We have developed pH-responsive, multifunctional nanopartides based on encapsulation of an antioxidant, tannic acid (TA), using flash nanoprecipitation, a polymer directed self-assembly method. Formation of insoluble coordination complexes of tannic acid and iron during mixing drives nanoparticle assembly. Tuning the core material to polymer ratio, the size of the nanoparticles can be readily tuned-between SO and 265 nm. The resulting nanopartide is pH-responsive i.e., stable at pH 7.4 and soluble under acidic conditions clue to the nature of the coordination complex. Further, the,coordination complex can be coprecipitated with other hydrophobic materials such as therapeutics or imaging agents. For example, coprecipitation With a hydrophobic fluorescent dye creates fluorescent nanopartides. In vitro, the nanopartides have low cytotoxicity and show antioxidant activity. Therefore, these particles may facilitate intracellular delivery of antioxidants.