A highly sensitive immunosensor based on immobilization of specific monoclonal antibodies onto gold nanoparticles (GNPs) attached to a modified gold electrode was developed for the determination of the presence of doxorubicin. Colloidal GNPs were coated on a gold electrode using the thiol groups of 1,6-hexanedithiol (HDT) as a cross-linker. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed for characterization of the various layers that were coated onto the electrode. The relative charge transfer resistance of the antibody-modified gold electrode changed linearly with the concentration of doxorubicin. The limit of detection (3 s(b)/m) was 0.83 pg mL(-1), and a linear range was obtained from 1.0 to 160.0 pg mL(-1) of doxorubicin. The affinity of doxorubicin for the immobilized antibody was determined to be 1.74 x 10(11) M-1. The sensitivity and specificity of the immunosensor was validated using doxorubicin-spiked human serum samples, and the results show that the immunosensor is a useful tool for screening picogram amounts of doxorubicin in the clinical laboratory. (C) 2011 Elsevier Ltd. All rights reserved.