Magnetic and Optical Properties of Isolated and Aggregated CoFe2O4

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C: Physical Processes in Nanomaterials and Nanostructures 2

4

Magnetic and Optical Properties of Isolated and Aggregated CoFeO Superparamagnetic Nanoparticles Studied by MCD Spectroscopy

Yulia Gromova, Vladimir G. Maslov, Mikhail A. Baranov, Raquel Serrano-García, Vera A. Kuznetsova, Finn Purcell-Milton, Yurii K. Gun'ko, Alexander V. Baranov, and Anatoly V. Fedorov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00829 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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The Journal of Physical Chemistry

Magnetic and Optical Properties of Isolated and Aggregated CoFe2O4 Superparamagnetic Nanoparticles Studied by MCD Spectroscopy Yulia A. Gromova,∗,† Vladimir G. Maslov,† Mikhail A. Baranov,† Raquel Serrano-García,‡ Vera A. Kuznetsova,‡,† Finn Purcell-Milton,‡ Yurii K. Gun’ko,‡ Alexander V. Baranov,† and Anatoly V. Fedorov† †ITMO University, St. Petersburg, 197101, Russia ‡University of Dublin, Trinity College, Dublin 2, Ireland E-mail: [email protected]

Abstract

Magnetic, optical, and structural properties of superparamagnetic CoFe2 O4 nanoparticles (MNPs), both isolated and in aggregates, are investigated by magnetic circular dichroism (MCD) spectroscopy. The MCD signal for MNPs is more than an order of magnitude greater than the signal for organic molecules, therefore, making this technique a very sensitive tool for the examination of MNP properties. MNP aggregation had a distinct effect upon the MCD signal intensity. Correlation between MCD signal intensity and MNP magnetization shows that MNP aggregation in colloidal solutions results in the changing of MNP magnetization with a maximum at the mean aggregate size of approximately 100 nm.

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Introduction Magnetic nanoparticles (MNPs) attract widespread attention due to their unique physical properties, which in most cases differ distinctly from the properties of the bulk material. 1 One of the most important features demonstrated by MNPs is superparamagnetism. In sufficiently small nanoparticles magnetic moments can flip randomly under the influence of temperature, therefore in the absence of an external magnetic field, there is no MNP remanence and the average magnetization of superparamagnetic MNP equates to zero. For example, for CoFe2 O4 , the superparamagnetic behavior observed at room temperature for MNPs with sizes below approximatly 10 nm. 2 This property makes superparamagnetic MNPs a perfect candidate for a biomedical applications 3–5 because the property of zero remanence obviates the risk of uncontrolled aggregation of the nanoparticles in the bloodstream or tissues. The drawback of using MNPs is that isolated MNPs are weakly magnetized, in contrast to the aggregated MNPs or volume materials. Therefore the control of MNP aggregation while maintaining their superparamagnetic properties remain an emerging challenge in MNP biomedical application. 6–8 The most popular approach to MNPs production is the co-precipitation of precursors in alkaline aqueous solution. 9–11 This approach is very simple and gives a high product yield. Also, MNPs synthesized by this method are biocompatible and free of toxic post-synthetic compounds. During the synthesis, MNPs can be capped by different hydrophilic ligands, and the use of chiral molecules for this purpose is an attractive option for biological applications. 12 Unfortunately, co-precipitated MNPs almost always exist in random size aggregates, 9,11,13,14 that may restrict their practical application. Moreover, aggregation may have substantial influence on MNP magnetic and optical properties. 15–17 However, techniques, routinely implemented for sample magnetization measurementÑŃ, normally require the use of solid or frozen samples that give a poor idea about sample properties in colloidal solutions. Therefore, the investigation of MNP magnetization in aggregates of different size in colloidal solutions is important but a nontrivial task. 2


