Water-dispersible superparamagnetic iron oxide nanoparticles (SPIONs) were synthesized by thermal decomposition of iron(III) acetylacetonate in the presence of triethylene glycol (TREG). The resulting TREG-coated SPIONs were not stable, undergoing agglomeration and loss of the TREG coating under prolonged storage at 37 °C or in the presence of increased saline concentrations. To avoid these problems, stable colloidal TREG-coated SPIONs were obtained by two different procedures: (i) dimercaptosuccinic acid (DMSA) ligand-exchange reactions to obtain DMSA-coated SPIONs and (ii) chemical modification of the TREG coating. Both procedures, but especially the DMSA exchange, increased the stability of the SPION suspension. Finally, the functionality of both types of particles for biological applications was demonstrated by conjugating a model antibody to the end carboxyl groups of the SPIONs and testing the immunoreactivity of the final antibody–particle conjugates by an enzyme-linked immunosorbent assay (ELISA).
A simple procedure is used to prepare anisometric iron oxide nanoparticlesexhibiting superparamagnetic behavior. The figure shows the nanorod/nanorice morphology of these novel structures. Superparamagnetism is engineered in these structures by favoring thermally activated processes by adopting small particle sizes, introducing defects, and doping with cations. Positively and negatively charged particles are obtained by varying the surface coating.
The thermodynamic properties of maghemite (γ-Fe 2 O 3 ) nanoparticles with size smaller than 10 nm had not been studied previously because of particle size limitations for samples synthesized by wet chemical methods. Laser-induced pyrolysis is a well-established method of producing maghemite with particle sizes smaller than 10 nm. Maghemite nanoparticles, obtained by this method and having a size range of 2-40 nm, were fully characterized and studied by solution calorimetry. The enthalpy of water adsorption was also measured. The surface enthalpy obtained from calorimetric data for the hydrated maghemite surface is 0.57 ( 0.10 J/m 2 and is in good agreement with previously reported values. The surface enthalpy for the dry, water-free surface is 0.71 ( 0.13 J/m 2 and is reported for the first time. The difference in the surface enthalpy for the dry surface between Rand γ-polymorphs of Fe 2 O 3 is similar to that between Rand γ-Al 2 O 3 . This large difference in surface enthalpy (∼1.2 J/m 2 ) creates an energy crossover so that fine-grained hematite is metastable relative to fine-grained maghemite at particle size <15 nm.
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