In this work, we prepared hollow Fe3O4 nanoparticles (Fe(3)O(4)HNPs) as nanoelectrocatalyst to obtain a multi-functionalized redox signal tag for high-performance thionine (Thi) catalysis toward ultrasensitive protein detection. Briefly, Au-Thi-Au with good conductivity and large surface area was prepared to enhance the immobilization amount of redox molecular (Thi). Simultaneously, to improve catalytic efficiency, Au-Thi-Au (redox molecule) was further anchored on the surface of Fe(3)O(4)HNPs (nanoelectrocatalyst). Obviously, the as-prepared Au-Thi-Au@ Fe(3)O(4)HNPs as signal tag combines the merits of electrochemical activity, catalytic activity, magnetic response as well as nanocarrier, which is employed in electrochemical immunoassay. Importantly, owing to the hollow structure and high activity site of Fe(3)O(4)HNPs, the signal tag of Au-Thi-Au@ Fe(3)O(4)HNPs exhibits high-performance Thi catalysis even in the absence of substrate, thus, giving a highly sensitive method for electrochemical detection of alpha fetoprotein (AFP) down to the 0.033 pg mL(-1). Compared with traditional enzyme-based signal amplification, the proposed immunosensor avoids the addition of any substrate in testing solution, and shows good stability, sensitivity and accuracy. (C) 2017 The Electrochemical Society. All rights reserved.