Researchers at Istanbul Technical University (ITU) have developed a molecular sensor that can detect extremely low concentrations of mercury through a visible colour change and near-infrared light emission.
Named NIR-2, the sensor responds to mercury(II) ions (Hg²⁺) in approximately 10 seconds, turning a yellow solution green while producing an intense near-infrared fluorescent signal. The technology could offer a simpler approach to mercury monitoring, including potential testing outside specialised laboratories.
The study, published in the journal Talanta, was conducted by researchers from Istanbul Technical University and Recep Tayyip Erdoğan University.
Selenium-Based Molecule Turns Mercury Detection Into a Visible Signal
Mercury contamination can be difficult to identify because even toxic trace amounts may not produce any visible change in water or food. Conventional detection methods generally require specialised laboratory equipment and trained personnel.
The NIR-2 sensor uses a selenium-based molecule that acts as a chemical switch. Under normal conditions, the molecule remains in a closed form and produces very little light.
When mercury(II) ions come into contact with the sensor, they interact strongly with selenium and trigger a structural change that opens the molecule. This reaction produces two detectable signals: a colour transition from yellow to green and intense near-infrared fluorescence.
The combination allows mercury detection through direct visual observation, camera-based analysis, or specialised fluorescence instruments. The near-infrared signal could also support cellular imaging.
“Our goal is not simply to detect smaller amounts of mercury(II) ions, but to make that detection easier to see and potentially easier to perform outside a specialized laboratory. A sensor that produces several readable signals gives us more options for how and where mercury could eventually be monitored,” said Prof. Ismail Yilmaz of ITU’s Department of Chemistry.
Sensor Detects Extremely Low Mercury Concentrations
The researchers evaluated NIR-2 using different analytical techniques to measure its sensitivity.
Using ultraviolet-visible spectroscopy, they detected mercury(II) ions at concentrations as low as 3.77 × 10⁻¹⁰ M. Fluorescence measurements achieved a detection limit of 4.54 × 10⁻⁹ M.
The sensor produced a response within approximately 10 seconds, with the resulting signal remaining stable for at least 20 minutes.
The team also investigated whether a smartphone camera could measure the colour change and estimate mercury concentrations without relying entirely on laboratory instruments.
Smartphone Camera Enables Mercury Measurement
For the smartphone-based experiment, researchers photographed samples using a Redmi Note 10S and analysed the red and green components of the resulting images.
As mercury concentrations increased, the sensor’s colour changed. By calculating the ratio between the red and green colour components, the researchers reduced the influence of variations in lighting and established a relationship between the camera readings and mercury levels.
The smartphone method detected Hg²⁺ concentrations ranging from 1.66 × 10⁻⁹ to 7.50 × 10⁻⁶ M. The researchers described this as the most sensitive smartphone-based mercury detection range reported at the time of the study.
For comparison, the US Environmental Protection Agency’s maximum contaminant level for inorganic mercury in drinking water is 2 parts per billion, equivalent to 0.002 mg/L. The smartphone method’s lower detection limit falls below this threshold under controlled experimental conditions.
Tests Conducted on Water, Food and Human Cells
The researchers also examined the sensor’s performance in samples beyond standard laboratory solutions.
Water samples included drinking water, tap water, and lake water. The team also tested strawberries, watermelon, and cherries, adding known quantities of mercury to assess the sensor’s ability to identify contamination.
Additional experiments explored visible colour responses in rice, corn, beans, and lentils. Although NIR-2 produced semi-quantitative readings in several food samples, its performance varied depending on the composition of each food.
The team further tested the sensor in three human lung cell lines. Cell viability remained above 90%, while fluorescence imaging confirmed that NIR-2 could produce a mercury-dependent signal inside the cells.
However, these results do not establish the sensor as a commercially available or ready-to-use consumer testing device.
Further Testing Needed Before Practical Deployment
Despite the promising results, NIR-2 still requires additional development before it can be used for routine environmental or food safety monitoring.
The experiments involved sample preparation and controlled lighting conditions, with many tests using samples deliberately spiked with known mercury concentrations. Further research will need to examine naturally contaminated samples, different environmental conditions, and practical field measurement systems.
The researchers also suggest that the selenium-based molecular design could provide a foundation for developing sensors capable of detecting other substances.
Rather than replacing specialised analytical laboratories, NIR-2 could eventually make sensitive mercury detection more accessible at locations where contamination needs to be investigated. Its combination of rapid response, visible colour change, smartphone compatibility, and near-infrared fluorescence offers several possible approaches for future monitoring applications.
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