Unveiling the Physicochemical Features of CoFe2O4 Nanoparticles

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Unveiling the Physicochemical Features of CoFe2O4 NPs Synthesized via a Variant Hydrothermal Method: NMR Relaxometric Properties Violetta Georgiadou, Vassilis Tangoulis, Ioannis Arvanitidis, Orestis Kalogirou, and Catherine Dendrinou-Samara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00717 • Publication Date (Web): 26 Mar 2015 Downloaded from http://pubs.acs.org on March 29, 2015

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The Journal of Physical Chemistry

Unveiling the Physicochemical Features of CoFe2O4 NPs Synthesized via a Variant Hydrothermal Method: NMR Relaxometric Properties Violetta Georgiadou,a Vassilis Tangoulis,a Ioannis Arvanitidis,b Orestis Kalogirou,b Catherine Dendrinou-Samara*a a

Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece. E-mail: [email protected] b Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece * Corresponding author, Email: [email protected], Tel: +30-2310-99-7876 http://users.auth.gr/samkat

ABSTRACT A series of CoFe2O4 nanoparticles were formed through a variant hydrothermal synthesis based in a self-assembly oil-water system in autoclaves at 200 °C in the presence of octadecylamine and the trivalent iron and cobalt acetylacetonates. The variation of the water content, the different valence of cobalt precursor (Co(II) and Co(III)) as well as Fe: Co precursor ratios (2:1 and 1:1) were studied. CoFe2O4 nanoparticles with a size range 9-16 nm of high crystallinity and enhanced saturation magnetization (~89 emu g-1) have been isolated and characterized. Raman spectroscopy provided information concerning the lattice strain, while incorporation of Co2+ at Td sites of the spinel indicated a different inversion degree (0.67-0.60) among the samples. EPR studies showed that EPR signal and spin relaxation process were size dependent and influenced by aggregation effects. CoFe2O4 nanoparticles were converted to dual agents via a reaction between the free amine groups of the organic coating and the sulfonyl group of the fluorescent dye sulforhodamine B acid chloride (SRB) and NMR relaxometric properties were measured. The relatively high transverse relaxivity values, r2 (232.0-130.3 mM-1 s-1) were attributed to nanocluster effects in aqueous suspensions in respect with the amount of SRB and encourage their potential application as versatile agents in theranostics.

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Keywords: acetylacetonate precursors, octadecylamine, electron paramagnetic resonance, Raman spectroscopy, NMR relaxivity

INTRODUCTION CoFe2O4 belongs to the group of spinel ferrites MxFe3-xO4, (where 0 ≤ x ≤ 1 and M = Fe, Co, Mn, Ni, or Zn) that are currently considered among the most successful inorganic nanoparticles for medical applications, including magnetic resonance imaging, magnetically guided drug delivery and hyperthermia cancer therapy, while also assumed to be biocompatible within certain threshold limits.1,2 The intrinsic properties of ferrite nanoparticles are greatly related to the nanoparticles size and the cation occupancy in the spinel given in the formula [Co1-iFei]A[CoiFe(2-i)]BO4, where the amounts in brackets represent the average cation occupancy of tetrahedral (A) sites and octahedral (B) sites and i is the inversion degree.3 Identifying the best combination of physical and chemical parameters for a specific application is a critical and on-going research process. Among several techniques, Raman spectroscopy is used for the study of the spinel lattice and electron paramagnetic resonance (EPR) is adopted to investigate further their magnetic properties. EPR studies on ferrite NPs show the existence of unpaired electrons, while on CoFe2O4 NPs are limited4,5 owing to the anisotropic nature of cobalt. Cobalt broadens the EPR spectrum, shades important features and complicates the interpretation in comparison to other spinels.6,7 EPR parameters such as the resonance field (Hr) and peakto-peak linewidth (∆Ηpp) are affected by the distribution of cations in the spinel.8 Furthermore, the size of the NPs is a factor that affects the resonance, since the ∆Ηpp linewidth is proportional to the particle volume.9 In an EPR spectrum, the magnetic dipole inter-particle interactions as well as super-exchange interactions through oxygen


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ions between cations in the spinel cell are the phenomena that define the g factor and the ∆Ηpp linewidth. Strong dipole magnetic interactions produce a wide ∆Ηpp linewidth and large g factor, while when the distance between the magnetic cations through the oxygen is shorter super-exchange interactions are stronger and ∆Ηpp and g factor are smaller.10 The amount of Co2+ ions in B sites affects the resonance linewidth due to the enhancement of spin-orbit interaction by the degenerate orbital ground state of Co2+.11 Apart from Co2+ ions, the Fe3+ ions have an effect on the EPR spectrum as well; the addition of Fe3+ ions to A sites increases the super-exchange interactions, contributing to the enhancement of the internal field and the decrease of the resonance magnetic field.12 The ∆Ηpp linewidth is used for the determination of the electron spin relaxation process, spin-lattice (T1) or spin-spin (T2) relaxation process, that restore the thermal equilibrium after it is disturbed at the resonance field (Hr). Hence, EPR spectroscopy can trace a variety of morphological and magnetic properties of CoFe2O4 NPs. It is well known that the preparation of ferrite nanoparticles for medical applications is a demanding task since certain features of the NPs should be among favorable ranges. For instance, monodispersed NPs with a range of size 10 < d < 30 nm with enhanced saturation magnetization 60 < Ms < 100 emu g−1 are needed while they have to be combined with appropriate surface properties.13 It is widely accepted that the size, composition, cation occupancy and surface properties of the NPs are influenced by the preparation methods.6, 14 The quest to control over these parameters is achieved by modifying traditional synthetic procedures.

