Abstract: The present invention provides a method for studying transport of an agent across a membrane comprising the steps a) providing at least one surface with a bilayer structure tethered to the surface, said bilayer structure comprising a detection volume, b) contacting the bilayer with at least one agent to be analysed, and c) detecting a change in refractive index in the detection volume resulting from transportation of the agent across the membrane. Further there is provided a device comprising a) at least one surface, b) at least one bilayer structure tethered to the surface, and c) at least one sensor capable of detecting a change in refractive index in a detection volume, wherein the bilayer structure encloses a first volume of the detection volume and wherein the volume not enclosed by the bilayer structure but within the detection volume is a second volume and wherein the ratio between the first volume and second volume is above about 0.001.