Furthermore, the advantages of good photostability, light stability, and adjustable absorption properties indicate that organic materials, especially π-conjugated organic semiconducting polymers, are an excellent choice for NIR-II fluorescence imaging. In contrast, organic material-based PA imaging contrast agents have good biocompatibility and can effectively avoid the toxicity of heavy metal ions to organisms ( Jiang et al., 2019). Although such kinds of inorganic agents have good performance in extinction coefficient and light stability, they cannot overcome the potential biological toxicity caused by heavy metal ions and metabolic problems. PA imaging contrast agents in the NIR-II window mainly focus on inorganic materials, including metallic nanoparticles, quantum dots, carbon materials, and rare-earth nanoparticles ( Cheng et al., 2020). PA imaging contrast agents in the NIR-II window have a relatively weak extinction ability and can improve SNR by reducing background fluorescence and photoacoustic signals. Due to obvious advantages in penetration depth and signal-to-noise ratio (SNR), PA imaging in the NIR-II window (1,000–1700 nm) has aroused increasing interest among clinicians and biomedical researchers. Depending on the wavelength, the near-infrared range includes the first (NIR-I, 650–950 nm) and the second near-infrared wavelength ranges (NIR-II, 1,000–1700 nm). Compared with ultraviolet and visible light, near-infrared light has relatively weaker interaction force in biological tissues, which is more conducive to clinical diagnosis ( Lyu et al., 2019). The excitation light source is an important factor affecting PA imaging. Therefore, PA imaging not only has the advantage of the sensitive light absorption in contrast to an optical method but also has the advantage of small acoustic scattering similar to an acoustic method, and exhibits better spatial resolution and imaging depth than traditional optical imaging ( Yin et al., 2021).

Finally, the generated ultrasonic signals are collected by a broadband ultrasonic transducer and converted into PA images. Second, the localized heat inside tissues undergoes transient thermoelastic expansion to generate ultrasonic waves. First, a safe non-ionizing laser pulse is used to irradiate the corresponding biological tissue, and the photon energy is converted into heat in a short time. The PA imaging process consists of three stages ( Hong et al., 2017). Photoacoustic (PA) imaging is a hybrid imaging technology based on light excitation and ultrasound detection, which is widely used in monitoring surgery, visualization of blood vessels, and early detection of disease biomarkers ( Zhen et al., 2021). The evolution process of BBT blocks provides a unique perspective for the design of high-performance NIR-II PA contrast agents.

This mini review summarizes the recent advances in the design strategy of BBT and its derivative-based semiconducting polymer nanoparticles for second near-infrared photoacoustic imaging. SPNs containing benzo(1,2- c 4,5- c′)bis(1,2,5)thiadiazole (BBT) units, as the most classic second near-infrared window (NIR-II, 1,000–1700 nm) PA contrast agents, can achieve light absorption in the NIR-II region, thereby effectively reducing light loss in biological tissues and improving imaging resolution. Semiconducting polymer nanoparticles (SPNs), as a new type of contrast agent, have the advantages of low toxicity, flexible structure adjustment, good photostability, and excellent photothermal conversion efficiency. It is still challenging to amplify their imaging ability and reduce the toxicity of inorganic materials and exogenous contrast agents. Photoacoustic (PA) imaging has received more and more attention on disease diagnosis and fundamental scientific research. 2Jiangxi Engineering Laboratory of Waterborne Coating, School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University, Nanchang, China.1Key Laboratory of Prevention and Treatment of Cardiovascular and Cerebrovascular Diseases, Ministry of Education, Key Laboratory of Biomaterials and Biofabrication in Tissue Engineering of Jiangxi Province, College of Medical Information Engineering, Gannan Medical University, Ganzhou, China.Xuelong Huang 1 Ning Lan 1 Yanfeng Zhang 1 Wei Zeng 1 Haifeng He 2* Xiuhong Liu 2*

Bbt online overview