In continuation of our efforts on the

preparation of ferrite NPs with enhanced magnetic properties for bioapplications, we presented recently, a variant hydrothermal method in the presence of a fatty amine that allowed us to isolate CoFe2O4 NPs with a variety of sizes, magnetization and surface morphology.15

In an attempt to investigate further this approach in this paper we



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modified the synthetic procedure with the decomposition of the trivalent cobalt precursor Co(acac)3 instead of the typical divalent Co(acac)2 to prepare CoFe2O4 NPs, and the newly synthesized NPs were physicochemically characterized and compared with the previously reported by us. The trivalent metal precursors are unstable compared to the divalent metal chelates and their dissociation occurs more or less simultaneously at lower temperatures,16 while the precise mechanism of the hydrothermal synthesis is more difficult to be predicted compared to synthesis in organic media and to our knowledge such a modification is reported for the first time. Thus, a series of hydrothermal experiments have been carried out in autoclaves at sub-critical region (200 °C) by dissociating Fe(acac)3 and Co(acac)3 in two different ratios 2:1 and 1:1 in the presence of octadecylamine (ODA) generating a self-assembly oilwater system. The water content has been differentiated to study the impact of the essentially immiscible system on the structure and properties of the resulting NPs. Beside the common characterization techniques (XRD, SEM, TEM, IR, TGA, VSM, SQUID) we went further by EPR and Raman spectroscopy to exploit the different properties of the NPs that were induced by the trivalent cobalt precursor. EPR studies of the samples showed that the EPR signal is influenced by aggregation or clustering effects and is size dependent, while the spin relaxation process is also affected by the particle size. Meanwhile, Raman spectroscopy was proven a useful tool for the estimation of the inversion degree and the strain state of the samples.

Finally, CoFe2O4 NPs were

converted to fluorophores via a reaction between the organic coating and the fluorescent dye SRB and the NMR transverse relaxometric properties of SRB modified NPs aqueous suspensions were also tested encouraging their potential performance as dual agents for theranostics.


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EXPERIMENTAL SECTION Chemicals: All the reagents were used without any further purification. Iron (III) acetylacetonate (Fe(acac)3) and octadecylamine (>90.0%) were purchased by Fluka, cobalt (III) acetylacetonate (Co(acac)3, ≥99.9%), and sulforhodamine B acid chloride (SRB, suitable for fluorescence, technical grade) were supplied by Aldrich, meso-2,3dimercaptosuccinic acid (DMSA, 98.0%) by Tokyo Chemical Industry, dimethyl sulfoxide (DMSO) was provided by Reag. Ph. Eur. Panreac Quimica SA, deuteriumoxide (D2O) was purchased by Deutero GmbH, 99.9%), ethanol (100%, 1% MEK) was from Bruggermann GmbH. Toluene (analytical reagent) and chloroform (analytical reagent) were from Chem. Lab NV. N,N-Dimethyl formamide (>99.5%) and ninhydrin (GR for analysis ACS, Reag. Ph Eur) were purchased by Merck. Deionized water was used for the fabrication of CoFe2O4 NPs and Milli pure water (< 3.0 MΩ, Millipore, MilliQ Gradient) was used for the modification of the NPs surface properties. Nanofilters Minisart RC 15 (Single use syringe filter Non-sterile RC-membrane, Pore Size: 0.20 µm) were from Sartorius Stedium Biotech GmbH. Characterization Techniques: Powder X-ray diffraction (XRD) was performed using a 2-cycle Rigaku Ultima + diffractometer (40 kV, 30 mA, CuKa radiation) with BraggBrentano geometry (detection limit 2% approximately). Conventional TEM images were obtained with a JEOL 100 CX microscope (TEM); stable dispersions of the NPs in toluene were prepared for transmission electron microscopy. Energy dispersive detector (EDS) integrated to a scanning electron microscopy instrument (SEM), JEOL 840A and an inductively coupled plasma optical emission spectroscope (ICP-OES, ICP Simultané VARIAN Vista Axial) were used for elemental analysis. Fourier transform infrared spectroscopy (280-4000 cm-1) was recorded using a Nicolet FTIR 6700 spectrometer with samples prepared as KBr pellets. Thermogravimetric analysis (TGA) was performed



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using SETA-RAM SetSys-1200 at a heating rate of 10 °C min-1 under N2 atmosphere. Magnetic measurements were acquired by a superconducting quantum interference device (Quantum Design MPMS-5 SQUID) and a vibrating sample magnetometer (1.2H/CF/ HT Oxford Instruments VSM). Raman spectra were recorded in the backscattering geometry using a LabRam ARAMIS spectrometer (HORIBA) equipped with a peltier-cooled charge coupled detector. The 632.8 nm line of a He-Ne laser was used for excitation, focused on the samples by means of a 100x objective at a power lower than 0.1 mW to avoid any laser heating induced effects. X-band EPR measurements were performed on an ESR900 Biospin Bruker at a range of temperature (5-280 K). 1H-NMR spectra of the isolated organic coatings of the samples were received in CDCl3 with TMS (500 MHz, Agilent Technologies); the isolation of the organic coating was achieved after the dissolution of the samples in CHCl3, that was followed by several cycles of sonication and centrifugation, until the supernatant was limpid. The supernatant from each washing cycle was removed and placed into glass vials. It was then filtered with the use of nanofilters to retain the dispersed NPs. The resulting pale yellow liquids were condensed, and CDCl3+TMS was added for their further spectroscopic studies. T2 relaxation time measurements were performed at a 500-MR NMR Spectrometer (500 MHz, Agilent Technologies), using a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, the metal concentrations of the aqueous suspension were estimated by inductively coupled plasma atomic emission spectroscopy (ICP-AES), Perkin-Elmer Optima 3100XL. UV–visible measurements were carried out by double beam UV-visible spectrophotometer U-2001 Hitachi. Fluorescence measurements were taken out at a Hitachi F-7000 Fluorescence spectrophotometer (exc.: 500 nm, em.: 520 nm, slit 2.5). Images under visible and fluorescent light of the modified with fluorescent dye nanoparticles were taken by 20x objective on a trinocular EPI fluorescence microscope (HBO illumination system model


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B-500 TiFL, Optika) equipped with a digital camera set (DIGI, 8 Mpixels) with optical adapter and measuring software. Preparation of CoFe2O4 Nanoparticles: Four samples were prepared namely CF1-4 and their preparation is summarized in Table 1S.

