As a result of a major inter-laboratory study, the standards body ASTM International
has been able to update its guidelines for a commonly used technique for measuring
the size of nanoparticles in solutions. The study, which was organized principally
by researchers from the National
Institute of Standards and Technology (NIST) and the Nanotechnology Characterization
Laboratory of the National Cancer Institute, enabled updated guidelines that
now include statistically evaluated data on the measurement precisions achieved
by a wide variety of laboratories applying the ASTM guide.
Data from the inter-laboratory comparison gathered from 26 different laboratories
will provide a valuable benchmark for labs measuring the sizes and size distribution
of nanoparticles suspended in fluids-one of the key measurements in nanotechnology
research, especially for biological applications, according to materials researcher
Vince Hackley, who led the NIST portion of the study.
Size is an important characteristic of nanoparticles in a variety of potential
uses, but particularly in biotech applications where they are being studied
for possible use in cancer therapies. The size of a nanoparticle can significantly
affect how cells respond to it. (See, for example "Study:
Cells Selectively Absorb Short Nanotubes," NIST Tech Beat, March 30,
2007.)
One widely used method for rapidly measuring the size profile of nanoparticles
in, say, a buffer solution, is photon correlation spectroscopy (PCS), sometimes
called "dynamic light scattering." The technique is powerful but
tricky. The basic idea is to pass a laser beam through the solution and then
to measure how rapidly the scattered light is fluctuating-faster moving
particles cause the light scattering to change more rapidly than slower moving
particles. If you know that, plus several basic parameters such as the viscosity
and temperature of the fluid, says Hackley, and you can control a number of
potential sources of error, then you can calculate meaningful size values for
the particles.
ASTM standard E2490 is a guide for doing just that. The goal of the ASTM-sponsored
study was to evaluate just how well a typical lab could expect to measure particle
size following the guide. "The study really assesses, in a sense, how
well people can apply these techniques given a fairly well-defined protocol
and a well-defined material," explains Hackley. Having a "well-defined
material" was a key factor, and one thing that made the experiment possible
was the release this past year of NIST's first nanoparticle reference
standards for the biomedical research community-NIST-certified solutions
of gold nanoparticles of three different diameters, a project also supported
by NCL. (See "NIST
Reference Materials Are 'Gold Standard' for Bio-Nanotech Research,
" NIST Tech Beat, Jan. 8, 2008.)
The inter-laboratory study required participating labs to measure particle
size distribution in five samples-the three NIST reference materials and
two solutions of dendrimers, a class of organic molecules that can be synthesized
within a very narrow size range. The labs used not only PCS, but also electron
and atomic force microscopy. The results were factored into precision and bias
tables that are now a part of the ASTM standard.
For more on the study and ASTM standard E2490, see the ASTM International release
"Extensive
Interlaboratory Study Incorporated into Revision of ASTM Nanotechnology Standard."