Broad-range flow velocimetry enabled by pulse-width-dependent luminescence of core-multishell upconversion nanoprobes

Lanthanide-doped upconversion nanoparticles (UCNPs) have garnered extensive attention in fundamental research and cutting-edge applications due to their unique optical properties. Particularly in sensing, UCNP-based fluorescent probes provide a versatile platform for microfluidic flow velocity calibration. However, designing nanoprobes with efficient luminescence modulation for broad-range flow velocimetry remains challenging. Herein, we engineered a core-multishell UCNP probe: NaGdF₄:Tm³⁺/Yb³⁺@NaGdF₄@NaGdF₄:Eu³⁺@NaYF₄, in which spatially isolated Tm³⁺ (blue) and Eu³⁺ (red) activators enable dual emissions. The intensity ratio between these channels exhibits a laser pulse-width-dependent behavior, enabling real-time dynamic optical modulation. Leveraging this mechanism, we showed fluid velocity assessment by dispersing nanoprobes in a fluid stream under fixed laser excitation. The flowing medium underwent flow-velocity-dependent effective excitation pulse width variations, establishing a quantitative emission ratio-velocity mapping for precise calibration. This paradigm advances flow velocimetry technology while significantly broadening the measurable velocity range via energy migration-mediated kinetics. This sensing paradigm not only advances fluid velocimetry techniques but also expands the multifunctional utility of core-multishell UCNPs in emerging photonic technologies.