1 Department of Chemistry and Biochemistry & Centre for Nanoscience Research, Concordia University, 7141 Sherbrooke W., Montreal, H4B 1R6, Canada.

Evaluation of particle dynamics at the nano- and microscale poses a challenge to the development of novel velocimetry techniques. Established optical methods implement external or internal calibrations of the emission profiles by varying the particle velocity and are limited to specific experimental conditions. The proposed multiemission particle velocimetry approach aims to introduce a new concept for a luminescent probe, which guarantees accurate velocity measurements at the microscale, independent of the particle concentration or experimental setup, and without need for calibration. The simplicity of these analyses relies on the intrinsic luminescence dynamics of core-shell upconverting nanoparticles. Upon excitation with a focused near-infrared pulsed laser, the nanoparticle emits photons at different wavelengths. The time interval between emissions from different excited states is independent of the local environment or particle velocity. The velocity of the particles is calculated by measuring the distance between the maxima of two different emissions and dividing it by the known difference in luminescence lifetimes. This method is demonstrated using simple digital imaging of nanoparticles flowing in 75-150 µm diameter capillaries. Using this novel approach typically results in a relative standard deviation of the experimental velocities of 5% or lower without any calibration.