Abstract: A method for detecting an analyte is described in which the simultaneously binding of two fusion proteins (i.e., a sandwich assay in solution) is used, bringing two halves of a split enzyme together to produce product, which is detected via a FRET-based biosensor. The method may incorporate an autocatalytic feedback loop that responds to enzymatic product by producing more product to provide ultrasensitive, bistable detection of analyte that is tunable over several orders of magnitude. This system is broadly applicable for protein and small molecule detection.