Normalization procedure for fluorescence images of RT nanosheets. (A) Bright-field (left) and fluorescence (right) images of RT nanosheets without cells. Fluorescence images of RT nanosheets without cells were averaged. The averaged image (image A, right) shows uniformity of the excitation light. (B) Top left: Bright-field image of HeLa cells on a RT nanosheet. Top right: Fluorescence image of the RT nanosheet in top left (image B). F.I. of image B in five regions of interest (ROIs) without cells was measured. F.I. of image A in the same five ROIs was also measured. (C) Image B (left) was divided by image A’ (middle) which had been obtained by adjusting the intensity of image A; hence each F.I. of five ROIs in image A’ was equal to that in image B. Therefore, the “normalized image” (right) shows the darkness of the RT nanosheet on which cells were present. The normalized images and their intensities with cells are shown in Fig. 2 and Fig. S8. a.u., arbitrary units.