A new bottom-up strategy is used to construct water-soluble organic/inorganic fluorescent unimolecular nanoparticles based on polyhedral oligomeric silsesquioxane (POSS) and conjugated oligoelectrolyte. Their high quantum yield, good cytocompatibility; and unique whole-cell permeability could serve as a light-harvesting energy donor to amplify the intracellular dye fluorescence for high-quality biological imaging through fluorescence resonance energy transfer (see image).