Ih320

For fluorescence imaging of HeLa cells incubated with GNR@SiO 2 @UCNPs, they were seeded in 6-well plates containing sterile cover slips and were kept in a sterile environment for 24 h. Then, cells were exposed to incubation with GNR@SiO 2 @UCNPs for 4 h at 37 °C. Subsequently, the cells were fixed with 3.5% formaldehyde in PBS for 20 min at 4 °C. Once they were washed three times with PBS, the cells were scanned using an in house designed confocal microscope (Fig. S1 †). Laser excitation was performed using a 980 nm diode laser coupled to a single mode fiber (LU0975M500 model from LUMICS GmbH). The 980 nm laser radiation was first collimated and then focused into the HeLa cancer cell using a 100× 0.85 NA microscope objective. The fluorescence generated by the GNR@SiO 2 @UCNPs incorporated into the cells was collected by the same microscope objective. This collected luminescence, after passing through different filters was spectrally analyzed by a monochromator (iH320, Horiba) and recorded with a high sensitivity Si CCD camera (Synapse, Horiba). The system was optimized for the analysis of the green emission of erbium (Er 3+ ) ions at approxi- mately 550 nm. The HeLa cells were mounted on a 0.1 µm resolution XY stage such that it was possible to scan the HeLa cells with respect to the 980 nm laser spot.