Dual-Mode Fluorescence Sensing of Multiple Oxidizing Analytes by Selective Oxidation of Poly(N-Phenyl Anthranilic Acid-Co-o-Phenylenediamine) Random Copolymers

Abstract

We report a dual-mode fluorescence sensing of multi-analytes like silver and permanganate ions by adopting a selective oxidation strategy in poly(N-phenyl anthranilic acid-co-o-phenylenediamine) copolymer. The P(NPA-co-OPD) copolymers were synthesized by in situ oxidative chemical polymerization of N-phenyl anthranilic acid (NPA) and o-phenylenediamine (OPD) comonomers using ferric chloride as oxidizing agent. Proton (¹H) and carbon (¹³C) nuclear magnetic resonance spectra and Fourier transform infrared spectra revealed the higher reactivity of NPA compared to OPD in P(NPA-co-OPD) copolymers. Monomer reactivity ratios calculated from the Fineman–Ross method also indicated the higher reactivity of NPA monomer in copolymerization. Powder X-ray diffractograms and differential scanning calorimetry (DSC) thermograms revealed the development of crystallinity in copolymers. Field emission scanning electron microscopic images (FE-SEM) and high-resolution transmission electron microscopic images (HR-TEM) revealed the morphological transformation of semi-micro-spherical-shaped poly-N-phenyl anthranilic acid and micro-rod-shaped poly-o-phenylenediamine (POPD) into micro-blocks/microrod-shaped P(NPA-co-OPD) copolymers. Poly-o-phenylenediamine (POPD) has a yellow emission, and poly-N-phenyl anthranilic acid (PNPA) has a blue emission in the neutralized medium. The blue emission was dominated in copolymers due to the higher PNPA fragments than the POPD fragments in the copolymer chain, except for P(NPA-co-OPD) 10:90, which has yellow emission due to the dominant OPD fragments in the copolymer. The fluorescence properties of copolymer P(NPA-co-OPD) 10:90 were selectively tuned from yellow to blue by adding AgNO₃, indicating the semi-oxidation of POPD fragments in the copolymer and, subsequently, blue to colorless by adding acidified KMnO₄, indicating complete oxidation of both POPD and PNPA fragments in the copolymer. The selective fluorescence response suggests the ability of the copolymer to distinguish between mild or strong oxidizing agents via selective oxidation.