The podand-type ligand L, based on a tertiary amine substituted by three pyridyl-6-phosphonic acid functions, forms hydrated complexes with Ln³⁺ cations. The luminescence properties of the YbL complex were studied in D₂O as a function of the pD and temperature. In basic conditions, increases in the luminescence quantum yield and the excited state lifetime of the Yb centered emission associated with the ²F_5/2 → ²F_7/2 transition were observed and attributed to a change in the hydration number from two water molecules in the first coordination sphere of Yb at acidic pH to a single one in basic conditions. Upon the addition of TbCl₃ salts to a solution containing the YbL complex in D₂O, heteropolynuclear Yb/Tb species formed, and excitation of the Yb at 980 nm resulted in the observation of the typical visible emission of Tb as a result of a cooperative upconversion (UC) photosensitization process. The UC was further evidenced by the quadratic dependence of the UC emission as a function of the laser power density.

Single-particle tracking (SPT) is a powerful technique to elucidate the behavior of biomolecules inside or on the surface of living cells in real time. SPT is based on labeling the component of interest with a phosphor or a light-scattering gold nanoparticle, and visualizing its motion by real time microscopy. However, SPT generally suffers from suboptimal performance of phosphors. For example, fluorescent dyes have poor photostability, while quantum dots suffer from sporadic blinking that hampers track acquisition and interpretation. Light-scattering gold nanoparticles require custom experimental setups and relatively large particle size for tracking purposes. In recent years, upconverting nanoparticles (UCNPs) have emerged as a promising phosphor that enables backgroundless anti-Stokes luminescence imaging. They are also attractive for SPT applications due to their extreme photostability and non-blinking luminescence. Previously, SPT has been used to monitor UCNP endocytosis in cells. However, targeted SPT with UCNPs that specifically bind to biomolecules of interest has not yet been achieved. In this work, we provided a first proof-of-concept experiment of targeted SPT with UCNPs and showed the superior performance of UCNPs compared to organic dyes and quantum dots. As a benchmark system to assess the performance of UCNPs, we tracked FcεRI receptors on the surface of RBL-2H3 cell membranes using UCNP-IgE conjugates and obtained good agreement with data from the literature. Importantly, the versatility of the biotin-streptavidin architecture of our system allows UCNPs to be used to track a wide range of targets. Overall, this report lays the foundation for UCNP applications in targeted SPT.

The physiological role of lipid asymmetry in intracellular membranes remains poorly understood. Here, we show that sphingomyelin (SM), typically confined to the lumen of the trans-Golgi network (TGN), is exposed on its cytoplasmic surface by the action of the Golgi-associated protein, Golgi-associated gamma-adaptin ear–containing adenosine 5′-diphosphate–ribosylation factor–binding protein 1 (GGA1). This exposure is driven by the GGA1 GAT domain, which induces lipid scrambling in a manner dependent on membrane curvature and cholesterol. SM exposure coincides with the exit of mannose 6-phosphate receptors from the TGN, a process essential for lysosomal enzyme trafficking. Furthermore, SM is transferred to autophagic membranes, where it facilitates autophagosome-lysosome fusion. These findings reveal a previously unrecognized role for lipid remodeling in membrane trafficking and autophagy.