Extraordinary Radical Scavenging of Transition Metal Dichalcogenides through Defect-Mediated Hydrogen Atom Transfer

Abstract

Transition metal dichalcogenides (TMDs), layered materials with strong in-plane bonding and weak out-of-plane interactions, offer a great number of opportunities in chemical and biological research. They show intriguing chemical, electrical, and optical properties when bulk TMDs with indirect band gaps reach monolayers with direct band gap. TMCs such as MoS2, WS2, MoSe2, and WSe2 have sizable bandgaps allowing their practical utilization in the field of antioxidation. This study introduces a new type of antioxidation material based on TMD nanosheets. TMD nanosheets retain their structure in aqueous phase due to the interaction with amphiphilic copolymer or immobilization in hydrogel matrix. By taking advantage of the defect-mediated hydrogen atom transfer, TMD nanosheets exhibit extraordinary radical scavenging activity. Further, we observe the stable and continuous radical scavenging activity in harsh conditions and coexistence with cells. First, we propose a transition metal dichalcogenide (TMD)-based antioxidation platform, in which radical scavenging is accomplished by the defect-mediated one-step hydrogen atom transfer (HAT) occurring on the nanosheets in water. To this end, the TMD nanosheets, including MoS2, WS2, MoSe2, and WSe2, are finely dispersed in water with the aid of an amphiphilic poly(ɛ-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) diblock copolymer, that envelops the nanosheets with a molecular layer of less than 1 nm thickness. We have then demonstrated that the PCL-b-PEO-stabilized TMD nanosheets show the extraordinarily enhanced and prolonged radical scavenging activity in water even under harsh storage conditions. Theoretical modeling studies on HAT have suggested that more favorable hydrogen association from chalcogen vacancies on the nanosheets dispersed in water can lead to the easier dissociation of hydrogen atoms with exothermicity by -0.43 ~ -1.33 eV, thus exhibiting such an outstanding radical scavenging performance. Then, we report on a WS2 nanosheet-immobilized hydrogel system that can inhibit oxidative stress on living cells. First, we fabricated a highly stable suspension of WS2 nanosheets as a radical scavenger by enveloping them with the amphiphilic poly(-caprolactone)-b-poly(ethylene oxide) copolymer (PCL-b-PEO) during in situ liquid exfoliation in an aqueous medium. After the PCL-b-PEO-enveloped WS2 nanosheets were embedded in the hydrogel matrix, they retained their characteristic optical properties. Intriguingly, the WS2 nanosheet-immobilized hydrogel particles exhibited sustainable radical scavenging performance without any deterioration in the original activity of the WS2 nanosheets, even after repeated use. This implies that hydrogen atoms dissociated from the chalcogen of the WS2 nanosheets effectively scavenged free radicals through the hydrogel mesh. Because of this unique behavior, the coexistence of the WS2 nanosheets with living cells in the hydrogel matrix improved cell viability up to 40%, which demonstrates that the WS2 nanosheets can suppress oxidative stress on living cells.