This paper reports an amplification procedure application of multi-functionalized gold nanoparticles (MFAuNPs) which was used for electrochemical impedance immunosensor. The modified electrode which was utilized for the affinity adsorption of protein A, was fabricated by stepwise immobilization of 1, 6-hexanedithiol, AuNPs, and IgG on the gold electrode via self-assembling technique. The interfacial properties of the modified electrodes were evaluated in the presence of Fe(CN)(6)(4-/3-) couple redox as a probe by cyclic voltammetry and electrochemical impedance spectroscopy. An equivalent circuit model with a constant phase element was used to interpret the obtained impedance spectra. The electrochemical behavior of the redox probe was affected by the accumulation of treated substances on the electrode surface. There is no significant increase in the electron transfer resistance after the binding of protein A to IgG on the electrode surface without amplification. The changes in the electron transfer resistance on the IgG-modified electrodes became more sensitive after the MFAuNPs were introduced for signal amplification. The increments of amplified impedance showed good correlation of the detection of protein A in the range of 5-1000 pg/mL with a detection limit of 1 pg/mL. The development of a rapid, facile, and sensitive amplification method capable to detect biomolecules in the picogram range, utilizing EIS measurement, may be achieved in this study. (C) 2008 Elsevier B.V. All rights reserved.