Distinct roles of covalent and non-covalent selenium functionalization in governing radical adsorption and redox catalysis on graphene oxide

Graphene oxide (GrO) is a promising platform for metal-free redox nanozymes, but the effect of selenium immobilization on catalytic mechanisms remains unclear. Herein, GrO was functionalized by covalent grafting of selenocysteine (SeC-GrO) or non-covalent π–π adsorption of ebselen (EbSe-GrO). SEM, EDS, FTIR, Raman, and LDI-MS confirmed successful selenium immobilization and distinct surface interactions. SeC-GrO exhibited the highest enzyme-mimetic activity, achieving 96% superoxide radical scavenging and 69.4% of native glutathione peroxidase activity, whereas EbSe-GrO showed superior DPPH scavenging. DFT calculations revealed that EbSe provides more favorable hydroxyl radical adsorption; however, experimental results indicate that adsorption thermodynamics alone does not determine catalytic performance. Instead, the immobilization strategy governs the balance between radical trapping and subsequent redox transformations. These findings establish a structure–activity relationship between selenium immobilization, radical adsorption, and redox catalysis, providing mechanistic guidance for designing graphene-based antioxidant nanozymes.