Academic journal article Environmental Health Perspectives

Toward Advancing Nano-Object Count Metrology: A Best Practice Framework

Academic journal article Environmental Health Perspectives

Toward Advancing Nano-Object Count Metrology: A Best Practice Framework

Article excerpt

Introduction

The unique properties of nanomaterials that make them attractive for a plethora of applications, including their use in microelectronics, catalysts, composite materials, and biotechnologies, also invoke concerns for equally unique human and environmental risks associated with the use of these materials. It is not surprising that numerous governing bodies and policy makers around the world have invoked, or are considering invoking, definitions that specify what constitutes a nanomaterial for regulatory purposes. A concern within the chemical industry is that several of these definitions precede the current measurement science and concessions regarding the strict interpretation of the definition in addition to technological advancements are needed to enable practical metrology.

Our intent here is not to review the global state of definitions and policies, but rather to highlight current technology and knowledge gaps that are hindering advances in this area and to propose a tiered approach for moving forward. The European Commission (EC)recommended definition of a nanomaterial (EC 2011a) is used as an important case example to highlight pertinent issues and challenges with regard to practical application of nano-object count-based metrics for categorizing materials as "nano" or "not nano."

The EC-adopted Definition of a Nanomaterial

The definition. On 18 October 2011, the EC recommended that nanomaterial be defined as comprising "natural, incidental or manufactured materials containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of particles in the number size distribution, one or more external dimensions is in the size range 1 nm-100 nm," where particles are defined as minute pieces of material with defined physical boundaries, an aggregate as a body of two or more particles that are strongly bound or fused together, and an agglomerate as a body of two or more particles that are weakly bound together by physical interactions (e.g., van der Waal forces) (EC 2011a). The application of volume-specific surface area (VSSA) [see Supplemental Material, "Volumetric Specific Surface Area (VSSA)--A Surface Area Approach," p. 3] was also acknowledged as an agglomerate-tolerant proxy (Kreyling et al. 2010) to identify potential materials; however, number size distributions are to prevail (EC 2011a, 2011b).

The EC-adopted definition refines the International Organization for Standardization (ISO) definition of nanomaterialas exclusively applicable to materials consisting essentially of hard particles [solid nano-objects, defined as a material with one, two, or three external dimensions in the nanoscale (ISO 2010)], excluding solvated and self-assembled soft particles such as proteins and micelles as well as macroscopic nanostructured materials.

The EC-adopted definition is an attempt to create a uniform interpretation for identifying nanomaterials using particle size as the only metric, and it is specifically intended to classify a material as a "nanomaterial" for legisl ative and policy purposes in the European Union. The Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR; http://ec.europa.eu/ health/scientific_committees/emerging/) has clearly expressed that classification as a nanomaterial does not imply that the material has a specific risk or new hazardous properties (EC 2011a, 2011b). Thus, the EC decided against a risk-based nanodefinition (Auffan et al. 2009) that would have addressed a much smaller number of materials based on other properties in addition to their size (Maynard 2011).

Challenges and implications. The EC-adopted definition poses multiple challenges in the area of particle metrology (Appendix 1). Given the current state of nano-object metrology, any given technique [including electron microscopy (see Supplemental Material, "Scanning Electron Microscopy (SEM)" and "Transmission Electron Microscopy (TEM)," pp. …

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