed equally to the manuscript.

‡

Current affiliation: Michelman Inc. 9080 Shell Road, Cincinnati, OH 45040

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Abstract Superparamagnetic nanoparticles with a high initial magnetic susceptibility χo are of great interest in a wide variety of chemical, biomedical, electronic and subsurface energy applications. In order to achieve the theoretically predicted increase in χo with the cube of the magnetic diameter, new synthetic techniques are needed to control the crystal structure, particularly for magnetite nanoparticles larger than 10 nm. Aqueous magnetite dispersions (Fe3O4) with a χo of 3.3 (dimensionless SI units) at 1.9 vol.%, over three to five fold greater than those reported previously, were produced in a one-pot synthesis at 210 °C and ambient pressure via thermal decomposition of Fe (II) acetate in triethylene glycol (TEG). The rapid nucleation and focused growth with an unusually high precursor-to-solvent molar ratio of 1:12 led to primary particles with a volume average diameter of 16 nm and low polydispersity according to TEM. The morphology was a mixture of stoichiometric and substoichiometric magnetite according to X-ray diffraction (XRD) and Mössbauer spectroscopy. The increase in χo with the cube of magnetic diameter as well as a saturation magnetization approaching the theoretical limit may be attributed to the highly crystalline structure and very small nonmagnetic layer (~ 1 nm) with disordered spin orientation on the surface. Keywords: magnetic susceptibility, superparamagnetic, magnetite, dispersion, nucleation and growth


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Introduction Magnetic nanoparticles (NPs) with high initial magnetic susceptibilities are of interest in diverse fields including magnetic separations for water purification 1-2, MRI contrast agents for biomedical imaging, 3 targeted drug delivery,4 magnetic hyperthermia treatment, and data storage.5-6 Recently magnetic NPs have been proposed as contrast agents in electromagnetic imaging of subsurface oil and gas reservoirs. 7-9 The spatial distribution of the nanoparticle dispersions flowing through a reservoir may be determined from the perturbations they produce in applied electromagnetic fields with low field strengths. Whereas numerous studies have investigated the magnetization of dry superparamagnetic nanoparticles, relatively few have reported initial magnetic susceptibilities χo at low fields (below 13 Oe or 1000 A/m), especially for aqueous dispersions.9-11 For non-interacting particles, the initial magnetic susceptibility (χo) is described by the Langevin equation in the limit where the applied field strength (H) goes to 0, 1214

Eq. 1 where µ 0 is the magnetic permeability in vacuum, Md is the bulk (solid) saturation magnetization, Dp is the magnetic diameter of the particle, Ɵ is the particle volume fraction, is Boltzmann constant, and T is the absolute temperature. At low particle concentrations as reported in this study (< 2 vol%), χo has been found to increase linearly with the nanoparticle volume fraction, whereas at high concentrations the behavior is perturbed by dipolar interactions.15 Magnetite (Fe3O4), iron oxide nanoparticles (IONPs) are widely utilized given that they are essentially non-toxic, earth abundant, relatively inexpensive and they have a relatively high saturation magnetization of 92.8 emu/g Fe3O4.15-16 For diameters smaller than ~20 to 30 nm at 25 o

C, magnetite is superparamagnetic, 10, 17 whereby the randomly oriented unpaired spins become

aligned reversibly in an external magnetic field, as a result of competition between thermal energy and the magnetic anisotropy energy. At an arbitrary reference concentration of 2 vol.% chosen for this study, χo is typically ~ 0.2 as seen for an aqueous dispersion of 8.6 nm magnetite11 and also for 15 nm magnetite NPs in cyclohexane.15 Recently, χo was found to increase from 0.04 to 0.96 ( ~2 vol.% basis) for an aqueous dispersion of magnetite as the primary particle size, measured by TEM, increased from 5 to 15 nm.9 However, the maximum Dp was 9.3 nm, well below the geometric diameter.


