Tm³⁺ heavily doped NIR-III bioprobe with 1 µm Stokes shift towards deep-tissue 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.