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International audience; In this work, Cu-based sulfides (chalcopyrite CuFeS2, mohite Cu2SnS3, tetrahedrite Cu12Sb4S13, mawsonite Cu6Fe2SnS8, and kesterite Cu2ZnSnS4) were synthesized by industrial milling in an eccentric vibratory mill to demonstrate the scalability of their synthesis. For a comparison, laboratory-scale milling in a planetary mill was performed. The properties of the obtained samples were characterized by X-ray diffraction and, in some cases, also by Mössbauer spectroscopy. For the densification of powders, the method of spark plasma sintering was applied to prepare suitable samples for thermoelectric (TE) characterization which created the core of this paper. A comparison of the figure-of-merit ZT, representative of the efficiency of thermoelectric performance, shows that the scaling process of mechanochemical synthesis leads to similar values as compared to using laboratory methods. This makes the cost-effective production of Cu-based sulfides as prospective energy materials for converting heat to electricity feasible. Several new concepts that have been developed involving combinations of natural and synthetic species (tetrahedrite) and nanocomposite formation (tetrahedrite/digenite, mawsonite/stannite) offer sustainable approaches in solid-state chemistry. Mechanochemical synthesis is selected as a simple, one-pot, and facile solid-state synthesis of thermoelectric materials with the capability to reduce, or even eliminate, solvents, toxic gases, and high temperatures with controllable enhanced yields. The synthesis is environmentally friendly and essentially waste-free. The obtained results illustrate the possibility of large-scale deployment of energy-related materials.

Our study targets some of the long-standing questions concerning the somewhat mysterious properties of chalcopyrite CuFeS2. We show that defect chemistry in connection with charge transfer within t...

Magnetic nanoparticles of e-Fe1.76 Ga0.24 O3 with the volume-weighted mean size of 17 nm were prepared by thermal treatment of a mesoporous silica template impregnated with metal nitrates and were coated with silica shell of four different thicknesses in the range 6-24 nm. The bare particles exhibited higher magnetization than the undoped compound, 22.4 Am2 kg-1 at 300 K, and were characterized by blocked state with the coercivity of 1.2 T at 300 K, being thus the very opposite of superparamagnetic iron oxides. The relaxometric study of the silica-coated samples at 0.47 T revealed promising properties for MRI, specifically, transverse relaxivity of 89-168 s-1 mmol(f.u.)-1 L depending on the shell thickness was observed. We investigated the effects of the silica-coated nanoparticles on human A549 and MCF-7 cells. Cell viability, proliferation, cell cycle distribution, and the arrangement of actin cytoskeleton were assessed, as well as formation and maturation of focal adhesions. Our study revealed that high concentrations of silica-coated particles with larger shell thicknesses of 16-24 nm interfere with the actin cytoskeletal networks, inducing thus morphological changes. Consequently, the focal adhesion areas were significantly decreased, resulting in impaired cell adhesion.

The geometrically frustrated diamond spin chain system has yielded materials with a diversity of interesting magnetic properties but is predominantly limited to compounds with single-spin components. Here, we report the compound [(CH

Magnetic nanoparticles of iron oxides have received extensive attention in the biomedical research, e.g. as prospective contrast agents for T2-weighted magnetic resonance imaging (MRI). The ability of a contrast agent to enhance the relaxation rate of 1H nuclei in its vicinity is quantitatively described by its relaxivity. Among the polymorphs of the iron(III) oxide, the nanoparticulate e-Fe2O3, distinguished by its high magnetocrystalline anisotropy resulting in a blocked state of single-domain particles up to the Curie temperature of ∼500 K, has never been studied as a contrast agent for MRI before. We analyzed r1 and r2 relaxivities of e-Fe2O3 nanoparticles coated with amorphous silica, particularly with the aim to determine their dependences on the external magnetic field, temperature, and thickness of the silica coating. The r2 relaxivity was interpreted within the motional averaging and static dephasing regimes. MRI images at 11.75 T confirmed high applicability of e-Fe2O3-based contrast agents in ultra-high fields. We present the first case study considering e-Fe2O3 nanomagnets for prospective application in biomedicine, in particular for MRI in ultra-high fields. The study is complemented by a thorough investigation of magnetic properties of the nanoparticles, revealing some interesting anomalies.

