 Abstract solid lipid nanoparticles, SLNs, are a promising lipid-based drug delivery system for the development of novel therapeutics. However, their characterization remains challenging due to the lack of suitable techniques. In this study, we introduce a new method based on stimulated Raman scattering, SRS, microscopy to image SLNs in vitro and in vivo. We demonstrate the feasibility of our method by imaging deuterated SLNs, which are bio-orthogonally labeled with a fluorescent dye. With this approach, we achieve ultra-high single-particle sensitivity, enabling us to count individual particles in vitro and in vivo. Furthermore, our method provides chemical specificity, allowing us to distinguish between different types of SLNs. Finally, our results show that our method has the potential to become a powerful tool for studying the mechanisms behind SLNs performance in drug delivery. This article was authored by Shachuling, XIN Gao, Mian Wei, and others. We are article.tv, links in the description below.