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Real-time color-tunable upconversion luminescence of lanthanide ions has recently attracted increasing attention. To date, at least two different excitation wavelengths are required to obtain tunable upconversion colors containing the three-primary-color components (Red-Green-Blue). In this work, for the first time, we demonstrate that it is possible achieving tunable three-primary-color upconversion luminescence using a single excitation wavelength, on the basis of the photon-order dependent uponversion nature. A core-shell-shell nanocrystal was synthesized, with rational designed compositions of Er/Yb and Tm/Yb in the core and the outermost shell, respectively, responsible for the green/red and blue emissions. By increasing the power density of the 980 nm continuous-wave excitation laser, the color of the emitted luminescence of the core-shell-shell nanocrystal evolved as green → red → blue, corresponding to 2 → 3 → 4-photons involved in the upconversion process.

Biological probes facilitate optical imaging and disease diagnosis and treatment. However, the large absorption and scattering loss in the tissue highly limit the depth during the application. In the present research, an NIR-I bioprobing system, which utilizes the Bessel beam to excite heavily doping nanocrystals, has been developed for deep tissue applications. On the one hand, the capillary mode selection method generates the Bessel excitation beam, lowering the excitation energy loss. On the other hand, a strong energy harvest of NaYbF4:90%Nd nanocrystals enables effective fluorescence and heat generation upon 800 nm excitation. By considering the advantages of Bessel excitation and heavily doping nanocrystals, up to ∼3 cm penetration depth for ex vivo bioimaging and the potential self-monitored photothermal treatment are demonstrated. The resultant bioprobing system allows deep tissue imaging and photothermal therapy, showcasing broad prospects in medical research and clinical applications.

Recently developed rare earth ions doped NIR-NIR bioprobes, with excitation and emission both falling in the NIR region, have attracted wide attention due to their deep-tissue penetration, high signal-to-noise ratio, and high imaging resolution. The current NIR-NIR bioprobes focused mainly on the 1.5 µm emission of Er3+. Here, we developed a novel bioprobe utilizing the 1.8 µm emission of Tm3+ upon 0.8 µm excitation. After an inert shell effectively suppressing the surface quenching effect, the strong cross relaxation 3H4 + 3H6 → 3F4 + 3F4 between heavily doped Tm3+ greatly improves the luminescence intensity at 1.8 µm. As a result, the formed Tm3+ based NIR-III bioprobe exhibits better penetration ability of the state-of-the-art Er3+ based NIR-III bioprobe, and holding an even larger Stokes shift beneficial for the multiplexed bioimaging and labeling applications.