A multistep procedure, employing akaganeite FeO(OH,Cl) as a precursor, was developed for the preparation of rod-like maghemite particles for medical applications. At first, akaganeite rods with length of several hundred nm and width of approximate to 85 nm were prepared under hydrothermal conditions and were subsequently coated with mesoporous silica. Such coating enabled to maintain the shape of rods during the following steps that involved structural transformation of akaganeite to maghemite, i.e. gamma-Fe2O3. Then the original protective coating was removed by alkaline leaching, bare maghemite rods were isolated, and their structure and ferrimagnetic order were characterized by X-ray diffraction, TEM inspection, Mossbauer spectroscopy and SQUID magnetometry. The magnetization of the bare maghemite rods, that were formed by elongated clusters of approximate to 10-20 nm crystallites, made 47.0 and 41.7 Am-2/kg in magnetic field of 3 T at 5 K and 300 K, respectively. The hysteresis loops of both the bare and coated products at 300 K and ZFC/FC studies showed that the maghemite particles were largely blocked at room temperature in spite of the small size of crystallites. Finally, the particles were equipped with standard silica coating for biological studies. An evaluation of cytotoxicity of this silica-coated product was performed on two cell lines, namely A549 and MCF-7. The viability of cells incubated with particles at the concentration of 0.10, 0.21 and 0.42 mmol(Fe)/L was determined after 24 h and 48 h of incubation, and the values normalized to the viability of negative control were generally higher than 95%. Moreover, the real-time monitoring of cell adhesion, proliferation, and cytotoxicity by an xCELLigence system during 72 h of the incubation with particles revealed only some decrease of the cell index for the MCF-7 cells at the high concentration. Pro přípravu tyčovitých maghemitových částic pro lékařské aplikace byl vyvinut vícestupňový postup využívající akaganeit FeO (OH, Cl) jako prekurzor. Nejprve byly za hydrotermálních podmínek připraveny akaganeitové tyče s délkou několika stovek nm a šířkou přibližně 85 nm a následně byly potaženy mesoporézním oxidem křemičitým. Takový povlak umožnil udržet tvar tyčinek v následujících krocích, které zahrnovaly strukturální přeměnu akaganeitu na maghemit, tj. gama-Fe203. Poté byl původní ochranný povlak odstraněn alkalickým vyluhováním, byly izolovány holé maghemitové tyčinky a jejich struktura a ferrimagnetické uspořádání byly charakterizovány rentgenovou difrakcí, TEM kontrolou, Mossbauerovou spektroskopií a SQUID magnetometrií. Magnetizace holých maghemitových tyčí, které byly vytvořeny protáhlými shluky přibližně 10 až 20 nm krystalů, vytvořila 47,0 a 41,7 Am-2 / kg v magnetickém poli 3 T při 5 K a 300 K. Hysterezní smyčky holých i potažených produktů při 300 K a ZFC / FC studie ukázaly, že částice maghemitu byly z velké části blokovány při pokojové teplotě navzdory malé velikosti krystalů. Nakonec byly částice potaženy silikou pro účely biologických testů. Vyhodnocení cytotoxicity bylo provedeno na dvou buněčných liniích, konkrétně A549 a MCF-7. Viabilita buněk inkubovaných s částicemi v koncentraci 0,10, 0,21 a 0,42 mmol (Fe) / l byla stanovena po 24 hodinách a 48 hodinách a hodnoty byly vztaženy k viabilitě negativní kontroly. Viabilita byla vyšší než 95%. Kromě toho, monitorování buněčné adheze, proliferace a cytotoxicity v reálném čase pomocí systému xCELLigence po dobu 72 hodin, ukázalo pouze nepatrné snížení buněčného indexu u buněčné linie MCF-7 při vysoké koncentraci částic.

Hydrothermal synthesis was employed to prepare three samples of Mn-Zn ferrite nanoparticles with the mean size of crystallites of 12–14 nm, whose compositions were accurately determined by X-ray fluorescence spectroscopy to Mn0.82Zn0.21Fe1.97O, Mn0.70Zn0.31Fe1.99O4, and Mn0.62Zn0.41Fe1.97O4. The X-ray diffraction and SQUID magnetometry were used to determine the spinel structure and ferrimagnetic properties of the samples. AC field heating experiments were performed on particles dispersed in glycerol to evaluate the specific absorption rate. Based on the superparamagnetic behaviour and magnetic hyperthermia data, the Mn0.62Zn0.41Fe1.97O4 phase was selected for further studies, including a more detailed characterization of the crystal and magnetic structure by neutron diffraction and Mossbauer spectroscopy, encapsulation into mesoporous silica, determination of the specific absorption rate of coated particles, and analysis of the porosity of the shell. Shortly, the Mn0.62Zn0.41Fe1.97O4 nanoparticles were found to possess high magnetization of M(10kOe) = 48.4 emu/g at room temperature and superparamagnetic behaviour at body temperature on the timescale of magnetic measurements. The coated product was formed by clusters of ferrite crystallites with overall mean size of 52 nm encapsulated into 22 nm thick mesoporous shell and showed heating efficiency of 57 W/g(Mn + Fe) in AC field of 970 kHz and amplitude of 8 mT. The mesoporous shell provided specific surface area of 670 m2/g, and its pores of an average diameter 3 nm occupied specific volume of 0.45 cm3/g. In a a preliminary study, the silica pores were loaded with rhodamine B as a model of a hydrophilic drug and subsequently enclosed by lauric acid that served as a temperature sensitive gatekeeper. The release of the model compound to aqueous medium was determined at room temperature and 60 °C by spectrophotometric analysis. Results have shown that amount of rhodamine B in pores achieved 15 mg per gram of silica-coated ferrite cores and its release was controlled by temperature.

