In vivo fluorescence imaging in the near-infrared region between 1500–1700 nm (NIR-IIb window) affords high spatial resolution, deep-tissue penetration, and diminished auto-fluorescence due to the suppressed scattering of long-wavelength photons and large fluorophore Stokes shifts. However, very few NIR-IIb fluorescent probes exist currently. Here, we report the synthesis of a down-conversion luminescent rare-earth nanocrystal with cerium doping (Er/Ce co-doped NaYbF4 nanocrystal core with an inert NaYF4 shell). Ce doping is found to suppress the up-conversion pathway while boosting down-conversion by ~9-fold to produce bright 1550 nm luminescence under 980 nm excitation. Optimization of the inert shell coating surrounding the core and hydrophilic surface functionalization minimize the luminescence quenching effect by water. The resulting biocompatible, bright 1550 nm emitting nanoparticles enable fast in vivo imaging of blood vasculature in the mouse brain and hindlimb in the NIR-IIb window with short exposure time of 20 ms for rare-earth based probes.

A new design for second near-infrared window (NIR-II) molecular fluorophores based on a shielding unit-donor-acceptor-donor-shielding unit (S-D-A-D-S) structure is reported. With 3,4-ethylenedioxy thiophene as the donor and fluorene as the shielding unit, the best performance fluorophores IR-FE and IR-FEP exhibit an emission quantum yield of 31% in toluene and 2.0% in water, respectively, representing the brightest organic dyes in NIR-II region reported so far.

The NIR-IIb (1500-1700 nm) window is ideal for deep-tissue optical imaging in mammals, but lacks bright and biocompatible probes. Here, we developed biocompatible cubic-phase (α-phase) erbium-based rare-earth nanoparticles (ErNPs) exhibiting bright downconversion luminescence at ~ 1600 nm for dynamic imaging of cancer immune-therapy in mice. We used ErNPs functionalized with cross-linked hydrophilic polymer layers attached to anti-PD-L1 antibody for molecular imaging of PD-L1 in a mouse model of colon cancer and achieved tumor to normal tissue signal ratios of ~ 40. The long luminescence lifetime of ErNPs (~ 4.6 ms) enabled simultaneous imaging of ErNPs and lead sulfide quantum dots (PbS QDs) emitting in the same ~ 1600 nm window. In vivo NIR-IIb molecular imaging of PD-L1 and CD8 revealed cytotoxic T *

Organic fluorophores have been widely used for biological imaging in the visible and the first near-infrared windows. However, their application in the second near-infrared window (NIR-II, 1000-1700 nm) is still limited mainly due to low fluorescence quantum yields (QYs). Here, we explore molecular engineering on the donor unit to develop high performance NIR-II fluorophores. The fluorophores are constructed by a shielding unit-donor(s)-acceptor-donor(s)-shielding unit structure. Thiophene is introduced as the second donor connected to the shielding unit, which can increase the conjugation length and red-shift the fluorescence emission. Alkyl thiophene is employed as the first donor connected to the acceptor unit. The bulky and hydrophobic alkyl thiophene donor affords larger distortion of the conjugated backbone and fewer interactions with water molecules compared to other donor units studied before. The molecular fluorophore IR-FTAP with octyl thiophene as the first donor and thiophene as the second donor exhibits fluorescence emission peaked at 1048 nm with a QY of 5.3% in aqueous solutions, one of the highest for molecular NIR-II fluorophore reported so far. Superior temporal and spatial resolutions have been demonstrated with IR-FTAP fluorophore for NIR-II imaging of the blood vessels of a mouse hindlimb.