To develop a fluorescent bioprobe for high-contrast deep tissue
fluorescence imaging, monochromatic 800 nm upconversion emissions
based on NaYF4: Yb3+, Tm3+ upconversion
nanoparticles are investigated. The ratio of I
800 to I
470, which is used to describe
the monochromaticity, showing exponential growth with the increase
of Tm3+ doping concentration in NaYF4: Yb3+, Tm3+ nanoparticles, can reach as high as 757
at 4% Tm3+. At such a doping level, the absolute quantum
efficiency can reach 3.9 × 10–3 as measured
by a calibrated integrating sphere, which is sufficient for fluorescence
imaging. High-contrast fluorescence phantom imaging was obtained by
adjusting monochromaticity of 800 nm upconversion emission under the
excitation of a 980 nm diode laser.
A special core/shell/shell structured nanoparticle NaYF4/NaYF4:Yb3+,Er3+/NaYF4 was designed and prepared to accurately investigate the effect of surroundings on rare earth centers.
The excited-state dynamics of widely used luminescence centers Yb3+ and Er3+ embedded in special core–shell–shell (CSS) structured NaYF4/NaYF4:Yb3+,Er3+/NaYF4 nanoparticles (NPs) are investigated. The CSS multi-layer structure is confirmed by transmission electron microscopy and the nonradiative relaxation coefficient (W) is investigated by time-resolved spectroscopy. The effect of surroundings on W for certain energy levels (Er3+: 2H11/2/4S3/2 and Yb3+: 2F5/2) is identified and W decreases exponentially upon increasing the shielding shell thickness of the prepared CSS NPs. In addition, it has been confirmed that the luminescence centers are nearly immune to surrounding quenching when the shielding shell thickness is boosted to ca. 4 nm.
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