Water soluble non-conjugated fluorescent polymers: aggregation induced emission, solid-state fluorescence, and sensor array applications

Abstract

In this work, two water soluble fluorescent non-conjugated polymers have been designed and synthesized. To confer water solubility, a synthetic strategy was made to incorporate secondary amine moieties and ester linkages in the backbone, and to make it fluorescent we have attached dansyl and naphthyl moieties. The polymers exhibited solid-state fluorescence which was confirmed by solid-state fluorescence spectroscopy and microscopy. They also exhibited aggregation induced emission (AIE) behaviour in a water–DMSO mixture which was confirmed by fluorescence, quantum yield, DLS, and lifetime analysis. Furthermore, the interaction of the polymers with different metal ions was studied with the aim of using it as a sensor. Among the 15 metal ions tested, it was found that the dansyl appended polymer gave distinct signals in the absorption and emission spectra for the addition of Cu²⁺, Co²⁺, Ag⁺, K⁺, Fe²⁺ and Fe³⁺ ions. These metals also showed discrete signals in the absorption spectra along with distinguishable colour changes such as Cu²⁺ – blue, Co²⁺ – pink, Ag⁺ – purple, Fe²⁺ – dark yellow and Fe³⁺ – light yellow on their addition. Further linear discriminant analysis (LDA) was performed to see the sensor array applications of the polymer and showed a discrete sensor array output and clear separation of clusters, validating the application of the polymer as a multi-analyte sensor. The separation and identification of the metals via LDA was also possible in nanomolar concentrations. Moreover, our strategy for developing a fluorescent polymer is also versatile for synthesising polymers with tuneable properties for diverse applications.