magnetite interface: Implications for the stability of aqueous dispersions and the magnetic properties of magnetite nanoparticles

Accepted Manuscript Title: Interactions at the CMC/magnetite interface: Implications for the stability of aqueous dispersions and the magnetic properties of magnetite nanoparticles Author: M. Maccarini A. Atrei C. Innocenti R. Barbucci PII: DOI: Reference:

S0927-7757(14)00709-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.08.026 COLSUA 19406

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

16-7-2014 25-8-2014 27-8-2014

Please cite this article as: M. Maccarini, A. Atrei, C. Innocenti, R. Barbucci, Interactions at the CMC/magnetite interface: Implications for the stability of aqueous dispersions and the magnetic properties of magnetite nanoparticles, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.08.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Highlights  Fe3O4 nanoparticles modified with CMC were prepared  They were prepared by mixing magnetite NPs separately prepared with CMC (ex-situ)  They were prepared by coprecitation in the presence of CMC (in-situ)  The in-situ method allows to obtained smaller aggregates of NPs  Interactions at the CMC/Fe3O4 interface explains the stability of the aqueous dispersions

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Interactions at the CMC/magnetite interface: Implications for the stability of aqueous dispersions and the magnetic properties of magnetite nanoparticles

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M. Maccarini1, A. Atrei1,2, , C. Innocenti3 and R. Barbucci2 Dipartimento di Biotecnologie, Chimica e Farmacia, Università di Siena, 53100 Siena, Italy

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CRISMA, 53034 Colle di Val d’Elsa, (Siena), Italy

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INSTM and Dipartimento di Chimica U. Schiff, Università di Firenze, 50019 Sesto F.no

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(Firenze), Italy

Abstract

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Magnetite NPs modified with CMC, a polysaccharide containing carboxylic groups derived from cellulose, were prepared. Two different methods were used: Addition of CMC to a dispersion of magnetite NPs previously synthesised (ex-situ preparation) and addition of NaOH to an aqueous

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solution of Fe(II) and Fe(III) in the presence of CMC (in-situ preparation). The aim of this study was to characterize in detail the interactions between magnetite NPs and CMC and to elucidate the effect of the polymer on the magnetite NP agglomeration. FTIR spectroscopy was used to shed light

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onto the nature of the interactions between CMC and Fe3O4 NPs. The morphological

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characterization of the NPs was carried out by FESEM. The size and the -potential of the NPs in various aqueous media were determined by DLS. XRD measurements indicate that the presence of

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CMC does not modify, within the uncertainty of the measurement, the size of the primary particle size (ca. 10 nm). FTIR spectra suggest that CMC chains are anchored to magnetite NPs via carboxylate groups interacting with iron ions at the surface. The FESEM images show that magnetite NPs prepared by the in-situ method form aggregates which are significantly smaller than those prepared by the ex-situ procedure. The FESEM images reveal a different morphology of the polymeric matrix between the CMC/magnetite NPs prepared following the two procedures. The hydrodynamic diameter of the CMC/magnetite NPs in water at neutral pH prepared in the presence of CMC is 280 nm. The -potential (ca. -80 mV) measured for the dispersions in water at neutral pH of CMC/magnetite NPs prepared according to the two methods explains their long-term stability. The magnetic behavior of the CMC/magnetite NPs can be explained considering the different size of the aggregates in the two kinds of sample.

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Corresponding author. e-mail: [email protected]

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1. Introduction The possibility to control the size and shape of nanoparticles (NPs), the degree of polydispersion and their aggregation is a key issue in the preparation of NP systems. This is particularly important for iron oxide NPs whose magnetic properties are determined, to a large extent, by their size and

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aggregation. In many applications iron oxide magnetic NPs, Fe3O4 (magnetite) and -Fe2O3 (maghemite), are used as aqueous dispersions [1, 2]. Iron oxides are usually prepared in aqueous environment by precipitation (and oxidation) from a Fe(II) solutions or by co-precipitation from a

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solution of Fe(II) and Fe(III) upon adding a base under controlled conditions [3-6]. This synthetic route is simpler than methods involving non-aqueous media and organo-metallic precursors.

