Chemical specificity in REDOX-responsive materials: the diverse effects of different Reactive Oxygen Species (ROS) on polysulfide nanoparticles

Abstract

REDOX responsive (nano)materials typically exhibit chemical changes in response to the presence and concentration of oxidants/reductants. Due to the complexity of biological environments, it is critical to ascertain whether the chemical response may depend on the chemical details of the stimulus, in addition to its REDOX potential, and whether chemically different responses can determine a different overall performance of the material. Here, we have used oxidation-sensitive materials, although these considerations can be extended also to reducible ones. In particular, we have used poly(propylene sulfide) (PPS) nanoparticles coated with a PEGylated emulsifier (Pluronic F127); inter alia, we here present also an improved preparative method. The nanoparticles were exposed to two Reactive Oxygen Species (ROS) typically encountered in inflammatory reactions, hydrogen peroxide (H₂O₂) and hypochlorite (ClO⁻); their response was evaluated with a variety of techniques, including diffusion NMR spectroscopy that allowed to separately characterize the chemically different colloidal species produced. The two oxidants triggered a different chemical response: H₂O₂ converted sulfides to sulfoxides, while ClO⁻ partially oxidized them further to sulfones. The different chemistry correlated to a different material response: H₂O₂ increased the polarity of the nanoparticles, causing them to swell in water and to release the surface PEGylated emulsifier; the uncoated oxidized particles still exhibited very low toxicity. On the contrary, ClO⁻ rapidly converted the nanoparticles into water-soluble, depolymerized fragments with a significantly higher toxicity. The take-home message is that it is more correct to discuss ‘smart’ materials in terms of an environmentally specific response to (REDOX) stimuli. Far from being a problem, this could open the way to more sophisticated and precisely targeted applications.