Photothermal properties. (A) Temperature change of pure water and water containing different concentrations of the 9T-PUNNC under NIR-II irradiation for 5 min. (B) Temperature change of the 9T-PUNNC in aqueous (200 µg mL-1) under NIR-II irradiation, the irradiation was turned off 390 sec after the initiation of irradiation (cooling period). (C) Linear time data versus -ln θ obtained from the cooling period shown in (B). (D) Cell viability of 4T1 cells treated by the 9T-PUNNC with or without NIR-II irradiation for 5 min. (E-H) Confocal images of 4T1 cells treated by the 9T-PUNNC at different concentrations (e, 0 µg mL-1; f, 25 µg mL-1; g, 50 µg mL-1; h, 100 µg mL-1) with a 5 min-exposure to NIR-II irradiation at pH = 7.4, and double-stained with calcein-AM/PI. Scale bar, 50 µm. The irradiation wavelength and power are 1064 nm and 0.8 W cm-2, respectively.