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

Abstract

Optical imaging has played a pivotal role in biology and medicine, but it faces challenges of relatively low tissue penetration and poor signal-to-background ratio due to light scattering and tissue autofluorescence. To overcome these issues, second near-infrared fluorescence, self-luminescence, and photoacoustic imaging have recently emerged, which utilize an optical region with reduced light-tissue interactions, eliminate real-time light excitation, and detect acoustic signals with negligible attenuation, respectively. Because there are only a few endogenous molecules absorbing or emitting above the visible region, development of contrast agents is essential for those deep-tissue optical imaging modalities. Organic semiconducting agents with π-conjugated frameworks can be synthesized to meet different optical imaging requirements due to their easy chemical modification and legible structure-property relation. Herein, the deep-tissue optical imaging applications of organic semiconducting agents including small-molecule agents and nanoparticle derivatives are summarized. In particular, the molecular engineering and nanoformulation approaches to further improve the tissue penetration and detection sensitivity of these optical imaging modalities are highlighted. Finally, current challenges and potential opportunities in this emerging subfield of biomedical imaging are discussed.

Ingestible Contrast Agents for Gastrointestinal Imaging

“…Therefore, InFroMs represent a novel nanoplatform that can be used for multimodal imaging of the intestine . Beyond that, other novel imaging strategies are adding new possibilities to the field of theranostics and molecular imaging …”

Organic Semiconductors for Photothermal Therapy and Photoacoustic Imaging

“…NIR‐II fluorescence imaging mainly uses photons in the wavelength range of 1000–1700 nm. NIR‐II fluorescence imaging suffers less from light scattering than NIR‐I fluorescence imaging, together with low tissue autofluorescence, so it gives better signal‐to‐noise ratios and penetration depths …”

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

“…Current PA imaging and PTT are mainly conducted utilizing the near‐infrared (NIR) light ranging from 750 to 1000 nm, which is defined as the first NIR window (NIR‐I) with a relatively low interaction with tissues . However, the tissue penetration depth of NIR‐I light is only around 1 cm, which cannot fulfill the requirement of the translational study of PA imaging and PTT.…”

“…Very recently, a new optical window with longer wavelengths ranging from 1000 to 1700 nm is termed as the second NIR (NIR‐II) window, which exhibits further reduced interaction with tissues compared with the NIR‐I light . In details, the incident light energy is mainly diminished through three processes after penetrating tissues, including reflection, scattering, and absorption by endogenous absorbers ( Figure a) .…”