The simple answer to this question is any particle less than 100 nm. But like most things in particle technology a more thorough discussion is required to achieve an unambiguous and complete response. The experts from the ISO and ASTM standards shown below provide additional nuances to the definition. The current agreement among the standards groups is that the scale from 1 – 100 nm defines the size range of a nanoparticle. Below 1 nm may be excluded in order to avoid calling clusters of atoms a particle, but the literature contains references to particles < 1 nm. Since particles are three dimensional the ASTM standard defines two or three dimensions must be between 1 – 100 nm. This provides for nanotubes with a diameter of 10 nm, but a length > 100 nm.
From ISO/TS 27687¹
The document lists various terms and definitions related to particles in the area of nanotechnologies. The definition described in the document for the nanoscale is:
Nanoscale: Size range from approximately from 1 nm to 100 nm.
This definition is accompanied by two notes:
Note 1: Properties that are not extrapolations from a larger size will typically, but not exclusively, be exhibited in this size range. For such properties the size limits are considered approximate.
Note 2: The lower limit in this definition (approximately 1 nm) is introduced to avoid single and small groups of atoms from being designated as nano-objects or elements of nanostructures, which might be implied by the absence of a lower limit.
From ASTM E2456-06²
This ASTM standard defines several terms including nanparticle, fine particle, ultrafine particle, and many others. The ASTM definition is also essentially 1-100nm, but mentions the number of dimensions and carries a note entitled "Discussion":
nanoparticle, n—in nanotechnology, a sub-classification of ultrafine particle with lengths in two or three dimensions greater than 0.001 micrometer (1 nanometer) and smaller than about 0.1 micrometer (100 nanometers) and which may or may not exhibit a size-related intensive property.
DISCUSSION—This term is a subject of controversy regarding the size range and the presence of a size-related property. Current usage emphasizes size and not properties in the definition. The length scale may be a hydrodynamic diameter or a geometric length appropriate to the intended use of the nanoparticle.
The European community has discussed the topic and issued a document Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) that offers a more complex approach. This document subdivides nanoparticles into three categories as explained below.
Category 1: size > 500 nm
If the size (e.g. mean, median, etc.) of the material is above 500 nm it is assumed that the size distribution at the lower end will most likely be above the designated lower threshold of 100 nm. This should be confirmed by determination of the size distribution. So, the need for further evaluation regarding possible nano-specific properties may be of lower priority and thus for the moment classical risk assessment should be performed taking into consideration the particulate nature of the material.
Category 2: 500 nm> size >100 nm
When the size is <500 nm it is more likely that part of the size distribution will be lower than 100 nm and that a material may be considered a nanomaterial and that therefore a more detailed characterization and nano-specific risk assessment will be necessary.
A nano-specific risk assessment should be undertaken if the characterization demonstrates that >0.15% (or any specified percentage) of the number size distribution is <100 nm. If these characteristics are not met, the need for further evaluation regarding possible nano-specific properties may be of lower priority and thus for the moment classical risk assessment should be performed taking into consideration the particulate nature of the material.
Category 3: 100 nm> size >1 nm