Tracing engineered nanomaterials in biological tissues using coherent anti-Stokes Raman scattering (CARS) microscopy - A critical review.

Nanomaterials (NMs) are used in an extremely diverse range of products and are increasingly entering the environment, driving a need to better understand their potential health effects in both humans and wildlife. A major challenge in nanoparticle (eco)toxicology is the ability to localise NMs post exposure, to enable more targeted biological effects analyses. A range of imaging techniques have been applied to do so, but they are limited, requiring either extensive processing of the material, staining or use of high intensity illumination that can lead to photo damage and/or have limited tissue penetration. Coherent anti-Stokes Raman scattering (CARS) microscopy is a label-free imaging technique, providing contrast based on the intrinsic molecular vibrations of a specimen, circumventing the need for chemical perturbation by exogenous labels. CARS uses near infra-red excitation wavelengths which allow microscopy at depths of several hundred microns in intact tissues and minimises photo-damage to live and delicate samples. Here we provide an overview of the CARS process and present a series of illustrative examples demonstrating its application for detecting NMs within biological tissues, ranging from isolated cells to whole organisms and including materials spanning metals to polymers. We highlight the advantages of this technique which include chemically selective live imaging and substantial depth penetration, but we also discuss its limitations when applied to nanotoxicology, which most notably include the lack of resolution for studies on single nanoparticles.