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Three commonly used methods for magnetite synthesis include aqueous co-precipitation of a mixture of Fe(II) and Fe(III) salts,18-19 arrested growth during decomposition of organometallic precursors in organic solvents at high temperatures to produce particles with hydrophobic surfaces, 17, 20-22 and solvothermal synthesis in polyols. The polyol approach combines key advantages from the other methods: (1) formation of hydrophilic particles as in aqueous syntheses and (2) rapid nucleation and focused growth from organometallic precursors step at elevated temperatures to achieve targeted sizes with relatively low polydispersities23 and high crystallinity. 24 The strong coordination of the concentrated polyol solvent to the Fe cations arrests growth and provides steric stabilization to control the particle size distribution. For triethylene glycol with a high boiling point of 285oC, Cai and Wan24 and Grabs et al. 25produced ~ 7 and 8 nm magnetite primary NPs, respectively, at 280 °C from Fe(acac)3, and Günay et al. 26 9.5 nm NPs from iron(II) acetate (Fe(OAc)2. To achieve substantially larger values of Dp to further increase χo, exquisite control of the nucleation and growth will be required to maintain high crystallinity and prevent formation of a disordered nonmagnetic layer on the surface.

Herein, we report magnetite NP dispersions with unprecedented high initial susceptibilities (3.30 at a concentration of 1.86 vol. %) by controlling the primary particle size and crystal structure during rapid thermal decomposition of iron (II) acetate in TEG at 210 °C at ambient pressure. The molar ratio of precursor-to-solvent was varied to produce high supersaturation for rapid nucleation and controlled growth in order to achieve high crystallinity and minimize the magnetic dead layer even with a large magnetic diameter up to 17 nm. Furthermore, the nanoparticles were found to be highly crystalline magnetite and substoichiometric magnetite, as characterized by Mössbauer spectroscopy, X-ray diffraction and the saturation magnetization. The nanoparticles were further stabilized with a thin layer of silica making them amenable for further functionalization.27-28

Results and Discussion Synthesis and characterization of IONPs The IONPs were synthesized by thermal decomposition of Fe(OAc)2 in TEG whereby the temperature was raised from ambient to 210 °C under an argon atmosphere. The molar ratio of Fe(OAc)2-to-TEG ratio was varied from 1:12, to 1:22 and 1:33 in an attempt to control the primary particle size. For each Fe(OAc)2:TEG ratio, the color of the reaction mixture changed 4 ACS Paragon Plus Environment

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from light brown to black at 180 °C in approximately 15 minutes after heating was initiated, and the temperature reached 210 °C and was maintained for two hours to allow crystal growth. After cooling to ambient temperature and purification to remove unbound TEG, the resulting TEG capped magnetic nanoparticles were readily dispersible in aqueous media resulting in a pH of 3.5 with an overall yield of >80 %.

Figure 1. Representative TEM images of TEG functionalized IONPs, (a-c) with different precursor ratios, 1:12 1:22 and 1:33 respectively, (d) IONPs at precursor ratio of 1:12, (e) high resolution TEM image of IONPs and the inset shows the lattice fringes (f) XRD spectra of IONPs with silica coating for a Fe(OAc)2-to-solvent ratio of 1:12 (Sample D in Table 1). As shown by TEM (Figures 1( a-c)), the diameter of the primary IONPs increased with increasing the precursor-to-solvent ratio. For the two lower molar ratios of Fe(OAc)2-to-TEG, the primary particle size was only ~10.8 nm, comparable to those reported previously at similar precursor:TEG ratios. 25-26 For the highest molar ratio of 1:12 the volume average diameter reached 15.8±1.5 nm (Figure 1c), still small enough to be in the superparamagnetic regime. For a range of Fe(OAc)2 initial masses, the primary particle diameter ranged from 14.3 to 17.6 for 5 ACS Paragon Plus Environment

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samples A-C in Table 1 and Figure S4. Furthermore, the hydrodynamic diameters obtained from DLS varied from 26 to 53 nm indicating the formation small aggregates.

Table 1. Magnetic properties of TEG-functionalized IONPs for different precursor amounts for a molar ratio of Fe(OAc)2]:TEG of 1:12. The Fe3O4 concentration is from FAAS. S

Concen-

a m p l

tration Fe(OAc)2

of

(g)

Fe3O4 (mg/mL

e

A

B

C

D

E

F

Primary Vol% of Fe3O4

Hydro-

Particle

dynamic

Diameter

Size (nm) from TEM (nm)

)

5.4

24.4

0.465

33

5.2

97.4

1.86

26

3.5

21.1

0.363

53

9.0

25.1

0.478

34

6.5

35.3

0.672

54

3.8

47.4

0.904

53

Sat.

