Surfactant structure effects on binding with oppositely charged polyelectrolytes observed by fluorescence of a pyrene probe and label

Abstract

Poly(2-(acrylamido)-2-methylpropanesulfonic acid) (PAMPS) and three copolymers containing about 40 mol% of AMPS and N,N-dimethylacrylamide with a single label of naphthalene or pyrene and with both labels were synthesized. The binding of cationic surfactants cetyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium chloride (DTAC), and nonionic surfactant octaethylene glycol monododecyl ether (C₁₂E₈) on these anionic polyelectrolytes was observed in dilute aqueous solutions with the relative emission intensity of excimer to monomer, I _E/I _m, of cationic probe 1-pyrenemethylamine hydrochloride (PyMeA·HCl), the nonradiative energy transfer (NRET), I _Py/I _Np, between the labels, the emission intensity ratio, I ₁/I ₃, of the first peak to the third peak and the fluorescence anisotropy, r, of the pyrene label. The binding with cationic surfactants made the polyelectrolyte chains much curlier, leading to an increase in I _E/I _M and I _Py/I _Np. The intermolecular NRET occurred at higher CTAB concentrations owing to the hydrophobic aggregation between CTAB tails bound on different polyelectrolyte chains, which induced an increase in r of the pyrene label. No intermolecular aggregation was observed from the intermolecular NRET and r of the pyrene label for DTAC-bound polyelectrolytes owing to its weaker hydrophobicity of 12 carbon atoms in the tail, shorter than that of CTAB. As shown with constant values of the previously mentioned photophysical parameters of the labels with increasing C₁₂E₈ concentration, there was no obvious binding for C₁₂E₈ on the anionic polyelectrolytes. However, the presence of PAMPS promoted the micelle formation for C₁₂E₈ below its critical micelle concentration as detected by the excimer emission of PyMeA·HCl probe.