Disclosed herein are nanoparticle immunoconjugates useful for therapeutics and/or diagnostics. The immunoconjugates have diameter (e.g., average diameter) no greater than 20 nanometers (e.g., as measured by dynamic light scattering (DLS) in aqueous solution, e.g., saline solution). In certain embodiments, the conjugates are silica-based nanoparticles with single chain antibody fragments attached thereto.