Lipid droplets (LDs) are intracellular lipid-rich organelles that regulate the storage of neutral lipids and were recently found to be involved in many physiological processes, metabolic disorders as well as diseases including obesity, diabetes and cancers. Herein we present a family of new fluorogenic merocyanine fluorophores based on an indolenine moiety and a dioxaborine barbiturate derivative. These so-called StatoMerocyanines (SMCy) span their fluorescence from yellow to the near infrared (NIR) in oil with an impressive fluorescence enhancement compared to aqueous media. Additionally, SMCy display remarkably high molar extinction coefficients (up to 390,000 M-1.cm-1) and high quantum yield values (up to 100%). All the members of this new family specifically stain the LDs in live cells with very low background noise. Unlike Nile Red, a well-known lipid droplet marker, SMCy dyes possess narrow absorption and emission bands in the visible thus allowing multicolor imaging. SMCy proved to be compatible with fixation and led to high quality 3D-images of lipid droplets in cells and tissues. Their high brightness allowed efficient tissue imaging of adipocytes and circulating LDs. Moreover their remarkably high two-photon absorption cross-section, especially SMCy5.5 (up to 13,300 GM) as well as their capacity to efficiently fluoresce in the NIR region led to two-photon multicolor tissue imaging (liver). Taking advantage of the available color palette, lipid droplets exchange between cells was tracked and imaged thus demonstrating intercellular communication.

Lipid droplets (LDs) are organelles that serve as the storage of intracellular neutral lipids. LDs regulate many physiological processes. They recently attracted attention after extensive studies showed their involvement in metabolic disorders and diseases such as obesity, diabetes, and cancer. Therefore, it is of the highest importance to have reliable imaging tools. In this review, we focus on recent advances in the development of selective fluorescent probes for LDs. Their photophysical properties are described, and their advantages and drawbacks in fluorescence imaging are discussed. At last, we review the reported applications using these probes including two-photon excitation, in vivo and tissue imaging, as well as LDs tracking.