The experiments were carried out in

autoclaves and employed different water content with a constant amount of octadecylamine (ODA). The samples CF1 and CF2 were prepared by the dissociation of 1.8 mmol Fe(acac)3 and 0.9 mmol Co(acac)3 in 13 mL deionized H2O in the presence of 0.269 g ODA, while the samples CF3 and CF4 were formed by 0.9 mmol Fe(acac)3 and 0.9 mmol Co(acac)3 in 3 mL deionized H2O and ODA (0.269 g). Temperature in all cases was elevated at 200 °C with a steady rate of 4 °C/min and remained stable for 24 h. After the 24 h reaction the autoclaves were left to cool down to room temperature with a steady rate (5 °C/min) and CoFe2O4 NPs were isolated by repeating centrifugation cycles (5000 rpm) with ethanol. Ninhydrin Colorimetric Assay: Aliquots of 0.1-0.6 mL of ODA (0.25 mg mL-1) in DMF were pipetted into a series of tubes.17 0.7 mL of ninhydrin solution in MeOH 0.06 M (10.7 mg mL-1) were added in each tube, mixed well and heated in a water bath at 100 °C for 5 min. The color of the solution changed with the heating and the tubes were left to cool down and then the content was transferred to a 5 mL volumetric flask and was diluted with DMF for the UV-Vis absorbance measurement. A series of NPs (CF1, CF3 and CF4) in DMF (0.25 mg mL-1) was prepared accordingly and 0.4 mL of each sample stock solution was pipetted into boiling tubes with 0.7 mL of ninhydrin solution following the same procedure described for ODA and UV-Vis measurements of the formed complex were recorded. Functionalization with sulforhodamine B acid chloride: The CF1, CF3 and CF4 samples became fluorescent probes by engaging SRB molecules on their surface. A



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solution of SRB in DMF (6.8x10–3 mM) was prepared (solution A). A solution of NPs (1 mg/mL) was prepared in 0.25 M phosphate buffer (pH 9.1; solution B).18 Solution B was added dropwise to solution A. The mixture was left to react in the dark for 2 h and then was washed repeatedly to remove the excess SRB. The fluorescence emission spectra were recorded for the isolated NPs@SRB. Aqueous suspensions of NPs@SRB were prepared for the NMR relaxometric measurements (0.8-0.2 mM), and the Fe+Co concentration was determined by ICP-AES analysis.

RESULTS AND DISCUSSION Structural Characterization Instead of the typical metal salts for a hydrothermal synthesis, the trivalent metal acetylacetonates have been used as metal sources, since there are no limits/preconditions in the reagent selection in sealed reactions. Four samples, CF1-CF4 were formed by Fe(acac)3 and Co(acac)3, while CF5 and CF6 have been synthesized by the decomposition of Co(acac)2, details are presented by us elsewhere,15 and are also included for comparison reasons in Table 1S. The impact of the precursor ratios (2:1 and 1:1) as well as the water content in the self assembled oil-water reaction system have been explored while no major change in the autogenous pressure (P1 = 192.86 kPa and P2 = 165.4 kPa for V1 = 13 and V2 = 3 mL respectively) instigated due to the small capacity of the autoclaves (23 mL). In so, the distinct properties of the resulted nanomaterial were ascribed to the different solubility and diffusion of the reacting species generated by the self-assembly oil-water system. Octadecylamine (ODA), a σ-donor fatty amine ligand that is in liquid state at 200 °C was constant in all preparations and assisted the otherwise difficult dissociation of the acac precursors in water due to the ability to offer electrons at elevated temperatures.19 The reductive environment in the reaction vessel was enhanced


by the formation of acetylacetone (Hacac) while the hydrolysis of Fe(acac)3 to Fe(OH)3 and then to β-FeOOH was promoted as well as the reduction of Co3+ to Co2+ (E0= +1.80 V)20 with the formation of Co(OH)2. The metal hydroxides led to pure phase spinel CoFe2O4 NPs21 for both ratios (2:1 and 1:1), except for sample CF2 where the higher H2O inhalt favored the Fe(OH)3 species and the side reaction of dehydration of β-FeOOH gave a small amount of hematite (α-Fe2O3) NPs.

440 531 214

70 620 533

422 511 116

60

400 331

50

202

40 311 222

30 220

20 111

10

1010

024

104

CF1

012

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Intensity (arb.units)

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CF2

CF3

CF4 10

20

30

40

50

60

70

2è (degrees)

Figure 1. XRD diagrams of the CF1-4 samples, (▼: cobalt ferrite, ▼: hematite)

The Powder X-ray diffraction diagrams of the samples show all the characteristic peaks of the cubic spinel structure of CoFe2O4 (pdf card no. 22-1086) (Figure 1). The average crystalline size of the samples was calculated by fitting the diffraction data with a pseudo-Voigt function (Jade6 Software) and found 16.0, 22.0, 13.0 and 12.5 nm for samples CF1, CF2, CF3 and CF4 respectively. The lattice constants of samples CF1-4 were also calculated and found 8.393(0), 8.394(0), 8.389(0), 8.391(1) Å respectively. The 9 ACS Paragon Plus Environment


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minor variation of lattice parameter from the value of bulk cobalt ferrite 8.3919 Å, indicated that the unit cell is under stress.22

Table 1. CoFe2O4 NPs structural and magnetic properties Size

Size

a

Ms

M1T

(emu/g) By SQUID

(emu/g) By VSM

(kOe)

Hc

Α/Α

Composition

(nm) by TEM

(nm) by XRD

lattice parameter of CoFe2O4 phase

CF1

Co0.86Fe2.14O4

14.0

16.0

8.393(0)

91a

85b

1.48

CF2

85% CoxFe3-xO4

11.0

22.0

8.394(0)

c

85b

1.80

CF3

CoFe2O4

8.70

13.0

8.389(0)

91a

69b

0.36

8.391(1) 8.405(0) 8.396(0)

 c c

b

0.70 0.29 0.93

CF4 CF5 CF6 a b c

(15% Fe2O3)

Co0.94Fe2.06O4 Co1.03Fe1.97O4 Co0.79Fe2.21O4

9.00 7.00 15.0

12.5 8.8 14.5

c

89 64b 87b

Saturation magnetization at an applied field of 4 T Magnetization at an applied field of 1 T

Magnetization of sample was not measured by SQUID

EDS analysis showed that CF3 and CF4 samples had similar composition with the generic formula of CoFe2O4, while sample CF1 was found iron rich by both EDS and ICP-OES analysis (Table 1). TEM analysis was used for the determination of the mean size, size distribution and morphology of the nanoparticles (Figure 2, Figure1S). The number weighted distributions were built by counting over 150 NPs for each sample and were fitted with a standard log-normal function. The NPs had a non spherical truncated shape and the mean particle size was 14.0 ± 0.02, 11.0 ± 0.024, 8.7 ± 0.026, 9.0 ± 0.03 nm for CF1-4 respectively. The similar size (~9 nm) for CF3 and CF4, can be attributed to the same growth of the NPs due to the similar decomposition rate of the Co(acac)3 and Fe(acac)3.23 In contrast, the high H2O inhalt in the case of the divalent cobalt precursor assisted its faster decomposition and resulted in larger NPs (CF6, 15 nm). It is worth mentioning, that the size distributions of the NPs were within reasonable values,


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overcoming the problem of typical hydrothermal procedures, where polydispersed NPs are formed as crystals tend to re-crystallize and result in a mixture of small and large NPs.

