Review of Long-Wavelength Optical and NIR Imaging Materials: Contrast Agents, Fluorophores, and Multifunctional Nano Carriers

TLDR

Long‑wavelength and near‑infrared (650–1450 nm) imaging is increasingly important for whole‑animal and deep‑tissue studies because this spectral window has minimal tissue absorption, enabling deep penetration and is supported by growing availability of targeted biological agents. The review focuses on imaging agents operating within the 650–1450 nm wavelength range. The authors survey a spectrum of imaging agents—including aqueous and insoluble, organic and inorganic, unimolecular and supramolecular constructs—and discuss how nanocarriers or supramolecular assemblies enable multimodal imaging. Nanoparticle‑based diagnostics offer longer circulation and brighter imaging than single‑molecule agents, driving rapid advances in NIR nanocarrier development.

Abstract

The importance of long wavelength and near infra-red (NIR) imaging has dramatically increased due to the desire to perform whole animal and deep tissue imaging. The adoption of NIR imaging is also growing rapidly due to the availability of targeted biological agents for diagnosis and basic medical research that can be imaged in vivo. The wavelength range of 650-1450 nm falls in the region of the spectrum with the lowest absorption in tissue and therefore enables the deepest tissue penetration. This is the wavelength range we focus on with this review. To operate effectively the imaging agents must both be excited and must emit in this long-wavelength window. We review the agents used both for imaging by absorption, scattering, and excitation (such as fluorescence). Imaging agents comprise both aqueous soluble and insoluble species, both organic and inorganic, and unimolecular and supramolecular constructs. The interest in multi-modal imaging, which involves delivery of actives, targeting, and imaging, requires nanocarriers or supramolecular assemblies. Nanoparticles for diagnostics also have advantages in increasing circulation time and increased imaging brightness relative to single molecule imaging agents. This has led to rapid advances in nanocarriers for long-wavelength, NIR imaging.