Superparamagnetic iron oxide nanoparticles (SPIONs) coated with azide groups were functionalized at the surface with biotin (biotin@SPIONs) and cysteine protease inhibitor E-64 (E-64@SPIONs) with the purpose of developing nanoparticle-based assays for identifying cysteine proteases in proteomes. Magnetite particles (ca. 6 nm) were synthesized by microwave-assisted thermal decomposition of iron acetylacetonate and subsequently functionalized following a click chemistry protocol to obtain biotin and E-64 labeled particulate systems. Successful surface modification and covalent attachment of functional groups and molecules were confirmed by FT-IR spectroscopy and thermal gravimetric analysis. The ability of the surface-grafted biotin terminal groups to specifically interact with streptavidin (either horseradish peroxidase [(HRP)-luminol-H2O2] or rhodamine) was confirmed by chemiluminescent assay. A quantitative assessment showed a capture limit of 0.55-1.65 μg protein/100 μg particles. Furthermore, E-64@SPIONs were successfully used to specifically label papain-like cysteine proteases from crude plant extracts. Owing to the simplicity and versatility of the technique, together with the superparamagnetic behavior of FeOx-nanoparticles, the results demonstrate that click chemistry on surface anchored azide group is a viable approach toward bioconjugations that can be extended to other nanoparticles surfaces with different functional groups to target specific therapeutic and diagnostic applications.