Intramolecular biosensors are disclosed, including PBP-based biosensors, comprising a ligand binding domain fused to donor and fluorescent moieties that permit detection and measurement of Fluorescence Resonance Energy Transfer upon binding ligand. At least one of the donor and fluorescent moieties may be internally fused to the biosensor such that both ends of the internally fused fluorophore are fixed. In addition, methods of improving the sensitivity of terminally fused biosensors are provided. The biosensors of the invention are useful for the detection and quantification of ligands in vivo and in culture.