Engineering the Infrared Luminescence and Photothermal Properties of Double-Shelled Rare-Earth-Doped Nanoparticles for Biomedical Applications

The nascent field of theranostics, which couples targeted therapy with diagnostics, has catalyzed efforts toward improved nanoprobe designs that facilitate both localized treatment and diagnostic imaging. Rare-earth-doped nanoparticles (RENPs) have emerged as a leading candidate for theranostics because of their versatile synthesis and modification chemistries, photostability, and relative safety. Furthermore, their bright, tunable fluorescence using near-infrared (NIR) excitation enables multispectral imaging with high signal-to-background ratios. In this work, we have synthesized double-shelled RENPs with tunable properties for optimal fluorescent imaging, photoacoustic imaging, and photothermal therapy. The properties of the double-shelled RENPs were tailored by controlling the density of rare-earth ions (i.e., activator or sensitizer) by using either a functional amorphous organic or a crystalline outermost shell. This study systematically analyzes the effects of the functional organic or inorganic outermost shell on the imaging and photothermal conversion properties of our RENPs. Despite the weaker infrared absorption enhancement, the functional organic outermost shell impregnated with a low density of rare-earth ions led to minimal reduction of fluorescence emissions. In contrast, the higher density of rare-earth ions in the inorganic shell led to higher infrared absorption and consequently significant reduction in emissions arising from the undesired optical attenuation. Inorganic shell thickness was therefore modified to reduce the deleterious attenuation, leading to brighter emissions that also enabled the in vitro SWIR detection of ∼2500 cells/cluster. Using the enhanced infrared properties that arise from this functional inorganic layer, which could be engineered to respond to either NIR or SWIR, we demonstrated that (1) bright SWIR emissions allowed detection of small cell clusters; (2) strong PA signals allowed clear visualization of particle distribution within tumors; and (3) strong photothermal effects resulted in localized elevated temperatures. Collectively, these results highlight the utility of these double-shelled RENPs as theranostic agents that are compatible with both photoacoustic or fluorescent imaging platforms.