Semiconducting polymer nanoparticles containing ether chain substituted bithiophene as a twisted donor unit for effective NIR-II photoacoustic imaging

Conjugated polymers with absorption in the second near-infrared (NIR-II) window are efficient materials for photoacoustic (PA) imaging by reducing the light interaction with tissues. The development of conjugated polymers with full spectral coverage of the NIR-II window is an effective way to achieve contrast agents with a high signal-to-noise ratio and tissue penetration depth. Herein, the twisted alkoxy-substituted bithiophene (TT) units are used as strong electron-donating units to construct PTT-DTBBT and PTT-DTPSe to yield NIR-II polymers. Results show that (i) PTT-DTBBT exhibits a strong absorption peak in the range of 1000–1700 nm, while the absorption of PTT-DTPSe basically does not extend into the NIR-II window; (ii) the TT-based polymer nanoparticles present good stability and biocompatibility, which is suitable for in vivo PA imaging application; (iii) using PTT-DTBBT nanoparticles as PA agent, the signal-to-noise ratio of in vivo PA imaging for mouse is significantly higher under NIR-II irradiation than that under NIR-I irradiation. Therefore, this work reveals that, accompanied with the outstanding advantages of low-cost and full NIR-II coverage, using ether chain substituted bithiophene as a twisted donor unit to construct NIR-II conjugated polymers is an effective strategy for developing NIR-II photoacoustic agents for in vivo bioimaging.