pH-Responsive diblock copolymers with two different fluorescent labels for simultaneous monitoring of micellar self-assembly and degree of protonation

Abstract

We report the synthesis of a novel amphiphilic pH-responsive diblock copolymer labeled with two different fluorophores. This copolymer comprises a water-soluble poly(glycerol monomethacrylate) [PGMA] block and a pH-responsive poly(2-(diisopropylamino)ethyl methacrylate) [PDPA] block. Pyrene methacrylate [PyMA] is statistically copolymerized with glycerol monomethacrylate (GMA) to introduce a suitable fluorescent label. The chain-ends of the PDPA block are labelled using cresyl violet perchlorate [CV] by exploiting the spin trap properties of this dye molecule. Below pH 6, fluorescence from both dye labels can be detected, but deprotonation of the PDPA block between pH 6 and 7 leads to strong attenuation of the CV fluorescence owing to formation of a charge transfer complex with the tertiary amine units in the PDPA block. Therefore, changes in the Cresyl Violet fluorescence intensity can be correlated to changes in PDPA protonation. Diblock copolymer self-assembly to form PDPA-core aggregates occurs under these conditions, leading to pyrene fluorescence at an excitation wavelength of 405 nm. This allows direct measurement of chain aggregation, whereas using pH-responsive dyes is simply a measure of the degree of protonation. Here we focus on the synthesis and characterisation of dual-labeled copolymers and their spectroscopic properties in different environments. Finally, we show that using CV as a spin trap provides a convenient and versatile route to fluorescently-labeled copolymers prepared by either RAFT or ATRP. Moreover, this cost-effective dye fluoresces in the red part of the visible spectrum at both neutral and acidic pH.