CF1

0

5

10

15

20

25

Size (nm)

30

35

20 nm

CF3

2

4

6

8

10

12

14

16

18

50 nm

20

Size (nm)

CF4

0

2

4

6

8

10

12

14

Size (nm)

16

18

20

22

24

50 nm

Figure 2. TEM images of the CF1, CF3 and CF4 samples.

The presence of the organic coating for the CF1-4 samples was certified by FT-IR and 1H-NMR spectroscopy and thermal analysis. The peak at ~1645 cm-1, common to all samples, was attributed to the carbonyl group of N-octadecylamide,24 which was formed by the nucleophilic attack of the amine group to the carbonyl group of acetylacetonate ligand (Figure 2S). Additionally, the N-H stretching at ~3290 cm-1 and the N-H wagging mode at ~724 cm-1 of ODA are present, while they are downshifted and of reduced intensity compared to the same features of the neat ODA (~3336 cm-1 Figure 3Sa),25-27 indicating that it is bound on the metal core.28

1

Η-NMR studies at the isolated organic

content (Figure 4S) showed the prominent resonances at δ (ppm): 5.15 (-NHCO), 4.133.23 (-CH2-NH2 and -CH2-NHCO-), 2.28 (CH3CO-), 1.55 (-CH2CH2NH-, -CH2CH2NH2), 1.26 (-CH2-), 087-0.83 (-CH3) attributed to the presence of both ODA (Fig. 2Sb) and Noctadecylamide. The intense chemical shift of the amide proton group of the organic coatings of CF1 and CF2 compared to the spectrum of CF5 (Figure 4S) indicated the larger amount of N-octadecylamide in these samples. The presence of different free -NH2 groups concentration was straightforward related to the synthetic conditions and was 11 ACS Paragon Plus Environment


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verified by performing a ninhydrin colorimetric assay (Figure 5S). The free -NH2 that correspond to 0.02 mg mL-1 NPs@DMF were 0.0049, 0.0029, 0.0051 and 0.0029 mg mL1

for CF1-4 respectively (Figure 5Sb). Hydrophobic interactions were developed between

the aliphatic chains of the amide molecules with the corresponding of amine and resulted to a micelle with different percentage of free amino groups according to the water content. Thus, the lower water content (3 mL) in case of CF1 and CF3 evoked the increase of free -NH2 groups concentration compared to the higher water inhalt (13 mL) for the CF2 and CF4 samples. Thermogravimetric data analysis of the samples was performed under nitrogen atmosphere (Figure 6S). The weight loss of organic coating was 59%, 37%, 35% and 47% for CF1-CF4 respectively, while found lower 33 and 32% for CF5 and CF6 respectively. Two to three steps of mass reduction were observed after 100 ˚C suggesting the existence of a bilayer structure surrounding the NPs29 and/or different binding sites of the functional groups30. The hydrocarbon chain decomposition occurred at 200∼450 ˚C, while the removal of the amine or amide group took place at higher temperatures owing to the bonding with the metal core.31 Apart from the common steps for all the samples as revealed by DTG analysis, the CF1 and CF4 samples showed an extra step of intense thermal loss at ~300 ˚C owing to the decomposition of an additional, outer, loose-jointed aliphatic layer. After all, a synthetic diversity between the samples can be stated; the creation of a bilayer is indicatory of the oil-water ratio in the self assemply reacting system that led to the formation of CoFe2O4 NPs through inverse micelles. Meanwhile, the dissociation of the trivalent cobalt precursor offered more acac ligands that resulted in a stronger presence of N-octadecylamide bound on the NPs surface compared to the samples prepared by the divalent precursor.


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CF1 CF2

Raman Intensity (arb. units)

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CF3 CF4

CF5 CF6 200

300

400

500

600

700

800

900

1000

-1

Raman Shift (cm )

Figure 3. Raman spectra of the samples CF1-6.

Raman spectroscopy was used to estimate the cation distribution and the strain state of the NPs. In Figure 3 the Raman spectra of the samples CF1-CF6 showed the five Raman active modes for cobalt ferrite, 3T2g+Eg+A1g predicted by the group theory analysis at the Γ point of the Brillouin zone of spinels.32 The low-frequency band at ~470 cm-1 is attributed to the oxygen motion around the Oh lattice sites (Fe-O and Co-O, T2g mode), whereas the higher frequency at ~685 cm-1 is assigned to the A1g symmetry and attributed to the motion of oxygen around metal ions in the tetrahedral sites (Td).33 The band at ~625 cm-1 is also attributed to the A1g mode (breathing of M-O), called as the A1g sub-band; this kind of splitting of A1g band is caused by partial cation redistribution.34 The frequencies and the assignment of the various Raman bands are summarized in Table 2. The CF5 and CF6 presented the same spectral profile (Figure 3), while, the CF1-4 samples formed by Co(acac)3, exhibited pronounced differences in the relative intensities 13 ACS Paragon Plus Environment


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of their Raman bands that are attributed to the cation occupancy.35 This is better illustrated in Figure 4a, where the relative integrated intensities of the two main peaks at ~682 cm-1 and ~474 cm-1 in the studied samples are presented. The band at ~625 cm-1 was more intense for the CF2 and CF3 samples and as it can be seen from Figure 4b, where the intensity ratio between the two A1g peaks (lower frequency to higher frequency band) is plotted for the studied samples, the sample CF3 exhibited the largest relative intensity of the A1g sub-band among the various samples studied, while the CF4 sample the smallest.

Figure 4. a) Integrated intensity ratio of the A1g to the band for the CF1-6 samples. b)

The relative intensity of the A1g sub-band with respect to the total intensity of the two A1g bands. Group A: samples prepared by Co(acac)2, groups B and C: samples prepared by Co(acac)3. The larger intensity of the A1g sub-band in group C indicates the higher Co2+ incorporation in Td sites.

