The functionalized magnetic nanoparticles

FeCl₃·6H₂O and FeCl₂·4H₂O, hexamethylene diisocyanate, tetraethyl orthosilicate (TEOS), Dextran and solvents were purchased from Sigma-Aldrich. Other chemicals were the analytic grade reagents commercially available and used without further purification. Aqueous solutions were prepared with distilled water.

The synthesis process of iron(II,III) oxide (Fe₃O₄) nanoparticles coated with dextran bearing chemically bounded 4-Amino-TEMPO (Fe₃O₄@SiO₂@Dextran-NH-TEMPO) consisted of several steps.

The mixture of 15 mL of 0.02 M FeCl₂ and 0.04 M FeCl₃ ([Fe²⁺]:[Fe³⁺ = 1:2) was stirred in a flask, and under a nitrogen. This was followed by the addition of 0.6 mL of 28% ammonia. Upon sonication for 10 min, the reaction mixture was heated to 80 °C for 60 min. The product was magnetically separated from the solution and washed until the solution reached neutral pH. In the next process, magnetic silica nanoparticles, Fe₃O₄@SiO₂, were prepared according to the Ströber method⁴⁰. This synthesis step has already been successfully used to obtain magnetic nanoparticles for a different purpose⁴¹. First, tetraethyl orthosilicate (TEOS) was dissolved in ethanol (0.1 mL TEOS per 1 mL of ethanol). TEOS solution (15 mL per 1 g of Fe₃O₄) was slowly added to a stable suspension of Fe₃O₄ (pH 11 adjusted by ammonia) and then the mixture was stirred overnight. The product Fe₃O₄@SiO₂ was magnetically collected, washed several times with water and finally dried at 60 °C.

Dextran functionalized by 4-Amino-TEMPO (Dextran-NH-TEMPO) was obtained as follows: a solution of 4-amino-2,2,6,6-tetramethylpiperidine-N-oxyl (4-Amino-TEMPO) (0.171 g) in acetone was added to a solution of hexamethylene diisocyanate (HDIS) (0.168 g) in acetone ([HDIS]: [4-amino-TEMPO] = 1: 1). After the addition was completed, the solution was stirred for 3 hours, and then dextran (0.120 g) was added. The system was stirred for 12 h.

Finally, Dextran-NH-TEMPO was used to cover and stabilize the magnetite particles. For this purpose the solvent from Dextran-NH-TEMPO solution was evaporated and 0.100 g of Fe₃O₄@SiO₂ in water was added. The mixture was intensively stirred for 1 hour and a stable product Fe₃O₄@SiO₂@Dextran-NH-TEMPO in water was obtained.

Synthesis of Fe₃O₄@SiO₂@Dextran-O-TEMPO was analogous whereby instead of 4-Amino-TEMPO, TEMPOL (O.172 g) was used.

The structures of Fe₃O₄@SiO₂@Dextran-NH (or O)-TEMPO are presented in Fig. 9.

The structure of Fe₃O₄@SiO₂@Dextran-NH-TEMPO and Fe₃O₄@SiO₂@ Dextran-O-TEMPO.