A fluorocarbonyl terminated ferrocene based redox probe was synthesized and characterized using infrared spectroscopy and nuclear magnetic resonance. It was then successfully grafted on various amorphous carbon nitride (a-CNx) samples (0.12 <= x <= 0.27) via the formation of a -CO-NH- type linkage with surface amine groups, whose spontaneous formation on the surface of a-CNx materials was thus proved and exploited. Cyclic voltammetry experiments were carried out in an acidic aqueous solution in the presence of this freely diffusing redox probe, in order to identify its electrochemical behavior. By comparison, CV experiments carried out on the modified a-CNx electrodes in the same blank electrolytic solution undoubtedly confirmed the presence of electroactive sub-monolayers, in spite of distortions of the resulting voltammograms. These latter were shown to reflect the influence of bulk atomic nitrogen content on the electrochemical reactivity of a-CNx materials. The grafted surface amine percentage deduced from the charge passed during oxidation of grafted ferrocene moieties was found to decrease from 6.5% for a-CN0.12 down to 1.1% for a-CN0.27. This trend is expected to reflect the trend in surface concentration of reactive amine groups as a function of bulk atomic nitrogen content. (C) 2014 Elsevier Ltd. All rights reserved.