Meanwhile, compared to the relatively low Φ (0.65%) of PPC5, Φ of PPC10, PPC15 and PPC20 were improved to 3.8% ~ 4.6% (Table ). The FL quantum yield is further improved to 8% for PPC79 NPs and 24% for PPC194 NPs. Such enhancement of Φ with an increase of PCL chain length could be attributed to the suppression of homo resonance energy transfer (HRET) due to the increased distance among J-aggregates. This hypothesis was further confirmed by the dynamic fluorescence results (Figure ), where FL lifetime of PPC5 (0.50 ns) is significantly shorter than those of PPC10~25 (1.14~1.94 ns, λem = 800 nm). Moreover, the FL lifetime of PPC79 (0.77 ns, λem = 760 nm) is much shorter than that of PPC194 (2.05 ns, λem = 760 nm), while the excitation wavelength was maintained as same as 635 nm. (Table ) The lifetimes of PPC5 and PPD2 NPs (where PCL is replaced with polydecalactone for PPD2) were measured to be 0.50 and 0.59 ns, respectively, which were lower than those of other PPCx NPs (Figure ). PPC25 also showed a dominant J-resonance emission despite only a small proportion of J-aggregates in the NPs, which could be attributed to the intra-particle Fӧrster resonance energy transfer (FRET) from monomeric state to J-aggregates. At x = 79 and 194, the J-band completely disappeared and the M-band turned dominant in the absorption spectra. The fluorescence images acquired by In Vivo Imaging System (IVIS) clearly show the strongest fluorescence of PPC5-25 NPs at 820 nm, compared to the brightest emission of PPC79-194 NPs at 760 nm (Figure ). A systematic summary of the optical properties of all the PPCx NPs is presented in Table .