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Magnetic circular dichroism (MCD) spectroscopy can be adopted as an alternative method for the study of MNP magnetic properties, because the intensity of the MCD signal is related to the magnetic moments of the compounds’ electronic energy levels. 18 MCD spectroscopy is considered as one of the most reliable and informative tools for the analysis and interpretation of the electronic spectra of various atomic and molecular systems. Valuable information about sample composition and structure can be obtained from MCD transitions unresolved by other spectroscopic techniques. 19 Previously MCD spectroscopy was used for the investigation of electronic structure of Fe3 O4 nanoparticles 19 and exploring the effect of Co doping in Fe2 O3 nanoparticles. 20 In the present work, we new introduce the application of MCD spectroscopy for the investigation and validation of aggregation, structure, and magnetization of CoFe2 O4 nanoparticles, which were synthesized by the co-precipitation method and capped by L- and D-cysteine (L-Cys-MNP and D-Cys-MNP, respectively).

Experimental Starting materials All chemical reagents were of analytical grade and used as purchased without further purification. L-cysteine (98.5%) and D-cysteine (99.8%), L- and D-cysteine hydrochlorides (≥98%), cobalt nitrate (Co(NO3 )2 ·6H2 O, ≥98%), iron(II) chloride (FeCl2 ·4H2 O, ≥99%), ammonium hydroxide (NH4 OH, ≥99.99%) were purchased from Sigma-Aldrich. Degassed Millipore water was also used as a solvent.

Synthesis of CoFe2 O4 nanoparticles CoFe2 O4 MNPs were synthesized following the co-precipitation method 21,22 with some modifications. In a 250 mL round bottomed flask, 180 mL of H2 O was heated up to 80◦ C and degassed under Argon. Simultaneously 0.58 g of Co(NO3 )2 · 6H2 O was dissolved in a sample tube containing 10 mL of Millipore water and 0.79 g of FeCl2 ·4H2 O was dissolved in 3



a separate sample tube containing 10 mL of Millipore water. L- or D-cysteine 0.04 g was added to these solutions. The solutions were then combined and added into the 180 mL of degassed Millipore water. The pH was adjusted to 11 using a solution of NH4 OH (0.88 M). The resulting solution was continuously degassed with argon for another 1 hour. In this synthesis Fe2+ ions were oxidized to Fe3+ by the nitrate ion from cobalt nitrate with the formation of dark precipitate of cobalt ferrite. 22–24 The precipitate was washed with ethanol and water until a neutral pH was reached and dried under vacuum.

MNP post synthesis treatment 5 mg of MNPs was dispersed in 2 ml of Millipore water and after addition of 5 vol.% of 0.01 M L- or D-cysteine hydrochloride the solution was sonicated during 15 min. The hydrochloride forms of cysteine have less coordination activity compared with the cysteine free form and therefore avoids MNP etching. Then solution was centrifugated at the speed of 16000 rpm (G-force ∼ 13000) for 0, 1, 3, 5 and 7 min to obtain aggregates of different mean sizes.

Characterization Transmission electron microscopy (TEM) was performed using an Titan electron microscope (FEI, Netherlands). TEM images were obtained under beam voltage of 300 kV. Scanning electron microscopy (SEM) was carried out using a Ultra plus (Zeiss, Germany). X-ray diffraction (XRD) analysis was implemented using a Siemens D500 diffractometer (Siemens AG, Germany). UV-Vis Spectroscopy was carried out using a UV-Probe 3600 spectrophotometer (Shimadzy, Japan). Circular dichroism (CD) and magnetic circular dichroism (MCD) Spectroscopy was carried out using a Jasco J-1500 CD spectrometer equipped with an electromagnetic unit, MCD-581, supplied constant magnetic field tunable from -1.5 to +1.5 T. The measurement of MCD spectra was performed at +0.08 and -0.08 T. 4


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The magnetic properties of MNP dry powder were studied using a custom-made Vibrating Sample Magnetometer (VSM) with a field up to 1 T. The particle size of samples were measured using dynamic light scattering (DLS) particle size analyzer Horiba, LB 550V (Horiba, Japan). Ultrasonic treatment was carried out using a Branson 2510 ultrasonic bath operating at 40 Hz.

