Detecting heavy metals in soil and water: New method for on-site analysis

Not all arsenic is the same

Arsenic is a dangerous heavy metal that can be found in soil and water. In addition to the quantity, the toxicity depends heavily on the chemical form in which the arsenic is present. Researchers must therefore distinguish precisely whether it is arsenic(III) or arsenic(V). "These two forms behave completely differently in the environment and also have different effects on health. Until now, however, measurements at this high level have been very expensive and complicated," explains Prof. Dr. Thomas Zentgraf from the Department of Physics at Paderborn University.

Currently, the so-called SERS method (SERS = "Surface-Enhanced Raman Scattering") is generally used to detect traces of arsenic. This is a highly sensitive analytical technique that combines conventional Raman spectroscopy with nanostructured metal surfaces. This amplifies the Raman signal of the molecules a million-fold, so that even the smallest traces of chemical or biological substances can be clearly identified. However, the method is preceded by complex manufacturing processes that often require special machines and expensive materials. In addition, sensors often have to be treated with chemicals in order to work reliably. "The evaluation of the data also often requires powerful computers and expensive measuring devices that cannot simply be used on site," adds Prof. Zentgraf.

Fast, inexpensive and can be used on site

The researchers have developed a new solution. They use a special structure, the "hole-sphere nanogap platform". They use gold nanoparticles that arrange themselves independently on a gold surface. They then treat the surface with heat and lightly etch it. This step replaces complicated lithography, i.e. structuring by means of exposure to light. The result is a platform that works extremely reliably. The researchers have shown that the measurement results hardly fluctuate. The method amplifies the light signal by a factor of 100 million. This means that even the smallest amounts of arsenic are clearly visible. As the structure is made entirely of metal, no other signals from the material interfere with the measurement. This makes the results very reliable.

The biggest advantage of the new method is its simplicity. It does not require any expensive machines or special chemicals. The platform works reliably even if the measuring devices are not of the highest precision. For example, the researchers have shown that they can even detect the arsenic variants using simple filters or smartphones. This makes the method ideal for use in the field, e.g. on construction sites or in agriculture.