Ortiz, G. et al. J. Am. Chem. Soc. 141, 6631–6638 (2019).

The ability to sense changes in membrane potential or voltage in electrically excitable cells such as neurons and cardiomyocytes is crucial for understanding their biological activities, and numerous dyes and genetically encoded sensors have been developed to monitor these changes. Ortiz et al. describe voltage-sensitive fluorescent indicators based on a carbofluorescein scaffold. To develop these probes, the researchers used chlorination and sulfonation of the carbofluorescein scaffold to lower the pKa and prevent cyclization to a nonabsorbing form, respectively. From there, they incorporated the chlorinated sulfone carbofluoresceins into phenylenevinylene molecular wire scaffolds for voltage sensing. The indicators show high sensitivity and are red-shifted relative to fluorescein, which makes them useful for two-color applications with green fluorophores. The researchers demonstrated the dyes’ performance on in vitro–cultured neurons and HEK cells, and they note that these dyes can be synthesized from readily available reagents.