Size-dependent downconversion near-infrared emission of NaYF4:Yb3+,Er3+ nanoparticles
Near-infrared-downconversion-near-infrared (NIR-DC-NIR) bioimaging based on lanthanide doped upconversion nanoparticles (UCNPs) has received much attention due to its deeper penetration, and higher contrast imaging and signal-to-noise ratio in biological tissues. The size of UCNPs determines the mechanism and rate of cell uptake of the nanoparticles and their ability to permeate through biological tissues. Herein, we experimentally and theoretically demonstrate downconversion-near-infrared (DC-NIR) emission behavior in different sized UCNPs ranging from 5–150 nm. Interestingly, 15–40 nm UCNPs have more effective DC-NIR emissions than 150 nm UCNPs and an extremely high excitation threshold, which is entirely different from the size-dependent upconversion-visible (UC-VIS) emissions usually observed in UCNPs. We also observed that the intensity ratio of the DC-NIR emission to the UC-VIS emission decreases with the increase of the particle size and the excitation power, attributed to the more efficient upconversion (UC) process. Finally, we further confirmed that the competition process between the UC population and non-radiative relaxation to the DC-NIR level plays a key role in size-independent DC-NIR emissions. Our discovery would provide guidance for optimizing and designing NIR-DC-NIR NPs for bioimaging applications.