Degradation of C ₆₀ by Hypochlorite: Possible Atomistic Scenarios Leading to the Opening of the Carbon Cage

We analyze the degradation of the C₆₀ fullerene under the action of hypochlorite (ClO⁻). The ClO⁻ anion, produced by the human myeloperoxidase (hMPO) enzyme, is highly reactive and known for destroying bacteria and degrading nanostructured materials. In particular, previous studies show that hMPO can biodegrade nC₆₀ nanoparticles in a short time, with hypochlorite playing a key role, though the exact mechanism is still unknown. In this work, we use density functional theory (DFT) calculations to investigate ClO⁻ adsorption on water-covered fullerenes. We find that there is a strong tendency of hypochlorite to dissociate rather than remain molecularly adsorbed near the hydrated C₆₀ surface. As a consequence of this reaction, the fullerene cage can be oxidized through the adsorption of carbonyl, epoxy, molecular O₂, and ClO groups preferentially located in close proximity on the carbon network, while individual chloride ions remain hydrated and stabilized in the aqueous environment. The formation of domains of chemisorbed oxygen species, as reported here, reduces the number of C═C double bonds in the cage, thereby decreasing the structural stability of C₆₀. Chlorination of the carbon surface is not energetically favored following ClO⁻ bond cleavage. Most interestingly, our calculations reveal that the oxidation of the fullerene surface is frequently accompanied by the breaking of C-C bonds beneath the oxidized regions, resulting in hole formation in the carbon cage. We performed simulations of NMR, UV-vis, and ECD spectroscopies, which reveal well-defined spectral features that could be very helpful in identifying the structural transformations and chemical composition reported here for these nanosized carbon materials. According to our proposed atomistic mechanism, the dissociative adsorption of hypochlorite at various regions on the carbon network, along with the formation of molecular islands composed of oxidizing species, may lead to a generalized porous morphology of the cage consistent with experimental observations of significant structural transformations in hMPO exposed C₆₀ solutions.