An apparatus for measuring electrical potentials of a liquid sample includes at least one field effect transistor having a source, a drain, and a gate, a substrate, and at least two intersecting nanowires of semiconductive material arranged on the substrate, each having a source and drain contact as a field effect transistor and a voltage applicator configured to apply a voltage between the respective source and drain contact. The cross section of the two nanowires has a shape of a triangle or a trapezium. A voltage applicator configured to apply a voltage to the substrate are arranged on the substrate. The nanowires are electrically insulated at least against the sample by a dielectric layer along their surface having a layer thickness between 5 and 40 nm, and at least one impurity is arranged in the dielectric layer as a bearing point which is capable of capturing charge carriers.