Quinoline-labeled poly(N-isopropylacrylamide): A selective polymeric luminescent sensor of cationic surfactants

Abstract

The effect of ionic surfactants, either anionic (sodium dodecyl sulfate, SDS) or cationic (hexadecyltrimethylammonium bromide, CTAB), on the luminescence response of a quinoline-labeled thermosensitive poly(N-isopropylacrylamide), P(NIPAM-co-SDPQ), is investigated in the present work. It is seen that at temperatures higher than the Lower Critical Solution Temperature (LCST) of the copolymer in aqueous solution, the emission peaks of both the unprotonated and the protonated form of the quinoline unit of P(ΝΙΡΑΜ-co-SDPQ) are observed even under acidic conditions. Upon association of P(NIPAM-co-SDPQ) with the anionic surfactant SDS, the emission spectrum of the copolymer is just slightly affected at temperatures below or above LCST. In contrast, under similar conditions, this copolymer exhibits a pronounced luminescence response upon association with CTAB. The emission peak of the unprotonated form of the quinoline unit is greatly enhanced, while the peak of the protonated form gradually disappears, upon association of P(NIPAM-co-SDPQ) with the cationic surfactant CTAB. Thus, above the critical aggregation concentration, the emitted color turns selectively from green to blue in the presence of CTAB, revealing that such polymeric structures are promising as selective luminescent sensors of cationic surfactants. This response seems to be general, since it is observed for a variety of cationic surfactants or binary surfactant mixtures, provided that mixed polymer/surfactant aggregates are formed and the net charge is cationic in the case of mixed micelles.