Nanoparticles in the environment as revealed by transmission electron microscopy: Detection, characterisation and activities

The characterisation of natural aquatic nanoparticles (especially in relation to flocculation processes, contaminant transport and biogeochemistry) has become an important field of environmental science. Ubiquitous colloid-size microbes and their nanoscale extracellular components affect the chemistry and physical properties of their surroundings in all habitable environments on Earth, thus affecting fundamentally the planet's geochemical systems. The adverse health effects of airborne particles, and the atmospheric deposition of particulate contaminants into surface waters, are well recognised environmental issues, with serious questions being posed about the biomedical effects of the nanoparticle component. There is a growing public health concern about nanoparticles in general, as a result of biomedical findings which reveal that atmospheric nanoparticles can present unanticipated toxicity and mechanisms for entering biological cells. The evolving analytical needs, issues, concerns and new facts call for improved means to detect and characterise environmental nanoparticles. Transmission electron microscopy (TEM) is making a major contribution. With foci on aquatic and airborne examples, this review presents literature highlighting nanoparticle relevance to environmental and public health. Common "species" of nanoparticles are described, while characterisation by TEM is considered in terms of apparatus, artifact minimisation and standard protocols for isolation and concentration. Evolving correlative microscopical approaches to characterisation are outlined, along with successful case studies involving heterogeneous environmental samples. Diverse activities of aquatic nanoparticles are featured, with reference to planetary-scale biogeochemical processes and water treatment. Informed speculation is presented on upcoming improvements to nanoparticle characterisation.