New dyes improve bioimaging in mice

Fluorescent dyes with emission wavelengths in the near-infrared region (NIR) can enable tissue imaging from deeper within the sample than dyes with shorter emission wavelengths. But dyes with emission wavelengths in the so-called NIR-II region, 1,000–1,700 nm, are often not as bright as researchers would like. Wen Shi and coworkers at the Chinese Academy of Sciences now report a family of xanthene-based dyes that fluoresce strongly in the NIR-II (J. Am. Chem. Soc. 2021, DOI: 10.1021/jacs.1c07711). These dyes, which the researchers call VIXs, have a xanthene core that is modified with para-substituted styryls. VIXs with nitrogen-bearing styryls fluoresce at longer wavelengths than VIXs with oxygen-bearing styryls. VIX-4, which fluoresces at wavelengths longer than 1,200 nm, is modified with julolidinestyryl. To demonstrate the dyes’ performance, the researchers used VIX-4 encapsulated in liposomes to image blood circulation in mice. They injected VIX-4 into the tail vein of mice and were able to watch the circulatory system light up as the dye progressed through the body. The dye was bright enough that clear images could be obtained with exposure times as low as 5 ms. The researchers were able to calculate blood-flow velocities and achieved spatial resolution that was good enough to differentiate between closely spaced femoral arteries and veins.