Calculated Magnetic

χo

Diameter

(SI)

(nm)

14.3 ± 2.2

16.3

17.6 ± 3.5

18.8

14.9 ± 2.1

18.2

16.0 ± 2.8

17.5

17.5 ± 3.1

19.5

14.6 ± 2.6

16.9

χo

Magne-

2 vol%

tization

basis

(emu/g Fe3O4)

Coating on the NPs

0.766

3.29

76.3

TEG

3.30

3.56

93.8

TEG

0.680

3.36

85.1

TEG

0.813

3.41

92.0

Silica

1.341

3.99

82.8

Silica

1.68

3.71

84.2

Silica

The decomposition of mechanism of Fe(OAc)2 in hydrophobic organic solvents goes through black wüstite (FeO) nanoparticles which are oxidized to form magnetite (Fe3O4).21,32,33 In the case of TEG, the mechanism is more complex, as TEG displaces acetate ions from the surface and adds hydroxyl groups that subsequently undergo condensation. 25, 29 In order to investigate the nucleation and growth stages, sample aliquots were withdrawn from the reaction mixture versus time and analyzed using TEM and DLS as shown in Figure S2 and Table S2, respectively. Before the color change at 150 oC, NPs were not yet present. When the reaction reached 210 oC, the small primary particle size of 6.5 - 8.5 nm and low polydispersity indicated a high nucleation


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rate and limited growth. An hour later it reached 10-12 nm and then 15-16 nm after two hours. The rapid nucleation was followed by growth and size focusing during the annealing process at 210 °C. The relatively low polydispersity of the unusually large 15.8 nm primary nanoparticles was favored by the short nucleation time window, during which the color changed. In addition, at high supersaturation, the higher growth rates for the smaller versus larger NPs leads to focusing of the size distribution to a moderate polydispersity. 30 Furthermore, the rapid growth rate of primary nuclei suppresses formation of secondary nucleation.29 The larger size may also be attributed to a lower ratio of capping agent TEG-to-Fe(OAc)2 as the capping agent arrests growth in addition to providing steric stabilization against aggregation. In contrast, the lower supersaturation for lower precursor-to-TEG ratios as presented in Figure 1(a-c) and Table S1 would lead to generation of a large number of nuclei and greater quenching of growth due to capping with TEG, thus resulting in the observed primary particles no larger than 11 nm. 25-26, 29 An increase in particle size was also observed with an increase in precursor-to-stabilizer ratio in the synthesis of iron oxide in benzyl ether, whereby the aliphatic amine stabilizing ligand was a mild reducing agent that increased the nucleation rate.31 Additional examples of this precursorto-stabilizer ratio effect include citrate stabilized precipitation of FeSO4 32 and co-precipitation of iron chlorides in aqueous media. 9 Powder XRD and Mössbauer spectroscopy measurements were performed to elucidate the crystal structure of the IONPs. The powder XRD revealed (Figure 1(f)) an inverse spinel magnetite crystalline phase as identified with the International Center for Diffraction Data (ICDD) card No. 19-0629. The diffraction pattern was indexed to a cubic spinel Fe3O4 structure, showing (220), (311), (400), (422), (511), and (440) reflections. The relatively sharp scattering peaks given the small particle size indicate a highly crystalline structure. According to the Scherrer equation for the (311) peak, =

, = 0.9, = 0.15418 , = 0.0108, = 17.7° cos

the calculated crystalline size was 13.4 nm, which is in good agreement with the primary particle size observed by TEM.


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Figure 2: Characterization of IONPs coated with silica. (a) Zeta potential of IONPs@SiO2 at different pHs. Electron microscopy imaging and elemental analyses of IONPs. (b) Bright-field HR-TEM image of IONPs@SiO2 (c) HAADF-STEM image of silica coated IONPs, (d) STEMEDS mapping of silica coated IONPs and (e-g) elemental mapping of the Fe, O and Si. To provide electrostatic stabilization, the IONPs were coated with a thin shell of silica via sol-gel processing with TEOS. The silica shell helps prevent oxidation, and provides a versatile platform for further functionalization. 33 The increase in the magnitude of the zeta potential of silica coated IONPs with pH in Figure 2(a) followed the expected behavior as silanol groups were deprotonated 34. A pH of 7 was chosen for studies of the silica-coated magnetite, such that the highly negative zeta potential provided strong electrostatic repulsion. The hydrodynamic diameter of the silica-coated particles typically changed less than 10% after storage for more than one month in deionized water. The approximate thickness of the silica layer determined by TEM was only ~1-2 nm (Figure 2(b, c)) such that the hydrodynamic diameter did not change significantly (Table 1 and Figure S4). The complementary suite of electron microscopy techniques and associated spectroscopic methods provide valuable insight into the core–shell 8 ACS Paragon Plus Environment

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structure of the silica coated IONPs. STEM-EDX mapping revealed the Fe rich core with a Si rich shell while O was present in both core and the shell with greater prevalence in the core (Figure 2(b-f)).

