NIR-II absorbing organic nanoagents for photoacoustic imaging and photothermal therapy

Near-infrared (NIR) absorbing materials hold great potential in biomedical applications, such as fluorescence imaging (FLI), photoacoustic imaging (PAI), photodynamic therapy (PDT), and photothermal therapy (PTT). Generally, these materials can be classified into two main categories based on their absorbing wavelengths: the first NIR (NIR-I) (~650–950 nm) absorbing materials and the second NIR (NIR-II) (~1000–1700 nm) absorbing materials. Due to the reduced absorption and scattering of NIR-II light in tissue compared to NIR-I light, NIR-II absorbing materials enable imaging and therapy with improved contrast and deepened penetration, which is in favor of practical applications. Various inorganic materials have been developed for NIR-II phototheranostics in recent years. However, the non-biodegradability and potential toxicity of these materials hinder their further clinical translation. Biocompatible organic materials with potential biodegradability as well as tailored optical property are thus more desired. In this review, we summarize the recent advances of NIR-II absorbing organic nanoagents (ONAs) based on small molecules (SMs) and conjugated polymers (CPs) for PAI and PTT and show our perspectives on future challenges and development of these materials.