Compounds of general formula I are used as labels in an electrochemical assay: (I) in which: Fc and Fc′ are substituted or unsubstituted ferrocenyl moieties, X is a C1 to C6 alkylene chain which is optionally interrupted by —O— or —NH—; Y is a C1 to C6 alkylene chain which is optionally interrupted by —O— or —NH—; Z is a C1 to C12 alkylene chain which may optionally be substituted and/or may optionally be interrupted by —O—, —S—, cycloalkyl, —CO—, —CON R1—, —NR1CO— or —NR1— in which R1 represents hydrogen or C1 to C4 alkyl; and R is a linker group. Compounds I are used to make labelled substrates, as well as functionalised compounds for making the labelled substrates.