Upconversion nanoparticles (UCNPs) have long been advertised as the nanocrystals that can do everything: bright, photostable, near-infrared excitable, and seemingly designed to solve all problems in bioimaging and nanomedicine. But anyone who has ever tried to take UCNPs from a flask to a functioning biological environment quickly learns that the real story starts - and often ends - at the surface. In this talk, I will guide you through a range of surface modification strategies we have explored, from ligand exchange to silica shells, from amphiphilic polymers to biomolecule conjugation. For each approach, I will highlight not only the shiny advantages but also the hidden traps, dead ends, and occasional heartbreaks that appear on the road to application.
The central focus will be on the different surface modification strategies themselves, what they enable, where they fail, and how they shape the interaction of UCNPs with biological environments. Over the years, we have learned that each step, from ligand exchange to covalent conjugation, brings its own set of challenges that only become apparent in practice. Careful characterization is a boring task, but it is the necessary compass. Without knowing what you really have in your hands, every subsequent experiment becomes increasingly uncontrollable and ends up in a kind of scientific lottery with poor odds and no reproducibility. By sharing our journey through successes, missteps, and occasional surprises, I aim to sketch a realistic picture of how UCNP surface chemistry can be tamed on the way to meaningful applications at biomembranes.