Crystalline structure characterization Table 2. Mössbauer parameters determined from the room temperature spectra of Sample D in Figure 4 Site

3+

A (tetrahedral Fe ) 3+

2+

B (octahedral Fe , Fe )

Isomer

Hyperfine

Area

shift

field

(%)

(mm/s)a

(T)

0.34

49.4

56

0.55

45.3

44

a. The isomer shift was calibrated to α-Fe foil.

Figure 3. Mössbauer spectroscopy of silica coated IONPs (Sample D in Table 1). Additional spectra are given in the supporting information.


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It is known that the bulk magnetite (Fe3O4) and maghemite (γ-Fe2O3) have nearly identical spinel structures with only about 1% difference in the cubic lattice constant, which is not discernable from the XRD patterns.35 Therefore, room temperature Mössbauer spectroscopy measurements were performed to further distinguish between magnetite (Fe3O4), substoichiometric magnetite and maghemite (γ-Fe2O3). The narrower sextet with the smaller isomer shift of 0.34 mm/s and the larger hyperfine field of 49.4 T was associated with the high spin Fe3+ ions occupying the tetrahedral A sites in the inverse spinel structure of magnetite. The broader sextet with the higher isomer shift of 0.55 mm/s and lower hyperfine filed of 45.3 T corresponded to octahedral B sites occupied by Fe3+ and Fe2+ ions. 36 The spectra reported here were fit to two discrete magnetic sextets (Figure 3) to yield hyperfine parameters listed in Table 2 and Table S4. The reproducibility was very good as indicated in the additional data for the other five samples provided in (Table S4 and Figure S3). The sample is therefore, identified as a mixture of Fe3O4 and substoichiometric magnetite. The lower isomer shift of sextet for site B (0.55 mm/s) of Sample D in the present study of magnetite as compared to the previously reported bulk value for stoichiometric Fe3O4 (0.66 mm/s) 37 could imply that the samples are nonstoichiometric with a lower content of Fe2+ ions than stoichiometric Fe3O4, or be attributed to the small size of synthesized nanoparticles. 38-39 The line broadening for room temperature spectra can be attributed to the size distribution of the nanoparticles. Magnetic properties of silica coated IONPs


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Figure 4. Magnetization curve of IONPs dispersion at (a) 10 kOe and (b) 3 Oe by VSM (Sample D in Table 1) for Fe(OAc)2:TEG of 1:12 at a concentration of 0.478 vol. % (c) linear correlation of initial susceptibility versus IONPs concentration The magnetic properties of the silica coated IONPs were measured by vibrating sample magnetometry (VSM) (Figure 4) where the sample was vibrated in a direction perpendicular to the magnetic field. The aqueous IONP dispersions were examined under both high (-10 kOe to +10 kOe) and low magnetic field to investigate the saturation magnetization and initial susceptibility, respectively. For the lowest precursor:TEG ratio of 1:33 and a diameter of 10.8 nm, the saturation magnetization was only 45 emu/g Fe3O4 and the normalized χo was 0.23 at 2 vol% (Table S1). At an unusually high molar ratio of Fe(OAc)2:TEG of 1:12 the average saturation magnetization was 85.8 ± 6.5 emu/g Fe3O4, approaching the theoretical value of bulk magnetite (92.7 emu/g Fe3O4) in contrast to the much smaller value for maghemite (γ−Fe2O3) of 70 emu/g. 40 Thus, the crystal structure indicated by the saturation magnetization was consistent with the Mössbauer results. A relatively uniform crystallinity over the entire cross section of the nanoparticles was further confirmed by HR-TEM with a measured d-spacing of 4.8 Å (Figure 1(d)), corresponding to the (111) planes of the Fe3O4 single crystal with cubic inverse spinel structure.41-43 The magnetic diameters (Table 1), calculated using the method from Chantrell et al. by assuming a lognormal size distribution for the ferrofluid12 as summarized recently9, are comparable to the sizes measured by TEM indicating a very small magnetic dead layer on the surface (< 1 nm), consistent with the high saturation magnetization and the HR- TEM. 12 ACS Paragon Plus Environment

