The present invention relates to a biosensor comprising a substrate with a coating system in which a Ruthenium complex and an enzyme is integrated. The enzyme is able to convert bioproducts, e.g. glucose, fructose or glycerol. The depletion of oxygen during these converting reactions can be monitored via the fluorescence of the Ruthenium complex. The inventive biosensor can be used in biotechnological processes, e.g. the synthesis of biofuels.