From chain collapse to new structures: spectroscopic properties of poly(3-thiophene acetic acid) upon binding by alkyl trimethylammonium bromide surfactants.

The binding of cationic surfactants with varying alkyl chain length to a regiorandom conjugated polyanion, poly(3-thiophene acetic acid) (PTAA), is studied in an aqueous buffer by using absorption and emission spectroscopies, photon correlation spectroscopy, isothermal titration calorimetry, and cryogenic transmission electron microscopy. We study the mixed solutions as a function of composition ratio R of surfactant molecules to monomer units molar concentrations, at low polymer concentration and in a very wide composition range (10(-6) < R < 10(2)) below the critical micellar concentration. Upon surfactant binding, the molecularly dispersed chains first collapse progressively and then form new structures as the mixed aggregates get enriched in surfactant. The collapse leads to a strong decrease of the conjugation length and to a blue shift of the absorption spectra by 30 to 50 nm. The new structures are responsible for a new intense emission band at about 600 nm, red-shifted by nearly 130 nm from the initial emission maximum of the polymer (~472 nm). As the surfactant tail becomes shorter, the blue shift of the absorption spectra and the intensity raise of the new emission are delayed to larger composition ratios while their variations become smoother functions of the surfactant concentration. These particular spectroscopic properties of PTAA seem related to its unique combination of a strongly hydrophobic backbone, a large ratio of contour length to persistence length, and an overall good aqueous solubility. Our results show that such features are well suited to design a colorimetric biosensor at small composition ratio, and a fluorescent biomarker at large composition ratio.