An optical nanoruler system based on a conjugated polyelectrolyte-silver nanoprism pair is developed for label-free protein detection by taking advantage of the metal-enhanced fluorescence effect of silver nanostructures. Antibody-antigen interactions induce a change in the metal-fluorophore distance, followed by the response of a fluorescent signal of the conjugated polyelectrolyte. The system is used to detect target antigens sensitively and selectively.