A device for sensing analyte concentration, and in particular glucose concentration, in vivo or in vitro is described. An optical conduit, preferably an optical fiber, has an optical system at the proximal end of the optical conduit. A sensing element is attached to the distal end of the optical conduit, and may include at least one binding protein adapted to bind with at least one target analyte. The sensing element further may also include at least one reporter group that undergoes a luminescence change with changing analyte concentrations. Optionally, the sensing element includes reference groups with luminescence properties that are substantially unchanged by variations in the analyte concentrations.