Interactions between polyacrylic acid (PAA) arms of star block copolymers through metal ions were investigated by fluorescence resonance energy transfer (FRET). Dendritic polyester (DPE) core was grafted with block copolymer arms of oligo (ethylene glycol) methyl ether acrylate (OEGA) and PAA. Fluorescein o-methacrylate (FMA) or methacryloxyethyl thiocarbamoyl rhodamine B (MTR) was introduced via copolymerization to the terminated PAA blocks. The unique copolymer structure enabled FRET between PAA chains without causing precipitation, which inevitably occurs for linear PAA. Addition of metal ions such as Zn2+, Ca2+, or Na+ induced FRET, the order of which was Zn2+ > Ca2+ >> Na+. FRET intensity was also correlated with concentration of each ion. Furthermore, in situ FRET measurement during the hydrogel formation of the star block copolymers was performed by mixing the star block copolymers and Zn2+. (c) 2018 Elsevier Ltd. All rights reserved.