Bright Near-Infrared π-Conjugated Oligomer Nanoparticles for Deep-Brain Three-Photon Microscopy Excited at the 1700 nm Window .

The development of three-photon fluorophores with 1700 nm excitation is pressingly desirable for imaging of tissue resided deep inside the brain. Herein, we report a designed and synthesized fluorescent molecule () for mouse brain imaging with three-photon microscopy at a record imaging depth. The molecule has a relatively high fluorescence brightness and has a near-infrared (NIR) maximum emission at 820 nm after integrating as water-dispersible nanoparticles ( NPs). Under 1720 nm excitation, NPs show a large three-photon action cross-section of 1.06 × 10 cm s/photon, which is more than twice that of the commonly used sulforhodamine 101 (SR101) dye. Benefiting from the high tissue penetration depths for both the long excitation in the second NIR window of 1720 nm and the emission wavelength in the first NIR window of 820 nm, a high brightness, and a large action cross-section of three-photon, NPs have good deep-brain imaging performance. Brain vasculatures of a mouse located at a depth of 1696 μm can be clearly resolved . With no observable cytotoxicity even in a high concentration, the present NPs suggest that fluorescent π-conjugated oligomers are of great potential in high-resolution 3PM imaging of deep-tissue.