Near-Infrared Fluorescent Nanomaterials for Bioimaging and Sensing

A great challenge in noninvasive biomedical imaging is the acquisition of images inside a biological system at the cellular level. Common modalities used today such as magnetic resonance or computed tomography imaging have the advantage that any part of a living organism can be imaged at any depth, but are limited to millimeter resolution and can usually not be employed e.g., for surgical guidance. Optical imaging techniques offer resolution on the 100 nanometer scale, but are limited by the strong attenuation of visible light by biological matter and are traditionally used to image on the surface. Near-infrared light in the biological windows can penetrate much deeper into biological samples, rendering fluorescence-based imaging a viable alternative. In the past two decades, many fluorescent nanomaterials have been developed to operate in the near infrared, yet only few materials emitting above 1000 nm exist and none are approved for clinical use. This review describes recent advances in the development and use of near-infrared fluorescent nanomaterials for biomedical imaging and sensing applications. The physical and chemical properties as well as the bioconjugation and application of materials such as organic fluorophores, semiconductor quantum dots, carbon-based materials, rare earth materials, and polymer particles are discussed.