Cuprous oxide and nano-chitosan composites (Cu2O@NCs) were synthesized by one-step reduction process and used as a highly sensitive electrochemical DNA sensor for detecting Hg2+ ions in aqueous solutions. The entire detection procedure of Hg2+ ions based on the developed electrochemical biosensor was investigated by electrochemical impedance, which shows that a single-stranded, thymine (T)-rich DNA was immobilized onto the surface of the composite electrode modified by Cu2O@NCs. The Cu2O@NCs modified with DNA exhibited significant sensitivity in detecting Hg2+ ions compared with pristine cuprous oxide and nano-chitosan. This behavior led to a high difference in charge-transfer resistance during Hg2+ ions detection. In Addition, Cu2O@NCs exhibited high sensitivity and stability for detecting Hg2+ ions with a detection limit of 0.15 nmol L (1) within the range of 1 to 100 nmol L (1) of Hg2+ ions. The fabricated Cu2O@NCs may be used as a novel biosensor for detecting heavy metal ions in water or the environment. (C) 2015 Elsevier Ltd. All rights reserved.