Accepted Manuscript Spectroscopic and quantum chemical studies of interaction between the alginic acid and Fe3O4 nanoparticles
Małgorzata Śmiłowicz, Katarzyna Pogorzelec-Glaser, Andrzej Łapiński, Rafał Motała, Marcin Grobela, Bartłomiej Andrzejewski PII: DOI: Reference:
S1386-1425(17)30242-1 doi: 10.1016/j.saa.2017.03.056 SAA 15035
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received date: Revised date: Accepted date:
28 September 2016 24 February 2017 24 March 2017
Please cite this article as: Małgorzata Śmiłowicz, Katarzyna Pogorzelec-Glaser, Andrzej Łapiński, Rafał Motała, Marcin Grobela, Bartłomiej Andrzejewski , Spectroscopic and quantum chemical studies of interaction between the alginic acid and Fe3O4 nanoparticles. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), doi: 10.1016/j.saa.2017.03.056
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ACCEPTED MANUSCRIPT Spectroscopic and quantum chemical studies of interaction between the alginic acid and Fe3O4 nanoparticles Małgorzata Śmiłowicza,b, Katarzyna Pogorzelec-Glasera, Andrzej Łapińskia,*, Rafał Motałac, Marcin Grobelac, Bartłomiej Andrzejewskia a
Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60179 Poznań, Poland Nanobiomedical Centre, Adam Mickiewicz University, Umultowska 85, 61-614 Poznań,
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b
Poland
Institute of Plant Protection - National Research Institute, W. Węgorka 20, 60-318 Poznań,
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c
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Poland
*Corresponding author at Institute of Molecular Physics, Polish Academy of Sciences, M.
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Smoluchowskiego 17, 60-179 Poznań, Poland Tel.: +48 61 86 95 210; fax: +48 61 86 84 524 E-mail address:
[email protected]
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Keywords: Nanocomposite; Magnetic nanoparticles; IR spectroscopy; DFT calculation
ABSTRACT
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In this work, we present the spectral investigation of the interactions between the coverage with alginic acid (AA) and nanoparticles for three different composites containing 74, 80, and
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88 wt. % of magnetite. These results show that the Fe3O4 nanoparticles are coated with the AA and indicate that there is an interaction between them. Moreover, we have investigated the thermal and magnetic properties of all investigated compounds. We show that bonding of
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alginic acid to the surface of magnetite results in better thermal stability of the polymer and in higher temperature of AA chains degradation. We find that for dense assembly of magnetite
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nanoparticles, at low temperatures, the intergranular coupling becomes much stronger than between nanoparticles dispersed in composites.
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1. Introduction New functional magnetic nanomaterials like nanoparticles, nanoshells, core-shell structures or nanoflowers can considerably revolutionize many areas of biology and medicine, both for in vitro and in vivo applications. Magnetic nanoparticles can be exploited similar to other nanoparticles, for intracellular hyperthermia, various biochemical sensors and for drug transporting [1]. However, they also take an extra advantage of manipulation with magnetic
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field for magnetic separation of living cells, as MRI contrast agents and also carriers for targeted drug delivery. Physical properties of nanomaterials are strongly modified compared
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to their bulk counterparts and they can be even intentionally tuned by means of size, shape
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and surface-to-volume ratio to obtain superparamagnetism, single magnetic domain state, and stable luminescence or desired optical properties. Very promising for future biomedical
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applications seems to be magnetite (Fe3O4) nanoparticles due to their relatively low toxicity, high biocompatibility and excellent magnetic and electric properties. It is also one of the beststudied nanomaterials which have been combined with various biopolymers.
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Magnetic nanoparticles have to be capped by means of surfactants, oxides [2-4] and various polymeric compounds [5-7] with some specific functional groups to modify properties
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of these particles, improve the stability of their suspensions and also to minimize their toxicity. Bare nanoparticles of magnetite are potentially more toxic than other iron oxides
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maghemite (γ-Fe2O3) or hematite (α-Fe2O3) probably because of higher ability to undergo oxidation reactions [8, 9]. Biotoxicity usually results from binding of serum proteins to the surface of oxide nanoparticles which modifies the composition of the cell medium [10] and
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the release of free iron can promote the formation of free radicals harmful to neural tissues [11]. Moreover, suspensions of bare nanoparticles are unstable and can precipitate in aqueous
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media impeding blood vessels. To overcome these problems many coatings of magnetic nanoparticles were so far tested. Häfeli et al. [12] observed a substantial reduction of cytotoxicity of nanoparticles capped with polyethylene oxide (PEO) when the molar mass of this polymer exceeded 2kDa. The coverage with dimercaptosuccinic acid (DMSA) almost eliminated the toxicity of maghemite nanoparticles [13] and a similar effect was also observed for capping with polyethylene glycol (PEG) [14]. On contrary, dextran-coated magnetite and maghemite exhibited similar high toxicity than bare nanoparticles even though this complex branched glucose polymer is widely used in medicine [15]. Therefore next generation of the capping polymers comprising chitosan, albumin, citric and alginic acid with toxicity even less than starch and PEG has been tested. Besides the type and the degree of surface coverage, the
ACCEPTED MANUSCRIPT toxicity defined as the rate of nanoparticles uptake by macrophages is also strongly influenced by the size effect. Bigger structures like 50 nm dextran coated and 4 μm polystyrene coated nanoparticles were shown to be safe for some medical applications [16]. The problem of proper capping becomes extremely crucial for magnetic drug targeting and resonance imaging because under the application of strong magnetic field nanoparticles exhibit enhanced toxicity. Bio-medical applications require also the colloidal stability of nanoparticle suspensions in different biological media like blood or plasma. Thus coverage should also
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provide a substantial repulsive force to prevent irreversible aggregation of nanoparticles under Van der Waals and dipolar magnetic interactions [17]. The most effective are steric
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stabilization resulting from entropic repulsive forces due to the entropic cost of long polymer
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chains compression when two particles approach one to another. The key factors steric repulsive forces are density and thickness of the polymeric coverage and conformation of
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polymer chains.
Very important issue is also the type of interactions between capping polymer and nanoparticles because it strongly alters the properties of polymeric coverage. This problem
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was studied, among others, for magnetite Fe3O4 nanoparticles embedded in various polymers. For instance, Deng et al. have investigated the interactive mechanism in Fe3O4/polypyrrole
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[18] and Fe3O4/polyaniline nanocomposites [19] and explained it assuming coupling between the electron lone pairs of N atom in polypyrrole or in polyaniline chains to the 3d orbitals of
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Fe atom, resulting in coordinate bonds. Li et al. [20] concluded that the interactive mechanism of the oleic molecular adsorbing on the surface of Fe3O4 nanoparticles is due to hydrogen bond or coordination linkage [21], whereas Zhang et al. reported that poly(methacrylic acid)
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adheres to Fe3O4 nanoparticles via coordination linkages between the carboxyl groups and atoms of iron [22]. According to Wei et al. [23], the driving force for the adhesion of
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hyperbranched aromatic polyamide on the surface of Fe3O4 nanoparticles may be attributed to the interaction between specific functional groups of the polyamide and Fe3O4 nanoparticles. The overall force of adhesion is due to the combination of various effects: the lone-pair electrons of the N atom in polymers coupling to the 3d orbit of Fe atom; the interaction between the carboxyl and iron atoms, and the hydrogen bonding between the aromatic polyamide and Fe3O4 surface. Vibrational spectroscopy seems to be a suitable tool for characterization of polymermagnetite systems. In the present studies, this method has been used to investigate the interactions between the coverage with alginic acid and nanoparticles for three different composites containing 74, 80, and 88 wt. % of magnetite.
ACCEPTED MANUSCRIPT 2. Experimental Section Magnetite nanoparticles about 20 nm in size were obtained by hydrothermal route with microwave activation as we described in an earlier study [40]. The nanocomposites were prepared from an aqueous dispersion of Fe3O4 nanoparticles and alginic acid (AA) which were next dried in the air at room temperature. We synthesized AA-Fe3O4 nanocomposites with three different weight concentrations of magnetite: 74, 80 and 88% to study the interaction between Fe3O4 and AA coverage, and to examine the influence of weight content nanoparticles as a core and alginic acid as a polymeric shell.
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of the magnetite in the composites. Fe3O4-AA nanocomposites were obtained using magnetite
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Differential Scanning Calorimetry (DSC) was used to study the thermal stability of the
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nanocomposites. DSC data were recorded with a Netzsch DSC-200 F3 calorimeter in the temperature range from 50 C to 500 C under N2. The measurements were performed on
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heating and cooling runs at a rate of 10 C /min.
FTIR spectroscopic studies of alginic acid (AA), Fe3O4, and AA-Fe3O4 nanocomposites with three different weight concentrations of magnetite: 74, 80, and 88 % in the frequency
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range from 400 to 4000 cm–1 were performed using FT-IR Bruker Equinox 55 spectrometer and a standard KBr pellets technique in transmission geometry. The resolution of the
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measurements was equal to 2 cm-1. For the analysis of the vibrational spectra, the program
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PeakFit v4.12 was used.
Magnetic properties of the set of AA-Fe3O4 nanocomposites were studied using a Vibrating Sample Magnetometer (VSM) probe installed on the Quantum Design Physical Property Measurement System (PPMS). Magnetization hysteresis loops M(H) were recorded
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for the temperatures 10 K, 100 K and 300 K and field range -2 T≤μ0H≤2 T. Magnetization dependences on temperature M(T) were recorded both for zero field cooled (ZFC) and field
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cooled procedures (FC) and the applied magnetic field 1 T. In the ZFC measurement the sample was first cooled to low temperature with no magnetic field, next the field was applied and the measurements were recorded on sample warming. In the FC procedure, the magnetization was measured on cooling the sample with applied magnetic field. 3. Details of calculations The quantum chemical calculations of different types of monomers of alginic acid were carried out by using the Gaussian 3.0 program with 6-311++G(d,p) basis set [40]. The hybrid gradient-corrected exchange functional proposed by Becke [44, 45] was combined with the gradient-corrected correlation functional of Lee, Yang, and Parr [46]. The geometries of molecules were fully optimized at the B3LYP/6-311++G(d,p) level of theory and their
ACCEPTED MANUSCRIPT optimized structures were checked by analysis of the harmonic vibrational frequencies to verify whether they are at minima, saddle points, or stationary points of higher orders. Only positive eigenvalues of the Hessian matrix were obtained, proving that the calculated geometry is a minimum on the potential energy surface. DFT orbitals were obtained by solving the Kohn-Sham equation, involving exchange and correlation terms. In order to obtain a good estimate of the experimental results, the computed frequencies were multiplied by a uniform factor 0.9613 [47]. It is worth to notice that frequencies computed with a quantum
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harmonic oscillator approximation tend to be higher than experimental ones. The mode description was performed by visual inspection of the individual modes using the GaussView
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program.
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4. Results and Discussion
Fig. 1 shows the absorption spectrum of alginic acid (AA) and simulated spectrum of one
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monomer of this polymer. Both calculated and experimental data are in good agreement with
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each other. It is a kind of challenge to performing vibrational analysis for this polymer.
Fig. 1. Absorbance spectrum of AA (upper panel) and theoretical IR spectrum calculated for
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the monomer of AA at the theory level B3LYP/6-311++G(d,p) (lower panel). Note: ν means stretching and δ bending vibrations; calculated frequencies of normal modes are multiplying by the scaling factor of 0.9613 [47]; the theoretical spectrum was graphically simulated by Gaussian curves having a half-width of 16 cm-1.
These data suggest that ab initio calculations for relatively short oligomers may provide valuable information regarding the interpretation of vibrational spectra of polymers. In the experimental spectrum of AA the strongest bands are observed at 3439, 2927, 1740, 1632, 1417, 1372, 1298, 1248, 1175, 1144, 1122, 1100, 1086, 1034, 948, 929, 878, 813, 788, 669,
ACCEPTED MANUSCRIPT and 605 cm-1. The interpretation of these features has been performed based on the literature [24, 25] and the quantum mechanical calculations carried out by us. The broad feature at around 3439 cm-1 is due to O-H stretching modes which are strongly affected by hydrogen bonding. The calculations performed for the monomer of AA predict the presence of eight O-H modes within the spectral region from 3743 till 3838 cm-1. The group of bands observed from 2790 till 3200 cm-1 is due to the C-H stretching vibrations, whereas the intense band at about 1740 cm-1 is associated with C=O stretching vibration in the free
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carboxyl group. Other strong features at 1632 and 1417 cm-1 correspond to the presence of the salified carboxyl group (antisymmetric and symmetric COO- stretching vibration) [26, 27].
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The existence of COO- moieties even in pure AA guarantees that it can interact with the
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surface of nanoparticles. In the region 1270-1400 cm-1, bands assigned to the C-H and O-H deformation vibration and bands at 1100 and 1034 cm-1 assigned to the C-O stretching
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vibration are present. In the region between 1200-950 cm-1 several normal modes are related to C-C-C, C-O-C bending and C-O, C-C stretching vibrations. In the spectra of alginic acid, the bands near 813 and 699 cm-1 are due to ring deformation modes. Fig. 2 shows the infrared
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spectra of alginic acid (AA), Fe3O4-AA composites containing 74, 80, 88 wt. % of magnetite, uncoated Fe3O4 nanoparticles. The spectra of composites are similar to each other and they
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are in good agreement with those reported by Unal et al. [28].
Fig. 2. Absorbance spectra of alginic acid (AA) (a), Fe3O4-AA composites containing 74 (b), 80 (c), 88 (d) wt. % of magnetite, and uncoated Fe3O4 nanoparticles (e). The vibrational spectra of investigated nanocomposites are the superposition of infrared active bands originating from AA and magnetite. The presence of the iron oxide nanoparticles is evidenced by the strong absorption bands at around 584 and 430 cm-1 and they can be
ACCEPTED MANUSCRIPT assigned to the metal-oxygen bond [29-36]. The first line corresponds to intrinsic stretching vibrations of the metal at the tetrahedral site (Fetetra-O), whereas the second one is due to octahedral-metal stretching (Feocta-O). In the spectra of Fe3O4-AA composites, certain differences in the positions of the bands are observed due to the interaction between the alginic acid and Fe3O4 nanoparticles, particularly in the spectral range from 1200 to 1900 cm-1 and below 800 cm-1. These results suggest for a certainty that the Fe3O4 nanoparticles are coated with the AA.
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Fig. 3 shows the DSC curves of alginic acid, three AA-Fe3O4 nanocomposites, and magnetite under N2. The thermogram of alginic acid reveals broad endothermic A and B
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peaks at temperature 94 C and 190 C, respectively.
Fig. 3. DSC curves of alginic acid (a), AA-Fe3O4 nanocomposites with three different weight
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concentrations of magnetite 74 % (b), 80 % (c), 88 % (d) and Fe3O4 (e) under N2. The first peak indicates a loss of water content in the polymer. The second peak is due to degradation of the polymer chains. The position of these peaks is in good agreement with what has been shown by Soares at al. [37]. They have reported that the dehydration process for alginic acid does not a significant influence on the DSC profiles when the heating rate is changing from 5-20 C min-1. The exothermic peak C appearing at temperatures 245 C indicates decomposition of the polymer chains [37], whereas peaks D, E, F, and G are related to magnetite. From the TGA measurements performed for Fe3O4 [38], it’s known that there
ACCEPTED MANUSCRIPT are three major weight losses for magnetite: one is in the temperature range from 170 C to 360 C with a value of 1.64% and from 360 C to 600 C with a weight loss of 0.70%. The third weight loss starts at 600 C and ends at 720 C with a value of 0.20%. The first weight loss is due to the decomposition of magnetite to non-stoichiometric magnetite, whereas the second and the third ones are related to the further decomposition of non-stoichiometric magnetite to another type of non-stoichiometric magnetite [38]. The DSC peaks observed for
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AA-Fe3O4 nanocomposites (D, E, F, and G) are in good agreement with these data. The position of peaks for all investigated samples is collected in Table 1. Bonding of alginic acid
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to the surface of magnetite results in better thermal stability of the polymer and in higher temperature of AA chains degradation (peak B) as a load of magnetite in the composite
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increases.
Peak A
alginic acid (AA)
AA-Fe3O4 (88 wt % Fe3O4)
Peak D
Peak E
245.4
209.1
243.5
275.5
307.6
74.1
211.4
249.8
273.3
304.7
73.8
225.7
252.9
270.5
300.0
267.9
299.4
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86.2
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magnetite (Fe3O4)
Peak C
190.4
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AA-Fe3O4 (74 wt % Fe3O4) AA-Fe3O4 (80 wt % Fe3O4)
94.2
Peak B
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Sample
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Table 1. DSC peaks (in C) observed for alginic acid, AA-Fe3O4 nanocomposites with three different weight concentrations of magnetite 74 %, 80 %, 88 % and Fe3O4.
To study the interaction of Fe3O4 nanoparticles with alginic acid we used vibrational spectroscopy. Comparing the IR spectra of Fe3O4-AA composites with that of Fe3O4 nanoparticles (see Fig. 2), it can be seen that the peak at 584 cm-1 assigned to Fetetra-O in the spectra of magnetite was red shifted to 567 cm-1 in the nanocomposite. Moreover, the position of this band is sensitive to the content of magnetite in the sample, what can provide evidence for the interaction being via bridging oxygens of carboxylate and the nanoparticle surface. By comparing study on the alginic acid and that of Fe3O4-AA nanocomposite, it can also be seen that the peak at 1740 cm-1 related to AA (Fig. 2, plot a) does not appear at the same position
ACCEPTED MANUSCRIPT in the spectra of nanocomposites (see Fig. 2, plots b,c,d), indicating that the Fe3O4 particles were interacting with AA. From the literature, it is known that another strong feature observed at about 1620 cm-1 corresponds to the presence of the salified carboxyl group (COO- stretching vibration) [26, 27]. In order to support this interpretation we have performed quantum chemical calculations for monomers of AA with two carboxyl groups (Fig. 4 a), with carboxyl (COOH) and salified
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carboxyl (COO-) groups (Fig. 4 b), and for two salified carboxyl (COO)22- groups (Fig. 4 c).
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Fig. 4. Optimized structure of monomers of AA with two carboxyl groups (a), with carboxyl (COOH) and salified carboxyl (COO-) groups (b), and for two salified carboxyl (COO)22-
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groups (c).
The corresponding scaled theoretical spectra and experimental data are shown in Fig. 5.
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The ratio of the integral intensity of 1740 and 1620 cm-1 bands suggests that COO- groups
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should be mainly observed in nanocomposites. Moreover, the peak observed for AA at 1629 cm-1 was red shifted to 1623, 1618, and 1616 cm-1 for Fe3O4-AA composites containing 74, 80, and 88 wt. % of magnetite, respectively. It is due to the adsorption of AA on the surface of
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Fe3O4 through their anchoring group (COO-). These changes are related to the complex formation between AA and Fe3O4. The relationship between the carbon-oxygen stretching
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frequencies of carboxylate complexes and the type of carboxylate coordination for aceto transition metal complexes has been investigated by Deacon and Phillips [42].
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Fig. 5. Absorbance spectra of AA and Fe3O4-AA composite containing 88 wt. % of magnetite
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(upper panel) and theoretical IR spectra calculated for monomers of AA with two carboxyl groups (a), with carboxyl and salified carboxyl groups (b), and for two salified carboxyl groups (c) (lower panel). Note: calculated frequencies of normal modes at the theory level
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B3LYP/6-311++G(d,p) are multiplying by the scaling factor of 0.9613 [47]; theoretical
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spectra were graphically simulated by Gaussian curves having a half-width of 16 cm-1.
By comparing the separation of the symmetric and antisymmetric stretching frequencies
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of salified carboxyl (COO-) groups (Δν) bond to transition metals with the separation for the
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sodium salt, they gave a set of rules for identifying the bonding mechanism [39]: (i) if there is C=O character in the spectrum and Δνadsorbed is greater than Δνsalt then the adsorbed structure is monodentate (Fig. 6 a), (ii) if there is no C=O character in the spectrum and Δνadsorbed is
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smaller than Δνsalt, then the adsorbed structure is bidentate chelating (Fig. 6 b), and (iii) if there is no C=O character in the spectrum and Δνadsorbed is similar to Δνsalt, then the adsorbed
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structure is bidentate bridging (Fig. 6 c).
Fig. 6. Possible modes of carboxylate metal complexation: monodentate (a), bidentate chelating (b), and bidentate bridging (c).
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For investigated Fe3O4-AA nanocomposites we can assume that there is no C=O character in the experimental spectra. The characteristic stretching frequencies include the carbonyl stretch (C=O) at 1735 cm-1 exhibit very low intensity. Moreover, for composites containing 74, 80, 88 wt. % of magnetite the separation of the symmetric and antisymmetric stretching frequencies of the carboxylate ion are 226, 216, and 208 cm-1, respectively. The differences in the separation bands are associated with different C-O-Fe bond lengths, which
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may indicate that the interaction between the nanoparticle and the polymer may depend on the content of magnetite. Theoretical calculations performed for monomers of AA with two free
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salified carboxyl (COO)22- groups show that such separation is equal Δνfree=301 cm-1
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(νas=1666 cm-1, νsym=1365 cm-1). This might suggest that for Fe3O4-AA nanocomposites the adsorbed structure can be the bidentate chelating type (there is no C=O character in the
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spectrum and Δν is smaller than Δνfree). This finding is in contradiction to the results published by Salafranca et al. [39] who performed theoretical calculations of various local crystal environments and corresponding electronic structures for bulk and bare magnetite
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nanoparticles and magnetite with bidentate bridging between its surface and oleic acid carboxyl groups. They, however, assumed that only one of six oxygen atoms is lost from
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surroundings of the octahedral iron. Our measurements suggest that degradation of surface nanoparticles due to the oxygen loss and modifications of magnetite crystal structure can be
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even more substantial. DSC measurements additionally confirm the interaction between the polymer and nanoparticles and its dependence on the concentration of magnetite. Fig. 3 and Table 1 show that the position of B peak, related to the degradation of the polymer chains,
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increases with a weight content of magnetite, which indicates that thermal stability of the Fe3O4-AA composites is better for a higher concentration of magnetite.
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AA-Fe3O4 composites of due to the high load of magnetite i.e. 74, 80 and 88wt% exhibited strong magnetic properties. Fig. 7 presents mass magnetization (magnetic moment per mass) dependence on temperature M(T) of three composite samples and powder of bare magnetite nanoparticles measured for cooling from 300 K to 4 K in applied magnetic field 1 T.
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Fig. 7. Magnetization dependence on temperature M(T) for AA-Fe3O4 nanocomposites with
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three different weight concentrations of magnetite 74 %, 80 %, 88 % and Fe3O4 measured for field cooling at 1 T. The magnetization was calculated with respect to the mass of magnetite
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in the samples.
If the magnetization is determined with respect to the mass of magnetite in the
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composites, as in Fig. 7, it attains about 63 Am2/kg at room temperature and is higher than the magnetization of Fe3O4 nanoparticles 50 Am2/kg. These both values are, however, much
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below the magnetization of bulk magnetite, which is ~92 Am2/kg. The diminished magnetization of bared magnetite nanoparticles results from alternated crystallographic
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structure at their surface due to loss of one of the six oxygen atoms surrounding the octahedral iron. In this structure, the in-plane surface oxygens ions are closer to the Fe ions, whereas the dx2-y2 orbitals of octahedral iron are shifted away from the Fermi level and become partially
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empty, which reduces magnetic moment. On the other hand, Salafranca et al. [39] showed that capping of magnetite nanoparticles with alginic acid allows supplementing missing oxygen atoms because of the chemical bonds formed between carboxyl groups of the acid and surface Fe ions the magnetite helping to restore of surface magnetism. Temperature dependence of magnetization of both Fe3O4 and Fe3O4-AA composites doesn’t satisfy the Bloch law: MS(T) = MS(0)(1-BT1.5). Actually, for the Fe3O4 nanoparticles with degraded surface and Fe3O4-AA composites with a large load of magnetite and thus with strong interparticle interactions, more appropriate seems to be M(T)~T1.9 dependence. At low temperature and for composite samples only, there occurs gradual upturn in magnetization due to the excitation of the spin waves in magnetite nanoparticles with restored, thanks to the
ACCEPTED MANUSCRIPT carboxylic group bonding, structure of the surface. The Verwey transition does appear neither in Fe3O4 nanoparticles nor in AA-Fe3O4 composites because the nanoparticles are in the superparamagnetic state. These effects were widely discussed in our earlier report [40]. The superparamagnetic behavior of magnetite nanoparticles can be also observed in magnetization curves M(H) recorded for AA-Fe3O4 nanocomposites with a various load of
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magnetite and Fe3O4 nanopowder (presented in Fig. 8).
Fig. 8. Magnetization dependence on applied field M(H) for AA-Fe3O4 nanocomposites with three different weight concentrations of magnetite 74 %, 80 %, 88 % and Fe3O4 measured at
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300 K. The magnetization was calculated with respect to the mass of magnetite in the
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samples.
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Also, in these measurements mass magnetization of composites determined with respect to magnetite content, exceeds magnetization of bare nanoparticles. Magnetization M(H) of composites and magnetite nanopowder saturates at relatively low magnetic field ~0.35 T and
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the values of coercivity Hc are very small. The coercive field Hc depends on the content of magnetite in composites and also on temperature, as presented in Fig. 9. For the high temperatures, 100 K and 300 K the coercive field Hc only weakly depends on magnetite content with a tendency to decrease as the magnetite load increases. The diminished magnitude of coercive field Hc in composites with a small amount of alginic acid matrix once again reflects reduced magnetic properties due to the degraded surface in uncapped nanoparticles of magnetite.
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Fig. 9. Coercive field Hc for AA-Fe3O4 nanocomposites with three different loads of
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magnetite 74 %, 80 %, 88 % and Fe3O4 measured at 10 K, 100K, and 300 K. Contrary to the weak decrease of Hc with the increase of magnetite load observed at high temperatures, the coercivity increases substantially for magnetite nanopowder at the
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temperature 10 K. This behavior can be explained assuming mutual interactions and coupling between closely packed magnetite nanoparticles. In this case, the coercivity Hc is described by a phenomenological formula [41]:
(1)
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0Hc exk 0Ha Neff Js .
where: αk and αex are the structure-dependent Kronmuller parameter and intergrain coupling, respectively. Neff is the magnetostatic parameter, Js represents the magnetization and Ha
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denotes the anisotropy field. For dense assembly of magnetite nanoparticles, at low temperatures, the intergranular coupling becomes much stronger than between nanoparticles
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dispersed in composites resulting in a large magnitude of coercivity observed in pure nanopowder.
