Precision tuning of rare-earth-doped upconversion nanoparticles via droplet-based microfluidic screening

Rare-earth-doped upconversion nanoparticles (UCNPs) show great promise in a range of applications, including biological imaging and sensing, solar cells and security inks. Although their emission color can be tuned widely as a function of host matrix and dopant composition, the high dimensionality of the associated parameter space and the sensitivity of emission to these parameters make optimization and precision tuning difficult. Herein, we present a new time-efficient synthetic route to NaYF4:Yb,Er,Tm UCNPs and a high-throughput microfluidic reactor to synthesize and precisely tune the emission characteristics of the particles in situ and in real time. We synthesize a range of particles with optimized emission intensity and wide color distribution by changing the doping degree of sensitizer Yb to provide for green-orange tunability, and the ratio of Tm–Er to give green–blue tunability. With the two tunable dimensions, we realize true white light emitting UCNPs based on optimized red, green and blue (RGB) emission ratios from a single composition NaYF4:Yb,Er,Tm nanocrystal—a demanding task for such materials—with CIE 1931 coordinates of (0.29, 0.34) and doping degrees of 60% Yb, 0.45% Er, and 1.05% Tm. Finally, we demonstrate the efficacy of these materials in a thin film format through the fabrication of an anti-counterfeit device.