The signal transduction of most target induced strand displacement-based assays relies on the conformational changes of aptamers, significantly limiting the general applications of these sensors. We report on a very simple and general sensor named signaling-probe displacement electrochemical aptamer-based sensor (SD-EAB), in which signal transduction is induced only by the affinity binding between an aptamer and its target and completely independent of the conformational state of the aptamer. A typical SD-EAB is comprised of a gold electrode immobilized with DNA duplexes formed between a thiolated capture probe (aptamer or its short complementary strand) and a redox tagged signaling probe (short complementary strand or aptamer). In the presence of target, the signaling probe is displaced and released from the electrode surface, leading to the decrease of current proportional to the logarithm of target concentrations. SD-EAB achieved the reagentless detection of kanamycin A with 7 orders of magnitude dynamic ranges (1 nM-10 mM). Amazingly, SD-EAB clearly differentiated kanamycin A from its structural analogues kanamycin B by showing opposite current change. In contrast, its counterpart, the label-free electrochemical impedance spectroscopy (EIS)-based sensor, was one thousand times less sensitive than SD-EAB and had a narrow dynamic range (1-100 mu M) due to its limited tolerance of nonspecific adsorption of kanamycin A. (C) 2015 Elsevier Ltd. All rights reserved.