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Dispersions of iron oxide in water at neutral pH are not stable since the point of zero charge of magnetite and maghemite is close to 7 [7]. A method to stabilize such dispersions consists in

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coating the surface of the NPs with hydrophilic polymeric layers which prevent aggregation processes [1, 8, 9]. Polymers or copolymers with tailored properties can be used for this purpose. The polymers can be simply adsorbed at the surface of the NPs or grafted to the NPs which where

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previously functionalized with suitable groups. When polyelectrolytes are used, an electrostatic contribution is added to the steric effect in stabilizing the NP dispersions [10]. Instead of mixing NPs with a polymer solution, NPs can be prepared in the presence of polymers [11-14]. If the

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polymers are capable to form complexes with the iron ions in solution and to bind the oxide NPs

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which form upon the addition of the base, it should be possible to influence the nucleation and growth of the oxide NPs and to reduce their aggregation. [13-16]. Moreover, it has been shown that

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polymeric networks in hydrogels provide a spatially confined environment for controlling the NPs size and their polydispersity [14, 17-21]. In the present work magnetite NPs modified with carboxymethylcellulose (CMC) were prepared and characterized. CMC is a non-toxic, biocompatible polymer largely used in food and pharmaceutical industries. The sodium salt of CMC is a water soluble derivative of cellulose with carboxymethyl groups (-CH2COO-) substituting some hydroxyl groups of cellulose. CMC was chosen since carboxylic groups are able to complex iron ions in solution and to bind oxide NPs when formed upon addition of the base [13, 16, 22]. Moreover, the functional groups not involved in the bonding with oxide NPs can be used to attach suitable molecules for targeted drug delivery. CMC stabilized dispersions of magnetite NPs embedded in hydrogels can be used to investigate the role of aggregation and polydispersity on the magnetic response of these hydrogels [23]. In view of these applications, a thorough investigation of the physicochemical properties of the CMC/Fe3O4 NPs is needed. The aim of this study was to characterize in detail the interactions between magnetite NPs and CMC and to investigate the effect of the polymer on NPs agglomeration. We 3

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investigated the CMC/magnetite NPs prepared both in the presence of CMC and by adding previously synthesized magnetite NPs to a CMC solution. Fourier Transform Infra Red (FTIR) spectroscopy and Field Emission Scanning Electron Microscopy (FESEM) were used to investigate the nature of the chemical interactions at the CMC-Fe3O4 interface and the morphology of the

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CMC/Fe3O4 NPs. The size of the CMC/magnetite NPs in the dispersions and the -potential in various aqueous environments were determined by means of dynamic light scattering (DLS). The magnetic properties of the CMC/magnetite NPs were investigated by measuring magnetization

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versus magnetic field at various temperatures, zero field cooling (ZFC) and field cooling (FC)

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curves.

2. Materials and methods

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2.1 Materials

FeCl3·6H2O (97%), FeSO4·(NH4)2SO4·6H2O (99%) and CMC (sodium salt) were purchased from Sigma-Aldrich and used as received. The average molecular weight of CMC was 700.000 Da and

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the degree of substitution 0.9 (number of CH2-COO- groups per anhydroglucose unit). Deionized (DI) water (milli-Q) was used in all experiments.

2.2 Synthesis of the CMC/magnetite nanoparticles

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Magnetite NPs modified with CMC were prepared by both adding previously prepared NPs to a

(in-situ synthesis).

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solution of CMC (ex-situ synthesis) and by coprecipitation of magnetite NPs in a solution on CMC

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For the ex-situ synthesis magnetite NPs were prepared according to the coprecipitation methods reported in the literature [3-6]. 20 ml of a solution 0.05 M of Fe(II) and 0.10 M of Fe(III) were prepared by dissolving the iron salts in DI water, deoxygenated by prolonged bubbling of N2 into it. The solution was brought at a temperature of 60°C and 12 ml of NaOH 1 M (prepared in deoxygenated water) were rapidly added. The colour of the solution changed immediately from orange to black signalling the formation of magnetite NPs. The dispersion of NPs was left under stirring for 15 minutes. NPs were allowed to sediment and separated from the solution with the help of a permanent magnet. The precipitate was extensively washed with DI. NPs were dried in a dry N2 flux and conserved in a dessicator. In order to prepare the CMC modified NPs (hereafter indicated as CMC@NP ex-situ), magnetite NPs were added to a 1% w/v CMC solution. The relative amounts of NPs and CMC were such to have a 1:1 molar ratio of Fe3O4 and COO- groups. For the in-situ synthesis, 25 ml of 1% w/v CMC solution were added to 20 ml of a solution 0.05 M of Fe(II) and 0.10 M of Fe(III) (both solutions deoxygenated) in the reaction vessel under N2 bubbling. After mixing the two solutions, a gel-like phase of orange color forms. This is probably a 4

