Probing ionic liquid based micellar structures using the photophysical properties of a plant alkaloid: spectroscopy and microscopy based approach

In this paper, we report the structural and dynamics aspect of self-assembled nano structures formed by an ionic liquid (IL) based surfactant, 1-butyl-3-methylimidazolium [Bmim] octyl Sulfate [Os] using photophysical properties of a well-known anti-cancer alkaloid, sanguinarine (SANG). SANG exists in two forms, iminium and alkanolamine, in aqueous solutions at equilibrium which gets disturbed upon addition of [Bmim][Os] IL. The anionic part of this IL is micellar in nature and induces the iminium form to convert into alkanolamine form after reaching critical micelle concentration (CMC) at 31–34 mM. The gradual conversion from iminium to alkanolamine with [Bmim][Os] exhibits interesting transition points at various concentrations of IL. Importantly, on adding IL, the increment in alkanolamine emission at 415 nm exhibited a different trend when compared with iminium emission at 565 nm. Only one transition point at CMC was seen by alkanolamine emission intensity whereas iminium fluorescence undergoes three different slopes of quenching. Atomic force microscopy (AFM) detected different morphologies at 10, 20 and 35 mM concentrations of [Bmim][Os]. Interestingly, time-resolved properties of alkanolamine showed an efficient way for detecting these premicellar transitions. Along with the alkanolamine lifetime component inside the micelle (2.9 ns), the appearance of a short component of fluorescence lifetime ∼ 1.6 ns was characterized as SANG-sulfate interactions using the [Bmim][HSO4]. Overall, three regions of premicellar and micellar states were observed based on spectroscopic analysis of iminium and alkanolamine forms. This kind of switching between prototropic forms of the drug by a surface-active IL (SAIL) is an important aspect for alkaloids and can find potential applications in drug loading and delivery methods.