Image: Clusters of roughly 30-nanometer gold nanoparticles imaged by transmission electron microscopy (Photo courtesy of NIST).

Extremely accurate pregnancy tests could be created by precisely measuring specific nanoparticle cluster-size agglomeration and the resulting characteristic light absorption.

Researchers at the US National Institute of Standards and Technology (NIST; Gaithersburg, MD, USA) developed the new technique by coating gold nanoparticles with antibodies to human chorionic gonadotropin (hCG), a hormone produced by an embryo following conception. The researchers first prepared samples of gold nanoparticles with a controlled distribution of sizes; they were then allowed to agglomerate in gradually growing clusters. The clumping process was then deactivated after varying lengths of time by adding a stabilizing agent, preventing further agglomeration.

Since only certain size clusters are optimal for this technique, analytical ultracentrifugation (AUC) was used to simultaneously sort the clusters by size and measure their light absorption. AUC 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 frequency is recorded; the larger the cluster, the more light is absorbed by lower frequencies. Measuring the absorption by frequency across the sample containers allowed the researchers both to watch the gradual separation of cluster sizes and to correlate absorbed frequencies with specific cluster sizes. The study was published on September 3, 2011, in ACS Nano.

“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,” said lead author biomedical engineer Justin Zook, PhD, of the NIST. “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.”
 

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