Biosensors fur use in detecting analytes, particularly small analytes such as those having a molecular weight of less than 5,000 Daltons, are disclosed which comprise a membrane and an electrode and a reservoir defined therebetween, the membrane having an inner layer proximate the electrode and an outer layer remote from the electrode comprising a closely packed array of amphiphilic molecules, a plurality or ionophores, and a plurality of membrane spanning lipids, the ionophores comprising first and second half membrane spanning monomers, the first half membrane spanning monomers being provided in the inner layer and being prevented from lateral diffusion within the membrane and the second half membrane spanning monomers being provided in the outer layer and being free to diffuse laterally within the membrane, the second half membrane spanning monomers having attached thereto a first receptor which is reactive with the small analyte, wherein a carrier to which is attached a plurality of the analyte is reversibly bound to the first receptor via the analyte. In an alternative embodiment, the second half membrane spanning monomers are attached through a carrier and/or linker group to at least one analyte, and a receptor is reversibly bound to the second half membrane spanning monomers via the analyte and said carrier and/or linker group.