The invention described herein relates to sterically stabilized colloidal constructs comprising preformed colloidal particles encapsulated within a polymeric shell. The constructs, which are controllably sized, are nanoparticles comprising hydrophobic elements, electrostatically charged particles with hydrophobic surfaces, hydrophobic inorganic nanostructures, and amphiphilic copolymers with hydrophobic domains and hydrophilic domains. The constructs are made by a process that allows for the simultaneous encapsulation of a preformed colloidal agent as well as a dissolved hydrophobic active within the core of the polymeric nanoparticle. Among the actives incorporated in various embodiments are organic fluorescent dyes, metal nanostructures and superparamagnetic materials for use in combined fluorescence, optical and magnetic resonance imaging applications, and hydrophobic drugs for therapeutic applications.