Superoxide dismutase mimicking nanocomposites based on immobilization of metal complexes on nanotubular carriers

Nanocomposites of antioxidant activity were developed by immobilization of a superoxide dismutase (SOD) mimicking metal complex on three types of nanotubular systems. First, bare halloysite nanotubes (h-HNT) and then, protamine sulfate (PSP) functionalized h-HNT (h-HNT-PSP) and finally polyimidazolium-based polymer (IP-2) coated h-HNT (h-HNT-IP-2) were used as carriers of copper(II)-citrate (Cu(Cit)) complexes in aqueous dispersions. Stable colloids were obtained at low Cu(Cit) concentrations in all systems, while particle aggregation occurred for h-HNT-PSP and h-HNT-IP-2 at elevated complex doses due to adsorption of Cu(Cit) and decrease in the magnitude of surface charges. Enzymatic assays showed that Cu(Cit) possesses significant SOD-like function in homogeneous solution. Upon immobilization on h-HNT-PSP and h-HNT-IP-2, higher superoxide radical anion scavenging activities were obtained than for the non-immobilized Cu(Cit) complex due to structural changes and to the possible interaction between the radicals and the oppositely charged carriers. The developed nanotubular systems are promising candidates as antioxidant agents in various processes, where natural or immobilized enzymes are not adequate choices giving rise to their high sensitivity to the environmental conditions and to the subsequent loss of enzymatic activity.