Enhancing the translational potential of photo-responsive covellite in medicine: solvent-free tuning of CuS nanoparticles LSPR and control of their stability in aqueous media by a carbonaceous shell

Covellite (CuS) nanoparticles (NPs) are versatile, non-noble metal plasmonic materials which have been recognized as promising candidates for therapeutical applications. However, the poor chemical stability of CuS NPs in aqueous media makes their use in medical applications challenging. While the application of coatings to reduce NPs degradation has been successfully proposed for other materials, in the case of CuS this strategy is limited by the loss of the plasmonic performance due to NPs chemical instability in the post-synthesis treatments. In the present study, we successfully obtained core-shell CuS-C nanocomposites with tailored size, enhanced stability, and superior photothermal performance, by applying a novel green synthetic strategy using water as solvent. CuS NPs were synthetized using polyvinylpyrrolidone (PVP) as stabilizer; in addition, a PVP concentration-dependent tunability of size and plasmon wavelength was demonstrated. The carbon shell was generated via hydrothermal carbonization without compromising the plasmonic performance of CuS NPs (52 % of photothermal conversion efficiency) and without inducing aggregation through a strict control of the dissolution/degradation process. The carbon coating slowed down the degradation of CuS NPs in biological fluids without inhibiting the redox activity evaluated as a generation of hydroxyl radicals by electron paramagnetic resonance spectroscopy. When tested in vitro for their cytotoxicity toward mesenchymal stem cells CuS@C appear non cytotoxic after 24 h of incubation up to 100 μg/mL. Moreover, the carbon shell inhibited the pro- proliferative effect observed for CuS NPs at low concentration after 48 h. Overall, the results prove the great potential of CuS-C nanocomposites as photothermal agents for cancer treatments.