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The magnetization for an extremely low magnetic field sweep from - 3 to +3 Oe on sample D at a concentration of 0.478 vol % [(Figure 4(b)] reached 8.0 emu/g Fe3O4 for the high 1:12 Fe(OAc)2:TEG ratio. It is remarkable that the magnetization was about one tenth of the saturation value under such a weak external field. From the slope of the magnetization curve and the correction for the volume fraction of 0.00478, χo was 0.813 (dimensionless SI units). In Table 1, a plot of χo versus particle volume fraction exhibited linear behavior, consistent with a previous study in this moderate concentration range,15 indicating absence of dipolar interactions, which appear at much larger concentrations. The linear correlation shown in Figure 4(c) validates our extrapolation of susceptibility data measured at different concentrations to the 2 vol. % basis for comparison with previous studies. For all of the samples in Table 1, the average χo, normalized to 2 vol. % was 3.55 ± 0.26 (dimensionless). As the Fe(OAc)2:TEG ratio increased from 1:22 to 1:11 (Table S1), the increase in χo from ~1 to ~3.5 is in semi-quantitative agreement with the predicted value of (15.8 nm/10.8 nm)3 = 3.1 according to the Langevin equation (Eq. 1). The ratio of 1:12 was near the optimum for χo as for a ratio of 1:6 the particle size increased above 25 nm leading to ferromagnetic behavior above the size limit for superparamagnetism.14 The saturation magnetization and χo did not change significantly with the cluster hydrodynamic diameter (Table 1) indicating a limited influence of dipolar interactions, perhaps due to the electrostatic stabilization by silica coating as well as the low concentration. As shown in Table 3, χo is > 6 fold higher than values obtained at similar concentrations for two of the best commercial ferrofluids, and over 3-5 fold higher than for previously reported magnetite dispersions at 2 vol. %. 11, 15

Table 3. Comparison of magnetic properties of synthesized IONP and commercial ferrofluid HydrodySample

Source

namic Diameter (nm)

Primary Particle Diameter from TEM (nm)

Concentration of Fe3O4 (mg/mL)

Saturation Vol. %

Measured

2 vol.

of

χo

% χo

zation

(SI)

(emu/g

Fe3O4

(SI)

Magneti-

Fe3O4)

Ferrofluid 1 FerroTec MSG W08

53

8.1 ± 1.9

45.2

0.861

0.255

0.59

99.1

Ferrofluid 2 FerroTec MSG W10

49

7.9 ± 2.0

35.8

0.683

0.213

0.62

97.8


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Given the various characterization techniques for the IONPs, we attribute the high χo to (i) the large diameters (15.8 ± 1.5 nm by TEM) of primary particles within the superparamagnetic regime, (ii) highly crystalline substoichiometric and magnetite phase as evidenced by XRD, Mössbauer spectroscopy and HR-TEM and (iii) the small nonmagnetic layer given the similarities in Dp versus the diameters from TEM (Table 1), within experimental uncertainty. The silica coating provides high colloidal stability yet the magnetic susceptibility per g of Fe3O4 does not decrease upon adding the silica layer (Table 1), as would often be the case with organic surfactants.44 To explore application of these core-shell IONPs for subsurface electromagnetic imaging of oil reservoirs, the silica coated IONPs were functionalized with amine using 3triethoxysilylpropylamine (APTES) and then grafted with a random copolymer of poly(AMPSco-AA) (poly(2-acrylamido-3-methylpropanesulfonate-co-acrylic acid)) by amide bond formation with the carboxylate AA groups using an approach with characterization reported previously.45-46 The polymer grafted IONPs exhibited unprecedented stability in high salinity (API brine, 8% NaCl and 2% CaCl2) and high temperature (120 oC) for over 4 weeks in that the hydrodynamic diameter by DLS remained approximately 50 nm. Further studies are underway and will be reported in a separate communication.

