NaBiF$_4$: Er$^{3+}$, Yb$^{3+}$ upconversion particle as a multi-functional bio-marker

Lanthanide-doped upconversion particles (UCPs) have revolutionized optical bioimaging platforms because of their excellent photostability, non-toxicity, and utilization of near-infrared excitation, which facilitates deep tissue penetration with negligible autofluorescence. 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 bismuth-doped UCPs address some of these challenges. Compared to the photoluminescence (PL) emission of the regular Yttrium doped UCPs, we observe a three-fold increment in the quantum yield of the overall emission of bismuth-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. Interestingly, 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 process involving three energy levels. In addition, the particle has a thickness under sub-diffraction, shows optical trapping ability, and potential of surface functionalization to enable specific conjugation with diverse biospecimens. These studies establish bismuth doped UCPs as an excellent candidate in accomplishing advanced biomarker operating with enhanced signal-to-noise ratio and sub-diffraction imaging capabilities.