We further characterized the temperature-dependence of the PA spectra of PPC10, PPC15, and PPD2 NPs as temperature-responsive PA probes for potential bioimaging applications (Figure ). The PA spectrum of PPC15 NPs shows two peaks at 710 and 770 nm, corresponding to monomeric and J-aggregate states, respectively (Figure ). Upon heating at 45 o C for 10 mins, the intensity of the PA signal at 770 nm decreased by 3.5-fold due to the disappearance of J-band. Similar results were obtained with PPC10 NPs. In contrast, PPD2 NPs exhibit no change in PA signal upon heating under the Given the fact that the photoacoustic contrast of chromophores is proportional to their photothermal conversion efficiency, PPD2 NPs, due to their relatively high photothermal conversion efficiency as described above, were chosen for in vivo photoacoustic imaging of tumor models under pulsed laser excitation at 770 nm. The PA intensity at 770 nm at tumor site was monitored at different time points after administration of PPD2 NPs (Figure ). A weak PA signal in the tumor region was observed at 0 h (i.e., immediately after the tail-vein injection of the NPs) and enhanced obviously at 12 h, indicating that NPs gradually accumulated in the tumor (Figure ).