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Lipid droplets (LDs) are simple intracellular storage sites for neutral lipids and exhibit important impact on many physiological processes. For example, the changes in the polar microenvironment inside LDs could affect physiological processes, such as lipid metabolism and storage, protein degradation, signal transduction, and enzyme catalysis. Herein, a new fluorescent chemo-sensor (Couoxo-LD) was formulated by our molecular design strategy. The probe could be applied to effectively label intracellular lipid droplets. Intriguingly, Couoxo-LD demonstrated positive sensitivity to both polarity and viscosity, which might be attributed to its D-π-A structure and the twisted rotational behavior of the carbon-carbon double bond (TICT). Additionally, Couoxo-LD was successfully implemented in cellular imaging due to its excellent selectivity, pH stability, and low biotoxicity. In HeLa cells, the co-localization curve between Couoxo-LD and commercial lipid droplet dyes overlapped at 0.93. The results indicated that the probe could selectively sense LDs in HeLa cells. Meanwhile, Couoxo-LD can be applied for in vivo imaging of zebrafish.

Fluorescence detection of H 2 S in living organisms is greatly advantageous because it is nondestructive and can be used for in situ analysis. We have constructed a novel rhodamine analogue dye (Rho630) by extending the conjugated system of rhodamine to create a novel cell-trappable H 2 S fluorescent probe Rho630-AM-H 2 S with red light emission. Its application for H 2 S fluorescence detection in living HeLa cells and zebrafish was investigated. As expected, Rho630-AM-H 2 S showed a huge fluorescence turn-on response of about 20-fold at 630 nm and good selectivity toward H 2 S in solution. An MTT assay demonstrated that the probe showed negligible cytotoxicity in the concentrations typically used in fluorescence imaging experiments. Cell imaging experiments revealed that compared with compound 4 without cell-trappable unit modification, Rho630-AM-H 2 S exhibited remarkably enhanced cell penetration ability, as an enormous fluorescence signal increase was observed at the red channel within 5 min after Rho630-AM-H 2 S was incubated with HeLa cells. Finally, the probe Rho630-AM-H 2 S was used to detect H 2 S in living HeLa cells and zebrafish with great fluorescence enhancement in the red channel. Graphical abstract.