New nanochemistry analytical tool based on quartz crystal
03 Dec 2010
A new chemical analysis technique developed by a research
group at the US National Institute of Standards and Technology (NIST)
uses the shifting ultrasonic pitch of a small quartz crystal to test the
purity of only a few micrograms of material.
Since it works with samples close to a thousand times smaller than
comparable commercial instruments, the new technique should be an
important addition to the growing arsenal of measurement tools for
nanotechnology, according to the NIST team.
As the objects of scientific research have become smaller — eg in
nanotechnology and gene therapy — scientists have been applying
considerable ingenuity to develop comparable instrumentation. This
new NIST technique is based on thermogravimetric analysis (TGA).
In TGA, a sample of material is heated, very slowly and
carefully, and changes in its mass are measured as the temperature
increases. The technique measures the reaction energy needed to
decompose, oxidize, dehydrate, or otherwise chemically change the
sample with heat.
TGA can be used, for example, to characterize complex biofuel
mixtures because the various components vaporize at different
temperatures. The purity of an organic sample can be tested by the
shape of a TGA plot because, again, different components will break
down or vaporize at different temperatures. Conventional TGA,
however, requires samples of several milligrams or more of material,
which makes it hard to measure very small, laboratory-scale powder
samples — such as nanoparticles — or very small surface chemistry
features such as thin films.
What's needed is an extremely sensitive microbalance to measure
the minute changes in mass. The NIST group found one in the quartz
crystal microbalance. Essentially it is a small piezoelectric disk
of quartz sandwiched between two electrodes. An alternating current
across the electrodes causes the crystal to vibrate at a stable and
precise ultrasonic frequency — the same principle as a quartz
crystal watch. Added mass (a microsample) lowers the resonant
frequency, which climbs back up as the microsample is heated and
A NIST researcher prepares quartz crystal
microbalance disks with samples of carbon nanotubes for for
microscale thermogravimetric analysis. Typical sample sizes are
about 2 microliters, or about 1 microgram.
In a new paper, the NIST materials science group demonstrates
that their microbalance TGA produces essentially the same results as
a conventional TGA instrument, but with samples about a thousand
times smaller . They can detect not only the characteristic
curves for carbon black, aluminium oxide and a sample organic fluid,
but also the more complex curves of mixtures.
"We started this work because we wanted to analyze the purity of
small carbon nanotube samples," explains analytical chemist
Elisabeth Mansfield. More recently, she says, they've applied the
technique to measuring the organic surface coatings biologists put
on gold nanoparticles to modify them for particular applications.
"Measuring how much material coats the particles surface is very
hard to do right now," she says, "It will be a really unique
application for this technique."
The prototype apparatus requires that the frequency measurements
be made in a separate step from the heating. Currently, the team is
at work integrating the microbalance disks with a heating element to
enable the process to be simultaneous.
1. E. Mansfield, A. Kar, T.P. Quinn and S.A. Hooker. Quartz
crystal microbalances for microscale thermogravimetric analysis.
Anal. Chem. Article ASAP, published online Nov. 16, 2010. DOI: