Improving Magnetoelectric Contactless Sensing and Actuation through

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C: Physical Processes in Nanomaterials and Nanostructures

Improving Magnetoelectric Contactless Sensing and Actuation Through Anisotropic Nanostructures Margarida M. Fernandes, Henrique Mora, Enrique Diaz Barriga Castro, Carlos Luna, Raquel Mendoza-Reséndez, Clarisse Ribeiro, Senentxu Lanceros-Mendez, and Pedro Martins J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04910 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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The Journal of Physical Chemistry

Improving Magnetoelectric Contactless Sensing and Actuation Through Anisotropic Nanostructures M. M. Fernandesa,b, H. Moraa, E. D. Barriga-Castroc, C. Lunac, R. Mendoza-Reséndezc, C. Ribeiroa,b, S. Lanceros-Mendeze,f,* and P. Martinsa,* a

Centro de Física, Universidade do Minho, 4710-057, Braga, Portugal, [email protected] b CEB - Centre of Biological Engineering, Universidade do Minho, Campus de Gualtar, Braga, Portugal c Centro de Investigación en Química Aplicada (CIQA), Saltillo, 25294 Coahuila, Mexico. d Universidad Autónoma de Nuevo Léon (UANL), Av. Universidad S/N, San Nicolas de los Garza, 66455 Nuevo Leon, Mexico e BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain f IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain.

ABSTRACT: Flexible polymer based magnetoelectric (ME) materials are developed based on novel CoFO nanoellipsoids and poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDFTrFE)]. The synthesized non-cytotoxic CoFO nanoellipsoids (270 nmx50nm) show high magnetization, ≈170 emu.g-1, high magnetostriction, ≈300 ppm, and magnetic anisotropy that, coupled to the piezoelectric response of P(VDF-TrFE), |d33|=22±1 pC.N-1, lead to an interfacial ME coupling (α) of 1.50 mV.cm-1.Oe-1. Further, nanoellipsoids orientation within the polymer matrix allow an anisotropic ME response of the CoFO/P(VDF-TrFE) composite. Such response is dependent on the angle between the DC magnetic field direction and the nanoellipsoids length direction. The proposed mechanism for the anisotropic behavior allows the tailoring of the ME response to contactless sensing applications.

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1. Introduction Nowadays, important technological advances are based on nanomaterials, mainly based on those physico-chemical properties that are fundamentally different from the ones in the bulk materials and largely dependent on their size, shape and surface chemistry1-4. Nanocrystals with zero-dimensional (0D)5 and one-dimensional (1D) structure1, 5, namely nanoparticles and nanowires/nanorods, respectively, exhibit specific electrical, optical, mechanical, and magnetic properties2. The later showing also anisotropic morphologies and responses1, which also depend on the size and shape of the materials5. Among the several nanomaterials, magnetic nanoparticles allow a great potential for technological applications in photonics, nanomedicine, electronics and data storage6-7. A common example, regarding applications, is the use of magnetic nanoparticles in microfluidic devices, where they can be controlled through external, inhomogeneous magnetic fields and detected by magnetoresistive sensors, due to their permanent magnetic moment8. With this scope, iron oxide nanoparticles have drawn much attention due to their application in various fields such as biosensors, gas sensors, magnetic resonance imaging (MRI), drug delivery systems and magnetic devices9, where they are components of ferrofluids, biomedical materials, catalysts or magnetic recording media, among others10-11. These applications are driven by the fact that those nanoparticles are one of the most studied nanomaterials due to their magnetic properties, low cost and, sometimes, biocompatibility12. The magnetic properties of fine particles strongly depend on surface effects, finite-size effects and interparticle interactions. Iron oxide is mostly found as FeO, Fe2O3 and Fe3O4, in which Fe2O3 has four known polymorph phases, α-Fe2O3, β-Fe2O3, γFe2O3 and ε-Fe2O39,

13-17

, being α-Fe2O3 and γ-Fe2O3 the most studied phases for

applications18. β-Fe2O3 and ε-Fe2O3 are typically synthesized in the laboratory, while α-Fe2O3 and γ-Fe2O3 also occur in nature. Overall, all of them can be synthesized as powders, thin films, composites, or coated particles17. Fe2O3 is the most common oxide of iron, which is an appropriate material for the general study of the polymorphism and the magnetic and 2 ACS Paragon Plus Environment

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structural phase transitions of nanoparticles

13, 19

. α-Fe2O3 is the most stable iron oxide20-21,

has a band gap of 2.2 eV 1, is nature-friendly22, shows high photocatalytic efficiency non-toxic22, inexpensive23, corrosion resistant

17, 22

23

, is

and it is relatively easy to synthesize by a

large variety of methods6, 17, shows antiferromagnetic properties13, 20, 24 below approximately 260 K and weak ferromagnetism14, 20 between 260 and 950 K

20

making it suitable for large

variety of technological applications in the areas of energy conversion and storage20, catalysts and pigments5, 20, gas sensors5, 20, optical devices and water purification20. Nanoparticles of α-Fe2O3 may have a considerable magnetic moment arising from surface spin canting due to competing exchange interactions in an incomplete coordination shell and the frustration of the antiferromagnetic order at the crystal surface25. This spin canting phenomenon in nanoparticles is dependent on chemical irregularities in the whole particle, specifically at the surface 25. Further, stoichiometric deviations and the frustration of the antiferromagnetic coupling appear at crystal surface where the lattice symmetry breaking occurs, yielding to superparamagnetic and spin-glass-like behaviours26,27. Numerous efforts have been implemented to obtain well-defined nanostructures of iron oxides, such as, nanorods, nanowires, nanotubes, nanobelts and nanocubes, as well as hollow and porous nanostructures, by a variety of solution chemistry routes and vapor-phase processes6, 20. Despite of the advantages of magnetic nanoparticles for specific applications6, there are some problems such as their tendency to aggregate. To overcome this problem, magnetic nanoparticles are prepared by a non-hydrolytic processes in organic additives at elevated temperatures. Further, to produce high-quality magnetic nanoparticles with welladjusted particle sizes and particle size distributions remains a challenge28. In this sense, coprecipitation, thermal decomposition, hydrothermal reactions, sol-gel reactions and microemulsions12 are being explored to produce magnetic nanoparticles with controlled size, chemical composition and shape.



Ferromagnetic elements such as cobalt (Co), manganese or nickel, can be used as doping materials for enhancement of magnetization of ferrite26. Also Co is more resistant to attack by mineral acids than iron27. But the most interesting aspects provided by Co in terms of properties, is the fact that it has a magnetic permeability about two-thirds of iron30 and has the highest-known Curie temperature (approximately 1121 ºC)27-28. These characteristics of Co give a strong reason for this metal to be used as a doping metal in composites, as a way to enhance the properties of the matrix material. It also increases the saturation of iron, provides high remanence and has an effect on anisotropy29. The magnetostriction of Co allows the production of nanocomposites with magnetoelectric (ME) effect, that can be of large interest in the development of sensors, actuators, micro-electrochemical systems, energy harvesters, transducers, resonators, and diodes, among others30-31. The ME effect consists on the coupling between the magnetic and electrical orders of matter32-34 in multiferroic or ME materials32, where a variation of the magnetization occurs by an applied electric field or vice versa30. In this context, polymer-based ME nanocomposites, constituted by piezoelectric polymers and magnetostrictive nanoparticles are an attractive concept, compared with single-phase or ceramic-based ME materials, due to the weak ME coupling at room temperature provided by most of the single-phase ME materials30

and the fragility of the ceramic-based ME

materials30. Most of the develop polymer based ME composites, based on magnetic nanoparticles embedded in a piezoelectric polymer are isotropic, due to the random dispersion of nearly spherical nanoparticles35-36. On the other hand, high anisotropic ME effect is critically necessary for ME single-axis sensors, allowing to determine both magnitude and direction of the magnetic field vector37. This approach on anisotropic polymer based ME magnetic sensors, despite having high applicability, has been only vaguely explored. Thus, δFeO(OH)/P(VDF-TrFE)37 and Co(II)Fe(III)O(OH)/P(VDF-TrFE)38 anisotropic ME response have been presented with a maximum ME coupling of 0.4 mV·cm-1·Oe-1 and 5.10 mV·cm−1·Oe−1 respectively. Anisotropy on ME materials are also relevant in the increasing 4 ACS Paragon Plus Environment

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area of piezoelectric39 and magnetoelectric40 tissue engineering applications as it can mimic the anisotropic contactless stimulus that occur on the alignment and structure of some body elements, such of hippocampal neural networks or muscle cells

41-43

. Nevertheless the

citotxicity on those anisotropic magnetic materials remains unreported. In the present work, novel polymer-based ME composites have been developed composed of CoFO nanoellipsoids and poly[(vinylidenefluoride-co-trifluoroethylene] [P(VDF-TrFE)]. P(VDF-TrFE) has been selected as piezoelectric component due to its highest piezoelectric response among polymers41-42 and CoFO particles were chosen as magnetostrictive material to induce high and anisotropic ME response.

2. Experimental section 2.1. CoFO particles preparation 2.1.1. Needle-like hematite (α-Fe2O3) precursors Needle-like hematite (α-Fe2O3) particles, used as precursor, were prepared by aging 0.1 M iron (III) perchlorate solutions (Fe(ClO4)3.9H2O, Fluka, >98 %) and sodium dihydrogen phosphate (NaH2PO4, Fluka, >97%) at 100 ºC in the presence of urea (Merck, >99%). During aging, the solutions were kept undisturbed in tightly capped Pyrex test tubes. The precipitates were separated from their mother solutions by centrifugation at 18,000 rpm and washed several times with doubly distilled water until a clear supernatant was observed. Finally, the so prepared solids were dried at 50 ºC before analyses. Hematite particles of 270 nm in length and axial ratio 5 were obtained by using 0.05 NaH2PO4/ Fe3+ molar ratio (sample H), according to reference45.

2.1.2. Cobalt coating and reduction of the hematite nanoparticles Cobalt-coated α-Fe2O3 (CoFO)particles were prepared by dispersing 5x10-3 moles of α-Fe2O3 nanoellipsoids in the form of powder in 150 ml of double distilled water under 5 ACS Paragon Plus Environment


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ultrasonication and adding 50 ml of a solution containing cobalt nitrate (Co(NO3)2.6H2O, Merck > 99%) in percentages of 10 or 30 wt.% of Co/Co+Fe. Then, the pH was adjusted to 10 with NH4OH solution and the sample kept under stirring for 1 hour. The resulting precipitates were washed with double distilled water and dried at 50 ºC for 12 h before reduction. The reduction of the cobalt-coated hematite particles to CoFO nanoellipsoids was carried out in two steps according to reference43. In the first step, Co-coated hematite particles were heated at 200 ºC in vacuum for three hours and then reduced to a Co-spinel under 1 atm. of hydrogen at 360 ºC for another 3 hours. The obtained samples were named as CoFO10 and CoFO30, respectively.

2.2. CoFO particles characterization 2.2.1. X-ray diffraction and microscopy Structural characterization was carried out by powder X-ray diffraction (XRD) using an X’pert Pro X-ray diffractometer (PANalytical) with Cu Kα radiation (λ =1.5418 Å). The mean coherence length (MCL), associated to the more intense diffraction peak of the detected crystalline phases was estimated from the full width at half maximum (FWHM) of the peak with the Scherrer equation: (1)

where λ is the X-ray wavelength, β is the broadening of the diffraction peak (after subtracting the instrumental broadening) and θB is the Bragg angle at which the maximum of the peak appears.47 The X-ray diffraction data were registered at room temperature in the interval between 20º and 90º (2θ) with a scan step size of 0.017°. The particle size, morphology and microstructure of the CoFO10 and CoFO30 nanoparticles were analyzed by conventional and high-resolution transmission electron microscopy (TEM and HRTEM, respectively), diffraction contrast (bright field and dark field) and selected-area electron diffraction (SAED) 6 ACS Paragon Plus Environment

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using a FEI-TITAN 80–300 kV microscope operated at 300 kV. Spectra of energy dispersive spectrometry (EDS) were recorded with an EDS analyzer attached to the TEM. For these characterizations, a drop of the colloidal sample was deposited on a lacey-carbon copper grid. There was no evidence of structural or chemical transformations of the samples during the TEM examination. The processing of the TEM micrographs and the analysis of the HRTEM images by fast Fourier transform (FFT) were carried out using the Digital Micrograph 3.7.0 software (Gatan Software, Inc, Pleasanton, CA, USA). Room temperature magnetic hysteresis loops were measured using an ADE 3473−70 Technologies vibrating sample magnetometer (VSM).

2.2.2. Cell culture and viability testing To determine the cytotoxicity of CoFO10 and CoFO30 samples towards mammalian cells, MC3T3-E1 pre-osteoblast cells (Riken bank) were used. The cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM, Gibco) containing 1 g/L glucose, 10 % Fetal Bovine Serum (FBS, Biochrom) and 1 % penicillin/streptomycin (P/S, Biochrom) at 37 °C in a 95 % humidified atmosphere containing 5 % CO2, according to the recommendations of the manufacturer. The culture medium was replaced every 2 days. At pre-confluence, cells were harvested using trypsin-EDTA (ATCC-30-2101, 0.25 % (w/v) trypsin/0.53 mM EDTA solution in Hank’s BSS without calcium or magnesium) and seeded in a 96 well plate assays, at a density of 3 × 104 cells/mL for 24 h. The cells were then exposed to different concentrations of CoFO10 and CoFO30 (serial dilutions of 2000, 1000, 500, 250, 125, 62.5, 31.3, 15.6, 7.8 and 3.9 µg/mL in DMEM) at a final volume of 150 µL and incubated at 37 ºC in a humidified atmosphere with 5 % CO2 for 24 h. Cells without any samples were used as reference (negative control - live cells) while 10 % (w/v) DMSO in contact with cells was used as positive control (dead cells). After each time-point, the samples 7 ACS Paragon Plus Environment


were removed and the cell viability analysed by a 3-(4,5-dimethylthiazol-2-yl)-2,5diphenyltetrazolium bromide (MTT) assay. The MTT assay measures the mitochondrial activity of the cells, which reflects the viable cell number. Fresh medium containing MTT was added to each well and after 3h of incubation the MTT crystals were dissolved with DMSO. The resulting optical density was measured at 570 nm, using 690 nm as a reference wavelength.

2.3. CoFO/P(VDF-TrFE) composite preparation The desired content of CoFO in relation to the P(VDF-TrFE) amount, 0.0036 in volume fraction-after solvent evaporation, has been added to DMF and then placed in ultrasound bath for 8 h to ensure an optimized dispersion of the CoFO nanoellipsoids. Such content was chosen once it allows both high ME response and flexible samples. 48,49 P(VDF-TrFE) polymer with a 70/30 molar composition was later added to the former mixture and mechanically stirred during 2 h with a Teflon stirrer in an ultrasound bath, aiming to avoid CoFO agglomeration during the mixing. Subsequently, the CoFO/P(VDF-TrFE) solution was spread on a clean glass substrate and submitted to solvent evaporation and polymer crystallization at 80 ºC (to obtain nonporous samples50), while placed between a pair of coils from an electromagnet to ensure the CoFO magnetic alignment alongside the length and thickness direction of the composite film. Samples with random CoFO filler orientation have also been also prepared. P(VDF-TrFE) crystallization was completed by cooling down composite films to room temperature (≈25ºC) and ≈50 µm CoFO/P(VDF-TrFE) free standing flexible films were then obtained after removing from the glass substrate.

2.4. CoFO/P(VDF-TrFE) composite characterization Composite’s (d33) piezoelectric response was evaluated with an wide range d33-meter (8000 model from APC International, Ltd.). Previously, the piezoelectric response of the samples 8 ACS Paragon Plus Environment

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was optimize by corona poling for 2h at 120 °C and 10 kV, and cooling down to room temperature (≈25ºC) under applied electric field, by using a previously optimized poling procedure48,51. The α33 ME coefficient was obtained by applying AC and DC magnetic fields along the length direction of the sample, which coincided with the direction of the alignment of the CoFO nanostructures. The 1 Oe amplitude AC magnetic field with frequencies ranging from 1 kHz to 75 kHz was controlled by a Agilent 33220A signal generator and delivered by two Helmholtz coils. It was found that the experimental resonance of the ME composite was ≈10kHz. An electromagnet powered with a Kepco BOP 20-20m bipolar power supply provided the DC magnetic field (HDC) with a maximum of 0.50 T. The highest ME response was obtained with an applied DC magnetic field of 0.25T. A Stanford Research Lock-in amplifier (SR530) was used to measure the produced ME voltage (∆V). The magnetic field was applied on both longitudinally and transversely aligned CoFO nanostructures, in parallel and perpendicular directions to the length of the CoFO nanoellipsoids. After that samples were rotated according to their vertical axis from 0 to 360º. The ME voltage coefficient (α) was calculated by: (2) where t is the thickness of the sample and HAC the intensity of the AC magnetic field. Before the measurement of the ME properties, CoFO/P(VDF-TrFE) samples were gold-coated with 5 mm diameter circular electrodes, by a Polaron SC502 sputter in both sides of CoFO/P(VDF-TrFE) composites. The magnetostriction of CoFO nanoellipsoids was determined by the proposed method reported on 44 and trough equations 3 and 4:



α  dS   =  dH  m × (1 − m ) ×  d 33 × EY × l × w    V V t  εo × ε  piezoelectric  dS   × BS  dH 

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(3)

λ =

(4)

where mV, EY, ɛ, ɛ0, l, t, w, BS and λ are the volume fraction of the CoFO nanoellipsoids, Young's modulus, relative permittivity, vacuum permittivity, length, thickness and width of the samples; and the value of magnetic field upon which the maximum magnetostriction (λ) saturation is reached, respectively.

3. Results and discussion 3.1. CoFO nanoellipsoids Figure 1 shows the XRD patterns of the prepared nanoellipsoids (samples CoFO10 and CoFO30) and the corresponding comparison with the standard data of hematite (α-Fe2O3, JCPDS file NO. 33-0664) and cobalt ferrite (CoFe2O, JCPDS file NO. 22-1086).


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Figure 1. XRD pattern of the samples CoFO10 and CoFO30 compared with the bulk patterns of hematite (JCPDS card NO. 33-0664) and cobalt iron oxide (JCPDS card NO. 22-1086).

The analysis of the XRD pattern obtained for sample CoFO10 indicates the presence of two crystalline phases: one of them corresponds to a nanocrystalline spinel cobalt ferrite and the other corresponds to the corundum structure of hematite (Figure 1). The quantification of the presence of the two phases from the relative diffraction peak intensities indicates a cobalt ferrite content of 81.4 ± 0.7 wt.% and a hematite content of 18.6 ± 2.1 wt.%. The MCL value obtained for the most intense peak was 23 ± 1 nm, and the values for the (220) peak of the spinel phase and the (104) peak of the hematite phase are 25 ± 1 nm and 29 ± 1 nm, respectively. The XRD pattern of sample CoFO30 shows well-defined peaks that can be indexed to a pure face center cubic structure of cobalt ferrite according to JCPDS card No. 221086 (Figure 1). The presence of hematite nanocrystals in this sample is no evidenced by



XRD.

Figure 2. a) Bright- and b) dark-field images of sample CoFO10. d) Bright- and d) dark-field images of sample CoFO30.

Low magnification bright- and dark-field TEM images (Figure 2) confirmed the uniformity and size of the nanoellipsoids of both samples, with average dimensions around of 270 x 50 nm in length and width, respectively, for both samples. Energy dispersive X-ray spectroscopic (EDS) measurements showed in Figure 3 confirm the presence of cobalt in the particle composition of both samples. In addition, traces


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of Si, Na and P indicate the adsorption of residual material from the reaction media. The Cu and C signals are attributed to the lacey-carbon of the TEM grid.

Figure 3. EDS patterns for samples a) CoFO10 and b) CoFO30. In agreement with the XRD results, selected area electron diffraction (SAED) patterns confirm the presence of both spinel ferrite and hematite phases in sample CoFO10 (Figure 4a).



Figure 4. Electron microscopy studies of sample CoFO10: a) SAED pattern, b) HRTEM image of a nanoellipsoid with hematite nanocrystals. The inset shows lattice fringes of the hematite {104}pod planes. c) HRTEM image of a pure Co-doped maghemite nanoellipsoid of the sample CoFO10 d) FFT image of image (c)). This pattern is indexed to the zone axis [110] ascribed to a maghemite-like structure.

High-resolution transmission electron microscopy analyses reveal that most of the nanoellipsoids of this sample are constituted by pure cobalt ferrite nanocrystals selfassembled sharing the same orientation into the nanoellipsoid (Figure 4b). In consequence, the Fast Fourier Transform (FFT) of these images (Figure 4c) are pseudo-monocrystalline 14 ACS Paragon Plus Environment

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patterns that can be ascribed to an axis zone of the spinel structure (Figure 4c). In a minor proportion, nanoellipsoids of sample CoFO10 present hematite nanocrystals. As an illustrative example, Figure 4c shows a HRTEM image of a nanoellipsoid where lattice fringes ascribed to the hematite structure are observed.

Figure 5. Electron microscopy studies of sample CoFO30: a) SAED pattern, b) HRTEM image of a nanoellipsoid. c) FFT of image (b)). The spots have been indexed to a cobalt ferrite structure.

Figure 5a shows a typical SAED pattern of sample CoFO30. In agreement with the XRD pattern (Figure 1), the observed spotty diffraction rings can be indexed to a spinel cobalt ferrite. On the other hand, the HRTEM images (Figure 5b) show lattice fringes with interplanar distances associated to a spinel ferrite (Figure 5c). However, although its presence was not detected by XRD, some nanoellipsoids present lattice fringes corresponding to hematite structure, but with an occurrence considerably less frequent than in sample CoFO30. It indicates a very minority presence of hematite in this sample.



200

b) CoFO10 CoFO30

100

Cell viability (%)

a)

Magnetization (emu.g-1)

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

100 0 -100 -200 -20000

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-10000

0

10000

20000

80 60 40 CoFO10 CoFO30

20 0

Magnetic Field (Oe)

1

10

100

1000

Concentration (µg/mL)

Figure 6. a) Room temperature hysteresis loops for the nanoellipsoid powders. b) In vitro dose dependent cytotoxicity assays of CoFO10 and CoFO30 nanoellipsoids after 24 hrs. contact with MC3T3-E1 osteoblasts (MTT assay). Cell viability is expressed as % compared to the control, and the error bars indicate the standard deviation of three independent replicate measurements.

Measurements of the magnetic properties of both CoFO nanoellipsoids can be found on Figure 6a. A similar magnetic response is observed on both nanoellipsoids types, nevertheless, as a result of the increased content of Cobalt and in agreement with the Slater– Pauling curve45-46, the maximum magnetization value is achieved on the CoFO30 nanoellipsoids (174 emu.g-1) when compared to the maximum magnetization value of the CoFO10 nanoellipsoids (165 emu.g-1). To assess the suitability of CoFO10 and CoFO30 nanoellipsoids for biomedical applications, such as treatment of bone cells and tissue40, it is important to evaluate their potential cytotoxicity towards the osteoblasts cell line. A dose-dependent cytotoxicity of CoFO10 and CoFO30 after 24 h exposure to MC3T3-E1 pre-osteoblast cell line is shown in Figure 6b. A partial loss of cell population viability is observed with increasing concentration. It is observed that CoFO10 nanoellipsoids are less toxic than CoFO30 nanoellipsoids showing 16 ACS Paragon Plus Environment

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an IC50 value of 1052 µg.mL-1 and 906 µg.mL-1 respectively, a parameter that determine the dose required to kill 50 % of cells. Both CoFO10 and CoFO30 nanoellipsoids reveal non cytotoxic behavior until 600 µg.ml-1, and show similar biocompatibility features than other Co/Fe/O based nanostructures already validated for biological experiments including magnetic resonance imaging and targeted drug delivery systems52.

3.2. CoFO/P(VDF-TrFE) nanocomposites Once CoFO10 and CoFO30 nanoellipsoids show similar magnetic responses (≈170 emu.g-1) and the CoFO10 nanoellipsoids show better cell viability (less cells killed and lower standard errors), CoFO10 nanoellipsoids were been incorporated into the P(VDF-TrFE) matrix and poled being obtained a piezoelectric coefficient d33=-28±1 pC.N-1. The anisotropic ME characterization was performed in agreement with35. It is represented as a function of the angle between the magnetic field direction and the sample length (Figure 7).


1.6 1.4

-1

-1

b)

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Trans Trans.Perp

HDC=2500 Oe HAC=1Oe

1.2

f=10kHz

1.0 0.8 0.6 0.4 0

45

90

135 180 225 270 315 360

angle (deg)

f=10kHz

1.0 0.8

1.6

-1

1.2

HAC=1Oe

d)

Long Long.Perp

1.4

-1

-1 -1

1.4

HDC=2500 Oe

α(mV.cm Oe )

1.6

c) α(mV.cm Oe )

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

α (mV.cm Oe )


1.2

HAC=1Oe

1.0

f=10kHz

0.6

0.4

0.4 45

90 135 180 225 270 315 360

HDC=2500 Oe

0.8

0.6

0

Alea Alea.P

0

45

angle (deg)

90 135 180 225 270 315 360

angle (deg)

Figure 7. a) Schematic representation of the anisotropic ME response characterization (The arrows represent the direction of sample’s rotation during the ME measurements: 0º to 360º). b)

Anisotropic ME response on the samples with nanoellipsoids aligned

transversally to the applied magnetic field. c) Anisotropic ME response on the samples with nanoellipsoids aligned longitudinally to the applied magnetic field. d) Isotropic ME response on samples with randomly oriented nanoellipsoids.

The magnetic field was applied on both longitudinally and transversely aligned CoFO nanostructures, in parallel and perpendicular directions to the length of the CoFO nanoellipsoids. After that samples were rotated according to their vertical axis (Figure 7a). A similar procedure was performed on non-aligned samples.


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It was observed that the highest ME responses (≈1.5 mV.cm-1.Oe-1) occurs when the DC magnetic field was applied parallel to the CoFO10 length direction alignment (0º, 180º and 360º). Consequently, the lowest ME responses (≈0.5 mV.cm-1.Oe-1) was observed when the magnetic field was applied perpendicularly to the alignment (90º and 270º). An intermediate ME response (≈0.8 mV.cm-1.Oe-1) was observed when the magnetic field was applied at angles of 45º, 135º, 225º and 315º relatively to the nanoellipsoids alignment. Regarding, the non-aligned samples (Figure 7d) a ME response of ≈0.7 mV.cm-1.Oe-1 is observed, independently of the magnetic field direction. Once for the CoFO10/P(VDF-TrFE) composite sample the magnetostrictive response is optimized along the length direction of the nanoellipsoids, when the magnetic field is applied along the perpendicular direction of the fillers, a low ME response is observed. The obtained ME response can be fully ascribed to the anisotropic behavior of the magnetostrictive coefficient in the CoFO fillers that strongly depend on the angle of the applied magnetic field38. Different to other studies38 when the magnetic field is applied perpendicularly to the nanostructures, the observed ME is not zero, such behavior can be explained by the cobalt coating that contrary to the nanoellipsoids are giving an isotropic contribution the ME response of the composite. Table 1. Comparison between the anisotropic ME response of the polymer-based ME materials reported in the literature. Composite

Magnetostriction coefficient

Cytotoxicity

ME coefficient

λ (ppm)

µg.mL-1

α (mV.cm .Oe )

Effect

-1

-1

Ref.

δ-FeO(OH)/ P(VDF-TrFE)

Rotation

0.51

Unreported

0.42

37

CoFeOOH/ P(VDF-TrFE)

Striction

507

Unreported

5.10

38

CoFO10/P(VDF-TrFE)

Striction

302

1052

1.50

Our

The values used on the CoFO10/P(VDF-TrFE) composite were: mV=0.0036, EY=0.90 GPa, d33=-21 pC.N-1, GPa, ɛ=10, l=12.5mm, t=50µm, w=6.5mm and BS=2500 Oe (198943 A.m-1).



Table 1 shows that the CoFO10 nanoellipsoids present a λE of the same order of magnitude of the high magnetostrictive CoFeOOH nanostructures. Such magnetostrictive response can be further improved by increasing the Co content on the nanoellipsoid structure. By changing the magnetostriction coefficient of the CoFO10 nanoellipsoids it is possible to control the actuation capability and ME response of the CoFO10/P(VDF-TrFE) composite making it useful for magnetoactive device applications and contactless biomedical applications such as cell culture39, tissue engineering40 and drug release6, 47.

4. Conclusions CoFO magnetostrictive nanoellipsoids (270 nmx50 nm) have been obtained by Co-coating hematite and subsequent hydrogen reduction in two steps. The obtained spinel CoFO nanoellipsoids revealed high magnetization (≈170 emu.g-1), high magnetostriction (302 ppm) and non-cytotoxicity after 24 h exposure to MC3T3-E1 preosteoblast cell line. Such nanostructures have been introduced to poly[(vinylidenefluoride-cotrifluoroethylene] [P(VDF-TrFE)] and a piezoelectric response of d33=-21±1 pC.N-1 and a maximum ME coupling (α) of 1.50 mV.cm-1.Oe-1 were obtained. Such highest ME response was observed when the DC magnetic field was applied parallel to the CoFO length direction alignment (0º, 180º and 360º) within the composite. The lowest ME responses (≈0.5 mV.cm1

.Oe-1) was observed when the magnetic field was applied perpendicularly to the alignment

(90º and 270º). The proposed anisotropic ME response is suitable for sensors and actuator applications as well as for anisotropic contactless stimuli of specific cell tissues such as of hippocampal neural networks or muscle cells. The magnetostriction coefficient of the nanoellipsoids and the ME response of the resulting polymer-based composites can be further improved by increasing the Co content on the nanoellipsoid structure.

Acknowledgements 20 ACS Paragon Plus Environment

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The authors thank the FCT- Fundação para a Ciência e Tecnologia- for financial support in the framework of the Strategic Funding UID/FIS/04650/2013 and under project PTDC/EEISII/5582/2014. P.M, C.R. and M.M.F. also thank the support from FCT (SFRH/BPD/96227/20130, SFRH/BPD/90870/2012 and SFRH/BPD/121464/2016 grants respectively). Funds provided by FCT in the framework of EuroNanoMed 2016 call, Project LungChek ENMed/0049/2016 are also gratefully acknowledged, as well as financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-3-R (AEI/FEDER, UE) (including the FEDER financial support) and from the Basque Government Industry Department under the ELKARTEK Program is also acknowledged. Financial support from the National Council of Science and Technology of Mexico (CONACYT) under research project CB12-179486 is also acknowledged. C. Luna thanks National Council of Science and Technology of Mexico (Consejo Nacional de Ciencia y Tecnología, CONACYT) for a sabbatical fellowship (ref. 215416).

References

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TOC Graphic


Improving Magnetoelectric Contactless Sensing and Actuation through

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