Hydrophobic silica xerogel as host for antioxidants

M. V. REVUELTA; M. B. FERNÁNDEZ VAN RAAP; P. MENDOZA ZÉLIS; F. H. SÁNCHEZ; G. R. CASTRO

La Plata

Conferencia; HK 2010 - Humboldt Kolleg; 2011

Physics Department - Faculty of Exact Sciences National University of La Plata

<!-- @page { margin: 0.79in } P { margin-bottom: 0.08in } -->The application of xerogels as a host matrix for bioactive substances has found an intensified interest in the last decade.Composite xerogels are promising tools for industrial applications in the areas of fortified food packaging and drug delivery because they are chemically inert and non-harmful to human body.The aims of the present work is to study hydrophobic silica xerogel as a carrier for drug delivery, using sensitive molecules like ascorbic acid (AA) to be applied in food packaging. AA is an essential nutrient in human diet with antioxidant properties as well. Free AA stability is very sensitive toenvironmental conditions such as temperature, light, oxygen, pH and water activity. Therefore ambient conditions cause a decrease in its biological activity. AA encapsulation and/or entrapment in an inert matrix are alternatives to keep AA activity protected and therefore for the development of AA-fortified food.AA was encapsulated by a sol-gel process in an organic-inorganic hybrid matrix. A one-step synthesis was optimised for the preparation of silica–drug composites by using tetraethyl orthosilicate (TEOS) and methyltrimethoxysilane (MTMS)as precursors at different molar ratios. The presence of methyl groups decrease the amount of silanol groups which results in a more hydrophobic silica surface.Characterization of AA loaded and unloaded xerogels were performed with FTIR and XRD. FTIR analysis of silica xerogels showed the characteristic vibrational peaks of SiO2 at 780 cm-1 (Si-O(b), bending mode) and 1070 cm-1 (Si-O(s), asymmetric stretching mode). However, when AA was entrapped in the xerogel, the carbonyl stretching (C=O(s)) mode of AA was shifted from 1754 cm-1 to 1738 cm-1 indicating a strong interaction with the matrix. In addition, a displacement of at least one hydroxyl group strectching (O-H(s)) mode was shifted from 3624 cm-1 to 3628 cm-1, that could be possibly because of H-bridge formation with SiO2 matrix. X-ray analysis showed two broad reflection lines at 2theta = 11 and 23 degrees corresponding to amorphous silica. For loaded xerogels the structure is expanded, the 11 degree peak shifts to lower angles. Results indicate that AA molecules were incorporated into the silica structure.Kinetics of AA release from silica xerogel granules was followed at 37ºC and 150 rpm. AA concentrations were quantified by high performance liquid chromatography (HPLC). The hidrophobic silica gel was very stable in aqueous medium pH 5. About 60% of AA was released from the dried-gel between 3 to 4 hours. The release of AA from dried-silica granules could be diffusion controlled. The effective diffusion constant of AA from dried-gel in aqueous solution obtained through Crank model was 4.12x10‐7 cm2/s.