Monochromatic Near-infrared to near-infrared upconversion nanoparticles for high-contrast fluorescence imaging

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 I₈₀₀ to I₄₇₀, 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.