Imaging metal oxide nanoparticles in biological structures with CARS microscopy

Metal oxide nanomaterials are being used for an increasing number of commercial applications, such as fillers, opacifiers, catalysts, semiconductors, cosmetics, microelectronics, and as drug delivery vehicles. The effects of these nanoparticles on the physiology of animals and in the environment are largely unknown and their potential associated health risks are currently a topic of hot debate. Information regarding the entry route of nanoparticles into exposed organisms and their subsequent localization within tissues and cells in the body are essential for understanding their biological impact. However, there is currently no imaging modality available that can simultaneously image these nanoparticles and the surrounding tissues without disturbing the biological structure. Due to their large nonlinear optical susceptibilities, which are enhanced by two-photon electronic resonance, metal oxides are efficient sources of coherent anti-Stokes Raman Scattering (CARS). We show that CARS microscopy can provide localization of metal oxide nanoparticles within biological structures at the cellular level. Nanoparticles of 20 - 70 nm in size were imaged within the fish gill; a structure that is a primary site of pollutant uptake into fish from the aquatic environment.