Stimulus-Responsive Supramolecular Aggregate Assembly of Auramine O Templated by Sulfated Cyclodextrin

Abstract

Self-aggregation of organic molecules is rarely seen with macrocyclic hosts like β-cyclodextrin, as they preferentially involve the formation of inclusion complexes with the guest molecule. In this contribution, we report the self-aggregation of a guest molecule induced by negatively charged sulfated β-cyclodextrin (SCD) to yield highly emissive aggregates of a recently projected amyloid marker dye, Auramine O (AuO). The SCD templated AuO aggregates display very different photophysics when compared to its reported behavior in a wide range of various chemical and biological environment but show a remarkable similarity with the recently reported photophysical behavior of AuO in human insulin fibrillar media, thus providing important insights into the molecular form of AuO responsible for its amyloid sensing ability. The self-assembled AuO aggregates formed in the presence of SCD display a significantly long excited-state lifetime, suggesting the retardation of the torsional relaxation of dye in the aggregated state, which otherwise leads to a very short excited-state lifetime for the monomeric form of the dye in the isolated form. Detailed time-resolved emission spectra (TRES) measurements show a dynamic Stokes shift suggesting excitonic migration within the AuO aggregates. The supramolecular aggregate assembly displays remarkable sensitivity to important external stimuli like temperature or ionic strength of the medium, pitching for its possible application in designing stimuli-responsive sensing schemes for important analytes.