A sensor system for measuring an electrical signal across a lipid bilayer for providing single molecule detection, the sensor system comprising a cell and an electrical reader unit which are connectable together, the electrical reader unit being sufficiently small to be hand-held and portable, wherein the cell defines two chambers, being a test chamber that is closed by a closure sheet and a secondary chamber that is closed, the test chamber having an inlet opening (30,32) for introduction of an aqueous test solution, the chambers being separated by a septum, the septum comprising: a membrane having an aperture capable of supporting a lipid bilayer and arranged between the chambers, the membrane having a pretreatment effective to increase the affinity of the membrane to a lipid, the aperture in the membrane having a diameter in at least one dimension which is 20μm or less; and on at least one side of the septum, a support sheet fixed to the membrane, the support sheet having a window which is aligned with the aperture in the membrane and of greater size than the aperture in the membrane, the cell has electrodes formed in each chamber for receiving an electrical signal developed between the chambers, the electrical reader unit has a reader circuit operative to measure an electrical signal developed between the chambers of the cell, and the cell and the reader unit are arranged to be connected together to provide electrical connection between the electrodes of the cell and the reader circuit of the electrical reader unit, wherein the test chamber has a depth between the septum and the closure sheet in the direction perpendicular to the septum selected such that when an aqueous test solution is introduced into the test chamber, an interface between the aqueous test solution and air in the test chamber across the depth of the test chamber is held by surface tension irrespective of the orientation of the cell. a membrane having an aperture capable of supporting a lipid bilayer and arranged between the chambers, the membrane having a pretreatment effective to increase the affinity of the membrane to a lipid, the aperture in the membrane having a diameter in at least one dimension which is 20μm or less; and on at least one side of the septum, a support sheet fixed to the membrane, the support sheet having a window which is aligned with the aperture in the membrane and of greater size than the aperture in the membrane, the cell has electrodes formed in each chamber for receiving an electrical signal developed between the chambers, the electrical reader unit has a reader circuit operative to measure an electrical signal developed between the chambers of the cell, and the cell and the reader unit are arranged to be connected together to provide electrical connection between the electrodes of the cell and the reader circuit of the electrical reader unit, wherein the test chamber has a depth between the septum and the closure sheet in the direction perpendicular to the septum selected such that when an aqueous test solution is introduced into the test chamber, an interface between the aqueous test solution and air in the test chamber across the depth of the test chamber is held by surface tension irrespective of the orientation of the cell.