Optical fibre sensor solves arsenic contamination simply

A new cost-effective tool paves the way for household water quality monitoring, helping combat arsenic contamination.

Researchers have developed a new optical sensor that provides a simple way to detect extremely low levels of arsenic in water in real-time. The technology could enable household testing for arsenic, empowering individuals to monitor their own water quality.

Arsenic contamination is a serious environmental and public health challenge affecting millions worldwide. Natural geological processes release arsenic from rocks and soil into groundwater, and mining, industrial waste disposal, and using arsenic-based pesticides can exacerbate the problem.

“Consuming arsenic-contaminated water can lead to severe health conditions, including arsenic poisoning and cancers of the skin, lung, kidney and bladder,” said lead researcher Sunil Khijwania from the Indian Institute of Technology Guwahati. “By creating a sensitive, selective, reusable and cost-effective sensor, 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.”

In the Optica Publishing Group journal Applied Optics, the researchers describe their new sensor, which uses optical fibre and an optical phenomenon known as localized surface plasmon resonance. They used it to detect arsenic levels as low as 0.09 parts per billion (ppb), 111 times lower than the maximum permissible limit of 10 ppb established by the World Health Organization. When tested on real drinking water samples from diverse locations and conditions, the sensor also exhibited reliable performance.

“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,” said Khijwania. “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.”

A user-friendly yet accurate sensor

Although conventional spectroscopy methods for detecting arsenic are highly accurate and sensitive, they tend to require complex, bulky, expensive equipment that is time-consuming and complicated. To fill this critical gap, the researchers developed an optical fibre sensor with a low detection limit that is cost-effective and user-friendly enough for routine arsenic monitoring in drinking water.

To make the new sensor, the 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. Due to the evanescent wave created by total internal reflection, a portion of the light travelling through the core extends into the surrounding fibre cladding. By removing the cladding in a small section of the fibre, the evanescent wave is exposed to the environment.

As light travels through the optical fibre, the evanescent wave interacts with gold nanoparticles, triggering localized surface plasmon resonance—a phenomenon in which electrons on the nanoparticle surface collectively oscillate in response to specific light wavelengths. If arsenic is present, it will bind to the nanocomposite, causing a measurable shift in the surface plasmon resonance wavelength and enabling the accurate detection of trace arsenic in water.

Thorough performance assessment

The researchers tested the sensor using varying concentrations of arsenic ion solutions, finding that it produced consistent and reliable detection of arsenic across the tested concentration range. After additional optimization, they tested other parameters, showing that the sensor produced consistent results during both low-to-high and high-to-low changes in arsenic ion concentration. It achieved a fast response time of just 0.5 seconds.

The sensor exhibited a maximum resolution of ± 0.058 ppb of arsenic and showed negligible variations in results for samples with identical arsenic concentrations analyzed on four separate days over 18 days. The researchers also compared sensor measurements to those obtained with inductively coupled plasma mass spectrometry (ICP-MS), commonly used for arsenic measurements. The sensor showed a relative percentage difference of less than 5%, indicating substantial agreement between the two methods.

To evaluate the sensor’s real-world applicability, the researchers tested it on drinking water samples collected from different locations in Guwahati, India. The sensor maintained reliable performance under these varied conditions.

“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,” said Khijwania. “In the long term, this new approach could be modified to create a new wave of affordable and accessible environmental monitoring tools.”

The researchers note that although the sensor is ready for real-world use in detecting arsenic, a less expensive and easier-to-use optical source and detector would need to be developed to enable widespread application.

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