(Left) A sample of a silver nanostructured film, deposited by the electroless technique on a glass plate with dimensions 2.5 x 7.5 cm. (right) A zoom in on the silver nanostructured electroless film at 1.5 µm x 1.5 µm by atomic force microscopy. Credit: Dominique Vouagner
Arsenic is a known cause of groundwater contamination in more than 100 countries, posing serious health risks, especially in developing countries. According to World Health Organization (WHO) recommendations, arsenic concentrations in drinking water should not exceed 10 ppb, requiring methods with low limits of detection such as induced coupled plasma-mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS), which are expensive, time-consuming and require sample pre-treatment. A team of French researchers, including scientists from the Centre national de la recherche scientifique (CNRS), have proposed a new sensitive method for trace arsenic detection based on surface-enhanced Raman spectroscopy (SERS), which could decrease the costs and increase the speed of arsenic testing in water, food and soil.
The team created two silver-coated SERS substrates for the detection of different arsenic species: one substrate was prepared through conventional thermal evaporation and the other was produced through an electroless process, in which the glass substrate slides were submerged in a series of reagents to facilitate deposition of a silver layer through a chemical reaction. The researchers found that the substrate produced through the electroless reaction was far more sensitive than the one fabricated through thermal evaporation, and that the electroless process was relatively simple, using safe starting compounds with low environmental toxicity, according to corresponding author Dominique Vouagner, of the Institut Matériaux Microélectronique Nanoscience de Provence (IM2NP) at the Université de Toulon.
The SERS substrates were used to test various concentrations of arsenic diluted in an aqueous solution, with concentrations ranging from 0.1 g/L (100 ppm) to 1 μg/L (1 ppb). Two species of arsenic, As(III) and As(V), were tested; differentiating between the species is important, as concentrations of the two tend to differ based on factors such as origin, seasonal rainfall and the course of the contaminated water. For example, As(III) is more common in groundwater, and As(III) is also more reactive and toxic than As(V). The researchers found that the substrate produced through thermal evaporation could detect As(III) down to 5 ppm but was unable to detect As(V). On the other hand, the electroless substrate could detect both types of arsenic down to just 1 ppb. This research was published in the Journal of Applied Physics.
“Because they’re less ‘noisy,’ optical detection systems are much more sensitive than electronic systems. At the same time, they’re less sensitive to parasitic electromagnetic fields,” noted co-author Bernard Dussardier, of the Institut de Physique de Nice at the Université Côte d’Azur and CNRS. “Also, the SERS technique allows direct physical-chemical property measurements, whereas electronic systems, and some other optical systems, are indirect.”
The use of SERS for sensitive arsenic detection could reduce the cost of testing, and because the method does not require sample pretreatment, it could be used for faster on-site field testing rather than necessitating time-consuming laboratory testing.