Molecular rotor bearing naphthol hydrazone Schiff base for sensing viscosity changes in solutions and biosystems

Viscosity is an important parameter in cells since it is closely related to a few severe human diseases. The development of fluorescent probes that can detect changes in intracellular viscosity has become an effective tool for monitoring the intracellular microenvironment. In this work, an efficient Schiff base molecular rotor, NaForm, is designed and synthesized by covalently linking a naphthol hydrazine derivative with the methyl 4-formylbenzoate group for the optical detection of viscosity changes. By increasing the viscosity of solvents, a significant enhancement of fluorescence for the probe at the yellow-green channel is observed. The probe also exhibits the advantages of high sensitivity and large Stokes shift of 128 nm. Both theoretical calculations and spectroscopic measurements suggest that the inhibition of chemical bond rotation between the hydrazine and the benzene ring leads to such a profound turn-on response. The successful applications of the probe in HeLa cells and zebrafish sufficiently demonstrate the privilege of NaForm as a practical visocosity-dependant chemosensor in aqueous samples and organisms. This chemosensor holds great promise for studying intracellular viscosity effects in various biological contexts.