Abstract
.Rare-earth-doped nanoparticles are one of the emerging probes for bioimaging due to their visible-to-near-infrared (NIR) upconversion emission via sequential single-photon absorption at NIR wavelengths. The NIR-excited upconversion property and high photostability make this probe appealing for deep tissue imaging. So far, upconversion nanoparticles include ytterbium ions () codoped with other rare earth ions, such as erbium () and thulium (). In these types of upconversion nanoparticles, through energy transfer from excited with continuous wave light at a wavelength of 980 nm, upconversion emission of the other rare earth dopants is induced. We have found that the use of the excitation of in the 1550-nm wavelength region allows us to perform deep tissue imaging with reduced degradation of spatial resolution. In this excitation–emission process, three and four photons of 1550-nm light are sequentially absorbed, and emits photons in the 550- and 660-nm wavelength regions. We demonstrate that, compared with the case using 980-nm wavelength excitation, the use of 1550-nm light enables us to moderate degradation of spatial resolution in deep tissue imaging due to the lower light scattering coefficient compared with 980-nm light. We also demonstrate that live cell imaging is feasible with this 1550 nm excitation.
Highlights
Fluorescence/luminescence imaging techniques play a key role in life sciences since they allow us to visualize structures and molecules in biological specimens with high spatial resolution and specificity
We report that using the excitation of Er3þdoped nanoparticles in the 1550-nm wavelength region allows us to perform deep tissue imaging with reduced degradation of spatial resolution
We demonstrated 1550-nm excitation of erbium (Er3þ)-doped nanoparticles for upconversion luminescence bioimaging for the first time
Summary
Fluorescence/luminescence imaging techniques play a key role in life sciences since they allow us to visualize structures and molecules in biological specimens with high spatial resolution and specificity. The combination of the high penetration depth of NIR light and the localization of the multiphoton excitation volume in the laser focus offers deep tissue imaging capability with cellular-level spatial resolution, optical sectioning capability, and high image contrast Another major approach for NIR deep tissue imaging is to use probes to bring about fluorescent/luminescent emission under single-photon excitation by NIR light. Recent advances in the development of NIR fluorescent/luminescent probes allow NIR imaging to be performed by using simple conventional fluorescence microscopes equipped with a continuous wave (CW) NIR laser or lamp sources Among these NIR probes, RE-doped nanoparticles, triplet– triplet annihilation nanoparticles, and so on have the feature of converting NIR photons to shorter wavelength photons.[7,8,9,10] The use of upconversion nanoparticles (UCNPs) as probes for imaging allows us to significantly reduce autofluorescence background from the samples and optics, such as the case of multiphoton excited fluorescence microscopy. To compare luminescence images of the same nanoparticles under 980 and 1550 nm excitation, we used NaYF4∶Er, Yb nanoparticles with diameters of 10 to 60 nm (74344, Sigma Aldrich), which can be excited at both wavelengths
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