Ionic thermoresponsive fluorescent polymers for detecting countercharged surfactants without phase separation

Abstract

Thermoresponsive polymers containing ionic and fluorescent groups were designed and synthesized for use as fluorescent probes to sense ionic surfactants without phase separation. The synthesized ionic thermoresponsive fluorescent polymers interact with countercharged surfactants, resulting in increases in their fluorescence intensities, even at temperatures lower than their phase-transition temperatures. The observed increases in the fluorescence intensities are attributable to the incorporation of fluorophores within the soluble aggregates formed by the polymers and ionic surfactants at temperatures lower than the phase-transition temperatures. The synergistic effects of the hydrophobic and ionic interactions between the two moieties lead to the formation of soluble aggregates, which enhance the fluorescence of the fluorophores bound to the polymers. The fluorescence intensities of cationic and anionic fluorescent polymer probes were found to be linearly dependent on the concentrations of sodium dodecyl sulfate (SDS) and zephilamine, respectively, at the 10−5 mol/L level. Moreover, the concentration of SDS, an anionic surfactant used commonly in detergents, as determined using the cationic fluorescent probe was in keeping with that determined by the conventional methylene blue method. Thus, it was confirmed that the synthesized thermoresponsive fluorescent polymers with ionic groups can function as probes for countercharged ionic surfactants in homogeneous aqueous media without any phase-separation processes, which require hazardous organic solvents.