Optical fibre detects arsenic contamination

Researchers at the Indian Institute of Technology Guwahati, India, have developed a novel tool that can detect arsenic in drinking water. The optical fibre sensor provides a simple method of real-time detection of extremely low levels of the toxic heavy metal.

Why is arsenic detection necessary?

Arsenic is a naturally occurring heavy metal that is found in the earth's crust. It is distributed throughout the environment and can be found in the air and water, as well as on land.

According to the World Health Organization, arsenic poses a serious health risk, because:

• It is naturally present at high levels in the groundwater of several countries
• It is highly toxic in its inorganic form
• Contaminated water used for drinking, food preparation and irrigation of food crops poses the greatest threat to public health from arsenic
• Long-term exposure to arsenic from drinking-water and food can cause cancer and skin lesions. It has also been associated with cardiovascular disease and diabetes. In utero and early childhood exposure has been linked to negative impacts on cognitive development and increased deaths in young adults
• The most important action in affected communities is the prevention of further exposure to arsenic by provision of a safe water supply.

In a 2022 report, the WHO stated that 'an estimated 140 million people in at least 70 countries have been drinking water containing arsenic at levels above the WHO provisional guideline value', and that as many as '220 million people are at risk of exposure to elevated arsenic concentrations in groundwater'.

The countries most exposed to arsenic include the USA and China, but others are located in Central America, South America and Asia, including India, Pakistan and Bangladesh.

Contamination of water occurs when natural geological processes release the arsenic from rocks and soil into groundwater. This process can be accelerated or brought on by industrial processes such as mining, industrial waste disposal and use of arsenic-based pesticides.

Research published in Applied Optics

The research undertaken by Professor Sunil Khijwania and PhD student Fatima Banoo at the Department of Physics, Indian Institute of Technology Guwahati was published in the Applied Optics journal under the title, Localized Surface Plasmon Resonance based Novel Optical Fiber Arsenic Ion Sensor Employing Al2O3/GO Nanocomposite.

The editors of the journal labelled the breakthrough, a 'new cost-effective tool paves the way for household water quality monitoring, helping combat arsenic contamination'. The technology has the potential to be used at the household level for personal water quality monitoring.

Lead researcher Sunil Khijwania told media: "Consuming arsenic-contaminated water can lead to severe health conditions including arsenic poisoning and cancers of the skin, lung, kidney and bladder. By creating a sensor that is sensitive, selective, reusable and cost-effective, we aim to address the need for a reliable and user-friendly tool for routine monitoring, helping to protect communities from the risks of arsenic exposure."

Sensors are being developed for home use that can detect a number of contaminants in drinking water, such as PFAS.

What can the sensor do?

The sensor uses an optical fibre and an optical phenomenon known as localised surface plasmon resonance. It has been used to detect arsenic levels as low as 0.09 parts per billion (ppb), which is 111 times lower than the maximum permissible limit of 10 ppb established by the World Health Organization.

Crucially, the sensor performed reliably when tested on real drinking water samples from diverse locations and conditions.

Khijwania added: "The highly sensitive sensor provides analysis within just 0.5 seconds and demonstrates a high degree of reusability, repeatability, stability and reliability, making it a powerful tool for monitoring and ensuring safer water quality. In the future, this technology could make it much easier for people to check whether their drinking water is safe, potentially saving lives by preventing exposure to harmful arsenic levels."

How does the sensor detect minute arsenic levels?

Arsenic can be detected using conventional spectroscopy methods, and while these are accurate and sensitive, they require expensive equipment that is time-consuming and complicated to use, prohibiting use within most home environments.

Khijwania and his team developed an optical fibre sensor that not only has a low detection limit but is also cost-effective and user-friendly enough for routine arsenic monitoring in drinking water.

Researchers coated the inside core of a fibre with gold nanoparticles and a thin layer of a unique nanocomposite composed of aluminium oxide and graphene oxide, which selectively binds to arsenic ions.

A portion of the light traveling through the core also extends into the surrounding fibre cladding due to the evanescent wave created by total internal reflection. By removing the cladding in a small section of the fibre, this wave is exposed to the environment.

As light travels through the optical fibre, the evanescent wave interacts with gold nanoparticles, triggering localised surface plasmon resonance. The researchers describe this as 'a phenomenon where electrons on the nanoparticle surface collectively oscillate in response to specific light wavelengths'.

Crucially, if arsenic is present, it will bind to the nanocomposite, causing a measurable shift in the surface plasmon resonance wavelength and enabling accurate detection of trace arsenic in water.

Developing a home-use application for the sensor

Real-world testing was performed on drinking water samples collected from different locations around the city of Guwahati. While the tests proved that the sensor was ready for real-world use, the researchers noted that wider adoption of the technology would only truly be possible with an application that was cheaper and easier to use in the home environment.

Khijwania added: "These investigations established that the proposed optical fibre sensor offers a highly sensitive, selective, fast, cost-effective, straightforward and easy solution for arsenic detection in real field conditions. In the long-term, this new approach could potentially be modified to create a new wave of affordable and accessible environmental monitoring tools."