LAUSR

Recently, fluorescence imaging has been extensively studied and used in life sciences and medical research due to its fast feedback, high sensitivity, and high spatiotemporal resolution.[1–4] In addition, it does not require hazard ionizing radiation, as compared to routine imaging modalities, such as computed tomography (CT),[5–8] positron emission tomography (PET),[7–9] and single-photon emission computed tomography (SPECT).[6,8,10] It is also known as a noninvasive technique, enabling the real-time acquisition of wide-field images. Despite many advantages, key issues in optical imaging lie in the high absorption and scattering of biological tissues as well as high autofluorescence (Figure 1A), leading to low tissue penetration depth and low signal-to-noise ratio (SNR). This is especially evident in the traditional visible range (VIS, 400–700 nm),[1,11,12] which is only feasible for superficial tissues imaging with penetration depth at ≈1 mm. Subsequently, imaging in the first near-infrared window (NIR-I, 700–900 nm),[11,13] traditionally defined as biological transparency window, shows an improved penetration depth with up to several millimeters in biological imaging due to the reduced absorption and scattering coefficient (Figure 1B,C), as photon scattering in biological tissue scaling with λ−ω (λ denotes the wavelength, ω = 0.22–1.68 depending on tissues).[11] Although fundamental researches Fluorescence-based imaging in the second near infrared window (NIR-II, 1000–1700 nm) is extensively used in both fundamental scientific research and clinical practice, owing to its advances of high sensitivity and high spatiotemporal resolution with increasing tissue penetration depths. Among several NIR-II fluorophores, recent accomplishments in biocompatible lanthanidebased luminescent nanomaterials have aroused great interest of researchers. This progress report summarizes recent progress in controlled synthesis of lanthanide-based NIR-II nanomaterials and their state-of-the-art in NIR-II biomedical imaging and biosensing applications. In addition, challenges and opportunities for this kind of novel NIR-II nanoprobes are also discussed. Fluorescence-Based Bioimaging