Figure 2. Temperature increase of pSGNs induced by surface plasmon resonance (SPR) caused by 808-nm NIR irradiation.
A. Temperature elevation curve of 1 mL of pSGNs-containing solution with culture medium (ratio 3:1, OD800 = 6) irradiated using a 2000-mW 808-nm NIR laser. The temperature change was monitored every 30 s using a thermocouple. The culture medium with pSGNs absorbed the energy of the NIR laser and caused the temperature to increase to nearly 30°C in 5 min, but the medium-only control did not. B. 1 × 105 mouse ovarian cancer ID8 cells were seeded into a 24-well plate and double-stained with Hochest 33342 and propidium iodide 18 h after NIR laser irradiation. Blue represents Hochest 33342 cell nuclei; red represents propidium iodide-stained dead cell nuclei. Cells that did not receive any treatment, only received NIR laser irradiation, and those that were only cocultured with pSGNs did not exhibit death. However, cell death was significant in the group treated with pSGNs and NIR laser irradiation. The size of the scale bar was 500 μm. C. Cell viability 18 h after NIR laser irradiation. The irradiation duration was set to when the medium temperature of the NIR + pSGNs cohort reached 42°C. The cell viability was determined using an MTT assay. Compared with all other groups, the cells that were cultured in pSGNs-containing medium and irradiated with the NIR laser exhibited significant cell death. D. Cell viability 18 h after the cultured cells were heat-treated in an oven for 10 min and with an NIR laser + pSGNs, achieving the same temperature. The viability for both groups was not significantly different at room temperature. However, the viability of cells treated with photothermal therapy was lower than that of cells heated using an oven at 35 and 42°C. These results demonstrated that heat generated with an NIR laser induced an SPR effect of pSGNs that can kill tumor cells in vitro. The culture medium heated in an oven to the same temperature did not exhibit significant cell death, implying that the local temperature surrounding the pSGNs stimulated by the NIR laser irradiation may be higher than that measured by the thermocouple and could effectively kill proximal tumor cells. The error bar in each chart represents the standard error. (P < 0.05#, <0.01*, <0.001**)