Multiphoton imaging techniques that convert low energy excitation to higher energy emission are widely used to improve signal over background, reduce scatter in subsurface imaging, and limit sample photodamage. Multiphoton imaging relies on luminescent probes able to efficiently sum the energies of 2 or more incident photons, as well as lasers powerful enough for multiphoton excitation. Lanthanide-doped upconverting nanoparticles have proven to be among the most efficient multiphoton probes, but even UCNPs with optimized lanthanide dopant levels require laser intensities that may be problematic for living systems. Here,1 we develop protein-sized, alloyed UCNPs (aUCNPs) that can be imaged at the single particle level at laser intensities below 300 W/cm2, over 300-fold lower than needed for comparably-sized doped UCNPs. Using single UCNP characterization and kinetic models of lanthanide energy transfer, we find that addition of inert epitaxial shells radically changes optimal lanthanide content from Yb3+, Er3+-doped NaYF4 nanocrystals to fully alloyed compositions. At high levels of the emitter Er3+, these ions can adopt a second role to enhance the effective aUCNP absorption cross-section by desaturating sensitizer Yb3+ or by absorbing photons directly. Core/shell aUCNPs are brighter than comparably sized doped UCNPs at all laser intensities tested, over 4 orders of magnitude. Core/shell aUCNPs 12 nm in total diameter can be imaged with strong contrast through several millimeters of tissue in live mice using a laser intensity of just 0.1 W/cm2, less than half of that of a standard laser pointer. aUCNPs open up the possibility of using both low irradiance and low-energy excitation wavelengths for non-destructive bioimaging experiments. Additional recent work about dye-sensitized UCNP emission2 and 5-micron-sized UCNP lasers3 will also be discussed.

Alloyed upconverting nanoparticles for multiphoton imaging and lasing at ultralow powers

2019 Spring National Meeting

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