A good example of the potential application of the work, says NIST
biomedical engineer Justin Zook, is in the development of nanoparticle
biosensors for ultrasensitive pregnancy tests. Gold nanoparticles
can be coated with antibodies to a hormone produced by an embryo
shortly after conception. Multiple gold nanoparticles can bind to each
hormone, forming clusters that have a different color from unclustered
gold nanoparticles. But only certain size clusters are optimal for this
measurement, so knowing how light absorbance changes with cluster size makes it easier to design the biosensors to result in just the right sized clusters.
Clusters of roughly 30-nanometer gold nanoparticles imaged by transmission electron microscopy.
The NIST team first prepared samples of gold nanoparticles—a
nanomaterial widely used in biology—in a standard cell culture solution,
using their previously developed technique for creating samples with a
controlled distribution of sizes. The particles are allowed to
agglomerate in gradually growing clusters and the clumping process is
"turned off" after varying lengths of time by adding a stabilizing agent
that prevents further agglomeration.
They then used a technique called analytical ultracentrifugation
(AUC) to simultaneously sort the clusters by size and measure their
light absorption. The centrifuge causes the nanoparticle clusters to
separate by size, the smaller, lighter clusters moving more slowly than
the larger ones. While this is happening, the sample containers are
repeatedly scanned with light and the amount of light passing through
the sample for each color or frequency is recorded. The larger the
cluster, the more light is absorbed by lower frequencies. Measuring the
absorption by frequency across the sample containers allows the
researchers both to watch the gradual separation of cluster sizes and to
correlate absorbed frequencies with specific cluster sizes.
Most previous measurements of absorption spectra for solutions of
nanoparticles were able only to measure the bulk spectra—the absorption
of all the different cluster sizes mixed together. AUC makes it possible
to measure the quantity and distribution of each nanoparticle cluster
without being confounded by other components in complex biological
mixtures, such as proteins. The technique previously had been used only
to make these measurements for single nanoparticles in solution. The
NIST researchers are the first to show that the procedure also works for
nanoparticle clusters.
From physorg
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But only certain size clusters are optimal for this measurement, so knowing how light absorbance changes with cluster size makes it easier to design the biosensors to result in just the right sized clusters. Nanoparticle Characterization Techniques
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