5. Conclusions
In this paper, we have presented the results of spectral, thermal and magnetic investigations of nanoparticles coverage with alginic acid. The capping of AA around the Fe3O4 nanoparticles and the interaction being via bridging oxygens of the carboxylate and the nanoparticle surface were confirmed by vibrational spectroscopy. IR spectra of Fe3O4-AA nanocomposites shows that the structure of adsorbed alginic acid can be the bidentate chelating type. Chemical bonding between alginic acid carboxyl groups and surface of Fe3O4 nanoparticles allows rebuilding crystal structure of magnetite and restoring magnetic
ACCEPTED MANUSCRIPT properties. Bare Fe3O4 nanoparticles and nanoparticles dispersed in AA-Fe3O4 composites exhibit superparamagnetic behavior with low coercivity and saturating field. The dependence of coercivity on magnetite load in composites can be explained in terms of the phenomenological model assuming mutual interactions and coupling between closely packed nanoparticles.
Acknowledgements
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This project was supported by the European Union - European Social Fund and Human Capital - National Cohesion Strategy. This project has been supported by National Science
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Centre (Project No. N N507 229040).
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Absorbance spectrum of AA (upper panel) and theoretical IR spectrum calculated for the monomer of AA at the theory level B3LYP/6-311++G(d,p) (lower panel). Note: ν means stretching and δ bending vibrations; calculated frequencies of normal modes are multiplying by the scaling factor of 0.9613 [47]; the theoretical spectrum was graphically simulated by Gaussian curves having a half-width of 16 cm-1. Fig. 2. Absorbance spectra of alginic acid (AA) (a), Fe3O4-AA composites containing 74 (b),
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80 (c), 88 (d) wt. % of magnetite, and uncoated Fe3O4 nanoparticles (e).
Fig. 3. DSC curves of alginic acid (a), AA-Fe3O4 nanocomposites with three different weight
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concentrations of magnetite 74 % (b), 80 % (c), 88 % (d) and Fe3O4 (e) under N2.
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Fig. 4. Optimized structure of monomers of AA with two carboxyl groups (a), with carboxyl (COOH) and salified carboxyl (COO-) groups (b), and for two salified carboxyl (COO)22-
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groups (c).
Fig. 5. Absorbance spectra of AA and Fe3O4-AA composite containing 88 wt. % of magnetite (upper panel) and theoretical IR spectra calculated for monomers of AA with two carboxyl
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groups (a), with carboxyl and salified carboxyl groups (b), and for two salified carboxyl groups (c) (lower panel). Note: calculated frequencies of normal modes at the theory level B3LYP/6-311++G(d,p) are multiplying by the scaling factor of 0.9613 [47]; theoretical
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spectra were graphically simulated by Gaussian curves having a half-width of 16 cm-1.
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Fig. 6. Possible modes of carboxylate metal complexation: monodentate (a), bidentate chelating (b), and bidentate bridging (c). Fig. 7. Magnetization dependence on temperature M(T) for AA-Fe3O4 nanocomposites with
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three different weight concentrations of magnetite 74 %, 80 %, 88 % and Fe3O4 measured for field cooling at 1 T. The magnetization was calculated with respect to the mass of magnetite
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in the samples.
Fig. 8. Magnetization dependence on applied field M(H) for AA-Fe3O4 nanocomposites with three different weight concentrations of magnetite 74 %, 80 %, 88 % and Fe3O4 measured at 300 K. The magnetization was calculated with respect to the mass of magnetite in the samples. Fig. 9. Coercive field Hc for AA-Fe3O4 nanocomposites with three different loads of magnetite 74 %, 80 %, 88 % and Fe3O4 measured at 10 K, 100K, and 300 K.
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights Spectral properties of nanocomposites have been discussed. Interactions between the coverage with AA and nanoparticles were investigated.
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The analysis of the position of IR bands provides evidence for interactions.
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Bonding of AA to the magnetite results in better thermal stability of the polymer.
