Highly selective ratiometric fluorescent probe for Au³⁺ and its application to bioimaging

The 4-propargylamino-1,8-naphthalimide based fluorescent probe 1 has been explored as a sensor for selective detection of Au³⁺. 4-Amino-1,8-naphthalimides, that possess typical intramolecular charge transfer (ICT) electronic characteristics, have been widely used as versatile platforms for fluorescent probes. The newly designed probe 1 contains a propargylamine moiety at C-4 of the naphthalimide chromophore that reacts with Au³⁺ to generate a product that has distinctly different electronic properties from 1. Specifically, the probe undergoes a remarkable change in its absorption spectrum upon addition of Au³⁺ that is associated with a distinct color change from yellow to light pink. In addition, a blue shift of ca. 56nm also takes place in the emission spectra of the probe. Consequently, 1 serves as a reaction-based sensor or so called chemodosimeter for Au³⁺. Importantly, surfactants enhance the rate of reaction of 1 with Au³⁺, thus, enhancing its use as a real time sensor. Finally, the results of studies probing its application to bioimaging of Au³⁺ in live cells show that the probe 1 has a unique ability to sense Au³⁺ in cells and, in particular, in lipid droplets within cells.