A facile, versatile, and one-pot sonochemical synthesis of polydopamine (PDA)-nanocapsules from dopamine is reported. The nanocapsules (227 ± 25 nm) can encapsulate hydrophobic substances while retaining the reactivity of PDA toward nucleophilic reactions, enabling facile surface modification for different applications. PDA nanocapsules are nontoxic to mammalian cells while Cu-containing PDA capsules demonstrate strong (99.9%) and rapid (15 min) bactericidal activity.