Photoluminescent polymer micelles with thermo-/pH-/metal responsibility and their features in selective optical sensing of Pd(ii) cations

Abstract

Photoluminescent polymers can be divided into two types of structures: one is the well-known conventional π-conjugated rigid chain polymers bearing π-conjugated chromophores in their side chains, and the other is the common flexible polymers without π-conjugated chromophores in their main or side chains but with a feature of clustering electron-rich and/or dipole groups in their main and/or side chains. In this work, we found a new photoluminescent polymer comprising theophylline (T) and imidazole (I) residues in a suitable ratio in the side chains on the common polystyrenic block (PVB-T/I). We synthesized a block copolymer (denoted as P2) consisting of hydrophobic PVB-T/I and hydrophilic poly(N-isopropylacrylamide), and we investigated its self-assembly into micelles and their micellar features, such as thermo-responsibility, fluorescence emission, pH, and metal ion-dependent photoluminescence, in detail. Especially, the micelles self-assembled from P2 showed intrinsic blue emission which was emitted from the charge transfer association between T and I residues in the intra-chains. Weakening the association by adjustment of the pH or addition of metal ions could evidently reduce the photoluminescence in the micellar state. Very interestingly, among many metal cations, only Pd²⁺, which can chelate strongly with theophylline, strongly quenched the photoluminescence from the micelles. Therefore, the polymer micelles functioned as an optical sensor for Pd(II) ion not only by spectroscopy but also with the naked eye.