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hydrogel formed by CMC with Fe(III) and/or Fe(II) as cross-linker. The mixture was heated to 60°C and 12 ml of NaOH 1M were rapidly added. Upon addition of the base, the gel-like phase dissolved leaving a black dispersion of magnetite NPs. The dispersion was left under stirring for 15 minutes. Magnetite NPs prepared by this method (hereafter indicated as CMC@NP in-situ) were

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separated from the reaction mixture by adding small volumes of acetone. Acetone, which works non-solvent, induces a phase separation of CMC/magnetite NPs from the liquid phase. The precipitate was washed with DI, redissolved and precipitated again several times until a neutral pH

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of the surnatant was obtained. Both the CMC@NP ex-situ and the CMC@NP in-situ samples were used as prepared or dried under a N2 flux. The iron content in the two kinds of sample was

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determined by spectrophotometry after dissolving the samples in concentrated hydrochloric acid [24]. The iron content was found to be around 50 weight % of Fe3O4 in both the CMC@NP ex-situ

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and the CMC@NP in-situ samples. The dispersions in water at neutral pH, at room temperature of both the CMC@NP ex-situ and the CMC@NP in-situ samples did not show any appreciable trace

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of precipitate for weeks after their preparation.

2.3 Characterization

FTIR spectra were measured using Spectrum BX spectrometer (Perkin Elmer) at a resolution of 4

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cm-1, in transmission mode, on samples finely grinded and dispersed in KBr pellets. X-ray

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Diffraction (XRD) measurements to determine the primary particle size were performed by means of a Philips PW1830 diffractometer using the Cu K radiation.

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A igma VP FESEM microscope (Zeiss, Germany) was used for the morphological characterization of the sample. The energy of the electron beam was in the range 2-10 keV and the “in lens” detector was used to collect the secondary electrons. Measurements were performed under high vacuum conditions. Samples were prepared by depositing drops of diluted dispersions on polished copper disks and then dried in air without metallization or graphitization. DLS and -potential measurements were performed on a Zetasizer Instrument Nano ZS 90 (Malvern Instrument Ltd, UK) The NPs dispersions were diluted ca. 1:10 with the proper dispersant and the pH was adjusted to the chosen value by the addition of 0.1M NaOH or 0.1M HCl solutions. Each NP dispersion was sonicated for 5 minutes immediately before the measurements. Measurements were performed at least in triplicate for each samples. Magnetization versus field intensity, ZFC/FC curves were measured by means of Superconducting Quantum Interference Device (SQUID) magnetometer from Quantum Design Ltd. operating with a maximum magnetic field of 50000 Oe in the 2-400 K temperature range. Hysteresis loops were measured cyclically varying the applied field from 50000 Oe. Magnetization curves at 300 K were measured from 5

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50000 to -300 Oe to check for possible remanence and coercivity. ZFC/FC were measured with an applied field of 50 Oe from 2.5 to 300 K, after having cooled the sample in absence (ZFC) or in the presence of the probe field.

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3. Results and discussion 3.1 FTIR

The FTIR spectra measured for CMC, magnetite NPs, CMC@Fe3O4 in-situ and CMC@Fe3O4 ex-

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situ samples are shown in Fig. 1. The spectrum of magnetite NP is characterized by the band at ca. 580 cm-1 due to the collective vibrations of Fe3O4 [25]. The peaks attributable to the bending (1630

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cm-1) and stretching vibrations (broad band at 3450 cm-1) of water molecules adsorbed on the NP surface are also visible. In the spectra of the Fe3O4 NPs modified with CMC, in addition to the band

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at 580 cm-1 of the oxide, the peaks due to CMC are present. There are significant changes in the position, relative intensity and width of the bands of CMC in the spectra of CMC, CMC@Fe3O4 insitu and CMC@Fe3O4 ex-situ. Considering the region 1200-1800 cm-1, we observed a broadening

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towards lower wavenumbers of the COO- asymmetric stretching peak (ca. 1600 cm -1) and a shift of the COO- symmetric stretching peak (ca. 1420 cm-1) in the spectrum of the CMC@Fe3O4 in-situ sample compared to that of the CMC@Fe3O4 ex-situ (Fig. 2). The appearance of the tail and the

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shift of the COO- peaks can be attributed to the interaction of the CMC with the magnetite NPs

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[26]. In order to determine the components contributing to the peaks, we performed a curve fitting analysis whose results are shown in Fig. 3. The attribution of the various components based of the