The bands at ~682 cm-1 and ~625 cm-1 can be ascribed to A1g (Fe3+-O) and A1g sub-band (Co2+-O) respectively.36 Therefore, for the CF2 and CF3 samples the relative intensity of the A1g (Td) band to the (Oh) is characteristic of the higher Co2+

incorporation in the Td sites, suggesting that more Co2+ ions moved to tetrahedral sites at the expense of the Fe3+ ions that displaced to octahedral sites (Figure 6a). For a rough


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estimation of Co2+ content in Td sites (δ) we have employed Eq.1 given by Nakagomi et al.37 for magnesium ferrite. According to Eq. 1, the Mg2+ content in Td sites of MgxFe3-xO4 NPs can be estimated by means of the formula: Α δ Raman ( Mg ) =

I Mg 2( I Mg + RI Fe )

,

(Eq. 1)

where R~0.5 is the relative oscillator strength of the Mg-O4[(12-n)Fe+nMg] bonds with respect to the Fe-O4-[(12-n)Fe+nMg] bonds (n is the number of bounded Mg ions), while IMg and IFe are the intensities of the A1g sub-band and A1g band respectively. Assuming the same R value (~0.5), δ was estimated (Co2+ content in Td sites) at 0.31, 0.37, 0.39, 0.31, 0.33, 0.32 for the samples CF1-6 respectively. The fitting of the experimental data using Lorentzian lineshapes is illustrated in Figure 7S. According to δ the inversion degree (i) was estimated by the formula: [Co x2−+ xi Fe xi3+ ] A [Co xi2 + Fe 33−+xi − x ] B O 4 ,38 where the composition (x) of each sample was also taken into account, while sample CF2 was considered as a stoichiometric ferrite. The i values obtained, in a decreasing order, are: 0.67 (CF4), 0.67 (CF5), 0.64 (CF1), 0.63 (CF2), 0.61 (CF3) and 0.60 (CF6). We would like to stress here, that even if we adopt the oppositely assignment of A1g sub-band to Fe3+-O and the main A1g band to Co2+-O in Td sites as very recently reported by S. Jovanovic et al.39 based on the XRD study of Kumar et al.40, very close values to those given above obtained as δ CoA 2+ was found 0.27-0.36.

Table 2. Raman and FT-IR bands of the cobalt ferrite NPs (CF1-6 samples) Assignment Raman FT-IR CF1

CF2

CF3

CF4

CF5

CF6

308

304

307

303

304

308

365

361

363

368

358

360

22

467

471

470

468

468

467

32

561

563

555

559

557

553

Eg

CF1

CF2 CF3

CF4

CF5

CF6



585

T1u (v1) A1g sub-band

627

628

625

630

626

625

A1g

686

688

686

685

684

683

576

583

594

585

586

The frequencies of the A1g (Td sites) (Figure 5a) and T22g (Oh sites) (Figure 5b) were plotted against the Co2+ content in Td and Oh sites, respectively.

The higher

frequencies of the A1g and T22g bands (blueshift) for the samples CF2 and CF3 that have the larger Co2+ content in Td sites indicates that the NPs are compressively strained.41 The ionic radii of cobalt and iron in the Td sites are 0.58 Å (Co2+) > 0.49 Å (Fe3+) and in the Oh sites 0.71 Å (Co2+) > 0.55 Å (Fe3+), as a result, upon the displacement of the divalent metal ions from tetrahedral to octahedral sites the unit cell expands, whereas the move of trivalent iron ions from octahedral sites to tetrahedral causes the contraction of the unit cell.42 However, the displacement of Co2+ ions has greater impact on the structure due to its larger ionic radii difference between the two sites compared to that of the Fe3+ ions. This could account for the higher frequencies of the Raman bands in samples CF2 and CF3, further supporting the assignment of the two A1g components adopted here.

a) 689

b) 471

CF2

CF2

688 470

CF3

469

2

685

CF1

-1

CF3

-1

686

T2g (cm )

687

A1g (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CF4

468

CF5

684

0.30

0.32

CF5

CF1

CF6

CF6

683

CF4

467 0.34

2+

0.36

δ, Co (Td)

0.38

0.40

0.45

0.50

0.55

0.60

0.65

0.70

2+

xi = x-δ, Co (Oh)

Figure 5. a) Frequency of the A1g band (Td) for the CF1-6 samples with respect to the Co2+ content in Td sites. b) Frequency of the T22g band with respect to the Co2+ content in Oh


Page 17 of 35

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sites. The higher values for the CF2 and CF3 samples are indicative for the presence of compressive stress.

The Raman data and their analysis are in agreement with the FT-IR data regarding the two out of four infrared-active modes (4T1u) of spinels predicted by group theory analysis.32 The different distances in the Td and the Oh positions of Fe3+-O-2 are reflected in the appearance of two separate bands in the low frequency region of the FT-IR spectra. Namely, the v1 band attributed to Td sites and the v2 ascribed to the Oh sites, located at ~580 and ~380 cm-1 respectively.43, 44 The two bands in the FT-IR spectrum of cobalt ferrite are broad and asymmetric and their position varies according to the synthetic conditions and the grain size.45 The downshift of the v1 stretching frequency observed in the magnified spectra area around ~500 cm-1 (Figure 2S) indicated the increase of the heavier Co2+ ions in the Td sites46 for the samples CF2 and CF3 (Table 2). The uneven incorporation of Co2+ into the spinel lattice was also regulated by the Fe/Co precursor ratio in combination with the water inhalt and is not driven only by the use of trivalent precursor as previously found.47

Magnetic Properties The Ms values (Table 1) of samples CF1 and CF3 were determined from SQUID measurements (Figure 8S) at an applied field of 4 T (300 K), while the sample CF3 was also measured at 5 K. The mass corrected Ms value for samples CF1 and CF3 at 300 K was 91 emu g-1 which is enhanced compared to that of the bulk material (~80 emu g-1)1. The Ms value of the CF3 sample measured at 5 K CF3 was 110 emu g-1. Additionally, VSM measurements on the same samples indicated that their magnetization is not saturated at 1 T (Figure 9S). For the CF2 and CF4 samples only VSM measurements at a



maximum field of 1 T were recorded.

