The adsorption of N-terminal domain of ciliate Euplotes octocarinatus centrin (N-EoCen) onto a glassy carbon (GC) electrode was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The adsorption process obeys Langmuir isotherm adsorption equation. Based on the adsorption, direct immobilization of N-EoCen onto the glassy carbon surface was used for construction of an N-EoCen-modified GC electrode. Then the electrode was used to probe the binding mode of europium ions with N-EoCen. The results show that with the increasing concentration of europium ions, the redox peaks of probe increase gradually. Simultaneously, the peak separation decrease and peak currents of the redox reaction increase. In addition, two binding sites in N-EoCen show no-equiv signal of CV change: the signal is slightly changed by the binding of the first europium ions and largely did by the binding of the second europium ions. It can be attributed to the change of conformation or aggregation of protein after binding of europium ions. It offers a viable model for illustrating the interaction of lanthanides with EoCen. Through the titration curve the process of combination can be explored. (C) 2013 Elsevier B.V. All rights reserved.