808 nm Laser Excited Upconverting Hydrophilized Fluid Velocimetric Probe with a Record Sensitivity

Abstract

Upconversion fluid velocimetry, a recently proposed technique, is promising for detecting in vivo blood flow velocity and monitoring microfluid systems due to its high sensitivity to low-flow velocities. However, the velocimetric nanoprobes reported so far remain too prototypical, with significant room for improving velocity sensitivity. Additionally, current nanoprobes are only 980 nm excitable and surface hydrophobic, which limits their practical applications. Herein, 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. Delightfully, 808 nm excitability accompanies the energy migration process, significantly extending the non-steady-state period of the upconversion process. The maximal absolute sensitivity and relative sensitivity reach 0.47 cm-1 s and 6.77% cm-1 s respectively, both of which are the highest records to date. This work will expedite the practical application and expand the availability of upconverting velocimetry techniques.