Excited-State Energy Transfer-Associated Dual Emission of Light-Emitting Polymers Containing Sulfonated Graphene Oxide for Sensing of pH, Co(II), and Bi(III)

Abstract

The design, synthesis, optimization, and development of an excited-state energy transfer (ESET)-assisted dual-light emission hybrid polymeric sensor are very much challenging, particularly when the polymer is purely aliphatic and bears nonconventional heteroatomic subfluorophores. In this work, aliphatic light-emitting polymers (LEPs) are synthesized from dimethylaminoethyl methacrylate and maleic acid monomers having −C(═O)OCH2–, −N(CH3)2, and −C(═O)OH/–C(═O)O– subfluorophores. In aliphatic LEPs, hydrogen bond associated strong supramolecular networks facilitate n → π* transmissions and dual excitation dual emission. The optimum incorporation of subfluorophores in LEP5 is supported by Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopies, thermogravimetric profiles, and aggregation enhanced emission (AEE) studies. Thereafter, the sulfonated graphene oxide (SGO) nanoparticle is incorporated in the optimum LEP5 to fabricate hybrid light-emitting polymers (HLEPs) having increased size/surface area, noncovalent interactions, and electronic distributions. In HLEPs, electron rich polar −C(═O)OH/–C(═O)O– and −SO3H/–SO3– functionalities increase hydrogen bonding and dipole–dipole interactions. Among HLEPs, HLEP3 having the maximum aggregating tendency and emission capacity is optimized by AEE studies and FTIR, Raman, powder X-ray diffraction (PXRD), and thermogravimetric analyses. In the aggregation-associated ESET phenomenon of HLEP3, the SGO nanoparticle acts as an energy acceptor. The ESET-associated single-excitation dual emissions are supported by the absorption and emission maxima of HLEP3-aggregate and HLEP3, respectively; excitation spectra; and average lifetimes in time correlated single photon count studies. The aggregations of HLEP3 are supported by dynamic light scattering (DLS) studies, scanning electron microscopy photomicrographs, and UV–vis spectra. The dual-emission phenomena of HLEP3 and its pH-sensitive subfluorophores −N(CH3)2, −C(═O)OH/–C(═O)O–, and −SO3H/–SO3– are responsible for precise ratiometric pH sensing within 8.0–11.0. The reversibility test and PXRD data of HLEP3 at different pH values indicate the stability of HLEP3 in acidic, neutral, and basic media. The dual-light emissions facilitate rapid sensing and detections of Co(II) and Bi(III) with limits of detection below the WHO-recommended values in both aqueous and nonaqueous media. The strong coordinations of Co(II) and Bi(III) with −C(═O)O–/–SO3H/–SO3– of HLEP3 and HLEP3-aggregate, respectively, are confirmed by UV–vis, FTIR, and Raman spectroscopies along with the DLS measurements.