Fluorogenic and Cell-Permeable Rhodamine Dyes for High-Contrast Live-Cell Protein Labeling in Bioimaging and Biosensing

The advancement of fluorescence microscopy techniques has opened up new opportunities for visualizing proteins and unraveling their functions in living biological systems. Small-molecule organic dyes, which possess exceptional photophysical properties, small size, and high photostability, serve as powerful fluorescent reporters in protein imaging. However, achieving high-contrast live-cell labeling of target proteins with conventional organic dyes remains a considerable challenge in bioimaging and biosensing due to their inadequate cell permeability and high background signal. Over the past decade, a novel generation of fluorogenic and cell-permeable dyes has been developed, which have substantially improved live-cell protein labeling by fine-tuning the reversible equilibrium between a cell-permeable, nonfluorescent spirocyclic state (unbound) and a fluorescent zwitterion (protein-bound) of rhodamines. In this review, we present the mechanism and design strategies of these fluorogenic and cell-permeable rhodamines, as well as their applications in bioimaging and biosensing.