Polyanionic amphiphilic polymer based supramolecular dye-host assembly: Highly selective turn–on probe for protamine sensing

Abstract

Here in we report a supramolecular dye-host assembly, forming through non-covalent interaction between a cationic dye, quinaldine red (QR), and polyanionic amphiphilic polymer, polystyrene sulfonate (PSS), as an efficient fluorescence turn-on probe for protamine (Pr) sensing. Formation of QR-PSS complex converts nonfluorescent QR into moderately fluorescent in nature, due to restriction in the structural agility of the dye. Interestingly, fluorescence from QR-PSS complex enhances further quite largely by the presence of Pr, attributing to the formation of QR-PSS-Pr ternary complex. Binding with multi-cationic Pr, polyanionic PSS experiences large charge neutralization, increasing its hydrophobic character. This leads to folding/coiling of PSS, creating several cavity-like patches in its modified structure. The QR molecules that had been initially bound to PSS surface and exposed to bulk aqueous phase, get incorporated suitably into these cavities, arresting the structural flexibility of the dyes significantly. Accordingly, there is a large reduction in the non-radiative de-excitation processes for the excited dye, triggering a noteworthy fluorescence turn-on for QR-PSS complex. This eventually allows us to estimate Pr concentration very effectively, with limit of detection as low as ∼14.8 nM in buffer solution and ∼0.18 μM in 1% human serum sample. Importantly, amongst all the reported dye-PSS systems, QR-PSS is found to be the first ever system that shows fluorescence turn-on response towards Pr. Consequently, present system provides several advantages like high signal to noise ratio, low background, high brightness, minimum possibility of false signal, etc., making it more effective than the previously reported dye-PSS systems.