Molecular layer-by-layer self-assembly and mercury sensing characteristics of novel brush polymers bearing thymine moieties

Two new brush polyoxyethylenes bearing thymine moieties at the bristle ends have been synthesized as model polymers in which the chemical loading of the thymine functional group into the polymer is maximized: poly(oxy(11- thyminoacetyloxyundecylthiomethyl)ethylene) (PECH(S)-T) and poly(oxy(11- thyminoacetyloxyundecylsulfonylmethyl)ethylene) (PECH(SO₂)-T). These brush polymers are thermally stable up to around 225 °C, and their glass transitions occur in the range 23-27 °C, but they have significantly different properties despite the similarity of their chemical structures. In particular, PECH(SO₂)-T films exhibit better performance in sensing mercury ions than PECH(S)-T films. These differences were found to originate in the differences between their morphological structures. The PECH(SO ₂)-T film has a multi-bilayer structure without interdigitation, in which the layers stack along the out-of-plane of the film and provide a thymine-rich surface. In contrast, the PECH(S)-T film is amorphous with a relatively low population of thymine moieties at the surface. This study demonstrated that a thymine-rich surface is required for recyclable thymine-based polymers to provide highly improved sensitivity and selectivity as well as full reversibility in the sensing of mercury ions. A thymine-rich surface can be achieved with a brush polymer bearing thymine moieties that can self-assemble into a multi-bilayer structure. Because of the thymine-rich surface, the PECH(SO₂)-T thin films even in only 6 nm thickness demonstrate the detection of mercury ions in aqueous solutions with a detection limit of 10^-6 M.