Anomalously Large Luminescence Modulation Induced by Trace Lanthanide Impurities in Alloyed Upconversion Nanocrystals

Achieving precise control over emission characteristics, such as wavelength and lifetime, is critical to unlocking the full potential of luminescent nanomaterials for diverse applications. In this work, we present a strategy for fine-tuning the optical properties of upconversion nanocrystals by engineering parts-per-million (ppm)-doping-level lanthanide impurities. We show that even trace impurities (∼10 ppm, fewer than 10 atoms per nanocrystal), which are only a hundredth of the conventionally studied doping levels and were previously considered negligible, serve as efficient energy traps in energy migration-based upconversion processes. By introducing controlled trapping centers via minimal impurity doping, we successfully regulate the upconversion emission colors and lifetimes with high precision. Moreover, we find that high-purity nanocrystals exhibit significantly greater lifetime changes in response to surface interactions, enabling an energy-transfer-based ultrasensitive spectrum and lifetime sensing. This approach facilitates the development of upconversion-based DNA sensors with detection limits over an order of magnitude lower than those of conventional methods, highlighting the potential of these nanocrystals as highly effective nanoprobes for interference-resistant biosensing in complex environments.