Results and discussion Characterization of MNP Cysteine stabilized CoFe2 O4 magnetic nanoparticles have been prepared using the well documented co-precipitation technique. 21 Figure 1a shows DLS data for L-cys- and D-cys-MNPs combined with example TEM micrograph of L-cys-MNPs shown on the insert. The TEM micrographs for D-cys-MNP are presented in Fig. S1 in the Supporting Materials. Examination of TEM micrograph of as-prepared CoF2 O4 nanoparticles together with the investigation of hydrodynamic diameters of MNPs dispersed in aqueous solution by DLS revealed that all MNPs exist in aggregates with average sizes over 100 nm. However, careful examination of TEM micrographs reviles that aggregates consists of spherical MNPs with sizes less than 10 nm. Powder XRD studies allowed the determination of the crystalline phase of the nanoparticles. Fig 1b displays a XRD pattern of the as-synthesized samples of L-cys-MNPs and D-cys-MNPs. The position and relative intensity of all of the diffraction peaks matched well with the standard inverse spinel phase of CoFe2 O4 . 25 The average MNP crystallite size was calculated using the Scherrer formula: 26

d=

kλ β sin θ 5


(1)


a

1 .0

D -c y s

L -c y s

L -c y s

0 .8 0 .6 0 .4

1 0 n m

0 .2 0 .0

(3 1 1 )

B In te n s ity , a r b .u .

In te n s ity , n o r m .

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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(4 4 0 ) (1 1 1 )

(2 2 0 )

D -c y s

(4 0 0 )

(5 1 1 ) (4 2 2 )

0

L -c y s 5 0 0

1 0 0 0

1 5 0 0

2 0

H y d r o d y n a m ic d ia m e te r , n m

3 0 4 0 5 0 6 0 2 T e th a , d e g re e

7 0

Figure 1: a) Hydrodynamic diameter and inserted TEM micrograph and b) XRD patterns of as-prepared CoFe2 O4 nanoparticles capped by L-cysteine (only in TEM image) and Dcysteine.

Figure 2: Magnetization curve versus magnetic field for as-prepared CoFe2 O4 nanoparticles capped by L-cysteine (1) and D-cysteine (2).

where: d – is the mean size of the crystalline domains; k =0.154 nm – is a the X-ray wavelength; β – is the line broadening at half the maximum intensity; θ – is the Bragg angle. According to the calculations, L-cys-MNPs and D-cys-MNPs have the average crystallite sizes of approximately 6.5±0.5 nm and 9.5±0.5 nm respectively. This difference in MNP crystalline size is repeatable from synthesis to synthesis and could be associated with differences in MNP crystallization and growth dynamics arising due to the presence of different levels of impurities in L- and D- cysteine. It is well known that size of nanoparticles is very sensitive to capping ligands concentrations and their purity. 27–30 Importantly this size 6


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difference between L-cys- and D-cys-MNPs is negligible when considering a MNP’s optical properties since electronic transitions in CoFe2 O4 MNPs are size independent in particles with diameters above 6 nm. 31 Room temperature magnetization measurements for MNPs are presented in Figure 2. The saturation magnetization of the as-prepared L-cys and D-cys coated CoFe2 O4 MNPs is determined to be 29 and 25 Am2 kg−1 respectively, much lower than for the bulk CoFe2 O4 , probably due to the presence of magnetically dead layer on MNP surface 32 and possible defects in crystal structure. 33 The samples did not show any coercivity and remanence, which is clear evidence of MNP superparamagnetism. As reported in literature, 2 superparamagnetic properties in CoFe2 O4 at room temperature observed in nanoparticles with diameters less 10 nm. Therefore the size of our MNPs should not exceed 10 nm to demonstrate superparamagnetic properties. Taking into account all of the examinations above, we suppose that as-prepared MNPs have sizes less than 10 nm but are observed stuck together in aggregates with average sizes over 100 nm.

