Due to the combination of tissue penetrating NIR (980 nm) excitation and anti-Stokes emission derived from multi-photon absorption, upconverting nanoparticles (UCNPs) have potential as novel contrast agents for autofluorescence-free 3D imaging of unsectioned tissue in optical emission computed tomography (optical-ECT). In this work, we discuss the synthesis and characterization of upconverting NaY0.78F4: Yb0.20, Er0.02 nanocrystals, and demonstrate their utility as contrast agents for optical-ECT. These UCNPs have a characteristic emission band centered at 540 nm, which lies in the spectral window in which conventional Si photodiodes offer maximal sensitivity. UCNPs were dispersed in 2.5% agarose samples at concentrations from 1 to 100ug/ml. These samples were then excited over a range of excitation power densities ranging from 0.3-1.39W/cm2 to characterize the UCNP emission under conditions relevant to optical-ECT imaging of real tissue. These NaY0.78F4: Yb0.20, Er0.02 nanocrystals were then imaged in an optically cleared mouse lung. Co-registered 3D transmission and emission images were reconstructed at an isotropic resolution of 25.8 um from the resulting 2D transmission and upconversion emission images. We show that NaY0.78F4: Yb0.20, Er0.02 nanocrystals are retained throughout the tissue clearing process and these compositions provide auto-fluorescent free imaging allowing for background-free reconstruction of the UCNP distribution within the mouse lung. This work demonstrates the use of UCNPs as a contrast agent for optical-ECT and represents the first use of UCNPs for 3D imaging of whole tissue using NIR excitation.

Design and use of upconverting NaYF4:Yb/Er nanocrystals for 3D tissue imaging in optical emission computed tomography

254th National Meeting (2017)

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