Ultra-high sensitivity fibre-optic refractometer for environmental and medical applications

For several decades, industrialization has significantly contributed to water pollution through the release of heavy metals. Factories and industrial plants discharge toxic heavy metals into rivers and lakes, posing serious risks to ecosystems and human health. The World Health Organization listed Cadmium (Cd) as one of the 10 chemicals of primary public concern due to its long biological half-life and toxicity. Excessive Cd exposure can lead to severe health problems such as hypertension, cardiovascular and cerebrovascular diseases, and an increased risk of various cancers. Detecting trace amounts of cadmium (Cd) in drinking water is crucial for ensuring public health and environmental safety, as even low concentrations can have profound health implications. Under the European Union (drinking water) regulations 2023, the maximum allowable concentration of dissolved cadmium in drinking water by the European Union is 5 ppb.
Conventional methods for detecting cadmium (Cd) ions in drinking water, such as atomic absorption spectroscopy and inductively coupled plasma mass spectrometry, often suffer from drawbacks like high cost, complexity, and long analysis times, making them unsuitable for rapid, on-site monitoring. Additionally, some methods require extensive sample preparation and can be limited by low sensitivity or poor resistance to interference. Fibre optic chemical sensors offer significant advantages due to their high sensitivity, immunity to electromagnetic interference, and capability for remote and multiplexed sensing over long distances.
This project’s primary objective was the development of a novel fibre optic sensor based on a hollow-core whispering gallery mode (WGM) optical resonator for detecting Cd ions in water in trace concentrations, and its proof-of-concept demonstration in laboratory conditions.
In the proposed sensing structure, a thin-walled silica capillary, whose inner walls are functionalized with gold nanoparticles, acts as the sensor head, which has the potential to be integrated with microfluidics.

Industrial activities have polluted water by releasing harmful heavy metals like cadmium (Cd), which poses serious health risks even at low levels. The European Union limits cadmium in drinking water to 5 parts per billion (ppb). Traditional detection methods are expensive and complicated, making quick testing difficult.

This project aims to develop a new fiber optic sensor using a hollow-core whispering gallery mode (WGM) optical resonator to quickly and sensitively detect low levels of cadmium in water. The sensor features a thin tube coated with gold nanoparticles and can potentially work with microfluidic systems.

The main outcomes of the project are:
1 A finite element simulation model for analysis of the performance of WGM cavities coated with plasmonic nanoparticles.
2. Fabrication and full characterisation of the proposed sensor, capable of detecting Cd ions with a limit of detection of 0.00000112 ppb.
3. The cross-sensitivity of the sensor was evaluated by infiltrating other interfering ions through the sensor. The observed responses confirm the selectivity of the developed sensor for Cd.