A spin-reorientation transition from a weakly ferromagnetic (WF) to an antiferromagnetic (AF) spin ordering in hematite (α-Fe2O3) during cooling occurs at Morin temperature (TM∼264 K for bulk). The transition is strongly size dependent and TM generally decreases with the decreasing volume of the particles. For particles smaller than approximately ∼20 nm, the Morin transition may be even suppressed and disappears entirely as near-surface spins deviate strongly from the antiferromagnetic easy axis. We report an investigation on nanoparticles prepared by hydrothermal method and sol–gel technique (in silica) of pure α-Fe2O3 phase as confirmed by XRD (space group R-3c, lattice parameters a = 5.038(2) A, c = 13.772(12) A) differing in the median size derived by TEM: 5.6 nm, 26 nm, 42 nm and 103 nm. By means of Mossbauer spectra acquired between 4.2 and 300 K, we determined the relative concentrations of magnetic phases (WF and AF) within the 57Fe enriched sample and searched for the best finite-scaling theoretical model (mean-field, 3D Heisenberg, Ising) describing the derived size dependence of Morin temperature of the nanoparticles with a log-normal size distribution. The comparison of relevant parameters derived from the fit of experimental data by theoretical model is consistent with the 3D Heisenberg model with scaling parameter λ = 1.4, Morin temperature of bulk material TM(∞) = 265(1) K and correlation length ξ0 = 8.1(2) nm or Ising model with λ = 1.6, TM(∞) = 265(1) K and ξ0 = 9.4(2) nm.

Magnetic nanoparticles of ε-Fe

Chalcopyrite CuFeS2 was shown to be a promising thermoelectric material. Considering thermoelectric efficiency, its relatively high and temperature weakly dependent power factor, economic affordability and ecological benignity is counterbalanced by a high lattice thermal conductivity. Thus it is highly desirable to lower the thermal conductivity of chalcopyrite thermoelectric material without deterioration of other thermoelectric characteristics. In our study, we demonstrate that mechanosynthesis followed by appropriate sintering enables to prepare such nanostructured ceramics with a favourable thermoelectric response. Our study shows that mechanosynthesis is a low-cost technological route for the production of thermoelectric chalcopyrite ceramics. Chalkopyrit CuFeS2 se ukázal jako slibný termoelektrický materiál. Vzhledem k termoelektrické účinnosti je její relativně vysoký a teplotně slabě závislý účiník, ekonomická dostupnost a ekologická neškodnost vyvážena vysokou tepelnou vodivostí mřížky. Proto je velmi žádoucí snížit tepelnou vodivost termoelektrického materiálu chalkopyritu bez zhoršení dalších termoelektrických vlastností. V naší studii dokazujeme, že mechanosyntéza následovaná vhodným slinováním umožňuje připravit takovou nanostrukturovanou keramiku s příznivou termoelektrickou odezvou. Naše studie ukazuje, že mechanosyntéza je levnou technologickou cestou pro výrobu termoelektrické keramiky z chalkopyritu.

Four types of magnetic nanoparticles were synthesized, namely, the La0.65Sr0.35MnO3and La0.80Na0.20MnO3 perovskite manganites and the Mn0.61Zn0.42Fe1.97O3 and Co0.44Zn0.70Fe1.86O4 spinel ferrites. Subsequent coating with silica provided typical core–shell particles with the mean shell thickness of 14–19 nm and the mean size of cores of 49–52 nm in the case of manganites and 26–33 nm in the case of ferrites. Moreover, two other surface modifications were employed for Mn–Zn ferrite nanoparticles, specifically coating with titania, leading to larger clusters with more complex morphology, and the stabilization by citrate anions. The products were characterized by powder X-ray diffraction and transmission electron microscopy, and their magnetic properties were probed by SQUID magnetometry. The main attention was devoted to the relaxometry on aqueous suspensions of particles in a magnetic field of 0.5 T. For distinct types of the prepared core–shell nanoparticles, the temperature dependence of transverse relaxivity was measured and the role of different relaxation regimes was analyzed.