Influence of post-synthetic treatment on MNP aggregates size DLS analysis of MNP colloidal solutions revealed that post-synthetic treatment of MNP ensemble leads to a decrease of MNP aggregate mean sizes and eventually yields single MNPs. Fig. 3 shows the L-cys-MNP aggregate size distribution after different centrifugation times at 16000 rpm (G-force ∼ 13000) for the aqueous MNP solution. Fig. 3 displays that the average size of the MNP aggregates according to the DLS data is 500, 300, 120, 70 and 8 nm for a centrifugation time of 0, 1, 3, 5 and 7 min. Thus, after 7 minutes of centrifugation, only nonaggregated MNPs remain in the solution. Obviously, that after shorter centrifugation times in solutions there are also nanoparticles smaller than the specified mean sizes, but their fraction is relatively small and further we will assume that the optical properties of the MNP are mainly determined by the average size of the 7



ensemble. The average size of aggregates obtained by analysis of SEM micrographs exceeds DLS data (results presented in Supporting matherials, Fig. S2). This fact can be attributed to the aggregation of MNPs during SEM sample preparation. 34 c '

d 5 0' n m 5 0 n m


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

e

0

1

0

d

b '

1 0 0 n m

c b

2 0 0 n m

a

2 5 0

5 0 0

7 5 0


Figure 3: Hydrodynamic diameter of L-cys-MNP aggregates after 0 (a), 1 (b), 3 (c), 5 (d) and 7 (e) minutes of centrifugation at 16000 rpm (G-force ∼ 13000) after 15 min sonication in presence excess of L-cys hydrochloride. On the top panel are presented SEM micro graphs of MNP aggregates after 1 (b’), 3 (c’) and 5 min (d’) of centrifugation.

Circular dichroism (CD) of MNPs It is well-known that the synthesis of colloidal semiconductor nanocrystals in the presence of chiral ligands leads to the formation of optically active nanostructures. Chirality in semiconductor nanoparticles can originate from (i) enantiomeric distortion of surface atoms during nanocrystals growth and (ii) the electronic coupling between molecular orbitals of chiral capping ligands and achiral nanocrystals. 35,36 In the case of CoFe2 O4 MNPs synthesis in the presence of chiral ligands, we also expect to produce an optically active material. In Fig. 4, the CD spectra of nonaggregated L-cys- and D-cys-MNPs are shown. The spectra demonstrate strong CD bands in the UV region. But a careful examination of data revealed that this band corresponds to chelate complexes of cysteine with Co2+ or Fe3+ ions detached from the MNP surface. Indeed, when MNPs solutions were filtered through MNP impenetrable membrane with 10 kDa pores, the solvent passed through membrane exhibited exactly the 8


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same CD bands as MNP solution before filtration (Fig. 4 spectra 3 and 4). Moreover, the observed CD bands completely coincide with a linear combination of CD bands from cysteine, and cysteine complexes with Fe3+ and Co2+ ions with concentrations 1.3 · 10−4 mol/l and 1 · 10−5 mol/l, respectively (for more details see the Supporting Materials). Thereby MNPs capped by chiral ligands themselves are optically inactive.

C D , m d e g

2 0 1 0 0 -1 0 -2 0

O p tic a l d e n s ity

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


0 .8 0 .6 0 .4 0 .2 0 .0

2 0 0

3 0 0

1 2 3 4

4 0 0 5 0 0 6 0 0 W a v e le n th , n m

7 0 0

8 0 0

Figure 4: CD (top) and absorption (bottom) spectra of aqueous solutions of nonaggregated L-cys-MNPs (1) and D-cys-MNPs (2) after post-synthetic treatment (7 minutes of centrifugation with G-force ∼ 13000 after 15 min sonication in presence excess of L-cys hydrochloride) and spectra of infiltrate passed through 10 kDa pore membrane after centrifugation of L-cys-MNPs (3) and D-cys-MNPs (4) solutions in concentrators.

