Dual mediator biosensor

A device for detecting the presence of an analyte in a sample, comprising

• (i) at least one electrode,
• (ii) an arsenite oxidase enzyme, and
• (iii) first and second redox mediators:
• wherein the first and second redox mediators are each independently selected from compounds which exist in two or more different redox states, and are selected from iron, ruthenium, cobalt or osmium complexes;
• wherein the first and second redox mediators are different;
• and
• wherein the arsenite oxidase enzyme is a molybdenum-containing arsenite oxidase Aio from NT-26, or is derived from Aio from NT-26;
• wherein the arsenite oxidase enzyme is modified to prevent translocation to the periplasm, wherein the enzyme comprises the native AioA subunit from NT-26, or a variant, homologue or derivative thereof having at least 60% sequence identity to SEQ ID NO:
• 4, and the native AioB subunit from NT-26 or a variant, homologue or derivative thereof having at least 60% sequence identity to SEQ ID NO:
• 3, wherein a portion of the native AioB subunit corresponding to the translocation signal sequence, or a functional fragment thereof, is modified.