Magnetic behaviour of nanoparticles deviates from their bulk equivalents not only due to finite-size and surface effects but it may be also affected by occurrence of metastable states, such as non-equilibrium cation distribution. The chemical composition along with the specific cation distribution are decisive for magnetic properties of spinel ferrites, the nanoparticles of which can be easily tailored to exhibit peculiar properties suitable for various applications. The present study is devoted to hydrothermally prepared Mn Zn ferrite nanoparticles; namely to five samples with approximate composition Mn1−xZnxFe2O4, where x = 0.21–0.63, that were prepared by a surfactant-free hydrothermal procedure at a rather low temperature of 180 °C. XRD measurements evidenced the cubic spinel structure and a gradual decrease of the mean crystallite size from 14 to 10 nm with increasing zinc content, which was further confirmed by TEM analysis. 57Fe Mossbauer spectroscopy and temperature-variable magnetic measurements provided valuable insight into the blocking behaviour of particles on two dramatically different time scales and demonstrated that Neel relaxation can be enhanced by increasing the zinc content. Nanoparticles of the selected composition Mn0.62Zn0.41Fe1.97O4 were subjected to neutron diffraction study at 2 K to determine the ferrimagnetic order and in combination with Mossbauer spectroscopy to analyse the cation distribution. The non-zero occupancy of Zn2+ in octahedral sites evidenced a metastable distribution, and the supplemental DFT study revealed that the distribution of Mn2+ is non-equilibrium as well.

Magnetic nanoparticles of e-Fe1.76Ga0.24O3 were prepared by thermal treatment of a mesoporous silica matrix impregnated with nitrates. The chosen Ga-doping enhanced magnetization and suppressed the low-temperature spin-reorientation transition typical for e-Fe2O3. Despite the small mean size of 11 nm, the nanoparticles were in the blocked state over the whole temperature range under study, unlike standard superparamagnetic contrast agents based on other iron oxides or ferrites. The role of Ga-doping in local magnetic properties of the epsilon polymorph of ferric oxide was probed by 57Fe Mossbauer spectroscopy. The particles were further coated with silica and their performance in MRI was tested both in relaxometry and ultra-high-field imaging. The obtained dependences of relaxivity on temperature and thickness of the coating were placed in the context of relevant theoretical models of particle-induced relaxation – motional averaging and static dephasing regimes.

Iron sulphide Fe3S4, known as the mineral greigite, has been prepared in two distinct nanocrystalline forms with the mean crystallite size of dXRD ≈ 30 and 80 nm via hydrothermal synthesis by using two different surfactants (oleic acid and cetyltrimethylammonium bromide). Their crystallinity and phase purity are probed by X-ray diffraction and Mossbauer spectroscopy. For detailed characterization of the ferrimagnetic state and its temperature evolution, the SQUID magnetometry, ferromagnetic resonance and the temperature dependence of 57Fe Mossbauer spectra are used. The blocking behaviour of the nanocrystalline samples is discussed with respect to magnetic and Mossbauer data. Moreover, special attention is given to the analysis of temperature-dependent data, which reveals that: first, spin reorientation associated with the change in easy axis from " open=" 110 to " open=" 111 on cooling occurs at ≈100 K, secondly, Verwey-like transition is not present.

The transverse relaxivity r2, characterizing the performance of negative contrast agents like magnetic nanoparticles in T2-weighted MRI, depends on multiple factors, most importantly on magnetic properties and the size of the particles, including both the size of magnetic cores and the effective size of whole particles present as individual entities in the suspension. The current study probes the transverse relaxivity of ferrimagnetic CoxZnyFe3-x-yO4 nanoparticles whose behavior varies from the blocked state, across the superparamagnetic regime to the vicinity of the paramagnetic state, while other parameters, namely the size of the magnetic cores (≈10 nm), the size of whole colloidally stable particles, and their geometry (≈30 nm-sized clusters of crystallites uniformly coated with ≈12 nm thick silica shell) are kept the same. Among the samples studied, the silica-coated clusters of Co0·66Zn0·47Fe1·87O4 crystallites show high values of r2 = 534 s−1 mmol(Me3O4)−1 L at room temperature in the magnetic field of 1.5 T. Importantly, the field and temperature dependences of transverse relaxivity are discussed in relation to relevant theoretical models, i.e., motional averaging and static dephasing regimes. The increase in r2 in higher magnetic fields is related to the notable paraprocess in the employed cores, whereas the gradual decrease of r2 with increasing temperature is dominated by the significant increase of the self-diffusion coefficient of water.

