Thioflavin T displays enhanced fluorescence selectively inside anionic micelles and mammalian cells.

Thioflavin T (ThT) has been widely employed to detect amyloid fibrils in tissues and recently in presence of SDS micelles. However, the contribution of membranes or micelles to ThT fluorescence has never been investigated. In this paper, we show for the first time that the anionic micellar microenvironment of SDS has a profound impact on the absorption and fluorescence spectra of ThT in sharp contrast to cationic (CTAB) and neutral micelles (Triton X-100 & Tween 20). Unlike CTAB or Triton X-100 or Tween 20 micelles, formation of SDS micelles shifts the lambdamax for ThT absorption from 412 nm in buffer to 428 nm inside the micelle, with a 28% increase in the peak molar absorptivity and an approximately 13 fold increase in ThT fluorescence (lambdamax=489 nm). Extending these observations to cell plasma membranes, we show that ThT can quickly enter and appear selectively fluorescent inside mammalian cells like BHK21 and HT29, against a dark background owing to negligible fluorescence from free ThT in aqueous medium. The above results suggest that ThT can be a useful probe for live cell imaging and for selectively labeling micelles on the basis of the charge in the polar headgroup.