Circulating tumor cells (CTCs), as potential cancer biomarkers, play a crucial role in cancer diagnosis, prognosis and treatment. However, there are still significant challenges due to their rarity. In this paper, a label-free and ultrasensitive electrochemical impedance cytosensor was developed by a galactosylated gold-nanoisland (Gal-GNI) biointerface for the capture and detection of rare cancer cells (HepG-2) in whole blood samples. By mimic the natural multivalent biorecognition systems, the biointerface was designed to capture the target cancer cells by the specific binding between galactosyl ligands of Gal-GNI and over-expressed asialoglycoprotein receptors (ASGP-R) on the surface of HepG-2. From the results of electrochemical impedance spectroscopy (EIS) and fluorescence spectroscopy, the nanostructured Gal-GNI provides an improved electron-transfer rate, specific detection for HepG-2, and good biocompatibility for viable captured cells. The capture of HepG-2 to the EIS cytosensor increased the electron-transfer resistance, with a good correlation with the logarithm of the concentration from 1.0 x 10(2) to 1.0 x 10(5) cells mL(-1), with a low detection limit of 30 cells mL(-1). This cytosensing strategy has good reproducibility with the RSD of 1.9% in blood samples, indicating that it can be used as a potential noninvasive assay tool for the early diagnosis and therapy of tumor. (C) 2016 Elsevier B.V. All rights reserved.