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peak assignment of CMC [26, 27] is reported in Table 1. The curve fitting analysis of the spectrum of the CMC@Fe3O4 in-situ sample reveals the presence of two components for the COOasymmetric and symmetric stretching vibrations. These components can be assigned to COObonded (1535 cm-1 and 1383 cm-1) and not bonded (1594 and 1416 cm-1) to the NPs. This is an indication that a part of the carboxylate groups of CMC (which is impossible to quantify reliably on the basis of the IR band intensities) is not bonded to magnetite NPs. In the spectrum of the CMC@Fe3O4 ex-situ sample the components corresponding to COO- bonded to NPs cannot be detected by curve fitting analysis, whereas the other components are essentially the same found for the CMC alone. These results can be explained considering the presence of an excess of CMC not directly bonded to the magnetite NPs which dominates the FTIR spectrum. This possibility is conceivable since CMC@Fe3O4 ex-situ samples were prepared by adding the magnetite NPs to the CMC solution without any separation of the product from the liquid, whereas the CMC@Fe3O4 insitu sample was separated from the reaction mixture and washed several times. In order to verify this hypothesis, CMC@Fe3O4 ex-situ NPs were separated magnetically from the dispersion and the 6

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precipitate washed several times to remove the CMC not bound to the NPs. The decrease of the intensity of the CMC bands in the spectrum measured after washing confirms that for the CMC@Fe3O4 ex-situ sample only part of the CMC is bonded to the oxide NP while the excess can be eliminated by washing with water. The curve fitting analysis of the sample after washing shows

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the presence of the components corresponding to COO- groups bonded to magnetite (Fig. 3, bottom). It is not straightforward to determine by IR spectroscopy whether the interactions between the carboxylate groups and the Fe3O4 NPs are non specific (i.e. electrostatic interactions, hydrogen

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bonding) or specific, namely the carboxylate groups are bonded to under coordinated iron ions at the surface of the oxide. Nonetheless, in the case of citrate adsorbed on TiO2 NPs the observation of

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broader and shifted bands was taken as an evidence for inner sphere adsorption [26, 28]. In the present case, there are additional experimental evidences (see below) which indicate specific,

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chemical interactions between COO- groups of CMC and the Fe3O4 NPs. On the hypothesis that carboxylate groups are coordinated to iron ions at the surface, the wavenumber difference between the asymmetric and symmetric stretching peaks measured for COO- bonded to oxide, a-s, can

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provide some hints about the coordination geometry [26, 29-31]. In the present case a-s 152-154 cm-1 for the CMC@Fe3O4 in-situ and washed CMC@Fe3O4 ex-situ sample. This a-s falls in the

3.2 XRD and FESEM

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bridge geometry [29, 30].

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range of values attributed to carboxylate groups bonded to metal oxide surface with a bidentate

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Diffractograms measured for the CMC@Fe3O4 ex-situ and the CMC@Fe3O4 in-situ are in agreement with those reported in many studies for magnetite and maghemite NPs. In fact, on the basis of the XRD it is not possible to distinguish between the Fe3O4 and -Fe2O3 which have the same structure and very similar lattice parameters (see JCPDS cards # 19-629, magnetite, and 391346, maghemite). From the width of the (311) peak (at 35.58°), by using the Scherrer equation, we determined a size of 9 ± 1 nm of the primary NPs for both samples. Hence, we can conclude that the presence of CMC during the precipitation of the magnetite NPs does not influence the size of the particles. On the contrary, in previous studies it was observed a dependence on the polymer concentration of the average diameter of NPs, the size decreasing upon increasing the polymer concentration [11, 13]. Si et al. [13] formed magnetite NPs in the presence of CMC but the molecular weight and the degree of substitution (65000 Da and 0.13, respectively) were very different from the ones of CMC employed in the present work. Moreover, magnetite NPs were prepared starting from a solution of Fe(II) by oxidation in air in the presence of CMC. Since the reaction mechanism for the formation of magnetite is rather complicated [5, 6] it is not surprising 7

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that differences in the coprecipitation method, the type of alkali and the reaction temperature may lead to a different dependence of the particle size on the polymer concentration. FESEM images show that magnetite NPs form smaller aggregates in the CMC@Fe3O4 in-situ sample than in the CMC@Fe3O4 ex-situ sample (Fig. 4a and 4b). By analyzing the images we found

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that the average size of the aggregates is 17 nm (with a standard deviation of 6 nm) and 50 nm (with a standard deviation of 16 nm) for the CMC@Fe3O4 in-situ and CMC@Fe3O4 ex-situ NPs, respectively. Magnetite seems to be present as single NPs or aggregates of a few particles in the

