A method of detecting the presence of an analyte in a sample, the method comprising contacting the sample with a carrier including a plurality of first ligands reactive with the analyte to allow binding of the analyte to the carrier ligands, contacting the carrier with a membrane comprising a first and second layer of a closely packed array of amphiphilic molecules and a plurality of ionophores comprising a first and second half membrane spanning monomers, the first half membrane spanning monomers being provided in the first layer and the second half membrane spanning monomers being provided in the second layer, the second half membrane spanning monomers being capable of lateral diffusion within the second layer independent of the first half membrane spanning monomers, the first half membrane spanning monomers being prevented from lateral diffusion in the first layer, and a second ligand provided on at least the second half membrane spanning monomers, said second ligand being reactive with the analyte or a portion thereof, the binding of the analyte to the second ligand causing a change in the relationship between the first half membrane spanning monomers and the second half membrane spanning monomers such that the flow of ions across the membrane via the ionophores is allowed or prevented, and measuring the impedance of the membrane.