The 100 nanometer (nm) upper limit for defining nanoparticles may be “potentially inappropriate” and does not reflect the “complexity of ecological interactions” with nanoparticles, according to a viewpoint article [PDF] published in Environmental Sciences and Technology and highlighted by ChemicalWatch. According to a team of Chinese and American scientists, threshold sizes for nanoparticles likely exist and thus, critical sizes for environmental impacts are also likely to exist and exceed, or at least differ from, the 100 nm definition.
Studies show that nanoparticle threshold sizes vary with composition and shape, probably due to “changes in excess surface energy and crystallography of [nanoparticles] as size decreases.” Threshold size ranges have been established for nanoparticles based on physicochemical properties including particle dissolution, contaminant adsorption, and suspension stability, for example. However, further research is required to determine threshold sizes for complex biological effects like accumulation and toxicity, as nanoparticles may have size-specific mechanisms for transformation, reactive oxygen species production, etc.
The scientists point out that if threshold sizes for nanoparticles’ environmental impacts exist, changes in the effects could not be predicted by larger particle sizes, even under the 100 nm definition. Thus, to better assess and minimize environmental risks from nanoparticles, the size and functionalization of nanoparticles in development should be designed appropriately if that nanoparticle is determined to have substantially increased ecotoxicological effects below a certain threshold.
The scientists call for more research “to assess the potential for threshold sizes for environmental impacts and to assess if [nanoparticles] larger than the threshold sizes have different impacts than bulk particles of the same composition.” The results of such research could better inform the current size definition of nanoparticles and affect methods for modeling environmental impacts and risks.