Versatile bioassay labels based on marker-loaded nanoparticles have been developed for sensitive biomarker detection. Redox markers have been loaded/attached to apoferritin and Si nanoparticles, respectively. The electroactive nanoparticles were used as labels for sensitive electrochemical immunoassay of protein biomarker. This method is ultrasensitive and its detection limit for TNF-a is 77 fM. The new nanoparticle labels could be applicable to other electrochemical bioassays, such as DNA, and is thus expected to lead to wide applications for protein diagnostics and for bioanalysis in general.