A water-soluble organometallic conjugated polyelectrolyte PI and its corresponding model complex M1 based on an aspartic acid-substituted fluorene spacer are reported, which possess good water solubility as well as intriguing fluorescent and phosphorescent dual-emissive properties in a completely organic-free aqueous medium at room temperature. A new colorimetric silver ion sensor based on PI is developed, which shows high selectivity and sensitivity for Ag+ ions in buffered water solution because of the Ag+-induced intersystem crossing from the singlet to triplet states. The obvious color change from colorless to yellow upon exposure to Ag+ ion is visible to the naked eyes and can be quantified colorimetrically by the visible absorption spectroscopic method. On the basis of the fluorescence intensity of PI obtained in the fluorescence titration curves, a linear relationship is observed in the Stern-Volmer plot at low concentrations (1-5 mu M), and the corresponding Stern-Volmer quenching constant (K-sv) of 1.9 x 10(5) M-1 for P1 is comparable to that obtained from the fluorescence titration studies. As determined by the Benesi-Hildebrand plot obtained from the absorption spectra, a 1:1 complex formation is anticipated between the Pt compound and Ag+ ion. The limit of detection is low at 0.5 mu M, i.e., at concentrations in the ppb range. The present study represents an original approach using a water-soluble organometallic conjugated polyelectrolyte for the accurate and rapid detection of trace amounts of Ag+ ion in pure water. It also establishes a new system featuring dual-emissive properties of platinum(H) acetylide-based conjugated polymers for chemosensing application.