Mention of arsenic poisoning usually brings to mind underhanded murder. However,
the danger of arsenic poisoning from contaminated drinking water is far greater.
Low concentrations of arsenic are found in nearly all soils and thus also in
ground water. About 140 million people worldwide possibly drink water that contains
arsenic concentrations above the WHO-recommended limit of 10 ppb (parts per
billion).
Researchers at Jackson State University (MS, USA) have now developed a new
approach for a rapid, easy, and highly sensitive arsenic test. As Paresh Chandra
Ray’s team reports in the journal Angewandte
Chemie, their method is based on the aggregation of gold nanoparticles,
and it selectively detects arsenic in drinking water down to concentrations
of 3 ppt (parts per trillion).
Countries like India, Bangladesh, and Thailand are primarily affected by ground
water with high arsenic concentrations. However, high concentrations of arsenic
have also been found in some areas of North and South America. Once detected,
the problem can fairly easily be addressed. Current analytical techniques are
time-consuming and require a series of enrichment steps.
The new process could now speed up and simplify arsenic analysis. The scientists
working with Ray have attached special organic molecules to the surfaces of
gold nanoparticles. These molecules act as “ligands” for arsenic,
meaning that they form a complex with it. Each arsenic ion can bind to three
ligands, which allows it to link together up to three gold particles. The higher
the arsenic concentration in the sample, the more strongly the gold particles
clump together and the number of bigger aggregates increases. The color of gold
nanoparticles in a liquid depends on their size. Whereas the arsenic-free gold
nanoparticles appear red, arsenic-induced aggregation causes the color to change
to blue. Concentrations down to 1 ppb can be detected with the naked eye by
means of the color change. Arsenic binds to the ligands much more strongly than
other metals; the researchers were able to increase this selectivity by attaching
three different ligands to the gold.
One very precise method for detecting minimal changes in particle size is dynamic
light scattering (DLS), in which laser light scattered by the particles is analyzed.
By using DLS, Ray and his co-workers were able to detect and quantify arsenic
concentrations as low as 3 ppt. In samples of well water from Bangladesh, the
team found 28 ppb arsenic; in water from taps in Jackson (Mississippi, USA)
they found 380 ppt.
More information: Paresh Chandra Ray, Use of Gold Nanoparticles in a Simple
Colorimetric and Ultrasensitive Dynamic Light Scattering Assay: Selective Detection
of Arsenic in Groundwater, Angewandte Chemie International Edition 2009, 48,
No. 51, 9668-9671, doi: 10.1002/anie.200903958
Posted November 25th, 2009
