Abstract
Photoacoustic imaging (PAI) is a fast growing deep-tissue imaging modality. However, light scattering and absorption in biological tissues limit imaging depth. Short near-infrared wavelengths (650 to 950nm) are widely used for PAI. Using longer near-infrared wavelengths reduces scattering. We demonstrate deep-tissue contrast-enhanced in vivo photoacoustic imaging at a wavelength of 1064nm. An ultranarrow bandgap semiconducting polymer poly (thienoisoindigo-alt-diketopyrrolopyrrole) (denoted as PIGD) is designed and demonstrated for imaging at 1064nm. By embedding colloidal nanoparticles (NPs) of PIGD in chicken-breast tissue, an imaging depth of ∼5 cm is achieved. Intravenous injection of PIGD NPs in living rats showed brain vascular images with ∼2 times higher contrast compared with the brain vascular images without any contrast agent. Thus, PIGD NPs as an NIR-II contrast agent opens new opportunities for both preclinical and clinical imaging of deep tissues with enhanced contrast.
Highlights
Photoacoustic tomography (PAT) is an emerging hybrid imaging modality that has found many demanding applications in both clinical and preclinical studies.[1,2,3,4,5] PAT combines high optical contrast and scalable ultrasound resolution in a single modality
In contrast to conventional cross-coupling techniques, such as Suzuki[43] coupling or Stille[44] coupling, which requires preactivation of C-H bonds, Direct arylation polymerization (DAP) technique enables the synthesis of semiconducting polymers in fewer steps
PIGD semiconducting polymer nanoparticles (SPNs) were prepared by a traditional method of nanoprecipitation.[45,46]
Summary
Photoacoustic tomography (PAT) is an emerging hybrid imaging modality that has found many demanding applications in both clinical and preclinical studies.[1,2,3,4,5] PAT combines high optical contrast and scalable ultrasound resolution in a single modality. At 1064-nm wavelengths more excitation energy ∼100 mJ∕cm can be used for imaging, as a result more light can reach deeper compared with the NIR-I window that allows only ∼30 mJ∕cm[2] at 800 nm This promising behavior motivated us to explore other semiconducting polymers with high absorption coefficient in the NIR-II region. Construct a myriad of narrow bandgap semiconducting polymers owing to their high structural planarity.[30,31,32] Most of them have shown outstanding performances in organic electronic devices, such as polymer solar cells[33,34] and field-effect transistors.[35,36] Very recently, we combined these two building blocks to construct a new semiconducting polymer PIGD, giving average molecular weight of 24.0 kDa and an optical bandgap as narrow as 0.8 eV.[37] The lower the optical bandgap of the SNPs, the longer wavelength of the light absorption by the SNPs. As a consequence, the deeper tissue-penetration depth is expected, which benefits PA imaging. The proof-of-concept application of PIGD NPs for NIR-II window PA imaging is demonstrated by imaging brain vasculatures in living rats
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