A fluorescent turn-on detection for nitric oxide in aqueous solution is developed using cationic conjugated polymers. The assay benefits from the sensitivity of optical signals from conjugated polymers and. the simplicity of fluorescence measurement techniques. The assay contains three elements: a cationic conjugated polymer that contains imidazole moieties, CU2+ ions, and the target nitric oxide. The highly fluorescent conjugated polymer coordinates to CU2+ ions through weak N center dot center dot center dot Cu interactions, and its fluorescence is efficiently quenched by a photoinduced electron transfer process ('off' state). In the presence of nitric oxide, the transformation of the paramagnetic CU2+ ion into a diamagnetic Cu2+ ion inhibits the quenching and, therefore, the fluorescence of the conjugate polymer is recovered ('on' state). Other biologically relevant reactive nitrogen species, such as NOBF4, NaNO2, and NaNO3 don't exhibit the fluorescence recovery of the conjugated polymer under the same conditions as nitric oxide. The cationic conjugated polymer/Cu-2, complex can thus be used as a platform to detect nitric oxide in aqueous solution with high sensitivity and selectivity.