Golden arsenic assessment

spectroscopyNOW has discussed previously the insidious environmental disaster of arsenic-contaminated drinking water on the Indian subcontinent is topic I've covered a couple of times previously.

Now, a new light-scattering technique that uses gold nanoparticles can selectively detect arsenic ions, without interference from dissolved alkali and alkali earth metals, improving on water tests based on Raman and SPR. UV-Vis spectroscopy was used in conjunction with tunnelling electron microscopy to characterise the nanoparticles themselves.

Across the Indian subcontinent, in India, Bangladesh, West Bengal, there are village drinking water wells heavily contaminated with soluble arsenic salts. Years of denial and political wrangling has led to the unsustainable position in which government officials condemn safe wells and leave affected sites unlabelled and hundreds of thousands if not millions of people are being slowly poisoned for want of a safe water supply.

The causes of the problem are manifold but put simply, bedrock that would normally covered with water becomes exposed either through natural causes or through agricultural demands for irrigation water. This leads to the oxidation of natural, but insoluble, arsenic salts in the rock, which then dissolve in the water when the rains return, filling wells and leaving villagers with no choice but to drink the "hellish water".

The issue is not, however, limited to the Indian sub-continent. An estimated 140 million people worldwide from Thailand to Arizona may be drinking water containing arsenic at concentrations above the recommended limit of 10 parts per billion advised by the World Health Organisation.

Finding a quick and easy way to test drinking water for arsenic is key to addressing the problem. Current analytical techniques are time-consuming and require a series of sample preparation and enrichment steps.

Paresh Chandra Ray and colleagues at Jackson State University, in Mississippi, USA, have now developed a new approach for a rapid, simple, and highly sensitive arsenic test. They report details in the latest issue of the journal Angewandte Chemie.

The novel method is based on the formation of gold nanoparticles aggregates in the presence of arsenic. The test can selectively detect arsenic ions in drinking water at concentrations as low as 3 parts per trillion (ppt) even in the presence of common metal ions.

Ray's technique involves attaching organic ligand molecules, glutathione, dithiothreitol, or cysteine, to the surface of the gold nanoparticles. These molecules chelate only arsenic ions to form a complex with each arsenic ion binding to a possible three ligands on different nanoparticles and hooking them together. The higher the arsenic concentration in the sample, the more strongly the gold particles clump together and the number of bigger aggregates increases.

By way of detection, it is the colour of gold nanoparticles suspended in a liquid that acts as the indicator and this depends on the size of the aggregates. Arsenic-free gold nanoparticles are red, whereas the arsenic-induced aggregates cause a colour to change to blue.

The team points out that concentrations as low as 1 ppb can be seen with the naked eye based on this obvious colour change. However, for lower concentrations a spectrophotometric approach to the analysis might be needed. However, the researchers explain that a very precise method for detecting minimal changes in particle size is dynamic light scattering (DLS). In this approach laser light is scattered by the particles and analysed to reveal the size of the aggregates, which is correlated with arsenic concentration in the sample.

By using DLS, Ray and his colleagues pushed the detection limit for their test from 1 ppb down to 3 ppt. They point out that tests on well water in Bangladesh have revealed arsenic concentrations as high as 28 ppb arsenic, which is well within in the visible test range, and obviously almost three times the WHO's safe limit. In contrast, tap water in the team's hometown of Jackson contains 380 ppt of dissolved arsenic, which is well below the safety limit.