Sometimes bigger isn’t better.
Researchers
at the U.S. Department of Energy’s Savannah River National Laboratory
have successfully shown that they can replace useful little particles of
monosodium titanate (MST) with even tinier nano-sized particles, making
them even more useful for a variety of applications.
MST
is an ion exchange material used to decontaminate radioactive and
industrial wastewater solutions, and has been shown to be an effective
way to deliver metals into living cells for some types of medical
treatment. Typically, MST, and a modified form known as mMST developed
by SRNL and Sandia National Laboratories, are in the form of fine
powders, spherically-shaped particles about 1 to 10 micrometers in
diameter (a micrometer is one-millionth of a meter).
“By
making each particle smaller,” says Dr. David Hobbs of SRNL, lead of
the research project, “you increase the amount of surface area, compared
to the overall volume of the particle. Since the particle surface is
where reactions take place, you’ve increased the MST’s working area.”
For
example, a 10-nm particle has a surface area-to-volume ratio that is
1,000 times that of a 10-micrometer particle. Thus, this project sought
to synthesize titanate materials that feature nano-scale particle sizes
(1 to 200 nm). After successfully synthesizing nanosize titanates, the
team investigated and found that the smaller particles do indeed
exhibit good ion exchange characteristics. They also serve as
photocatalysts for the decomposition of organic contaminants and are
effective platforms for the delivery of therapeutic metals.
Dr.
Hobbs and his partners in the project examined three methods of
producing nano-sized particles, resulting in three different shapes.
One is a sol-gel method, similar to the process used to produce
“normal” micron-sized MST particles, but using surfactants and dilute
concentrations of reactive chemicals to control particle size. This
method resulted in spherical particles about 100 to 150 nm in diameter.
A
second method started with typical micron-sized particles, then
delaminated and “unzipped” them to produce fibrous particles about 10 nm
in diameter and 100 to 150 nm long. The third method, which had been
previously reported in the scientific literature, was a hydrothermal
technique that produced nanotubes with a diameter of about 10 nm and
lengths of about 100 to 500 nm.
The
team had considerable expertise in working with MST, having previously
modified it with peroxide to form mMST, which exhibits enhanced
performance in removing certain contaminants from radioactive waste and
delivering metals for medical treatment. Nanosize MST produced by all
three methods was successfully converted to the peroxide-modified form. As with micron-sized titanates, the peroxide-modified nanosize
titanates exhibit a yellow color. The intensity of the yellow color
appeared less intense with the hydrothermally produced nanotubes,
suggesting the chemically resistant surface of the nanotubes may limit
conversion to mMST.
Testing
confirmed that the materials function as effective ion exchangers. For
example, the spherical nanoMST and nanotube samples and their respective
peroxide-modified forms remove strontium and actinides from alkaline
high-level waste radioactive waste. Under weakly acidic conditions, the
nanosize titanates and peroxotitanates removed more than 90% of 17
different metal ions.
The
“unzipped” titanates and their peroxide-modified forms proved to be
particularly good photocatalysts for the decomposition of organic
contaminants.
Screening
in vitro tests showed that both nano-size and micron-size
metal-exchanged titanates inhibit the growth of a number of oral cancer
and bacterial cell lines. The mechanism of inhibition is not known, but
preliminary scanning electron microscopy results suggest that the
titanates may be interacting directly with the wall of the nucleus to
deliver sufficient metal ion concentration to the cell nucleus to
inhibit cell replication.
In
addition to Dr. Hobbs, the team included M. C. Elvington, M. H. Tosten,
K. M. L. Taylor-Pashow of SRNL; J. Wataha of the University of
Washington; and M. D. Nyman of Sandia National Laboratories.
This
work was funded under SRNL’s Laboratory Directed Research &
Development program, which supports highly innovative and exploratory
research aligned with the Laboratory's priorities.
SRNL
is DOE’s applied research and development national laboratory at the
Savannah River Site. SRNL puts science to work to support DOE and the
nation in the areas of environmental stewardship, national security, and
clean energy. The management and operating contractor for SRS and SRNL
is Savannah River Nuclear Solutions, LLC.
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