However, even at 1 T they presented high

magnetization values (85 and 89 emu g-1, respectively) well above from the Ms value of the bulk and among the highest with respect to previously reported values for hydrothermally formed CoFe2O4 NPs48-51 (Figure 9S). The enhanced magnetization is indicative of a normal spinel but it could also be attributed to the binding of the σ-donor ligand on the surface of the NPs that decrease the crystal field splitting and thus favors the uplift of the surface layer magnetocrystalline anisotropy and/or to spin disorder layer structure. Nevertheless, it would be expected that samples CF1 and CF4 would exhibit a lower magnetization value, owing to the larger i value as it was evidenced by Raman spectroscopy. This aspect was further investigated by EPR spectroscopy. The coercive field (Hc) values are 1.48 and 0.36 kOe for CF1 and CF3 respectively, which is significantly lower than Hc value of the bulk material (5.4 kOe),52 attributed to the pinning of organic donors,53 which decrease the spin disorder, but large enough to exceed the superparamagnetic limit due to the size range of the NPs (9-14 nm) and basic anisotropy parameters such as surface and shape anisotropy.54 CF1 presents larger Hc compared to CF3 as expected from its relative larger size.55 Hc of such order of magnitude is in accordance with those reported for CoFe2O4 NPs isolated through hydrothermal synthesis.56

b)

0.5

Magnetization (emu)

a) Magnetization (emu)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

FC 0.3

0.2

0.1

0.07 0.06

FC 0.05 0.04 0.03 0.02 0.01

CF1

ZFC 0

50

100

150

200

250

300

CF3

ZFC

0.00

0.0

0

50

Temperature (K)

100

150

200

250

300

Temperature (K)

Figure 6. ZFC/FC measurements of a) CF1 and b) CF3. 18 ACS Paragon Plus Environment

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ZFC/FC measurements were performed for the CF1 and CF3 samples on a superconducting quantum interference device in the temperature range of 10-300 under a magnetic field of 50 Oe K and are presented in Figure 6. The ZFC-FC curves of the CF1 sample (Figure 6a) did not coincide with each other at 300 K indicating that the NPs are still magnetically blocked at RT and not in a superparamagnetic state evident also by the coercive field at VSM and SQUID measurements. Thus, the blocking temperature of the samples could not be estimated in this range of temperature (10-300K).57 The Hc of CF1 is large enough even at RT and as a result magnetization is not decreased probably due to the large mean size of the NPs, whereas for sample CF3 it is smaller and magnetization exhibited a decrease at ~300 K (Figure 6b). Actually, for both samples, the NPs are large enough to be magnetically blocked at 300 K and show the typical magnetic behavior of a bulk ferrimagnetic material.

Electron paramagnetic resonance EPR spectroscopy was employed to investigate further the magnetic features of the CF1- CF6 samples. The X-band EPR spectra of the CF2, CF3 and CF6 samples within a temperature range of 10K up to RT are given in Figure 7, where the presence of two broad signals is obvious in all spectra was attributed to Fe3+-Co2+ ionic interactions. In the spectrum of CF3 sample, the lower field signal is narrower upon heating owing to the smaller size (< 10 nm) of the nanoparticles compared to CF2 and CF6 (11 and 16 nm respectively).



Signal (arb. units.)

a)

30 10 K 20 K 45 K 95 K 140 K 170 K 195 K

20

10

0 213 K 245 K 265 K

-10

CF2 -20 0

1000

2000

3000

4000

5000

6000

Field (G) b)

50 10 K 22 K 45 K 65 K 70 K 95 K

Signal (arb. units)

40 30 20 10 0

155 K 170 K 180 K 210 K 274 K

-10 -20 -30

CF3

-40 0

1000

2000

3000

4000

5000

6000

Field (G) c) Signal (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

30

10 K 20 K 50 K 110 K 128 K 200 K

20 10 0

205 K 230 K 270 K

-10

CF6

-20 0

1000

2000

3000

4000

5000

6000

Field (G)

Figure 7. EPR signal of samples a) CF2, b) CF3 and c) CF6 with variable temperature.

The peak-to-peak linewidth (∆Ηpp) (Figure 8a) becomes narrower with a small temperature dependence for samples CF2 and CF6 at least in the temperature range 5-280 K. The ∆Ηpp tends to decrease upon heating in the low temperature region (~20-170 K) for the CF3 sample due to super-exchange interactions between unlike spins. Nevertheless, when temperature was further increased it behaved similarly with CF2 and CF6. The EPR resonance of these three samples resembles that of a suspension’s EPR


Page 21 of 35

behavior, where the inter-particle interactions are negligible and do not contribute to the signal broadening due to high dilution that promotes the particles isolation.58 The g value is affected by the same phenomena as ∆Ηpp. Reduction of linewidth may cause reduction of ∆Ε (Eq. 2).

∆Ε = hv = g × β × Ηr ,

(Eq. 2)

where g is the constant of proportionality between the frequency and the field at which resonance occurs and is proportional to the magnetic moment of the molecule being studied. We have estimated the g-value from EPR spectra by using the formula of Poole and Farach (Eq. 3)59

g=

h×v , µΒ × Ηr

(Eq. 3)

where Hr is the resonance field, ν the frequency and µ Β the Bohr magneton. The shift of the main signal to g~2 with increase of temperature reflects the magnetic moment weakening (Figure 8b) and corresponds to strongly interacting Fe3+ and Co2+ ions.60

a) 2000

b)

1900

CF2

1800

3.4

1700

CF3

3.2

1600

CF6

3.0

1500 1400

11 nm

1300

15 nm

1200

g factor

ÄÇpp (G)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CF2 CF3 CF6

2.8 2.6

8.7 nm 2.4

1100

8.7 nm

11 nm

1000

2.2

15 nm

900 0

50

100

150

200

250

300

0

50

Temperature (K)

100

150

200

250

300

Temperature (K)

Figure 8. a) Variation of peak to peak linewidth (∆ΗPP) with temperature of the samples CF2 (--), CF3 (-▼-) and CF6 (-●-); b) g factor variation with temperature for CF2 (--), CF3 (-▼-) and CF6 (-●-) samples.



The first derivative of the X-band EPR spectrum of the CF5 sample is shown in Figure 11. The small values of ∆Ηpp at low temperature region showed negligible dipolar interactions among the small sized nanoparticles (7 nm)10,

61

as seen in Figure 9. This

dissimilar behavior of the CF5 sample at low temperature compared to CF2, CF3 and CF6 was related to the magnetic crystalline anisotropy combined with the random orientations of the particles.

