EPA-Expo-Box (A Toolbox for Exposure Assessors)
A defining characteristic of nanomaterials is their size; nanomaterials are typically defined as particles or other substances that are less than 100 nanometers in at least one dimension.
The small size of nanomaterials influences the way that these substances behave in the environment because of quantum effects and surface-to-volume ratios. Two important aspects of the behavior and characteristics of small particles that affect exposures are described here.
- Physics of Small Particles. At the nano scale, "quantum effects" dictate the physical and chemical properties of the material, and these properties can be very different than those of the non-nanoscale-sized material. Quantum mechanics describe the physics behind particle and atom interactions.
- An example of quantum mechanics resulting in unique characteristics of nanomaterials is nanoscale gold. Unlike non-nanoscale-sized gold, which interacts with light to create the radiant yellow color it is prized for, nanoscale gold appears red or purple. The nanoscale size translates to unique optical properties because the movement of the gold electrons are restricted, and therefore interactions with light change.
- Other unique properties that can result from quantum effects include enhanced reactivity, improved catalysis, tunable wavelength sensing ability, increased mechanical strength, electrical conductivity, and stronger oxidative and reductive potential.
- Surface-to-Volume Ratio. In addition to quantum mechanics, a high surface-to-volume ratio influences nanomaterial behavior, resulting in a proportionally greater amount of the material at the "surface" to interact with the surrounding environment. (The image below illustrates this concept.) Transport, deposition, and uptake mechanisms which rely on surface interactions and size factors will therefore differ compared to larger compounds. The surface of a nanomaterial is also important because many engineered nanomaterials can be “functionalized”, meaning they are altered in a way that changes reactivity with other molecules or interaction with the environment—for example, through the addition of reactive surface structures such as amine groups or carboxyl groups.
In addition to the physics of small size particles and high surface-to-volume ratio of nanomaterials, it is also important to consider the following when characterizing exposure:
- Shape of the nanoscale material
- Surface area and surface-to-volume and/or surface-to-mass ratios of individual particles
- Distribution and agglomeration (bundling) of individual nanoscale particles
- Surface functionalization (i.e., presence of other substances bound to the surface of a nanoparticle) and the resulting changes in both chemistry and surface topography
- Impurities within a solution
- State (solid, liquid, or gas) and dispersion/suspension in media
- Hydrophobicity and solubility
- Dissolution kinetics