Broadband Absorbing Semiconducting Polymer Nanoparticles for Photoacoustic Imaging in Second Near-Infrared Window

TLDR

Photoacoustic imaging is promising, but most work uses the first near‑infrared window (650–950 nm) and few exploit the second window (1000–1700 nm) because suitable contrast agents are lacking. The authors synthesize semiconducting polymer nanoparticles (SPN‑II) that absorb across both NIR‑I and NIR‑II and use them for PA imaging in the second window. SPN‑II’s broadband absorption allows direct comparison of PA signals at 750 nm and 1064 nm, demonstrating feasibility of dual‑wavelength imaging. At 3 cm depth, PA images at 1064 nm show a 1.4‑fold higher SNR than at 750 nm, and in vivo rat brain imaging yields a 1.5‑fold SNR improvement, establishing SPN‑II as the first broadband organic contrast agent and highlighting NIR‑II’s superiority over NIR‑I.

Abstract

Photoacoustic (PA) imaging holds great promise for preclinical research and clinical practice. However, most studies rely on the laser wavelength in the first near-infrared (NIR) window (NIR-I, 650–950 nm), while few studies have been exploited in the second NIR window (NIR-II, 1000–1700 nm), mainly due to the lack of NIR-II absorbing contrast agents. We herein report the synthesis of a broadband absorbing PA contrast agent based on semiconducting polymer nanoparticles (SPN-II) and apply it for PA imaging in NIR-II window. SPN-II can absorb in both NIR-I and NIR-II regions, providing the feasibility to directly compare PA imaging at 750 nm with that at 1064 nm. Because of the weaker background PA signals from biological tissues in NIR-II window, the signal-to-noise ratio (SNR) of SPN-II resulted PA images at 1064 nm can be 1.4-times higher than that at 750 nm when comparing at the imaging depth of 3 cm. The proof-of-concept application of NIR-II PA imaging is demonstrated in in vivo imaging of brain vasculature in living rats, which showed 1.5-times higher SNR as compared with NIR-I PA imaging. Our study not only introduces the first broadband absorbing organic contrast agent that is applicable for PA imaging in both NIR-I and NIR-II windows but also reveals the advantages of NIR-II over NIR-I in PA imaging.