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CMC@Fe3O4 in-situ sample. By adding Fe3O4 NPs prepared ex-situ to the solution of CMC, there is a reduction of the size of the aggregates compared to that of the aggregates of NPs in water (see

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figure 4c). The CMC@Fe3O4 ex-situ NPs are less dispersed than the CMC@Fe3O4 in-situ NPs and the smaller interfacial area may explain why the contribution in the FTIR spectra of the COO-

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groups bonded to magnetite is weaker in the former than in the latter case. For the CMC@Fe3O4 insitu sample, the NPs seem to be dispersed within the matrix rather than coated by the polymer (Figure 5a and 5b). The morphology of the polymer matrix appears similar to that of the hydrogel-

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like phase of CMC and Fe(II)/Fe(III) (Fig. 5c) from which the magnetite NPs are formed by adding the alkali. Although the morphology of the polymer matrices may change after drying, the effect should be comparable for the droplets of the CMC@Fe3O4 in-situ dispersion and for those of the

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CMC Fe(II)/Fe(III) gel-like phase. These results suggest that around the magnetite NPs, the

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hydrogel structure is kept even after the precipitation of the magnetite NPs. The gel-like structure of CMC can explain the resistance towards washing of the polymer coating for the CMC/Fe3O4 NPs

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prepared in the presence of CMC compared to those prepared by mixing CMC and Fe3O4 NPs synthesised ex-situ. The inclusion of CMC polymer chains inside magnetite NPs during the in-situ preparation, which can contribute to anchor CMC to the NPs, cannot be ruled out.

3.3 DLS

The -potentials determined by means of DLS for the dispersions in various media of CMC, bare magnetite NPs, CMC@Fe3O4 ex-situ (as prepared and after washing) and CMC@Fe3O4 in-situ NPs are reported in Table 2. The -potentials measured for the CMC modified NPs are close to the values determined for CMC solutions in the corresponding media. The trend of the -potentials in the various media is that expected on the basis of surface charge of the oxide NPs, protonation of COO- groups and ionic strength. The rather negative -potentials of CMC/Fe3O4 NPs prepared insitu and ex-situ explains the stability of their dispersions in water at pH close to neutrality. Since we have electrostatic and steric contributions to the energy barrier that the NPs must overcome to aggregate, this stabilization mechanism is referred to as “electrosteric” [10]. The negative value (but 8

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close to zero) -potential at pH 2 for the CMC@Fe3O4 in-situ dispersion provides a further evidence that the bonding of CMC to the magnetite NPs occurs via chemical, specific interactions. A positive value, close to that of the bare Fe3O4 NPs, would be expected if the COOH groups interact via hydrogen bonding with the NPs because of the protonation of the basic sites on the particle surface. Moreover, these measurements show that the bonding of COO- to undercoordinated iron atoms at

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the surface is resistant to the hydrolysis at neutral and acid pH. At pH 2 the -potential is positive (+13 mV) for the dispersion prepared with the CMC@Fe3O4 ex-situ NPs which were washed to

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remove of the excess of CMC. This observation can be explained considering the removal of CMC by washing and the protonation of the sites on the fraction of NP surface not covered by the

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polymer. At pH 9 it is not possible to discriminate whether the negative -potential is due to the carboxylate groups or to OH- groups which have substituted the carboxylates at the surface of the

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oxide NPs.

The hydrodynamic diameter (Z-average value) determined for the CMC@Fe3O4 in-situ NPs in DI water is 280 ± 20 nm. The hydrodynamic diameter does not vary significantly changing the pH or

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the ionic strength of the dispersion medium. The polydispersity index for the CMC@Fe3O4 in-situ NPs is around 0.6, indicating that the aggregates in solution are polydispersed. The hydrodynamic

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diameters determined for the CMC@Fe3O4 ex-situ samples are much larger (for instance ca. 2300

3.4 Magnetic properties

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nm in DI water) than for the CMC@Fe3O4 in-situ.

