808 nm laser excited upconverting hydrophilized fluid velocimetric probe with a record sensitivity

Abstract

Upconversion fluid velocimetry, an emerging technique, has great potential for detecting in vivo blood flow velocity and monitoring microfluid systems, due to its high sensitivity to low-flow velocities. However, existing velocimetric nanoprobes remain prototypical, with significant room for improving the velocity sensitivity. Additionally, most current nanoprobes are limited by 980 nm excitation and hydrophobic surfaces, which limits their practical applications. Here, we propose an elaborate design of core-multi-shell fluoride nanocrystals (NaYF₄:30Gd/20Yb/2Ho/10Ce@NaYF₄:10Yb@NaNdF₄:20Y/20Yb@NaYF₄) and further conduct surface hydrophilization. Satisfyingly, 808 nm excitability accompanies the energy migration process, significantly prolonging the non-steady-state period of the upconversion process. The maximum absolute sensitivity and relative sensitivity reach 0.47 cm⁻¹ s and 6.77% cm⁻¹ s, respectively, both of which are the highest values recorded to date. This work will expedite the practical application and expand the availability of upconversion velocimetry techniques.