Temperature measurement for rat hippocampal neurons. (A) Fluorescence image of Fluo4 in neurons on an EuTTA nanosheet. Three somas (S1, S2, and S3) and two neurites (N1 and N2) were analyzed. Image averaged from 900 frames (for 23.4 s) during observation. (B) Time course of changes in the F.I. of Fluo4 (top) and EuTTA (bottom). See Video 8. (C) Top: Time course of changes in average Fluo4 F.I. in somas and neurites on EuTTA nanosheets. Bottom: Time course of changes in average EuTTA F.I. for somas and neurites. Number of somas and neurites, 53 and 48, respectively (n = 38). The same neurons were used in top and bottom. (D) Superimposed data of Fluo4 (top) and EuTTA (bottom). Data of 20 stimuli were superimposed. A calibration bar of 0.05°C was calculated from the temperature sensitivity of EuTTA nanosheets (i.e., −3.42%/°C; cf. Fig. 1 C). Arrows in B, C, and D indicate the points at which electrical stimuli were applied. a.u., arbitrary units.