Two polarity-sensitive fluorescent probes with large Stokes shifts for locating in lipid droplets and lysosomes in cells and zebrafish imaging

Lipid droplets (LDs) and lysosomes, vital subcellular organelles in cells, played irreplaceable roles in the areas involved in the synthesis of cell membranes, the movement of substances and the transfer of energy. Moreover, the imbalance of LDs could cause the occurrence of certain diseases, such as cardiovascular disease, liver cancer, atherosclerosis and diabetes. LDs and lysosomes interact in some biological processes. In this work, two polaritysensitive fluorescent probes (NNap-LD-I, NNap-LD-II) were developed and synthesized by simple 1,8-naphthalimide scaffold modifications, which exhibited large Stokes shifts (173 nm and 190 nm), viscosity, pH stability and interference resistance. Notably, they were biocompatible and showed good targeting for bioimaging. LDs, lysosomes in HepG2 cells and zebrafish could be marked by bright yellow and red fluorescence after incubation with probes (NNap-LD-I, NNap-LD-II). In addition, HepG2 cells were stained for NNap-LD-I and NNap-LD-II to monitor the corresponding reduction in the number of LDs in starvation-treated cells. Similarly, the targeting of the probes to the lysosomes was diminished during imaging after the cells were treated with chloroquine (CQ), resulting in a reduction in lamellar fluorescence. These remarkable characteristics suggested that the probes NNap-LD-I and NNap-LD-II were quite efficient fluorescent probes with good visualization of LDs and lysosomes.