Rajesh Regmi

Effects of fatty acid surfactants on the magnetic and magnetohydrodynamic properties of ferrofluids

We prepared Fe3O4 magnetic nanoparticles having diameters of approximately 12 nm by chemical coprecipitation, which were coated with three different fatty acid surfactants: oleic acid, lauric acid, and myristic acid. From x-ray diffraction, transmission electron microscopy, and Mossbauer spectroscopy measurements we confirmed that Fe3O4 is the only phase present in the samples. The zero field cooled magnetization curves for the nanoparticles exhibit broad peaks, consistent with superparamagnetic blocking for the polydisperse samples, and a saturation magnetization smaller than that for bulk Fe3O4. Although there are minimal differences in the magnetic properties of the nanoparticles having different surfactants, we find significant changes in the hydrodynamic response depending on chain length. Hyperthermia measurements show considerably larger response for oleic acid-coated samples, while magneto-optical studies indicate that these samples have slower dynamics of aggregation under the influence of a dc f...

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Composite nanoparticles of aerosol OT-alginate hydrogel loaded with Fe3O4 and rhodamine 6G (R6G) in average diameter between 25 nm and 50 nm were synthesized by emulsification-cross-linking process and characterized by transmission electron microscopy (TEM) and SQUID magnetometery measurements. TEM measurements show that the Fe3O4 particles are uniformly distributed within AOT-alginate nanoparticles. The Ca2+ cross-linked AOT-alginate nanoparticles loaded with Fe3O4 and rhodamine showed superparamagnetic behavior at room temperature with saturation magnetization of ~50 emu/g of Fe3 O4. Similar behavior was exhibited by Fe2+ cross-linked AOT-alginate nanoparticles loaded with Fe3O4 and rhodamine except it showed twice the magnetization of Ca2+ crosslinked counterpart. A decrease in the amount of rhodamine loading for the composite nanoparticle systems as compared to the bare AOT-alginate nanoparticles was found.

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We parameterized the temperature dependent magnetic dissipation of iron oxide nanoparticles fixed in a frozen aqueous solution in an ac magnetic field. The magnetic power dissipated can be modeled by considering only Neel relaxation. This dissipation increased monotonically with temperature, increasing by approximately 50% between −40 °C and −10 °C. These experimental results provide quantitative confirmation for the Neel model of magnetic dissipative heating for nanoparticles rigidly confined in a solid matrix. We also find substantial temperature dependence in the magnetic dissipation of nanoparticles suspended in a liquid, which has important consequences for potential applications of magnetic nanoparticles for hyperthermia.

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Small changes in the synthesis of MnAs nanoparticles lead to materials with distinct behavior. Samples prepared by slow heating to 523 K (type-A) exhibit the characteristic magnetostructural transition from the ferromagnetic hexagonal (α) to the paramagnetic orthorhombic (β) phase of bulk MnAs at Tp = 312 K, whereas those prepared by rapid nucleation at 603 K (type-B) adopt the β structure at room temperature and exhibit anomalous magnetic properties. The behavior of type-B nanoparticles is due to P-incorporation (up to 3%), attributed to reaction of the solvent (trioctylphosphine oxide). P-incorporation results in a decrease in the unit cell volume (∼1%) and shifts Tp below room temperature. Temperature-dependent X-ray diffraction reveals a large region of phase-coexistence, up to 90 K, which may reflect small differences in Tp from particle-to-particle within the nearly monodisperse sample. The large coexistence range coupled to the thermal hysteresis results in process-dependent phase mixtures. As-prepared type-B samples exhibiting the β structure at room temperature convert to a mixture of α and β after the sample has been cooled to 77 K and rewarmed to room temperature. This change is reflected in the magnetic response, which shows an increased moment and a shift in the temperature hysteresis loop after cooling. The proportion of α present at room temperature can also be augmented by application of an external magnetic field. Both doped (type-B) and undoped (type-A) MnAs nanoparticles show significant thermal hysteresis narrowing relative to their bulk phases, suggesting that formation of nanoparticles may be an effective method to reduce thermal losses in magnetic refrigeration applications.

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PURPOSE The purpose of this work is to develop a method for accurately quantifying effective magnetic moments of spherical-like small objects from magnetic resonance imaging (MRI). A standard 3D gradient echo sequence with only one echo time is intended for our approach to measure the effective magnetic moment of a given object of interest. METHODS Our method sums over complex MR signals around the object and equates those sums to equations derived from the magnetostatic theory. With those equations, our method is able to determine the center of the object with subpixel precision. By rewriting those equations, the effective magnetic moment of the object becomes the only unknown to be solved. Each quantified effective magnetic moment has an uncertainty that is derived from the error propagation method. If the volume of the object can be measured from spin echo images, the susceptibility difference between the object and its surrounding can be further quantified from the effective magnetic moment. Numerical simulations, a variety of glass beads in phantom studies with different MR imaging parameters from a 1.5T machine, and measurements from a SQUID (superconducting quantum interference device) based magnetometer have been conducted to test the robustness of our method. RESULTS Quantified effective magnetic moments and susceptibility differences from different imaging parameters and methods all agree with each other within two standard deviations of estimated uncertainties. CONCLUSION An MRI method is developed to accurately quantify the effective magnetic moment of a given small object of interest. Most results are accurate within 10% of true values, and roughly half of the total results are accurate within 5% of true values using very reasonable imaging parameters. Our method is minimally affected by the partial volume, dephasing, and phase aliasing effects. Our next goal is to apply this method to in vivo studies.

Incorporated AOT-Alginate Nanoparticles for Drug Delivery

MnAs exhibits a large magnetocaloric effect associated with the first-order phase transition at 315 K, making it a promising phase for near-room-temperature magnetic refrigeration technologies. Optimization of the properties to expand the temperature range of operation (by adjusting the phase transition temperature) and reduce hysteresis losses can be achieved by phosphorus-doping and nanostructuring, respectively. The synthesis of P-doped MnAs as discrete nanoparticles by rapid injection synthesis has been previously reported, but suffers from extensive polydispersity and an inability to independently control size and dopant concentration. In the present work, a kinetic analysis of the P-doped MnAs nanoparticle formation by temporal correlation of particle volume, P-dopant concentration, and particle size to monomer concentration, is undertaken. Narrow polydispersity samples can only be obtained by rapid quenching from the reaction temperature, achieved by injection of the hot solution into cold chloroform; cooling in the flask leads to polydispersity characteristic of Ostwald ripening. When isolated at high temperature, particles initially grow at high monomer concentrations, achieving volumes of ca. 740 nm3 by 10 min, and then decrease in volume by more than a factor of two by 30 min reaction time, at which point both the size and residual monomer concentration (ca. 20%) remain constant, suggesting formation of an equilibrium or an unreactive byproduct. Simultaneously, the concentration of P incorporated is found to decrease over the time of the reaction from 5–7% initially to a nearly constant concentration of <2% by 60 min. These data suggest that P is incorporated preferentially at the nucleation stage, but is lost over time. This detailed understanding of MnAs particle formation and P-inclusion enables independent assessment of the role of size, polydispersity, and dopant concentration on magnetic properties.