A facile synthesis of NaYF4:Yb3+/Er3+ nanoparticles with tunable multicolor upconversion luminescence properties for cell imaging

The ability to manipulate the upconversion luminescence of lanthanide-ion doped fluoride upconversion nanoparticles (UCNPs) is critical and highly desired due to their wide applications in color displays, multiplexing bioassays and multicolor imaging. Here, we developed a facile strategy for simultaneously tuning color output and enhancing upconversion emission of Yb3+/Er3+ doped fluoride UCNPs, based on adjusting the crystalline phases and particle size. With the transformation of crystalline phases from α, a transition state of α mixed β, and finally to β phase, the upconversion emission color output changed from red, yellow, and finally turned green. That is, the UCNPs' emission is sensitive to the crystallite size and phase. Moreover, the fluorescence intensity of the obtained UCNPs became stronger with the crystalline phase changing from a to β. Three kinds of UCNPs including α-UCNPs, α mixed β-UCNPs, and β-UCNPs were used for cell imaging, while the transition state phase α mixed β-UCNPs was the better candidate.