Antioxidant Activity of SiO₂@{Sericin} Hybrids: A Comparable OH-Radical and DPPH-Radical Scavenging Study

A new class of hybrid materials was developed via covalent grafting of sericin, a silk-derived protein, onto SiO₂ particles to assess their antioxidant properties. Two variants of SiO₂@sericin hybrids were synthesized, with 10% ({SiO₂@sericin_10}) or 20% ({SiO₂@sericin_20}) sericin loadings. An in tandem analysis of their antioxidant efficiency was performed against OH and DPPH radicals. The experimental results demonstrate that the {SiO₂@sericin} hybrids exhibit significantly enhanced antioxidant activity compared with sericin in aqueous solution. Specifically, 1 g of the {SiO₂@sericin_10} hybrid quenches 308 μmol of DPPH radicals and 120 μmol of ^●OH, whereas aqueous sericin in solution quenched only 85 μmol of DPPH and 53 μmol of ^●OH. IR, Raman, BET, and DLS data collectively indicate that the interfacial topography of sericin on the SiO₂ surface is highly dependent on its loading concentration. At the high loading (20%), sericin forms a hermetic coating over the SiO₂ nanoparticles, resulting in steric hindrance that restricts the accessibility of antioxidant functional groups. In contrast, at the optimized 10% loading, a greater proportion of sericin's antioxidant moieties remains accessible for radical scavenging. This interfacial topography effect is reflected in the antioxidant's activity. One gram of {SiO₂@sericin_20} quenches 202 μmol of DPPH radicals and 100 μmol of ^●OH, which are smaller than the amount of {SiO₂@sericin_10}. These findings reinforce our previous conclusion that covalent grafting of organic molecules bearing antioxidant functionalities onto SiO₂ surfaces is an effective strategy to enhance radical scavenging efficiency, applicable to both hydroxyl-radical and DPPH (hydrogen atom transfer) quenching mechanisms.