Ferrocenophane derivatives 3 act as electrochemical sensors of Mg2+ ions: a new redox peak appears in the CVs anodically shifted compared to the E1/2 of the free receptors by 268-382 mV. Receptors 3a, 3c and 3d are selective for Mg2+, whilst 3b is also responsive to Ca2+ ion (302 mV shift). Receptor 3c also gave a highly visual output response in the presence of Mg2+ consisting of a deep purple colour.