Semiconducting Polymer Nanoparticles for Centimeters‐Deep Photoacoustic Imaging in the Second Near‐Infrared Window

Abstract

Thienoisoindigo-based semiconducting polymer with a strong near-infrared absorbance is synthesized and its water-dispersed nanoparticles (TSPNs) are investigated as a contrast agent for photoacoustic (PA) imaging in the second near-infrared (NIR-II) window (1000-1350 nm). The TSPNs generate a strong PA signal in the NIR-II optical window, where background signals from endogenous contrast agents, including blood and lipid, are at the local minima. By embedding a TSPN-containing tube in chicken-breast tissue, an imaging depth of more than 5 cm at 1064 nm excitation is achieved with a contrast-agent concentration as low as 40 µg mL . The TSPNs under the skin or in the tumor are clearly visualized at 1100 and 1300 nm, with negligible interference from the tissue background. TSPN as a PA contrast in the NIR-II window opens new opportunities for biomedical imaging of deep tissues with improved contrast.

Organic Semiconducting Agents for Deep‐Tissue Molecular Imaging: Second Near‐Infrared Fluorescence, Self‐Luminescence, and Photoacoustics

“…Bioluminescence relies on the enzymatic reaction to generate photons, and thus involves the enzyme and substrate, while chemiluminescence is produced by chemical reactions between substrates and often reactive species . Different from bioluminescence and chemiluminescence triggered by addition of a substrate into the system, afterglow imaging requires preirradiation of light to generate the chemical or energy defects and detects photons released from those defects after cessation of light irradiation (Table ) …”

“…Although it had been well validated that fluorescence imaging in the NIR‐II window outperformed that in the NIR‐I window, it was unclear if shifting PA imaging from the NIR‐I to the NIR‐II window could further improve the imaging performance. Due to the lack of NIR‐II absorbing contrast agents, PA imaging was often carried out in the NIR‐I window mainly …”

Second Near‐Infrared Absorbing Agents for Photoacoustic Imaging and Photothermal Therapy

“…The rational design of molecular structures to increase the absorption at the designated region has been reported to enhance PA signals of contrast agents. Mei and co‐workers adopted a planar backbone in the polymer design to improve the light harvest capability of a thienoisoindigo‐based polymer (SP3, Figure ) . Such a SP3‐formed SPN (SPN3) showed a strong absorption at 1200 nm with the mass extinction coefficient of 35 ± 5 mL mg −1 cm −1 , which was comparable with that of NIR dyes and diketopyrrolopyrrole‐based polymers in the NIR‐I window.…”

Polymer Dots as Effective Phototheranostic Agents

“…(e) In vivo PA/US imaging of TSPN in mouse tumor (dashed circles) without injection of TSPNs (upper) and with injection of TSPNs (lower). Reproduced with permission from ref . Copyright 2017 Wiley‐VCH Verlag GmbH & Co. KGaA.…”