Rapid exchange luminescence (REL) for high sensitivity detection

A molecular switch, comprising a binding domain for a ligand, a framework and a signaling apparatus, wherein said signaling apparatus comprises a long-lived emitter molecule and short range quencher molecule located along said framework and having changeable positions relative to one another, such that a difference is detectable in a fluorescent signal upon change in conformation between two predominantly populated conformational states of said switch, wherein one conformational state binds the ligand, and wherein there is interchange between these two conformational states that is rapid compared to the emission lifetime of the long-lived emitter.