2.8. Synthesis of Nanoparticles

The synthesis and characterization of monodisperse cobalt ferrite nanoparticles (CFNPs) have been previously described [21]. Cobalt acetylacetonate and Iron acetylacetonate react with the nonionic surfactant Triton-X100, the synthesis of nanoparticles of Fe₂O₄ magnetite doped with cobalt was carried out by the process of Thermal Decomposition (TD), which involves the reaction of a precursor compound that occurs by increasing the temperature (roughly 250–270 °C) to produce two or more compounds. TD of iron(III) acetylacetonate 1 mmol (Sigma 97%) precursor plus cobalt acetylacetonate 0.5 mmol (Sigma Aldrich 97%, San Louis, MO, USA) plus an organic surfactant as Triton X-100 (0.1 M).

The mixture was placed in a volumetric flask connected to a cooling system driven by ice water and a pump to circulate the cold water inside a refrigerant tube and allow the reaction to be carried out for the required time under Nitrogen (N₂) atmosphere, during 10 min of exposure and a temperature of 260 °C, the solution was heated for 60 min until it acquires the characteristic dark color of magnetite.

Approximately 15 mL solution was left at room temperature for 24 h and subsequently centrifuged in 5 cycles at 8000 RPM, the first three adding 5 mL of methanol 98% and the last two cycles adding 5 mL water milli Q 800 Ohm.

The characterization showed the presence of CoFe₂O₄ nanoparticles of 5 nm on average with superparamagnetic properties. The morphology and size of cobalt ferrite nanoparticle were characterized using transmission electron microscopy (TEM JEM-ARM200F, Jeol, Tokyo, Japan) and HAADF-STEM (High-Angle Annular Dark-Field scanning Transmission Electron Microscopy). In order to reduce the toxicity of the cobalt-iron nanoparticle, a gold coating was formed on the nanoparticle. The coating is fundamental to get a uniform sample and enhanced biological capacities, otherwise, CoFe₂O₄ would be toxic for “In Vitro’’ or animal models. The gold coating also drives for an enhanced half-life in the system.

The synthesis of Au seed solution (Solution 1) was prepared by using 0.5 mL (1 M, CTAB), 1 mL (50 mM, L-Ascorbic acid), and 100 µL (1 M, HAuCl₄ solution). The whole complex was sonicated for 30 min. The freshly prepared gold seed solution was used for Au shell coating. The preparation of cobalt ferrite nanoparticles was used as solution 2. Solutions 1 y 2 were used to synthesize the core-shell. Initially, 1 part of solution 2 was added dropwise to solution 1. This solution mixture was stirred for 3 h continuously until there was a color change. Then, these core-shell nanoparticles were magnetically separated using a magnet and they were washed twice with a mixture of hexane and ethanol to obtain high purity NPs excluding excess gold NPs. The washed NPs were centrifuged again twice to remove excess Triton X-100 from the solution.