Learning form Nature to improve the heat generation of iron-oxide nanoparticles for magnetic hyperthermia applications.

The performance of magnetic nanoparticles is intimately entwined with their structure, mean size and magnetic anisotropy. Besides, ensembles offer a unique way of engineering the magnetic response by modifying the strength of the dipolar interactions between particles. Here we report on an experimental and theoretical analysis of magnetic hyperthermia, a rapidly developing technique in medical research and oncology. Experimentally, we demonstrate that single-domain cubic iron oxide particles resembling bacterial magnetosomes have superior magnetic heating efficiency compared to spherical particles of similar sizes. Monte Carlo simulations at the atomic level corroborate the larger anisotropy of the cubic particles in comparison with the spherical ones, thus evidencing the beneficial role of surface anisotropy in the improved heating power. Moreover we establish a quantitative link between the particle assembling, the interactions and the heating properties. This knowledge opens new perspectives for improved hyperthermia, an alternative to conventional cancer therapies

Effect of divalent metal hydroxide solubility product on the size of ferrite nanoparticles

We study the effect of divalent metal hydroxide solubility product on the size and magnetic properties of nanoparticles formed during co-precipitation. We synthesized ferrite nanoparticles by varying the solubility product from 10<SUP>−13</SUP> to 10<SUP>−17</SUP> by using different divalent cations of Mn, Co, Fe and Zn, where the average particle size decreased from 29.1 to 8.9 nm. The Mn, Co and Fe ferrites were magnetic in nature with saturation magnetization of 44.6, 47.38 and 56.19 emu/g respectively, whereas the Zn ferrite was paramagnetic. The increase in particle size observed with increasing solubility product of divalent metal hydroxide is in agreement with the nucleation theory

Effect of digestion time and alkali addition rate on physical properties of magnetite nanoparticles

We investigate the effect of digestion time and alkali addition rate on the size and magnetic properties of precipitated magnetite nanoparticles. It is observed that the time required to complete the growth process for magnetite nanocrystals is very short (~300 s), compared to long digestion times (20-190 min) required for MnO and CdSe nanocrystals. The rapid growth of magnetite nanoparticles suggests that Oswald ripening is insignificant during the precipitation stage, due to the low solubility of the oxides and the domination of a solid-state reaction where high electron mobility between Fe2+ and Fe3+ ions drives a local cubic close-packed ordering. During the growth stage (0-300 s), the increase in the particle size is nominal (6.7-8.2 nm). The effect of alkali addition rate on particle size reveals that the nanocrystal size decreases with increasing alkali addition rate. The particle size decreases from 11 to 6.8 nm as the alkali addition rate is increased from 1 to 80 mL/s. During the size decrease, the lattice parameter decreases from 0.838 to 0.835 nm, which is attributed to an increase in the amount of Fe3+ atoms at the surface due to oxidation. As the alkali addition rate increases, the solution reaches supersaturation state rapidly leading to the formation of large number of initial nuclei at the nucleation stage, resulting in large number of particles with smaller size. When alkali addition rate is increased from 1 to 80 mL/s, the saturation magnetization of the particles decreases from 60 to 46 emu/g due to the reduced particle size

Three distinct scenarios under polymer, surfactant, and colloidal interaction

Force measurement between emulsion droplets in the presence of a neutral polymer, poly(vinyl alcohol), and an ionic surfactant, sodium dodecyl sulfate, reveals that the interaction between polymer, surfactant, and colloid can lead to three distinct scenarios, depending on the sequence of adsorption of polymer and surfactant onto the colloidal interface. In the first two cases, where the colloidal interface is adsorbed with or without surfactant molecules, polymer−surfactant complexation occurs in the bulk phase but without being adsorbed at the interface. Under the above condition, the repulsive force between colloidal droplets is not significantly altered by polymer−surfactant complexes. In the third case, where the polymer is preadsorbed at the colloidal interface, polymer−surfactant interaction leads to dramatic changes in repulsive forces due to conformational changes of polymers at the interface, enhancing the stability of the colloid considerably

Effect of thermal annealing under vacuum on the crystal structure, size, and magnetic properties of ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles

In this paper, we report the variations in the crystal structure, average particle size, and magnetic properties of ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles on thermal annealing, using in situ high temperature x-ray diffraction (XRD). Fine powder of ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles with an average particle size of 9.3 nm, prepared through coprecipitation technique, has been used in these studies. The powder is heated from room temperature to 1000°C, under vacuum in steps of 100°C and the XRD pattern is recorded in situ. A sudden drop in the lattice parameter from 8.478 to 8.468 Å is observed at 800°C, above which it increases with increasing temperature. After annealing at 1000°C, the lattice parameter reduces from 8.441 to 8.399 Å and the magnetization value increases from 5 to 62 emu/g, suggesting the possibility of a conversion of the cubic structured ZnFe<SUB>2</SUB>O<SUB>4</SUB> from normal to inverse spinel structure due to canting of ions between the tetrahedral and octahedral interstitial sites. During annealing, the Zn<SUP>2+</SUP> ions move from tetrahedral site to octahedral site whereas Fe<SUP>3+</SUP> ions redistribute within the octahedral and tetrahedral sites in order to reduce the strain. The increase in the average particle size from 9 to 27 nm, after the thermal annealing at 1000°C, can be attributed to coalescence phenomenon, which starts at 600°C. The estimated value of the activation energy of ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanoparticles during the growth is 18.207 kJ/mol

X-ray diffraction-based characterization of magnetic nanoparticles in presence of goethite and correlation with magnetic properties

Nanoparticles synthesized using co-precipitation technique from iron salt solutions at different pH and temperatures are found to consist of magnetite, magnetite-goethite mixtures and goethite particles depending on process conditions. The synthesized particles are characterized using X-ray diffraction (XRD) technique and their magnetic properties are evaluated using a vibrating sample magnetometer (VSM). As properties of these nanoparticles are essentially dependent on various phases present, a detailed phase analysis and quantification of the magnetite phase in magnetite-goethite mixed system has been carried out using XRD technique. For quantification, a calibration graph has been made using samples of magnetite and goethite mixed in different weight percentages. Using this calibration graph, magnetite weight percentages have been estimated in mixed particles produced by co-precipitation from iron salt solutions at various pH values. A good correlation between saturation magnetization values and magnetite weight percentage has been observed in these mixed particles. When the synthesized particles are pure magnetite, the saturation magnetization value is found to depend on the diameter of the particles, estimated using XRD and VSM data