Conclusions Aqueous magnetite dispersions (Fe3O4) with an average χo of 3.3 at 1.9 vol. %, over three to five fold greater than those reported previously, were synthesized at 210 °C and ambient pressure via thermal decomposition of Fe (II) acetate in triethylene glycol (TEG). For the unusually high precursor-to-solvent ratio of 1:12, the high supersaturation produced rapid nucleation and focused growth leading to relatively uniform large primary particles with an average diameter of 15.8 nm and high crystallinity, which enabled the large χo. High crystallinity was present throughout the nanoparticle cross-section with a very small nonmagnetic layer on the surface based on: (1) the similarity in the geometric (TEM) and magnetic diameters Dp, (2) direct observation by HR-TEM, (3) the saturation magnetization, which approached the theoretical value for bulk magnetite, and (4) the increase in χo with Dp3. From Mössbauer spectroscopy, a substoichiometric phase was identified along with magnetite. The silica shells on the nanoparticle surfaces provided colloidal stabilization and prevented loss in χo from spin disorder, as well as a versatile platform for further functionalization. Aqueous dispersions of superparamagnetic nanoparticles with extremely high values of χo and saturation magnetization are of interest in a wide variety of practical applications. 14 ACS Paragon Plus Environment

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Materials and Methods Materials Fe(OAc)2 was purchased from Alpha Aesar and stored under argon. Triethylene glycol (C6H14O4), tetraethyl orthosilicate (TEOS) [Si(OC2H6)4] were purchased from Sigma Aldrich, USA, and used without further purification. Synthesis of TEG-coated iron oxide nanoclusters. IONPs were synthesized by modifying the procedure of Günay et al. 26 IONPs were prepared by the thermal decomposition of Fe(OAc)2 (light brown powder) in the presence of TEG at 210 °C in an inert argon atmosphere with an overhead mechanical stirrer (15 mm x 46 mm PTFE blade, PTFE stirrer bearing). The mechanical stirrer was used to avoid aggregation that could take place in the case of a submerged magnetic stir bar. In a typical synthesis, 5 grams (28.8 mM) of iron (II) acetate were mixed with 50 mL (366 mM) TEG (molar ratio of 1:12) in a 250 mL three-neck round bottom flask equipped with condenser, mechanical stirrer and a heating mantle. The mixture was purged with argon gas for at least 20 min before ramping the temperature up to 210 °C in 20 min, and then refluxing at 210 °C for two hours. After cooling to room temperature, the IONPs were purified by copious washing with a mixture of DI water/ethanol/ethyl acetate followed by centrifugation to separate particles from solution. The procedure was repeated at least three times to remove excess TEG. The pellets were redispersed in 1wt. % nitric acid to aid dispersion well below the isoelectric point and immediately precipitated with centrifugation at 10,000 rpm, and purified as described immediately above. The loosely agglomerated pellets were redispersed with DI water to form a stable colloidal dispersion at pH 3.5 (from residual acid) at a final concentration of ~10 mg/mL. Silica coating of IONPs The silica coating was performed by modifying the procedure of Kralj et al. 33 TEG coated IONPs were dispersed in pH 12 alkaline media at a concentration of 0.1 wt. %. TEOS was diluted with ethanol at volumetric ratio of 1:9 and the mixture was added to the IONPs dispersion over 3.5 hours by a syringe pump during vigorous stirring. The mass ratio of IONPs to TEOS was 0.05. The particles were collected with a strong magnet and washed with copious DI water. The aqueous dispersion was probe sonicated and centrifuged at 3500 rpm to remove big aggregates. The dispersion was finally concentrated by centrifugal filtration or tangential flow filtration (TFF). Characterization Flame atomic absorption spectroscopy (FAAS) The concentration of Fe in the dispersion was measured on a GBC 908AA flame atomic absorption spectrometer (GBC Scientific Equipment Pty Ltd). All measurements were conducted at 242.8 nm using an air-acetylene flame. 100 µL of IONP dispersion (20 mg/mL) was digested in 900 µL of 12 M HCl overnight; the volume ratio of 15 ACS Paragon Plus Environment

