Tunable power-dependent upconversion emission of co-doped Er$^{3+}$--Tm$^{3+}$ nanoparticles for bioimaging

Abstract

Lanthanide-doped upconversion nanoparticles (UCNPs) have emerged as promising platforms for bioimaging and theranostics owing to their unique ability to convert near-infrared (NIR) excitation into visible emission. Controlling their emission behavior is critical for both mechanistic understanding and biomedical translation. In this work, we synthesized NaYF\textsubscript{4}: Yb\textsuperscript{3+}, Er\textsuperscript{3+}, Tm\textsuperscript{3+} UCNPs via a hydrothermal route and systematically investigated their power-dependent luminescence. Log--log slope analysis clarified photon participation in different emission bands, revealed competition between Yb\textsuperscript{3+}$\longrightarrow$Er\textsuperscript{3+} and Yb\textsuperscript{3+}$\longrightarrow$Tm\textsuperscript{3+} pathways, and identified saturation effects at higher excitation power. Based on these insights, the nanoparticles were functionalized with silica--TPGS to improve colloidal stability, dispersibility, and biocompatibility. Preliminary biological evaluation with MCF-7 breast cancer cells demonstrated efficient uptake, bright intracellular luminescence, and dose-dependent cytotoxicity (GI$_{50} = 0.26 \pm 0.02$~ppm). These findings highlight excitation-power control as a powerful strategy for tailoring UCNP emission properties, laying the foundation for advanced applications in ratiometric sensing, multicolor imaging, and cancer theranostics.