An electrochemical biosensor for quantifying one or more redox species in a liquid sample comprising a background electrochemical activity, wherein said biosensor comprises: (i) an electrode system; (ii) a filter means comprising a sample loading position; and (iii) a charge transfer surface between said filter means and said electrode system; wherein said filter means separates said sample loading position from said charge transfer surface by an amount of filter material capable of providing a tangential peak height for said background electrochemical activity of less than 30% of the tangential peak height for said redox species; and said biosensor comprises an intermediate layer between said filter means and said charge transfer surface, wherein said intermediate layer has a lower energy requirement to transfer said liquid sample to said charge transfer surface than said filter material.