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HCl: nanoparticle dispersion was 9:1. After digestion the samples were then diluted with 1 wt. % HNO3 to a concentration between 1 and 5 ppm of Fe. Nanoparticles that were coated with silica were filtered through a 20 nm pore size syringe filter (Whatman ANOTOP 25). The FAAS was calibrated with ferric nitrate nonahydrate solutions in 1% nitric acid at concentrations from 1 to 5 ppm of Fe. The calibration was repeated every 10 samples. The concentration of the original suspension was calculated in the unit of mg/mL Fe3O4 based upon the measured Fe content measurements. Dynamic light scattering (DLS) Hydrodynamic diameters (DH) of aqueous dispersions of the asprepared iron oxide clusters were measured with a Brookhaven ZetaPALS instrument at a 90o scattering angle. The auto-correlation functions were fit with CONTIN routine to give volumeaveraged size distributions. All measurements were made over a period of 2 min and at least three measurements were performed on each sample and averaged. The concentration of IONPs for DLS samples were adjusted to obtain a count rate of ~500 kcps (~0.1 wt. %). The hydrodynamic diameter reported here is the peak value of the main peak that accounts for >70% of cluster volume. After silica coating the hydrodynamic diameter changed less than 10% in one month of storage at pH above 7.5 Zeta potential measurements Electrophoretic mobility was measured with a Brookhaven Zeta PALS instrument at a 15o scattering angle at room temperature in 10 mM KCl solution (Debye length = 3 nm). 10 measurements with 30 electrode cycles for each run were performed and averaged. Zeta potential was obtained from the electrophoretic mobility using the Smoluchowski model. X-ray diffraction (XRD) X-ray diffraction was obtained on a Rigaku R-Axis Spider diffractometer with an image plate detector using Cu Kα radiation (λ = 1.54 Å). Samples were prepared on nylon loops and scanned for 10 min rotated at a rate of 10 o/min. Transmission electron microscopy (TEM) TEM images were acquired on a FEI Tecnai Spirit Bio Twin TEM operated at 80 kV. The samples were prepared by drop casting a dilute aqueous suspension of IONPs onto 400 mesh Formvar coated copper grids, blotting and drying in air. Image analysis was done with Image J from 200 primary particles at various spots on the grid, being careful to avoid cases where primary particles on top of each other produced larger sizes that were not included. The diameter was averaged on a volume basis. High-resolution transmission microscopy (HR-TEM) and HAADF-STEM images were obtained on a field emission JEOL 2010F TEM operated at 200 kV. Scanning transmission electron microscopy Energy dispersive X-ray spectroscopy (STEM-EDX) mapping was done on a JEOL 16 ACS Paragon Plus Environment

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2010F equipped with an Oxford X-MaxN 80TLE solid state detector. The STEM probe size was ~1 nm and drift correction was performed during the mapping using AutoLock in Oxford’s Aztec software while the EDS maps were obtained with acquisition times of more than 1 min. The microscope was operated at 200 kV accelerating voltage. Vibrating sample magnetometer (VSM) The initial susceptibility of magnetic fluid samples was measured with a VSM (Microsense model EZ7) under a magnetic field with intensity swept between -3 to +3 Oe at a rate of 0.15 Oe/s at ambient temperature. Saturation magnetization was measured under a -10 kOe to +10 kOe range swept at a rate of 250 Oe/s. For each measurement, 40 µL magnetic particle solution was loaded into cylindrical crucibles and was measured at 300 K at a DC field with vibration frequency of 75 Hz. Values reported do not include normalization by demagnetization factor due to shape of the sample-holder. Mössbauer spectroscopy Room temperature Mössbauer spectroscopy measurements were performed with the facilities at both Argonne National Lab and MIT. The Argonne National Lab facilities had a 57Co source in Rh matrix mounted on a constant acceleration drive. The spectrometer was calibrated using an iron foil. A VORTEX detector with 150 eV resolution was used to discriminate the 14.4 keV radiation. Data analysis was performed using in-house software. Facilities at MIT had a 57Co source in Rh matrix mounted on a constant acceleration drive. 57Fe-enriched metallic iron foil was used for velocity calibration. Fits of the data were calculated by the WMOSS plot and fit program, version 2.5. All samples of IONPs were dried and the sample powders were placed between two layers of Kapton tape. All isomer shifts are given with respect to metallic α-Fe at room temperature. Supporting information: Synthesis results for different precursor ratios, effect of time on particle size, Mossbauer parameters of various samples, and TEM of volume distributions. Conflict of interest The authors declare no competing financial interests. Acknowledgments This work was supported by the Advanced Energy Consortium: http://www.beg.utexas.edu/aec/. Member companies include BHP Billiton, ExxonMobil, Repsol, Sandia National Laboratories, Shell, Total, and US Department of Energy. It was also supported by the Department of Energy (DOE) Center for Subsurface Energy Security, and the Welch Foundation (F-1319). We thank Karalee Jarvis, at Texas Materials Institute (TMI) for assistance with HRTEM and STEM-EDX.


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copolymers on Berea sandstone in high salinity brine. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2017, 520 (Supplement C), 257-267.


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