Nanomaterials in Water: Detection and Characterization

Introduction. The manufacturing of nanomaterials (MNMs) and their incorporation in consumer products, such as cosmetics, pharmaceuticals, textiles, food, packaging, and many other sectors of our everyday life has created the necessity for methods of detection tailored to monitor these materials. The application of analytical techniques for the detection and characterization of nanomaterials is termed nanometrology and is critical for the development of new MNMs, quality control in production lines, development and enforcement of regulations, and environmental monitoring. Each of these applications calls for different nanometrology strategies, due to differences in nanomaterial concentrations and the composition of the surrounding matrix. This article focuses on environmental monitoring in aqueous systems, whose main characteristics are the low concentrations of nanomaterials and the complexity of the water matrix.

MNMs are expected to be released into surface waters gradually, following the use and degradation of nano-enabled products. This gradual release, combined with the inevitable dilution into large and mobile waters results in extremely low concentrations of these materials, when compared to natural suspended particulate matter (SPM). Natural biogeochemical activity produces various organic and inorganic products, such as minerals, dissolved organic matter, extracellular polymeric substances, etc., whose composition and morphology may exhibit significant variations depending on the location and season. Despite the very low concentration of MNMs compared to their natural analogues in SPM, both natural and manufactured nanomaterials are likely to play a critical role in the environment, as they may act as transport vectors of toxic compounds and pathogens and play a key role in bio-geochemical reactions of metals.

Furthermore, as nanotechnology continues to penetrate industrial and consumer products, the concentrations of MNMs in the environment will inevitably rise. Having developed the tools for detection with high sensitivity in terms of concentration, will allow tracking MNMs to their source, identifying locations were they accumulate, and understanding their role and impact in the environment. It is, therefore, critical to develop methods for detecting these materials in the natural environment, at the early stage of nano-enabled products development.

Although several nanometrology techniques are available for analyzing aqueous nanomaterial suspensions, this article offers an overview on those techniques that are currently applicable in the context of environmental monitoring. Inevitably, techniques that have been proven to work with artificial samples of nanomaterials (especially X-ray based techniques), but have not yet been applied to environmental samples are not presented here. Each of these techniques requires a dedicated sample preparation protocol, which is addressed in another article of this issue. Here, we focus on the technological characteristics and analytical handles that may allow for the identification and characterization of MNMs in natural water systems, which inevitably also applies to naturally occurring nanomaterials. A more detailed description is given for those techniques that are more promising for application in environmental samples.

The main challenge of environmental nanometrology is detecting an often small number of MNMs in samples that contain a multitude of natural particles and organic compounds. In comparison to MNMs, natural particles in water systems have been studied for decades. Due to the expectedly high ratio of natural to manufactured nanoparticles, techniques capable to differentiate between the two are particularly useful for environmental nanometrology. Unfortunately, there is not a single universal method that can be used to quantify and characterize nanoparticles in the aquatic environment. However, there are several analytical approaches available with different degrees of effectiveness to characterize nanoparticles in complex systems and in many cases, the application of complementary techniques can produce the information needed.