NaBiF₄:Er³⁺,Yb³⁺ upconversion particle as a multi-functional probe

Lanthanide-doped upconversion particles (UCPs) have revolutionized optical bioimaging platforms because of their excellent photostability, non-toxicity, and utilization of near-infrared excitation. However, it remains a challenge to achieve high-contrast and sub-diffraction imaging in noisy biological media, without using a high-power laser. Here, we report various protocols applied to NaBiF₄:Er³⁺,Yb³⁺ UCPs to address some of these challenges. Compared to the photoluminescence (PL) emission of the regular NaYF₄:Er³⁺,Yb³⁺ UCPs, we observe a three-fold increment in the relative PL intensity of the overall emission of NaBiF₄:Er³⁺,Yb³⁺ UCPs, and a four-fold increment, specifically, in red emission. Leveraging this advantage, we devise a protocol employing two infrared wavelengths, 975 nm and 1064 nm, to selectively control the PL emission. It is here that we mention that the 1064 nm is off-resonance of the UCP, but is enabled by the broadband absorption of the material. Our results reveal two distinct regimes in which PL can be systematically quenched or enhanced, by adjusting the 975 nm laser power. We model the overall dynamics as a simplified stimulated emission depletion like process involving three energy levels. In addition, the particle has a thickness under sub-diffraction, shows optical trapping ability, and has potential for surface functionalization to enable specific conjugation with diverse biospecimens.