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Magnetization versus magnetic field intensity curves at 2.5 K and 300 K were measured for the dried CMC@Fe3O4 ex-situ and CMC@Fe3O4 in-situ samples. The magnetization curves (normalized to the respective saturation magnetization) are shown in Fig. 6 and the results of these measurements are summarized in Table 3. In both cases the saturation magnetization reaches the saturation even though with a slightly different approach. The saturation values are very similar for the two samples and within the range of values measured for Fe3O4 NPs of comparable size, although smaller than for bulk magnetite (92 emu/g). The difference between the saturation magnetization of the CMC@Fe3O4 ex-situ and CMC@Fe3O4 in-situ samples is smaller at 300 K than at 2.5 K. At room temperature both samples did not show any hysteresis loop whereas, at low temperature, a remanence magnetization was measured, as expected for NPs of this size. Remanence magnetization and coercive field for both samples are similar to those reported for magnetite NPs of similar size. The very small variations of the magnetic parameters can be ascribed to preparation reproducibility of NPs, interactions among them and with the CMC. In particular, a

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modified contribution of the particle surface can account for the differences in the saturation approach and in the variation of the saturation value with temperature. The ZFC and FC curves for the dried CMC@Fe3O4 ex-situ and CMC@Fe3O4 in-situ samples are shown in Fig. 7. These curves are characteristic of superparamagnetic materials but the blocking

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temperatures (Tb) are rather different: 170 K and ca. 300 K for the CMC@Fe3O4 in-situ CMC@Fe3O4 ex-situ samples, respectively. Since the size of the primary NPs is the same in the two samples, the different Tb can be explained considering the different size of the aggregates. An

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increase of Tb is related to an increase of the magnetic dipole-dipole interactions [32, 33]. An increasing number of particles in the cluster intensifies the interaction and induces an upward shift

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of Tb [33]. Hence, the observed variation of Tb between CMC@Fe3O4 in-situ and CMC@Fe3O4 exsitu samples can be explained taking into account the presence of larger magnetite particle

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aggregates in the latter than in the former sample.

4. Conclusions

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The comparison of the results obtained for CMC/magnetite NPs prepared following the in-situ and ex-situ methods shows that the size of the primary particles of Fe3O4 is not influenced by the polymer. However, the aggregation of the NPs is significantly different for the two samples. The

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CMC@Fe3O4 in-situ sample contains single NPs or aggregates of few particles whereas larger

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aggregates are present in the CMC@Fe3O4 ex-situ sample. FTIR and -potential measurements suggest a specific interaction of the carboxylate groups with iron ions at the surface of the oxide

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NPs. The polymer coating of the CMC@Fe3O4 in-situ NPs is resistant to dilution and washing whereas a loss of CMC is observed for the CMC@Fe3O4 ex-situ NPs. A possible explanation of these results is that the polymer chains around the magnetite NPs keep a cross-linked structure similar to that they had in the hydrogel phase formed by CMC and Fe(II) /Fe(III) from which NPs were precipitated. According to this interpretation, magnetite NPs formed by the in-situ synthesis precipitate inside the meshes of the CMC-Fe(II)/Fe(III) gel-like phase and are bonded via the carboxylate groups to the polymer network. The CMC@Fe3O4 in-situ dispersions can be considered as “nanogel” of CMC with the magnetite NPs acting as cross-linkers. The nanogel structure of the CMC may play an important role in the colloidal stability of dispersions of magnetite NPs. The formation of the nanogel is only a hypothesis which should be verified in further studies.

Acknowledgments This research was financially supported by the FIRB project (RBAP11ZJFA). The authors are grateful to Michael Gregorkiewitz, Università di Siena, for the XRD measurements. 10

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[14] S. Kaihara, Y. Suzuki and K. Fujimoto, In situ synthesis of polysaccharide nanoparticles via polyion complex of carboxymethyl cellulose and chitosan, Colloids and Surfaces B: Biointerfaces 85 (2011) 343-348. [15] P.R. Chang, J. Yu, X. Ma, D. P. Anderson, Polysaccharides as stabilizers for the synthesis of

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magnetic nanoparticles, Carbohydrate Polymers 83 (2011) 640-644. [16] J.F. Luna-Martinez, E Reyes-Melo, V. GonzalezGonzalez, C. Guerriero-Salazar, A. TorresCastro, S. Sepulveda-Guzamn, Synthesis and Characterization of a Magnetic Hybrid Material

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Consisting of Iron Oxide in a Carboxymethylcellulose Matrix, J. Appl. Polym. Sci. 127 (2013) 2325-2331.

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[17] A.L. Daniel-da-Silva, T. Trinidade, B. J. Goodfellow, B.F.O. Costa, R. N. Correia, A.M. Gil, In situ synthesis of magnetite nanoparticles in carrageenan gels, Biomacromolecules 8 (2007) 2350-

an

2357.

