A continuous synthesis method for the less than 10 nm core–shell upconversion nanoparticles was developed via coiled tube embedded flask reactors and a flow solvothermal co-participation reaction up to 300 ℃. Fast nucleation of hexagonal nanocrystals in less than 9 min residence time was achieved owing to the excellent heating ability of the reactors, and a two-step reaction strategy was created for the synthesis of β-NaYF₄: Gd, Yb, Er/Ho/Tm@NaYF₄ particles without intermediate purification.

As an important upconversion illuminant material, LiREF₄ nanocrystals were efficiently synthesized in a continuous reactor with the assistant of a new precursor solution. The employed trioctylamine solvent in the solution had a strong interaction with HF, and helped to avoid the generation of unnecessary solid components as LiF and NH₄REF₄ during the reaction. A silicon carbide reactor was developed to carry out the synthesis reaction, where LiYF₄:Yb,Er/Ho/Tm quickly nucleated in 5 min at 300 °C. The reaction time to successfully prepare 15 nm sized nanocrystals was less than 30 min, and the space-time yield of the flow synthesis method was 14.8 times that of a control group batch reaction. The prepared nanocrystals had a strong illuminant ability, which could find its use in the area of security mark printing.