Synthesis of Organic-Inorganic Hybrid Materials

SiO₂/CGA and SiO₂/PEG/CGA organic–inorganic hybrids materials were synthesized by the sol-gel method according to a procedure reported in a recent study [34 (link)]. A solution of tetraethyl orthosilicate (TEOS, reagent grade, 98%, Sigma Aldrich, Steinheim am Albuch, Germany) was used as a precursor of the SiO₂ inorganic matrix. TEOS, ethanol 99% (Sigma Aldrich, Steinheim am Albuch, Germany), nitric acid (solution 65%, Sigma Aldrich, Steinheim am Albuch, Germany), and water were added in a solution under stirring; nitric acid was used to promote the kinetics of hydrolysis and condensation reactions. The molar ratios among the reagents in the obtained solution are: EtOH/TEOS = 6.2, TEOS/HNO₃ = 1.7, H₂O/TEOS = 6.
The hybrid materials were obtained using ethanol solutions of CGA at different percentages (5 wt %, 10 wt %, 15 wt %, 20 wt %), while a high amount of polyethylene glycol (MW = 400, Sigma Aldrich) (PEG 50 wt %) dissolved in ethanol was added to the pure silica matrix (to reach 50 wt %) before preparing the SiO₂/PEG/CGA materials. The different prepared solutions were put at room temperature until the gel was obtained, and then left into an oven at 40°C to allow for the removal of the residue solvent, thus avoiding the thermal degradation of the drug. The flow chart of the sol-gel process is shown in Figure 1.