Line broadening was caused as the averaging effect of thermal

fluctuation of magnetization was reduced and the direction of magnetization was blocked, at first in bigger and progressively in smaller particles.62 The increase of temperature provided thermal energy to the atoms, increased the ions motion and resulted in stronger super-exchange interactions through the oxygen ions.63

10

Signal (arb.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

CF5

0 5K 10 K 15 K 45 70 K 100 K 130 K 210 K 245 K 272 K

-10 -20 -30 -40 0

1000 2000 3000 4000 5000 6000 7000 8000

Field (G)

Figure 9. EPR Signal of CF5 sample in a temperature range of 5 K-272 K.

The ∆Ηpp broadening with the increase of temperature up to ~300 K (Figure 10) could be associated with a kind of phase transition, although, ZFC/FC measurements were not performed for this sample it could be assumed that due to its small size, it has a lower blocking temperature, Tb than the larger NPs, e.g. CF3 has a Tb >300 K. The g factor of the CF5 sample (Figure 10 inset) remained stable at low temperature (5-30 K) and then slightly increased between 30 and 80 K. With further increase of temperature the g factor significantly decreased and reached the value ~2.4 at 270 K. This has been observed 22 ACS Paragon Plus Environment

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before for zinc ferrite nanoparticles of 10 nm and it was attributed to the dominancy of surface energy on the magnetodynamics of nanoparticles below a critical diameter, hence, the large specific area of the sample caused a nonlinear variation of the g-value in respect with temperature.64 4500

CF5

4000 3500 3000 2500 2000 g factor

ÄÇpp (G)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1500 1000

3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 2.4

500

0

50

100 150 200 250 300

Temperature (K)

0 0

50

100

150

200

250

300

Temperature (K)

Figure 10. Peak-to-peak linewidth variation with temperature of the CF5 (--) sample; inset: g factor variation with temperature of CF5 (--).

EPR spectroscopy also provided the effective uniaxial magnetic anisotropy (Keff) by Eq. 3,7 based on the assumption of negligible inter-particle interactions. K eff =

MS 2

 2πv   − Η R  ,  γ0 

(Eq. 4)

Keff was calculated at RT for the samples where the inter-particle magnetic

interactions are insignificant and was found Keff (105 erg/cm3): 0.80, 0.70 and 0.75 for CF2, CF3 and CF6 respectively. The samples exhibited smaller anisotropy than the recently reported Keff for CoFe2O4 NPs of 10 nm that exhibited a broad resonance at RT (Keff = 3.7x105 erg/cm3).7



4

Signal (arb. units.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

10 K 20 K

2 0 -2 -4 -6

CF1

-8 0

2000

4000

6000

8000

10000

Field (G)

Figure 11. EPR signal of the CF1 sample at 10 and 20 K.

For the CF1 sample it was possible to receive EPR signals in the temperature range of 5-20 K (Figure 11), after that temperature the signal became extremely broad. At very low temperatures (< 10 K) many resonances both in low and high magnetic fields were observed, that could be attributed to strong Fe3+-Co2+ interactions. Above 20 K the signal was severely broadened and became invisible in X-Band. The CF4 sample did not exhibit an EPR signal at least in the measured temperature range (5-300 K), which is not uncommon for cobalt ferrite nanoparticles.6 Additional studies are required in order to set a trend for the incorporation of Co2+ in Td sites and the EPR signal of CoFe2O4 NPs. However, an important aspect on the absence of resonance in a wide range of temperature for these two samples could be related to the signal linewidth broadening as a consequence of aggregation.65 This could also account for the enhanced magnetization observed for the CF1 and CF4 samples and can be assigned to exchange coupling and/or dipolar interactions.

Assumingly, this

behavior can emerge by the interaction of the free -NH2 groups on the NPs surface with the metal core of the next moiety through electron donation resulting in an extended network that shows a collective magnetic behavior similar to that of cluster-assembled nanoparticles.66, 67 24 ACS Paragon Plus Environment

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EPR Relaxation Phenomena For the better understanding of the phenomena related to the magnetic behavior of the NPs, EPR spectroscopy was used to study the relaxation mechanism. The spin relaxation process is a function of static magnetic field and depends on the rate at which microwave energy can be absorbed and dissipated via relaxation processes either by spinlattice (T1) or spin-spin relaxation (T2) process. The spin-spin relaxation process is the energy difference (∆Ε) transferred to neighboring electrons and the relaxation time (T2) (Figure 12) can be determined from the ∆Ηpp linewidth according to Eq. 5.

1 π × g × β × ∆H = , T2 h

(Eq. 5)

where ∆Η is the full-width-at-half-maximum of the absorption curve ( = √3

).

16

CF3

14

CF5 CF6

12

-12 T2 (10 s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CF2

10 8 6 4 2 0

50

100

150

200

250

300

Temperature (K)

Figure 12. Spin-spin relaxation time in respect with temperature for the CF2, CF3, CF5 and CF6 samples

Figure 12 displays that the CF2, CF3 and CF6 samples presented the same behavior attributed to the spin-spin relaxation process (T2), that increased with the increase of temperature ascribed to the superparamagnetic relaxation at the surface of the NPs. In case of CF5 (Figure 10), ∆Η decreases rapidly as the temperature decreases. 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

This behavior is observed when the linewidth (∆Η) is determined by the spin-lattice (T1) relaxation process,

since the involved spins are like and the magnetic exchange

interaction causes the resonance lines to broaden with the increase of temperature. On the contrary, when the magnetic exchange interaction takes place between unlike spins, the resonance lines become narrower with the increase of temperature as in CF2, CF3 and CF6 samples (Figure 8a).68

Hydrophilic CoFe2O4 NPs and NMR Relaxometry CoFe2O4 NPs have been proposed previously as T2 contrast agents.69,70 The dispersion of hydrophobic MNPs in water for potential MRI applications is a prerequisite for in vitro and in vivo applications. The presence of free -NH2 groups, which was confirmed by ninhydrin colorimetric assay, was advantageous since it enabled the conversion of the hydrophobic NPs to hydrophilic through the attachment of the fluorescent dye SRB. The modification with SRB involved the attachment of the dye through the formation of a sulfonamide bond between the -NH2 groups on the surface of the NPs and the sulfonyl group of SRB (NPs@SRB).


