Controlled Phase Transition of the Onion-like Carbon Nanostructure for Photothermal Cancer Therapy

Carbon-based nanomaterials are viewed as preferred candidates as photothermal agents in tumor photothermal therapy due to their tailored structure and excellent biocompatibility. Herein, onion-like carbon (OLC) with controlled sp2/sp3-hybridized carbon ratios was explored as an effective photothermal reagent. The OLC was synthesized through thermal annealing of nanodiamonds, with an increase of the sp2 carbon content as the annealing temperature increased. The obtained OLC demonstrated high photothermal efficiency under 808 nm laser irradiation with a maximum value of 74.36% at an annealing temperature of 1400 °C. We suggest that a well-regulated amount of sp2 carbon in the OLC provided an abundance of conjugated π bonds, leading to enhanced light absorption and electron–phonon coupling. In addition, the OLC was conjugated with polyethylene glycol (PEG) and fluorescent molecules (FITC) to visualize the cellular uptake in HCT 116. In vivo tests demonstrated that the OLC-PEG-FITC exhibited good biocompatibility and achieved photothermal killing of cancer cells upon near-infrared laser irradiation. After treatment with the OLC-based photothermal agent, tumor tissues in tumor-bearing mice were effectively ablated. Taken together, these findings confirm the potential of the developed OLC as a photothermal agent for tumor therapy.