2.1. Synthesis of the Iron Oxide Nanoparticles

Magnetite particles of 35 nm (Fe₃O₄-35nm) are synthesized by oxidative precipitation of FeSO₄ in the presence of ethanol [37]. The FeSO₄ precipitation and the subsequent aging are carried out in a globe box to avoid the formation of other undesirable secondary iron phases due to an oxidizing atmosphere. To synthesize the Fe₃O₄-35nm sample, two solutions were prepared: a basic solution consisting on a mixture of 25 mL NaNO₃ (2 M) and NaOH (4.2 M), 88 mL of distilled water and 62.5 mL of ethanol, and an acid solution formed by 13.9 g of FeSO₄·7H₂O dissolved in 50 mL of H₂SO₄ (0.01 M). N₂ gas was previously passed through all the solutions to ensure that only Fe₃O₄ was present in the final precipitate. The basic solution was rapidly added to the acid solution and stirred for 15 min, forming a turquoise-colored compound known as green rust. When the precipitation was completed, green rust was introduced in a jacketed glass bottle previously heated to 90 °C, and the system was closed and undisturbed at this temperature for 24 h. Aging time was fixed at 24 h in order to reach conditions near equilibrium. At this point, the sample was cooled down at room temperature and the solid was separated by magnetic decantation and washed several times with distilled water.

To synthesize magnetite nanoparticles of 14 nm in diameter (Fe₃O₄-14nm), 75 mL of a NH₄OH solution (25%) was rapidly added to a solution of FeCl₂ (0.175 M) and FeCl₃ (0.334 M). The addition was carried out rapidly at room temperature under vigorous stirring. Later, the sample was washed three times with distilled water using magnetic decantation.

To obtain the largest nanoparticles (Fe₃O₄-350nm), additional time was added as much as possible. Thus, urea (CO(NH₂)₂) was used as a base instead of NH₄OH. The slow hydrolytic degradation of urea in acidic conditions generated ammonia (NH₃), which increased the reaction pH very slowly, leading to the precipitation of large Fe₃O₄ nanoparticles. The reaction vessel was a 100 mL Duran^® glass laboratory bottle, which can be used as a low-temperature hydrothermal reactor. An amount of 4.8 g of urea was added to 70 mL of distilled water and stirred vigorously. Then, 2.7 g de FeCl₃·6H₂O was added and, once it was totally dissolved, 1.06 g of FeCl₂·4H₂O was added to the mixture. To synthesize this sample, N₂ was bubbled through the solution during the precursor’s addition to reduce the presence of oxygen as much as possible, which produces the oxidation of the iron intermediates and induces the formation of other iron phases different from Fe₃O₄. Then, the bottle was closed and introduced in a pre-heated oil bath at 90 °C for 48 h. Sample was under magnetic stirring during the whole process. After this time, the sample was cooled down to room temperature and the resulting black precipitate consisting of Fe₃O₄ particles was decanted using magnetic settling and washed several times until the supernatant was totally clear and transparent.

The other types of particles have been synthesized using coprecipitation of a mixture of the Fe(II) and Fe(III) salts in alkaline medium as previously described by other authors in detail [38]. The size of the particles can be controlled by the addition rate and order and also by the aging time and temperature (among others).

In order to enhance the colloidal properties of the particles and to oxidize Fe₃O₄ nanoparticles to γ-Fe₂O₃, which is more stable at room temperature and atmosphere, an acid treatment was carried out [39]. This treatment consisted of three steps: First, the Fe₃O₄ nanoparticles previously synthesized were mixed with 300 mL of HNO₃ (2 M) and stirred for 15 min. In a second step, the supernatant was removed and Fe(NO₃)₃ 1 M (75 mL) and distilled water (130 mL) were added. The mixture was stirred and heated up to the boiling point for 30 min. Then, it was cooled down to room temperature. Finally, in a third step, the supernatant was removed and another 300 mL of HNO₃ (2 M) were added. The mixture was stirred for 15 min, the supernatant was removed and particles were washed three times with acetone and redispersed in distilled water. Acetone wastes were removed with a rotary evaporator. The maghemite nanoparticles obtained as a result of the oxidation of magnetite were named as γFe₂O₃-8nm. Longer stirring times during the third step led to the smallest nanoparticles (sample γFe₂O₃-6nm) as the particle surface was partially dissolved by nitric acid.

Slight modifications of this synthesis protocol led to larger nanoparticles. Thus, to synthesize the γFe₂O₃-12nm sample, Fe(II)-Fe(III) solution was added to the basic solution as slow as possible (drop by drop). In addition, the aging time was increased from 5 min to 1 h and the aging temperature was fixed to 90 °C. Afterwards, the oxidizing acid treatment was carried out as explained previously [10].