[18] R. Hernández, J. Sacristán, A. Nogales, T. A. Ezquerra, C. Mijangos, Structural Organization of Iron Oxide Nanoparticles Synthesized Inside Hybrid Polymer Gels Derived from Alginate

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Studied with Small-Angle X-ray Scattering, Langmuir 25 (2009) 13212-13218. [19] R. Hernández, J. Sacristán, L. Asín. T.E. Torres, M.R. Ibarra, G.F. Goya, C. Mijangos, Magnetic Hydroges Derived from Polysaccharides with Improved Specific Power Absorption:

te

12007.

d

Potential Devices for Remotely Triggered Drug Delivery, J. Phys. Chem. B 114 (2010) 12002-

[20] R. Hernández, J. Sacristán, A. Nogales, M. Fernández, T. A. Ezquerra, C. Mijangos Structure

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and viscoelastic properties of hybrid ferrogels with iron oxide nanoparticles synthesized in situ, Soft Matter 6 (2010) 3910-3917

[21] S.K. Suh, K. Yuet, D. K. Hwang, K.W. Bong, P.S. Doyle, T.A. Hatton, Synthesis of nonspherical superparamagnetic particles: in situ coprecipitation of magnetic nanoparticles in microgels prepared by stop-flow lithography. J. Am. Chem. Soc. 134 (2012) 7337-7343. [22] W.M. Hosny, Metal chelates with some cellulose derivatives: V. Synthesis and characterization of some iron (III) complexes with cellulose ethers. Polymers International 42 (1997) 157-162. [23] M. Uva, D. Pasqui, L. Mencuccini, S. Fedi, R. Barbucci, Influence of alternating and static magnetic fields on drug release from hybrid hydrogels containing magnetic nanoparticles, Journal of Biomaterials and Nanobiotechnology, 5 (2014) 116-127. [24] M. Song, Y. Zhang, S. Hu, L. Song, J. Dong, Z. Chen and N. Gu, Influence of morphology and surface exchange reaction on magnetic properties of monodisperse magnetite nanoparticles, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 408 (2012) 114-121.

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[25] A.M. Jubb, H.C. Allen, Vibrational spectroscopic characterization of hematite, maghemite and magnetite thin films produced by vapor deposition, Applied materials and interfaces 2 (2010) 28042812 [26] L.T. Cuba-Chiem, L. Huynh, J. Ralston, D.A. Beattie, In situ particle film ATR FTIR

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spectroscopy of carboxymethylcellulose adsorption on talc: binding mechanism, pH effects and adsorption kinetics, Langmuir 24 (2008) 8036-8044.

graft copolymer, Carbohydrates Polymers 57 (2004) 379-387.

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[27] D.R. Biswall, R.P. Singh, Characterisation of carboxymethyl cellulose and polyacrylamide

[28] I.A. Mundunkotuwa, V.H. Grassian, Citric acid adsorption on TiO2 nanoparticles in aqueous

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suspensions at acidic and circumneutral pH: Surface coverage, surface speciation, and its impact on nanoparticle-nanoparticle interactions. J. Am. Chem. Soc. 132 (2010) 14986-14994.

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[29] K. Dobson, A.J. McQuillan, In situ infrared spectroscopic analysis of the adsorption of aliphatic carboxylic acids to TiO2, ZrO2, Al2O3 and Ta2O5, Spectrochimica Acta A 55 (1999) 13951405.

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[30] L.J. Kirwan, P.D. Fawell, W. van Bronswijk, In situ FTIR-ATR examination of Poly(acrylic acid) adsorbed onto hematite at low pH, Langmuir, 19 (2003) 5802-5802 [31] Q. Qu, H. Geng, R. Peng, Q. Cui, X. Gu, F. Li, M. Wang, Chemically binding carboxylic acids

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onto TiO2 nanoparticles with adjustable coverage by solvothermal strategy, Langmuir 26 (2010)

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Effenberger, L.M. Rossi, H.R. Rechenberg, R.D. Zysler, Heat generation in agglomerated ferrite nanoparticles in an alternatine field, J. Phys. D: Appl. Phys. 46 (2013) 045002 [33] K. Mandel, F. Hutter, C. Gellermann, G. Sextl, Stabilization effects of superparamagnetic nanoparticles on clustering in nanocomposite microparticles and on magnetic behaviour, J. of Magnetism and Magnetic Materials, 331 (2013) 269-275.

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Figure captions Figure 1 FTIR spectra measured for samples of (from top to bottom): CMC, CMC@Fe3O4 ex-situ, CMC@Fe3O4 in-situ and bare Fe3O4 NPs. An offset has been added to the absorbance of each

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spectrum for a better reading.

