Novel Bio-imaging Tools Based on the Precise Design of Functional Fluorescence Probes

Fluorescence imaging is one of the most powerful techniques currently available for continuous observation of intracellular processes in living cells and tissues, and so it would be useful to have a large toolset of fluorescence probes suitable for visualizing bioactive molecules or biological phenomena. However, design strategies for small molecule-based fluorescence probes have been limited. Therefore, we are interested in establishing novel rational design strategies for developing functional fluorescence probes. For this purpose, we have focused on the intramolecular spirocyclization reaction of rhodamine derivatives bearing intramolecular nucleophiles as a fluorescence switching mechanism; these derivatives exist in thermal equilibrium between a fluorescent open form and a non-fluorescent spirocyclic form. By utilizing this reaction, we have successfully developed various kinds of fluorescence probes. This review first describes the design of our recently developed fluorescence probes, and then introduces their applications for visualizing the activities of reporter enzymes, for in vivo cancer imaging, and for super-resolution imaging. We believe these probes can serve as key tools to provide unprecedented insights into a wide range of biological phenomena.