An acidic medium-compatible deep-near-infrared dye for in vivo imaging

In vivo imaging in the deep near-infrared (NIR) spectral region, that is, beyond 800 nm, has become popular due to its penetration depth. While imaging of the neutral medium/tissue has been repeatedly showcased, imaging of the high-acidic medium remains challenging partly because of the high-lying HOMO orbital and hence a high pK_a of the electron-donating group of the NIR fluorophores. We devised a novel electron-donating group (D6) with which we further synthesized ECJ. ECJ exhibits an absorption wavelength beyond 900 nm and is fluorescent. Its pK_a was found to be lower than zero, rendering it suitable for bioimaging of a highly-acidic medium. Its potential for practical applications was showcased in proof-of-concept in vivo imaging with a mouse model.