Intraluminal Gold Nanoparticle-Loaded Liposomes Enable High-Resolution Nanoscale Mapping of Intracellular Delivery Pathways.

Understanding how lipid nanocarriers behave inside cells is key to improving nucleic acid and protein therapies. However, direct visualization of intracellular trafficking mechanisms is challenging because liposomes possess low intrinsic electron density and highly dynamic and flexible structural features. In this study, we present Lipo-Gold, a hybrid nanosystem consisting of clinically relevant liposomes containing multiple intraluminal small gold nanoparticles (AuNPs) and demonstrate its utility for nanoscale-resolution investigation of intracellular delivery pathways. Using an optimized adaptation of a stepwise in situ reduction strategy, specifically tailored to a cholesterol-containing lipid formulation, we generated multiple nonspace-filling AuNPs (15-20 nm) within each vesicle without altering the bilayer structure, surface charge, or colloidal stability, and overall vesicle architecture. Comprehensive physicochemical characterization, including dynamic light scattering, nanoparticle tracking analysis, and transmission electron microscopy (TEM), along with cellular uptake investigations by flow cytometry and confocal microscopy, demonstrates that Lipo-Gold retains the same biological identity and cellular interaction profile as the corresponding unmodified liposomes. In HeLa cells, Lipo-Gold also exhibits similar uptake kinetics and intracellular trafficking behavior comparable to unloaded vesicles. The intraluminal AuNPs generate strong electron contrast, enabling direct visualization of intracellular nanoscale transport events, including endocytic uptake, vesicle maturation, and subcellular confinement. Correlative light-electron microscopy (CLEM) further enabled spatial overlap between fluorescent liposome signals and electron-dense AuNP clusters, providing a multimodal imaging platform with nanometric structural resolution. Across all examined sections, AuNPs remained confined to membrane-bound endosomal compartments, with no evidence of cytosolic dispersion, consistent with the expected behavior of anionic, nonfusogenic liposomes. By combining fluorescence tracking with high-resolution structural imaging while preserving native liposome-cell interactions, Lipo-Gold offers a valuable tool for investigating intracellular delivery barriers and for guiding the rational development of next-generation lipid-based therapeutics.