Neuronal tau protein is one of the biomarkers of neurodegeneration which remains without a cure and differential diagnosis. Hence, novel methods are needed for detection of tau protein. Toward this aim, the detection of full-length tau441 protein was achieved using the electrochemical immunoassay based on direct immobilization of anti-tau antibodies on Au surface by using an active N-hydroxysuccinimide ester coupling chemistry (Ab-Au). The electrochemical impedance spectroscopy was used to monitor tau441 binding to Ab-Au in the presence [Fe(CN)(6)](3-/4-) redox probe. The reduction in the charge-transfer resistance, R-ct, was observed upon tau441 binding to Ab-Au. Detection of tau441 was observed in mu M to nM range with high specificity and selectivity. The electrochemical immunosensor exhibited minimal non-specific binding to bovine serum albumin. Tau441 did not bind to Au surface functionalized with non-tau-specific antibody MOPC-21. The electrochemical biosensor was optimized by screening various anti-tau antibodies. The tau441 binding to anti-tau antibodies was also measured using Enzyme-Linked Immunosorbent Assay (ELISA). The label-free electrochemical impedance spectroscopy immunosensor may be used for selective detection of tau protein in biological fluids, as a biomarker of neurodegeneration. (C) The Author(s) 2018. Published by ECS.