Calero-Díaz del Castillo, Victoria L.

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    Synthesis and characterization of cobalt-substituted ferrite nanoparticles using reverse micelles
    (2005-09) Calero-Díaz del Castillo, Victoria L.; Rinaldi, Carlos; College of Engineering; Perales-Perez, Oscar; Torres-Lugo, Madeline; Department of Chemical Engineering
    With the objective of developing a magnetic nanoparticle based sensor, we have synthesized cobalt-substituted ferrite particles using reverse micelles. Reverse micelles have been used to control the nanoparticle size. Cobalt ferrite was chosen due to its high anisotropy constant which assures that the relaxation mechanism is Brownina. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (CoxFe3-xO4). Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) spectra show bands at ~560 and ~400 cm-1, confirming the metal-oxygen bond characteristic of ferrites. Transmission electron microscopy shows that the average size of the particles was ~3 nm with a geometric deviation of ~0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a = 8.388Å for Co0.61Fe0.39O4, which is near that of CoFe2O4 ( a = 8.39 Å). Magnetic properties were determined using a Superconducting Quantum Interference Device (SQUID). Coercivities (Hc) higher than 8 kOe were observed at 5 K, whereas at 300K the particles showed superparamagnetic behavior. The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field. We obtained an expression relating χ ' and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of ~106 kerg/cm3 were determined, using the Debye model, whereas anisotropy constants in the order of ~107 kerg/cm3 were calculated assuming Ωτ =1 at the temperature peak of the in-phase component of the susceptibility curve.
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    Synthesis and characterization of magnetic nanoparticles for use as sensors
    (2009-06) Calero-Díaz del Castillo, Victoria L.; Rinaldi, Carlos; College of Engineering; Perales-Perez, Oscar J.; Torres-Lugo, Madeline; Hernandez, Samuel P.; Department of Chemical Engineering; Cruz Pol, Astrid
    A comprehensive study of physical and magnetic properties of cobalt substituted ferrite and magnetite nanoparticles synthesized by thermal-decomposition was carried out. Initially, a systematic study of the effect of synthesis conditions on final size, crystalline structure, and composition of cobalt substituted ferrite nanoparticles synthesized by thermal-decomposition was carried out. Using a statistical design of experiments, the impact of the temperature and time during nucleation and growth stages on final particle size, lattice parameter, and Fe/Co ratio was determined. Based on DC and AC susceptibility measurements, the effect of sample preparation on determination of the anisotropy constant of magnetite nanoparticles was studied. Magnetite nanoparticles synthesized by the thermal-decomposition method were fixed in a poly(styrene-divinylbenzene) matrix at 0.1%, 1%, and 6%(w/w). ZFC curves and the out-of-phase component of the dynamic susceptibility were obtained for each of sample and using Néel and Vogel-Fulcher models, the anisotropy constant was determined. The effect of particle size on the anisotropy constant of magnetite nanoparticles was also studied. In this case, magnetite with different diameter were synthesized by vary the synthesis conditions during thermal-decomposition method. Magnetite nanoparticles were fixed in poly(styrene-divinylbenzene) at 0.1% (w/w) and ZFC curves and AC susceptibility measurements were carried out Finally, cobalt ferrite nanoparticles synthesized by the thermal-decomposition method were tested as possible sensors. Cobalt ferrite nanoparticles were functionalized with biotin using a COO- -silane coupling agent. Using AC measurements, avidin molecules at a concentration of 1 μM in aqueous solution could be detected by monitoring the change in Brownian relaxation time after the attachment of avidin molecule to biotinylized cobalt ferrite nanoparticle. This analysis required a sample volume of 100 μl and on 8 μg of the protein to be detected.