To learn the impact of aqueous environmental species on the property of the isolated core of a water-soluble unimolecular micelle (UIM), a guest dye of erythrosine B (EB) is used as a probe to map the dynamic microenvironment of the UIM. PEGylation of branched polyethylenimine (PEI) with oxirane-functionalized polyethylene glycol (PEG) leads to a UIM of PEI@PEG, and the core is further chemically engineered. The resulting UIMs can irreversibly encapsulate EB exclusively by the core. It is found that the stacking of EB molecules is dependent on the electronic microenvironment of the UIMs, where a polar and ionic core favors a twist stacking of EB, but a less polar core results in an unprecedented parallel stacking of EB. Spectral analysis shows that EB is encapsulated along with its counter ion of Na+, and an exterior ion can cause dehydration of the UIMs but can hardly enter the UIMs; moreover, ion exchange through the PEG shell is actually allowed. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 566-573