Detection sensitivity and detection depth of LGO:Cr nanoparticles-based imaging. (A) Detection sensitivity of PEI-LGO:Cr-based cell labeling. Elevated numbers (~5, 50, 100, 500, and 1,000) of PEI-LGO:Cr labeled RAW264.7 cells were subcutaneously injected onto the back of a nude mouse. The cells were fixed and irradiated by a 254 nm UV lamp for 5 min before the injection. (B,C) Dorsal and ventral views, respectively, of a nude mouse subcutaneously implanted with 32 µg UV pre-irradiated PEI-LGO:Cr nanoparticles. (D,E) Dorsal and ventral views, respectively, of a nude mouse subcutaneously implanted with 32 µg QDs (emission at 705 nm). The QDs were excited by 570 nm light while imaging. (F) S/N ratios in (B) and (D). (G) Detection depth study using pork as a model tissue. 15 μg UV pre-irradiated LGO:Cr nanoparticles were injected into 2.5 cm depth inside the pork, and the image was taken at 10 min after the injection. The images were taken on an IVIS imaging system in the bioluminescence mode for LGO:Cr nanoparticles and in the fluorescence mode for QDs. The exposure time is 5 min in (A), 2 min in (B), (C) and (G), and 1 s in (D) and (E).