A polarity-sensitive naphthalimide-based red-emissive fluorescent probe for live-cell imaging

Polarity is a crucial physicochemical parameter that regulates essential cellular processes such as cell division, signaling, and migration. Fluctuations in cellular polarity are strongly associated with pathological conditions, including cancer. In this study, we developed a polarity-sensitive fluorescent probe, Nap-P, for live-cell imaging applications. The probe showed an absorption shift in UV-visible spectra from 429 nm to 446 nm as the solvent polarity increased from dioxane (ET₃₀ = 36) to water (ET₃₀ = 63.3). In dioxane-water and toluene-DMSO mixtures, increasing polarity induced a red shift in emission maxima, accompanied by decreased fluorescence intensity at 590 nm, confirming its polarity sensitivity. Importantly, Nap-P showed negligible sensitivity toward viscosity and various biologically relevant analytes. Cellular imaging studies demonstrated uniform intracellular distribution, enabling real-time monitoring of polarity fluctuations under starvation and oxidative stress conditions in cervical cancer (SiHa) and normal (3T3-L1) cells. Under stress conditions, cancer cells exhibited reduced fluorescence relative to normal cells, suggesting increased intracellular polarity due to lipid depletion and oxidative degradation of lipid droplets. These findings highlight Nap-P as a promising probe for live-cell polarity imaging and mapping.