New sensor makes invisible traces of mercury easier to see

Istanbul, Türkiye – Mercury contamination presents an obvious challenge: it is invisible to a glass of water or a piece of fruit can seem perfectly normal to the naked eye, even when trace amounts of toxic mercury are present. Detecting such minute levels usually requires specialized laboratory tools and trained experts. Researchers at Istanbul Technical University (ITU) have created a molecule that provides an alternative method to detect mercury. When mercury(II) ions are present, the sensor changes color and starts emitting near-infrared light, transforming an invisible contaminant into a measurable signal.

Named NIR-2, the sensor responded to mercury(II) ions in about 10 seconds and could identify very low concentrations of the metal ions. The team also demonstrated that its color change could be measured using a smartphone camera, offering a potential way to perform simpler mercury testing outside specialized labs. The study was published in Talanta by researchers from ITU and Recep Tayyip Erdoğan University.

Making mercury turn on the light

The new sensor functions like a tiny warning light that remains off until mercury(II) ion is detected. At its core, NIR-2 contains a selenium-based molecule. Normally, this molecule is in a closed form and emits minimal light. Mercury(II) ion, however, strongly attracts selenium. When mercury(II) ion contacts the sensor, it pulls on the molecular lock, causing the structure to open. This small chemical change results in a noticeable change: the solution transitions from yellow to green and emits intense near-infrared light. Essentially, mercury(II) triggers the alarm. What sets NIR-2 apart is its design around this selenium-based molecular switch. The same reaction produces both a visible color change and a near-infrared fluorescent signal. The sensor offers rapid response, high sensitivity, and multiple methods for reading results. The color change can be seen directly or captured with a camera, while the near-infrared signal can be detected using lab instruments and used for 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 the Department of Chemistry at Istanbul Technical University.

From laboratory instruments to a phone camera

Using UV–visible spectroscopy, the researchers detected mercury(II) ions at concentrations as low as 3.77 × 10⁻¹⁰ M, while fluorescence measurements reached a detection limit of 4.54 × 10⁻⁹ M. The response was observed within approximately 10 seconds and remained stable for at least 20 minutes. They then explored whether a smartphone could turn this visible color change into a quantitative measurement.

As mercury(II) ion levels increased, the sensor's color shifted. Researchers photographed the samples with a Redmi Note 10S and analyzed the red and green color components. By comparing the ratio of these colors, they minimized the impact of lighting variations and related the data to mercury(II) ion concentrations. The smartphone method detected Hg²⁺ from 1.66 × 10⁻⁹ to 7.50 × 10⁻⁶ M, which the authors consider the most sensitive smartphone-based Hg²⁺ detection range reported so far. For context, the U.S. EPA's maximum contaminant level for inorganic mercury in drinking water is 2 parts per billion, or 0.002 mg/L. The lower detection limit of the smartphone approach falls below this threshold, demonstrating high sensitivity in controlled experimental conditions.

Testing water, food and living cells

The researchers also examined whether NIR-2 could perform in more complex samples beyond simple laboratory solutions. They tested drinking water, tap water, lake water, as well as strawberries, watermelon, and cherries. These samples were intentionally spiked with known mercury levels to assess the sensor’s accuracy in detecting those amounts.

Additionally, the team investigated whether visible color changes could quickly indicate mercury contamination in rice, corn, beans, and lentils. While the sensor produced semi-quantitative color responses in several of these foods, its effectiveness varied with different food matrices. NIR-2 was also tested in three human lung cell lines, where cell viability stayed above 90%, and fluorescence imaging confirmed that the sensor could generate a mercury-dependent signal inside cells. However, these findings do not yet establish NIR-2 as a ready-to-use consumer mercury detector.

Sample preparation was still necessary, measurements were taken under controlled lighting conditions, and most experiments involved samples with known mercury amounts added by researchers. Further testing with naturally contaminated samples, diverse environmental conditions, and field measurement systems is required before it can be practically deployed.

Bringing mercury detection closer to where contamination happens

The long-term impact of this research may be less about replacing analytical labs and more about bringing sensitive chemistry closer to the locations where contamination must be monitored. Currently, detecting tiny mercury amounts involves sampling, transporting to a lab, and analyzing with specialized equipment. A molecular sensor that reacts swiftly, within seconds, and provides multiple readable signals, ranging from a visual color change to near-infrared fluorescence, could bridge this gap.

The researchers also propose that this molecular design could serve as a foundation for creating sensors for other substances. A glass of water with trace mercury levels might appear identical to a clean one, but NIR-2 demonstrates how molecular chemistry can change that, by producing a detectable signal without making the mercury itself visible, enabling measurement and potential use outside the lab.