The invention relates to a biosensor comprising an electrically conductive substrate, with a first layer comprising Ruthenium Purple formed on the substrate, a second layer comprising polyaniline or a derivative thereof comprising one or more non-polar substituents formed on the first layer, and a third layer comprising one or more enzymes trapped within a matrix formed on the second layer. The biosensor is for use in the detection of analytes such as purines and derivatives thereof, particularly in the detection of hypoxanthine.