Silver Ion-Induced Reversible Turn-Off Fluorescence in Redox States of Poly-o-phenylenediamine

In the present work, we report the sensing of silver ions by yellow fluorescence turn-off in poly-o-phenylenediamine (POPD) solution via an oxidative mechanism. Poly-o-phenylenediamine was synthesized by chemical oxidative polymerization using ferric chloride as an oxidizing agent in ethanol solvent. Proton nuclear magnetic resonance spectra of the POPD revealed two types of protons corresponding to benzenoid and quinonoid structures in semioxidized POPD. Fourier transform infrared (FT-IR) spectra confirmed the presence of benzenoid and quinoid absorption bands in poly-o-phenylenediamine. Powder X-ray diffraction (PXRD) patterns revealed sharp peaks at 2θ values of 10.54 and 26.70°, indicating the crystalline nature of the polymer. Matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) analysis indicated the presence of 5–6 repeating monomeric units in POPD. POPD has intense yellow fluorescence in water, and its fluorescence emission was quenched upon oxidation with AgNO3. Fluorescence spectra and naked-eye detection studies have revealed that an analyte to polymer molar ratio of 10 was obtained for fluorescence quenching. The POPD polymer has shown oxidative fluorescence turn-off and reductive fluorescence turn-on with specific redox analytes, indicating reversible structures of POPD. The cyclic voltammogram of POPD indicated the presence of electrochemically reversible redox states, particularly in the negative voltage sweep in the polymer solution. The oxidized POPD was nonfluorescent, and fluorescence emission reappeared upon adding NaBH4 to the oxidized POPD. Thus, reversible fluorescence turn-off and turn-on in poly-o-phenylenediamine may provide an attractive strategy for sensing various oxidizing/reducing analytes in water.