Controlled thermal decomposition of zinc and iron acetylacetonates in the presence of oleic acid and oleylamine provided surfactant-capped magnetic nanoparticles with narrow size distribution and the mean diameter of ≈15 nm. The combined study by XRD, XRF and Mossbauer spectroscopy revealed three important features of the as-prepared nanoparticles. First, the actual ratio of Zn:Fe was considerably lower in the product compared to the initial ratio of metal precursors (0.14 vs. 0.50). Second, a pure stoichiometric Zn-doped magnetite system, specifically of the composition Zn0.37Fe2.63O4, with no signatures of oxidation to maghemite was formed. Third, Zn2+ ions were distributed at both tetrahedral and octahedral sites, and the observed preference for the tetrahedral site was only twice as high as for the octahedral site. Furthermore, carbon-coated nanoparticles were achieved by pyrolysis of the surfactants at 500 °C, providing a potential sorbent of organic pollutants with room-temperature magnetization as high as 79.1 emu g−1 and very low carbon content of 5 wt%. The thermal treatment, albeit intended only for the carbonization of surfactants, did alter also the non-equilibrium cation distribution toward the equilibrium one by the relocation of a considerable fraction of the octahedrally coordinated Zn2+ to the tetrahedral sites. Preliminary experiments with magnetic solid-phase extraction of β-estradiol from aqueous solutions evidenced applicability and reusability of the carbon-coated product in the separation of steroid pollutants.

Most magnetic materials studied as negative contrast agents for magnetic resonance imaging (MRI) enter the superparamagnetic state with decreasing size of their particles, which is distinguished by thermal fluctuations of particle magnetic moment. In the present study, a novel type of contrast agents based on non-superparamagnetic nanoparticles (nanomagnets) of aluminum-doped epsilon polymorph of Fe2O3 is suggested and used as a model system to elucidate the role of magnetic blocking and other factors in the transverse relaxation of water. Specifically, the dependence of the transverse relaxivity r2 on the magnetic field (0.47–11.75 T), temperature (278−348 K), magnetization, surface modification (silica or citrate) and thickness of silica coating (6−21 nm) is analyzed for the e-AlxFe2-xO3 nanoparticles with x = 0.23 and the median size of 21 nm. These nanoparticles evinced higher magnetization than the undoped material and were in the blocked state in the temperature range under study. Further, irreversible magnetization processes were revealed by SQUID magnetometry in aqueous suspension of coated clusters of magnetic crystallites that resulted in the field dependence of r2. The temperature dependence of r2 was interpreted by the combination of two different regimes – motional averaging and static dephasing regimes. Slow diffusion of water molecules inside the silica shells was suggested to explain only moderate decrease of r2 with increasing the coating thickness. Moreover, the performance of the contrast agent was demonstrated not only in ultra-high-field MRI at 11.75 T but also by imaging in vivo on a mouse model at 1 T. Finally, preliminary evaluation of cytotoxicity on rat mesenchymal stem cells did not reveal any significant effects. Due to their low toxicity and high relaxivity, e-AlxFe2-xO3 nanoparticles present a promising starting material for further biomedical applications involving MRI, multimodal contrast agents and theranostic carriers.

Magnetic nanoparticles have found broad applications in medicine, especially for cell targeting and transport, and as contrast agents in MRI. Our samples of 𝜖-Fe2O3 nanoparticles were prepared by annealing in silica matrix, which was leached off and the bare particles were then coated with amorphous silica layers of various thicknesses. The distribution of particle sizes was determined from the TEM pictures giving the average size ∼20 nm and the thickness of silica coating ∼5; 8; 12; 19 nm. The particles were further characterized by the XRPD and DC magnetic measurements. The nanoparticles consisted mainly of 𝜖-Fe2O3 with admixtures of ∼1 % of the α phase and less than 1 % of the γ phase. The hysteresis loops displayed coercivities of ∼2 T at room temperature. The parameters of hyperfine interactions were derived from transmission Mossbauer spectra. Observed differences of hyperfine fields for nanoparticles in the matrix and the bare ones are ascribed to strains produced during cooling of the composite. This interpretation is supported by slight changes of their lattice parameters and increase of the elementary cell volume deduced from XRD. The temperature dependence of the magnetization indicated a two-step magnetic transition of the 𝜖-Fe2O3 nanoparticles spread between ∼85 K and ∼150 K, which is slightly modified by remanent tensile stresses in the case of nanoparticles in the matrix. The subsequent coating of the bare particles by silica produced no further change in hyperfine parameters, which indicates that this procedure does not modify magnetic properties of nanoparticles.