Sandwich-structured upconversion nanoparticles with tunable color for multiplexed cell labeling

The need for a more efficient biological label to meet their burgeoning utility in rapidly developing multiplexing applications may be realized through the recent advent of upconversion nanoparticles (UCNs). UCNs fabricated to-date, however, are either not displaying strong fluorescence or have limited available colors. Here, we report on fabricating sandwich-structured UCNs with a NaYbF4 matrix sandwiched between two NaYF4 layers. Such sandwich design allows for efficient absorption of the excitation energy by the absorber ion-rich NaYbF4 layer that then transfers it to the adjacent NaYF4 layers on either side for an improved fluorescence efficiency. By doping different emitters into each of the shells and adjusting their thickness, different color output tunable based on the RGB color model were obtained. In this study, multicolor UCNs with strong emission intensity have been facilely synthesized and used for multiplex detection of three subcellular targets with a single near-infrared excitation wavelength.