Engineering upconverting core–shell nano-probe for spectral responsive fluid velocimetry

Particle velocimetry based on the temporal feature of upconversion luminescent nanocrystals is a newly-raising fluid velocimetry. Exploiting the availability to low flow rate fluid and exempting redundance external calibration (achieving once calibration for all) are highly expected and challenging. Herein, an engineered core–shell nano-probe, NaYF₄:Yb/Ho/Ce@NaGdF₄, was proposed, in which the Ce³⁺ ions were utilized to manipulate the upconversion dynamic of Ho³⁺. Through optimization, a superior sensitive against low-speed flow is achieved, and the external calibrations before each operation can be avoided. Application demonstrations were conducted on a fluid circulation system with controllable flow rate. The fluid velocity was monitored successfully, no matter it is permanent, or cyclically variating (imitating the in vivo arterial blood). Moreover, this velocimetric route is competent in spatial scanning for handling the spatially inhomogeneous velocity field. Such sensing nanomaterial and fluid velocimetric method exhibit promising application potential in human blood velocimetry, industrial control, or environmental monitoring.