Magnetic circular dichroism (MCD) of nonaggregated MNP In contrast to CD, MCD spectroscopy is suitable for the investigation of a much more broad class of objects including compounds containing paramagnetic ions like Co2+ and Fe3+ . Indeed, the application of magnetic fields induces intensive MCD signals in MNPs, with no difference for L-cys- and D-cys- capped MNPs observed. Therefore, we will further only concentrate on the properties of L-cys-MNPs. The MCD spectrum of the nonaggregated L-cys-MNPs is shown in Fig. 5. To avoid distortion of the MCD spectra by the CD signal arising from the complexes of cysteine with Co2+ and Fe3+ ions detached from the MNP surface, the spectra registered at zero magnetic field was subtracted from the MCD spectrum. The spectrum is adequately fitted with six 9



Gaussian bands with fitting parameters listed in Table 1. It should be noted that the ratio of intensities of MCD bands are slightly changed for different sets of samples. This could be caused by the different amount of surface ions detached by cysteine during MNP postsynthetic treatment. Importantly, the observed signals refer exclusively to the MNPs, since the cysteine complexes with Co2+ and Fe3+ do not exhibit MCD signal under magnetic field of 0.08 T used in the experiments (data is shown in the Supporting Materials, Fig. S4). W a v e n u m b e r s , 1 0 3 ⋅c m −1 2 9 2 2 1 8 1 5 1 3

4 0 3 0 e

a

d b

2 0 M C D , m d e 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

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c

1 0 0 -1 0

1

-2 0 2 f

-3 0

3 0 0

4 5 0 6 0 0 W a v e le n th , n m

7 5 0

Figure 5: Experimental MCD spectrum (1) of individual L-cys-MNPs measured at +0.08 T magnetic field and its Gaussian band fitting. The bands marked with letters from a to f. The sum of the deconvoluted spectra (2) is also presented.

Table 1: The main MCD transitions in CoFe2 O4 between 800 nm and 200 nm are presented with parentheses denoting tetrahedral coordination and square brackets indicating octahedral coordination. No. a b c d e f

Peak nm 685 550 425 351 303 256

position, FWHM, cm−1 cm−1 14600 3145 18180 4195 23530 5970 28490 508 33000 6290 39060 9435

Intensity, arb.units 23 17 8 25 28 -30

Transition interpretation (Co2+ ): 4 A2 [Co2+ ]t2g ([Fe3+ ]):6 A1 ([Fe3+ ]):6 A1 ([Fe3+ ]):6 A1 ([Fe3+ ]):6 t1

→ 4 T1 (P ) → [Fe3+ ]t2g → 4 E,4 A1 (4G ) → 4 T2 (4 D) → 4 T1 (4 P ) → 2 t2g

Ref. 37 37 38 38 38 38

Two electronic transitions at the longer wavelengths are attributed to transition involving Co2+ ions 37 . The first band at 685 nm can be assigned to the paramagnetic crystal field (CF) transition from the ground 4 A2 to the excited 4 T1 (P ) state of Co2+ in tetrahedral 10


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coordination. The second band at 550 nm corresponds to the intervalence charge transfer transition (IVCT) from [Co2+ ]t2g state to [Fe3+ ]t2g , involving ions in the octahedral sites. Therefore MCD sample investigation revealed that Co2+ cations occupy positions with both tetrahedral and octahedral coordination. The position of other bands matches well with the transitions observed in maghemite (γ-Fe2 O 3 ) 38 . The 430 nm, 350 nm and 303 nm bands correspond to the 6 A1 →4 E,4 A1 (4 G), 6 A1 →4 T2 (4 D) and 6 A1 →4 T1 (4 P ) ligand field transitions respectively. However, according to the literature, the band at 303 nm also can be attributed to the double excitation of magnetically coupled two adjacent Fe3+ cations: 6

