Highly bluish-white light emissive and redox active conjugated poly-N-phenyl anthranilic acid polymer fluoroprobe for analytical sensing

Abstract

An efficient fluorescent and redox type conjugated poly-N-phenyl anthranilic acid (PNPA) was synthesised via chemical oxidative polymerisation of N-phenyl anthranilic acid (NPA) using FeCl3 as oxidising agent in ethanol medium. The polymer in sulphuric acid solution denoted as PNPA-H have highly intense bluish-white fluorescence. The intense bluish-white light emission of polymer was quenched upon the addition of oxidising analytes like Ce4+, MnO4− and Cr2O72- ions. The fluorescence quenching concentration at which the above three analytes oxidises the polymer were determined from three independent methods like naked eye fluorescence detection, UV–visible spectroscopy and spectrofluorometry. The mechanism of fluorescence quenching has been explained on the basis of the oxidation of diphenyl benzidine dicarboxylic acid repeating units of PNPA-H (in reduced form) into non-fluorescent diphenyl diquinoid dicarboxylic acid units (in oxidised form) by the oxidising analytes. The mole ratio plot of [analyte]/[polymer] against fluorescence intensity have revealed different stoichiometry for a particular analyte leading to quenching of fluorescence of polymer, which have striking influence on the redox potential of the analytes. The limit of detection (LOD) for naked eye fluorescence quenching was found to be 0.5 μM, 0.75 μM, and 25 μM for Ce4+, MnO4− and Cr2O72- respectively and sensitivity of quenching action was obtained highest for MnO4− ions and least for Cr2O72- ions from stern-volmer plots. The oxidised and non-fluorescent diphenyl diquinoid dicarboxylic acid units of PNPA-H have been reduced back to fluorescent diphenyl benzidine dicarboxylic units with reducing biomolecules like ascorbic acid, which indicate the redox reversibility of the system.