Figure 2

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Comparison of the spectra measured for the CMC@Fe3O4 ex-situ and the CMC@Fe3O4 in-situ

samples in the wavenumber region of asymmetric and symmetric stretching modes of the COO-

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stretching peaks (ca. 1600 cm-1) have the same height.

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groups of CMC. The absorbance of the spectra is normalized in such a way that the asymmetric

Figure 3.

Result of the curve fitting analysis of the spectra of the CMC@Fe3O4 in-situ (top), the

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CMC@Fe3O4 ex-situ (middle) and CMC@Fe3O4 ex-situ after washing (bottom) samples. The attribution of the various components is reported in Table 1. The number of components was determined on the basis of the components contributing to the spectrum of CMC. In the curve

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fitting procedure peak positions, peak areas, full width half maximum values, mixing of Gaussian

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Figure 4.

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and Lorentzian lineshapes were optimized.

FESEM images measured for: a) CMC@Fe3O4 in-situ sample, b) CMC@Fe3O4 ex-situ sample, c) aggregates of bare Fe3O4 NPs.

Figure 5.

FESEM images measured for: a) and b) CMC@Fe3O4 in-situ sample at two different enlargements, c) dried gel-like phase of CMC and Fe(II)/Fe(III).

Figure 6. Magnetization normalized to the respective saturation magnetization versus field for the dried CMC@Fe3O4 ex-situ and CMC@Fe3O4 in-situ samples. Main panel: curves measured at 300K. Inset: Details of the low field data measured at 2.5K.

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Figure 7. Zero field cooling (ZFC) and Field Cooling (FC) curves measured for the CMC@Fe3O4 ex-situ

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sample and for CMC@Fe3O4 in-situ sample.

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Table 1 Peak position (cm-1), assignment and colour of corresponding curve in figure 3.

Assignment

Color of curve in figure 3

1638

H2O bending

Blue

1594

COO- asymmetric stretching

Magenta

1535

COO- asymmetric stretching

Red

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(bonded to NPs)

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Peak position (cm-1)

-CCH and -OCH bending

Green

1416

COO- symmetric stretching

Orange

1383

COO- symmetric stretching

Pink

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1462

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(bonded to NPs) 1375

-CH and OH coupled bending

Olive green

1320

-CH2- wagging in

Violet

Cyan

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-CO stretching

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1268

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O-CH2-COO- groups

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Table 2 -potentials (mV) measured for CMC, bare magnetite NPs, CMC@Fe3O4 ex-situ (as prepared and after washing) and CMC@Fe3O4 in-situ NPs. The measurements were performed in DI water, in

dispersions). The uncertainty in the measurements is ± 5 mV.

Fe3O4 NP

CMC@Fe3O4 ex-situ

DI water NaCl 0.01 M pH 2

-75 -45 -4 -80

13 -7 35 -55

-90 -48 -4 -90

CMC@Fe3O4 in-situ

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-70 -50 -4 -68

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pH 9

CMC@Fe3O4 ex-situ (after washing) -48 -23 13 -63

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CMC

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Dispersing medium

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NaCl solution 0.01 M, at pH 2 and 9 (by adding HCl and NaOH, respectively, to the NPs

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Table 3 Magnetic properties measured for the CMC@Fe3O4 ex-situ and CMC@Fe3O4 in-situ samples. Tb is the blocking temperature, estimated from the maximum of the ZFC curve; Hc is the coercive field,

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Ms is the saturation magnetization, estimated form the magnetization reached at 50000 Oe. Mr is the remanent magnetization normalized to the corresponding Ms, at 2.5 K. At 300 K Mr and Hc are

Hc (Oe)

CMC@Fe3O4 ex-situ

300

408

Ms at 2.5 K (emu/g Fe3O4) 66

CMC@Fe3O4 in-situ

170

360

71

Ms at 300 K (emu/g Fe3O4) 59

Mr at 2.5 K 0.39

60

0.39

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Tb( K)

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sample

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zero, within the sensitivity of the measurements.

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*Graphical Abstract (for review)

Fe(II) / Fe(III)

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CMC

OH-

CMC + OH-

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ex-situ

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Figure 1

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Figure 2

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Figure 3

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Figure4

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100 nm

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a

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100 nm

b

200 nm

c

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Figure 5

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200 nm

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a

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1 µm

b

2 µm

c

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Figure 6

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Figure 7

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