A1 +6 A1 →4 T1 (4 G) +4 E,4 A1 (4 G). The last band at 256 nm can readily be assigned

to ligand-to-metal (Fe3+ ) charge transfer (LMCT) transition 6 t1u →2 t2g . It should be noted that ligand field states of tetrahedrally coordinated Fe3+ ions have energies which are similar to the analogous states of octahedrally coordinated Fe3+ . Therefore, bands observed in the spectra could not be unambiguously attributable to Fe3+ cations in octahedral or tetrahedral coordination. It should be noted that the full width at half maximum (FWHM) of all transition are twofold broadened compared with the bulk material. This effect is probably due to a large impact of disordered surface layer or defects in the MNP core. 39,40

MCD field dependence It was found that the MCD of MNPs is very sensitive even to a low magnetic field. The dependence of the normalized MCD intensity on applied magnetic field for different transitions are shown in Fig. 6. MCD measurements in the magnetic field less 0.1 T do not lead to the aggregation of MNPs, confirmed by absorption spectra which had no changes during measurements (spectra presented in the Supporting Materials, Fig. S5). MCD transition intensities grow with the increase of magnetic fields and all observed curves are superimposed with each other. As is seen from Fig. 6, the dependence of MCD intensity on magnetic field completely coincides with the magnetization curve. This result means that room temperature measurement of the MCD intensities as a function of magnetic fields provides optically 11



1 5

6 8 5 n m 5 5 0 n m 3 5 1 n m

0 .5

1 0 5

0 .0

0 -5

-0 .5

-1 0 M a g n e tiz a tio n - 1 5

-1 .0 -0 .1 0

-0 .0 5 0 .0 0 0 .0 5 M a g n e tic fie ld , T

2

M C D

M a g n e tiz a tio n , A m

1 .0

/k g

detected magnetization data.

M C D in te n s ity , n o r m .

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 .1 0

Figure 6: Normalized MCD intensity at 685, 550 and 351 nm under changing magnetic field for nonaggregated L-cys-MNPs superimposed on MNP magnetization curve from Fig. 2

MCD of MNP aggregates As was mentioned in the introduction, aggregation of MNPs might change their magnetic and optical properties. Dipolar interactions between neighboring MNPs is the most expected reason of these effects. 15 Size dependence of magnetic G-factor (ratio of MCD signal in units of absorption to absorption at given wavelength normalized on the applied magnetic field) is presented in Fig. 7. The values of G-factor for MNP transitions are more than in order of magnitude exceed the value for organic molecule. 41,42 The different behavior of G-factor for MCD transitions involving Co2+ and Fe3+ ions is observed. In the case of Co2+ transitions, the G-factor grows with increasing aggregate size and G-factor decreases when aggregate size becomes over 100 nm. For Fe3+ transitions, growing in G-factor is not observed. As was shown above, the MCD signal dependence on applied magnetic field follows the MNP magnetization curve. Therefore all changes in MCD spectra intensity are possible to be interpreted according to MNP magnetic susceptibility change. In small aggregates (below 100 nm) MNPs localized close to each other, form spherical aggregates (Fig. 3, top panel, d’). In these aggregates, the dipolar interaction could align 12


A g g r e g a te s iz e , n m

1 0 0

2 0 0

3 0 0

4 0 0

0

1 0 0

2 0 0

3 0 0

4 0 0

0 .4

0 .2 0

0 .3 0 .1 0 0 .2 6 8 5 n m

0 .1

( C o

0 .0

2 +

) : 4A

4 2

3 5 1 n m

( [F e

-1

0 .0 6

3 +

]) : 6A 1

4

T 1( P )

5 5 0 n m [C o 2+]t

T 2( 4D )

