Nanoparticle-based biodetection commonly employs monoclonal antibodies (mAbs) for targeting. Although several types of conjugates have been used for biomarker labeling, the large size of mAbs limits the number of ligands per nanoparticle, impedes their intratumoral distribution, and limits intracellular penetration. Furthermore, the conditions of mAb conjugation using conventional techniques provide nanoprobes with irregular orientation of mAbs on the nanoparticle surface and often provoke mAb unfolding. Here, we have developed a protocol to engineer ultrasmall diagnostic nanoprobes through directional conjugation of semiconductor quantum dots (QDs) with 13-kDa single-domain antibodies (sdAbs) derived from llama immunoglobulin G (IgG). sdAbs are conjugated with QDs in a highly oriented manner via an additional cysteine residue specifically integrated into the sdAb C-terminus. The resultant nanoprobes are <12 nm in diameter, ten times smaller in volume compared to the known alternatives. They have been proved highly efficient in flow cytometry and immunuhistochemical diagnosis. This approach is easy to extend to other semiconductor and plasmonic nanoparticles. In general, sdAb-QD bioconjugation, quality control and characterization take 3 days.