Monitoring the confinement of methylene blue in pyrene labeled poly(acrylic acid)

This work was devoted to investigate the aggregation of methylene blue (MB) with pyrene labeled poly(acrylic acid) (PAA25Py3) by using the measurements of UV-vis absorption, steady-state and time-resolved fluorescence. For comparison, the effect of unlabeled poly(acrylic acid) (PAA25) on the self-association of MB was also evaluated. The spectral positions of the absorption and emission bands of MB as well as the pyrene monomer and excimer emissions were used to assess the local changes of polymer/MB microenvironment. The binding mechanism and thermodynamic analysis for the interaction between MB and pyrene labeled poly(acrylic acid) were evaluated by fluorescence quenching measurements. Moreover, the strategy of switching of pyrene excimer/monomer emission was used to manage the self-aggregation of the pH-responsive fluorescent polymer with phenothiazinium dye. The UV-vis absorption and fluorescence results showed that low amounts of polymers provoked the self-association of MB. At high concentrations of polymer, the MB monomer is stabilized. The labeling of polymer with pyrene induced a dramatic changing of optical properties suggesting that a peculiar process occurred between PAA25Py3 and MB. At low quencher concentrations, both steady-state and time-resolved fluorescence data indicated a static quenching process. The thermodynamic parameters (∆H, ∆S and ∆G) and the efficiency of pyrene excimer formation revealed that the association of MB with the fluorescent polymer occurred spontaneously due to non-covalent hetero π-π stacking, hydrophobic and electrostatic interactions. The study highlights the key contribution of the π-π stacking interactions in the capturing of methylene blue in pyrene labeled poly(acrylic acid). The outcome may be useful in biomedical applications or in wastewater treatment.