Successful further development of superhigh-constrast upconversion (UC) bioimaging requires addressing the existing paradox: 980 nm laser light is used to excite upconversion nanoparticles (UCNPs), while 980 nm light has strong optical absorption of water and biological specimens. The overheating caused by 980 nm excitation laser light in UC bioimaging is computationally and experimentally investigated for the first time. A new promising excitation approach for better near-infrared to near-infrared (NIR-to-NIR) UC photoluminescence in vitro or in vivo imaging is proposed employing a cost-effective 915 nm laser. This novel laser excitation method provides drastically less heating of the biological specimen and larger imaging depth in the animals or tissues due to quite low water absorption. Experimentally obtained thermal-graphic maps of the mouse in response to the laser heating are investigated to demonstrate the less heating advantage of the 915 nm laser. Our tissue phantom experiments and simulations verified that the 915 nm laser is superior to the 980 nm laser for deep tissue imaging. A novel and facile strategy for surface functionalization is utilized to render UCNPs hydrophilic, stable, and cell targeting. These as-prepared UCNPs were characterized by TEM, emission spectroscopy, XRD, FTIR, and zeta potential. Specifically targeting UCNPs excited with a 915 nm laser have shown very high contrast UC bioimaging. Highly stable DSPE-mPEG-5000-encapsulated UCNPs were injected into mice to perform in vivo imaging. Imaging and spectroscopy analysis of UC photoluminescence demonstrated that a 915 nm laser can serve as a new promising excitation light for UC animal imaging.
We demonstrate that tridoping with Li+ ions enhances
the visible green and red upconversion (UC) emissions in Er3+/Yb3+-codoped Y2O3 nanocrystals
by up to half of the bulk counterpart, i.e., about 2 orders of magnitude
higher than previous results. X-ray diffraction and decay time investigations
give evidence that tridoping with Li+ ions can tailor the
local crystal field of the Y2O3 host lattice.
Theoretical calculations illustrate well that a significant UC intensity
enhancement arises from the synthesized tailoring effect induced by
the Li+ ions, which increase lifetimes in the intermediate 4I11/2 (Er) and 2F5/2 (Yb)
states, increase optically active sites in the Y2O3 host lattice, and dissociate the Yb3+ and Er3+ ion clusters in the nanocrytals. The general theoretical
description of the visible UC radiations shows that the Yb3+ ion sensitization and the tailoring effect induced by the Li+ ions are two independent enhancement mechanisms, which is
expected to lead to an increasing number of photonic and biomedical
applications in the future.
Upconversion (UC) emission tuning from green to red in monodisperse NaYF(4):Yb(3+)/Ho(3+) nanocrystals was successfully achieved by tridoping with Ce(3+) ions under diode laser excitation of 970 nm. It is proposed that two efficient cross-relaxation processes, 5S2/5F4(Ho) + 2F(5/2)(Ce) --> 5F5(Ho) + 2F(7/2)(Ce) and 5I6(Ho) + 2F(5/2)(Ce) --> 5I7(Ho) + 2F(7/2)(Ce)between Ho(3+) and Ce(3+) ions, have been employed to select UC pathways to tune the UC radiation. Theoretical investigations based on steady-state equations demonstrate the proposed UC mechanisms and explain well the observed linear increase of the UC red-to-green intensity ratio with the increment of Ce(3+) ion concentrations.
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