Page 27 of 35

3000

2000

1000

a) CF1

CF1@SRB

b) CF3

CF3@SRB

CF4

CF4@SRB SRB 3000

2000

1000

520

-1

Wavenumbers (cm )

547 nm

Emission (normalized, arbitr. units)

Transmittance

CF1@SRB CF3@SRB CF4@SRB unmodified NPs

540

560

580

600

620

Wavelength (nm)

Figure 13. a) FT-IR spectra of samples CF1, CF3 and CF4 () and the corresponding spectra after their modification with SRB (); b) Fluorescence measurements of the isolated SRB modified NPs (CF1@SRB, CF3@SRB and CF4@SRB).

b)

60 40

Magnetization (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CF3 CF3@SRB

20 0 -20 -40 -60 -10000

-5000

0

5000

10000

Applied Magnetic Field (Oe)

Figure 14. a) Image of CF3@SRB NPs in DMF i) by optical microscope and ii) under UV lamp (ex. 475 nm, em. 535 nm); b) Magnetization at 1 T of sample CF3 without a TGA correction (-●-) and magnetization of CF3@SRB without TGA correction (). 27 ACS Paragon Plus Environment


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Page 28 of 35

FT-IR spectroscopy and fluorescence emission measurements of the isolated NPs@SRB (~547 nm) certified the presence of SRB on the nanoparticles (Figure 13). Additionally, images in an optical microscope under UV light also confirmed the modification of the NPs to fluorophore agents, while VSM measurements on the modified NPs showed no change in magnetization compared to the unmodified NPs as it was expected due to the small molecular weight of SRB (Figure 14). A simplified equation typical for the calculation of R2 with respect to the metal ion concentration (C) of an aqueous suspension of magnetic NPs is given in Eq. 6.

R2 =

1 1 = 0 + r2 ⋅ C T2 T2

(Eq. 6)

The efficiency of the NPs to act as contrast agents is determined in terms of the 0 transverse relaxivity (r2) according to Eq. 5,71 in which T2 is the proton relaxation time of

pure water, and C is the concentration of the contrast agent. The transverse relaxivity, r2, is the slope of the curve fitted to the plot of concentration (C) against R2 (C, R2). All measurements showed monoexponential decay, which is characteristic of magnetic compounds that enhance the water proton relaxivity by diffusion effects. The relaxivity values were found 232.0, 167.4 and 130.3 mM-1 s-1 (Figure 15) for CF1@SRB, CF3@SRB and CF4@SRB respectively.

The signal intensity was weakened with a

certain R2 rate and fundamentally depends on the corresponding Ms values and the NPs size while the nature of the surface coating around the magnetic core is crucial since it controls the diffusion of water protons.72 The higher value of r2 found for CF1 and CF3 compared to CF4 is attributed to the formation of aggregates in aqueous solution, supported also by the optical images as a result of the presence of more SRB molecules on the surface of the NPs in consequence with the greater amount of -NH2 groups. Within


Page 29 of 35

these nanoclusters, concerned as magnetized spheres, intercrystal interactions produce a high magnetic field gradient and consequently a dominant r2 effect as previously reported by Gillis et al.73 180

CF1 SRB -1 -1 r2=232.0 mM s

160 140

CF3 SRB -1 -1 r2=167.4 mM s

120 100 -1

R2 (s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CF4 SRB -1 -1 r2=130.3 mM s

80 60 40 20 0 -20 0.0

0.2

0.4

0.6

0.8

Metal Ion Concentration ([Fe+Co] mM)

Figure 15. Relaxivity of aqueous solutions of CF1(■), CF3() and CF4 (●) modified with SRB

CONCLUSIONS Developing novel general greener synthesis routes covering a wide range of metal oxide phases that can be adapted for custom size and surface modifications are important. The adopted variant hydrothermal synthesis created by a self assembly oil in water system was proven an efficient approach for the isolation of pure phase CoFe2O4 NPs with a range of size and enhanced Ms values, favorable for bioapplications. The beneficial presence of fatty amine (ODA) in combination with acetylacetonate origin of the precursors favored a bilayer surrounding the NPs or an extended network, with free amino groups that can be exploited further for the modification of the NPs to fluorophore agents and/or drug carriers. Additionally, the use of the trivalent cobalt acetylacetonate precursor gave rise to variations of Co2+ amount in Td and Oh sites in the spinel cell as indicated from Raman spectroscopy while the expansion/contraction of the spinel cell can easily be



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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probed. EPR spectroscopy albeit is not an easy method in case of cobalt ferrite, in the present study it was shown that the EPR signal and the spin relaxation process is size dependent, while it is influenced by aggregation or clustering effects. Also, EPR spectroscopy gave an estimation of the uniaxial magnetic anisotropy of the samples. The combination of both Raman and EPR spectroscopy allowed the comprehension of the morphological and magnetic properties of the resulting CoFe2O4 NPs.

ACKNOWLEDGEMENTS This research has been co-financed by the European Union, European Social Fund (ESF) and by the Greek National Strategic Reference Framework (NSRF) (Operational Program “Education and Lifelong Learning” of the Research Funding Program “Thales”). The authors would like to thank A. N. Papadopoulos, Ass. Proffessor in Alexander Technological Educational Institute of Thessaloniki (ATEITH), Greece for the concession of the EPR facility.

ASSOCIATED CONTENT Supporting Information: TEM image of sample CF2, FT-IR spectra of samples CF1-4, FT-IR and 1H-NMR spectra of free ODA, 1H-NMR spectra of the organic coatings of CF1, CF2 and CF5, absorbance spectra of Ruhemann’s purple for samples CF1-4 after ninhydrin colorimetric assay, thermogravimetric analysis of the CF1-4 samples, fitted Raman spectra of the CF1-6 samples, SQUID measurements of sample CF3 and CF1 and VSM measurements of samples CF1-4 with correction by TGA data. This information is available free of charge via the Internet at http://pubs.acs.org

REFERENCES


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CoFe2O4 nanoparticles (16-9 nm) were hydrothermally synthesized through a water/octadecylamine self-assembly system acetylacetonate precursor ratio.

in respect to

oil/water ratio and

Raman spectroscopy and Electron paramagnetic

resonance were used for further characterization.

The magnetic nanoparticles were

converted to fluorescent agents and NMR relaxometric properties were studied.


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50x49mm (300 x 300 DPI)

ACS Paragon Plus Environment

Unveiling the Physicochemical Features of CoFe2O4 Nanoparticles

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