Antioxidant and Antiradical SiO2 Nanoparticles Covalently Functionalized with Gallic Acid

Gallic acid (GA) and its derivatives are natural polyphenolic substances widely used as antioxidants in nutrients, medicine and polymers. Here, nanoantioxidant materials are engineered by covalently grafting GA on SiO2 nanoparticles (NPs). A proof-of-concept is provided herein, using four types of well-characterized SiO2 NPs of specific surface area (SSA) 96-352 m(2)/g. All such hybrid SiO2-GA NPs had the same surface density of GA molecules (similar to 1 GA per nm(2)). The radical-scavenging capacity (RSC) of the SiO2-GA NPs was quantified in comparison with pure GA based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH center dot) radical method, using electron paramagnetic resonance (EPR) and UV-vis spectroscopy. The scavenging of DPPH radicals by these nanoantioxidant SiO2-GA NPs showed mixed-phase kinetics: An initial fast-phase (t(1/2) <1 min) corresponding to a H-Atom Transfer (HAT) mechanism, followed by a slow-phase attributed to secondary radical radical reactions. The slow-reactions resulted in radical-induced NP agglomeration, that was more prominent for high-SSA NPs. After their interaction with DPPH radicals, the nanoantioxidant particles can be reused by simple washing with no impairment of their RSC.