( [F e

2 5 6 n m 3 +

[F e

2 g

]) : 6 t

1 u

3 +

2 t

]t

2 g

2

0 .0 5 0 .0 0 0 .0 6

-1

G -fa c to r, T

-1

0 .1 5

0 .0 4

0 .0 4

0 .0 2

0 .0 2

0 .0 0 0

1 0 0

2 0 0

3 0 0

4 0 0

0

A g r e g a te s iz e , n m

1 0 0

2 0 0

3 0 0

4 0 0

-1

0 .5

0

G -fa c to r, T


G -fa c to r,T

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


G -fa c to r, T

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0 .0 0


Figure 7: Dependence of G-factor on aggregate size for individual L-cys-MNPs and their aggregates in units of T−1 for different transitions. The G-factor magnitude for 256 nm band is presented in absolute values the magnetic moments of neighboring MNPs and reduce spin canting in the MNP surface dead layer 40 . These factors will increase the magnetic susceptibility and G-factor of MNPs in spherical aggregates. But as can be seen in Fig. 7, G-factor increases only for transitions involving Co2+ . The possible reason for this different behavior of G-factor for MCD transitions involving Co2+ and Fe3+ ions in aggregates below 100 nm could be associated with formation of Co-rich MNP surface layer during post-synthetic MNPs treatment. Indeed, cysteine detaches Fe3+ ions from the MNP surface more favorably then Co2+ iones, meaning that surface dead layers mostly consists of Co-ions. In this case, the alignment of magnetic moments in surface dead layers under dipolar interaction will have much more significant influence on transitions involving Co2+ ions. Aggregates over 100 nm consist of a number of weakly interacting smaller round shaped aggregates, as seen in TEM images (Fig. 3, top panel b’, c’). If MNP easy axis in such 13



aggregates is randomly oriented, a magnetic susceptibility of particles will be smaller than that in individual MNPs. This effect is due to a high magnetic anisotropy of CoFe2 O4 which makes alignment of the magnetic moments with the MNP’s easy axis energetically favorable. 16 In randomly oriented aggregates, particles are unable to align their easy axes with the direction of magnetic fields by means of rotation diffusion, that could be a reason for the decreasing of magnetic susceptibility and G-factor in aggregates over 100 nm.

Conclusion

The magnetic and optical properties of the CoFe2 O4 MNPs synthesized by the co-precipitation method and coated by L- and D-cysteine were investigated using various instrumental techniques including MCD. A proposed new procedure for post-synthetic MNP treatment allows obtaining both isolated MNPs and their tightly bonded spherical shaped aggregates. We have found that MNP aggregation had a distinct effect upon the resulting MCD signal intensity, showing an unequal behavior for transitions involving Co2+ and Fe3+ . Established correlation between MCD signal intensity and MNP magnetization allowed us to track change in the magnetic properties of the MNP aggregates with increasing aggregate size. We have demonstrated that MCD spectroscopy is a powerful tool for studying magnetic and structural properties of the CoFe2 O4 MNPs. This research opens the way for the development of new methodology for using MCD spectroscopy for a quantitative estimation of MNP magnetization in colloidal solutions that should be useful for further optimization of magnetic properties of MNP clusters highly demanded in biomedical and other important applications. 14


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Associated content Supporting Materials TEM micrographs of as-prepared CoFe2 O4 nanoparticles capped by D-cysteine; histogram of size distribution from TEM measurement of L-cys-MNP aggregates; method of preparation and CD, MCD and absorption spectra of L-cysteine complexes with Fe3+ and Co2+ ions; absorption spectra of MNPs aggregates used for G-factor calculation.

Autor information Competing Interests The authors declare that they have no competing interests.

Acknowledgements This work was funded by RFBR Project 17-52-50004. Yu.A.G. thanks also the Ministry of Education and Science of the Russian Federation for support via the Scholarships of the President of the Russian Federation for Young Scientists and Graduate Students.

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