Near-IR absorbing Upconverting Nanocrystals (UCNCs) have garnered interest as in vitro and in vivo Vis/NIR imaging agents. Efficient upconversion to higher energy UV emissive states, however, has not been reported in UCNCs of a biologically relevant 10 nm size regime, thereby limiting the potential for NIR-sensitized nanotherapeutic drug activation. In this work, 10 nm, cubic phase NaYbF4, with a 1.2% Thulium codopant, is shown to exhibit dominant 349 nm UV emission at 20 W/cm2 laser density, exciting with a 980 nm CW laser. UV fluorescence is further enhanced by ~2-fold with the inclusion of a Gallium codopant. Power dependence studies detecting fluorescence in the UV-Vis-NIR regime elucidate the evolution of the upconversion mechanism through Tm3+-Tm3+ cross relaxation pathways increasing from low (0.5 W/cm2) to high laser fluence, achieving saturation at 60 W/cm2. X-ray photoelectron spectroscopic investigations suggest tuning of the local crystal field environment surrounding the Yb3+ sensitizer ion in the presence of the spectroscopically innocent Ga3+ codopant. In kind, Rietveld refinement of high resolution X-ray diffraction data shows a constriction of unit cell geometry maximized at an optimal 15% Ga3+ dopant concentration, resulting in the greatest UV emission enhancement. Shelling of these NaYbF4 (1.2% Tm/15% Ga) cores with 2 nm thick NaYF4 affords decreased fluorescence quenching at the lanthanide-rich nanocrystal surface and further enhance UV output, thereby generating a sub-15 nm UCNC capable of efficient NIR-to-UV upconversion.?

Enhancement of dominant UV emission in 10 nm gallium-codoped, upconverting nanocrystals

246th National Meeting (2013)

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