To confirm the important role of the covalent bonding between PCL and PPcy in the precipitation-induced J-aggregation, we conducted a control experiment by encapsulating physically blended P0 and PCL37 with PCL37-b-POEGMA60 into NPs in water. The feed ratio between P0 and PCL37 is approximately 1:3.3 (by mass), consistent with the theoretical molecular weight ratio between P0 and PCL block in PPC20. A typical absorption feature of the dimeric and monomeric state of PPcy and a blueish green color of dispersion were observed. In contrast to PPC5 NPs, the NPs composed of P0 physically blended with blank PCL37 displayed the typical H-aggregate and monomeric bands (denoted as H-and M-band, respectively) with λmax,abs at 688 and 748 nm, respectively (Figure ), and weak fluorescence (Figure ). These results suggest that the covalent bonding between PPcy and PCL is crucial in the formation of J-aggregates in the colloidal NPs.