Microwave-assisted green synthesis of some nanoconjugated copolymers: characterisation and fluorescence quenching studies with bovine serum albumin

Copolymerization is an effective technique to design conjugated polymers with properties higher than individual homopolymers, as the composition and the desired chemical properties can be controlled by the judicious choice of co-monomers and polymerization techniques. With a view to explore the influence of microwave-assisted copolymerization on the spectral and fluorescence properties of some functionalized monomers, the present work reports for the first time, the synthesis of conjugated copolymers of poly(1-naphthylamine), poly(o-phenylenediamine), and poly(o-anisidine) under microwave irradiation. Fourier transform infrared spectroscopy confirmed random copolymerization, while ultraviolet-visible studies revealed the variation in polaronic states upon copolymerization. High crystallinity was achieved through the formation of a distorted orthorhombic lattice and controlled morphology via microwave-assisted synthesis which was confirmed by X-ray diffraction and transmission electron microscopy analysis. This behaviour was explained on the basis of variable orientations adopted by the conducting polymer chains. A water soluble homopolymer and a copolymer were tested for the deactivation of bovine serum albumin's fluorescence and the former was found to effectively quench the fluorescence emission of the latter. Quenching occurred through formation of an intermolecular complex and was initiated by photoinduced electron transfer and was observed to be static in nature. The quenching rate constant, k(q), for POPD and POPD-co-PNA was found to be 1.08 x 10(14) L M-1 s(-1) and 2.0 x 10(14) L M-1 s(-1), revealing a much higher quenching rate constant k(q) for the copolymer than the homopolymer. Likewise, the binding constants, K-a, for a homopolymer and a copolymer were found to be 3.98 x 10(6) L mol(-1) and 1.26 x 10(7) L mol(-1), revealing a much higher binding constant for the copolymer POPD-co-PNA than the homopolymer POPD. Confocal microscopy revealed widely distributed binding of the copolymer with the tryptophan residues in the protein scaffold. These results reveal that the copolymer holds potential for use in bioimaging and also as a protein sensor.