Abstract
Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting multiple near-infrared (NIR) photons into shorter-wavelength one by utilizing real long-lived, ladder-like energy levels of lanthanide ions. Such unique anti-Stokes emissions are immune to the auto-fluorescence background interference and derive from moderate irradiation compared to conventional multi-photon fluorescence, ideally suitable for imaging applications, coupled with improved imaging depth by NIR excitation light, excellent photostability, multicolor sharp-band emissions and tunable long emission lifetimes. However, the critical concentration quenching on upconversion luminescence generally observed in UCNPs sets barriers to obtain bright high doping UCNPs at low excitation power density, which hampers this promising luminescence probe applying to long-term live cell imaging. We explore the mechanism of Tm3+ concentration quenching from ensemble and single-particle levels to design the new generation sub-25-nm ultra-bright highly Tm3+-doped UCNPs for high-contrast imaging at limited excitation power density condition. This achievement provides a new strategy for developing small-sized bright highly activator-doped UCNPs and will broaden the applications of UCNPs in microscopic imaging.
Highlights
Optical upconversion uses two or more NIR photons to generate emissions at shorter wavelength in a multiple-step excitation process [1]
Optical measurements were done at room temperature with 100 ng/mL upconversion nanoparticles (UCNPs) dispersed in hexane that were spin-coated onto a coverslip and a fiber coupled laser diode with tunable output power at 980 nm was used as the pump source
The diffraction peaks are indexed to the standard data of hexagonal phase NaYF4 (Figure 1d), which is consistent with the shape of nanoparticles in Transmission electron microscopic (TEM) image
Summary
Optical upconversion uses two or more NIR photons to generate emissions at shorter wavelength in a multiple-step excitation process [1]. This luminescence facilitates background-free imaging in deep tissue and many other applications, such as optical sensing, photovoltaics and anti-counterfeiting [2,3]. The weak UCL caused by concentration quenching of highly lanthanide activator ion doping remains one major thorny problem in the application of UCNPs as single-molecule bio-imaging probes, especially for frequently-used Tm3+ [4]. We elaborately devised a core-shell-shell-shell structure UCNPs, NaYbF4@NaYF4:x% Tm3+@NaYbF4@NaYF4, highly sensitized by Yb3+ and realized that the optimal doping concentration of Tm3+ boosts to 70 mol% at 100 kW/cm of 980 nm excitation. Nanomaterials 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/nanomaterials
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