This disclosure relates to systems, devices, and methods of sensing an analyte. An implantable sensor may be contacted with a test sample under conditions that permit a binding protein and a ligand of the sensor to interact in an analyte-dependent manner to produce an analyte-dependent signal, and (b) detecting the analyte-dependent signal with a detector. A binding protein may reversibly bind an analyte and/or a ligand. A binding protein may have a higher binding affinity for an analyte than for a ligand. A binding protein and a ligand may each include a fluorophore, the absorption and/or emission properties of which may change in an analyte-dependent manner. A binding protein and/or a ligand may be bound to an active or inactive substrate. Some embodiments of systems, devices, and methods may be practiced in vitro, in situ, and/or in vivo. Systems and/or devices of the disclosure may be configured to be wearable.